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At least 523 records · Page 29Linked to original sources
PubMed2000

Paraspinal muscle response to electrical vestibular stimulation.

Galvanic (electrical) vestibular stimulation (GVS) has been used to study the role of the vestibular system in postural control by inducing postural sway in standing subjects. The purpose of this study was to determine the timing and pattern of activation in the paraspinal muscles in response to GVS and to compare these responses with those in the muscles of the lower leg. Binaural-bipolar GVS was applied to the skin overlying the mastoid processes of 10 subjects while they stood on a force plate with their eyes closed. The stimulus consisted of a 0.6 mA 5-pulse sequence. Each pulse lasted for 2 s, followed by 4 s of rest. The centre of pressure (COP) vs. time for each trial was calculated from the reaction forces and moments. Surface electromyographic (EMG) signals from the paraspinal and gastrocnemius muscles were recorded bilaterally. The EMG signals were rectified and integrated (iEMG). The iEMG from the muscles on the cathodal side of the body were then subtracted from the iEMG of the anodal side muscles, to yield a differential EMG (dEMG). Both the paraspinal and gastrocnemius muscles became activated in response to the stimulus. The pattern of activation was consistent with the changes observed in the centre of pressure. The primary response in both muscles acted to move the body toward the anode. This primary response began at 74 +/- 20 ms in the paraspinal muscles and at 118 +/- 18 ms in the gastrocnemius. A second component of the response began at 232 +/- 27 ms in the paraspinal muscles and 262 +/- 54 ms in the gastrocnemius muscles. This second phase of the response was opposite in direction to the primary response and was responsible for decelerating the body and maintaining the deviated position of the centre of mass over the base of support. Following the termination of the stimulus, the opposite pattern of muscle activation in both the paraspinal and the gastrocnemius muscles was observed. The results of this study suggest that the paraspinal muscles may play a significant role in the frontal plane response to vestibular stimulation during stance in humans.

Adult↗
PubMed1989

Increased brain water content in pseudotumour cerebri measured by magnetic resonance imaging of brain water self diffusion.

Brain water self diffusion was investigated by magnetic resonance scanning in 7 patients fulfilling conventional diagnostic criteria for pseudotumour cerebri. Quantitative diffusion measurements were obtained using single spin echo pulse sequences with pulsed magnetic field gradients of different magnitude. In all patients the diffusion images showed an increased diffusion in various brain regions when compared with the diffusion coefficients for corresponding regions in healthy subjects. In 3 pseudotumour patients the increased self diffusion was localized to the periventricular regions, while 4 patients had increased diffusion in the whole brain. The findings indicate the presence of increased brain water content both intra- and extracellularly suggesting that patients with pseudotumour have two defects of pathogenetical significance: intracellular water accumulation and increased resistance to cerebrospinal fluid (CSF) outflow leading to an interstitial oedema.

Adult↗
PubMedDate not supplied

An index system for comparative parameter weighting in MR imaging.

An analytic method for comparative parameter weighting in magnetic resonance (MR) imaging has been developed using the concept of "fractional sensitivity." This new approach results in easily calculated indexes for T1, T2, and hydrogen weighting. This index system enables quantitative comparisons to be made between MR studies that have been performed at various field strengths, using different pulse sequences and pulse timing intervals.

Brain↗
PubMed2000

Solid state coherent transient measurements using hard optical pulses

An isolated and spectrally narrow absorptive feature is prepared via a novel spectral hole burning process in an inhomogeneously broadened optical transition in Eu(3+):Y(2)SiO5. With the narrow feature it is shown that it is feasible to apply complex optical pulse sequences analogous to rf pulse sequences used extensively in NMR.

Journal Article↗
PubMed2000

A concept for a research tool for experiments with cochlear implant users.

