Physics of Imaging

Question 1

Treatment of Contrast Media Reaction

Answer 1

Question 2

Contrast media reaction overview

Answer 2

Acute reactions to intravenous contrast can be divided into allergic-type and non-allergic-type mechanisms. Both allergic-type and non-allergic-type reactions can range in severity from mild and self-limited to severe and life-threatening.

Patients with asthmas are at increased risk of an allergic reaction to contrast medium.

A seafood or shellfish allergy is not associated with allergic reaction to contrast.

Mild nausea, sensation of warmth, and flushing are considered physiologic and are not adverse reactions to contrast.

Question 3

Mild contrast reactions

Answer 3

A mild contrast reaction is self-limited and does not require medical management.

A vasovagal reaction to intravenous contrast is rare and is characterized by bradycardia and hypotension. Mild vasovagal reactions are usually self-limited and are not allergic in etiology.

Urticarial reactions are mild allergic-type reactions, and include hives and mild angioedema. The symptoms of mild angioedema include scratchy throat, slight tongue or facial swelling, and sneezing, and generally do not require medical management.

Question 4

Moderate contrast reactions

Answer 4

Moderate contrast reactions are not immediately life-threatening but may require medical management.

Moderate allergic-type reactions include severe urticaria, bronchospasm, moderate tongue/facial swelling, and transient hypotension with tachycardia.

Moderate non-allergic-type reactions include significant vasovagal reaction, pulmonary edema, bronchospasm, and limited seizure.

Question 5

Severe contrast reactions

Answer 5

Severe contrast reactions to intravenous iodinated contrast may be immediately life-threatening.

Allergic-type severe reactions include anaphylaxis and angioedema. Symptoms may be varied and include altered mental status, respiratory distress, diffuse erythema, severe hypotension, or cardiac arrest.

Non-allergic-type severe reactions include severe pulmonary edema, severe bronchospasm, and severe seizure.

Question 6

Premedication to prevent contrast reaction

Answer 6

In a patient with a known contrast allerg, a repeat contrast reaction is most likely to be similar to the prior reaction. However, the repeat reaction may be either more or less severe. Therefore, if IV contrast is necessary for a patient who has had a previous reaction, a premedication regimen is recommended, although contrast reactions may occur despite premedication. Intravenous contrast is generally contraindicated in patients who have had a prior severe allergic-type reaction.

Elective premedication: Prednisone 50 mg PO at 13 hours, 7 horus, and 1 hour before the exam, plus diphenhydramine 50 mg (IV or PO) 1 hour before.

Emergent premedication: Hydrocorisone 200 mg IV Q4h, 1-2 times prior to administration of IV contrast. Diphenhydramine is given 1 hour prior. Note that IV steroids have not been shown to be effective when given sooner than 4 hours before the contrast administration.

Question 7

Risk of contrast-induced nephropathy (CIN)

Answer 7

Contrast-induced nephropathy (CIN) is a decrease in renal function of unkown etiology following the intravascular (venous or arterial) administration of iodinated contrast.

The most important risk factor for development of CIN is preexisting renal insufficiency.

For patients with eGFR <30 ml/min/1.73m2, the risk of CIN is between 7.8 and 12.1%.

For patients with eGFR <30 and <45, the risk of CIN is between 2.9 and 9.8%.

For patients with eGFR>45 and <60, the risk of CIN is between 0 and 2.5%.

The development of CIN in patients with normal renal function (eGFR >60 ml/min/1.73m2) is exceptionally rare.

Note that gadolinium-based contrast media are not known to cause contrast-induced nephropathy.

Patients with mulitple myeloma are at increased risk of irreversible renal failure after receiving high-osmolality contrast media from tubular protein precipitation. There are no data on the risk of the low or iso-osmolar contrast agents in current use.

Question 8

Prevention of contrast nephropathy

Answer 8

The main prevention strategies against CIN are to use the minimal dose of contrast possible and to adequately hydrate the patient. The use of sodium bicabonate adn N-acetylcysteine has been previously advocated but the effectiveness of these agents has not been proven.

Patients with an eGFR >30 and <60 typically receive approximately 2/3 the standard contrast dose. Administration of intravenous contrast to a patient with an eGFR <30 would require a careful assessment of risks and benefits on a case by case basis.

