Time-harmonic magnetic resonance elastography of the normal feline brain

Abstract

Imaging of the mechanical properties of in vivo brain tissue could eventually lead to non-invasive diagnosis of hydrocephalus, Alzheimer's disease and other pathologies known to alter the intracranial environment. The purpose of this work is to (1) use time-harmonic magnetic resonance elastography (MRE) to estimate the mechanical property distribution of cerebral tissue in the normal feline brain and (2) compare the recovered properties of grey and white matter. Various in vivo and ex vivo brain tissue property measurement strategies have led to the highly variable results that have been reported in the literature. MR elastography is an imaging technique that can estimate mechanical properties of tissue non-invasively and in vivo. Data was acquired in 14 felines and elastic parameters were estimated using a globo-regional nonlinear image reconstruction algorithm. Results fell within the range of values reported in the literature and showed a mean shear modulus across the subject group of 7-8 kPa with all but one animal falling within 5-15 kPa. White matter was statistically stiffer (p < 0.01) than grey matter by about 1 kPa on a per subject basis. To the best of our knowledge, the results reported represent the most extensive set of estimates in the in vivo brain which have been based on MRE acquisition of the three-dimensional displacement field coupled to volumetric shear modulus image reconstruction achieved through nonlinear parameter estimation. However, the inter-subject variation in mean shear modulus indicates the need for further study, including the possibility of applying more advanced models to estimate the relevant tissue mechanical properties from the data.

Mechanical restriction of intracortical vessel dilation by brain tissue sculpts the hemodynamic response

“…Published estimates of bulk shear moduli for brain tissue span a wide range, from 1.5 to 19 kPa (Pattison et al 2010; Weaver et al 2012), yielding calculated Young’s moduli between 4–55 kPa. We used a value in the middle of this range, the average shear modulus measured in (Pattison et al 2010), 7 kPa, for our calculations, giving 20.3kPa for the Young’s modulus of brain tissue. The moduli obtained for brain tissue in vivo using magnetic resonance elastography are 10–100× larger than those obtained from ex vivo slices (Franze 2011).…”

Phantom evaluations of nonlinear inversion MR elastography

“…Magnetic resonance elastography (MRE) is an emerging imaging technique that quantifies mechanical properties of tissue, and has been applied to the breast [1, 2], liver [3, 4], heart [5, 6], and brain [7, 8]. For example, alterations in brain parenchyma caused by the demyelinating disease multiple sclerosis have been reported to decrease tissue stiffness.…”

Sample interval modulation for the simultaneous acquisition of displacement vector data in magnetic resonance elastography: theory and application

“…A recent study demonstrated that the acquisition of all displacement components of a multifrequency vibration is very useful for increasing the spatial resolution and the quality of MRE-derived elastograms by applying a new least square error-based reconstruction method to 3D MRE data (Hirsch et al 2013a). There have been other developments in both multifrequency and monofrequency MRE that propose the acquisition of three components of the displacement vector (Murphy et al 2011, Hirsch et al 2013b, Johnson et al 2013, Pattison et al 2010, Qin et al 2013, Hirsch et al 2013, Romano et al 2012, Yasar et al 2013a). …”

“…The array of images is then further processed to create elastograms. When performing MRE, depending on the specific approach, an acquisition block of four (Murphy et al 2011, Yin et al 2007) to eight (Pattison et al 2010, Yasar et al 2013b, Zhang et al 2011) individual phase-difference images is acquired in order to determine the complex wave image for one sensitization direction. For recording all components of the displacement vector, this acquisition block is typically repeated twice with MEGs applied along the remaining two coordinate axes—resulting in a total of 12–24 phase-difference images.…”