Biomedical subjects

Peter Jezzard

Publications and source records attributed to Peter Jezzard.

29 records · Page 2Linked to original sources

PubMed2003

Theoretical and experimental evaluation of detached endcaps for 3 T birdcage coils.

The use of detached endcaps for 3 T birdcage coils was investigated both theoretically and experimentally. Finite difference time domain analysis, along with workbench and MRI techniques, were used to map the radiofrequency (RF) B(1) distribution along the coil axis with and without an endcap. Without an endcap the measured B(1) value at the service end of the birdcage was only 45% of the value at the coil's center. This was improved to 85% with a detached endcap of maximum achievable diameter (375 mm), positioned 4 mm from the RF shield. The B(1) field distribution on the patient side of the coil was unaffected by the presence of the endcap. The dependence of the B(1) distribution as a function of endcap diameter was also investigated. Surprisingly, simulations and experiments show that there is an optimum ratio of endcap-to-birdcage coil diameter (approximately 1.08) that gives the best B(1) homogeneity. In the human head the optimized endcap, positioned 16 mm from the RF shield, improves the MRI signal amplitude from 55% to 85% of maximum toward the service end. This novel endcap design is easy to implement with existing birdcage coils, and could prove useful when flexibility in access to the RF coil is required.

Equipment Design↗

PubMed2003

Selective arterial spin labeling (SASL): perfusion territory mapping of selected feeding arteries tagged using two-dimensional radiofrequency pulses.

To date, most perfusion magnetic resonance imaging (MRI) methods using arterial spin labeling (ASL) have employed slab-selective inversion pulses or continuous labeling within a plane in order to obtain maps derived from all major blood vessels entering the brain. However, there is great potential for gaining additional information on the territories perfused by the major vessels if individual feeding arteries could be tagged. This study demonstrates noninvasive arterial perfusion territory maps obtained using two-dimensional (2D) selective inversion pulses. This method is designated "selective ASL" (SASL). The SASL method was used to tag the major arteries below the circle of Willis. A combination of 2D selective tagging and multislice readout allows perfusion territories to be clearly visualized, with likely applications to cerebrovascular disease and stroke.

Brain↗

PubMed2003

Utilization of an intra-oral diamagnetic passive shim in functional MRI of the inferior frontal cortex.

Due to the presence of gross magnetic susceptibility artifacts, functional MRI (fMRI) has proved problematic in studies of the human inferior frontal cortex (IFC). There is a strong desire, therefore, to employ techniques that mitigate susceptibility artifacts in the IFC while preserving the imaging parameters of an fMRI study. It has been shown that the use of a single, strongly diamagnetic, intra-oral passive shim significantly improves the homogeneity of the static magnetic field (B(0)) and, as a result, alleviates the susceptibility artifacts within the IFC. In this study, practical issues regarding the use of an intra-oral passive shim are examined. We investigated B(0) instabilities within the IFC resulting from subject head motion in order to calculate the effects of an intra-oral passive shim on the temporal variance of an EPI time series. These studies show that the addition of an intra-oral passive shim improves both B(0) homogeneity and signal stability, and increases sensitivity to functional activation.

Artifacts↗

PubMed2003

Protocol to determine the optimal intraoral passive shim for minimisation of susceptibility artifact in human inferior frontal cortex.

At higher static magnetic field (B(0)) strengths (>/=3 T), the study of human inferior frontal cortex (IFC) when utilising a variety of MRI techniques is severely hampered by the presence of susceptibility artifacts. This is particularly the case for blood oxygenation level-dependent functional MRI, where large signal voids are generally encountered in the frontal lobes. A previous study described an approach to artifact correction involving a mouth insert consisting of a prototype diamagnetic passive shim [Magn. Reson. Med. 48 (2002), 906]. Here we extend that method by investigating the effect of five different intraoral passive shims on B(0) homogeneity and echoplanar imaging susceptibility artifacts within the brain, and particularly the IFC, of six subjects. The optimal passive shim is shown to be subject- and study-specific, providing an average reduction in mean absolute B(0) offset within the IFC of 57%, along with a concomitant reduction in echoplanar susceptibility artifact. All subjects were at ease while wearing the intraoral shims. A 4-min in vivo protocol to determine the optimal passive shim from the available set, utilising intrinsic structural and B(0) subject data, is described and shown to be accurate and reliable.

Artifacts↗

PubMed2002

Thalamic neurodegeneration in multiple sclerosis.

Multiple sclerosis is still regarded primarily as a disease of the white matter. However, recent evidence suggests that there may be significant involvement of gray matter. Here, we have used magnetic resonance imaging and magnetic resonance spectroscopy in vivo and histopathology postmortem to estimate thalamic neuronal loss in patients with multiple sclerosis. Our results show that neuronal loss in multiple sclerosis can be substantial (30-35% reduction). We conclude that a neurodegenerative pathology may make a major contribution to the genesis of symptoms in multiple sclerosis.

Adult↗

PubMed2002

Characterization and reduction of gradient-induced eddy currents in the RF shield of a TEM resonator.