The standard dose of intravenous iodinated contrast can generally be given to patients on dialysis. Careful attention should be paid to the volume status in these patients as theoretically the osmotic load increases intravascular volume.

Question 9

Iodinated contrast and pheochromocytoma

Answer 9

It is safe to administer nonionic contrast media to patients with pheochromocytoma. Prior studies showed an increased in serum catecholamines after high-osmolality contrast agents, which are no longer in current use.

Question 10

Iodinated contrast and thyroid uptake

Answer 10

Thyroid gland uptake of I-131 is reduced to about 50% one week after iodinated contrast injection. Therefore, if radioactive I-131 therapy is planned, iodinated contrast should be avoided for a few weeks prior to I-131 therapy.

Question 11

Metformin and intravenous contrast

Answer 11

Metformin is an oral anti-hyperglycemic agent that decreases hepatic glucose production and increases peripheral glucose uptake. Although exceptionally rare, there is an increased risk of metformin-associated lactic acidosis in patients receiving intravenous iodinated contrast, thought to be due to CIN and the resultant accumulation of metformin.

There is no need ot discontinue metformin in patients with normal renal function. In patients with multiple comorbidities, metformin should be discontinued at time of contrast administration and withheld for 48 hours.

Note that it is not necessary to discontinue metformin prior to gadolinium-based contrast.

Question 12

Iodinated contrast and pregnancy

Answer 12

Iodinated contrast crosses the placenta and enters the fetal circulation. No mutagenic or teratogenic effects have been observed; however, no controlled studies in pregnant patients have been performed.

It is acceptable to administer iodinated contrast to a pregnant patient if medically necessary. It is recommended that a pregnant patient sign an informed consent form prior to undergoing an examination involving ionizing radiation and iodinated contrast.

Question 13

Iodinated contrast and breast feeding

Answer 13

The plasma half-life of iodinated contrast is approximately 2 hours. Less than 1% of the administered maternal dose of iodinated contrast is excreted in the breast milk within 24 hours of maternal administration, and 1% of that dose may be absorbed by the infant’s gastrointestinal tract. The total infant absorbed dose is therefore approximately 0.01% of the administered maternal dose.

Breast feeding mothers do not need to halt breast feeding. If the mother is concerned, abstaining from breast feeding for 24 hours (while pumping and discarding milk) would result in effectively zero fetal dose.

Question 14

Contrast extravasation

Answer 14

Extravasation is the leakage of contrast into the soft tissues at the injection site. Although the risk of extravasation is not related to the injection flow rate, the use of automatic injectors can lead to a large extravasated volume of contrast media.

Iodinated contrast is toxic to the soft tissues and skin, although serious adverse events are relatively rare following extravasation. The most common serious injury due to contrast extravasation is compartment syndrome. Less commonly, skin ulceration and tissue necrosis can occur.

All patients with extravasation should be evaluated by the radiologist. Elevation of the extremity to decrease capillary hydrostatic pressure has been recommended, but is without supporting data. There is no evidence favoring warm or cold compresses, and both are used commonly.

Surgical consultation should be obtained for progressive swelling and pain, altered tissue perforation (decreased capillary refill), change in sensation, or skin ulceration or blistering.

Question 15

Immediate adverse reactions to gadolinium-based contrast

Answer 15

Both mild and severe adverse reactions to gadolinium-based contrast are much more rare compared to iodinated contrast. Most adverse reactions to gadolinium-based contrast are mild such as nausea, vomiting, headache, or pain at the injection site.

Allergic-type reactions to gadolinium are rare, seen in 0.004% to 0.7%.

Serious anaphylactic reactions are exceeding rare (<0.01%).

Question 16

Contrast extravasation

Answer 16

Gadolinium-based contrast agents are much less toxic to the skin and soft tissues compared to iodinated contrast.

Evaluation and treatment of extravasation is similar to both types of contrast.

Question 17

Nephrogenic systemic fibrosis (NSF)

Answer 17

Nephrogenic systemic fibrosis (NSF) is a highly morbid disease characterized by diffuse fibrosis of the skin and subcutaneous tissues, which may also involve the visceral organs.

NSF is strongly associated with gadolinium exposure in patients with reduced renal function. The exact mechanism for development of NSF is unkown, but may involve dissociation of toxic free gadolinium in patients with reduced renal clearance. The free gadolinium may bind phosphate and precipitate tissues, inducing a fibrotic reaction.

