Optimal parallel-hole collimation for lesion detection and activity estimation in Ga-67 imaging

Gallium-67 plays an important role in the field of nuclear medicine, in particular for evaluation and staging of lymphoma patients. However, optimal collimation has not yet been designed specifically for this radionuclide. Instead, most nuclear medicine clinics use generic, medium-energy (ME) collimators which may be suboptimal for Ga-67. Collimator-design methods based on empirical rules, such as specification of an allowable level of single-septal penetration (SSP) at a fixed energy, are especially inappropriate for Ga-67, which is characterized by an abundance of high-energy contaminant photons that scatter in the patient, collimator, and/or detector, and ire then detected within one of the three energy windows (93, 185, 300 keV) normally used for imaging. Furthermore, lead X-rays produced in the collimator represent another significant source of contamination in the 93-keV window. We have optimized the design of collimation tailored for Ga-67 imaging, based on relevant clinical imaging tasks and a realistic simulation of photon transport in a phantom, collimator, and detector. Collimator designs were compared on the basis of performance in lesion defection, as predicted by a three-channel Hotelling observer (CHO), as well as in tumor and background activity estimation (EST), by computing task-specific signal-to-noise ratios (SNR). For each collimator design and task, SNR values were averaged over five spherical lesion sizes (1-3 cm diam.) at each of five locations within a 45-cm-diameter low-activity background cylinder containing a 10-cm-diameter spherical ´organ´ with a higher activity concentration. The optimal values of collimator lead content were 22.0 and 23.8 g/cm², respectively, for CHO and EST, while the optimal geometric resolution values at 23.5 cm were 1.8 and 1.6 cm FWHM, respectively. The resolution of a commercially available ME low-penetration collimator is 1.9 cm at this distance. The CHO, EST, and commercial collimators would allow, respectively, 7.3%, 5.8%, and 5.2% SSP at 300 keV.