Magnetic Resonance Imaging

Editorial Board

2010-04-01

Quantitative SENSE-MRSI of the human brain

David Bonekamp, Mari A. Smith, He Zhu, Peter B. Barker — 2010-01-04

Abstract: Purpose: To develop a method for estimating metabolite concentrations using phased-array coils and sensitivity-encoded (SENSE) magnetic resonance spectroscopic images (MRSI) of the human brain.Materials and Methods: The method is based on the phantom replacement technique and uses receive coil sensitivity maps and body-coil loading factors to account for receive B1 inhomogeneity and variable coil loading, respectively. Corrections for cerebrospinal fluid content from the MRSI voxel were also applied, and the total protocol scan time was less than 15 min. The method was applied to 10 normal human volunteers using a multislice 2D-MRSI sequence at 3 T, and seven different brain regions were quantified.Results: N-Acetyl aspartate (NAA) concentrations varied from 9.7 to 14.7 mM, creatine (Cr) varied from 6.6 to 10.6 mM and choline (Cho) varied from 1.6 to 3.0 mM, in good general agreement with prior literature values.Conclusions: Quantitative SENSE-MRSI of the human brain is routinely possible using an adapted phantom-replacement technique. The method may also be applied to other MRSI techniques, including conventional phase encoding, with phased-array receiver coils, provided that coil sensitivity profiles can be measured.

Quantitative multivoxel proton chemical shift imaging of the breast

2010-01-14

Abstract: The study of focal pathology by single-voxel magnetic resonance spectroscopy (MRS) is hampered by the impossibility to study tissue heterogeneity or compare the metabolite signals in breast lesion directly to those in unaffected tissue. Multivoxel MRS studies, while potentially allowing for truly quantitative tissue characterization, have up to now also been far from quantitative with, for example, the signal-to-noise ratio of the choline (Cho) signal serving as measure of tumor activity. Shown in this study is that in a standard clinical setting with a regular 1.5-T magnetic resonance scanner, it is possible to perform quantitative multivoxel MRS. With the use of literature values for the T1 and T2 relaxation times of Cho and water in fibroglandular breast tissue and tumors, one can determine the concentrations of Cho in different tumor compartments and surrounding tissues in two brief multivoxel MRS measurements. This opens excellent perspectives to quantitative diagnostic and follow-up studies of focal pathology such as lesions suspected of breast cancer.

Diffusion tensor magnetic resonance imaging of the normal breast

2010-01-11

Abstract: Purpose: The objective of this study was to evaluate diffusion anisotropy of the breast parenchyma and assess the range and repeatability of diffusion tensor imaging (DTI) parameters in normal breast tissue.Materials and Methods: The study was approved by our institutional review board and included 12 healthy females (median age, 36 years). Diffusion tensor imaging was performed at 1.5 T using a diffusion-weighted echo planar imaging sequence. Diffusion tensor imaging parameters including tensor eigenvalues (λ1, λ2, λ3), fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were measured for anterior, central and posterior breast regions.Results: Mean normal breast DTI measures were λ1=2.51×10−3 mm2/s, λ2=1.89×10−3 mm2/s, λ3=1.39×10−3 mm2/s, ADC=1.95±0.24×10−3 mm2/s and FA=0.29±0.05 for b=600 s/mm2. Significant regional differences were observed for both FA and ADC (P<.05), with higher ADC in the central breast and higher FA in the posterior breast. Comparison of DTI values calculated using b=0, 600 s/mm2 vs. b=0, 1000 s/mm2, showed significant differences in ADC (P<.001), but not FA. Repeatability assessment produced within-subject coefficient of variations of 4.5% for ADC and 11.4% for FA measures.Conclusion: This study demonstrates anisotropy of water diffusion in normal breast tissue and establishes a normative range of breast FA values. Attention to the influence of breast region and b value on breast DTI measurements may be important for clinical interpretation and standardization of techniques.

