Simultaneous imaging of myocardial motion and chamber blood flow with SPAMM n' EGGS (Spatial Modulation of Magnetization With Encoded Gradients for Gauging Speed)

Simultaneous quantification of myocardial and blood flow velocities based on duplex mode ultrasound imaging

Background

Ultrasound imaging of the heart is a commonly used clinical tool to assess cardiac function. The basis for this analysis is the quantification of cardiac blood flow and myocardial velocities. These are typically measured using different imaging modes and on different cardiac cycles. However, due to beat-to-beat variations such as irregular heart rhythm and transient events, simultaneous acquisition is preferred. There exists specialized ultrasound systems for this purpose; however, it would be beneficial if this could be achieved using conventional ultrasound systems due to their wide availability. The conventional Duplex mode ultrasound allows simultaneous acquisition, however at a highly reduced spatial and temporal resolution.

Methods

The aim of this work was to present and evaluate the performance of a novel method to recover myocardial tissue velocity using conventional Duplex ultrasound imaging, and to demonstrate its feasibility for the assessment of simultaneous blood flow and myocardial velocity in-vivo. The essence of the method was the estimation of the axial phase shift of robust echogenic structures between subsequent image frames. The performance of the method was evaluated on synthetic tissue mimicking B-mode image sequences at different frame rates (20–60 Hz) and tissue velocities (peak velocities 5-15cm/s), using cardiac deformation and displacement characteristics. The performance was also compared to a standard 2-D speckle tracking technique.

Results

The method had an overall high performance at frame rates above 25 Hz, with less than 15% error of the peak diastolic velocity, and less than 10 ms peak timing error. The method showed superior performance compared to the 2-D tracking technique at frame rates below 50 Hz. The in-vivo quantification of simultaneous blood flow and myocardial tissue velocities verified the echocardiographic patterns and features of healthy subjects and the specific patient group.

Conclusions

A novel myocardial velocity quantification method was presented and high performance at frame rates above 25 Hz was shown. In-vivo quantification of simultaneous myocardial and blood flow velocities was feasible using the proposed method and conventional Duplex mode imaging. We propose that the methodology is suitable for retrospective as well as prospective studies on the mechanics and hemodynamics of the heart.

Assessment of early diastolic strain-velocity temporal relationships using spatial modulation of magnetization with polarity alternating velocity encoding (SPAMM-PAV)

A novel MR imaging technique, spatial modulation of magnetization with polarity alternating velocity encoding (SPAMM-PAV), is presented to simultaneously examine the left ventricular early diastolic temporal relationships between myocardial deformation and intra-cavity hemodynamics with a high temporal resolution of 14 ms. This approach is initially evaluated in a dynamic flow and tissue mimicking phantom. A comparison of regional longitudinal strains and intra-cavity pressure differences (integration of computed in-plane pressure gradients within a selected region) in relation to mitral valve inflow velocities is performed in eight normal volunteers. Our results demonstrate that apical regions have higher strain rates (0.145 ± 0.005 %/ms) during the acceleration period of rapid filling compared to mid-ventricular (0.114 ± 0.007 %/ms) and basal regions (0.088 ± 0.009 %/ms), and apical strain curves plateau at peak mitral inflow velocity. This pattern is reversed during the deceleration period, when the strain-rates in the basal regions are the highest (0.027 ± 0.003 %/ms) due to ongoing basal stretching. A positive base-to-apex gradient in peak pressure difference is observed during acceleration, followed by a negative base-to-apex gradient during deceleration. These studies shed insight into the regional volumetric and pressure difference changes in the left ventricle during early diastolic filling.

Imaging of wall motion coupled with blood flow velocity in the heart and vessels in vivo: a feasibility study

