- [Show abstract] [Hide abstract] ABSTRACT: Quantitative MRI and MRS are increasingly important as non-invasive outcome measures in therapy development for Duchenne muscular dystrophy (DMD). Many studies have focussed on individual measures such as fat fraction and metabolite levels in relation to age and functionality, but much less attention has been given to how these indices relate to each other. Here, we assessed spatially resolved metabolic changes in leg muscles of DMD patients, and classified muscles according to the degree of fat replacement compared with healthy controls. Quantitative MRI (three-point Dixon and multi-spin echo without fat suppression and a tri-exponential fit) and 2D-CSI (31) P MRS scans were obtained from 18 DMD patients and 12 healthy controls using a 3 T and a 7 T MR scanner. Metabolite levels, T2 values and fat fraction were individually assessed for five lower leg muscles. In muscles with extensive fat replacement, phosphodiester over adenosine triphosphate (PDE/ATP), inorganic phosphate over phosphocreatine, intracellular tissue pH and T2 were significantly increased compared with healthy controls. In contrast, in muscles without extensive fat replacement, only PDE/ATP and T2 values were significantly elevated. Overall, our results show that PDE levels and T2 values increase prior to the occurrence of fat replacement and remain elevated in later stages of the disease. This suggests that these individual measures could not only function as early markers for muscle damage but also reflect potentially reversible pathology in the more advanced stages.
- [Show abstract] [Hide abstract] ABSTRACT: Purpose: To compare methods for estimating B0 maps used in retrospective correction of high-resolution anatomical images at ultra-high field strength. The B0 maps were obtained using three methods: (1) 1D navigators and coil sensitivities, (2) field probe (FP) data and a low-order spherical harmonics model, and (3) FP data and a training-based model. Methods: Data from nine subjects were acquired while they performed activities inducing B0 field fluctuations. Estimated B0 fields were compared with reference data, and the reductions of artifacts were compared in corrected T2* images. Results: Reduction of sum-of-squares difference relative to a reference image was evaluated, and Method 1 yielded the largest artifact reduction: 27 ± 15%, 20 ± 18% (mean ± 1 standard deviation) for deep breathing and combined deep breathing and hand motion activities. Method 3 performed almost as well (24 ± 18%, 15 ± 17%), provided that adequate training data were used, and Method 2 gave a similar result (21 ± 16%, 19 ± 17%). Conclusion: This study confirms that all of the investigated methods can be used in retrospective image correction. In terms of image quality, Method 1 had a small advantage, whereas the FP-based methods measured the B0 field slightly more accurately. The specific strengths and weaknesses of FPs and navigators should therefore be considered when determining which B0 -estimation method to use. Magn Reson Med, 2016. © 2016 International Society for Magnetic Resonance in Medicine.
- [Show abstract] [Hide abstract] ABSTRACT: Background and purpose: Early markers for cerebral amyloid angiopathy are largely unknown. We aimed to identify which magnetic resonance imaging (MRI) (performed at 7 and 3T) and cognitive markers are an early sign in (pre) symptomatic subjects with hereditary cerebral hemorrhage with amyloidosis-Dutch type. Methods: Twenty-seven DNA-proven Dutch-type mutation carriers (15 symptomatic and 12 presymptomatic) (mean age of 45.9 years) and 33 controls (mean age of 45.6 years) were included. 7T and 3T MRI was performed, cerebral amyloid angiopathy and small-vessel disease type MRI markers were estimated, and cognitive performance was assessed. Univariate general linear modeling analysis was used to assess the association between MRI markers and cognitive performance on the one hand and on the other, mutation status, adjusted for age, sex, and education. Results: In symptomatic patients, all established cerebral amyloid angiopathy MRI markers (microbleeds, intracerebral hemorrhages, subarachnoid hemorrhages, superficial siderosis, microinfarcts, volume of white matter hyperintensities, and dilated perivascular spaces in centrum semiovale) were increased compared with controls (P<0.05). In presymptomatic subjects, the prevalence of microinfarcts and median volume of white matter hyperintensities were increased in comparison to controls (P<0.05). Symptomatic patients performed worse on all cognitive domains, whereas presymptomatic subjects did not show differences in comparison with controls (P<0.05). Conclusions: White matter hyperintensities and microinfarcts are more prevalent among presymptomatic subjects and precede cognitive and neuropsychiatric symptoms and intracerebral hemorrhages.
