LSO PET/SPECT spatial resolution: critical on-line DOI rebinning methods and results (original) (raw)
Related papers
Depth of interaction calibration for PET detectors with dual-ended readout by PSAPDs
Physics in Medicine and Biology, 2009
Many laboratories develop depth-encoding detectors to improve the trade-off between spatial resolution and sensitivity in positron emission tomography (PET) scanners. One challenge in implementing these detectors is the need to calibrate the depth of interaction (DOI) response for the large numbers of detector elements in a scanner. In this work, we evaluate two different methods, a linear detector calibration and a linear crystal calibration, for determining DOI calibration parameters. Both methods can use measurements from any source distribution and location, or even the intrinsic lutetium oxyorthosilicate (LSO) background activity, and are therefore well suited for use in a depth-encoding PET scanner. The methods were evaluated by measuring detector and crystal DOI responses for all eight detectors in a prototype depth-encoding PET scanner. The detectors utilize dual-ended readout of LSO scintillator arrays with position-sensitive avalanche photodiodes (PSAPDs). The LSO arrays have 7 × 7 elements, with a crystal size of 0.92 × 0.92 × 20 mm3 and pitch of 1.0 mm. The arrays are read out by two 8 × 8 mm2 area PSAPDs placed at opposite ends of the arrays. DOI is measured by the ratio of the amplitude of the total energy signals measured by the two PSAPDs. Small variations were observed in the DOI responses of different crystals within an array as well as DOI responses for different arrays. A slightly nonlinear dependence of the DOI ratio on depth was observed and the nonlinearity was larger for the corner and edge crystals. The DOI calibration parameters were obtained from the DOI responses measured in a singles mode. The average error between the calibrated DOI and the known DOI was 0.8 mm if a linear detector DOI calibration was used and 0.5 mm if a linear crystal DOI calibration was used. A line source phantom and a hot rod phantom were scanned on the prototype PET scanner. DOI measurement significantly improved the image spatial resolution no matter which DOI calibration method was used. A linear crystal DOI calibration provided slightly better image spatial resolution compared with a linear detector DOI calibration.
IEEE Transactions on Nuclear Science, 2012
We have previously reported on continuous miniature crystal element (cMiCE) PET detectors that provide depth of interaction (DOI) positioning capability. A key component of the design is the use of a statistics-based positioning (SBP) method for 3D event positioning. The Cramer-Rao lower bound (CRLB) expresses limits on the estimate variances for a set of deterministic parameters. We examine the CRLB as a useful metric to evaluate the performance of our SBP algorithm and to quickly compare the best possible resolution when investigating new detector designs. In this work, the CRLB is first reported based upon experimental results from a cMiCE detector using a 50×50×15-mm 3 LYSO crystal readout by a 64-channel PMT (Hamamatsu H8500) on the exit surface of the crystal. The X/Y resolution is relatively close to the CRLB, while the DOI resolution is more than double the CRLB even after correcting for beam diameter and finite X (i.e., reference DOI position) resolution of the detector. The positioning performance of the cMiCE detector with the same design was also evaluated through simulation. Similar with the experimental results, the difference between the CRLB and measured spatial resolution is bigger in DOI direction than in X/Y direction. Another simulation study was conducted to investigate what causes the difference between the measured spatial resolution and the CRLB. The cMiCE detector with novel sensor-on-entrancesurface (SES) design was modeled as a 49.2×49.2×15-mm 3 LYSO crystal readout by a 12×12 array of 3.8×3.8-mm 2 silicon photomultiplier (SiPM) elements with 4.1-mm center-to-center spacing on the entrance surface of the crystal. The results show that there are two main causes to account for the differences between the spatial resolution and the CRLB. First, Compton scatter in the crystal degrades the spatial resolution. The DOI resolution is degraded more than the X/Y resolution since small angle scatter is preferred. Second, our maximum likelihood (ML) clustering algorithm also has limitations when developing 3D look up tables during detector calibration.
