Karel Deprez

Rapid additive manufacturing of MR compatible multipinhole collimators with selective laser melting of tungsten powder

PURPOSE The construction of complex collimators with a high number of oblique pinholes is very labor intensive, expensive or is sometimes impossible with the current available techniques (drilling, milling or electric discharge machining). All these techniques are subtractive: one starts from solid plates and the material at the position of the pinholes is removed. The authors used a novel technique for collimator construction, called metal additive manufacturing. This process starts with a solid piece of tungsten on which a first layer of tungsten powder is melted. Each subsequent layer is then melted on the previous layer. This melting is done by selective laser melting at the locations where the CAD design file defines solid material. METHODS A complex collimator with 20 loftholes with 500 μm diameter pinhole opening was designed and produced (16 mm thick and 70 × 52 mm(2) transverse size). The density was determined, the production accuracy was measured (GOM ATOS II Triple Scan, Nikon AZ100M microscope, Olympus IMT200 microscope). Point source measurements were done by mounting the collimator on a SPECT detector. Because there is increasing interest in dual-modality SPECT-MR imaging, the collimator was also positioned in a 7T MRI scanner (Bruker Pharmascan). A uniform phantom was acquired using T1, T2, and T2* sequences to check for artifacts or distortion of the phantom images due to the collimator presence. Additionally, three tungsten sample pieces (250, 500, and 750 μm thick) were produced. The density, attenuation (140 keV beam), and uniformity (GE eXplore Locus SP micro-CT) of these samples were measured. RESULTS The density of the collimator was equal to 17.31 ± 0.10 g∕cm(3) (89.92% of pure tungsten). The production accuracy ranges from -260 to +650 μm. The aperture positions have a mean deviation of 5 μm, the maximum deviation was 174 μm and the minimum deviation was -122 μm. The mean aperture diameter is 464 ± 19 μm. The calculated and measured sensitivity and resolution of point sources at different positions in the field-of-view agree well. The measured and expected attenuation of the three sample pieces are in a good agreement. There was no influence of the 7T magnetic field on the collimator (which is paramagnetic) and minimal distortion was noticed on the MR scan of the uniform phantom. CONCLUSIONS Additive manufacturing is a very promising technique for the production of complex multipinhole collimators and may also be used for producing other complex collimators. The cost of this technique is only related to the amount of powder needed and the time it takes to have the collimator built. The timeframe from design to collimator production is significantly reduced.

Characterization of a SPECT pinhole collimator for optimal detector usage (the lofthole)

In single-photon emission computed tomography (SPECT), multi-pinhole collimation is often employed nowadays. Most multi-pinhole collimators avoid overlap (multiplexing) of the projections on the detector. This can be done by using additional shielding or by spacing the pinholes far enough apart. Using additional shielding has the drawback that it increases weight, design complexity and cost. Spacing the pinholes far enough apart results in sub-optimal detector usage, the valuable detector area is not entirely used. This is due to the circular projections of pinholes on the detector; these ellipses can not be tiled with high detector coverage. To overcome this we designed a new pinhole geometry, the lofthole, that has a rectangular projection on the detector. The lofthole has a circular aperture and a rectangular entrance/exit opening. Sensitivity formulae have been derived for pinholes and loftholes. These formulae take the penumbra effect into account; the proposed formulae do not take penetration into account. The derived formulae are valid for geometries where the field-of-view and the sensitivity of the aperture are solely limited by the exit window. A flood map measurement was performed to compare the rectangular projection of a lofthole with the circular projection of a pinhole. Finally, measurements were done to compare the amount of penetration of pinholes with the amount of penetration of a lofthole. A square lofthole collimator has less penetration than a knife-edge pinhole collimator that irradiates the same rectangular detector area with full coverage. A multi-lofthole collimator allows high detector coverage without using additional shielding. An additional advantage is the lower amount of penetration.

Design and performance of a compact and stationary microSPECT system.

