Relationship between Shear Stiffness Measured by MR Elastography and Perfusion Metrics Measured by Perfusion CT of Meningiomas

BACKGROUND AND PURPOSE

When managing meningiomas, intraoperative tumor consistency and histologic subtype are indispensable factors influencing operative strategy. The purposes of this study were the following: 1) to investigate the correlation between stiffness assessed with MR elastography and perfusion metrics from perfusion CT, 2) to evaluate whether MR elastography and perfusion CT could predict intraoperative tumor consistency, and 3) to explore the predictive value of stiffness and perfusion metrics in distinguishing among histologic subtypes of meningioma.

MATERIALS AND METHODS

Mean tumor stiffness and relative perfusion metrics (blood flow, blood volume, and MTT) were calculated (relative to normal brain tissue) for 14 patients with meningiomas who underwent MR elastography and perfusion CT before surgery (cohort 1). Intraoperative tumor consistency was graded by a neurosurgeon in 18 patients (cohort 2, comprising the 14 patients from cohort 1 plus 4 additional patients). The correlation between tumor stiffness and perfusion metrics was evaluated in cohort 1, as was the ability of perfusion metrics to predict intraoperative tumor consistency and discriminate histologic subtypes. Cohort 2 was analyzed for the ability of stiffness to determine intraoperative tumor consistency and histologic subtypes.

RESULTS

The relative MTT was inversely correlated with stiffness (P = .006). Tumor stiffness was positively correlated with intraoperative tumor consistency (P = .01), while perfusion metrics were not. Relative MTT significantly discriminated transitional meningioma from meningothelial meningioma (P = .04), while stiffness did not significantly differentiate any histologic subtypes.

CONCLUSIONS

In meningioma, tumor stiffness may be useful to predict intraoperative tumor consistency, while relative MTT may potentially correlate with tumor stiffness and differentiate transitional meningioma from meningothelial meningioma.

Waveguide effects and implications for cardiac magnetic resonance elastography: A finite element study

Magnetic resonance elastography (MRE) is increasingly being applied to thin or small structures in which wave propagation is dominated by waveguide effects, which can substantially bias stiffness results with common processing approaches. The purpose of this work was to investigate the importance of such biases and artifacts on MRE inversion results in: (i) various idealized 2D and 3D geometries with one or more dimensions that are small relative to the shear wavelength; and (ii) a realistic cardiac geometry. Finite element models were created using simple 2D geometries as well as a simplified and a realistic 3D cardiac geometry, and simulated displacements acquired by MRE from harmonic excitations from 60 to 220 Hz across a range of frequencies. The displacement wave fields were inverted with direct inversion of the Helmholtz equation with and without the application of bandpass filtering and/or the curl operator to the displacement field. In all geometries considered, and at all frequencies considered, strong biases and artifacts were present in inversion results when the curl operator was not applied. Bandpass filtering without the curl was not sufficient to yield accurate recovery. In the 3D geometries, strong biases and artifacts were present in 2D inversions even when the curl was applied, while only 3D inversions with application of the curl yielded accurate recovery of the complex shear modulus. These results establish that taking the curl of the wave field and performing a full 3D inversion are both necessary steps for accurate estimation of the shear modulus both in simple thin-walled or small structures and in a realistic cardiac geometry when using simple inversions that neglect the hydrostatic pressure term. In practice, sufficient wave amplitude, signal-to-noise ratio, and resolution will be required to achieve accurate results.

Brain stiffens post mortem

Alterations in brain rheology are increasingly recognized as a diagnostic marker for various neurological conditions. Magnetic resonance elastography now allows us to assess brain rheology repeatably, reproducibly, and non-invasively in vivo. Recent elastography studies suggest that brain stiffness decreases one percent per year during normal aging, and is significantly reduced in Alzheimer’s disease and multiple sclerosis. While existing studies successfully compare brain stiffnesses across different populations, they fail to provide insight into changes within the same brain. Here we characterize rheological alterations in one and the same brain under extreme metabolic changes: alive and dead. Strikingly, the storage and loss moduli of the cerebrum increased by 26% and 60% within only three minutes post mortem and continued to increase by 40% and 103% within 45 minutes. Immediate post mortem stiffening displayed pronounced regional variations; it was largest in the corpus callosum and smallest in the brainstem. We postulate that post mortem stiffening is a manifestation of alterations in polarization, oxidation, perfusion, and metabolism immediately after death. Our results suggest that the stiffness of our brain–unlike any other organ–is a dynamic property that is highly sensitive to the metabolic environment Our findings emphasize the importance of characterizing brain tissue in vivo and question the relevance of ex vivo brain tissue testing as a whole. Knowing the true stiffness of the living brain has important consequences in diagnosing neurological conditions, planning neurosurgical procedures, and modeling the brain’s response to high impact loading.

MR Elastography Analysis of Glioma Stiffness and IDH1-Mutation Status

BACKGROUND AND PURPOSE

Our aim was to noninvasively evaluate gliomas with MR elastography to characterize the relationship of tumor stiffness with tumor grade and mutations in the isocitrate dehydrogenase 1 (IDH1) gene.

MATERIALS AND METHODS

Tumor stiffness properties were prospectively quantified in 18 patients (mean age, 42 years; 6 women) with histologically proved gliomas using MR elastography from 2014 to 2016. Images were acquired on a 3T MR imaging unit with a vibration frequency of 60 Hz. Tumor stiffness was compared with unaffected contralateral white matter, across tumor grade, and by IDH1-mutation status. The performance of the use of tumor stiffness to predict tumor grade and IDH1 mutation was evaluated with the Wilcoxon rank sum, 1-way ANOVA, and Tukey-Kramer tests.

