Specifications such as pinhole aperture size and number of pinholes open can be adjusted based on individual patients and applications (92). The first step to develop a clinical SPECT/MRI system is similar to what has been done for PET/MRI, where a PET insert was designed to be utilized with an existing MRI system. where n is the number of pinholes. The system spatial resolution of a single pinhole collimator can be described by Eq. The system employs ellipsoidal CsI scintillation crystals with 6 mm thickness, 80 mm diameter and 120 mm height (Figure 25). Pinhole-to-ROI distance ranges from 15 to 22 cm as the detector moves around the patient. Characterization of a multi-pinhole molecular breast tomosynthesis scanner. Based on theoretical prediction, the MPGP collimators target resolution is set to 12 mm and sensitivity is calculated as 0.037% for the CFOV, while the target resolution and sensitivity of MPHR collimator are 8 mm and 0.014%, respectively. A 49-pinhole SPECT system was designed and investigated by simulations for use as a computer-aided robotic system for head, breast, thorax, arms and legs imaging in radiation therapy (Figure 20) by Bowsher et al. Funk et al. Si et al. See: https://creativecommons.org/licenses/by-nc-nd/4.0/. The UCSF 20-pinhole SPECT collimator (courtesy of Dr. Youngho Seo). Furthermore, Korevaar et al. MPH cardiac SPECT significantly reduces the acquisition time, thus increasing the patient throughput and improving the patient comfort compared to the use of a conventional parallel hole collimator. Yan et al. The intrinsic spatial resolution is the same as the detector pixel size of 2.46 mm, while the system resolution is 6.1 mm at 10 cm from the collimator. Knik et al. All 54 pinholes distributed over three rings are focused on the complete data volume [CDV: the spherical object shown (A) in the FOV]. Their experimental results showed that the efficiency of this collimator is 1.6103 which is 22 times of that of LEHR (7.2105) with 2.8 times resolution loss at 19.3 cm from the collimator. Besides the very successful application in small animal imaging, there has been a resurgence of the use of MPH collimators for clinical cardiac and brain studies, as well as other small field-of-view applications. Accomplishing this for SPECT/MRI is still being researched for clinical brain imaging (98-100). Review of SPECT collimator selection, optimization, and fabrication for clinical and preclinical imaging. Projections from the pinhole collimator may suffer more from the DOI effect compared to other collimators when the incident angles are large. During this process, determination of the incident angle of gamma-photons on the detector is crucial. Monte Carlo simulations of pinhole imaging accelerated by kernel-based forced detection. They performed GATE simulations with various phantoms to evaluate the system performance. Busca et al. (A) Transparent plates compress the breast and a VOI is selected using optical cameras. (66) performed simulation studies to quantify the spatial resolution loss due to the DOI effect for 140 keV gamma rays. In these systems, 75 gold circular pinholes were arranged in a cylindrical-shaped collimator and there is another outer lead cylinder with square holes that act as a shielding for the detector, which completely solves the problem of multiplexing as discussed in the next section. (C) Top view showing the 0.3-mm square-pinhole aperture, located 2 mm below the opening on the top, and 5 mm above the large opening on the bottom [courtesy of Moore et al. GEANT4 application for tomographic emission (GATE) (90) simulations for the four scanners were performed to evaluate the systems in terms of sensitivity and spatial resolution by using a point source placed at the CFOV. The improved system can reach 3.5 mm resolution for a clinical 99mTc activity concentration in an 11-minute scan, while the smallest spheres visible in breast phantoms were 6 mm in diameter for the same scan time. (A) Semitransparent design drawing of a rectangular pinhole insert, showing the double-pyramid-shaped inside walls, which open up above and below the plane of the small square aperture. (85). Deprez K, Pato LR, Vandenberghe S, Van Holen R. Characterization of a SPECT pinhole collimator for optimal detector usage (the lofthole). official website and that any information you provide is encrypted When gamma rays interact with the detector, the exact location of interaction through the detector along the depth direction is usually unknown. 