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Unprecedented SNR Efficiency in Prostate DWI By Combining Ultra-Strong Gradients and Spiral Readouts1Cardiff University Brain Research Imaging Centre, Cardiff University, Cardiff, United Kingdom, 2Medical Radiation Physics, Lund University, Lund, Sweden, 3Leeds Institute of Cardiovascular and Metabolic Medicine, University of Leeds, Leeds, United Kingdom, 4Siemens Healthcare Ltd, Camberly, United Kingdom, 5Siemens Healthcare GmbH, Erlangen, Germany, 6Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands
Synopsis
Keywords: Diffusion Acquisition, Prostate, Field monitoring
Motivation: Prostate DWI with high b-values holds promise for microstructural tissue characterization but is notoriously SNR-deprived.
Goal(s): (i) To boost the SNR of prostate DWI; (ii) to reduce artefacts resulting from scanner imperfections that are exacerbated by the methods used to increase the SNR.
Approach: Spirals and strong gradients (≤300mT/m) are used to attain short TEs and thus high SNR. An expanded encoding model including measured static and dynamic fields is deployed to obtain high image quality.
Results: The approach is demonstrated in a healthy subject and a patient diagnosed with prostate cancer. It delivers higher SNR and improved cancerous lesion conspicuity.
Impact: We provide the demonstration of prostate DWI with ultra-strong gradients and spiral readouts. Using high b-values and short echo times enhances lesion conspicuity and holds potential for early and non-invasive disease detection.
Introduction
Ultra-high b-value DWI offers exceptional MRI contrast for in-depth characterization of tissue microstructure, holding promise for advancing the early detection of prostate cancer (PCa)1,2 and, in turn, reduction of biopsies3. Unfortunately, clinical DWI currently only allows for low b-values due to limited gradient amplitudes leading to prolonged echo times (TE) and thus low signal-to-noise ratio (SNR), - all of which limit diagnostic efficacy. Advancements in gradient hardware4–9 enabled probing brain tissue properties at different diffusion time scales at higher SNR10–12, resulting in robust characterisation of key brain tissue properties13–17. Besides, spiral trajectories18 allow for shorter TE than conventional EPI19, resulting in additional SNR increase20. This work demonstrates considerable improvement of prostate DWI by combining 300mT/m gradients for diffusion-encoding21 with spiral readouts for spatial encoding and accounting for field perturbations in the image reconstruction22.Methods
Participants: Ethical approval for the study was obtained. One healthy control (51 yo) and one patient (53 yo, GS 3+3 PCa) were scanned with written consent.Data acquisition: Images were acquired on a 3T Connectom research-only scanner (Siemens Healthcare, Erlangen, Germany).
Multi-echo GRE images were acquired for the estimation of static B0 maps and receive coil sensitivities (voxel size = 3.07 x 3.07 x 5mm3).
A PGSE sequence with the flexibility of using arbitrary readouts ("research sequence") (Fig. 1A) was used to acquire DWI along 15 non-collinear directions distributed on a sphere at b = [0, 0.05, 0.5, 1.5, 2, 3]ms/μm2 (Gmax = 273mT/m, SRmax = 110T/m/s, δ = 5ms, ∆ = 25ms) and TEs of 53 and 35ms for EPI and spiral, respectively, TR = 3s, voxel volume = 1.3 x 1.3 x 5mm3, and 18 slices. EPI and spiral readouts were matched in length and k-space coverage (Fig. 1B.; for both: Gmax = 39.11mT/m, SRmax=186.24T/m/s, for EPI: undersampling factor R = 2, partial Fourier factor = 6/8, phase-encoding = anterior-posterior, and for spiral: R = 2.24). In addition, DWI with the vendor’s PGSE EPI sequence were acquired. Due to limits in the number of samples per segment across sequence implementations, dwell times were: 2.80µs (EPI), 2.70µs (spiral), and 1.6µs (vendor’s EPI). Each DWI protocol took 4 minutes, 45sec.
