Simultaneous noninvasive determination of regional myocardial perfusion and oxygen content in rabbits: toward direct measurement of myocardial oxygen consumption at MR imaging

Impaired Myocardial Oxygenation Response to Stress in Patients With Chronic Kidney Disease

BACKGROUND

Coronary artery disease and left ventricular hypertrophy are prevalent in the chronic kidney disease (CKD) and renal transplant (RT) population. Advances in cardiovascular magnetic resonance (CMR) with blood oxygen level-dependent (BOLD) technique provides capability to assess myocardial oxygenation as a measure of ischemia. We hypothesized that the myocardial oxygenation response to stress would be impaired in CKD and RT patients.

METHODS AND RESULTS

Fifty-three subjects (23 subjects with CKD, 10 RT recipients, 10 hypertensive (HT) controls, and 10 normal controls without known coronary artery disease) underwent CMR scanning. All groups had cine and BOLD CMR at 3 T. The RT and HT groups also had late gadolinium CMR to assess infarction/replacement fibrosis. The CKD group underwent 2-dimensional echocardiography strain to assess fibrosis. Myocardial oxygenation was measured at rest and under stress with adenosine (140 μg/kg per minute) using BOLD signal intensity. A total of 2898 myocardial segments (1200 segments in CKD patients, 552 segments in RT, 480 segments in HT, and 666 segments in normal controls) were compared using linear mixed modeling. Diabetes mellitus (P=0.47) and hypertension (P=0.57) were similar between CKD, RT, and HT groups. The mean BOLD signal intensity change was significantly lower in the CKD and RT groups compared to HT controls and normal controls (-0.89±10.63% in CKD versus 5.66±7.87% in RT versus 15.54±9.58% in HT controls versus 16.19±11.11% in normal controls, P<0.0001). BOLD signal intensity change was associated with estimated glomerular filtration rate (β=0.16, 95% CI=0.10 to 0.22, P<0.0001). Left ventricular mass index and left ventricular septal wall diameter were similar between the CKD predialysis, RT, and HT groups. None of the CKD patients had impaired global longitudinal strain and none of the RT group had late gadolinium hyperenhancement.

CONCLUSIONS

Myocardial oxygenation response to stress is impaired in CKD patients and RT recipients without known coronary artery disease, and unlikely to be solely accounted for by the presence of diabetes mellitus, left ventricular hypertrophy, or myocardial scarring. The impaired myocardial oxygenation in CKD patients may be associated with declining renal function. Noncontrast BOLD CMR is a promising tool for detecting myocardial ischemia in the CKD population.

Computational estimation of tricarboxylic acid cycle fluxes using noisy NMR data from cardiac biopsies

Background

The aerobic energy metabolism of cardiac muscle cells is of major importance for the contractile function of the heart. Because energy metabolism is very heterogeneously distributed in heart tissue, especially during coronary disease, a method to quantify metabolic fluxes in small tissue samples is desirable. Taking tissue biopsies after infusion of substrates labeled with stable carbon isotopes makes this possible in animal experiments. However, the appreciable noise level in NMR spectra of extracted tissue samples makes computational estimation of metabolic fluxes challenging and a good method to define confidence regions was not yet available.

Results

Here we present a computational analysis method for nuclear magnetic resonance (NMR) measurements of tricarboxylic acid (TCA) cycle metabolites. The method was validated using measurements on extracts of single tissue biopsies taken from porcine heart in vivo. Isotopic enrichment of glutamate was measured by NMR spectroscopy in tissue samples taken at a single time point after the timed infusion of ¹³C labeled substrates for the TCA cycle. The NMR intensities for glutamate were analyzed with a computational model describing carbon transitions in the TCA cycle and carbon exchange with amino acids. The model dynamics depended on five flux parameters, which were optimized to fit the NMR measurements. To determine confidence regions for the estimated fluxes, we used the Metropolis-Hastings algorithm for Markov chain Monte Carlo (MCMC) sampling to generate extensive ensembles of feasible flux combinations that describe the data within measurement precision limits. To validate our method, we compared myocardial oxygen consumption calculated from the TCA cycle flux with in vivo blood gas measurements for 38 hearts under several experimental conditions, e.g. during coronary artery narrowing.

Conclusions

Despite the appreciable NMR noise level, the oxygen consumption in the tissue samples, estimated from the NMR spectra, correlates with blood-gas oxygen uptake measurements for the whole heart. The MCMC method provides confidence regions for the estimated metabolic fluxes in single cardiac biopsies, taking the quantified measurement noise level and the nonlinear dependencies between parameters fully into account.

