Physical characteristics of low and medium energy collimators for i imaging and simultaneous dual-isotope imaging

Septal penetration of high energy photons may degrade I images obtained with a low energy collimator. We evaluated the physical characteristics of a low energy, high resolution (LEHR) collimator, special LEHR (SLEHR) collimator, and medium energy collimator for I studies. The cross-talk of ^99mTc and I into the Tl window was also examined. Sensitivity and spatial resolution were measured with each collimator. Point sources of ^99mTc and I were imaged at various source-collimator distances using multiple energy windows, and the effects of collimator choice on energy spectrum and spatial distribution of photopeak counts were assessed. For ^99mTc, both sensitivity and resolution were similar with the low energy collimators, and higher sensitivity and lower resolution were observed with the medium energy collimator. For I, the full width at tenth maximum was larger for I than for ^99mTc when using the LEHR collimator. Acquisitions with multiple energy windows revealed severe degradation due to septal penetration in imaging I with the LEHR collimator, especially at short distances. The degradation was reduced with the SLEHR collimator and further with the medium energy collimator. In both ^99mTc and I imaging, cross-talk into the Tl window was larger at shorter distances and the largest with the LEHR collimator. In conclusion, variation in collimator geometry causes differences in the effect of septal penetration on I images and in cross-talk into the Tl window. The SLEHR collimator may be suitable for use in high resolution I imaging and simultaneous ^99mTc/Tl imaging. Use of the medium energy collimator appears to be preferable in quantitative I studies.