Amir Pourmoghaddas

Respiratory phase alignment improves blood‐flow quantification in Rb82 PET myocardial perfusion imaging

PURPOSE Positron emission tomography (PET) is considered the gold standard for measuring myocardial blood flow in vivo but it is known that respiratory motion can lead to misalignment of the PET and computed tomography (CT) data sets and introduce artifacts in the CT-based attenuation correction (AC) of images. In addition, respiratory motion blurs the PET image and degrades spatial resolution. The purpose of this study is to evaluate the combined effect of respiratory motion compensation (MC) and accurate attenuation correction on relative and absolute blood flow imaging of the heart. METHODS Dynamic (82)Rb-PET acquisitions were generated for a homogeneous tracer distribution in the heart using an anthropomorphic computer phantom and a Monte Carlo simulator. Attenuation correction was done using three different approaches in which the PET data were corrected by: (1) a respiratory-gated CT map with each respiratory phase of the PET scan corrected by its corresponding CT phase (matched); (2) a time-averaged attenuation map (avg); or (3) an attenuation map generated from the maximum CT-number of every voxel over the respiratory cycle (max). Motion compensated was done using an automated rigid-body registration algorithm that aligned all of the phases of the respiratory-gated PET data after AC. The corrected dynamic PET data were then processed by inhouse kinetic analysis software to generate 3D maps of blood flow. Polar maps of the blood-flow for each CT-AC method with and without MC were compared to the truth using a 17-segment model. The same comparison was performed on data from a pig study. RESULTS Motion compensation significantly reduced the segmental mean percentage error (sMPE) in all cases (p < 0.01 for matched CTAC and avg CTAC and p = 0.03 for max CTAC). MC significantly increased image uniformity in the case of matched and avg CTAC (p < 0.01, p = 0.04, respectively) with the best improvement coming for matched CTAC. Without MC, there were no significant differences between the three CTAC approaches. With MC, matched CTAC had significantly smaller mean absolute sMPE (p < 0.01 vs avg CTAC; p < 0.01 vs max CTAC) and improved uniformity (p = 0.05 vs avg CTAC; p < 0.01 vs max CTAC). The results were supported with a pig study. CONCLUSIONS Without MC, there was no significant difference between the three CTAC methods for measuring blood flow. With MC, the matched CTAC approach was significantly better, reducing the mean difference from truth by 6% in the simulated data and improving uniformity by 5%.

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.

Validation of a Multimodality Flow Phantom and Its Application for Assessment of Dynamic SPECT and PET Technologies

Simple and robust techniques are lacking to assess performance of flow quantification using dynamic imaging. We therefore developed a method to qualify flow quantification technologies using a physical compartment exchange phantom and image analysis tool. We validate and demonstrate utility of this method using dynamic PET and SPECT. Dynamic image sequences were acquired on two PET/CT and a cardiac dedicated SPECT (with and without attenuation and scatter corrections) systems. A two-compartment exchange model was fit to image derived time-activity curves to quantify flow rates. Flowmeter measured flow rates (20-300 mL/min) were set prior to imaging and were used as reference truth to which image derived flow rates were compared. Both PET cameras had excellent agreement with truth (r2> 0.94). High-end PET had no significant bias (p > 0.05) while lower-end PET had minimal slope bias (wash-in and wash-out slopes were 1.02 and 1.01) but no significant reduction in precision relative to high-end PET (<;15% vs. <;14% limits of agreement, p > 0.3). SPECT (without scatter and attenuation corrections) slope biases were noted (0.85 and 1.32) and attributed to camera saturation in early time frames. Analysis of wash-out rates from non-saturated, late time frames resulted in excellent agreement with truth (r2= 0.98, slope = 0.97). Attenuation and scatter corrections did not significantly impact SPECT performance. The proposed phantom, software and quality assurance paradigm can be used to qualify imaging instrumentation and protocols for quantification of kinetic rate parameters using dynamic imaging.

Quantitative accuracy of SPECT imaging with a dedicated cardiac camera: Physical phantom experiments

Recently, there has been increased interest in dedicated cardiac SPECT scanners with multi-pinhole designs and improved detector technology. However, the pinhole collimator design introduces position-dependent attenuation, sensitivity and resolution variations. Variations in attenuation patterns and energy-spectrum responses can also inhibit accurate measurement of activity distributions. Simple correction methods for these effects are easily implemented however their level of accuracy is unclear. In this study, we assess the quantitative accuracy and reproducibility of absolute activity measurements made using easily implemented correction techniques applied to controlled physical phantom experiments.