In vivo imaging of the human brain at 1.5 T with 0.6-mm isotropic resolution

2010-02-01

Abstract: We present high-resolution in vivo anatomical scans with 3D whole-brain coverage and an isotropic resolution of 0.6 mm, obtained at a clinical field of 1.5 T. The data are acquired in 10 independent scans over two sessions using a 3D magnetization-prepared, gradient echo sequence, modified to output phase images in addition to magnitude images. The independent scans are coregistered to correct for head motion, prior to performing complex averaging. The resolution of the final, averaged image, is found to be equal to the nominal one.The separation between the distribution of gray-scale values characterizing the gray and white matter, respectively, is substantially improved over single-scan images.Complex and magnitude averaging are compared and found to deliver similar results for regions with a high initial signal-to-noise ratio (SNR) within the brain. However, complex averaging is strongly recommended for quantitative applications or for studies where regions of low initial SNR are important.To summarize, a method for high-resolution in vivo anatomical imaging at a clinical field strength is demonstrated and is recommended for brain mapping. The method can also be applied at higher fields with a reduced acquisition time.

SNR-optimized myocardial perfusion imaging using parallel acquisition for effective density-weighted saturation recovery imaging

2010-01-22

Abstract: The concept of density-weighted imaging and parallel acquisition for effective density-weighted (PLANED) imaging was transferred to saturation recovery (SR) sequences, in order to increase the SNR in first-pass myocardial perfusion imaging. Filtering in combination with density-weighted imaging allows SNR-optimized data weighting and the free choice of the corresponding spatial response function (SRF) simultaneously. This method was evaluated in simulations and applied successfully to phantom and in vivo first-pass myocardial perfusion studies. Unfiltered, Cartesian sampled images were compared to images acquired with SR-PLANED, which has been adjusted to result in an identical SRF as the Cartesian imaging. SNR-optimized SR-PLANED imaging improved the SNR up to 15% without changing acquisition time, the SRF or the field of view (FOV). The presented method provides high image quality and optimized SNR for first-pass myocardial perfusion imaging.

An fMRI study of magnitude comparison and exact addition in children

2010-02-01

Abstract: By contrast to the adult literature, in which a consistent parietofrontal network for number processing has been identified, the data from studies of number processing in children have been less consistent, probably due to differences in study design and control conditions. Number processing was examined using functional magnetic resonance imaging in 18 right-handed children (8–12 years) from the Cape Coloured community in Cape Town, South Africa, using Proximity Judgment and Exact Addition (EA) tasks. The findings were consistent with the hypothesis that, as in adults, the anterior horizontal intraparietal sulcus (HIPS) plays a major role in the representation and manipulation of quantity in children. The posterior medial frontal cortex, believed to be involved in performance monitoring in more complex arithmetic manipulations in adults, was extensively activated even for relatively simple symbolic number processing in the children. Other areas activated to a greater degree in the children included the left precentral sulcus, which may mediate number knowledge and, for EA, the head of the caudate nucleus, which is part of a fronto-subcortical circuit involved in the behavioral execution of sequences. Two regions that have been linked to number processing in adults — the angular gyrus and posterior superior parietal lobule — were not activated in the children. The data are consistent with the inference that although the functional specialization of the anterior HIPS may increase as symbolic number processing becomes increasingly automatic, this region and other elements of the parietofrontal network identified in adults are already reliably and robustly activated by middle childhood.

Assessment of cardiac iron by MRI susceptometry and R2* in patients with thalassemia

2010-01-11

Abstract: A magnetic resonance imaging cardiac magnetic susceptometry (MRI-CS) technique for assessing cardiac tissue iron concentration based on phase mapping was developed. Normal control subjects (n=9) and thalassemia patients (n=13) receiving long-term blood transfusion therapy underwent MRI-CS and MRI measurements of the cardiac relaxation rate R2*. Using MRI-CS, subepicardium and subendocardium iron concentrations were quantified exploiting the hemosiderin/ferritin iron specific magnetic susceptibility. The average of subepicardium and subendocardium iron concentrations and R2* of the septum were found to be strongly correlated (r=0.96, P<.0001), and linear regression analysis yielded CIC (μg Fe/gwet tissue)=(6.4±0.4)·R2* septum (s−1) − (120±40). The results demonstrated that septal R2* indeed measures cardiac iron level.

