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

Coronary Computed Tomography vs. Cardiac Magnetic Resonance Imaging in the Evaluation of Coronary Artery Disease

Coronary artery disease (CAD) represents a widespread burden to both individual and public health, steadily rising across the globe. The current guidelines recommend non-invasive anatomical or functional testing prior to invasive procedures. Both coronary computed tomography angiography (cCTA) and stress cardiac magnetic resonance imaging (CMR) are appropriate imaging modalities, which are increasingly used in these patients. Both exhibit excellent safety profiles and high diagnostic accuracy. In the last decade, cCTA image quality has improved, radiation exposure has decreased and functional information such as CT-derived fractional flow reserve or perfusion can complement anatomic evaluation. CMR has become more robust and faster, and advances have been made in functional assessment and tissue characterization allowing for earlier and better risk stratification. This review compares both imaging modalities regarding their strengths and weaknesses in the assessment of CAD and aims to give physicians rationales to select the most appropriate modality for individual patients.

[Oxygenation-sensitive cardiac magnetic resonance imaging]

BACKGROUND

Oxygenation-sensitive cardiac magnetic resonance imaging (OS-CMR) is an evolving cardiac imaging technique offering new perspectives to understand, predict, and diagnose cardiac pathologies.

OBJECTIVES

To provide an overview of the basic principles of OS-CMR, the current diagnostic applications and how it may aid in future diagnostic challenges.

MATERIALS AND METHODS

Description, analysis, and interpretation of the current literature on basic research and applicational studies in both humans and animals assessing OS-CMR.

RESULTS

OS-CMR is based on the paramagnetic properties of deoxygenated hemoglobin, which is visualized by a T2*-sensitive sequence. The measured signal correlates with the oxygenation of the myocardium and can analyze vascular function during pharmacological vasodilation or vasoactive breathing exercises (hyperventilation, apnea). The herewith triggered changes in myocardial oxygenation and oxygenation reserve can be used to identify relevant stenoses in coronary artery disease. Other areas of application involve myocardial hypertrophy, microvascular dysfunction, and pulmonary hypertension.

CONCLUSION

A broad number of applications for the clinical use of OS-CMR exist so far, especially in combination with breathing exercises. OS-CMR can be conducted medication- and needle-free. Limitations involve the current lack of clinically approved, automated evaluation tools and the unavailability of vendor- and site-independent normal values.

Assessment of Myocardial Function During Blood Pressure Manipulations Using Feature Tracking Cardiovascular Magnetic Resonance

Background: Coronary autoregulation is a feedback system, which maintains near-constant myocardial blood flow over a range of mean arterial pressure (MAP). Yet in emergency or peri-operative situations, hypotensive or hypertensive episodes may quickly arise. It is not yet established how rapid blood pressure changes outside of the autoregulation zone (ARZ) impact left (LV) and right ventricular (RV) function. Using cardiovascular magnetic resonance (CMR) imaging, measurements of myocardial tissue oxygenation and ventricular systolic and diastolic function can comprehensively assess the heart throughout a range of changing blood pressures.

Design and methods: In 10 anesthetized swine, MAP was varied in steps of 10–15 mmHg from 29 to 196 mmHg using phenylephrine and urapidil inside a 3-Tesla MRI scanner. At each MAP level, oxygenation-sensitive (OS) cine images along with arterial and coronary sinus blood gas samples were obtained and blood flow was measured from a surgically implanted flow probe on the left anterior descending coronary artery. Using CMR feature tracking-software, LV and RV circumferential systolic and diastolic strain parameters were measured from the myocardial oxygenation cines.

Results: LV and RV peak strain are compromised both below the lower limit (LV: Δ1.2 ± 0.4%, RV: Δ4.4 ± 1.2%, p < 0.001) and above the upper limit (LV: Δ2.1 ± 0.4, RV: Δ5.4 ± 1.4, p < 0.001) of the ARZ in comparison to a baseline of 70 mmHg. LV strain demonstrates a non-linear relationship with invasive and non-invasive measures of oxygenation. Specifically for the LV at hypotensive levels below the ARZ, systolic dysfunction is related to myocardial deoxygenation (β = −0.216, p = 0.036) in OS-CMR and both systolic and diastolic dysfunction are linked to reduced coronary blood flow (peak strain: β = −0.028, p = 0.047, early diastolic strain rate: β = 0.026, p = 0.002). These relationships were not observed at hypertensive levels.

Conclusion: In an animal model, biventricular function is compromised outside the coronary autoregulatory zone. Dysfunction at pressures below the lower limit is likely caused by insufficient blood flow and tissue deoxygenation. Conversely, hypertension-induced systolic and diastolic dysfunction points to high afterload as a cause. These findings from an experimental model are translatable to the clinical peri-operative environment in which myocardial deformation may have the potential to guide blood pressure management, in particular at varying individual autoregulation thresholds.

Randomized controlled trial of perhexiline on regression of left ventricular hypertrophy in patients with symptomatic hypertrophic cardiomyopathy (RESOLVE-HCM trial)

BACKGROUND

The presence and extent of left ventricular hypertrophy (LVH) is a major determinant of symptoms in patients with hypertrophic cardiomyopathy (HCM). There is increasing evidence to suggest that myocardial energetic impairment represents a central mechanism leading to LVH in HCM. There is currently a significant unmet need for disease-modifying therapy that regresses LVH in HCM patients. Perhexiline, a potent carnitine palmitoyl transferase-1 (CPT-1) inhibitor, improves myocardial energetics in HCM, and has the potential to reduce LVH in HCM.

OBJECTIVE

The primary objective is to evaluate the effects of perhexiline treatment on the extent of LVH, in symptomatic HCM patients with at least moderate LVH.

METHODS/DESIGN

RESOLVE-HCM is a prospective, multicenter double-blind placebo-controlled randomized trial enrolling symptomatic HCM patients with at least moderate LVH. Sixty patients will be randomized to receive either perhexiline or matching placebo. The primary endpoint is change in LVH, assessed utilizing cardiovascular magnetic resonance (CMR) imaging, after 12-months treatment with perhexiline.

SUMMARY

RESOLVE-HCM will provide novel information on the utility of perhexiline in regression of LVH in symptomatic HCM patients. A positive result would lead to the design of a Phase 3 clinical trial addressing long-term effects of perhexiline on risk of heart failure and mortality in HCM patients.

The Potential of Oxygenation-Sensitive CMR in Heart Failure

PURPOSE OF REVIEW

Cardiac magnetic resonance imaging (CMR) use in the context of heart failure (HF) has increased over the last decade as it is able to provide detailed, quantitative information on function, morphology, and myocardial tissue composition. Furthermore, oxygenation-sensitive CMR (OS-CMR) has emerged as a CMR imaging method capable of monitoring changes of myocardial oxygenation without the use of exogenous contrast agents.

RECENT FINDINGS

The contributions of OS-CMR to the investigation of patients with HF includes not only a fully quantitative assessment of cardiac morphology, function, and tissue characteristics, but also high-resolution information on both endothelium-dependent and endothelium-independent vascular function as assessed through changes of myocardial oxygenation. In patients with heart failure, OS-CMR can provide deep phenotyping on the status and important associated pathophysiology as a one-stop, needle-free diagnostic imaging test.

Effect of inhaled oxygen concentration on 129 Xe chemical shift of red blood cells in rat lungs

PURPOSE

To investigate the dependence of dissolved ¹²⁹ Xe chemical shift on the fraction of inhaled oxygen, F_i O₂ , in the lungs of healthy rats.