2007 IEEE Nuclear Science Symposium Conference Record, 2007
Present Positron Emission Tomography (PET) detectors suffer from degradation of the spatial resolution due to the lack of depth-of-interaction (DOI) information leading to uncertainty in deducing the Lines of Response (LOR) between coincident events. The Centre for Medical Radiation Physics at the University of Wollongong has developed a novel detector module which will provide depth of interaction information while retaining the sensitivity of current scanners. This will result in superior imaging together with the ability to locate smaller lesions. This work focuses on preliminary investigations of the suitability of replacing the bulky scintillator crystals and photomultiplier tubes of traditional PET detector modules with compact 3 × 3 × 3 mm 3 LYSO scintillator crystals individually coupled to Si photdetectors.
Characterization of a 64 channel PET detector using photodiodes for crystal identification
IEEE Transactions on Nuclear Science, 1997
element detector module and still yield a practical design. In this paper we report results of measurements made on a 64 element detector module made with LSO scintillator crystal. We present performance results for a prototype PET detector module consisting of 64 LSO scintillator crystals (3x3x20 mm) coupled on one end to a single photomultiplier tube and on the opposite end to a 64 pixel array of 3 mm square silicon photodiodes (typical pixel parameters are 5 pF capacitance, 300 pA dark current, and 73% quantum efficiency at 415 nm). The photomultiplier tube (PMT) provides an accurate timing pulse and energy threshold for all crystals in the module, the silicon photodiodes (PD) identify the crystal of interaction, the sum (PD+PMT) provides a total energy signal, and the PD/(PD+PMT) ratio determines the depth of interaction. With 32 of the channels instrumented, the detector module correctly identifies the crystal of interaction (where "correct" includes the adjacent 4 crystals) 79±4% of the time with high detection efficiency. The timing resolution for a single LSO detector module is 750 ps fwhm, while its pulse height resolution at 511 keV is 24±3% fwhm. The depth of interaction (DOI) measurement resolution is 8±1 mm fwhm. We prefer this ratio based technique to determine the depth of interaction because: (a) it is less sensitive to patient Compton scatter than approaches that use a single photodetector [3]; (b) it obtains better DOI resolution than approaches that use physically segmented crystals [4-6]; and (c) it has a smaller deadtime-surface area product than approaches that use large scintillator plates [7].
A High Resolution Monolithic Crystal, DOI, MR Compatible, PET Detector
2012
The principle objective of this proposal is to develop a positron emission tomography (PET) detector with depth-of-interaction (DOI) positioning capability that will achieve state of the art spatial resolution and sensitivity performance for small animal PET imaging. When arranged in a ring or box detector geometry, the proposed detector module will support <1 mm 3 image resolution and >15% absolute detection efficiency. The detector will also be compatible with operation in a MR scanner to support simultaneous multi-modality imaging. The detector design will utilize a thick, monolithic crystal scintillator readout by a two-dimensional array of silicon photomultiplier (SiPM) devices using a novel sensor on the entrance surface (SES) design. Our hypothesis is that our single-ended readout SES design will provide an effective DOI positioning performance equivalent to more expensive dual-ended readout techniques and at a significantly lower cost. Our monolithic crystal design will also lead to a significantly lower cost system. It is our goal to design a detector with state of the art performance but at a price point that is affordable so the technology can be disseminated to many laboratories. A second hypothesis is that using SiPM arrays, the detector will be able to operate in a MR scanner without any degradation in performance to support simultaneous PET/MR imaging. Having a co-registered MR image will assist in radiotracer localization and may also be used for partial volume corrections to improve radiotracer uptake quantitation. The far reaching goal of this research is to develop technology for medical research that will lead to improvements in human health care. Executive Summary This project focused on the development of a positron emission detector that can provide three dimensional positioning within the detector. A novel aspect of the design was the placement of photosensors on the entrance surface of the crystal detector versus conventional placement of the back surface of the detector. Our hypothesis was that using the sensor on the entrance surface (SES) design, our detector would provide better X, Y and Z positioning localization in the detector than using conventional bad sided readout. To facilitate our design, we needed to use two-dimensional arrays of silicon photomultipler (SiPM) devices. SiPMs are an emerging technology being looked at as a possible replacement for photomultiplier tubes. SiPMs are solid state devices made using CMOS processing techniques that are very low attenuating for the photons produced in positron emission tomography. Because of their low attenuation characteristic they can be placed on the front surface of a detector without affecting the incoming flux of photons. In addition to their low attenuation characteristics, they also have electronic and amplification characteristics similar to bulkier photomultipliers the current photosensor