Blood Oxygen Level-dependent Cardiovascular Magnetic Resonance Research Articles

Myocardial oxygenation is impaired in advanced chronic kidney disease and renal transplant patients

Background Coronary artery disease (CAD) and left ventricular hypertrophy are prevalent in the chronic kidney disease (CKD) and renal transplant population. Advances in cardiovascular magnetic resonance (CMR) with the blood oxygen level-dependent (BOLD) technique provides unprecedented capability to assess myocardial oxygenation as a measure of ischaemia. We hypothesised that myocardial oxygenation would be reduced in advanced CKD and renal transplant patients and may provide a novel strategy for assessing myocardial ischaemia. Methods We prospectively studied 20 advanced CKD subjects (8 dialysis group with median eGFR 9.5 (range 5-37) ml/ min and 12 CKD group with median eGFR 14 (range 818) ml/min), 8 renal transplant (RT) recipients with median eGFR 74.5 (range 57-114) ml/min and 7 hypertensive (HT) controls with median eGFR 107 (range 57144) ml/min. All patients were asymptomatic for CAD and none had prior history of CAD. All groups had cine and BOLD CMR at 3T, and RT and HT groups also had late gadolinium CMR to assess infarction/replacement fibrosis. CKD group additionally underwent 2D echocardiography strain to assess fibrosis. Myocardial oxygenation was measured at rest and under stress with adenosine (140 µg/kg/min) using BOLD Signal Intensity (SI). Analyses were performed using linear mixed models. Results

Ex-Vivo Validation of Blood Oxygen Level-Dependent Cardiovascular Magnetic Resonance (BOLD-CMR): the Role of Haemodilution of Arterial and Venous Blood on Signal Intensity

BACKGROUND: Oxygenation-sensitive cardiovascular magnetic resonance (OS-CMR) becomes more and more utilized in the field, thus it is important to assess confounders that may alter image results. Contrast changes are mainly caused by changes of deoxyhaemoglobin (dHb), and thus are dependent on haemoglobin concentration (cHb) and on oxygen saturation (sO2). Importantly, during long or repeated experiments, changes in fluid status may alter cHb, potentially introducing further variability. The primary objective of this in vitro study was to investigate the relationship between Hb, sO2 and changes in signal intensity (SI). METHODS: Arterial blood of dogs ventilated with 100% oxygen was obtained to study the singular effect of haemodilution, while venous blood was used to assess the combined effects of both haemodilution and desaturation. The blood was diluted in 10% increments with isotonic saline up to 50%. Oxygenation-sensitive T2* weighted images were obtained using a cardiac sequence at 3 Tesla and SI was compared to the blood gas analysis of all samples. RESULTS: Arterial blood remained fully saturated at each dilution while venous blood showed a decrease in absolute dHb content and an increase in sO2 with increasing dilutions, which was significantly correlated to changes in SI (R1⁄40.62; p<0.001). Both the arterial and venous blood demonstrated a decrease in cHb levels as dilutions increased. There was a corresponding negative correlation between SI and cHb for venous (R1⁄40.43; p1⁄40.009) and arterial blood (R1⁄40.53; p1⁄40.002). Arterial blood had a significant elevation difference in SI in comparison to venous samples (p<0.001) at the same dilutions. A drop in cHb of 23% venous (p1⁄40.035) or 28% arterial blood (p1⁄40.041) was required to yield a significant difference in SI at 10% dilution increments. CONCLUSION: With decreasing haemoglobin concentration at physiological levels, the SI in oxygenation-sensitive images changes despite similar oxygenation. Therefore, in experiments with suspected changes in fluid status, signal intensity changes in oxygenation-sensitive CMR images may have to be corrected for changes in haemoglobin concentration. 089 ALTERATIONS OF MYOCARDIAL OXYGENATION DURING VOLUNTARY APNEA IN PATIENTS WITH OBSTRUCTIVE SLEEP APNEA SYNDROME: A STUDY USING OXYGENATION-SENSITIVE CARDIOVASCULAR MR

