To achieve higher spatial resolution in MR elastography (MRE), it has been reported that the combination of SS-SE-EPI with a spatially selective excitation efficiently reduce FOV in the phase encoding direction without wrap around artifacts thereby increase the spatial resolution [1]. This local excitation technique is called FOCUS (FOV Optimized and Constrained Undistorted Single-shot) that may extend the application for relatively small organs such as the pancreas [2]. In this study, we sought to optimize FOCUS-MRE to the pancreas using homogenous phantom and a volunteer.

1) To compare the shear stiffness value of the homegeneous phantom between conventional-MRE and FOCUS-MRE .

2) To optimize the parameters for FOCUS-MRE.

3) To optimize the form of FOV placement for the pancreatic FOCUS-MRE.

In this study, we used a 2D echo-planar RF pulse instead of spatial spectral RF pulse for excitation in the SS-SE-EPI sequence. Only the region of interest in the phase FOV direction can be excited without incurring aliasing artifacts, as a result of which fewer phase encodings are required to achieve the desired spatial resolution compared to the conventional-MRE. All examinations were conducted on a clinical MR scanner (Discovery 750w 3.0T, GE Healthcare,) with body AA and PA phased array coil with 32 elements. The parameters for each method were as follows: TR/TE = 1000/53.4-77.3 msec (conentional-MRE) and 1000/59.5-77.5 msec (FOCUS-MRE), motion-encoding gradients (MEG) direction = z. In both cases, all MRE images were acquired axial with three slices. The wave images and elastograms were automatically generated by MR-Touch system. We used the polyacrylamide gel (70 wt% glycerin) phantom for standardwith storage modulus (G') of 3.1 kPa (rheometer) [3]. The phantom is 16*18cm cylindrical shape. The acoustic driver was placed on the top of the phantom. 1) Shear stiffness was measured on the phantom for the conventional-MRE and FOCUS-MRE varying MEG and driver frequencies. 2) In FOCUS-MRE, various size and the position of FOV placement was tested. 3) Using FOCUS-MRE, pancreatic MRE was tested on a normal volunteer.1) In conventional-MRE, the larger the driver frequency was, the higher the measured shear stiffness. In FOCUS-MRE, the measured stiffness were about 8% lower than that of normal-MRE. In both MRE, the difference of MEG did not affect shear stiffness (Table 1). 2) Figure 1&2 show how elastogram and wave images vary according to the forms of the FOV placement with FOCUS-MRE. As for FOCUS-MRE, when the whole FOV was placed inside of the phantom (AB), the elastogram suffered from inhomogeneity and the artifacts. Waves were not parallel on the wave images. When the FOV was placed off-set including the remote side of the phantom (CDEF), the elastograms were more homogeneous and the waves were parallel on the wave images. When the FOV was placed off-set including the lateral half side of the phantom, liner artifact occurred in the edge of magnitude image and this artifact seemed to affect the wave image and elastogram. 3) Figure 3&4 show the result of volunteer study for the pancreatic MRE. When the whole FOV was placed inside the body (J), the elastogram and wave image was unsatisfactory similar to the phantom study (AB). When the FOV was placed including the lateral side of the body (K), the artifact occured in some cases (arrow ; magnitude image of K) and the analysis was seems to be affected (under-bar ; elastogram and wave image of K). This phenomenon was reproduced similar to the phantom study (GH). When the FOV was placed off-set including the proximal and remote side of the body (LM like phantom study CE), the obtained image of elastogram and wave image were better than others (JK). In the phantom study, the measured stiffness value of FOCUS-MRE is about 8% lower than those of conventional-MRE; however, the values were closer to that measured with rheometer. In FOCUS-MRE, FOV should be placed off-set including proximal or remote side of the object, furthermore, inclusion of the lateral side of the object is not recommended, because it may result in the artifacts and the resultant stiffness values may be affected. Pancreatic MRE using FOCUS-MRE, FOV may be better placed off-set including ventral or dorsal side of the body.