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Quantitative Bremsstrahlung SPECT Imaging: Attenuation-Corrected Activity Determination

Department of Radiation Oncology, Cooper Hospital/University Medical Center, Camden, New Jersey

Correspondence: For correspondence or reprints contact: Jeffry A. Siegel, PhD, Dept. of Radiation Oncology, Cooper Hospital/University Medical Center, One Cooper Plaza, Camden, NJ 08103.

ABSTRACT

Bremsstrahlung SPECT imaging and activity quantitation have been performed using ³²P-chromic phosphate. Methods: Attenuation correction was applied to the reconstructed transverse SPECT slices using a commercially available first-order postprocessing algorithm. The patient's body contour was defined through the use of four externally placed sources and attenuation correction was then performed with an experimentally determined effective linear attenuation coefficient for ³²P. Phantom studies were performed to determine the activity needed in the four external sources and also to validate absolute activity analysis on the reconstructed SPECT slices. A computer algorithm was written to facilitate ROI activity determination based on a fixed threshold method. Four cancer patients enrolled in clinical Phase I protocols were injected with 2.5 million particles of macroaggregated albumin followed by colloidal ³²P-chromic phosphate by direct interstitial injection into the tumor-bearing region under CT guidance. The in vivo ³²P activity distribution was restricted to a small volume with minimal background activity. SPECT images were obtained in these patients and the activity of ³²P present in the tumors was calculated from their attenuation-corrected reconstructed SPECT slices. Results: The effective linear attenuation coefficient for ³²P was determined to be 0.13 cm^–1. A fixed 39% threshold was best for activity calculation since it provided the best correlation between known and measured activity levels in the phantom. The calculated activities were within 16.9% of the actual activities in the patients studied. Conclusion: Accurate quantitative bremsstrahlung SPECT imaging with a commercially available postprocessing attenuation correction algorithm can be performed in a clinical setting.

Key Words: bremsstrahlung • quantitative SPECT imaging • phosphorus-32 • attenuation correction • radionuclide therapy

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