Characterization of dynamic collimation mechanisms for helical CT scans with direct measurements

Abstract

Dynamic collimation is an important dose reduction mechanism for helical CT scans, especially for modern wide-beam scanner models. Its implementation and efficacy need to be studied to optimize CT scan protocols and to reduce unnecessary patient dose. The purpose of this study is to evaluate dynamic beam collimation for modern wide-beam CT scanners with direct measurements and to estimate the efficacy for dose reduction. By using a linear-array solid state detector, primary x-ray beam coverage was measured for four CT scanner models: GE Revolution CT, Siemens Somatom Force, Philips iQon, and GE LightSpeed VCT. Supported independently from patient table, the detector remained stationary at the isocenter during helical scans. Data lines were recorded every 0.24 ms throughout one entire helical scan, with a spatial resolution of 0.8 mm along the craniocaudal direction. The measurements were repeated for various scan parameters related to dynamic collimation, including beam collimation width, pitch, rotation time, and scan length. The recorded beam coverage area was used as a surrogate to total primary dose, to model different dynamic collimation mechanisms. The directly measured total radiation range was compared to table travel distance and nominal scan length which equals to the ratio between DLP and CTDIvol. Equations to calculate the percentage dose reduction with dynamic collimation were derived for different mechanisms. Three different dynamic collimation mechanisms were revealed and related linear model parameters were reported for different helical scan parameters. The nominal scan length used to calculate DLP was shown to vary for different dynamic collimation mechanisms. For typical head and abdomen scans with nominal scan lengths of 17.5 cm and 25 cm, percentage dose reduction from dynamic collimation ranged from 2% to 32%. In conclusion, with direct measurements of primary x-ray beam coverage, dynamic collimation mechanisms and related dose reduction effects were characterized for four modern wide-beam CT scanners.

Validation of a deterministic linear Boltzmann transport equation solver for rapid CT dose computation using physical dose measurements in pediatric phantoms

“…Further details on how Acuros modeled bowtie filters are provided in previous work 19 . The overrange collimation was modeled so that portions of the X‐ray beam were blocked at the beginning and end of the helical scan, limiting the exposure beyond the field of view, according to the scheme described by Yang et al 27 . for the GE Discovery 750 HD.…”

“…Further details on how Acuros modeled bowtie filters are provided in previous work. 19 The overrange collimation was modeled so that portions of the X-ray beam were blocked at the beginning and end of the helical scan, limiting the exposure beyond the field of view, according to the scheme described by Yang et al 27 for the GE Discovery 750 HD. This effect is implemented in Acuros by discretizing the spectral fluence of the X-ray beams across the cone angle and is affected by the discretization in view angle.…”

Deterministic linear Boltzmann transport equation solver for patient‐specific CT dose estimation: Comparison against a Monte Carlo benchmark for realistic scanner configurations and patient models

“…The overrange region is the scan length that extends beyond the volume of the reconstructed image that is required for helical scanning and that results in unnecessary dose to the patient. To avoid this exposure, some CT scanners implement overrange collimation, for which the beam is collimated across the cone angle at the beginning and at the end of a helical scan, whereas other scanners, such as the GE Revolution, maintain a constant aperture of the beam across the helical scan 25 . Furthermore, different scanners can be characterized by different collimation patterns, as is the case for Siemens Somatom Force and Philips iQon, which both implement the same overrange collimation but, for example, differently from the GE Lightspeed VCT and GE Discovery 750HD 25 .…”

“…To avoid this exposure, some CT scanners implement overrange collimation, for which the beam is collimated across the cone angle at the beginning and at the end of a helical scan, whereas other scanners, such as the GE Revolution, maintain a constant aperture of the beam across the helical scan 25 . Furthermore, different scanners can be characterized by different collimation patterns, as is the case for Siemens Somatom Force and Philips iQon, which both implement the same overrange collimation but, for example, differently from the GE Lightspeed VCT and GE Discovery 750HD 25 . In this work, the collimation scheme of the Siemens Somatom Force/Philips iQon scanners is adopted.…”

“…25 Furthermore, different scanners can be characterized by different collimation patterns, as is the case for Siemens Somatom Force and Philips iQon, which both implement the same overrange collimation but, for example, differently from the GE Lightspeed VCT and GE Discovery 750HD. 25 In this work, the collimation scheme of the Siemens Somatom Force/Philips iQon scanners is adopted. For the first view of the scan, the overrange collimation limits the exposure coverage to half of the detector rows, that is, for a nominal collimation of 4 cm at the isocenter, exposure is limited to 2 cm for the first view.…”