- [Show abstract] [Hide abstract] ABSTRACT: Duchenne muscular dystrophy is caused by dystrophin gene mutations which lead to the absence of the protein dystrophin. A significant proportion of patients suffer from learning and behavioral disabilities, in addition to muscle weakness. We have previously shown that these patients have a smaller total brain and grey matter volume, and altered white matter microstructure compared to healthy controls. Patients with more distal gene mutations, predicted to affect dystrophin isoform Dp140 and Dp427, showed greater grey matter reduction. Now, we studied if cerebral blood flow in Duchenne muscular dystrophy patients is altered, since cerebral expression of dystrophin also occurs in vascular endothelial cells and astrocytes associated with cerebral vasculature. T1-weighted anatomical, and pseudo-continuous arterial spin labeling cerebral blood flow images were obtained from 26 patients and 19 age-matched controls (ages 8-18 years) on a 3 tesla MRI scanner. Group comparisons of cerebral blood flow were made with and without correcting for grey matter volume using partial volume correction. Results showed that patients had a lower cerebral blood flow than controls (40.0±6.4 and 47.8±6.3 mL/100g/min respectively, p=0.0002). This reduction was independent of grey matter volume, suggesting that they are two different aspects of the pathophysiology. Cerebral blood flow was lowest in patients lacking Dp140. There was no difference in CBF between ambulant and non-ambulant patients. Only three patients showed a reduced left ventricular ejection fraction. No correlation between cerebral blood flow and age was found. Our results indicate cerebral perfusion is reduced in Duchenne muscular dystrophy patients independent of the reduced grey matter volume.
- [Show abstract] [Hide abstract] ABSTRACT: Diffusion-tensor imaging and single voxel diffusion-weighted magnetic resonance spectroscopy were used at 7T to explore in vivo age-related microstructural changes in the corpus callosum. Sixteen healthy elderly (age range 60-71 years) and 13 healthy younger controls (age range 23-32 years) were included in the study. In healthy elderly, we found lower water fractional anisotropy and higher water mean diffusivity and radial diffusivity in the corpus callosum, indicating the onset of demyelination processes with healthy aging. These changes were not associated with a concomitant significant difference in the cytosolic diffusivity of the intra-axonal metabolite N-acetylaspartate (p = 0.12), the latter representing a pure measure of intra-axonal integrity. It was concluded that the possible intra-axonal changes associated with normal aging processes are below the detection level of diffusion-weighted magnetic resonance spectroscopy in our experiment (e.g., smaller than 10%) in the age range investigated. Lower axial diffusivity of total creatine was observed in the elderly group (p = 0.058), possibly linked to a dysfunction in the energy metabolism associated with a deficit in myelin synthesis.
- [Show abstract] [Hide abstract] ABSTRACT: Purpose: To implement an on-line monitoring system to detect eye blinks during ocular MRI using field probes, and to reacquire corrupted k-space lines by means of an automatic feedback system integrated with the MR scanner. Methods: Six healthy subjects were scanned on a 7 Tesla MRI whole-body system using a custom-built receive coil. Subjects were asked to blink multiple times during the MR-scan. The local magnetic field changes were detected with an external fluorine-based field probe which was positioned close to the eye. The eye blink produces a field shift greater than a threshold level, this was communicated in real-time to the MR system which immediately reacquired the motion-corrupted k-space lines. Results: The uncorrected images, using the original motion-corrupted data, showed severe artifacts, whereas the corrected images, using the reacquired data, provided an image quality similar to images acquired without blinks. Conclusion: Field probes can successfully detect eye blinks during MRI scans. By automatically reacquiring the eye blink-corrupted data, high quality MR-images of the eye can be acquired. Magn Reson Med, 2016. © 2016 International Society for Magnetic Resonance in Medicine.
