Partial volume correction incorporating Rb-82 positron range for quantitative myocardial perfusion PET based on systolic-diastolic activity ratios and phantom measurements

Impaired coronary flow reserve in patients with supra-normal left ventricular ejection fraction at rest

PURPOSE

Recently, a "U" hazard ratio curve between resting left ventricular ejection fraction (LVEF) and prognosis has been observed in patients referred for routine clinical echocardiograms. The present study sought to explore whether a similar "U" curve existed between resting LVEF and coronary flow reserve (CFR) in patients without severe cardiovascular disease (CVD) and whether impaired CFR played a role in the adverse outcome of patients with supra-normal LVEF (snLVEF, LVEF ≥ 65%).

METHODS

Two hundred ten consecutive patients (mean age 52.3 ± 9.3 years, 104 women) without severe CVD underwent clinically indicated rest/dipyridamole stress electrocardiography (ECG)-gated 13 N-ammonia positron emission tomography/computed tomography (PET/CT). Major adverse cardiac events (MACE) were followed up for 27.3 ± 9.5 months, including heart failure, late revascularization, re-hospitalization, and re-coronary angiography for any cardiac reason. Clinical characteristics, corrected CFR (cCFR), and MACE were compared among the three groups categorized by resting LVEF detected by PET/CT. Dose-response analyses using restricted cubic spline (RCS) functions, multivariate logistic regression, and Kaplan-Meier survival analysis were conducted to evaluate the relationship between resting LVEF and CFR/outcome.

RESULTS

An inverted "U" curve existed between resting LVEF and cCFR (p = 0.06). Both patients with snLVEF (n = 38) and with reduced LVEF (rLVEF, LVEF < 55%) (n = 66) displayed a higher incidence of reduced cCFR than those with normal LVEF (nLVEF, 55% ≤ LVEF < 65%) (n = 106) (57.9% vs 54.5% vs 34.3%, p < 0.01, respectively). Both snLVEF (p < 0.01) and rLVEF (p < 0.05) remained independent predictors for reduced cCFR after multivariable adjustment. Patients with snLVEF encountered more MACE than those with nLVEF (10.5% vs 0.9%, log-rank p = 0.01).

CONCLUSIONS

Patients with snLVEF are prone to impaired cCFR, which may be related to the adverse prognosis. Further investigations are warranted to explore its underlying pathological mechanism and clinical significance.

Pitfalls in quantitative myocardial PET perfusion I: Myocardial partial volume correction

BACKGROUND

PET quantitative myocardial perfusion requires correction for partial volume loss due to one-dimensional LV wall thickness smaller than scanner resolution.

METHODS

We aimed to assess accuracy of risk stratification for death, MI, or revascularization after PET using partial volume corrections derived from two-dimensional ACR and three-dimensional NEMA phantoms for 3987 diagnostic rest-stress perfusion PETs and 187 MACE events. NEMA, ACR, and Tree phantoms were imaged with Rb-82 or F-18 for size-dependent partial volume loss. Perfusion and Coronary Flow Capacity were recalculated using different ACR- and NEMA-derived partial volume corrections compared by Kolmogorov-Smirnov statistics to standard perfusion metrics with established correlations with MACE.

RESULTS

Partial volume corrections based on two-dimensional ACR rods (two equal radii) and three-dimensional NEMA spheres (three equal radii) over estimate partial volume corrections, quantitative perfusion, and Coronary Flow Capacity by 50% to 150% over perfusion metrics with one-dimensional partial volume correction, thereby substantially impairing correct risk stratification.

CONCLUSIONS

ACR (2-dimensional) and NEMA (3-dimensional) phantoms overestimate partial volume corrections for 1-dimensional LV wall thickness and myocardial perfusion that are corrected with a simple equation that correlates with MACE for optimal risk stratification applicable to most PET-CT scanners for quantifying myocardial perfusion.

Pitfalls in quantitative myocardial PET perfusion II: Arterial input function

RATIONALE

We aimed to define the impact of variable arterial input function on myocardial perfusion severity that may misguide interventional decisions and relates to limited capacity of 3D PET for high-count arterial input function of standard bolus R-82.

