A custom mixed-signal CMOS integrated circuit for high performance PET tomograph front-end applications

Ullisch

2006

IEEE Trans. Nucl. Sci.

Abstract--The CPS Accel is a commercial PET camera based on a block detector with 64 LSO scintillator crystals (each 6.75 x 6.75 x 25 mm) read out with 4 photomultiplier tubes. The excellent timing resolution of LSO suggests that this camera might be used for time-of-flight (TOF) PET, thereby reducing the statistical noise significantly. Although the Accel achieves 3 ns coincidence resolution (a factor of two better than BGObased PET cameras), its timing resolution is nearly an order of magnitude worse than that demonstrated with individual LSO crystals. This paper quantifies the effect on the timing of each component in the Accel timing chain to identify which components most limit the camera's timing resolution. The components in the timing chain are: the scintillator crystal, the photomultiplier tube (PMT), the constant fraction discriminator (CFD), and the time to digital converter (TDC). To measure the contribution of each component, we construct a single crystal test system with high-performance versions of these components. This system achieves 221 ps fwhm coincidence timing resolution, which is used as a baseline measurement. One of the high-performance components is replaced by a production component, the coincidence timing resolution is remeasured, and the difference between measurements is the contribution of that (production) component. We find that the contributions of the TDC, CFD, PMT, and scintillator are 2000 ps, 1354 ps, 422 ps, and 326 ps fwhm respectively, and that the overall timing resolution scales like the square root of the amount of scintillation light detected by the PMT. Based on these measurements we predict that the limit for the coincidence timing resolution in a practical, commercial, LSO-based PET camera is 528 ps fwhm.

Section: Discussionmentioning

confidence: 99%

Factors influencing timing resolution in a commercial LSO PET camera

Ullisch

2006

IEEE Trans. Nucl. Sci.

“…A true CFD uses an analog delay line with frequency response of ~10 GHz, and these are difficult to incorporate into an ASIC. Non-delay line CFD ASICs have been developed [32,43,44], but it is not clear that these have the requisite timing accuracy. In addition, such an ASIC needs to include a high precision TDC and the rest of the electronic circuitry needed for a PET camera (gain adjustment, energy windowing, crystal identification, event formatting, test pulsing, etc.…”

Section: Electronicsmentioning

confidence: 99%

Recent advances and future advances in time-of-flight PET

Nuclear Instruments and Methods in Physics Research Section A:

2007

179

Simple theory predicts that the statistical noise variance in PET can be reduced by an order of magnitude by using time-of-flight (TOF) information. This reduction can be obtained by improving the coincidence timing resolution, and so would be achievable in clinical, whole body studies using with PET systems that differ little from existing cameras. The potential impact of this development is large, especially for oncology studies in large patients, where it is sorely needed. TOF PET was extensively studied in the 1980's but died away in the 1990's, as it was impossible to reliably achieve sufficient timing resolution without sacrificing other important PET performance aspects, such as spatial resolution and efficiency. Recent advances in technology (scintillators, photodetectors, and high speed electronics) have renewed interest in TOF PET, which is experiencing a rebirth. However, there is still much to be done, both in instrumentation development and evaluating the true benefits of TOF in modern clinical PET. This paper looks at what has been accomplished and what needs to be done before time-of-flight PET can reach its full potential.

“…A timing estimator is generated via an analog sum of the four PMT signals, then converting this analog signal into a digital timing signal using a CFD [3][4][5][6][7][8]. Differences in the propagation times between the 4 PMTs affect the leading edge of the summed signal and thus affect the timing resolution.…”

Section: Introductionmentioning

confidence: 99%

Multi-CFD Timing Estimators for PET Block Detectors

Ullisch

2007

IEEE Trans. Nucl. Sci.

Abstract-In a conventional PET system with block detectors, a timing estimator is created by generating the analog sum of the signals from the four photomultiplier tubes (PMT) in a module and discriminating the sum with a single constant fraction discriminator (CFD). The differences in the propagation time between the PMTs in the module can potentially degrade the timing resolution of the module. While this degradation is probably too small to affect performance in conventional PET imaging, it may impact the timing inaccuracy for time-of-flight PET systems (which have higher timing resolution requirements). Using a separate CFD for each PMT would allow for propagation time differences to be removed through calibration and correction in software.In this paper we investigate and quantify the timing resolution achievable when the signal from each of the 4 PMTs is digitized by a separate CFD. Several methods are explored for both obtaining values for the propagation time differences between the PMTs and combining the four arrival times to form a single timing estimator. We find that the propagation time correction factors are best derived through an exhaustive search, and that the "weighted average" method provides the best timing estimator. Using these methods, the timing resolution achieved with 4 CFDs (1052 ± 82 ps) is equivalent to the timing resolution with the conventional single CFD setup (1067 ± 158 ps).