- [Show abstract] [Hide abstract] ABSTRACT: Improvements are proposed for practical design and use of high permittivity materials in high field neuroimaging in three different areas: (i) a simple formula to predict the permittivity of tri-component aqueous-based perovskite suspensions with relative permittivities between 110 and 300, (ii) characterization of addition of a hydroxyethyl-cellulose gelling agent to improve the long-term stability and material properties of "dielectric pads", and (iii) investigation of the integration of, for example, headphones into the dielectric pads to increase patient comfort within tightly-fitting receive coil arrays.
- [Show abstract] [Hide abstract] ABSTRACT: Purpose: Even when guided by SPECT/CT planning of nodal resection in the head-and-neck area is challenging due to the many critical anatomical structures present within the surgical field. In this study the potential of a (SPECT/)MRI-based surgical planning method was explored. Hereby MRI increases the identification of SNs within clustered lymph nodes (LNs) and vital structures located adjacent to the SN (such as cranial nerve branches). Method and patients: SPECT/CT and pathology reports from 100 head-and-neck melanoma and 40 oral cavity cancer patients were retrospectively assessed for SN locations in levels I–V and degree of nodal clustering. A diffusion-weighted-preparation magnetic resonance neurography (MRN) sequence was used in eight healthy volunteers to detect LNs and peripheral nerves. Results: In 15% of patients clustered nodes were retrospectively shown to be present at the location where the SN was identified on SPECT/CT (level IIA: 37.2%, level IIB: 21.6% and level III: 15.5%). With MRN, improved LN delineation enabled discrimination of individual LNs within a cluster. Uniquely, this MRI technology also provided insight in LN distribution (23.2 ± 4 LNs per subject) and size (range 21–372 mm 3), and enabled non-invasive assessment of anatomical variances in the location of the LNs and facial nerves. Conclusion: Diffusion-weighted-preparation MRN enabled improved delineation of LNs and their surrounding delicate anatomical structures in the areas that most often harbor SNs in the head-and-neck. Based on our findings a combined SPECT/MRI approach is envisioned for future surgical planning of complex SN resections in this region.
- [Show abstract] [Hide abstract] ABSTRACT: Purpose: To design and characterize dual-segment dielectric resonators (DR) and assess their performance for in vivo imaging and localized proton spectroscopy at 7 Tesla. Methods: Annular DRs operating in the HEM11 mode at ultrahigh field have advantage of a simple design with an intrinsically circularly polarized homogeneous mode. However, practical realization for extremity imaging requires a splittable design for patient comfort and positioning. Splitting the annulus segments breaks the resonator symmetry and the circularly polarized distribution is lost. Here, we show that one can restore the electromagnetic modes by incorporating copper connectors between the segments. By designing the connectors such that two modes become frequency degenerate, one can also generate circularly polarized modes in a half-annular resonator. Electromagnetic simulations were performed and phantom and in vivo experiments conducted using custom-built DRs. Results: Electromagnetic simulations showed that using copper interconnectors, the resonant modes could be restored. This was confirmed in phantom experiments, as well as in vivo images of the human knee using a dual-segment splittable annular DR and images and localized spectra of the human calf muscle acquired using a dual-segment half-annular resonator. Conclusion: A new approach incorporating copper connectors into the DR structure allows efficient operation of splittable DRs.
