Evaluation of very highly pixellated crystal blocks with SiPM readout as candidates for PET/MR detectors in a small animal PET insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. In this study we report the readout performance of SensL SiPM arrays in terms of their ability to resolve all elements of pixellated lutetium oxyorthosilicate (LYSO) crystals their energy resolution, and coincidence response function. Two SensL SPMarray-4 were used as light sensors. Two LYSO crystal blocks consisting of a arrays of 8 78 1.2 mm 7 1.2 mm 7 6.0 mm crystals on the lower layer and 7 77 1.2 mm 7 1.2 mm 7 4.0 mm crystals on the upper layer (which is offset by 1/2 the crystal width) were mounted on the SensL arrays. A second study was performed with single layer arrays of 8 78 1.2 mm 7 1.2 mm 7 10.0 mm crystals. All of the crystals in the dual layer block were easily identified with a peak to valley ratio of 7.2 while the single layer blocks had a peak to valley ratio of 11.2. The crystal dots were more uniformly spaced in the dual layer arrays suggesting that the inter-layer connection affords some useful light sharing. The average energy resolution for all 113 crystals was 16.3\ub12.3%. The coincidence response to a 0.25 mm Na-22 source in plastic was 0.97 \ub10.12 mm for normally incident gamma rays with the detectors 120 mm apart. These results indicate that it should be possible to achieve a spatial resolution of about one millimetre near the center of the field of view. \ua9 2012 IEEE.Peer reviewed: YesNRC publication: Ye

Simulation guided optimization of Dual Layer Offset detector design for use in small animal PET

A small animal PET insert for use inside of a small MR bore is currently in the design phase. The small diameter of the tomograph results in an increased need to collect Depth of Interaction (DOI) information to mitigate the parallax error. The tomograph will use a Dual Layer Offset block design to collect DOI information. Of critical importance to the design of the block is the depth into the block where the front layer is separated from the back layer. With the total thickness of scintillating crystal limited to 10 mm, GATE simulations with a single block were run with the front layer thickness ranging from 1 mm to 10 mm (single layer). These simulations characterized the block's ability to accurately locate the radial coordinate of the first interaction location of a `single'. It was found that a split between front and back that is roughly even minimized the mispositioning of the radial coordinate of first interaction. To estimate the reconstructed resolution and resolution uniformity obtainable with this block design, coincidence data from a full tomograph were simulated. Data were reconstructed using Filtered Back Projection. Data were also reconstructed with the DOI information discarded to estimate the improvement in resolution uniformity obtained with this block design.Peer reviewed: YesNRC publication: Ye

Comparison of single and dual layer detector blocks for pre-clinical MRI\u2013PET

Dual or multi-layer crystal blocks have been proposed to minimise the radial blurring effect in PET scanners with small ring diameters. We measured two potential PET detector blocks' performance in a configuration which would allow 16 blocks in a ring which could be inserted in a small animal 7T MRI scanner. Two crystal sizes, 1.60 71.60 mm\ub2 and 1.20 71.20 mm\ub2, were investigated. Single layer blocks had 10 or 12 mm deep crystals, the dual layer blocks had 4 mm deep crystals on the top layer and 6 mm deep crystals on the bottom layer. The crystals in the dual layer blocks are offset by \ubd of the crystal pitch to allow for purely geometric crystal identification. Both were read out with SensL 4 74 SiPM arrays. The software identifies 64 crystals in the single layer and either 85 or 113 crystals in the dual layer array, (either 49 or 64 in the lower layers and 36 or 49 in the upper layers). All the crystals were clearly visible in the crystal identification images and their resolvability indexes (average FWHM/crystal separation) were shown to range from 0.29 for the best single layer block to 0.33 for the densest dual layer block. The best coincidence response FWHM was 0.95 mm for the densest block at the centre of the field. This degraded to 1.83 mm at a simulated radial offset of 16 mm from the centre, while the single layer crystals blurred this result to 3.4 mm. The energy resolution was 16.4\ub12.2% averaged over the 113 crystals of the densest block.Peer reviewed: YesNRC publication: Ye