Patients wtih end-stage renal disease (eGFR <30 ml/min/1.73 m2) have between 1% and 7% chance of developing NSF even after a single exposure to a gadolinium-containing contrast agent. There has been only one established case of NSF developing in a patient with an eGFR >30.

In general, gadolinium-based contrast should not be given to patients on renal dialysis or with eGFR < 30. Although it is exceedingly rare for NSF to develop in a patient with eGFR between 30 and 59, eGFR may fluctuate daily in these patients. For this reason, caution should be employed for patients on the lower end of this spectrum.

Different brands and formulations of gadolinium-baed contrast are associated with varying rates of NSF.

Question 18

Gadolinium-based contrast and pregnancy

Answer 18

Gadolinium based contrast should not be administered during pregnancy. Gadolinium-based contrast crosses the placenta. Although never demonstrated to cause harm, gadolinium chelates may accumulate in the amniotic fluid and remain there indefinitely, with risk of dissociation of toxic free gadolinium ion.

Question 19

Gadolinium-based contrast and breast feeding

Answer 19

The plasma half-life of gadolinium-based contrast is 2 hours. Less than 0.04% of the administered maternal dose is excreted in the breast milk within 24 hours of maternal administration, and 1% of that dose may be absorbed by the infant’s gastrointestinal tract. The total infant absorbed dose of gadolinium is therefore approximately 0.0004% of the administered dose.

It is likely safe for the mother to continue breast feeding. There is no information to suggest that oral ingestion of such a tiny ammount of gadolinium-containing contrast may be harmful to the fetus.

Question 20

Measuring radiation

Answer 20

Exposure is the charge of electrons liberated per unit mass of air. Exposure is measured in Coulombs/kg.

Air kerma (kinetic energy released per mass) describes the incident X-ray beam intensity as the kinetic energy transferred from uncharged particles (photons) to charged particles (electrons). Air kerma is measured in Gray (J/kg).

Air Kerma (Gy) can be converted to absorbed dose (also quantified in Gy) by the R factor, which depends on kV and the atomic number (Z) of the adsorbing tissue. Bone (Z=12) absorbs much more energy than soft tissue (Z~7.6). At 10 mGy air kerma, bone absorbs 40 Gy, tissue absorbs 11 Gy.

The equivalent dose (expressed in Sievert; Sv) is athe absorbed dose in Gy multiplied by a radiation weighting factor (Wr). Wr depends on the linear energy transfer (LET) of the type of radiation. For diagnostic radiology using X-rays, Wr=1. An alpha particle has a high LET.

The effective dose (also expressed in Sv) is an estimation of radiation exposure that takes into account the equivalent dose to all organs exposed and each organ’s radiosensitivity.

Effective dose = the sum of the absorbed dose (Gy) * Wr * Wt for all organs exposed. Wt is the tissue weighting factor, which varies from 0.12 for radiosensitive organs (e.g. bone marrow, colon, lung, breast), to 0.01 for less sensitive organs (e.g. bone, brain, and skin).

Question 21

Radiation units

Answer 21

SI (Systeme International) units including Gray and Sievert are almost universally used rather than the older non-SI units. One notable exception is the Curie, which is still commonly used in nuclear medicine (typical dosing ranges are in millicuries).

SI (Systeme International) units

• Gray quantifies both absorbed dose and air kerma (exposure) as energy absorbed per unit mass.
• 1 Gy = 1 J/kg
• 1 Gy = 100 Rad
• Sievert (Sv) is used to quantify equivalent dose and effective dose
• 1 Sv = 100 Rem
• A Becquerel (Bq) is only used in nuclear medicine for radioactive materials
• 1 Bq = 1 disintegration per second.

Non-SI units

• 1 Rad (radiation absorbed dose) = 10 mGy
• 1 Rem = 1 mSv
• 1 Curie = 37 million Becquerels = 3.7 e10 disintegrations per second.

Question 22

X-ray generator

Answer 22

X-ray photons are generated when high-energy electrons hit a target at the anode side of the circuit. For general radiography and CT, the target is made of tungsten (atomic symbol W). 99% of the electrons’ kinectic energy is converted to heat, and 1% is converted to X-rays.

