Navigator‐free metabolite‐cycled proton spectroscopy of the heart

Purpose Respiratory gating in cardiac water‐suppressed (WS) proton spectroscopy leads to long and unpredictable scan times. Metabolite cycling allows to perform frequency and phase correction on the water signal and, hence, offers an approach to navigator‐free cardiac spectroscopy with fixed scan time. The objective of the present study was to develop and implement navigator‐free metabolite‐cycled cardiac proton spectroscopy (MC nonav) and compare it with standard navigator‐gated WS (WS nav) and navigator‐free WS (WS nonav) measurements for the assessment of triglyceride‐to‐water ratios (TG/W) and creatine‐to‐water ratios (CR/W) in the intraventricular septum of the in vivo heart. Methods Navigator‐free metabolite‐cycled spectroscopy was implemented on a clinical 1.5T system. In vivo measurements were performed on 10 young and 5 older healthy volunteers to assess signal‐to‐noise ratio efficiency as well as TG/W and CR/W and the relative Cramér‐Rao lower bounds for CR. The performance of the metabolite‐cycled sequence was verified using simulations. Results On average, scan times of MC nonav were 3.4 times shorter compared with WS nav, while no significant bias for TG/W was observed (coefficient of variation = 14.0%). signal‐to‐noise ratio efficiency of both TG and CR increased for MC nonav compared with WS nav. Relative Cramér‐Rao lower bounds of CR decreased for MC nonav. Overall spectral quality was found comparable between MC nonav and WS nav, while it was inferior for WS nonav. Conclusion Navigator‐free metabolite‐cycled cardiac proton spectroscopy offers 3.4‐fold accelerated assessment of TG/W and CR/W in the heart with preserved spectral quality when compared with navigator‐gated WS scans

Seventy-one-year-old woman followed with CMR during the course of systemic vasculitis.

CLINICAL INTRODUCTION A 71-year-old woman was diagnosed with systemic vasculitis in 2009 and received specific therapy (corticosteroids, cytostatic agents or rituximab in differing order). A transthoracic echocardiography (TTE) in 2013 for follow-up purposes of complications showed hyperechogenic endocardium of the whole left ventricular (LV) apex and cardiac MRI (CMR) was followed to further examine this finding. Using CMR, an apical LV thrombus was detected along with abnormal signal intensities within the endocardium (figure 1A). As a consequence, the existing medication was extended by phenprocoumon to treat the LV thrombus. For tracking the pathologic image features as well as for follow-up of the LV thrombus, serial CMRs were performed at our hospital from 2013 to 2018 (figure 1A). Because of the increasing hyperintense (white arrowheads) and hypointense (red arrowheads) signal spots in cine images of the apex (figure 1A), differently weighted sequences in short-axis view were acquired in the CMR of 2018 (figure 1B). QUESTION Which of the following is the most likely diagnosis in CMR?Microvascular ischaemiaCircular oedemaEndomyocardial fibrosisHaemorrhageApical lipoma

Unipolar MR elastography: Theory, numerical analysis and implementation

In MR elastography (MRE), zeroth moment balanced motion-encoding gradients (MEGs) are incorporated into MRI sequences to induce a phase shift proportional to the local displacement caused by external actuation. To maximize the signal-to-noise ratio (SNR), fractional encoding is employed, i.e., the MEG duration is reduced below the wave period. Here, gradients encode primarily the velocity of the motion-reducing encoding efficiency. Thus, in GRE-MRE, T2 * decay and motion sensitivity have to be balanced, imposing a lower limit on repetition times (TRs). We propose to use a single trapezoidal gradient, a "unipolar gradient", to directly encode spin displacement. Such gradients cannot be used in conventional sequences as they exhibit a large zeroth moment and dephase magnetization. By time-reversing a spoiled SSFP sequence, the spoiling gradient becomes an efficient unipolar MEG. The proposed "unipolar MRE" technique benefits from this approach in three ways: first, displacement encoding is split over multiple TRs increasing motion sensitivity; second, spoiler and MEG coincide, allowing a reduction in TR; third, motion sensitivity of a typical unipolar lobe is of an order of magnitude higher than a bipolar MEG of equal duration. In this work, motion encoding using unipolar MRE is analyzed using the extended phase graph (EPG) formalism with a periodic motion propagator. As an approximation, the two-transverse TR approximation for diffusion-weighted SSFP is extended to incorporate cyclic motion. A complex encoding efficiency metric is introduced to compare the displacement fields of unipolar and conventional GRE-MRE sequences in both magnitude and phase. The derived theoretical encoding equations are used to characterize the proposed sequence using an extensive parameter study. Unipolar MRE is validated against conventional GRE-MRE in a phantom study showing excellent agreement between measured displacement fields. In addition, unipolar MRE yields significantly increased octahedral shear strain-SNR relative to conventional GRE-MRE and allows for the recovery of high stiffness inclusions, where conventional GRE-MRE fails

Determinants of myocardial function characterized by CMR-derived strain parameters in left ventricular non-compaction cardiomyopathy

