Scanner Development & Characterization

The advent of silicon photomultipliers (SiPMs) has introduced the possibility of increased detector performance in commercial whole-body PET scanners. The primary advantage of these photodetectors is the ability to couple a single SiPM channel directly to a single pixel of PET scintillator that is typically 4 mm wide (one-to-one coupled detector design). We performed simulation studies to evaluate the impact of three different event positioning algorithms in such detectors: (i) a weighted energy centroid positioning (Anger logic), (ii) identifying the crystal with maximum energy deposition (1st max crystal), and (iii) identifying the crystal with the second highest energy deposition (2nd max crystal). Detector simulations performed with LSO crystals indicate reduced positioning errors when using the 2nd max crystal positioning algorithm. These studies are performed over a range of crystal cross-sections varying from 1  ×  1 mm² to 4  ×  4 mm²as well as crystal thickness of 1 cm to 3 cm. System simulations were performed for a whole-body PET scanner (85 cm ring diameter) with a long axial FOV (70 cm long) and show an improvement in reconstructed spatial resolution for a point source when using the 2nd max crystal positioning algorithm. Finally, we observe a 30–40% gain in contrast recovery coefficient values for 1 and 0.5 cm diameter spheres when using the 2nd max crystal positioning algorithm compared to the 1st max crystal positioning algorithm. These results show that there is an advantage to implementing the 2nd max crystal positioning algorithm in a new generation of PET scanners using one-to-one coupled detector design with lutetium based crystals, including LSO, LYSO or scintillators that have similar density and effective atomic number as LSO.

We are designing a dual-resolution pre-clinical SPECT system based on square-pinhole apertures for use in applications with a small field-of-view (FOV), such as cardiac imaging of mice. Square pinholes allow for increased sensitivity due to more efficient projection tiling on the detector compared to circular pinholes. Aperture fabrication techniques cannot produce a perfect square, giving the square pinholes some amount of roundedness at the corners. This work investigates how this roundedness affects the physical properties of projection images in terms of spatial resolution. Different pinhole fullacceptance angles and roundedness values were simulated. To facilitate a fair comparison, properties of the non-square pinholes were manipulated to yield pinholes with approximately the same sensitivity (to within 0.1%) and FOV (to within 0.5%) as those of the square pinholes, subsequently referred to as matched apertures. The aperture size (flat-to-flat edge length) of each nonsquare aperture was increased until its sensitivity was approximately equal to that of the square pinhole. Next, the full acceptance angle was increased until the FOV of each non-square aperture was approximately equivalent to that of the square pinhole. Sensitivity was calculated to include both the geometric and penetrative sensitivity of a point source, as well as the packing faction of the multi-pinhole collimator. Using the sensitivity-matched and FOV-matched apertures, spatial resolution was estimated. For the 0.3mm, 0.5mm, and 1mm edge-length square apertures studied, the full-width at half-maximum widened as pinhole shape changed from square to circle, while full-width tenth-maximum showed little change. These results indicate that a perfect square pinhole shape is more desirable than a rounded-square pinhole with regard to spatial resolution when sensitivity and FOV-matched pinholes are compared.

Waveform sampling is an appealing technique for instruments requiring precision time and pulse-height measurements. Sampling each photomultiplier tube (PMT) waveform at oscilloscope-like rates of several gigasamples per second enables one to process PMT signals digitally, which in turn makes it straightforward to optimize timing resolution and amplitude (energy and position) resolution in response to calibration effects, pile-up effects, and other systematic sources of waveform variation. We describe a system design and preliminary implementation that neatly maps waveform-sampling technology onto the LaPET prototype whole-body time-of-flight PET scanner that serves as the platform for testing this new technology.

This paper describes the design and performance of a high spatial resolution positron emission tomography (PET) detector with time-of-flight capabilities. With an emphasis on high spatial resolution and sensitivity, we initially evaluated the performance of several 1.5 ×1.5 and 2.0 ×2.0 mm ² and 12-15 mm long LYSO crystals read out by several appropriately sized PMTs. Experiments to evaluate the impact of reflector on detector performance were performed and the final detector consisted of a 32 ×32 array of 1.5 ×1.5 ×15 mm ³ LYSO crystals packed with a diffuse reflector and read out by a single Hamamatsu 64 channel multi-anode PMT. Such a design made it compact, modular and offered a cost-effective solution to obtaining excellent energy and timing resolution. To minimize the number of readout signals, a compact front-end readout electronics that summed anode signals along each of the orthogonal directions was also developed. Experimental evaluation of detector performance demonstrates clear discrimination of the crystals within the detector. An average energy resolution (FWHM) of 12.7 ±2.6% and average coincidence timing resolution (FWHM) of 348 ps was measured, demonstrating suitability for use in the development of a high spatial resolution time-of-flight scanner for dedicated breast PET imaging.

