RT Journal Article SR Electronic T1 Dead time correction method for long axial field-of-view, whole-body PET scanner JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 458 OP 458 VO 60 IS supplement 1 A1 Tang, Songsong A1 liu, yilin A1 wang, jianxun A1 zhao, yong A1 fan, xin A1 dong, yun YR 2019 UL http://jnm.snmjournals.org/content/60/supplement_1/458.abstract AB 458Objectives: PET scanners require a minimum amount of time to deal with each event. Since radioactive decay is a random process, there is also a certain probability that more than two events occur within this minimum time, which means some events may be lost, and the probability can become significant at high count rates. This phenomenon causes the result that the linear response of PET system is compromised as photo flux increases. To obtain quantitative accuracy images, dead time correction is necessary. Up to now, with the appearance of 3D long axial field-of-view (FOV), whole-body PET scanner, the dead time effect will become more complicated because of the extremely high sensitivity and the obviously non-uniform count rate distribution inside the whole FOV. The dead time correction should be done more carefully to obtain quantitative accuracy and uniform images. In this paper, we propose a dead time correction method for long axial FOV PET scanner using a decaying source experiment of a specific long uniform phantom. Methods: Up to now, the latest long axial FOV, whole-body PET scanner offers extremely high sensitivity, simulation study shows that a sensitivity gain of 30~40 times of current scanners would be obtained. High sensitivity causes a substantial increase in the burden of count rate and the dead time effect will be more notable, long axial FOV means that the count rate distribution would be obviously non-uniform inside the whole FOV and the dead time effect will be location dependent. In this paper, to estimate the dead time behavior of the whole scanner, a long uniform source decaying experiment was made. To correct location dependent dead time effect online, detector block-pair based correction factors are applied. We assume that singles count rate loss is the main source of the dead time effect and this hypothesis is proved to be reasonable by our results. The simple non-paralyzable response model is selected to describe the overall effect of singles count rate. Result: The system used in the experiment is UIH/UExplorer which contains as many as eight units. Each unit contains 84 detector rings, with a total of 672 detector rings. Each detector ring contains 840 detectors. The axial FOV and ring diameter of the scanner are 1948 mm and 786 mm respectively. The singles count rate for each detector block is recorded in on-line scan. To obtain the dead time correction LUT, a long uniform phantom was used as shown in fig.1. To verify the dead time correction LUT, a combinant of three traditional scatter phantoms was used as shown in fig. 2. The total length of the polyethylene cylinder is 210 cm. The initial activity was about 25 mCi, in order to achieve count rates beyond the expected peak of the noise equivalent count rate (NECR). Fig.3 shows the accuracy result. Data processing was mainly according to the NEMA NU2-2012 protocol. The first and last ten slices were excluded from this evaluation and the thickness of each slice is 2.85 mm. The relative count rate error at the activity concentration of the NECR peak (8.6 kBq/ml) was about 6.0%. Conclusion: The aim of our study is to create a feasible method for the dead time calibration and dead time correction on-line of whole-body PET scanner. Our results show that a simple non-paralyzable model could describe the count rate behavior of the system properly and achieve accuracy result within 6%.