Assessment of significant coronary artery stenosis using blood oxygen level dependent cardiovascular magnetic resonance (BOLD-CMR)

Summary Oxygenation-sensitive CMR at 1.5T can be used to identify functionally significant coronary artery stenosis, by demonstrating a blunted response to adenosine induced hyperaemia. Image quality remains a limitation. Background Using the magnetic properties of hemoglobin, changes in myocardial tissue oxygenation can be detected with blood oxygen level dependent (BOLD) cardiovascular MRI (CMR). The study aim was to assess whether BOLD-CMR images can detect an abnormal myocardial tissue response to adenosine infusion in patients with CAD, when compared to fractional flow reserve (FFR). Methods Patients undergoing clinically indicated coronary angiography underwent BOLD CMR scans using a clinical 1.5T scanner. Three short axis BOLD cine images were captured at baseline and during adenosine-induced coronary hyperemia. The mean segmental percent signal intensity (SI) changes were calculated between baseline and hyperemia in the subendocardial myocardium using the 16-segment model. Segments were defined as ischemic or non-ischemic by FFR (cut-off <0.80). The segment with the lowest BOLD SI percent change per patient was used for analysis. Results Thirty-two patients were enrolled, 5 patients were excluded due to incomplete CMR, leaving 27 patients (age 61 ± 10 years) for analysis. There were 864 myocardial segments (baseline and adenosine) available for analysis, 289 were subtended by a coronary artery with an available FFR value. Eighty-two segments (28%) were excluded due to pre-defined criteria for poor image quality, 67% were apical. From the remaining 20 patients, 7 had ischemic FFR values and 13 had non ischemic FFR values. Using the segment with the lowest % BOLD SI change per patient there was a significant difference between ischemic -6.49% ± -8.65% and non ischemic 4.21 ± 4.94% (p=0.0023) patients. Using a cut off value of 1.1% SI change the sensitivity is 86%, specificity 69%, positive predictive value 0.6 and negative predictive value 0.9.

582 Change in myocardial oxygenation during mild hypo- and hypercapnia: A blood oxygen level dependent cardiovascular magnetic resonance (BOLD-CMR) study in healthy volunteers

BOLD-CMR uses de-oxygenated hemoglobin (Hb) as an endogenous contrast. The signal intensity (SI) is determined by the amount of de-oxygenated Hb, blood volume and blood flow. The aim of this study was to investigate whether BOLD-CMR can detect changes in myocardial oxygenation during mild hypo- and hypercapnia in healthy individuals. Fifteen healthy volunteers were scanned with a BOLD sensitive modified SSFP cine sequence on a clinical 1.5T scanner (hypocapnia n = 8, hypercapnia n = 7). Continuous BOLD cines were acquired during 1 min of apnea in the hypercapnic group, whereas images were acquired prior to and post 1 min of hyperventilation in the hypocapnic group. SI was measured and averaged over all phases of the cardiac cycle, and also averaged between two acquisitions before and again after the breath hold. Immediately before and after image acquisition capillary blood gases were measured. One subject was excluded in the hypocapnia group due to motion artifacts. The hypocapnia groups showed a mean SI drop of 6.1% (P < 0.001, n = 7) after 1 min of hyperventilation (fig. 1a) while a mean SI increase of 7.1% (P < 0.001, n = 7) was observed over 1 min of apnea (fig. 1b). PCO2 dropped significantly from 27.9 ± 4.1 mmHg to 22.0 ± 3.2 mmHg (P = 0.02) after hyperventilation. Although there was a slight increase in PCO2 post breath-hold, it was not significant. PO2 did not change in either group. Our data suggest that BOLD-CMR can detect changes of myocardial oxygenation related to vasoconstriction/vasodilatation during hypo- and hypercapnia.

