Oz. Ostapiak et al., MEGAVOLTAGE IMAGING WITH LOW Z-TARGETS - IMPLEMENTATION AND CHARACTERIZATION OF AN INVESTIGATIONAL SYSTEM, Medical physics, 25(10), 1998, pp. 1910-1918
The poor quality of stereotactic radiotherapy portal images is a limit
ing factor in precise image registration. To alleviate this problem, a
low atomic number (Z) target was implemented on our Siemens MXE linea
r accelerator. This investigational system was used to assess the perf
ormance of various target materials by filming an aluminum contrast ob
ject. Beryllium, carbon and conventional target materials were studied
. The bremsstrahlung spectra of these materials were simulated using M
onte Carlo techniques. These spectra were used to calculate the depend
ence of narrow beam contrast on phantom thickness for verification of
the data measured from film. A Monte Carlo simulation of the beryllium
spectrum in a wide beam geometry was used to evaluate the effect of p
hantom-to-film distance on contrast. Although the same degree of contr
ast improvement with distance was not realized in practice, the improv
ement in image quality rivaled that achieved using a scatter reduction
grid. A comparison of conventional localization images of the head an
d neck of an anthropomorphic phantom with images produced with a beryl
lium or carbon target and a mammography film and screen system support
s earlier suggestions that the technique is clinically useful. (C) 199
8 American Association of Physicists in Medicine. [S0094-2405(98)01010
-4].