MRI study on volume effects of coronary emboli on myocardial function, perfusion and viability. Int J Cardiol. 2013;165:93-99. [PMID: 21872947 DOI: 10.1016/j.ijcard.2011.07.096]

Periprocedural Myocardial Injury: Pathophysiology, Prognosis, and Prevention

The definition and clinical implications of myocardial infarction occurring in the setting of percutaneous coronary intervention have been the subject of unresolved controversy. The definitions of periprocedural myocardial infarction (PMI) are many and have evolved over recent years. Additionally, the recent advancement of different imaging modalities has provided useful information on a patients' pre-procedural risk of myocardial infarction. Nonetheless, questions on the benefit of different approaches to prevent PMI and their practical implementation remain open. This review aims to address these questions and to provide a current and contemporary perspective.

Decreased atrioventricular plane displacement after acute myocardial infarction yields a concomitant decrease in stroke volume

Acute myocardial infarction (AMI) can progress to heart failure, which has a poor prognosis. Normally, 60% of stroke volume (SV) is attributed to the longitudinal ventricular shortening and lengthening evident in the atrioventricular plane displacement (AVPD) during the cardiac cycle, but there is no information on how the relationship changes between SV and AVPD before and after AMI. Therefore, the aim of this study was to determine how SV depends on AVPD before and after AMI in two swine models. Serial cardiac magnetic resonance imaging was carried out before and 1–2 h after AMI in a microembolization model (n = 12) and an ischemia-reperfusion model (n = 14). A subset of pigs (n = 7) were additionally imaged at 24 h and at 7 days. Cine and late gadolinium enhancement images were analyzed for cardiac function, AVPD measurements and infarct size estimation, respectively. AVPD decreased (P < 0.05) in all myocardial regions after AMI, with a concomitant SV decrease (P < 0.001). The ischemia-reperfusion model affected SV to a higher degree and had a larger AVPD decrease than the microembolization model (−29 ± 14% vs. −15 ± 18%; P < 0.05). Wall thickening decreased in infarcted areas (P < 0.001), and A-wave AVPD remained unchanged (P = 0.93) whereas E-wave AVPD decreased (P < 0.001) after AMI. We conclude that AVPD is coupled to SV independent of infarct type but likely to a greater degree in ischemia-reperfusion infarcts compared with microembolization infarcts. AMI reduces diastolic early filling AVPD but not AVPD from atrial contraction. These findings shed light on the physiological significance of atrioventricular plane motion when assessing acute and subacute myocardial infarction.

NEW & NOTEWORTHY The link between cardiac longitudinal motion, measured as atrioventricular plane displacement (AVPD), and stroke volume (SV) is investigated in swine after acute myocardial infarction (AMI). This cardiac magnetic resonance study demonstrates a close coupling between AVPD and SV before and after AMI in an experimental setting and demonstrates that this connection is present in ischemia-reperfusion and microembolization infarcts, acutely and during the first week. Furthermore, AVPD is equally and persistently depressed in infarcted and remote myocardium after AMI.

Intradialytic Cardiac Magnetic Resonance Imaging to Assess Cardiovascular Responses in a Short-Term Trial of Hemodiafiltration and Hemodialysis

Hemodynamic stress during hemodialysis (HD) results in recurrent segmental ischemic injury (myocardial stunning) that drives cumulative cardiac damage. We performed a fully comprehensive study of the cardiovascular effect of dialysis sessions using intradialytic cardiac magnetic resonance imaging (MRI) to examine the comparative acute effects of standard HD versus hemodiafiltration (HDF) in stable patients. We randomly allocated 12 patients on HD (ages 32-72 years old) to either HD or HDF. Patients were stabilized on a modality for 2 weeks before undergoing serial cardiac MRI assessment during dialysis. Patients then crossed over to the other modality and were rescanned after 2 weeks. Cardiac MRI measurements included cardiac index, stroke volume index, global and regional contractile function (myocardial strain), coronary artery flow, and myocardial perfusion. Patients had mean±SEM ultrafiltration rates of 3.8±2.9 ml/kg per hour during HD and 4.4±2.5 ml/kg per hour during HDF (P=0.29), and both modalities provided a similar degree of cooling. All measures of systolic contractile function fell during HD and HDF, with partial recovery after dialysis. All patients experienced some degree of segmental left ventricular dysfunction, with severity proportional to ultrafiltration rate and BP reduction. Myocardial perfusion decreased significantly during HD and HDF. Treatment modality did not influence any of the cardiovascular responses to dialysis. In conclusion, in this randomized, crossover study, there was no significant difference in the cardiovascular response to HDF or HD with cooled dialysate as assessed with intradialytic MRI.

