Wolfgang G. Rehwald

Rapid Detection of Myocardial Infarction by Subsecond, Free-Breathing Delayed Contrast-Enhancement Cardiovascular Magnetic Resonance

Background— An ultrafast, delayed contrast-enhancement cardiovascular magnetic resonance technique that can acquire subsecond, “snapshot” images during free breathing (subsecond) is becoming widely available. This technique provides myocardial infarction (MI) imaging with complete left ventricular coverage in <30 seconds. However, the accuracy of this technique is unknown. Methods and Results— We prospectively compared subsecond imaging with routine breath-hold delayed contrast-enhancement cardiovascular magnetic resonance (standard) in consecutive patients. Two cohorts with unambiguous standards of truth were prespecified: (1) patients with documented prior MI (n=135) and (2) patients without MI and with low likelihood of coronary disease (lowest Framingham risk category; n=103). Scans were scored masked to identity and clinical information. Sensitivity, specificity, and accuracy of subsecond imaging for MI diagnosis were 87%, 96%, and 91%, respectively. Compared with the standard technique (98%, 100%, 99%), the subsecond technique had modestly reduced sensitivity (P=0.0001), but specificity was excellent. Missed infarcts were generally small or subendocardial (87%). Overall, regional transmural extent of infarction scores were highly concordant (2083/2294; 91%); however, 51 of 337 regions (15%) considered predominantly infarcted (>50% transmural extent of infarction) by the standard technique were considered viable (≤25% transmural extent of infarction) by the subsecond technique. Quantitative analysis demonstrated moderately reduced contrast-to-noise ratios for subsecond imaging between infarct and remote myocardium (12.0±7.2 versus 20.1±6.6; P<0.0001) and infarct and left ventricular cavity (−2.5±2.7 versus 3.6±3.7; P<0.0001). Conclusions— MI can be rapidly detected by subsecond delayed contrast-enhancement cardiovascular magnetic resonance during free breathing with high accuracy. This technique could be considered the preferred approach in patients who are more acutely ill or unable to hold their breath. However, compared with standard imaging, sensitivity is mildly reduced, and the transmural extent of infarction may be underestimated.

Relationship of T2-Weighted MRI Myocardial Hyperintensity and the Ischemic Area-At-Risk

RATIONALE After acute myocardial infarction (MI), delineating the area-at-risk (AAR) is crucial for measuring how much, if any, ischemic myocardium has been salvaged. T2-weighted MRI is promoted as an excellent method to delineate the AAR. However, the evidence supporting the validity of this method to measure the AAR is indirect, and it has never been validated with direct anatomic measurements. OBJECTIVE To determine whether T2-weighted MRI delineates the AAR. METHODS AND RESULTS Twenty-one canines and 24 patients with acute MI were studied. We compared bright-blood and black-blood T2-weighted MRI with images of the AAR and MI by histopathology in canines and with MI by in vivo delayed-enhancement MRI in canines and patients. Abnormal regions on MRI and pathology were compared by (a) quantitative measurement of the transmural-extent of the abnormality and (b) picture matching of contours. We found no relationship between the transmural-extent of T2-hyperintense regions and that of the AAR (bright-blood-T2: r=0.06, P=0.69; black-blood-T2: r=0.01, P=0.97). Instead, there was a strong correlation with that of infarction (bright-blood-T2: r=0.94, P<0.0001; black-blood-T2: r=0.95, P<0.0001). Additionally, contour analysis demonstrated a fingerprint match of T2-hyperintense regions with the intricate contour of infarcted regions by delayed-enhancement MRI. Similarly, in patients there was a close correspondence between contours of T2-hyperintense and infarcted regions, and the transmural-extent of these regions were highly correlated (bright-blood-T2: r=0.82, P<0.0001; black-blood-T2: r=0.83, P<0.0001). CONCLUSION T2-weighted MRI does not depict the AAR. Accordingly, T2-weighted MRI should not be used to measure myocardial salvage, either to inform patient management decisions or to evaluate novel therapies for acute MI.