APEX, an acronym for computer Application for Psycho-Electrical eXperiments, is a user friendly tool used to conduct psychophysical experiments and to investigate new speech coding algorithms with cochlear implant users. Most common psychophysical experiments can be easily programmed and all stimuli can be easily created without any knowledge of computer programing. The pulsatile stimuli are composed off-line using custom-made MATLAB (Registered trademark of The Mathworks, Inc., http://www.mathworks.com) functions and are stored on hard disk or CD ROM. These functions convert either a speech signal into a pulse sequence or generate any sequence of pulses based on the parameters specified by the experimenter. The APEX personal computer (PC) software reads a text file which specifies the experiment and the stimuli, controls the experiment, delivers the stimuli to the subject through a digital signal processor (DSP) board, collects the responses via a computer mouse or a graphics tablet, and writes the results to the same file. At present, the APEX system is implemented for the LAURA (Registered trademark of Philips Hearing Implants) cochlear implant. However, the concept-and many parts of the system-is portable to any other device. Also, psycho-acoustical experiments can be conducted by presenting the stimuli acoustically through a sound card.

Cochlear Implants↗
PubMed1989

[The preliminary experience of MRI diagnoses in multiple sclerosis: a report of 3 cases].

In this report, 3 patients with suspected MS were studied by MRI. All patients were performed with spin-echo (SE) pulse sequence. A pulse repetition time (TR) of 1800 msec. and echo time (TE) of 33, 66, 99, 132 msec. producing a T2-weighted image. The result showed that MS is characterized by numerous long T2 lesions in the paraventricular white matter. Brain stem lesions were seed in all 3 patients. MRI is a very useful diagnostic tool for diagnosis of MS.

Adult↗
PubMed1989

[Brain and spinal tumors: magnetic resonance imaging using gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)].

Magnetic resonance imaging (MRI) was performed in 17 patients (with 24 tumors) before and after intravenous administration of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA). Contrast enhancement was seen in 16 patients. The image pattern of enhancement is the same as that of CT. With 0.1 mmol/kg intravenous bolus injection, the best enhancement can keep for one hour. In brain, the degree of enhancement was greater than that seen with CT in 7 tumours, equal to it in 12 tumors, and less in zero. MRI distinguished between tumour and peritumoral edema to the same extent as did CT. No side-effects were encountered and there was no significant change in regular urine and blood tests after administration of Gd-DTPA. T1-weighted images of spin echo pulse sequence are the best pulse sequence to observe the enhancement. Gd-DTPA is of considerable value in MRI of the brain and spine.

Adolescent↗
PubMed1984

Magnetic resonance imaging. Part I--physical principles.

Magnetic resonance (MR) imaging is the most complex imaging technology available to clinicians. Whereas most imaging technologies depict differences in one, or occasionally two, tissue characteristics, MR imaging has five tissue variables-spin density, T(1) and T(2) relaxation times and flow and spectral shifts-from which to construct its images. These variables can be combined in various ways by selecting pulse sequences and pulse times to emphasize any desired combination of tissue characteristics in the image. This selection is determined by the user of the MR system before imaging data are collected. If the selection is not optimal, the imaging process must be repeated at a cost of time and resources. The optimal selection of MR imaging procedures and the proper interpretation of the resultant images require a thorough understanding of the basic principles of MR imaging. Included in this understanding should be at least the rudiments of how an MR imaging signal is produced and why it decays with time; the significance of relaxation constants; the principles of scanning methods such as saturation recovery, inversion recovery and spin echo; how data obtained by these methods are used to form an image, and how the imaging data are complied by multi-slice and volumetric processes. In selecting an MR imaging unit, information about different magnet designs (resistive, superconductive and permanent) is useful. Although no bioeffects are thought to be associated with an MR imaging examination, some knowledge of the attempts to identify bioeffects is helpful in alleviating concern in patients.

Humans↗
PubMed1984

Magnetic resonance imaging. Part II--Clinical applications.

Magnetic resonance (MR) imaging is the most promising new technology to appear in the clinical imaging arena since the advent of x-ray transmission computed tomography in the early 1970s. Five independent tissue characteristics (spin density, spin-lattice and spin-spin relaxation times, flow and spectral shift information) are accessible to MR imaging, and their relative influence in the magnetic resonance image can be varied by appropriate selection of pulse sequences and pulse times. All major organ systems appear to be amenable to MR imaging, and some are revealed with superior definition compared with their appearance in images obtained by alternate imaging technologies. Of particular interest is the superior contrast resolution in MR images of the brain and spinal cord, and the absence of bone- and motion-induced artifacts in images of the abdomen and pelvis. Applications of MR imaging to the heart and great vessels are just developing, as are new types of contrast agents for use in MR imaging. In vivo chemical spectroscopic measurements by magnetic resonance are heralded by some investigators as the most significant contribution that magnetic resonance will make ultimately to clinical diagnosis. At present, the number of MR imaging units is extremely low, and clinical studies are proceeding at a slow rate. Nevertheless, it is possible to provide a preliminary evaluation of the usefulness of MR imaging in a variety of clinical applications. This article is such an evaluation, tempered by the acknowledgement that much additional work remains to be done.

