Finally, if a word or phrase doesn’t add to the meaning of the report, delete it. Ra-diologists are not meant to create a mood, develop a complex story line, or vividly paint a character. Our aim is to eciently and eectively interpret the visual evi-dence presented in order to support an accurate clinical diagnosis and help direct treatment.
◆Grading and Measuring
When a measurement is necessary to es-tablish a diagnosis, or to provide the ap-proximate size of a mass, it should be recorded in either one dimension (round masses/cysts) or three dimensions (oblong or complex masses). Specific measure-
3
1 Introduction to Emergency Imaging
◆CT Window and Level
CT uses X-rays to create a three-dimen-sional density map of the patient. As an X-ray source rotates around the patient, the table supporting the patient slides through the scanner, perpendicular to the beam, and density data consisting of overlapping “voxels,” or tiny volume units, is acquired based on the X-ray attenuation as it pass-es through the patient. This data is con-verted into sectional images in any plane, along a curved path, or as one of several types of three-dimensional reformations. The tissue density value for each voxel is measured in Houndsfield units (HU), after Godfrey Houndsfield, one of the inven-tors of CT. The densities of air and distilled water are defined as – 1,000 and 0 HU, re-spectively. While there is no upper limit, medical scanners use a scale of – 1,024 to + 3,171 (Table 1.1).
The CT “window” is a representation of the range of tissue densities visible on an image. The window width defines which densities are distributed over the visible grayscale; everything outside of the win-dow is either black or white. The maxi-mum CT window includes all measurable densities and therefore extends over 4,096 HU, more than the ~ 700 shades of gray that humans can distinguish under optimal conditions. The level indicates the center of the window selected and is usually close to the density of the tissue being examined. Consider the following example.
If one wants to evaluate the skull base and calvarium, one assigns a level close to
happen to be the most common diseases one will encounter in any but the most specialized practices. Since the first major step toward independence as a radiologist is beginning overnight call, it makes sense that one’s work for the first 6 to 12 months of residency should be directed at prepara-tion for that experience.
Some suggestions for learning in the first year:
• Interpret (and report) as many studies as you can by yourself. Watching other radiologists report studies is a poor substitute for coming to your own conclusions before reviewing them with a more experienced radiologist.
• Read about the diseases you encounter during the workday. Keep a list.
• Follow up uncertain findings (and some of the ones you feel certain about). Speak with clinicians when you can; check pathology and laboratory reports on patients whose studies you have interpreted to be sure of your impression.
• Learn radiographic, CT, ultrasound, and magnetic resonance (MR) anatomy as it applies to image interpretation.
• Practice “taking cases” with a colleague on a regular basis and from the beginning of training. You will learn the most from your fellow residents.
• Read other radiologists’ reports, and pay attention to how clearly they express themselves. In your own reports, strive to develop concise written descriptions and terse, clear analysis of your findings.
• Learn the basic physical principles of radiography, CT, ultrasound, MR imaging (MRI), and nuclear medicine.
• Become acquainted with all common and many not-so-common emergent conditions.
• Know the imaging indications and the most appropriate studies for evaluating common conditions.
• Learn the various protocols for CT and MRI examinations and how to modify them to optimize a particular study.
Table 1.1 Densities of various tissues
TissueDensity (HU)
Air
– 1,000
Lung
– 500
Fat
– 100 to – 50
Water
0
CSF
15
Soft tissue
10–60
Blood
30–45
Bone
700–3,000
4Emergency Imaging
some patients are at increased risk for ad-verse, allergic-like reactions to contrast material. Such reactions range from mild to life-threatening; they can be mitigated by premedication, which reduces the inci-dence and severity of mild and moderate reactions and theoretically reduces the in-cidence of severe ones. The following dis-cussion and recommendations are based on the New York University Medical Center and Bellevue Hospital Radiology Depart-ment practice policies.
0 and a window of ~ 4,000. In this case, all 4,000 densities are distributed over ~ 700 distinguishable shades of gray. Soft tis-sues have densities between – 100 and 300, so their densities will be mapped to a relatively small number of gray tones in the middle of the window and will be in-distinguishable from each other. The vari-ous densities of bone (cortical bone, bone marrow, trabeculae) are distributed over a much larger number of HU, so they will be visible in detail.
To optimally study the subtle dier-ences in densities of the brain, on the other hand, one can a set a level of 30, the den-sity of the brain in HU, and a window of 80, a relatively narrow setting. In this case, only 160 densities are distributed over the visible range, permitting visualization of blood, scalp, white matter, gray matter, and cerebrospinal fluid (CSF). With narrow windows, bone detail is limited, and all bone elements appear white. With these settings fat (– 70) and air (– 1,000) are also both outside of the window margins and will appear black. Widening the window to 150 would increase the visibility of fat as distinct from air (Fig. 1.1).
◆ Approximate Radiation Doses for Emergency Studies (mSv)
It is useful to have a sense of the relative radiation doses for common examinations, in comparison to natural background ra-diation, which is approximately 3–4 mil-lisieverts (mSv) per year. Radiation doses above 10 mSv are associated with increased risk of cancer, with a 5% increased cancer risk at doses above 1,000 mSv (Table 1.2).
◆ CT Contrast Administration, Risks, and Adverse Reactions
Contrast material for CT examinations is administered via intravenous (IV) cath-eter. Rapid administration for arterial and venous visualization (as in pulmonary embolism, mesenteric ischemia, aortic dissection) generally requires a well-func-tioning, large-gauge peripheral catheter or central introducer sheath. In addition,
Table 1.2 Approximate radiation doses for emergency studies
Whole body dosesmSv
Dental X-ray
0.005
Background radiation, 1 day0.01
Flight across United States0.04
Chest