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High-pass filtering is required for the removal of background field inhomogeneities in magnetic resonance phase images. This high-pass filtering smooths across boundaries between areas with large differences in phase. The most prominent boundary is the surface of the brain where areas with large phase values inside the brain are located close to areas outside the brain where the phase is, on average, zero. Cortical areas, which are of great interest in brain MRI, are therefore often degraded by high-pass filtering. Here, we propose the use of the bilateral filter for the high-pass filtering step. The bilateral filter is essentially a Gaussian filter that stops smoothing at boundaries. We show that the bilateral filter improves image quality at the brain's surface, without sacrificing contrast within the brain.

BACKGROUND AND PURPOSE: On clinical MR images, the subthalamic nuclei (STN) are poorly delineated from adjacent structures, impeding safe direct targeting for placement of electrodes in the treatment of Parkinson disease. Susceptibility-weighted MR phase imaging offers improved contrast and spatial resolution at reduced imaging times relative to clinically used T2-weighted spin-echo imaging for STN visualization. Our purpose was to assess STN visibility by using phase imaging, comparing phase and magnitude images obtained concurrently by using susceptibility-weighted imaging (SWI). The goal was to identify an efficient scanning protocol for high-quality phase images of STN. MATERIALS AND METHODS: Seventy-eight SWI scans were acquired at 3T by using different TEs and acceleration factors. STN visibility and delimitation from adjacent structures were scored from 0 (not interpretable) to 5 (excellent). Regression analyses assessed the relationship of STN visibility to scanning parameters RESULTS: STN were identified at all studied TEs on phase images. Visibility and delimitation of STN were consistently superior on phase images compared with magnitude images. Good visualization (score of ≥4) of STN on phase imaging occurred at a mean TE of 20.0 ms and a sensitivity encoding (SENSE) of 1.40. Scores of STN visualization on phase images were dependent on SENSE (P < .002) and TE (P < .031). Good delimitation of the STN on phase imaging occurred at a mean TE of 21.6 ms and a SENSE of 1.36. CONCLUSIONS: Visualization and delimitation of STN was superior on phase images and was achieved at 3T in

Purpose To perform systematic investigations on parameter selection of a dual-echo sequence (ToF-SWI) for combined 3D time-of-flight (ToF) angiography and susceptibility weighted imaging (SWI). Materials and Methods ToF-SWI was implemented on 1.5 T and 3 T MR scanners with complete 3D first-order flow compensation of the second echo. The efficiency of flow compensating the SWI echo was studied based on phantom and in vivo examinations. Arterial and venous contrasts were examined in volunteers as a function of flip angle and compared with additionally acquired single-echo ToF and single-echo SWI data. Results Complete flow compensation is required to reduce arterial contamination in the SWI part caused by signal voids. A ramped flip angle of 20° depicted arteries best while venous contrast was preserved. Comparing ToF-SWI with single-echo ToF demonstrated arteries with similar quality and delineated all major arteries equally well. Venous delineation was degraded due to lower SNR associated with the thinner slabs used with ToF-SWI compared to single-echo SWI acquisition. Conclusion A dual-echo sequence (ToF-SWI) with full flow compensation of the second echo in a single scan is feasible. This sequence allows simultaneous visualization of intrinsically coregistered arteries and veins without spatial mis-registration of vessels caused by oblique flow and with minimal signal loss in arteries. J. Magn. Reson. Imaging 2009;29:1478–1484. © 2009 Wiley-Liss, Inc.

This article provides an overview of relaxation times and their application to normal brain and brain and cord affected by multiple sclerosis. The goal is to provide readers with an intuitive understanding of what influences relaxation times, how relaxation times can be accurately measured, and how they provide specific information about the pathology of MS. The article summarizes significant results from relaxation time studies in the normal human brain and cord and from people who have multiple sclerosis. It also reports on studies that have compared relaxation time results with results from other MR techniques.

