Biomedical subjects

Richard Bowtell

Publications and source records attributed to Richard Bowtell.

10 recordsLinked to original sources

PubMed2006

Sensitivity to local dipole fields in the CRAZED experiment: an approach to bright spot MRI.

Local dipole fields such as those created by small iron-oxide particles are used to produce regions of low intensity (dark contrast) in many molecular magnetic resonance imaging applications. We have investigated, with computer simulations and experiments at 17.6 T, how the COSY revamped with asymmetric z-gradient echo detection (CRAZED) experiment that selects intermolecular double-quantum coherences can also be used to visualize such local dipole fields. Application of the coherence-selection gradient pulses parallel to the main magnetic field produced similar, dark contrast as conventional gradient echo imaging. Application of the gradient along the magic angle leads to total loss of signal intensity in homogeneous samples. In the presence of local dipole fields, the contrast was inverted and bright signals from the dipoles were observed over a very low background. Both simulations and experiments showed that the signal strongly decreased when a phase-cycle suppressing single-quantum coherences was employed. Therefore, we conclude that most of the signal comes from directly refocused magnetization or intermolecular single-quantum coherences. Finally, we demonstrate that bright contrast from local dipole fields can also be obtained, when the pair of coherence-selection gradient pulses is deliberately mismatched. Both methods allowed visualization of local dipole fields in phantoms in experimental times of about 3 min.

Journal Article↗

PubMed2006

Echo-shifted multislice EPI for high-speed fMRI.

The advantages of event-related functional Magnetic Resonance Imaging (fMRI) and the increasing use of fMRI in cognitive experiments are both driving the development of techniques that allow images sensitive to the blood oxygen level-dependent effect to be acquired at ever-higher temporal resolution. Here, we present a technique based on the use of echo shifting (ES) in conjunction with a multislice (MS) echo planar imaging (EPI) readout, which allows T2*-weighted images to be generated with a repetition time per slice that is less than the echo time (TE). Using this ES-MS-EPI approach, it is shown that images with a TE of 40 ms can be acquired with an acquisition time per slice of only 27 ms. The utility of the MS-ES-EPI sequence is demonstrated in a visual-motor, event-related fMRI study in which nine-slice image volumes are acquired continuously at a rate of 4.1 Hz. The sequence is shown to produce reliable activation associated with both visual stimuli and motor actions.

Brain Mapping↗

PubMed2006

Improved echo volumar imaging (EVI) for functional MRI.

Echo volumar imaging (EVI) is a 3D modification of echo-planar imaging (EPI) that allows data from an entire volume to be acquired following a single RF excitation. EVI provides a high rate of volumar data acquisition, which is advantageous for functional MRI (fMRI). However, few studies to date have applied EVI to fMRI, since because of gradient hardware limitations EVI generally has to be used with long sampling times, resulting in high sensitivity to susceptibility-induced distortions. In this study we modified the EVI sequence to improve its suitability for fMRI. The sampling time is reduced by the use of a high gradient-switching frequency, a small number of echoes, and outer volume suppression (OVS); rewind gradients ameliorate Nyquist ghosting; and phase correction via a calibration scan reduces ghosting and distortion. It is shown that the modified EVI sequence allows fMRI data to be acquired with a temporal resolution of 167 ms.

Brain↗

PubMed2005

Hemispherical gradient coils for magnetic resonance imaging.

Hemispherical gradient coils offer an open geometry that is well suited to imaging of the human brain. The windings of a hemispherical gradient coil are on average closer to the target region than those of a comparable cylindrical coil, and consequently hemispherical coils can produce higher efficiency at fixed inductance. The mathematical formalism needed for the design of hemispherical gradient coils is described here, including expressions relating the current distribution on the hemisphere to the magnetic field generated, as well as the stored energy and power dissipation. In addition, expressions for the torque experienced by the current distribution in the presence of the main magnetic field have been derived and used to develop an approach allowing the design of torque-balanced, hemispherical transverse gradient coils. Hemispherical coil designs suitable for brain imaging are presented and shown to have improved performance compared with their cylindrical counterparts. Small, prototype, hemispherical z- and x-gradient coils have been constructed and tested in phase-mapping experiments at 3 T. The experimental results show good agreement with theoretical predictions, validating the mathematical expressions used in the coil design process. The formalism also allows the design of coils wound on a complete spherical surface, and the performance of such coils is additionally described here.

