Blurring and MTF determination of a radiotherapy EPID using the edge spread function

We present a method based in the study of the edge spread function (ESF) obtained by using a solid metallic block image to measure the resolution of a radiotherapy EPID. With this simple method it is possible to determine the effective focal spot size of a LINAC, the intrinsic unsharpness of the portal imaging device and also its MTF response, which it is not usually tested in routine quality controls since an specific phantom is needed. Results for the spatial resolution are compared to those calculated with a bar phantom of the same material. This procedure can be implemented in the routine EPID quality control in the clinical practice. Measurements were made with the Beamview TI EPID of a Siemens Primus linac. A focal spot size of 1.057 and 1.484 mm (X, Y directions) and a f/sub 50/ at isocenter of 0.272 and 0.304 lp/mm (X,Y directions) were obtained.