Real‐time 4D phase unwrapping applied to magnetic resonance elastography

Barnhill, Eric; Kennedy, Paul; Johnson, Curtis L.; Mada, Marius; Roberts, Neil

doi:10.1002/mrm.25332

Cited by 37 publications

(37 citation statements)

References 32 publications

(51 reference statements)

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“…Any assessment of the performance of a phase unwrapping method is dependent on the features of the test object, as well as how the method is implemented and the choice of parameters such as thresholds, seed voxels etc. The reader is referred to References , and for other comparative evaluations in a broad range of contexts.…”

Section: Phase Unwrappingmentioning

confidence: 99%

An illustrated comparison of processing methods for MR phase imaging and QSM: combining array coil signals and phase unwrapping

et al. 2016

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Phase imaging benefits from strong susceptibility effects at very high field and the high SNR afforded by multi-channel coils. Combining the information from coils is not trivial, however, as the phase that originates in local field effects (the source of interesting contrast) is modified by the inhomogeneous sensitivity of each coil. This has historically been addressed by referencing individual coil sensitivities to that of a volume coil, but alternative approaches are required for ultra-high field systems in which no such coil is available. An additional challenge in phase imaging is that the phase which develops up to the echo time is “wrapped” into a range of 2π radians. Phase wraps need to be removed in order to reveal the underlying phase distribution of interest. Beginning with coil combination with a homogeneous reference volume coil - the Roemer approach – which can be applied at 3 T and lower field strengths, we review alternative methods for combining single-echo and multi-echo phase images where no such reference coil is available. These are applied to high resolution data acquired at 7 Tesla and their effectiveness assessed via an index of agreement between phase values over channels and the contrast-to-noise ratio in combined images. The Virtual Receiver Coil and ‘COMPOSER’ approaches were both found to be computationally efficient and effective. The main features of spatial and temporal phase unwrapping methods are reviewed, placing particular emphasis on recent developments in temporal phase unwrapping and Laplacian approaches. The features and performance of these are illustrated in application to simulated and high resolution in-vivo data. Temporal unwrapping was the fastest of the methods tested and Laplacian the most robust in images with low signal-to-noise ratio.

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Section: Phase Unwrappingmentioning

confidence: 99%

An illustrated comparison of processing methods for MR phase imaging and QSM: combining array coil signals and phase unwrapping

et al. 2016

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“…In the present analysis, we assumed that no phase unwrapping is applied prior to decoding. As multiple wave phase offsets per encoding direction are acquired, the unwrapping of at least one phase wrap is possible . As the encoding schemes only measure different projections of the displacement field, phase wrapping and subsequent unwrapping of each encoding direction can be applied prior to decoding.…”

Section: Discussionmentioning

confidence: 99%

“…As multiple wave phase offsets per encoding direction are acquired, the unwrapping of at least one phase wrap is possible. [44][45][46][47] As the encoding schemes only measure different projections of the displacement field, phase wrapping and subsequent unwrapping of each encoding direction can be applied prior to decoding. Hence, unwrapping equally extends the dynamic range of all the present schemes (π → (2n + 1)π in Equations 11 and 15, with n being the number of unwrappable phase wraps), and the DNR comparison, as well as the two discussed limits, remain valid in the presence of phase wraps.…”

