Time-resolved contrast-enhanced MR angiography with single-echo Dixon fat suppression

Abstract

Purpose: Dixon-based fat suppression has recently gained interest for dynamic contrast-enhanced MRI, but multi-echo techniques require longer scan times and reduce temporal resolution compared to single-echo alternatives without fat suppression. The purpose of this work is to demonstrate accelerated single-echo Dixon imaging with high spatial and temporal resolution.

Theory and methods: Real-valued water and fat images can be obtained from a single measurement if the shared initial phase and that due to ΔB0 are assumed known a priori. An expression for simultaneous sensitivity encoding (SENSE) unfolding and fat-water separation is derived for the general undersampling case, and simplified under the special case of uniform Cartesian undersampling. In vivo experiments were performed in extremities and brain with SENSE acceleration factors of up to R = 8.

Results: Single-echo Dixon reconstruction of highly undersampled data was successfully demonstrated. Dynamic contrast-enhanced water and fat images provided high spatial and temporal resolution dynamic images with image update times shorter than previous single-echo Dixon work.

Conclusion: Time-resolved contrast-enhanced MRI with single-echo Dixon fat suppression shows high image quality, improved vessel delineation, and reduced sensitivity to motion when compared to time-subtraction methods.

Keywords: contrast-enhanced MR angiography; CE-MRA; Dixon; constrained phase; fat-water; single echo.

Figure 1

(a) An N3 pCAPR acquisition and view-sharing scheme as used in this work. Schematics of the acquisition and reconstruction for time-resolved contrast-enhanced imaging with single-echo Dixon and time-subtraction are shown in (b) and (c), respectively. In this work, the same data from the contrast-enhanced acquisition was used for both the single-echo Dixon and time-subtraction reconstructions.

Figure 2

Relative SNR vs. TE for single-echo Dixon imaging at 3T assuming a 440 Hz chemical shift for fat excluding any effects of ${\mathrm{T}}_2^{\ast }$ decay. The maximum SNR corresponds to echo times that allow fat to dephase to odd multiples of π/2 (e.g. π/2, 3π/2, …). At 3.0T, a window of about ±0.2 ms around each optimal TE provides >85% of the max SNR.

Figure 3

MIPs (a, b) and individual coronal partitions (c, d) from the single-echo Dixon (a, c) and time-subtraction (b, d) CE-MRA reconstructions. Red arrowheads show a motion-induced subtraction artifact in the partitioned images in (d), which manifests as shading in (b). The Dixon partitions and MIPs (a, c) are unaffected. Yellow arrows show that small superficial subcutaneous veins are well seen in the Dixon CE-MRA images (d). Enlargements of portions of the images (dashed and dotted boxes) are shown in (e)–(h).

Figure 4

MIPs of a hand study reconstructed with single-echo Dixon (water images, a) and time subtraction (b). Enlargements of the portion of the images within the dashed box are shown in (c). Note the excellent depiction of the vasculature in both time series.

Figure 5

Brain images reconstructed with a single-echo Dixon reconstruction (a) and time subtraction (b). Note the high SNR of the single-echo Dixon images compared to the subtraction images, and good depiction of the cerebral vasculature. Enlargements of the portion within the dashed box from each of the enhanced time frames are shown in (c) and (d).

Figure 6

Brain MIPs reconstructed from data acquired with an echo time of TE=1.7ms (a) and TE=2.8ms (b). Both echo times provide near optimal SNR for single-echo Dixon, but the shorter echo time allows for a shorter TR and thus a shorter image update time (4.5 vs. 6 seconds) and more images acquired within the same window of time.