3D iteratively reconstructed spatial resolution map and sensitivity characterization of a dedicated cardiac SPECT camera

Characterization of myocardial perfusion imaging systems - an extension of quality metrics

PURPOSE

The aim of this study was to evaluate the performance of myocardial perfusion imaging (MPI) systems in detecting perfusion defects (PDs). The defect perfusion index (DPI) was introduced to extend and further advance the current MPI quality metrics.

METHODS

An anthropomorphic phantom simulating normal and pathological myocardial perfusion conditions was imaged by various NaI-crystal detector systems with and without corrections for scatter (SC) and attenuation (AC) (Symbia, Symbia + SC, Symbia IQ + SCAC, Symbia IQ), and cadmium-zinc-telluride detector systems without corrections (DSPECT, D530c). The extent of PD and the summed score (SS) were obtained by comparing polar maps with ad hoc normal databases created for each MPI system by using phantom polar maps with normal perfusion. The segmental uptake (SU) and the global uniformity (GU) were evaluated. The DPI was calculated on segments included in the PD to minimize attenuation artifacts outside the PD. The 17 segmental model was used.

RESULTS

The highest level of uniformity of polar map was obtained for Symbia IQ + SCAC. D530c showed the highest extent of PD and dependence of the extent on the PD position. It showed in general the lowest SU values and the highest GU due to attenuation artifacts. Nevertheless, D530c outperforms other MPI systems in terms of PD detection, showing the highest DPI value. DSPECT system showed the lowest SS value, and DPI values comparable to NaI-crystal detector systems.

CONCLUSION

The DPI can be evaluated to investigate the intrinsic ability of MPI systems to detect PDs, whatever the quantitative post-processing software used.

Myocardial flow reserve estimation with contemporary CZT-SPECT and 99mTc-tracers lacks precision for routine clinical application

BACKGROUND

PET myocardial flow reserve (MFR) has established diagnostic and prognostic value. Technological advances have now enabled SPECT MFR quantification. We investigated whether SPECT MFR precision is sufficient for clinical categorization of patients.

METHODS

Validation studies vs invasive flow measurements and PET MFR were reviewed to determine global SPECT MFR thresholds. Studies vs PET and a SPECT MFR repeatability study were used to establish imprecision in SPECT MFR measurements as the standard deviation of the difference between SPECT and PET MFR, or test-retest SPECT MFR. Simulations were used to evaluate the impact of SPECT MFR imprecision on confidence of clinically relevant categorization.

RESULTS

Based on validation studies, the typical PET MFR categories were used for SPECT MFR classification (< 1.5, 1.5-2.0, > 2.0). Imprecision vs PET MFR ranged from 0.556 to 0.829, and test-retest imprecision was 0.781-0.878. Simulations showed correct classification of up to only 34% of patients when 1.5 ≤ true MFR ≤ 2.0. Categorization with high confidence (> 80%) was only achieved for extreme MFR values (< 1.0 or > 2.5), with correct classification in only 15% of patients in a typical lab with MFR of 1.8 ± 0.5.

CONCLUSIONS

Current SPECT-derived estimates of MFR lack precision and require further optimization for clinical risk stratification.

Dual-layer collimator for improved spatial resolution in SPECT with CZT camera: an analytical and Monte Carlo study

Purpose. Current hole matching pixel detector (HMPD) collimators for SPECT imaging exist in two configurations: one hole per pixel (1HMPD) or four holes per pixel (4HMPD). The aim of this study was to assess the performance of a dual-layer collimator made by stacking up these two collimator types (1H/4HMDP) for low- and medium energy gamma emitters. Method. Analytical equations describing geometrical efficiency and full width at half maximum (FWHM) of the 1H/4HMDP collimator were derived. In addition, a fast dedicated Monte Carlo (MC) code neglecting scattering and designed for the collimator geometry was developed to assess the collimator’s point spread function and to simulate planar and SPECT acquisitions. Results. A relative agreement between analytical equations and MC simulations better than 3% was observed for the efficiency and for the FWHM. The length of the two layers was optimized to get the best spatial resolution while keeping the geometrical efficiency equal to that of the 45 mm length 1HMPD collimator. An optimized combination of the 1H/4HMPD configuration with respective hole lengths of 20 and 13 mm has been derived. For source-collimator distances above 5 cm and equal collimator geometrical efficiency, the spatial resolution of this optimal 1H/4HMDP collimator supersedes that of the 45 mm length 1HMPD collimator, and that of the 19.1 mm length 4HMPD collimator. This improvement was observed in simulations of bar phantom planar images and of hot rods phantom SPECT. Remarkably, the spatial resolution was preserved along the whole radial range within the Jaszczak phantom. Conclusion. The 1H/4HMDP collimator is a promising solution for CZT SPECT imaging of low- and medium energy emitters.