METHODS

The chemical shifts of ¹²⁹ Xe dissolved in red blood cells, δ_RBC , and blood plasma and/or tissue, δ_Plasma , were measured using MRS in 12 Sprague Dawley rats mechanically ventilated at F_i O₂ values of 0.14, 0.19, and 0.22. Regional effects on the chemical shifts were controlled using a chemical shift saturation recovery sequence with a fixed delay time. MRS was also performed at an F_i CO₂ value of 0.085 to investigate the potential effect of the vascular response on δ_RBC and δ_Plasma .

RESULTS

δ_RBC increased with decreasing F_i O₂ (P = .0002), and δ_Plasma showed no dependence on F_i O₂ (P = .23). δ_RBC at F_i CO₂ = 0 (210.7 ppm ± 0.1) and at F_i CO₂ = 0.085 (210.6 ppm ± 0.2) were not significantly different (P = .67). δ_Plasma at F_i CO₂ = 0 (196.9 ppm ± 0.3) and at F_i CO₂ = 0.085 (197.0 ppm ± 0.1) were also not significantly different (P = .81).

CONCLUSION

Rat lung δ_RBC showed an inverse relationship to F_i O₂ , opposite to the relationship previously demonstrated for in vitro human blood. Rat lung δ_RBC did not depend on F_i CO₂ .

Cardiovascular magnetic resonance stress and rest T1-mapping using regadenoson for detection of ischemic heart disease compared to healthy controls

Background

Adenosine stress T1-mapping on cardiovascular magnetic resonance (CMR) can differentiate between normal, ischemic, infarcted, and remote myocardial tissue classes without the need for contrast agents. Regadenoson, a selective coronary vasodilator, is often used in stress perfusion imaging when adenosine is contra-indicated, and has advantages in ease of administration, safety profile, and clinical workflow. We aimed to characterize the regadenoson stress T1-mapping response in healthy individuals, and to investigate its ability to differentiate between myocardial tissue classes in patients with coronary artery disease (CAD).

Methods

Eleven healthy controls and 25 patients with CAD underwent regadenoson stress perfusion CMR, as well as rest and stress ShMOLLI T1-mapping. Native T1 values and stress T1 reactivity were derived for normal myocardium in healthy controls and for different myocardial tissue classes in patients with CAD.

Results

Healthy controls had normal myocardial native T1 values at rest (931 ± 22 ms) with significant global regadenoson stress T1 reactivity (δT1 = 8.2 ± 0.8% relative to baseline; p < 0.0001). Infarcted myocardium had significantly higher resting T1 (1215 ± 115 ms) than ischemic, remote, and normal myocardium (all p < 0.0001) with an abolished stress T1 response (δT1 = −0.8% [IQR: −1.9–0.5]). Ischemic myocardium had elevated resting T1 compared to normal (964 ± 57 ms; p < 0.01) with an abolished stress T1 response (δT1 = 0.5 ± 1.6%). Remote myocardium in patients had comparable resting T1 to normal (949 ms [IQR: 915–973]; p = 0.06) with blunted stress reactivity (δT1 = 4.3% [IQR: 3.1–6.3]; p < 0.0001).

Conclusions

Healthy controls demonstrate significant stress T1 reactivity during regadenoson stress. Regadenoson stress and rest T1-mapping is a viable alternative to adenosine and exercise for the assessment of CAD and can distinguish between normal, ischemic, infarcted, and remote myocardium.

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Regadenoson has advantages over adenosine in terms of administration, safety profile, and clinical workflow.

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There are distinct tissue characteristics for normal, ischemic, infarcted, and remote myocardium.

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Healthy controls demonstrate significant stress T1 reactivity during vasodilator stress.

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Regadenoson stress T1-mapping can distinguish between different myocardial tissue classes.

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Regadenoson stress T1-mapping is a viable alternative to adenosine and exercise for the assessment of coronary artery disease.

Right ventricular myocardial deoxygenation in patients with pulmonary artery hypertension

BACKGROUND

In pulmonary arterial hypertension (PAH), progressive right ventricular (RV) dysfunction is believed to be largely secondary to RV ischaemia. A recent pilot study has demonstrated the feasibility of Oxygen-sensitive (OS) cardiovascular magnetic resonance (CMR) to detect in-vivo RV myocardial oxygenation. The aims of the present study therefore, were to assess the prevalence of RV myocardial ischaemia and relationship with RV myocardial interstitial changes in PAH patients with non-obstructive coronaries, and corelate with functional and haemodynamic parameters.

METHODS

We prospectively recruited 42 patients with right heart catheter (RHC) proven PAH and 11 healthy age matched controls. The CMR examination involved standard functional imaging, OS-CMR imaging and native T1 mapping. An ΔOS-CMR signal intensity (SI) index (stress/rest signal intensity) was acquired at RV anterior, RV free-wall and RV inferior segments. T1 maps were acquired using Shortened Modified Look-Locker Inversion recovery (ShMOLLI) at the inferior RV segment.

RESULTS

The inferior RV ΔOS-CMR SI index was significantly lower in PAH patients compared with healthy controls (9.5 (- 7.4-42.8) vs 12.5 (9-24.6)%, p = 0.02). The inferior RV ΔOS-CMR SI had a significant correlation to RV inferior wall thickness (r = - 0.7, p < 0.001) and RHC mean pulmonary artery pressure (mPAP) (r = - 0.4, p = 0.02). Compared to healthy controls, patients with PAH had higher native T1 in the inferior RV wall: 1303 (1107-1612) vs 1232 (1159-1288)ms, p = 0.049. In addition, there was a significant difference in the inferior RV T1 values between the idiopathic PAH and systemic sclerosis associated PAH patients: 1242 (1107-1612) vs 1386 (1219-1552)ms, p = 0.007.

CONCLUSION

Blunted OS-CMR SI suggests the presence of in-vivo microvascular RV dysfunction in PAH patients. The native T1 in the inferior RV segments is significantly increased in the PAH patients, particularly among the systemic sclerosis associated PAH group.

Role of Cardiovascular Magnetic Resonance in Ischemic Cardiomyopathy

Ischemic heart disease is the most common cause of cardiovascular morbidity and mortality. Cardiac magnetic resonance (CMR) improves on other noninvasive modalities in detection, assessment, and prognostication of ischemic heart disease. The incorporation of CMR in clinical trials allows for smaller patient samples without the sacrifice of power needed to demonstrate clinical efficacy. CMR can accurately quantify infarct acuity, size, and complications; guide therapy; and prognosticate recovery. Timing of revascularization remains the holy grail of ischemic heart disease, and viability assessment using CMR may be the missing link needed to help reduce morbidity and mortality associated with the disease.

Refractory Angina: From Pathophysiology to New Therapeutic Nonpharmacological Technologies

Despite optimal combination of guideline-directed anti-ischemic therapies and myocardial revascularization, a substantial proportion of patients with stable coronary artery disease continues to experience disabling symptoms and is often referred as "no-option." The appraisal of the pathways linking ischemia to symptom perception indicates a complex model of heart-brain interactions in the generation of the subjective anginal experience and inspired novel approaches that may be clinically effective in alleviating the angina burden of this population. Conversely, the prevailing ischemia-centered view of angina, with the focus on traditional myocardial revascularization as the sole option to address ischemia on top of medical therapy, hinders the experimental characterization and broad-scale clinical implementation of strongly needed therapeutic options. The interventionist, often the first physician to establish the diagnosis of refractory angina pectoris (RAP) following coronary angiography, should be aware of the numerous emerging technologies with the potential to improve quality of life in the growing population of RAP patients. This review describes the current landscape and the future perspectives on nonpharmacological treatment technologies for patients with RAP, with a view on the underlying physiopathological rationale and current clinical evidence.

