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why is contrast media used in CT?

because radiographic contrast of structures may be difficult to differentiate (similar appearance, HU values ex windowing)


role of tech with contrast administration

special knowledge and application of:

  • patient assessment
  • preparation
  • administration of contrast media
  • precautions


contrast media

An agent that temporarily enhances differences between anatomical structures


  • Viewing
  • Localization
  • Differentiation


Types of contrast media

Main forms:

  • Liquid (most common in CT)
  • Gas/ air
  • Paste/Powder (barium, add liquid)


  • Dependant on attenuation value of enhanced structure




Classifications of contrast media

  • Negative contrast appears black or dark

Example: Air & Gases (CO2)

  • Positive contrast appears white or light

Example: Iodinated Solutions & Some Barium Sulfate Solutions (IV injections)

  • Double contrast studies can use a combination of contrast agents to help distinguish between structures on a CT scan


Selection Criteria for contrast media

The use of contrast material in any given study is determined by evaluating a variety of factors:

Radiographic appearance

  • Necessary for diagnosis


  • Equipment limitations
  • Type of contrast agent


  • Contraindications
  • Condition

Administration Route

  • Example: Barium can only be used for GI tract imaging

********* NEVER inject barium into veins


types of contrast media used in CT

  • air
  • carbon dioxide (preferred over air)
  • water
  • barium sulfate solutions (formulated for CT usage)
  • iodinated water-soluble solutions (most commonly used in CT imaging)


why air and gases are used in CT

air and gases:

  • are non-toxic and provide unobstructed visualization of anatomical structures
  • distends GI tract (colon and stomach) for improved pathology detection
  • readily absorbed by the body

advantage of carbon dioxide:

  • insufflation
  • absorption
  • patient comfort


usage and administration of air and gas

air and gas can be used for the following exams:

  • virtual colonoscopies
  • GI studies
  • arthrograms
  • myelograms

air and gases can be administered:

  • via injection
  • orally
  • rectally


why water is used in CT


  • patients will not present with allergies, and it is cheap and easily accessible
  • waters low density will not impede 3D reformatting


  • poor bowel distention
  • rapid transit time


usage and administration of water

water can be used for the following exams:

  • evaluation of the pancreas
  • GI studies
  • gastric neoplasms

water is typically administered:

  • orally


why barium sulfate is used in CT:

Barium sulfate solutions:

  • specially formulated radiopaque agents use 1 to 3% concentration for CT usage
  • 2 forms can be used to improve anatomic visualization:
  1. liquid: low concentration and viscosity
  2. paste: high viscosity for esophageal studies
  • can cling to bowel walls
  • ex: Readi-cat


  • can cause streaks
  • needs to be diluted
  • allergies can occur
  • poor mucosal evaluation
  • complications
  • contraindications



"unique" CT barium sulfate solution:

  • low HU (classifications can vary)
  • 0.1% barium sulfate concentration
  • resembles water

advantages over water:

  • better bowel distension
  • slower transit time
  • improved bowel wall and mucosa visualization



usage and administration of barium sulfate solutions

can be used for:

  • GI studies

Barium solutions are typically administered:

  • orally (more common)
  • rectally


why iodinated media is used in CT

iodinated water-soluble solutions:

  • provide differential enhancement, increasing visualization of a variety of structures
  • there are many non-ionic forms that can be used (osmolality and viscosity is variable)
  • not metabolized


  • differential enhancement
  • allergic-like reactions
  • poor mucosal coating
  • contraindicatioons
  • diarrhea


usage and administration of iondinated contrast media

iodinated contrast media can be used for:

  • GI studies
  • arthrograms
  • postmyelograms
  • vascular and arterial studies
  • solid organ contrast enhancement

Iodinated agents can be administered:

  • orally
  • rectally
  • via injection
  • IV


iodinated oral contrast media

  • oral conrast media is used for CT scans investigating pathology in the GI tract
  • most commonly Telebrix, Lactulose, or Gastrografin are used
  • Normal responses to iodinated oral contrast can include the onset of diarrhea either immediately while ingesting the oral contrast or post-procedure


injected and IV iodinated contrast media

  • IV contrast is the most common administration method in CT
  • IV contrast is used when imaging the arterial or venous circulation system as well as the solid organs of the chest, abdomen or pelvis
  • Optiray, Visipaque or Omnipaque will most commonly be used for IV and injection administration methods


administration of contrast media

contrast media can be introduced into the body in the following ways:

  • IV (veins and arteries)
  • Oral (by mouth or nasogastric tube)
  • rectal (via specialized catheters)
  • injection (intra-articular or intrathecal)

there are pros and cons to each method


rectal administration

  • one some occasions, rectal contrast is required for CT scans
  • the contrast is administered using an enema tip that is attached to a catheter
  • only useful for large bowel visualization
  • typically all contrast media except water will be administered rectally


injection administration

  • useful for post-arthrogram and intrathecal imaging
  • the contrast is administered outside of the CT department using a needle

-injection of contrast into a joint or space surrounding the spinal cord is accomplished under fluoroscopic guidance

  • CT imaging provides visualization of the soft tissue and bony anatomy
  • special considerations must be followed for post myelogram CT imaging
  • barium and water cannot be administered

**there are additional precautions for intrathecal iodinated agents


administration of oral contrast

  • oral contrasts are diluted with water (concentration of oral contrast used can vary)
  • patients drink the oral contrast in intervals over a period of time(this allows the contrast to fill the entire GI tract, the length of time the pt is required to ingest the contrast is department dependent; usually a couple of hours)

-pts are required to be NPO for these exams (diabetic pts should be monitored closely)

  • all forms of contrast can be administered via this method


intravenous contrast media

iodinated agents are used for IV administration

  • non-ionic contrast has become the only contrast media to be used for intravascular injection

-adverse effects are uncommon

-most commonly used: Optiray, and Omnipaque

  • HOCM (4-5 times higher incidence of adverse effects vs LOCM)
  • LOCM (more expensive than HOCM)
  • IOCM (example: Visipaque; is the most expensive)


intravenous contrast media

Contrast is NOT metabolized

  • excreted by the kidneys (causes stress to the kidneys and urinary tract)
  • blood work must be done to determine safety (must have "normal" ranges)
  • GFR has become the gold standard

-measures renal function

-site dependent protocols determine kidney function

  1. pts with normal renal function will eliminate half the dosage of contrast from their body within 2 hours; with complete elimination within 24 hours
  2. impaired renal function can increase half life to 30 hours


blood work-safety indicators

(SeCr) Creatinine overestimates GFR

  • due to metabolism of creatinine in muscle

-this causes creatinine levels to be higher in:

-men vs women

-younger vs older patients (age)

-blacks vs whites (race)

-other considerations that affect SeCR readings:

-malnourished patients/muscle wasting decrease SeCR

-protein (consumption of cooked meat) increase SeCR

  • despite SeCR limitations; it remains a fast and inexpensive way to assess renal function (should be used in conjunction with GFR)


iodinated contrast safety considerations

impossible to predict which patients will have an adverse reaction to IV contrast

  • iodinated contrast agents are one of the most widely used of all medications
  • also considered one of the safest (fatal reactions are rare)
  • excessive doses of contrast can be toxic:
  • safe dose limits
  • over doses can be fatal (affects the pulmonary and cardiovascular systems)
  • hydration can affect safe dose limits
  • upper limit= 64gI (grams of iodine)
  • pediatric patient dosages calculated by wieght: 2mL/kg
  • common practice is to limit the pt to 1 contrast injection per 48 hours


iodinated contrast safety considerations: pregnancy 

  • iodinated contrast can cross the placenta and enter the fetus
  • studies unable to prove if risk exists


iodinated contrast safety considerations: lactation

  • contrast media can be excreted into breast milk
  • infant absorbed dose = 0.01%
  • safe to continue breastfeeding
  • option to abstain for 24 hrs 


adverse effects of iodinated contrast:

iodinated contrast reactions can be broadly categorized as:

  1. subjective/normal
  2. chemotoxic
  3. idiosyncratic


subjective (normal) responses to IV contrast

Normal responses to injection of IV contrast (aka side effects):

  • feeling of heat/ warmth (voiding sensation)
  • mild flushing
  • metallic taste
  • nausea and/or vomiting

symptoms are generally very brief

  • anxiety may increase probability of reactions
  • patients should be informed of normal responses prior to administration of contrast


chemotoxic responses to IV contrast

result from physiochemical properties of the contrast media, the dose and the speed of injection

  • inludes hemodynamic disturbances and injuries to organs and vessels
  • examples:

-pain at injection site

-contrast induced nephropathy (CIN)

  1. a condition or disease affecting the kidney
  2. CIN is the 3rd leading cause of ARF (acute renal failure)



acute impairment of renal function resulting from the administraton of IV contrast

  • elevated SeCR within 24 hrs of contrast administration

-may take up to 48 hrs for SeCR levels to rise

-levels should return to normal within 7-10 days

  • incidence of CIN is:

-1-6% for low risk pts

-50% for high risk pts

  • may cause the need for temporary or chronic dialysis treatments
  • increases risk of death from non-renal causes


CIN risk factors:

the following conditions and factors predispose pts to an increased likelihood of developing CIN:

Diabetes Mellitus:

  • considered a low risk (increase of 0.6%)
  • high risk if coupled with a preexisting renal impairment ( increase of 19.7%)

Volume of contrast material:

  • direct relationship
  • increased volume= increased risk
  • allow 48 hrs between procedures requiring contrast material



other CIN risk factors


  • increases risk
  • affiliated with the higher incidence of CIN in pts presenting with multiple myeloma

special considerations:

  • other possible risk factors for CIN include:
  • age
  • gender (increased incidence in men)
  • atherosclerotic disease


prevention of CIN

methods recommended for preventing CIN:

  • use of LOCM or IOCM (IOCM may be the safest contrast media to use)
  • hydration (all pts should be well hydrated prior to exam, high risk pts should recieve IV fluids prior to contrast administration)
  • dose (use the smallest amount of contrast media possible, min 48 hrs must elapse between administration)
  • temporarily discontinue medication (ex Metformin)


idiosyncratic responses to IV contrast

include all other forms of reactions

  • unpredictable and can occur within 1 hour of contrast administration

-delayed reactions have been reported between 1 hr and 1 week following contrast media injection

-most adverse reactions occur within minutes

-the majority are non-life threatening events

  • are unrelated to dose

-"allergic like"

-children have a lower incidence


idiosyncratic responses to IV contrast

the most important method of reducing the pts risk of developing an idiosyncratic reaction to contrast media is to avoid using HOCM

  • patients who are at greater risk of developing a reaction should be monitored for 1 hr
  • premedication may also be used to decrease risk

idiosyncratic reactions are classified as:

  • mild
  • moderate
  • severe


mild and moderate idiosyncratic reactions:

are acute reactions and include the following signs and symtoms:

  • itchy skin
  • hives (uticaria), or other skin rash
  • nasal congestion, sneezing, watery eyes
  • coughing with possible laryngeal swelling
  • peripheral tingling
  • tachycardia or bradycardia ( >100 beats/min or < 60 beats/min)
  • hypotension
  • feeling of fullness or tightness of chest, mouth or throat
  • feeling of anxiety or nervousness


responding to mild/moderate reactions

what can you do:

  • stop the injection and the exam
  • calm and reassure the pt (elevate pts legs for hypotension and comfort)
  • apply cool compress to itchy areas
  • observe the pt for signs/symptoms of increased distress or changes
  • document details of reaction in pts electronic profile, on the requisition and in the pts chart
  • obtain medical assistance (consult with radiologist and or physician/nurse to determine necessary observation of pt prior to pt discharge) 


severe reactions

are potentially or immediately life threatening and include the following signs/symptoms:

  • abrupt onset
  • bradycardia (less than 50 beats per min)
  • hypotension (decrease in BP)
  • severe dyspnea
  • cardiac arrhythmias
  • laryngeal swelling
  • possible convulsions/seizures
  • loss of consciousness
  • respiratory arrest or cardiac arrest



responding to severe reactions

what can you do:

  • call a code (fast response of RTs is necessary to maintain pts airway)
  • ensure integrity of IV site, which may be used to give medication to treat the reaction
  • calm and reassure the pt
  • prepare: oxygen, suction, crash cart
  • have the pts history ready and available
  • be ready to assist the physicians


Documenting Contrast Reactions

If an adverse reaction occurs, the following information should be documented:

  • Amount & type of contrast used
  • Signs & symptoms of the reaction
  • Interventions or medications given during the reaction

          -Include the patients response to treatment

  • Final outcome

          -Was the patient sent home, or admitted to the hospital

  • Update patient information in computer system

          -Requisition should reflect the sensitivity


Risk Factors Continued

Some risk factors predispose pt.’s to an ↑ likelihood of developing an adverse reaction


  • 3X more risk
  • In some cases, an asthma attack can be triggered by the stress on the body that a contrast injection can trigger
  • If the patient uses an inhaler, the patient should have the inhaler nearby in the event of an asthma attack

Allergies to food, drugs, or other substances

(Example: Hayfever or Eczema)

  • 2X higher risk
  • Allergies to iodine containing foods or skin preps pose an equivalent risk as other food allergies

Special considerations:

  • Beta-blockers can impair response to a contrast reaction treatment

           -*They do not ↑ the risk of idiosyncratic reactions

           -May be considered a contra-indication


Risk Factors of IV Contrast

Previous Contrast Reaction

11X greater risk

  • Reaction rate is higher for pt.'s with a previous contrast-reaction

          -*Most important predictor of relative risk factor

          -Test injections are not recommended

  • Important to assess the severity of the prior reaction

           -Pt. may not be allowed to have another injection of contrast

           -Pt. may be given an antihistamine (e.g., Benadryl) prior to the injection of contrast to negate the risk of an allergic reaction

           -Your job as a tech is to bring all the relevant information to the radiologists attention so they can decide if the scan with contrast can be performed, & with which type of contrast agent, or if an alternative exam should be performed


Contraindications to IV Contrast

Patient has kidney disease or is in renal failure

     -IV contrast is filtered through the kidneys, therefore special consideration must be taken for patients who already have compromised kidney function

  • Considerations:

     -Blood tests can be performed to check the patients renal function prior to the CT scan to ensure the patients kidneys will be able to adequately filter the foreign molecules of the contrast




Contraindications to IV contrast

Dialysis & Contrast Media

  • Patients who are on temporary dialysis should not be given contrast media

           -Could lead to chronic kidney failure

           -In-patients may have the CT if arrangements have been made with unit/ward for the patient to go for dialysis treatment post CT scan

  • Patients who present with end-stage renal failure may undergo contrast media exams

          -Cannot increase extent of preexisting kidney damage

          -Contrast media can stay in the blood for prolonged periods of time

          -Contrast media is efficiently removed from the blood by hemodialysis & peritoneal dialysis


Contraindications to IV contrast

Patient has history of diabetes & is taking Metformin

  • Diabetic patients can still have contrast-enhanced CT scans, but precautions must be taken as these patients have a predisposition for renal complications

           -Metformin lowers blood sugar

  • If renal dysfunction occurs, metformin can accumulate & cause lactic acidosis

            -These medications should be withheld for 48 hours post-contrast injection

            -They should not be resumed until a blood test indicates normal renal function

  • Diabetic patients who may be fasting prior to their CT scan should be prioritized and watched to avoid diabetic shock


Contraindications to IV contrast

Patient has heart disease or hypertension

  • A bolus contrast injection causes vasodilation

          -These pt.’s can have weakened & narrowed blood vessels

          -A bolus injection can cause changes in BP & cardiac output, which is potentially dangerous for these patients