Physiological Basis for Potassium (39K) Magnetic Resonance Imaging of the Heart

The potassium cation (K+) is fundamentally involved in myocyte metabolism. To explore the potential utility of direct MRI of the most abundant natural isotope of potassium, 39K, we compared 39K magnetic resonance (MR) image intensity with regional myocardial K+ concentrations after irreversible injury. Rabbits were subjected either to 40 minutes of in situ coronary artery occlusion and 1 hour of reperfusion (n=26) or to 24 hours of permanent occlusion (n=4). The hearts were then isolated and imaged by 39K MRI (n=10), or tissue samples were analyzed for regional 39K content by MR spectroscopy (n=9), K+ and Na+ concentrations by atomic emission spectroscopy (inductively coupled plasma atomic emission spectroscopy; n=5), or intracellular K+ content by electron probe x-ray microanalysis (n=6). Three-dimensional 39K MR images of the isolated hearts were acquired in 44 minutes with 3 x 3 x 3-mm resolution. 39K MR image intensity was reduced in infarcted regions (51.7+/-4. 8% of remote; P<0.001). The circumferential extent and location of regions of reduced 39K image intensity were correlated with those of infarcted regions defined histologically (r=0.97 and r=0.98, respectively). Compared with remote regions, tissue analysis revealed that infarcted regions had reduced 39K concentration (by MR spectroscopy, 40.5+/-9.3% of remote; P<0.001), reduced potassium-to-sodium ratio (by inductively coupled plasma atomic emission spectroscopy, 20.7+/-2.1% of remote; P<0.01), and reduced intracellular potassium (by electron probe x-ray microanalysis, K+ peak-to-background ratio 0.95+/-0.32 versus 2.86+/-1.10, respectively; P<0.01). We acquired the first 39K MR images of hearts subjected to infarction. In the pathophysiologies examined, potassium (39K) MR image intensity primarily reflects regional intracellular K+ concentrations.

Motion and flow insensitive adiabatic T2 -preparation module for cardiac MR imaging at 3 Tesla.

A versatile method for generating T2‐weighting is a T2‐preparation module, which has been used successfully for cardiac imaging at 1.5T. Although it has been applied at 3T, higher fields (B0 ≥ 3T) can degrade B0 and B1 homogeneity and result in nonuniform magnetization preparation. For cardiac imaging, blood flow and cardiac motion may further impair magnetization preparation. In this study, a novel T2‐preparation module containing multiple adiabatic B1‐insensitive refocusing pulses is introduced and compared with three previously described modules [(a) composite MLEV4, (b) modified BIR‐4 (mBIR‐4), and (c) Silver‐Hoult–pair]. In the static phantom, the proposed module provided similar or better B0 and B1 insensitivity than the other modules. In human subjects (n = 21), quantitative measurement of image signal coefficient of variation, reflecting overall image inhomogeneity, was lower for the proposed module (0.10) than for MLEV4 (0.15, P < 0.0001), mBIR‐4 (0.27, P < 0.0001), and Silver‐Hoult–pair (0.14, P = 0.001) modules. Similarly, qualitative analysis revealed that the proposed module had the best image quality scores and ranking (both, P < 0.0001). In conclusion, we present a new T2‐preparation module, which is shown to be robust for cardiac imaging at 3T in comparison with existing methods. Magn Reson Med 70:1360–1368, 2013.

Cardiovascular MRI: its current and future use in clinical practice

Cardiovascular magnetic resonance (CMR) imaging is a comprehensive clinical tool for assessing a large variety of cardiovascular diseases. Using the clinical service of the Duke Cardiovascular Magnetic Resonance Center as an example, we describe how to perform image contractile function, myocardial perfusion at stress and rest, myocardial viability, cardiovascular morphology, vascular anatomy and blood flow tests. The emergence of successful dedicated CMR services presents an opportunity to optimize patient throughput by streamlining the user interface of CMR scanners, standardizing the viewing format and reporting software, and customizing training programs to focus on the standardized CMR approaches. Accordingly, we discuss potential pathways to create these standards. Finally, we discuss several promising new CMR techniques we expect will complement existing clinical procedures.