8600 Rockville Pike The sensitivity for the tilted pinholes can be given as follows: where dSeff is the sensitivity effective pinhole diameter corrected for penetration effect (49), n is a normal vector coincident with the tilted pinhole axis, (x1,y1,z1) is the pinhole aperture center and (x2,y2,z2) is an arbitrary point along the pinhole axis. Before Left: (A) pod geometry. Photon cross sections, attenuation coefficients, and energy absorption coefficients from 10 keV to 100 GeV 1969 National Bureau of Standards (U.S.) Rep. NSRDS-NBS 29. Jaszczak RJ, Li J, Wang H, Zalutsky MR, Coleman RE. Bowen JD, Huang Q, Ellin JR, Lee TC, Shrestha U, Gullberg GT, Seo Y. The increasing clinical need for dedicated diagnostic systems with improved performance for imaging specific organs, e.g., the heart and brain, has led to the development of single photon emission computed tomography (SPECT) systems with better resolution, sensitivity and throughput (1-12) than standard general-purpose SPECT systems. Left: experimental setup of the 9-pinhole MPI SPECT system with a torso phantom. Collimators with larger holes allow more photons to reach the detector, but increasing the hole diameter decreases the spatial resolution since the collimator is less selective with regard to where the photons come from. It compares the projections of a spherical phantom with one-pinhole and four different cases of six-pinhole collimator with varying degrees of multiplexing, truncation, and utilization of detectors. Ljungberg M. In: Ljungberg M, Strand SE, King MA, editors. (10) designed and evaluated two MPH collimators for brain SPECT to image cerebral blood flow and striatum (Figure 17) based on a GE Inifinia Hawkeye scanner. The measured FWHM is 8.2 mm, which is 23% better compared to LEHR with the same imaging distance of 22 cm, while the sensitivity is 0.017%. King MA, Mukherjee JM, Konik A, Zubal IG, Dey J, Licho R. Design of a multi-pinhole collimator for I-123 DaTscan imaging on dual-headed SPECT systems in combination with a fan-beam collimator. On the other hand, along with the existing cerebral blood flow tracers, new brain SPECT tracers for Alzheimers and Parkinsons diseases are either already available as in the case of I-123 labeled DaTscan (101), or are on the horizon, such as Tau imaging (102). The first one is a two-sphere phantom comprised of two spherical sources with a diameter of 5 mm, one located at the CFOV and the other located 20 mm offset from the CFOV. The second one is a four-rod phantom comprises of four cylindrical rods that are located 30 mm offset from the center. Physical and clinical evaluation of high-resolution thyroid pinhole tomography. To the best of our knowledge, there has been no study published in the literature on the analytic modeling of the resolution of a titled pinhole for MPH collimators. The pendant breast is compressed by transparent compression plates and imaged by two tungsten collimators with 10 mm thickness, each with 63 pinholes (Figure 21). The effects of object activity distribution on multiplexing multi-pinhole SPECT. The role of DAT-SPECT in movement disorders. Ono M, Watanabe H, Kitada A, Matsumura K, Ihara M, Saji H. Highly selective tau-SPECT imaging probes for detection of neurofibrillary tangles in Alzheimer's disease. For instance, FOCDC is shift-invariant for low and medium energy, which is good for reducing reconstruction time. Single photon emission computed tomography (SPECT), multi-pinhole, collimator, brain, cardiac. Learn more Onboard functional and molecular imaging: A design investigation for robotic multipinhole SPECT. (C) Comparison of resolution of G-SPECT-I equipped with a 38 cm collimator with a dual head Siemens Symbia system at equal scan time and equal dose based on physical experiments (with permission of MILabs B.V. Utrecht, The Netherlands). Imaging 131I tracers is challenging because of the high energy photons (primary emission at 364 keV and secondary emissions at 284, 637, and 723 keV), which cause more penetration near the pinhole aperture. Hwang et al. (B) After the scan volume selection, the collimator plates and gamma detectors move into scanning position (arrows). Even though SPECT/CT hybrid systems have been available for nearly two decades, the combination of a SPECT system with MRI is still in progress due to technical challenges including the need for MRI-compatible components of SPECT and driving applications. To correct this geometric change, a virtual object plane parallel to the tilted aperture plane was set such that the geometric sensitivity and penetration path length in the conventional equation can be applied with respect to this