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Results for “Diffusion MRI”

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At least 253 records · Page 14Linked to original sources
PubMed2000

The effect of bulk susceptibility on murine snapshot imaging at 7.0 T: a comparison of snapshot imaging techniques.

The sensitivity of several single-shot imaging techniques to local field gradients (LFGs) generated by discontinuities in bulk magnetic susceptibility (BMS) were compared in mouse brain at 7.0 T. At high field, differences in BMS can cause substantial signal attenuation and image distortion. Because susceptibility effects scale with B(o), mouse brain single-shot imaging at high field is particularly susceptible to the effects of LFGs. The spin-echo techniques GRASE and RARE were found to have a decreased sensitivity to LFGs compared to echo-planar imaging (EPI). Images obtained using EPI and SE-EPI exhibit severe signal attenuation in regions of high LFGs such as near air-tissue interfaces and at the brain edges. In applications such as functional MRI and diffusion MRI, GRASE and RARE are likely to provide more comprehensive whole brain coverage in mouse brain at high field than EPI techniques, which are likely to image regions of the brain with strong LFGs with low signal to noise, reducing the probability of detecting significant physiologically based changes.

Animals↗
PubMed2003

Emergent stenting to treat patients with carotid artery dissection: clinically and radiologically directed therapeutic decision making.

BACKGROUND AND PURPOSE: Carotid artery dissection (CAD) is a common cause of stroke in young patients. Clots formed at the low-flow zone in the false arterial lumen may give rise to distal emboli, and the mural hematoma may eventually occlude the artery. Anticoagulation is currently the accepted treatment, but it is unknown which patients will improve spontaneously, which will respond to anticoagulation, and which will have an exacerbation of ischemic symptoms despite therapy. Endovascular treatment of CAD may be an attractive alternative to anticoagulation, and methods of identifying patients who stand to benefit from such therapy need to be established. METHODS: We present here 3 cases of spontaneous symptomatic CAD in which endovascular stenting procedures were performed on the basis of a paradigm aimed at identifying potentially salvageable but at-risk brain tissue by combining clinical with MRI (diffusion and perfusion) and angiographic data. RESULTS: Diffusion-perfusion MRI mismatches and/or evidence of cerebral ischemia on angiographic parenchymography were identified in all patients. They did not respond to anticoagulation, were therefore treated with endovascular stents, and had excellent outcomes. CONCLUSIONS: Endovascular stent placement may be an immediate, effective, and safe method of restoring vessel lumen integrity. It may be considered for selected patients who are clinically symptomatic despite anticoagulant treatment and in whom neuroimaging methods suggest that the neurological signs originate from a viable, hypoperfused, ischemic penumbra.

Angiography↗
PubMed2006

New developments in MRI for target volume delineation in radiotherapy.

MRI is being increasingly used in oncology for staging, assessing tumour response and also for treatment planning in radiotherapy. Both conformal and intensity-modulated radiotherapy requires improved means of defining target volumes for treatment planning in order to achieve its intended benefits. MRI can add to the radiotherapy treatment planning (RTP) process by providing excellent and improved characterization of soft tissues compared with CT. Together with its multiplanar capability and increased imaging functionality, these advantages for target volume delineation outweigh its drawbacks of lacking electron density information and potential image distortion. Efficient MR distortion assessment and correction algorithms together with image co-registration and fusion programs can overcome these limitations and permit its use for RTP. MRI developments using new contrast media, such as ultrasmall superparamagnetic iron oxide particles for abnormal lymph node identification, techniques such as dynamic contrast enhanced MRI and diffusion MRI to better characterize tissue and tumour regions as well as ultrafast volumetric or cine MR sequences to define temporal patterns of target and organ at risk deformity and variations in spatial location have all increased the scope and utility of MRI for RTP. Information from these MR developments may permit treatment individualization, strategies of dose escalation and image-guided radiotherapy. These developments will be reviewed to assess their current and potential use for RTP and precision high dose radiotherapy.

Contrast Media↗
PubMed2006

The clinical perspective of large scale projects: a case study of multiparametric MR imaging of pediatric brain tumors.

