High-resolution SPECT System Research Articles

Convolutional neural network based attenuation correction for 123I-FP-CIT SPECT with focused striatum imaging

SPECT imaging with 123I-FP-CIT is used for diagnosis of neurodegenerative disorders like Parkinson’s disease. Attenuation correction (AC) can be useful for quantitative analysis of 123I-FP-CIT SPECT. Ideally, AC would be performed based on attenuation maps (-maps) derived from perfectly registered CT scans. Such -maps, however, are most times not available and possible errors in image registration can induce quantitative inaccuracies in AC corrected SPECT images. Earlier, we showed that a convolutional neural network (CNN) based approach allows to estimate SPECT-aligned -maps for full brain perfusion imaging using only emission data. Here we investigate the feasibility of similar CNN methods for axially focused 123I-FP-CIT scans. We tested our approach on a high-resolution multi-pinhole prototype clinical SPECT system in a Monte Carlo simulation study. Three CNNs that estimate -maps in a voxel-wise, patch-wise and image-wise manner were investigated. As the added value of AC on clinical 123I-FP-CIT scans is still debatable, the impact of AC was also reported to check in which cases CNN based AC could be beneficial. AC using the ground truth -maps (GT-AC) and CNN estimated -maps (CNN-AC) were compared with the case when no AC was done (No-AC). Results show that the effect of using GT-AC versus CNN-AC or No-AC on striatal shape and symmetry is minimal. Specific binding ratios (SBRs) from localized regions show a deviation from GT-AC 2.5% for all three CNN-ACs while No-AC systematically underestimates SBRs by 13.1%. A strong correlation ( 0.99) was obtained between GT-AC based SBRs and SBRs from CNN-ACs and No-AC. Absolute quantification (in kBq ml−1) shows a deviation from GT-AC within 2.2% for all three CNN-ACs and of 71.7% for No-AC. To conclude, all three CNNs show comparable performance in accurate -map estimation and 123I-FP-CIT quantification. CNN-estimated -map can be a promising substitute for CT-based -map.

Theory and realization of a 2D high resolution and high sensitivity SPECT system with an angle-encoding attenuator pattern

The camera of the conventional SPECT system requires a collimator to allow incoming photons from a specific range of incident angle to reach the detector. It is the major factor that determines the spatial resolution of the camera. Moreover, it also greatly reduces the number of detected photons and hence increases statistical fluctuations in the acquired image data. The goal of this paper is to propose a theory and design for a novel high resolution and high sensitivity SPECT system without conventional collimators. The key is to resolve the incident photons from all directional angles and detected by every detector bin. Special ‘attenuators’ were designed to ‘encode’ the incoming photons from different directions similar to coded aperture to form projection data for image reconstruction. Each encoded angular pattern of detected photons was recorded as one measurement. Different angular patterns were achieved by changing the configurations of the attenuators so that angular pattern of different measurements or measurement matrix (MM) is invertible, which guarantee a unique reconstructed image. In simulation, the attenuators were fitted on a virtual full-ring gamma camera, as an alternative to the collimators in conventional SPECT systems. To evaluate the performance of the new SPECT system, analytical simulated projection data in 2D scenario were generated from the XCAT phantom. Noisy simulation using 100 noise realizations suggests that the new attenuator design provides much improved image quality in terms of contrast-noise trade-offs (~30% improvement). The results suggest that the new design of using attenuators to replace collimator is feasible and could potentially improve sensitivity without sacrificing resolution in today’s SPECT systems.

Inflammation imaging of atherosclerosis in Apo-E-deficient mice using a 99mTc-labeled dual-domain cytokine ligand

Interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) play a critical role in initiating and accelerating atherosclerosis. This study evaluated the imaging properties of 99mTc-TNFR2-Fc-IL-1RA (99mTc-TFI), a dual-domain cytokine radioligand that targets TNF-α and IL-1β pathways, in assessing atherosclerosis development in apolipoprotein-E-deficient (ApoE−/−) mice. MethodsThe feasibility and specificity of detecting atherosclerosis with 99mTc-TFI SPECT imaging were investigated in ApoE−/− and ApoE+/+ mice. Fifty-four ApoE−/− mice were fed either an atherogenic diet (AGD) or a normal diet (ND) beginning at 5weeks of age. Eighteen Apo-E wild-type (ApoE+/+) mice were fed an ND. Two groups of ApoE−/− mice (n=12 each group) on AGD and ND were imaged three times with 99mTc-TFI and a high-resolution SPECT system at 20–25, 30–40, and 48–52 weeks to study the evolution of atherosclerotic plaque. ResultsFocal radioactive accumulations in the aortic arch region were observed in the ApoE−/− mice (n=12) on AGD but not in the ApoE+/+ mice on ND (n=10). Apo-E−/− mice on ND (n=11) exhibited lower radioactive uptake than ApoE−/− mice on AGD (P<0.05). Co-injection of an excess of cold ligand with 99mTc-TFI resulted in significant reduction of 99mTc-TFI uptake in the ApoE−/− mice on AGD. Longitudinal studies showed that 99mTc-TFI uptake in the aortas of ApoE−/− mice progressively increased from 20 to 48weeks. Real-time PCR assays demonstrated that atherosclerotic aortas expressed significantly higher IL-1β and TNF-α than the aortas from wild-type controls. ConclusionsAtherosclerotic plaques were detected by 99mTc-TFI imaging in ApoE−/− mice. 99mTc-TFI is promising for specific detection of inflammatory response in atherosclerotic plaques.

