Amy E. Perkins

Design of a lanthanum bromide detector for time-of-flight PET

Recent improvements in the growth and packaging of lanthanum bromide (LaBr/sub 3/), in addition to its superb intrinsic properties of high light output, excellent energy resolution, and fast decay time, make it a viable detection material for a positron emission tomography (PET) scanner based on time-of-flight (TOF). We have utilized theoretical simulations and experimental measurements to investigate the design and performance of pixelated LaBr/sub 3/ Anger-logic detectors suitable for use in a TOF PET scanner. Our results indicate that excellent energy resolution can be obtained from individual as well as multicrystal arrays of LaBr/sub 3/ in a 4 mm/spl times/4 mm /spl times/ 30 mm geometry. Measured energy resolutions (at 511 keV) of 4.1% for a single crystal and an average of 5.1% for an array of 100 crystals have been achieved with our best samples. Both simulations and experimental measurements of an Anger-logic based detector consisting of the LaBr/sub 3/ crystal array coupled to a continuous light guide and seven photomultiplier tubes (PMTs), have resulted in the ability to clearly discriminate 511 keV interactions in each crystal. We have measured coincidence time resolutions for both 0.5% and 5.0% cerium-doped LaBr/sub 3/ and found that the higher level of Ce-doping yielded superior results with little to no degradation in light output or energy resolution. The time resolution for a single 5.0% Ce-doped LaBr/sub 3/ crystal (4 mm /spl times/ 4 mm /spl times/ 30 mm) coupled directly to a PMT was measured to be 275 ps full-width at half-maximum (FWHM). With an array of 100 crystals coupled to a light guide and seven PMT cluster an average time resolution of 290 ps FWHM was obtained by summing the signals from the PMT cluster. Ultimately, two 5.0% Ce-doped LaBr/sub 3/ Anger-logic detectors placed in coincidence yielded a time resolution of 313 ps FWHM.

Design evaluation of A-PET: A high sensitivity animal PET camera

In recent years it has been shown that PET is capable of obtaining in vivo metabolic images of small animals. These serve as models to study the development and progress of diseases within humans. Imaging small animals requires not only image resolution better than 2 mm, but also high sensitivity in order to image ligands with low specific activity or radiochemical yields. Toward achieving these goals, we have developed a discrete 2 /spl times/ 2 /spl times/ 10 mm/sup 3/ GSO Anger-logic detector for use in a high resolution, high sensitivity, and high count-rate animal PET scanner. This detector uses relatively large 19 mm diameter photomultiplier tubes (PMT), but nevertheless achieves good spatial and energy resolution. The scanner (A-PET) has a port diameter of 21 cm, transverse field-of-view of 12.8 cm, axial length of 11.6 cm, and operates in 3-D volume imaging mode. The absolute coincidence sensitivity is 1.3% for a point source. Due to the use of large PMTs in an Anger design, the encoding ratio (number of crystals/PMT) is high, which reduces the complexity and leads to a cost-effective scanner. Simulation results show that this scanner can achieve high NEC rates for small cylindrical phantoms due to its high sensitivity and low dead-time. Initial measurements show that our design goals for spatial resolution and sensitivity were realized in the prototype scanner.

The imaging performance of a LaBr3-based PET scanner

A prototype time-of-flight (TOF) PET scanner based on cerium-doped lanthanum bromide [LaBr(3) (5% Ce)] has been developed. LaBr(3) has a high light output, excellent energy resolution and fast timing properties that have been predicted to lead to good image quality. Intrinsic performance measurements of spatial resolution, sensitivity and scatter fraction demonstrate good conventional PET performance; the results agree with previous simulation studies. Phantom measurements show the excellent image quality achievable with the prototype system. Phantom measurements and corresponding simulations show a faster and more uniform convergence rate, as well as more uniform quantification, for TOF reconstruction of the data, which have 375 ps intrinsic timing resolution, compared to non-TOF images. Measurements and simulations of a hot and cold sphere phantom show that the 7% energy resolution helps to mitigate residual errors in the scatter estimate because a high energy threshold (>480 keV) can be used to restrict the amount of scatter accepted without a loss of true events. Preliminary results with incorporation of a model of detector blurring in the iterative reconstruction algorithm not only show improved contrast recovery but also point out the importance of an accurate resolution model of the tails of LaBr(3)s point spread function. The LaBr(3) TOF-PET scanner demonstrated the impact of superior timing and energy resolutions on image quality.

