Myocardial blood flow: Putting it into clinical perspective

Hemodynamic Insights into Combined Fractional Flow Reserve and Instantaneous Wave-Free Ratio Assessment Through Quantitative [15O]H2O PET Myocardial Perfusion Imaging

In patients evaluated for obstructive coronary artery disease (CAD), guidelines recommend using either fractional flow reserve (FFR) or instantaneous wave-free ratio (iFR) to guide coronary revascularization decision-making. The hemodynamic significance of lesions with discordant FFR and iFR measurements is debated. This study compared [15O]H2O PET-derived absolute myocardial perfusion between vessels with concordant and discordant FFR and iFR measurements. Methods: We included 197 patients suspected of obstructive CAD who had undergone [15O]H2O PET perfusion imaging and combined FFR/iFR interrogation in 468 vessels. Resting myocardial blood flow (MBF), hyperemic MBF, and coronary flow reserve (CFR) were compared among 4 groups: FFR low/iFR low (n = 79), FFR high/iFR low (n = 22), FFR low/iFR high (n = 22), and FFR high/iFR high (n = 345). Predefined [15O]H2O PET thresholds for ischemia were 2.3 mL·min-1·g-1 or less for hyperemic MBF and 2.5 or less for CFR. Results: Hyperemic MBF was lower in the concordant low (2.09 ± 0.67 mL·min-1·g-1), FFR high/iFR low (2.41 ± 0.80 mL·min-1·g-1), and FFR low/iFR high (2.40 ± 0.69 mL·min-1·g-1) groups compared with the concordant high group (2.91 ± 0.84 mL·min-1·g-1) (P < 0.001, P = 0.004, and P < 0.001, respectively). A lower CFR was observed in the concordant low (2.37 ± 0.76) and FFR high/iFR low (2.64 ± 0.84) groups compared with the concordant high group (3.35 ± 1.07, P < 0.01 for both). However, for vessels with either low FFR or low iFR, quantitative hyperemic MBF and CFR values exceeded the ischemic threshold in 38% and 49%, respectively. In addition, resting MBF exhibited a negative correlation with iFR (P < 0.001) and was associated with FFR low/iFR high discordance compared with concordant low FFR/low iFR measurements, independent of clinical and angiographic characteristics, as well as hyperemic MBF (odds ratio [OR], 0.41; 95% CI, 0.26-0.65; P < 0.001). Conclusion: We found reduced myocardial perfusion in vessels with concordant low and discordant FFR/iFR measurements. However, FFR/iFR combinations often inaccurately classified vessels as either ischemic or nonischemic when compared with hyperemic MBF and CFR. Furthermore, a lower resting MBF was associated with a higher iFR and the occurrence of FFR low/iFR high discordance. Our study showed that although combined FFR/iFR assessment can be useful to estimate the hemodynamic significance of coronary lesions, these pressure-derived indices provide a limited approximation of [15O]H2O PET-derived quantitative myocardial perfusion as the physiologic standard of CAD severity.

Positron emission tomography myocardial perfusion imaging (PET MPI) findings predictive of post-liver transplant major adverse cardiac events

Positron emission tomography myocardial perfusion imaging (PET MPI) is a noninvasive diagnostic test capable of detecting coronary artery disease, structural heart disease, and myocardial flow reserve (MFR). We aimed to determine the prognostic utility of PET MPI to predict post-liver transplant (LT) major adverse cardiac events (MACE). Among the 215 LT candidates that completed PET MPI between 2015 and 2020, 84 underwent LT and had 4 biomarker variables of clinical interest on pre-LT PET MPI (summed stress and difference scores, resting left ventricular ejection fraction, global MFR). Post-LT MACE were defined as acute coronary syndrome, heart failure, sustained arrhythmia, or cardiac arrest within the first 12 months post-LT. Cox regression models were constructed to determine associations between PET MPI variable/s and post-LT MACE. The median LT recipient age was 58 years, 71% were male, 49% had NAFLD, 63% reported prior smoking, 51% had hypertension, and 38% had diabetes mellitus. A total of 20 MACE occurred in 16 patients (19%) at a median of 61.5 days post-LT. One-year survival of MACE patients was significantly lower than those without MACE (54% vs. 98%, p =0.001). On multivariate analysis, reduced global MFR ≤1.38 was associated with a higher risk of MACE [HR=3.42 (1.23-9.47), p =0.019], and every % reduction in left ventricular ejection fraction was associated with an 8.6% higher risk of MACE [HR=0.92 (0.86-0.98), p =0.012]. Nearly 20% of LT recipients experienced MACE within the first 12 months of LT. Reduced global MFR and reduced resting left ventricular ejection fraction on PET MPI among LT candidates were associated with increased risk of post-LT MACE. Awareness of these PET-MPI parameters may help improve cardiac risk stratification of LT candidates if confirmed in future studies.

Prognostic value of combined coronary angiography-derived IMR and myocardial perfusion imaging by CZT SPECT in INOCA

BACKGROUND

A significant proportion of ischemia with non-obstructive coronary artery disease (INOCA) demonstrate coronary microvascular dysfunction (CMD), a condition associated with abnormal myocardial perfusion imaging (MPI) and adverse outcomes. Coronary angiography-derived index of microvascular resistance (caIMR) is a novel non-invasive technique to assess CMD. We aimed to investigate the prognostic value of combined caIMR and MPI by CZT SPECT in INOCA patients.

