Coronary microvascular resistance: methods for its quantification in humans

Quantitative relationship between coronary artery calcium score and hyperemic myocardial blood flow as assessed by hybrid 15O-water PET/CT imaging in patients evaluated for coronary artery disease

BACKGROUND

The incremental value of CAC over traditional risk factors to predict coronary vasodilator dysfunction and inherent myocardial blood flow (MBF) impairment is only scarcely documented (MBF). The aim of this study was therefore to evaluate the relationship between CAC content, hyperemic MBF, and coronary flow reserve (CFR) in patients undergoing hybrid (15)O-water PET/CT imaging.

METHODS

We evaluated 173 (mean age 56 ± 10, 78 men) patients with a low to intermediate likelihood for coronary artery disease (CAD), without a documented history of CAD, undergoing vasodilator stress (15)O-water PET/CT and CAC scoring. Obstructive coronary artery disease was excluded by means of invasive (n = 44) or CT-based coronary angiography (n = 129).

RESULTS

91 of 173 patients (52%) had a CAC score of zero. Of those with CAC, the CAC score was 0.1-99.9, 100-399.9, and ≥400 in 31%, 12%, and 5% of patients, respectively. Global CAC score showed significant inverse correlation with hyperemic MBF (r = -0.32, P < .001). With increasing CAC score, there was a decline in hyperemic MBF on a per-patient basis [3.70, 3.30, 2.68, and 2.53 mL · min(-1) · g(-1), with total CAC score of 0, 0.1-99.9, 100-399.9, and ≥400, respectively (P < .001)]. CFR showed a stepwise decline with increasing levels of CAC (3.70, 3.32, 2.94, and 2.93, P < .05). Multivariate analysis, including age, BMI, and CAD risk factors, revealed that only age, male gender, BMI, and hypercholesterolemia were associated with reduced stress perfusion. Furthermore, only diabetes and age were independently associated with CFR.

CONCLUSION

In patients without significant obstructive CAD, a greater CAC burden is associated with a decreased hyperemic MBF and CFR. However, this association disappeared after adjustment for traditional CAD risk factors. These results suggest that CAC does not add incremental value regarding hyperemic MBF and CFR over established CAD risk factors in patients without obstructive CAD.

Quantification of myocardial blood flow with 82Rb positron emission tomography: clinical validation with 15O-water

Purpose

Quantification of myocardial blood flow (MBF) with generator-produced ⁸²Rb is an attractive alternative for centres without an on-site cyclotron. Our aim was to validate ⁸²Rb-measured MBF in relation to that measured using ¹⁵O-water, as a tracer 100% of which can be extracted from the circulation even at high flow rates, in healthy control subject and patients with mild coronary artery disease (CAD).

Methods

MBF was measured at rest and during adenosine-induced hyperaemia with ⁸²Rb and ¹⁵O-water PET in 33 participants (22 control subjects, aged 30 ± 13 years; 11 CAD patients without transmural infarction, aged 60 ± 13 years). A one-tissue compartment ⁸²Rb model with ventricular spillover correction was used. The ⁸²Rb flow-dependent extraction rate was derived from ¹⁵O-water measurements in a subset of 11 control subjects. Myocardial flow reserve (MFR) was defined as the hyperaemic/rest MBF. Pearson’s correlation r, Bland-Altman 95% limits of agreement (LoA), and Lin’s concordance correlation ρ_c (measuring both precision and accuracy) were used.

Results

Over the entire MBF range (0.66–4.7 ml/min/g), concordance was excellent for MBF (r = 0.90, [⁸²Rb–¹⁵O-water] mean difference ± SD = 0.04 ± 0.66 ml/min/g, LoA = −1.26 to 1.33 ml/min/g, ρ_c = 0.88) and MFR (range 1.79–5.81, r = 0.83, mean difference = 0.14 ± 0.58, LoA = −0.99 to 1.28, ρ_c = 0.82). Hyperaemic MBF was reduced in CAD patients compared with the subset of 11 control subjects (2.53 ± 0.74 vs. 3.62 ± 0.68 ml/min/g, p = 0.002, for ¹⁵O-water; 2.53 ± 1.01 vs. 3.82 ± 1.21 ml/min/g, p = 0.013, for ⁸²Rb) and this was paralleled by a lower MFR (2.65 ± 0.62 vs. 3.79 ± 0.98, p = 0.004, for ¹⁵O-water; 2.85 ± 0.91 vs. 3.88 ± 0.91, p = 0.012, for ⁸²Rb). Myocardial perfusion was homogeneous in 1,114 of 1,122 segments (99.3%) and there were no differences in MBF among the coronary artery territories (p > 0.31).