  • Vasodilatation can also cause a vasovagal reaction

          -Important to have a baseline blood pressure for patients who suffer from high BP in the event they feel unwell post injection


contraindications to IV contrast


  • A pt. history of hyperthyroidism must be brought to the radiologists attention prior to the exam

          -Patients may need to be monitored by an endocrinologist post-exam

  • Contrast media can ↑ thyroid hormone levels in these patients

          -Could result in a fatal thyroid storm

          -Requires CT scans to be conducted no sooner than 2 weeks prior to a nuclear medicine thyroid uptake study


contraindications to IV contrast

CNS (central nervous system) disorders

  • Iodinated contrast agents cannot cross an intact BBB (blood brain barrier)
  • Patients with diseases that disrupt the BBB are at an increased risk for developing seizures
  • Metastasis

           -Higher risk of seizure

  • Primary brain tumors

            -Lower risk of seizure

  • Risk of seizure can be reduced:

            -Oral dose of diazepam 30 minutes prior to contrast media administration


contrast type: iodinated water-soluble

structures demonstrated:

  • blood vessels
  • solid organs
  • joints
  • spinal canal
  • GI studies (bowel)


  • injection
  • oral
  • rectal
  • intravenous

Appearance on images:

  • white; positive contrast


contrast type: Barium sulfate solutions

Structures Demonstrated:

  • bowel
  • stomach
  • esophagus


  • oral
  • rectal

Appearance on images:

  • white; positive contrast

** VoLumen can be considered negative contrast


contrast type: Air

Structures demonstrated:

  • bowel
  • stomach
  • joint spaces
  • spinal canal


  • injection
  • oral
  • rectal

Appearance on images:

  • black; negative contrast


contrast type: gas

structures demonstrated:

  • bowel
  • stomach
  • joint spaces
  • spinal canal


  • injection
  • oral
  • rectal

Appearance on Images:

  • black; negative contrast


contrast type: water

structures demonstrated:

  • pancreas
  • stomach
  • bowel


  • oral

appearance on images:

  • dark grey; negative contrast


What is the active molecule in water-soluble iodinated contrast? What gives it its appearance on CT images?

Iodine is the active molecule. It has an atomic number of 53 which is heavier than the compounds of the body so it absorbs more radiation than the organs and soft tissues surrounding it in the body. Iodine is considered radiopaque


Describe the differences between ionic and non-ionic contrast.

Ionic – molecules dissociate in solution, resulting in higher osmolality. This increases the risk of adverse reactions in patients. Slightly less expensive.

Non-ionic – molecules remain intact in solution, resulting in lower osmolality. This reduces the risk of adverse reactions in patients and is favourable for injection in CT. Slightly more expensive.


Define osmolality

Osmolality refers to the number of particles in solution per kg of water. Acts as a method of comparing the properties of IV contrast media with blood. Lower osmolality reduces the risk of adverse reactions.


Define viscosity

Viscosity refers to the thickness or friction of a liquid as it flows. High viscosity liquids reduce flow rate. Viscosity is determined by the number of particles in solution, the size of the particles and the relationships between the particles. Agents with higher iodine concentrations are more viscous.


What are 3 ways MRT’s can manage IV injections of high-viscosity contrast media?

- ↑ injection pressure, flow rate, & injection time
- ↑ needle bore size
- Warm contrast media to body temperature to reduce the viscosity


What are the benefits of using an isomolar contrast agent? What are the drawbacks? List one common trade name for an isomolar contrast agent

Benefits – osmolality is equal to blood, reducing the toxic effect and risk of anaphylaxis
Drawbacks – higher cost
Trade Name - Visipaque


What are the normal side effects of an IV contrast injection?

Feeling of warmth (voiding sensation), flush feeling, metallic taste, nausea or vomiting, are normal. Symptoms last briefly


Why should you warn the patients of the normal side effects prior to injection?

For compliance & ↓ pt. anxiety. If the patient moves, image quality will be severely degraded


What is an alternative to using barium sulphate to enhance the GI tract?

Iodinated water-soluble contrast agents, such as Telebrix, Lactulose, or Gastografin


Why is it important to assess the patient’s renal function prior to injection of contrast media?

Impaired renal function reduces the patient’s ability to eliminate the contrast and could result in CIN


What are some methods to assess a patient’s renal function?

  • Creatinine (SeCR)
  • GFR = Glomerular Filtration Rate.


What are 5 common patient risk factors that a technologist should screen patients for prior to contrast injection? Explain why each condition is a risk factor.

Diabetes – predisposition to renal failure or compromised renal function

High blood pressure / hypertension – injection of contrast causes vasodilation and could result in elevated blood pressure

Previous contrast injection – ensure a toxic dose of contrast is not delivered. Patients should only receive 1 dose per 48 hours

Allergies – risk of adverse reactions if allergic to contrast media, food & other substances (hayfever & Eczema)

Asthma – predisposed to adverse reactions


What diabetes medication is a contraindication to contrast injection? If the technologist discovers their patient is taking this medication, what is the correct course of action prior to injection of contrast?

Glucophage or Metformin (oral diabetes medication). Patients may be instructed to temporarily discontinue use prior to injection of contrast, and obtain a blood test 48 hours post injection to ensure their creatine, or GFR is within normal limits before resuming medication.


If a patient is known to have an allergy to contrast media, what are some possible ways to manage this history prior to injection of contrast?

  • Communicate with the patient, and determine severity/symptoms of prior reaction
  • Consult radiologist
  • Antihistamine could be administered prior to injection (dependent on severity of previous reaction(s)
  • Non-ionic or isomolar contrast may be indicated, or dose may be altered to reduce risk factors or toxicity


IV Access

Stable IV access is necessary for contrast media administration

  • IV line can be established in the CT department
  • Or the IV line could be pre-established

             -Indwelling peripheral catheters


  • AC or large forearm vein is preferred when using a pressure injector

             -Bending at the IV site may cause the injection to fail

             -Flow rates need to be ↓ if accessing a smaller vein and a smaller gauge is required

                    -Flexible plastic cannulas need to be used with mechanical injections


Pre-existing Vascular Access

When a pt. arrives in the CT department with an existing indwelling peripheral venous catheter, it must be carefully evaluated before it can be used to administer contrast media

  • Location
  • Age
  • Un-accessed connecting hub or port

               -Okay if accessed by saline or dextrose in water

  • No redness, blanching or swelling in the surrounding skin


Central Venous Access Device (CVAD)

Designed to deliver medications & fluids directly into the SVC (superior vena cava), IVC (inferior vena cava), or RA (right atrium)

  • Used for days, weeks, months, or years
  • Durable

          -Not as easily blocked or infected

  • May contain 1-3 lumens

          -Each should be used as an independent catheter port

                -Prevents mixing of meds

          -Catheters can have open or closed ends


Central Venous Access Device (CVAD)

PICC’s (peripherally inserted central catheter)

  • Are open ended catheters
  • Must be clamped when not in use
  • Should be flushed with heparinized saline to maintain the catheters patency between uses

Close ended catheters:

  • Contain a valve that controls fluid flow & prevents reflux of blood into the catheter
  • Only requires a saline flush to maintain patency



Only specially designed PICCs can handle a mechanical injection

  • Make sure you verify it can be used
  • If not, start a separate IV

          -Must establish standard peripheral IV’s for contrast admin.