RESULTS

Gliomas were softer than healthy brain parenchyma, 2.2 kPa compared with 3.3 kPa (P < .001), with grade IV tumors softer than grade II. Tumors with an IDH1 mutation were significantly stiffer than those with wild type IDH1, 2.5 kPa versus 1.6 kPa, respectively (P = .007).

CONCLUSIONS

MR elastography demonstrated that not only were gliomas softer than normal brain but the degree of softening was directly correlated with tumor grade and IDH1-mutation status. Noninvasive determination of tumor grade and IDH1 mutation may result in improved stratification of patients for different treatment options and the evaluation of novel therapeutics. This work reports on the emerging field of "mechanogenomics": the identification of genetic features such as IDH1 mutation using intrinsic biomechanical information.

Traumatic Trabecular Lesions Observed on MR Imaging of the Knee

Examinations carried out on 302 consecutive patients with MR of the knee between January 1988 and March 1989 were reviewed for detection of trabecular lesion. Twenty-seven patients found presenting trabecular lesion were further reviewed with specific reference to their activity level and need for specific therapy to determine the clinical significance of the trabecular lesion. Twenty-one of the trabecular lesions were in the femur, 5 were in the tibia, and one was in the fibula. Three of them were associated with a direct trauma, 12 with a valgus type injury, 3 with pure rotation mechanism, and 5 with a combination of valgus and rotation. In 17 cases trabecular lesion was a single finding, in 10 cases it was associated with some ligamentous tear. At the follow-up visit, 26 of the 27 patients with trabecular lesion had no symptoms, and the patient with moderate knee symptoms had had similar knee symptoms prior to the accident due to an osteochondral defect. We conclude that a trabecular lesion in an MR image is a benign bone change associated with knee trauma which heals without sequelae.

MR Elastography Demonstrates Increased Brain Stiffness in Normal Pressure Hydrocephalus

BACKGROUND AND PURPOSE

Normal pressure hydrocephalus is a reversible neurologic disorder characterized by a triad of cognitive impairment, gait abnormality, and urinary incontinence that is commonly treated with ventriculoperitoneal shunt placement. However, multiple overlapping symptoms often make it difficult to differentiate normal pressure hydrocephalus from other types of dementia, and improved diagnostic techniques would help patient management. MR elastography is a novel diagnostic tool that could potentially identify patients with normal pressure hydrocephalus. The purpose of this study was to assess brain stiffness changes in patients with normal pressure hydrocephalus compared with age- and sex-matched cognitively healthy individuals.

MATERIALS AND METHODS

MR elastography was performed on 10 patients with normal pressure hydrocephalus and 21 age- and sex-matched volunteers with no known neurologic disorders. Image acquisition was conducted on a 3T MR imaging scanner. Shear waves with 60-Hz vibration frequency were transmitted into the brain by a pillowlike passive driver. A novel postprocessing technique resistant to noise and edge artifacts was implemented to determine regional brain stiffness. The Wilcoxon rank sum test and linear regression were used for statistical analysis.

RESULTS

A significant increase in stiffness was observed in the cerebrum (P = .001), occipital lobe (P < .001), parietal lobe (P = .001), and the temporal lobe (P = .02) in the normal pressure hydrocephalus group compared with healthy controls. However, no significant difference was noted in other regions of the brain, including the frontal lobe (P = .07), deep gray and white matter (P = .43), or cerebellum (P = .20).

CONCLUSIONS

This study demonstrates increased brain stiffness in patients with normal pressure hydrocephalus compared with age- and sex-matched healthy controls; these findings should motivate future studies investigating the use of MR elastography for this condition and the efficacy of shunt therapy.

Calculation of shear stiffness in noise dominated magnetic resonance elastography data based on principal frequency estimation

Magnetic resonance elastography (MRE) is a non-invasive phase-contrast-based method for quantifying the shear stiffness of biological tissues. Synchronous application of a shear wave source and motion encoding gradient waveforms within the MRE pulse sequence enable visualization of the propagating shear wave throughout the medium under investigation. Encoded shear wave-induced displacements are then processed to calculate the local shear stiffness of each voxel. An important consideration in local shear stiffness estimates is that the algorithms employed typically calculate shear stiffness using relatively high signal-to-noise ratio (SNR) MRE images and have difficulties at an extremely low SNR. A new method of estimating shear stiffness based on the principal spatial frequency of the shear wave displacement map is presented. Finite element simulations were performed to assess the relative insensitivity of this approach to decreases in SNR. Additionally, ex vivo experiments were conducted on normal rat lungs to assess the robustness of this approach in low SNR biological tissue. Simulation and experimental results indicate that calculation of shear stiffness by the principal frequency method is less sensitive to extremely low SNR than previously reported MRE inversion methods but at the expense of loss of spatial information within the region of interest from which the principal frequency estimate is derived.

Shear modulus decomposition algorithm in magnetic resonance elastography

Magnetic resonance elastography (MRE) is an imaging modality capable of visualizing the elastic properties of an object using magnetic resonance imaging (MRI) measurements of transverse acoustic strain waves induced in the object by a harmonically oscillating mechanical vibration. Various algorithms have been designed to determine the mechanical properties of the object under the assumptions of linear elasticity, isotropic and local homogeneity. One of the challenging problems in MRE is to reduce the noise effects and to maintain contrast in the reconstructed shear modulus images. In this paper, we propose a new algorithm designed to reduce the degree of noise amplification in the reconstructed shear modulus images without the assumption of local homogeneity. Investigating the relation between the measured displacement data and the stress wave vector, the proposed algorithm uses an iterative reconstruction formula based on a decomposition of the stress wave vector. Numerical simulation experiments and real experiments with agarose gel phantoms and human liver data demonstrate that the proposed algorithm is more robust to noise compared to standard inversion algorithms and stably determines the shear modulus.

