Cine and tagged magnetic resonance imaging in short-term stunned versus necrotic myocardium

Subendocardial contractile impairment in chronic ischemic myocardium: assessment by strain analysis of 3T tagged CMR

BACKGROUND

The purpose of this study was to quantify myocardial strain on the subendocardial and epicardial layers of the left ventricle (LV) using tagged cardiovascular magnetic resonance (CMR) and to investigate the transmural degree of contractile impairment in the chronic ischemic myocardium.

METHODS

3T tagged CMR was performed at rest in 12 patients with severe coronary artery disease who had been scheduled for coronary artery bypass grafting. Circumferential strain (C-strain) at end-systole on subendocardial and epicardial layers was measured using the short-axis tagged images of the LV and available software (Intag; Osirix). The myocardial segment was divided into stenotic and non-stenotic segments by invasive coronary angiography, and ischemic and non-ischemic segments by stress myocardial perfusion scintigraphy. The difference in C-strain between the two groups was analyzed using the Mann-Whitney U-test. The diagnostic capability of C-strain was analyzed using receiver operating characteristics analysis.

RESULTS

The absolute subendocardial C-strain was significantly lower for stenotic (-7.5 ± 12.6%) than non-stenotic segment (-18.8 ± 10.2%, p < 0.0001). There was no difference in epicardial C-strain between the two groups. Use of cutoff thresholds for subendocardial C-strain differentiated stenotic segments from non-stenotic segments with a sensitivity of 77%, a specificity of 70%, and areas under the curve (AUC) of 0.76. The absolute subendocardial C-strain was significantly lower for ischemic (-6.7 ± 13.1%) than non-ischemic segments (-21.6 ± 7.0%, p < 0.0001). The absolute epicardial C-strain was also significantly lower for ischemic (-5.1 ± 7.8%) than non-ischemic segments (-9.6 ± 9.1%, p < 0.05). Use of cutoff thresholds for subendocardial C-strain differentiated ischemic segments from non-ischemic segments with sensitivities of 86%, specificities of 84%, and AUC of 0.86.

CONCLUSIONS

Analysis of tagged CMR can non-invasively demonstrate predominant impairment of subendocardial strain in the chronic ischemic myocardium at rest.

Magnetic resonance imaging/magnetic resonance spectroscopy biomarkers evaluation of stunned myocardium in canine model

OBJECTIVES

To evaluate whether dynamic alterations in high-energy phosphate (HEP) occur in postischemic "stunned" myocardium (SM) in canine model and to investigate the correlation between HEP and cardiac function, using cine magnetic resonance imaging (cine-MRI) and phosphorus-31 magnetic resonance spectroscopy (31P-MRS).

MATERIALS AND METHODS

Dogs (n = 13) underwent cine MRI and 31P-MRS at 60 minutes, 8 days after 10 minutes full left anterior descending occlusion followed by reperfusion. The same MRI/MRS experiments were repeated on 5 reference animals (dogs without ischemic reperfusion) at the same time points to serve as internal reference myocardium (RM). After MR data acquisitions, the SM dogs (n = 3 at 60 minutes; n = 10 at 60 minutes and day 8) and RM dogs (n = 5) were euthanized and myocardial tissues were sampled for histologic study by triphenyltetrazolium chloride staining, hematoxylin and eosin staining, and electron microscopic examination.

RESULTS

The myocardial stunning at 60 minutes was confirmed by electron microscopy examinations from the 3 randomly chosen animals with SM. The phosphocreatine (PCr)/β- adenosine triphosphate (ATP) ratio of SM was significantly lower at 60 minutes than that at day 8 (1.07 ± 0.20 vs. 1.97 ± 0.28, P < 0.05). However, no significant difference was found between 60 minutes and day 8 in RM group (1.91 ± 0.14 at 60 minutes vs. 1.89 ± 0.16 at day 8, P > 0.05). At 60 minutes, the PCr/β-ATP ratio has significant difference between SM and RM groups; while at day 8, the ratio shows no significant difference between the 2 groups. The same results were obtained for left ventricle ejection fraction (LVEF). In SM group, LVEF has good correlation with myocardial PCr/β-ATP ratios at 60 minutes (R2 = 0.71, P < 0.05) and at day 8 (R2 = 0.73, P < 0.05), respectively.