Quantitative SPECT/CT for dosimetry of peptide receptor radionuclide therapy

Neuroendocrine tumors (NETs) are uncommon malignancies of increasing incidence and prevalence. As these slow growing tumors usually overexpress somatostatin receptors (SSTRs), the use of ⁶⁸Ga-DOTA-peptides (gallium-68 chelated with dodecane tetra-acetic acid to somatostatin), which bind to the SSTRs, allows for PET based imaging and selection of patients for peptide receptor radionuclide therapy (PRRT). PRRT with radiolabeled somatostatin analogues such as ¹⁷⁷Lu-DOTATATE (lutetium-177-[DOTA,Tyr³]-octreotate), is mainly used for the treatment of metastatic or inoperable NETs. However, PRRT is generally administered at a fixed injected activity in order not to exceed dose limits in critical organs, which is suboptimal given the variability in radiopharmaceutical uptake among patients. Advances in SPECT (single photon emission computed tomography) imaging enable the absolute quantitative measure of the true radiopharmaceutical distribution providing for PRRT dosimetry in each patient. Personalized PRRT based on patient-specific dosimetry could improve therapeutic efficacy by optimizing effective tumor absorbed dose while limiting treatment related radiotoxicity.

Noise heterogeneity in attenuation-corrected cardiac SPECT images increases perfusion value uncertainty near the base of the heart

BACKGROUND

Dedicated cardiac SPECT cameras which employ multi-pinhole detectors have variable photon sensitivity within the camera's field-of-view such that a lower number of photon counts is typically detected from the base of the heart than from the apex. Consequently, the noise in a reconstructed image is expected to be higher at the base than at the apex of the heart.

METHODS

Patient emission images were resampled to create statistical replicates which were reconstructed with and without attenuation correction. Noise images were computed using one standard deviation of the replicated images. These were evaluated for 93 patients with normal study results, each imaged with both a dual-headed parallel-hole camera and a multi-pinhole camera. Statistics for a normal database (NDB) of images from the 93 patients were also calculated.

RESULTS

Image noise (1.7-fold) and NDB uncertainty (1.3-fold) increase significantly from the apex-to-the base of the heart in attenuation-corrected multi-pinhole SPECT images. The differences for non-attenuation-corrected images or those acquired with a parallel-hole camera were not significant.

CONCLUSIONS

For best interpretation of attenuation-corrected images acquired with multi-pinhole cameras, knowledge of NDB uncertainty gradients should be taken into consideration.

Recent advances in cardiac SPECT instrumentation and imaging methods

Cardiac SPECT continues to play a critical role in detecting and managing cardiovascular disease, in particularly coronary artery disease (CAD) (Jaarsma et al 2012 J. Am. Coll. Cardiol. 59 1719-28), (Agostini et al 2016 Eur. J. Nucl. Med. Mol. Imaging 43 2423-32). While conventional dual-head SPECT scanners using parallel-hole collimators and scintillation crystals with photomultiplier tubes are still the workhorse of cardiac SPECT, they have the limitations of low photon sensitivity (~130 count s^-1 MBq^-1), poor image resolution (~15 mm) (Imbert et al 2012 J. Nucl. Med. 53 1897-903), relatively long acquisition time, inefficient use of the detector, high radiation dose, etc. Recently our field observed an exciting growth of new developments of dedicated cardiac scanners and collimators, as well as novel imaging algorithms for quantitative cardiac SPECT. These developments have opened doors to new applications with potential clinical impact, including ultra-low-dose imaging, absolute quantification of myocardial blood flow (MBF) and coronary flow reserve (CFR), multi-radionuclide imaging, and improved image quality as a result of attenuation, scatter, motion, and partial volume corrections (PVCs). In this article, we review the recent advances in cardiac SPECT instrumentation and imaging methods. This review mainly focuses on the most recent developments published since 2012 and points to the future of cardiac SPECT from an imaging physics perspective.

