Biomedical subjects

Geetha V Menon

Publications and source records attributed to Geetha V Menon.

3 recordsLinked to original sources

PubMed2004

Quality assurance measurements of a-Si EPID performance.

The performance stability of a Varian aS500 amorphous silicon (a-Si) electronic portal imaging device (EPID) was monitored over an 18-month period using a variety of standard quality assurance (QA) tests. The tests were selected to provide ongoing information about image quality and dose response from the time of EPID acceptance into clinical service. To evaluate imaging performance, we made spatial resolution and contrast measurements using both PortalVision and QC-3V phantoms for 6- and 15-MV photon beams at repetition rates of 100, 300, and 400 MU/min in standard scanning mode. To assess operational stability for dosimetry applications, we measured central axis radiation response and beam pulse variability for the same image acquisition modes. Using the QC-3V phantom, values for the critical frequency of 0.435 +/- 0.005 lp/mm for 6 MV and 0.382 +/- 0.003 lp/mm for 15 MV were obtained. The contrast-to-noise ratio was found to be approximately 20% higher for the lower photon energy. Beam pulse variability remained within the tolerance of 3% set by the manufacturer. The central axis pixel response of the EPID remained constant within +/-1% over a 5-month period for the 6-MV beam, but fell approximately 4% over the same period for the 15-MV beam. The Varian aS500 EPID studied exhibited consistent image quality and a stable radiation response. These characteristics render it suitable for quantitative applications such as clinical dose measurement.

Calibration↗

PubMed2004

Compensator thickness verification using an a-Si EPID.

Electronic portal imaging devices (EPIDs) are being increasingly employed to make therapy verification and dose measurements in the clinic. In this work, we investigate the use of an amorphous silicon (a-Si) EPID to verify the accuracy of compensator fabrication and mounting. Compensator thickness estimates on a two-dimensional grid were calculated from the primary component of transmission obtained by subtracting a modeled scatter component from the total transmission measured with the EPID. The primary component was related to the thickness via an exponential relation that includes beam hardening. Implementation of the method involved determination of: (i) a calibration curve relating EPID pixel values to energy fluence for open and attenuated fields, which was found to be linear for open fields but to have a small quadratic component for attenuated beams; (ii) EPID scatter factors to account for field size effects, which exhibited a small dependence on compensator thickness and field size; (iii) the attenuation coefficient of the steel shot compensator material, which varied slightly with off-axis distance and field size, and (iv) an analytical model to predict scatter from the compensator, which was calculated to be <4% at the standard EPID imaging distance of 140 cm. Thickness distributions were then measured for several types of attenuators including flat, test, and clinical compensators. Although uncertainties associated with compensator manufacturing were non-negligible and made assessment of thickness measurement uncertainty difficult, we estimate the latter to be approximately 0.5 mm for steel shot compensators of thickness <4 cm.

Algorithms↗

PubMed2003

Compensator quality control with an amorphous silicon EPID.

The calibration and quality control of compensators is conventionally performed with an ion chamber in a water-equivalent phantom. In our center, the compensator factor and four off-axis fluence ratios are measured to verify the central axis beam modulation and orientation of the compensator. Here we report the investigation of an alternative technique for compensator quality control using an amorphous silicon electronic portal imaging device (a-Si EPID). Preliminary experiments were performed to identify appropriate EPID operating parameters for this relative dosimetric study and also to quantify EPID operation. The pixel value versus energy fluence response of the EPID for both open and compensated fields was then determined, and expressed via calibration curves. For open fields the response was seen to be linear, whereas for compensated fields it exhibited a small quadratic component. To account for field size effects, we measured EPID scatter factors. These exhibited small but non-negligible dependencies on compensator thickness and source-detector distance. Finally, a number of test and clinical compensators were evaluated to assess the suitability of the EPID for compensator quality control. Our results indicate that the a-Si EPID can measure clinical compensator factors and off-axis energy fluence ratios to within 2% of values measured by a Farmer chamber on average, and so is a suitable ion chamber replacement.

Calibration↗