Time delay correction method for PET-based tumor tracking

The world´s first open type PET, named OpenPET, is being developed at the National Institute Radiological Sciences. We are aiming to employ the OpenPET in radiotherapy and to track the respiratory motion of a tumor in the thoracoabdominal region of a patient. By using PET images, we expect that the tumor itself can be directly visualized without using radio-opaque markers, but this goal has not been achieved yet. Our demonstration results using a small prototype OpenPET system showed that the system can output reconstructed images at about 2 frames per second with about a 2 s delay; this delay is mainly due to the reconstruction calculation time. In this study, we developed a time delay correction method for tumor tracking for the OpenPET. Since it is difficult to correct the time delay using only the tumor location 2 s before, we introduced another sensor to acquire the respiration phase for correction. In the proposed method, the relationship between tumor motion and the additional sensor output signal was calculated by support vector regression (SVR) and the time delay was corrected by using the regression line which represents the relationship. We simulated this time delay correction method with computer-generated PET images which had practical respiration motions obtained from clinical MRI. As a result, we could track the tumor within almost 1.5 mm mean error when we assumed the use of a belt type respiratory motion sensor. We also found from the simulation that disturbance of the relationship can be detected by gates. Thus we could avoid the tracking errors which are caused by disturbance of the correlation.