MR elastography of the lung with hyperpolarized 3He

MR elastography (MRE) is a phase contrast-based technique for spatially mapping the mechanical properties of tissue-like materials. While hyperpolarized noble gases such as helium-3 ((3)He) have proven to be an ideal contrast mechanism for imaging of the lung using conventional MR techniques, their applicability for lung MRE is unknown, due to the fact that gases do not support shear. In this study, we report on the application of MRE to an ex vivo porcine lung specimen inflated with a hyperpolarized noble gas. Unlike proton MRE, shear wave propagation is encoded into the gas entrapped within the alveolar spaces rather than the parenchyma itself. These data provide first evidence of the technical feasibility of MRE of the lung using hyperpolarized noble gases.

Shear waves induced by moving needle in MR elastography

Magnetic resonance elastography (MRE) is a phase contrast-based method for observing shear wave propagation in a material to determine its stiffness. The objective of this study was to determine whether shear waves suitable for MRE could be induced using a moving acupuncture needle. Tissue-simulating bovine gel phantom and a 0.4 mm diameter acupuncture needle were used in the experiment. The results showed that observable shear waves could be induced in the gel phantom by cyclic needle motion. The observed wavelength varied with excitation frequency, as expected. Generating shear waves using moving needles may be a useful tool to study the basic mechanism of acupuncture with MRE. Further study will be conducted to observe the wave motion in inhomogeneous media and acupuncture-induced effects in in-vivo studies.

Magnetic resonance elastography of the lung: technical feasibility

Magnetic resonance elastography (MRE) is a phase-contrast technique that can spatially map shear stiffness within tissue-like materials. To date, however, MRE of the lung has been too technically challenging-primarily because of signal-to-noise ratio (SNR) limitations and phase instability. We describe an approach in which shear wave propagation is not encoded into the phase of the MR signal of a material, but rather from the signal arising from a polarized noble gas encapsulated within. To determine the feasibility of the approach, three experiments were performed. First, to establish whether shear wave propagation within lung parenchyma can be visualized with phase-contrast MR techniques, MRE was performed on excised porcine lungs inflated with room air. Second, a phantom consisting of open-cell foam filled with thermally polarized (3)He gas was imaged with MRE to determine whether shear wave propagation can be encoded by the gas. Third, preliminary evidence of the feasibility of MRE in vivo was obtained by using a longitudinal driver on the chest of a normal volunteer to generate shear waves in the lung. The results suggest that MRE in combination with hyperpolarized noble gases is potentially useful for noninvasively assessing the regional elastic properties of lung parenchyma, and merits further investigation.

Vascular wall elasticity measurement by magnetic resonance imaging

The goal of this current study was to determine whether an MRI-based elastography (MRE) method can visualize and assess propagating mechanical waves within fluid-filled vessels and to investigate the feasibility of measuring the elastic properties of vessel walls and quantitatively assessing stenotic lesions by using MRE. The ability to measure the Young's modulus-wall thickness product was tested using a thin-walled latex vessel model. Also tested in vessel models was the ability to quantitate the degree of stenosis by measuring transmitted and reflected mechanical waves. This method was then applied to ex vivo porcine models and in vivo human arteries to further test its feasibility. The results provide preliminary evidence that MRE can be used to quantitatively assess the stiffness of blood vessels, and provide a non-morphologic method to measure stenosis. With further development, it is possible that the method can be implemented in vivo.

Determination and analysis of guided wave propagation using magnetic resonance elastography

We present a novel extension of standard magnetic resonance elastography (MRE) measurement and analysis methods, which is applicable in cases where the medium is characterized by waveguides or fiber bundles (i.e., muscle) leading to constrained propagation of elastic waves. As a demonstration of this new method, MRI is utilized to identify the pathways of the individual fibers of a stalk of celery, and 3D MRE is then performed throughout the volume containing the celery fibers for a measurement of the displacements. A Helmholtz decomposition is performed permitting a separation of the displacements into longitudinal and transverse components, and an application of a hybrid Radon transform permits a spectral decomposition of wave propagation along the fibers. Dot product projections between these elastic displacements measured in the global coordinate system and three Frenet vectors representing the tangent and two corresponding orthogonal vectors along any particular fiber orientation yield the displacement contributions to wave propagation along the fiber as if it were a waveguide. A sliding window spatial Fourier transform is then performed along the length of each fiber to obtain dispersion images that portray space-wavenumber profiles. Therefore, this method can permit localized tracking and characterization of wave types, velocities, and coupling along arbitrarily oriented fibers.

Needle shear wave driver for magnetic resonance elastography

Magnetic resonance elastography (MRE) is capable of quantitatively depicting the mechanical properties of tissues in vivo. In contrast to mechanical excitation at the surface of the tissue, the method proposed in this study describes shear waves produced by an inserted needle. The results demonstrate that MRE performed with the needle driver provides shear stiffness estimates that correlate well with those obtained using mechanical testing. Comparisons between MRE acquisitions obtained with surface and needle drivers yielded similar results in general. However, the well-defined wave propagation pattern provided by the needle driver in a target region can reduce section orientation-related error in wavelength estimation that occurs with surface drivers in 2D MRE acquisitions. Preliminary testing of the device was performed on animals. This study demonstrates that the needle driver is an effective option that offers advantages over surface drivers for obtaining accurate stiffness estimates in targeted regions that are accessible by the needle.

