Site qualification and clinical interpretation standards for 99mTc-SPECT perfusion imaging in a multi-center study of MITNEC (Medical Imaging Trials Network of Canada)

BACKGROUND

Qualification and interpretation standards are essential for establishing 99mTc-SPECT MPI accuracy vs. alternative modalities.

METHODS

Rest-stress 99mTc-SPECT phantom scans were acquired on 35 cameras. LV defects were quantified with summed stress (SSS) and difference scores (SDS) at 2 core labs. SDS ≥ 2 in the right coronary artery (RCA) was the qualifying standard. Twenty rest (R)-stress (S) patient images were acquired on qualified cameras and interpreted by core labs. Global scoring differences > 3 between labs or discordant clinical interpretations underwent review. Scoring, interpretation, image quality, and diagnostic parameter agreement were assessed.

RESULTS

Phantom scans: visual scoring confirmed RCA-ischemia on all cameras. Regional SSS, SDS agreement was moderate to very good: ICC-r = 0.57, 0.84. Patient scans: 90% of global SSS, 85% of SDS differences were ≤ 3. Regional SSS, SDS agreement: ICC-r = 0.87, 0.86, and global abnormal (SSS ≥ 4) and ischemic (SDS ≥ 2) interpretation: ICC-r = 0.90 were excellent. Clinical interpretation agreement was 100% following review. Image quality agreement was 70%. Automated metrics also agreed: ischemic total perfusion deficit ICC-r = 0.75, reversible perfusion defect, transient ischemic dilation, and S-R LV ejection fraction ICC-r ≥ 0.90.

CONCLUSION

Quantitative scoring and interpretation of scans were highly repeatable with site qualification and clinical interpretation standardization, indicating that dual-core lab interpretation is appropriate to determine 99mTc-SPECT MPI accuracy.

Reproducible Quantification of Regional Sympathetic Denervation with [11C]meta-Hydroxyephedrine PET Imaging

BACKGROUND

Regional cardiac sympathetic denervation is predictive of sudden cardiac arrest in patients with ischemic cardiomyopathy. The reproducibility of denervation scores between automated software programs has not been evaluated. This study seeks to (1) compare the inter-rater reliability of regional denervation measurements using two analysis programs: FlowQuant® and Corridor4DM®; (2) evaluate test-retest repeatability of regional denervation scores.

METHODS

N = 190 dynamic [11C]meta-hydroxyephedrine (HED) PET scans were reviewed from the PAREPET trial in ischemic cardiomyopathy patients with reduced left ventricular ejection fraction(LVEF ≤ 35%). N = 12 scans were excluded due to non-diagnostic quality. N = 178 scans were analyzed using FlowQuant and Corridor4DM software, each by two observers. Test-retest scans from N = 20 patients with stable heart failure were utilized for test-retest analysis. Denervation scores were defined as extent × severity of relative uptake defects in LV regions with < 75% of maximal uptake. Results were evaluated using intraclass correlation coefficient (ICC) and Bland-Altman coefficient of repeatability (RPC).

RESULTS

Inter-observer, inter-software, and test-retest ICC values were excellent (ICC = 94% to 99%) and measurement variability was small (RPC < 11%). Mean differences between observers ranged .2% to 1.1% for Corridor4DM (P = .28), FlowQuant (P < .001), and between software programs (P < .001). Kaplan-Meier analysis demonstrated HED scores from both programs were predictive of SCA.

CONCLUSION

Inter-rater reliability for both analysis programs was excellent and test-retest repeatability was consistent. The minimal difference in scores between FlowQuant and Corridor4DM supports their use in future trials.

Increased myocardial oxygen consumption rates are associated with maladaptive right ventricular remodeling and decreased event-free survival in heart failure patients

BACKGROUND

Reduced left ventricular (LV) function is associated with increased myocardial oxygen consumption rate (MVO2) and altered sympathetic activity, the role of which is not well described in right ventricular (RV) dysfunction.

