Viscoelastic property detection by elastic displacement NMR measurements

Magnetic Resonance Elastography of the Brain

Magnetic resonance elastography (MRE) is an emerging noninvasive technique, an alternative to palpation for quantitative assessment of biomechanical properties of tissue. In MRE, tissue stiffness information is obtained by a 3-step process, propagating mechanical waves in the tissues, measuring the wave propagation using modified magnetic resonance (MR) pulse sequences, and generating the quantitative stiffness maps from the MR images. MRE is clinically used in patients with liver diseases, whereas its applications in other organs are still being investigated. At present, the pediatric studies are in the initial stage and preliminary results promise to provide additional information about tissue characteristics.

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

SampLe Interval Modulation-magnetic resonance elastography (SLIM-MRE) is introduced for simultaneously encoding all three displacement projections of a monofrequency vibration into the MR signal phase. In SLIM-MRE, the individual displacement components are observed using different sample intervals. In doing so, the components are modulated with different apparent frequencies in the MR signal phase expressed as a harmonic function of the start time of the motion encoding gradients and can thus be decomposed by applying a Fourier transform to the sampled multidirectional MR phases. In this work, the theoretical foundations of SLIM-MRE are presented and the new idea is implemented using a high field (11.7 T) vertical bore magnetic resonance imaging system on an inhomogeneous agarose gel phantom sample. The local frequency estimation-derived stiffness values were the same within the error margins for both the new SLIM-MRE method and for conventional MRE, while the number of temporally-resolved MRE experiments needed for each study was reduced from three to one. In this work, we present for the first time, monofrequency displacement data along three sensitization directions that were acquired simultaneously and stored in the same k-space.

Review of MR elastography applications and recent developments

The technique of MR elastography (MRE) has emerged as a useful modality for quantitatively imaging the mechanical properties of soft tissues in vivo. Recently, MRE has been introduced as a clinical tool for evaluating chronic liver disease, but many other potential applications are being explored. These applications include measuring tissue changes associated with diseases of the liver, breast, brain, heart, and skeletal muscle including both focal lesions (e.g., hepatic, breast, and brain tumors) and diffuse diseases (e.g., fibrosis and multiple sclerosis). The purpose of this review article is to summarize some of the recent developments of MRE and to highlight some emerging applications.

Non-invasive measurement of brain viscoelasticity using magnetic resonance elastography

The purpose of this work was to develop magnetic resonance elastography (MRE) for the fast and reproducible measurement of spatially averaged viscoelastic constants of living human brain. The technique was based on a phase-sensitive echo planar imaging acquisition. Motion encoding was orthogonal to the image plane and synchronized to intracranial shear vibrations at driving frequencies of 25 and 50 Hz induced by a head-rocker actuator. Ten time-resolved phase-difference wave images were recorded within 60 s and analyzed for shear stiffness and shear viscosity. Six healthy volunteers (six men; mean age 34.5 years; age range 25-44 years) underwent 23-39 follow-up MRE studies over a period of 6 months. Interindividual mean +/- SD shear moduli and shear viscosities were found to be 1.17 +/- 0.03 kPa and 3.1 +/- 0.4 Pas for 25 Hz and 1.56 +/- 0.07 kPa and 3.4 +/- 0.2 Pas for 50 Hz, respectively (P < or = 0.01). The intraindividual range of shear modulus data was 1.01-1.31 kPa (25 Hz) and 1.33-1.77 kPa (50 Hz). The observed modulus dispersion indicates a limited applicability of Voigt's model to explain viscoelastic behavior of brain parenchyma within the applied frequency range. The narrow distribution of data within small confidence intervals demonstrates excellent reproducibility of the experimental protocol. The results are necessary as reference data for future comparisons between healthy and pathological human brain viscoelastic data.

Fractional encoding of harmonic motions in MR elastography

In MR elastography (MRE) shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency fv as the mechanical vibration. As a result, both the repetition time (TR) and echo time (TE) of such an MRE sequence are greater than the vibration period 1/fv. This causes long acquisition times and considerable signal dephasing in tissue with short transverse relaxation times. Here we propose a reverse concept with TR<or=1/fv which we call "fractional" MRE, i.e., only a fraction of one vibration cycle per TR, can be used for motion sensitization. The benefit of fractional MRE is twofold: 1) acquisition times in seconds can be achieved for a single-phase difference wave image, and 2) materials that combine low elasticity, high viscosity, and short T2* relaxation times show an increased phase-to-noise ratio (PNR). A twofold increase of the phase signal is predicted for liver-like materials. Volunteer studies performed in liver and biceps show the benefit of fractional MRE. Furthermore, we demonstrate the feasibility of the technique for in vivo myocardial MRE by visualizing transverse wave propagation in the interventricular septum (IVS).