Susceptibility-weighted (SW) magnetic resonance (MR) imaging provides high-resolution, distortion-free blood oxygen level-dependent (BOLD) data for assessment of cerebral veins, blood products, and brain lesions. Currently, reconstruction of SW imaging data is not implemented on all MR imaging systems or is restricted in terms of parameter adjustments. New developments in SW imaging have been implemented into a graphical user interface (GUI), which is named GUIBOLD. The GUI was designed for imaging system-independent off-line data reconstruction with interactive setting of parameters on the basis of k-space data and Digital Imaging and Communications in Medicine images. GUIBOLD is capable of presenting magnitude, unwrapped phase, and SW images in different orientations and parallel projections with various rendering methods and region-of-interest-based data analysis tools. Moreover, GUIBOLD affords easy and comprehensive data reconstruction possibilities for venographic and arterial imaging and anatomic phase imaging. As a direct application, differentiation between cavernous and calcified lesions on the basis of their magnetic susceptibility on phase images was performed. GUIBOLD widens the range of potential applications of SW imaging and makes it more accessible for use in the clinical routine as well as in medical research.

This review discusses how magnetic resonance techniques are used to provide specific information about the pathology of multiple sclerosis.

The MR signal decay in gradient echo sequences includes signal loss due to spin dephasing caused by static magnetic field inhomogeneities. This decay can be calculated for different geometries of the susceptibility distribution, such as spheres, cylinders, or cylinder networks. In particular, the model of an infinitely long cylinder is a good approximation for single straight blood vessels. Blood oxygenation and blood volume fraction are important parameters, which influence the signal in a characteristic way. In this work the signal decays for a single cylindrical vessel were investigated and evaluated in simulations, phantom measurements as well as in vivo measurements of small single veins in the human brain by using a 3D multiecho gradient echo sequence. Good agreement between simulations and phantom experiments was obtained for different experimental settings. Based on the simulations, physiologically consistent values of venous blood oxygenation level, Y, were extracted from the in vivo measurements of different veins and volunteers (Y = 0.55 ± 0.02). The methods ability to measure changes in venous blood oxygenation induced by carbogen breathing was demonstrated in one volunteer, where an increase from Y ≈ 0.5 to Y ≈ 0.7 was observed. Magn Reson Med 58:1035–1044, 2007. © 2007 Wiley-Liss, Inc.

We present a fully automated phase unwrapping algorithm (Phi UN) which is optimized for high-resolution magnetic resonance imaging data. The algorithm is a region growing method and uses separate quality maps for seed finding and unwrapping which are retrieved from the full complex information of the data. We compared our algorithm with an established method in various phantom and in vivo data and found a very good agreement between the results of both techniques. Phi UN, however, was significantly faster at low signal to noise ratio (SNR) and data with a more complex phase topography, making it particularly suitable for applications with low SNR and high spatial resolution. Phi UN is freely available to the scientific community.

To enhance susceptibility-related contrast of magnetic resonance images, the phase of susceptibility weighted data needs to be corrected for background inhomogeneities and phase wraps caused by them. Current methods either use homodyne filtering or a combination of phase unwrapping and high pass filtering. The drawback of homodyne filtering is incomplete elimination of phase wraps in areas with steep phase topography produced by background inhomogeneities of the static magnetic field. The disadvantage of phase unwrapping is that it requires subsequent high pass filtering, which introduces artifacts in areas with very steep transitions, such as areas near interfaces between parenchyma and bone or air. A method is proposed that reduces the artifacts associated with high pass filtering without sacrificing the advantages of phase unwrapping. This technique is demonstrated with phantom data at 1.5 T and with human data at 1.5, 3 and 7 T.

Susceptibility-weighted MR imaging (SWI) has become a non-invasive diagnostic modality for functional MR imaging (fMRI) of the brain and also for the imaging of tumors, injuries, malformations or microhemorrhages. SWI often enables detection of otherwise subtle abnormalities or provides additional relevant information when combined with routine MR imaging. The purpose of this article is to illustrate the potential of SWI in the discrimination of paramagnetic and diamagnetic brain lesions in neuroradiological applications.