Section: Discussionmentioning

confidence: 99%

Analysis and improvement of motion encoding in magnetic resonance elastography

et al. 2018

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Magnetic resonance elastography (MRE) utilizes phase contrast magnetic resonance imaging (MRI), which is phase locked to externally generated mechanical vibrations, to measure the three-dimensional wave displacement field. At least four measurements with linear-independent encoding directions are necessary to correct for spurious phase contributions if effects from imaging gradients are non-negligible. In MRE, three encoding schemes have been used: unbalanced four- and six-point and balanced four-point ('tetrahedral') encoding. The first two sensitize to motion with orthogonal gradients, with the four-point method acquiring a single reference scan without motion sensitization, whereas three additional scans with inverted gradients are used with six-point encoding, leading to two-fold higher displacement-to-noise ratio (DNR) and 50% longer scan duration. Balanced four-point (tetrahedral) encoding encodes along the four diagonals of a cube, with one direction serving as a reference for the other three encoding directions, similar to four-point encoding. The objective of this work is to introduce a theoretical framework to compare different motion sensitization strategies with respect to their motion encoding efficiency in two fundamental encoding limits, the gradient strength limit and the dynamic range limit, which are both placed in relation to conventional gradient recalled echo (GRE)- and spin echo (SE)-based MRE sequences. We apply the framework to the three aforementioned schemes and show that the motion encoding efficiency of unbalanced four- and six-point encoding schemes in the gradient-limited regime can be increased by a factor of 1.5 when using all physical gradient channels concurrently. Furthermore, it is demonstrated that reversing the direction of the reference in balanced four-point (tetrahedral) encoding results in the Hadamard encoding scheme, which leads to increased DNR by 2 compared with balanced four-point encoding and 2.8 compared with unbalanced four-point encoding. As an example, we show that optimal encoding can be utilized to reduce the acquisition time of standard liver MRE in vivo from four to two breath holds.

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“…MRE data analysis was performed using phase unwrapping46 and inversion software called ESP47 running in MatLab (MathWorks, Natick, MA) with several simplifying assumptions including viscoelasticity, local homogeneity, and mechanical isotropy. These assumptions allow calculation of the complex modulus G* from the temporally Fourier‐transformed 3D displacement field U using the Helmholtz‐type equation48:

G^{*} = - ρ ω^{2} \frac{U}{Δ U}

…”

Section: Methodsmentioning

confidence: 99%

MR elastography measurement of the effect of passive warmup prior to eccentric exercise on thigh muscle mechanical properties

Kennedy

Macgregor

Barnhill

et al. 2017

Magnetic Resonance Imaging

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PurposeTo investigate the effect of warmup by application of the thermal agent Deep Heat (DH) on muscle mechanical properties using magnetic resonance elastography (MRE) at 3T before and after exercise‐induced muscle damage (EIMD).Materials and MethodsTwenty male participants performed an individualized protocol designed to induce EIMD in the quadriceps. DH was applied to the thigh in 50% of the participants before exercise. MRE, T 2‐weighted MRI, maximal voluntary contraction (MVC), creatine kinase (CK) concentration, and muscle soreness were measured before and after the protocol to assess EIMD effects. Five participants were excluded: four having not experienced EIMD and one due to incidental findings.ResultsTotal workload performed during the EIMD protocol was greater in the DH group than the control group (P < 0.03), despite no significant differences in baseline MVC (P = 0.23). Shear stiffness |G*| increased in the rectus femoris (RF) muscle in both groups (P < 0.03); however, DH was not a significant between‐group factor (P = 0.15). MVC values returned to baseline faster in the DH group (5 days) than the control group (7 days). Participants who displayed hyperintensity on T 2‐weighted images had a greater stiffness increase following damage than those without: RF; 0.61 kPa vs. 0.15 kPa, P < 0.006, vastus intermedius; 0.34 kPa vs. 0.03 kPa, P = 0.06.ConclusionEIMD produces increased muscle stiffness as measured by MRE, with the change in |G*| significantly increased when T 2 hyperintensity was present. DH did not affect CK concentration or soreness; however, DH participants produced greater workload during the EIMD protocol and exhibited accelerated MVC recovery. Level of Evidence: 1 Technical Efficacy: Stage 2J. Magn. Reson. Imaging 2017;46:1115–1127.

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Real‐time 4D phase unwrapping applied to magnetic resonance elastography

Abstract: PhaseTools enables the substantial increase of signal-to-noise in MRE experiments at negligible additional computational cost. PhaseTools is freely released with this study, making robust real-time phase unwrapping available to any group using phase-based imaging.

Cited by 37 publications

References 32 publications

An illustrated comparison of processing methods for MR phase imaging and QSM: combining array coil signals and phase unwrapping

An illustrated comparison of processing methods for MR phase imaging and QSM: combining array coil signals and phase unwrapping

Analysis and improvement of motion encoding in magnetic resonance elastography

MR elastography measurement of the effect of passive warmup prior to eccentric exercise on thigh muscle mechanical properties

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