Assessment of Significant Coronary Artery Stenosis Using Blood Oxygen Level Dependent Cardiovascular Magnetic Resonance (BOLD-CMR)

Assessment of Significant Coronary Artery Stenosis Using Blood Oxygen Level Dependent Cardiovascular Magnetic Resonance (BOLD-CMR)

Relationship Between Regional Myocardial Oxygenation and Perfusion in Patients With Coronary Artery Disease

Background— It is recognized that the interplay between myocardial ischemia, perfusion, and oxygenation in the setting of coronary artery disease (CAD) is complex and that myocardial oxygenation and perfusion may become dissociated. Blood oxygen level–dependent (BOLD) cardiovascular magnetic resonance (CMR) has the potential to noninvasively measure myocardial oxygenation, whereas positron emission tomography (PET) with oxygen-15 labeled water is the gold standard technique for myocardial blood flow quantification. Thus, we sought to apply BOLD CMR at 3 T and oxygen-15–labeled water PET in patients with CAD and normal volunteers to better understand the relationship between regional myocardial oxygenation and blood flow during vasodilator stress. Methods and Results— Twenty-two patients (age, 62�8 years; 16 men) with CAD (at least 1 stenosis ≥50% on quantitative coronary angiography) and 10 normal volunteers (age, 58�6 years; 6 men) underwent 3-T BOLD CMR and PET. For BOLD CMR, 4 to 6 midventricular short-axis images were acquired at rest and during adenosine stress (140 μg/kg/min). Using PET with oxygen-15–labeled water, myocardial blood flow was measured at baseline and during adenosine in the same slices. BOLD images were divided into 6 segments, and mean signal intensities calculated. Taking ≥50% stenosis on quantitative coronary angiography as the gold standard, cutoff values for stress myocardial blood flow (&lt;2.45 mL/min/g; AUC, 0.83) and BOLD signal intensity change (&lt;3.74%; AUC, 0.78) were determined to define ischemic segments. BOLD CMR and PET agreed on the presence or absence of ischemia in 18 of the 22 patients (82%) and in all normal subjects. On a per-segment analysis, 40% of myocardial segments with stress myocardial blood flow below the cutoff of 2.45 mL/min/g did not show deoxygenation, whereas 88% of segments with normal perfusion also had normal oxygenation measurements. Conclusions— Regional myocardial perfusion and oxygenation may be dissociated, indicating that in patients with CAD, reduced perfusion does not always lead to deoxygenation.

To BOLDly Go Where Positron Emission Tomography Has Been Before

Atherosclerotic disease remains concealed for many years and becomes clinically overt only when the growth of atherosclerotic plaques and the adverse remodeling of the arterial wall reach a critical stage that results in impairment of blood flow, ischemia, and angina. Large epicardial arteries have a diameter ranging from a few millimeters to ≈500 μm and are visible at coronary angiography. Prearterioles (diameter from ≈500 to ≈100 μm) and arterioles (diameter <100 μm) are beyond the resolution of current angiographic systems and hence are not visible at angiography. Each compartment is regulated by distinct mechanisms, and vascular resistance is distributed in series along the coronary vascular bed.1 The oxygen supply to the myocardium is determined by arterial oxygen saturation and myocardial extraction, which are relatively fixed in normal perfusion conditions. At constant distending pressure, variations of flow in epicardial coronary arteries can be achieved by means of intracoronary injection of arteriolar vasodilators.2 Near maximal hyperemia can be achieved using coronary vasodilators such as adenosine or dipyridamole, which induce vasodilatation, mainly in the coronary microcirculation. The functional severity of a stenosis cannot be estimated by anatomic imaging such as x-ray coronary angiography or multislice computed tomography, and, in addition, diffuse atherosclerosis and extensive arterial remodeling may contribute to the dissociation of anatomic and functional measurements of coronary stenosis severity. Therefore, the functional significance of coronary stenoses can only be evaluated by measures of coronary flow, and commonly the flow reserve, which is the ratio of nonregulated maximum (hyperemic) and autoregulated (resting) flow. This characterization requires accurate quantitative measurements of pressure and/or flow and can be estimated invasively as fractional flow reserve2 or measured noninvasively with positron emission tomography (PET).3 Articles see p 32 and 41 The relationship between resting and hyperemic myocardial blood flow and severity of coronary …