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Improved fast multi-slice myocardial T1 mapping (FAST1) for full left ventricular coverage in three breathholds
Li Huang1, Radhouene Neji1,2, Muhammad Sohaib Nazir1, John Whitaker1, Filippo Bosio1, Amedeo Chiribiri1, Reza Razavi1, and Sébastien Roujol1

1School of Biomedical Engineering and Imaging Sciences, Faculty of Life Sciences and Medicine, King's College London, London, United Kingdom, 2MR Research Collaborations, Siemens Healthcare Limited, Frimley, United Kingdom

Synopsis

Modified Look-Locker inversion recovery (MOLLI) is a commonly used approach for myocardial T1 mapping, and yields high precision as well as reproducibility. This technique requires one breathhold per slice, resulting in prolonged examination time when full left ventricular coverage is required. Our previously developed fast multi-slice myocardial T1 mapping (FAST1) technique showed potential for time-efficient full left ventricular coverage. In this work, the FAST1 sequence is further improved using Bloch-equation-based fitting for higher T1 accuracy and heart-rate independence. Compared to MOLLI, the proposed new FAST1 can yield higher T1 accuracy, enhanced tolerance to T2 and heart-rate variations, as well as similar repeatability and T1 map quality at 1.5T.

Introduction

Myocardial T1 mapping shows promise for assessing cardiomyopathies1. The modified Look-Locker inversion recovery (MOLLI)2,3 technique is commonly used and yields high precision as well as reproducibility4-6. In MOLLI and its variations such as ShMOLLI7, 7-13 images are acquired in a breathhold of 9-17 heartbeats to generate a single T1 map. Therefore, their applications for full left ventricular coverage is limited. Fast multi-slice myocardial T1 mapping (FAST1)8 has been recently proposed for time-efficient full left ventricular coverage. In this work, FAST1 is further improved for increased accuracy as well as enhanced tolerance to heart rate (HR) variation and slice cross-talk.

Methods

(1) Sequence: FAST1 enables the acquisition of five slices within a single breathhold, as shown in Fig. 1. For each slice, a slice-selective inversion pulse is followed by the acquisition of two ECG-triggered images. Compared to our previous work8, newly inserted recovery heartbeats (HBs) allow further increase of the inversion pulse slice thickness without promoting slice cross-talk. The inversion slice thickness over imaging slice thickness ratio (RTHK) was optimized to 4 in this work. The number of recovery HBs (NR) was determined to allow for a recovery delay of TRD≥6s between the inversion pulses for adjacent slices, allowing for a >98% recovery of the longitudinal magnetization of myocardium in the worst-case scenario (defined as a slice being fully inverted by an adjacent slice inversion).

(2) Reconstruction: T1 maps were reconstructed using a dictionary matching approach. The signal dictionary was generated using Bloch equation simulations of FAST1 for a 1-4000ms T1 range as well as T1-dependent slice profiles of the inversion and excitation pulses in the presence of typical B0/B1 inhomogeneities (±150Hz/80-100%) and myocardial T2=45ms. During dictionary matching, the measured signal was polarity-restored using a phase-sensitive inversion recovery approach8,9 and then scaled to each dictionary entry8. This reconstruction was compared to the original FAST1 reconstruction, where the signal dictionary was generated using a one-parameter model defined as S(TI)=1-(1+δ)e-TI/T1 with δ as the inversion factor of the inversion pulse8,10.

(3) Numerical simulations: The proposed and original FAST1 approaches were compared to MOLLI by numerical simulations in terms of accuracy and precision for a range of physiologically reasonable myocardial T1 times (300-1500ms), T2 times (30-60ms) and HRs (50-110bpm), together with a practical signal-to-noise ratio (SNR) of 50.

(4) Experiments: FAST1 and conventional 5-(3)-3 MOLLI were performed on a 1.5T scanner (MAGNETOM Aera, Siemens Healthcare, Erlangen, Germany) in phantom11, 9 healthy volunteers and 17 patients. Both sequences used the same 2D bSSFP parameters: TR/TE/α=2.70ms/1.12ms/35°, FOV=360×306mm2, pixel size=1.4×2.1mm2, slice thickness=8mm, GRAPPA factor=2, partial Fourier factor=7/8, bandwidth=1085Hz/px, first TI=100ms. For phantom study, simulated HRs of 40-120bpm were used. For in-vivo studies, slices were prescribed in the short-axis orientation, and FAST1/MOLLI were performed three times each to generate 15/3 slices. Five and two scan repetitions were performed in phantom and healthy volunteer studies, respectively.

(5) Analyses: T1 times/spatial variability/repeatability of FAST1 and MOLLI were measured in each vial (phantom) and each myocardial segment12 (healthy volunteers). Subject-wise T1 times were computed in each patient. Subjective assessment of map quality was performed by consensus of two experienced cardiac MRI readers using a 4-point-scale scoring system (1-non-diagnostic: severe artifacts/4-excellent: no artifacts).

