Ultrashort echo time and zero echo time MRI at 7T

Abstract

Objective: Zero echo time (ZTE) and ultrashort echo time (UTE) pulse sequences for MRI offer unique advantages of being able to detect signal from rapidly decaying short-T2 tissue components. In this paper, we applied 3D ZTE and UTE pulse sequences at 7T to assess differences between these methods.

Materials and methods: We matched the ZTE and UTE pulse sequences closely in terms of readout trajectories and image contrast. Our ZTE used the water- and fat-suppressed solid-state proton projection imaging method to fill the center of k-space. Images from healthy volunteers obtained at 7T were compared qualitatively, as well as with SNR and CNR measurements for various ultrashort, short, and long-T2 tissues.

Results: We measured nearly identical contrast-to-noise and signal-to-noise ratios (CNR/SNR) in similar scan times between the two approaches for ultrashort, short, and long-T2 components in the brain, knee and ankle. In our protocol, we observed gradient fidelity artifacts in UTE, and our chosen flip angle and readout also resulted in shading artifacts in ZTE due to inadvertent spatial selectivity. These can be corrected by advanced reconstruction methods or with different chosen protocol parameters.

Conclusion: The applied ZTE and UTE pulse sequences achieved similar contrast and SNR efficiency for volumetric imaging of ultrashort-T2 components. Key differences include that ZTE is limited to volumetric imaging, but has substantially reduced acoustic noise levels during the scan. Meanwhile, UTE has higher acoustic noise levels and greater sensitivity to gradient fidelity, but offers more flexibility in image contrast and volume selection.

Keywords: Magnetic resonance imaging; Musculoskeletal system; Neuroimaging.

Figure 1

(a,b) A single TR of the 3D UTE and ZTE pulse sequences used, with the corresponding k-space coverage. The missing central k-space in ZTE was filled using low-resolution projections (9). Sequence timing diagram for the (c) knee, ankle, and (d) brain studies including periodically applied fat suppression and inversion recovery pulses.

Figure 2

Resolution phantom experiments acquired with different center frequencies. These demonstrate the off-resonance sensitivity of both techniques, where phase accumulation during the radial readouts results in an isotropic blurring and ringing artifact. This can be seen by the increased signal in the center of the resolution phantom circles, which should be void of signal. These artifacts are nearly identical between ZTE and UTE.

Figure 3

In vivo brain imaging results in a healthy volunteer at 1.1 mm isotropic resolution with fat-suppression pulses and IR preparation. UTE and ZTE demonstrated similar gray/white matter contrast. Both detected signal from the cortical bone in the skull (dashed orange arrows). Signal was also seen from the RF coil and foam padding (wide green arrows), with typically more signal in the ZTE images. In this study, ZTE suffered from some shading and signal dropout artifacts near the edges of the FOV (thin red arrows), although this can be alleviated through improved sequence and reconstruction methods as discussed in the text.

Figure 4

In vivo ankle imaging results in a healthy volunteer at 0.7 mm isotropic resolution with fat-suppression pulses. UTE and ZTE demonstrated similar contrast for ultrashort-T2 components in tendons (dashed yellow arrows). In particular, both showed a fascicular structure in the Achilles tendon (dashed orange arrows). ZTE showed increased signal from foam padding (wide green arrows). Some failure of the fat suppression pulses was observed, particularly in the heel (thin red arrows).

Figure 5

In vivo knee imaging results in a healthy volunteer at 0.8 mm isotropic resolution with fat-suppression pulses. UTE and ZTE demonstrated similar contrast for connective tissues such as the patellar ligament (dashed orange arrows) as well as for other ligaments, cartilage, and the meniscus. Similarly to the brain results (Fig. 3), ZTE suffered from some shading and signal dropout artifacts (thin red arrows). Methods for correcting these artifacts are included in the Discussion.

Figure 6

Sample raw data from UTE and ZTE acquisitions in the (a) brain, (b) ankle, and (c) knee. The k-space data is normalized to pixel values, such that the maximum extent in k-space is ±0.5 1/pixel, and only up to a radius of 0.1 is shown. The ZTE data includes the low-resolution projection data required to fill in the center of k-space for the WASPI technique up to a radius of k ≈ 0.01 1/pixel.

Figure 7

SNR and CNR comparison of different tissue types between the ZTE and UTE images in the brain (N=5), ankle (N=5) and knee (N=3). There was no statistically significant difference between ZTE and UTE SNR (p > 0.05) for all tissues shown except for White Matter (p < 0.05). There was no statistically significant difference between the ZTE and UTE CNRs we examined (p > 0.05).

Figure 8

In vivo brain imaging results in multiple sclerosis patients. The IR SPGR images were acquired at 1 mm isotropic resolution, while the IR UTE and ZTE were acquired at 1.1 mm isotropic resolution with fat-suppression pulses and IR preparation (same parameters as the volunteer images, e.g. in Fig. 3). The UTE and ZTE images clearly depict suspicious lesions (arrows).

Figure 9

Theoretical SNR of the UTE and ZTE sequences used in this study with transmit/receive switching delays (“T/R delay”) of 70 μs (UTE) and 8 μs (ZTE), as well for an optimized UTE sequence with 8 μs delay. This point spread function simulation accounts for differences due to T2 relaxation during this delay and the readout. In the sub-millisecond T2 regime, there are a larger expected ZTE signals compared to the UTE sequence used in this study, but this expected increase becomes much small with an optimized UTE sequence.