Positron emission tomography (PET) is a tool for metabolic imaging that has been utilized since the earliest days of nuclear medicine. A key component of such imaging systems is the detector modules—an area of research and development with a long, rich history. Development of detectors for PET has often seen the migration of technologies, originally developed for high energy physics experiments, into prototype PET detectors. Of the many areas explored, some detector designs go on to be incorporated into prototype scanner systems and a few of these may go on to be seen in commercial scanners. There has been a steady, often very diverse development of prototype detectors, and the pace has accelerated with the increased use of PET in clinical studies (currently driven by PET/CT scanners) and the rapid proliferation of pre-clinical PET scanners for academic and commercial research applications. Most of these efforts are focused on scintillator-based detectors, although various alternatives continue to be considered. For example, wire chambers have been investigated many times over the years and more recently various solid-state devices have appeared in PET detector designs for very high spatial resolution applications. But even with scintillators, there have been a wide variety of designs and solutions investigated as developers search for solutions that offer very high spatial resolution, fast timing, high sensitivity and are yet cost effective. In this review, we will explore some of the recent developments in the quest for better PET detector technology.

https://iopscience.iop.org/article/10.1088/0031-9155/53/17/R01/pdf

Recent developments in PET detector technology.

We present a clustering method to extract the depth of interaction (DOI) information from an 8 mm thick crystal version of our continuous miniature crystal element (cMiCE) small animal PET detector. This clustering method, based on the maximum-likelihood (ML) method, can effectively build look-up tables (LUT) for different DOI regions. Combined with our statistics-based positioning (SBP) method, which uses a LUT searching algorithm based on the ML method and two-dimensional mean–variance LUTs of light responses from each photomultiplier channel with respect to different gamma ray interaction positions, the position of interaction and DOI can be estimated simultaneously. Data simulated using DETECT2000 were used to help validate our approach. An experiment using our cMiCE detector was designed to evaluate the performance. Two and four DOI region clustering were applied to the simulated data. Two DOI regions were used for the experimental data. The misclassification rate for simulated data is about 3.5% for two DOI regions and 10.2% for four DOI regions. For the experimental data, the rate is estimated to be ~25%. By using multi-DOI LUTs, we also observed improvement of the detector spatial resolution, especially for the corner region of the crystal. These results show that our ML clustering method is a consistent and reliable way to characterize DOI in a continuous crystal detector without requiring any modifications to the crystal or detector front end electronics. The ability to characterize the depth-dependent light response function from measured data is a major step forward in developing practical detectors with DOI positioning capability.

https://faculty.washington.edu/rmiyaoka/Miyaoka_Publications/Ling_Depth_PhysMedBiol_2007.pdf

Depth of interaction decoding of a continuous crystal detector module.

Over the past decade, many small animal PET scanners have been developed (1–12), and this technology has played a very important role in the rapidly growing field of molecular imaging. High sensitivity is needed to increase signal to noise ratio of the images in order to reliably detect lower levels of radiotracer uptake and to reduce the injected dose (reducing radiation dose to the subject) (13) and scan time (increasing temporal resolution for dynamic studies). High spatial resolution is required to detect small structures and lesions and to improve quantification by reducing the partial volume effect. A compromise between sensitivity and spatial resolution always exists in small animal PET scanner design due to depth of interaction (DOI) effects. Most designs use a large detector ring radius (relative to the subject to be studied) and/or a short crystal length in order to reduce the DOI effects to acceptable levels. But in both cases the sensitivity of the scanner is sacrificed.

For this reason, much attention in recent years has focused on detector designs with depth-encoding ability. Multi-layer detectors consisting of crystal layers with different scintillation light decay times (14–16), with different reflector arrangements (17, 18), and using a position shift of half a crystal for different layers (19), have been proposed and successfully implemented to measure DOI. The DOI resolution obtained by these methods is limited by the number of layers that can be decoded, and is typically on the order of 5–10 mm. As a rule of thumb, to eliminate the DOI effect completely, the DOI resolution needs to be on the order of the detector width itself. For small animal PET, the detector widths are typically 0.5–2 mm (8, 10, 20, 21). Therefore, although these designs help reduce DOI-related degradation of spatial resolution, they are still not good enough to allow detectors to be placed in very close proximity to the subject. Thus the classic trade-off between resolution and sensitivity remains, with scanners utilizing thinner detectors and larger ring diameters than desirable. The larger ring diameter also has the undesirable effect of increasing the cost per unit sensitivity.

It is therefore important to develop detectors for small animal PET imaging with DOI resolution that approaches the detector width. A promising approach for obtaining higher DOI resolution is to read out the scintillation light by placing photodetectors at both ends of the crystal array and use the ratio of the two photodetector signals to measure the DOI (22–26). An alternative approach being used to estimate DOI for monolithic scintillator crystals measures the distribution of the scintillation light signal on a position-sensitive photodetector arrays (27, 28). In this case events occurring nearer the photodetector produce a narrower spread of scintillation light on the detectors than events occurring far from the photodetector, thus providing a basis for discriminating based on depth. Both of these approaches provide continuous DOI measurement rather than the discrete DOI information provided by the layered approach described earlier.