Real time measurement of myocardial oxygen dynamics during cardiac ischemia-reperfusion of rats

Because oxygen plays a critical role in the pathophysiology of myocardial injury during subsequent reperfusion, as well as ischemia, the accurate measurement of myocardial oxygen tension is crucial for the assessment of myocardial viability by ischemia-reperfusion (IR) injury. Therefore, we utilized a sol-gel derived electrochemical oxygen microsensor to monitor changes in oxygen tension during myocardial ischemia-reperfusion. We also analyzed differences in oxygen tension recovery in post-ischemic myocardium depending on ischemic time to investigate the correlation between recovery parameters for oxygen tension and the severity of IR injury. An oxygen sensor was built using a xerogel-modified platinum microsensor and a coiled Ag/AgCl reference electrode. Rat hearts were randomly divided into 5 groups: control (0 min ischemia), I-10 (10 min ischemia), I-20 (20 min ischemia), I-30 (30 min ischemia), and I-40 (40 min ischemia) groups (n = 3 per group, respectively). After the induction of ischemia, reperfusion was performed for 60 min. As soon as the ischemia was initiated, oxygen tension rapidly declined to near zero levels. When reperfusion was initiated, the changes in oxygen tension depended on ischemic time. The normalized peak level of oxygen tension during the reperfusion episode was 188 ± 27 in group I-10, 120 ± 24 in group I-20, 12.5 ± 10.6 in group I-30, and 1.24 ± 1.09 in group I-40 (p < 0.001, n = 3, respectively). After 60 min of reperfusion, the normalized restoration level was 129 ± 30 in group I-10, 88 ± 4 in group I-20, 3.40 ± 4.82 in group I-30, and 0.99 ± 0.94 in group I-40 (p < 0.001, n = 3, respectively). The maximum and restoration values of oxygen tension in groups I-30 and I-40 after reperfusion were lower than pre-ischemic values. In particular, oxygen tension in the I-40 group was not recovered at all. These results were also demonstrated by TTC staining. We suggest that these recovery parameters could be utilized as an index of tissue injury and severity of ischemia. Therefore, quantitative measurements of oxygen tension dynamics in the myocardium would be helpful for evaluation of the cardioprotective effects of therapeutic treatments such as drug administration.

Myocardial oxygenation in coronary artery disease: insights from blood oxygen level-dependent magnetic resonance imaging at 3 tesla

OBJECTIVES

The purpose of this study was to assess the diagnostic accuracy of blood oxygen-level dependent (BOLD) MRI in suspected coronary artery disease (CAD).

BACKGROUND

By exploiting the paramagnetic properties of deoxyhemoglobin, BOLD magnetic resonance imaging can detect myocardial ischemia. We applied BOLD imaging and first-pass perfusion techniques to: 1) examine the pathophysiological relationship between coronary stenosis, perfusion, ventricular scar, and myocardial oxygenation; and 2) evaluate the diagnostic performance of BOLD imaging in the clinical setting.

METHODS

BOLD and first-pass perfusion images were acquired at rest and stress (4 to 5 min intravenous adenosine, 140 μg/kg/min) and assessed quantitatively (using a BOLD signal intensity index [stress/resting signal intensity], and absolute quantification of perfusion by model-independent deconvolution). A BOLD signal intensity index threshold to identify ischemic myocardium was first determined in a derivation arm (25 CAD patients and 20 healthy volunteers). To determine diagnostic performance, this was then applied in a separate group comprising 60 patients with suspected CAD referred for diagnostic angiography.

RESULTS

Prospective evaluation of BOLD imaging yielded an accuracy of 84%, a sensitivity of 92%, and a specificity of 72% for detecting myocardial ischemia and 86%, 92%, and 72%, respectively, for identifying significant coronary stenosis. Segment-based analysis revealed evidence of dissociation between oxygenation and perfusion (r = -0.26), with a weaker correlation of quantitative coronary angiography with myocardial oxygenation (r = -0.20) than with perfusion (r = -0.40; p = 0.005 for difference). Hypertension increased the odds of an abnormal BOLD response, but diabetes mellitus, hypercholesterolemia, and the presence of ventricular scar were not associated with significant deoxygenation.

CONCLUSIONS

BOLD imaging provides valuable insights into the pathophysiology of CAD; myocardial hypoperfusion is not necessarily commensurate with deoxygenation. In the clinical setting, BOLD imaging achieves favorable accuracy for identifying the anatomic and functional significance of CAD.