- [Show abstract] [Hide abstract] ABSTRACT: Systemic lupus erythematosus is an inflammatory autoimmune disease with multi-organ involvement. Central nervous system involvement in systemic lupus erythematosus is common and results in several neurological and psychiatric symptoms that are poorly linked to standard magnetic resonance imaging outcome. Magnetic resonance imaging methods sensitive to tissue microstructural changes, such as diffusion tensor imaging and magnetization transfer imaging, show some correlation with neuropsychiatric systemic lupus erythematosus (NPSLE) symptoms. Histological examination of NPSLE brains reveals presence of cerebral oedema, loss of neurons and myelinated axons, microglial proliferation and reactive astrocytosis, microinfacrts and diffuse ischaemic changes, all of which can affect both diffusion tensor imaging and magnetization transfer imaging in a non-specific manner. Here we investigated the underlying cell-type specific microstructural alterations in the brain of patients with systemic lupus erythematosus with and without a history of central nervous system involvement. We did so combining diffusion tensor imaging with diffusion-weighted magnetic resonance spectroscopy, a powerful tool capable of characterizing cell-specific cytomorphological changes based on diffusion of intracellular metabolites. We used a 7 T magnetic resonance imaging scanner to acquire T1-weighted images, diffusion tensor imaging datasets, and single volume diffusion-weighted magnetic resonance spectroscopy data from the anterior body of the corpus callosum of 13 patients with systemic lupus erythematosus with past NPSLE, 16 patients with systemic lupus erythematosus without past NPSLE, and 19 healthy control subjects. Group comparisons were made between patients with systemic lupus erythematosus with/without past NPSLE and healthy controls on diffusion tensor imaging metrics and on diffusion coefficients of three brain metabolites: the exclusively neuronal/axonal N-acetylaspartate, and the predominantly glial creatine + phosphocreatine and choline compounds. In patients with systemic lupus erythematosus with past NPSLE, significantly higher diffusion tensor imaging mean and radial diffusivities were accompanied by a significantly higher intracellular diffusion of total creatine (0.202 ± 0.032 μm2/ms, P = 0.018) and total choline (0.142 ± 0.031 μm2/ms, P = 0.044) compared to healthy controls (0.171 ± 0.024 μm2/ms, 0.124 ± 0.018 μm2/ms, respectively). Total N-acetylaspartate, total creatine and total choline diffusion values from all patients with systemic lupus erythematosus correlated positively with systemic lupus erythematosus disease activity index score (P = 0.033, P = 0.040, P = 0.008, respectively). Our results indicate that intracellular alterations, and in particular changes in glia, as evidenced by increase in the average diffusivities of total choline and total creatine, correlate with systemic lupus erythematosus activity. The higher diffusivity of total creatine and total choline in patients with NPSLE, as well as the positive correlation of these diffusivities with the systemic lupus erythematosus disease activity index are in line with cytomorphological changes in reactive glia, suggesting that the diffusivities of choline compounds and of total creatine are potentially unique markers for glial reactivity in response to inflammation.
- [Show abstract] [Hide abstract] ABSTRACT: Purpose: Arteriolar cerebral-blood-volume (CBVa) is an important perfusion parameter that can be measured using inflow-based vascular-space-occupancy (iVASO) MRI without exogenous contrast agent administration. The purpose of this study is to assess the potential diagnostic value of CBVa in brain tumor patients by comparing it with total-CBV (including arterial, capillary and venous vessels) measured by dynamic-susceptibility-contrast (DSC) MRI. Materials and methods: Twelve brain tumor patients were scanned using iVASO (on 7T as part of a research project) and DSC (on 3T as part of routine clinical protocols) MRI. Region-of-interest analysis was performed to compare the resulting perfusion measures between tumoral and contralateral regions, and to evaluate their associations with tumor grades. Results: CBVa measured by iVASO MRI significantly correlated with WHO grade (ρ = 0.37, P = 0.04). Total-CBV measured by DSC MRI showed a trend of correlation with WHO grade (ρ = 0.28, P = 0.5). The signal-to-noise ratio was comparable (P > 0.1) between the two methods, while the contrast-to-noise ratio between tumoral and contralateral regions was higher in iVASO-CBVa than DSC-CBV in WHO II/III patients (P < 0.05) but comparable in WHO IV patients (P > 0.1). A trend of positive correlation between DSC-CBV and iVASO-CBVa was observed (R(2) = 0.28, P = 0.07). Conclusion: In this initial patient study, CBVa demonstrated a stronger correlation with WHO grade than total-CBV. Further investigation with a larger cohort is warranted to validate whether CBVa can be a better classifier than total-CBV for the stratification of brain tumors, and whether iVASO MRI can be a useful alternative method for the assessment of tumor perfusion, especially when exogenous contrast agent administration is difficult in certain patient populations. J. Magn. Reson. Imaging 2016.