METHODS

Consecutive 151 patients with chest pain and < 50% coronary stenosis who underwent coronary angiography and MPI within 3 months were enrolled. caIMR was calculated by computational pressure-flow dynamics. CMD was defined as caIMR ≥ 25. The endpoint was major adverse cardiac events (MACE: cardiovascular death, nonfatal myocardial infarction, revascularization, angina-related rehospitalization, heart failure, and stroke).

RESULTS

Of all INOCA patients, CMD was present in 93 (61.6%) patients. The prevalence of abnormal MPI was significantly higher in CMD compared with non-CMD patients (40.9% vs 13.8%, P < .001). CMD showed a higher risk of MACE than non-CMD patients. Patients with both CMD and abnormal MPI had the worst prognosis, followed by patients with CMD and normal MPI (log-rank P < .001). Cox regression analysis identified CMD (HR 3.121, 95%CI 1.221-7.974, P = .017) and MPI (HR 2.704, 95%CI 1.030-7.099, P = .043) as predictive of MACE. The prognostic value of INOCA patients enhanced significantly by adding CMD and MPI to the model with clinical risk factors (AUC = 0.777 vs 0.686, P = .030).

CONCLUSION

caIMR-derived CMD is associated with increased risk of MACE among INOCA patients. Patients with abnormalities on both caIMR and MPI had the worse outcomes.

Human Heart Anoxia and Reperfusion Tissue (HEART) Model for the Rapid Study of Exosome Bound miRNA Expression As Biomarkers for Myocardial Infarction

Current biomarkers for myocardial infarction (MI) diagnosis are typically late markers released upon cell death, incapable of distinguishing between ischemic and reperfusion injury and can be symptoms of other pathologies. Circulating microRNAs (miRNAs) have recently been proposed as alternative biomarkers for MI diagnosis; however, detecting the changes in the human cardiac miRNA profile during MI is extremely difficult. Here, to study the changes in miRNA levels during acute MI, a heart-on-chip model with a cardiac channel, containing human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes in human heart decellularized matrix and collagen, and a vascular channel, containing hiPSC-derived endothelial cells, is developed. This model is exposed to anoxia followed by normoxia to mimic ischemia and reperfusion, respectively. Using a highly sensitive miRNA biosensor that the authors developed, the exact same increase in miR-1, miR-208b, and miR-499 levels in the MI-on-chip and the time-matched human blood plasma samples collected before and after ischemia and reperfusion, is shown. That the surface marker profile of exosomes in the engineered model changes in response to ischemic and reperfusion injury, which can be used as biomarkers to detect MI, is also shown. Hence, the MI-on-chip model developed here can be used in biomarker discovery.

The diagnostic value of SPECT CZT quantitative myocardial blood flow in high-risk patients

BACKGROUND

The objective of this study was to evaluate the accuracy of global MBF and MFR quantitation performed by myocardial perfusion scintigraphy (MPS) for the detection of multivessel coronary artery disease (CAD).

METHODS

52 CAD patients underwent CZT MPS, with the evaluation of MBF and MFR, followed by invasive coronary angiography (ICA). According to MPS and ICA results, all patients were divided into three groups: (1) non-obstructive CAD and normal MPS scan (control group) (n = 7), (2) one vessel disease (1VD) (n = 16), (3) multivessel disease (MVD) (n = 29).

RESULTS

Global absolute MBF and MFR were significantly reduced in MVD patients as compared to those with 1VD [0.93 (IQR 0.76; 1.39) vs 1.94 (1.37; 2.21) mL·min-1·g-1, P = .00012] and [1.4 (IQR 1.02; 1.85) vs 2.3 (1.8; 2.67), P = . 0 004], respectively. The Syntax score correlated with global stress MBF (ρ = - 0.64; P < .0001) and MFR (ρ = - 0.53; P = .0003). ROC analysis showed higher sensitivity and specificity for stress MBF and MFR compared with semiquantitative MPS stress evaluation. Multivariate regression analysis showed that only stress MBF [OR (95% CI) 0.59 (0.42-0.82); P < .0003] was an independent predictor of MVD.

CONCLUSIONS

Quantitative myocardial blood flow values assessed with the use of CZT camera may identify high-risk patients, such as those with multivessel disease.

The Utility of Noninvasive PET/CT Myocardial Perfusion Imaging in Adult Liver Transplant Candidates

Background.

The optimal cardiovascular (CV) risk stratification in liver transplant (LT) candidates remains unclear. The aim of this study was to evaluate concordance of findings between dobutamine stress echocardiography (DSE), positron emission tomography/computed tomography myocardial perfusion imaging (PET/CT MPI), and left heart catheterization in adult LT candidates.

Methods.

Data on 234 consecutive adult LT candidates from February 2015 to June 2018 with PET/CT MPI were reviewed. Adverse CV outcomes were adjudicated via chart review by a board-certified cardiologist.

Results.

Median age was 60.8, body mass index 30.2 kg/m², and model of end-stage liver disease–sodium 14; 61% were male, and 54% had diabetes. Thirty-seven percent had nonalcoholic steatohepatitis and 29% alcohol-related liver disease. Sixty-five percent of patients had a DSE, of which 41% were nondiagnostic. No factors were independently associated with having a nondiagnostic DSE. The median global myocardial flow reserve correlated positively with hemoglobin and negatively with model of end-stage liver disease–sodium, age, ejection fraction, and body mass index. Moderate/high-risk MPIs were associated with older age and known CV disease. In patients with 2 cardiac testing modalities, findings were concordant in 87%. Eleven of 53 LT recipients experienced an adverse CV outcome, but no independent predictors were identified for this outcome.