Brain Mapping↗

PubMed2004

Initial attempts at directly detecting alpha wave activity in the brain using MRI.

The "direct detection" of neuronal activity by MRI could offer improved spatial and temporal resolution compared to the blood oxygenation level-dependent (BOLD) effect. Here we describe initial attempts to use MRI to detect directly the neuronal currents resulting from spontaneous alpha wave activity, which have previously been shown to generate the largest extracranial magnetic fields. Experiments were successfully carried out on four subjects at 3 T. A single slice was imaged at a rate of 25 images per second under two conditions. The first (in darkness with eyes-closed) was chosen to promote alpha wave activity, while the second (eyes-open viewing a visual stimulus) was chosen to suppress it. The fluctuations of the phase and magnitude of the resulting MR image data were frequency analysed, and tested for the signature of both alpha wave activity and neuronal activity evoked by the visual stimulus. Regions were found that consistently showed elevated power in fluctuations of the phase of the MR signal, in the frequency range of alpha waves, during the eyes-closed condition. It was conservatively assumed that if oscillations occurred at the same frequency in the magnitude signal from the same region or at the same frequency in the phase or magnitude signal from other regions overlying large vessels or cerebrospinal fluid (CSF), then the phase changes were not due to neuronal activity related to alpha waves. Using these criteria the data obtained were consistent with direct detection of alpha wave activity in three of the four volunteers. No significant MR signal fluctuations due to evoked activity were identified.

Adult↗

PubMed2003

Abnormal cortical sensory activation in dystonia: an fMRI study.

Despite the obvious motor manifestations of focal dystonia, it is recognised that the sensory system plays an important role in this condition. This functional magnetic resonance imaging study examines the sensory representations of individual digits both within the subregions of the primary sensory cortex (SI) and in other nonprimary sensory areas. Patients with focal dystonia and controls were scanned during vibrotactile stimulation of both the index (digit 2) and little (digit 5) fingers of their dominant hand (which was the affected hand in all the dystonic subjects). The activation maps obtained were analysed for location, size, and magnitude of activation and three-dimensional (3-D) orientation of digit representations. Data from both groups were compared. There were significant differences in the average 3-D separation between the two digit representations in area 1 of SI between subject groups (9.6 +/- 1.2 mm for controls and 4.1 +/- 0.2 mm for dystonic subjects). There were also strong trends for reversed ordering of the representation of the two digits in both the secondary sensory cortex and posterior parietal area between the two groups. In addition, in dystonic subjects, there was significant under activation in the secondary somatosensory cortex (SII/area 40) for both digits and in the posterior parietal area for digit 5. These results indicate the presence of widespread activation abnormalities in the cortical sensory system in dystonia.

Adolescent↗

PubMed2003

MRI detection of weak magnetic fields due to an extended current dipole in a conducting sphere: a model for direct detection of neuronal currents in the brain.

To investigate the feasibility of direct MR detection of neuronal activity in the brain, neuronal current flow was modeled as an extended current dipole located in a conducting sphere. The spatially varying magnetic field induced within the sphere by such a dipole was calculated, including its form close to and within the current source. The predicted field variation was experimentally verified by measurements of the variation in phase of the MR signal in a sphere containing a model dipole. The effects of the calculated magnetic field distributions on the phase and magnitude of the signal in MR images were explored. The minimum detectable dipole strength under normal experimental conditions was calculated to be about 4.5 nAm, which is similar in magnitude to dipole strengths from evoked neuronal activity, and is an order of magnitude smaller than dipole strengths expected from spontaneous activity. This minimum detectable dipole strength increases with increasing spatial extent of the primary current distribution. In the experimental work, the effects of a field of [1.1 +/- 0.5] x 10(-10) T strength were detected, corresponding to the maximum net field caused by a dipole of 6.3 nAm strength with a spatial extent of 3 x 3 x 2 mm(3).