Brain Diseases↗
PubMed1993

High-resolution variable flip angle 3D MR imaging of trabecular microstructure in vivo.

Two conceptually related variable-flip-angle 3D spin-echo pulse sequences were designed for imaging at voxel sizes of 2-5 x 10(-3) mm3 corresponding to pixel areas of less than 100 x 100 microns2 and section thicknesses on the order of 300-400 microns on a conventional 1.5 T MR imaging system equipped with 1 G/cm imaging field gradients, providing 12 sections in 10 min imaging time. The pulse sequences make use of the concept of restoring longitudinal magnetization inverted by the 180 degrees phase reversal pulse and are derivatives of pulse sequences previously dubbed "FATE" and "RASEE." It is shown that even in the small-voxel regime (< 10(-2) mm3 voxel size) and at echo times on the order of 10 ms, gradient echo images are sensitive to intrinsic fields causing artifactual boundary effects, including signal loss from intravoxel phase scrambling and spatial mismapping. At this resolution the variable flip-angle spin-echo pulse sequences are demonstrated to be better suited for imaging magnetically heterogeneous systems such as trabecular bone microstructure in vivo. These pulse sequences are found to be substantially less sensitive to distortions from magnetic dipole fields occurring at the boundaries of two phases of different magnetic permeability.

Bone and Bones↗
PubMed1990

The role of ferritin and hemosiderin in the MR appearance of cerebral hemorrhage: a histopathologic biochemical study in rats.

A rat model of cerebral hemorrhage using stereotaxic injection of blood into the right basal ganglia was developed to investigate the influence of iron metabolism on the appearance of cerebral hemorrhage on MR images. Images of in vitro fixed brain sections stained specifically for different iron-storage substances, ferritin and hemosiderin, created by digitization of the pathology sections using an Eikonix CCD camera, were compared with the in vivo MR images of late-phase hematomas. Areas of the pathologic and MR features of the lesions were quantitatively correlated. The single-slice MR images were obtained with the use of T1- and T2-weighted spin-echo pulse sequences, as well as T2-weighted spin-echo pulse sequences in which the 180 degrees refocusing pulse was offset from the center of the echo time; this was termed an asymmetric spin-echo pulse sequence. The symmetric and asymmetric T2-weighted images allowed the calculation of line-width images, which emphasize line broadening from intravoxel magnetic field inhomogeneities that arise from the presence of iron-containing substances. From biochemical and histochemical staining, we conclude that at least two iron-storage substances are present in the late phase of resolving cerebral hematomas. Ferritin has a wider distribution than hemosiderin, showing a similar distribution to the MR signal changes of the calculated line-width images. Line-width mapping is a sensitive means of detecting magnetic field inhomogeneities caused by the magnetic susceptibility differences introduced by the aggregation of these iron-storage substances.

Animals↗
PubMedDate not supplied

The role of ferritin and hemosiderin in the MR appearance of cerebral hemorrhage: a histopathologic biochemical study in rats.

A rat model of cerebral hemorrhage using stereotaxic injection of blood into the right basal ganglia was developed to investigate the influence of iron metabolism on the appearance of cerebral hemorrhage on MR images. Images of in vitro fixed brain sections stained specifically for different iron-storage substances, ferritin and hemosiderin, created by digitization of the pathology sections using an Eikonix CCD camera, were compared with the in vivo MR images of late-phase hematomas. Areas of the pathologic and MR features of the lesions were quantitatively correlated. The single-slice MR images were obtained with the use of T1- and T2-weighted spin-echo pulse sequences, as well as T2-weighted spin-echo pulse sequences in which the 180 degrees refocusing pulse was offset from the center of the echo time; this was termed an asymmetric spin-echo pulse sequence. The symmetric and asymmetric T2-weighted images allowed the calculation of line-width images, which emphasize line broadening from intravoxel magnetic field inhomogeneities that arise from the presence of iron-containing substances. From biochemical and histochemical staining, we conclude that at least two iron-storage substances are present in the late phase of resolving cerebral hematomas. Ferritin has a wider distribution than hemosiderin, showing a similar distribution to the MR signal changes of the calculated line-width images. Line-width mapping is a sensitive means of detecting magnetic field inhomogeneities caused by the magnetic susceptibility differences introduced by the aggregation of these iron-storage substances.