Resolution-recovery-embedded image reconstruction for a high-resolution animal SPECT system

The small-animal High-Resolution SPECT (HiReSPECT) is a dedicated dual-head gamma camera recently designed and developed in our laboratory for imaging of murine models. Each detector is composed of an array of 1.2 × 1.2 mm2 (pitch) pixelated CsI(Na) crystals. Two position-sensitive photomultiplier tubes (H8500) are coupled to each head's crystal. In this paper, we report on a resolution-recovery-embedded image reconstruction code applicable to the system and present the experimental results achieved using different phantoms and mouse scans. Collimator-detector response functions (CDRFs) were measured via a pixel-driven method using capillary sources at finite distances from the head within the field of view (FOV). CDRFs were then fitted by independent Gaussian functions. Thereafter, linear interpolations were applied to the standard deviation (σ) values of the fitted Gaussians, yielding a continuous map of CDRF at varying distances from the head. A rotation-based maximum-likelihood expectation maximization (MLEM) method was used for reconstruction. A fast rotation algorithm was developed to rotate the image matrix according to the desired angle by means of pre-generated rotation maps. The experiments demonstrated improved resolution utilizing our resolution-recovery-embedded image reconstruction. While the full-width at half-maximum (FWHM) radial and tangential resolution measurements of the system were over 2 mm in nearly all positions within the FOV without resolution recovery, reaching around 2.5 mm in some locations, they fell below 1.8 mm everywhere within the FOV using the resolution-recovery algorithm. The noise performance of the system was also acceptable; the standard deviation of the average counts per voxel in the reconstructed images was 6.6% and 8.3% without and with resolution recovery, respectively.

Three-dimensional quantitation of regional cerebral blood flow in mice using a high-resolution pinhole SPECT system and 123I-iodoamphetamine

This study is intended to evaluate the feasibility of using a high-resolution pinhole SPECT system and iodine-123-N-isopropyl-4-iodoamphetamine ((123)I-IMP) for three-dimensional (3D) absolute quantitation of regional cerebral blood flow (rCBF) in mice. The pinhole SPECT system consists of a rotating stage and a pinhole collimator attached to a clinical gamma camera. The collimator's focal length is 251 mm. Phantom studies were performed to evaluate sensitivity and full-width half-maximum (FWHM) spatial resolution. The aperture-to-object distance was 15 mm. Six mice were studied. Cerebral infarctions were induced by ligating and disconnecting the distal portion of the left middle cerebral artery. Ex vivo SPECT studies were performed using harvested brains and skulls. The CBF volumetric image was computed using the standardized input function. Excellent spatial resolution of 0.9-mm FWHM and uniform sensitivity throughout the 3D volume were demonstrated in the phantom experiments. The CBF images showed a defect in the infarcted areas and a reduction of CBF values in the infarcted region as compared with the control region. This study demonstrated the feasibility of the 3D quantitation of rCBF in mice using a high-resolution pinhole SPECT system and (123)I-IMP.

Design for a high-resolution small-animal SPECT system using pixellated Si(Li) detectors for in vivo /sup 125/I imaging

We propose a design for a high-resolution SPECT system for in vivo /sup 125/I imaging in small animal using pixelated lithium-drifted silicon (Si(Li)) detectors. The detectors have high detection efficiency (>90%), good energy resolution (<15% FWHM), and good intrinsic spatial resolution (/spl sim/1 mm FWHM). The SPECT system has a dual head detector geometry with the distance between the detectors ranging 30-50 mm to allow for the shortest imaging distance between the mouse and the detectors. The detectors, each with an active area of 64 mm/spl times/40 mm (64/spl times/40 array of 1 mm/sup 2/ pixels and a 6 mm thick Si(Li) detector), are mounted on a rotating gantry with an axial field-of-view of 64 mm. Using a high-resolution parallel-hole collimator, the expected spatial resolution is 1.6 mm FWHM at an imaging distance of 20 mm, and sensitivity is 6.7 cps//spl mu/Ci. /sup 125/I is a readily available radioisotope with a long half-life of 59.4 days and it is commonly used to label biological compounds in molecular biology. Conventional gamma cameras are not optimized to detect the low emission energies (27 to 35 keV) of /sup 125/I. However, Si(Li) detector provides an ideal solution for detecting the low-energy emissions of /sup 125/I. In addition, a SPECT system based on Si(Li) detector can potentially be less expensive than other detector technology because of its ease of operation and the availability of many resources for processing silicon. This will increase the availability of small-animal SPECT systems in biomedical research, thereby providing an important tool to facilitate the research in mouse biology.