METHODS AND RESULTS

33 patients with left heart failure were assessed for RV function/size using echocardiography. Positron emission tomography (PET) was used to measure 11C-acetate clearance rate (kmono), 11C-hydroxyephedrine (11C-HED) standardized uptake value (SUV), and retention rate. RV MVO2 was estimated from kmono. 11C-HED SUV and retention indicated sympathetic neuronal function. A composite clinical endpoint was defined as unplanned cardiac hospitalization within 5 years. Patients with (n = 10) or without (n = 23) RV dysfunction were comparable in terms of sex (male: 70.0 vs 69.5%), LV ejection fraction (39.6 ± 9.0 vs 38.6 ± 9.4%), and systemic hypertension (70.0 vs 78.3%). RV dysfunction patients were older (70.9 ± 13.5 vs 59.4 ± 11.5 years; P = .03) and had a higher prevalence of pulmonary hypertension (60.0% vs 13.0%; P = .01). RV dysfunction was associated with increased RV MVO2 (.106 ± .042 vs .068 ± .031 mL/min/g; P = .02) and decreased 11C-HED SUV and retention (6.05 ± .53 vs 7.40 ± 1.39 g/mL (P < .001) and .08 ± .02 vs .11 ± .03 mL/min/g (P < .001), respectively). Patients with an RV MVO2 above the median had a shorter event-free survival (hazard ratio = 5.47; P = .01). Patients who died within the 5-year follow-up period showed a trend (not statistically significant) for higher RV MVO2 (.120 ± .026 vs .074 ± .038 mL/min/g; P = .05).

CONCLUSIONS

RV dysfunction is associated with increased oxygen consumption (also characterized by a higher risk for cardiac events) and impaired RV sympathetic function.

Validation of multiparametric rubidium-82 PET myocardial blood flow quantification for cardiac allograft vasculopathy surveillance

BACKGROUND

We previously demonstrated high diagnostic accuracy of Rubidium-82 positron emission tomography (PET) myocardial blood flow (MBF) quantification for CAV. The purpose of this study was to validate multiparametric PET detection of CAV by combined rate-pressure-product-corrected myocardial flow reserve (cMFR), stress MBF, and coronary vascular resistance (CVR) assessment.

METHODS AND RESULTS

Diagnostic CAV cut-offs of cMFR < 2.9, stress MBF < 2.3, CVR > 55 determined in a previous study (derivation) were assessed in heart transplant recipients referred for coronary angiography and intravascular ultrasound (IVUS) (validation). CAV was defined as International Society of Heart and Lung Transplantation CAV1-3 on angiography; and maximal intimal thickness ≥ 0.5 mm on IVUS. Eighty patients (derivation n = 40, validation n = 40) were included: 80% male, mean age 54±14 years, 4.5±5.6 years post transplant. The prevalence of CAV was 44% on angiography and 78% on IVUS. Combined PET cMFR < 2.9, stress MBF < 2.3, CVR > 55 CAV assessment yielded high 88% (specificity 75%) and 83% (specificity 40%) sensitivity for ≥ 1 abnormal parameter and high 88% (sensitivity 59%) and 90% (sensitivity 43%) specificity for 3 abnormal parameters, in the derivation and validation cohorts, respectively.

CONCLUSION

We validate the diagnostic accuracy of multiparametric PET flow quantification by cMFR, stress MBF, and CVR for CAV.

Internal validation of myocardial flow reserve PET imaging using stress/rest myocardial activity ratios with Rb-82 and N-13-ammonia

BACKGROUND

Myocardial flow reserve (MFR) measurement provides incremental diagnostic and prognostic information. The objective of the current study was to investigate the application of a simplified model for the estimation of MFR using only the stress/rest myocardial activity ratio (MAR) in patients undergoing rest-stress cardiac PET MPI.