Detection of bone metastases using diffusion weighted magnetic resonance imaging: comparison with 11C-methionine PET and bone scintigraphy

2010-01-28

Abstract: Purpose: We evaluated the ability of diffusion-weighted imaging (DWI) to detect bone metastasis by comparing the results obtained using this modality with those obtained using 11C-methionine (MET) positron emission tomography (PET) and bone scintigraphy.Materials and methods: This retrospective study involved 29 patients with bone metastasis. DWI was obtained using a single-shot echo planar imaging (EPI) sequence with fat suppression using a short inversion time inversion recovery sequence. The detection capabilities of DWI for bone metastases were compared with those of whole body MET PET (in 19 patients) and 99mTc-methylene diphosphonate bone scintigraphy (in 15 patients).Results: Among the 19 patients who were diagnosed using DWI and PET, the PET identified 39 bone metastases, while the DWI identified 60 metastases out of 69 metastases revealed with conventional magnetic resonance imaging (MRI). Among the 15 patients who were diagnosed using DWI and bone scintigraphy, the bone scintigraphy identified 18 bone metastases, while the DWI identified 72 metastases out of 78 metastases revealed with conventional MRI. The overall bone metastasis detection rates were 56.5% for PET, 23.1% for bone scintigraphy and 92.3% for DWI.Conclusion: DWI is a very sensitive method for detecting bone metastasis and is superior to MET PET and bone scintigraphy in terms of its detection capabilities.

Exploring vision-related acupuncture point specificity with multivoxel pattern analysis

Linling Li, Wei Qin, Lijun Bai, Jie Tian — 2010-01-14

Abstract: Acupoint specificity is one of the central issues of functional magnetic resonance imaging (fMRI) studies of acupuncture and has been under discussed. However, strong and consistent proof has not been provided for the existence of acupoint specificity, and unsuitable analysis approach applied could be the reason. We observed that previous researches of acupoint specificity were mostly based on model-based methods which were limited to make exploration of acupoint specificity because of the inaccurate specified prior. Here we applied multi-voxel pattern analysis (MVPA) to investigate the specificity of brain activation patterns induced by acupuncture stimulations at a vision-related acupoint (GB37) and a nearby nonacupoint (NAP). Results showed that multiple brain areas could differentiate the central neural response patterns induced by acupuncture stimulation at these two sites with higher accuracy above the chance level. These regions included occipital cortex, limbic-cerebellar areas and somatosensory cortex. Our results support that the characteristic neural response patterns of brain cortex to the acupuncture stimulation at GB37 and a nearby NAP could differ from each other effectively with the application of MVPA approach.

EEG topography-specific BOLD changes: a continuous EEG-fMRI study in a patient with focal epilepsy

2010-01-14

Abstract: Blood oxygenation level dependent (BOLD) response related to interictal activity was evaluated in a patient with post-traumatic focal epilepsy at repeated continuous electroencephalogram (EEG)-functional magnetic resonance imaging examinations. Lateralized interictal EEG activity induced a main cluster of activation co-localized with the anatomical lesion. Spreading of EEG interictal activity to both frontal lobes evoked bilateral clusters of activation indicating that topography of BOLD response might depend on the spatial distribution of epileptiform activity.