- [Show abstract] [Hide abstract] ABSTRACT: Purpose: To optimize data acquisition parameters in cardiac proton MR spectroscopy, and to evaluate the intra- and intersession variability in myocardial triglyceride content. Materials and methods: Data acquisition parameters at 3 Tesla (T) were optimized and reproducibility measured using, in total, 49 healthy subjects. The signal-to-noise-ratio (SNR) and the variance in metabolite amplitude between averages were measured for: (i) global versus local power optimization; (ii) static magnetic field (B0 ) shimming performed during free-breathing or within breathholds; (iii) post R-wave peak measurement times between 50 and 900 ms; (iv) without respiratory compensation, with breathholds and with navigator triggering; and (v) frequency selective excitation, Chemical Shift Selective (CHESS) and Multiply Optimized Insensitive Suppression Train (MOIST) water suppression techniques. Using the optimized parameters intra- and intersession myocardial triglyceride content reproducibility was measured. Two cardiac proton spectra were acquired with the same parameters and compared (intrasession reproducibility) after which the subject was removed from the scanner and placed back in the scanner and a third spectrum was acquired which was compared with the first measurement (intersession reproducibility). Results: Local power optimization increased SNR on average by 22% compared with global power optimization (P = 0.0002). The average linewidth was not significantly different for pencil beam B0 shimming using free-breathing or breathholds (19.1 Hz versus 17.5 Hz; P = 0.15). The highest signal stability occurred at a cardiac trigger delay around 240 ms. The mean amplitude variation was significantly lower for breathholds versus free-breathing (P = 0.03) and for navigator triggering versus free-breathing (P = 0.03) as well as for navigator triggering versus breathhold (P = 0.02). The mean residual water signal using CHESS (1.1%, P = 0.01) or MOIST (0.7%, P = 0.01) water suppression was significantly lower than using frequency selective excitation water suppression (7.0%). Using the optimized parameters an intrasession limits of agreement of the myocardial triglyceride content of -0.11% to +0.04%, and an intersession of -0.15% to +0.9%, were achieved. The coefficient of variation was 5% for the intrasession reproducibility and 6.5% for the intersession reproducibility. Conclusion: Using approaches designed to optimize SNR and minimize the variation in inter-average signal intensities and frequencies/phases, a protocol was developed to perform cardiac MR spectroscopy on a clinical 3T system with high reproducibility. J. Magn. Reson. Imaging 2016.
- [Show abstract] [Hide abstract] ABSTRACT: The fractional derivative operator provides a mathematical means to concisely describe a heterogeneous and relatively complex system that exhibits non-local, power-law behavior. Discretization of a fractional partial differential equation is non-trivial and computationally intensive. Furthermore, the closed form solution, particularly in the case of the fractional time derivative, comes in the form of a convergent power series of the Mittag-Leffler type, which involves considerable computational burden for accurate and efficient calculation. Here, we extend the Padé global approximation technique in order to accurately compute the Mittag-Leffler function and its inverse operation. Furthermore, we introduce the use of the Padé technique for the partial derivatives of the Mittag-Leffler function, which holistically eliminates the need for the calculation of any convergent power series when fitting to data sets. The Padé approximations provide great utility, particularly in the field of diffusion-weighted magnetic resonance imaging, in which Mittag-Leffler function and its partial derivatives are computed on a voxel-wise basis, with each data set containing on the order of 1e7 voxels. Finally, the Padé approximation codes are provided freely for those interested in modeling and fitting data for processes encompassed by this convergent power series.
- [Show abstract] [Hide abstract] ABSTRACT: Methods: Recent studies have shown that an annular cylinder filled with water can serve as dielectric resonator for proton MRI of the extremities at 7 T. Using a very high permittivity material such as BaTiO3, this type of dielectric resonator can potentially be designed for lower gyromagnetic ratio nuclei. Combining this with a remote antenna for proton imaging, an alternative method for dual frequency imaging at ultra-high field has been implemented. Results: 3D electromagnetic simulations were performed to examine the efficiency of the dielectric resonator. The new dielectric resonator was constructed for 31P acquisition at 121 MHz on a human 7 T MRI system. Phantom and in-vivo scans demonstrated the feasibility of the setup, although the current sensitivity of the dielectric resonator is only-half that of an equivalently-sized birdcage. Conclusion: The new approach offers a simple implementation for dual nuclei imaging at ultra-high field, with several possibilities for further increases in sensitivity. Significance: Utilizing high permittivity materials enables very simple designs for high field RF coils: in the current configuration the interactions between the proton and phosphorous resonators are very low.