Left ventricular ischemia in pre-capillary pulmonary hypertension: a cardiovascular magnetic resonance study

Background

Prognosis in pulmonary arterial hypertension (PAH) is largely dependent on right ventricular (RV) function. However, recent studies have suggested the presence of left ventricular (LV) dysfunction in PAH patients. The potential role of LV ischemia, as a contributor to progressive LV dysfunction, has not been systematically studied in PAH. We aim to assess the presence and extent of LV myocardial ischemia in patients with known PH and without obstructive coronary artery disease (CAD), using oxygen-sensitive (OS) cardiovascular magnetic resonance (CMR) and stress/rest CMR T1 mapping.

Methods

We prospectively recruited 28 patients with right heart catheter-proven PH and no significant CAD, 8 patients with known CAD and 11 normal age-matched controls (NC). OS-CMR images were acquired using a T2* sequence and T1 maps were acquired using Shortened Modified Look-Locker Inversion recovery (ShMOLLI) at rest and adenosine-induced stress vasodilatation; ΔOS-CMR signal intensity (SI) index (stress/rest SI) and ΔT1 reactivity (stress-rest/rest T1 mapping) were calculated.

Results

Global LV ΔOS SI index was significantly lower in PH patients compared with controls (11.1%±6.7% vs. 20.5%±10.5%, P=0.016), as was ΔT1 reactivity (5.2%±4.5% vs. 8.0%±2.9%, P=0.047). The ischemic segments of CAD patients had comparable ΔOS SI (10.3%±6.4% vs. 11.1%±6.7%, P=0.773) to PH patients, but lower ΔT1 reactivity (1.1%±4.2% vs. 5.2%±4.5%, P=0.036).

Conclusions

Decreased OS-CMR SI and T1 reactivity signify the presence of impaired myocardial oxygenation and vasodilatory response in PH patients. Given their unobstructed epicardial coronary arteries, this is likely secondary to coronary microvascular dysfunction (CMD).

Cardiac Imaging for Coronary Heart Disease Risk Stratification in Chronic Kidney Disease

Chronic kidney disease (CKD), defined as dysfunction of the glomerular filtration apparatus, is an independent risk factor for the development of coronary artery disease (CAD). Patients with CKD are at a substantially higher risk of cardiovascular mortality compared with the age- and sex-adjusted general population with normal kidney function. The risk of CAD and mortality in patients with CKD is correlated with the degree of renal dysfunction including presence of microalbuminuria. A greater cardiovascular risk, albeit lower than for patients receiving dialysis, persists even after kidney transplantation. Congestive heart failure, commonly caused by CAD, also accounts for a significant portion of the cardiovascular-related events observed in CKD. The optimal strategy for the evaluation of CAD in patients with CKD, particularly before renal transplantation, remains a topic of contention spanning over several decades. Although the evaluation of coexisting cardiac disease in patients with CKD is desirable, severe renal dysfunction limits the use of radiographic and magnetic resonance contrast agents due to concerns regarding contrast-induced nephropathy and nephrogenic systemic sclerosis, respectively. In addition, many patients with CKD have extensive and premature (often medial) calcification disproportionate to the severity of obstructive CAD, thereby limiting the diagnostic value of computed tomography angiography. As such, echocardiography, non-contrast-enhanced magnetic resonance, nuclear myocardial perfusion, and metabolic imaging offer a variety of approaches to assess obstructive CAD and cardiomyopathy of advanced CKD without the need for nephrotoxic contrast agents.

Stress cardiac MRI in stable coronary artery disease

PURPOSE OF REVIEW

Non-invasive testing is often the first step in the evaluation of stable coronary artery disease (CAD). Stress cardiac magnetic resonance imaging (CMR) is an established modality with high diagnostic accuracy and prognostic value. This review will focus on the recent advances in understanding how stress CMR can help guide patient care.

RECENT FINDINGS

Diagnostic accuracy of stress CMR has been validated against coronary angiography with fractional flow reserve (FFR) in patients with stable CAD. Large registry data have shown stress CMR to have important prognostic importance and that its cost-effectiveness compares favorably to alternatives. In patients with stable CAD, guidance using a CMR based strategy led to equivalent outcomes when compared to coronary angiography with FFR.

SUMMARY

In persons with stable CAD, Stress CMR is an accurate and cost-effective imaging modality that should be considered in patients at intermediate pre-test probability of CAD. Prognostic studies have shown it to have excellent negative predictive value and that it can safely serve as a "gatekeeper" for invasive angiography.

Heart Rate-Independent 3D Myocardial Blood Oxygen Level-Dependent MRI at 3.0 T with Simultaneous 13N-Ammonia PET Validation

Background Despite advances, blood oxygen level-dependent (BOLD) cardiac MRI for myocardial perfusion is limited by inadequate spatial coverage, imaging speed, multiple breath holds, and imaging artifacts, particularly at 3.0 T. Purpose To develop and validate a robust, contrast agent-unenhanced, free-breathing three-dimensional (3D) cardiac MRI approach for reliably examining changes in myocardial perfusion between rest and adenosine stress. Materials and Methods A heart rate-independent, free-breathing 3D T2 mapping technique at 3.0 T that can be completed within the period of adenosine stress (≤4 minutes) was developed by using computer simulations, ex vivo heart preparations, and dogs. Studies in dogs were performed with and without coronary stenosis and validated with simultaneously acquired nitrogen 13 (13N) ammonia PET perfusion in a clinical PET/MRI system. The MRI approach was also prospectively evaluated in healthy human volunteers (from January 2017 to September 2017). Myocardial BOLD responses (MBRs) between normal and ischemic myocardium were compared with mixed model analysis. Results Dogs (n = 10; weight range, 20-25 kg; mongrel dogs) and healthy human volunteers (n = 10; age range, 22-53 years; seven men) were evaluated. In healthy dogs, T2 MRI at adenosine stress was greater than at rest (mean rest vs stress, 38.7 msec ± 2.5 [standard deviation] vs 45.4 msec ± 3.3, respectively; MBR, 1.19 ± 0.08; both, P < .001). At the same conditions, mean rest versus stress PET perfusion was 1.1 mL/mg/min ± 0.11 versus 2.3 mL/mg/min ± 0.82, respectively (P < .001); myocardial perfusion reserve (MPR) was 2.4 ± 0.82 (P < .001). The BOLD response and PET MPR were positively correlated (R = 0.67; P < .001). In dogs with coronary stenosis, perfusion anomalies were detected on the basis of MBR (normal vs ischemic, 1.09 ± 0.05 vs 1.00 ± 0.04, respectively; P < .001) and MPR (normal vs ischemic, 2.7 ± 0.08 vs 1.7 ± 1.1, respectively; P < .001). Human volunteers showed increased myocardial T2 at stress (rest vs stress, 44.5 msec ± 2.6 vs 49.0 msec ± 5.5, respectively; P = .004; MBR, 1.1 msec ± 8.08). Conclusion This three-dimensional cardiac blood oxygen level-dependent (BOLD) MRI approach overcame key limitations associated with conventional cardiac BOLD MRI by enabling whole-heart coverage within the standard duration of adenosine infusion, and increased the magnitude and reliability of BOLD contrast, which may be performed without requiring breath holds. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Almeida in this issue.