  • Or, decrease injection rate & perform a hand injection NOT mechanical

Accessing & de-accessing an implanted port requires special training & is beyond the scope of a CT tech

  • Example: Tunneled CVC (central venous catheters)



Only sterile devices or needles are used to access CVADs

  • Disinfect the injection caps & dry prior to use
  • Before administering ANY substance the patency of the central line must be verified

             -Demonstrate blood aspiration

                     -Most sites use a 10mL saline syringe

              -Contraindications to injecting contrast media:

                     -Inability to demonstrate blood aspiration can indicate catheter malposition or occlusion

                     -Resistance to flushing a CVAD may cause the catheter to rupture

                     -*Dialysis catheters should never be used for contrast administration


Basic Principles of IV Contrast Administration

Protocols establish proper injection parameters:

  • Goal is to select parameters that consistently improve images & facilitate reproducible studies
  1. Volume & Concentration
  2. Flow rate(s)
  3. Pressure limits
  4. Timing of injection & scan acquisitions

Documentation is required:

  • Name of agent used
  • Dose
  • Flow rate(s)
  • Injection site


Injection Methods

Method of injecting IV contrast depends on:

  • Vascular access
  • Type of exam
  • Exams clinical indications

Can be injected via the following methods:

  • Drip infusion
  • Hand
  • Mechanical injector (Most common)



Mechanical Injectors

(Aka. Power Injector, Pressure Injector or Pump)

  • Used to deliver a bolus of contrast in a short period of time
  • Provide reproducible levels of contrast media enhancement


  • Connection tubing must be capable of withstanding the pressure of the injection
  • Air bubbles in the syringes & tubing must be cleared prior to the final connection is made to the patient

           -Prevents possibility of potentially fatal air emboli

  • Patency of the established IV site must be checked with blood aspiration, resistance and saline flush prior to use


Mechanical Injector Risks


  • Infiltration: contrast media leaking into surrounding tissues
  • extravasation: contrast media accidentally injected into surrounding tissue (IV becomes dislodged)


  • pain at injection site
  • swelling at injection site
  • possible hematoma


  • ensure pts arm is straight and IV line is not kinked
  • ensure proper dose is delivered
  • ensure a stable vein by flushing IV with saline prior to injection
  • choose a large enough vein for injection (consider rate and volume)
  • choose a large bore IV catheter


  • stop injection if pt complains of pain at injection site or if you notice swelling at injection site
  • remove needle immediately, maintain pressure on the vein to prevent hematoma
  • apply cold pack to help reduce pain


Mechanical Injector Components

  • Warming device(s)
  • Syringe(s)


  • Pressure mechanism
  • Control panel


Mechanical Injector Components: warming device

Warming Device:

  • Attaches to a syringe
  • Maintains temperature of the contrast agent at or near body temperature (37oC)

         -Does not heat the contrast agent

               -Contrast agent must be warmed prior to being drawn into the pressure injector syringe


Mechanical Injector Components: Syringes


  • Removable, pre-sterilized & disposable
  • Common sizes:


  • Attaches to the top of the “pressure mechanism”
  • Contains the pistons for contrast loading & delivery


Mechanical Injector Components: Pressure Mechanism

Pressure Mechanism:

  • The ‘body’ of the system
  • Ceiling or floor mounted

           -Mobile (can be used on either side of the pt.)

  • Consists of an electromechanical motor

           -Used to drive piston movement (Controls the precise delivery of contrast)

  • Safety devices built-in

         -Programmable pressure limits

         -Audible alarm


Mechanical Injector Components: control panel

Control Panel:

Enables technologist manipulation of injection parameters, such as:

  • Flow rate
  • Volume
  • Pressure Limit (PSI; pounds per square inch)

Stores injection parameter protocols into their memory

  • Consistent & reproducible
  • Can also provide a patients injection history


Timing of IV Contrast

  • Timing is very important for injections, so the desired anatomy can be demonstrated

                -Blood flow dilutes the contrast media very quickly

  • Software allows the injector to be electronically connected to the imaging equipment, synchronizing the injection with image acquisition

                -Usually built into the software and scanning parameters


General Phases of Tissue Enhancement

Three phases:

  1. Bolus
  2. Non-equilibrium
  3. Equilibrium

Phases are classified by rate of contrast enhancement

  • Helps determine ideal time between the initial administration of contrast media & scan acquisition


Bolus Phase

(Aka. Arterial phase)

  • Immediately follows an IV bolus injection
  • IVC & Aorta have a difference of 30 or more HU’s
  • Arterial structures are filled with contrast

               -Venous structures are not yet enhanced

  • CT angiography images are taken in this phase
  • Usually takes 15→22 seconds to reach peak contrast enhancement

              -Plateau typically lasts 10→15 seconds

              -Imaging usually occurs during the plateau


Enhancement Considerations

Most organs have an arterial blood supply

  • Examples: pancreas, bowel, & bladder

           -Peak enhancement of these organs occurs about 5→15 seconds after peak aortic enhancement

  • Kidneys are an exception

           -They also excrete contrast

           -Peak enhancement is 80→120 seconds after injection

                    -A scan delay should be used between liver and kidney scans

Liver has a dual blood supply

  • Portal vein & hepatic artery

           -Portal venous scans of the liver occur 60 seconds after a bolus injection

                    -Includes arterial & venous enhancement


Enhancement Considerations

Routine brain scanning

  • For metastases or primary central nervous system tumors
  • Enhancement is due to BBB disruptions, not the vascularity of the tissue

             -Injection rate is of no importance

             -Scan delay is an important consideration

                 -4 minutes or longer

Non routine brain scans:

  • CT angiography or CT brain perfusion scans

                 -Must follow routine contrast injection protocols


Non-equilibrium Phase

(Aka. Venous phase)

  • Follows the arterial phase (Approx. 1mins after bolus phase)
  • Has a 10→30 HU difference Aka. AVID (arteriovenous iodine difference)
  • Contrast is still brighter in the arteries than the parenchyma of organs, but venous structures are now opacified
  • Lasts approx. 1mins (Depends on volume & flow rate of contrast agent)
  • Most routine (non-angiography) images are acquired in this phase


Equilibrium Phase

(Aka. Delayed phase)

  • Begins 2mins after bolus phase
  • Contrast media is emptied from the arteries, diluted from the veins & soaked the organ parenchyma
  • AVID of less than 10HU
  • Worst phase for liver scans


Timing Factors

The exact timing of the start and end of each of the three phases are affected by the following factors:

  • Pharmacokinetic
  • Patient
  • Equipment


Pharmacokinetic Factors

Are controllable and include:

  • Contrast media:
  1. Volume *****
  2. Flow Rate *****
  3. Flow Duration
  4. Scan delay time
  5. Total scan time
  • Contrast media characteristics
  1. Osmolality
  2. Viscosity
  3. Concentration

***** = affect peak enhancement time


IV Contrast Media Concentration

When using a higher concentration:

  • Compensate with a lower injection rate & decreased volume
  1. To maintain adequate enhancement
  2. Higher injection rates ↑ risk of extravasation
  3. Higher volumes also challenge the stability of the IV (Particularly when small catheter gauges are used)
  • Good for CT angiography studies
  • Viscosity is higher
  • Streak artifacts are more likely to appear


Time-Density Curves

Depict effects of aortic & hepatic enhancement

   -Aortic enhancement effects are more pronounced

  • For constant injection rates:
  1. ↑ contrast volume = ↑ peak enhancement & ↑ time to reach peak
  2. Overall level of enhancement is also maintained longer
  • For constant volumes/doses of contrast:
  1. ↑ flow rate = ↓ time to peak enhancement
  2. Scan delay should be adjusted to flow rate

     -Flow rates are typically uniphasic

     -Can be manipulated to change characteristics of the time-density curve (aka. Bolus Shaping)

     -Prolongs peak enhancement, ↓ need for precise scan timing


Patient Factors

The following patient factors affect contrast enhancement:

  • age
  • gender
  • weight
  • height
  • cardiovascular status (cardiac output)
  • renal function
  • presence of disease

Injection parameters can be adjusted to compensate for enhancement effects caused by patient factors



Equipment Factors

CT Scanner capabilities affect scan timing & contrast administration

  • Faster scanners:

          -Delay between injection of contrast media & scan acquisition must be increased (Ensures scanning will occur during peak enhancement)

           -↓ scan time = ↑ scan delay

           -May enable the use of a smaller volume of contrast media

                  -However, if volume is ↓ peak enhancement will also ↓

(Injection rate or concentration must be increased to compensate)


Automated Injection Triggering

Two methods:

