CT brain perfusion: A static phantom study of contrast-to-noise ratio and radiation dose.

INTRODUCTION: Computed tomography perfusion (CTP) is increasingly employed in the diagnosis and management of ischaemic stroke but radiation dose can be significant and optimising contrast-to-noise ratio (CNR) is challenging. This study aimed to quantify and optimise the balance between CNR as a surrogate for image quality and radiation dose. METHODS: A perspex head phantom with vials of dilute contrast agent was scanned using a Siemens Definition Flash 128-slice scanner. The CTP protocol exposure parameters were adjusted over 70-120 kVp and 150-285 mAs. Measurements were obtained for the average dose per slice, Hounsfield Units (HU) for iodinated contrast agent, and the image noise for background regions of perspex. The CNR was measured as a function of the volumetric CT dose index (CTDIvol) and kVp. RESULTS: A change from 120 to 80 kVp, achieved the same CNR with 60% reduction in dose. Alternatively, for the same dose, the change from 120 to 80 kVp improved CNR by +58%. A change from 80 to 70 kVp while operating at the same CNR, led to 13% reduction in dose. Alternatively, maintaining the same dose while changing from 80 to 70 kVp improved the CNR by +7%. CONCLUSION: Lower beam energies achieved the same CNR with less dose, or improved CNR at the same dose. A reduction from 80 kVp to 70 kVp may be clinically useful to optimise CTP acquisitions.

Reference dosimetry at the Australian Synchrotron's imaging and medical beamline using free-air ionization chamber measurements and theoretical predictions of air kerma rate and half value layer.

PURPOSE: Novel, preclinical radiotherapy modalities are being developed at synchrotrons around the world, most notably stereotactic synchrotron radiation therapy and microbeam radiotherapy at the European Synchrotron Radiation Facility in Grenoble, France. The imaging and medical beamline (IMBL) at the Australian Synchrotron has recently become available for preclinical radiotherapy and imaging research with clinical trials, a distinct possibility in the coming years. The aim of this present study was to accurately characterize the synchrotron-generated x-ray beam for the purposes of air kerma-based absolute dosimetry. METHODS: The authors used a theoretical model of the energy spectrum from the wiggler source and validated this model by comparing the transmission through copper absorbers (0.1-3.0 mm) against real measurements conducted at the beamline. The authors used a low energy free air ionization chamber (LEFAC) from the Australian Radiation Protection and Nuclear Safety Agency and a commercially available free air chamber (ADC-105) for the measurements. The dimensions of these two chambers are different from one another requiring careful consideration of correction factors. RESULTS: Measured and calculated half value layer (HVL) and air kerma rates differed by less than 3% for the LEFAC when the ion chamber readings were corrected for electron energy loss and ion recombination. The agreement between measured and predicted air kerma rates was less satisfactory for the ADC-105 chamber, however. The LEFAC and ADC measurements produced a first half value layer of 0.405 ± 0.015 and 0.412 ± 0.016 mm Cu, respectively, compared to the theoretical prediction of 0.427 ± 0.012 mm Cu. The theoretical model based upon a spectrum calculator derived a mean beam energy of 61.4 keV with a first half value layer of approximately 30 mm in water. CONCLUSIONS: The authors showed in this study their ability to verify the predicted air kerma rate and x-ray attenuation curve on the IMBL using a simple experimental method, namely, HVL measurements. The HVL measurements strongly supports the x-ray beam spectrum, which in turn has a profound effect on x-ray dosimetry.

Justifying the use of abdominal wall computed tomographic angiography in deep inferior epigastric artery perforator flap planning.

Abdominal wall computed tomography angiography (CTA) is used to guide preoperative planning and intraoperative technique for deep inferior epigastric artery perforator free flap breast reconstructive surgery. This study considers the amount of radiation delivered to the patient, outlining how scan parameters can be optimized to minimize the radiation exposure whilst maintaining image quality. Results of scan parameters and dose reports for 34 patients undergoing abdominal wall CTA are compared with those patients undergoing routine abdominal computed tomography. The links between computed tomography dose quantities are explained, including their conversion to effective dose (in mSv) and risk as the probability for inducing deterministic effects (eg, skin burns) and stochastic effects (ie, cancer induction). The mean effective dose by using our technique for routine abdominal computed tomography is 9.9 and for abdominal wall CTA is 6.0 mSv. All doses are far below the thresholds for deterministic effects to the skin. Abdominal wall CTA can be justified before major reconstructive surgery if the surgeon believes that the very low estimated risk of fatal radiation-induced cancer (1 in 4270 for 6 mSv) is outweighed by the benefits.