Feasibility of oxygen sensitive cardiac magnetic resonance of the right ventricle in pulmonary artery hypertension

Background

Progressive right ventricular (RV) dysfunction in pulmonary arterial hypertension (PAH) which is contributed by RV ischemia leads to adverse clinical outcomes. Oxygen-sensitive (OS) cardiovascular magnetic resonance (CMR) has been used to determine the in vivo myocardial oxygenation of the left ventricle (LV). The aims of the present study were therefore to determine the feasibility of RV targeted rest/stress OS-CMR imaging in PAH patients and healthy volunteers.

Methods

We prospectively recruited 20 patients with right heart catheter proven PAH and 9 healthy age matched controls (NC). The CMR examination involved standard functional imaging and OS-CMR imaging. An OS-CMR signal intensity (SI) index (stress/rest SI) was acquired at RV anterior, RV free-wall and RV inferior segments. In the LV, the OS-CMR SI index was acquired globally.

Results

Reliable OS SI changes were only obtained from the RV inferior segment. As RV dysfunction in PAH is a global process, hence this segment was used in both patients and NC for further comparison. RV OS-CMR SI change between rest and stress in the NC was 17%±5% (mean ± SD). Nine of 20 (45%) of the PAH patients had a mean OS SI change of less than 9% (or ≥2 SD different from the mean values in NC). Overall, RV OS SI index between the PAH patients and NC was 11%±9% vs. 17%±5% (P=0.045) in the RV inferior segment. In the LV, the global OS-CMR SI index between the PAH patients and NC was 11%±7% vs. 21%±9% (P=0.019). There was a strong correlation between RV Inf OS-CMR SI and LV OS-CMR SI (r=0.86, P<0.001).

Conclusions

In this small pilot study, pharmacological induced OS-CMR is a feasible and safe technique to identify and study myocardial oxygenation in the RV of PAH patients.

Analysis of physiological noise in quantitative cardiac magnetic resonance

Purpose

To determine the impact of imaging parameters on the temporal signal-to-noise ratio (TSNR) of quantitative cardiac magnetic resonance (MR) in humans, and to determine applicability of the physiological noise covariance (PNC) model for physiological noise (PN).

Methods

We conducted MRI experiments in four healthy volunteers, and obtained series of short-axis cardiac images acquired with snapshot balanced steady-state free precession (bSSFP) and snapshot gradient echo (GRE) using a broad range of spatial resolutions and parallel imaging acceleration factors commonly used in quantitative cardiac MR. We measured regional SNR and TSNR in these datasets and fit the measurements to the PNC model for PN, which assumes that PN scales with signal strength.

Results

The relationship between SNR and TSNR in human cardiac MR without contrast preparation was well modeled by the PNC model. SNR consistently decreased as the spatial resolution (matrix size) and acceleration factor (R) increased for both GRE and bSSFP imaging. TSNR varied linearly with SNR using GRE imaging, when SNR was low (SNR < 20), and approached an asymptotic limit using bSSFP imaging, when SNR was high (SNR > 40).

Conclusions

The PNC model can be used to guide the choice of matrix size and acceleration factor to optimize TSNR in stable contrast cardiac MR, such as T2-prepared Blood-Oxygen-Level-Dependent (BOLD) and several variants of Arterial Spin Labeled (ASL) cardiac MR.

Early detection of heart function abnormality by native T1: a comparison of two T1 quantification methods

Objective

To compare the robustness of native T1 mapping using mean and median pixel-wise quantification methods.

Methods

Fifty-seven consecutive patients without overt signs of heart failure were examined in clinical routine for suspicion of cardiomyopathy. MRI included the acquisition of native T1 maps by a motion-corrected modified Look-Locker inversion recovery sequence at 1.5 T. Heart function status according to four established volumetric left ventricular (LV) cardio MRI parameter thresholds was used for retrospective separation into subgroups of normal (n = 26) or abnormal heart function (n = 31). Statistical normality of pixel-wise T1 was tested on each myocardial segment and mean and median segmental T1 values were assessed.

Results

Segments with normally distributed pixel-wise T1 (57/58%) showed no difference between mean and median quantification in either patient group, while differences were highly significant (p < 0.001) for the respective 43/42% non-normally distributed segments. Heart function differentiation between two patient groups was significant in 14 myocardial segments (p < 0.001–0.040) by median quantification compared with six (p < 0.001–0.042) by using the mean. The differences by median quantification were observed between the native T1 values of the three coronary artery territories of normal heart function patients (p = 0.023) and insignificantly in the abnormal patients (p = 0.053).

Conclusion

Median quantification increases the robustness of myocardial native T1 definition, regardless of statistical normality of the data. Compared with the currently prevailing method of mean quantification, differentiation between LV segments and coronary artery territories is better and allows for earlier detection of heart function impairment.

Key Points

• Median pixel-wise quantification of native T1 maps is robust and can be applied regardless of the statistical distribution of data points.

• Median quantification is more sensitive to early heart function abnormality compared with mean quantification.

• The new method yields significant native T1 value differentiation between the three coronary artery territories.

Electronic supplementary material

The online version of this article (10.1007/s00330-019-06364-9) contains supplementary material, which is available to authorized users.

Gadolinium-Free Cardiac MR Stress T1-Mapping to Distinguish Epicardial From Microvascular Coronary Disease

BACKGROUND

Novel cardiac magnetic resonance (CMR) stress T1 mapping can detect ischemia and myocardial blood volume changes without contrast agents and may be a more comprehensive ischemia biomarker than myocardial blood flow.

OBJECTIVES

This study describes the performance of the first prospective validation of stress T1 mapping against invasive coronary measurements for detecting obstructive epicardial coronary artery disease (CAD), defined by fractional flow reserve (FFR <0.8), and coronary microvascular dysfunction, defined by FFR ≥0.8 and the index of microcirculatory resistance (IMR ≥25 U), compared with first-pass perfusion imaging.

METHODS

Ninety subjects (60 patients with angina; 30 healthy control subjects) underwent CMR (1.5- and 3-T) to assess left ventricular function (cine), ischemia (adenosine stress/rest T1 mapping and perfusion), and infarction (late gadolinium enhancement). FFR and IMR were assessed ≤7 days post-CMR. Stress and rest images were analyzed blinded to other information.

RESULTS

Normal myocardial T1 reactivity (ΔT1) was 6.2 ± 0.4% (1.5-T) and 6.2 ± 1.3% (3-T). Ischemic viable myocardium downstream of obstructive CAD showed near-abolished T1 reactivity (ΔT1 = 0.7 ± 0.7%). Myocardium downstream of nonobstructive coronary arteries with microvascular dysfunction showed less-blunted T1 reactivity (ΔT1 = 3.0 ± 0.9%). Stress T1 mapping significantly outperformed gadolinium-based first-pass perfusion, including absolute quantification of myocardial blood flow, for detecting obstructive CAD (area under the receiver-operating characteristic curve: 0.97 ± 0.02 vs. 0.91 ± 0.03, respectively; p < 0.001). A ΔT1 of 1.5% accurately detected obstructive CAD (sensitivity: 93%; specificity: 95%; p < 0.001), whereas a less-blunted ΔT1 of 4.0% accurately detected microvascular dysfunction (area under the receiver-operating characteristic curve: 0.95 ± 0.03; sensitivity: 94%; specificity: 94%: p < 0.001).

