Background

Regional changes in diastolic and systolic properties after myocardial infarction contribute to adverse left ventricular (LV) remodeling. Regional function is currently assessed using load-dependent measures such as slice ejection fraction (sEF), wall motion abnormalities, or strain imaging. However, load-independent measures of cardiac function may be useful in the study of the infarction-induced remodeling.

Methods

In this study, we used a recently validated 2-dimensional (2D) real-time magnetic resonance imaging (MRI) technique to evaluate regional variations in load-independent slice-by-slice measures of systolic and diastolic function and compared the values to a load-dependent measure in 11 sheep at rest and during inotropic agent infusion.

Results

Slice-derived ejection fraction (sEF) was greater in the apex relative to the midventricular and basal regions, and inotropic infusion increased sEF in the base more than in the apex and midventricle. Slice-derived ESPVR (sESPVR) in the apex was significantly lower than in the midventricle and the base, and inotropic infusion increased sESPVR in the apical slices more than in the midventricle. Similarly, slice-derived volume-axis intercept V0 (sV0) was higher in the base relative to the midventricle and apex. sEDPVR did not demonstrate significant regional variations, but inotropic infusion resulted in a small increase in the apex.

Conclusions

In conclusion, acquisition of slice-derived load-independent measures demonstrated variations that contradict those observed with load-dependent sEF. The approach may provide advanced slice-based measures of function during the LV remodeling process and aid in the development of therapies.

Purpose

Develop self-gated MRI for distinct heartbeat morphologies in subjects with arrhythmias.

Methods

Golden angle radial data was obtained in seven sinus and eight arrhythmias subjects. An image-based cardiac navigator was derived from single-shot images, distinct beat types were identified, and images were reconstructed for repeated morphologies. Image sharpness, contrast, and volume variation were quantified and compared with self-gated MRI. Images were scored for image quality and artifacts. Hemodynamic parameters were computed for each distinct beat morphology in bigeminy and trigeminy subjects and for sinus beats in patients with infrequent premature ventricular contractions.

Results

Images of distinct beat types were reconstructed except for two patients with infrequent premature ventricular contractions. Image contrast and sharpness were similar to sinus self-gated images (contrast = 0.45 ± 0.13 and 0.43 ± 0.15; sharpness = 0.21 ± 0.11 and 0.20 ± 0.05). Visual scoring was highest in self-gated images (4.1 ± 0.3) compared with real-time (3.9 ± 0.4) and ECG-gated cine (3.4 ± 1.5). ECG-gated cine had less artifacts than self-gating (2.3 ± 0.7 and 2.1 ± 0.2), but was affected by misgating in two subjects. Among arrhythmia subjects, post-extrasystole/sinus (58.1 ± 8.6 mL) and interrupted sinus (61.4 ± 5.9 mL) stroke volume was higher than extrasystole (32.0 ± 16.5 mL; P < 0.02).

Conclusion

Self-gated imaging can reconstruct images during ectopy and allowed for quantification of hemodynamic function of different beat morphologies. Magn Reson Med 78:678-688, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Background

The evolution of T1ρ and of other endogenous contrast methods (T2, T1) in the first month after reperfused myocardial infarction (MI) is uncertain. We conducted a study of reperfused MI in pigs to serially monitor T1ρ, T2 and T1 relaxation, scar size and transmurality at 1 and 4 weeks post-MI.

Methods

Ten Yorkshire swine underwent 90 min of occlusion of the circumflex artery and reperfusion. T1ρ, T2 and native T1 maps and late gadolinium enhanced (LGE) cardiovascular magnetic resonance (CMR) data were collected at 1 week (n = 10) and 4 weeks (n = 5). Semi-automatic FWHM (full width half maximum) thresholding was used to assess scar size and transmurality and compared to histology. Relaxation times and contrast-to-noise ratio were compared in healthy and remote myocardium at 1 and 4 weeks. Linear regression and Bland-Altman was performed to compare infarct size and transmurality.

Results

Relaxation time differences between infarcted and remote myocardial tissue were ∆T1 (infarct-remote) = 421.3 ± 108.8 (1 week) and 480.0 ± 33.2 ms (4 week), ∆T1ρ = 68.1 ± 11.6 and 74.3 ± 14.2, and ∆T2 = 51.0 ± 10.1 and 59.2 ± 11.4 ms. Contrast-to-noise ratio was CNR_T1 = 7.0 ± 3.5 (1 week) and 6.9 ± 2.4 (4 week), CNR_T1ρ = 12.0 ± 6.2 and 12.3 ± 3.2, and CNR_T2 = 8.0 ± 3.6 and 10.3 ± 5.8. Infarct size was not significantly different for T1ρ, T1 and T2 compared to LGE (p = 0.14) and significantly decreased from 1 to 4 weeks (p < 0.01). Individual infarct size changes were ∆T1ρ = -3.8%, ∆T1 = -3.5% and ∆LGE = -2.8% from 1 - 4 weeks, but there was no observed change in infarct size for T2 or histologically.

Conclusions

T1ρ was highly correlated with alterations left ventricle (LV) pathology at 1 and 4 weeks post-MI and therefore it may be a useful method endogenous contrast imaging of infarction.