METHODS

We used GE Discovery-ST 16 slice PET-CT, serial 2D and 3D acquisitions of variable Rb-82 dose in a dynamic circulating arterial function model, static resolution and uniformity phantoms, and in patients with dipyridamole stress to quantify per-pixel rest and stress cc·min-1·g-1, CFR and CFC with (+) and (-) 10% simulated change in arterial input.

RESULTS

For intermediate, border zone severity of stress perfusion, CFR and CFC comprising 7% of 3987 cases, simulated arterial input variability of ± 10% may cause over or underestimation of perfusion severity altering interventional decisions. In phantom tests, current 3D PET has capacity for quantifying high activity of arterial input and high-count per-pixel values of perfusion metrics per artery or branches.

CONCLUSIONS

Accurate, reproducible arterial input function is essential for at least 7% of patients at thresholds of perfusion severity for optimally guiding interventions and providing high-activity regional per-pixel perfusion metrics by 3D PET for displaying complex quantitative perfusion readily understood ("owned") by interventionalists to guide procedures.

The utility of 82Rb PET for myocardial viability assessment: Comparison with perfusion-metabolism 82Rb-18F-FDG PET

BACKGROUND

82Rb kinetics may distinguish scar from viable but dysfunctional (hibernating) myocardium. We sought to define the relationship between 82Rb kinetics and myocardial viability compared with conventional 82Rb and 18F-fluorodeoxyglucose (FDG) perfusion-metabolism PET imaging.

METHODS

Consecutive patients (N = 120) referred for evaluation of myocardial viability prior to revascularization and normal volunteers (N = 37) were reviewed. Dynamic 82Rb 3D PET data were acquired at rest. 18F-FDG 3D PET data were acquired after metabolic preparation using a standardized hyperinsulinemic-euglycemic clamp. 82Rb kinetic parameters K1, k2, and partition coefficient (KP) were estimated by compartmental modeling RESULTS: Segmental 82Rb k2 and KP differed significantly between scarred and hibernating segments identified by Rb-FDG perfusion-metabolism (k2, 0.42 ± 0.25 vs. 0.22 ± 0.09 min-1; P < .0001; KP, 1.33 ± 0.62 vs. 2.25 ± 0.98 ml/g; P < .0001). As compared to Rb-FDG analysis, segmental Rb KP had a c-index, sensitivity and specificity of 0.809, 76% and 84%, respectively, for distinguishing hibernating and scarred segments. Segmental k2 performed similarly, but with lower specificity (75%, P < .001) CONCLUSIONS: In this pilot study, 82Rb kinetic parameters k2 and KP, which are readily estimated using a compartmental model commonly used for myocardial blood flow, reliably differentiated hibernating myocardium and scar. Further study is necessary to evaluate their clinical utility for predicting benefit after revascularization.

Improving PET Quantification of Small Animal [68Ga]DOTA-Labeled PET/CT Studies by Using a CT-Based Positron Range Correction

PURPOSE

Image quality of positron emission tomography (PET) tracers that emits high-energy positrons, such as Ga-68, Rb-82, or I-124, is significantly affected by positron range (PR) effects. PR effects are especially important in small animal PET studies, since they can limit spatial resolution and quantitative accuracy of the images. Since generators accessibility has made Ga-68 tracers wide available, the aim of this study is to show how the quantitative results of [⁶⁸Ga]DOTA-labeled PET/X-ray computed tomography (CT) imaging of neuroendocrine tumors in mice can be improved using positron range correction (PRC).

PROCEDURES

Eighteen scans in 12 mice were evaluated, with three different models of tumors: PC12, AR42J, and meningiomas. In addition, three different [⁶⁸Ga]DOTA-labeled radiotracers were used to evaluate the PRC with different tracer distributions: [⁶⁸Ga]DOTANOC, [⁶⁸Ga]DOTATOC, and [⁶⁸Ga]DOTATATE. Two PRC methods were evaluated: a tissue-dependent (TD-PRC) and a tissue-dependent spatially-variant correction (TDSV-PRC). Taking a region in the liver as reference, the tissue-to-liver ratio values for tumor tissue (TLR_tumor), lung (TLR_lung), and necrotic areas within the tumors (TLR_necrotic) and their respective relative variations (ΔTLR) were evaluated.

