FIG. 296-4. Slice misregistration caused by a patient's breathing. Each consecutive slice was 10 mm or more inferior, yet the second image appears the most superior. Lesions as large as 1 cm can be missed as a result of slice misregistration.
Because spiral scans are acquired at a slight slant, computer software must adjust for the angle by averaging the data. This statistical method of processing the data is referred to as interpolation (linear and nonlinear). The evolution of mathematical software processes enabling missing data to be estimated in helical data sets has also been critical to the advancement and success of spiral CT.3 There is, however, a small decrease in resolution inherent to interpolation ( Fig, 2.96.-5).
FIG. 296-5. Spiral scans are acquired at a slight slant because of the continuous motion of the x-ray tube and table. By interpolating the data, the computer then creates an image that is not slanted. This process increases the effective slice thickness and causes some loss of image resolution. The more pronounced the slant, the more interpolation is required. The slant is affected by selected slice thickness and pitch.
The term pitch refers to the relation of table speed to slice thickness. If pitch is set at 1:1, the table will move at a speed that allows the gantry to rotate once for every slice thickness chosen. For example, with slice thickness at 5 mm, the table moves at a speed that allows the gantry to rotate once every 5 mm of table travel. If the pitch is adjusted to 2:1 and the slice thickness is maintained at 5 mm, the tube rotates only once for every 10 mm of table motion. Pitch is sometimes adjusted toward 2:1, which, in essence, "stretches the spring" and less data are acquired.1 Pitch is usually set at close to 1:1 but may be increased when it is important to cover a long anatomic area in a very short time, as with CT angiography.
Spiral CT scanning has optimized the delivery of intravenous (IV) contrast. The rapid speed of the scanning technique allows for much finer control over which phase of IV contrast enhancement is imaged and allows for the possibility of obtaining more information from a single bolus of contrast. For example, in the abdomen, images can be obtained during the hepatic arterial, portal venous, and equilibrium phases. Delayed images are also easily and rapidly obtained when enhancement characteristics of certain lesions need to be visualized or when the urinary excretory phase must be imaged. This capability is widely utilized in trauma where major organ lacerations and active bleeding sites are best visualized during early phases of contrast enhancement, while bladder perforation may be better imaged on delayed contrast imaging.
The major advantages of helical scanning over conventional scanning are
1. Volumetric data acquisition is acquired rapidly.
2. Less contrast material is needed because of increased scanning speed.
3. Images can be retrospectively reconstructed at any desired interval or thickness or may be overlapping without rescanning the patient.
4. Respiratory, cardiac, and other motion artifacts are reduced.
5. The continuous nature of the data allows high-quality three-dimensional and multiplanar reconstruction.
Although the advantages of spiral CT predominate, there are some disadvantages:
1. If pitch (relation of table speed to slice thickness) is increased from 1:1 toward 2:1, image resolution can be lost.
2. When scanning very large patients, there will be increased image noise if the milliampere setting cannot be maintained. Scanners vary regarding weight limitations, although the average maximum patient weight is approximately 350 lb.
3. Some image resolution may be lost due to the interpolation required to process spiral data.
4. Contrast material injection must be timed precisely, although this problem has been minimized with the use of established contrast protocols and power injectors.
5. Children and uncooperative adults need sedation and thus, close monitoring.
Spiral CT scanners continue to have the capability of acquiring axial images. For example, at some institutions, CT protocols for cervical spine trauma use a spiral CT scanner to obtain axial images because of the potential improvement in image resolution in this anatomic area.
An important issue raised by the advent of spiral CT is what constitutes an appropriate examination archive. At this point, because of the enormous space required to store all the information, most CT scanners delete the raw data soon after the scan is acquired. Thus, it is important to select the reconstruction interval quickly and appropriately so that the opportunity is not lost to request additional reformatted images before the raw data is deleted.
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