Investig Magn Reson Imaging.  2019 Dec;23(4):328-340. 10.13104/imri.2019.23.4.328.

Generating Motion- and Distortion-Free Local Field Map Using 3D Ultrashort TE MRI: Comparison with Tâ‚‚* Mapping

• Jeong K
• Thapa B
• Han BS
• Kim D
• Jeong EK

• KMID: 2468050
• DOI: http://doi.org/10.13104/imri.2019.23.4.328

Abstract

PURPOSE
To generate phase images with free of motion-induced artifact and susceptibility-induced distortion using 3D radial ultrashort TE (UTE) MRI.
MATERIALS AND METHODS
The field map was theoretically derived by solving Laplace's equation with appropriate boundary conditions, and used to simulate the image distortion in conventional spin-warp MRI. Manufacturer's 3D radial imaging sequence was modified to acquire maximum number of radial spokes in a given time, by removing the spoiler gradient and sampling during both rampup and rampdown gradient. Spoke direction randomly jumps so that a readout gradient acts as a spoiling gradient for the previous spoke. The custom raw data was reconstructed using a homemade image reconstruction software, which is programmed using Python language. The method was applied to a phantom and in-vivo human brain and abdomen. The performance of UTE was compared with 3D GRE for phase mapping. Local phase mapping was compared with Tâ‚‚* mapping using UTE.
RESULTS
The phase map using UTE mimics true field-map, which was theoretically calculated, while that using 3D GRE revealed both motion-induced artifact and geometric distortion. Motion-free imaging is particularly crucial for application of phase mapping for abdomen MRI, which typically requires multiple breathold acquisitions. The air pockets, which are caught within the digestive pathway, induce spatially varying and large background field. Tâ‚‚* map, that was calculated using UTE data, suffers from non-uniform Tâ‚‚* value due to this background field, while does not appear in the local phase map of UTE data.
CONCLUSION
Phase map generated using UTE mimicked the true field map even when non-zero susceptibility objects were present. Phase map generated by 3D GRE did not accurately mimic the true field map when non-zero susceptibility objects were present due to the significant field distortion as theoretically calculated. Nonetheless, UTE allows for phase maps to be free of susceptibility-induced distortion without the use of any post-processing protocols.

Keyword

Ultrashort TE; UTE; Fieldmap; Phasemap; Quantitative susceptibility mapping; Susceptibility imaging

MeSH Terms

Abdomen
Artifacts
Boidae
Brain
Humans
Image Processing, Computer-Assisted
Magnetic Resonance Imaging*
Methods

Figure

• 

  Fig. 1. Numerical simulation of (a-d: top row) local field variation and (e-h: bottom row) geometric distortion in the typical FT MRI images for (a, b, e, f). a solid sphere and (c, d, g, h). a solid cylinder with Δχ = +16 ppm. The cylinder in (c, d, g, h) is oriented perpendicular to the external static magnetic field Bo.

• 

  Fig. 2. Pulse sequence diagram for a double FID acquisition. The smooth-varying spoke order is randomly shuffled so that the current readout gradient (spkn) plays as a spoiling gradient for the previous spoke (spkn-1). The flip angle is set to an Ernst angle for the T1 of the tissue of our interest.

• 

  Fig. 3. Front-end of GUI-based reconstruction software. The software is very flexible to set the reconstruction as well as the acquisition parameters. It can selectively reconstruct specific loop counters, such as receive channels, echoes, and repetitions. The current software is capable to reconstruct the UTE raw data from Siemens MRI system with VB and VD version.

• 

  Fig. 4. Magnitude and phase images of (a, b: top row) UTE with TE = 0.07 and 2.07 ms, and (c, d: bottom row) 3D GRE with TE = 1.6 and 3.6 ms on a phantom filled with MnCl2/water solution and aluminum and two plastic tubings from top to bottom. Note the distorted and asymmetric magnitude and phase images around the aluminum rod and motion-artifact in GRE images in (c, d), indicated by downward arrows.

• 

  Fig. 5. Local phase images of a phantom filled with corn starch gel with aluminum rod, a vial filled with high-concentration Gd-DTPA solution, multiple spherical air bubbles, measured using (a: Lt) UTE and (b: Rt) 3D GRE. Note that all local phases are symmetric in UTE phase maps as pointed by arrows, while they mimic the distorted images in Figure 1.

• 

  Fig. 6. (a, c: first and third columns) Local phase maps, and (b, d: second and forth columns) T2* maps of in-vivo brain and abdomen of a healthy volunteer. The same data set with TE = 0.07, 2.07, and 4.07 ms was used for T2* and TE = 0.07 and 4.07 ms for local phase mappings. Non-uniformity in the T2* map is clearly visible at the frontal lobe brain (b) and upper part of liver close to lung space and kidney (d).

• 

  Fig. 7. (a: Lt) Local phase and (b: Rt) QSM maps of abdomen from a healthy volunteer using UTE with nSpks = 131,072, TR = 3.60 ms, TE = 0.070, 2.57 ms, 1 mm3, α = 4.5°. QSM map was calculated using the FT method with Fth = 2.

• 

  Fig. 8. Local field maps of a kidney dialysis patient in (a: Lt) sagittal, (b: Mid) coronal, (c: Rt) axial planes.

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