Proton-Echo-Planar-Spectroscopic-Imaging (PEPSI) in Human Breast

INTRODUCTION MR single voxel spectroscopy (MRS) to measure Choline in breast tumors has shown promise as a diagnostic adjunct to dynamic contrast-enhanced MRI exam. MRS improved the sensitivity, specificity, and accuracy for all readers, and improved the inter observer agreement between the readers [1,6]. However, single voxel spectroscopic techniques do not allow characterization of lesion heterogeneity and multi-focal lesions. Feasibility of MR spectroscopic imaging (MRSI) was demonstrated by Jacobs et al. [2]. However, conventional MRSI is time consuming and artifacts due to chest wall motion and overwhelming lipid resonances make spectral quantification challenging. Here, we describe the use of 2D and 3D Proton-Echo-Planar-Spectroscopic-Imaging (PEPSI) [3,4] in the breast to increase encoding speed and volume coverage. A navigator acquisition was integrated into the water suppression module to monitor frequency shifts from chest wall motion. Preliminary results demonstrate high quality 2D and 3D mapping of lipid resonances. METHOD The PEPSI pulse sequence consists of the following modules: water suppression with integrated navigator data acquisition, outer volume suppression, spin-echo RF excitation with MEGA lipid suppression [5], echo-planar readout with 1024 readout gradients, 1087 Hz of spectral width and 2 Hz digital spectral resolution. Spatially resolved navigator data acquisition is performed for each phase encoding step. An echoplanar readout train with 64 readout gradients is integrated into the WET module after the water suppression RF pulse and before the dephasing gradient pulse (Fig.1). Readout gradients are orthogonal to the imaging slice, enabling measurement of frequency shifts in slices parallel to the imaging slice. In vivo experiments in 3 healthy volunteers were performed on 3T MR scanners (Tim Trio, Siemens Medical Solutions) equipped with 4-channel breast coil or 8-channel body array. Automated shimming was performed across the entire PEPSI slice or slab. Non-water suppressed 2D PEPSI data were acquired with TR/TE: 1s/11ms, 64×64 matrix, 286 mm FOV and slice thickness of 5mm, resulting in a voxel size of 0.1 cc. Non-water suppressed 3D PEPSI data were acquired with TR/TE: 0.63s/11ms, 64x64x8 matrix, FOV of 286×286×55, 40 mm slab, resulting in a voxel size of 0.14 cc. Waterand lipid-suppressed data were acquired with TR/TE 3s/75ms, 8 averages, 32x32 matrix, 320 mm FOV, 15 mm slice, resulting in 1.5 cc voxel size. Raw data were reconstructed as described previously [4]. NWS scans with identical pulse sequence parameters and single average were used as a reference for phase, frequency and eddy current correction. Spectral quantification was performed using LCModel with simulated basis set integrated in LCModel.