Time-Delay Correction Method for PET-Based Tumor Tracking

The world´s first open-type positron emission tomography (PET), named OpenPET, is being developed at the National Institute of Radiological Sciences, Japan. 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. Our demonstration results using a small prototype OpenPET system showed that the system can output reconstructed images at about two frames per second with about a 2-s delay; this delay is mainly due to the reconstruction calculation time. In this paper, 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 at 2 s before, we assumed the introduction of another high-speed sensor to acquire the respiration phase for correction. In the proposed method, the relationship between the tumor motion and the additional sensor output signal is calculated by support vector regression and the time delay is corrected by using the obtained regression line. We simulated this time-delay correction method with computer-generated PET images which had actual respiration motions obtained from clinical magnetic resonance imaging. As a result, we could track the tumor with 1.20 ± 0.91 mm mean error when we assumed the use of a belt-type respiratory motion sensor.