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.

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.

Magnetic resonance elastography by direct visualization of propagating acoustic strain waves.

A nuclear magnetic resonance imaging (MRI) method is presented for quantitatively mapping the physical response of a material to harmonic mechanical excitation. The resulting images allow calculation of regional mechanical properties. Measurements of shear modulus obtained with the MRI technique in gel materials correlate with independent measurements of static shear modulus. The results indicate that displacement patterns corresponding to cyclic displacements smaller than 200 nanometers can be measured. The findings suggest the feasibility of a medical imaging technique for delineating elasticity and other mechanical properties of tissue.

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.

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 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.

Magnetic resonance imaging in pediatric hematology/oncology. Part I. Basic technology.

Magnetic resonance imaging (MRI) is a promising new diagnostic modality that is well suited for the evaluation of children with hematological or oncological diagnosis. The side effects of ionizing radiation are avoided, the tomographic pathological anatomy in three orthogonal planes can be obtained, and differences between normal and abnormal tissues are often present. In order to present our preliminary clinical experience with MRI in pediatric hematology and oncology, the historical background of MRI, the technique, and possibilities for tissue characterization are reviewed.

Magnetic resonance imaging in pediatric hematology/oncology. Part II. Illustrative cases and assessment of technique.

Preliminary clinical experience with magnetic resonance imaging (MRI) in 28 pediatric patients with 20 different hematological diseases, benign tumors, or malignant neoplasms is presented. The clinical results are presented in the form of case presentations that are discussed in the context of alternative diagnostic imaging modalities. Also discussed are the known biological effects, or lack thereof, the need for sedation, the effect of motion, the effect of MRI on foreign metallic objects, the financial considerations, and the trends for the future of MRI. This imaging modality has many unique merits. Present difficulties should be overcome by future innovations, making MRI even more efficacious for the diagnosis of blood diseases and cancer in children.

Deep branch ulnar neuropathy due to giant cell tumor: report of a case.

Electrophysiologic studies indicated a lesion of the deep branch of the ulnar nerve in the palm of a 42-year-old man with painless weakness and wasting of ulnar-innervated intrinsic hand muscles. Magnetic resonance imaging of this region demonstrated a soft-tissue mass in the palm distal to Guyon's canal. Surgical exploration disclosed a giant cell tumor of the tendon sheath, compressing the deep branch of the ulnar nerve.

Magnetic resonance imaging of the heart: current status.

The favorable technical capabilities of magnetic resonance imaging (MRI) make it well suited for delineating the heart and great vessels. The clinical applications of cardiac MRI have gradually expanded in the past several years. Currently, the most important applications use the modality to provide detailed morphologic information, especially about surgical lesions of the heart. Particularly noteworthy have been applications of MRI in preoperative assessment of patients with certain types of congenital heart disease, cardiac masses, and lesions of the great vessels. Other roles for the modality such as in functional cardiac imaging and nuclear magnetic resonance spectroscopy remain attractive, but these have not yet entered the domain of routine clinical practice.

Magnetic resonance imaging of vascular rings.

This case report illustrates the ability of magnetic resonance imaging (MRI) to define the abnormal aortic anatomy associated with a vascular ring malformation. After a brief review of the clinical and pathologic aspects of vascular rings, the unique characteristics of MRI that make it particularly well suited for cardiovascular imaging are discussed. A comparison between MRI and other currently used imaging modalities emphasizes their differences and indicates some of the advantages of MRI of the cardiovascular system. We recommend MRI as the technique of choice for patients suspected of having a vascular ring and predict that MRI will play an important role in the diagnosis of cardiovascular abnormalities in the future.

The "broken ring" sign in magnetic resonance imaging of partial anomalous pulmonary venous connection to the superior vena cava.

In two patients with partial anomalous pulmonary venous connection to the superior vena cava, we attempted to demonstrate the pertinent anatomic abnormalities by using magnetic resonance imaging. The results show that magnetic resonance imaging is an ideal cardiovascular imaging modality. The observation of a "broken ring" structure in the magnetic resonance image of the superior vena cava is proposed as a diagnostic sign of partial anomalous pulmonary venous connection to the superior vena cava.

Inoperable plasma cell granuloma of the heart: spontaneous decrease in size during an 11-month period.

Plasma cell granuloma occurs in children, typically as an intrapulmonary mass. Surgical excision is the treatment of choice and is usually curative. We report an atypical and unresectable plasma cell granuloma that occurred asymptomatically in the heart of a child and spontaneously decreased in size by 40% during an 11-month period. Thus, plasma cell granuloma should be considered in the differential diagnosis of any child who has a cardiac mass. Observation should be considered a treatment option because this case demonstrated that the cardiac mass can spontaneously recede without therapy.

