Measurement of the interactions of low energy gamma rays with dense metals for applications in nuclear cardiology collimators

Objectives The objective of this project was to compare the transmission of 140 keV Tc-99m and 70-80 keV Tl-201 photons separately and simultaneously for non-Pb high density metals to Pb for application in gamma camera collimators. There is a difficulty in trying to simultaneously image the lower Tl-201 energy photons in the presence of the higher energy Tc-99m photons when using a standard Pb collimator because K-shell interactions between the Tc-99m photons and Pb create down-scatter in the Tl signal region. Correction of the down-scatter problem by replacing the Pb in the gamma camera collimator with a suitably dense metal but with scatter characteristics that differ from Pb would allow the clinician to take advantage of a simultaneous dual-isotope approach, increasing the detectability of reversible myocardium defects.

Methods 1. Use NIST table data to identify high density metals that meet the criteria for reduced Tc-99m down-scatter while maintaining primary attenuation characteristics similar to Pb. 2. Use a multi-channel analyzer to record spectra obtained for Tl-201 and Tc-99m using selected metal attenuators. 3. Measure and compare energy characteristics of spectra using simple integrative techniques. 4. Compare percentage of down scatter differences and attenuation characteristics between Pb and the selected high density metals.

Results 28 metals were initially identified as having suitable characteristics to meet the project objective. Of these, Yb, Lu, Hf, Ta and Bi displayed spectra that met the objective.

Conclusions Yb, Lu, Hf, and Ta met the project objective by moving any K-shell interference below the Tl-201 70-80 keV range, while still providing dense attenuation for the Tc-99m 140 keV photons. Bi met the project objective by moving the K-shell interference above the Tl-201 70-80 keV range, while still providing dense attenuation for the Tc-99m 140 keV photons. These results indicate that it is feasible to produce gamma camera collimators that could facilitate simultaneous dual isotope cardiac imaging.