Spatiotemporal Encoding (SPEN) Based Breast Diffusion MRI at Sub-Millimeter Resolution with High Immunity to Artifacts
1Radiology, Sheba Medical Center, Israel
2Chemical Physics, Weizmann Institute of Science, Israel
PURPOSE: To design and clinically examine spatio-temporal encoding (SPEN) based pulse sequence capable of delivering full-coverage high-resolution ultra-fast diffusion breast images and to compare it vis-à-vis SE-EPI reference.
METHODS: This IRB-approved study included 5 healthy volunteers and 21 breast cancer patients. Axial images were acquired at 3T (Siemens, Trio) using a 4-channels breast coil. MRI protocol included T2-weighted, subtracted dynamic-contrast-enhanced T1-weighted and two diffusion sequences comparing twice-refocused SE-EPI, against the novel double-bipolar SPEN sequence. Several developments were implemented in the SPEN protocol, namely: a) multi band pulses covering both breasts in a single shot, resulting in shorter scan times with improved signal-noise-ratio; b) multi-shot SPEN interleaving along the low-bandwidth axis in a reference-less, motion-immune fashion; and c) partial Fourier transform along the readout. The first of these features enables low-SAR full-organ coverage, while the latter two yield sub-millimeter resolution. Diffusion measurements were applied using 500 s/mm2 b-value, applied in 3 orthogonal and 12 directions. Spatial resolutions were 2.0×2.0×2.5 mm (for both breasts) with single-shot DWI SPEN and SE-EPI, and 0.66×0.87×1 mm when SPEN was set to perform “zoomed” single breast DTI mapping. SPEN ADC maps were obtained after suitably correcting the b-values to account for all the non-diffusion-related, imaging gradients.
RESULTS: Comparisons between the DW-images confirmed that many artifacts arising in SE-EPI were systematically diminished in SPEN. Several examples showed that unlike SE-EPI, the b-zero images clearly demonstrated that SPEN MRI was free from axial artifacts and from ghosting problems surrounding and overlapping with the breast’s regions of interest, thanks to its use of a relatively high acquisition bandwidth. Also noticed was SPEN’s ability to separate fat from fibroglandular tissues, and its better representation of the anatomical features when compared against a T2-image reference. For both SE-EPI and SPEN, ADC maps obtained after masking background signal and non-fibro-glandular tissues clearly illustrate the lesions with their typically slower diffusion values. In the higher resolution images arising from SPEN vs conventional resolution SE-EPI DWI, SPEN images preserved the original shape of the lesion and consequently, SPEN’s ADC values became more reliable and faithful to the anatomy than SE-EPI’s.
CONCLUSIONS: Extensive results demonstrate SPEN’s new capabilities to deliver reliable ADC maps at sub-mm resolution using conventional imaging acquisition hardware, while showing significantly higher image qualities and reduction of artifacts than SE-EPI. Reductions in ADCs due to the presence of malignancies can then be rapidly and non-invasively diagnosed by single-shot or interleaved SPEN MRI.