Radiofrequency (RF) shields that surround MRI transmit/receive coils should provide effective RF screening, without introducing unwanted eddy currents induced by gradient switching. Results are presented from a detailed examination of an effective RF shield design for a prototype transverse electromagnetic (TEM) resonator suitable for use at 3 Tesla. It was found that effective RF shielding and low eddy current sensitivity could be achieved by axial segmentation (gap width = 2.4 mm) of a relatively thick (35 microm) copper shield, etched on a kapton polyimide substrate. This design has two main advantages: first, it makes the TEM less sensitive to the external environment and RF interference; and second, it makes the RF shield mechanically robust and easy to handle and assemble.

Electromagnetic Phenomena↗

PubMed2002

Sensitivity-encoded single-shot spiral imaging for reduced susceptibility artifacts in BOLD fMRI.

Sensitivity encoding (SENSE) with iterative image reconstruction was used to shorten the readout duration in single-shot spiral imaging by a factor of 2. This enabled susceptibility-related blurring and signal loss artifacts to be reduced and spatial resolution to be improved. As a beneficial side effect, the gradient duty cycle was also reduced. The spiral SENSE technique was applied to functional MRI (fMRI) with blood oxygen level-dependent (BOLD) contrast and compared to a conventional spiral acquisition. Stimulation experiments were performed in seven volunteers using motor, visual, and taste paradigms. The signal-to-noise ratio (SNR) and signal-to-fluctuation-noise ratio (SFNR) of the SENSE acquisitions were reduced by 20% and 13%, respectively, with respect to the longer readout. The overall activation detected was comparable to that of the conventional spiral acquisition, even though difficulties in reproducing the stimulation response hampered the evaluation. In some cases, the application of SENSE enabled recovery of activation in regions affected by signal loss due to field inhomogeneity.

Artifacts↗

PubMed2002

Optimization of static field homogeneity in human brain using diamagnetic passive shims.

These preliminary studies demonstrate that static field inhomogeneity in the human inferior frontal cortex (IFC) is significantly diminished through placement of a small amount of strongly diamagnetic material in the roof of the mouth. As a result, susceptibility-related image artifacts in this region, as observed in blood oxygen level dependent (BOLD) functional MRI (fMRI), are considerably decreased without compromising the spatial or temporal resolution of the study. Simulations of the static field utilizing perturbation theory are shown, which support the experimental results. The limitations and possible future developments of the technique are described. The application of diamagnetic passive shimming on other regions of the brain is also discussed. Routine use of the proposed method within fMRI studies is practicable through subject-specific optimization of the technique utilizing the simulation algorithm.

Brain↗

PubMed2002

A method for determining venous contribution to BOLD contrast sensory activation.

While BOLD contrast reflects hemodynamic changes within capillaries serving neural tissue, it also has a venous component. Studies that have determined the relation of large blood vessels to the activation map indicate that veins are the source of the largest response, and the most delayed in time. It would be informative if the location of these large veins could be extracted from the properties of the functional responses, since vessels are not visible in BOLD contrast images. The present study describes a method for investigating whether measures taken from the functional response can reliably predict vein location, or at least be useful in down-weighting the venous contribution to the activation response, and illustrates this method using data from one subject. We combined fMRI at 3 Tesla with high-resolution anatomic imaging and MR venography to test whether the intrinsic properties of activation time courses corresponded to tissue type. Measures were taken from a gamma fit to the functional response. Mean magnitude showed a significant effect of tissue type (p < 0.001) where CSF > veins approximately gray matter > white matter. Mean delays displayed the same ranking across tissue types (p < 0.001), except that veins > gray matter. However, measures for all tissue types were distributed across an overlapping range. A logistic regression model correctly discriminated 72% of the veins from gray matter in the absence of independent information of macroscopic vessels (ROC = 0.72). While tissue classification was not perfect for this subject, weighting the T contrast by the predicted probabilities materially reduced the venous component to the activation map.

Adult↗

PubMed2002

A critical period for right hemisphere recruitment in American Sign Language processing.

Signed languages such as American Sign Language (ASL) are natural languages that are formally similar to spoken languages, and thus present an opportunity to examine the effects of language structure and modality on the neural organization for language. Native learners of spoken languages show predominantly left-lateralized patterns of neural activation for language processing, whereas native learners of ASL show extensive right hemisphere (RH) and LH activation. We demonstrate that the RH angular gyrus is active during ASL processing only in native signers (hearing, ASL-English bilinguals) but not in those who acquired ASL after puberty (hearing, native English speakers). This is the first demonstration of a 'sensitive' or 'critical' period for language in an RH structure. This has implications for language acquisition and for understanding age-related changes in neuroplasticity more generally.

Adult↗

PubMed2002

Fast, fully automated global and local magnetic field optimization for fMRI of the human brain.

The aim of this novel technique is to allow researchers, particularly those operating at high static magnetic field strengths on fMRI applications, to tailor the static magnetic field within the brain. The optimum solution for their experimental needs is reached, utilizing the full potential of the active shims at their disposal. The method for shimming human brain, which incorporates automatic brain segmentation to remove nonbrain tissue from the optimization routine, is presented and validated. The technique is fast, robust, and accurate, achieving the global minimum to a static field homogeneity function of the in vivo brain. Both global and specified local regions of the brain can be selected on which to optimize the shims without requiring skilled intervention. The effectiveness of the automated local shim is demonstrated in an olfactory fMRI study where significant activations in the orbitofrontal cortex were very clear when the above method was employed.

Adult↗