RESULTS

All TLR values in the PRC images were significantly different (p < 0.05) than the ones from non-PRC images. The relative differences of the tumor TLR values, respect to the case with no PRC, were ΔTLR_tumor 87 ± 41 % (TD-PRC) and 85 ± 46 % (TDSV-PRC). TLR_lung decreased when applying PRC, being this effect more remarkable for the TDSV-PRC method, with relative differences respect to no PRC: ΔTLR_lung = - 45 ± 24 (TD-PRC), - 55 ± 18 (TDSV-PRC). TLR_necrotic values also decreased when using PRC, with more noticeable differences for TD-PRC: ΔTLR_necrotic = - 52 ± 6 (TD-PRC), - 48 ± 8 (TDSV-PRC).

CONCLUSION

The PRC methods proposed provide a significant quantitative improvement in [⁶⁸Ga]DOTA-labeled PET/CT imaging of mice with neuroendocrine tumors, hence demonstrating that these techniques could also ameliorate the deleterious effect of the positron range in clinical PET imaging.

The effect of coronary revascularization on regional myocardial blood flow as assessed by stress positron emission tomography

OBJECTIVES

We examined whether regional improvement in stress myocardial blood flow (sMBF) following angiography-guided coronary revascularization depends on the existence of a perfusion defect on positron emission tomography (PET).

BACKGROUND

Percent stenosis on coronary angiography often is the main factor when deciding whether to perform revascularization, but it does not reliably relate to maximum sMBF. PET is a validated method of assessing sMBF.

METHODS

19 patients (79% M, 65 ± 12 years) underwent PET stress before and after revascularization (17 PCI, 2 CABG). Pre- and post-revascularization sMBF for each left ventricular quadrant (anterior, septal, lateral, and inferior) was stratified by the presence or absence of a baseline perfusion defect on PET and whether that region was revascularized.

RESULTS

Intervention was performed on 40 of 76 quadrants. When a baseline perfusion defect existed in a region that was revascularized (n = 26), post-revascularization flow increased by 0.6 ± 0.7 cc/min/g (1.2 ± 0.4 vs 1.7 ± 0.8, P < 0.001). When no defect existed but revascularization was performed (n = 14), sMBF did not change significantly (1.7 ± 0.3 vs 1.5 ± 0.4 cc/min/g, P = 0.16). In regions without a defect that were not revascularized (n = 29), sMBF did not significantly change (2.0 ± 0.6 vs 1.9 ± 0.7, P = 0.7).

CONCLUSIONS

When a stress-induced perfusion defect exists on PET, revascularization improves sMBF in that region. When there is no such defect, sMBF shows no net change, whether or not intervention is performed in that area. PET stress may be useful for identifying areas of myocardium that could benefit from revascularization, and also areas in which intervention is unlikely to yield improvement in myocardial blood flow.

Rb-82 PET/CT left ventricular mass-to-volume ratios

Left ventricular (LV) mass:volume ratios indexed to body size (Mi/Vi) provide risk stratification for cardiac events. We sought to determine whether Rb-82 PET mass and volume indices are similar to MRI normal values for low likelihood subjects, and whether abnormal indices are related to abnormal myocardial blood flow (MBF). Data were analyzed retrospectively for 194 patients referred for rest/stress Rb-82 PET. LV EF, volume and mass values were calculated and mass:volume ratios were indexed to patients' height and weight. MBF was computed from the first pass dynamic component of PET data. 53 patients at low likelihood of CAD had PET Mi/Vi = 1.35 ± 0.27, consistent with the MRI literature range of 1.0-1.5. Compared to patients with normal indexed volume (Vi), patients with abnormally high Vi had lower rest MBF (0.56 ± 0.24 vs 0.93 ± 0.57 ml/g/min, p = 0.0001), and lower stress MBF (0.97 ± 0.52 vs. 1.83 ± 0.96 ml/g/min, p < 0.0001). Stress EF < 50% predicted abnormal Vi with 90% accuracy. Patients with Mi/Vi < 1.0 had abnormally low rest EF (45 ± 16% vs. 60 ± 15%, p < 0.0001) and low rest MBF (0.58 ± 0.25 vs. 0.96 ± 0.59 ml/g/min, p < 0.0001). In our study population, abnormal LV volume and mass correlated with lower rest and stress MBF and EF, suggesting that the pathophysiologic explanation of these patients' increased risk is more extensive obstructive CAD.

