In conventional (nonhelical) CT scanning, the image acquisition is as follows: scan a slice, move the patient, scan a slice, etc. Thus, there are two distinct parts to the process: turn on the x-ray beam, usually for only about 1 to 2 s, and then move the patient to the next table position. Patient motion to the next position may take an additional 2 to 4 s. Therefore, each slice can take approximately 5 s (1-s scan and 4 s of motion), of which only 20 percent (1 s/5 s) is actually imaging time.
In its most basic form, helical CT consists of scanning a patient continuously while the patient is being continuously moved through the CT gantry. In this mode, the x-ray beam is on for perhaps 20 s or more, and the patient is moved for this same duration. The efficiency of scanning is 100 percent. During the scanning process, the x-ray beam source is continuously rotating around the patient, tracing a helical, or a helix, along the patient's body. Implementing this is actually quite demanding of the CT scanner, its computer, and the electronic components. For this reason, helical CT remains a relatively new and developing technology.
Helical CT data sets are truly 3D. Instead of scanning a series of slices, a volume data set is acquired. This results in
• Vastly improved multiplanar and 3D reconstructions. Since data sets are acquired in a breath hold, respiratory misregistration is eliminated. Data appear "smoother" with fewer "stair-step" artifacts in sagittal and coronal reconstructions.
• Reconstruction of slices at arbitrary intervals. Because of the 3D nature of helical scanning, images can be reconstructed at small intervals, e.g., 2 to 3 mm. This reduces stair-step artifacts in 3D reconstructions.
• Rapid acquisition. Complete helical data sets of a patient's examination are typically acquired within 20 to 30 s, and the patient is then done with the examination.
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