Quantitative analysis of myocardial perfusion in rabbits by transthoracic real-time myocardial contrast echocardiography

To evaluate the feasibility of real-time myocardial contrast echocardiography (RTMCE) by quantitative analysis of myocardial perfusion in rabbits, transthoracic RTMCE was performed in 10 healthy rabbits by using continuous infusion of SonoVue into the auricular vein. The short axis view at the papillary muscle level was obtained. The duration of the time that the contrast took to appear in right heart, left heart and myocardium was recorded. The regional myocardial signal intensity (SI) versus refilling time plots were fitted to an exponential function: y(t) =A(1-e(-beta(t-t0))) + C, where y is SI at any given time, A is the SI plateau that reflects myocardial blood volume, and beta is the slope of the refilling curve that reflects myocardial microbubble velocity. The A, beta and Axbeta values at different infusion rate of SonoVue were analyzed and the A, beta and Axbeta values in each segment in the short axis view at the papillary muscle level were compared. All the animal experiments were successful and high-quality images were obtained. The best intravenous infusion rate for SonoVue was 30 mL/h. The contrast appeared in right heart, left heart and myocardium at 7.5+/-2.2 s, 9.1+/-2.4 s and 12.2+/-1.6 s respectively. After 16.6+/-2.3s, myocardial opacification reached a steady state. The mean A, beta and Axbeta value in the short axis view at the papillary muscle level were 9.8+/-3.0 dB, 1.4+/-0.5 s(-1) and 13.5+/-3.6 dBxs(-1) respectively. A, beta and Axbeta values showed no significant differences among 6 segments. It was suggested that RTMCE was feasible for quantitative analysis of myocardial perfusion in rabbits. It provides a non-invasive method to evaluate the myocardial perfusion in rabbit disease models.

Assessment of myocardial oxygen extraction fraction and perfusion reserve with BOLD imaging in a canine model with coronary artery stenosis

PURPOSE

To determine the feasibility of T2-weighted BOLD imaging for estimating regional myocardial oxygen extraction fraction (OEF) and approximating perfusion reserve (MPR) simultaneously in a canine model with moderate coronary artery stenosis.

MATERIALS AND METHODS

Eight mongrel dogs with moderate coronary artery stenosis underwent BOLD imaging at rest and during dipyridamole-induced hyperemia, using a turbo spin echo (TSE) sequence. Based on a two-compartment model, myocardial OEF(hyperemia) was calculated with the corresponding T2. MPR could be approximated based on Fick's law.

RESULTS

During responsive hyperemia, a regional hypointensity was observed in the abnormally perfused myocardium, reflecting a relatively smaller myocardial T2 increase (3.06% +/- 2.74%, in contrast to 10.19% +/- 4.12% in the normal region). The average OEFs in the normally and abnormally perfused myocardial territories were 0.21 +/- 0.11 and 0.43 +/- 0.12, respectively. For the MPR approximated from the BOLD imaging, a strong correlation (R = 0.9) in the normal myocardium and a good correlation (R = 0.6) distal to the stenosis were obtained compared to microsphere results.

CONCLUSION

In a canine model with moderate coronary artery stenosis, TSE-based BOLD imaging can quantitatively estimate the regional OEF(hyperemia) and approximate the MPR, and can distinguish segments perfused by defected coronary artery.

Magnetic resonance imaging of regional cardiac function in the mouse

In this paper we introduce an improved harmonic phase (HARP) analysis for complementary spatial modulation of magnetization (CSPAMM) tagging of the mouse left ventricular wall, which enables the determination of regional displacement fields with the same resolution as the corresponding CINE anatomical images. CINE MRI was used to measure global function, such as the ejection fraction. The method was tested on two healthy mouse hearts and two mouse hearts with a myocardial infarction, which was induced by a ligation of the left anterior descending coronary artery. We show that the regional displacement fields can be determined. The mean circumferential strain for the left ventricular wall of one of the healthy mice was -0.09 +/- 0.04 (mean +/- standard deviation), while for one of the infarcted mouse hearts strains of -0.02 +/- 0.02 and -0.10 +/- 0.03 were found in the infarcted and remote regions, respectively.

Relationship of apparent myocardial T2 and oxygenation: towards quantification of myocardial oxygen extraction fraction

PURPOSE

To explore the relationship of myocardial T(2) and oxygenation for the quantification of myocardial oxygen extraction fraction (OEF).

MATERIALS AND METHODS

A proposed myocardial T(2)-OEF relationship was evaluated by computer simulation and in nine normal dogs in vivo. The relationship was based on a simplified two-compartment T(2) model. In the dogs, dipyridamole was infused intravenously to increase blood flow and change in myocardial oxygen content. The accuracy of the measurement in myocardial OEF in vivo by magnetic resonance imaging (MRI) was determined by arterial and coronary sinus blood sampling.

RESULTS

Global myocardial T(2) increased 16.1% from rest to the peak of dipyridamole-induced vasodilation (44.6 +/- 2.1 msec vs. 51.4 +/- 2.1 msec, P < 0.001). Corresponding OEF measured by arterial and venous (AV) sampling decreased from 0.64 +/- 0.15 at rest to 0.18 +/- 0.08 during the dipyridamole vasodilation, whereas OEF calculated by MRI at the peak effect of dipyridamole was 20 +/- 4%. Global myocardial OEF measured dynamically by MRI showed a strong correlation with OEF measured by blood sampling (correlation coefficient (CC) = 0.83) during pharmacologic vasodilation.

