Comparison of simultaneous dual-isotope multipinhole SPECT with rotational SPECT in a group of patients with coronary artery disease

The clinical utilities of multi-pinhole single photon emission computed tomography

Single photon emission computed tomography (SPECT) is an important imaging modality for various applications in nuclear medicine. The use of multi-pinhole (MPH) collimators can provide superior resolution-sensitivity trade-off when imaging small field-of-view compared to conventional parallel-hole and fan-beam collimators. 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. This article reviews the basic principles of MPH collimators and introduces currently available and proposed clinical MPH SPECT systems.

Novel SPECT Technologies and Approaches in Cardiac Imaging

Recent novel approaches in myocardial perfusion single photon emission CT (SPECT) have been facilitated by new dedicated high-efficiency hardware with solid-state detectors and optimized collimators. New protocols include very low-dose (1 mSv) stress-only, two-position imaging to mitigate attenuation artifacts, and simultaneous dual-isotope imaging. Attenuation correction can be performed by specialized low-dose systems or by previously obtained CT coronary calcium scans. Hybrid protocols using CT angiography have been proposed. Image quality improvements have been demonstrated by novel reconstructions and motion correction. Fast SPECT acquisition facilitates dynamic flow and early function measurements. Image processing algorithms have become automated with virtually unsupervised extraction of quantitative imaging variables. This automation facilitates integration with clinical variables derived by machine learning to predict patient outcome or diagnosis. In this review, we describe new imaging protocols made possible by the new hardware developments. We also discuss several novel software approaches for the quantification and interpretation of myocardial perfusion SPECT scans.

Segmented slant hole collimator for stationary cardiac SPECT: Monte Carlo simulations

PURPOSE

This work is a preliminary study of a stationary cardiac SPECT system. The goal of this research is to propose a stationary cardiac SPECT system using segmented slant-hole collimators and to perform computer simulations to test the feasibility. Compared to the rotational SPECT, a stationary system has a benefit of acquiring temporally consistent projections. The most challenging issue in building a stationary system is to provide sufficient projection view-angles.

METHODS

A GATE (GEANT4 application for tomographic emission) Monte Carlo model was developed to simulate a two-detector stationary cardiac SPECT that uses segmented slant-hole collimators. Each detector contains seven segmented slant-hole sections that slant to a common volume at the rotation center. Consequently, 14 view-angles over 180° were acquired without any gantry rotation. The NCAT phantom was used for data generation and a tailored maximum-likelihood expectation-maximization algorithm was used for image reconstruction. Effects of limited number of view-angles and data truncation were carefully evaluated in the paper.

RESULTS

Simulation results indicated that the proposed segmented slant-hole stationary cardiac SPECT system is able to acquire sufficient data for cardiac imaging without a loss of image quality, even when the uptakes in the liver and kidneys are high. Seven views are acquired simultaneously at each detector, leading to 5-fold sensitivity gain over the conventional dual-head system at the same total acquisition time, which in turn increases the signal-to-noise ratio by 19%. The segmented slant-hole SPECT system also showed a good performance in lesion detection. In our prototype system, a short hole-length was used to reduce the dead zone between neighboring collimator segments. The measured sensitivity gain is about 17-fold over the conventional dual-head system.

CONCLUSIONS

The gate Monte Carlo simulations confirm the feasibility of the proposed stationary cardiac SPECT system with segmented slant-hole collimators. The proposed collimator consists of combined parallel and slant holes, and the image on the detector is not reduced in size.

Design and evaluation of an adaptive multipinhole collimator for high-performance clinical and preclinical imaging

OBJECTIVE

Pinhole single-photon emission computed tomography provides superior trade-off between resolution and detection efficiency as compared with conventional parallel-hole collimators for imaging small objects. This study aims to design and evaluate an optimized adaptive multipinhole (MPH) collimator for improved clinical myocardial perfusion single-photon emission computed tomography imaging (MPI) and preclinical small-animal imaging (SAI) of rats based on a clinical scanner.

