Computed Tomography

Computed tomography (CT) is currently the workhorse of radiology. Recent technical developments permit extremely fast volume scans that may serve to generate two-dimensional slices in all possible orientations as well as sophisticated three-dimensional reconstructions (Fig. 3.3). The radiation dose, however, remains high and continues to require a very strict indication for every intended CT.

Working Principle

In computed tomography the x-ray tube continuously rotates around the cranio-caudal axis of the patient. A beam of radiation passes through the body and hits a ring or a moving ring segment of detectors. The incoming radiation is continuously registered, the signal is digitized and fed into a data matrix taking into account the varying beam angulations (Fig. 3.4). The data matrix can then be transformed into an output image. In today's modern CT machines the tube rotation continues as the patient is fed through the ringlike CT gantry, thus generating not single slice scans but spiral volume scans of larger body

Table 3.1 Attenuation of different body components

Body component

Hounsfield units (HU)

Bone

1000 to 2000

Thrombus

60 to 100

Liver

50 to 70

Spleen

40 to 50

Kidney

25 to 45

White brain matter

20 to 35

Gray brain matter

35 to 45

Water

-5 to 5

Fat

-100 to -25

Lung

-1000 to -400

I Working Principle of Computed Tomography

Computed Tomography Principle Computed Tomography Principle

Rotating tube

Patient

Stationary detector rinc

Virtual rotating detector field

Fig. 3.4 a The x-ray tube rotates continuously around the longitudinal axis of the patient. A rotating curved detector field opposite to the tube registers the attenuated fan beam after it has passed through the patient. Taking into account the tube position at each time point of measurement, the resulting attenuation values are fed into a data matrix and further computed to create an image. b This is a modern volume CT scanner (by GE Medical Systems).

b segments. For each picture element (pixel) the attenuation of the radiation is calculated and expressed as Hounsfield units (HU) (Table 3.1). Water has, by definition, a Hounsfield unit value of 0.

Contrast Media

Contrast media are used in CT to visualize vessels and the vascularization of different organ systems. They attenuate radiation because of their high atomic number (e.g., iodine and barium). Contrast media containing gadolinium (which also has a high atomic number) normally intended for use in magnetic resonance tomography could theoretically also be used in CT if the administration of iodine is contraindicated. They are, however, incredibly expensive and not registered for this use yet. To better appreciate the inside of hollow viscera, iodine or barium contrast media are also given orally or instilled into the rectum.

f Fat and air are always black in CT; bone cortex and high-i atomic-number contrast media are always white.

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