METHODS AND RESULTS

Rest and dipyridamole stress dynamic PET imaging was performed in consecutive patients using 82Rb or 13NH3 (n = 250 each). Reference standard MFR was quantified using a standard one-tissue compartment model. Stress/rest myocardial activity ratio (MAR) was calculated using the LV-mean activity from 2 to 6 minutes post-injection. Simplified estimates of MFR (MFREST) were then calculated using an inverse power function. For 13NH3, there was good correlation between MFR and MFREST values (R = 0.63), with similar results for 82Rb (R = 0.73). There was no bias in the MFREST values with either tracer. The overall diagnostic performance of MFREST for detection of MFR < 2 was good with ROC area under the curve (AUC) = 83.2 ± 1.2% for 13NH3 and AUC = 90.4 ± 0.7% for 82Rb.

CONCLUSION

MFR was estimated with good accuracy using 82Rb and 13NH3 with a simplified method that relies only on stress/rest activity ratios. This novel approach does not require dynamic imaging or tracer kinetic modeling. It may be useful for routine quality assurance of PET MFR measurements, or in scanners where full dynamic imaging and tracer kinetic modeling is not feasible for technical or logistical reasons.

Positron Emission Tomography Imaging of Regional Versus Global Myocardial Sympathetic Activity to Improve Risk Stratification in Patients With Ischemic Cardiomyopathy

BACKGROUND

Current risk assessment approaches fail to identify the majority of patients at risk of sudden cardiac arrest (SCA). Noninvasive imaging of the cardiac sympathetic nervous system using single-photon emission computed tomography and positron emission tomography offers the potential for refining SCA risk assessment. While various [11C]meta-hydroxyephedrine quantification parameters have been proposed, it is currently unknown whether regional denervation or global innervation yields greater SCA risk discrimination. The aim of the study was to determine whether the global innervation parameters yield any independent and additive prognostic value over the regional denervation alone.

METHODS

In a post hoc competing-risks analysis of the PAREPET trial (Prediction of Arrhythmic Events With Positron Emission Tomography), we compared global innervation and regional denervation parameters using the norepinephrine analog [11C]meta-hydroxyephedrine for SCA risk discrimination. Patients with ischemic cardiomyopathy (n=174) eligible for an implantable cardioverter-defibrillator for the primary prevention of SCA were recruited into the trial. [11C]meta-hydroxyephedrine uptake and clearance rates were measured to assess global (left ventricle mean) retention index and volume of distribution. Regional defects were quantified as the percentage of the left ventricle having values <75% of the maximum.

RESULTS

During a median follow-up of 4.2 years, there were 56 cardiac-related deaths, of which 26 were SCAs. For any given regional denervation volume, there was substantial heterogeneity in global innervation scores. Global retention index and distribution volume did not decrease until regional defects exceeded 40% left ventricle. Global scale parameters, retention index, and distribution volume (area under the curve=0.61, P=0.034, P=0.046, respectively), yielded inferior SCA risk discrimination compared to regional heterogeneity (area under the curve=0.74).

CONCLUSIONS

Regional denervation volume has superior cause-specific mortality prediction for SCA versus global parameters of sympathetic innervation. These results have widespread implications for future cardiac sympathetic imaging, which will greatly simplify innervation analysis. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT01400334.

[11C]meta-hydroxyephedrine PET evaluation in experimental pulmonary arterial hypertension: Effects of carvedilol of right ventricular sympathetic function

BACKGROUND

Little is known about the sequelae of chronic sympathetic nervous system (SNS) activation in patients with pulmonary arterial hypertension (PAH) and right heart failure (RHF). We aimed to, (1) validate the use of [11C]-meta-hydroxyephedrine (HED) for assessing right ventricular (RV) SNS integrity, and (2) determine the effects of β-receptor blockade on ventricular function and myocardial SNS activity in a PAH rat model.

METHODS

PAH was induced in male Sprague-Dawley rats (N = 36) using the Sugen+chronic hypoxia model. At week 5 post-injection, PAH rats were randomized to carvedilol (15 mg·kg-1·day-1 oral; N = 16) or vehicle (N = 16) for 4 weeks. Myocardial SNS function was assessed with HED positron emission tomography(PET).