Multi-detector CT and MRI of microembolized myocardial infarct: monitoring of left ventricular function, perfusion, and myocardial viability in a swine model

BACKGROUND

Patients with acute myocardial infarct (MI) show additional damage after coronary interventions.

PURPOSE

To longitudinally quantify structural and functional changes in the left ventricle (LV) subjected to microembolized MI using multidisciplinary computed tomography (MDCT) and independent reference methods.

MATERIAL AND METHODS

Swine (n = 20) served as controls (group I) or were subjected to a combination of coronary occlusion, microembolization, and reperfusion and imaged at 3 days (group II) or 3 days and 5 weeks (group III). LV volumes, perfusion, and MI mass were quantified on cine, perfusion, and delayed contrast enhancement (DE) MDCT. MRI, cardiac injury biomarkers, histochemical and histopathologic stains were used as independent references.

RESULTS

MDCT showed a reduction in ejection fraction and increased end systolic volume (31 ± 2% and 82 ± 3 mL, respectively) of group III compared with I (48 ± 2% and 57 ± 1 mL, respectively). It also demonstrated perfusion deficits in microembolized MI and peri-infarcts. DE-MDCT delineated microvascular obstruction (MVO) zones embedded in acute microembolized MI and microinfarct specks resulting from persistent MVO by deposited microemboli in microvessels of peri-infarct zone. Bland-Altman test showed close agreements between the extents of microembolized MI measured on DE-MDCT, DE-MRI, and histochemical TTC staining, but not between these modalities and microscopy. MI resorption was evident between 3 days and 5 weeks (13.4 ± 0.5 g and 9.8 ± 0.5 g, P < 0.017) and histologic examination revealed incomplete healing. Injury biomarkers were increased after intervention.

CONCLUSION

MDCT can longitudinally quantify regional perfusion deficits, LV dysfunction, and resorption of microembolized MI. MDCT or MRI can be used alternatively after coronary interventions in cases of contraindications for one modality or the other.

Cardiac MR imaging: current status and future direction

Coronary artery disease is currently a worldwide epidemic with increasing impact on healthcare systems. Magnetic resonance imaging (MRI) sequences give complementary information on LV function, regional perfusion, angiogenesis, myocardial viability and orientations of myocytes. T2-weighted short-tau inversion recovery (T2-STIR), fat suppression and black blood sequences have been frequently used for detecting edematous area at risk (AAR) of infarction. T2 mapping, however, indicated that the edematous reaction in acute myocardial infarct (AMI) is not stable and warranted the use of edematous area in evaluating therapies. On the other hand, cine MRI demonstrated reproducible data on LV function in healthy volunteers and LV remodeling in patients. Noninvasive first pass perfusion, using exogenous tracer (gadolinium-based contrast media) and arterial spin labeling MRI, using endogenous tracer (water), are sensitive and useful techniques for evaluating myocardial perfusion and angiogenesis. Recently, new strategies have been developed to quantify myocardial viability using T1-mapping and equilibrium contrast enhanced MR techniques because existing delayed contrast enhancement MRI (DE-MRI) sequences are limited in detecting patchy microinfarct and diffuse fibrosis. These new techniques were successfully used for characterizing diffuse myocardial fibrosis associated with myocarditis, amyloidosis, sarcoidosis heart failure, aortic hypertrophic cardiomyopathy, congenital heart disease, restrictive cardiomyopathy, arrhythmogenic right ventricular dysplasia and hypertension). Diffusion MRI provides information regarding microscopic tissue structure, while diffusion tensor imaging (DTI) helps to characterize the myocardium and monitor the process of LV remodeling after AMI. Novel trends in hybrid imaging, such as cardiac positron emission tomography (PET)/MRI and optical imaging/MRI, are recently under intensive investigation. With the promise of higher spatial-temporal resolution and 3D coverage in the near future, cardiac MRI will be an indispensible tool in the diagnosis of cardiac diseases, coronary intervention and myocardial therapeutic delivery.

