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Spiral CT
In conventional CT technique, first a slice is made of the desired area, after which the table moves up a little. In this way the patient is imaged slice by slice (step-by-step). Around 1990, the ‘slip ring’ technique was developed where the x-ray tube and detector ring rotate and continue scanning without interruption. This led to the so-called spiral CT where the scanner table moves with constant speed through the ring with the rotating x-ray tube and detectors. This generates a helix/spiral-shaped pattern (fig. 7).
Figure 7. Spiral CT technique.
The significant benefit of spiral CT is the shorter scan time. The patient can be scanned during one breathing instruction. Another benefit are the overlapping intervals, improving the visualization of small lesions and counteracting the undesired partial volume effect. In the partial volume effect, two different structures are located in the same voxel; the average of both densities will then be converted into a gray tone (particularly thicker slices). Very subtle abnormalities will have only negligible impact on mean density, rendering them undetectable. A drawback of spiral CT is the longer time needed to create image reconstructions. New CT scanners, however, are becoming increasingly fast in processing the information obtained.
Another drawback of spiral CT are the specific spiral CT artifacts. The spiral technique is used frequently in the above-described multislice CT scanners. In some cases conventional step-by-step technique is still used, as in HRCT tests or intervention procedures. HRCT stands for high resolution CT and is used to create thin CT slices (1-2 mm) of the chest which are reconstructed with high resolution and powerful enlargement (this is used for interstitial lung disease in particular).