RESULTS

With increasing PAH disease severity, immunohistochemistry confirmed selective sympathetic denervation within the RV and sparing of parasympathetic nerves. These findings were confirmed on PET with a significant negative relationship between HED volume of distribution(DV) and right ventricular systolic pressure (RVSP) in the RV (r = -0.90, p = 0.0003). Carvedilol did not reduce hemodynamic severity compared to vehicle. RV ejection fraction (EF) was lower in both PAH groups compared to control (p < 0.05), and was not further reduced by carvedilol. Carvedilol improved SNS function in the LV with significant increases in the HED DV, and decreased tracer washout in the LV (p < 0.05) but not RV.

CONCLUSIONS

PAH disease severity correlated with a reduction in HED DV in the RV. This was associated with selective sympathetic denervation. Late carvedilol treatment did not lead to recovery of RV function. These results support the role of HED imaging in assessing SNS innervation in a failing right ventricle.

Persistent Lung Inflammation After Clinical Resolution of Community-Acquired Pneumonia as Measured by 18FDG-PET/CT Imaging

BACKGROUND

Survivors of community-acquired pneumonia (CAP) are at increased risk of cardiovascular disease, cognitive and functional decline, and death, but the mechanisms remain unknown.

RESEARCH QUESTION

Do CAP survivors have evidence of increased inflammatory activity in their lung parenchyma on 2-deoxy-2-[¹⁸F]fluoro-d-glucose (¹⁸FDG)-PET/CT imaging after clinical resolution of infection?

STUDY DESIGN AND METHODS

We obtained ¹⁸FDG-PET/CT scans from 22 CAP survivors during their hospitalization with pneumonia (acute CAP) and 30 to 45 days after hospital discharge (post-CAP). In each set of scans, we assessed the lungs for foci of increased ¹⁸FDG uptake by visual interpretation and by total pulmonary glycolytic activity (tPGA), a background-corrected measure of total metabolic activity (as measured by ¹⁸FDG uptake). We also measured, post-CAP, the glycolytic activity of CAP survivor lung areas with volumes similar to the areas in 28 matched historical control subjects without pneumonia.

RESULTS

Overall, 68% of CAP survivors (95% CI, 45%-85%) had distinct residual areas of increased ¹⁸FDG uptake in their post-CAP studies. tPGA decreased from 821.5 (SD, 1,140.2) in the acute CAP period to 80.0 (SD, 81.4) in the post-CAP period (P = .006). The tPGA post-CAP was significantly higher than that in lung areas of similar volume in control subjects (80.0 [SD, 81.4] vs -19.4 [SD, 5.9]; P < .001).

INTERPRETATION

An important proportion of CAP survivors have persistent pulmonary foci of increased inflammatory activity beyond resolution of their infection. As inflammation contributes to cardiovascular disease, cognitive decline, functional waning, and mortality risk in the general population, this finding provides a plausible mechanism for the increased morbidity and mortality that have been observed post-CAP.

Comparison of myocardial blood flow and flow reserve with dobutamine and dipyridamole stress using rubidium-82 positron emission tomography

BACKGROUND

The objective of this study was to compare the hyperemic myocardial blood flow (MBF) and myocardial flow reserve (MFR) obtained with dobutamine to those of dipyridamole in patients referred for myocardial perfusion imaging (MPI) using 82Rb positron emission tomography.

METHODS

One hundred and fifty-six patients who underwent a 82Rb PET MPI study with dobutamine stress were included. A matching cohort of patients who underwent a 82Rb PET MPI study with dipyridamole stress was created, accounting for sex, age, history of coronary artery disease (CAD), prior revascularization, CAD risk factors, body mass index, and MPI interpretation.

RESULTS

Global rest MBF (median [interquartile range] 0.84 [0.64-1.00] vs 0.69 [0.59-0.85]), stress MBF (2.36 [1.73-3.08] vs 1.66 [1.25-2.06]), MFR (2.75 [2.19-3.64] vs 2.29 [1.78-2.84]), and corrected MFR (2.85 [2.14-3.64] vs 2.20 [1.65-2.75]) were all significantly higher (P < 0.0001) in the dobutamine cohort compared to the dipyridamole cohort.

CONCLUSION

The results of this study suggest that dobutamine produces higher MBF compared to dipyridamole in a representative population referred to nuclear cardiology laboratories.