virtual object plane. Salvado D, Erlandsson K, Bousse A, Occhipinti M, Busca P, Fiorini C, Hutton B. Collimator design for a brain SPECT/MRI insert. The axis of each pinhole has a different angle [courtesy of Ogawa et al. Photons being detected with a larger incident angle would be more blurred out (red point spread function) compared to those with a smaller incident angle (blue point spread function). The researchers fixed the target spatial resolution at 6 mm in the CFOV and then maximized the volume sensitivity by varying the collimator radius, aperture size and number of loftholes. Design and performance of a multi-pinhole collimation device for small animal imaging with clinical SPECT and SPECTCT scanners. Performance evaluation and quantitative accuracy of multipinhole nanoSPECT/CT scanner for theranostic Lu-177 imaging. This design consists of three hexagonal detector rings: caudal ring with 9 detectors, middle ring with 9 detectors, and quasi-vertex ring with 5 detectors. Design and evaluation of an adaptive multipinhole collimator for high-performance clinical and preclinical imaging. The system resolution was set to 810 mm, which is similar to that of a brain SPECT scanner and the number of detectors was arranged to maximize the sensitivity (0.0180.032%) (98). (8) designed a 14-pinhole cardiac collimator based on a target resolution of 1 cm, FOV of 20 cm and H-mode rotational acquisition for a clinical SPECT scanner by maximizing the sensitivity. Beekman et al. [5]. The Discovery NM 530c is a commercially available system dedicated for MPI, developed by GE Healthcare, Haifa, Israel (4). Evaluation of stationary and semi-stationary acquisitions in a dual-head multi-pinhole collimator for myocardial perfusion SPECT. Quantification of the multiplexing effects in multi-pinhole small animal SPECT: a simulation study. The potential of MPH SPECT imaging for other applications, e.g., breast, thyroid and pediatric applications is being explored. Bowsher J, Yan S, Roper J, Giles W, Yin FF. Aguiar P, Pino F, Silva-Rodriguez J, Pavia J, Ros D, Ruibal A, El Bitar Z. Analytical, experimental, and Monte Carlo system response matrix for pinhole SPECT reconstruction. In their designs, the patient lies prone on a specially designed bed with the scanner underneath. They estimated that the in-plane (normal to the axis of rotation) resolution loss was 0.320.63 mm FWHM and the cross-plane (aligned with the axis of rotation) resolution loss was 0.210.42 mm FWHM. The ring is formed by 9 large FOV cameras using 595 mm 472 mm NaI (Tl) scintillation crystals. The pinholes are arranged such that the distances between pinhole centers are 6.2 and 5.8 cm for column and rows, respectively. Effect of overlapping projections on reconstruction image quality in multipinhole SPECT. Technical considerations such as pinhole aperture design, effect of multiplexing, sampling and depth of interaction (DOI) are firstly presented in the next section. The system design parameters are: 19.3 cm radius of rotation (ROR), 0.75 cm aperture diameter, 31.5 cm system radius, 65.4 acceptance angle and 21 cm FOV. Measurement of absolute myocardial blood flow in humans using dynamic cardiac SPECT and 99mTc-tetrofosmin: Method and validation. Multi-pinhole (MPH) collimators have been proposed to address resolution-sensitivity tradeoff issues together with sampling issues (33). Quantification of myocardial perfusion reserve using dynamic SPECT imaging in humans: a feasibility study. (95) investigated a novel cardiac SPECT system consisting of 21 pinhole collimators, each coupled to a hemi-ellipsoid detector. (95)]. This stationary system has the potential to be integrated with an MRI system. Imaging distance and focal length were set at 150 and 50 mm, respectively, resulting in a magnification of 1/3. Nemoto K. Brain perfusion SPECT in Alzheimers disease. (35) showed that the artifacts reduced when either helical sampling (bed movement in constant intervals between angular steps) or order-subsets sampling (bed movement in a repeating sequence to different axial positions) were utilized, since both methods provide improved axial sampling. Simulations were performed with the base configuration of a single-central pinhole per detector, and the usage of temporal shuttering to close that aperture and open two other apertures on either side of the central pinhole in the axial direction. Development and application of high-sensitivity and high-resolution fully 3D SPECT imaging techniques using two different collimator designs.
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