CONCLUSIONS

The HEP alterations were confirmed by 31P-MRS in SM and there is a good correlation between PCr/β-ATP ratio and LVEF for SM at 60 minutes and recovered myocardium at day 8. The combined MRS/MRI method offers the potential to systematically assess the cardiac function, morphology, and metabolism of SM. These MRS/MRI biomarker datasets could be used to dynamically monitor therapeutic efficiency and predict cardiac events.

3D versus 2D dynamic 82Rb myocardial blood flow imaging in a canine model of stunned and infarcted myocardium

PURPOSE

Previous studies have shown the ability of rubidium-82 ((82)Rb) positron emission tomography (PET) imaging to quantitatively measure myocardial blood flow (MBF), many of which are performed using two-dimensional (2D) imaging. Three-dimensional (3D) imaging provides increased sensitivity and may result in decreased costs owing to a reduction in the required injected activity of radiotracer. This study compares 2D and 3D (82)Rb PET MBF results obtained in the same imaging session.

METHODS

Three-dimensional and 2D (82)Rb perfusion imaging was performed in canines on a GE Discovery LS PET/CT scanner at rest and during hyperemia in stunned and infarcted tissue. MBF (ml/min/g) was determined using a 1-compartment model and an extraction correction of the uptake rate and analyzed using a standard 17-segment model.

RESULTS

A strong, significant correlation was present (rho = 0.95, P<0.0001). Average 3D MBF values were slightly lower at rest and higher during stress versus 2D. MBF results in normal, stunned, and infarcted tissue differed by 7% on average and significant increases in MBF from rest to hyperemia were noted with both the techniques.

CONCLUSION

These results imply that MBF results obtained in 3D are comparable with traditional 2D imaging. Therefore, it may be possible to use 3D imaging with lower administered activity, helping to reduce costs and patient dose without compromising quantitative information.

Cine and tagged cardiovascular magnetic resonance imaging in normal rat at 1.5 T: a rest and stress study

BACKGROUND

The purpose of this study was to measure regional contractile function in the normal rat using cardiac cine and tagged cardiovascular magnetic resonance (CMR) during incremental low doses of dobutamine and at rest.

METHODS

Five rats were investigated for invasive left ventricle pressure measurements and five additional rats were imaged on a clinical 1.5 T MR system using a cine sequence (11-20 phases per cycle, 0.28/0.28/2 mm) and a C-SPAMM tag sequence (18-25 phases per cycle, 0.63/1.79/3 mm, tag spacing 1.25 mm). For each slice, wall thickening (WT) and circumferential strains (CS) were calculated at rest and at stress (2.5, 5 and 10 microg/min/kg of dobutamine).

RESULTS

Good cine and tagged images were obtained in all the rats even at higher heart rate (300-440 bpm). Ejection fraction and left ventricular (LV) end-systolic volume showed significant changes after each dobutamine perfusion dose (p < 0.001). Tagged CMR had the capacity to resolve the CS transmural gradient and showed a significant increase of both WT and CS at stress compared to rest. Intra and interobserver study showed less variability for the tagged technique. In rats in which a LV catheter was placed, dobutamine produced a significant increase of heart rate, LV dP/dtmax and LV pressure significantly already at the lowest infusion dose.

CONCLUSION

Robust cardiac cine and tagging CMR measurements can be obtained in the rat under incremental dobutamine stress using a clinical 1.5 T MR scanner.