Our approach is to measure DOI by using dual-ended readout of finely pixelated lutetium oxyorthosilicate (LSO) arrays with two position sensitive avalanche photodiodes (PSAPDs) (29–31). Our previous results on individual detector modules have shown that with such a detector at room temperature, crystals arrays down to 1 mm could be resolved and that an energy resolution of about 15% FWHM and a uniform DOI resolution of 3–4 mm FWHM (including the radiation beam width of more than 2 mm) could be achieved (32). In this work the imaging performance of a prototype PET scanner consisting of eight detector modules arranged in two detector plates is evaluated. First, results obtained from individual detectors used within the scanner, including measurements of the flood histograms calculated by using the energy signals from just one PSAPD or both PSAPDs, the performance of the detectors at different temperatures, DOI resolution measured with a smaller radiation beam width of 0.7 mm, and the timing resolution of the PSAPD are presented. Secondly imaging studies are performed with the prototype scanner to measure the effect of the DOI information on reconstructed image resolution.

https://jnm.snmjournals.org/content/jnumed/49/7/1132.full.pdf

A Prototype PET Scanner with DOI-Encoding Detectors

This paper presents a study of possible models to describe the relation between the scintillation light point-of-origin and the measured photo detector pixel signals in monolithic scintillation crystals From these models the X, Y and depth of interaction (DOI) coordinates can be estimated simultaneously by nonlinear least-square fitting The method depends only on the information embedded in the signals of individual events, and therefore does not need any prior position training or calibration Three possible distributions of the light sources were evaluated: an exact solid-angle-based distribution, an approximate solid-angle distribution and an extended approximate solid-angle-based distribution which includes internal reflection at side and bottom surfaces The performance of the general model using these three distributions was studied using Monte Carlo simulated data of a 20 × 20 × 10 mm lutetium oxyorthosilicate (Lu2SiO5 or LSO) block read out by 2 Hamamatsu S8550 avalanche photo diode arrays The approximate solid-angle-based model had the best compromise between resolution and simplicity This model was also evaluated using experimental data by positioning a narrow 12 mm full width at half maximum (FWHM) beam of 511 keV photons at known positions on the 20 × 20 × 10 mm LSO block An average intrinsic resolution in the X-direction of 14 mm FWHM was obtained for positions covering the complete block The intrinsic DOI resolution was estimated at 26 mm FWHM

http://iopscience.iop.org/article/10.1088/0031-9155/55/21/012/pdf

Nonlinear least-squares modeling of 3D interaction position in a monolithic scintillator block.

High-energy (> 100 keV) photon detectors are often made thick relative to their lateral resolution in order to improve their photon-detection efficiency. To avoid issues of parallax and increased signal variance that result from random interaction depth, we must determine the 3D interaction position in the imaging detector. With this goal in mind, we examine a method of calibrating response statistics of a thick-detector gamma camera to produce a maximum-likelihood estimate of 3D interaction position. We parameterize the mean detector response as a function of 3D position, and we estimate these parameters by maximizing their likelihood given prior knowledge of the pathlength distribution and a complete list of camera signals for an ensemble of gamma-ray interactions. Furthermore, we describe an iterative method for removing multiple-interaction events from our calibration data and for refining our calibration of the mean detector response to single interactions. We demonstrate this calibration method with simulated gamma-camera data. We then show that the resulting calibration is accurate and can be used to produce unbiased estimates of 3D interaction position.

Calibration Method for ML Estimation of 3D Interaction Position in a Thick Gamma-Ray Detector

We have studied a new and inexpensive method of measuring the depth of interaction (DOI) in /spl gamma/-ray detectors with large-sized scintillation crystals. This method takes advantage of the strong correlation between the width of the undisturbed light-distribution in continuous crystals and the /spl gamma/-ray's DOI. In order to quantify the dependence of the distribution's width with respect to the DOI, we first studied an analytical model of the light-distribution and tested it by means of Monte Carlo (MC) simulations of the light transport inside the crystal. Further we present an inexpensive modification of the commonly used charge division circuit that allows analog and instantaneous computation of the light-distribution's second moment without affecting the determination of the centroid. This redesigned resistor network is based on the position-sensitive proportional counter (DPC) readout and allows, together with position sensitive photo-detectors, the additional measurement of the light-distribution's standard-deviation /spl sigma/. We tested the proposed circuit using the design-tool OrCAD and found the signal sufficiently large for digitalization. Finally, we conducted MC simulations of a realistic Positron Emission Tomography (PET) detector module setup that mimic a continuous Lu/sub 2/SiO/sub 5/:Ce/sup 3+/ (LSO) crystal of dimensions 40/spl times/40/spl times/10 mm/sup 3/ together with the new large area position-sensitive photo multiplier tube (PSPMT) H8500 from Hamamatsu. The influence of Compton scattering on the DOI determination was also estimated by MC simulations. Altogether, we obtained /spl les/ 5 mm DOI resolution. PACS: 87.57.Ce, 87.58.Fg, 87.62.+n, 07.85.-m.

N. Pavon

Papers

Depth of /spl gamma/-ray interaction within continuous crystals from the width of its scintillation light-distribution