Cardiac-gated steady-state multifrequency magnetic resonance elastography of the brain: Effect of cerebral arterial pulsation on brain viscoelasticity

Abstract

In-vivo brain viscoelasticity measured by magnetic resonance elastography (MRE) is a sensitive imaging marker for long-term biophysical changes in brain tissue due to aging and disease; however, it is still unknown whether MRE can reveal short-term periodic alterations of brain viscoelasticity related to cerebral arterial pulsation (CAP). We developed cardiac-gated steady-state MRE (ssMRE) with spiral readout and stroboscopic sampling of continuously induced mechanical vibrations in the brain at 20, 31.25, and 40 Hz frequencies. Maps of magnitude |G*| and phase ϕ of the complex shear modulus were generated by multifrequency dual visco-elasto inversion with a temporal resolution of 40 ms over 4 s. The method was tested in 12 healthy volunteers. During cerebral systole, |G*| decreased by 6.6 ± 1.9% (56 ± 22 Pa, p < 0.001, mean ± SD), whereas ϕ increased by 0.5 ± 0.5% (0.006 ± 0.005 rad, p = 0.002). The effect size of CAP-induced softening slightly decreased with age by 0.10 ± 0.05% per year ( p = 0.04), indicating lower cerebral vascular compliance in older individuals. Our data show for the first time that the brain softens and becomes more viscous during systole, possibly due to an effect of CAP-induced arterial expansion and increased blood volume on effective-medium tissue properties. This sensitivity to vascular-solid tissue interactions makes ssMRE potentially useful for detection of cerebral vascular disease.

Real‐time MR elastography for viscoelasticity quantification in skeletal muscle during dynamic exercises

“…Unlike in our previous work, we captured shear waves only at a single‐drive frequency. Therefore, multifrequency inversion as proposed by Guo et al could not be applied .…”

“…To this end, the method captures 2 components of harmonic motion fields in an interleaved fashion by utilizing the principle of stroboscopic undersampling of harmonic vibrations (SUV). SUV was recently proposed for MRE of the brain as a method of sampling relatively fast tissue oscillations by a sequence of relatively slow encoding steps . Sampling of harmonic vibrations with a frequency larger than the frame rate of the sequence causes aliasing.…”

“…However, in SUV the aliased frequency is well defined because both sequence timing and the harmonic vibration period are precisely known. In our earlier work on SUV in the brain, we performed k‐space acquisition across several shots in conjunction with cardiac triggering, which required additional resynchronization of the encoding sequence and external motion …”

“…SUV was recently proposed for MRE of the brain as a method of sampling relatively fast tissue oscillations by a sequence of relatively slow encoding steps. 31 Sampling of harmonic vibrations with a frequency larger than the frame rate of the sequence causes aliasing. However, in SUV the aliased frequency is well defined because both sequence timing and the harmonic vibration period are precisely known.…”

“…In our earlier work on SUV in the brain, we performed k-space acquisition across several shots in conjunction with cardiac triggering, which required additional resynchronization of the encoding sequence and external motion. 31 In the present work, we extend the concept of SUV to ungated single-shot MRE, allowing us to capture nonperiodic viscoelasticity changes during dynamic voluntary activation of skeletal muscles. Our hypothesis is that we can measure the changes in viscoelastic parameters induced in different groups of skeletal muscles of the lower extremity upon plantar flexion and dorsiflexion.…”