Current developments in medical information technologies provide the clinical researcher with overwhelming amounts of data that need to be retrieved, organized, analyzed, and shared using secure, efficient, and robust protocols. The development of a local research database can provide an infrastructure for improved data management and detailed data analysis. For example, a pediatric brain tumor database of magnetic resonance imaging data, including conventional MRI imaging, hemodynamic MRI, diffusion weighted MRI and MR spectroscopic imaging, combined with neuropathological and neurological evaluation data, will significantly enhance the assessment and treatment of pediatric brain tumor patients. Furthermore, a negotiation system by which different clinical research facilities can share and combine data will permit re-analyses and meta-analyses of large data arrays that are beyond the focus or time constraints of the original researchers. Such a system will greatly enhance the utility of different data sets to a wide array of scientists. At present, efforts to organize medical data locally and between different sites is limited by diversity, interoperability, security, and accountability difficulties.

Brain Neoplasms↗
PubMed2004

Diffusion-weighted MRI for the "small stuff": the details of acute cerebral ischaemia.

Diffusion-weighted MRI (DWI) has already had a substantial effect on the diagnosis of patients with ischaemic stroke. It provides in vivo pathological information and allows the differentiation of acute stroke from chronic stroke and from non-specific white-matter lesions. The high contrast of the acute DWI lesion against the dark background facilitates the detection of lesions even when they are 1 mm or less in diameter. Small lesions, which are undetectable by other means, include small lacunar infarcts, punctate cortical infarcts, and DWI bright dots in patients with transient ischaemic attacks (TIA). The latter constitute remnants or "footprints" of recent ischaemia and confirm the clinical TIA syndrome as ischaemic. Because of these attributes, DWI not only confirms the clinical diagnosis, but also facilitates the recognition of certain patterns of ischaemia, thereby providing clues to the underlying aetiology. DWI is becoming an important technique for optimum management of patients.

Brain↗
PubMed2004

Use of fractional anisotropy value by diffusion tensor MRI for preoperative diagnosis of astrocytic tumors: case report.

The fractional anisotropy (FA) value calculated by diffusion tensor MRI can indicate the degree of directionality of water diffusion in astrocytic tumors. Here, we report a case of anaplastic astrocytoma in which FA proved invaluable for the preoperative differential diagnosis. A 60-year-old man complained of headache, and underwent routine neuroimaging and DTI. The routine images suggested a low-grade glioma in the left temporal lobe, based on lack of enhancement on MRI with contrast medium and lack of tumor staining on angiograms, whereas FA value was very high. Based on these findings, a preoperative diagnosis of high-grade glioma was suspected. The surgical specimen exhibited the histological features of anaplastic astrocytoma with a high density of spindle shaped cells and low vascularity. In this report, we discuss the relationship between FA and other characteristics of the present tumor, and discuss the utility of FA measurement in astrocytic tumors.

Anisotropy↗
PubMed2002

Diffusion-weighted MRI in the characterization of soft-tissue tumors.

PURPOSE: To explore the potential of perfusion-corrected diffusion-weighted magnetic resonance imaging (MRI) in characterizing soft-tissue tumors. METHODS AND MATERIALS: Diffusion-weighted MRI was performed in 23 histologically proven soft-tissue masses using a diffusion-weighted spin-echo sequence with diffusion gradient strengths yielding five b-values (0-701 seconds/mm(2)). True diffusion coefficients and perfusion fractions were estimated and compared with apparent diffusion coefficients (ADCs). RESULTS: ADC values of all tumors, subcutaneous fat, and muscle were significantly higher than true diffusion coefficients, indicating a contribution of perfusion to the ADC. True diffusion coefficients of malignant tumors (1.08 x 10(-3) mm(2)/second) were significantly lower than those of benign masses (1.71 x 10(-3) mm(2)/second), whereas ADC values between these groups were not significantly different. CONCLUSION: Perfusion-corrected diffusion-weighted MRI has potential in differentiating benign from malignant soft-tissue masses.

Adolescent↗
PubMed2003

Diffusion-weighted MRI of intracerebral hemorrhage clinically undifferentiated from ischemic stroke.

This report describes diffusion-weighted MRI findings of some intracerebral hemorrhages clinically undifferentiated from ischemic stroke. We treated patients with characteristic features of intracerebral hemorrhages that could distinguish themselves from ischemic lesion on diffusion-weighted imaging. Therefore, we think diffusion-weighted imaging could be an emergency screening tool for ischemic stroke as an alternative to computed tomography, and the EP should be familiar with the diffusion-weighted imaging findings of intracerebral hemorrhage as well as cerebral infarction.