MRI monitoring of function, perfusion and viability in microembolized moderately ischemic myocardium

Assessment of microembolization after coronary interventions is clinically challenging, thus we longitudinally investigated microemboli effects on moderately ischemic myocardium using MRI and histopathology. Twenty-four pigs (8/group) were divided into: group I (no intervention), group II (45 min LAD occlusion) and group III (45 min LAD occlusion with microembolization). Cine, perfusion and delayed contrast enhanced MRI (DE-MRI), using 1.5T MRI, were used for assessment at 3 days and 5 weeks. Triphenyltetrazolium-chloride (TTC) and Masson-trichrome were used as gold standard references for macro and microscopic quantification of myocardial infarction (MI). Cine MRI showed differential increase in end systolic volume (1.3 ± 0.08 ml/kg group II and 1.6 ± 0.1 ml/kg group III) and decrease in ejection fraction (45 ± 2 and 36 ± 2%, respectively) compared with controls at 3 days (2.1 ± 0.1 ml ESV and 50 ± 1% EF, P < 0.05). At 5 weeks group III, but not II, showed persistent perfusion deficits, wall thinning in the LAD territory and compensatory hypertrophy in remote myocardium. DE-MRI MI at 3 days was significantly smaller in group II (3.3 ± 2.2 g) than III (9.8 ± 0.6 g), at 5 weeks, MI were smaller by 60% (1.3 ± 0.9 g) and 22% (7.7 ± 0.5 g), respectively. TTC MI was similar to DE-MRI in group II (1.6 ± 1.0 g) and III (9.2 ± 1.6 g), but not microscopy (2.8 ± 0.4 and 10.5 ± 1.5 g, respectively). The effects of moderate ischemia with and without microembolization on myocardium could be differentiated using multiple MRI sequences. MRI demonstrated that microemboli in moderately ischemic myocardium, but not solely ischemia, prolonged ventricular dysfunction, created perfusion deficits, poor infarct resorption and enhanced compensatory hypertrophy, while moderate ischemia alone caused minor LV changes.

Magnetic resonance imaging and multi-detector computed tomography assessment of extracellular compartment in ischemic and non-ischemic myocardial pathologies

Myocardial pathologies are major causes of morbidity and mortality worldwide. Early detection of loss of cellular integrity and expansion in extracellular volume (ECV) in myocardium is critical to initiate effective treatment. The three compartments in healthy myocardium are: intravascular (approximately 10% of tissue volume), interstitium (approximately 15%) and intracellular (approximately 75%). Myocardial cells, fibroblasts and vascular endothelial/smooth muscle cells represent intracellular compartment and the main proteins in the interstitium are types I/III collagens. Microscopic studies have shown that expansion of ECV is an important feature of diffuse physiologic fibrosis (e.g., aging and obesity) and pathologic fibrosis [heart failure, aortic valve disease, hypertrophic cardiomyopathy, myocarditis, dilated cardiomyopathy, amyloidosis, congenital heart disease, aortic stenosis, restrictive cardiomyopathy (hypereosinophilic and idiopathic types), arrythmogenic right ventricular dysplasia and hypertension]. This review addresses recent advances in measuring of ECV in ischemic and non-ischemic myocardial pathologies. Magnetic resonance imaging (MRI) has the ability to characterize tissue proton relaxation times (T1, T2, and T2*). Proton relaxation times reflect the physical and chemical environments of water protons in myocardium. Delayed contrast enhanced-MRI (DE-MRI) and multi-detector computed tomography (DE-MDCT) demonstrated hyper-enhanced infarct, hypo-enhanced microvascular obstruction zone and moderately enhanced peri-infarct zone, but are limited for visualizing diffuse fibrosis and patchy microinfarct despite the increase in ECV. ECV can be measured on equilibrium contrast enhanced MRI/MDCT and MRI longitudinal relaxation time mapping. Equilibrium contrast enhanced MRI/MDCT and MRI T1 mapping is currently used, but at a lower scale, as an alternative to invasive sub-endomyocardial biopsies to eliminate the need for anesthesia, coronary catheterization and possibility of tissue sampling error. Similar to delayed contrast enhancement, equilibrium contrast enhanced MRI/MDCT and T1 mapping is completely noninvasive and may play a specialized role in diagnosis of subclinical and other myocardial pathologies. DE-MRI and when T1-mapping demonstrated sub-epicardium, sub-endocardial and patchy mid-myocardial enhancement in myocarditis, Behcet’s disease and sarcoidosis, respectively. Furthermore, recent studies showed that the combined technique of cine, T2-weighted and DE-MRI technique has high diagnostic accuracy for detecting myocarditis. When the tomographic techniques are coupled with myocardial perfusion and left ventricular function they can provide valuable information on the progression of myocardial pathologies and effectiveness of new therapies.