MR tracking of magnetically labeled mesenchymal stem cells in rats with liver fibrosis

2010-01-22

Abstract: Purpose: In vivo magnetic resonance (MR) tracking of magnetically labeled bone marrow mesenchymal stem cells (BMSCs) administered via the mesenteric vein to rats with liver fibrosis.Materials and Methods: Rat BMSCs were labeled with superparamagnetic iron oxide (SPIO) and the characteristics of the BMSCs after labeling were investigated. Eighteen rats with CCL4-induced liver fibrosis were randomized to three groups to receive SPIO-labeled BMSCs (BMSC-labeled group), cell-free SPIO (SPIO group), or unlabeled BMSCs (control group). MR imaging of the liver was performed at different time points, and signal-to-noise ratio (SNR) of the liver was measured. In vivo distribution of delivered BMSCs was assessed by histological analysis.Results: Labeling of BMSCs with SPIO did not significantly alter cell viability and proliferation activity. In BMSC-labeled group, the liver SNR immediately decreased from 8.56±0.26 to 3.53±0.41 at 1 h post injection and remained at a significantly lower level till 12 days (P<.05 versus the level before). By contrast, the liver SNR of the SPIO group almost recovered to the preinjection level (P=.125) at 3 days after a transient decrease. In control group, the liver SNR demonstrated no significant difference at the tested time points. Additionally, Prussian blue-positive cells were mainly distributed in the liver parenchyma, especially in injured areas.Conclusion: The magnetically labeled BMSCs infused through the mesenteric vein can be detected in the fibrotic liver of rats using in vivo MR imaging up to 12 days after injection.

Initial in vivo rodent sodium and proton MR imaging at 21.1 T

Victor D. Schepkin, William W. Brey, Peter L. Gor'kov, Samuel C. Grant — 2010-01-04

Abstract: The first in vivo sodium and proton magnetic resonance (MR) images and localized spectra of rodents were attained using the wide bore (105 mm) high resolution 21.1-T magnet, built and operated at the National High Magnetic Field Laboratory (Tallahassee, FL, USA). Head images of normal mice (C57BL/6J) and Fisher rats (∼250 g) were acquired with custom designed radiofrequency probes at frequencies of 237/900 MHz for sodium and proton, respectively. Sodium MR imaging resolutions of ∼0.125 μl for mouse and rat heads were achieved by using a 3D back-projection pulse sequence. A gain in SNR of ∼3 for sodium and ∼2 times for proton were found relative to corresponding MR images acquired at 9.4 T. 3D Fast Low Angle Shot (FLASH) proton mouse images (50×50×50 μm3) were acquired in 90 min and corresponding rat images (100×100×100 μm3) within a total time of 120 min. Both in vivo large rodent MR imaging and localized spectroscopy at the extremely high field of 21.1 T are feasible and demonstrate improved resolution and sensitivity valuable for structural and functional brain analysis.

Quantitative evaluation of optimal imaging parameters for single-cell detection in MRI using simulation

Ali-Reza Mohammadi-Nejad, Gholam-Ali Hossein-Zadeh, Hamid Soltanian-Zadeh — 2010-01-11

Abstract: Super-paramagnetic iron oxide (SPIO) nanoparticles are actively investigated to enhance disease detection through molecular imaging using magnetic resonance imaging (MRI). Detection of the cells labeled by SPIO depends on the MRI protocols and pulse sequence parameters that can be optimized. To evaluate the sensitivity and specificity of the image acquisition methods and to obtain optimal imaging parameters for single-cell detection, we further developed an MRI simulator. The simulator models an object (tissue) at a microscopic level to evaluate effects of spatial distribution and concentration of nanoparticles on the resulting image. In this study, the simulator was used to evaluate and compare imaging of the labeled cells by the gradient-echo (GE), true-FISP [fast imaging employing steady-state acquisition (FIESTA)] and echo-planar imaging (EPI) pulse sequences. Effects of the imaging and object parameters, such as field strength, imaging protocol and pulse sequence parameters, imaging resolution, cell iron load, position of SPIO within the voxel and cell division within the voxel, were investigated in the work. The results suggest that true-FISP has the highest sensitivity for single-cell detection by MRI.