Imaging performance of a-PET: a small animal PET camera

The evolution of positron emission tomography (PET) imaging for small animals has led to the development of dedicated PET scanner designs with high resolution and sensitivity. The animal PET scanner achieves these goals for imaging small animals such as mice and rats. The scanner uses a pixelated Anger-logic detector for discriminating 2 /spl times/ 2 /spl times/ 10 mm/sup 3/ crystals with 19-mm-diameter photomultiplier tubes. With a 19.7-cm ring diameter, the scanner has an axial length of 11.9 cm and operates exclusively in three-dimensional imaging mode, leading to very high sensitivity. Measurements show that the scanner design achieves a spatial resolution of 1.9 mm at the center of the field-of-view. Initially designed with gadolinium orthosilicate but changed to lutetium-yttrium orthosilicate, the scanner now achieves a sensitivity of 3.6% for a point source at the center of the field-of-view with an energy window of 250-665 keV. Iterative image reconstruction, together with accurate data corrections for scatter, random, and attenuation, are incorporated to achieve high-quality images and quantitative data. These results are demonstrated through our contrast recovery measurements as well as sample animal studies.

Characterization of a time-of-flight PET scanner based on lanthanum bromide

A proto-type time-of-flight (TOF) 3D PET scanner based on lanthanum bromide detectors has been developed. The LaBr/sub 3/(5%Ce) Anger-logic detectors in this new scanner use 4/spl times/4/spl times/30 mm pixels and continuous light-guide coupled to a hexagonal array of 50-mm PMTs. The scanner consists of 24 modules with a 93-cm detector diameter and 25-cm axial field-of-view. Initial characterization of scanner performance has been performed, including energy and timing performance. We currently measure an overall system energy resolution of 7.5% and a system timing resolution is 460 ps, although we expect these results to improve eventually when the electronics are fully optimized. Since there are not yet standard tests to quantify the benefit of TOF, we designed two phantoms with hot and cold spheres in 27-cm and 35-cm diameter vessels to evaluate the TOF performance as a function of body size. The data from this scanner are reconstructed with a fully 3D list-mode iterative TOF algorithm with all data corrections incorporated into the system model. We find that TOF reconstruction reduces the noise and background variability, especially for the larger phantom representing a large patient. In addition, TOF improves detail and contrast of the spheres (lesions), especially the smallest 10-mm sphere. The TOF reconstruction reaches convergence faster than the non-TOF reconstruction, and the rate of convergence is seen to be more insensitive to object size. These results indicate that TOF will help improve image quality and potentially reduce scan time with clinical patients.

Determination of Accuracy and Precision of Lesion Uptake Measurements in Human Subjects with Time-of-Flight PET

Inclusion of time-of-flight (TOF) information in PET reconstructions has been demonstrated to improve image quality through better signal-to-noise ratios, faster convergence, better lesion detectability, and better image uniformity. The goal of this work was to assess the impact of TOF information on the accuracy and precision of quantitative measurements of activity uptake in small lesions in clinical studies. Methods: Data from small (10-mm diameter) spheres were merged with list-mode data from 6 healthy volunteers after injection of 18F-FDG. Six spheres having known activity uptake with respect to the average whole-body uptake were embedded in both the liver and the lung of the subject’s data. Images were reconstructed with TOF information and without TOF information (non-TOF reconstruction). The measured uptake was compared with the known activity; variability was measured across 60 bootstrapped replicates of the merged data, across the 6 spheres within a given organ, and across all spheres in all subjects. Results: The average uptake across all spheres and subjects was approximately 50% higher in the lung and 20% higher in the liver with TOF reconstruction than with non-TOF reconstruction at comparable noise levels. The variabilities across replicates, across spheres within an organ, and across all spheres and subjects were 20%–30% lower with TOF reconstruction than with non-TOF reconstruction in the lung; in the liver, the variabilities were 10%–20% lower with TOF reconstruction than with non-TOF reconstruction. Conclusion: TOF reconstruction leads to more accurate and precise measurements, both within a subject and across subjects, of the activity in small lesions under clinical conditions.

Time of flight coincidence timing calibration techniques using radioactive sources

The coincidence timing offset must be measured accurately and robustly in order to elicit the best performance for a time of flight (TOF) PET scanner. The proposed calibration methods involve measuring the timing differences between multiple coincidence line pairs. The absolute time biases for each pixel on the detector are assumed to be independent which greatly reduces the number of counts that are needed for a good estimate. It is not a requirement that each pixel on the detector be measured in coincidence with every other pixel on the detector, but that the coincidences involve a reasonably large number of different pixels on the opposite side. The intrinsic timing offset for each detector crystal pixel is unfolded by forming the average of the offset values of each crystal pixel with opposing pixels to average out crystal variations and photomultiplier (PMT) timing differences. In this work, a comparison is made of different timing calibration techniques using radioactive sources, specifically a rotating line source and a point source in a scattering block. The calibrations using the different methods are performed on a prototype TOF scanner and the results are presented