METHODS

The target resolution and field of view was set to be 1/20 cm for MPI and 0.15/5 cm for SAI, respectively. We determined the design parameters by maximizing the detection efficiency based on system constraints. Point source simulations using Geant4 Application for Emission Tomography were performed for different collimator-to-center of field of view distances to assess the detection efficiency and resolution trade-off. The XCAT phantom with Tc-99m sestamibi distribution and the four-dimensional mouse whole-body phantom with Tc-99m methylene diphosphonate distribution were used to generate noise-free and noisy projections using a three-dimensional analytical MPH projector. Projections were reconstructed using a three-dimensional MPH ordered-subset expectation maximization algorithm. Noise and bias were assessed on the reconstructed images for different collimators.

RESULTS

The design parameters are (i) 14 pinholes with 3.42 mm aperture size, 14.5 cm collimator-to-detector distance for MPI; (ii) six pinholes with an aperture size of 0.94 mm, 21.2 cm collimator-to-detector distance for SAI. For MPI, the projected full width at half maximum values were 10.68 and 8.19 mm for low energy high resolution (LEHR) and MPH, respectively, whereas MPH had double detection efficiency. For SAI, the projected full width at half maximum values for LEHR and MPH were 4.93 and 1.20 mm, respectively, whereas the detection efficiency of MPH showed 17.5% improvement as compared with LEHR. The noise-bias trade-off improved for MPH as compared with LEHR for both MPI and SAI. The proposed collimator will have adjustable collimator-to-detector distances - that is, 14.5 cm for MPI and 21.2 cm for SAI.

CONCLUSION

The new collimator yields substantial improvement in image quality as compared with current MPI using LEHR with extra capability for SAI, bridging the clinical and preclinical imaging based on the same platform.

Stress thallium-201/rest technetium-99m sequential dual-isotope high-speed myocardial perfusion imaging validation versus invasive coronary angiography

BACKGROUND

Recent advances in nuclear myocardial perfusion imaging (MPI) have made it possible to develop a dual-isotope protocol for high-speed acquisition with image quality and radiation delivery comparable to that obtained with conventional single isotope protocols. So far, no study has compared dual-isotope high-speed MPI to invasive coronary angiography (ICA) in a large cohort using a Cadmium-zinc-telluride SPECT system.

METHODS

Over a 1-year period (May 2011 to April 2012), 1366 patients underwent dual-isotope high-speed MPI. Patients with ICA within 3 months after dual-isotope high-speed MPI were included together with patients with a low likelihood of coronary artery disease (CAD) in order to assess normalcy rate. Global summed stress score (SSS) and summed rest score (SRS) were calculated, and ICA results were analyzed independently. The main end point was a patient-based assessment of the diagnostic performance of dual-isotope high-speed MPI in detecting or ruling out significant CAD (>70% reduction in lumen diameter).

RESULTS

Inclusion criteria were fulfilled for 214 patients (143 men; age 60 ± 14 years; ICA, n = 104; low likelihood for CAD, n = 110). An exercise stress test was performed in 62% of patients and a pharmacological stress test was performed with either dipyridamole (32%) or dobutamine (6%). Average examination duration was 22.4 ± 4.5 minutes. Mean SSS, SRS, and SDS were 8.0 ± 4.9, 3.1 ± 4.3, and 5.0 ± 3.2, respectively. Prevalence of angiographic CAD was 75%. ICA detected stenosis in the left main trunk, left anterior descending artery, left circumflex artery, and right coronary artery in 4, 33, 31, and 42 patients, respectively. Sensitivity of dual-isotope high-speed MPI was 94%, normalcy rate was 92%, and accuracy was 83% for detecting CAD.

CONCLUSION

Dual-isotope high-speed MPI is reliable at detecting or ruling out CAD. NCT01785589.

Geometric Calibration and Image Reconstruction for a Segmented Slant-Hole Stationary Cardiac SPECT System

A dedicated stationary cardiac single-photon emission computed tomography (SPECT) system with a novel segmented slant-hole collimator has been developed. The goal of this paper is to calibrate this new imaging geometry with a point source.

