EVALUATION OF EMISSION-TRANSMISSION REGISTRATION IN THORACIC PET

The intent of this investigation was to quantitate the amount of misre gistration between PEF emission and transmission scans of the thorax t hat occurs in a normal clinical environment. Methods: The data from 17 FDG myocardial studies were evaluated. Prior to injection, a transmis sion study was acquired for 15 min using a Ge-68/Ga-68 ring source. Th e location of the cross-hairs from a laser alignment system was marked on the patient who was then removed from the scanner and injected wit h 10 mCi of FDG. After 45 min, the patient was placed back on the tabl e and repositioned with the previously placed marks and a 15-min emiss ion scan was acquired. The outline of the lungs on both the transmissi on and emission images was manually segmented. Both attenuation-correc ted and noncorrected emission images were evaluated and the one that p rovided clearer visualization of the outline of the lungs was chosen f or segmentation. The segmented contours of the transmission and emissi on scans were then registered with the method described by Pelizzari e t al, using the transmission image as the ''head'' and the emission im age as the ''hat.'' The allowable transformations were x and y shifts and rotation in the transverse plane. Results: Shifts in the x-axis av eraged 2.4 mm (range: 0.2-7.3 mm, 80% less than 3.3 mm) with shifts in the y-axis averaging 2.6 mm (range: 0.1-8.7 mm, 80% less than 2.4 mm) and rotations in the transverse plane averaging 1.6 degrees (range: 0 .2 to 5.1 degrees, 80% less than 2.4 degrees). A phantom study indicat ed that the accuracy of this method of evaluating misregistration was 2.35 mm and 1.81 mm in the x and y directions, respectively. Conclusio n: Our preliminary evaluation indicates that careful application of la ser alignment is an adequate method of registration in most cases.