The effect of CT-based attenuation correction on the automatic perfusion score of myocardial perfusion imaging using a dedicated cardiac solid-state CZT SPECT/CT

BACKGROUND

Data regarding cardiac cadmium-zinc-telluride (CZT)-specific augmented databases and their impact on CT-based attenuation correction (AC) perfusion scores in myocardial perfusion imaging (MPI) were obtained on a multiple-pinhole CZT SPECT/CT.

METHODS AND RESULTS

Summed stress (SSS) and rest scores (SRS) were measured using automated software in three independent patient groups: group 1 (n = 80) underwent MPI on both CZT and conventional sodium iodide (NaI) devices, group 2 (n = 80) with low coronary artery disease likelihood and normal MPI provided reference CZT databases; and group 3 (n = 152) served to compare AC and non-AC (NAC) scores on CZT. Group 1 CZT and NaI scores gave a significant 1:1 linear correlation for CZT scores referenced to the custom database vs NaI scores referenced to the default database, but these were not concordant when CZT scores were referenced to the default database. AC significantly decreased average SSS and SRS in men vs NAC, 4.29 ± 6.30 vs 5.37 ± 7.26 (P < 0.001) and 2.37 ± 4.72 vs 3.13 ± 5.85 (P < 0.001), but not in women, 2.28 ± 3.42 vs 2.28 ± 3.08 (p NS) and 0.46 ± 1.51 vs 0.61 ± 1.86, (p NS), respectively.

CONCLUSIONS

Specifically designed databases for solid-state CZT cardiac SPECT provide accurate quantitation of perfusion scores concordant with those previously validated for conventional SPECT. AC and NAC CZT scores differed significantly, especially in men.

Quantification of intramyocardial blood volume with 99mTc-RBC SPECT-CT imaging: A preclinical study

BACKGROUND

Currently, there is no established non-invasive imaging approach to directly evaluate myocardial microcirculatory function in order to diagnose microvascular disease independent of co-existing epicardial disease. In this work, we developed a methodological framework for quantification of intramyocardial blood volume (IMBV) as a novel index of microcirculatory function with SPECT/CT imaging of 99mTc-labeled red blood cells (RBCs).

METHODS

Dual-gated myocardial SPECT/CT equilibrium imaging of 99mTc-RBCs was performed on twelve canines under resting conditions. Five correction schemes were studied: cardiac gating with no other corrections (CG), CG with attenuation correction (CG + AC), CG + AC with scatter correction (CG + AC + SC), dual cardiorespiratory gating with AC + SC (DG + AC + SC), and DG + AC + SC with partial volume correction (DG + AC + SC + PVC). Quantification of IMBV using each approach was evaluated in comparison to those obtained from all corrections. The in vivo SPECT estimates of IMBV values were validated against those obtained from ex vivo microCT imaging of the casted hearts.

RESULTS

The estimated IMBV with all corrections was 0.15 ± 0.03 for the end-diastolic phase and 0.11 ± 0.03 for the end-systolic phase. The cycle-dependent change in IMBV (ΔIMBV) with all corrections was 23.9 ± 8.6%. Schemes that applied no correction or partial correction resulted in significant over-estimation of IMBV and significant under-underestimation of ΔIMBV. Estimates of IMBV and ΔIMBV using all corrections were consistent with values reported in the literature using invasive techniques. In vivo SPECT estimates of IMBV strongly correlated (R2 ≥ 0.70) with ex vivo measures for the various correction schemes, while the fully corrected scheme yielded the smallest bias.

CONCLUSIONS

Non-invasive quantification of IMBV is feasible using 99mTc-RBCs SPECT/CT imaging, however, requires full compensation of physical degradation factors.

