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.
From the excellent Trauma Professionals Blog:
CT Scan Images Simplified
Ever wonder what is going on when you drag your mouse across a CT
image, or when you change the “window” settings of an image from lung to
abdomen? It all has to do with the way CT generated xray information is
displayed, and how your eyes and brain perceive it.
Let’s get down to basics. The first thing needed is
to understand the concept of radiodensity. The CT scanner uses a set of
software algorithms to determine the amount of x-radiation absorbed by
every element in a plane of tissue. Each of these elements is
represented by a pixel on the video display, and the density (amount of x-radiation absorbed) is measured in Hounsfield units.
This scale was developed by Sir Godfrey Hounsfield, who set the
radiodensity of water at 0, and air at -1000. The scale extends in the
positive direction to about +4000, which represents very dense metals.
See the table for the density of common substances on CT.
When you view a CT scan on a video display, two important numbers are displayed on screen. The first is the window width (W),
which describes the range of Hounsfield units displayed. The maximum
window width possible is usually about 2000, but our eyes are not
capable of seeing this many shades. Actually, we can really only
distinguish about 16 shades of gray. So the window width is divided by
16, and each group of Hounsfield values is converted to one of 16 shades
of gray. The lowest Hounsfield numbers in the window range are shown as
black, and the highest are white.
The second important number is the window level (L). This is the Hounsfield number in the center of the window width. So let’s look at some typical examples of W/L settings.
The abdomen contains mostly soft tissue, which is just a little
denser than water. So most of the abdominal contents have Hounsfield
values from 0 to 100 or so. A typical abdominal scan W/L setting is
350/50. This means that a total range of 350 different densities are
displayed, centered on a density of 50 Hounsfield units ( range is -125
to 225 HU). Each difference of 22 HU will show up as a different shade
of gray. So this narrow window allows us to distinguish relatively
subtle differences in density.
The chest cavities are primarily air-filled, and the lungs are very
low density. So it makes sense that a typical lung W/L setting is
1500/-500. The window ranges from -1250 to +250 HU, and a wider range of
94 HU represents one shade of gray. This is typical of body regions
with a wider range of densities.
Finally, bone windows are usually 2000/250. This window is centered
above the usual tissue densities, and is very wide so that it shows a
wide range of densities in only 16 shades of gray. Thus, the contrast
appears very low.
On most displays, the window width increases as you drag the mouse to
the right. This increases the range of densities in a shade of gray,
thus decreasing the overall amount of contrast in the image. Dragging
the mouse down decreases the window level, moving it toward the air end
of the spectrum. This allows you to center your window on the type of
tissue you are interested in viewing and adjust your ability to
distinguish objects with a lot or only a little contrast (see table
I apologize to my radiology colleagues in advance for this simplistic
explanation. Trauma professionals have minimal exposure (pun intended)
to the physics and details of radiographic imaging. We are much more
interested in effectively using this technology to save our patients’
Pearls for PE
More PEs in caudad vessels Look for contrast leak around embolism Use the patients 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 ReactionsUSE A LOW OSMOLALITY CONTRAST AGENTthen Prednisone 50mg PO or hydrocortisone 200mg IV 13 hours, 7 hours, and 1 hour before contrastplus Diphenhydramine 50mg IV/IM/PO 1 hour before contrastOR Methyprednisolone 32 mg PO 6-12 hours and 2 hours before contrastOR(emergency only) Hydrocortisone 200mg IV 1 hour before contrast and every 4 hours thereafter plus diphenhydramine until procedure completed
IV contrast: allergy and nephrotoxicityAllergy 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, 4th 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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