The high light output and fast decay time of LaBr ₃ :Ce scintillation detectors leads to excellent timing performance. To realize the potential of timing resolution with LaBr ₃ :Ce we have investigated the performance with SiPMs, which enable 1-to-1 coupling to individual crystals, and which have been optimized for the near-ultraviolet (NUV) scintillation light emission of LaBr ₃ :Ce. Coincidence timing resolution (CTR) of 100 ps was measured for a 4 ×4 ×5 mm ³ LaBr ₃ :30%Ce crystal directly coupled to a large-area 4 ×4 mm ² NUV-SiPM. Results show very little dependence on temperature, in the range of - 20 ^° to 20 ^° C, and bias voltage, from 2 V to 5 V over breakdown. Optimal performance was achieved at an over-voltage (OV) range of 3 V - 5 V, at which high gain and high photon detection efficiency are achieved. Though saturation was evident at 511 keV, an energy resolution of 6.8% was measured after correcting for non-linearity. We also measured a CTR of 110 ps for a 4 ×4 ×5 mm ³ LaBr3:5%Ce crystal and 245 ps for a 4 ×4 ×30 mm ³ LaBr ₃ :5%Ce crystal using the NUV-SIPM. The poorer timing measurement for the 30-mm long crystal is due mainly to a systematic shift in the time pick-off as a function of the depth-of-interaction. The excellent temperature stability, fast rise time, high gain, and low noise of the NUV-SiPM make it a practical and highly appealing photodetector for the readout of a LaBr ₃ :Ce TOF-PET detector.

Positron emission tomography (PET) detectors based on continuous scintillation crystals can achieve very good performance and have a number of practical advantages compared to detectors based on a pixelated array of crystals. Our goal is to develop a thick continuous detector with high energy and spatial resolution, along with high γ -photon capture efficiency. We examine the performance of two crystal blocks: a 46 × 46 × 14 mm ³ and a 48×48×25 mm ³ block of LYSO (Lutetium Yttrium Orthosilicate). Using Maximum Likelihood (ML) positioning based upon the light response function (LRF) in the 14 mm thick crystal, we measure a spatial resolution of 3 mm in the central region of the crystal with degradation near the edges due to reflections off the crystal sides. We also show that we can match the spatial resolution achieved using a 14 mm thick crystal by using a 25 mm thick crystal with slots cut into the gamma entrance surface to narrow the LRF. We also find that we can improve the spatial resolution performance near the detector edges by reducing the reflectivity of the crystal sides, albeit with some loss in energy resolution.

Clinical TOF PET systems achieve detection efficiency using thick crystals, typically of thickness 2-3 cm. The resulting dispersion in interaction depths degrades spatial resolution for increasing radial positions due to parallax error. Furthermore, interaction depth dispersion results in time pickoff dispersion and thus in degraded timing resolution, and is therefore of added concern in TOF scanners. Using fast signal digitization, we characterize the timing performance, pulse shape and light output of LaBr₃:Ce, CeBr ₃ and LYSO. Coincidence timing resolution is shown to degrade by ~50 ps/cm for scintillator pixels of constant cross section and increasing length. By controlling irradiation depth in a scintillator pixel, we show that DOI-dependence of time pickoff is a significant factor in the loss of timing performance in thick detectors. Using the correlated DOI-dependence of time pickoff and charge collection, we apply a charge-based correction to the time pickoff, obtaining improved coincidence timing resolution of <; 200 ps for a uniform 4 × 4 × 30 mm³ LaBr₃ pixel. In order to obtain both DOI identification and improved timing resolution, we design a two layer LaBr₃[5%Ce]/LaBr₃ [30%Ce] detector of total size 4 × 4 × 30 mm³ , exploiting the dependence of scintillator rise time on [Ce] in LaBr₃:Ce . Using signal rise time to determine interaction layer, excellent interaction layer discrimination is achieved, while maintaining coincidence timing resolution of <; 250 ps and energy resolution <; 7% using a R4998 PMT. Excellent layer separation and timing performance is measured with several other commercially-available TOF photodetectors, demonstrating the practicality of this design. These results indicate the feasibility of rise time discrimination as a technique for measuring event DOI while maintaining sensitivity, timing and energy performance, in a well-known detector architecture.