AbstractBackgroundSegmented cine cardiac MRI combines data from multiple heartbeats to achieve high spatiotemporal resolution cardiac images, yet predefined k-space segmentation trajectories can lead to suboptimal k-space sampling. In this work, we developed and evaluated an autonomous and closed-loop control system for radial k-space sampling to increase sampling uniformity.MethodsThe closed-loop system autonomously selects radial k-space sampling trajectory during live segmented cine MRI and attempts to optimize angular sampling uniformity by selecting views in regions of k-space that were not previously well-sampled. Sampling uniformity and robustness to arrhythmias was assessed using ECG data acquired from 10 normal subjects in an MRI scanner. The approach was then implemented with a fast gradient echo sequence on a whole-body clinical MRI scanner and imaging was performed in 4 healthy volunteers. The closed-loop k-space trajectory was compared to random, uniformly distributed and golden angle view trajectories via measurement of k-space uniformity and the point spread function. Lastly, an arrhythmic dataset was used to evaluate a potential application of the approach.ResultsThe autonomous trajectory increased k-space sampling uniformity by 13±7%, main lobe point spread function (PSF) signal intensity by 14±6%, and reduced ringing relative to golden angle sampling. When implemented, the autonomous pulse sequence prescribed radial view angles faster than the scan TR (0.98 ± 0.02 ms, maximum = 1.38 ms) and increased k-space sampling mean uniformity by 5±12%, decreased uniformity variability by 45±14%, and increased PSF signal ratio by 5±5% relative to golden angle sampling.ConclusionThe closed-loop approach enables near-uniform radial sampling in a segmented acquisition approach which was higher than predetermined golden-angle radial sampling. This can be utilized to increase the sampling or decrease the temporal footprint of an acquisition and the closed-loop framework has the potential to be applied to patients with complex heart rhythms.

Background

Segmented cine cardiac MRI combines data from multiple heartbeats to achieve high spatiotemporal resolution cardiac images, yet predefined k-space segmentation trajectories can lead to suboptimal k-space sampling. In this work, we developed and evaluated an autonomous and closed-loop control system for radial k-space sampling (ARKS) to increase sampling uniformity.

Methods

The closed-loop system autonomously selects radial k-space sampling trajectory during live segmented cine MRI and attempts to optimize angular sampling uniformity by selecting views in regions of k-space that were not previously well-sampled. Sampling uniformity and the ability to detect cardiac phase in vivo was assessed using ECG data acquired from 10 normal subjects in an MRI scanner. The approach was then implemented with a fast gradient echo sequence on a whole-body clinical MRI scanner and imaging was performed in 4 healthy volunteers. The closed-loop k-space trajectory was compared to random, uniformly distributed and golden angle view trajectories via measurement of k-space uniformity and the point spread function. Lastly, an arrhythmic dataset was used to evaluate a potential application of the approach.

Results

The autonomous trajectory increased k-space sampling uniformity by 15±7%, main lobe point spread function (PSF) signal intensity by 6±4%, and reduced ringing relative to golden angle sampling. When implemented, the autonomous pulse sequence prescribed radial view angles faster than the scan TR (0.98 ± 0.01 ms, maximum = 1.38 ms) and increased k-space sampling mean uniformity by 10±11%, decreased uniformity variability by 44±12%, and increased PSF signal ratio by 6±6% relative to golden angle sampling.

Conclusion

The closed-loop approach enables near-uniform radial sampling in a segmented acquisition approach which was higher than predetermined golden-angle radial sampling. This can be utilized to increase the sampling or decrease the temporal footprint of an acquisition and the closed-loop framework has the potential to be applied to patients with complex heart rhythms.

Background

Late gadolinium enhanced (LGE) cardiovascular magnetic resonance (CMR) is frequently used to evaluate myocardial viability, estimate total infarct size and transmurality, but is not always straightforward is and contraindicated in patients with renal failure because of the risk of nephrogenic systemic fibrosis. T2- and T1-weighted CMR alone is however relatively insensitive to chronic myocardial infarction (MI) in the absence of a contrast agent. The objective of this manuscript is to explore T1ρ-weighted rotating frame CMR techniques for infarct characterization without contrast agents. We hypothesize that T1ρ CMR accurately measures infarct size in chronic MI on account of a large change in T1ρ relaxation time between scar and myocardium.

Methods

7Yorkshire swine underwent CMR at 8 weeks post-surgical induction of apical or posterolateral myocardial infarction. Late gadolinium enhanced and T1ρ CMR were performed at high resolution to visualize MI. T1ρ-weighted imaging was performed with a B₁  = 500 Hz spin lock pulse on a 3 T clinical MR scanner. Following sacrifice, the heart was excised and infarct size was calculated by optical planimetry. Infarct size was calculated for all three methods (LGE, T1ρ and planimetry) and statistical analysis was performed. T1ρ relaxation time maps were computed from multiple T1ρ-weighted images at varying spin lock duration.

Results

Mean infarct contrast-to-noise ratio (CNR) in LGE and T1ρ CMR was 2.8 ± 0.1 and 2.7 ± 0.1. The variation in signal intensity of tissues was found to be, in order of decreasing signal intensity, LV blood, fat and edema, infarct and healthy myocardium. Infarct size measured by T1ρ CMR (21.1% ± 1.4%) was not significantly different from LGE CMR (22.2% ± 1.5%) or planimetry (21.1% ± 2.7%; p < 0.05).T1ρ relaxation times were T1ρinfarct = 91.7 ms in the infarct and T1ρremote = 47.2 ms in the remote myocardium.

Conclusions

T1ρ-weighted imaging using long spin locking pulses enables high discrimination between infarct and myocardium. T1ρ CMR may be useful to visualizing MI without the need for exogenous contrast agents for a wide range of clinical cardiac applications such as to distinguish edema and scar tissue and tissue characterization of myocarditis and ventricular fibrosis.