Unipolar MR elastography: Theory, numerical analysis and implementation

Abstract

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.

Accelerated multislice MRI with patterned excitation

“…This has been exploited with techniques like DENSE (displacement encoding with stimulated echoes) and SSFP for the measurement of brain pulsations and elastography. 8,[12][13][14] Next we show that this may be more efficiently done with the PME concept using an SE approach (Figure 1B).…”

“…The use of large crusher gradients in SE and STE MRI also increases sensitivity to subtle tissue displacement and has particularly strong effects on image phase. This has been exploited with techniques like DENSE (displacement encoding with stimulated echoes) and SSFP for the measurement of brain pulsations and elastography 8,12–14 . Next we show that this may be more efficiently done with the PME concept using an SE approach (Figure 1B).…”

“…On the other hand, SSFP (steady state free precession) is a class of techniques [4][5][6][7][8][9] that performs both functions concurrently and has improved time-efficiency; however, the contrast generated is often more difficult to interpret and generally suboptimal. This is in part due to the various types of NMR echoes that contribute to the SSFP signal and determine image contrast.…”

Viscoelasticity Imaging of Biological Tissues and Single Cells Using Shear Wave Propagation

“…On the acquisition side, significant amount of effort has been put into MR sequence developments to reduce scan time while preserving 3D motion encoding and signal amplitude. Although equipping an MR sequence with bipolar magnetic field gradients prevails, recent implementations took advantage of MR sequence inherent gradients to encode motion, thus keeping the timing shorter than conventional use of MEG [397]. As in any MRI scan, artefacts may occur due to patient motion.…”