Ga-67 imaging with scintillation camera: the selection of collimator

Ga-67 scintiphotos made with 300 and 360 keV collimators (LFOV) were compared to determine which collimator performed better. One target was a modified Rollo phantom; the other an Anger focal photon-deficient phantom. Three energy windows were used to bracket the 93-, 185-, and 300-keV peaks of Ga-67. The MTF and contrast efficiency functions of the 360 keV collimator were found higher than those of the 300 keV; on the other hand, the latter one was graded higher by a performance index. The modified Rollo phantom images were studied by four observers to determine how well photon-deficient lesions could be seen. From their findings the "lesion-detecting ability" of each collimator was calculated--i.e., the number of lesions with decreased activity detected divided by the number actually present in the phantom. This quotient was found higher for the 360 keV than for the other one. The advantage of the 360 keV collimator was also suggested by a study of clinical Ga-67 scintigrams of a liver with focal defects.