Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system

Abstract

Purpose Diffusion weighted imaging (DWI) is commonly limited by low signal‐to‐noise ratio (SNR) as well as motion artifacts. To address this limitation, a method that allows to maximize the achievable signal yield and increase the resolution in motion robust single‐shot DWI is presented. Methods DWI was performed on a 3T scanner equipped with a recently developed gradient insert (gradient strength: 200 mT/m, slew rate: 600 T/m/s). To further shorten the echo time, Stejskal‐Tanner diffusion encoding with a single‐shot spiral readout was implemented. To allow non‐Cartesian image reconstruction using such strong and fast gradients, the characterization of eddy current and concomitant field effects was performed based on field‐camera measurements. Results An echo time of only 19 ms was achieved for a b‐factor of 1000 s/mm2. An in‐plane resolution of 0.68 mm was encoded by a single‐shot spiral readout of 40.5 ms using 3‐fold undersampling. The resulting images did not suffer from off‐resonance artifacts and T2∗ blurring that are common to single‐shot images acquired with regular gradient systems. Conclusion Spiral diffusion imaging using a head gradient system, together with an accurate characterization of the encoding process allows for a substantial reduction of the echo time, and improves the achievable resolution in motion‐insensitive single‐shot DWI.

On the signal‐to‐noise ratio benefit of spiral acquisition in diffusion MRI

“…Besides the readout strategy, high‐amplitude gradients with high duty cycle are thus prime means of improving the SNR in dMRI 56‐59 . For instance, the TE of diffusion imaging with b = 1000 s/mm 2 has recently been brought down to 19 ms, using G max = 200 mT/m, slew‐rate max = 600 mT/m/ms, and spiral acquisition 9 …”

“…On the hardware side, this is best achieved by stronger gradients, which reduce the time required for diffusion encoding. [7][8][9] On the acquisition side, one widely used strategy is partial-Fourier encoding, which reduces the echo time (TE) of echo-planar imaging (EPI) readouts. The partial-Fourier approach is very effective at mitigating T 2 decay but has been reported to incur a certain degree of sensitivity to cardiac pulsation [10][11][12] and table vibration.…”

“…Most efforts to improve the SNR of dMRI aim to limit signal loss due to T 2 decay. On the hardware side, this is best achieved by stronger gradients, which reduce the time required for diffusion encoding 7‐9 . On the acquisition side, one widely used strategy is partial‐Fourier encoding, which reduces the echo time (TE) of echo‐planar imaging (EPI) readouts.…”

Distortion‐free, high‐isotropic‐resolution diffusion MRI with gSlider BUDA‐EPI and multicoil dynamic B0 shimming

“…This could be used together with the optimized shim procedure so that it would allow us to retain most of the shim performance when compared with the brain-only dynamic shimming. Future work will explore the use of ultrahigh performance gradients, such as in Foo et al 59 and Wilm et al, 60 to reduce the echo spacing, as well as the use of higher channel-count AC/DC shim array for further B 0 inhomogeneity mitigation. These improvements should further reduce the noise penalty and allow for higher per-shot acceleration, which would be necessary to limit T * F I G U R E 7 A, The 1/g-factor maps of single-shot EPI, BUDA-EPI without and with dynamic shim, and with perfect shim (no B 0 inhomogeneity), all at MB × R inplane = 2 × 5.…”

SNR-efficient distortion-free diffusion relaxometry imaging using ACcelerated Echo-train shifted EPTI (ACE-EPTI)

“…At high resolution, the benefit of TE reduction using echo-train shifting over the conventional acquisition is more prominent, since the long echo train before the spin-echo with large echo spacing will lead to long TE in ss-EPI. The benefit of minimizing the TE has also been studied in many previous works, such as center-out spiral (49,50), and center-out EPI (51). One challenge of using a center-out trajectory in conventional single-shot acquisition is that the fast T 2 * decay would lead to more image blurring that needs to be corrected.…”