Conclusion

Quantification of MBF with ⁸²Rb with a newly derived correction for the nonlinear extraction function was validated against MBF measured using ¹⁵O-water in control subjects and patients with mild CAD, where it was found to be accurate at high flow rates. ⁸²Rb-derived MBF estimates seem robust for clinical research, advancing a step further towards its implementation in clinical routine.

When Collateral Supply is Accounted For Epicardial Stenosis Does Not Increase Microvascular Resistance

Background—

The relationship between epicardial stenosis and microvascular resistance remains controversial. Exploring the relationship is critical, as many tools used in interventional cardiology imply minimal and constant resistance. However, variable collateralization may impact well on these measures. We hypothesized that when collateral supply was accounted for, microvascular resistance would be independent of epicardial stenosis.

Methods and Results—

Forty patients with stable angina were studied before and following percutaneous intervention. A temperature and pressure sensing guide wire was used to derive microvascular resistance using the index of microcirculatory resistance (IMR), defined as the hyperemic distal pressure multiplied by the hyperemic mean transit time. Lesion severity was assessed using fractional flow reserve. For comparison, evaluation of an angiographically normal reference vessel from the same subject also was undertaken. Both simple IMR (sIMR) and IMR corrected for collateral flow (cIMR) were calculated. When collateral supply was not accounted for, there was a significant difference in IMR values between the culprit, the post PCI, and nonculprit values (culprit sIMR 26.68±2.06, nonculprit sIMR 18.37±1.89, P =0.002; post percutaneous intervention sIMR 18.5±1.94 versus culprit sIMR 26.68±2.06, P <0.0001). However, when collateral supply was accounted for there was no difference observed (cIMR 16.96±1.78 versus nonculprit sIMR 18.37±1.89, P =0.52; post percutaneous intervention sIMR 18.5±1.94 versus cIMR 16.96±1.78, P =0.42).

Conclusions—

When collateral supply is accounted for, epicardial stenosis does not increase microvascular resistance in patients with stable angina.

Increased basal coronary blood flow as a cause of reduced coronary flow reserve in diabetic patients

A reduced coronary flow reserve (CFR) has been demonstrated in diabetes, but the underlying mechanisms are unknown. We assessed thermodilution-derived CFR after 5-min intravenous adenosine infusion through a pressure-temperature sensor-tipped wire in 30 coronary arteries without significant lumen reduction in 30 patients: 13 with and 17 without a history of diabetes. We determined CFR as the ratio of basal and hyperemic mean transit times (Tmn); fractional flow reserve (FFR) as the ratio of distal and proximal pressures at maximal hyperemia to exclude local macrovascular disease; and an index of microvascular resistance (IMR) as the distal coronary pressure at maximal hyperemia divided by the inverse of the hyperemic Tmn. We also assessed insulin resistance by the homeostasis model assessment (HOMA) index. FFR was normal in all investigated arteries. CFR was significantly lower in diabetic vs. nondiabetic patients [median (interquartile range): 2.2 (1.4–3.2) vs. 4.1 (2.7–4.4); P = 0.02]. Basal Tmn was lower in diabetic vs. nondiabetic subjects [median (interquartile range): 0.53 (0.25–0.71) vs. 0.64 (0.50–1.17); P = 0.04], while hyperemic Tmn and IMR were similar. We found significant correlations at linear regression analysis between logCFR and the HOMA index ( r2 = 0.35; P = 0.0005) and between basal Tmn and the HOMA index ( r2 = 0.44; P < 0.0001). In conclusion, compared with nondiabetic subjects, CFR is lower in patients with diabetes and epicardial coronary arteries free of severe stenosis, because of increased basal coronary flow, while hyperemic coronary flow is similar. Basal coronary flow relates to insulin resistance, suggesting a key role of cellular metabolism in the regulation of coronary blood flow.