Action Potentials↗

PubMed2003

Using the vector potential in evaluating the likelihood of peripheral nerve stimulation due to switched magnetic field gradients.

The time-varying magnetic field gradients used in MRI can cause peripheral nerve stimulation (PNS) in human subjects, as a result of the electric fields induced in tissue. The local electric field, E, is given by E = - partial differential A/ partial differential t - nabla phi where A, is the vector potential and phi is the scalar electric potential generated by charges accumulated at boundaries between regions of different conductivity. Difficulties in calculating phi have led some investigators to use - partial differential A/ partial differential t alone as a predictor of the induced field. Here the spatial variation of - partial differential A/ partial differential t and E is investigated for the case of a simple spherical conductor exposed to time-varying gradients produced by two different gradient coils that generate identical internal magnetic fields, but very different vector potentials. The results indicate that the temporal derivative of A bears little relation to the induced electric field, and that consequently neglecting the effect of the scalar potential introduces significant errors in estimating the likelihood of PNS.

Computer Simulation↗

PubMed2002

Functional neuroimaging studies of human somatosensory cortex.

Two studies were carried out to assess the applicability of echoplanar fMRI at 3.0 T to the analysis of somatosensory mechanisms in humans. Vibrotactile stimulation of the tips of digits two and five reliably generated significant clusters of activation in primary (SI) and secondary (SII) somatosensory cortex, area 43, the pre-central gyrus, posterior insula, posterior parietal cortex and posterior cingulate. Separation of these responses by digit in SI was possible in all subjects and the activation sites reflected the known lateral position of the representation of digit 2 relative to that of digit 5. A second study employed microneurographic techniques in which individual median-nerve mechanoreceptive afferents were isolated, physiologically characterized, and microstimulated in conjunction with fMRI. Hemodynamic responses, observed in every case, were robust, focal, and physiologically orderly. These techniques will enable more detailed studies of the representation of the body surface in human somatosensory cortex, the relationship of that organization to short-term plasticity in responses to natural tactile stimuli, and effects of stimulus patterning and unimodal/cross-modal attentional manipulations. They also present unique opportunities to investigate the basic physiology of the BOLD effect, and to optimize the operating characteristics of two important human functional neuroimaging modalities-high-field fMRI and high-resolution EEG-in an unusually specific and well-characterized neurophysiological setting.

Brain Mapping↗

PubMed2002

1H detected (13)C echo planar imaging.

Cyclic J cross polarisation (CYCLCROP) is a sensitive method for the noninvasive monitoring of (13)C distributions and fluxes. The PRAWN rotating frame Hartmann-Hahn mixing sequence ameliorates problems associated with sensitivity to Hartmann-Hahn mismatch and reduces RF power deposition. The combination of CYCLCROP with echo planar imaging (EPI) for spatial encoding of the proton detected carbon signal allows efficient use of the available signal to be made, permitting a significant improvement in the temporal resolution of any study. We report here on some initial experiments to demonstrate the feasibility of echo planar proton detected (13)C imaging using CYCLCROP based upon the PRAWN module, including the application of the technique to the measurement of transport and accumulation of (13)C-labelled sucrose in a castor bean seedling. Two methods that can be used to eliminate the effect of the J-splitting in the EP images are presented. In addition, a fast, image-based B(1) field-mapping method which may be used to quantitatively map the low frequency RF field in a dual resonant ((13)C/(1)H) probe is presented. The technique utilises the above described imaging method, permitting fully quantitative, 64x64 axial field maps to be generated in about a minute.

Carbon Isotopes↗