Purpose: To characterize the dynamic collimation mechanisms during helical scans from four representative wide-beam CT scanners and evaluate the impact on dose reduction for over scan.
Methods: Primary CT beam coverage along the z-direction was directly measured with a linear detector (0.8f3-512, Detection Technology). This GOS-based detector has a pixel size of 0.8x0.7 mmÂ², an effective length of 51.2 cm, and readout speed up to 0.24 ms per line. The detector was supported on an independent platform to remain stationary at scanner isocenter during helical scans. The primary x-ray beam range was recorded on the detector throughout helical scan acquisitions, including any dynamic adjustment of the collimation at the start and end of each scan. Measurements were performed on four modern wide-beam CT scanners (GE Revolution, Siemens Force, Philips iQon, and GE VCT), with different helical scan parameters, including beam collimation width, pitch, rotation time, and scan length. The recorded beam coverage area was used as a surrogate for the overall primary radiation dose to model different dynamic collimation mechanisms. Percentage dose reduction with dynamic collimation was calculated.
Results: Three of the four scanner models (Siemens Force, Philips iQon, and GE VCT) implemented dynamic collimation and one does not (GE Revolution). Siemens Force and Philips iQon have very similar mechanisms, while GE VCT implemented quite differently. The difference in the implementation has an impact on dose reduction and the calculation of scanner reported DLP. For an abdomen scan with nominal scan length of 25 cm, pitch close to 1, and the widest total collimation width from each scanner, the percentage dose reduction from dynamic collimation was 20% for Siemens Force, 12% for Philips iQon, and 7% for GE VCT.
Conclusion: Dynamic collimation mechanisms for four modern wide-beam CT scanners were studied with direct measurements of the primary x-ray beam range.