PURPOSE Over the last ten years, there has been an extensive growth in the development of microSPECT imagers. Most of the systems are based on the combination of conventional, relatively large gamma cameras with poor intrinsic spatial resolution and multipinhole collimators working in large magnification mode. Spatial resolutions range from 0.58 to 0.76 mm while peak sensitivities vary from 0.06% to 0.4%. While pushing the limits of performance is of major importance, the authors believe that there is a need for smaller and less complex systems that bring along a reduced cost. While low footprint and low-cost systems can make microSPECT available to more researchers, the ease of operation and calibration and low maintenance cost are additional factors that can facilitate the use of microSPECT in molecular imaging. In this paper, the authors simulate the performance of a microSPECT imager that combines high space-bandwidth detectors and pinholes with truncated projection, resulting in a small and stationary system. METHODS A system optimization algorithm is used to determine the optimal SPECT systems, given our high resolutions detectors and a fixed field-of-view. These optimal system geometries are then used to simulate a Defrise disk phantom and a hot rod phantom. Finally, a MOBY mouse phantom, with realistic concentrations of Tc99m-tetrofosmin is simulated. RESULTS Results show that the authors can successfully reconstruct a Defrise disk phantom of 24 mm in diameter without any rotating system components or translation of the object. Reconstructed spatial resolution is approximately 800 μm while the peak sensitivity is 0.23%. Finally, the simulation of the MOBY mouse phantom shows that the authors can accurately reconstruct mouse images. CONCLUSIONS These results show that pinholes with truncated projections can be used in small magnification or minification mode to obtain a compact and stationary microSPECT system. The authors showed that they can reach state-of-the-art system performance and can successfully reconstruct images with realistic noise levels in a preclinical context. Such a system can be useful for dynamic SPECT imaging.

FlexiSPECT: A SPECT System Consisting of a Compact High-Resolution Scintillation Detector (SPECTatress) and a Lofthole Collimator

This article describes a compact single-photon emission computed tomography (SPECT) system that consists of a high-resolution detector combined with a lofthole collimator. The detector is based on a NaI(Tl) scintillator, a position sensitive photomultiplier (PSPMT), dedicated read-out electronics that digitize all PSPMT anodes and finally, a maximum likelihood algorithm for position estimation. The collimator has a new pinhole geometry, called the lofthole. Our choice of magnification (1.06 for the mouse setup, 0.63 for the rat setup) results in a small system with a footprint of 45 cm × 25 cm. Design and measurements of both the detector and the SPECT system are shown. Detector measurements with a beam source have been done to investigate the spatial and energy resolution of the detector. Two SPECT setups have been made, one that fits rat-size phantoms and one that fits mouse-size phantoms. On both setups we have done measurements of a Derenzo phantom and a uniformity phantom. The results show that the detector resolution is 1.1 mm and energy resolution is 9.3% in the center of the detector. With the tomographic rat setup we are able to distinguish the 2.4 mm hot rods in a Derenzo phantom. The mouse setup allows us to distinguish the 1.6 mm rods. This demonstrates the SPECT capabilities of this compact prototype scanner.

The lofthole: A novel shaped pinhole geometry for optimal detector usage without multiplexing and without additional shielding

Multi-pinhole collimator based SPECT systems are nowadays used for pre-clinical and clinical imaging.

Fast calibration of SPECT monolithic scintillation detectors using un-collimated sources

Monolithic scintillation detectors for positron emission tomography and single-photon emission computed tomography (SPECT) imaging have many advantages over pixelated detectors. The use of monolithic crystals allows for reducing the scintillator cost per unit volume and increasing the sensitivity along with the energy and timing resolution of the detector. In addition, on thick detectors the depth-of-interaction can be determined without additional hardware. However, costly and complex calibration procedures have been proposed to achieve optimal detector performance for monolithic detectors. This hampers their use in commercial systems. There is thus, a need for simple calibration routines that can be performed on assembled systems. The main goal of this work is to develop a simplified calibration procedure based on acquired training data. In comparison with other methods that use training data acquired with beam sources attached to robotic stages, the proposed method uses a static un-collimated activity source with simple geometry acquiring in a reasonable time. Once the data are acquired, the calibration of the detector is accomplished in three steps: energy calibration based on the k-means clustering method, self-organization based on the self-organizing maps algorithm, and distortion correction based on the Monge-Kantorovich grid adaptation. The proposed calibration method was validated for 2D positioning using a SPECT detector. Similar results were obtained by comparison with an existing calibration method (maximum likelihood estimation). In conclusion, we proposed a novel calibration method for monolithic scintillation detectors that greatly simplifies their use with optimal performance in SPECT systems.