Conclusions.

Results of different cardiac risk-stratification modalities were concordant across modalities the majority of the time in LT candidates, although these findings were not independently correlated with risk of post-LT CV outcomes. Given the high rates of nondiagnostic DSEs in this population, PET/CT MPI may be the preferred CV risk-stratification modality in older patients and those with known CV disease.

The diagnosis and prognosis of coronary microvascular disease using PET/CT

OBJECTIVE

This study aimed to evaluate the diagnostic and prognostic value of 13N-NH3·H2O positron emission tomography combined with computed tomography (PET/CT) for non-obstructive coronary microvascular disease (CMVD).

METHODS

A retrospective analysis was conducted on 70 patients with clinically suspected non-obstructive CMVD (35 males, 35 females) between March 2017 and August 2019. The average age of the patients was 53.32±7.82 years. The patients underwent 13N-NH3·H2O PET/CT and were divided into two groups based on diagnostic criteria: a CMVD group and a non-CMVD group. They were then followed up for 180-1,095 days. Data were analyzed using an χ 2 test, the logistic regression model, the multiple linear regression model, the Kaplan-Meier method, the Cox proportional hazards regression model, and a receiver operating characteristic (ROC) curve.

RESULTS

(1) The incidence of cardiovascular family history and a high calcification score (11-400) was higher in the CMVD group than in the non-CMVD group (58.8% vs. 20.8% and 29.4% vs. 5.7%, respectively; P <  0.05 for all), stress myocardial blood flow (MBF) and coronary flow reserve (CFR) values were lower in the CMVD group than in the non-CMVD group (2.280±0.693 vs. 3.641±1.365 and 2.142±0.339 vs. 3.700±1.123, respectively), and calcification score was higher in the CMVD group than in the non-CMVD group (110.18±165.07 vs. 13.21±41.68, respectively; P <  0.05 for all). Gender and diabetes were risk factors for stress MBF reduction (β= 1.287 and β= -0.636, respectively), calcification score and hypertension were risk factors for CFR reduction (β= -0.004 and β= -0.654, respectively), and hypertension, family history, and calcification score were risk factors in the CMVD group (OR = 7.323, OR = 5.108, OR = 1.012, respectively; P <  0.05 for all). (2) The prognosis of patients with CFR <  2.5 was worse than that of patients with CFR≥2.5 (x2 value: 27.404, P <  0.001). The risk of adverse cardiovascular events in diabetic patients was also increased (β= 0.328, P <  0.001). When CFR was set to 2.595, the prognostic sensitivity was 94% and the specificity was 80%.

CONCLUSION

The technology of 13N-NH3·H2O PET/CT can be used for the diagnosis and prognosis of non-obstructive CMVD. Cardiovascular risk factors are related to the occurrence and prognosis of CMVD.

Coronary artery calcium burden, carotid atherosclerotic plaque burden, and myocardial blood flow in patients with end-stage renal disease: A non-invasive imaging study combining PET/CT and 3D ultrasound

BACKGROUND

Imaging-based measures of atherosclerosis such as coronary artery calcium score (CACS) and coronary flow reserve (CFR) as well as carotid atherosclerotic plaque burden (cPB) are predictors of cardiovascular events in the general population. The objective of this study was to correlate CACS, cPB, myocardial blood flow (MBF), and CFR in patients with end-stage renal disease (ESRD).

METHODS AND RESULTS

39 patients (mean age 53 ± 12 years) with ESRD prior to kidney transplantation were enrolled. MBF and CFR were quantified at baseline and under hyperemia by 13N-NH3-PET/CT. CACS was calculated from low-dose CT scans acquired for PET attenuation correction. cPB was assessed by 3D ultrasound. Uni- and multivariate regression analyses between these and clinical parameters were performed. Median follow-up time for clinical events was 4.4 years. Kaplan-Meier survival estimates with log-rank test were performed with regards to cardiovascular (CV) events and death of any cause. CACS and cPB were associated in ESRD patients (r = 0.48; p ≤ 0.01). While cPB correlated with age (r = 0.43; p < 0.01), CACS did not. MBFstress was negatively associated with age (r = 0.44; p < 0.01) and time on dialysis (r = 0.42; p < 0.01). There were negative correlations between MBFstress and CACS (r = - 0.62; p < 0.001) and between MBFstress and cPB (r = - 0.43; p < 0.01). Age and CACS were the strongest predictors for MBFstress. CFR was impaired (< 2.0) in eight patients who also presented with higher cPB and higher CACS compared to those with a CFR > 2.0 (p = 0.06 and p = 0.4). In contrast to MBFstress, there was neither a significant correlation between CFR and CACS (r = - 0.2; p = 0.91) nor between CFR and cPB (r = - 0.1; p = 0.55). CV event-free survival was associated with reduced CFR and MBFstress (p = 0.001 and p < 0.001) but not with cPB or CACS.

CONCLUSIONS

CACS, cPB, and MBFstress are associated in patients with ESRD. Atherosclerosis is earlier detected by MBFstress than by CFR. CV event-free survival is associated with impaired CFR and MBFstress.