Model-based PRFS thermometry using fat as the internal reference and the extended Prony algorithm for model fitting

Xinyi Pan, Cheng Li, Kui Ying, Dehe Weng, Wen Qin, Kuncheng Li — 2010-02-05

Abstract: A model-based proton resonance frequency shift (PRFS) thermometry method was developed to significantly reduce the temperature quantification errors encountered in the conventional phase mapping method and the spatiotemporal limitations of the spectroscopic thermometry method. Spectral data acquired using multi-echo gradient echo (GRE) is fit into a two-component signal model containing temperature information and fat is used as the internal reference. The noniterative extended Prony algorithm is used for the signal fitting and frequency estimate. Monte Carlo simulations demonstrate the advantages of the method for optimal water-fat separation and temperature estimation accuracy. Phantom experiments demonstrate that the model-based method effectively reduces the interscan motion effects and frequency disturbances due to the main field drift. The thermometry result of ex vivo goose liver experiment with high intensity focused ultrasound (HIFU) heating was also presented in the paper to indicate the feasibility of the model-based method in real tissue.

A method for automatic identification of water and fat images from a symmetrically sampled dual-echo Dixon technique

2010-01-11

Abstract: Sampling water and fat signals symmetrically (i.e., at 0° and 180° relative phase angles) in a dual-echo Dixon technique offers high intrinsic tolerance to phase fluctuations in postprocessing and maximum signal-to-noise performance for the separated water and fat images. However, identification of which image is water and which image is fat after their separation is not possible based on the phase information alone. In this work, we proposed a semiempirical automatic image identification method that is based on the intrinsic asymmetry between the water and fat chemical shift spectra. Specifically, the approximately bimodal feature of the fat spectra and the observation that most in vivo tissues are either predominantly water or predominantly fat are used to construct a spectrum-based algorithm. Additional refinement is accomplished by considering the spatial distribution of the tissues that may have a coexistence of water and fat. The final improved algorithm was tested on a total of 131 three-dimensional patient datasets collected from different scanners and found to yield correct water and fat identification in all datasets.

Double-spin-echo diffusion weighting with a modified eddy current adjustment

Jürgen Finsterbusch — 2010-01-14

Abstract: Magnetic field inhomogeneities like eddy current-related gradient fields cause geometric distortions in echo-planar imaging (EPI). This in particular affects diffusion-weighted imaging where these distortions vary with the direction of the diffusion weighting and hamper the accurate determination of diffusion parameters. The double-spin-echo preparation often used aims to reduce the cumulative eddy current effect by adjusting the diffusion-weighting gradient pulse durations to the time constant of the dominant eddy current contribution. However, eddy currents with a variety of time constants may be present and cause residual distortions. Here, a modification is proposed where the two bipolar gradient pairs of the preparation are adjusted independently to different time constants. At the expense of a slightly prolonged echo time, residual geometric distortions and correspondingly increased values of the diffusion anisotropy can be reduced as is demonstrated in phantoms and the human brain. Thus, it may help to improve the reliability of diffusion-weighted EPI.

Improved B0 field map estimation for high field EPI

2010-02-05

Abstract: Echo planar imaging (EPI) is an ultrafast magnetic resonance imaging (MRI) technique that allows one to acquire a 2D image in about 100 ms. Unfortunately, the standard EPI images suffer from substantial geometric distortions, mainly originating from susceptibility differences in adjacent tissues. To reduce EPI distortions, correction methods based on a field map, which is a map of the off-resonance frequencies, have been developed. In this work, a nonlinear least squares estimator is used to optimize the estimation of the field map of the B0 field. The model of the EPI and reference data includes parameters for the phase evolution, the complex magnitude, the relaxation of the MRI signal and the EPI-specific phase difference between odd and even echoes, and from these parameters, additional corrections might be computed. The reference data required to estimate the field map can be acquired with a modified EPI-sequence. The proposed method is tested on simulated as well as experimental data and proves to be significantly more robust against noise, compared to the previously suggested method.