Adult↗
PubMed2003

Investigation of the microstructure of the isolated rat heart: a comparison between T*2- and diffusion-weighted MRI.

Myocardial fiber structure can be determined with diffusion-weighted (DW) MRI as well as with high-resolution T*(2) imaging. The purpose of the present study was twofold: to provide a more quantitative description of T*(2)-based myocardial fiber contrast, and to compare the T*(2)-based fiber structure with high-resolution (78 microm in-plane, 1-mm slice thickness) DW images of the isolated rat heart at 11.75 T. This study demonstrates that the static dephasing regime is responsible for visualization of myocardial microstructure, and that the dynamic dephasing regime can be neglected. In comparison with DW experiments, T*(2) mapping and DW images yield almost equivalent information on myocardial fiber structure.

Animals↗
PubMed2003

Diffusion-weighted MRI and ADC mapping in FK506 neurotoxicity.

FK506 is a newly developed potent immunosuppressant for preventing rejection after organ transplantation. However, FK506 can induce central nervous system toxicity. Until now the pathogenic mechanism of FK506 neurotoxicity was unclear. We report the findings of diffusion-weighted MRI and apparent diffusion coefficient (ADC) mapping of a FK506 neurotoxicity patient who showed increased signal intensities in both parieto-occipital lobes on T(2) weighted images, diffusion-weighted images and ADC maps. These findings suggest that a vasogenic oedema rather than a cytotoxic oedema may play a pivotal role in FK506 neurotoxicity pathogenesis.

Adult↗
PubMed2004

Diffusion-weighted MRI and 99mTc-HMPAO SPECT in delayed relapsing type of carbon monoxide poisoning: evidence of delayed cytotoxic edema.

BACKGROUND: Carbon monoxide (CO) is a common cause of poisoning, and its sequelae include a progressive (25%) and a delayed relapsing form (75%). We report the diffusion-weighted MRI (DWI) findings in the delayed relapsing form of CO poisoning and characterize the types of edema. METHODS: From November 1, 2000 to June 1, 2003, 5 consecutive patients (2 men, 3 women, range of age: 54-67 years), who had the delayed relapsing type of CO poisoning, underwent DWI, conventional MRI, MR angiography and SPECT. CO poisoning was diagnosed by the presence of a typical clinical history, an abnormally increased level of serum carboxyhemoglobin and MRI findings. Apparent diffusion coefficient (ADC) values were measured in all of the abnormal lesions with visual inspection of DWI and T(2)-weighted echo-planar imaging. RESULTS: DWI showed high signal intensities in bilateral periventricular white matter, in the splenium of the corpus callosum, in internal capsules, and brainstem showing moderately decreased ADC values. In the globus pallidus, the ADC values were rather increased with low signal intensities on DWI. Brain SPECT showed decreased perfusion in bilateral white matter and some parts of the cerebral cortex, which correlated well with the DWI findings. CONCLUSIONS: We suggest that prominent, symmetric restricted diffusion can occur in periventricular white matter, brainstem, and corpus callosum after the delayed relapsing type of CO poisoning. Delayed cytotoxic edema can occur in this setting, which provides a new guidance for the pathogenesis of CO poisoning and the differential diagnosis of white matter diseases.

Aged↗
PubMed2000

Diffusion tensor MRI of the spinal cord.

Apparent diffusion tensor (ADT) measurements on the spinal cord using a pulsed-field-gradient (PFG) multi-shot echo-planar imaging (EPI) sequence are presented. In a study of 10 healthy volunteers, the obtained rotationally invariant anisotropy information is compared to the results obtained by the rotationally dependent methods. The water diffusivity in the direction parallel to the fibers was found to be almost 2.5 times higher than the average diffusivity in directions perpendicular to the fibers and showed cylindrically symmetric anisotropy characteristics. The influence of partial volume effects and the point spread function on the measured results was evaluated, and it was the concluded that a resolution of 1 mm in the read and phase directions is required to obtain unbiased values. Possible clinical implications were demonstrated by investigating the diffusion characteristics of 10 patients suffering from narrowing of the cervical canal. The changes in the diffusion characteristics were found to be large enough to allow a robust detection of diffusion changes in the spine, even in cases in which conventional T(2) and T(1) weighted images were unable to detect any lesion.

Anisotropy↗
PubMed2004

Direct estimation of the fiber orientation density function from diffusion-weighted MRI data using spherical deconvolution.