Effects of two patient-specific dosing protocols on measurement of myocardial blood flow with 3D 82Rb cardiac PET

PURPOSE

Clinical measurement of myocardial blood flow (MBF) has emerged as an important component of routine PET-CT assessment of myocardial perfusion in patients with known or suspected coronary artery disease. Although multiple society guidelines recommend patient-specific dosing, there is a lack of studies evaluating the efficacy of patient-specific dosing for quantitative MBF accuracy.

METHODS

Two patient-specific dosing protocols (weight- and BMI-adjusted) were retrospectively evaluated in 435 consecutive clinical patients referred for PET myocardial perfusion assessment. MBF was estimated at rest and after regadenoson-induced hyperemia. The effect of dosing protocol on dose reduction, PET scanner saturation, relative perfusion, and image quality was compared. The effect of PET saturation on the accuracy of MBF and myocardial flow reserve (MFR) in remote myocardium was assessed with multivariable linear regression.

RESULTS

BMI-adjusted dosing was associated with lower administered ⁸²Rb activities (1036.0 ± 274 vs. 1147 ± 274 MBq, p = 0.003) and lower PET scanner saturation incidence (28 vs. 38%, p = 0.006) and severity (median saturation severity index 0.219 ± 0.33 vs. 0.397 ± 0.59%, p = 0.018) compared to weight-adjusted dosing. PET saturation that occurred with either dosing protocol was moderate and resulted in modest remote MBF and MFR biases ranging from 2 to 9% after adjusting for patient age, sex, BMI, rate-pressure product, and LV ejection fraction. No adverse effects of BMI dose adjustment were observed in relative perfusion assessment or image quality.

CONCLUSIONS

Patient-specific dosing according to BMI is an effective method for guideline-directed dose reduction while maintaining image quality and accuracy for routine MBF and MFR quantification.

Validation of regional myocardial blood flow quantification using three-dimensional PET with rubidium-82: repeatability and comparison with two-dimensional PET data acquisition

INTRODUCTION

Three-dimensional (3D) data acquisition is now standard on PET/computed tomography scanners. The aim of this study was to evaluate the repeatability of myocardial blood flow (MBF) estimation with rubidium-82 (Rb) 3D PET and to validate regional MBF measurements by comparison with two-dimensional (2D) PET.

PATIENTS AND METHODS

Fifteen healthy individuals (31.6 ± 11.4 years old) were enrolled for the evaluation of the short-term repeatability of rest 3D MBF quantification. Another 19 healthy individuals (35.3 ± 12.6 years old) underwent rest and pharmacological stress PET using 2D and 3D data acquisition within a 1-month interval. The injected dose was 1500 MBq for 2D and 555 MBq for 3D PET acquisition.

RESULTS

MBF at rest showed good repeatability [whole left ventricular MBF; 0.54 ± 0.13 vs. 0.52 ± 0.13 mL/min/g, P = 0.98]. Rest MBF, stress MBF, and myocardial flow reserve (MFR) were not significantly different between 3D and 2D data acquisition. 3D MBF correlated well with 2D MBF over a wide flow range for both whole left ventricular (r = 0.97, P < 0.0001) and regional values (r = 0.61, P < 0.0001).

CONCLUSION

MBF measured with 3D PET showed very good test-retest repeatability. Whole left ventricular and regional MBF measurements obtained using lower Rb-dose 3D PET were highly correlated over a wide range with those from 2D PET. Therefore, MBF with 3D PET can be applied using a lower Rb dosage in clinical settings with reduced radiation exposure.

Coronary Microvascular Dysfunction: Clinical Considerations and Noninvasive Diagnosis

Chest pain in patients without obstructive coronary artery disease has been realized as a frequent problem encountered in clinical practice. Invasive flow investigations have suggested that up to two-thirds of patients with nonobstructive coronary atherosclerosis may have microvascular dysfunction (MVD). Positron emission tomography myocardial perfusion imaging in conjunction with tracer-kinetic modeling enables the concurrent quantification of myocardial blood flow (MBF) in milliliters per minute per gram of tissue. This allows the assessment of hyperemic MBFs and myocardial flow reserve for the noninvasive identification and characterization of MVD as an important functional substrate for angina symptoms amenable to intensified and individualized medical intervention with nitrates, calcium-channel blockers, statins, angiotensin-converting enzyme inhibitors, and/or angiotensin II type 1 receptor blockers. Recent investigations suggest that cardiac magnetic resonance and computed tomography may also be suitable for the noninvasive detection of MVD. Whether intensified and individualized treatment related improvement or even normalization of hyperemic MBF and/or myocardial flow reserve may lead to a persistent reduction in angina symptoms and/or improved cardiovascular outcome as compared to standard care, deserves further testing in large-scale randomized clinical trials.

Impact of improved attenuation correction on 18F-FDG PET/MR hybrid imaging of the heart

PURPOSE

The aim of this study was to evaluate and quantify the effect of improved attenuation correction (AC) including bone segmentation and truncation correction on 18F-Fluordesoxyglucose cardiac positron emission tomography/magnetic resonance (PET/MR) imaging.