Our recent measurements with pixelated LaBr₃ Anger-logic detectors for use in time-of-flight (TOF) PET have demonstrated excellent energy resolution (5.1% at 511 keV) and coincidence time resolution (313 ps full width at half maximum, FWHM) with small prototype configurations . A full size detector module suitable for a whole-body 3D PET scanner has been constructed based on the prototype designs and consists of 1620 4x4x30 mm³ LaBr₃ crystals. We have utilized simulations to guide experimental measurements with the goal of optimizing energy and time resolution in evaluating triggering configurations and pulse shaping needed in a full system. Experimental measurements with the detector module indicate energy and time resolution consistent with our earlier prototypes when measured at low count rate. At very high count rate the energy, time and spatial resolution degrade due to pulse pileup. While it is possible to reduce pulse pileup by using smaller photomultiplier tubes (i.e., 39 mm instead of 50 mm diameter), we are trying to limit the total number of PMTs needed for a full-scale PET scanner with a large axial field-of-view. Therefore, we have designed and tested a pulse shaping circuit to improve the detector response and performance at high count rate. Simulations of a complete LaBr₃ scanner indicate significant improvements in noise equivalent count rate (NEC) and spatial resolution can be achieved using pulse shaping.

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.

A new detector design using pixelated NaI(Tl) crystals has been evaluated for use in positron emission tomography (PET). This detector uses 4/spl times/4/spl times/30 mm/sup 3/ NaI(Tl) pixels coupled via a lightguide (1.4 cm thick) to an array of 39 mm diameter photomultiplier tubes (PMTs) in an Anger detector design. Our measurements show that the high light output of NaI(Tl) leads to a good discrimination of the 4 mm NaI(Tl) pixels for varying integration times of 220, 120, and 60 ns. The crystal-to-crystal variation in the measured energy for a central group of nine crystals is 1.1%, while the average energy resolution at 511 keV varies from 8.6% to 10.7% as the integration time decreases from 220 to 60 ns. Additionally, the measured light response function (LRF) of the pixelated detector is much narrower than that of the continuous, curve-plate plate C-PET detectors, even though the pixelated crystal is thicker. High count-rate simulations for a whole-body scanner with pixelated NaI(Tl) detectors but based on the C-PET geometry (diameter of 90 cm and an axial field-of-view of 25 cm) predict more than a doubling of the peak NEC rate over the C-PET scanner with curved, continuous detectors. This is due to the increased sensitivity of thicker crystals and reduced deadtime achieved by the pixelated detector design. Thus, a pixelated NaI(Tl) detector based scanner offers significant improvements in both the spatial resolution and count-rate performance over the current whole-body C-PET scanner.

The main thrust for this work is the investigation and design of a PET scanner based on new Lanthanum Halide scintillators. In 3-D PET the major limitations are scanner dead-time and ability to reject randoms and scatter. Therefore, to reach the full potential of 3-D PET requires a scintillator with good timing resolution and good energy resolution. The new Lanthanum Halide scintillators have very fast decay and very high light output which leads to timing resolution and energy resolution that are both superlative. For application to PET, we have constructed pixels with dimensions 4 x 4 x 30 mm³, and have measured energy resolution of 5% (fwhm) at 511 keV and a timing resolution (fwhm) of 420ps, in coincidence with a plastic scintillator. An Anger-logic PET detector has been designed with pixels of these dimensions, coupled through a light-guide to an array of PMTs. Simulations have been performed which demonstrate very uniform energy resolution and very good pixel identification using 39 mm PMTs, although consideration will be given to larger 50 mm PMTs, as well. We have also simulated the timing resolution of this detector to be 440 ps, depending on the light-guide configuration. Using this detector as a basis of a new LaBr₃ PET scanner with 90 cm diameter and 25 cm axial extent, we predict a sensitivity of 1400 kcps/mCi/cc and a peak NEC count-rate of 120 kcps using the NEMA NU2-2001 standard. The predicted NEC is higher than existing GSO and LSO-based scanners, which has been achieved by using a very high Energy Lower Level Discriminator (ELLD = 470 keV) to take full advantage of the excellent energy resolution of LaBr₃. The relative advantage of the LaBr₃ scanner is shown to increase for a phantom with larger diameter which better simulates a heavy patient. Further, the excellent timing resolution opens the possibility of measuring time-of-flight with sufficient accuracy to reduce the noise propagation during image reconstruction, thus leading to a significant gain in signal-to-noise. Assuming a timing resolution of 500 ps, we can expect the effective NEC to increase by a factor of 3 for a thin patient (20 cm diameter) and a factor 6 for a very heavy patient (40 cm diameter). Therefore, the combination of excellent energy resolution and timing resolution with LaBr₃ can potentially lead to a very significant improvement in PET performance.

