A phantom study to evaluate the effect of patient weight on SPECT-CT alignment in brain imaging.

Objectives Multi-modality SPECT-CT allows for the localization of the physiologic uptake of administered radiopharmaceuticals relative to the anatomical structures as visualized by CT (fusion). Furthermore, the CT may be utilized for attenuation correction techniques. Both fusion and properly applied attenuation correction require that the SPECT and CT volumes be accurately aligned in 3D space. Alignment errors between the mechanically joined SPECT and CT scans are corrected by calibrating the combined system. In contrast to whole body imaging, when imaging the brain, the patient table is cantilevered into the gantry. This may result in a vertical shift (Y) between the acquired SPECT and CT volumes. This project evaluates the degree of SPECT to CT misalignment as patient weight increases.

Methods An 8 ml hollow sphere containing 16 uCi of Tc-99m and a Hoffman brain phantom containing 2.4 mCi of Tc-99m were prepared. Both phantoms were included in the field of view and imaged with a Siemens Symbia T16 SPECT-CT system. Placed in the patient brain extension, the phantoms were imaged alone and with table weights to simulate a 94.1 kg, 80.0 kg, 70.0 kg, and 59.5 kg patient. Brain SPECT-CT images were reconstructed, fused and the extent of misalignment evaluated using a Siemens MI Applications system.

Results Re-alignment of the CT to SPECT volumes resulted in vertical (Y) shifts of approximately 9 mm for the phantom only, and 11 mm, 12 mm, 10 mm and 10 mm for a 94.1 kg, 80.0 kg, 70.0 kg and 59.5 kg patient, respectively.

Conclusions The findings indicate that increased patient weight has a negligible effect on SPECT to CT volume alignment in brain imaging, but the misalignment itself is significant and must be taken into account.