Quantitative blood flow evaluation of vasodilation-stress compared with dobutamine-stress in patients with end-stage liver disease using 82Rb PET/CT

BACKGROUND

Our aim was to determine if end-stage liver disease (ESLD) is associated with an attenuated response to vasodilator-stress or dobutamine-stress using 82Rb-PET MPI with blood flow quantification.

METHODS AND RESULTS

Pre-liver transplant patients who had a normal dipyridamole-stress (n = 27) or dobutamine-stress (n = 26) 82Rb PET/CT MPI study with no identifiable coronary artery calcium were identified retrospectively and compared to a prospectively identified low-risk of liver disease dipyridamole-stress control group (n = 20). The dipyridamole-stress liver disease group had a lower myocardial flow reserve (MFR) (1.89 ± 0.79) than the control group (2.79 ± 0.96, P < .05). The dobutamine-stress group had a higher MFR than both other groups (3.69 ± 1.49, P < .05). A moderate negative correlation between MELD score and MFR was demonstrated for the dipyridamole-stress liver disease group (r = - 0.473, P < .05). This correlation was not observed for the dobutamine-stress liver disease group (r = - 0.253, P = .21). The liver failure group as a whole (n = 53) had a higher resting myocardial blood flow (0.97 ± 0.33 mL/min/g) than the control group (0.82 ± 0.26, P < .05).

CONCLUSION

Dipyridamole demonstrates an attenuated vasodilatory response in ESLD patients compared to a non-ESLD control group related to higher resting blood flow and comparatively reduced stress blood flow. Dobutamine does not demonstrate this effect implying it may be the preferred pharmacologic MPI stress agent for ESLD patients.

Effect of proton pump inhibitors on Rubidium-82 gastric uptake using positron emission tomography myocardial perfusion imaging

BACKGROUND

Rb-82 positron emission tomography (PET) myocardial perfusion imaging (MPI) is a robust tool for the evaluation of coronary artery disease (CAD). However, gastric uptake and spillover can be seen in 10% of Rb-82 PET MPI studies, commonly affecting the inferior wall, and can preclude the accurate identification of myocardial ischemia. We sought to understand the relationship between Rb-82 gastric uptake and the use of proton pump inhibitors (PPI).

METHODS

600 consecutive patients who presented for a clinically indicated Rb-82 PET MPI study were prospectively enrolled. In addition to the clinical history, PPI use was ascertained (medication, dose, frequency and duration of use, and time of last dose). Patients were categorized as PPI and non-PPI users. Rb-82 uptake in the gastrium, myocardium, and liver were measured at rest. Absolute uptake values and gastric:hepatic ratios were compared in PPI and non-PPI users.

RESULT

Of 600 enrolled patients, 181 (30.2%) patients were using PPI. The gastric Rb-82 uptake in PPI users was 23% higher than non-PPI users (146 ± 52 kBq/cc vs 119 ± 40 kBq/cc, respectively; P < 0.001). The resting gastric:hepatic Rb-82 uptake ratio was also 23% higher in PPI vs non-PPI users (2.7 ± 1.0 vs 2.2 ± 0.8, respectively; P < 0.001).

CONCLUSION

The gastric uptake of Rb-82 appears to be greater in patients actively using PPI and may identify a group who might be at greater risk of non-diagnostic Rb-82 PET MPI.

Rubidium-82 generator yield and efficiency for PET perfusion imaging: Comparison of two clinical systems

INTRODUCTION

Strontium-82/Rubidium-82 (82Sr/82Rb) generators are used widely for positron emission tomography (PET) imaging of myocardial perfusion. In this study, the 82Rb isotope yield and production efficiency of two FDA-approved 82Sr/82Rb generators were compared.

METHODS

N = 515 sequential daily quality assurance (QA) reports from 9 CardioGen-82® and 9 RUBY-FILL® generators were reviewed over a period of 2 years. A series of test elutions was performed at different flow-rates on the RUBY-FILL® system to determine an empirical correction-factor used to convert CardioGen-82® daily QA values of 82Rb activity (dose-calibrator 'maximum' of 50 mL elution at 50 mL·min-1) to RUBY-FILL® equivalent values (integrated 'total' of 35 mL elution at 20 mL·min-1). The generator yield (82Rb) and production efficiency (82Rb yield/82Sr parent activity) were measured and compared after this conversion to a common scale.