Performance analysis of a high-sensitivity multi-pinhole cardiac SPECT system with hemi-ellipsoid detectors

PURPOSE

Single-photon emission computed tomography (SPECT) is a noninvasive imaging modality, used in myocardial perfusion imaging. The challenges facing the majority of clinical SPECT systems are low sensitivity, poor resolution, and the relatively high radiation dose to the patient. New generation systems (GE Discovery, DSPECT) dedicated to cardiac imaging improve sensitivity by a factor of 5-8. This improvement can be used to decrease acquisition time and/or dose. However, in the case of ultra-low dose (~3 mCi) injections, acquisition times are still significantly long, taking 10-12 min. The purpose of this work is to investigate a new gamma camera design with 21 hemi-ellipsoid detectors each with a pinhole collimator for cardiac SPECT for further improvement in sensitivity and resolution and reduced patient exposures and imaging times.

METHODS

To evaluate the resolution of our hemi-ellipsoid system, GATE Monte-Carlo simulations were performed on point-sources, rod-sources, and NCAT phantoms. For average full-width-half-maximum (FWHM) equivalence with base flat-detector, the pinhole-diameter for the curved hemi-ellipsoid detector was found to be 8.68 mm, an operating pinhole-diameter nominally expected to be ~3 times more sensitive than state-of-the-art systems. Rod-sources equally spaced within the region of interest were acquired with a 21-detector system and reconstructed with our multi-pinhole (MPH) iterative OSEM algorithm with collimator resolution recovery. The results were compared with the results of a state-of-the-art system (GE Discovery) available in the literature. The system was also evaluated using the mathematical anthropomorphic NCAT (NURBS-based Cardiac Torso; Segars et al. IEEE Trans Nucl Sci. 1999;46:503-506) phantom with a full (clinical)-dose acquisition (25 mCi) for 2 min and an ultra-low dose acquisition of 3 mCi for 5.44 min. The estimated left ventricle (LV) counts were compared with the available literature on a state-of-the-art system (DSPECT). FWHM of the LV wall on MPH-OSEM-reconstructed images with collimator resolution recovery was estimated.

RESULTS

On acquired rod-sources, the average resolution (FWHM) after reconstruction with resolution recovery in the entire region of interest (ROI) for cardiac imaging was on the average 4.44 mm (±2.84), compared to 6.9 mm (±1 mm) reported for GE Discovery (Kennedy et al., J Nucl Cardiol. 2014:21:443-452). For NCAT studies, improved sensitivity allowed a full-dose (25 mCi) 2-min acquisition (Ell8.68mmFD) which yielded 3.79 M LV counts. This is ~3.35 times higher compared to 1.13 M LV counts acquired in 2 min for clinical full dose for state-of-the-art DSPECT. The increased sensitivity also allowed an ultra-low dose acquisition protocol (Ell8.68 mmULD), 3 mCi (eight times less injected dose) in 5.44 min. This ultra-low dose protocol yielded ~1.23 M LV counts which was comparable to the full-dose 2-min acquisition for DSPECT. The estimated NCAT average FWHM at the LV wall after 12 iterations of the OSEM reconstruction was 4.95 and 5.66 mm around the mid-short-axis slices for Ell8.68mmFD and Ell8.68mmULD, respectively.

CONCLUSION

Our Monte-Carlo simulation studies and reconstruction suggest using (inverted wineglass sized) hemi-ellipsoid detectors with pinhole collimators can increase the sensitivity ~3.35 times over the new generation of dedicated cardiac SPECT systems, while also improving the reconstructed resolution for rod-sources with an average of 4.44 mm in region of interest. The extra sensitivity may be used for ultra-low dose imaging (3 mCi) at ~5.44 min for comparable clinical counts as state-of-the-art systems.

Noise suppressed partial volume correction for cardiac SPECT/CT

PURPOSE

Partial volume correction (PVC) methods typically improve quantification at the expense of increased image noise and reduced reproducibility. In this study, the authors developed a novel voxel-based PVC method that incorporates anatomical knowledge to improve quantification while suppressing noise for cardiac SPECT/CT imaging.