Noninvasive Assessment of Blood Flow Based on Magnetic Resonance Global Coherent Free Precession

Background—Magnetic resonance global coherent free precession (GCFP) is a new technique that produces cine projection angiograms directly analogous to those of x-ray angiography noninvasively and without a contrast agent. In this study, we compared GCFP blood flow with “gold standards” to determine the accuracy of noninvasive GCFP blood flow measurements. Methods and Results—The relationship between GCFP blood flow and true blood flow defined by invasive ultrasonic flow probe and by phase contrast velocity encoded MRI (VENC) was studied in anesthetized dogs (n=6). Blood flow was controlled by use of a hydraulic occluder around the left iliac artery. GCFP images were acquired by selectively exciting the abdominal aorta and visualizing temporal blood flow into the iliac arteries. GCFP flow was similar to ultrasonic blood flow at baseline (131.3±44.8 versus 114.8±34.2 mL/min), during occlusion (10.8±5.1 versus 6.5±7.2 mL/min), during reactive hyperemia (191.4±100.7 versus 260.3±138.7 mL/min), during the new resting state (135.5±52.4 versus 117.8±24.1 mL/min), and during partial occlusion (61.4±36.4 versus 49.3±13.1 mL/min, P=NS for all). Results comparing GCFP flow with VENC were similar. Statistical analysis revealed that GCFP flow was related to mean blood flow assessed by the flow probe (P<0.0001) and by VENC (P<0.0001). In the control right iliac artery, conversely, GCFP measurements were unaffected throughout all left iliac interventions (P=NS). Conclusions—GCFP blood flow is linearly related to true blood flow for a straight, cylindrical blood vessel without branches. Although more complex geometries imply a qualitative rather than a quantitative relationship, the data nevertheless suggest that GCFP may serve as the basis for a new form of noninvasive stress testing.

Respiratory Motion and Cardiac Arrhythmia Effects on Diagnostic Accuracy of Myocardial Delayed-enhanced MR Imaging in Canines

PURPOSE To prospectively compare in canines the diagnostic accuracy for myocardial infarction (MI) of standard delayed-enhancement (DE) magnetic resonance (MR) imaging versus that of subsecond DE MR imaging with and without breath holding and/or cardiac arrhythmia, with histologic findings or absence of surgical creation of MI as the reference standard. MATERIALS AND METHODS This study was approved by the Institutional Animal Care and Use Committee; 21 canines were imaged with one standard and two subsecond DE MR techniques in four conditions: condition 1, breath holding and steady gating; 2, non-breath holding and steady gating; 3, breath holding and irregular heart rhythm; and 4, non-breath holding and irregular heart rhythm. Images were randomized and scored for diagnostic accuracy, image quality, and observer confidence. Sensitivity, specificity, and diagnostic accuracy for MI detection were calculated for each technique and clinical condition separately. The chi(2), paired t, and McNemar tests were used for comparisons. RESULTS Fifteen dogs had MIs. Among conditions 2-4, differences were not significant (P > .05); data were pooled and referred to as group B. Condition 1 was group A. Accuracy, image quality, and observer confidence, respectively, for standard DE MR imaging were 96%, 3.7 +/- 0.8, and 2.7 +/- 0.6 in group A but only 74%, 2.4 +/- 0.8, and 1.8 +/- 0.7 in group B (P < or = .004 for each). Corresponding scores for subsecond techniques were unaffected by respiratory motion and/or arrhythmia. Subsecond techniques had higher accuracy (82% and 86% vs 74%), better image quality (3.9 +/- 0.7 and 3.2 +/- 0.8 vs 2.4 +/- 0.8), and greater confidence (2.4 +/- 0.7 and 2.1 +/- 0.7 vs 1.8 +/- 0.7) (P < or = .0002 for each) than standard DE MR imaging. In group A, standard performed better than subsecond DE MR imaging. CONCLUSION Standard DE MR imaging is appropriate for MI detection with breath holding and regular heart rhythm, while subsecond techniques are appropriate with an irregular heart rhythm and when breath holding is not possible.

