Spectrally selective excitation for improved DWI APT and MRS of the breast
Target Audience: Investigators interested in development and application of quantitative MRI/MRS of breast cancer Purpose: Though diagnostic breast MRI can provide higher sensitivity than mammography [1] considerable research efforts have been devoted to improving MRI's comparatively low specificity. A multi modality breast exam including diffusion weighted imaging (DWI) [2] amide proton transfer (APT) [3] and magnetic resonance spectroscopy (MRS) [4] is of interest because these methods have established several biomarkers for lesions elsewhere in the body such as the brain. However, techniques that work well in the brain may face additional challenges and require significant modifications to be successfully used in the breast. For example, lipid suppression presents an area where proven approaches in the brain often fail because of more heterogeneous water fat distribution in the breast. Recently, water only excitation was implemented for dynamic contrast enhanced (DCE) MRI of the breast [5] showing superior results to conventional lipid suppression. This concept can be applied to all methods in a multi modality breast MR exam. Here, we present our initial experience combining spectrally selective excitation with DWI, APT and MRS in the breast. Methods: All in vivo scans were performed on a 3T whole body scanner with a 16 channel phased array breast coil (Achieva, Philips Healthcare, Best, the Netherlands) Spectrally selective excitation was implemented with three spin echo (SE) based pulse sequences. Unlike the gradient echo sequence [5] where the only RF pulse was modified to be spectrally and spatially selective, SE based sequences allowed us to test the same concept with frequency selective excitation and slice selective refocusing pulses. Specifically, for DWI, a sinc gaussian (SG) pulse with a bandwidth (BW) of 200 Hz was combined with two adiabatic full passage (AFP) pulses (5.8 ms duration and 2.5 kHz BW) The arrangement of the diffusion gradients was adapted to one extra 180 pulse and increased crusher gradients (Fig. 1A) The rest of the parameters were: TE/TR=76ms/1s, slice thickness=3mm, FOV=(192 mm) (FHxAP) resolution=(2 mm) b factors=0, 50, 600 s/mm, NSA=10, scan time=1:41 min. For the APT sequence, the same SG pulse replaced lipid suppression and slice selective excitation in a newly developed CEST 3D sequence [6] with gradient spin echo (GRASE) readout (Fig. 1B) The other parameters were: TE/TR=36 ms/2.5 s, FOV=160x160x40mm (APxFHxRL) resolution= 2x2x4mm; RF saturation=200msx4 with a B1 of 2 mT, full z spectrum at 26 offsets=M0, 0 6 ppm in 0.5 ppm step size, scan time=4:28 min. For MRS, a standard PRESS sequence was modified so that three slice selective refocusing pulses defined a volume and a frequency selective 90 pulse specified a desired spectral region to excite (Fig. 4A) (This sequence was denoted 'PRESS with Excitation Restriction' or PRESSER. The PRESSER sequence was intended to excite a small spectral window around the Choline (Cho) resonance frequency. As a biomarker for breast cancer, Cho is difficult to detect in healthy breast tissues. As we have yet to optimize the PRESSER sequence for Cho detection, it was used as a tool to validate the excitation profile of the SG pulse of the DWI and APT sequences in vivo. A sagittal volume was prescribed to match the coverage of DWI or APT