A high resolution scintillator based SPECT detector with digital pulse processing (SPECTatress)

SPECT scanners using multi-pinhole collimators benefit from modular detectors having a high spatial resolution. Such detectors can be placed closer to the collimator and perpendicular to the pinhole axis (thereby limiting DOI spatial resolution degradation). Current clinical gamma ray cameras have a large area and a poor spatial resolution. This proceeding describes the architecture of SPECTatress, a modular high resolution gamma camera. A brief summary of the electronics and results obtained with the center-of-gravity positioning algorithm have been presented previously (Imaging 2010 conference, Stockholm). Here we present a detailed overview of the pre-amplifiers and the algorithms used in the FPGA. Results using a GPU accelerated MLE algorithm are shown. This measurement indicates a spatial resolution of 1.1mm FWHM in the center of the detector and an energy resolution of 9% FWHM.

Performance characterization of a compact SPECT detector based on dSiPMs and monolithic LYSO

Silicon photomultipliers (SiPM) are a promising alternative light sensor for the classic photomultiplier tubes currently used as single photon emission computed tomography (SPECT) detectors due to their compact dimensions and smaller pixelization. Furthermore, these SiPMs are also known to be MR-hard which brings on new possibilities for simultaneous SPECT-MR imaging. In contrast to other research on SiPMs which is mainly focussed on PET applications, we will focus on SPECT imaging. The detector used in this work, consists of a dSiPM (DPC-3200-22-44, Philips Digital Photon Counting) optically coupled to a monolithic LYSO scintillator of 2mm thick. Using a collimated 57Co source, a filter was developed to remove the dark count events from the spectrum and a comparison was made between the detection efficiency of the DPC-based detector and a PSPMT-based detector. Furthermore, the intrinsic spatial resolution was determined using both a resolution collimator and beam source measurements. The same beam source measurements were also used to calculate the energy resolution across the detector. Finally, the count rate performance of the DPC was investigated by measuring a decaying 99mTc source in front of the detector. Measurements showed that, despite the presence of dark count events in the spectrum, the DPCs have approximately the same detection efficiency as our gold standard, the PSPMT. Furthermore, we demonstrate that the detector has an intrinsic spatial resolution of 0.486mm and an energy resolution of 22.3 %. This work demonstrates the usefulness of the DPCs as SPECT detectors.

Design of a static full-ring multi-pinhole collimator for brain SPECT

In clinical practice, brain SPECT is mostly performed using a dual-head SPECT scanner with fan-beam or parallel-beam collimators rotating around the patients head. The resolution of such a system is typically about 6-8 mm, which is rather poor to image the complex structures of the human brain. We developed a non-rotating multi-pinhole collimator for brain SPECT imaging with a resolution of 4 mm. A full-ring geometry allows for complete transaxial sampling. This enables the use of a stationary collimator. The collimator is a tungsten ring with two rows of pinholes. Each pinhole can individually be opened or closed with shutters. A sequence of shutter movements is performed to obtain an acquisition setup that simulates a rotational movement. The collimator is designed for the LaPET system (a PET detector ring made of 24 LaBr3 detectors) and is optimized to maximize the system performance, resulting in a collimator radius of 145 mm and a pinhole diameter of 2 mm. This system has a sensitivity that is 4 times lower than a dual-head system with LEHR parallel-beam collimators. However, the resolution is 2 times better, a trade-off that is supported by Muehllehner [1]. Monte-Carlo simulated projections of a resolution phantom are successfully reconstructed and the resulting image shows that a resolution of 4 mm is indeed achieved.