Nonlinear viscoelastic properties of tissue assessed by ultrasound

A technique to assess qualitatively the presence of higher-order viscoelastic parameters is presented. Low-frequency, monochromatic elastic waves are emitted into the material via an external vibrator. The resulting steady-state motion is detected in real time via an ultra fast ultrasound system using classical, one-dimensional (1-D) ultrasound speckle correlation for motion estimation. Total data acquisition lasts only for about 250 ms. The spectrum of the temporal displacement data at each image point is used for analysis. The presence of nonlinear effects is detected by inspection of the ratio of the second harmonics amplitude with respect to the total amplitude summed up to the second harmonic. Results from a polyacrylamide-based phantom indicate a linear response (i.e., the absence of higher harmonics) for this type of material at 65 Hz mechanical vibration frequency and about 100 microm amplitude. A lesion, artificially created by injection of glutaraldehyde into a beef specimen, shows the development of higher harmonics at the location of injection as a function of time. The presence of upper harmonics is clearly evident at the location of a malignant lesion within a mastectomy.

Shear wave group velocity inversion in MR elastography of human skeletal muscle

In vivo quantification of the anisotropic shear elasticity of soft tissue is an appealing objective of elastography techniques because elastic anisotropy can potentially provide specific information about structural alterations in diseased tissue. Here a method is introduced and applied to MR elastography (MRE) of skeletal muscle. With this method one can elucidate anisotropy by means of two shear moduli (one parallel and one perpendicular to the muscle fiber direction). The technique is based on group velocity inversion applied to bulk shear waves, which is achieved by an automatic analysis of wave-phase gradients on a spatiotemporal scale. The shear moduli are then accessed by analyzing the directional dependence of the shear wave speed using analytic expressions of group velocities in k-space, which are numerically mapped to real space. The method is demonstrated by MRE experiments on the biceps muscle of five volunteers, resulting in 5.5+/-0.9 kPa and 29.3+/-6.2 kPa (P<0.05) for the medians of the perpendicular and parallel shear moduli, respectively. The proposed technique combines fast steady-state free precession (SSFP) MRE experiments and fully automated processing of anisotropic wave data, and is thus an interesting MRI modality for aiding clinical diagnosis.

MRI virtual biopsies: analysis of an explanted endovascular device and perspectives for the future

Information that can be obtained by magnetic resonance imaging (MRI) of explanted endovascular devices must be validated as this method is non-destructive. Histology of such a device together with its encroached tissues can be elegantly performed after polymethymethacrylate (PMMA) embedding, but this approach requires destruction of the specimen. The issue is therefore to determine if the MRI is sufficient to fully validate an explanted device based upon the characterization of an explanted specimen. An AneuRx device deployed percutaneously 25 months earlier in a 75-year-old patient was removed en bloc at autopsy together with the surrounding aneurysmal sac and segments of the upstream and downstream arteries. Macroscopic pictures were taken and a slice of the cross-section was processed for histology after polymethylmethacrylate (PMMA) embedding. For the magnetic resonance imaging investigation, the device was inserted in a Biospec 4.7 T MRI system with a 20 mm diameter birdcage resonator used for both emission and reception. A Spin-Echo (SE) was used to acquire both T1 proton density (PD) and T2 weighted images. A gradient-echo (GE) sampling of a free induction decay (GESFID) was used to generate multiple GE images using a single excitation pulse so that four images at different TE were obtained in the same acquisition. The selected explanted device was outstandingly well-healed compared to most devices harvested from humans. No inflammatory process was observed in contact or at distance of the materials. In MRI T1 images display no specific contrast and were homogeneous in the different tissues. The contrast was improved on proton density weighed images. On the T2 weighed images, the different areas were well identified. The diffusion images displayed in the surrounding B region had the greatest diffusion coefficient and the greatest anisotropy. The MRI analysis of the explanted AneuRx device illustrates the possibilities of this technique to characterize the interaction of the endovascular graft with the surrounding tissues. MRI is a breakthrough to investigate explanted medical devices but it also can be advantageously used in vivo to obtain virtual biopsies, because real biopsies to determine the 3 Bs (biocompatibility, biofunctionality and bioresilience) cannot be carried out as they could obviously initiate infection and degradation of the foreign materials.

Imaging anisotropic and viscous properties of breast tissue by magnetic resonance-elastography

MR-elastography is a new technique for assessing the viscoelastic properties of tissue. One current focus of elastography is the provision of new physical parameters for improving the specificity in breast cancer diagnosis. This analysis describes a technique to extend the reconstruction to anisotropic elastic properties in terms of a so-called transversely isotropic model. Viscosity is treated as being isotropic. The particular model chosen for the anisotropy is appealing because it is capable of describing elastic shear anisotropy of parallel fibers. The dependence of the reconstruction on the particular choice of Poisson's ratio is eliminated by extracting the compressional displacement contribution using the Helmholtz-Hodge decomposition. Results are presented for simulations, a polyvinyl alcohol breast phantom, excised beef muscle, and measurements in two patients with breast lesions (invasive ductal carcinoma and fibroadenoma). The results show enhanced anisotropic and viscous properties inside the lesions and an indication for preferred fiber orientation.

