2767

Cardiac T1 Mapping Using True Hybrid Inversion and Saturation Recovery

Glenn S. Slavin¹, Anne Menini², Haonan Wang³, and Anja C.S. Brau⁴

¹GE Healthcare, Bethesda, MD, United States, ²GE Global Research, Munich, Germany, ³GE Healthcare, Waukesha, WI, United States, ⁴GE Healthcare, Menlo Park, CA, United States

Synopsis

Despite limited accuracy, MOLLI remains popular for T1 mapping due to high precision and visual quality of the maps. This is due primarily to the large dynamic range afforded by the inversion-recovery (IR) acquisition. Methods using saturation-recovery (SR) have better T1 accuracy but have relatively poor precision compared with MOLLI. This work presents a true hybrid IR/SR acquisition that targets the optimal regions of both the IR and SR relaxation curves, employs a novel method for maximizing dynamic range, and uses an improved sampling strategy. The proposed hybrid method combines the accuracy of single-point SR with the precision of IR.

Background

The goal of T1 mapping is to measure tissue T1 with the highest accuracy and precision possible. Despite systematic underestimation of T1 and dependence on T2, heart rate (HR), and other imaging parameters resulting from the Look-Locker readout, MOLLI (1) remains popular owing to its high precision and visual quality of T1 maps. This is due primarily to the large dynamic range afforded by the inversion-recovery (IR) acquisition. Single-point approaches, such as SASHA, SAPPHIRE, and SMART1Map (2-4), achieve T1 accuracy by using parameter-independent acquisitions. However, precision has been relatively poor for these methods, especially those using saturation recovery (SR), compared with MOLLI. This is attributable to the limited range of saturation delay times (TS), limited number of TSs, and reduced dynamic range. In this work, we propose a new method for cardiac T1 mapping that uses a true hybrid IR/SR acquisition designed to combine the accuracy of single-point SR with the precision of IR. The proposed hybrid technique uses repeated data point sampling (5,6) and employs novel means to maximize the dynamic range and to selectively sample the high-signal regions of the IR and SR relaxation curves.

Methods

The hybrid acquisition takes advantage of the fact that the IR and SR curves have different relative magnitudes during the recovery process. Data points are acquired only on the thick curve in Figure 1, using IR when the IR curve has higher signal and using SR when the SR curve has higher signal. The hybrid IR/SR pulse sequence is shown in Figure 2 and requires 13 heartbeats. The first heartbeat is used to acquire data at an “infinite” delay time. The next five heartbeats repeatedly sample a data point on the IR curve. To maximize the dynamic range for IR, the longest possible TS and shortest inversion time (TI) are used. Unlike other SR methods, where the maximum intra-heartbeat TS is limited by the trigger delay (TD), a longer TS is achieved here by placing the saturation pulse immediately after the preceding readout. The final seven heartbeats repeatedly sample a data point on the SR curve. To generate T1 maps, data is simultaneously fit to two equations, depending on the acquisition:

$$$A-Be^{-(\frac{TS}{T1})}$$$ for SR and $$$A-2Be^{-(\frac{TI}{T1})}+Ae^{-(\frac{TS+TI}{T1})}$$$ for IR (7).

Because TS is heart-rate dependent, all heartbeats are measured to ensure accurate TS values. Monte Carlo simulations of a native myocardial scan at 1.5T with a constant HR of 60 bpm were performed to compare the precision of the hybrid sequence to that of SASHA, SAPPHIRE, SMART1Map, and MOLLI. Phantom experiments on a 1.5T system were conducted to validate the simulations (T1=1200ms, TR/TE=2.8/1.2ms, FA=65°, TD=674ms, readout=168ms). To assess HR sensitivity, the hybrid sequence was also evaluated with HR that varied arbitrarily throughout the scan (range: 41-86 bpm). Each phantom scan was performed 10 times. Volunteer scans were also performed with both MOLLI and the hybrid sequence.

Results

The simulation and scan results are shown in Table 1. In the simulations, SASHA and SMART1Map had the lowest precision, with SAPPHIRE higher by approximately 25%. The hybrid IR/SR method should provide an additional 25% improvement in precision over SAPPHIRE. Phantom scans showed very good agreement with the simulations. The hybrid sequence acquired with variable HR (CV=0.978%) did not result in any loss of precision versus constant HR (CV=1.009%). Volunteer images are shown in Figure 3. MOLLI has higher precision, whereas hybrid IR/SR has higher accuracy. The T1 inaccuracy of MOLLI results in a greater difference between measured blood and myocardial T1, leading to the improved appearance in Figure 3b.