METHODS

In the proposed method, the SPECT images were first reconstructed using anatomical-based maximum a posteriori (AMAP) with Bowsher's prior to penalize noise while preserving boundaries. A sequential voxel-by-voxel PVC approach (Yang's method) was then applied on the AMAP reconstruction using a template response. This template response was obtained by forward projecting a template derived from a contrast-enhanced CT image, and then reconstructed using AMAP to model the partial volume effects (PVEs) introduced by both the system resolution and the smoothing applied during reconstruction. To evaluate the proposed noise suppressed PVC (NS-PVC), the authors first simulated two types of cardiac SPECT studies: a (99m)Tc-tetrofosmin myocardial perfusion scan and a (99m)Tc-labeled red blood cell (RBC) scan on a dedicated cardiac multiple pinhole SPECT/CT at both high and low count levels. The authors then applied the proposed method on a canine equilibrium blood pool study following injection with (99m)Tc-RBCs at different count levels by rebinning the list-mode data into shorter acquisitions. The proposed method was compared to MLEM reconstruction without PVC, two conventional PVC methods, including Yang's method and multitarget correction (MTC) applied on the MLEM reconstruction, and AMAP reconstruction without PVC.

RESULTS

The results showed that the Yang's method improved quantification, however, yielded increased noise and reduced reproducibility in the regions with higher activity. MTC corrected for PVE on high count data with amplified noise, although yielded the worst performance among all the methods tested on low-count data. AMAP effectively suppressed noise and reduced the spill-in effect in the low activity regions. However it was unable to reduce the spill-out effect in high activity regions. NS-PVC yielded superior performance in terms of both quantitative assessment and visual image quality while improving reproducibility.

CONCLUSIONS

The results suggest that NS-PVC may be a promising PVC algorithm for application in low-dose protocols, and in gated and dynamic cardiac studies with low counts.

Analytically based photon scatter modeling for a multipinhole cardiac SPECT camera

PURPOSE

Dedicated cardiac SPECT scanners have improved performance over standard gamma cameras allowing reductions in acquisition times and/or injected activity. One approach to improving performance has been to use pinhole collimators, but this can cause position-dependent variations in attenuation, sensitivity, and spatial resolution. CT attenuation correction (AC) and an accurate system model can compensate for many of these effects; however, scatter correction (SC) remains an outstanding issue. In addition, in cameras using cadmium-zinc-telluride-based detectors, a large portion of unscattered photons is detected with reduced energy (low-energy tail). Consequently, application of energy-based SC approaches in these cameras leads to a higher increase in noise than with standard cameras due to the subtraction of true counts detected in the low-energy tail. Model-based approaches with parallel-hole collimator systems accurately calculate scatter based on the physics of photon interactions in the patient and camera and generate lower-noise estimates of scatter than energy-based SC. In this study, the accuracy of a model-based SC method was assessed using physical phantom studies on the GE-Discovery NM530c and its performance was compared to a dual energy window (DEW)-SC method.

METHODS

The analytical photon distribution (APD) method was used to calculate the distribution of probabilities that emitted photons will scatter in the surrounding scattering medium and be subsequently detected. APD scatter calculations for ^99mTc-SPECT (140 ± 14 keV) were validated with point-source measurements and 15 anthropomorphic cardiac-torso phantom experiments and varying levels of extra-cardiac activity causing scatter inside the heart. The activity inserted into the myocardial compartment of the phantom was first measured using a dose calibrator. CT images were acquired on an Infinia Hawkeye (GE Healthcare) SPECT/CT and coregistered with emission data for AC. For comparison, DEW scatter projections (120 ± 6 keV ) were also extracted from the acquired list-mode SPECT data. Either APD or DEW scatter projections were subtracted from corresponding 140 keV measured projections and then reconstructed with AC (APD-SC and DEW-SC). Quantitative accuracy of the activity measured in the heart for the APD-SC and DEW-SC images was assessed against dose calibrator measurements. The difference between modeled and acquired projections was measured as the root-mean-squared-error (RMSE). APD-modeled projections for a clinical cardiac study were also evaluated.