METHODS

PET data of 32 cardiac PET/MR datasets were reconstructed with three different AC-maps (1. Dixon-VIBE only, 2. HUGE truncation correction and bone segmentation, 3. MLAA). The Dixon-VIBE AC-maps served as reference of reconstructed PET data. 17-segment short-axis polar plots of the left ventricle were analyzed regarding the impact of each of the three AC methods on PET quantification in cardiac PET/MR imaging. Non-AC PET images were segmented to specify the amount of truncation in the Dixon-VIBE AC-map serving as a reference. All AC-maps were evaluated for artifacts.

RESULTS

Using HUGE + bone AC results in a homogeneous gain of ca. 6% and for MLAA 8% of PET signal distribution across the myocardium of the left ventricle over all patients compared to Dixon-VIBE AC only. Maximal relative differences up to 18% were observed in segment 17 (apex). The body volume truncation of -12.7 ± 7.1% compared to the segmented non-AC PET images using the Dixon-VIBE AC method was reduced to -1.9 ± 3.9% using HUGE and 7.8 ± 8.3% using MLAA. In each patient, a systematic overestimation in AC-map volume was observed when applying MLAA. Quantitative impact of artifacts showed regional differences up to 6% within single segments of the myocardium.

CONCLUSIONS

Improved AC including bone segmentation and truncation correction in cardiac PET/MR imaging is important to ensure best possible diagnostic quality and PET quantification. The results exhibited an overestimation of AC-map volume using MLAA, while HUGE resulted in a more realistic body contouring. Incorporation of bone segmentation into the Dixon-VIBE AC-map resulted in homogeneous gain in PET signal distribution across the myocardium. The majority of observed AC-map artifacts did not significantly affect the quantitative assessment of the myocardium.

Myocardial bridging of the left anterior descending coronary artery is associated with reduced myocardial perfusion reserve: a 13N-ammonia PET study

Myocardial Bridging (MB) refers to the band of myocardium that abnormally overlies a segment of a coronary artery. This paper quantitatively evaluates the influence of MB of the left anterior descending artery (LAD) on myocardial perfusion of the entire left ventricle. We studied 131 consecutive patients who underwent hybrid rest/stress 13N-ammonia positron emission tomography (PET) and coronary computed tomography angiography (CCTA) due to suspected myocardial ischemia. Patients with previous myocardial infarction and/or significant coronary artery disease (≥ 50% stenosis) were excluded. Myocardial perfusion measurements were compared between patients with and without LAD-MB. Additionally, we evaluated the relationship between anatomical characteristics (length and depth) of LAD-MB and myocardial perfusion measurements. 17 (13%) patients presented a single LAD-MB. Global myocardial perfusion reserve (MPR) was lower in patients with LAD-MB than in patients without LAD-MB (1.9 ± 0.5 vs. 2.3 ± 0.6, p < 0.01). Global stress myocardial blood flow (MBF) was similar in patients with and without LAD-MB (2.2 ± 0.4 vs. 2.3 ± 0.7 ml/g/min, p = 0.40). Global rest MBF was higher in patients with LAD-MB than in patients without LAD-MB (1.2 ± 0.3 vs. 1.0 ± 0.2 ml/g/min, p < 0.01). Global rest MBF, stress MBF, and MPR quantifications were similar in patients with superficial and deep LAD-MB (all p = NS). We did not find any correlation between length and global rest MBF, stress MBF nor MPR (r = - 0.14, p = 0.59; r = 0.44, p = 0.07; and r = 0.45, p = 0.07 respectively). Quantitative myocardial perfusion suggests that LAD-MB may be related to impaired perfusion reserve, an indicator of microvascular dysfunction. Anatomical characteristics of LAD-MB were not related to changes in myocardial perfusion.

Moving into the next era of PET myocardial perfusion imaging: introduction of novel 18F-labeled tracers

The heart failure epidemic continues to rise with coronary artery disease as one of its main causes. Novel concepts for risk stratification to guide the referring cardiologist towards revascularization procedures are of significant value. Myocardial perfusion imaging using single-photon emission computed tomography (SPECT) agents has demonstrated high accuracy for the detection of clinically relevant stenoses. With positron emission tomography (PET) becoming more widely available, mainly due to its diagnostic performance in oncology, perfusion imaging with that modality is more practical than in the past and overcomes existing limitations of SPECT MPI. Advantages of PET include more reliable quantification of absolute myocardial blood flow, the routine use of computed tomography for attenuation correction, a higher spatiotemporal resolution and a higher count sensitivity. Current PET radiotracers such as rubidium-82 (half-life, 76 s), oxygen-15 water (2 min) or nitrogen-13 ammonia (10 min) are labeled with radionuclides with very short half-lives, necessitating that stress imaging is performed under pharmacological vasodilator stress instead of exercise testing. However, with the introduction of novel ¹⁸F-labeled MPI PET radiotracers (half-life, 110 min), the intrinsic advantages of PET can be combined with exercise testing. Additional advantages of those radiotracers include, but are not limited to: potentially improved cost-effectiveness due to the use of pre-existing delivery systems and superior imaging qualities, mainly due to the shortest positron range among available PET MPI probes. In the present review, widely used PET MPI radiotracers will be reviewed and potential novel ¹⁸F-labeled perfusion radiotracers will be discussed.