Detection of acoustic waves by NMR using a radiofrequency field gradient

A B(1) field gradient-based method previously described for the detection of mechanical vibrations has been applied to detect oscillatory motions in condensed matter originated from acoustic waves. A ladder-shaped coil generating a quasi-constant RF-field gradient was associated with a motion-encoding NMR sequence consisting in a repetitive binomial 13;31; RF pulse train (stroboscopic acquisition). The NMR response of a gel phantom subject to acoustic wave excitation in the 20-200 Hz range was investigated. Results showed a linear relationship between the NMR signal and the wave amplitude and a spectroscopic selectivity of the NMR sequence with respect to the input acoustic frequency. Spin displacements as short as a few tens of nanometers were able to be detected with this method.

MR detection of mechanical vibrations using a radiofrequency field gradient

A new method for NMR characterization of mechanical waves, based upon radiofrequency field gradient for motion encoding, is proposed. A binomial B1 gradient excitation scheme was used to visualize the mobile spins undergoing a periodic transverse mechanical excitation. A simple model was designed to simulate the NMR signal as a function of the wave frequency excitation and the periodicity of the NMR pulse sequence. The preliminary results were obtained on a gel phantom at low vibration frequencies (0-200 Hz) by using a ladder-shaped coil generating a nearly constant RF field gradient along a specific known direction. For very small displacements and/or B1 gradients, the NMR signal measured on a gel phantom was proportional to the vibration amplitude and the pulse sequence was shown to be selective with respect to the vibration frequency. A good estimation of the direction of vibrations was obtained by varying the angle between the motion direction and the B1 gradient. The method and its use in parallel to more conventional MR elastography techniques are discussed. The presented approach might be of interest for noninvasive investigation of elastic properties of soft tissues and other materials.

A new fast and unsynchronized method for MRI of viscoelastic properties of soft tissues

Quantitative measurement of mechanical properties of biologic tissues may have several applications for diagnosis or biomechanic modeling in sports medicine, traumatology, or computer-guided surgery. The magnetic resonance imaging (MRI) methods previously tested for these applications all required synchronization between MRI acquisition pulses and the mechanical stimulation. A new unsynchronized method operating with no prior knowledge of intensity, direction, and frequency of the mechanical waves is proposed. A specifically modified SPAMM (SPAtial Modulation of Magnetization) sequence has been used, operating on a 0.2-T MRI system. The experimental results obtained on test objects fit well with theoretical calculations. The new proposed method is very fast (a less than 5-second acquisition time) for routine clinical use.

Magnetic resonance imaging of ultrasound fields: gradient characteristics

Phase-contrast magnetic resonance imaging (MRI) is used to image particle displacements arising from a 0.515-MHZ focused ultrasound (US) field. The technique used a phase-locked, self-resonant gradient matched to the US frequency in conjunction with a spin-echo sequence to generate phase images of US-induced displacement parallel to the US propagation direction. The gradient design was numerically optimized to provide maximum linearity and magnitude while minimizing gradient inductance. The windings were fabricated of Litz wire to minimize resistive losses and mounted in an oil-cooled imaging chamber. When driven by a resonance power supply, a peak magnetic field gradient of 0.40 T/m was attained with a peak current of 20 amp in a volume of 53 cm(3), achieving stable oscillation at the required US frequency. Clear detection of the nanometer scale particle motions of the US field was achieved and allowed quantitative, noninvasive visualization of the entire US field. While the required gradient slew rate for US detection is beyond that recommended for in vivo application, this imaging method opens new possibilities for in vitro or ex vivo research in the study of the interaction of US with tissue.

An overlapping subzone technique for MR-based elastic property reconstruction

A finite element-based nonlinear inversion scheme for magnetic resonance (MR) elastography is detailed. The algorithm operates on small overlapping subzones of the total region of interest, processed in a hierarchical order as determined by progressive error minimization. This zoned approach allows for a high degree of spatial discretization, taking advantage of the data-rich environment afforded by the MR. The inversion technique is tested in simulation under high-noise conditions (15% random noise applied to the displacement data) with both complicated user-defined stiffness distributions and realistic tissue geometries obtained by thresholding MR image slices. In both cases the process has proved successful and has been capable of discerning small inclusions near 4 mm in diameter. Magn Reson Med 42:779-786, 1999.

Magnetic resonance imaging of ultrasonic fields

A nuclear magnetic resonance imaging (MRI) method is described that allows noninvasive, quantitative mapping of medical ultrasound (US) fields in tissue. Application of a resonant magnetic field gradient operating at the US frequency permits detection of nanometer motions associated with ultrasound, and allows direct measurement of absolute pressure, intensity, and speed of sound. By altering gradient timing, the propagation of US fields in time and space can be observed; this enables tracking of US scattering phenomena in a tissue-equivalent medium. An experimental apparatus was constructed that combined a 515-kHz focused US transducer configured with its focus in the center of a small-bore oscillating gradient. This provided an oscillating gradient with a peak gradient strength of 0.40 T/m over a useable imaging volume of 61 cm3. When used in conjunction with 1.5-T clinical MRI system, this apparatus allowed the clear visualization of the focused US field within this volume and its propagation with time. Current limits of sensitivity indicate a noise equivalent sensitivity of 3.8 nm in displacement amplitude, 19 kPa in pressure amplitude and 12 mW/cm2 in acoustic intensity. These studies indicate that MRI can provide a new, noninvasive method for US exposimetry and the basic study of ultrasound biophysics in tissue.