To optimize and validate a fully-integrated version of modified Look-Locker inversion-recovery (MOLLI) for clinical single-breathhold cardiac T1 mapping.
A MOLLI variant allowing direct access to all pulse sequence parameters was implemented on a 1.5T MR system. Varying four critical sequence parameters, MOLLI was performed in eight gadolinium-doped agarose gel phantoms at different simulated heart rates. T1 values were derived for each variant and compared to nominal T1 values. Based on the results, MOLLI was performed in midcavity short-axis views of 20 healthy volunteers pre- and post-Gd-DTPA.
In phantoms, a readout flip angle of 35 degrees , minimum TI of 100 msec, TI increment of 80 msec, and use of three pausing heart cycles allowed for most accurate and least heart rate-dependent T1 measurements. Using this pulse sequence scheme in humans, T1 relaxation times in normal myocardium were comparable to data from previous studies, and showed narrow ranges both pre- and postcontrast without heart rate dependency.
We present an optimized implementation of MOLLI for fast T1 mapping with high spatial resolution, which can be integrated into routine imaging protocols. T1 accuracy is superior to the original set of pulse sequence parameters and heart rate dependency is avoided.
"" Conventional MOLLI " , based on the optimised original MOLLI sequence, was chosen as a standard for comparisons—to evaluate any improvements relative to this widely reported method. The sequence consisted of three IR blocks, in a 3b(3b)3b(3b)5b arrangement, with b's indicating images were acquired one-per-beat, with a three beat pause between consecutive IR blocks. "
[Show abstract][Hide abstract] ABSTRACT: Background: The purpose of this work was to evaluate different magnetisation preparation and readout sequences for modified Look-Locker inversion recovery (MOLLI) towards improved T1 mapping in the heart. Elements investigated include: catalysation sequences to prepare the magnetisation before readout, alternate k-space trajectories, a spoiled gradient recalled echo (GRE) readout, and a 5b(3b)3b MOLLI sampling scheme (‘b’ denoting beats).
Methods: Conventional 3b(3b)3b(3b)5b MOLLI with a linear k-space trajectory was compared to four variants in simulations, in vitro and in vivo (at 3T). Variants were centric conventional MOLLI, centric-paired conventional MOLLI, linear 5b(3b)3b MOLLI and spoiled GRE MOLLI. Each of these was applied with three magnetisation catalysation methods, and T1 measurement accuracy and precision were evaluated in simulations via a Monte Carlo algorithm, in a set of calibrated phantoms, and in ten healthy volunteers. Contrast-to-noise, heart rate dependence and B1+ dependence were also evaluated.
Results: A linear k-space trajectory was superior in vitro to centric and centric-paired trajectories. Of the catalysation methods, preparation of transverse magnetisation only—using a linearly increasing flip angle catalysation—improved MOLLI T1 measurement accuracy, precision, and map quality versus methods that include catalysation of the longitudinal magnetisation. The 5b(3b)3b MOLLI scheme offered comparable native T1 measurement accuracy and precision to conventional MOLLI, despite its shortened acquisition.
Conclusions: MOLLI T1 measurement accuracy, precision, and map quality depends on the method of catalysation of magnetisation prior to image acquisition, as well as on the readout method and MOLLI sampling scheme used.
Magnetic Resonance Imaging 02/2015; DOI:10.1016/j.mri.2015.02.004 · 2.09 Impact Factor
"After recent technical improvements, T1 measurement (multi-breath-hold or multiple images requiring curve fitting and processing) has been replaced by T1 mapping. In a single breath-hold, using various approaches, a T1 colour relaxation map is made [11–13]. Within the map, each given pixel value directly corresponds its underlying relaxation time that can be seen (in colour) or more formally measured, standardized, calibrated to histology [14••, 15, 16], compared across diseases and with normal reference ranges . "
[Show abstract][Hide abstract] ABSTRACT: Heart failure (HF) is a major and growing cause of morbidity and mortality. Despite initial successes, there have been few recent therapeutic advances. A better understanding of HF pathophysiology is needed with renewed focus on the myocardium itself. A new imaging technique is now available that holds promise. T1 mapping is a cardiovascular magnetic resonance (CMR) technique for non-invasive myocardial tissue characterization. T1 alters with disease. Pre-contrast (native) T1 changes with a number of processes such as fibrosis, edema and infiltrations. If a post contrast scan is also done, the extracellular volume fraction (ECV) can be measured, a direct measure of the interstitium and its reciprocal, the cell volume. This dichotomy is fundamental - and now measurable promising more targeted therapy and new insights into disease biology.
Current Cardiovascular Imaging Reports 09/2014; 7(9):9287. DOI:10.1007/s12410-014-9287-8
"Short-axis tagged images were acquired at basal, mid and apical ventricular levels using a segmented k-space fast gradient echo sequence with spatial modulation of magnetization in orthogonal planes. A single-shot modified Look Locker inversion recovery (MOLLI) sequence was acquired in short-axis at mid-ventricular level before contrast agent was administered, and 15-minutes after the contrast agent ‘top-up’ (see below) . Same day haematocrit was measured. "
[Show abstract][Hide abstract] ABSTRACT: Background
Serial surveillance endomyocardial biopsies are performed in patients who have recently undergone heart transplantation in order to detect acute cardiac allograft rejection (ACAR) before symptoms occur, however the biopsy process is associated with a number of limitations. This study aimed to prospectively and longitudinally evaluate the performance of multiparametric cardiovascular magnetic resonance (CMR) for detecting and monitoring ACAR in the early phase post-transplant, and characterize graft recovery following transplantation.
All patients receiving a heart transplant at a single UK centre over a period of 25 months were approached within one month of transplantation. Multiparametric CMR was prospectively performed on the same day as biopsy on four separate occasions (6 weeks, 10 weeks, 15 weeks and 20 weeks post-transplant). CMR included assessment of global and regional ventricular function, myocardial tissue characterization (T1 mapping, T2 mapping, extracellular volume, LGE) and pixel-wise absolute myocardial blood flow quantification. CMR parameters were compared with biopsy findings. As is standard, grade 2R or higher ACAR was considered significant.
88 CMR-matched biopsies were performed in 22 patients. Eight (9%) biopsies in 5 patients demonstrated significant ACAR. Significant ACAR was associated with a reduction in circumferential strain (−12.7 ± 2.5% vs. -13.7 ± 3.6%, p = 0.047) but there was considerable overlap between groups. Whilst trends were observed between ACAR and proposed CMR markers of oedema, particularly after adjusting for primary graft dysfunction, differences were not significant. Significant improvements were seen in markers of graft structure and contractility, oedema and microvascular function over the period studied, although few parameters normalised.
This study provides novel insight into the myocardial injury associated with transplantation, and its recovery, however multiparametric CMR was not able to accurately detect ACAR during the early phase post-transplantation.
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