Correlation between magnetic resonance imaging of the heart and cardiac anatomy.

An understanding of anatomy forms the cornerstone for accurate interpretations of pathologic alterations. In this article, we present cardiac magnetic resonance images and the corresponding sections of normal hearts obtained at autopsy and cut in planes parallel and perpendicular to the ventricular septum in addition to the standard anatomic orthogonal planes (coronal, sagittal, and transverse). This correlation demonstrates the ability of magnetic resonance imaging to display cardiac anatomy accurately and noninvasively. Because magnetic resonance imaging provides excellent contrast between flowing blood and cardiac walls and has the capacity to provide direct images in multiple planes without inherent difficulties, this procedure has advantages over other currently available imaging techniques.

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.

Translational molecular self-diffusion in magnetic resonance imaging. I. Effects on observed spin-spin relaxation.

The reduced T2 (spin-spin) relaxation times (T2obs less than 200 ms) measured on pure fluids on our 0.35T magnetic resonance imagers stimulated an investigation into this phenomenon. The cause for the short T2obs of fluids was found to be translational molecular self-diffusion of hydrogen nuclei through the pulsed slice-selective magnetic gradient in the imagers. Similar reductions in biological tissue T2obs were also attributed to molecular self-diffusion.

Translational molecular self-diffusion in magnetic resonance imaging. II. Measurement of the self-diffusion coefficient.

By varying slice-selective gradients in successive data acquisitions, the first in vitro measurements of molecular self-diffusion coefficients were performed in a magnetic resonance imager at 0.35 Tesla. Reasonably accurate measurements were found by the MRI method in comparison with 2.3 T NMR spectrometer measurements on the same samples, and in comparison to reported literature values. Thus, in addition to T1, T2, mobile proton density, flow velocity, magnetic susceptibility, and chemical shift, molecular self-diffusion coefficients are now added to the list of biophysical parameters measurable by magnetic resonance imaging in the noninvasive characterization of biological systems.

Magnetic resonance imaging in the diagnosis of acute renal allograft rejection and its differentiation from acute tubular necrosis. Experimental study in the dog.

This study was designed to evaluate the potential utility of magnetic resonance imaging (MRI) for the diagnosis of acute renal allograft rejection and its differentiation from acute tubular necrosis (ATN). Eighteen canines were used. Five animals served as controls. ATN was induced in six animals by cross-clamping of the left renal artery for 90 minutes. In order to study acute renal allograft rejection, seven animals were subjected to exchange allograft transplantation of the left kidney. MRI was performed with a 0.35T superconductive magnet. A double spin-echo technique was used with varying TR and TE parameters. The spin echo images were analyzed for morphology, signal intensity, T1 and T2 relaxation times, and spin density. The most useful MRI criteria for the diagnosis of ATN and acute rejection were found to be the renal size, the intensity difference between cortex and medulla (corticomedullary contrast), and the T1 relaxation time of the cortex. Normal kidneys showed maximal corticomedullary contrast (19% +/-2) on images obtained with TR = 0.5 sec and TE = 28 msec. Cortical T1 relaxation time was 551 msec + /-73. In the ATN group, the kidneys were slightly swollen (P = ns) and the corticomedullary contrast (11% + /-3) was reduced by 42% (P less than .01). T1 of the cortex (689 + /-142) was increased by 25% (P less than .10). In acute rejection, significant renal enlargement was noted (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Diradical nitroxyl spin label contrast agents for magnetic resonance imaging. A comparison of relaxation effectiveness.

The proton relaxation enhancement characteristics of seven potential MRI contrast agents containing two nitroxyl spin labels per molecule (diradicals) were compared with eight similar agents with only one spin label per molecule (monoradicals). Diradical nitroxyls were evaluated to test the hypothesis that multiple paramagnetic centers in one molecule will result in stronger proton relaxation enhancement characteristics, allowing effective contrast enhancement at lower molar concentrations and thus a reduced osmotic load and greater safety. The acute toxicity of these agents is believed to be largely related to osmotic load. Five of seven diradical nitroxyls tested had spin-lattice relaxivities that were substantially greater than all eight of the monoradicals tested. The spin-spin relaxation properties of these agents and other pertinent characteristics are favorable for contrast enhancement. The results indicate that diradical nitroxyl spin labels may be used advantageously for the design of safer, more effective MRI contrast agents.