Design and simulation of a high-resolution stationary SPECT system for small animals

Exciting new SPECT systems can be created by combining pinhole imaging with compact high-resolution gamma cameras. These new systems are able to solve the problem of the limited sensitivity-resolution trade-off that hampers contemporary small animal SPECT. The design presented here (U-SPECT-III) uses a set of detectors placed in a polygonal configuration and a cylindrical collimator that contains 135 pinholes arranged in nine rings. Each ring contains 15 gold pinhole apertures that focus on the centre of the cylinder. A non-overlapping projection is acquired via each pinhole. Consequently, when a mouse brain is placed in the central field-of-view, each voxel in the cerebrum can be observed via 130 to 135 different pinholes simultaneously. A method for high-resolution scintillation detection is described that eliminates the depth-of-interaction problem encountered with pinhole cameras, and is expected to provide intrinsic detector resolutions better than 150 µm. By means of simulations U-SPECT-III is compared to a simulated dual pinhole SPECT (DP-SPECT) system with a pixelated array consisting of 2.0 × 2.0 mm NaI crystals. Analytic calculations indicate that the proposed U-SPECT-III system yields an almost four times higher linear and about sixty times higher volumetric system resolution than DP-SPECT, when the systems are compared at matching system sensitivity. In addition, it should be possible to achieve a 15 up to 30 times higher sensitivity with U-SPECT-III when the systems are compared at equal resolution. Simulated images of a digital mouse-brain phantom show much more detail with U-SPECT-III than with DP-SPECT. In a resolution phantom, 0.3 mm diameter cold rods are clearly visible with U-SPECT-III, whereas with DP-SPECT the smallest visible rods are about 0.6–0.8 mm. Furthermore, with U-SPECT-III, the image deformations outside the central plane of reconstruction that hamper conventional pinhole SPECT are strongly suppressed. Simulation results indicate that future pinhole SPECT systems are likely to bring about significant improvements in radio-molecular imaging of small animals.

Imaging recognition of multidrug resistance in human breast tumors using 99mTc-labeled monocationic agents and a high-resolution stationary SPECT system

Imaging recognition of multidrug-resistance by 99mTc-labeled sestamibi, tetrofosmin and furifosmin in mice bearing human breast tumors was evaluated using a high-resolution SPECT, FASTSPECT. Imaging results showed that the washout rates in drug-resistant MCF7/D40 tumors were significantly greater than that in drug-sensitive MCF7/S tumors. Furifosmin exhibited greater washout from both MCF7/S and MCF7/D40 than sestamibi, while tetrofosmin washout was greater than sestamibi in MCF7/D40 only. Feasibility of the monocationic agents for characterizing MDR expression was well clarified with FASTSPECT imaging.

Imaging of striatal dopamine transporters in rat brain with single pinhole SPECT and co-aligned MRI is highly reproducible

A recently developed pinhole high-resolution SPECT system was used to measure striatal to non-specific binding ratios in rats (n = 9), after injection of the dopamine transporter ligand 123I-FP-CIT, and to assess its test/retest reproducibility. For co-alignment purposes, the rat brain was imaged on a 1.5 Tesla clinical MRI scanner using a specially developed surface coil. The SPECT images showed clear striatal uptake. On the MR images, cerebral and extra-cerebral structures could be easily delineated. The mean striatal to non-specific [ 123I]FP-CIT binding ratios of the test/retest studies were 1.7 ± 0.2 and 1.6 ± 0.2, respectively. The test/retest variability was approximately 9%. We conclude that the assessment of striatal [ 123I]FP-CIT binding ratios in rats is highly reproducible.

Ultra high resolution pinhole SPECT for small animal study

The aim of the study is to develop an ultra high-resolution pinhole SPECT system and analyze the performance by simulations. The pinhole SPECT was conducted with a Toshiba GCA-7200 A gamma camera. The diameter of the pinhole was 1 mm. Data were acquired with a 128/spl times/128 matrix, and the distance from the center of rotation (COR) to the pinhole was 20 mm [case 1] or 28 mm [case 2]. The data were collected at 4 degree increments over 360 degrees. The Feldkamp method was used for image reconstruction. Before the image reconstruction, the authors corrected the COR of the pinhole collimator. The imaging voxel was 0.27/spl times/0.27/spl times/0.27 mm/sup 3/ [case 1] or 0.4/spl times/0.4/spl times/0.4 mm/sup 3/ [case 2]. From the experiments with a resolution phantom the spatial resolution (FWHM) was 1.1 mm [case 1] or 1.3 mm [case 2]. The pinhole SPECT was also examined for in vivo imaging for studies of the heart in mice which were injected with Tc-99m-Tetrofosmin intravenously. In the reconstructed image the heart muscle of the mouse was clearly visualized. The authors also investigated the spatial resolution of the reconstructed image by means of simulations.