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Biomedical subjects

P Jezzard

Publications and source records attributed to P Jezzard.

At least 19 recordsLinked to original sources
PubMed2003

Thalamic neurodegeneration in relapsing-remitting multiple sclerosis.

OBJECTIVE: To define the extent of neuronal injury and loss in thalamic gray matter in patients with relapsing-remitting (RR) MS and to characterize how these neuronal pathologic changes are related to disease duration. METHODS: The authors studied 14 patients with RRMS (Expanded Disability Status Scale score, mean 3.25, range 2.0 to 6.0) and 14 (8 men, 6 women) age-matched healthy controls. Structural MR and MRS studies were performed in a single scanning session using a 3T MR system. RESULTS: N-acetylaspartate (NAA) concentrations (a measure of the apparent neuronal density) were decreased approximately 11% in the thalami of the patients with RRMS relative to controls (p < 0.05). The patients with RRMS also had an almost 25% lower mean normalized thalamic volume than controls (p < 0.005). Decreases in thalamic NAA concentration correlated strongly with thalamic volume loss for individual patients (r = 0.85, p < 0.01). Both the NAA concentration (r = -0.48, p = 0.044) and normalized thalamic volume (r = -0.60, p = 0.01) were correlated inversely with disease duration. There was a trend for a correlation between the thalamic NAA/creatine (Cr) ratio and the NAA/Cr in the frontal normal-appearing white matter (r = 0.56, p < 0.08). CONCLUSIONS: The reduction of both NAA concentration and thalamic volume suggests that a neurodegenerative component may contribute to the pathology of MS even in the earlier RR stage. The trend toward a relationship between thalamic NAA/Cr and distant normal-appearing white matter changes implies that there may be a common mechanism for the white matter axonal loss and thalamic neuronal injury.

Adult↗
PubMed2001

Rapid T(1) mapping using multislice echo planar imaging.

Determination of neurological pathology in white matter disease can be made in a semiquantitative way from T(1)- or T(2)-weighted images. A higher level of quantification based on measured T(1) or T(2) values has been either limited to specific regions of interest or to low-resolution maps. Higher-resolution T(1) maps have proved difficult to obtain due to the excessively long scan times required using conventional techniques. In this study, clinically acceptable images are obtained by using single-shot echo planar imaging (EPI) with an acquisition scheme that maximizes signal-to-noise while minimizing the scan time. Magn Reson Med 45:630-634, 2001.

Adult↗
PubMed2001

Radio frequency magnetic field mapping of a 3 Tesla birdcage coil: experimental and theoretical dependence on sample properties.

The RF B(1) distribution was studied, theoretically and experimentally, in phantoms and in the head of volunteers using a 3 T MRI system equipped with a birdcage coil. Agreement between numerical simulation and experiment demonstrates that B(1) distortion at high field can be explained with 3D full-Maxwell calculations. It was found that the B(1) distribution in the transverse plane is strongly dependent on the dielectric properties of the sample. We show that this is a consequence of RF penetration effects combined with RF standing wave effects. In contrast, along the birdcage coil z-axis the B(1) distribution is determined mainly by the coil geometry. In the transverse plane, the region of B(1) uniformity (within 10% of the maximum) was 15 cm with oil, 6 cm with distilled water, 11 cm with saline, and 10 cm in the head. Along z the B(1) uniformity was 9 cm with phantoms and 7 cm in the head.

Brain Mapping↗
PubMed2001

Compensating for B(1) inhomogeneity using active transmit power modulation.

The effect of poor B(1) homogeneity on MRI images not only affects the appearance of the images, but produces difficulty in automated segmentation and in certain quantification methods. While improved RF coil design is the first line in reducing such artifact, compensation methods can significantly improve the quality of images. Existing methods of compensation typically apply a filter during the image reconstruction. Here a method is presented that compensates for part of the inhomogeneity by actively modulating the RF transmit power as a function of slice position. The method is demonstrated both quantitatively on a phantom and qualitatively on a human brain.

Brain↗
PubMed1999

Sources of distortion in functional MRI data.