Although high-altitude exposure can lead to neurocognitive impairment, even upon return to sea level, it remains unclear the extent to which brain volume and regional cerebral vascular reactivity (CVR) are altered following high-altitude exposure. The purpose of this study was to simultaneously determine the effect of 3 weeks at 5050 m on: (1) structural brain alterations; and (2) regional CVR after returning to sea level for 1 week. Healthy human volunteers (n = 6) underwent baseline and follow-up structural and functional magnetic resonance imaging (MRI) at rest and during a CVR protocol (end-tidal PCO2 reduced by -10, -5 and increased by +5, +10, and +15 mmHg from baseline). CVR maps (% mmHg(-1)) were generated using BOLD MRI and brain volumes were estimated. Following return to sea level, whole-brain volume and gray matter volume was reduced by 0.4 ± 0.3% (P < 0.01) and 2.6 ± 1.0% (P < 0.001), respectively; white matter was unchanged. Global gray matter CVR and white matter CVR were unchanged following return to sea level, but CVR was selectively increased (P < 0.05) in the brainstem (+30 ± 12%), hippocampus (+12 ± 3%), and thalamus (+10 ± 3%). These changes were the result of improvement and/or reversal of negative CVR to positive CVR in these regions. Three weeks of high-altitude exposure is reflected in loss of gray matter volume and improvements in negative CVR.

Objectives Socially disadvantaged people experience greater risk for illnesses that may contribute to premature death. This study aimed to evaluate the impact of treatable illnesses on mortality among adults living in precarious housing. Design A prospective cohort based in a community sample. Setting A socially disadvantaged neighbourhood in Vancouver, Canada. Participants Adults (N=371) living in single room occupancy hotels or recruited from the Downtown Community Court and followed for median 3.8 years. Main outcome measures Participants were assessed for physical and mental illnesses for which treatment is currently available. We compared cohort mortality rates with 2009 Canadian rates. Left-truncated Cox proportional hazards modelling with age as the time scale was used to assess risk factors for earlier mortality. Results During 1269 person-years of observation, 31/371 (8%) of participants died. Compared with age-matched and sex-matched Canadians, the standardised mortality ratio was 8.29 (95% CI 5.83 to 11.79). Compared with those that had cleared the virus, active hepatitis C infection was a significant predictor for hepatic fibrosis adjusting for alcohol dependence and age (OR=2.96, CI 1.37 to 7.08). Among participants

OBJECTIVES: We sought to determine the optimal dose of a contrast agent with known high relaxivity on 1.5 and 3 Tesla scanners that would achieve the best compromise between image quality and scan time for the clinical application of contrast-enhanced susceptibility-weighted imaging (CE-SWI). METHODS: Pre- and postcontrast SWI was performed with different contrast agent doses (0.05, 0.1, and 0.2 mmol/kg gadobenate dimeglumine) at both 1.5 and 3 T in 6 healthy volunteers, resulting in 72 examinations. Venograms were created from minimum intensity projection reconstructions over specified deep white matter volumes to enhance the visual appearance of connected venous structures. Three independent radiologists blindly rated the visibility of the veins on a continuous scale of 1 to 10. A general linear model was used for statistical evaluation, with fixed effects of the contrast agent dose, the field strength, the rater and the patients as a random effect. RESULTS: With CE-SWI, we found significant differences in the visibility of the deep veins dependent on the contrast media dose (P = 0.02). At 3 T, the visibility of deep venous vessels, with regard to susceptibility effect, image quality, and scan time reduction after a standard contrast agent dose 0.1 mmol/kg was significantly better than that achieved with 0.05 mmol/kg. The visibility was considered equal with 0.1 mmol/kg of the contrast agent to the precontrast images and a dose of 0.2 mmol/kg. At 1.5 T, no significant difference was found between the 4 contrast agent doses. We found no difference in the visibility of the veins with the shorter sequences at 3 T compared with the sequences at 1.5 T. CONCLUSIONS: Only a standard dose (0.1 mmol/kg) of gadobenate dimeglumine is required to achieve the optimum susceptibility effect and image quality at 3 T, together with a reduced scan time. This result can be attributed to the higher relaxivity of gadobenate dimeglumine, compared with conventional gadolinium chelates.