The authors are designing a new detector for PET scanners using discrete 4 x 4 x 10 mm³ GSO(Ce) crystals on a continuous lightguide with 39 mm photomultiplier tubes. The lightguide is designed to optimize identification of the 4 mm crystals while also minimizing the detector dead area for a given event, thereby reducing pulse pileup at high count rates. The authors' simulations show that a 1.8 cm thick lightguide produces a narrow Light Response Function (LRF) with good crystal discrimination. The LRF describes the spread of light from the crystal to the PMT array. Further improvement is achieved by extending the lightguide to 2.3 cm in thickness, but with 0.5 cm slots cut in its front surface, which results in reduced tails of the LRF The slotted lightguide also minimizes the spatial dependence of event positioning on different depths of interaction of the gamma ray. Experiments are in general agreement with the simulations. The good spatial resolution and narrow LRF of such a detector will result in high performance for a PET scanner which is based on the CSO detector.

In positron tomographs using a small number of position-sensitive detectors, each detector must operate at high singles event rates, especially during dynamic studies. To enable the PENN-PET tomograph to perform studies involving high data rates, the high count rate behaviour of the position-sensitive scintillation detector used in the tomograph was investigated at singles rates in excess of 2 million counts per second (MCPS). Detector dead-time, minimised through the use of pulse clipping (clipping time, 120ns), is a maximum of 20% at the highest data rates. At 2 MCPS and 240 ns pulse integration time, the full width at half maximum of the point spread function (PSF) worsens by approximately 20% over its low count rate value of 5.2 mm. Furthermore, at high count rates, pulse pile-up produces long tails in the PSF along the detector’s long axis. These tails were reduced or eliminated through the use of a shortened pulse integration time (160 ns instead of 240ns), an upper level energy discriminator and a local centroid event positioning algorithm. Detector performance was characterised for different combinations of these event processing techniques, and the mechanisms by which pulse pile-up distorts the high count rate PSF were investigated using computer simulations. With the incorporation of the high count rate event processing techniques, the detector’s count rate capability enables the PENN-PET tomograph to handle most current imaging protocols.

The spatial resolution of a NaI(Tl), 25 mm thick bar detector designed for use in positron emission tomography has been studied. The position along the 500 mm long detector is determined from the centroid of the light distribution in the crystal as measured by a linear array of photomultiplier tubes. A Monte Carlo computer simulation was performed to investigate the factors limiting the spatial resolution. The program allowed the authors to study the effect of various phototube configurations and crystal surfaces. Since the resolution is affected by the width of the light distribution, the authors studied the effect of sharpening the distribution by modifying the front crystal surface with grooves cut perpendicular to the long axis of the crystal and by using nonlinear preamplifiers. The simulation predicts a spatial resolution (FWHM) of 3 mm with this crystal. Experimental measurements of spatial resolution were performed concurrently with the simulations. In particular, a modified grooved crystal was measured to have 4.0 mm spatial resolution, an improvement over the original crystal without grooves. With delay line pulse shortening, which increases the count rate capability of the detector, the grooved crystal was measured to have 5.5 mm spatial resolution.

One of the major limitations of the Anger scintillation camera as it exists today is its limited resolution capability. At the higher gamma-ray energies for example, the 511-keV gamma radiation emitted after positron decay -- the resolution of the camera system is largely determined by the collimator. How ever, at the lower energies -- for example, at 140 keV which corresponds to the gamma-ray energy emitted by the widely used ^99mTc -- the system resolution could be improved significantly if the camera had a higher intrinsic resolution. We have therefore in- vestigated the intrinsic resolution limitation in order to improve the device.