Assessment of segmental myocardial viability using regional 2-dimensional strain echocardiography

We determined whether 2-dimensional strain echocardiography can identify viable from infarcted myocardium in a rat ischemia-reperfusion model. A total of 16 male Sprague-Dawley rats underwent left anterior descending coronary artery occlusion for 12 or 30 minutes followed by 60-minute reperfusion. Short-axis 2-dimensional strain echocardiography was performed at the mid-ventricle 60 minutes post-reperfusion. Post-sacrifice, triphenyl tetrazolium chloride was infused to the coronary circulation. Regional end-systolic radial and circumferential strain, and time to peak strain, were measured using software in all 96 segments and correlated with areas of infarct in corresponding histologic slices. Segments with greater than 50% area of infarct had lower end-systolic radial and circumferential strain and longer time to peak strain versus areas with 50% or less strain or no infarct. Extent of infarct correlates with radial and circumferential strain. End-systolic radial strain less than 2% has 88% sensitivity and 95% specificity for detecting infarcted area greater than 50%. Two-dimensional strain echocardiography-derived strain is useful in distinguishing infarcted from viable myocardium.

Normal and pathological NCAT image and phantom data based on physiologically realistic left ventricle finite-element models

The four-dimensional (4-D) NURBS-based cardiac-torso (NCAT) phantom, which provides a realistic model of the normal human anatomy and cardiac and respiratory motions, is used in medical imaging research to evaluate and improve imaging devices and techniques, especially dynamic cardiac applications. One limitation of the phantom is that it lacks the ability to accurately simulate altered functions of the heart that result from cardiac pathologies such as coronary artery disease (CAD). The goal of this work was to enhance the 4-D NCAT phantom by incorporating a physiologically based, finite-element (FE) mechanical model of the left ventricle (LV) to simulate both normal and abnormal cardiac motions. The geometry of the FE mechanical model was based on gated high-resolution X-ray multislice computed tomography (MSCT) data of a healthy male subject. The myocardial wall was represented as a transversely isotropic hyperelastic material, with the fiber angle varying from -90 degrees at the epicardial surface, through 0 degrees at the midwall, to 90 degrees at the endocardial surface. A time-varying elastance model was used to simulate fiber contraction, and physiological intraventricular systolic pressure-time curves were applied to simulate the cardiac motion over the entire cardiac cycle. To demonstrate the ability of the FE mechanical model to accurately simulate the normal cardiac motion as well as the abnormal motions indicative of CAD, a normal case and two pathologic cases were simulated and analyzed. In the first pathologic model, a subendocardial anterior ischemic region was defined. A second model was created with a transmural ischemic region defined in the same location. The FE-based deformations were incorporated into the 4-D NCAT cardiac model through the control points that define the cardiac structures in the phantom which were set to move according to the predictions of the mechanical model. A simulation study was performed using the FE-NCAT combination to investigate how the differences in contractile function between the subendocardial and transmural infarcts manifest themselves in myocardial Single photon emission computed tomography (SPECT) images. The normal FE model produced strain distributions that were consistent with those reported in the literature and a motion consistent with that defined in the normal 4-D NCAT beating heart model based on tagged magnetic resonance imaging (MRI) data. The addition of a subendocardial ischemic region changed the average transmural circumferential strain from a contractile value of -0.09 to a tensile value of 0.02. The addition of a transmural ischemic region changed average circumferential strain to a value of 0.13, which is consistent with data reported in the literature. Model results demonstrated differences in contractile function between subendocardial and transmural infarcts and how these differences in function are documented in simulated myocardial SPECT images produced using the 4-D NCAT phantom. Compared with the original NCAT beating heart model, the FE mechanical model produced a more accurate simulation for the cardiac motion abnormalities. Such a model, when incorporated into the 4-D NCAT phantom, has great potential for use in cardiac imaging research. With its enhanced physiologically based cardiac model, the 4-D NCAT phantom can be used to simulate realistic, predictive imaging data of a patient population with varying whole-body anatomy and with varying healthy and diseased states of the heart that will provide a known truth from which to evaluate and improve existing and emerging 4-D imaging techniques used in the diagnosis of cardiac disease.