Spatio-temporal directional filtering for improved inversion of MR elastography images

Dynamic magnetic resonance elastography can visualize and measure propagating shear waves in tissue-like materials subjected to harmonic mechanical excitation. This allows the calculation of local values of material parameters such as shear modulus and attenuation. Various inversion algorithms to perform such calculations have been proposed, but they are sensitive to areas of low displacement amplitude (and hence low SNR) that result from interference patterns due to reflection and refraction. A spatio-temporal directional filter applied as a pre-processing step can separate interfering waves so they can be processed separately. Weighted combinations of inversions from such directionally separated data sets can significantly improve reconstructions of shear modulus and attenuation.

Spatio-temporal directional filtering for improved inversion of MR elastography images

Dynamic magnetic resonance elastography can visualize and measure propagating shear waves in tissue-like materials subjected to harmonic mechanical excitation. This allows the calculation of local values of material parameters such as shear modulus and attenuation. Various inversion algorithms to perform such calculations have been proposed, but they are sensitive to areas of low displacement amplitude (and hence low SNR) that result from interference patterns due to reflection and refraction. A spatio-temporal directional filter applied as a pre-processing step can separate interfering waves so they can be processed separately. Weighted combinations of inversions from such directionally separated data sets can significantly improve reconstructions of shear modulus and attenuation.

Magnetic resonance elastography: non-invasive mapping of tissue elasticity

Magnetic resonance elastography (MRE) is a phase-contrast-based MRI imaging technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation. The data acquired allows the calculation of local quantitative values of shear modulus and the generation of images that depict tissue elasticity or stiffness. This is significant because palpation, a physical examination that assesses the stiffness of tissue, can be an effective method of detecting tumors, but is restricted to parts of the body that are accessible to the physician's hand. MRE shows promise as a potential technique for 'palpation by imaging', with possible applications in tumor detection (particularly in breast, liver, kidney and prostate), characterization of disease, and assessment of rehabilitation (particularly in muscle). We describe MRE in the context of other recent techniques for imaging elasticity, discuss the processing algorithms for elasticity reconstruction and the issues and assumptions they involve, and present recent ex vivo and in vivo results.

Complex-valued stiffness reconstruction for magnetic resonance elastography by algebraic inversion of the differential equation

Noninvasive quantitation of the mechanical properties of tissue could improve early detection of pathology. Previously a method for detecting displacement from propagating shear waves using a phase-contrast MRI technique was developed. In this work it is demonstrated how a collection of data representing the full vector displacement field could be used to potentially estimate the full complex stiffness tensor. An algebraic inversion approach useful for piece-wise homogeneous materials is described in detail for the general isotropic case, which is then specialized to incompressible materials as a model for tissue. Results of the inversion approach are presented for simulated and experimental phantom data that show the technique can be used to obtain shear wave-speed and attenuation in regions where there is sufficient signal-to-noise ratio in the displacement and its second spatial derivatives. The sensitivity to noise is higher in the attenuation estimates than the shear wave-speed estimates. Magn Reson Med 45:299-310, 2001.

Magnetic resonance elastography of skeletal muscle

While the contractile properties of skeletal muscle have been studied extensively, relatively little is known about the elastic properties of muscle in vivo. Magnetic resonance elastography (MRE) is a phase contrast-based method for observing shear waves propagating in a material to determine its stiffness. In this work, MRE is applied to skeletal muscle under load to quantify the change in stiffness with loading. A mathematical model of muscle is developed that predicts a linear relationship between shear stiffness and muscle load. The MRE technique was applied to bovine muscle specimens (N = 10) and human biceps brachii in vivo (N = 5). Muscle stiffness increased linearly for both passive tension (14.5 +/- 1.77 kPa/kg) and active tension, in which the increase in stiffness was dependent upon muscle size, as predicted by the model. A means of noninvasively assessing the viscoelastic pro-perties of skeletal muscle in vivo may provide a useful method for studying muscle biomechanics in health and disease.

Autocorrection of three-dimensional time-of-flight MR angiography of the Circle of Willis

OBJECTIVE

The purpose of this study was to investigate the efficacy of a retrospective adaptive motion correction technique known as autocorrection for reducing motion-induced artifacts in high-resolution three-dimensional time-of-flight MR angiography of the circle of Willis.

MATERIALS AND METHODS

Ten consecutive volunteers were imaged with an unenhanced gradient-recalled echo three-dimensional time-of-flight MR angiography sequence of the circle of Willis. Each volunteer was asked to rotate approximately 2 degrees after completion of one third and one half of the acquisition in the axial, sagittal, and oblique planes (45 degrees to the axial and sagittal planes). A single static data set was also acquired for each volunteer. Unprocessed and autocorrected maximum-intensity-projection images were reviewed as blinded image pairs by six radiologists and were compared on a five-point image quality scale.

RESULTS

Mean improvement in image quality after autocorrection was 1.4 (p < 0.0001), 1.1 (p < 0.0001), and 0.2 (p = 0.003) observer points (maximum value, 2.0), respectively, for examinations corrupted by motion in the axial, oblique, and sagittal planes. All three axes had statistically significant improvement in image quality compared with the uncorrected images. Changes in image quality after the application of the autocorrection algorithm to static angiogram data were not statistically significant (mean change in score = -0.13 points; p = 0.29).

CONCLUSION

Autocorrection can reduce artifacts in motion-corrupted MR angiography of the circle of Willis without distorting motion-free examinations.

