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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