- [Show abstract] [Hide abstract] ABSTRACT: Objectives: To assess the tumour dimensions in uveal melanoma patients using 7-T ocular MRI and compare these values with conventional ultrasound imaging to provide improved information for treatment options. Materials and methods: Ten uveal melanoma patients were examined on a 7-T MRI system using a custom-built eye coil and dedicated 3D scan sequences to minimise eye-motion-induced image artefacts. The maximum tumour prominence was estimated from the three-dimensional images and compared with the standard clinical evaluation from 2D ultrasound images. Results: The MRI protocols resulted in high-resolution motion-free images of the eye in which the tumour and surrounding tissues could clearly be discriminated. For eight of the ten patients the MR images showed a slightly different value of tumour prominence (average 1.0 mm difference) compared to the ultrasound measurements, which can be attributed to the oblique cuts through the tumour made by the ultrasound. For two of these patients the more accurate results from the MR images changed the treatment plan, with the smaller tumour dimensions making them eligible for eye-preserving therapy. Conclusion: High-field ocular MRI can yield a more accurate measurement of the tumour dimensions than conventional ultrasound, which can result in significant changes in the prescribed treatment.
- [Show abstract] [Hide abstract] ABSTRACT: Ultra-high field MRI is an area of great interest for clinical research and basic science due to the increased signal-to-noise, spatial resolution and magnetic-susceptibility-based contrast. However, the fact that the electromagnetic wavelength in tissue is comparable to the relevant body dimensions means that the uniformity of the excitation field is much poorer than at lower field strengths. In addition to techniques such as transmit arrays, one simple but effective method to counteract this effect is to use high permittivity "pads". Very high permittivities enable thinner, flexible pads to be used, but the limiting factor is wavelength effects within the pads themselves, which can lead to image artifacts. So far, all studies have used simple continuous rectangular/circular pad geometries. In this work we investigate how the wavelength effects can be partially mitigated utilizing shaped pad with holes. Several arrangements have been simulated, including low order pre-fractal geometries, which maintain the overall coverage of the pad, but can provide better image homogeneity in the region of interest or higher sensitivity depending on the setup. Experimental data in the form of in-vivo human images at 7 T were acquired to validate the simulation results.
- [Show abstract] [Hide abstract] ABSTRACT: This study presents a novel method for 3D imaging of tubular structures, such as blood vessels and catheters. With this method, the 3D object is reconstructed from three mutually exclusive orthogonal projections of the same volume. This triplanar projection imaging (TPI) was evaluated on a phantom filled with T1-shortening, Gd-based contrast agent embedded into a matrix. The projected structures were segmented out on each one of the projections and a backprojection algorithm was used to generate a rendering of the tubular structure in 3D. The accuracy of the method was investigated by comparing the centerline of the 3D tubular structure generated from TPI with the centerline extracted from a multislice set of images of the same volume. The two tubular structures were well matched in 3D. With further optimization and reduction of acquisition time, this method can be used for 3D fast imaging of interventional tools or blood vessels with applications in interventional MRI.
Chapter: Electromagnetic modelling[Show abstract] [Hide abstract] ABSTRACT: The efficiencies of the designs of the different coils used to produce the static, gradient, and radiofrequency magnetic fields, as well as assessing the various interactions between these fields and the subject being imaged, are fundamental to the performance and safety of MRI. Here we review the basic field equations for these types of interactions and how they can be used to simulate MRI performance and safety.
Urbana UniversityUrbana, Illinois, United States
University of Illinois, Urbana-Champaign
Urbana, Illinois, United States
- Department of Electrical and Computer Engineering
University of FloridaGainesville, Florida, United States
William Penn UniversityWorcester, Massachusetts, United States
University of Illinois at Chicago
Chicago, IL, United States
- Department of Bioengineering