Animals↗
PubMed1999

Interscanner variation in brain MR lesion load measurements in multiple sclerosis using conventional spin-echo, rapid relaxation-enhanced, and fast-FLAIR sequences.

BACKGROUND AND PURPOSE: Different MR pulse sequences have been proposed for measuring multiple sclerosis (MS)-related abnormalities. The reproducibility of measured brain MS lesion volumes was compared for MR images performed using different scanners and different pulse sequences. METHODS: Nine patients with relapsing-remitting MS were each imaged on two scanners and, on each occasion, dual-echo conventional spin-echo, dual-echo rapid-acquisition relaxation-enhanced (RARE), and fast fluid-attenuated inversion recovery (fast-FLAIR) images were obtained. The lesion volume present on each image was evaluated three times by a single observer in random order, using a local thresholding technique. RESULTS: The mean lesion volumes present on fast-FLAIR images were significantly higher than those measured on dual-echo conventional spin-echo and RARE images. The mean intraobserver coefficients of variation for the different sequences and scanners ranged from 3.0% to 4.2% (no statistically significant difference). For each of the sequences, the use of different scanners introduced a variability that was higher than the intraobserver variability: the interscanner coefficient of variation was 7.4% for conventional spin-echo, 9.5% for RARE, and 18.5% for fast-FLAIR images. CONCLUSION: Our study confirms that the use of different scanners significantly influences lesion loads measured from MR images of patients with MS and establishes that newer sequences are more susceptible to measurement variability. It also indicates that, if newer sequences are to be used in clinical trials, careful standardization is needed.

Adult↗
PubMed1993

High-field MR microscopy using fast spin-echoes.

Fast spin-echo imaging has been investigated with attention to the requirements and opportunities for high-field MR microscopy. Two- and three-dimensional versions were implemented at 2.0 T, 7.1 T, and 9.4 T. At these fields, at least eight echoes were collectable with a 10 ms TE from fixed tissue specimens and living animals, giving an eightfold improvement in imaging efficiency. To reduce the phase-encoding gradient amplitude and its duty cycle, a modified pulse sequence with phase accumulation was developed. Images obtained using this pulse sequence exhibited comparable signal-to-noise (SNR) to those obtained from the conventional fast spin-echo pulse sequences. Signal losses due to imperfections in RF pulses and lack of phase rewinders were offset in this sequence by reduced diffusion losses incurred with the gradients required for MR microscopy. Image SNR, contrast, edge effects and spatial resolution for three k-space sampling schemes were studied experimentally and theoretically. One method of sampling k-space, 4-GROUP FSE, was found particularly useful in producing varied T2 contrast at high field. Two-dimensional images of tissue specimens were obtained in a total acquisition time of 1 to 2 min with in-plane resolution between 30 to 70 microns, and 3D images with 256(3) arrays were acquired from fixed rat brain tissue (isotropic voxel = 70 microns) and a living rat (isotropic voxel = 117 microns) in approximately 4.5 h.

Animals↗
PubMedDate not supplied

[Transformation of hemoglobulin into methemoglobulin during heating of blood: its role in the treatment of vascular lesions using an Nd: YAG (1.06 m) laser treatment of vascular lesions].