Clinical significance of PET-CT coronary flow reserve in diagnosis of non-obstructive coronary artery disease

OBJECTIVE

To improve current knowledge of coronary flow reserve and non-obstructive coronary artery disease in terms of definition, features and clinical implications of measurement of coronary flow reserve (CFR), is an integrated measure of focal, diffuse, and small vessel coronary artery disease, can also be explained as a calculated ratio of hyperaemic to rest absolute myocardial blood flow. Non-obstructive coronary artery disease is defined as atherosclerotic plaque that does not obstruct blood flow or result in anginal symptoms. We also aimed at knowing the significance of PET in diagnosing coronary microvascular disease.

RESULTS

In our study 92% were between 41 and 60 years. 83% were males and 17% females, more patients had hypertension about 50%, few had diabetes mellitus about 16%, while those with both hypertension and diabetes mellitus were 17%. About 83% had ST segment and T wave changes on ECG. All patients and controls had normal coronaries on coronary angiography TIMI 3 flow. On further investigation by Positron emission tomography/CT we found out 58% had abnormal CFR and 42% had normal coronary flow reserve. Our findings indicate PET/CT coronary flow reserve concept provides a platform for the diagnosis of non-obstructive coronary artery disease in patients with signs and symptoms of ischemia without angiographic obstructive CAD.

Computational Assessment of Blood Flow Heterogeneity in Peritoneal Dialysis Patients' Cardiac Ventricles

Dialysis prolongs life but augments cardiovascular mortality. Imaging data suggests that dialysis increases myocardial blood flow (BF) heterogeneity, but its causes remain poorly understood. A biophysical model of human coronary vasculature was used to explain the imaging observations, and highlight causes of coronary BF heterogeneity. Post-dialysis CT images from patients under control, pharmacological stress (adenosine), therapy (cooled dialysate), and adenosine and cooled dialysate conditions were obtained. The data presented disparate phenotypes. To dissect vascular mechanisms, a 3D human vasculature model based on known experimental coronary morphometry and a space filling algorithm was implemented. Steady state simulations were performed to investigate the effects of altered aortic pressure and blood vessel diameters on myocardial BF heterogeneity. Imaging showed that stress and therapy potentially increased mean and total BF, while reducing heterogeneity. BF histograms of one patient showed multi-modality. Using the model, it was found that total coronary BF increased as coronary perfusion pressure was increased. BF heterogeneity was differentially affected by large or small vessel blocking. BF heterogeneity was found to be inversely related to small blood vessel diameters. Simulation of large artery stenosis indicates that BF became heterogeneous (increase relative dispersion) and gave multi-modal histograms. The total transmural BF as well as transmural BF heterogeneity reduced due to large artery stenosis, generating large patches of very low BF regions downstream. Blocking of arteries at various orders showed that blocking larger arteries results in multi-modal BF histograms and large patches of low BF, whereas smaller artery blocking results in augmented relative dispersion and fractal dimension. Transmural heterogeneity was also affected. Finally, the effects of augmented aortic pressure in the presence of blood vessel blocking shows differential effects on BF heterogeneity as well as transmural BF. Improved aortic blood pressure may improve total BF. Stress and therapy may be effective if they dilate small vessels. A potential cause for the observed complex BF distributions (multi-modal BF histograms) may indicate existing large vessel stenosis. The intuitive BF heterogeneity methods used can be readily used in clinical studies. Further development of the model and methods will permit personalized assessment of patient BF status.

PET-measured longitudinal flow gradient correlates with invasive fractional flow reserve in CAD patients

Aims

We aimed to evaluate whether a PET-determined longitudinal decrease in myocardial blood flow (MBF) or gradient, assumed as a more specific flow parameter for epicardial resistance, correlates with invasively measured fractional flow reserve (FFR) in coronary artery disease (CAD) patients.

Methods and Results

In 29 patients with suspected or known CAD, myocardial perfusion and MBF in mL/g/min was determined with 13N-ammonia PET/CT during regadenoson stimulation and at rest, and corresponding myocardial flow reserve (MFR = MBF stress/MBF rest) was calculated. MBF parameters were assessed in the myocardial region with stress-related perfusion defect and with stenosis ≥50% (Region 1), without defect but with stenosis ≥50% (Region 2), or without stenosis ≥50% (Region 3). Hyperaemic MBFs were significantly lower in the mid-distal than in the mid-left ventricular myocardium in Regions 1-3 [median and IQ range: 1.57 (1.24, 1.84) vs. 1.87 (1.61, 2.00), and 1.23 (1.11, 1.86) vs. 1.89 (1.80, 1.97), and 1.78 (1.48, 2.00) vs. 1.94 (1.84, 2.05) mL/g/min, P < 0.0001]. Resulting longitudinal MBF gradient during hyperaemic flows was more pronounced in Region 2 than in Regions 1 and 3, respectively [-0.46 (-0.70, -0.10) vs. -0.17 (-0.29, -0.11) and -0.15 (-0.25, -0.09) mL/g/min, respectively, P < 0.01]. There was a significant correlation between the hyperaemic longitudinal MBF gradient and FFR (r = 0.95; P < 0.0001), while this association was less pronounced for corresponding MFR (r = 0.50; P = 0.006).

Conclusion

The observed close correlation between a longitudinal MBF gradient during hyperaemic flows and invasively measured FFR suggests the longitudinal flow gradient as an emerging non-invasive index of flow-limiting CAD.