  1. Test Bolus
  2. Bolus Triggering
  • Useful for vascular studies
  • Ensures imaging during peak enhancement regardless of patient:
  1. Age
  2. Disease
  3. Cardiac output


Test Bolus

Injection of a 10→20mL IV bolus of contrast media

  • Test injection is delivered at the same rate as the diagnostic scan
  • Trial “scans” are performed (Using the lowest mA setting possible)

             -2 second intervals are common @ the same anatomical location

             -10→15 scan slices acquired

         -Determines the length of time from injection to peak enhancement

              -Identifies ideal duration of delay between injection & scan acquisition

              -Better imaging results if 3 seconds are added to this calculated delay


Bolus Triggering

Aka. Bolus tracking, or Automated triggering

More efficient than Test Bolus technique

  • Uses the contrast bolus itself to initiate scan acquisition

A single cross-sectional slice is used to place an ROI

  • Uses a series of low-dose “scans” to monitor contrast bolus progression over the selected ROI (Aka. “Monitor scans”)
  • Once the desired HU is achieved (computer automatically triggers or the technologist) will manually trigger the scan acquisition

           -Table needs time to move the patient into the appropriate “start” position (Can cause a 3→9 second delay)



Bolus Triggering: Limitations/Special Considerations

Limitations/Special Considerations:

  • Delay between last monitor scan & the start of scanning may ↓ accuracy of imaging during peak aortic enhancement

           -Compensate for delay by triggering scan acquisition early

  • Technologist is not able to stay in the room with the patient during contrast administration

           -↑ risk of extravasation

                       -A test injection with saline should be performed prior to contrast administration

           -Bolus Triggering should only be used for vascular studies


QA vs. QC

Quality Assurance is a management program used to ensure excellence in healthcare

  • A systematic collection & evaluation of data
  • QA focuses on preventing defects or deficiencies
  • emphasis on human factors that lead to variation in quality pt care

Quality Control is part of the QA program that deals with techniques used in monitoring and maintaining the technical elements of the systems that affect the quality of the image

  • QC deals with instrumentation and equipment
  • Focuses on detecting deficiencies and defects


What is QC?

The part of the QA program that tests the performance of the CT scanner against an industry or machine standard

  • The goal of QC is to ensure all acquired images are diagnostic with minimal dose to the patient

            -QC is an important part of radiation protection


To maintain high performance standards for patients

  • Regulatory bodies demand a standard be maintained due to the potential harm to patients and the public from x-rays


Benefits of QC tests

  • QC ensures a scanners performance and accuracy
  • Image quality issues can be solved faster by evaluating the QC results
  • QC tests are also valuable for service engineers as the problems are often apparent in the results


QC Schedule

For a QC program to run smoothly:

  • QC tests need to be performed on a regular basis

        -May be required to be performed daily, weekly, monthly or yearly

  • Results need to be recorded & maintained

         -Log all test results

  • Results must be interpreted promptly

          -Comparing QC results to both the specifications of the scanner and previous results

               -Results can demonstrate if the scanners performance is degrading (Analysis of results designate required response)



Steps in QC

1. Acceptance testing:

(Usually done by a physicist)

  • Ensures the scanner was delivered as ordered
  • Tests the stability, integrity & safety of the scanner

Acceptance tests include:

  • Slice thickness verification
  • CT number linearity
  • Spatial and contrast resolution
  • Noise
  • Dose output

2. Routine Performance Evaluation:

(Can be performed by the technologist and/or the physicist)

  • Can be done daily, weekly, monthly and yearly
  • Monitors the components of a CT scanner that affect image quality and dose

3. Error Correction:

  • If the values of the QC tests are outside the acceptance or tolerance limits, the scanner needs to be serviced


Who performs the QC Testing?

  • Authorities such as the American College or Radiology (ACR), the International Atomic Energy Agency (IAEA), and Health Canada determine which QC procedures are to be performed by the technologist, and which will be performed by medical physicists
  • The manufacturer of the scanner may also recommend evaluative procedures


What Tests are Done When?

More advanced QC tests:

  • Less important more time consuming tests are usually performed less frequently


  • The most complex tests are usually performed semi-annually or annually

              -These tests are usually performed by a physicist and may require the use of QC phantoms

QC tests and their acceptance limits are also outlined in the Canadian SC 35


Three Mandates of QC

Must be performed on a regular basis

  • Schedule of QC testing is a compromise between pros & cons

          -Scanner downtime, regulatory requirements & expected benefits

  • Prompt interpretation of measurements

          -Must recognize when the CT scanner is not operating within acceptable limits

  • Consistent & accurate record keeping

          -Results should be maintained for as long as the CT scanner is active (about 10yrs)


Discerning Acceptable Limits

Perhaps more important than the actual value of the measured variable is a change in the variable between measurements

  • Two performance standards for QC test results:

1. Acceptable (essential)

  • Indicates that performance must be within certain tolerances
  • If not, the equipment should not be used

2. Achievable (desirable)

  • Indicates the level of performance that should be attained under favorable circumstances
  • The level at which a facility should work if feasible


QC Equipment

Phantoms are regularly used in QC testing

Types of phantoms:

  1. Image performance: (image quality, subjective ex contast and spatial resolution)
  2. Geometric: (ensure object shape, size, and positions are being represented accurately)
  3. Dosimetry: (ensures ALARA principles are being adhered to)
  4. Quantitative: (objective, statistical measurements)


ACR Phantom

(Aka. American College of Radiology Phantom)

Allows for testing of:

  • Positioning and alignment
  • CT number accuracy
  • Slice thickness
  • Low contrast detectability
  • Image uniformity
  • Spatial resolution
  • Inter and intra place distance measurement accuracy


Techniques for QC Measurements

Depends on the type of CT scanner and the test being performed

Suggestions when choosing technique combinations:

  • Select one or two representative techniques
  • Use techniques that match a frequently used clinical technique
  • Head or body technique
  • Technique should remain the same for a specific test from day to day


Test Priority and Frequency

Test priority classifications:

  1. Essential
  2. Desirable

Test frequency considerations:

  • Replacement of major components, equipment service or adjustments

                  -Requires immediate testing

  • If used for accurately localizing tissue (biopsies & radiation therapy treatment)

                 -↑ frequency of appropriate tests

  • Complexity of tests

                 -↓ frequency of test completion


Test Frequency

Daily QC tests (as outlined by the Canadian SC 35) include:

  • Equipment Warm Up

(Usually a predefined protocol built into the scanner as per the manufacturers specifications)

  • Meter Operation
  • Equipment Conditions
  • Visual inspection of electronic display devices

             -Ensures the integrity of radiation protection considerations are maintained

             -Frequency depends on regulatory body

                      -ACR states monthly, IAEA states performance by physicist, CAMRT states daily technologist responsibility


Daily QC Procedures: equipment warm-up

Equipment Warm-up:

  • There is a warm up period before the system performs optimally

          -Systems that have no “downtime” still require warm-up every 24hours

           -Must be consistent, occurring at the same time of day

  • May include the boot-up of the system and operating system
  • Warms up the x-ray tube
  • Calibrates the detectors


Daily QC procedures: Visual Inspection Checklist

A visual inspection may include the following:

  • Checking the integrity of cables & wires
  • Checking the WW & WL display
  • Checking for appropriate warning labels & service records
  • Room cleanliness/safety concerns
  • Checking to ensure all equipment is functioning


  • Table height & position indicators
  • Gantry angulation indicators
  • Laser localization lights
  • Intercom system


what are the weekly QC tests?

  • CT number accuracy
  • visual inspection of cleanliness
  • noise
  • CT uniformity


what are the monthly QC tests?

  • calibration of CT numbers
  • electronic display performance
  • CT number linearity
  • CT slice thickness


what are the quarterly QC tests?

  • CT pt support movement
  • low contrast detectability
  • spatial resolution
  • interlocks


what are the semi-annual QC tests?

