{"id":5318,"date":"2011-07-14T20:25:26","date_gmt":"2011-07-15T00:25:26","guid":{"rendered":"http:\/\/crashtext.org\/misc\/ct-scan-info.htm\/"},"modified":"2012-01-25T13:39:57","modified_gmt":"2012-01-25T18:39:57","slug":"ct-scan-info","status":"publish","type":"post","link":"https:\/\/crashingpatient.com\/imaging\/ct-scan-info.htm\/","title":{"rendered":"CT Scan Info"},"content":{"rendered":"

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CT Scan Info<\/p>\n

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.<\/p>\n

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From the excellent Trauma Professionals Blog<\/a>:
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\"CT<\/div>\n
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CT Scan Images Simplified<\/strong><\/p>\n

Ever wonder what is going on when you drag your mouse across a CT
\nimage, or when you change the \u201cwindow\u201d settings of an image from lung to
\n abdomen? It all has to do with the way CT generated xray information is
\n displayed, and how your eyes and brain perceive it.<\/p>\n

Let\u2019s get down to basics.<\/strong> The first thing needed is
\nto understand the concept of radiodensity. The CT scanner uses a set of
\nsoftware algorithms to determine the amount of x-radiation absorbed by
\nevery element in a plane of tissue. Each of these elements is
\nrepresented by a pixel on the video display, and the density (amount of x-radiation absorbed) is measured in Hounsfield units<\/strong>.
\n This scale was developed by Sir Godfrey Hounsfield, who set the
\nradiodensity of water at 0, and air at -1000. The scale extends in the
\npositive direction to about +4000, which represents very dense metals.
\nSee the table for the density of common substances on CT.<\/p>\n

When you view a CT scan on a video display, two important numbers are displayed on screen. The first is the window width (W)<\/strong>,
\n which describes the range of Hounsfield units displayed. The maximum
\nwindow width possible is usually about 2000, but our eyes are not
\ncapable of seeing this many shades. Actually, we can really only
\ndistinguish about 16 shades of gray. So the window width is divided by
\n16, and each group of Hounsfield values is converted to one of 16 shades
\n of gray. The lowest Hounsfield numbers in the window range are shown as
\n black, and the highest are white.<\/p>\n

The second important number is the window level (L)<\/strong>. This is the Hounsfield number in the center of the window width. So let\u2019s look at some typical examples of W\/L settings.<\/p>\n

The abdomen contains mostly soft tissue, which is just a little
\ndenser than water. So most of the abdominal contents have Hounsfield
\nvalues from 0 to 100 or so. A typical abdominal scan W\/L setting is
\n350\/50. This means that a total range of 350 different densities are
\ndisplayed, centered on a density of 50 Hounsfield units ( range is -125
\nto 225 HU). Each difference of 22 HU will show up as a different shade
\nof gray. So this narrow window allows us to distinguish relatively
\nsubtle differences in density.<\/p>\n

The chest cavities are primarily air-filled, and the lungs are very
\nlow density. So it makes sense that a typical lung W\/L setting is
\n1500\/-500. The window ranges from -1250 to +250 HU, and a wider range of
\n 94 HU represents one shade of gray. This is typical of body regions
\nwith a wider range of densities.<\/p>\n

Finally, bone windows are usually 2000\/250. This window is centered
\nabove the usual tissue densities, and is very wide so that it shows a
\nwide range of densities in only 16 shades of gray. Thus, the contrast
\nappears very low.<\/p>\n

On most displays, the window width increases as you drag the mouse to
\n the right. This increases the range of densities in a shade of gray,
\nthus decreasing the overall amount of contrast in the image. Dragging
\nthe mouse down decreases the window level, moving it toward the air end
\nof the spectrum. This allows you to center your window on the type of
\ntissue you are interested in viewing and adjust your ability to
\ndistinguish objects with a lot or only a little contrast (see table
\nabove).<\/p>\n

I apologize to my radiology colleagues in advance for this simplistic
\n explanation. Trauma professionals have minimal exposure (pun intended)
\nto the physics and details of radiographic imaging. We are much more
\ninterested in effectively using this technology to save our patients\u2019
\nlives.<\/p>\n<\/div>\n

Pearls for PE<\/b><\/p>\n

\u0095 More PEs in caudad vessels \t\t\u0095 Look for contrast leak around embolism \t\t\u0095 Use the patient\u0092s pain location as an aid \t\t\u0095 Check lung windows \t\t <\/p>\n

Pitfalls for PE<\/b><\/p>\n

\u0095 Motion \u0096 false negative \t\t\u0095 Poor bolus \u0096 false negative or positive \t\t\u0095 Lymph nodes \u0096 false positive \t\t\u0095 Tachypnea \u0096 false negative \t\t\u0095 Slow scanner\u0096 false negative \t\t\u0095 Obese patient\u0096 false negative \t\t\u0095 External compression\u0096 false positive \t\t <\/p>\n

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Algorithm for evaluating CT for PE<\/b><\/p>\n

1. Choose vascular window setting \t\t2. Assess contrast bolus quality \t\t3. Assess for motion artifact \t\t4. Locate the main pulmonary artery and inspect for saddle embolism \t\t5. Moving cephalad\/caudad, inspect each large order vessel for emboli \t\t <\/p>\n

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Pitfalls in Aortic Imaging<\/b><\/p>\n