X-RAY SOURCES OF MEDICAL LINEAR ACCELERATORS - FOCAL AND EXTRA-FOCAL RADIATION

A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal s pot in medical linear accelerators. Using this technique, we have meas ured the focal spots in a total of nine accelerators, including (i) tw o Varian Clinac 2100c's, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac-25's, (iii) two AECL Therac 6's, (iv) a Siemens KD-2, (v) a Var ian Clinac 600c (4 MV), and (vi) an AECL Therac-20. Some of these foca l spots were monitored for changes over a 2-yr period. It has been fou nd that (i) the size and shape of the source spot varies greatly betwe en accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same d esign, the focal spots are very similar. In addition to the measuremen ts of the focal spot, a new technique for measuring the magnitude and distribution of extra-focal radiation originating from the linear acce lerator head (flattening filter, primary collimator) has also been dev eloped. The extra-focal radiation produced by a Varian Clinac 2100c ac celerator was measured using this technique and it was found that the extra-focal radiation accounts for as much as 8% of the total photon f luence reaching the isocenter. The majority (75%) of this extra-focal radiation originates from within a circle 6 cm in diameter at the targ et plane. The source MTFs for each of the measured focal spots have be en calculated in order to assess their influence on the spatial resolu tion of verification images. The limiting spatial resolution (i.e., 10 % modulation) for all the source MTFs is 1. 8 mm-1 or greater when use d for transmission radiography at a magnification of 1.2. The extra-fo cal radiation, which produces a low-frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accele rators and image magnification, but also on the field size used when f orming an image.