CONCLUSIONS

CMR stress T1 mapping accurately detected and differentiated between obstructive epicardial CAD and microvascular dysfunction, without contrast agents or radiation.

T1 reactivity as an imaging biomarker in myocardial tissue characterization discriminating normal, ischemic and infarcted myocardium

To demonstrate the potential for differentiating normal and diseased myocardium without Gadolinium using rest and stress T1-mapping. Patients undergoing 1.5T magnetic resonance imaging (MRI) as part of clinical work-up due to suspicion of coronary artery disease (CAD) were included. Adenosine stress perfusion MRI and late gadolinium enhancement (LGE) imaging were performed to identify ischemic and infarcted myocardium. Patients were retrospectively categorized into an ischemic, infarct and control group based on conventional acquisitions. Patient with both ischemic and infarcted myocardium were excluded. A total of 64 patients were included: ten with myocardial ischemia, 15 with myocardial infarction, and 39 controls. A native Modified Look-Locker Inversion Recovery (MOLLI) T1-mapping acquisition was performed at rest and stress. Pixel-wise myocardial T1-maps were acquired in short-axis view with inline motion-correction. Short-axis T1-maps were manually contoured using conservative septal sampling. Regions of interest were sampled in ischemic and infarcted areas detected on perfusion and LGE images. T1 reactivity was calculated as the percentage difference in T1 values between rest and stress. Remote myocardium was defined as myocardium without defects in the ischemic and infarcted group whereas normal myocardium is found in the control group only. Native T1-values were significantly higher in infarcted myocardium in rest and stress [median 1044 ms (interquartile range (IQR) 985–1076) and 1053 ms (IQR 989–1088)] compared to ischemic myocardium [median 961 ms (IQR 939–988) and 958 ms (IQR 945–988)]. T1-reactivity was significantly lower in ischemic and infarcted myocardium [median 0.00% (IQR − 0.18 to 0.16) and 0.41% (IQR 0.09–0.86)] compared to remote myocardium [median 3.54% (IQR 1.48–5.78) and 3.21% (IQR 1.95–4.79)]. Rest-stress T1-mapping is able to distinguish between normal, ischemic, infarcted and remote myocardium using native T1-values and T1-reactivity, and holds potential as an imaging biomarker for tissue characterization in MRI.

Assessment of myocardial oxygenation, strain, and diastology in MYBPC3-related hypertrophic cardiomyopathy: a cardiovascular magnetic resonance and echocardiography study

Aims

Myocardial oxygenation is impaired in hypertrophic cardiomyopathy (HCM) patients with left ventricular hypertrophy (LVH), and possibly also in HCM gene carriers without LVH. Whether these oxygenation changes are also associated with abnormalities in diastolic function or left ventricular (LV) strain are unknown.

Methods and results

We evaluated 60 subjects: 20 MYBPC3 gene positive patients with LVH (G+LVH+), 18 MYBPC3 gene positive without LVH (G+LVH−), 11 gene negative siblings (G−), and 11 normal controls (NC). All subjects underwent 2D transthoracic echocardiography and cardiovascular magnetic resonance imaging for assessment of ventricular volumes, mass, and myocardial oxygenation at rest and adenosine stress using the blood oxygen level dependent (BOLD) technique. Maximal septal thickness was 20 mm in the G+LVH+ group, vs. 9 mm for the G+LVH− group. As expected, the G+LVH+ group had a more blunted myocardial oxygenation response to stress when compared with the G+LVH− group (−5% ± 3% vs. 2% ± 4%, P < 0.05), G− siblings (−5% ± 3% vs. 11% ± 4%, P < 0.0001) and NC (−5% ± 3% vs. 15% ± 4%, P < 0.0001). A blunted BOLD response to stress was also seen in G+LVH− subjects when compared with gene negative siblings (2% ± 4% vs. 11% ± 4%, P < 0.05) and NC (15% ± 4%, P < 0.050). G+LVH+ patients exhibited abnormal diastolic function including lower Eʹ, higher E to Eʹ ratio and greater left atrial area compared with the G+LVH− subjects who all had normal values for these indices.

Conclusion

Myocardial deoxygenation during stress is observed in MYBPC3 HCM patients, even in the presence of normal LV diastolic function, LV global longitudinal strain, and LV wall thickness.

Imaging the myocardial ischemic cascade

Non-invasive imaging plays a growing role in the diagnosis and management of ischemic heart disease from its earliest manifestations of endothelial dysfunction to myocardial infarction along the myocardial ischemic cascade. Experts representing the North American Society for Cardiovascular Imaging and the European Society of Cardiac Radiology have worked together to organize the role of non-invasive imaging along the framework of the ischemic cascade. The current status of non-invasive imaging for ischemic heart disease is reviewed along with the role of imaging for guiding surgical planning. The issue of cost effectiveness is also considered. Preclinical disease is primarily assessed through the coronary artery calcium score and used for risk assessment. Once the patient becomes symptomatic, other imaging tests including echocardiography, CCTA, SPECT, PET and CMR may be useful. CCTA appears to be a cost-effective gatekeeper. Post infarction CMR and PET are the preferred modalities. Imaging is increasingly used for surgical planning of patients who may require coronary artery bypass.

Diagnosis of Microvascular Angina Using Cardiac Magnetic Resonance

BACKGROUND

In patients with angina and nonobstructive coronary artery disease (NOCAD), confirming symptoms due to coronary microvascular dysfunction (CMD) remains challenging. Cardiac magnetic resonance (CMR) assesses myocardial perfusion with high spatial resolution and is widely used for diagnosing obstructive coronary artery disease (CAD).

OBJECTIVES

The goal of this study was to validate CMR for diagnosing microvascular angina in patients with NOCAD, compared with patients with obstructive CAD and correlated to the index of microcirculatory resistance (IMR) during invasive coronary angiography.

METHODS

Fifty patients with angina (65 ± 9 years of age) and 20 age-matched healthy control subjects underwent adenosine stress CMR (1.5- and 3-T) to assess left ventricular function, inducible ischemia (myocardial perfusion reserve index [MPRI]; myocardial blood flow [MBF]), and infarction (late gadolinium enhancement). During subsequent angiography within 7 days, 28 patients had obstructive CAD (fractional flow reserve [FFR] ≤0.8) and 22 patients had NOCAD (FFR >0.8) who underwent 3-vessel IMR measurements.

RESULTS

In patients with NOCAD, myocardium with IMR <25 U had normal MPRI (1.9 ± 0.4 vs. controls 2.0 ± 0.3; p = 0.49); myocardium with IMR ≥25 U had significantly impaired MPRI, similar to ischemic myocardium downstream of obstructive CAD (1.2 ± 0.3 vs. 1.2 ± 0.4; p = 0.61). An MPRI of 1.4 accurately detected impaired perfusion related to CMD (IMR ≥25 U; FFR >0.8) (area under the curve: 0.90; specificity: 95%; sensitivity: 89%; p < 0.001). Impaired MPRI in patients with NOCAD was driven by impaired augmentation of MBF during stress, with normal resting MBF. Myocardium with FFR >0.8 and normal IMR (<25 U) still had blunted stress MBF, suggesting mild CMD, which was distinguishable from control subjects by using a stress MBF threshold of 2.3 ml/min/g with 100% positive predictive value.

