CT Scan Info

Emedhome: On CT, structures are assigned a Hounsfield unit number representing their relative density. Air is assigned a value of -1000, water 0, and bone +1000. Fat, being less dense than water but more dense than air, has a value of approximately -50. Soft tissues such as muscle are somewhat denser than water and have an approximate value of +40. A grayscale is then assigned, with the densest structures appearing white and the least dense appearing black. This grayscale can be shifted to accentuate anatomic detail. For example, if the user is interested in viewing details of bone, the computer reassigns the entire grayscale to values just below +1000 Hounsfield units, allowing differentiation of subtle detail within bone. Detail of other structures is not visible on this setting, because all structures that are significantly less dense than bone appear completely black. If the user is interested in viewing lung detail, the grayscale is assigned to values near -1000 Hounsfield units, to accentuate details of low density lung. Details of bone would be obscured on this setting, because all structures that are significantly denser than air would appear completely white.

Pearls for PE

• More PEs in caudad vessels
• Look for contrast leak around embolism
• Use the patient’s pain location as an aid
• Check lung windows

Pitfalls for PE

• Motion – false negative
• Poor bolus – false negative or positive
• Lymph nodes – false positive
• Tachypnea – false negative
• Slow scanner– false negative
• Obese patient– false negative
• External compression– false positive

Algorithm for evaluating CT for PE

1. Choose vascular window setting
2. Assess contrast bolus quality
3. Assess for motion artifact
4. Locate the main pulmonary artery and inspect for saddle embolism
5. Moving cephalad/caudad, inspect each large order vessel for emboli

Pitfalls in Aortic Imaging

Poor contrast bolus
Aortic motion artifact
Normal anatomic variants

Algorithm for evaluating CT for aortic pathology

1. Choose vascular window setting

2. Assess contrast bolus quality

3. Assess for motion artifact

4. Inspect the ascending aorta, arch, descending aorta, and branch vessels:

Suspect dissection or trauma when the aortic contour is not smooth
Identify intimal flaps
Inspect for extravasating contrast
Look for difference in contrast density in true and false lumens

Proposed Rapid Pretreatment Protocol to Prevent Allergic Contrast Reactions

USE A LOW OSMOLALITY CONTRAST AGENT

then

Prednisone 50mg PO or hydrocortisone 200mg IV
13 hours, 7 hours, and 1 hour before contrast

plus

Diphenhydramine 50mg IV/IM/PO
1 hour before contrast

OR

Methyprednisolone 32 mg PO
6-12 hours and 2 hours before contrast

OR

(emergency only)

Hydrocortisone 200mg IV 1 hour before contrast and every 4 hours thereafter plus diphenhydramine until procedure completed

IV contrast: allergy and nephrotoxicity

Allergy to iodinated contrast agents is relatively rare, with an incidence of 3-15% for mild reactions but only 0.004 to 0.04% for very severe reactions. Fatal reactions occur in only 1 in 170,000 (45). Risk factors for allergy include asthma (6 to 10 fold risk) and severe allergies to any other substance. Seafood allergies do not appear to constitute a specific additional risk factor, although severe allergy to seafood, peanuts, or any other substance carries a risk. Seafood allergies are thought to be mediated by proteins in seafood, not iodine. For a variety of reasons, new low osmolality contrast agents have a lower potential for allergic reaction (5 times lower for mild reactions, 10 times lower for severe reactions) and should be considered for high-risk patients. These agents are somewhat more expensive than standard high osmolality agents (approximately $40 per patient), which has prevented their universal use (46). Most institutions will have these agents readily available upon request. Anaphylactoid reactions, which are not true allergy but have similar presentation and emergency treatment, may also occur (47).

Pre-treatment to prevent allergic reaction can be performed, but most regimens require 12-24 hours of pre-treatment and are impractical in the emergency department. The American College of Radiology recommends a minimum of 6 hours between steroid and contrast administration, whether steroids are administered orally or intravenously. A rapid pre-treatment protocol beginning 1 hour before contrast has been described (see text box) (45,48,49). Pretreatment would be appropriate for patients who report mild or moderate prior contrast reactions, or for those at high risk such as patients with severe asthma or prior anaphylaxis to other antigens such as peanuts or shellfish. If possible, contrast should be avoided entirely in patients with severe prior contrast reactions, as breakthrough severe reactions can occur despite pre-medication and may be severe in 24% of cases (50), so alternative methods of diagnosis should be considered in high-risk patients.

(45) Morcos SK, Thomsen HS. Adverse reactions to iodinated contrast media. Eur Radiol (2001) 11: 1267-1275.

(46) Valls C, Andia E, Sanchez A, Moreno V. Selective use of low-osmolaltiy contrast media in computed tomography. Eur Radiol (2003) 13: 2000-2005.

(47) Manual on Contrast Media, 4^th Edition. American College of Radiology. 1998.

(48) Greenberger PA, Halwig JM, Patterson R, Wallemark CB. Emergency administration of radiocontrast media in high-risk patients. J Allergy Clin Immunol. 1986 Apr;77(4):630-4.

(49) Lasser EC, Berry CC, Mishkin MM, Williamson B, Zheutlin N, Silverman JM. Pretreatment with corticosteroids to prevent adverse reactions to nonionic contrast media. AJR Am J Roentgenol. 1994 Mar;162(3):523-6.

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