T(Rho) and magnetization transfer and INvErsion recovery (TRAMINER)-prepared imaging: A novel contrast-enhanced flow-independent dark-blood technique for the evaluation of myocardial late gadolinium enhancement in patients with myocardial infarction.

To evaluate a new dark‐blood late gadolinium enhancement (LGE) technique called “T(Rho) And Magnetization transfer and INvErsion Recovery” (TRAMINER) for the ability to detect myocardial LGE versus standard “bright‐blood” inversion recovery (SIR) imaging.

Flow-Independent Dark-blood DeLayed Enhancement (FIDDLE): validation of a novel black blood technique for the diagnosis of myocardial infarction

Background A fundamental component of the CMR exam is contrast enhanced imaging, which is crucial for delineating diseased from normal tissue. Unfortunately, diseased tissue adjacent to vasculature often remains hidden since there is poor contrast between hyperenhanced tissue and bright blood-pool. Conventional black-blood double-IR methods are not a solution; these were not designed to function after contrast administration since they rely on the long native T1 of blood (~2s at 3T) and adequate blood flow within this time period. We introduce a novel Flow-Independent Dark-blood DeLayed Enhancement technique (FIDDLE) that allows visualization of tissue contrast-enhancement while suppressing blood-pool signal. We validate FIDDLE in an animal model of myocardial infarction (MI) and demonstrate feasibility in patients.

Left ventricular function assessment using a fast 3D gradient echo pulse sequence: comparison to standard multi-breath hold 2D steady state free precession imaging and accounting for papillary muscles and trabeculations.

Objective Papillary muscles and trabeculae for ventricular function analysis are known to signifi cantly contribute to accurate volume and mass measurements. Fast imaging techniques such as three-dimensional steady-state free precession (3D SSFP) are increasingly being used to speed up imaging time, but sacrifi ce spatial resolution. It is unknown whether 3D SSFP, despite its reduced spatial resolution, allows for exact delineation of papillary muscles and trabeculations. We therefore compared 3D SSFP ventricular function measurements to those measured from standard multi-breath hold two-dimensional steady-state free precession cine images (standard 2D SSFP). Methods and results 14 healthy subjects and 14 patients with impaired left ventricular function underwent 1.5 Tesla cine imaging. A stack of short axis images covering the left ventricle was acquired with 2D SSFP and 3D SSFP. Left ventricular volumes, ejection fraction, and mass were determined. Analysis was performed by substracting papillary muscles and trabeculae from left ventricular volumes. In addition, reproducibility was assessed. EDV, ESV, EF, and mass were not signifi cantly diff erent between 2D SSFP and 3D SSFP (mean diff erence healthy subjects: -0.06 ± 3.2 ml, 0.54 ± 2.2 ml, -0.45 ± 1.8%, and 1.13 ± 0.8 g, respectively; patients: 1.36 ± 2.8 ml, -0.15 ± 3.5 ml, 0.86 ± 2.5%, and 0.91 ± 0.9 g, respectively; P≥ 0.095). Intra- and interobserver variability was not diff erent for 2D SSFP (P≥ 0.64 and P≥ 0.397) and 3D SSFP (P≥ 0.53 and P≥ 0.47). Conclusions Diff erences in volumes, EF, and mass measurements between 3D SSFP and standard 2D SSFP are very small, and not statistically signifi cant. 3D SSFP may be used for accurate ventricular function assessment when papillary muscles and trabeculations are to be taken into account.