CONCLUSIONS

In angina patients with NOCAD, CMR can objectively and noninvasively assess microvascular angina. A CMR-based combined diagnostic pathway for both epicardial and microvascular CAD deserves further clinical validation.

Magnetic resonance imaging for characterizing myocardial diseases

The National Institute of Health defined cardiomyopathy as diseases of the heart muscle. These myocardial diseases have different etiology, structure and treatment. This review highlights the key imaging features of different myocardial diseases. It provides information on myocardial structure/orientation, perfusion, function and viability in diseases related to cardiomyopathy. The standard cardiac magnetic resonance imaging (MRI) sequences can reveal insight on left ventricular (LV) mass, volumes and regional contractile function in all types of cardiomyopathy diseases. Contrast enhanced MRI sequences allow visualization of different infarct patterns and sizes. Enhancement of myocardial inflammation and infarct (location, transmurality and pattern) on contrast enhanced MRI have been used to highlight the key differences in myocardial diseases, predict recovery of function and healing. The common feature in many forms of cardiomyopathy is the presence of diffuse-fibrosis. Currently, imaging sequences generating the most interest in cardiomyopathy include myocardial strain analysis, tissue mapping (T₁, T_2, T₂*) and extracellular volume (ECV) estimation techniques. MRI sequences have the potential to decode the etiology by showing various patterns of infarct and diffuse fibrosis in myocarditis, amyloidosis, sarcoidosis, hypertrophic cardiomyopathy due to aortic stenosis, restrictive cardiomyopathy, arrythmogenic right ventricular dysplasia and hypertension. Integrated PET/MRI system may add in the future more information for the diagnosis and progression of cardiomyopathy diseases. With the promise of high spatial/temporal resolution and 3D coverage, MRI will be an indispensible tool in diagnosis and monitoring the benefits of new therapies designed to treat myocardial diseases.

Assessment of stable coronary artery disease by cardiovascular magnetic resonance imaging: Current and emerging techniques

Coronary artery disease (CAD) is a leading cause of death and disability worldwide. Cardiovascular magnetic resonance (CMR) is established in clinical practice guidelines with a growing evidence base supporting its use to aid the diagnosis and management of patients with suspected or established CAD. CMR is a multi-parametric imaging modality that yields high spatial resolution images that can be acquired in any plane for the assessment of global and regional cardiac function, myocardial perfusion and viability, tissue characterisation and coronary artery anatomy, all within a single study protocol and without exposure to ionising radiation. Advances in technology and acquisition techniques continue to progress the utility of CMR across a wide spectrum of cardiovascular disease, and the publication of large scale clinical trials continues to strengthen the role of CMR in daily cardiology practice. This article aims to review current practice and explore the future directions of multi-parametric CMR imaging in the investigation of stable CAD.

Imaging oxygen metabolism with hyperpolarized magnetic resonance: a novel approach for the examination of cardiac and renal function

Every tissue in the body critically depends on meeting its energetic demands with sufficient oxygen supply. Oxygen supply/demand imbalances underlie the diseases that inflict the greatest socio-economic burden globally. The purpose of this review is to examine how hyperpolarized contrast media, used in combination with MR data acquisition methods, may advance our ability to assess oxygen metabolism non-invasively and thus improve management of clinical disease. We first introduce the concept of hyperpolarization and how hyperpolarized contrast media have been practically implemented to achieve translational and clinical research. We will then analyse how incorporating hyperpolarized contrast media could enable realization of unmet technical needs in clinical practice. We will focus on imaging cardiac and renal oxygen metabolism, as both organs have unique physiological demands to satisfy their requirements for tissue oxygenation, their dysfunction plays a fundamental role in society’s most prevalent diseases, and each organ presents unique imaging challenges. It is our aim that this review attracts a multi-disciplinary audience and sparks collaborations that utilize an exciting, emergent technology to advance our ability to treat patients adversely affected by an oxygen supply/demand mismatch.

Cardiac energetics, oxygenation, and perfusion during increased workload in patients with type 2 diabetes mellitus

AIMS

Patients with type 2 diabetes mellitus (T2DM) are known to have impaired resting myocardial energetics and impaired myocardial perfusion reserve, even in the absence of obstructive epicardial coronary artery disease (CAD). Whether or not the pre-existing energetic deficit is exacerbated by exercise, and whether the impaired myocardial perfusion causes deoxygenation and further energetic derangement during exercise stress, is uncertain.

METHODS AND RESULTS

Thirty-one T2DM patients, on oral antidiabetic therapies with a mean HBA1c of 7.4 ± 1.3%, and 17 matched controls underwent adenosine stress cardiovascular magnetic resonance for assessment of perfusion [myocardial perfusion reserve index (MPRI)] and oxygenation [blood-oxygen level-dependent (BOLD) signal intensity change (SIΔ)]. Cardiac phosphorus-MR spectroscopy was performed at rest and during leg exercise. Significant CAD (>50% coronary stenosis) was excluded in all patients by coronary computed tomographic angiography. Resting phosphocreatine to ATP (PCr/ATP) was reduced by 17% in patients (1.74 ± 0.26, P = 0.001), compared with controls (2.07 ± 0.35); during exercise, there was a further 12% reduction in PCr/ATP (P = 0.005) in T2DM patients, but no change in controls. Myocardial perfusion and oxygenation were decreased in T2DM (MPRI 1.61 ± 0.43 vs. 2.11 ± 0.68 in controls, P = 0.002; BOLD SIΔ 7.3 ± 7.8 vs. 17.1 ± 7.2% in controls, P < 0.001). Exercise PCr/ATP correlated with MPRI (r = 0.50, P = 0.001) and BOLD SIΔ (r = 0.32, P = 0.025), but there were no correlations between rest PCr/ATP and MPRI or BOLD SIΔ.

CONCLUSION

The pre-existing energetic deficit in diabetic cardiomyopathy is exacerbated by exercise; stress PCr/ATP correlates with impaired perfusion and oxygenation. Our findings suggest that, in diabetes, coronary microvascular dysfunction exacerbates derangement of cardiac energetics under conditions of increased workload.

Breathing Maneuvers as a Vasoactive Stimulus for Detecting Inducible Myocardial Ischemia - An Experimental Cardiovascular Magnetic Resonance Study

BACKGROUND

Breathing maneuvers can elicit a similar vascular response as vasodilatory agents like adenosine; yet, their potential diagnostic utility in the presence of coronary artery stenosis is unknown. The objective of the study is to investigate if breathing maneuvers can non-invasively detect inducible ischemia in an experimental animal model when the myocardium is imaged with oxygenation-sensitive cardiovascular magnetic resonance (OS-CMR).

METHODS AND FINDINGS

In 11 anesthetised swine with experimentally induced significant stenosis (fractional flow reserve <0.75) of the left anterior descending coronary artery (LAD) and 9 control animals, OS-CMR at 3T was performed during two different breathing maneuvers, a long breath-hold; and a combined maneuver of 60s of hyperventilation followed by a long breath-hold. The resulting change of myocardial oxygenation was compared to the invasive measurements of coronary blood flow, blood gases, and oxygen extraction. In control animals, all breathing maneuvers could significantly alter coronary blood flow as hyperventilation decreased coronary blood flow by 34±23%. A long breath-hold alone led to an increase of 97±88%, while the increase was 346±327% (p<0.001), when the long breath-hold was performed after hyperventilation. In stenosis animals, the coronary blood flow response was attenuated after both hyperventilation and the following breath-hold. This was matched by the observed oxygenation response as breath-holds following hyperventilation consistently yielded a significant difference in the signal of the MRI images between the perfusion territory of the stenosis LAD and remote myocardium. There was no difference between the coronary territories during the other breathing maneuvers or in the control group at any point.