No-reflow phenomenon and endothelial glycocalyx of microcirculation

The progress in reperfusion therapy dictated the necessity for developing new tools and procedures for adjacent/additional therapy of acute cardiovascular disorders. The adjacent therapy is targeted on the damage of the microcirculation, leading to the unfavorable prognosis for the patients. The no-reflow phenomenon holds special place in the multifactorial etiology of the microcirculation disorders, offering a new challenge in treating the patients associated with ST-segment elevation on ECG at myocardial infarction. One of the numerous causes of no-reflow, the influence of the endothelial glycocalyx of the microcirculation, is analyzed. The results obtained in the studies of the endothelial glycocalyx ultrastructure are generalized, the effect that the fragments of the glycocalyx glycosaminoglycans have on the function of the vascular wall is demonstrated. The trends in searching for correlations between the thickness of the capillary glycocalyx and the cardiovascular disease risk are noted.

Carriers of the hypertrophic cardiomyopathy MYBPC3 mutation are characterized by reduced myocardial efficiency in the absence of hypertrophy and microvascular dysfunction

AIMS

Next to left ventricular (LV) hypertrophy, hypertrophic cardiomyopathy (HCM) is characterized by microvascular dysfunction and reduced myocardial external efficiency (MEE). Insights into the presence of these abnormalities as early markers of disease are of clinical importance in risk stratification, and development of therapeutic approaches. Therefore, the aim was to investigate myocardial perfusion and energetics in genotype-positive, phenotype-negative HCM subjects (carriers).

METHODS AND RESULTS

Fifteen carriers of an MYBPC3 mutation underwent [(15)O]water positron emission tomography (PET) to assess myocardial blood flow (MBF). [(11)C]acetate PET was performed to obtain myocardial oxygen consumption (MVO(2)). By use of cardiovascular magnetic resonance imaging, LV volumes and mass were defined to calculate MEE, i.e. the ratio between external work and MVO(2). Eleven healthy, genotype-negative, family relatives underwent similar scanning protocols to serve as a control group. Left ventricular mass was comparable between carriers and controls (93 ± 25 vs. 99 ± 21 g, P= 0.85), as was MBF at rest (1.19 ± 0.34 vs. 1.18 ± 0.32 mL min(-1) g(-1), P= 0.92), and during hyperaemia (3.87 ± 0.75 vs. 3.96 ± 0.86 mL min(-1) g(-1), P= 0.77). Myocardial oxygen consumption averaged 0.137 ± 0.057 mL min(-1) g(-1) in carriers and was not significantly different from controls (0.125 ± 0.043 mL min(-1) g(-1), P= 0.29). Cardiac work, however, was slightly reduced in carriers (7398 ± 1384 vs. 9139 ± 2484 mmHg mL in controls, P= 0.08). As a consequence, MEE was significantly decreased in carriers (27 ± 10 vs. 36 ± 8% in controls, P= 0.02).

CONCLUSION

Carriers display reduced myocardial work generation in relation to oxygen consumption, in the absence of hypertrophy and flow abnormalities. Hence, impaired myocardial energetics may constitute a primary component of HCM pathogenesis.

Regulation of the human coronary microcirculation

Atherosclerosis of conduit epicardial arteries is the principal culprit behind the complications of coronary heart disease, but a growing body of literature indicates that the coronary microcirculation also contributes substantially to the pathophysiology of cardiovascular disease. An understanding of mechanisms regulating microvascular function in humans is an essential foundation for understanding the role in disease, especially since these regulatory mechanisms vary substantially across species and vascular beds. In fact all subjects whose coronary tissue was used in the studies described have medical conditions that warrant cardiac surgery, thus relevance to the normal human must be inferential and is based on tissue from subjects without known arteriosclerotic disease. This review will focus on recent advances in the physiological and pathological mechanisms of coronary microcirculatory control, describing a robust plasticity in maintaining endothelial control over dilation, including mechanisms that are most relevant to the human heart. This article is part of a Special Issue entitled "Coronary Blood Flow".

QT Dispersion is Not Increased in Familial Mediterranean Fever

Familial Mediterranean fever (FMF) is an autoimmune disease inherited as an autosomal recessive trait and is characterized by recurrent attacks of fever and sterile polyserositis. This study examined electrocardiographic ventricular repolarization parameters (QT interval and QT dispersion) in 38 FMF patients and 35 healthy controls. The QT interval was measured manually from the onset of QRS to the end of the T wave (return to the TP baseline). QT dispersion was defined as the difference between the maximum and minimum QT values, and corrected QT was calculated according to the Bazett formula. There were no significant differences between FMF patients and healthy control subjects in any parameter of ventricular repolarization; hence QT dispersion was not affected by FMF. Electrocardiographic assessment of QT interval and QT dispersion are, therefore, of little value for the evaluation of cardiac impairment and risk of arrhythmia in FMF patients.