Evaluation of high density pixilated crystal blocks with SiPM readout as candidates for PET/MR detectors in a small animal PET insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET/MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. We tested individual dual-layer blocks of pixilated lutetium oxy-orthosilicate (LYSO) coupled to SensL 4 74 SiPM arrays in terms of their ability to resolve all elements using resolvability index (RI) defined by the FWHM of the crystal response function divided by the separation. Our crystal blocks had 49 1.67 7 1.67 7 6.0 mm\ub3 crystals on the bottom layer and 36 1.67 7 1.67 7 4.0 mm\ub3 crystals in the top layer which was offset by \ubd of the crystal pitch. All 85 crystals were well resolved: RI=0.46 compared with RI=.41 for a conventional pre-clinical PET scanner's block with 40% lower crystal density. A pair of crystal blocks mounted on translation stages scanned a 0.25 mm \ub2\ub2Na source in 60 0.25 mm steps with the detectors angulated as if there were 16 blocks in a ring. The FWHM near the centre of the field of view was 1.31 mm and FWTM was 2.7 mm.Peer reviewed: YesNRC publication: Ye

Development of a PET Scanner for Simultaneously Imaging Small Animals with MRI and PET

Recently, positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of cancer. Combined PET and X-ray computed tomography (PET-CT) scanners are now the modality of choice in cancer treatment planning. More recently, the combination of PET and magnetic resonance imaging (MRI) is being explored in many sites. Combining PET and MRI has presented many challenges since the photo-multiplier tubes (PMT) in PET do not function in high magnetic fields, and conventional PET detectors distort MRI images. Solid state light sensors like avalanche photo-diodes (APDs) and more recently silicon photo-multipliers (SiPMs) are much less sensitive to magnetic fields thus easing the compatibility issues. This paper presents the results of a group of Canadian scientists who are developing a PET detector ring which fits inside a high field small animal MRI scanner with the goal of providing simultaneous PET and MRI images of small rodents used in pre-clinical medical research. We discuss the evolution of both the crystal blocks (which detect annihilation photons from positron decay) and the SiPM array performance in the last four years which together combine to deliver significant system performance in terms of speed, energy and timing resolution.Medicine, Faculty ofScience, Faculty ofNon UBCPhysics and Astronomy, Department ofRadiology, Department ofReviewedFacult

Fetal hemodynamics and cardiac streaming assessed by 4D flow cardiovascular magnetic resonance in fetal sheep

Abstract Background To date it has not been possible to obtain a comprehensive 3D assessment of fetal hemodynamics because of the technical challenges inherent in imaging small cardiac structures, movement of the fetus during data acquisition, and the difficulty of fusing data from multiple cardiac cycles when a cardiac gating signal is absent. Here we propose the combination of volumetric velocity-sensitive cardiovascular magnetic resonance imaging (“4D flow” CMR) and a specialized animal preparation (catheters to monitor fetal heart rate, anesthesia to immobilize mother and fetus) to examine fetal sheep cardiac hemodynamics in utero. Methods Ten pregnant Merino sheep underwent surgery to implant arterial catheters in the target fetuses. Anesthetized ewes underwent 4D flow CMR with acquisition at 3 T for fetal whole-heart coverage with 1.2–1.5 mm spatial resolution and 45–62 ms temporal resolution. Flow was measured in the heart and major vessels, and particle traces were used to visualize circulatory patterns in fetal cardiovascular shunts. Conservation of mass was used to test internal 4D flow consistency, and comparison to standard 2D phase contrast (PC) CMR was performed for validation. Results Streaming of blood from the ductus venosus through the foramen ovale was visualized. Flow waveforms in the major thoracic vessels and shunts displayed normal arterial and venous patterns. Combined ventricular output (CVO) was 546 mL/min per kg, and the distribution of flows (%CVO) were comparable to values obtained using other methods. Internal 4D flow consistency across 23 measurement locations was established with differences of 14.2 ± 12.1%. Compared with 2D PC CMR, 4D flow showed a strong correlation (R2 = 0.85) but underestimated flow (bias = − 21.88 mL/min per kg, p < 0.05). Conclusions The combination of fetal surgical preparation and 4D flow CMR enables characterization and quantification of complex flow patterns in utero. Visualized streaming of blood through normal physiological shunts confirms the complex mechanism of substrate delivery to the fetal heart and brain. Besides offering insight into normal physiology, this technology has the potential to qualitatively characterize complex flow patterns in congenital heart disease phenotypes in a large animal model, which can support the development of new interventions to improve outcomes in this population