Room: Exhibit Hall | Forum 8
Purpose: The modulation transfer function (MTF) of a megavoltage imager is conventionally measured using highly attenuating metal jaws to create a slit 100-200 microns on the panel. A Fourier transform of this line spread function (LSF) yields the MTF. Given the burdensome expense and inconvenience of this measurement, we sought to adapt the method of characterizing kilovoltage imager MTF for megavoltage panels. The standard IEC procedure for kilovoltage devices involves irradiating a thin, angled metal foil to create a step function. This image is processed to create an oversampled edge spread function, which can be differentiated to create the LSF.
Methods: Measurements were conducted with a non-FFF 6 MV beam on a Varian TrueBeam. Two different imaging panels were analyzed: the standard Varian AS-1200 and LKH-5 (Collimated Holes Inc., Campbell, CA), an experimental EPID with a high efficiency scintillating glass. MTF was measured: (1) with a slit phantom comprised of a pair of tungsten blocks totaling 75kg with shims creating a 100 micron opening; and (2) with an edge phantom comprised of a 0.5mm thick 10cmx10cm, 83.5gm, piece of tantalum. Because the foil attenuates less than 5% of the beam, photon flux was increased by averaging more frames to abate the loss in contrast-to-noise ratio. The edge phantom was placed 10cm in front of the EPID, to eliminate error caused by forward scattered photons off the phantom. A baseline image of the empty foil holder is also acquired and subtracted from the image. An offset is added to zero out the ESF tails.
Results: MTF plots for the AS-1200 and LKH-5 were generated. Good agreement was found between the slit phantom and edge phantom results.
Conclusion: When measuring MTF for megavoltage EPIDs, an edge phantom can effectively substitute for a slit phantom, substantially reducing the overhead for megavoltage EPID characterization.
Funding Support, Disclosures, and Conflict of Interest: NIH/NCI R01CA188446