Clinical Utility of Enhanced Relative Activity Recovery on Systolic Myocardial Perfusion SPECT: Lessons from PET

SPECT and PET myocardial perfusion images show greater myocardial intensity and homogeneity in systole than diastole because of greater systolic myocardial thickness, less partial volume loss, and enhanced activity recovery. Consequently, conventional myocardial perfusion images obtained from whole cardiac cycles have lower myocardial intensity and greater heterogeneity than systolic images. Considering relative activity distribution on SPECT systolic images may add clinical utility to whole-cycle images and wall motion.

METHODS

Patients undergoing coronary angiogram within 4 mo after SPECT myocardial perfusion imaging were reviewed. Images were interpreted by 2 masked interpreters using a 17-segment, 5-point scale to determine summed rest scores (SSS), summed stress scores, and summed difference scores on conventional and systolic images in 603 patients (55.6% no coronary artery disease [no-CAD] and 44.4% CAD). Studies were considered normal when the SSS was less than 4 and summed difference score was less than 2.

RESULTS

In the no-CAD group, systolic SSS was lower than SSS from conventional images (2 ± 2.3 vs. 3 ± 2.6, P < 0.001). In contrast, SSS derived from systolic and conventional images were not different in the obstructive CAD group (9.1 ± 7.6 vs. 9.2 ± 7.4, P = 0.559). When systolic images were considered, true-negative studies increased from 27.2% to 43.3% (P < 0.001) whereas false-positive studies decreased from 28.4% to 12.3% (P < 0.001). True-positive (38% vs. 37.2%, P = 0.505) and false-negative studies (6.5% vs. 7%, P = 0.450) were not significantly changed. Diagnostic accuracy increased from 65.2% to 80.8% (P < 0.001).

CONCLUSION

For gated SPECT myocardial perfusion imaging, when relative activity distribution on systolic images was considered, false-positive studies were reduced and diagnostic accuracy was improved.

Basal hyperaemia is the primary abnormality of perfusion in Takotsubo cardiomyopathy: a quantitative cardiac perfusion positron emission tomography study

AIMS

Takotsubo cardiomyopathy (TTC) is characterized by acute completely reversible regional left ventricle (LV) akinesia and decreased tracer uptake in the akinetic region on semi-quantitative perfusion imaging. The latter may be due to normoperfusion of the akinetic mid/apical area and basal hyperperfusion. Our aim was to examine abnormalities of perfusion in TTC, and we hypothesized that basal hyperperfusion is the primary perfusion abnormality in the acute state.

METHOD AND RESULTS

Twenty-five patients were diagnosed with TTC due to (i) acute onset of symptoms, (ii) typical apical ballooning, (iii) absence of significant coronary disease, and (iv) complete remission on 4-month follow-up. The patients underwent coronary angiography (CAG), echocardiography, cardiac magnetic resonance imaging (CMR), and (13)NH3/(82)Rb positron emission tomography (PET) in the acute state and-except CAG-on follow-up. Patients initially had severe heart failure, mid/apical oedema but no infarction, and a rise in cardiac biomarkers. On initial perfusion PET imaging, eight patients appeared to have normal, whereas 17 patients had impaired LV perfusion. In the latter, flow in the basal region was increased in the acute state (1.5 ± 0.1 vs. 1.2 ± 0.1 mL/g/minRPP-corrected, P < 0.01), whereas midventricular (1.7 ± 0.1 vs. 1.6 ± 0.1 mL/g/minRPP-corrected, P = 0.21) and apical (1.4 ± 0.1 vs. 1.5 ± 0.1 mL/g/minRPP-corrected, P = 0.36) flow was unchanged between acute and follow-up, and within normal range.

CONCLUSION

Our results suggest an abnormal LV perfusion distribution in the acute state of TTC with basal hyperperfusion and a normoperfused akinetic region. The proportion of patients without visualized perfusion abnormalities in the acute state may represent a subgroup with fast remission.