Multimodality Image Fusion for Coronary Artery Disease Detection: Concepts and Latest Developments

The debate on the role of anatomy and function in the assessment of coronary artery disease has been progressing for decades. While each imaging modality brings its own strengths and weaknesses, a multimodality image fusion approach combining an anatomical acquisition with a functional one has the potential of providing all the complementary information necessary to select the proper treatment. The technology has been available to physicians for a decade, but the recent introduction of positron emission tomography-derived absolute myocardial blood flow has further advanced the case for an image fusion diagnostic approach.

PET measurements of myocardial blood flow post myocardial infarction: Relationship to invasive and cardiac magnetic resonance studies and potential clinical applications

This review focuses on clinical studies concerning assessment of coronary microvascular and conduit vessel function primarily in the context of acute and sub acute myocardial infarction (MI). The ability of quantitative PET measurements of myocardial blood flow (MBF) to delineate underlying pathophysiology and assist in clinical decision making in this setting is discussed. Likewise, considered are physiological metrics fractional flow reserve, coronary flow reserve, index of microvascular resistance (FFR, CFR, IMR) obtained from invasive studies performed in the cardiac catheterization laboratory, typically at the time of PCI for MI. The role both of invasive studies and cardiac magnetic resonance (CMR) imaging in assessing microvascular function, a key determinant of prognosis, is reviewed. The interface between quantitative PET MBF measurements and underlying pathophysiology, as demonstrated both by invasive and CMR methodology, is discussed in the context of optimal interpretation of the quantitative PET MBF exam and its potential clinical applications.

Thrombospondin-1, Free Radicals, and the Coronary Microcirculation: The Aging Conundrum

SIGNIFICANCE

Successful matching of cardiac metabolism to perfusion is accomplished primarily through vasodilation of the coronary resistance arterioles, but the mechanism that achieves this effect changes significantly as aging progresses and involves the contribution of reactive oxygen species (ROS). Recent Advances: A matricellular protein, thrombospondin-1 (Thbs-1), has been shown to be a prolific contributor to the production and modulation of ROS in large conductance vessels and in the peripheral circulation. Recently, the presence of physiologically relevant circulating Thbs-1 levels was proven to also disrupt vasodilation to nitric oxide (NO) in coronary arterioles from aged animals, negatively impacting coronary blood flow reserve.

CRITICAL ISSUES

This review seeks to reconcile how ROS can be successfully utilized as a substrate to mediate vasoreactivity in the coronary microcirculation as "normal" aging progresses, but will also examine how Thbs-1-induced ROS production leads to dysfunctional perfusion and eventual ischemia and why this is more of a concern in advancing age.

FUTURE DIRECTIONS

Current therapies that may effectively disrupt Thbs-1 and its receptor CD47 in the vascular wall and areas for future exploration will be discussed. Antioxid. Redox Signal. 27, 785-801.

Integrated Noninvasive Physiological Assessment of Coronary Circulatory Function and Impact on Cardiovascular Mortality in Patients With Stable Coronary Artery Disease

BACKGROUND

It is suggested that the integration of maximal myocardial blood flow (MBF) and coronary flow reserve (CFR), termed coronary flow capacity, allows for comprehensive evaluation of patients with known or suspected stable coronary artery disease. Because management decisions are predicated on clinical risk, we sought to determine the independent and integrated value of maximal MBF and CFR for predicting cardiovascular death.

METHODS

MBF and CFR were quantified in 4029 consecutive patients (median age 66 years, 50.5% women) referred for rest/stress myocardial perfusion positron emission tomography scans from January 2006 to December 2013. The primary outcome was cardiovascular mortality. Maximal MBF <1.8 mL·g^-1·min^-1 and CFR<2 were considered impaired. Four patient groups were identified based on the concordant or discordant impairment of maximal MBF or CFR. Association of maximal MBF and CFR with cardiovascular death was assessed using Cox and Poisson regression analyses.

RESULTS

A total of 392 (9.7%) cardiovascular deaths occurred over a median follow-up of 5.6 years. CFR was a stronger predictor of cardiovascular mortality than maximal MBF beyond traditional cardiovascular risk factors, left ventricular ejection fraction, myocardial scar and ischemia, rate-pressure product, type of radiotracer or stress agent used, and revascularization after scan (adjusted hazard ratio, 1.79; 95% confidence interval [CI], 1.38-2.31; P<0.001 per unit decrease in CFR after adjustment for maximal MBF and clinical covariates; and adjusted hazard ratio, 1.03; 95% CI, 0.84-1.27; P=0.8 per unit decrease in maximal MBF after adjustment for CFR and clinical covariates). In univariable analyses, patients with concordant impairment of CFR and maximal MBF had high cardiovascular mortality of 3.3% (95% CI, 2.9-3.7) per year. Patients with impaired CFR but preserved maximal MBF had an intermediate cardiovascular mortality of 1.7% (95% CI, 1.3-2.1) per year. These patients were predominantly women (70%). Patients with preserved CFR but impaired maximal MBF had low cardiovascular mortality of 0.9% (95% CI, 0.6-1.6) per year. Patients with concordantly preserved CFR and maximal MBF had the lowest cardiovascular mortality of 0.4% (95 CI, 0.3-0.6) per year. In multivariable analysis, the cardiovascular mortality risk gradient across the 4 concordant or discordant categories was independently driven by impaired CFR irrespective of impairment in maximal MBF.