CONCLUSION

In an experimental animal model, the response to a combined breathing maneuver of hyperventilation with subsequent breath-holding is blunted in myocardium subject to significant coronary artery stenosis. This maneuver may allow for detecting severe coronary artery stenosis and have a significant clinical potential as a non-pharmacological method for diagnostic testing in patients with suspected coronary artery disease.

The impact of hematocrit on oxygenation-sensitive cardiovascular magnetic resonance

BACKGROUND

Oxygenation-sensitive (OS) Cardiovascular Magnetic Resonance (CMR) is a promising utility in the diagnosis of heart disease. Contrast in OS-CMR images is generated through deoxyhemoglobin in the tissue, which is negatively correlated with the signal intensity (SI). Thus, changing hematocrit levels may be a confounder in the interpretation of OS-CMR results. We hypothesized that hemodilution confounds the observed signal intensity in OS-CMR images.

METHODS

Venous and arterial blood from five pigs was diluted with lactated Ringer solution in 10 % increments to 50 %. The changes in signal intensity (SI) were compared to changes in blood gases and hemoglobin concentration. We performed an OS-CMR scan in 21 healthy volunteers using vasoactive breathing stimuli at baseline, which was then repeated after rapid infusion of 1 L of lactated Ringer's solution within 5-8 min. Changes of SI were measured and compared between the hydration states.

RESULTS

The % change in SI from baseline for arterial (r = -0.67, p < 0.0001) and venous blood (r = -0.55, p = 0.002) were negatively correlated with the changes in hemoglobin (Hb). SI changes in venous blood were also associated with SO2 (r = 0.68, p < 0.0001) and deoxyHb concentration (-0.65, p < 0.0001). In healthy volunteers, rapid infusion resulted in a significant drop in the hemoglobin concentration (142.5 ± 15.2 g/L vs. 128.8 ± 15.2 g/L; p < 0.0001). Baseline myocardial SI increased by 3.0 ± 5.7 % (p = 0.026) following rapid infusion, and in males there was a strong association between the change in hemoglobin concentration and % changes in SI (r = 0.82, p = 0.002). After hyperhydration, the SI response after hyperventilation was attenuated (HV, p = 0.037), as was the maximum SI increase during apnea (p = 0.012). The extent of SI attenuation was correlated with the reduction in hemoglobin concentration at the end of apnea (r = 0.55, p = 0.012) for all subjects and at maximal SI (r = 0.63, p = 0.037) and the end of breath-hold (r = 0.68, p = 0.016) for males only.

CONCLUSION

In dynamic studies using oxygenation-sensitive CMR, the hematocrit level affects baseline signal intensity and the observed signal intensity response. Thus, the hydration status of the patient may be a confounder for OS-CMR image analysis.

Cardiovascular Imaging: The Past and the Future, Perspectives in Computed Tomography and Magnetic Resonance Imaging

Today's noninvasive imaging of the cardiovascular system has revolutionized the approach to various diseases and has substantially affected prognostic information. Cardiovascular magnetic resonance (MR) and computed tomographic (CT) imaging are at center stage of these approaches, although 5 decades ago, these technologies were unheard of. Both modalities had their inception in the 1970s with a primary focus on noncardiovascular applications. The technical development of the various decades, however, substantially pushed the envelope for cardiovascular MR and CT applications. Within the past 10-15 years, MR and CT technologies have pushed each other in cardiac applications; and without the "rival" modality, neither one would likely not have reached its potential today. This view on the history of MR and CT in the field of cardiovascular applications provides insight into the story of success of applications that once have been ideas only but are at prime time today.

Free-breathing, motion-corrected, highly efficient whole heart T2 mapping at 3T with hybrid radial-cartesian trajectory

PURPOSE

To develop and test a time-efficient, free-breathing, whole heart T2 mapping technique at 3.0T.

METHODS

ECG-triggered three-dimensional (3D) images were acquired with different T2 preparations at 3.0T during free breathing. Respiratory motion was corrected with a navigator-guided motion correction framework at near perfect efficiency. Image intensities were fit to a monoexponential function to derive myocardial T2 maps. The proposed 3D, free breathing, motion-corrected (3D-FB-MoCo) approach was studied in ex vivo canine hearts and kidneys, healthy volunteers, and canine subjects with acute myocardial infarction (AMI).

RESULTS

Ex vivo T2 values from proposed 3D T2 -prep gradient echo were not different from two-dimensional (2D) spin echo (P = 0.7) and T2 -prep balanced steady-state free precession (bSSFP) (P = 0.7). In healthy volunteers, compared with 3D-FB-MoCo and breath-held 2D T2 -prep bSSFP (2D-BH), non-motion-corrected (3D-FB-Non-MoCo) myocardial T2 was longer, had a larger coefficient of variation (COV), and had a lower image quality (IQ) score (T2 = 40.3 ms, COV = 38%, and IQ = 2.3; all P < 0.05). Conversely, the mean and COV and IQ of 3D-FB-MoCo (T2 = 37.7 ms, COV = 17%, and IQ = 3.5) and 2D-BH (T2 = 38.0 ms, COV = 15%, and IQ = 3.8) were not different (P = 0.99, P = 0.74, and P = 0.14, respectively). In AMI, T2 values and edema volumes from 3D-FB-MoCo and 2D-BH were closely correlated (R(2) = 0.88 and 0.96, respectively).

CONCLUSION

The proposed whole heart T2 mapping approach can be performed within 5 min with similar accuracy to that of the 2D-BH T2 mapping approach.

Adenosine Stress and Rest T1 Mapping Can Differentiate Between Ischemic, Infarcted, Remote, and Normal Myocardium Without the Need for Gadolinium Contrast Agents

Objectives

The aim of this study was to evaluate the potential of T1 mapping at rest and during adenosine stress as a novel method for ischemia detection without the use of gadolinium contrast.

Background

In chronic coronary artery disease (CAD), accurate detection of ischemia is important because targeted revascularization improves clinical outcomes. Myocardial blood volume (MBV) may be a more comprehensive marker of ischemia than myocardial blood flow. T1 mapping using cardiac magnetic resonance (CMR) is highly sensitive to changes in myocardial water content, including MBV. We propose that T1 mapping at rest and during adenosine vasodilatory stress can detect MBV changes in normal and diseased myocardium in CAD.

Methods

Twenty normal controls (10 at 1.5-T; 10 at 3.0-T) and 10 CAD patients (1.5-T) underwent conventional CMR to assess for left ventricular function (cine), infarction (late gadolinium enhancement [LGE]) and ischemia (myocardial perfusion reserve index [MPRI] on first-pass perfusion imaging during adenosine stress). These were compared to novel pre-contrast stress/rest T1 mapping using the Shortened Modified Look-Locker Inversion recovery technique, which is heart rate independent. T1 values were derived for normal myocardium in controls and for infarcted, ischemic, and remote myocardium in CAD patients.