RESULTS

At the start of clinical use, the system reported 82Rb activity from daily QA was lower for CardioGen-82® vs RUBY-FILL® (2.3 ± 0.2 vs 3.0 ± 0.2 GBq, P < 0.001) despite having similar 82Sr activity. Dose-calibrator 'maximum' (CardioGen-82®) values were found to under-estimate the integrated 'total' (RUBY-FILL®) activity by ~ 24% at 50 mL·min-1. When these data were used to convert the CardioGen-82 values to a common measurement scale (integrated total activity) the CardioGen-82® efficiency remained slightly lower than the RUBY-FILL® system on average (88 ± 4% vs 95 ± 4%, P < 0.001). The efficiency of 82Rb production improved for both systems over the respective periods of clinical use.

CONCLUSIONS

82Rb generator yield was significantly under-estimated using the CardioGen-82® vs RUBY-FILL® daily QA procedure. When generator yield was expressed as the integrated total activity for both systems, the estimated 82Rb production efficiency of the CardioGen-82® system was ~ 7% lower than RUBY-FILL® over the full period of clinical use.

Reproducibility of cardiac magnetic resonance imaging in patients referred for the assessment of cardiac sarcoidosis; implications for clinical practice

Cardiac sarcoidosis (CS) is an increasingly recognized condition, but cardiac magnetic resonance (CMR) image interpretation in these patients may be challenging as findings are often non-specific. The main objective of this study was to investigate the inter-reader agreement for the overall interpretation of CMR for the diagnosis of CS in an experienced reference center and investigate factors that may lead to discrepancies between readers. Consecutive patients undergoing CMR imaging to investigate for CS were included. CMR images were independently reviewed by two readers, blinded to all clinical, imaging and demographic information. The readers classified each scan as "consistent with cardiac sarcoidosis", "not consistent with cardiac sarcoidosis" or "indeterminate". Inter-reader agreement was assessed using κ-statistics. When there was disagreement on the overall interpretation, a third reader reviewed the images. Also, two readers independently commented on the presence of edema, presence of LGE (both ventricles) and quantified the extent of left ventricular LGE. 87 patients (43 women, mean age 54.3 ± 12.2 years) were included in the study. There was agreement regarding the overall interpretation in 72 of 87 (83%) CMR scans. The κ value was 0.64, indicating moderate agreement. There was similar moderate agreement in the interpretation of LGE parameters. In an experienced referral center, we found moderate agreement between readers in the interpretation of CMR in patients with suspected CS. Physicians should be aware of this inter-observer variability in interpretation of CMR studies in patients with suspected CS.

Motion tracking of low-activity fiducial markers using adaptive region of interest with list-mode positron emission tomography

PURPOSE

Motion compensated positron emission tomography (PET) imaging requires detecting and monitoring of patient body motion. We developed a semiautomatic list-mode method to track the three-dimensional (3D) motion of fiducial positron-emitting markers during PET imaging.

METHODS

A previously developed motion tracking method using positron-emitting markers (PeTrack) was enhanced to work with PET imaging. A novel combination of filtering methods was developed to reject physiological tracer background, which would drown out the events from the marker if unfiltered. The most critical filter rejects events whose line-of-response (LOR) is outside an adaptive region of interest (ADROI). The size of ROI was optimized by exploiting the distinct differences between the distributions of events from background and marker. The ADROI PeTrack method was evaluated with Monte Carlo and phantom studies. A 92.5-kBq 22 Na marker moving sinusoidally in 3D was simulated with Monte Carlo methods. The simulated events were combined with list-mode data from cardiac PET imaging patients to evaluate the performance of the tracking. In phantom studies, three 22 Na markers were placed on a dynamic torso phantom with an initial activity of 680 MBq of 82 Rb in its cardiac insert. The motion of the markers was tracked while the phantom simulated various types of patient motion. Motion correction on an event-by-event basis of the list-mode data was then applied and images were reconstructed.