  • accuracy of gantry tilt
  • patient dose
  • CT accuracy of automatic positioning of tomographic plane


what are the annual QC tests?

  • radiation dose profile
  • protective equiptment integrity
  • general preventative maintenance
  • electronic display device performance
  • radiation dose
  • scout image
  • CT number dependence on phantom position


number accuracy

(weekly QC test)

equipment needed:

  • uniform water phantom

How is the test done:

  • conditions of a typical axial head and a typical axial body scan

What is the purpose:

  • evaluate the accuracy of the CT number of water

SC35 Acceptance limits:

  • must be in the range of 0 (plus or minus 4HU) (ex -4 to 4)


CT noise

(weekly QC test)

Equipment needed:

  • uniform phantom

How is the test done:

  1. scan phantom
  2. measure variation in CT numbers from mean value
  3. perform again under condition of head scan and body scan

What is the purpose:

  • measures noise, variation in HU
  • noise is calculated by the variation of CT numbers from a mean value in a defined area in the image of the phantom
  • its magnitude is equal to the SD of the CT number values within the ROI
  • SD within ROI must be plus or minus 10%

SC 35 acceptance limits:

  • plus or minus 10% of 0.2HU from baseline (whichever is larger)
  • the baseline must not deviate from the manufacturers specifications by more than plus or minus 15% (of the linear attenuation coefficient of water)


CT uniformity

(weekly QC test)

Equipment needed:

  • CT machine
  • body phantom
  • head phantom

How is the test done:

  • Calculated by the mean CT number in the center of the image subtracting the mean CT number at the periphery of the image

What is the purpose:

  • to check the consistency of the CT number of an image of a homogenous material across the scan field

SC 35 Acceptance limits:

  • the difference between the mean CT number at the center of the phantom and the periphery must not exceed 2HU from the established baseline values
  • the basline uniformity of CT number for water must not be greater than plus or minus 5HU from the center of the phantom to the periphery

*Capping: high value in center, low in the edges

*Cupping: high at the edges, low in the center

(they have to do with CT uniformity)


Visual inspection of cleanliness

(weekly QC test)

Equiptment needed:

  • cleaning supplies (dust rag)

How is the test done:

  • inspect for dust or dirt on or near image reception area

What is the purpose:

  • so the dust and dirt do not negatively affect image quality

SC 35 acceptance limits:

  • clean off dust and dirt or anything that comes in contact with the IR (subjective)


CT slice thickness

(monthly QC test)

Equipment needed:

  • test device containing ramps, thin disks or beads

How is the test done:

  • using a test device containing one or two ramps positioned at an angle to the scan plane

What is the purpose:

  • evaluation of the tomographic section thickness

SC 35 Acceptance limits:

  • 2mm or more slice thickness: plus or minus 1mm
  • 2mm-1mm slice thickness: plus or minus 50%
  • <1mm slice thickness: plus or minus 0.5mm


Calibration of CT number

(monthly QC test)

Equipment needed:

  • water filled uniform phantom

How is the test done:

  • the mean CT number and standard deviation should be calculated for a 2-3cm sqaured area of water and air in the reconstructed image
  • the mean CT number and SD should be measured at all clinically used voltage settings

What is the purpose:

  • to measure the mean CT number and standard deviation

SC 35 Acceptance limits:

  • CT number for water must be 0 (plus or minus 4HU)
  • CT number for air must be -1000 (plus or minus 10HU)


Electronic device display performance

(monthly QC test)

Equipment needed:

  • test pattern generator
  • laser film printer

How is the test done:

  • using a test pattern such as the SMPTE or TG18 test pattern

What is the purpose:

  • evaluating the performance of electronic display devices used to view images

SC 35 acceptance limits:

  • subjective


CT number linearity

(monthly QC test)

Equipment needed:

  • quantitive phantom containing uniform objects of known materials with a wide range of CT numbers

How is the test done:

  • perform scan of phantom, compare numbers obtained with CT numbers provided by phantom manufacturer and with previously measured values

What is the purpose:

  • ensures the CT scanners CT numbers are within established limits (set by manufacturer)

SC35 acceptance limits:

  • CT numbers checked over the CT range of -1000 to 1000
  • acceptable limits acieved when values "form a straight line"


CT patient support movement

(Quarterly QC test)

Equipment needed:

  • phantom, table/gantry

How is the test done:

  • phantom is placed on couch and is moved in and out of gantry
  • test is performed using an even weight load of no more than 135kg
  • table is moved a distace of 30cm (maximum scan increment) and returned to start position, must not exceed 1mm

What is the purpose:

  • ensure scan interval = the scan width to image the entire volume of the pt (for contiguous scans)

SC35 acceptance limits:

  • plus or minus 1mm of intended movement of pt in and out of gantry



Spatial Resolution

(Quarterly QC test)

Equipment needed:

  • high contrast wire (2mm in diameter or less) in a tube of minimally attenuating material
  • bar pattern test device
  • test device with repeating pattern holes

How is the test done:

  • with MTF curves, visual assessment of bars or lines, using region of interest measurements

What is the purpose:

  • evaluating spatial resolution

SC35 Acceptance limits:

  • MTF curve must be within 0.5 lp/cm or plus or minus 15% of established baseline value
  • visual assessment has a limiting high contrast resolution of 5 lp/cm or more or spacing of 1mm or less


low contrast detectability

(Quarterly QC test)

Equipment needed:

  • CT spatial resolution test device
  • a phantom of objects of varying size

How is the test done:

  • done using a phantom with <1% or (10HU) difference to surrounding material
  • subjective test

What is the purpose:

  • evaluate low-contrast resolution

SC35 acceptance limits:

  • proper phantom
  • subjective acceptance limit
  • value determined as the smallest sized object seen at a specified contrast level to the background when imaged under specific conditions


CT accuracy of automatic positioning of tomographic plane

(Semi-annual QC test)

Equipment needed:

  • scout localizer image

How is the test done:

  • the location of the actual scan plane is compard with the scanned projection radiograph scout localization

What is the purpose:

  • evaluating accuracy of the automatic position of the tomographic plane
  • assess scout images, ensures accuracy of localizer with the digital display location

SC35 acceptable limits:

  • reference lines indicating tomographic sections must not differ from true position by more than 2mm with the gantry in vertical position (the scan projection radiograph must be plus or minus 2mm of the actual scan plane)


accuracy of gantry tilt

(semi-annual QC test)

equipment needed:

  • pre-packaged film
  • protractor

How is the test done:

  • expose film
  • place upright and parallel to sagittal laser at various gantry tilt angles
  • at least 3 irradiations need to be made on the film: no tilt, 2 extreme angles
  • angles measured should be plus or minus 3 degrees of computer display

What is the purpose:

  • ensures physical tilt of gantry corresponds to tilt angle indicated on display

SC35 acceptable limits:

  • plus or minus 3 degrees of computer display


patient dose

(semi-annual QC test)

equipment needed:

  • dosimetry phantom (head and body)
  • dosimeter
  • CT dose probes

How is test done:

  • dosimetry phantom placed on pt table without any additional attenuating material

what is the purpose:

  • evaluating patient dose

SC35 evaluation criteria:

  • must be within plus or minus 20% of the established baseline values


laser light accuracy

(semi-annual QC test)

equipment needed:

  • 1mm pin
  • pencil
  • film

how is test done:

  • Axial: using a needle to puncture holes in film at the positions of the laser lights and exposing the film using the smallest available scan width. The difference between the exposed areas on the film and the locations of the pin pricks must be less than plus or minus 2mm
  • Sagittal/coronal: place a thin absorber (pencil) centered in the tomographic plane at the intersection of the sagittal and coronal positioning light fields. The intersection localization lights must indicate the center of the FOV. The midline of the table should be coincident with the sagittal scan alignment light. The results must be within the manufacturers recommended values and tolerances. A limit of plus or minus 5mm should be achievable

What is the purpose:

  • axial: ensures the location of the radiation beam relative to anatomy is correct
  • Sagittal and coronal: centers the anatomic structures in the SFOV

SC35 acceptable limits:

  • underlined them for both


CT number dependence on phantom position

(annual QC test)

Equipment needed:

  • water filled phantom

how is the test done:

  • scan the water filled phantom in varied clinically relevant positions on the pt support

What is the purpose:

  • to ensure HU accuracy for vaired patient positions within the gantry

SC35 acceptable limits:

  • plus or minus 5HU


Radiation dose profile

(annual QC test)

Equipment needed:

  • scanning microdensitometer
  • pre-packaged film

How is the test done:

  • measuring the value either by the density profile of the resultant film or computed from the digital image

What is the purpose:

  • variation of the intensity of the x-ray beam in the Z-plane (section thickness), to ensure collimation does not exceed prescribed scan width

SC35 acceptable limits:

  • should not exceed prescibed scan width by more than manufacturers specifications
  • scan width: full width half maximum of radiation dose profile


General preventative maintenance:

(annual QC test)

equipment needed:

  • x-ray equipment and accessories

How is the test done:

  • inspection for structural integrity, cleanliness, ease of movement for all components and any other procedures recommended by the manufacturers

What is the purpose:

  • to prolong the life of the equipment

SC35 acceptable limits:

  • N/A


Protective equipment:

(annual QC test)

equipment needed:

  • radiographic equipment

How is the test done:

  • take an x-ray of equipment

What is the purpose:

  • looking for defective areas of equipment

SC35 acceptable limits:

  • defective area greater than 670mm squared is NOT acceptable (lead apron)
  • defective area greater than 5mm diameter circle is NOT acceptable (thyroid/reproductive shielding)


Radiation dose and scout image

(annual QC test)

Equipment needed:

  • dosimeter
  • CT dosimetry phantom
  • (head and body phantoms)
  • CT dose probes

How is test done:

  • take a scout image of the phantom and record dose recieved

What is the purpose:

  • evaluate dose patient recieves

SC35 acceptance limits:

  • plus or minus 20% of the nominal value


what is acceptance testing?

basically verifies compliance, that is, whether the CT equipment meets manufacturers specifications (usually as outlined in the hospital request proposal to purchase a CT scanner) and ensuring that it operates efficiently in terms of various outputs, such as image quality requirements and dose output

  • generally includes that the features ordered are delivered and there is mechanical and electrical integrity, stability, safety of various components (ex interlocks and power drives) and CT dose and imaging performance


what types of test(s) will acceptance testing include?

  • verification of slice thickness
  • CT number linearity
  • uniformity
  • spatial and contrast resolution
  • noise
  • dose output


who performs acceptance testing?

  • qualified medical physicists
  • others with a firm knowledge of how CT scanners work


according to the SC35, which CT QC tests need to be performed daily?

  • equipment warm up
  • meters operation
  • equipment conditions
  • overall visual assessment of electronic display devices


what types of phantoms are regularly used in CT QC tests?

  • image performance
  • geometric
  • dosimetry
  • quantitive


what is the intended purpose of an ACR phantom?

based on solid water construction to ensure stability and reproducibility of QC tests conducted over time

  • an important feature of this phantom is that it consists of 4 modules which are designed to measure positioning accuracy, CT number accuracy, slice thickness, light accuracy alignment, low contrast resolution, CT number uniformity, and high contrast resolution


what test(s) can an ACR phantom be used for?

  • CT number accuracy
  • low contrast resolution
  • slice thickness
  • light accuracy alignment
  • CT number uniformity
  • high contrast resolution (spatial resolution)
  • CT number linearity
  • radiation dose


what is the SC35 recommended frequency of ACR phantom tests (inclue linearity)

  • light accuracy alignment:
  • CT spatial resolution:
  • CT low contrast:
  • image uniformity:
  • noise:
  • CT number accuracy:
  • slice thickness:
  • CT linearity:

  • light accuracy alignment: semi-annual
  • CT spatial resolution: quarterly
  • CT low contrast: quarterly
  • image uniformity: weekly
  • noise: weekly
  • CT number accuracy: weekly
  • slice thickness: monthly
  • CT linearity: monthly


what are the acceptance limits for ACR phantom tests (include linearity):

  • light accuracy alignment:
  • CT spatial resolution:
  • CT low contrast:
  • image uniformity:
  • noise:
  • CT number accuracy:
  • slice thickness:
  • CT linearity:

  • light accuracy alignment: within 2mm
  • CT spatial resolution: 0.5 lp/cm or plus or minus 15% of baseline
  • CT low contrast: <1%or 10HU contrast to surrounding material
  • image uniformity: plus or minus 2HU from the center ROI to any at periphery
  • noise: SD within the ROI must be plus or minus 10% or 0.2HU
  • CT number accuracy: water = 0 (plus or minus 4HU)
  • slice thickness: AST > or equal to 2mm; IT should not vary plus or minus 1mm. AST 2 to 1mm; IT should not vary plus or minus 50%. AST < 1mm; IT should not vary plus or minus 0.5mm
  • CT linearity: -1000 to 1000 CT numbers checked out of this range


who is the author of the safety code 35?

Health canada


according to the safety code 35, what is the goal of a quality assurance program?

ensure timely diagnosis and treatment at minimum dose to pts and staff


according to the safety code 35, what is not a required step in the establishment of quality control procedures?

inspection by regulatory bodies


acceptance testing is required to ensure

verify compliance with performance specifications of the CT scanner


acceptance testing of a CT system includes evaluation of the following with the exception of:

  1. x-ray tube voltage
  2. spatial resolution
  3. MTF
  4. tomographic section thickness

3. MTF


what is an example of a quality control test that should be performed daily?

equipment warm up


CT number accuracy test is performed to evaluate the accuracy of the CT number of?



the quality control test evaluating CT noise is measured by comparing the magnitude of the ______ of CT numbers with a region of interest

standard deviation


the quality control procedure evaluating CT uniformity requires a phantom with?

a homogenous material across the scan field


the number of ROIs required for evaluating CT uniformity is?



acceptable CT uniformity values result in no more than a ____HU variation from an ROI placed in the center of the water phantom to those placed at the periphery

plus or minus 2HU


CT slice thickness requires a phantom that contains:

ramps or step wedge


acceptable CT tomographic section thickness for slices less than 1mm should not vary more than plus or minus ____from the established baseline



calibration of CT numbers for water must be 0 (plus or minus 4HU) and _____ for air

-1000 plus or minus 10HU


CT linearity evaluates the relationship between CT numbers and 

linear attenuation coefficients


evaluation of CT linearity requires a phantom that:

contains objects with a wide range of CT numbers


when CT linearity CT numbers are plotted against the linear attenuation coefficients, acceptable results are demonstrated as a

straight line up or down graph


Evaluation of interlocks is performed to ensure:

x-rays are unable to be emitted if the door is open


CT spatial resolution is also known as:

high contrast resolution


CT spatial resolution can be performed by:

both a technologist and a physicist


CT spatial resolution may be evaluated by measuring:

  • modulated transfer function
  • line pairs/cm
  • repeating patterns of holes
  • bars or lines


CT low contrast resolution requires a phantom that contains objects with less than ____ contrast to the surrounding material

1% or 10HU


true/false: phantoms used for evaluation of contrast resolution should be scanned consistently at the same mAs setting



laser lights are located ______ the CT gantry and used for patient positioning and alignment

both inside and outside of the gantry


true/false: the axial scan localization light is tested to indicate the beam relative to the external anatomical structures of the pt



true/false: the intersection of the sagittal and coronal scan localization lights indicate the diameter of the scan field of view



CT radiation dose measurements are performed by a

medical physicist


CT Exposures

CT imaging produces higher doses vs. general x-ray procedures

  • Quantum Mottle compromises image quality:

           -Small objects are harder to image

           -Thinner slices require more photons

           -Low-contrast resolution

The decision to perform a CT must be calculated via Risk vs. Benefit (Benefit must outweigh the risk)