Multidetector CT measurement of myocardial extracellular volume in acute patchy and contiguous infarction: validation with microscopic measurement

PURPOSE

To provide proof of concept that expansion of myocardial extracellular volume (MECV), measured at contrast material-enhanced multidetector computed tomography (CT), can be used as a (a) marker for viability based on histologic confirmation and (b) predictor of severity of myocardial injury.

MATERIALS AND METHODS

Animals cared for in compliance with Institutional Animal Care and Use Committee served as controls (group 1, n = 6) or were subjected to microinfarction by using 16-mm(3) (60 000 count) microemboli (group 2) and 32-mm(3) (120 000 count) microemboli (group 3), contiguous infarct with left anterior descending artery (LAD) occlusion followed by reperfusion (group 4), or the combination of LAD occlusion and 32-mm(3) microemboli followed by reperfusion (group 5) (n = 7 per group). MECV calculations were based on regional measurements of signal attenuation at contrast-enhanced multidetector CT and counterstaining of infarct at microscopy. Two-way analysis of variance and Student t tests were used to determine significant differences (P < .05). Data were presented as means ± standard deviations.

RESULTS

Mean signal attenuation at equilibrium state of contrast media distribution (10 minutes) was significantly different among blood (137 HU ± 10), myocardial muscle (77 HU ± 12, P < .05), and skeletal muscle (35 HU ± 12, P < .05). Patchy microinfarct, contiguous infarct, and microinfarct with preexisting contiguous infarct can be differentiated on the basis of mean MECV (24% ± 3 [group 1] vs 36% ± 3 [group 2], P < .01, and 55% ± 5 [group 4], 56% ± 4 [group 5] vs 41% ± 3 [group 3], P < .05). Microscopy measurements confirmed multidetector CT quantitative measurements and differences in patterns of infarct caused by obstruction of major and minor coronary arteries. Regression analysis revealed excellent correlation between regional MECV using multidetector CT and microscopy (r(2) = 0.92).

CONCLUSION

Contrast-enhanced multidetector CT is a suitable noninvasive imaging technique for assessing MECV in acute patchy and contiguous infarct caused by obstruction of major and minor coronary vessels.

MRI demonstrates a decrease in myocardial infarct healing and increase in compensatory ventricular hypertrophy following mechanical microvascular obstruction

PURPOSE

To provide direct evidence that mechanical obstruction of microvessels inhibits infarct resorption (healing) and enhances left ventricular (LV) remodeling using MRI.

MATERIALS AND METHODS

Animals (n = 20 pigs) served as controls (group I) or were subjected to either 90 min left anterior descending (LAD) coronary artery occlusion/reperfusion (group II) or 90 min LAD occlusion/ microemboli delivery/reperfusion (group III). MRI (cine and delayed contrast enhanced MRI, DE-MRI) was performed at 3 days and 5 weeks after interventions and used for assessing LV function, mass, and extent of myocardial damage and microvascular obstruction (MVO) using semi-automated threshold method.