RESULTS

Simulation results show that the background rejection methods can significantly suppress the tracer background and increase the fraction of marker events by a factor of up to 2500. A 92.5-kBq marker can be tracked in 3D at a frequency of 2.0 Hz with an accuracy of 0.8 mm and a precision of 0.3 mm. The phantom study experimentally confirms that the algorithm can track various types of motion. The relative accuracy of the experimental tracking is 0.26 ± 0.14 mm. Motion-corrected images from the phantom study show reduced blurring.

CONCLUSIONS

An algorithm and background rejection methods were developed that can track the 3D motion of low-activity positron-emitting markers during PET imaging. The motion information may be used for motion-compensated PET imaging.

Correction to: PET imaging of sympathetic innervation with [18F]Flubrobenguane vs [11C]mHED in a patient with ischemic cardiomyopathy

At time of initial publication, the USAN Council had assigned the generic name for LMI1195 as Flurobenguan. However, the Council has since changed and finalized this compound name as Flubrobenguane which is recommended as the generic name to be used in the future.

PET and SPECT Tracers for Myocardial Perfusion Imaging

Coronary artery disease has been the leading cause of death since the 1960s, which has motivated the research and development of myocardial perfusion imaging (MPI) agents for early diagnosis and to guide treatment. MPI with SPECT has been the clinical workhorse for MPI, but over the past two decades PET MPI is experiencing growth due to enhanced image quality that results in superior diagnostic accuracy over SPECT. Furthermore, dynamic PET imaging of the tracer distribution process from time of tracer administration to tracer accumulation in the myocardium has enabled routine quantification of myocardial blood flow (MBF) and myocardial flow reserve (MFR) in absolute units. MBF and MFR incrementally improve diagnostic and prognostic accuracy over MPI alone. In some cases (eg, rubidium PET imaging with pharmacologic stress) MPI, MBF, and MFR can be acquired simultaneously without incremental cost, radiation exposure, or significant processing time. Nuclear cardiology clinics have been looking to incorporate MBF quantification into clinical routine, but traditional SPECT and MPI tracers are inadequate for this challenge. Cardiac dedicated SPECT scanners can also perform dynamic imaging and have stimulated research into MBF quantification using SPECT tracers. New perfusion tracers must be tailored for emerging clinical needs (including MBF quantification), technical capabilities of imaging instrumentation, market constraints, and supply chain feasibility. Because these conditions have been evolving, tracers previously considered inferior may be reconsidered for future applications and some recently developed tracers may be suboptimal. This article reviews current, clinically-available tracers and those under development showing greatest potential. It discusses for each tracer the rationale for development, physiological mechanism of uptake by the myocardium, published evaluation results and development state. Finally, it gauges the suitability of each tracer for clinical application. The article demonstrates an acceleration in the pace of perfusion radiotracer development due to better understanding of the relevant physiology, better chemistry tools and small animal imaging. Consequently, bad tracers may fail faster and with less wasted investment, and good tracers may translate more efficiently from bench to bedside.

Clinical comparison of the positron emission tracking (PeTrack) algorithm with the real-time position management system for respiratory gating in cardiac positron emission tomography

PURPOSE

A data-driven motion tracking system was developed for respiratory gating in positron emission tomography (PET)/computed tomography (CT) studies. The positron emission tracking system (PeTrack) estimates the position of a low-activity fiducial marker placed on the patient during imaging. The aim of this study was to compare the performance of PeTrack against that of the real-time position management (RPM) system as applied to respiratory gating in cardiac PET/CT studies.