Nanoparticle diffusion measures bulk clot permeability

A clot's function is to achieve hemostasis by resisting fluid flow. Permeability is the measurement of a clot's hemostatic potential. It is sensitive to a wide range of biochemical parameters and pathologies. In this work, we consider the hydrodynamic phenomenon that reduces the mobility of fluid near the fiber surfaces. This no-slip boundary condition both defines the gel's permeability and suppresses nanoparticle diffusion in gel interstices. Here we report that, unlike previous work where steric effects also hindered diffusion, our system-nanoparticles in fibrin gel-was subject exclusively to hydrodynamic diffusion suppression. This result enabled an automated, high-throughput permeability assay that used small clot volumes. Permeability was derived from nanoparticle diffusion using the effective medium theory, and showed one-to-one correlation with measured permeability. This technique measured permeability without quantifying gel structure, and may therefore prove useful for characterizing similar materials (e.g., extracellular matrix) where structure is uncontrolled during polymerization and difficult to measure subsequently. We also report that PEGylation reduced, but did not eliminate, the population of immobile particles. We studied the forces required to pull stuck PEG particles free to confirm that the attachment is a result of neither covalent nor strong electrostatic binding, and discuss the relevance of this force scale to particle transport through physiological clots.

Coronary pressure-flow relations as basis for the understanding of coronary physiology

Recent technological advancements in the area of intracoronary physiology, as well as non-invasive contrast perfusion imaging, allow to make clinical decisions with respect to percutaneous coronary interventions and to identify microcirculatory coronary pathophysiology. The basic characteristics of coronary hemodynamics, as described by pressure-flow relations in the normal and diseased heart, need to be understood for a proper interpretation of these physiological measurements. Especially the hyperemic coronary pressure-flow relation, as well as the influence of cardiac function on it, bears great clinical significance. The interaction of a coronary stenosis with the coronary pressure-flow relation can be understood from the stenosis pressure drop-flow velocity relationship. Based on these relationships the clinically applied concepts of coronary flow velocity reserve, fractional flow reserve, stenosis resistance and microvascular resistance are discussed. Attention is further paid to the heterogeneous nature of myocardial perfusion, the vulnerability of the subendocardium and the role of collateral flow on hyperemic coronary pressure-flow relations. This article is part of a Special Issue entitled "Coronary Blood Flow".

Feasibility of subendocardial and subepicardial myocardial perfusion measurements in healthy normals with (15)O-labeled water and positron emission tomography

BACKGROUND

Positron emission tomography (PET) enables robust and reproducible measurements of myocardial blood flow (MBF). However, the relatively limited resolution of PET till recently prohibited distinction between the subendocardial and the subepicardial layers in non-hypertrophied myocardium. Recent developments in hard- and software, however, have enabled to identify a transmural gradient difference in animal experiments. The aim of this study is to determine the feasibility of subendocardial and subepicardial MBF in normal human hearts assessed with (15)O-labeled water PET.

METHODS

Twenty-seven healthy subjects (mean age 41 ± 13 years; 11 men) were studied with (15)O-labeled water PET to quantify resting and hyperaemic (adenosine) MBF at a subendocardial and subepicardial level. In addition, cardiac magnetic resonance imaging was performed to determine left ventricular (LV) volumes and function.

RESULTS

Mean rest MBF was 1.46 ± 0.49 in the subendocardium, and 1.14 ± 0.342 mL · min(-1) · g(-1) in the subepicardium (P < .001). MBF during vasodilation was augmented to a greater extent at the subepicardial level (subendocardium vs subepicardium: 3.88 ± 0.86 vs 4.14 ± 0.88 mL · min(-1) · g(-1), P = .013). The endocardial-to-epicardial MBF ratio decreased significantly during hyperaemia (1.35 ± 0.23 to 1.12 ± 0.20, P < .001). Hyperaemic transmural MBF was inversely correlated with left ventricular end-diastolic volume index (LVEDVI) (r (2) = 0.41, P = .0003), with greater impact however at the subendocardial level.

CONCLUSIONS

(15)O-labeled water PET enables MBF measurements with distinction of the subendocardial and subepicardial layers in the normal human heart and correlates with LVEDVI. This PET technique may prove useful in evaluating patients with signs of ischaemia due to coronary artery disease or microvascular dysfunction.

