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Effects of patient motion in coincidence studies on hybrid PET/SPECT system

Patient motion frequently occurs during coincidence imaging studies performed on hybrid PET/SPECT systems due to long acquisition times (30-50 min). The influence of patient motion on image quality and a motion correction method with hybrid PET/SPECT systems were investigated on a Picker AXIS dual head system. The evaluation was done using phantoms made of point, line and spherical sources filled with F-18 solution and inserted in a cylinder. Translations of 0.5 to 4.0 cm in the axial direction, and rotations from 5/spl deg/ to 30/spl deg/ at various times during the acquisition were applied. SNR, object contrast, FWHM and FWTM were calculated. Object deformation caused by motion was expressed as the ratio of the moments of inertia corresponding to the principal axes. The motion correction method applied is based on acquiring multiple fast scans and comparing them for motion. Scans prior to and after rotation and translation of the phantom were reconstructed separately, and combined using image registration. Changes in position of the phantom caused a decrease in contrast and resolution, as well as mispositioning and splitting of the sources. Motion in the axial direction splits the source in two parts that can appear several slices apart depending on the size of the motion. FWHM increased from 8.4 mm to 11.3 mm, and FWTM increased from 14.6 mm to 21.2 mm when 30/spl deg/ rotation was applied. The deformation of the spherical sources changed from 7% (no motion) up to 91% over the of motions applied in this study. Detection and correction of motion from coincidence data is a complex problem. Since motion at a given angular position of the camera can affect 20-40 projections, recording pre and post motion events in the same image. The proposed correction method successfully corrects for arbitrary motion, however further work is required to evaluate performance and quantitative accuracy with clinical data.