The mechanical property and geometry changes as a result of cardiovascular disease affect both the wall motion and blood flow in the heart and vessels, whereas the latter two are also coupled and therefore continuously influence one another. Simultaneous and registered imaging of both cardiovascular wall motion and blood velocity may thus contribute to more complete computational models of cardiovascular mechanical and fluid dynamics as well as provide additional diagnostic information. The objective of this paper was to determine the feasibility of imaging cardiovascular wall motion coupled with blood flow in vivo. Normal (n = 6) and infarcted (n = 5) murine left ventricles, and normal (n = 5) and aneurysmal (n = 4) murine abdominal aortas, were imaged in longitudinal views with a 30-MHz ultrasound probe. Using electrocardiogram (ECG) gating, 2-D radio-frequency (RF) data were acquired at a frame rate of 8 kHz. The axial wall velocity and blood velocity were estimated using a speckle-tracking technique. Spatially and temporally registered imaging of both cardiovascular wall motion and blood flow was shown to be feasible. Reduced wall motion was detected in the infarcted region, whereas vortex flow patterns were imaged in diastolic phases of both normal and infarcted left ventricles. The myocardial wall motion and blood flow were found to be more synchronous in the normal heart, where the blood moves toward the anteroseptal wall after the mitral valve opens (i.e., rapid filling phase), and the anteroseptal wall simultaneously undergoes outward motion. In the infarcted heart, however, in the rapid filling phase, the basal anteroseptal wall starts moving about 20 ms before the mitral valve opens and the blood enters the left ventricle. In the normal aorta, the wall motion and blood velocity were uniform and synchronous. In the aneurysmal aorta, reduced and spatially varied wall motion and vortex flow patterns in the aneurysmal sac were found. The wall motion and blood velocity were thus less synchronous in the aneurysmal aorta. Cardiovascular wall motion and blood flow were both imaged in mice in vivo. This dual information may provide important insights for the diagnosis of cardiovascular disease as well as essential parameters for its biomechanical modeling.

Imaging left ventricular tissue mechanics and hemodynamics during supine bicycle exercise using a combined tagging and phase-contrast MRI pulse sequence

Imaging the left ventricular mechanical and hemodynamic response to the stress of exercise may offer early prognosis in select patients with cardiac disease. Here, we demonstrate the feasibility of obtaining simultaneous measurements of longitudinal strain and transvalvular blood velocity during supine bicycle exercise stress in a wide bore magnetic resonance scanner. Combining information from the two datasets, we observe that although the time to peak strain (33.28 ± 1.86 versus 25.7 ± 2.12 as % of R-R interval) and time to peak mitral inflow velocity (44.37 ± 5.21 versus 35.5 ± 4.19 as % of R-R interval) from R-wave of the QRS complex occurred earlier during stress, the time from peak strain to peak mitral inflow velocity was not statistically different (16.5 ± 3.23 versus 13.4 ± 3.06). Further, the percentage of longitudinal relaxation at peak mitral inflow velocity was higher during stress (63.5 ± 7.72 versus 84.32 ± 6.24). These results suggest that although diastole is shortened, early diastolic filling efficiency is augmented during exercise stress in normal volunteers in an effort to maintain stroke volume.

Mitral cerclage annuloplasty, a novel transcatheter treatment for secondary mitral valve regurgitation: initial results in swine

OBJECTIVES

We developed and tested a novel transcatheter circumferential annuloplasty technique to reduce mitral regurgitation in porcine ischemic cardiomyopathy.

BACKGROUND

Catheter-based annuloplasty for secondary mitral regurgitation exploits the proximity of the coronary sinus to the mitral annulus, but is limited by anatomic variants and coronary artery entrapment.

METHODS

The procedure, "cerclage annuloplasty," is guided by magnetic resonance imaging (MRI) roadmaps fused with live X-ray. A coronary sinus guidewire traverses a short segment of the basal septal myocardium to re-enter the right heart where it is exchanged for a suture. Tension is applied interactively during imaging and secured with a locking device.

RESULTS

We found 2 feasible suture pathways from the great cardiac vein across the interventricular septum to create cerclage. Right ventricular septal re-entry required shorter fluoroscopy times than right atrial re-entry, which entailed a longer intramyocardial traversal but did not cross the tricuspid valve. Graded tension progressively reduced septal-lateral annular diameter, but not end-systolic elastance or regional myocardial function. A simple arch-like device protected entrapped coronary arteries from compression even during supratherapeutic tension. Cerclage reduced mitral regurgitation fraction (from 22.8 +/- 12.7% to 7.2 +/- 4.4%, p = 0.04) by slice tracking velocity-encoded MRI. Flexible cerclage reduced annular size but preserved annular motion. Cerclage also displaced the posterior annulus toward the papillary muscles. Cerclage introduced reciprocal constraint to the left ventricular outflow tract and mitral annulus that enhanced leaflet coaptation. A sample of human coronary venograms and computed tomography angiograms suggested that most have suitable venous anatomy for cerclage.

CONCLUSIONS

Transcatheter mitral cerclage annuloplasty acutely reduces mitral regurgitation in porcine ischemic cardiomyopathy. Entrapped coronary arteries can be protected. MRI provided insight into the mechanism of cerclage action.