utilized in most PET detectors. Thus this research adds to the knowledge of three-dimensional positioning detectors for PET and also how best to use SiPM technology. One of the exciting aspects of the design is that it has the potential to improve performance without adding cost to the detector. Our preliminary investigations indicated that a 20-25% improvement in positioning resolution could be achieved by just changing the location of the readout sensors. To get similar three-dimensional positioning performance other researchers have proposed methods that double the cost of sensors and readout electronics. In regards to SiPMs, they have the potential to be very economically priced. Current device are made in low quantities; however, the basic fabrication process is similar to CMOS processing and should be very economical as production scales increase. Thus we believe the technology we are proposing should be economical enough to be a practical design alternative for future high resolution PET detector systems. While this project is mainly focused on a new PET detector design, it can potentially have far reaching benefits to the public as PET has emerged as a very important medical imaging technology. PET is already improving health care in oncology, cardiology and neurology. Improvements to PET instrumentation can lead to even better diagnosis and also potentially broader access to the public. Report on Accomplishment of Specific Aims There were three specific aims proposed in this research project. They are listed below. Specific Aim 1-Investigate via computer simulation the intrinsic spatial resolution in X, Y and Z of a detector utilizing of a thick, monolithic crystal coupled to a 2-D array of SiPM devices. Specific Aim 2-Develop prototype detector electronics including FPGA implementation to support real time processing of event data.
IEEE Transactions on Nuclear Science, 2000
Monolithic scintillator detectors have been shown to provide good performance and to have various practical advantages for use in PET systems. Excellent results for the gamma photon interaction position determination in these detectors have been obtained by means of the -nearest neighbor ( -NN) method. However, the practical use of monolithic scintillator detectors and the -NN method is hampered by the extensive calibration measurements and the long computation times. Therefore, several modified -NN methods are investigated that facilitate as well as accelerate the calibration procedure, make the estimation algorithm more efficient, and reduce the number of reference events needed to obtain a given lateral -resolution. These improved methods utilize the information contained in the calibration data more effectively. The alternative approaches were tested on a dataset measured with a SiPM-array-based monolithic LYSO detector. It appears that, depending on the number of reference events, to better spatial resolution can be obtained compared to the standard approach. Moreover, the methods amongst these that are equivalent to calibrating with a line source may allow for much faster and easier collection of the reference data. Finally, some of the improved methods yield essentially the same spatial resolution as the standard method using times less reference data, greatly reducing the time needed for both calibration and interaction position computation. Thus, using the improvements proposed in this work, the high spatial resolution obtainable with the -NN method may come within practical reach and, furthermore, the calibration may no longer be a limiting factor for the application of monolithic scintillator detectors in PET scanners.
Physics in Medicine and Biology, 2009
Small animal PET scanners may be improved by increasing the sensitivity, improving the spatial resolution and improving the uniformity of the spatial resolution across the field of view. This may be achieved by using PET detectors based on crystal elements that are thin in the axial and transaxial directions and long in the radial direction, and by employing depth of interaction (DOI) encoding to minimize the parallax error. With DOI detectors, the diameter of the ring of the PET scanner may also be decreased. This minimizes the number of detectors required to achieve the same solid angle coverage as a scanner with a larger ring diameter and minimizes errors due to non-collinearity of the annihilation photons. In this study, we characterize prototype PET detectors that are finely pixelated with individual LSO crystal element sizes of 0.5 mm × 0.5 mm × 20 mm and 0.7 mm × 0.7 mm × 20 mm, read out at both ends by position sensitive avalanche photodiodes (PSAPDs). Both a specular reflector and a diffuse reflector were evaluated. The detectors were characterized based on the ability to clearly resolve the individual crystal elements, the DOI resolution and the energy resolution. Our results indicate that a scanner based on any of the four detector designs would offer improved spatial resolution and more uniform spatial resolution compared to present day small animal PET scanners. The greatest improvements to spatial resolution will be achieved when the detectors employing the 0.5 mm × 0.5 mm × 20 mm crystals are used. Monte Carlo simulations were performed to demonstrate that 2 mm DOI resolution is adequate to ensure uniform spatial resolution for a small animal PET scanner geometry using these detectors. The sensitivity of such a scanner was also simulated using Monte Carlo simulations and was shown to be greater than 10% for a four ring scanner with an inner diameter of 6 cm, employing 20 detectors per scanner ring.