Results

Normal myocardium in controls (normal wall motion, MPRI, no LGE) showed normal resting T1 (954 ± 19 ms at 1.5-T; 1,189 ± 34 ms at 3.0-T) and significant positive T1 reactivity during adenosine stress compared to baseline (6.2 ± 0.5% at 1.5-T; 6.3 ± 1.1% at 3.0-T; all p < 0.0001). Infarcted myocardium showed the highest resting T1 of all tissue classes (1,442 ± 84 ms), without significant T1 reactivity (0.2 ± 1.5%). Ischemic myocardium showed elevated resting T1 compared to normal (987 ± 17 ms; p < 0.001) without significant T1 reactivity (0.2 ± 0.8%). Remote myocardium, although having comparable resting T1 to normal (955 ± 17 ms; p = 0.92), showed blunted T1 reactivity (3.9 ± 0.6%; p < 0.001).

Conclusions

T1 mapping at rest and during adenosine stress can differentiate between normal, infarcted, ischemic, and remote myocardium with distinctive T1 profiles. Stress/rest T1 mapping holds promise for ischemia detection without the need for gadolinium contrast.

No Evidence of Myocardial Oxygen Deprivation in Nonischemic Heart Failure

Whether the myocardium in nonischemic heart failure experiences oxygen limitation remains a long-standing controversy. We addressed this question in patients with dilated cardiomyopathy (DCM) using a dual approach. First, we tested the changes in myocardial oxygenation between rest and stress states, using oxygenation-sensitive cardiovascular magnetic resonance. Second, we sought to assess the functional consequences of oxygen limitation at rest by measuring myocardial energetics before and after short-term oxygen supplementation.

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.

Cardiac imaging in patients with chronic kidney disease

Patients with chronic kidney disease (CKD) carry a high cardiovascular risk. In this patient group, cardiac structure and function are frequently abnormal and 74% of patients with CKD stage 5 have left ventricular hypertrophy (LVH) at the initiation of renal replacement therapy. Cardiac changes, such as LVH and impaired left ventricular systolic function, have been associated with an unfavourable prognosis. Despite the prevalence of underlying cardiac abnormalities, symptoms may not manifest in many patients. Fortunately, a range of available and emerging cardiac imaging tools may assist with diagnosing and stratifying the risk and severity of heart disease in patients with CKD. Moreover, many of these techniques provide a better understanding of the pathophysiology of cardiac abnormalities in patients with renal disease. Knowledge of the currently available cardiac imaging modalities might help nephrologists to choose the most appropriate investigative tool based on individual patient circumstances. This Review describes established and emerging cardiac imaging modalities in this context, and compares their use in CKD patients with their use in the general population.

Cardiovascular magnetic resonance in ischemic heart disease

Ischemic heart disease is the major cause of death in developed countries. Recently, cardiovascular magnetic resonance (CMR) has appeared as a powerful technique for diagnosis and prognosis of ischemia, as well as for postischemic therapy follow-up. The objective of this chapter is to provide an overview of the role of CMR in assessing ischemic myocardium. It reviews the most recent studies in this field and includes CMR parameters that are already well established in the clinical setting as well as promising or emerging parameters in clinical use.

Myocardial perfusion and oxygenation are impaired during stress in severe aortic stenosis and correlate with impaired energetics and subclinical left ventricular dysfunction

BACKGROUND

Left ventricular (LV) hypertrophy in aortic stenosis (AS) is characterized by reduced myocardial perfusion reserve due to coronary microvascular dysfunction. However, whether this hypoperfusion leads to tissue deoxygenation is unknown. We aimed to assess myocardial oxygenation in severe AS without obstructive coronary artery disease, and to investigate its association with myocardial energetics and function.

METHODS

Twenty-eight patients with isolated severe AS and 15 controls underwent cardiovascular magnetic resonance (CMR) for assessment of perfusion (myocardial perfusion reserve index-MPRI) and oxygenation (blood-oxygen level dependent-BOLD signal intensity-SI change) during adenosine stress. LV circumferential strain and phosphocreatine/adenosine triphosphate (PCr/ATP) ratios were assessed using tagging CMR and 31P MR spectroscopy, respectively.

RESULTS

AS patients had reduced MPRI (1.1 ± 0.3 vs. controls 1.7 ± 0.3, p < 0.001) and BOLD SI change during stress (5.1 ± 8.9% vs. controls 18.2 ± 10.1%, p = 0.001), as well as reduced PCr/ATP (1.45 ± 0.21 vs. 2.00 ± 0.25, p < 0.001) and LV strain (-16.4 ± 2.7% vs. controls -21.3 ± 1.9%, p < 0.001). Both perfusion reserve and oxygenation showed positive correlations with energetics and LV strain. Furthermore, impaired energetics correlated with reduced strain. Eight months post aortic valve replacement (AVR) (n = 14), perfusion (MPRI 1.6 ± 0.5), oxygenation (BOLD SI change 15.6 ± 7.0%), energetics (PCr/ATP 1.86 ± 0.48) and circumferential strain (-19.4 ± 2.5%) improved significantly.

CONCLUSIONS

Severe AS is characterized by impaired perfusion reserve and oxygenation which are related to the degree of derangement in energetics and associated LV dysfunction. These changes are reversible on relief of pressure overload and hypertrophy regression. Strategies aimed at improving oxygen demand-supply balance to preserve myocardial energetics and LV function are promising future therapies.

Assessment of myocardial reactivity to controlled hypercapnia with free-breathing T2-prepared cardiac blood oxygen level-dependent MR imaging

PURPOSE

To examine whether controlled and tolerable levels of hypercapnia may be an alternative to adenosine, a routinely used coronary vasodilator, in healthy human subjects and animals.

MATERIALS AND METHODS

Human studies were approved by the institutional review board and were HIPAA compliant. Eighteen subjects had end-tidal partial pressure of carbon dioxide (PetCO2) increased by 10 mm Hg, and myocardial perfusion was monitored with myocardial blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging. Animal studies were approved by the institutional animal care and use committee. Anesthetized canines with (n = 7) and without (n = 7) induced stenosis of the left anterior descending artery (LAD) underwent vasodilator challenges with hypercapnia and adenosine. LAD coronary blood flow velocity and free-breathing myocardial BOLD MR responses were measured at each intervention. Appropriate statistical tests were performed to evaluate measured quantitative changes in all parameters of interest in response to changes in partial pressure of carbon dioxide.

RESULTS

Changes in myocardial BOLD MR signal were equivalent to reported changes with adenosine (11.2% ± 10.6 [hypercapnia, 10 mm Hg] vs 12% ± 12.3 [adenosine]; P = .75). In intact canines, there was a sigmoidal relationship between BOLD MR response and PetCO2 with most of the response occurring over a 10 mm Hg span. BOLD MR (17% ± 14 [hypercapnia] vs 14% ± 24 [adenosine]; P = .80) and coronary blood flow velocity (21% ± 16 [hypercapnia] vs 26% ± 27 [adenosine]; P > .99) responses were similar to that of adenosine infusion. BOLD MR signal changes in canines with LAD stenosis during hypercapnia and adenosine infusion were not different (1% ± 4 [hypercapnia] vs 6% ± 4 [adenosine]; P = .12).

CONCLUSION

Free-breathing T2-prepared myocardial BOLD MR imaging showed that hypercapnia of 10 mm Hg may provide a cardiac hyperemic stimulus similar to adenosine.