PURPOSE To employ a high resolution blood oxygenation level dependent (BOLD) method called susceptibility weighted imaging (SWI) together with the breathing of carbogen to investigate the response of cerebral tumors to this breathing gas and to assess tumor anatomy at high resolution. METHODS Five patients with cerebral tumors (four glioblastoma multiforme, one astrocytoma [WHO grade II]) were studied using a susceptibility weighted 3D gradient echo, first order velocity compensated sequence (TE = 45 ms, TR = 67 ms, alpha = 25 degrees , FOV = 256 x 192 x 64 mm(3), typical matrix = 512 x 192 x 64), on a 1.5 T MR scanner while they were breathing air and carbogen. Signal changes between the two breathing conditions were investigated. RESULTS The glioblastomas showed strong but heterogeneous signal changes between carbogen and air breathing, with changes between + 22.4 +/- 4.9 % at the perimeter of the tumors and - 5.0 +/- 0.4 % in peritumoral areas that appeared hyperintense on T (2)-weighted images. The astrocytoma displayed a signal decrease during carbogen breathing (- 4.1 +/- 0.1 % to - 6.8 +/- 0.3 % in peritumoral areas that correspond to hyperintense regions on T (2)-weighted images, and - 3.1 +/- 0.1 % in the tumor-center). CONCLUSIONS SWI provides high resolution images of cerebral anatomy and venous vascularization. Combined with hypercapnia it allows for regional assessment of tumor activity.

Purpose To develop and to evaluate an advanced image acquisition and analysis method for collecting T1-weighted dynamic 3D MR mammography data sets by using a combined in-phase (IP) and opposed-phase (OP) imaging procedure. Materials and Methods 3D MR mammography data sets were acquired by applying an interleaved gradient-echo OP and IP imaging sequence during administration of contrast agent. A phantom data set, two volunteer breast data sets, and six patient breast data sets were recorded. Subtraction of dynamic in-phase magnitude images was performed for clinical assessment. In addition, the magnitude subtraction (SIPOP) as well as the complex subtraction (cSIPOP) of the IP and OP magnitude and phase images were considered. Results The detection of small lesions, lesion boundaries, and tumor offshoots in fatty tissue was improved by the subtraction of IP and OP images without the risk of signal cancellation due to partial volume effects. Conclusion Dynamic MR mammography acquisition of IP and OP images in combination with appropriate data processing yields important supplementary information that can support routinely applied diagnostics of breast lesions that are fully embedded in fatty tissue by only marginally increasing acquisition time. J. Magn. Reson. Imaging 2005;21:565–575. © 2005 Wiley-Liss, Inc.

There is interest in applying novel methods to dynamic MR mammography (MRM). One such possibility is to administer an exogenous hyperoxic contrast agent, such as carbogen (95-98% O2 and 2-5% CO2) or pure oxygen (100% O2). We report our first experiences with these agents in a patient with an invasive lobular carcinoma. Fourteen dynamic series were acquired with an rf-spoiled 2D multislice gradient echo sequence, including three measurements while breathing air, four measurements with 100% oxygen, three measurements with air and four measurements with carbogen. Afterwards, 0.1 mmol/kg bw of Gd-DTPA was administered to obtain dynamic T1-weighted double-echo 3D axial gradient echo images (TR/TE1/TE2/alpha=7.8 ms/2 ms/4.76 ms/15 degrees) every 90 s up to 4.5 min after injection. The lesion was well delineated on the contrast-enhanced images, contrary to magnitude images reconstructed from the raw data sets acquired during air/oxygen/carbogen breathing. A ROI-based median-filtered signal-time course revealed a tumor signal increase of roughly 15% between scans acquired during air and oxygen breathing. Though preliminary, these first results are encouraging concerning the exploration of these alternative contrast agents in MRM in greater detail.

Measurements of cerebral perfusion using dynamic susceptibility contrast magnetic resonance imaging rely on the assumption of isotropic vascular architecture. However, a considerable fraction of vessels runs in parallel with white matter tracts. Here, we investigate the effects of tissue orientation on dynamic susceptibility contrast magnetic resonance imaging. Tissue orientation was measured using diffusion tensor imaging and dynamic susceptibility contrast was performed with gradient echo planar imaging. Perfusion parameters and the raw dynamic susceptibility contrast signals were correlated with tissue orientation. Additionally, numerical simulations were performed for a range of vascular volumes of both the isotropic vascular bed and anisotropic vessel components, as well as for a range of contrast agent concentrations. The effect of the contrast agent was much larger in white matter tissue perpendicular to the main magnetic field compared to white matter parallel to the main magnetic field. In addition, cerebral blood flow and cerebral blood volume were affected in the same way with angle-dependent variations of up to 130%. Mean transit time and time to maximum of the residual curve exhibited weak orientation dependency of 10%. Numerical simulations agreed with the measured data, showing that one-third of the white matter vascular volume is comprised of vessels running in parallel with the fibre tracts.