METHODS

The list-mode data of 35 patients that were referred for ⁸² Rb myocardial perfusion studies were retrospectively processed with PeTrack to generate respiratory motion signals and triggers. Fifty acquisitions from the initial cohort, conducted under physiologic rest and stress, were considered for analysis. Respiratory-gated reconstructions were performed using reconstruction software provided by the vendor. The respiratory signals and triggers of the gating systems were compared using quantitative measurements of the respiratory signal correlation, median, and interquartiles range (IQR) of observed respiratory rates and the relative frequencies of respiratory cycle outliers. Quantitative measurements of left-ventricular wall thicknesses and motion due to respiration were also compared. Real-time position management signals were also retrospectively processed using the trigger detection method of PeTrack for a third comparator ("RPMretro") that allowed direct comparison of the motion tracking quality independently of differences in the trigger detection methods. The comparison of PeTrack to the original RPM data represent a practical comparison of the two systems, whereas that of PeTrack and RPMretro represents an equal comparison of the two. Nongated images were also reconstructed to provide reference left-ventricular wall thicknesses. LV wall thickness and motion measurements were repeated for a subset of cases with motion ≥7 mm as image artifacts were expected to be more severe in these cases.

RESULTS

A significant correlation (P < 0.05) was observed between the RPM and PeTrack respiratory signals in 45/50 acquisitions; the mean correlation coefficient was 0.43. Similar results were found between PeTrack and RPMretro. No significant difference was observed between the RPM and PeTrack with respect to median respiratory rates and the percentage of respiratory cycles outliers. Respiratory rate variability (IQR) was significantly higher with PeTrack vs RPM (P = 0.002) and RPMretro (P = 0.04). Both PeTrack and RPM had a significant increase in the percentage of respiratory rate outliers compared to RPMretro (P < 0.001 and P = 0.001, respectively). All methods indicated significant differences in LV thickness compared to nongated images (P < 0.02). LV thickness was significantly larger for PeTrack compared to RPMretro in the highest motion subset (P = 0.009). Images gated with RPMretro showed significant increases in motion compared to both PeTrack (P < 0.001) and prospective RPM (P = 0.002). In the subset of highest motion cases, the difference between RPM and RPMretro was no longer present.

CONCLUSIONS

The data-driven PeTrack algorithm performed similarly to the well-established RPM system for respiratory gating of ⁸² Rb cardiac perfusion PET/CT studies. Real-time position management performance improved after retrospective processing and led to enhanced performance compared to both PeTrack and prospective RPM. With further development PeTrack has the potential to reduce the need for ancillary hardware systems to monitor respiratory motion.

Kernel-Based Reconstruction of C-11-Hydroxyephedrine Cardiac PET Images of the Sympathetic Nervous System

Image reconstruction for positron emission tomography (PET) can be challenging and the resulting image typically has high noise. The kernel-based reconstruction method [1], incorporates prior anatomic information in the reconstruction algorithm to reduce noise while preserving resolution. Prior information is incorporated in the reconstruction algorithm by means of spatial kernels originally used in machine learning. In this paper, the kernel-based method is used to reconstruct PET images of sympathetic innervation in the heart. The resulting images are compared with standard Ordered Subset Expectation Maximization (OSEM) reconstructed images qualitatively and quantitatively using data from 6 human subjects. The kernel-based method demonstrated superior SNR with preserved contrast and accuracy compared to OSEM.

Phase analysis of gated PET in the evaluation of mechanical ventricular synchrony: A narrative overview

Noninvasive imaging modalities offer the possibility to dynamically evaluate cardiac motion during the cardiac cycle by means of ECG-gated acquisitions. Such motion characterization along with orientation, segmentation preprocessing, and ultimately, phase analysis, can provide quantitative estimates of ventricular mechanical synchrony. Current evidence on the role of mechanical synchrony evaluation is mainly available for echocardiography and gated single-photon emission computed tomography, but less is known about the utilization of gated positron emission tomography (PET). Although data available are sparse, there is indication that mechanical synchrony evaluation can be of diagnostic and prognostic values in patients with known or suspected coronary artery disease-related myocardial ischemia, prediction of response to cardiac resynchronization therapy, and estimation of risk for adverse cardiac events in patients' heart failure. As such, the evaluation of mechanical ventricular synchrony through phase analysis of gated acquisitions represents a value addition to modern cardiac PET imaging modality, which warrants further research and development in the evaluation of patients with cardiovascular disease.