Spiral computed tomography (or helical computed tomography) is a computed tomography (CT) technology in which the source and detector travel along a helical path relative to the object. Typical implementations involve moving the patient couch through the bore of the scanner whilst the gantry rotates. Spiral CT can achieve improved image resolution for a given radiation dose, compared to individual slice acquisition. Most modern hospitals currently use spiral CT scanners.
Willi Kalender is credited with invention of the technique, and uses the term spiral CT. Kalender argues that the terms spiral and helical are synonymous and equally acceptable.
There are a class of image artifacts specific to helical acquisition.
Single-slice and multi-slice helical CT
Since its invention by Kalender in the 1980s, helical scan CT machines have steadily increased the number of rows of detectors (slices) they deploy. The prototype 16 multi-slice scanner was introduced in 2001 and in 2004, 64 multislice scanners are on the market. These can produce an image in less than a second and thus can obtain images of the heart and its blood vessels (coronary vessels) as if frozen in time.
In order to illuminate multiple rows of detector elements in a multi-slice scanner, the x-ray source must emit a beam which is divergent along the axial direction (i.e. a cone beam instead of a fan beam).
Pitch
A helical CT beam trajectory is characterized by its pitch, which is equal to the table feed distance over one gantry rotation divided by the section collimation. When pitch is greater than 1, the radiation dose for a given axial field-of-view is decreased compared to conventional CT. At high pitches there is, however, a trade-off in terms of noise and longitudinal resolution.
Helical (or spiral) cone beam computed tomography
In cone-beam computed tomography (commonly abbreviated CBCT), the X-ray beam is conical.
Helical (or spiral) cone beam computed tomography is a type of three-dimensional computed tomography (CT) in which the source (usually of X-rays) describes a helical trajectory relative to the object while a two-dimensional array of detectors measures the transmitted radiation on part of a cone of rays emanating from the source.
In practical helical cone beam X-ray CT machines, the source and array of detectors are mounted on a rotating gantry while the patient is moved axially at a uniform rate. Earlier X-ray CT scanners imaged one slice at a time by rotating source and one-dimensional array of detectors while the patient remained static. The helical scan method reduces the X-ray dose to the patient required for a given resolution while scanning more quickly. This is however at the cost of greater mathematical complexity in the reconstruction of the image from the measurements.
References
External links
- Interview with Kalender
- Principles of Computerized Tomographic Imaging
