Scientists and clinicians who are interested in myocardial tissue characterization.

Myocardial T1 mapping is a non-invasive and important tool for quantitative myocardium characterization, which has been shown great potential in comprehensive disease assessment[1]. Nowadays, myocardial T1 mapping methods, such as MOLLI[2], shMOLLI[3], SASHA[4] and SAPPHIRE[5], are generally implemented with 2D single shot imaging technique along with breath hold, resulting in limited image coverage and imaging resolution. Besides, due to use of long acquisition window, the cardiac motion and imperfect breath holding may cause image blurring and misregistration among the T1-weighted images, which finally results in the degraded quality on T1 maps[6].

In this study, we developed a free-breathing, three-dimensional (3D) T1 mapping method at 3T based on the saturation-prepared, inversion-prepared radiofrequency-spoiled gradient echo (SPGR) sequence, which could largely overcome the limitations of conventional 2D techniques.

All imaging studies were performed on a 3T MR system (Achieva TX, Philips Healthcare, Best, Netherlands). Cardiac studies used a 32-channel phased array (InVivo, Gainesville, Florida, USA) and phantom studies used an 8-channel head coil.

Sequence design: A series of 3D volumes were acquired in an interleaved fashion during diastolic cardiac phase, as shown in Figure 1A. Such interleaved acquisition enabled reducing misregistration among the volumes. Four different T1 weightings were achieved by using non-selective saturation prepulses with different delay (T_SAT) to SPGR readout. An additional inversion prepulse with a short delay (T_INV) was inserted right before the readout of the first volume imaging, for purpose of extending the magnetization T1 recovery dynamic range. In order to achieve the fully recovered magnetization, five or six idle heartbeats were skipped before the last volume was acquired. To remove time limits imposed by breath-holds and achieve desired spatial resolution and coverage, a pencil beam respiratory navigator was used for respiratory compensation. As for postprocessing of this proposed sequence, pixel-wised 3D T1 maps were calculated using a 3 parameters model: s(i) = Aα(T_INV, T1)+Bβ(T_SAT,i, T1)*exp(T_d/T1), where i was the i^th T1-weighted volume,T_d was T_INV or T_{SAT, i} (depending on whether there was an inversion prepusle or not in the corresponding imaging volume).

Phantom study: Gel phantoms (n=13) were prepared using Gd-DTPA (Magnevist, Berlex) and Agarose (Sigma-Aldrich) to approximate T1 values between 300-2200ms. Reference T1 values were measured using the IR prepared single-echo spin echo sequence (IR-SE) with TI ranging from 100ms to 3000ms. A TR of 10s was used in order for complete magnetization recovery. Imaging voxel size was 2×2×8mm³ and a simulated heart rate 60bpm was employed. Correlation between the T1 values from the 3D T1 mapping and that from spin echo was performed.

Normal Human Subjects Study: Under the local Institutional Review Board approved protocol, 3 healthy volunteers without known cardiovascular dysfunction were enrolled and scaned with the proposed 3D T1 mapping sequence. Typical imaging parameters were: FOV 300×300×80mm³, voxel size 2×2×8mm³, FA 18°, TR/TE 3.28/1.0ms with a 5/8 fractional readout, acquisition window about 100ms. Total scan time for T1 mapping was around ~10minutes with about 40% navigator efficiency and without using any parallel imaging.

Excellent correlation betwwen the proposed and reference method for the T1 values typically observed in myocardium pre and post contrast imaging was obtained (R² = 0.99 and regression slope 0.93, Figure 2). Homogeneous T1 maps were obtained from in-vivo study. Representative short axis view (SAX) T1 map from one healthy volunteer was shown in Figure 3A. Ten SAX T1 maps covering the whole left ventricle were reformatted to long axis view as shown in Figure 3B. Mean and standard deviation of T1 values obtained with the proposed sequence in normal myocardium was1508.4±34.5ms.In this study, we presented a novel 3D free-breathing T1 mapping sequence at 3T that achieves whole heart coverage with excellent homogeneity. Measured normal myocardium T1 values are comparable to those previously published [7,8]. The 3D T1 maps should allow for higher resolution myocardial tissue characterization. By combining with parallel imaging or the state-of-the-art fast imaging techniques, higher spatial resolution and more efficient imaging acquisition could be achieved in the future.