CONCLUSIONS

CFR is a stronger predictor of cardiovascular mortality than maximal MBF. Concordant and discordant categories based on integrating CFR and maximal MBF identify unique prognostic phenotypes of patients with known or suspected coronary artery disease.

Positron Emission Tomography-Determined Hyperemic Flow, Myocardial Flow Reserve, and Flow Gradient-Quo Vadis?

Positron emission tomography/computed tomography (PET/CT) applied with positron-emitting flow tracers such as ¹³N-ammonia and ⁸²Rubidium enables the quantification of both myocardial perfusion and myocardial blood flow (MBF) in milliliters per gram per minute for coronary artery disease (CAD) detection and characterization. The detection of a regional myocardial perfusion defect during vasomotor stress commonly identifies the culprit lesion or most severe epicardial narrowing, whereas adding regional hyperemic MBFs, myocardial flow reserve (MFR), and/or longitudinal flow decrease may also signify less severe but flow-limiting stenosis in multivessel CAD. The addition of regional hyperemic flow parameters, therefore, may afford a comprehensive identification and characterization of flow-limiting effects of multivessel CAD. The non-specific origin of decreases in hyperemic MBFs and MFR, however, prompts an evaluation and interpretation of regional flow in the appropriate context with the presence of obstructive CAD. Conversely, initial results of the assessment of a longitudinal hyperemic flow gradient suggest this novel flow parameter to be specifically related to increases in CAD caused epicardial resistance. The concurrent assessment of myocardial perfusion and several hyperemic flow parameters with PET/CT may indeed open novel avenues of precision medicine to guide coronary revascularization procedures that may potentially lead to a further improvement in cardiovascular outcomes in CAD patients.

Cardiovascular PET/MR imaging: Quo Vadis?

With the recent advent of PET/MRI scanners, the combination of molecular imaging with a variety of known and novel PET radiotracers, the high spatial resolution of MRI, and its potential for multi-parametric imaging are anticipated to increase the diagnostic accuracy in cardiovascular disease detection, while providing novel mechanistic insights into the initiation and progression of the disease state. For the time being, cardiac PET/MRI emerges as potential clinical tool in the identification and characterization of infiltrative cardiac diseases, such as sarcoidosis, acute or chronic myocarditis, and cardiac tumors, respectively. The application of PET/MRI in conjunction with various radiotracer probes in the identification of the vulnerable atherosclerotic plaque also holds much promise but needs further translation and validation in clinical investigations. The combination of molecular imaging and creation of multi-parametric imaging maps with PET/MRI, however, are likely to set new horizons to develop predictive parameters for myocardial recovery and treatment response in ischemic and non-ischemic cardiomyopathy patients. Molecular imaging and multi-parametric imaging in cardiovascular disease with PET/MRI at current stage are at its infancy but bear a bright future.

Coronary microvascular dysfunction in women with nonobstructive ischemic heart disease as assessed by positron emission tomography

Traditional approaches for risk assessment of ischemic heart disease (IHD) are based on the physiological consequences of an epicardial coronary stenosis. Of note, normal coronary arteries or nonobstructive coronary artery disease (CAD) is a common finding in women with signs and symptoms of ischemia. Therefore, assessment of risk based on a coronary stenosis approach may fail in women. Positron emission tomography (PET) quantifies absolute myocardial blood flow (MBF) which may help to elucidate other mechanisms involved such as endothelial dysfunction and alterations in the smooth muscle cell relaxation responsible for IHD in women. The objective of the present review is to describe the current state of the art of PET imaging in assessing IHD in women with nonobstructive CAD.

Physiological levels of thrombospondin-1 decrease NO-dependent vasodilation in coronary microvessels from aged rats

Aging and cardiovascular disease are associated with the loss of nitric oxide (NO) signaling and a decline in the ability to increase coronary blood flow reserve (CFR). Thrombospondin-1 (Thbs-1), through binding of CD47, has been shown to limit NO-dependent vasodilation in peripheral vascular beds via formation of superoxide (O2 (-)). The present study tests the hypothesis that, similar to the peripheral vasculature, blocking CD47 will improve NO-mediated vasoreactivity in coronary arterioles from aged individuals, resulting in improved CFR. Isolated coronary arterioles from young (4 mo) or old (24 mo) female Fischer 344 rats were challenged with the NO donor, DEA-NONO-ate (1 × 10(-7) to 1 × 10(-4) M), and vessel relaxation and O2 (-) production was measured before and after Thbs-1, αCD47, and/or Tempol and catalase exposure. In vivo CFR was determined in anesthetized rats (1-3% isoflurane-balance O2) via injected microspheres following control IgG or αCD47 treatment (45 min). Isolated coronary arterioles from young and old rats relax similarly to exogenous NO, but addition of 2.2 nM Thbs-1 inhibited NO-mediated vasodilation by 24% in old rats, whereas young vessels were unaffected. Thbs-1 increased O2 (-) production in coronary arterioles from rats of both ages, but this was exaggerated in old rats. The addition of CD47 blocking antibody completely restored NO-dependent vasodilation in isolated arterioles from aged rats and attenuated O2 (-) production. Furthermore, αCD47 treatment increased CFR from 9.6 ± 9.3 (IgG) to 84.0 ± 23% in the left ventricle in intact, aged animals. These findings suggest that the influence of Thbs-1 and CD47 on coronary perfusion increases with aging and may be therapeutically targeted to reverse coronary microvascular dysfunction.