Complex-valued stiffness reconstruction for magnetic resonance elastography by algebraic inversion of the differential equation

Noninvasive quantitation of the mechanical properties of tissue could improve early detection of pathology. Previously a method for detecting displacement from propagating shear waves using a phase-contrast MRI technique was developed. In this work it is demonstrated how a collection of data representing the full vector displacement field could be used to potentially estimate the full complex stiffness tensor. An algebraic inversion approach useful for piece-wise homogeneous materials is described in detail for the general isotropic case, which is then specialized to incompressible materials as a model for tissue. Results of the inversion approach are presented for simulated and experimental phantom data that show the technique can be used to obtain shear wave-speed and attenuation in regions where there is sufficient signal-to-noise ratio in the displacement and its second spatial derivatives. The sensitivity to noise is higher in the attenuation estimates than the shear wave-speed estimates. Magn Reson Med 45:299-310, 2001.

Assessment of thermal tissue ablation with MR elastography

An important part of thermal ablation therapy is the assessment of the spatial extent of tissue coagulation. In this work, the mechanical properties of thermally-ablated tissue were quantitatively evaluated using magnetic resonance elastography (MRE). This study shows that the mechanical properties of focused ultrasound ablated tissue are significantly different from normal tissue, and the difference can be imaged and measured using MRE. Repeated experiments revealed a reproducible pattern of tissue mechanical property change during thermal ablation in ex vivo bovine muscle. This pattern may reflect changes in intrinsic tissue structure and could be used to evaluate tissue coagulation during thermal ablation therapy. Magn Reson Med 45:80-87, 2001.

Simultaneous image acquisition utilizing hybrid body and phased array receiver coils

In clinical MR imaging the design and selection of receiver coil is an important step in ensuring the highest image quality. Often this choice is based on selecting a receiver coil characterized by high spatial uniformity such as the body and head volume receiver coils or a surface coil (or array of coils) that provide high signal-to-noise ratio (SNR). In the past, it has been difficult to accomplish both high SNR and spatial uniformity as both coil types achieve one of these characteristics at the expense of the other. The purpose of this study was to achieve both high SNR and spatial uniformity through the simultaneous acquisition of the MR signal using the body and a surface coil array. Results indicate that this hybrid system can provide uniformity and SNR values comparable to those achieved by the body and surface coil arrays, respectively.

Autocorrection in MR imaging: adaptive motion correction without navigator echoes

A technique for automatic retrospective correction of motion artifacts on magnetic resonance (MR) images was developed that uses only the raw (complex) data from the MR imager and requires no knowledge of patient motion during the acquisition. The algorithm was tested on coronal images of the rotator cuff in a series of 144 patients, and the improvements in image quality were similar to those achieved with navigator echoes. The results demonstrate that autocorrection can significantly reduce motion artifacts in a technically demanding MR imaging application.

Tissue characterization using magnetic resonance elastography: preliminary results

The well-documented effectiveness of palpation as a diagnostic technique for detecting cancer and other diseases has provided motivation for developing imaging techniques for noninvasively evaluating the mechanical properties of tissue. A recently described approach for elasticity imaging, using propagating acoustic shear waves and phase-contrast MRI, has been called magnetic resonance elastography (MRE). The purpose of this work was to conduct preliminary studies to define methods for using MRE as a tool for addressing the paucity of quantitative tissue mechanical property data in the literature. Fresh animal liver and kidney tissue specimens were evaluated with MRE at multiple shear wave frequencies. The influence of specimen temperature and orientation on measurements of stiffness was studied in skeletal muscle. The results demonstrated that all of the materials tested (liver, kidney, muscle and tissue-simulating gel) exhibit systematic dependence of shear stiffness on shear rate. These data are consistent with a viscoelastic model of tissue mechanical properties, allowing calculation of two independent tissue properties from multiple-frequency MRE data: shear modulus and shear viscosity. The shear stiffness of tissue can be substantially affected by specimen temperature. The results also demonstrated evidence of shear anisotropy in skeletal muscle but not liver tissue. The measured shear stiffness in skeletal muscle was found to depend on both the direction of propagation and polarization of the shear waves.

Three-dimensional contrast-enhanced MR angiography with real-time fluoroscopic triggering: design specifications and technical reliability in 330 patient studies

Technical reliability was determined for triggering three-dimensional (3D) contrast material-enhanced magnetic resonance (MR) angiography with MR fluoroscopy. Technical requirements for high reliability were also identified. Reliability was evaluated in 330 consecutive patient studies of the neck, thorax, abdomen, and pelvis. Contrast material arrival was detected fluoroscopically in 325 of the 330 studies (98.5%), and the 3D sequence was successfully triggered in 321 of 330 studies (97.3%). Fluoroscopic triggering of centrically encoded 3D MR angiographic acquisitions is a highly reliable means of obtaining 3D MR angiograms with high spatial resolution.

Rapid autocorrection using prescan navigator echoes

Autocorrection is an adaptive motion correction algorithm that does not require an in vivo measurement of the motion record. A novel method for ensuring convergence of this algorithm when motion is severe is presented. A limited number of navigator echoes are acquired before the imaging sequence to obtain a "snapshot" of the object. Phase differences between the navigator and image k-space data are used as an estimate of motion-induced phase shifts in the image, followed by autocorrection. In phantom data a six-fold reduction in computation time compared to autocorrection alone was realized. These results indicate that this navigator/autocorrection combination may be useful for reducing motion artifacts and computation time for MR exams when motion along the image phase encoding axis is severe.

Acoustic shear-wave imaging using echo ultrasound compared to magnetic resonance elastography

We compare a previously developed phase contrast-based magnetic resonance imaging technique (MRE) to a phase-based ultrasound (US) method for measuring small cyclic displacements (submicrometer level) caused by propagating acoustic shear waves in tissue-like media. Our preliminary experiments with gelatin phantoms show that acoustic shear-wave propagation can be measured with US, and we speculate that this technique could find applications in medical imaging.