Monolithic scintillator PET detectors with intrinsic depth-of-interaction correction
Physics in Medicine and Biology, 2009
We developed positron emission tomography (PET) detectors based on monolithic scintillation crystals and position-sensitive light sensors. Intrinsic depth-of-interaction (DOI) correction is achieved by deriving the entry points of annihilation photons on the front surface of the crystal from the light sensor signals. Here we characterize the next generation of these detectors, consisting of a 20 mm thick rectangular or trapezoidal LYSO:Ce crystal read out on the front and the back (double-sided readout, DSR) by Hamamatsu S8550SPL avalanche photodiode (APD) arrays optimized for DSR. The full width at half maximum (FWHM) of the detector point-spread function (PSF) obtained with a rectangular crystal at normal incidence equals ∼1.05 mm at the detector centre, after correction for the ∼0.9 mm diameter test beam of annihilation photons. Resolution losses of several tenths of a mm occur near the crystal edges. Furthermore, trapezoidal crystals perform almost equally well as rectangular ones, while improving system sensitivity. Due to the highly accurate DOI correction of all detectors, the spatial resolution remains essentially constant for angles of incidence of up to at least 30 • . Energy resolutions of ∼11% FWHM are measured, with a fraction of events of up to 75% in the full-energy peak. The coincidence timing resolution is estimated to be 2.8 ns FWHM. The good spatial, energy and timing resolutions, together with the excellent DOI correction and high detection efficiency of our detectors, are expected to facilitate high and uniform PET system resolution.
Physics in Medicine and Biology, 2010
Improvements to current small animal PET scanners can be made by improving the sensitivity and the spatial resolution of the scanner. In the past, efforts have been made to minimize the crystal dimensions in the axial and transaxial directions to improve the spatial resolution and to increase the crystal length to improve the sensitivity of the scanner. We have designed tapered PET detectors with the purpose of reducing the gaps between detector modules and optimizing the sensitivity of a future generation small animal PET scanner. In this work, we investigate spatial resolution and sensitivity of a scanner based on tapered detector elements using Monte Carlo simulations. For tapered detector elements more scintillation material is used per detector resulting in a higher sensitivity of the scanner. However, since the detector elements are not uniform in size, degradation in spatial resolution is also expected. To investigate characteristics of tapered PET detectors the spatial resolution and sensitivity of a one-ring scanner were simulated for a system based on traditional cuboid detectors and a scanner based on tapered detectors. Additionally, the effect of depth of interaction (DOI) resolution on the spatial resolution for the traditional and tapered detectors was evaluated.
A practical method for depth of interaction determination in monolithic scintillator PET detectors
Physics in Medicine and Biology, 2011
Several new methods for determining the depth of interaction (DOI) of annihilation photons in monolithic scintillator detectors with singlesided, multi-pixel readout are investigated. The aim is to develop a DOI decoding method that allows for practical implementation in a positron emission tomography system. Specifically, calibration data, obtained with perpendicularly incident gamma photons only, are being used. Furthermore, neither detector modifications nor a priori knowledge of the light transport and/or signal variances is required. For this purpose, a clustering approach is utilized in combination with different parameters correlated with the DOI, such as the degree of similarity to a set of reference light distributions, the measured intensity on the sensor pixel(s) closest to the interaction position and the peak intensity of the measured light distribution. The proposed methods were tested experimentally on a detector comprised of a 20 mm × 20 mm × 12 mm polished LYSO:Ce crystal coupled to a 4 × 4 multi-anode photomultiplier. The method based on the linearly interpolated measured intensities on the sensor pixels closest to the estimated (x, y)-coordinate outperformed the other methods, yielding DOI resolutions between ∼1 and ∼4.5 mm FWHM depending on the DOI, the (x, y) resolution and the amount of reference data used.