Uranium pinhole collimators for I-131 SPECT brain tumor 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 radiolabelled monoclonal antibodies. High resolution is required for imaging residual tumor tissue, although for therapeutic doses, collimator sensitivity is less important. The resolution of uranium and tungsten pinhole apertures was compared in planar and SPECT images of I-131 sources. The planar sensitivity was studied on- and off-axis. The 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. Collimator sensitivity is higher for tungsten than for uranium, indicating significantly lower edge penetration for uranium than for tungsten. On-axis SPECT (14.4 cm radius) scans also show a higher resolution for a 2 mm uranium pinhole (7.8 mm FWHM) than for a 2 mm tungsten pinhole (9.5 mm FWHM). The background radiation from a uranium collimator (102 cts/sec) is acceptably low compared to expected clinical count rates