{"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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<\/a><\/a><\/a><\/a><\/a><\/a><\/p>\n From the excellent Trauma Professionals Blog<\/a>: CT Scan Images Simplified<\/strong><\/p>\n Ever wonder what is going on when you drag your mouse across a CT Let\u2019s get down to basics.<\/strong> The first thing needed is 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>, 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 The chest cavities are primarily air-filled, and the lungs are very Finally, bone windows are usually 2000\/250. This window is centered On most displays, the window width increases as you drag the mouse to I apologize to my radiology colleagues in advance for this simplistic 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 <\/p>\n 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 <\/p>\n <\/p>\n <\/a><\/a><\/a><\/a><\/a><\/a><\/p>\n <\/p>\n <\/p>\n Pitfalls in Aortic Imaging<\/b><\/p>\n Algorithm for \t\tevaluating CT for aortic pathology<\/b><\/p>\n <\/p>\n 1. Choose vascular \t\twindow setting<\/p>\n 2. Assess contrast \t\tbolus quality<\/p>\n 3. Assess for motion \t\tartifact<\/p>\n 4. Inspect the \t\tascending aorta, arch, descending aorta, and branch vessels:<\/p>\n <\/p>\n Proposed Rapid Pretreatment Protocol to Prevent Allergic Contrast ReactionsUSE A LOW OSMOLALITY CONTRAST AGENTthen<\/b> Prednisone 50mg PO or hydrocortisone 200mg IV 13 hours, 7 hours, and 1 hour before contrastplus<\/b> Diphenhydramine 50mg IV\/IM\/PO 1 hour before contrastOR<\/b> Methyprednisolone 32 mg PO 6-12 hours and 2 hours before contrastOR(emergency only)<\/b> Hydrocortisone 200mg IV 1 hour before contrast and every 4 hours thereafter plus diphenhydramine until procedure completed <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n IV contrast: allergy and nephrotoxicity<\/b>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)<\/a>. 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)<\/a>. 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)<\/a>. 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)<\/a>. 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)<\/a>, so alternative methods of diagnosis should be considered in high-risk patients.<\/p>\n <\/p>\n (45)<\/a> Morcos SK, Thomsen HS. Adverse reactions to iodinated contrast media. Eur Radiol (2001) 11: 1267-1275. (46)<\/a> 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)<\/a> Manual on Contrast Media, 4th Edition. American College of Radiology. 1998. (48)<\/a> 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)<\/a> 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. <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n | | | <\/p>\n","protected":false},"excerpt":{"rendered":" Array<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[6],"tags":[],"yoast_head":"\n
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\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
\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
\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
\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
\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
\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
\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
\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\n
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