METHODS

Unlike the commercially available dedicated cardiac SPECT systems, which are specialized and can be used only to image the heart, our proposed cardiac system is based on a conventional SPECT system but with a segmented slant-hole collimator replacing the collimator. For a dual-head SPECT system, 2 segmented collimators, each with 7 sections, are arranged in an L-shaped configuration such that they can produce a complete cardiac SPECT image with only one gantry position. A calibration method was developed to estimate the geometric parameters of each collimator section as well as the detector rotation radius, under the assumption that the point source location is calculated using the central-section data. With a point source located off the rotation axis, geometric parameters for each collimator section can be estimated independently. The parameters estimated individually are further improved by a joint objective function that uses all collimator sections simultaneously and incorporates the collimator symmetry information.

RESULTS

Estimation results and images reconstructed from estimated parameters are presented for both simulated and real data acquired from a prototype collimator. The calibration accuracy was validated by computer simulations with an error of about 0.1° for the slant angles and about 1 mm for the rotation radius. Reconstructions of a heart-insert phantom did not show any image artifacts of inaccurate geometric parameters.

CONCLUSION

Compared with the detector's intrinsic resolution, the estimation error is small and can be ignored. Therefore, the accuracy of the calibration is sufficient for cardiac SPECT imaging.

Advances in pinhole and multi-pinhole collimators for single photon emission computed tomography imaging

The collimator in single photon emission computed tomography (SPECT), is an important part of the imaging chain. One of the most important collimators that used in research, preclinical study, small animal, and organ imaging is the pinhole collimator. Pinhole collimator can improve the tradeoff between sensitivity and resolution in comparison with conventional parallel-hole collimator and facilities diagnosis. However, a major problem with pinhole collimator is a small field of view (FOV). Multi-pinhole collimator has been investigated in order to increase the sensitivity and FOV with a preserved spatial resolution. The geometry of pinhole and multi-pinhole collimators is a critical factor in the image quality and plays a key role in SPECT imaging. The issue of the material and geometry for pinhole and multi-pinhole collimators have been a controversial and much disputed subject within the field of SPECT imaging. On the other hand, recent developments in collimator optimization have heightened the need for appropriate reconstruction algorithms for pinhole SPECT imaging. Therefore, iterative reconstruction algorithms were introduced to minimize the undesirable effect on image quality. Current researches have focused on geometry and configuration of pinhole and multi-pinhole collimation rather than reconstruction algorithm. The lofthole and multi-lofthole collimator are samples of novel designs. The purpose of this paper is to provide a review on recent researches in the pinhole and multi-pinhole collimators for SPECT imaging.

Design and performance evaluation of a 20-aperture multipinhole collimator for myocardial perfusion imaging applications

Single photon emission computed tomography (SPECT) myocardial perfusion imaging remains a critical tool in the diagnosis of coronary artery disease. However, after more than three decades of use, photon detection efficiency remains poor and unchanged. This is due to the continued reliance on parallel-hole collimators first introduced in 1964. These collimators possess poor geometric efficiency. Here we present the performance evaluation results of a newly designed multipinhole collimator with 20 pinhole apertures (PH20) for commercial SPECT systems. Computer simulations and numerical observer studies were used to assess the noise, bias and diagnostic imaging performance of a PH20 collimator in comparison with those of a low energy high resolution (LEHR) parallel-hole collimator. Ray-driven projector/backprojector pairs were used to model SPECT imaging acquisitions, including simulation of noiseless projection data and performing MLEM/OSEM image reconstructions. Poisson noise was added to noiseless projections for realistic projection data. Noise and bias performance were investigated for five mathematical cardiac and torso (MCAT) phantom anatomies imaged at two gantry orbit positions (19.5 and 25.0 cm). PH20 and LEHR images were reconstructed with 300 MLEM iterations and 30 OSEM iterations (ten subsets), respectively. Diagnostic imaging performance was assessed by a receiver operating characteristic (ROC) analysis performed on a single MCAT phantom; however, in this case PH20 images were reconstructed with 75 pixel-based OSEM iterations (four subsets). Four PH20 projection views from two positions of a dual-head camera acquisition and 60 LEHR projections were simulated for all studies. At uniformly-imposed resolution of 12.5 mm, significant improvements in SNR and diagnostic sensitivity (represented by the area under the ROC curve, or AUC) were realized when PH20 collimators are substituted for LEHR parallel-hole collimators. SNR improves by factors of 1.94-2.34 for the five patient anatomies and two orbital positions studied. For the ROC analysis the PH20 AUC is larger than the LEHR AUC with a p-value of 0.0067. Bias performance, however, decreases with the use of PH20 collimators. Systematic analyses showed PH20 collimators present improved diagnostic imaging performance over LEHR collimators, requiring only collimator exchange on existing SPECT cameras for their use.