Prospective multiaxial motion correction for fMRI

Corruption of the image time series due to interimage head motion limits the clinical utility of functional MRI. This paper presents a method for real-time prospective correction of rotation and translation in all six degrees of rigid body motion. By incorporating an orbital navigator (ONAV) echo for each of the sagittal, axial, and coronal planes into the fMRI pulse sequence, rotation and translation can be measured and the spatial orientation of the image acquisition sequence that follows can be corrected prospectively in as little as 160 msec. Testing of the method using a computerized motion phantom capable of performing complex multiaxial motion showed subdegree rotational and submillimeter translational accuracy over a range of +/-8 degrees and +/-8 mm of motion. In vivo images demonstrate correction of simultaneous through-plane and in-plane motion and improved detection of fMRI activation in the presence of head motion.

Retrospective adaptive motion correction for navigator-gated 3D coronary MR angiography

Retrospective adaptive motion correction (AMC) was developed for reducing effects of residual respiration in real-time navigator-gated three-dimensional (3D) coronary magnetic resonance (MR) angiography. In both motion phantom and in vivo experiments, AMC improved image sharpness of coronary arteries. This navigator-based technique combining adaptive correction and real-time gating is potentially an efficient and effective motion reduction method for 3D coronary MR angiography.

Image metric-based correction (autocorrection) of motion effects: analysis of image metrics

Magnetic resonance (MR) imaging of the shoulder necessitates high spatial and contrast resolution resulting in long acquisition times, predisposing these images to degradation due to motion. Autocorrection is a new motion correction algorithm that attempts to deduce motion during imaging by calculating a metric that reflects image quality and searching for motion values that optimize this metric. The purpose of this work is to report on the evaluation of 24 metrics for use in autocorrection of MR images of the rotator cuff. Raw data from 164 clinical coronal rotator cuff exams acquired with interleaved navigator echoes were used. Four observers then scored the original and corrected images based on the presence of any motion-induced artifacts. Changes in metric values before and after navigator-based adaptive motion correction were correlated with changes in observer score using a least-squares linear regression model. Based on this analysis, the metric that exhibited the strongest relationship with observer ratings of MR shoulder images was the entropy of the one-dimensional gradient along the phase-encoding direction. We speculate (and show preliminary evidence) that this metric will be useful not only for autocorrection of shoulder MR images but also for autocorrection of other MR exams.

Dual-echo breathhold T(2)-weighted fast spin echo MR imaging of liver lesions

The purpose of this study was to develop a multi-shot dual-echo breathhold fast spin echo technique (DFSE) and compare it with conventional spin echo (T2SE) for T(2)-weighted MR imaging of liver lesions. The DFSE acquisition (EffTE1/EffTE2/TR = 66/143/2100 ms) imaged 5 sections per 17 s breathhold. T2SE imaging (TE1/TE2/TR = 60/120/2500 ms) required 16:55 (min:s) for 14 sections. Both techniques used a receive-only phased-array abdominal multicoil and provided 192 x 256 effective resolution. The results showed first and second echo relative DFSE/T2SE contrast values for 27 representative lesions (15 consecutive patients) were 1.08 +/- 0.05 and 1.16 +/- 0.09 (mean +/- STD mean), respectively. Corresponding CNR values were 1.12 +/- 0.09 and 0.97 +/- 0.12. Overall DFSE was comparable-to-superior to T2SE for lesion sizing and image artifact. DFSE lesion detection was inferior to T2SE's in several patient studies because of decreased conspicuity of lesions located near multicoil edges and because of poor breathhold-to-breathhold reproducibility and lack of breathholding. However both DFSE (and T2SE) provided lesion detection rated to be of diagnostic quality for all patient studies. In conclusion, we found that DFSE provides diagnostically useful dual-echo T(2)-weighted MR liver images in a greatly decreased acquisition time.

MR imaging of shear waves generated by focused ultrasound

This study has shown that magnetic resonance elastography (MRE) can detect shear waves excited by focused ultrasound (FUS) in both gel phantoms and ex vivo muscle. Good agreement was shown between the shear modulus measured from MRE images generated using FUS and that using previously reported MRE techniques. The shear wave displacement amplitude at the FUS focus was studied and found to be proportional with both FUS ultrasonic pulse intensity and the FUS modulation pulse period over the range tested.

Reliability of water proton chemical shift temperature calibration for focused ultrasound ablation therapy

Our purpose in this work was to assess the reliability of the calibration coefficient for magnetic resonance water proton chemical shift temperature mapping. Over a six month period, the calibration coefficient was measured 15 times in several different phantoms. A highly linear relationship between water proton chemical shift and temperature change was found. The average temperature calibration coefficient determined from all studies was 0.009+/-0.001 ppm/degrees C. Four of the 15 studies were conducted on the same day using the same phantom. The average temperature calibration coefficient of these four studies was 0.0096+/-0.0001 ppm/degrees C.

Contrast-enhanced 3D MR breathhold imaging of porcine coronary arteries using fluoroscopic localization and bolus triggering

The purpose of this study was to develop cardiac-gated contrast-enhanced 3D MRA for imaging the coronary arteries of pigs. Each major coronary artery was imaged individually in a single 3D slab in one breathhold. To permit acquisition within a breathhold, a limited number of partitions (12-16) were collected in a single, oblique, thin 3D slab. Typical resolution of the acquisition was 0.8 (X) x 1.6 (Y) x 1.6 (Z) mm. MR fluoroscopic localization was used to establish the 3D double-oblique orientation. Real-time MR fluoroscopy was also used to instantaneously trigger the 3D scan after detection in the aortic root of the intravenously administered contrast bolus. Six pigs were used in the study. Each pig was scanned on two separate days. Images routinely show the majority of the length of the three principal coronary arteries. Magn Reson Med 42:1159-1165, 1999.

