Assessment of magnetic resonance velocity mapping of global ventricular function during dobutamine infusion in coronary artery disease

Simplified detection of myocardial ischemia by seismocardiography. Differentiation between causes of altered myocardial function

Seismocardiography (SCG) is a noninvasive technique for recording cardiac vibrations. Changes in these waves have been correlated with chronic and acute alterations in myocardial function. This analysis is complex and clinical integration limited. The current study aimed to simplify the utilization of SCG by fast Fourier transformation for a reliable discrimination between different intra- and postoperative causes of hypotension (i.e., myocardial ischemia or hypovolemia). We operated on nine pigs and recorded SCG at baseline, at hypovolemia (occlusion of the inferior vena cava), and at ischemia (occlusion of the right coronary artery). In conclusion, SCG enables detection and differentiation of ischemia and hypovolemia as important causes of altered myocardial function during and after surgery. Thus, this simple and noninvasive diagnostic tool may be used intra- and postoperatively to identify patients at risk.

CMR of Ventricular Function

Cardiovascular magnetic resonance (CMR) is the reference standard for the assessment of ventricular dimensions, function, and mass in terms of accuracy and reproducibility. It has been thoroughly validated both ex vivo and against other imaging techniques. Measurements are highly accurate and no geometrical assumptions need to be made about the ventricle. A routine ventricular dataset of images can be acquired in less than 5 minutes and analyzed in about the same time. The field is rapidly advancing with increasing automation and simplification in both image acquisition and analysis. Using parallel and real time imaging techniques, good quality data can be obtained even in patients who are unable to hold their breath. While providing useful information in all patients with suspected heart failure, CMR should particularly be considered in those with poor echo windows, where it can also be combined with myocardial stress. Tagging techniques can provide highly detailed information about myocardial torsion and strain for individual myocardial segments. In a research environment, the very high degree of interscan reproducibility can dramatically reduce the number of patients needed to perform clinical trials.

Cardiovascular magnetic resonance in the quantitative assessment of left ventricular mass, volumes and contractile function

Cardiovascular magnetic resonance is a well validated, highly accurate and reproducible technique for the assessment of ventricular volumes, function and mass. State of the art cardiovascular magnetic resonance practice is capable of a ventricular assessment that includes not only systolic but also diastolic function. Thus, it provides an insight into the complex changes in ventricular morphology, physiology and function in cardiovascular disease. This has produced great interest not only in its clinical utilization but also as an important research tool. As refinement of the technique continues to incorporate hardware and software developments, the technique becomes quicker, more accurate and easier to analyse. Here, we review recent developments and current practice.

Simultaneous measurement of blood and myocardial velocity in the rat heart by phase contrast MRI using sparseq-space sampling

PURPOSE

To measure cardiac blood flow patterns and ventricular wall velocities through the cardiac cycle in anesthetized Wistar Kyoto (WKY) rats.

MATERIALS AND METHODS

A gradient-echo cine pulse sequence incorporating pulsed field gradients (PFGs) provided phase contrast (PC) motion encoding. We achieved a range of velocity sensitivity that was sufficient to measure simultaneously the large flow velocities within the cardiac chambers and aortic outflow tract (up to 70 cm s(-1) during systole), and the comparatively small velocities of the cardiac wall (0-3 cm s(-1)). A scheme of sparsely sampling q-space combined with a probability-based method of velocity calculation permitted such measurements along three orthogonal axes, and yielded velocity vector maps in all four chambers of the heart and the aorta, in both longitudinal and transverse sections, for up to 12 time-points in the cardiac cycle.

RESULTS

Left ventricular systole was associated with a symmetrical laminar flow pattern along the cardiac axis, with no appearance of turbulence. In contrast, blood showed a swirling motion within the right ventricle (RV) in the region of the pulmonary outflow tract. During left ventricular diastole a plume of blood entered the left ventricle (LV) from the left atrium. The ventricular flow patterns could also be correlated with measurements of left ventricular wall motion. The greatest velocities of the ventricular walls occurred in the transverse cardiac plane and were maximal during diastolic refilling. The cardiac wall motion in the longitudinal axis demonstrated a caudal-apical movement that may also contribute to diastolic refilling.

CONCLUSION

The successful measurements of blood and myocardial velocity during normal myocardial function may be extended to quantify pathological cardiac changes in animal models of human cardiac disease.

Noninvasive determination of coronary blood flow velocity with cardiovascular magnetic resonance in patients after stent deployment

BACKGROUND

In patients with coronary artery stents, no direct noninvasive coronary artery imaging is possible with magnetic resonance (MR). A well-established method for the assessment of the functional significance of a coronary lesion is the measurement of coronary flow reserve by invasive intracoronary Doppler. The purpose of the study was to determine coronary flow velocity reserve (CFVR) with MR after stent deployment.