  • Patient’s must be aware of the risk
  • ALARA principles must be maintained


Relevant Doses

  1. Exposure: the amount of radiation the patient is exposed to
  2. Absorbed Dose: the amount of radiation that is absorbed by the patient
  3. Effective Dose: used to measure the risk of partial body exposure from the equivalent whole body dose

              -Effective dose takes into account the type of radiation and the exposed tissues


Important Doses to Consider

  • Absorbed dose (Gy) is used to determine at what threshold a deterministic effect will occur
  • Effective dose (Sv) is used when considering the probability that a stochastic effect may occur


Stochastic Effects

  • These effects occur by chance
  • The probability of an adverse effect increases with an increase in dose
  • There is no dose threshold
  • The severity of the adverse effect is not dependent on the dose (adverse effects can still happen at low doses)
  • Usually appear years after exposure
  • Examples include cancer and genetic effects
  • measured using LNT dose response model (linear-non threshold radiation dose)


Deterministic Effects

  • There is a dose threshold after which an adverse effect will happen
  • The severity of the adverse effect is related to the dose (increases in dose increase the severity of the effect)
  • Because there is a dose threshold, radiation protection methods are put into place to reduce the risk to patients as well as radiation workers
  • Examples include skin erythema, necrosis, epilation and cataracts


Dose Distribution

In CT the dose distribution is more uniform

  • Dose spread becomes less uniform as the patient thickness increases (and the SFOV)

             -Entrance skin dose is greater than dose at the center of the patient

             -Caused by: Partial Shielding


Factors Affecting Dose in CT

Factors are directly controlled by the technologist, or inherently built into the scanner

      -Balance between Image Quality & Dose

Factors you should understand:

  • Technique (kVp, mA)
  • Collimation
  • Pitch
  • Centering
  • Over-ranging
  • Beam Geometry
  • Repeat Scans
  • Filtration
  • Detector Efficiency
  • Scan Field Diameter
  • Localizers
  • Slice Width & Spacing
  • Patient size & part thickness


How Dose Affects Image Quality

  • The CT dose also affects image quality
  • ALARA principle: as low as reasonably achievable to maintain diagnostic image quality
  • Image quality factors that are affected by dose include:


          -Spatial Resolution

          -Contrast Resolution


Automatic Tube Current Modulation

  • “AEC” for CT scanners
  • Optimizes the dose to the patient while maintaining image quality regardless of patient size or tissue attenuation
  • Automatically adapts the mA in both the “z” and “x-y” axis
  • Uses information from the scout image to determine the appropriate mA during the scan
  • A predefined level of image quality is defined by the vendor and protocolled by the users


Continued Dose Concerns

Advancements such as ATCM have reduced doses to patients, however:

  • New technology has also increased the number of clinical applications of CT

          -CT Screening procedures

          -CT guided procedures

          -Pediatric imaging (Due to scan speed)

  • These advancements have resulted in:

           -More CT exams

           -Increased patient doses


Management of Dose in CT

Utilizing the 3 cardinal rules of radiation protection

1. time

2. distance

3. shielding


Dose Management


  • Limit the number of scans
  • Adjust Pitch


  • From the source of radiation (for patients)
  • From the patient (for technologists)

            -Dose is inversely proportional to the square of the distance

-Overall amount of scatter radiation in CT is low, and the distance it travels is short


Dose Management

Patient Shielding:

  • Less beneficial in CT due to the amount of scatter

           -Most scatter is internal & is unaffected by surface shielding

Bismuth shielding:

Used for anterior dose-reduction

  • In-plane shielding

          -Causes problems with ATCM

          -Must be outside SFOV during Scout acquisition

  • Lead alternative

Lead shielding:

Used to enhance perception of safety

  • Out-of-plane shielding

           -Must not be in the SFOV

           -No effect on image quality

           -Must be 360 degrees around the patient


Dose Management

Shielding the Public & Personnel:

As per the SC35:

  • Secondary shielding is required

           -Room walls (to protect the public)

           -Based on equipment not workload values

  • Lead shielding is required for any person who must remain in the room during scan acquisitions

            -Areas of greatest radiation exposure are near the gantry aperture

             -0.35mm of lead required for kVp’s of 100→150


How can we Reduce Dose in CT?

Technologists can use the following guidelines:

  • Appropriate patient selection

           -CT referrals are reviewed by the radiologist

  • Limit multiphase scans (i.e., with and without contrast)
  • Develop protocols based on the patient’s body habitus (i.e., small, medium, & large patients)

            -Using the lowest appropriate mAs

            -Adjusting the Pitch when appropriate


Pediatric Considerations

The law of Bergonie and Tribondeau states that the radiosensitivity of a tissue is dependent on its metabolic state

  • Pediatric patients are therefore more sensitive to radiation exposure because their cells are rapidly dividing
  • Children have a longer life expectancy than adults

           -Results in a larger window for potential adverse effects to radiation exposure


How can we Reduce Pediatric Dose?

  • Have separate protocols for pediatric CT exams

         -Customize the exam

               -Education plays an important role in reducing dose to pediatric patients

  • Use CT only when clinically indicated

              -Consider using other modalities (e.g., MRI, ultrasound)

  • Apply ALARA Principles

              -In some cases, lower resolution scans can still be diagnostic and save patient dose

              -Limit the need for multiphase scans (i.e., with and without contrast)

              -Consider patient shielding


Factor: Iterative Reconstruction


This method can (↓) pt. dose by 50% compared to back-projection methods


Factor: Technique (mA)


This should be (↓) for smaller sized patients


Factor: Technique (kVp)


Will increase dose to pediatric patients if not (↓) from an adult protocol


Factor: (↑) Pitch


Moves the patient through the gantry faster, (↓) pt. dose by 33%


Factor: (↓) SFOV


Creates a more uniform dose distribution


Factor: Localizer

Are very low dose & are used to assist in (↓) total scan dose


Factor: Radiation Beam Geometry


A 180 degree arc would (↓) dose, however, a 3600 arc is typically used for (↑) image quality


Factor: Over-ranging


Applies to helical scans & (↑) pt. dose


Factor: Collimation


Beam width must be (↑) so that the penumbra extends beyond the active detectors (↑) pt. dose


Factor: Repeat Scans


Multiphase enhancement studies fall under this category, and (↑) pt. dose


Factor: Filtration


Designed to absorbed low energy photons which (↓) absorbed dose & (↑) image quality


Factor: Centering


Important when using ATCM. If not perfect, image quality is compromised due to (↑) image noise


Factor: Detector Efficiency


Scintillation systems have (↑) absorption efficiency, which (↓) pt. dose & improves image quality



Factor: (↓) Slice Width & (↓) Spacing


  • If the same volume of tissue was scanned:
  • Taking more slices would (↑) pt. dose
  • Overlap would (↑) pt. dose


Which 3 factors have an indirect effect on dose? What do they have a direct effect on?

All 3 factors have a direct effect on Image Quality

  • Noise
  • Contrast Resolution
  • Spatial Resolution


What is the difference between stochastic and deterministic effects?


  • No threshold dose
  • Radiation protection modeled after this belief (no safe dose)
  • Cause late effects (genetic & carcinogenic)


  • Threshold dose
  • A specific amount of radiation is required to produce an ill effect
  • Cause early effects


Describe each of the following & identify at least one limitation for each:

  • In-plane shielding
  • Out-of-plane shielding
  • Partial shielding

In-plane shielding: Bismuth shielding that is used inside the SFOV; Must not be used during Scout acquisition as it may affect the ATCM and ↑ pt. dose

Out-of-plane shielding: Lead shielding that is used to cover anatomical areas that are not being scanned during CT acquisition; must be wrapped around the patient in a full 3600 to be effective.

Partial shielding: Peripheral body parts that shield more centrally located organs from (entrance) radiation exposure; smaller body parts that lack partial shielding receive higher doses during CT scanning.