TY - JOUR

T1 - Unipolar MR elastography

T2 - Theory, numerical analysis and implementation

AU - Guenthner, Christian

AU - Sethi, Sweta

AU - Troelstra, Marian

AU - van Gorkum, Robbert J.H.

AU - Gastl, Mareike

AU - Sinkus, Ralph

AU - Kozerke, Sebastian

PY - 2020/1/1

Y1 - 2020/1/1

N2 - In MR elastography (MRE), zeroth moment balanced motion-encoding gradients (MEGs) are incorporated into MRI sequences to induce a phase shift proportional to the local displacement caused by external actuation. To maximize the signal-to-noise ratio (SNR), fractional encoding is employed, i.e., the MEG duration is reduced below the wave period. Here, gradients encode primarily the velocity of the motion-reducing encoding efficiency. Thus, in GRE-MRE, T2* decay and motion sensitivity have to be balanced, imposing a lower limit on repetition times (TRs). We propose to use a single trapezoidal gradient, a “unipolar gradient”, to directly encode spin displacement. Such gradients cannot be used in conventional sequences as they exhibit a large zeroth moment and dephase magnetization. By time-reversing a spoiled SSFP sequence, the spoiling gradient becomes an efficient unipolar MEG. The proposed “unipolar MRE” technique benefits from this approach in three ways: first, displacement encoding is split over multiple TRs increasing motion sensitivity; second, spoiler and MEG coincide, allowing a reduction in TR; third, motion sensitivity of a typical unipolar lobe is of an order of magnitude higher than a bipolar MEG of equal duration. In this work, motion encoding using unipolar MRE is analyzed using the extended phase graph (EPG) formalism with a periodic motion propagator. As an approximation, the two-transverse TR approximation for diffusion-weighted SSFP is extended to incorporate cyclic motion. A complex encoding efficiency metric is introduced to compare the displacement fields of unipolar and conventional GRE-MRE sequences in both magnitude and phase. The derived theoretical encoding equations are used to characterize the proposed sequence using an extensive parameter study. Unipolar MRE is validated against conventional GRE-MRE in a phantom study showing excellent agreement between measured displacement fields. In addition, unipolar MRE yields significantly increased octahedral shear strain-SNR relative to conventional GRE-MRE and allows for the recovery of high stiffness inclusions, where conventional GRE-MRE fails.

AB - In MR elastography (MRE), zeroth moment balanced motion-encoding gradients (MEGs) are incorporated into MRI sequences to induce a phase shift proportional to the local displacement caused by external actuation. To maximize the signal-to-noise ratio (SNR), fractional encoding is employed, i.e., the MEG duration is reduced below the wave period. Here, gradients encode primarily the velocity of the motion-reducing encoding efficiency. Thus, in GRE-MRE, T2* decay and motion sensitivity have to be balanced, imposing a lower limit on repetition times (TRs). We propose to use a single trapezoidal gradient, a “unipolar gradient”, to directly encode spin displacement. Such gradients cannot be used in conventional sequences as they exhibit a large zeroth moment and dephase magnetization. By time-reversing a spoiled SSFP sequence, the spoiling gradient becomes an efficient unipolar MEG. The proposed “unipolar MRE” technique benefits from this approach in three ways: first, displacement encoding is split over multiple TRs increasing motion sensitivity; second, spoiler and MEG coincide, allowing a reduction in TR; third, motion sensitivity of a typical unipolar lobe is of an order of magnitude higher than a bipolar MEG of equal duration. In this work, motion encoding using unipolar MRE is analyzed using the extended phase graph (EPG) formalism with a periodic motion propagator. As an approximation, the two-transverse TR approximation for diffusion-weighted SSFP is extended to incorporate cyclic motion. A complex encoding efficiency metric is introduced to compare the displacement fields of unipolar and conventional GRE-MRE sequences in both magnitude and phase. The derived theoretical encoding equations are used to characterize the proposed sequence using an extensive parameter study. Unipolar MRE is validated against conventional GRE-MRE in a phantom study showing excellent agreement between measured displacement fields. In addition, unipolar MRE yields significantly increased octahedral shear strain-SNR relative to conventional GRE-MRE and allows for the recovery of high stiffness inclusions, where conventional GRE-MRE fails.

KW - diffusion-weighted imaging

KW - MR elastography

KW - sampling strategies

UR - http://www.scopus.com/inward/record.url?scp=85074941492&partnerID=8YFLogxK

U2 - 10.1002/nbm.4138

DO - 10.1002/nbm.4138

M3 - Article

C2 - 31664745

AN - SCOPUS:85074941492

SN - 0952-3480

VL - 33

JO - NMR in Biomedicine

JF - NMR in Biomedicine

IS - 1

M1 - e4138

ER -