Contrast Media in Medical Imaging Flashcards
(34 cards)
Why are contrast agents used in medical imaging?
What needs to be considered before administering contrast and why?
- they are used as they enhance images and give additional information about certain pathologies
- they are associated with some adverse effects / complications
- it is important to question whether the required information could be sought via a different imaging modality / without contrast
What are the 5 most commonly used contrast agents?
- iodine-based agents
- barium salts
- gadolinium
- air
- water
What is the difference between a “positive” and a “negative” contrast agent?
What are double contrast techniques and why are they used?
“Positive” agents:
- these block the transmission of x-rays through tissues
- pathologies often appear as filling defects
“Negative” agents:
- these include air and water and they distend structures, allowing for greater X-ray transmission
Double contrast techniques:
- this uses both types of agent and gives excellent detail of the gut mucosa
How can iodine-based agents be administered?
What are the 3 most common uses?
- they can be administered IV, for purposes such as:
- CTPA and CT head
- imaging the vasculature (e.g. coronary angiogram)
- imaging the urinary tract
- they can also be administered orally for CT scanning the abdomen
- preparations are taken PO approx 1 hour before imaging
When might water soluble iodine-based contrast be used for GI studies?
- water soluble iodine-based contrast can be used for GI studies instead of barium if there is a risk of peritoneal contamination
- e.g. fistula, suspected colonic obstruction
What is shown in this image?
Why is timing important?

CT pulmonary angiogram (CTPA) using iodine contrast
- timing is important as images need to be acquired at the point that the contrast is in the arterial circulation (arterial phase)
What is the technique used in a coronary angiogram?
What type of contrast is used?
- coronary angiography uses iodinated contrast
- the technique used is fluoroscopy, which is real time imaging using X-rays
- the image can be processed to make the vessels clearer - digital subtraction angiograhy

What other types of investigation use iodinated contrast?
- micturating cystourethrogram
- hysterosalpingogram
- intravenous pyelogram / urography
- sialogram
- cholangiography

What is the imaging modality of choice for imaging the urinary tract for renal calculi?
non-contrast CT
- CT is more sensitive for renal calculi
- the radiation dose between CT and X-ray is similar, and other pathologies may be seen on CT

What are the adverse effects of iodinated agents?
- hypersensitivity / allergic reactions
- anaphylaxis
- bronchospasm
- angio-oedema
- urticarial reactions
- minor side effects, such as nausea
- contrast-induced nephropathy (CIN)
- caution should be taken in thyroid disease
What is contrast-induced nephropathy (CIN)?
When does it occur and why?
- acute renal impairment that occurs within 3 days of contrast administration, in the absence of other precipitating causes
- it occurs secondary to the toxic effects of contrast on the renal tubular epithelium
- renal ischaemia is also implicated
Who is more at risk of the side effects of iodinated contrast?
- there are some conditions that increase the risk of an allergic / adverse reaction to iodine-based contrast materials
- previous adverse reactions to iodine-based contrast
- history of asthma
- history of allergy
- heart disease
- renal disease
- dehydration
What conditions are associated with an increased risk for an adverse reaction to iodinated contrast?
- history of a previous reaction to iodinated contrast (200x increased risk)
- hyperthyroidism
- family history of hypersensitivity reaction to iodinated contrast
- allergic diseases, including asthma
- previous history of multiple allergies
- allergic diseases and allergies are not contraindications to the use of contrast, but the risk of severe contrast reactions is elevated
- a more detailed history about the allergies should be obtained
What are some other medical conditions are associated with putting someone at increased risk of contrast administration?
- asthma
- renal problems
- diabetes mellitus
- metformin therapy
If someone has had a previous allergic reaction to contrast, what steps should be taken prior to administration?
- need to determine the exact nature of the previous reaction and the specific compound used on that occasion
- re-examine the need for contrast and consider an unenhanced study or other methods of investigation
- assess the risk-benefit ratio of the procedure, as a non-diagnostic examination may be more detrimental to the patient than the risk from contrast exposure
- if the injection is deemed necessary, use a different contrast compound to the one previously used
What factors increase the risk of AKI in patients receiving iodinated contrast?
- chronic kidney disease (eGFR < 40 ml/min/1.73m3
- heart failure
- renal transplant
- age 75 or over
- hypovolaemia
- increasing dose of contrast or repeated administration of contrast
- intra-arterial adminsitration of contrast
In what groups of people should precaution with iodinated agents be taken?
- people with a history of allergic reactions / anaphylaxis
- people with pre-existing renal disease - this often includes people with diabetes
- people who take nephrotoxic drugs (there are lots - check the BNF in doubt)
- preventative measures should be taken in those with moderate-severe renal disease
What preventative measures can be taken to reduce risk of CIN in those with moderate to severe renal failure?
- the dose of contrast medium should be minimised, taking into account the indication and body weight
- iodine-based contrast should be maximised by using a saline flush for intravenous volume expansion
- consider stopping ACEi and angiotensin II receptor blockers when eGFR < 40
- maintain a hydration regime post contrast administration
What decisions need to be made before contrast is given?
How much contrast is given?
- how much contrast should be given?
- how fast should the contrast be given?
- when should the scan be performed?
- a standard dose is given in adults, unless they are very small/large but it is weight-dependent in children
- how fast the contrast is given depends on the bore of the cannula used
- when the scan is performed depends on what is needed to be seen in the image
What are the 3 contrast phases and when are they used?
Arterial phase:
- used to look at arteries, including the pulmonary arteries
- useful for identifying GI bleeds, ?PE and in CT angiograms
Portal venous phase:
- used to look at abdominopelvic viscera and venous structures
Delayed phase:
- this allows tissues to enhance, which is good for many cancers
- it can be used to see if contrast “washes out” quickly or not, which can identify adrenal lesions, parathyroid adenoma and liver lesions
- used to look at the urinary tract as it needs time to be excreted
When is barium used as a contrast agent?
- it is a “positive” contrast agent that is used widely in fluoroscopic imaging of the GI tract
- barium contrast agent coats the luminal surface of the gut

How is barium administered?
How are images taken and what pathologies is this good for visualising?
- barium is administered PO or PR
- real-time X-ray images are then taken (fluoroscopy)
- this can give functional, dynamic information as well as structural / anatomical information
- it is good for visualising pathologies such as malignancy and polyps**, as well as visualising the **terminal ileum in Crohn’s disease
What are the drawbacks of using barium salts?
What are the potential complications?
- patients must have adequate bowel preparation for these studies
- barium contrast is dense, so CT abdomen is contraindicated for several days after barium enema
- barium studies deliver a large radiation dose to the patient
- potential complications are chemical pneumonitis or peritonitis