Results

(1) Simulation study: The proposed FAST1 led to superior accuracy and reduced HR dependence to MOLLI and the original FAST1 approach (Fig. 2). However, the proposed and original FAST1 techniques had ~77% and ~46% inferior precision to MOLLI, respectively.

(2) Phantom study: The proposed FAST1/original FAST1/MOLLI yielded accuracy of -26±5ms/-73±53ms/-56±36ms, spatial variability of 9±6ms/8±4ms/6±4ms, repeatability of 2±1ms/1±1ms/1±0ms and HR sensitivity of 3±2ms/19±23ms/4±4ms, respectively (Fig. 3).

(3) In-vivo studies: Fig. 4 and Fig. 5 present the results of healthy volunteer and patient studies, respectively. Compared to MOLLI, the proposed FAST1 yielded increased native T1 times (1016±0ms vs. 987±23ms, p<0.0001), higher spatial variability (66±10ms vs. 46±7ms, p<0.0001) and similar repeatability (18±6ms vs. 14±5ms, p=0.10) in healthy volunteers. High Pearson correlation coefficients were found between native/post-contrast T1 times using both techniques in patients (0.92/0.98). Similar T1 map quality of FAST1 and MOLLI was obtained (3.8±0.2 vs. 3.7±0.3, p=0.13 in healthy volunteers and 3.5±0.5 vs. 3.3±0.6, p=0.32 in patients, respectively).

Discussion

The proposed enhanced FAST1 led to improved T1 accuracy and T2/HR insensitivity. The validation of FAST1 in a larger patient cohort will be the focus of future work. Extension of FAST1 to higher fields (≥3T) remains to be investigated.

Conclusion

Compared to MOLLI, the improved FAST1 enables whole-left-ventricle T1 mapping in three breathholds with higher T1 accuracy and tolerance to T2/HR variations as well as similar repeatability and map quality at 1.5T.

Acknowledgements

This work was supported by the Health Innovation Challenge Fund (grant number HICF-R10-698), a parallel funding partnership between the Department of Health and the Wellcome Trust, the Wellcome Engineering and Physical Sciences Research Council (EPSRC) Centre for Medical Engineering at King's College London (WT 203148/Z/16/Z) and the EPSRC grant (EP/R010935/1). This research was also supported by the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy's and St Thomas' National Health Service (NHS) Foundation Trust in partnership with King's College London, and by the NIHR Healthcare Technology Co-operative for Cardiovascular Disease at Guy’s and St Thomas' NHS Foundation Trust. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.

References

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Figures

Fig. 1. FAST1 sequence diagram. Five two-heartbeat imaging blocks, in each two ECG-triggered images following slice-selective inversion are acquired, constitute a single-breathhold five-slice sequence. Partial volume effects introduced by respiratory and cardiac motions between inversion and acquisition are minimized by two means: (i) the minimum first TI; (ii) thickened tagging, which is described using RTHK as the ratio of inversion to excitation slice thickness. Slice cross-talk is minimized using two strategies: (i) slice interleaving; (ii) insertion of NR HBs between the imaging blocks #5 and #2, ensuring a minimum recovery delay TRD between all adjacent slice pairs.

Fig. 2. Results of the simulation study. T1-T2 dependent and T1-HR dependent accuracy and precision of FAST1 and MOLLI in the presence of added noise are shown, respectively. Accuracy is demonstrated in the form of T1 mean difference to actual T1 over noise-introduced signals, and precision is presented in the form of T1 standard deviation over noise-introduced signals.

Fig. 3. Results of the phantom study. Region of interest (ROI) analysis was performed in six vials with T2~45ms (reference T1/T2 values as the horizontal axes) in phantom. Accuracy (average of T1 mean difference in ROI to reference T1 over repetitive scans), spatial variability (average of T1 standard deviation in ROI over repetitive scans) and repeatability (standard deviation of T1 mean in ROI over repetitive scans) as well as HR sensitivity (T1 standard deviation in ROI over scans using different HRs) are shown.

Fig. 4. Results of the healthy volunteer study. (a) Representative example native T1 maps of a healthy volunteer using the proposed FAST1 and MOLLI. (b) Segmental native myocardial T1 times, spatial variability and repeatability obtained across all healthy volunteers using the proposed FAST1 and MOLLI, respectively.

Fig. 5. Results of the healthy volunteer study. (a) Representative example native and post-contrast T1 maps of two patients using the proposed FAST1 and MOLLI sequences. (b) Pearson correlation analysis between the proposed FAST1 and MOLLI for both native and post-contrast T1 mapping.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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