MR-guided focused-ultrasound ablation system: determination of precision and accuracy in preparation for clinical trials

The authors analyzed the accuracy and precision of focal-spot placement with a magnetic resonance-guided, focused-ultrasound system. Average absolute accuracy errors ranged from 0.2 to 1.0 mm, and average absolute individual precision errors ranged from 0.2 to 0.3 mm. To prevent damage to vital structures, single sonications and sonication grids should be placed beyond approximately 2 and 1 mm, respectively.

Two-dimensional multishot echo-planar coronary MR angiography

This work presents a two-dimensional (2D) multishot echo-planar imaging (EPI) technique for magnetic resonance angiography (MRA) of individual coronary arteries in a 17-heartbeat breath-hold. Conventional 2D and 3D segmented gradient-echo (GRE) coronary MRA requires repetitive excitation of the same slice or slab within each cardiac cycle, which can result in reduced blood signal and in motion artifacts. Two-dimensional multishot EPI can address these limitations by eliminating multiple excitations per cardiac cycle, using large flip-angle excitations, markedly reducing the data acquisition window, and performing oblique multislice 2D imaging. The goal of this study was to assess the feasibility of breath-hold 2D multishot EPI for multislice coronary MRA and to demonstrate its reliability by consistently acquiring high-quality images of the coronary arteries in a series of 16 volunteers.

Infraclavicular brachial plexus block: parasagittal anatomy important to the coracoid technique

Infraclavicular brachial plexus block is a technique well suited to prolonged continuous catheter use. We used a coracoid approach to this block to create an easily understood technique. We reviewed the magnetic resonance images of the brachial plexus from 20 male and 20 female patients. Using scout films, the parasagittal section 2 cm medial to the coracoid process was identified. Along this oblique section, we located a point approximately 2 cm caudad to the coracoid process on the skin of the anterior chest wall. From this point, we determined simulated needle direction to contact the neurovascular bundle and measured depth. At the skin entry site, the direct posterior insertion of a needle will make contact with the cords of the brachial plexus where they surround the second part of the axillary artery in all images. The mean (range) distance (depth along the needle shaft) from the skin to the anterior wall of the axillary artery was 4.24 +/- 1.49 cm (2.25-7.75 cm) in men and 4.01 +/- 1.29 cm (2.25-6.5 cm) in women. Hopefully, this study will facilitate the use of this block.

IMPLICATIONS

We sought a consistent, palpable landmark for facilitation of the infraclavicular brachial plexus block. We used magnetic resonance images of the brachial plexus to determine the depth and needle orientation needed to contact the brachial plexus. Hopefully, this study will facilitate the use of this block.

Peroneal tendon injuries

Injury to the peroneal tendons is a frequently overlooked cause of persistent lateral ankle pain after trauma. Peroneal tendon anatomy, biomechanics, diagnostic studies, and traumatic disorders were reviewed.

A prospective approach to correct for inter-image head rotation in fMRI

Global head motion occurring between successive image acquisitions during a functional MRI time series can corrupt the signal of physiologic brain activation, potentially invalidating interpretation of the final activation map from that particular fMRI time series. By approximating the head as a rigid body, multiaxial global head motion can be decomposed into orthogonal linear and rotational components. This paper describes a method using orbital navigator echoes to provide prospective correction for both through-plane and in-plane inter-image head rotation in functional MRI. The dynamic detection and correction of rotation can be performed in <100 ms. Phantom experiments demonstrate accurate correction of rotational motion over a range of +/-0.36 degrees to +/-12 degrees. Imaging studies in volunteers document the feasibility of real-time prospective correction of rotational motion in vivo. Using a modified receiver operating characteristic method, motion-corrected functional MRI sensorimotor studies incorporating deliberate head rotations are shown to be superior to functional MRI time series acquired under similar conditions but without motion correction.

The shoulder: adaptive motion correction of MR images

PURPOSE

To evaluate an adaptive-motion-correction technique to reduce global motion in shoulder magnetic resonance (MR) images.

MATERIALS AND METHODS

In the adaptive-motion-correction technique, interleaved navigator echoes are used to provide a measure of view-to-view displacement along the craniocaudal direction for each image echo in the acquisition. The information is then retrospectively applied to the k-space data to correct for global shoulder motion. This algorithm was evaluated in a series of 143 consecutive patient shoulder examinations by comparing the original image set for each patient with the same image set after retrospective correction by means of this algorithm.

RESULTS

The average amplitude of craniocaudal motion was 1.4 mm. Image degradation due to motion was apparent in 100 (70%) of the 143 examinations. Application of the adaptive-motion-correction technique improved image quality in 73 (73%) of these 100 examinations or 51% of all 143 examinations.

CONCLUSION

Adaptive motion correction improved image quality in approximately three-quarters of the examinations in which motion was present.

Fluoroscopically triggered contrast-enhanced three-dimensional MR angiography with elliptical centric view order: application to the renal arteries

PURPOSE

To determine the reliability of obtaining arterial-phase, contrast-material-enhanced three-dimensional (3D) magnetic resonance (MR) angiograms of the renal arteries by using a technique that combines two-dimensional real-time MR fluoroscopy and a 3D MR angiographic acquisition with elliptical centric view order.