90% of X-rays are produced from bremsstrahlung (nuclear field interaction). The maximum keV (energy) of the X-ray spectum is the kV of the generator. The average keV ~1/3 max.

10% of X-rays are produced from characteristic radiation. A characteristic X-ray is produced when a high energy electron knocks a K-shell electron out of orbit.

Tungsten: Atomic weight = 74; K-edge = 70 keV; characteristic X-rays <70 keV.

Question 23

Effect of kV on tube output

Answer 23

X-ray production is proportional to (kV)².

In practice, changing kv is complicated as change in kV causes a change in characteristic X-ray, shifts spectrum to the right, and adds photons.

Increase in kV by 15% = increase in photons by 100%.

In general, increasing kV will decrease dose and decrease contrast when automatic exposure control is used.

Question 24

Heel effect

Answer 24

The heel effect is due to attenuation at the anode, causing fewer X-rays at the anode side.

The main contributor to the heel effect is the anode angle (typical anode angle ~15 degrees).

Decrease anode angle = increase heel effect.

Question 25

X-ray interactions with matter

Answer 25

(All of these interactions deal with incoming photons)

Cohereent scatter: No exchange of energy, no change in frequency, and no contribution to patient dose. Contributes less than 5% of X-ray interactions.

Compton scatter: Proportional to (electron density)/E. Scattered photons go in all directions. Compton scatter dominates at >25 keV in soft tissue and >40 keV in bone.

E = energy of incoming photon (keV).

Photoelectric effect: Proportional to Z³/E³. Photoelectric effect dominates at <25 keV in soft tissue and <40keV in bone.

Question 26

Linear attenuation coefficient

Answer 26

N = N0 * e(-ut)

N0 = initial number of photons. N = transmitted number of photons after thickness t. e(-ut) = fraction transmitted.

If u is “small” (<0.1/cm), then u is the proportion fo photons that interact with matter. (This is only true for weakly attenuating material).

If u is “large” (>0.1/cm), then the formula above is used. (Example: If u=0.5/cm then e(-ut) = 61% transmitted.

Question 27

Mass attenuation coefficient

Answer 27

Mass attenuation coefficient=(u/p), where p=density.

Because density is accounted for in the formulat above, the mass attenuation coefficient is independent of density.

Question 28

Scatter and grids

Answer 28

In radiography, the typical scatter: primary ratio is 5-10:1.

Scatter decreases contrast.

Grid ratio = height/width

Typically grid ratios in radiography are between 8 and 12 (the height of the grid is 8-12 times the width of the grid elements).

About 70% of primary radiation passes through the grid.

Bucky factor is the releative increase dose due to grid=incident/transmitted radiation. The Bucky factor should not be confused with the grid ratio. Typical Bucky factor is 5-10 (e.g., increase MAs from 40 -> 200).

Increase kV will increase scatter (Compton effects dominante at higher kV).

Grids not used for extremity radiography (bone is increased Z and not very thick).

Question 29

Beam quality and half value layer (HVL)

Answer 29

A “high quality” beam has low-energy photons filtered out.

The half-value layer (HVL) is a measurement of the beam quality and is the thickness of material that attenuates 50% of the incident energy.

Aluminum (Al) is the standard material for measuring HVL.

Typical HVL for mammography is 0.3 mm Al.

Typical HVL for radiography is 3 mm Al. State regulations require beam quality: HVL >2.5 mm Al at 80 kVp.

Typical HVL for CT is 8-9 mm Al.

Question 30

Film opitical density (OD) and characteristic curves

Answer 30

Film optical density (OD) = log10(I0/It) = log10(incident/transmitted light intensity)

OD of 1 -> 10% photons transmitted through film

OD of 2 -> 1% transmitted

OD of 3 -> 0.1% transmitted

Film “looks good” if the average OD is ~1.5. This occurs after approximately 5 uGy photons hit the film/screen.

A characteristic curve logarithmically plots the relationship between radiation exposure (air kerma) and film optical density. The toe is the low-exposure region and the shoulder is the high exposure region. Fog is the baseline low-level darkening of film that occurs in the absence of radiation exposure.

Latitude is the range of air kerma with satifactory film density. A high-latitude film is ideal for imaging body parts with a wide variation of X-ray transmission (e.g., a chest radiograph). A high-contrast (low-latitude) film is ideal for accentuating image contrast in tissues with low subject contrast (e.g. mammography).