Uranium pinhole collimators for I-131 SPECT imaging

Pinhole apertures made of depleted uranium were evaluated for high-resolution SPECT brain tumor imaging of therapeutic doses (30--100 mCi) of intratumorally administered I-131 radiolabeled monoclonal antibodies. High resolution is required for imaging residual tumor tissue, although for therapeutic doses, high collimator sensitivity is less important. Resolution of uranium and tungsten pinhole apertures was compared in planar and SPECT images of I-131 sources. Planar sensitivity was studied on- and off-axis. On-axis planar resolution for uranium pinholes is higher than that of tungsten pinholes for all cases examined; full-width-half-max (FWHM) is narrower for a 3 mm uranium pinhole than for a 1 mm tungsten pinhole, indicating significantly lower penetration at pinhole edges for uranium than for tungsten. Collimator sensitivity is higher for tungsten than for uranium, which also indicates higher edge penetration for tungsten. On-axis SPECT (14.4 cm radius) scans also show better resolution for a 2 mm uranium pinhole (7.8 mm FWHM) than for a 2 mm tungsten pinhole (9.5 mm FWHM). Background radiation from a uranium collimator (102 cts/sec) is acceptably low compared to expected clinical count rates