Functional magnetic resonance image (fMRI) experiments rely on the ability to detect subtle signal changes in magnetic resonance image time series. Any areas of signal change that correlate with the neurological stimulus can then be identified and compared with a corresponding high-resolution anatomical scan. This report reviews some of the several artefacts that are frequently present in fMRI data, degrading their quality and hence their interpretation. In particular, the effects of magnetic field inhomogeneities are described, both on echo planar imaging (EPI) data and on spiral imaging data. The modulation of these distortions as the subject moves in the magnet is described. The effects of gradient coil nonlinearities and EPI ghost correction schemes are also discussed.

Artifacts↗
PubMed1998

Hemispheric specialization for English and ASL: left invariance-right variability.

Functional magnetic resonance imaging (fMRI) was used to compare the cerebral organization during sentence processing in English and in American sign language (ASL). Classical language areas within the left hemisphere were recruited by both English in native speakers and ASL in native signers. This suggests a bias of the left hemisphere to process natural languages independently of the modality through which language is perceived. Furthermore, in contrast to English, ASL strongly recruited right hemisphere structures. This was true irrespective of whether the native signers were deaf or hearing. Thus, the specific processing requirements of the language also in part determine the organization of the language systems of the brain.

Adult↗
PubMed1998

The acquisition of skilled motor performance: fast and slow experience-driven changes in primary motor cortex.

Behavioral and neurophysiological studies suggest that skill learning can be mediated by discrete, experience-driven changes within specific neural representations subserving the performance of the trained task. We have shown that a few minutes of daily practice on a sequential finger opposition task induced large, incremental performance gains over a few weeks of training. These gains did not generalize to the contralateral hand nor to a matched sequence of identical component movements, suggesting that a lateralized representation of the learned sequence of movements evolved through practice. This interpretation was supported by functional MRI data showing that a more extensive representation of the trained sequence emerged in primary motor cortex after 3 weeks of training. The imaging data, however, also indicated important changes occurring in primary motor cortex during the initial scanning sessions, which we proposed may reflect the setting up of a task-specific motor processing routine. Here we provide behavioral and functional MRI data on experience-dependent changes induced by a limited amount of repetitions within the first imaging session. We show that this limited training experience can be sufficient to trigger performance gains that require time to become evident. We propose that skilled motor performance is acquired in several stages: "fast" learning, an initial, within-session improvement phase, followed by a period of consolidation of several hours duration, and then "slow" learning, consisting of delayed, incremental gains in performance emerging after continued practice. This time course may reflect basic mechanisms of neuronal plasticity in the adult brain that subserve the acquisition and retention of many different skills.

Adult↗
PubMed1998

Cerebral organization for language in deaf and hearing subjects: biological constraints and effects of experience.

Cerebral organization during sentence processing in English and in American Sign Language (ASL) was characterized by employing functional magnetic resonance imaging (fMRI) at 4 T. Effects of deafness, age of language acquisition, and bilingualism were assessed by comparing results from (i) normally hearing, monolingual, native speakers of English, (ii) congenitally, genetically deaf, native signers of ASL who learned English late and through the visual modality, and (iii) normally hearing bilinguals who were native signers of ASL and speakers of English. All groups, hearing and deaf, processing their native language, English or ASL, displayed strong and repeated activation within classical language areas of the left hemisphere. Deaf subjects reading English did not display activation in these regions. These results suggest that the early acquisition of a natural language is important in the expression of the strong bias for these areas to mediate language, independently of the form of the language. In addition, native signers, hearing and deaf, displayed extensive activation of homologous areas within the right hemisphere, indicating that the specific processing requirements of the language also in part determine the organization of the language systems of the brain.

Adult↗
PubMed1998

Characterization of and correction for eddy current artifacts in echo planar diffusion imaging.