MATERIALS AND METHODS

Twenty-five consecutive patients suspected of having renal artery disease were evaluated with the fluoroscopically triggered technique by using a mean dose of 0.18 mmol/kg gadoteridol. Left renal vein suppression, inferior vena cava suppression, motion artifact, and image quality for depiction of the renal arteries were each evaluated on a five-point scale (1 = best). The findings were compared with those of another 25 consecutive patients who underwent conventional gadolinium-enhanced 3D MR angiography.

RESULTS

The fluoroscopically triggered technique produced 4.6 times less left renal vein enhancement than did the conventional method (P < .01). With the fluoroscopically triggered technique, visualization of the renal arteries was adequate for diagnosis in 24 patients (96%) and the overall result (score of 1-3 for all criteria) was of good quality in 22 patients (88%).

CONCLUSION

With this fluoroscopically triggered MR angiographic technique, high-quality, arterial phase, relatively motion immune angiograms can be routinely obtained.

Systemic gadolinium toxicity in patients with renal insufficiency and renal failure: retrospective analysis of an initial experience

OBJECTIVE

To study the possible deleterious systemic effects of gadolinium in patients with impaired renal function.

DESIGN

We retrospectively analyzed the routine laboratory data and clinical course of patients who had undergone a gadolinium-enhanced magnetic resonance imaging (MRI) examination of the brain and spine and had evidence of impaired glomerular filtration.

MATERIAL AND METHODS

Between October 1988 and October 1992, 15,830 patients underwent gadolinium-enhanced MRI at our institution, 151 of whom had a serum creatinine value of more than 2 mg/dL. The clinical records of these 151 patients were thoroughly examined for the period from 3 days before to 30 days after the gadolinium-enhanced MRI examination. All data were analyzed in an attempt to detect any adverse events that could be related to free gadolinium as a result of dissociation from the chelating agent due to prolonged elimination times (that is, increased serum creatinine concentrations). In addition, we calculated the 90-day mortality rate for both the study group and a matched control population of 80 patients who had undergone MRI of the brain and spine before gadolinium was available.

RESULTS

The overall incidence of adverse events in the study group was 3.6%. No event was severe or life threatening--nausea and rash occurred in two patients each, and seizure and headache occurred in one patient each. These findings were not significantly different from those in previous studies performed in populations with normal elimination times. Moreover, no significant difference was noted in the 90-day mortality rate (14.6% of the study group) in comparison with that in the control group (13.8%).

CONCLUSION

On the basis of this initial retrospective analysis, we were unable to detect any clinical deleterious effects of administration of gadolinium for MRI examination in patients with impaired renal function. Further investigation with prospective studies is needed to confirm these initial retrospective findings.

3D MR angiography of pulmonary arteries using real-time navigator gating and magnetization preparation

An ECG-triggered magnetization-prepared segmented 3D fast gradient echo sequence was developed to perform pulmonary arterial MR angiography. A selective inversion recovery pulse was used in the magnetization preparation to suppress venous vasculature. A real-time gating technique based on navigator echoes was implemented to reduce respiration effects. Pencil-beam navigator echoes were acquired immediately before and after the readout train and processed in real-time to dynamically measure the diaphragm position, which was used to control data acquisition with an accept-or-reject-reacquire logic. In a study of 10 volunteers, a gated 3D acquisition with 28 slices required on average approximately 4 min of acquisition time, and six to seven segmental arteries related to the interlobar trunk of the pulmonary artery were depicted. The use of SIR pulse reduced venous signal by 99%. The gated acquisitions were superior to the ungated acquisitions (n = 10, P < 0.005). The real-time navigator gating technique is effective for reduction of respiration effects and thereby makes high resolution 3D MRA of the pulmonary arteries feasible.

Cardiac magnetic resonance fluoroscopy

A technique is described for high speed interactive imaging of the heart with either white or black blood contrast. Thirty-two views of a segmented, magnetization-prepared gradient echo sequence are acquired during diastole. Using three-quarter partial Fourier sampling, data for a complete 128 x 128 image are acquired in three cardiac cycles. High speed reconstruction provides an image update of each cardiac cycle 159 ms after measurement. An independent graphical user interface facilitates interactive control of section localization and contrast by permitting pulse sequence parameter modification during scanning. The efficiency and image quality of the cardiac MR fluoroscopy technique were evaluated in 11 subjects. Compared with the conventional graphic prescription method, the cardiac fluoroscopy technique provides an approximate eightfold reduction in the time required to obtain subject-specific double oblique sections. Image quality for these scout acquisitions performed during free breathing was sufficient to identify small cardiac structures.

Dynamic MR digital subtraction angiography using contrast enhancement, fast data acquisition, and complex subtraction

A dynamic MR angiography technique, MR digital subtraction angiography (MR DSA), is proposed using fast acquisition, contrast enhancement, and complex subtraction. When a bolus of contrast is injected into a patient, data acquisition begins, dynamically acquiring a thick slab using a fast gradient echo sequence for 10-100 s. Similar to x-ray DSA, a mask is selected from the images without contrast enhancement, and later images are subtracted from the mask to generate angiograms. Complex subtraction is used to overcome the partial volume effects related to the phase difference between the flowing and stationary magnetization in a voxel. Vessel signal is the enhancement of flow magnetization resulting from the contrast bolus. MR DSA was performed in 28 patients, including vessels in the lungs, brains, legs, abdomen, and pelvis. All targeted vessels were well depicted with MR DSA. Corresponding dynamic information (contrast arrival time ta and duration of the arterial phase tav) was measured: ta/tav = 3.4/4.7 s for the lung, 10.3/4.9 s for the brain, 12.8/19.3 for the aorta, 15.2/12.6 s for the leg. MR DSA can provide dynamic angiographic images using a very short acquisition time.