Assessment of coronary microcirculation in a swine animal model

Coronary microvascular dysfunction has important prognostic implications. Several hemodynamic indexes, such as coronary flow reserve (CFR), microvascular resistance, and zero-flow pressure (P(zf)), were used to establish the most reliable index to assess coronary microcirculation. Fifteen swine were instrumented with a flow probe, and a pressure wire was advanced into the distal left anterior descending artery. Adenosine was used to produce maximum hyperemia. Microspheres were used to create microvascular dysfunction. An occluder was used to produce stenosis. Blood flow from the probe (Q(p)), aortic pressure, distal coronary pressure, and right atrium pressure were recorded. Angiographic flow (Q(a)) was calculated using a time-density curve. Flow probe-based CFR and angiographic CFR were calculated using Q(p) and Q(a), respectively. Flow probe-based (NMR(qh)) and angiographic normalized microvascular resistance (NMR(ah)) were determined using Q(p) and Q(a), respectively, during hyperemia. P(zf) was calculated using Q(p) and distal coronary pressure. Two series of receiver operating characteristic curves were generated: normal epicardial artery model (N model) and stenosis model (S model). The areas under the receiver operating characteristic curves for flow probe-based CFR, angiographic CFR, NMR(qh), NMR(ah), and P(zf) were 0.855, 0.836, 0.976, 0.956, and 0.855 in N model and 0.737, 0.700, 0.935, 0.889, and 0.698 in S model. Both NMR(qh) and NMR(ah) were significantly more reliable than CFR and P(zf) in detecting the microvascular deterioration. Compared with CFR and P(zf), NMR provided a more accurate assessment of microcirculation. This improved accuracy was more prevalent when stenosis existed. Moreover, NMR(ah) is potentially a less invasive method for assessing coronary microcirculation.

Relation of coronary microvascular dysfunction in hypertrophic cardiomyopathy to contractile dysfunction independent from myocardial injury

We studied the spatial relations among hyperemic myocardial blood flow (hMBF), contractile function, and morphologic tissue alterations in 19 patients with hypertrophic cardiomyopathy (HC). All patients were studied with oxygen-15 water positron emission tomography during rest and adenosine administration to assess myocardial perfusion. Cardiovascular magnetic resonance was performed to derive delayed contrast-enhanced images and to calculate contractile function (E(cc)) with tissue tagging. Eleven healthy subjects underwent similar positron emission tomographic and cardiovascular magnetic resonance scanning protocols and served as a control group. In the HC group, hMBF averaged 2.46 ± 0.91 ml/min/g and mean E(cc) was -14.7 ± 3.4%, which were decreased compared to the control group (3.97 ± 1.48 ml/min/g and -17.7 ± 3.2%, respectively, p <0.001 for the 2 comparisons). Delayed contrast enhancement (DCE) was present only in patients with HC, averaging 6.2 ± 10.3% of left ventricular mass. In the HC group, E(cc) and DCE in the septum (-13.7 ± 3.6% and 10.2 ± 13.6%) significantly differed from the lateral wall (-16.0 ± 2.8% and 2.4 ± 5.9%, p <0.001 for the 2 comparisons). In general, hMBF and E(cc) were decreased in segments displaying DCE compared to nonenhanced segments (p <0.001 for the comparisons). In the HC group, univariate analysis revealed relations of hMBF to E(cc) (r = -0.45, p <0.001) and DCE (r = -0.31, p <0.001). Multivariate analysis revealed that E(cc) was independently related to hMBF (beta -0.37, p <0.001) and DCE (beta 0.28, p <0.001). In conclusion, in HC hMBF is impaired and related to contractile function independent from presence of DCE. When present, DCE reflected a progressed disease state as characterized by an increased perfusion deficit and contractile dysfunction.