Magnetic resonance diffusion imaging is potentially an important tool for the noninvasive characterization of normal and pathological tissue. The technique, however, is prone to a number of artifacts that can severely affect its ability to provide clinically useful information. In this study, the problem of eddy current-induced geometric distortions that occur in diffusion images acquired with echo planar sequences was addressed. These geometric distortions produce artifacts in computed maps of diffusion parameters and are caused by misalignments in the individual diffusion-weighted images that comprise the diffusion data set. A new approach is presented to characterize and calibrate the eddy current effects, enabling the eddy current distortions to be corrected in sets of interleaved (or snapshot) echo planar diffusion images. Correction is achieved by acquiring one-dimensional field maps in the read and phase encode direction for each slice and each diffusion step. The method is then demonstrated through the correction of distortions in diffusion images of the human brain. It is shown that by using the eddy current correction scheme outlined, the eddy current-induced artifacts in the diffusion-weighted images are almost completely eliminated. In addition, there is a significant improvement in the quality of the resulting diffusion tensor maps.

Artifacts↗
PubMed1997

Correction for vascular artifacts in cerebral blood flow values measured by using arterial spin tagging techniques.

"Vascular" artifacts can have substantial effects on human cerebral blood flow values calculated by using arterial spin tagging approaches. One vascular artifact arises from the contribution of "tagged" arterial water spins to the observed change in brain water MR signal. This artifact can be reduced if large bipolar gradients are used to "crush" the MR signal from moving arterial water spins. A second vascular artifact arises from relaxation of "tagged" arterial blood during transit from the tagging plane to the capillary exchange site in the imaging slice. This artifact can be corrected if the arterial transit times are measured by using "dynamic" spin tagging approaches. The mean transit time from the tagging plane to capillary exchange sites in a gray matter region of interest was calculated to be approximately 0.94 s. Cerebral blood flow values calculated for seven normal volunteers agree reasonably well with values calculated by using radioactive tracer approaches.

Adult↗
PubMed1997

An in vivo model for functional MRI in cat visual cortex.

A protocol is described for obtaining functional magnetic resonance images in anesthetized cat brain based on the blood oxygenation level dependent (BOLD) contrast mechanism. A visual stimulus was used, which consisted of a high-contrast drifting grating, whose speed and spatial frequency was optimized for cat area 18 (V2). Experiments were conducted at 4.7 Tesla using a gradient echo EPI sequence with a 29-ms echo time, yielding signal changes of between 0.7% and 2% in area 18.

Animals↗
PubMed1997

Simultaneous measurement of DeltaR2 and DeltaR2* in cat brain during hypoxia and hypercapnia.

One of the most important issues in blood-oxygen-level-dependent (BOLD)-based brain functional magnetic resonance imaging is the understanding of the vascular structures that are responsible for the signal changes observed. The T2*-related signal changes observed during variations in susceptibility-induced magnetic field gradients are a function both of non-refocusable mechanisms, such as diffusion, and of refocusable effects such as field inhomogeneities. Conversely, T2-related signal changes are only a function of non-refocusable effects. It has been suggested that T2-weighted images could be less sensitive to blood susceptibility changes in a macrovascular environment than T2*-weighted images and could thus be more accurate in identifying the "activation" of the parenchyma rather than "draining vein" effects. In this study we use hypoxia and hypercapnia challenges in cats to provide a change in blood deoxyhemoglobin concentration (as a model for classic BOLD changes and not as a model for neuronal activation). A combined gradient echo and spin echo echo-planar-imaging (EPI) pulse sequence was used to map DeltaR2 (i.e., Delta(1/T2)) and DeltaR2* (i.e., Delta(1/T2*)) changes during the challenges. Our experiments demonstrate that: (i) the acquisition of T2-weighted EPI data does not in itself differentiate signal changes in the parenchyma from those occurring in regions around larger vessels, but that (ii) the simultaneous acquisition of T2- and T2*-weighted images could be useful in identifying microvascular regions in gray matter by analyzing the ratio DeltaR2/DeltaR2*. This value seems independent of the degree of deoxyhemoglobin concentration change, but is related to properties of the vascular environment. We suggest a possible application of the results to the study of brain function in humans.

Animals↗
PubMed1996

Perfusion imaging with compensation for asymmetric magnetization transfer effects.