RESULTS

Persistent MVO in the core of contiguous infarct was larger and more frequent (n = 8/8) in group III than group II (4/8) on DE-MRI at 3 days. Furthermore, patchy microinfarct, as a result of microembolization, was visible as hyperenhanced zone at the borders of the contiguous infarct. The reduction in ejection fraction and increase in LV volumes on cine MRI were greater in group III than group II at 3 days and 5 weeks, which may be attributed to the slow infarct resorption, MVO extents and patchy microinfarct at the borders.

CONCLUSION

This MRI study illustrates the recently raised conjecture that MVO delays/inhibits infarct resorption (healing), accentuates LV hypertrophy and pathological remodeling.

MRI quantification of left ventricular function in microinfarct versus large infarct in swine model

To quantify, using MRI, the acute impacts of defined volume and sizes of coronary microemboli on myocardial viability and left ventricular (LV) function and to use LAD occlusion/reperfusion, as a reference. A total of 28 farm pigs were used in this study. Eight animals were used as controls. Successful coronary interventions were performed under X-ray fluoroscopy in 16 pigs to induce microinfarct (delivery of 16 mm(3) of 40-120 μm) and large infarct (90 min LAD occlusion/reperfusion). On day 3, animals were imaged using contrast enhanced (in beating and non-beating hearts) and cine MRI for evaluating LV viability and function, respectively. Microscopy and cardiac injury enzymes were used to confirm the presence of necrosis. Myocardial damage was smaller after microembolization than occlusion/reperfusion (6.5 ± 0.6%LV mass vs. 12.6 ± 1.2%, P < 0.001). The increase in LV end-systolic volume and decreases in ejection fraction, cardiac output and regional systolic wall thickening, however, were comparable between groups, but significantly differed from controls. MRI also demonstrated microvascular obstruction after microembolization and occlusion/reperfusion as hyperenhanced and hypoenhanced regions, respectively. Microscopic examination revealed patchy necrosis, inflammatory cell infiltration, but no intramyocardial hemorrhage after microembolization and extensive intramyocardial hemorrhage and transmural damage in the occlusion/reperfusion group. Cardiac injury enzymes confirmed presence of myocardial damage in animals with interventions. Coronary microemboli have acute impact on LV function compared to control animals. Despite the difference in myocardial damage, global and regional LV dysfunction after microembolization was comparable to occlusion/reperfusion. This MR study suggests that the pattern of myocardial damage plays a role in LV dysfunction.

Value of MR contrast media in image-guided body interventions

In the past few years, there have been multiple advances in magnetic resonance (MR) instrumentation, in vivo devices, real-time imaging sequences and interventional procedures with new therapies. More recently, interventionists have started to use minimally invasive image-guided procedures and local therapies, which reduce the pain from conventional surgery and increase drug effectiveness, respectively. Local therapy also reduces the systemic dose and eliminates the toxic side effects of some drugs to other organs. The success of MR-guided procedures depends on visualization of the targets in 3D and precise deployment of ablation catheters, local therapies and devices. MR contrast media provide a wealth of tissue contrast and allows 3D and 4D image acquisitions. After the development of fast imaging sequences, the clinical applications of MR contrast media have been substantially expanded to include pre- during- and post-interventions. Prior to intervention, MR contrast media have the potential to localize and delineate pathologic tissues of vital organs, such as the brain, heart, breast, kidney, prostate, liver and uterus. They also offer other options such as labeling therapeutic agents or cells. During intervention, these agents have the capability to map blood vessels and enhance the contrast between the endovascular guidewire/catheters/devices, blood and tissues as well as direct therapies to the target. Furthermore, labeling therapeutic agents or cells aids in visualizing their delivery sites and tracking their tissue distribution. After intervention, MR contrast media have been used for assessing the efficacy of ablation and therapies. It should be noted that most image-guided procedures are under preclinical research and development. It can be concluded that MR contrast media have great value in preclinical and some clinical interventional procedures. Future applications of MR contrast media in image-guided procedures depend on their safety, tolerability, tissue specificity and effectiveness in demonstrating success of the interventions and therapies.