Effects of alcohol septal ablation on coronary microvascular function and myocardial energetics in hypertrophic obstructive cardiomyopathy

This study investigated the effects of alcohol septal ablation (ASA) on microcirculatory function and myocardial energetics in patients with hypertrophic cardiomyopathy (HCM) and left ventricular outflow tract (LVOT) obstruction. In 15 HCM patients who underwent ASA, echocardiography was performed before and 6 mo after the procedure to assess the LVOT gradient (LVOTG). Additionally, [(15)O]water PET was performed to obtain resting myocardial blood flow (MBF) and coronary vasodilator reserve (CVR). Changes in LV mass (LVM) and volumes were assessed by cardiovascular magnetic resonance imaging. Myocardial oxygen consumption (MVo(2)) was evaluated by [(11)C]acetate PET in a subset of seven patients to calculate myocardial external efficiency (MEE). After ASA, peak LVOTG decreased from 41 ± 32 to 23 ± 19 mmHg (P = 0.04), as well as LVM (215 ± 74 to 169 ± 63 g; P < 0.001). MBF remained unchanged (0.94 ± 0.23 to 0.98 ± 0.15 ml·min(-1)·g(-1); P = 0.45), whereas CVR increased (2.55 ± 1.23 to 3.05 ± 1.24; P = 0.05). Preoperatively, the endo-to-epicardial MBF ratio was lower during hyperemia compared with rest (0.80 ± 0.18 vs. 1.18 ± 0.15; P < 0.001). After ASA, the endo-to-epicardial hyperemic (h)MBF ratio increased to 1.03 ± 0.26 (P = 0.02). ΔCVR was correlated to ΔLVOTG (r = -0.82; P < 0.001) and ΔLVM (r = -0.54; P = 0.04). MEE increased from 15 ± 6 to 20 ± 9% (P = 0.04). Coronary microvascular dysfunction in obstructive HCM is at least in part reversible by relief of LVOT obstruction. After ASA, hMBF and CVR increased predominantly in the subendocardium. The improvement in CVR was closely correlated to the absolute reduction in peak LVOTG, suggesting a pronounced effect of LV loading conditions on microvascular function of the subendocardium. Furthermore, ASA has favorable effects on myocardial energetics.

Quantification of coronary microvascular resistance using angiographic images for volumetric blood flow measurement: in vivo validation

Structural coronary microcirculation abnormalities are important prognostic determinants in clinical settings. However, an assessment of microvascular resistance (MR) requires a velocity wire. A first-pass distribution analysis technique to measure volumetric blood flow has been previously validated. The aim of this study was the in vivo validation of the MR measurement technique using first-pass distribution analysis. Twelve anesthetized swine were instrumented with a transit-time ultrasound flow probe on the proximal segment of the left anterior descending coronary artery (LAD). Microspheres were injected into the LAD to create a model of microvascular dysfunction. Adenosine (400 μg·kg(-1)·min(-1)) was used to produce maximum hyperemia. A region of interest in the LAD arterial bed was drawn to generate time-density curves using angiographic images. Volumetric blood flow measurements (Q(a)) were made using a time-density curve and the assumption that blood was momentarily replaced with contrast agent during the injection. Blood flow from the flow probe (Q(p)), coronary pressure (P(a)), and right atrium pressure (P(v)) were continuously recorded. Flow probe-based normalized MR (NMR(p)) and angiography-based normalized MR (NMR(a)) were calculated using Q(p) and Q(a), respectively. In 258 measurements, Q(a) showed a strong correlation with the gold standard Q(p) (Q(a) = 0.90 Q(p) + 6.6 ml/min, r(2) = 0.91, P < 0.0001). NMR(a) correlated linearly with NMR(p) (NMR(a) = 0.90 NMR(p) + 0.02 mmHg·ml(-1)·min(-1), r(2) = 0.91, P < 0.0001). Additionally, the Bland-Altman analysis showed a close agreement between NMR(a) and NMR(p). In conclusion, a technique based on angiographic image data for quantifying NMR was validated using a swine model. This study provides a method to measure NMR without using a velocity wire, which can potentially be used to evaluate microvascular conditions during coronary arteriography.

Stress perfusion imaging using cardiovascular magnetic resonance: a review

Stress perfusion CMR can provide both excellent diagnostic and important prognostic information in the context of a comprehensive assessment of cardiac anatomy and function. This coupled with the high spatial resolution, and the lack of both attenuation artefacts and ionising radiation, make CMR stress perfusion imaging a highly attractive stress imaging modality. It is now in routine use in many centres, and shows promise in evaluating patients with clinical problems beyond those of epicardial coronary disease.

Myocardial perfusion imaging after coronary artery bypass surgery using cardiovascular magnetic resonance: a validation study

BACKGROUND

Absolute quantification of perfusion with cardiovascular magnetic resonance has not previously been applied in patients with coronary artery bypass grafting (CABG). Owing to increased contrast bolus dispersion due to the greater distance of travel through a bypass graft, this approach may result in systematic underestimation of myocardial blood flow (MBF). As resting MBF remains normal in segments supplied by noncritical coronary stenosis (<85%), measurement of perfusion in such territories may be utilized to reveal systematic error in the quantification of MBF. The objective of this study was to test whether absolute quantification of perfusion with cardiovascular magnetic resonance systematically underestimates MBF in segments subtended by bypass grafts.

