Effect of registration errors between transmission and emission scans on a SPECT system using sequential scanning

The purpose of this study was to evaluate the effects of patient motion on nonuniform attenuation correction of cardiac SPECT when the transmission and emission scans were performed sequentially. By using a sequential protocol rather than doing the scans simultaneously, contamination from the emission scan into the transmission scan could be eliminated, but registration of the two scans become a concern.

Transmission and emission scans were acquired using both an anthropomorphic thorax phantom containing a cardiac insert and a human volunteer. The types of motion considered were transverse shifts, axial shifts and rotations that occur in the time period between the transmission and emission scan. For this study, the various types of motion were simulated in the data. Both the transmission and emission data were reconstructed using filtered backprojection. A single-iteration Chang algorithm, modified for nonuniform attenuation correction, was used to further process the emission data. To evaluate the effects of motion errors, circumferential profiles, all normalized to the same scale, were generated for each case. The cardiac images reconstructed using registered data were considered references. Error profiles were generated by subtracting misaligned images from the reference and then normalizing the difference by the reference. For comparison purposes, an error profile was generated for the case in which no attenuation correction was performed.

It was found that, for transverse and axial shifts of 2.9 cm, the normalized myocardial SPECT activity was decreased in certain regions of the heart by 20%-35%. For a 12 degrees rotational shift, the error was on the order of 10%-20%, compared to a normalized variation of 20%-25% in the image with no attenuation correction.

The results indicate that registration errors of 2-3 cm can seriously affect image quality in both the phantom and human images.