Design of a lanthanum bromide detector for TOF PET

Recent improvements in the growth and packaging of lanthanum bromide (LaBr/sub 3/) in addition to its superb intrinsic properties of high light output, excellent energy resolution, and fast decay time, make it a viable detection material for a PET scanner based on time-of-flight (TOF). We have utilized theoretical simulations and experimental measurements to investigate the design and performance of pixilated LaBr/sub 3/ Anger-logic detectors suitable for use in a TOF PET scanner. Our results indicate that excellent energy resolution can be obtained from individual as well as multicrystal arrays of LaBr/sub 3/ in a 4/spl times/4/spl times/30 mm/sup 3/ geometry. Measured energy resolutions (at 511 keV) of 4.3% for a single crystal and an average of 5.31% for an array of 100 crystals have been achieved. Both simulations and experimental measurements of an Anger-logic based detector consisting of the LaBr/sub 3/ crystal array coupled to a continuous light guide and seven PMTs, have resulted in the ability to clearly discriminate 511 keV interactions in each crystal. We have measured coincidence time resolutions for both 0.5% and 5.0% Ce-doped LaBr/sub 3/ and found that the higher level of Ce-doping yielded superior results with little to no degradation in light output or energy resolution. The time resolution for a single 5.0% Ce-doped LaBr/sub 3/ crystal coupled directly to a PMT was measured to be 275 ps. With a 25 crystal array coupled to a light guide and seven PMT cluster an average time resolution of 308 ps was obtained by summing the signals from the PMT cluster. Ultimately, 5.0% and 0.5% Ce-doped LaBr/sub 3/ Anger-logic detectors placed in coincidence yielded a time resolution of approximately 505 ps.

Imaging performance of a LaBr 3 -based time-of-flight PET scanner

There has recently been renewed interest in time-of-flight (TOF) PET due to the availability of fast scintillators that also have high light output and high stopping power, as well as cost-effective fast photomultiplier tubes and stable electronics. Early results with these TOF-PET systems have shown both an improved contrast/noise trade-off and faster convergence compared with reconstructions without TOF information. Simulations have predicted further improvement in imaging performance with better timing resolution. A prototype whole-body PET scanner incorporating Ce-doped LaBr3 crystals and specialized timing circuitry to take advantage of the scintillator’s fast timing characteristics has recently been completed. The intrinsic performance of the scanner has been measured. The average energy resolution over all crystals is 6.5%, and the system timing resolution is 375 ps. The scatter fraction for 20-, 27-, and 35-cm diameter cylinders is 21, 27, and 32%, respectively, for a 485-keV lower energy threshold. The average spatial resolution is 5.8 mm at 1 cm and 6.5 mm at 10 cm. Resolution modeling has been incorporated into the list-mode TOF iterative algorithm. Simulation studies were carried out to measure the relative impact of timing resolution, energy resolution and lower energy threshold, and spatial resolution modeling on TOF-PET imaging performance as characterized by the contrast/noise trade-off. It was found that improved timing resolution leads to faster, more uniform convergence with less variability in quantification as a function of either radial position or local activity environment. Better energy resolution allows for the use of a tighter energy window, which leads to fewer accepted scatter events and improved quantitative accuracy. Resolution modeling improves contrast recovery at the cost of slower convergence; further work is needed to define an accurate model of the point spread function for the LaBr3 system. The superior timing and energy resolutions appear to mitigate the loss of spatial resolution that arises from the lower stopping power of the crystal.

Performance measurements of a pixelated NaI(Tl) PET scanner

A prototype positron emission tomography (PET) scanner for whole body imaging with 4 mm /spl times/ 4 mm /spl times/ 30 mm NaI(Tl) crystals and Anger-logic readout has been built and tested. The scanner is composed of 36,540 NaI(Tl) pixels which are coupled to an optically continuous lightguide and a hexagonal closed packed array of 39 mm photomultiplier tubes. The scanner is designed with a crystal-to-crystal diameter of 89 cm and an axial field of view (AFOV) of 25 cm. The main goals of this study are: 1) to overcome the count-rate limitation of the continuous NaI(Tl) scanner (C-PET); 2) to improve the spatial resolution and image contrast by using small pixels; and 3) to eliminate the relatively large data gaps between the detectors in the continuous NaI(Tl) scanner. The use of pixelated crystals allows the light spread to be controlled by the proper lightguide design, thereby reducing the spreading of the light relative to a continuous crystal. A two-dimensional position histogram obtained using the pixelated NaI(Tl) scanner shows very good crystal discrimination due to the high light output of NaI(Tl).