Biomedical subjects

F B Mohamed

Publications and source records attributed to F B Mohamed.

12 recordsLinked to original sources

PubMed2001

Increased differentiation of intracranial white matter lesions by multispectral 3D-tissue segmentation: preliminary results.

MRI is a very sensitive imaging modality, however with relatively low specificity. The aim of this work was to determine the potential of image post-processing using 3D-tissue segmentation technique for identification and quantitative characterization of intracranial lesions primarily in the white matter. Forty subjects participated in this study: 28 patients with brain multiple sclerosis (MS), 6 patients with subcortical ischemic vascular dementia (SIVD), and 6 patients with lacunar white matter infarcts (LI). In routine MR imaging these pathologies may be almost indistinguishable. The 3D-tissue segmentation technique used in this study was based on three input MR images (T(1), T(2)-weighted, and proton density). A modified k-Nearest-Neighbor (k-NN) algorithm optimized for maximum computation speed and high quality segmentation was utilized. In MS lesions, two very distinct subsets were classified using this procedure. Based on the results of segmentation one subset probably represent gliosis, and the other edema and demyelination. In SIVD, the segmented images demonstrated homogeneity, which differentiates SIVD from the heterogeneity observed in MS. This homogeneity was in agreement with the general histological findings. The LI changes pathophysiologically from subacute to chronic. The segmented images closely correlated with these changes, showing a central area of necrosis with cyst formation surrounded by an area that appears like reactive gliosis. In the chronic state, the cyst intensity was similar to that of CSF, while in the subacute stage, the peripheral rim was more prominent. Regional brain lesion load were also obtained on one MS patient to demonstrate the potential use of this technique for lesion load measurements. The majority of lesions were identified in the parietal and occipital lobes. The follow-up study showed qualitatively and quantitatively that the calculated MS load increase was associated with brain atrophy represented by an increase in CSF volume as well as decrease in "normal" brain tissue volumes. Importantly, these results were consistent with the patient's clinical evolution of the disease after a six-month period. In conclusion, these results show there is a potential application for a 3D tissue segmentation technique to characterize white matter lesions with similar intensities on T(2)-weighted MR images. The proposed methodology warrants further clinical investigation and evaluation in a large patient population.

Adult↗

PubMed2000

Functional localization of a "Time Keeper" function separate from attentional resources and task strategy.

The functional neuroanatomy of time estimation has not been well-documented. This research investigated the fMRI measured brain response to an explicit, prospective time interval production (TIP) task. The study tested for the presence of brain activity reflecting a primary time keeper function, distinct from the brain systems involved either in conscious strategies to monitor time or attentional resource and other cognitive processes to accomplish the task. In the TIP task participants were given a time interval and asked to indicate when it elapsed. Two control tasks (counting forwards, backwards) were administered, in addition to a dual task format of the TIP task. Whole brain images were collected at 1.5 Tesla. Analyses (n = 6) yielded a statistical parametric map (SPM ¿z¿) reflecting time keeping and not strategy (counting, number manipulation) or attention resource utilization. Additional SPM ¿z¿s involving activation associated with the accuracy and magnitude the of time estimation response are presented. Results revealed lateral cerebellar and inferior temporal lobe activation were associated with primary time keeping. Behavioral data provided evidence that the procedures for the explicit time judgements did not occur automatically and utilized controlled processes. Activation sites associated with accuracy, magnitude, and the dual task provided indications of the other structures involved in time estimation that implemented task components related to controlled processing. The data are consistent with prior proposals that the cerebellum is a repository of codes for time processing, but also implicate temporal lobe structures for this type of time estimation task.

Adult↗

PubMed1999

Optimization of tissue segmentation of brain MR images based on multispectral 3D feature maps.

The purpose of this work was to optimize and increase the accuracy of tissue segmentation of the brain magnetic resonance (MR) images based on multispectral 3D feature maps. We used three sets of MR images as input to the in-house developed semi-automated 3D tissue segmentation algorithm: proton density (PD) and T2-weighted fast spin echo and, T1-weighted spin echo. First, to eliminate the random noise, non-linear anisotropic diffusion type filtering was applied to all the images. Second, to reduce the nonuniformity of the images, we devised and applied a correction algorithm based on uniform phantoms. Following these steps, the qualified observer "seeded" (identified training points) the tissue of interest. To reduce the operator dependent errors, cluster optimization was also used; this clustering algorithm identifies the densest clusters pertaining to the tissues. Finally, the images were segmented using k-NN (k-Nearest Neighborhood) algorithm and a stack of color-coded segmented images were created along with the connectivity algorithm to generate the entire surface of the brain. The application of pre-processing optimization steps substantially improved the 3D tissue segmentation methodology.

Anisotropy↗

PubMed1998

Rapid ventricular pacing in a pacemaker patient undergoing magnetic resonance imaging.