Diffusion-weighted magnetic resonance imaging can provide information related to the arrangement of white matter fibers. The diffusion tensor is the model most commonly used to derive the orientation of the fibers within a voxel. However, this model has been shown to fail in regions containing several fiber populations with distinct orientations. A number of alternative models have been suggested, such as multiple tensor fitting, q-space, and Q-ball imaging. However, each of these has inherent limitations. In this study, we propose a novel method for estimating the fiber orientation distribution directly from high angular resolution diffusion-weighted MR data without the need for prior assumptions regarding the number of fiber populations present. We assume that all white matter fiber bundles in the brain share identical diffusion characteristics, thus implicitly assigning any differences in diffusion anisotropy to partial volume effects. The diffusion-weighted signal attenuation measured over the surface of a sphere can then be expressed as the convolution over the sphere of a response function (the diffusion-weighted attenuation profile for a typical fiber bundle) with the fiber orientation density function (ODF). The fiber ODF (the distribution of fiber orientations within the voxel) can therefore be obtained using spherical deconvolution. The properties of the technique are demonstrated using simulations and on data acquired from a volunteer using a standard 1.5-T clinical scanner. The technique can recover the fiber ODF in regions of multiple fiber crossing and holds promise for applications such as tractography.

Algorithms↗
PubMed2000

Reducing motion artefacts in diffusion-weighted MRI of the brain: efficacy of navigator echo correction and pulse triggering.

Diffusion-weighted MRI (DWI) is extremely sensitive to motion of the object being examined. Pulse triggering and navigator echo correction are methods for reducing motion artefacts which can be combined with conventional DWI sequences. Implementation of these methods in imaging sequences with a readout of one, three, or five echoes is presented and imaging results compared in a study of five healthy volunteers. As an objective measure for motion-induced image artefacts, the "artefacticity" of an image is defined. Pulse triggering and navigator echo correction significantly improve image quality and provide a technique for high-quality DWI on standard imagers without improved gradient hardware.

Adult↗
PubMed2006

Atypical abdominal paediatric lymphangiomatosis: diagnosis aided by diffusion-weighted MRI.

We report a 4-year-old child with a mesenteric mass, which on ultrasound, CT and conventional MRI appeared solid, raising lymphoma as a possible diagnosis. Diffusion weighted MRI (DW-MRI), however, suggested a low-cellularity lesion, making lymphoma less likely. Biopsy confirmed lymphangioma. DW-MRI may be a useful adjunct to conventional imaging, even in the abdomen.

Abdominal Neoplasms↗
PubMed2003

Diffusion-weighted MRI and the evaluation of spinal cord axonal integrity following injury and treatment.

Diffusion-based magnetic resonance imaging (MRI) (DWI) has been shown experimentally to detect both injury and functionally significant neuroprotection of injured spinal cord white matter that would otherwise go undetected with conventional MRI techniques. The diffusion of water in the central nervous system (CNS) is thought to be affected by both its location (intracellular or extracellular), and by diffusion barriers formed by cell membranes and myelin sheaths. There is, however, controversy concerning how to obtain, interpret, and present DWI data. Computer simulations and MR microscopy have been helpful in resolving some of these issues, as well as determining exact histologic correlates to DWI findings.

Animals↗
PubMed2003

[Early diagnosis of bacterial brain abscesses: interest of diffusion-weighted MRI].

Three cases of bacterial brain abscesses, in immunocompetent patients, are reported. In all these cases, the diffusion-weighted magnetic resonance (MRI) with apparent diffusion coefficient (ADC) map has permitted an early diagnosis and a rapid treatment. This emergency MRI showed in the three cases a low signal on TI-weighted images, a high signal on T2-weighted and echo-planar images, and a decrease of ADC (0.36- 0.49 x 10(-3) mm2/s). So, this new MRI technique provides an available and rapid element in the brain abscess diagnosis which often remains a complex clinical and radiological diagnosis.

Actinomycosis↗
PubMed2000

An alternative explanation of the origin of the signal in diffusion-weighted MRI.

I formulated an alternative explanation for the origin of the signal in diffusion-weighted MRI (DWI) and tested it in a simple experimental model using gelatin. My findings indicate that the signal in DWI is influenced by the passage of water from the sol to the gel state, independently of the presence of cell structures or membrane-dependent diffusion.

Body Water↗