Structural MRI scans were acquired using a 2D T2-weighted TSE sequence in the axial plane (voxel-size = 0.625 x 0.625 x 3mm3). The magnetic field dynamics were monitored using an NMR field-camera (Skope Magnetic Resonance Technologies)23.
Data reconstruction: The DW EPI data acquired with the vendor’s PGSE sequence were reconstructed using a GRAPPA-based reconstruction (vendor). The data acquired with the research sequence were reconstructed using an expanded encoding model 22,24 (SENSE-based approach)25 including higher order field dynamics (up to 3rd-order spherical harmonics and 2nd-order concomitant fields26,27) and static B0-inhomogeneities (skope-i).
Data processing: Correction of gradient-non-uniformity in images28,29 and B-matrices30.
Analysis: SNR and SNR gain of spiral over EPI was calculated using the 'pseudo multiple replica method'20,31. Mean diffusivity (MD), fractional anisotropy (FA), mean, axial and radial kurtosis (MK,AK and RK) were computed32.
Results
Fig. 2. shows mean b = 0ms/µm2 images. Fine anatomical features are sharper when accounting for measured field perturbations in the reconstruction (2nd and 3rd column) and align better with the morphology in the T2-weighted image.The SNR maps (Fig. 3) confirm that employing the spiral readout as an alternative to EPI results in higher SNR. The median SNRgain ranges between 30% and 44%, depending on the b-value. The SNR gain was even higher in the PCa lesion.
The powder-averaged DW-images for both subjects (Fig. 4) show enhanced tissue-to-background contrast across all b-values in spiral compared to EPI. The conspicuity of cancerous lesions in the PCa patient is improved on high b-value images (4th to 6th column).
Quantitative maps obtained from DKI (Fig. 5.) show more noise-bias when obtained from dMRI with EPI than with spirals (e.g., elevated MD, higher FA in the transitional zone), but nevertheless excellent conspicuity of the cancerous lesion regardless of the readout employed (lower MD, and higher MK, AK, and, minorly, RK).
Discussion & Conclusions
We devised advanced field sensing and image reconstruction techniques for the prostate. Thereby, we achieved high DWI quality even at high b-values. We deployed these methods for spiral readouts, which provide shorter TEs and thus higher SNR20,33. The combination of strong gradients and spiral readouts unlocks sampling at short diffusion times and short TEs and can potentially lead to improved differentiation between low GS cancer and healthy tissue34,35 and to accurate characterisation of short T2 compartments (e.g., stroma)36. As stronger gradients and alternative readouts become more readily available, they can enable comprehensive characterization of the prostate in multidimensional space36–39.Acknowledgements
This work was supported by a Wellcome Trust Investigator Award (096646/Z/11/Z), a Wellcome Trust Strategic Award (104943/Z/14/Z), an EPSRC equipment grant (EP/M029778/1), and Siemens Healthcare Limited grant to DKJ. CMWT is supported by a Sir Henry Wellcome Fellowship (215944/Z/19/Z) and a Veni grant (17331) from the Dutch Research Council (NWO).
The authors thank Ralph Kimmlingen, Radhouene Neji, Andrew Dewdney, Eva Eberlein and Ludwig Eberler (Siemens Healthineers), Christian Mirkes and Bertram Wilm (Skope Magnetic Resonance Technologies), Filip Szczepankiewicz (Medical Radiation Physics, Lund University), and Kieran Foley MD (School of Medicine, Cardiff University) for technical and scientific support.
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Figures
Figure 1:
A) Sketch of the research-purpose PGSE sequences: Single-shot PF EPI and single-shot spiral. The vendor’s sequence has additional navigators for phase correction40 (light grey dashed lines). ADC & RF: Spectral fat saturation, excitation and refocusing pulses and ADC. G: Slice selective and refocusing (light grey), diffusion (maroon), and readout gradients (EPI and spiral in teal and violet, respectively), trigger for dynamic field camera (Tx, dark grey).
B) Parametric view of readout trains with matched k-space area (teal + yellow areas = purple area).
