A DISCUSSION OF CT SCANNER GEOMETRY THIRD AND FOURTH GENERATION SCANNERS

Background:

  1. Development of slip ring technology
  2. Leading the evolution of the CT scanner up to the late 1980s/early 1990s into generations (III and IV)
  3. Specific determination of the geometrical arrangement and type of movement of the x-ray tube and detectors.

Basic scheme

image

Topic of differences

  1. Beam Geometry
  2. Data collection method
  3. Detector
  4. Scatter rejection
  5. Focal spot magnification
  6. Image reconstruction
  7. Spatial resolution

Beam Geometry

Both third and fourth generation scanners utilise wide fan beam geometry rather than narrow fan beam geometry, which is used in second generation scanners and pencil beam geometry that is used in first generation.

The fan beam in third generation CT uses a fan where the apex of the fan is the x-ray tube, and rays fan out from the x-ray focal spot to the multiple detectors positioned on the detector array. The fan beam in fourth generation CT, however, uses a fan where each detector is the apex of the fan, and the rays acquired by each detector fan out to different positions of the x-ray source.

In third generation system, multiple detectors are aligned along the arc of a circle whose centre is the x-ray tube focal spot. The detectors are always perfectly aligned with the x-ray tube. The x-ray beam is collimated into a fan beam. Both the x-ray tube and detectors rotate about the patient (rotate-rotate geometry) in concentric circles whose centres approximately coincide with the centre of the patient.

Data collection method

the third generation fan data are acquired by the detector array simultaneously, in one instance of time. Each time the total detector array is sampled, a view is obtained. Conversely, the fourth generation fan beam data are acquired by a single detector over the period of time that is required for the x-ray tube to rotate through the arc angle of the fan.

with third generation scanners, the number of projections that are acquired are dependent on how frequently the detector array is sampled. In fourth generation scanners, however, the number of projections is determined by the number of detectors that are in the detector array around the patient. For example, in a 1200 detector system, 1200 projections can be used.

Detector:

Most modern CT systems use either Xenon detectors or solid state scintillation detectors.

The later is used on all fourth generation CT scanners and is used on many third generation systems. The former is only used for third generation system.

Xenon detectors cannot be used for fourth generation scanners because fourth generation detectors have to record measurements as the x-ray tube sweeps over a wide angle. Xenon detectors need to be positioned in a fixed orientation with the respect to the x-ray source due to their high directionality source.

Callibration of detector

Third generation detectors are always in the scanner beam and therefore cannot be individually calibrated during a scan. The systems are calibrated by removing all objects from within the gantry and performing a calibration routine. In fourth generation geometry, detector calibration occurs prior to and after the passing of the fan beam on every revolution.

In fourth generation scanners, the orientation between each detector and the x-ray beam changes as the x-ray tube rotates around, and therefore no collimation or grid system can be used. Consequently, third generation scanners have better in plane scatter rejection than do fourth generation systems.

In fourth generation system, there is no mechanical support on the post-patient side of beam. This means that in the slice thickness dimension, there is no post-patient collimation, which can serve to limit the detection of scattered radiation. In fourth generation scanner, scatter has to be removed mathematically.

Image Reconstruction

In third generation, the first fan beam is acquired just moments after the x-ray acquisition starts. This means that the computer can start the necessary mathematical computations on the acquired data while it is coming from the detectors, then speeding up the reconstruction time.

In fourth generation scanners, each fan of information is acquired over a sizeable period of time, as the x-ray tube sweeps through the arc of the fan. This does not permit for the simultaneous mathematical processing of the data, which can slow the reconstruction time.

Spatial resolution

In third generation scanner, the sampling interval between ray sums is dictated by the distance between each individual detector element and the number of detectors. Therefore, the spatial resolution in this system is depend on how closely the rays are spaced. This means that if each detector is placed closely, spatial resolution improves.

In contrast, in fourth generation system, however, spatial resolution is dependent on how frequently the individual detector element is sampled. The more frequently the detector is sampled, the more rays that result

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