The effects of off-resonance radio-frequency irradiation on the intensity of the MR signal from water protons in the cat brain are asymmetric around the chemical shift of the water signal. This asymmetry, which could arise from a shift in the magnetization transfer spectrum approximately 1.5 ppm upfield from the solvent water signal, must be taken into account to compensate for magnetization transfer effects inherent in arterial spin tagging approaches that use a single radio-frequency coil. Two approaches that either correct for, or circumvent, the apparent upfield shift of the magnetization transfer spectrum are presented, and a perfusion image of the cat brain, using flow-induced adiabatic inversion of arterial water protons, is presented. Other problems in obtaining quantitative cerebral blood flow values using the arterial spin tagging approach are discussed.

Animals↗
PubMed1996

Evaluation of methemoglobin as an autologous intravascular MRI contrast agent.

Methemoglobin (MetHb) was evaluated as an intravascular paramagnetic contrast agent. Methemoglobin formation was induced by 4-dimethylaminophenol (4-DMAP), causing a reduction in blood T2* in vitro. The 4-DMAP generated metHb with a time constant of 62 s. A 4-DMAP bolus did not decrease measurably the signal intensity in the in vivo rabbit kidney in the first pass. At steady state, a MetHb concentration of 24.8 +/- 2.3% resulted in a signal decrease of 9.2 +/- 2.6% in the kidney. Methemoglobin is an effective vascular T2* relaxation agent, but the formation of MetHb by 4-DMAP is too slow for first-pass imaging. A more effective conversion agent resulting in a bolus of at least 25% MetHb within 5 s would result in a detectable first-pass signal and a viable contrast technique.

4-Aminopyridine↗
PubMed1996

Perfusion imaging of the human brain at 1.5 T using a single-shot EPI spin tagging approach.

Single-shot echo planar imaging (EPI) techniques have been applied, in conjunction with arterial spin tagging approaches, to obtain images of cerebral blood flow in a single axial slice in the human brain. Serial studies demonstrate that cerebral blood flow images acquired in 8 min are reproducible, with a statistical precision of approximately +/-10 cc/100 g/min. The average value of cerebral blood flow in the slice is 51 +/- 11 cc/100 g/min for six normal subjects. The cerebral blood flow images contain two types of artifact, probably due to arterial and venous blood volume contributions, which must be overcome before the arterial spin tagging approach can be used for routine clinical studies.

Artifacts↗
PubMed1996

Technical foundations and pitfalls of clinical fMRI.

Magnetic resonance imaging (MRI) has become an established and invaluable tool in the diagnosis of numerous diseases through its ability to show pathologic contrast in images of soft tissue. More recently, MRI has found application in the study of organ function, principally in the brain and heart. This article deals with MRI imaging of brain function and describes some of the techniques that allow physiological parameters such as cerebral blood volume, cerebral blood oxygenation, and cerebral perfusion to be determined. Additionally, some of the potentially confounding influences in these experiments are discussed.

Brain↗
PubMed1996

MR relaxation times in human brain: measurement at 4 T.

PURPOSE: To determine the values for relaxation times in human brain for magnetic resonance (MR) imaging at 4 T. MATERIALS AND METHODS: T1 measurements were made with a progressive saturation sequence, an implementation of the Look-Locker sequence, and an inversion-recovery (IR) interleaved echo-planar imaging (IEPI) sequence. T2 measurements were made with a standard spin-echo (SE) sequence and an SE IEPI sequence. RESULTS: The T1 measurements yielded values of 1,724 msec +/- 51 for gray matter, 1,043 msec +/- 27 for white matter, and 4,550 msec +/- 800 msec for cerebrospinal fluid. The deep gray matter regions had T1 values of 1,458 +/- 38 (caudate nucleus) and 1,372 +/- 60 (putamen). The T2 measurements yielded results of 63 msec +/- 6.2 for gray matter and 49.8 msec +/- 2.2 for white matter. CONCLUSION: The T1 values measured at 4 T show a higher value than predicted from extrapolation at lower field strengths. The T2 measurements showed a slight decrease in values over those measured at lower-field strength. The gain in signal-to-noise ratio from the higher field strength may be substantially offset by these altered relaxation time values to a degree that is sequence dependent.

Adult↗