A tailored ML-EM algorithm for reconstruction of truncated projection data using few view angles

Dedicated cardiac single photon emission computed tomography (SPECT) systems have the advantage of high speed and sensitivity at no loss, or even a gain, in resolution. The potential drawbacks of these dedicated systems are data truncation by the small field of view (FOV) and the lack of view angles. Serious artifacts, including streaks outside the FOV and distortion in the FOV, are introduced to the reconstruction when using the traditional emission data maximum-likelihood expectation-maximization (ML-EM) algorithm to reconstruct images from the truncated data with a small number of views. In this note, we propose a tailored ML-EM algorithm to suppress the artifacts caused by data truncation and insufficient angular sampling by reducing the image updating step sizes for the pixels outside the FOV. As a consequence, the convergence speed for the pixels outside the FOV is decelerated. We applied the proposed algorithm to truncated analytical data, Monte Carlo simulation data and real emission data with different numbers of views. The computer simulation results show that the tailored ML-EM algorithm outperforms the conventional ML-EM algorithm in terms of streak artifacts and distortion suppression for reconstruction from truncated projection data with a small number of views.

Simultaneous dual-radionuclide myocardial perfusion imaging with a solid-state dedicated cardiac camera

PURPOSE

We compared simultaneous dual-radionuclide (DR) stress and rest myocardial perfusion imaging (MPI) with a novel solid-state cardiac camera and a conventional SPECT camera with separate stress and rest acquisitions.

METHODS

Of 27 consecutive patients recruited, 24 (64.5+/-11.8 years of age, 16 men) were injected with 74 MBq of (201)Tl (rest) and 250 MBq (99m)Tc-MIBI (stress). Conventional MPI acquisition times for stress and rest are 21 min and 16 min, respectively. Rest (201)Tl for 6 min and simultaneous DR 15-min list mode gated scans were performed on a D-SPECT cardiac scanner. In 11 patients DR D-SPECT was performed first and in 13 patients conventional stress (99m)Tc-MIBI SPECT imaging was performed followed by DR D-SPECT. The DR D-SPECT data were processed using a spill-over and scatter correction method. DR D-SPECT images were compared with rest (201)Tl D-SPECT and with conventional SPECT images by visual analysis employing the 17-segment model and a five-point scale (0 normal, 4 absent) to calculate the summed stress and rest scores. Image quality was assessed on a four-point scale (1 poor, 4 very good) and gut activity was assessed on a four-point scale (0 none, 3 high).

RESULTS

Conventional MPI studies were abnormal at stress in 17 patients and at rest in 9 patients. In the 17 abnormal stress studies DR D-SPECT MPI showed 113 abnormal segments and conventional MPI showed 93 abnormal segments. In the nine abnormal rest studies DR D-SPECT showed 45 abnormal segments and conventional MPI showed 48 abnormal segments. The summed stress and rest scores on conventional SPECT and DR D-SPECT were highly correlated (r=0.9790 and 0.9694, respectively). The summed scores of rest (201)Tl D-SPECT and DR-DSPECT were also highly correlated (r=0.9968, p<0.0001 for all). In six patients stress perfusion defects were significantly larger on stress DR D-SPECT images, and five of these patients were imaged earlier by D-SPECT than by conventional SPECT.

CONCLUSION

Fast and high-quality simultaneous DR MPI is feasible with D-SPECT in a single imaging session with comparable diagnostic performance and image quality to conventional SPECT and to a separate rest (201)Tl D-SPECT acquisition.

