Robin Simpson

MR assessment of regional myocardial mechanics

Regional myocardial function can be measured by several MR techniques including tissue tagging, phase velocity mapping, and more recently, displacement encoding with stimulated echoes (DENSE) and strain encoding (SENC). Each of these techniques was developed separately and has undergone significant change since its original implementation. As a result, in the current literature, the common features and the differences between the techniques and what they measure are often unclear and confusing. This review article delivers an extensively referenced introductory text which clarifies the current methodology from the starting point of the Bloch equations. By doing this in a consistent way for each method, the similarities and differences between them are highlighted. In addition, their capabilities and limitations are discussed, together with their relative advantages and disadvantages. While the focus is on sequence design and development, the principal parameters measured by each technique are also summarized, together with brief results, with the reader being directed to the extensive literature on data processing and clinical applications for more detail. J. Magn. Reson. Imaging 2013;37:576–599.

Spiral tissue phase velocity mapping in a breath-hold with non-cartesian SENSE.

Tissue phase velocity mapping (TPVM) is capable of reproducibly measuring regional myocardial velocities. However acquisition durations of navigator gated techniques are long and unpredictable while current breath‐hold techniques have low temporal resolution. This study presents a spiral TPVM technique which acquires high resolution data within a clinically acceptable breath‐hold duration.

Retrogated spiral 3-directional myocardial phase velocity mapping in a single breath-hold

Background Myocardial phase velocity mapping studies have generally been acquired using Cartesian k-space coverage and respiratory gating [1,2]. Acquisition durations for high temporal resolution studies are therefore long and unpredictable and the use of navigators and prospective cardiac gating results in ‘dead-times’ in the cardiac cycle where imaging cannot be performed. We have developed a technique which combines highly efficient spiral k-space coverage with retrospective cardiac gating for 3D velocity mapping over the entire cardiac cycle within a breath-hold. The feasibility for rapid assessment of myocardial motion is demonstrated.

High resolution spiral myocardial phase velocity mapping (PVM) of the entire cardiac cycle

Background Three-directional PVM is capable of measuring regional myocardial velocities. Current techniques use Cartesian k-space coverage, and navigator-gated high spatial and temporal resolution acquisitions are long [1,2]. In addition, they use prospective ECG-gating and analysis of the full cardiac cycle is not possible. The aim of this study is to develop a high temporal and spatial resolution PVM technique using efficient spiral k-space coverage and retrospective ECG-gating which will allow detailed analysis of the entire cardiac cycle, including atrial systole which accounts for 20-30% of leftventricular filling in healthy motion [3]

Breath-hold spiral tissue phase velocity mapping (TPVM) with non-Cartesian SENSE.

high temporal resolution TPVM data over the entire NIHR CBRU, Imperial College, London, UK Figure 1 a) Global velocities from 9 short axis slices in one volunteer (each curve represents velocity averaged over a slice). In longitudinal and radial directions one systolic (SL/SR), one early diastolic (DL/DR) and one atrial systolic (ASL/ASR) peak is seen. Longitudinal peaks reduce from base to apex, while radial peaks do not. Two systolic circumferential peaks (C1 and C2) are seen, as well as a diastolic peak (C3) which is negative at base but positive at apex. b) Peak and TTP global velocity values in the mid slice for the healthy volunteers (mean ± SD values shown by red bars), early DCM patient (green crosses) and late DCM patient (blue crosses). Healthy values show small standard deviations (although circumferential peaks are more variable). C3 is not shown as it is very slice-position dependent not always seen in the mid slice. The early stage patient shows normal longitudinal values but reduced radial values, whereas the late stage patient shows reduced peak values in all directions. Simpson et al. Journal of Cardiovascular Magnetic Resonance 2014, 16(Suppl 1):P40 http://www.jcmr-online.com/content/16/S1/P40

Accelerating spiral tissue phase velocity mapping without affecting peak velocity measurements.

Background TPVM is a promising technique for measuring myocardial mechanics [1] but long scan times have limited its clinical application, despite acceleration techniques such as view-sharing [2] or k-t BLAST [3] being used. Cartesian work using k-t BLAST found that accelerations greater than a factor of 2 affected peak velocity measurements, possibly because of temporal smoothing inherent in the technique [3]. Recently spiral trajectories have been shown to be capable of greatly reducing scan duration in comparison with similar resolution Cartesian sequences [4]. A new spiral TPVM sequence which uses the Gadgetron [5] GPU implementation of non Cartesian SENSE has been developed: this abstract compares peak velocities measured with different acceleration factors.

Retrospective gating leads to more accurate velocity measurements than prospective gating in spiral phase velocity mapping.

Background Spiral phase velocity mapping (PVM) has been used to assess both blood flow [1] and myocardial velocities [2]. Data is usually acquired over several heartbeats which, will not be exactly the same due to physiological RR interval (RR) variations. The way in which the data from different cycles is combined depends on the method of ECG gating. This simulation study investigates the effect of RR variation on the velocities measured for prospective (pro) and retrospective (retro) gating.

Application of through-time spiral GRAPPA to phase velocity mapping (PVM)

Background PVM is an established technique for measuring blood [1] and myocardial velocities [2]. However, long scan times can restrict its application. Peak-GRAPPA has been used to accelerate Cartesian PVM up to a factor of 6 (R = 6) without degrading peak velocity measurements [3], however use of efficient k-space trajectories could allow higher temporal resolution (TR) in similar scan time. Through-time spiral GRAPPA allows highly accelerated spiral data to be reconstructed using throughtime calibration information [4]. By collecting multiple repetitions of fully sampled calibration data, which can be collected without gating during free breathing, through-time information can be used to generate GRAPPA weights specific to the local undersampling in the non-Cartesian data. This abstract presents preliminary work on a single volunteer to apply this reconstruction to myocardial spiral PVM data.

Right ventricular (RV) velocity measurements using high resolution spiral myocardial phase velocity mapping (PVM)

Background PVM is capable of accurately and reproducibly measuring myocardial velocities in the LV [1]. However, analyzing RV motion is more difficult because of the thinness of the free wall and its asymmetrical geometry. Currently, the most commonly used technique for assessing RV velocities and function is Tissue Doppler Imaging (TDI) where the high temporal resolution allows the detailed analysis of fine features of motion (small peaks in velocity during isovolumic contraction (IC) and isovolumic relaxation (IR) [2], for example). However TDI is restricted by inadequate acoustic windows and cannot comprehensively assess velocities over the entire RV. This study aims to establish that high resolution spiral PVM is capable of reproducibly measuring RV free wall velocities and that, consequently, it has a future role in assessing RV function.

Reproducibility of peak and time to peak velocity measurements with a high resolution spiral phase velocity mapping (PVM) sequence

Background Myocardial PVM may be used to assess peak velocities and the times to those peaks (TTP) in order to characterise healthy and pathological regional heart motion [1]. However data on the reproducibility of the technique is limited. A previous breathhold (low resolution) study investigated reproducibility for peak radial and circumferential velocities [2]; however that of TTP measurements was not presented. A higher resolution, navigator-gated study showed that MR had better reproducibility than tissue Doppler imaging for peak and TTP velocity measurements, but only longitudinal velocities were considered [3]. All previous studies have used prospective cardiac gating and late diastolic (atrial systole) peak and TTP velocities have not been investigated. This study presents a comprehensive assessment of the reproducibility of myocardial PVM for assessing peak and TTP velocities throughout the entire cardiac cycle. Methods