Brain mechanical property measurement using MRE with intrinsic activation

Abstract

Problem Addressed Many pathologies alter the mechanical properties of tissue. Magnetic resonance elastography (MRE) has been developed to noninvasively characterize these quantities in vivo. Typically, small vibrations are induced in the tissue of interest with an external mechanical actuator. The resulting displacements are measured with phase contrast sequences and are then used to estimate the underlying mechanical property distribution. Several MRE studies have quantified brain tissue properties. However, the cranium and meninges, especially the dura, are very effective at damping externally applied vibrations from penetrating deeply into the brain. Here, we report a method, termed ‘intrinsic activation’, that eliminates the requirement for external vibrations by measuring the motion generated by natural blood vessel pulsation. Methodology A retrospectively gated phase contrast MR angiography sequence was used to record the tissue velocity at eight phases of the cardiac cycle. The velocities were numerically integrated via the Fourier transform to produce the harmonic displacements at each position within the brain. The displacements were then reconstructed into images of the shear modulus based on both linear elastic and poroelastic models. Results, Significance and Potential Impact The mechanical properties produced fall within the range of brain tissue estimates reported in the literature and, equally important, the technique yielded highly reproducible results. The mean shear modulus was 8.1 kPa for linear elastic reconstructions and 2.4 kPa for poroelastic reconstructions where fluid pressure carries a portion of the stress. Gross structures of the brain were visualized, particularly in the poroelastic reconstructions. Intra-subject variability was significantly less than the inter-subject variability in a study of 6 asymptomatic individuals. Further, larger changes in mechanical properties were observed in individuals when examined over time than when the MRE procedures were repeated on the same day. Cardiac pulsation, termed intrinsic activation, produces sufficient motion to allow mechanical properties to be recovered. The poroelastic model is more consistent with the measured data from brain at low frequencies than the linear elastic model. Intrinsic activation allows MR elastography to be performed without a device shaking the head so the patient notices no differences between it and the other sequences in an MR examination.

Compression-sensitive magnetic resonance elastography

“…The principal component is directed towards the feet, since this is the only direction that is not obstructed by the cranium. This leads to the funnel-shaped pattern which is consistent with both the data acquired in this study and previous results by Greitz et al [29] and Weaver et al [100].…”

“…Further applications include prostate [39], female breast [53], brain [21,32,44,60,100], kidney [82], muscle [14,65] and additional preclinical research on animal models.…”

Intracranial biomechanics following cortical contusion in live rats

“…However, the time scale of changes is similar. Another novel MRE method eliminates the requirement for external vibrations by measuring the brain motions generated from blood vessel pulsations 32 but has not yet been applied to brain injury situations. When MRE becomes applicable to physiologically unstable patients such as brain trauma victims, it will be important to have data such as those presented herein.…”

Patient-Activated Three-Dimensional Multifrequency Magnetic Resonance Elastography for High-Resolution Mechanical Imaging of the Liver and Spleen

“…Potential applications of high clinical relevance include the kidney [40], prostate [41,42], and cervix [43,44]. A further perspective of abdominal 3D-MMRE is the noninvasive assessment of tissue pressure by evaluating volumetric strain [19] or poroelastic parameters [45]. Applications in patients are warranted for the ultimate determination of the clinical value of the proposed respiratory gating method.…”