A fast cardiac gamma camera with dynamic SPECT capabilities: design, system validation and future potential

PURPOSE

The goal of this study is to present the Discovery NM 530c (DNM), a cardiac SPECT camera, interfacing multi-pinhole collimators with solid-state modules, aiming at slashing acquisition time without jeopardizing quality. DNM resembles PET since it enables 3-D SPECT without detector motion. We further envision how these novel capabilities may help with current and future challenges of cardiac imaging.

METHODS

DNM sensitivity, spatial resolution (SR) and energy resolution (ER), count rate response, cardiac uniformity and cardiac defect contrast were measured and compared to a dedicated cardiac, dual-head standard SPECT (S-SPECT) camera.

RESULTS

DNM sensitivity was more than threefold higher while SR was notably better. Significantly, SR was the same for (99m)Tc and (201)Tl. ER was improved on DNM and allowed good separation of (99m)Tc and (123)I spectral peaks. Count rate remained linear on DNM up to 612 kcps, while S-SPECT showed severe dead time limitations. Phantom studies revealed comparable uniformity and defect contrast, notwithstanding significantly shorter acquisition time for the DNM. First patient images, including dynamic SPECT, are also presented.

CONCLUSION

DNM is raising the bar for expedition and upgrade of practice. It features high sensitivity as well as improved SR, temporal resolution and ER. It enables reduction of acquisition time and fast protocols. Importantly, it is potentially capable of dynamic 3-D acquisition. The new technology is potentially upgradeable and may become a milestone in the evolution of nuclear cardiology as it assumes its key role in molecular imaging of the heart.

New Imaging Protocols for New Single Photon Emission CT Technologies

Nuclear cardiology practitioners have several new technologies available with which to perform myocardial perfusion single photon emission CT (MPS). These include dedicated small-footprint cardiac scanners, new stationary or semi-stationary three-dimensional detectors, and advanced software algorithms for optimal image reconstruction. These new technologies have been employed to reduce imaging time and radiation exposure. They require less technologist and camera time and offer improved patient comfort. They have potential for the overall cost reduction of MPS and at the same time for improved accuracy by increased resolution, or accurate attenuation correction. Furthermore, these new technologies offer potential for new protocols such as simultaneous dual isotope, new combinations of isotopes, stress only MPS, or dynamic first-pass imaging. In addition, new imaging technologies in coronary CT angiography (CCTA) allow novel hybrid stress only MPS/CCTA protocols with reduced radiation burden. Additional developments further improving efficiency and diagnostic accuracy of MPS are on the horizon.

Novel solid-state-detector dedicated cardiac camera for fast myocardial perfusion imaging: multicenter comparison with standard dual detector cameras

OBJECTIVE

To compare the diagnostic performance of a new dedicated ultrafast solid-state cardiac camera (Discovery NM 530c [DNM]) with standard dual detector cameras (S-SPECT) in myocardial perfusion imaging. The primary goal was a per-patient analysis of diagnostic performance of the DNM using S-SPECT as the reference standard.

METHODS AND RESULTS

In total, 168 patients underwent one-day Tc-99m tetrofosmin rest/stress myocardial perfusion SPECT. DNM and S-SPECT images were obtained with the same injected doses. The DNM camera uses an array of cadmium zinc telluride pixilated detectors and a multipinhole collimator simultaneously imaging all cardiac views with no moving parts. Rest and stress acquisition times were 4 and 2 minutes for DNM and 14 and 12 minutes for S-SPECT. Two blinded readers independently interpreted all scans on a patient level and on a vascular territory level using a standard five-point scale. Interobserver differences were resolved by a third observer. Agreement between DNM and S-SPECT for presence or absence of myocardial perfusion defects on a per-patient analysis was 91.9% and 92.5%, respectively. Correlation coefficients of rest and stress left ventricular ejection fractions were 0.87 (P < .01) and 0.90 (P < .01).

CONCLUSION

The diagnostic performance of DNM is comparable to that of S-SPECT on a per-patient basis. However, superior image quality can be achieved with significantly shorter acquisition times with DNM because of improved count sensitivity and image contrast over S-SPECT.