RESULTS

APD-modeled projections showed good agreement with SPECT measurements and had reduced noise compared to DEW scatter estimates. APD-SC reduced mean error in activity measurement compared to DEW-SC in images and the reduction was statistically significant where the scatter fraction (SF) was large (mean SF = 28.5%, T-test p = 0.007). APD-SC reduced measurement uncertainties as well; however, the difference was not found to be statistically significant (F-test p > 0.5). RMSE comparisons showed that elevated levels of scatter did not significantly contribute to a change in RMSE (p > 0.2).

CONCLUSIONS

Model-based APD scatter estimation is feasible for dedicated cardiac SPECT scanners with pinhole collimators. APD-SC images performed better than DEW-SC images and improved the accuracy of activity measurement in high-scatter scenarios.

Recent Advances in Nuclear Cardiology

Nuclear cardiology is one of the major fields of nuclear medicine practice. Myocardial perfusion studies using single-photon emission computed tomography (SPECT) have played a crucial role in the management of coronary artery diseases. Positron emission tomography (PET) has also been considered an important tool for the assessment of myocardial viability and perfusion. However, the recent development of computed tomography (CT)/magnetic resonance imaging (MRI) technologies and growing concerns about the radiation exposure of patients remain serious challenges for nuclear cardiology. In response to these challenges, remarkable achievements and improvements are currently in progress in the field of myocardial perfusion imaging regarding the applicable software and hardware. Additionally, myocardial perfusion positron emission tomography (PET) is receiving increasing attention owing to its unique capability of absolute myocardial blood flow estimation. An F-18-labeled perfusion agent for PET is under clinical trial with promising interim results. The applications of F-18 fluorodeoxyglucose (FDG) and F-18 sodium fluoride (NaF) to cardiovascular diseases have revealed details on the basic pathophysiology of ischemic heart diseases. PET/MRI seems to be particularly promising for nuclear cardiology in the future. Restrictive diseases, such as cardiac sarcoidosis and amyloidosis, are effectively evaluated using a variety of nuclear imaging tools. Considering these advances, the current challenges of nuclear cardiology will become opportunities if more collaborative efforts are devoted to this exciting field of nuclear medicine.

The impact of system matrix dimension on small FOV SPECT reconstruction with truncated projections

PURPOSE

A dedicated cardiac hybrid single photon emission computed tomography (SPECT)/CT scanner that uses cadmium zinc telluride detectors and multiple pinhole collimators for stationary acquisition offers many advantages. However, the impact of the reconstruction system matrix (SM) dimension on the reconstructed image quality from truncated projections and 19 angular samples acquired on this scanner has not been extensively investigated. In this study, the authors aimed to investigate the impact of the dimensions of SM and the use of body contour derived from adjunctive CT imaging as an object support in reconstruction on this scanner, in relation to background extracardiac activity.

METHODS

The authors first simulated a generic SPECT/CT system to image four NCAT phantoms with various levels of extracardiac activity and compared the reconstructions using SM in different dimensions and with/without body contour as a support for quantitative evaluations. The authors then compared the reconstructions of 18 patient studies, which were acquired on a GE Discovery NM570c scanner following injection of different radiotracers, including (99m)Tc-Tetrofosmin and (123)I-mIBG, comparing the scanner's default SM that incompletely covers the body with a large SM that incorporates a patient specific full body contour.

RESULTS

The simulation studies showed that the reconstructions using a SM that only partially covers the body yielded artifacts on the edge of the field of view (FOV), overestimation of activity and increased nonuniformity in the blood pool for the phantoms with higher relative levels of extracardiac activity. However, the impact on the quantitative accuracy in the high activity region, such as the myocardium, was subtle. On the other hand, an excessively large SM that enclosed the entire body alleviated the artifacts and reduced overestimation in the blood pool, but yielded slight underestimation in myocardium and defect regions. The reconstruction using the larger SM with body contour yielded the most quantitatively accurate results in all the regions of interest for a range of uptake levels in the extracardiac regions. In patient studies, the SM incorporating patient specific body contour minimized extracardiac artifacts, yielded similar myocardial activity, lower blood pool activity, and subsequently improved myocardium-to-blood pool contrast (p < 0.0001) by an average of 7% (range 0%-18%) across all the patients, compared to the reconstructions using the scanner's default SM.

