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Introduction to CT of the Thorax: Chest CT Techniques


S piral (helical) computed tomography (CT) allows the entire chest to be imaged in a few seconds or less (i.e., during a single breath hold), with volumetric acquisition of scan data. Two- and three-dimensional reformations may be performed if desired. Because scanning is rapid, contrast agents can be injected quickly, excellent vascular opacification can be achieved, and reduced volumes of contrast agent can be used.

Multidetector CT (MDCT) scanners have multiple parallel rows of X-ray detectors (an ever-increasing number, now exceeding 250 in some scanners, and capable of reconstructing more than 500 slices at a time). With MDCT, each of the detector rows records data independently as the gantry rotates; consequently, a volume of the patient (e.g., up to 16 cm along the longitudinal axis, or z -axis, with a 256-detector scanner) is imaged with each gantry rotation. With large-area detector scanners, scanning of a volume may be performed without table motion; this is most useful for cardiac imaging. The gantry rotation time is 0.5 seconds or less.

Spiral Chest CT: General Principles

The specific protocols used for chest CT depend on the scanner used, the scanner manufacturer, and the reason for the study. However, several general principles apply to all chest scans ( Table 1.1 ).

TABLE 1.1
Chest CT: General Principles

  • Scan levels: Lung apices to the posterior costophrenic angles

  • Patient position: Supine; prone scans sometimes used for diagnosis of lung disease of pleural effusions

  • Lung volume: Full inspiration, single breath hold; expiratory scans sometimes used to diagnose air trapping or bronchial abnormalities

  • Gantry rotation time 0.5 seconds or less in most instances

  • Scan duration: 2.5 seconds or less for the thorax, with use of multiple-detector CT and fast scanning

  • Detector width: Usually the thinnest detectors (e.g., 0.625 mm or less) for image acquisition

  • Pitch (table excursion): Depends on tolerable image noise; increased if noise is OK; decreased if there is a desire for high resolution

  • Reconstruction algorithm: High-resolution algorithm used for most studies; standard or soft-tissue algorithm usually used for vascular studies

  • Two- or three-dimensional reconstructions: Not routine; occasionally useful for lung, airway, or vascular studies

  • Contrast agents: Intravenous contrast agent injection in some cases; oral contrast agents only for gastrointestinal abnormalities

Scan Levels

Chest CT is usually performed from a level just above the lung apices (near the suprasternal notch) to the level of the posterior costophrenic angles; these scans also encompass the diaphragm and the upper abdomen. The distance (or volume) needed to cover the thorax (usually 25–30 cm) is determined by a preliminary projection scan (e.g., a “scout view”).

Patient Position

Routinely, patients are scanned supine. Prone scans may be obtained for high-resolution CT (HRCT) or to assess movement of pleural fluid collections. The patient may also be positioned prone for biopsy of posterior lung lesions or drainage of pleural fluid collections.

Lung Volume

Scans are routinely obtained after a full inspiration (i.e., at total lung capacity) and during suspended respiration. Postexpiratory scans may be performed in some cases (particularly on HRCT) to assess air trapping.

Detector Row and Slice Thickness

Scan data are usually acquired with the thinnest detector width available on the scanner (detector rows on most scanners range from 0.25 to 0.625 mm), and the reconstructed slice thickness used for scan interpretation is determined by the indication for the scan. For example, if data are recorded with 0.625-mm detectors, slices can be reconstructed at any thickness from 0.625 to 5 mm for viewing. Thin slices are required for some specific indications, whereas thicker slices are quicker to interpret and do not occupy as much memory when they are stored.

Most chest scans are reconstructed with a 1- to 1.25-mm thickness. When one is viewing a study reconstructed with 2.5- or 5-mm-thick slices, if the scan data were collected using thinner detectors, and if the scan data are still available (they are usually preserved on the scanner disk for a day or two), you can have thinner slices reconstructed at a later time.

Usually slices are reconstructed at an interval equal to the slice thickness (e.g., 1.25 mm) to provide a volumetric data set. On occasion, scans are reconstructed at overlapping levels (e.g., 1.25-mm slices reconstructed at 0.625-mm intervals), although this is not generally necessary.

Pitch (Table Excursion)

The term pitch refers to the distance the table travels during a complete gantry rotation divided by the width of all the detectors used (e.g., table excursion/detector width × number of detector rows). With MDCT, pitch usually ranges from 1 to 2. The higher the pitch, the faster the scan, but images are generally noisier, spatial resolution is reduced somewhat, and the effective slice thickness (the thickness of the patient that is actually imaged) is increased.

Keep in mind that with the spiral technique the actual thickness of the slice you view (i.e., “effective slice thickness”) may be greater than the slice thickness you select (e.g., 1.25 mm), depending on the pitch or table excursion during gantry rotation; the greater the pitch, the greater the effective slice thickness. Thus there is a trade-off; with a higher pitch, the study is quicker but the scans are not quite as good.

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