C-SPECT - a Clinical Cardiac SPECT/Tct Platform: Design Concepts and Performance Potential

Because of scarcity of photons emitted from the heart, clinical cardiac SPECT imaging is mainly limited by photon statistics. The sub-optimal detection efficiency of current SPECT systems not only limits the quality of clinical cardiac SPECT imaging but also makes more advanced potential applications difficult to be realized. We propose a high-performance system platform - C-SPECT, which has its sampling geometry optimized for detection of emitted photons in quality and quantity. The C-SPECT has a stationary C-shaped gantry that surrounds the left-front side of a patient's thorax. The stationary C-shaped collimator and detector systems in the gantry provide effective and efficient detection and sampling of photon emission. For cardiac imaging, the C-SPECT platform could achieve 2 to 4 times the system geometric efficiency of conventional SPECT systems at the same sampling resolution. This platform also includes an integrated transmission CT for attenuation correction. The ability of C-SPECT systems to perform sequential high-quality emission and transmission imaging could bring cost-effective high-performance to clinical imaging. In addition, a C-SPECT system could provide high detection efficiency to accommodate fast acquisition rate for gated and dynamic cardiac imaging. This paper describes the design concepts and performance potential of C-SPECT, and illustrates how these concepts can be implemented in a basic system.

Targeting murine heart and brain: visualisation conditions for multi-pinhole SPECT with (99m)Tc- and (123)I-labelled probes

PURPOSE

The study serves to optimise conditions for multi-pinhole SPECT small animal imaging of (123)I- and (99m)Tc-labelled radiopharmaceuticals with different distributions in murine heart and brain and to investigate detection and dose range thresholds for verification of differences in tracer uptake.

METHODS

A Triad 88/Trionix system with three 6-pinhole collimators was used for investigation of dose requirements for imaging of the dopamine D(2) receptor ligand [(123)I]IBZM and the cerebral perfusion tracer [(99m)Tc]HMPAO (1.2-0.4 MBq/g body weight) in healthy mice. The fatty acid [(123)I]IPPA (0.94 +/- 0.05 MBq/g body weight) and the perfusion tracer [(99m)Tc]sestamibi (3.8 +/- 0.45 MBq/g body weight) were applied to cardiomyopathic mice overexpressing the prostaglandin EP(3) receptor.

RESULTS

In vivo imaging and in vitro data revealed 45 kBq total cerebral uptake and 201 kBq cardiac uptake as thresholds for visualisation of striatal [(123)I]IBZM and of cardiac [(99m)Tc]sestamibi using 100 and 150 s acquisition time, respectively. Alterations of maximal cerebral uptake of [(123)I]IBZM by >20% (116 kBq) were verified with the prerequisite of 50% striatal of total uptake. The labelling with [(99m)Tc]sestamibi revealed a 30% lower uptake in cardiomyopathic hearts compared to wild types. [(123)I]IPPA uptake could be visualised at activity doses of 0.8 MBq/g body weight.

CONCLUSION

Multi-pinhole SPECT enables detection of alterations of the cerebral uptake of (123)I- and (99m)Tc-labelled tracers in an appropriate dose range in murine models targeting physiological processes in brain and heart. The thresholds of detection for differences in the tracer uptake determined under the conditions of our experiments well reflect distinctions in molar activity and uptake characteristics of the tracers.

Multidivergent-beam stationary cardiac SPECT

This article develops a stationary cardiac single photon emission computed tomography (SPECT) system using a novel multidivergent-beam collimator. This stationary SPECT system is inexpensive to build, small, and able to acquire true dynamic SPECT data. Stationary cardiac SPECT systems with multipinhole technology already exist. The proposed approach is to replace the multipinhole collimators with the originally designed multidivergent-beam collimators. The motivation for replacing the pinhole technology by divergent-beam technology is based on the following facts. The resolution/sensitivity trade-off for the pinhole is excellent (good resolution and good sensitivity) only in small object (e.g., small animal) imaging when it operates in the image magnifying mode. However, in large object (e.g., human) imaging, the resolution/sensitivity trade-off is poor (poor resolution and poor sensitivity) when the pinhole operates in the image reducing mode. In a stationary system, the number of angular views is limited; thus, image reduction is necessary to obtain more view angles. In this image reducing situation, divergent-beam collimation is able to provide better resolution and detection sensitivity than pinhole collimation. Computer simulations are carried out to verified the claims.