Quantitative imaging of iodine-131 distributions in brain tumors with pinhole SPECT: a phantom study

A method of quantitatively imaging 131I distributions in brain tumors from intratumoral administration of activity was developed and investigated using pinhole SPECT of brain tumor phantoms.

Pinhole SPECT sensitivity and resolution were characterized using 131I point-source acquisitions with high-resolution lead (1.4-mm diameter aperture) and tungsten (1.0-mm diameter aperture) pinhole inserts. SPECT scans were obtained from brain tumor phantoms in a water-filled cylinder. The tumor phantoms consisted of spheres filled with an 131I solution to model intratumoral administration of radiolabeled monoclonal antibodies. Two spheres were 20.5 and 97 ml, and two other concentric spheres modeled a tumor with a high-activity shell (71.5 ml) and a low-activity core (21 ml). The collimator focal length was 16 cm and the distance from the pinhole to the center of rotation was 13 cm. The filtered backprojection reconstruction algorithm incorporated scatter and attenuation compensation. SPECT tumor activities and concentrations were estimated using scaling factors from reference point-source scans.

System sensitivities for point sources at the center of rotation were 28.4 cts/sec(-1) MBq(-1) (lead insert) and 13.6 cts/sec(-1) MBq(-1) (tungsten insert). SPECT resolutions (FWHM) at the center of rotation were 8.1-11.9 mm (lead) and 6.7-10.3 mm (tungsten). Total tumor activity estimates from SPECT were within 17% of the true activities. SPECT activity concentration estimates in small regions of interest (ROIs) averaged -20% for the 20.5-ml sphere, -11% for the 97-ml sphere, -39% for the shell and +20% for the core of the shell-core phantom. Activity spillover due to limited spatial resolution and the tails of the system response functions biased the estimates. The shell-to-core activity concentration ratio of 4.1 was better estimated with the tungsten insert (2.3) than with the lead insert (1.9) due to better resolution.

Pinhole SPECT is a promising technique for imaging and quantifying total 131I activity in regions the size of brain tumors. Relative errors were greater for activity concentration estimates in small ROIs than for total activity estimates.