Magnetic resonance imaging (MRI) generates potent electromagnetic forces in the form of a static, gradient, or pulsed radiofrequency magnetic field that can result in pacemaker malfunction. This report documents a case of rapid cardiac pacing during MRI in a patient with a dual chamber pacemaker. Although the mechanism of rapid cardiac pacing is unclear, it was directly related to radiofrequency pulsing. We postulated that the lead acts as an antenna for radiofrequency energy that interacts with the pacemaker's output circuit, thus, causing cardiac pacing at a cycle length representing a multiple of the repetition time; or perhaps rapid pacing is related to induced currents generated between the MRI unit and the pacing lead.

Aged↗

PubMed1998

Carotid artery bifurcation: multiple-flip-angle, three-dimensional, time-of-flight MR angiographic technique.

Multiple-flip-angle, three-dimensional, time-of-flight magnetic resonance (MR) angiography was performed in vitro and in vivo in the carotid artery bifurcation. Composite MR angiographic images were created from multiple data sets. Compared with images obtained with a low (20 degrees) flip angle, composite images obtained with flip angles of 20 degrees and 60 degrees demonstrated improved image quality and statistically significant improvement in signal-to-noise ratio (P < .05).

Adult↗

PubMed1997

Paraganglioma of the cauda equina with associated intramedullary cyst: MR findings.

The MR imaging features of a paraganglioma of the cauda equina with associated spinal cord cysts are presented. MR imaging showed the tumor to be isointense with the spinal cord on all pulse sequences and to enhance homogeneously. The intramedullary cysts had increased signal intensity on proton density- and T2-weighted images, and involved the cervical and thoracic regions.

Cauda Equina↗

PubMed1996

Carotid magnetic resonance angiography: improved image quality with dual 3-inch surface coils.

Magnetic resonance angiography (MRA) hs inherent artifacts due to variation in velocity and direction of flowing blood in the carotid bulb and regions of stenosis. We examined the efficiency of dual 3-inch surface coils to delineate carotid artery flow better. Carotid MRA was performed on ten healthy volunteers and six patients, on a 1.5 T system. A special adapter was constructed to use with 3-inch (receive-only) coils, which were placed over the carotid bifurcations. Routine anterior neck coils were also used. Contiguous axial two-dimensional (45/8.7, 1.5 mm, flip angle 60 degrees) time-of-flight sequences were used. Image matrix was 256 x 256 with two signals averaged and acquisition time 6-10 min. These images were postprocessed and reformatted into angiographic views using a maximum intensity projection algorithm. Computer simulation of carotid artery blood flow throughout the cardiac cycle based on vessel contours derived from digital subtraction angiography was carried out by finite element analysis. Improved definition of vessel margin, particularly at the carotid bifurcation, and substantially increased signal-to-background ratio of flowing blood were obtained with 3-in-chcoils. Apparent loss of signal in the carotid bulb was diminished. In one patient, contiguous flow throughout a high-grade stenosis was well defined, with the surface coil method, while drop-off of signal was observed with routine neck coil imaging.

Adult↗

PubMed1993

Use of Ferrum in MRI of lung parenchyma and pulmonary embolism.

MRI of lung parenchyma and pulmonary embolism (PE) remains challenging. "Ferrum," a ferric hydroxide sucrose complex used clinically for iron deficiency anemia for more than 40 years, was investigated as a negative MRI contrast agent in five rabbits bearing experimental PE as well as in five normal volunteers. Clots were prepared by spontaneous coagulation of 0.1 ml In-111 labeled autologous red blood cells and introduced through the jugular vein. Scintigraphic imaging permitted anatomical localization of PE in vivo and thereby served as a control for MR imaging. MRI was performed on a 1.5 T GE Signa scanner before and after induction of PE, and before and after the injection of Ferrum. T1-weighted images were obtained continuously for up to 90 min using varying doses of Ferrum. In five normal human volunteers, a single dose of 100 mg each was administered. T1- and T2-weighted spin-echo and gradient-echo images of lung parenchyma were repeatedly obtained before and after agent administration. In rabbit, Ferrum remained in circulation for several hours where it shortened both T1 and T2 of blood, improving the contrast between PE and lung parenchyma (i.e., intravascular compartment). A dose of 3 mg/kg was enough to increase the contrast-to-noise ratio (CNR) between PE and lung parenchyma by almost three fold, substantially improving lesion detectability. CNR increased up to five-fold when the dose was increased up to 20 mg/kg at which point CNR reached a plateau. In humans, T2-weighted spin-echo sequence appeared to be most sensitive to changes in signal-to-noise ratio (SNR) of normal lung parenchyma. Within 60 min after injection of 100 mg of iron, SNR dropped by 34% (p < .025). However, 24 hr later, SNR returned to almost normal. Ferrum increased the contrast between PE and lung parenchyma in the rabbit and decreased the parenchymal SNR in humans in nontoxic doses. These results suggest that Ferrum is worthy of further investigation of PE imaging in humans.

