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Tomographic Measurement of X-Ray Beam Spot Profiles Using a Rotating Edge

J Star-Lack

J Star-Lack1*, T Etmektzoglou1, R Davidi2, E Abel1, M Sun1, G Virshup1, K Kielar2, R Fahrig2, (1) Varian Medical Systems, Palo Alto, CA, (2) Stanford University, Stanford, CA

TH-C-BRA-12 Thursday 10:30:00 AM - 12:30:00 PM Room: Ballroom A

Purpose: X-ray beam spot size and shape are critical performance determinants of an imaging or treatment system. However, quantitative assessment of such spot profiles can prove difficult, particularly at MV energies. We have developed a novel and convenient tomographic spot measurement technique that uses a rotating edge phantom. The method can be applied to x-ray systems equipped with flat panel imagers.

Methods: Data were acquired at 10MV and 6MV on a Varian TrueBeam system. A 0.5 mm thick tantalum sheet (attenuation ~5%) was placed on the system's rotatable treatment head with an edge abutting the axis of rotation. A total of 144 projections, each 10MU, were acquired at 2.5 degree steps. For each projection, a line-spread function (LSF) is generated by differentiating the measured edge spread function. For sufficiently high source magnifications, the LSF, taken at a given rotation angle, is a tomographic projection of the x-ray beam spot at that angle. The LSF's were then assembled into a sinogram and the corresponding spot profile was reconstructed using a parallel-beam CT algorithm. The reconstructed profiles were compared to those measured using a film-based 'spot camera' made from a 15 cm thick tungsten cylinder pierced with an array of small holes. Monte Carlo simulations of the edge and the spot camera experiments were also performed.

Results: The edge technique produced profiles similar to those from the spot camera yet with higher resolution (0.17mm vs. 0.25mm). These results were confirmed by Monte Carlo simulations. The measured FWHM of the 10 MV spot was 1.6 mm. The 6 MV spot was slightly asymmetric with an average FWHM of 1.5 mm.

Conclusions: A thin rotating edge with low attenuation can be used to accurately and conveniently measure x-ray beam spot profiles.

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