CONCLUSIONS

Their results demonstrate that using a large SM that incorporates a CT derived body contour in the reconstruction could improve quantitative accuracy within the FOV for clinical studies with high extracardiac activity.

Anatomical-based partial volume correction for low-dose dedicated cardiac SPECT/CT

Due to the limited spatial resolution, partial volume effect has been a major degrading factor on quantitative accuracy in emission tomography systems. This study aims to investigate the performance of several anatomical-based partial volume correction (PVC) methods for a dedicated cardiac SPECT/CT system (GE Discovery NM/CT 570c) with focused field-of-view over a clinically relevant range of high and low count levels for two different radiotracer distributions. These PVC methods include perturbation geometry transfer matrix (pGTM), pGTM followed by multi-target correction (MTC), pGTM with known concentration in blood pool, the former followed by MTC and our newly proposed methods, which perform the MTC method iteratively, where the mean values in all regions are estimated and updated by the MTC-corrected images each time in the iterative process. The NCAT phantom was simulated for cardiovascular imaging with (99m)Tc-tetrofosmin, a myocardial perfusion agent, and (99m)Tc-red blood cell (RBC), a pure intravascular imaging agent. Images were acquired at six different count levels to investigate the performance of PVC methods in both high and low count levels for low-dose applications. We performed two large animal in vivo cardiac imaging experiments following injection of (99m)Tc-RBC for evaluation of intramyocardial blood volume (IMBV). The simulation results showed our proposed iterative methods provide superior performance than other existing PVC methods in terms of image quality, quantitative accuracy, and reproducibility (standard deviation), particularly for low-count data. The iterative approaches are robust for both (99m)Tc-tetrofosmin perfusion imaging and (99m)Tc-RBC imaging of IMBV and blood pool activity even at low count levels. The animal study results indicated the effectiveness of PVC to correct the overestimation of IMBV due to blood pool contamination. In conclusion, the iterative PVC methods can achieve more accurate quantification, particularly for low count cardiac SPECT studies, typically obtained from low-dose protocols, gated studies, and dynamic applications.

The effect of object size on the sensitivity of single photon emission computed tomography: comparison of two CZT cardiac cameras and an Anger scintillation camera

Background

Heart sizes vary greatly across the spectrum of patients referred for myocardial perfusion imaging. We therefore performed a phantom study to explore under controlled circumstances how count rates change when different volumes containing the same amount of activity are scanned. Two dedicated cadmium-zinc-telluride cameras, the D-SPECT (Spectrum Dynamics, Caesarea, Israel) and Discovery 530c (D530c, GE Healthcare, Haifa, Israel), and the conventional SPECT Anger (A-SPECT, GE Healthcare, Haifa, Israel) camera are included in the study.

Methods

Different heart sizes were represented by syringes of various column heights mimicking a range of cardiac diameters. Syringes with fixed activity were scanned at five different volumes by successively adding non-radioactive water to the syringes. This procedure was repeated five times on each of the three cameras. Raw count rates were recorded for each scan to determine whether count rates changed with syringe column height.

Results

Using mixed-effect regression modeling, a linear relationship was found between count rate and water column height. For the D-SPECT, D530c, and A-SPECT, the changes in count rate for each centimeter increase in water column height were −1.75, +0.28, and −0.022 kilocounts per min per MBq, respectively (95% confidence intervals −1.89 to −1.61, 0.19 to 0.36, and −0.035 to −0.009); all effects are significantly different from each other and significantly different from zero. Average coefficients of variation were 0.080, 0.028, and 0.009.

Conclusions

The D-SPECT demonstrated a significant progressive increase in count rate related to decreasing size of the imaged object. D530c count rate increased slightly with increasing column height. The Anger SPECT showed minimally increased count rates with decreasing column height, an order of magnitude smaller than the D-SPECT based on their relative coefficients of variation.

Electronic supplementary material

The online version of this article (doi:10.1186/s40658-014-0097-5) contains supplementary material, which is available to authorized users.