Myelin water imaging (MWI) has been suggested as a sensitive and specific approach for imaging myelin^[1]. Recently, a new high quality MWI method, direct visualization of short transverse relaxation time component (ViSTa)^[2,3] has been developed. The signal characteristics of ViSTa have shown that the signal is primarily from short T₂* in the range of myelin water. Voxel-wise correlation analysis demonstrated a high degree of similarity between the 3D-ViSTa apparent MWF (aMWF^[4] due to T₁-weighting) and conventional MWF^[5,6]. Moreover, it was successfully delineating multiple sclerosis (MS) lesions with significantly reduced signal level^[2,3]. When the lesions are so small, however, it has been difficult to identify them particularly at lower field strength. Stronger magnetic field strength leads to a higher signal-to-noise ratio (SNR) and allowing for high-resolution MR imaging. In this study, we proposed a 3D-ViSTa acquisition scheme that uses an MPRAGE^[7] readout and demonstrated the feasibility of high-resolution whole brain MS patient scan using ViSTa at 7T.Three controls and an MS patient were scanned using a 7T MRI (Siemens; IRB approved) with a 32-receive channel head coil (Nova Medical). To cover a 3D volume, a new ViSTa sequence for 7T was developed (Fig. 1A). The readout was implemented using a 3D-MPRAGE^[7] acquisition. A pair of adiabatic inversion RF pulses (duration = 10.24 ms and bandwidth = 1 kHz) was used to reduce B₁ sensitivity. The three timing intervals (TI₁, TI₂, and TD) were optimized to have maximum suppression in the wide range of long T₁ signals (|M_xy|/M₀ < 0.65% for 1050 < T₁ < 4000ms) while retaining short T₁ signals (|M_xy|/M₀ < 70.0% for T₁ < 180ms). The optimization was conducted assuming axonal and extracellular water has a T1 longer than 1000ms^[8,9] and considering additional longitudinal magnetization evolution in the MPRAGE readout blocks^[10] (flip angle = 7°, echo spacing = 5.5ms). The scan parameters were as follows: 176 slices, FOV = 192×192×176mm3, resolution = (1.0mm)³-isotropic, TR/TE = 2923/1.89ms, TI₁/TI₂/TD = 1248/388/1287ms, flip angle = 7°, partial k-space in phase and slice = 7/8 and 6/8, GRAPPA factor = 3 and scan time = 3.58 min. To generate an aMWF map, for quantification, a FLASH sequence with the same readout as ViSTa was acquired using a TR of 1000ms. The scan time was 1.22 min. The ViSTa data were divided by the FLASH data and then scaled by correction factor which described in our previous work^[2-4]. To verify the short T₂* characteristics of ViSTa, a single slice, covering the same slice position as previously studied^[2,3], were acquired using multi-echo 2D-ViSTa^[2,3]. The same scan parameters (except TR/TI₁/TI₂/TD/TE = 2960/1090/790/1080/1.4ms) and analysis as in [2] were used. ROIs were manually drawn at the internal capsule and splenium areas.

Because of increased T₁ at 7T, ViSTa timings have been modified. The optimal inversion timing that satisfied the condition was TI₁/TI₂/TD=1248/388/1287ms. The resulting transverse magnetization is shown in Fig. 1B. The double inversion pulse enabled signal suppression over a wide range of long T₁. When the T₂* spectra were investigated (Fig. 2), both ROIs show that the signal is predominately from the short T₂* in the range of myelin water(< 20ms). This suggests that the ViSTa signal at 7T is from myelin water.

Figure 3 shows a quantitative ViSTa aMWF map. It reveals higher MWFs in CC, OR, SCC, GCC, SLF, and IC areas. This map shows a similar (but smaller) MWF distribution to a conventional MWF map^[6] and ViSTa aMWF map using 3T^[4]. Venous in-flow is noticeable in the ViSTa image (red arrows).

The 7T ViSTa illustrates high resolution aMWF map. In the expanded ViSTa map, shown in Fig. 4B, structure such as optic radiation is clearly delineating.

Fig. 5 shows FLAIR, T1-MP2RAGE^[10], and ViSTa images from an MS patient. The MS lesions show hypo and hyper signal in the T₁-weighted and FLAIR images respectively (arrows). In the corresponding areas, the ViSTa image shows significantly reduced signal levels successfully.

In this study, we assessed 3D-ViSTa sequence for 7T MR imaging which can cover the whole brain. In some regions, the 7T ViSTa sequence suffered from non-uniform inversion and excitation due to B₁ inhomogeneity. Further research is needed to improve or correct non-uniform B₁ profile. Due to incomplete knowledge of the constant in the scaling factor^[2-4] ViSTa shows a smaller MWF. However, taking advantage of higher SNR at 7T, ViSTa using 7T MRI provided high-resolution and high-quality myelin water images generating a whole brain volume in clinically reasonable time. And the method successfully detected demyelinated MS lesions. Hence, the method has potential clinical utility.