CONCLUSION

When combined with vasodilator stress, assessment of OEF may provide a putative measure of myocardial flow reserve, allowing consecutive monitoring of myocardial dose-responses to a variety of interventions and offering a new tool for the detection of coronary artery disease.

Perfusion imaging using arterial spin labeling

Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

MR imaging in ischemic heart disease

This article reviews the current MR imaging literature with respect to ischemic heart disease and focuses on the clinical practicalities of cardiac MR imaging today.

Susceptibility-sensitive magnetic resonance imaging detects human myocardium supplied by a stenotic coronary artery without a contrast agent

OBJECTIVES

Evaluation of the severity of a coronary artery stenosis is of paramount importance for therapy. A relevant stenosis provokes post-stenotic microvascular dilation with capillary recruitment. This autoregulatory response was investigated in the present study by use of susceptibility-sensitive magnetic resonance imaging (MRI) without contrast agents.

BACKGROUND

Functional alterations of the microvascular system may be studied noninvasively and without a contrast agent by susceptibility-sensitive MRI, which is based on the paramagnetic property of deoxyhemoglobin. This effect, also referred to as the "blood oxygenation level-dependent (BOLD) effect," is investigated by phase relaxation (T(2)*) measurements.

METHODS

In patients (n = 16) with single-vessel coronary artery disease, no history of myocardial infarction, normal left ventricular function at rest, and a positive stress echocardiogram, the susceptibility-sensitive parameter T(2)* was assessed in the myocardium.

RESULTS

In regions associated with the stenotic artery, T(2)* was significantly lower than in residual myocardium (p < 0.01). This difference in T(2)* increased after application of the vasodilator dipyridamole (p < 0.001). In patients being re-investigated after therapeutic interventions, the microvascular dilation was partly removed.

CONCLUSIONS

For the first time, we could show that myocardial BOLD MRI detects post-stenotic capillary recruitment dependent on coronary artery stenosis.

Assessment of regional differences in myocardial blood flow using T2-weighted 3D BOLD imaging

The feasibility of detecting regional differences in myocardial blood flow based on the blood oxygen level-dependent (BOLD) effect was evaluated in vivo in dogs (N = 9) using a 3D T2-prepared segmented gradient-echo sequence at 1.5 T. Regional differences in myocardial blood flow were created by administering adenosine through a catheter placed in the left circumflex coronary artery (LCX). The difference in the R2 (1/T2) relaxation rate between the left ventricular myocardial region supplied by the LCX and regions supplied by the left anterior descending coronary artery (LAD) or septal artery during adenosine administration was correlated to the corresponding regional myocardial blood flow difference determined using fluorescent microspheres. A correlation coefficient of 0.80 was found between the MR BOLD measurements and the myocardial flow assessment. Our results show that the sequence used in this study allows fast 3D BOLD imaging of the heart, and is a promising technique for detecting regional myocardial perfusion differences.

Magnetic resonance imaging in coronary heart disease

Modern level of cardiac magnetic resonance imaging (MRI) development already allows its routine use (with proper indications) in coronary heart disease patients for studies of heart morphology and functions, performance of stress tests for evaluation of myocardial perfusion and contractile function. Coronary MRA and some other new MR techniques are close to its wide-scale clinical application. It has been shown that cardiac MRI is a valuable tool for detection of postinfarction scars, aneurysms, pseudoaneurysms, septal defects, mural thrombi and valvular regurgitations. Due to intrinsic advantages of the method it is of special value when these pathological conditions cannot be fully confirmed or excluded with echocardiography. MRI is recognized as the best imaging method for quantification of myocardial thickness, myocardial mass, systolic myocardial thickening, chamber volumes, ejection fraction and other parameters of global and regional systolic and diastolic function. MRI is used in studies of cardiac remodeling in postinfarction patients. The most attractive areas for cardiovascular applications of MRI are assessment of myocardial perfusion and non-invasive coronary angiography. Substantial progress has been achieved in these directions. There are some other new developments in studies of coronary artery disease with MRI. High-resolution MR is used for imaging and quantification of atherosclerotic plaque composition in vivo. Intravascular MR devices suitable for performing imaging-guided balloon angioplasty are created. But before MRI will be widely accepted by the medical community as a important cardiovascular imaging modality several important problems have to be solved. Further technical advances are necessary for clinical implementation of all major diagnostic capabilities of cardiac MRI. The subjective obstacles for growth of clinical applications of cardiac MRI are lack of understanding of its possibilities and benefits both by clinicians and radiologists themselves. So proper training of specialists and promotion of this promising modality among the medical community are necessary.