Clinical modifications are usually observed during the irradiation of a blood vessel with a laser. These modifications are due to the heating of the blood. Photothermal modifications of oxy-hemoglobin (Hb-O(2)) and des-oxyhemoglobin (Hb) induce formation of met-hemoglobin (Met-Hb), not normally present in healthy human blood. In the near-infrared, the optical absorption of Met-Hb is 3 to 4 times higher that the other chemical constituents of blood. Consequently, after the conformational change of the molecule of heme, the near-infrared (1.06 microm) absorption is considerably enhanced. In order to improve the Nd: YAG laser treatment of vascular lesions, using a non uniform pulse sequence appears to be a much better and safer solution than using a single pulse. The non uniform pulse sequence consists of 3 pulses. The first one induces the optical modification of the blood. After a few hundreds of milliseconds, a 2(nd) and a 3(rd), carrying an energy 3 to 4 times lower than the 1(st) one, are applied. These two pulses maintained a constant temperature inside the vessel. In order to achieve the complete destruction of the vessel, the total duration of the non uniform pulse sequence is similar to the thermal relaxation time of the blood vessel. In conclusion; a better knowledge of the laser tissue interaction process can lead to a better use of a laser. In this particular case, the efficiency of the Nd: YAG laser treatment of vascular lesions can be considerably improved while preserving non-targeted structures.

Hemoglobins↗
PubMed2005

MRI-based technique for determining nonuniform deformations throughout the volume of articular cartilage explants.

Articular cartilage is critical to the normal function of diarthrodial joints. Despite the importance of the tissue and the prevalence of cartilage degeneration (e.g., osteoarthritis), the technology required to noninvasively describe nonuniform deformations throughout the volume of the tissue has not been available until recently. The objectives of the work reported in this paper were to 1) describe a noninvasive technique (termed the cartilage deformation by tag registration (CDTR) technique) to determine nonuniform deformations in articular cartilage explants with the use of specialized MRI tagging and image processing methods, 2) evaluate the strain error of the CDTR technique using a custom MRI-compatible phantom material, and 3) demonstrate the applicability of the CDTR technique to articular cartilage by determining 3D strain fields throughout the volume of a bovine articular cartilage explant. A custom MRI pulse sequence was designed to tag and image articular cartilage explants at 7 Tesla in undeformed and deformed states during the application of multiple load cycles. The custom pulse sequence incorporated the "delays alternating with nutations for tailored excitation" (DANTE) pulse sequence to apply tags. This was followed by a "fast spin echo" (FSE) pulse sequence to create images of the tags. The error analysis using the phantom material indicated that deformations can be determined with an error, defined as the strain precision, better than 0.83% strain. When this technique was applied to a single articular cartilage explant loaded in unconfined compression, hetereogeneous deformations throughout the volume of the tissue were evident. This technique potentially can be applied to determine normal cartilage deformations, analyze degenerated cartilage, and evaluate cartilage surgical repair and treatment methodologies. In addition, this technique may be applied to other soft tissues that can be appropriately imaged by MR.

Animals↗
PubMedDate not supplied

Assessment of cardiovascular anatomy in patients with congenital heart disease by magnetic resonance imaging.

The following discussion addresses the assessment of cardiovascular anatomy in patients with congenital heart disease by magnetic resonance (MR). The focus of this review is on the techniques of performing the MR examination. In particular, individual pulse sequences are described and illustrated with their strengths and weaknesses. Imaging strategies using the described pulse sequences are proposed. The pulse sequences described are widely available on most MR scanners. Therefore, the proposed imaging strategies are clinically proven to be simple and effective ways to perform cardiac MR examination for the assessment of cardiovascular anatomy in patients with congenital heart disease. Functional imaging, such as flow analysis and ventricular function assessment, are discussed elsewhere in this issue.

Adolescent↗
PubMed1998

[Efficacy of the fluid attenuated inversion recovery (FLAIR) sequence of MRI as a preoperative diagnosis of hippocampal sclerosis].

A newly advanced MRI pulse sequence, the FLAIR (fluid attenuated inversion recovery) imaging, in which a long TE spin echo sequence is used with suppression of the CSF with an inversion pulse, displays the CSF space as a no-signal intensity area. There have been only a few reports on the FLAIR pulse sequence of temporal lobe epilepsy (TLE) as yet. We examined 9 cases of intractable TLE by FLAIR images and analyzed the advantages and disadvantages of the FLAIR pulse sequence for decision making on temporal lobectomy. All patients underwent anterior temporal lobectomy with hippocampectomy, and the diagnoses were confirmed histologically after surgery. Abnormally high T2 signals (HT2S) were more conspicuous with the FLAIR sequence than with any of the conventional sequences. Tilted axial plane, orientated along to the long axis of the hippocampal body, clearly demonstrated hippocampal atrophy (HA). Selection of a FLAIR sequence into the routine MR examination of patients with TLE is recommended.

Adolescent↗