Quantitative analysis of spatial distortions of diffusion techniques at 3T

2010-01-22

Abstract: Diffusion has been widely adopted in the clinical setting to study the microstructural tissue changes in conjunction with anatomic imaging and metabolic imaging to offer insights on the status of the tissue injury or lesion. However, geometric distortions caused by magnetic susceptibility effects, eddy currents and gradient imperfections greatly affect the clinical utility of the diffusion images. Several diffusion methods have been proposed in the recent years to obtain diffusion parameters with increased accuracy. In most cases, the comparisons to the clinical standard echo-planar imaging (EPI) diffusion are done visually without quantitative measurements. In this study, we present three simple, complementary quantitative methods of nonrigid image registration and shape analyses for evaluating spatial distortions on magnetic resonance images with application in comparing single-shot fast spin-echo (SSFSE) and EPI based diffusion measurements. These methods have confirmed the SSFSE based diffusion method is less distorted than the EPI based one, which is generally accepted through visual inspection.

Incidental abdominal aortic aneurysm on lumbosacral magneticresonance imaging — a case series

Alex J. Trompeter, Guy P. Paremain — 2010-01-11

Abstract: Magnetic resonance imaging (MRI) is commonly used as part of the assessment of patients presenting with leg/back pain to the orthopedic spinal outpatient clinic. Abdominal aortic aneurysm (AAA) can cause symptoms often similar to those of spinal stenosis. We report a case series of four patients who had incidental AAA detected on lumbosacral MRI. All patients were suffering from degenerative spinal disease and had been referred to the orthopedic spinal clinic. After history, examination and review of the imaging, all patients were referred to a vascular surgeon, and three were found to be completely asymptomatic from their aneurysm. One patient required open repair with an aortic graft due to the size of the aneurysm, although his symptoms were attributable to his spinal disease. All patients still required management of their degenerative spinal disease after their vascular review. We can find no other case reports of AAA as an incidental finding on lumbosacral MRI. This case series highlights the importance of looking at all aspects of our imaging and remembering the nonspinal causes of back and leg pains. Furthermore, in the presence of AAA when managing patients in the orthopedic outpatient setting, the authors recommend vascular review before offering orthopedic interventional management options to these patients.

Magnetic Resonance Imaging

Editorial Board

Contents

Quantitative SENSE-MRSI of the human brain

Quantitative multivoxel proton chemical shift imaging of the breast

Diffusion tensor magnetic resonance imaging of the normal breast

In vivo imaging of the human brain at 1.5 T with 0.6-mm isotropic resolution

SNR-optimized myocardial perfusion imaging using parallel acquisition for effective density-weighted saturation recovery imaging

An fMRI study of magnitude comparison and exact addition in children

Assessment of cardiac iron by MRI susceptometry and R2* in patients with thalassemia

Detection of bone metastases using diffusion weighted magnetic resonance imaging: comparison with 11C-methionine PET and bone scintigraphy

Exploring vision-related acupuncture point specificity with multivoxel pattern analysis

EEG topography-specific BOLD changes: a continuous EEG-fMRI study in a patient with focal epilepsy

MR tracking of magnetically labeled mesenchymal stem cells in rats with liver fibrosis

Initial in vivo rodent sodium and proton MR imaging at 21.1 T

Quantitative evaluation of optimal imaging parameters for single-cell detection in MRI using simulation

Model-based PRFS thermometry using fat as the internal reference and the extended Prony algorithm for model fitting

A method for automatic identification of water and fat images from a symmetrically sampled dual-echo Dixon technique

Double-spin-echo diffusion weighting with a modified eddy current adjustment

Improved B0 field map estimation for high field EPI

Quantitative analysis of spatial distortions of diffusion techniques at 3T

Incidental abdominal aortic aneurysm on lumbosacral magneticresonance imaging — a case series