Aged↗

PubMed1991

Comparison of Kaopectate with barium for negative and positive enteric contrast at MR imaging.

In an evaluation of safe, inexpensive, orally administered contrast agents for use at magnetic resonance imaging, different concentrations of the clay agents kaolin-pectate and attapulgite were compared in an in vitro phantom by performing spin-echo imaging with variable echo times to estimate T2 relaxation time. Additional phantoms containing from 0% to 100% attapulgite or barium preparations were compared by using spin-echo and spoiled-gradient-echo techniques to estimate T1, T2, and T2* relaxation times. In vivo spin-echo and gradient-echo images were obtained of 10 healthy volunteers and of five patients with pancreatic disease after oral administration of 12-16 oz of attapulgite preparation. T1 and T2 relaxation times obtained with use of attapulgite were slightly lower than those obtained with kaolin-pectate at both 50% and 25% concentrations. Compared with barium, attapulgite had shorter T1, T2, and T2* relaxation times, and concentration-dependent relaxivity was greater by nearly an order of magnitude. All subjects were able to drink the attapulgite, and the only complaints were of bad taste and mildly loose stools.

Barium Sulfate↗

PubMedDate not supplied

Respiratory and postprandial changes in portal flow rate: assessment by phase contrast MR imaging.

The objective of this study was to investigate the variability of several different phase contrast (PC) magnetic resonance techniques and duplex Doppler imaging (DDI) for measuring portal flow rate (PFR) with different patterns of respiration before and after a standard meal. PFR was measured in nine normal volunteers in both fasting and postprandial states, using both 2D and cine PC MR techniques. Two-dimensional PC technique was performed with and without breath-hold (BH). Four different patterns of BH (relaxed maximal inspiration, expiration, Valsalva maneuver, and Müller or reversed Valsalva) were used. Cine PC technique was performed without BH. DDI measurement of PFR was done in the fasting state with relaxed inspiration. PFR measurements made by means of PC MR techniques varied, depending on BH method, and were systematically less than those obtained by DDI. PFR measured during inspiration was significantly less than that measured during expiration and other respiratory maneuvers (P < .03). PFR increased significantly after a meal for PC and Doppler measurements, with a range of mean increases of 24 to 74%. Two-dimensional PC MR imaging techniques yielded differing measurements of PFR, depending on whether they were done with or without BH, as well as on the pattern of BH. PC mean measures of PFR were consistently less than those of DDI. The physiologic inspiratory decreases and postprandial increases of the PFR were evident from 2D and cine PC MRI techniques, as well as with DDI.

Adult↗

PubMedDate not supplied

A simple method to improve image nonuniformity of brain MR images at the edges of a head coil.

One of the major sources of image nonuniformity in the high field MR scanners is the radiofrequency (RF) coil inhomogeneity. It degrades conspicuity of lesion(s) in the MR images of the brain and surrounding tissues and reduces accuracy of image postprocessing particularly at the edges of the coil. In this investigation, we have devised and tested a simple method to correct for nonuniformity of MR images of the brain at the edges of the RF head coil. Initially, a cylindrical oil phantom, which fit exactly in the head coil, was scanned on a 1.5 T imager. Then, a correction algorithm identified a reference pixel value in the phantom at the most homogeneous region of the RF coil. Next, every pixel inside the phantom was normalized relative to this reference value. The resulting set of coefficients or "correction matrices" was obtained for different types of MR contrast agent. Finally, brain MR images of normal subjects and multiple sclerosis patients were acquired and processed by the corresponding correction matrices obtained with different pulse sequences. Application of correction matrices to brain MR images showed a gain in pixel intensity particularly in the slices at the edge of the coil.

Algorithms↗

PubMedDate not supplied

Investigation of alternating and continuous experimental task designs during single finger opposition fMRI: a comparative study.

The purpose of this study was to empirically investigate and compare the effects of alternating and continuous experimental task designs on blood oxygenation level dependent (BOLD) signal contrast. Six healthy volunteers underwent single-finger opposition functional magnetic resonance imaging (fMRI) using T2*-weighted echo planar imaging technique on a 1.5 T MR scanner. Two different acquisition patterns were tested: alternating (ABABAB) and continuous (AAABBB), rest: A, activation: B. The BOLD signal contrast within a primary motor cortex region of interest (ROI) was evaluated using normalized t-values (z-scores) and mean region of interest (ROI) intensity for the two patterns. Analysis of variance (ANOVA) on ROI mean z-score and signal intensities demonstrate that the alternating pattern of administering rest and activation epochs produced a more robust statistical difference than a continuous pattern. The results showed that different patterns of acquisition yield differences in the BOLD signal at field strength of 1.5 T, and that an alternating task design can be considered more optimal than a continuous task design.

Adolescent↗