METHODS AND RESULTS

The study population comprised 28 patients undergoing elective CABG for treatment of multivessel coronary artery disease. Eligible patients had angiographic evidence of at least 1 myocardial segment subtended by a noncritically stenosed coronary artery (<85%). Subjects were studied at 1.5 T, with evaluation of resting MBF using model-independent deconvolution. Analyses were confined to myocardial segments subtended by native coronary arteries with <85% stenosis at baseline, and MBF was compared in grafted and ungrafted segments before and after revascularization. A total of 249 segments were subtended by coronary arteries with <85% stenosis at baseline. After revascularization, there was no significant difference in MBF in ungrafted (0.82±0.19 mL/min/g) versus grafted segments (0.82±0.15 mL/min/g, P=0.57). In the latter, MBF after revascularization did not change significantly from baseline (0.86±0.20 mL/min/g, P=0.82).

CONCLUSIONS

Model-independent deconvolution analysis does not systematically underestimate blood flow in graft-subtended territories, justifying the use of this methodology to evaluate myocardial perfusion in patients with CABG.

Theoretical models for coronary vascular biomechanics: progress & challenges

A key aim of the cardiac Physiome Project is to develop theoretical models to simulate the functional behaviour of the heart under physiological and pathophysiological conditions. Heart function is critically dependent on the delivery of an adequate blood supply to the myocardium via the coronary vasculature. Key to this critical function of the coronary vasculature is system dynamics that emerge via the interactions of the numerous constituent components at a range of spatial and temporal scales. Here, we focus on several components for which theoretical approaches can be applied, including vascular structure and mechanics, blood flow and mass transport, flow regulation, angiogenesis and vascular remodelling, and vascular cellular mechanics. For each component, we summarise the current state of the art in model development, and discuss areas requiring further research. We highlight the major challenges associated with integrating the component models to develop a computational tool that can ultimately be used to simulate the responses of the coronary vascular system to changing demands and to diseases and therapies.

Low-dose quantitative myocardial blood flow imaging using 15O-water and PET without attenuation correction

Misalignment between PET and low-dose CT (LD-CT) can cause severe artifacts in cardiac PET/CT because of attenuation-correction errors, even when using slow or cine LD-CT. Myocardial blood flow (MBF), as measured by (15)O-water, can be determined from the rate of (15)O-water washout from myocardial tissue, which is independent of tissue attenuation. The purpose of the present study was to assess the accuracy of these MBF measurements in the absence of attenuation correction.

METHODS

Twenty-five patients referred for evaluation of myocardial perfusion underwent 6-min rest and adenosine stress PET scans after the administration of 370 MBq of (15)O-water; both scans were followed by slow LD-CT. Data were acquired on a PET/CT scanner and reconstructed by a 3-dimensional row-action maximum likelihood algorithm both with (CTAC) and without (NAC) attenuation correction. An ascending aorta volume of interest was used as input function. MBF and coronary flow reserve (CFR) were calculated for 17 myocardial segments using nonlinear regression of the standard single-tissue-compartment model with corrections for left and right ventricular spillover and perfusable tissue fraction.

RESULTS

High correlation (r(2) = 0.99 and 0.97, with slopes of 0.96 and 0.91 for rest and stress, respectively) and excellent agreement (intraclass correlation coefficient [ICC], 1.00 and 0.98) between NAC- and CTAC-based MBF values were found. Absolute rest and stress MBF values were 3% and 8%, respectively, lower for NAC scans. The correlation coefficient between all NAC and CTAC CFR values was 0.95 (ICC, 0.95; slope, 0.92) and 0.97 (ICC, 0.99; slope, 1.01) when only CFR values below 2 were considered. Deviations between CTAC and NAC values were smallest for basal segments and increased toward the apex.

CONCLUSION

MBF and CFR can be measured accurately using (15)O-water and PET without correcting for attenuation, reducing the effective dose to the patient to 0.8 mSv for a complete rest-stress protocol. This dose is an order of magnitude lower than typical values for (82)Rb, (99m)Tc-methoxyisobutylisonitrile, or CT perfusion scans.