Clinical presentation of left ventricular non-compaction cardiomyopathy (LVNC) can be heterogeneous from asymptomatic expression to congestive heart failure. Deformation indices assessed by cardiovascular magnetic resonance (CMR) can determine subclinical alterations of myocardial function and have been reported to be more sensitive to functional changes than ejection fraction. The objective of the present study was to investigate the determinants of myocardial deformation indices in patients with LVNC. Twenty patients with LVNC (44.7 ± 14.0 years) and twenty age- and gender-matched controls (49.1 ± 12.4 years) underwent functional CMR imaging using an ECG-triggered steady state-free-precession sequence (SSFP). Deformation indices derived with a feature tracking algorithm were calculated including end-systolic global longitudinal strain (GLS), circumferential strain (GCS), longitudinal and circumferential strain rate (SR$_{ll}$ and SR$_{cc}$). Twist and rotation were determined using an in-house developed post-processing pipeline. Global deformation indices (GLS, GCS, SR$_{ll}$ and SR$_{cc}$) were significantly lower in patients with LVNC compared to healthy controls (all, p < 0.01), especially for midventricular and apical regions. Apical rotation and twist were impaired for LVNC (p = 0.007 and p = 0.012), but basal rotation was preserved. Deformation indices of strain, strain rate and twist correlated well with parameters of the non-compacted myocardium, but not with the total myocardial mass or the thinning of the compacted myocardium, e.g. r = 0.595 between GLS and the non-compacted mass (p < 0.001). In conclusion, CMR deformation indices are reduced in patients with LVNC especially in affected midventricular and apical slices. The impairment of all strain and twist parameters correlates well with the extent of non-compacted myocardium

Cardiac- versus diaphragm-based respiratory navigation for proton spectroscopy of the heart

OBJECTIVES To study inter-individual differences of the relation between diaphragm and heart motion, the objective of the present study was to implement respiratory navigation on the heart and compare it against the established method of navigator gating on the diaphragm for single-voxel cardiac H-MRS. MATERIALS AND METHODS H-MRS was performed on a 1.5T system in 19 healthy volunteers of mixed age (range 24-75 years). Spectra were recorded in a 6-8 ml voxel in the ventricular septum using a PRESS (point-resolved spectroscopy) sequence and ECG gating. Water-unsuppressed data acquired with pencil beam navigation on the heart were compared to data with navigation on the diaphragm. Water-suppressed data were obtained to assess triglyceride-to-water ratios. RESULTS Water phase and amplitude fluctuations for cardiac versus diaphragm navigation did not reveal significant differences. Both navigator positions provided comparable triglyceride-to-water ratios and gating efficiencies (coefficient of variation (CoV) 7.0%). The cardiac navigator showed a good reproducibility (CoV 5.2%). DISCUSSION Respiratory navigation on the heart does not convey an advantage over diaphragm-based navigator gating for cardiac H-MRS, but also no disadvantage. Consequently, cardiac and diaphragm respiratory navigation may be used interchangeably

Retrospective phase-based gating for cardiac proton spectroscopy with fixed scan time

BACKGROUND Respiratory motion is a major limiting factor for spectral quality and duration of in vivo proton MR spectroscopy of the heart. Prospective navigator gating is frequently applied to minimize the effects of respiratory motion, but scan durations are subject-dependent and hence difficult to predict. PURPOSE To implement cardiac proton MRS with fixed scan time by employing retrospective phase-based gating and to compare the proposed method to conventional navigator-gated MRS. STUDY TYPE Prospective. SUBJECTS Eighteen healthy volunteers (29.7 ± 7.8 years). FIELD STRENGTH/SEQUENCE 1.5, navigator-gated (16 averages without, 96 with water suppression [WS]) data acquisition as reference and navigator-free data acquisition with a fixed scan time (48 without WS, 304 with WS), cardiac-triggered point-resolved spectroscopy (PRESS). ASSESSMENT Navigator-free data acquisition with retrospective phase-based gating was compared with prospective navigator-gating. Navigator-free acquisition was repeated in 10 subjects to assess reproducibility. Scan time was assessed for prospective and retrospective gating. Retrospective phase-based gating was performed using a threshold based on the standard deviation (SD) of individual water (W) and triglyceride (TG) phases. STATISTICAL TESTS T-tests and Bland-Altman analysis. RESULTS The duration of the prospective navigator-gated scans ranged from 6:09 minutes to 21:50 minutes (mean 10:05 ± 3:46 min, gating efficiency 40.4 ± 10.5%), while data acquisition for retrospective phase-based gating had a fixed scan time of 11:44 minutes. Retrospective phase-based gating using a threshold of 1 × SD yielded a gating efficiency of 72.7 ± 4.3% and a coefficient of variation (CoV) of triglyceride-to-water ratios of 9.8% compared with the navigated reference. The intrasubject reproducibility of retrospective gating revealed a CoV of 9.5%. DATA CONCLUSION Cardiac proton MRS employing retrospective phase-based gating is feasible and provides reproducible assessment of TG/W in a fixed scan time. Since scan time is independent of respiratory motion, retrospective phase-based gating offers an approach to motion compensation with predictable exam time for proton MRS of the heart. LEVEL OF EVIDENCE 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1973-1981