METHODS AND RESULTS

Thirty-eight patients after successful PTCA and stent deployment were included. CFVR was measured perpendicular to the artery distal to the stent using phase-contrast velocity quantification at rest and during adenosine-stimulated hyperemia with a 1.5T MR tomograph (ACS NT, Philips). Measurements were repeated after 3 months and compared with invasive coronary angiography. In 18 patients, additional invasive Doppler flow measurements were obtained. CFVR could be determined in 29 of 38 (76%) of the patients. After 3 months, significant differences were obtained between coronary arteries with and without restenosis. Using a threshold of 1.2, a sensitivity of 83% with a specificity of 94% was achieved for > or =75% stenoses. CFVR with CMR was similar to Doppler results (r=0.87), with a mean relative difference of 7.5%.

CONCLUSIONS

In patients with preserved coronary microcirculating vasoreactivity that are suitable for MR coronary angiography and flow assessments, CMR measures of coronary blood flow velocities reserve may be used to detect in-stent restenosis.

The seismocardiogram as magnetic-field-compatible alternative to the electrocardiogram for cardiac stress monitoring

Seismocardiography (SCG) is a methodology derived from the field of seismology to measure non-invasively compression waves generated by myocardial motion. The use of SCG is proposed here as method to monitor cardiac function during stress testing and magnetic resonance imaging (MRI). It is shown that recording of the seismocardiogram in subjects undergoing an MRI study is not compromised by flow-induced electrical voltages that can severely distort the electrocardiogram. Electromagnetic compatibility of the seismocardiograph device with the MRI scanner hardware was tested. Experiments in an animal model proved that changes in the SCG waveform during different levels of LAD stenosis correlate with the changes seen in LV function with cine MRI. The changes in the SCG waveform characteristic for the onset of ischemia were observed almost immediately after the creation of a flow-limiting stenosis. This novel application of SCG to monitor changes in peak acceleration from cardiac blood flow and wall motion may represent a solution to the long-standing and important problem of monitoring patients for signs of myocardial ischemia during MRI.

Noninvasive determination of coronary blood flow velocity with magnetic resonance imaging: comparison of breath-hold and navigator techniques with intravascular ultrasound

The aim of this study was to evaluate two different magnetic resonance (MR) techniques for the noninvasive assessment of intracoronary blood flow. Coronary blood flow velocities were measured invasively in 26 angiographically normal segments of 12 patients. Noninvasive measurements were performed in identical segments with two MR techniques using a 1.5 T MR tomograph (ACS NT, Philips). A single breath-hold technique (temporal resolution: 140 msec) and a similar non-breath-hold technique with prospective navigator correction and improved temporal resolution (45 msec) were used. Maximal coronary flow velocities determined by MR correlated closely with invasive measurements (breath-hold: r = 0.70; navigator: r = 0.86); however, a significant underestimation of the MR measurements was found (slope = 0.33 and 0.37). The relative difference from the invasive method was lower for the navigator technique compared with the breath-hold technique (P<0.02). Both MR techniques allow the determination of coronary blood flow velocities. The higher temporal resolution and shorter acquisition window of navigator-corrected non-breath-hold techniques lead to increased accuracy. This approach is a further step toward the diagnostic use of MR flow measurements in coronary artery disease.

Breath-hold dobutamine magnetic resonance myocardial tagging: normal left ventricular response

Analysis of the changes in myocardial deformation produced by adrenergic stress has been limited by the imaging techniques used. We used rapid magnetic resonance imaging (MRI) myocardial tagging to map the dose-dependent response to incremental dobutamine in the normal human left ventricle. Thirteen volunteers underwent breath-hold tagged cine MRI during dobutamine infusion. Images were acquired throughout systole to a peak dose of 20 microg/kg/min. End-systolic percent circumferential shortening (%S) was measured at 3 transmural locations and 4 circumferential locations at 3 long-axis positions. Mean circumferential shortening velocity (CSV) was also calculated at each location and dose. Mean %S reached a maximum of 26 +/- 3% at 10 microg/kg/min compared with 21 +/- 4% at baseline (p <0.003). Peak %S was reached by 10 microg/kg/min before a significant increase in heart rate or blood pressure and was unchanged at higher doses. In contrast, CSV increased linearly with dobutamine dose from 4.4 +/- 0.9 mm/s at baseline to 9.8 +/- 1.4 mm/s at 20 microg/kg/min (p <0.0001). Breath-hold tagged dobutamine MRI is safe and effective in detecting regional and transmural changes in function during incremental dobutamine. CSV increased continuously across the dobutamine dose range. At low dose (< or =10 microg/kg/min) %S increased without any change in blood pressure or heart rate. Maintenance of peak %S beyond 10 microg/kg/min in the presence of decreasing systolic intervals resulted from a continued increase in CSV. Thus, CSV may be the preferred measure of contractile function during dobutamine stimulation in human myocardium.

Cardiac imaging: present status and future trends

Cardiac imaging has developed rapidly in a number of different areas, to the extent that the clinical practice of cardiology is highly image dependent. Advances in cross-sectional and three-dimensional imaging using computed tomography and magnetic resonance imaging have now found a significant role in demonstrating cardiac anatomy and function alongside the more traditional role of ultrasound which with its intravascular use is becoming a part of intravascular cardiac therapy. The imaging of myocardial perfusion by nuclear medicine techniques including single photon emission tomography and positron emission tomography has grown to add substantial clinical and academic information on cardiac blood flow and metabolism.