Chapter 12

### BASICS OF COMPUTED TOMOGRAPHY

The basic single-detector computed tomography (CT) scanner operates with a single row of detectors mounted opposite an x-ray source on a rotating gantry through which a patient moves during image acquisition. In 1992, the first dual-section helical CT scanner was introduced, which had two rows of detectors.1 In subsequent years, technology has continued to improve, with additional rows of detectors added to the rotating gantry. Currently, multidetector scanners with a single x-ray source are available with up to 128 rows of detectors. Scanners with two x-ray sources have 256 or more detectors.

A multidetector CT (MDCT) scanner houses parallel rows of detectors aligned along the long axis of the patient on a rotating gantry. These detectors can be of equal width, matrix detectors, or unequal width, adaptive array detectors. Multiple rows of detectors allow a larger length of the patient to be covered per gantry rotation, which means that CT scans can be performed faster and using higher x-ray tube current. This improves temporal resolution, decreases motion artifacts, and decreases noise. The more the detectors, the faster the scan can be performed. A conventional single detector CT scanner makes a complete 360-degree rotation in approximately 1 second. Multidetector scanners have faster gantry rotation speeds and obtain multiple sections per revolution. For example, a scanner with four rows of detectors is actually eight times faster than its single detector counterpart.

In addition, thinner sections can be acquired with multiple detectors, improving spatial resolution. Thinner sections also allow for the creation of multiplanar reformatted (MPR) images, maximum intensity projection (MIP) images, and three-dimensional reformatted images. MPR images are created by fusing data collected from the multiple rows of detectors. This data can be reformatted into coronal, sagittal, and oblique planes. Interpolation algorithms can also be used to average overlapping data points and further decrease noise in the resultant images.2

#### Contrast

Intravenous (IV) contrast administration is at the cornerstone of imaging the venous and lymphatic systems. Faster scan times allow for improved vascular concentration of contrast and better separation of arterial and venous phases of imaging. The timing of the CT scan depends on the vascular system or organ of interest. There are three techniques for determining the appropriate delay to maximize contrast opacification. The first is bolus tracking, which involves low-dose (low tube current) monitoring scans through the level of interest. The actual scan begins when a predetermined Hounsfield unit threshold is reached. The second technique involves use of a test bolus. A small quantity of contrast, the test bolus, is administered, and low dose monitoring scans are again obtained through a predetermined region of interest. The time it takes for the test bolus to reach the region of interest is used to determine the scan delay after the actual, full dose of contrast. The third and final technique is to simply use an approximation based ...

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