Discussion

The results demonstrate that the hybrid method has the best combination of accuracy and precision compared with existing breath-held T1 mapping techniques. Although SAPPHIRE also uses both inversion and saturation pulses, the proposed method is expected to outperform SAPPHIRE due to a longer TS for greater dynamic range, true hybrid IR/SR sampling for maximizing SNR, and improved sampling point selection. Additionally, unlike other methods, the maximum TS of the hybrid sequence is independent of TD because the saturation pulse is decoupled from the ECG trigger; precision is therefore maintained even for systolic imaging. Precision is also expected to be tolerant of HR variation. In fact, long arrhythmias can improve precision. While MOLLI still has somewhat better precision than the hybrid approach, MOLLI cannot provide the accuracy and objectivity of true T1 measurements typical of single-point acquisitions. Although the repeated sampling point strategy used in this work was validated for SR acquisitions, further investigation is required to determine the optimal strategy for the hybrid acquisition.

Acknowledgements

No acknowledgement found.

References

1. Messroghli DR et al. Modified Look-Locker Inversion Recovery (MOLLI) for High-Resolution T1 Mapping of the Heart. Magn Reson Med 2004; 52:141-146

2. Chow K et al. Saturation Recovery Single-Shot Acquisition (SASHA) for Myocardial T1 Mapping. Magn Reson Med 2014;71:2082-2095

3. Weingartner S et al. Combined Saturation/Inversion Recovery Sequences for Improved Evaluation of Scar and Diffuse Fibrosis in Patients with Arrhythmia or Heart Rate Variability. Magn Reson Med 2014; 71:1024-1034

4. Slavin GS et al. Breath-Held Myocardial T1 Mapping Using Multiple Single-Point Saturation Recovery. Proc Intl Soc Mag. Reson Med 2012; 20:1244

5. Kellman P et al. Optimized saturation recovery protocols for T1-mapping in the heart: influence of sampling strategies on precision. J Cardiovasc Magn Reson 2014; 16:55

6. Akçakaya M et al. On the Selection of Sampling Points for Myocardial T1 Mapping. Magn Reson Med 2014; 73:1741-1753

7. Deichmann R et al. Optimisation of the 3D MDEFT sequence for anatomical brain imaging: technical implications at 1.5 and 3T. NeuroImage 2004; 21:757-767

Figures

Figure 1. IR and SR magnitude curves. Black circles represent sample points at delay times t. The thicker curve shows the higher available magnetization M_z across the range of delay times. The hybrid approach acquires data only along this curve, as opposed to being restricted to either the IR (blue) or SR (orange) curve. For IR acquisitions, the maximum TS and minimum TI are used to maximize dynamic range by providing the most negative possible value for M_z.

Figure 2. Hybrid IR/SR pulse sequence diagram. Heartbeat 1 (HB 1) acquires data with an “infinite” delay time. HB 2-6 constitute the IR component. A saturation pulse is played out immediately after each bSSFP readout to set M_z=0 and to allow the maximum TS. An inversion pulse is played immediately prior to the subsequent readout (minimum TI) to maximize dynamic range. This data point is acquired five times. HB 7-13 constitute the SR component, where a saturation pulse is played out after every readout. This data point is acquired seven times.

Figure 3. T1 color maps for Hybrid IR/SR and MOLLI in a volunteer. T1 was measured around the left ventricle. (a) Hybrid IR/SR and (b) MOLLI with myocardium displayed in green. (c) MOLLI using the same color scale as in (a). CV=coefficient of variation. TR/TE 3.7/1.6ms, 35cm FOV, 192x128 matrix, FA 65°, bSSFP readout=179ms, average HR=68bpm. For IR: TI=9ms, average TS=618ms. For SR: average TS=699ms.

Table 1. Comparison of precision of Hybrid IR/SR with four other commonly used T1 mapping techniques. SMART1Map, SASHA (13-heartbeat), and SAPPHIRE are single-point SR methods; MOLLI (17-heartbeat) is Look-Locker IR. Results are for simulations and phantom scans at 1.5T. Precision is reported as the coefficient of variation (CV), calculated as the standard deviation divided by the mean of the measured T1. *SAPPHIRE pulse sequence was not available for scanning.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)

2767