Biomedical subjects

R W Bowtell

Publications and source records attributed to R W Bowtell.

At least 19 recordsLinked to original sources

PubMed2003

Investigating the BOLD effect during infusion of Gd-DTPA using rapid T2* mapping.

This work aimed to investigate the effects of administering Gd-DTPA on the BOLD effect, and to determine the feasibility of using this approach to measure fractional changes in blood volume and blood oxygenation on neuronal activation during a visual paradigm. A linear relationship between cortical R(2)(*) and the intravascular concentration of Gd-DTPA was demonstrated. The change in R(2)(*) in the visual cortex on activation at 3 T (in the absence of Gd-DTPA) was found to be 1.38 +/- 0.31 s(-1) (N = 4). The fractional change in total blood volume during visual activation was calculated to be 28% +/- 7% (N = 4). The absolute increase in venous blood oxygenation on activation (DeltaY) was estimated to be 21% +/- 4% (N = 4) (assuming HCT = 0.4, resting blood oxygenation = 60%, fraction of volume change that was venous = 36%, and the resting venous fraction of the blood volume = 70%). Simulations showed that the estimated change in venous blood oxygenation was sensitive to the assumed venous blood fraction, and that the estimated fractional change in blood oxygenation was insensitive to whether the change in blood volume occurred in the arterial or venous network.

Adult↗

PubMed2001

Functional magnetic resonance imaging measurements of sound-level encoding in the absence of background scanner noise.

Effects of sound level on auditory cortical activation are seen in neuroimaging data. However, factors such as the cortical response to the intense ambient scanner noise and to the bandwidth of the acoustic stimuli will both confound precise quantification and interpretation of such sound-level effects. The present study used temporally "sparse" imaging to reduce effects of scanner noise. To achieve control for stimulus bandwidth, three schemes were compared for sound-level matching across bandwidth: component level, root-mean-square power and loudness. The calculation of the loudness match was based on the model reported by Moore and Glasberg [Acta Acust. 82, 335-345 (1996)]. Ten normally hearing volunteers were scanned using functional magnetic resonance imaging (tMRI) while listening to a 300-Hz tone presented at six different sound levels between 66 and 91 dB SPL and a harmonic-complex tone (F0= 186 Hz) presented at 65 and 85 dB SPL. This range of sound levels encompassed all three bases of sound-level matching. Activation in the superior temporal gyrus, induced by each of the eight tone conditions relative to a quiet baseline condition, was quantified as to extent and magnitude. Sound level had a small, but significant, effect on the extent of activation for the pure tone, but not for the harmonic-complex tone, while it had a significant effect on the response magnitude for both types of stimulus. Response magnitude increased linearly as a function of sound level for the full range of levels for the pure tone. The harmonic-complex tone produced greater activation than the pure tone, irrespective of the matching scheme for sound level, indicating that bandwidth had a greater effect on the pattern of auditory activation than sound level. Nevertheless, when the data were collapsed across stimulus class, extent and magnitude were significantly correlated with the loudness scale (measured in phons), but not with the intensity scale (measured in SPL). We therefore recommend the loudness formula as the most appropriate basis of matching sound level to control for loudness effects when cortical responses to other stimulus attributes, such as stimulus class, are the principal concern.

Adult↗

PubMed2000

In vivo intravoxel incoherent motion measurements in the human placenta using echo-planar imaging at 0.5 T.

This paper presents the first in vivo measurements of intravoxel incoherent motion in the human placenta, obtained using the pulsed gradient spin echo (PGSE) sequence. The aims of this study were two-fold. The first was to provide an initial estimate of the values of the IVIM parameters in this organ, which are currently unknown. The second aim was then to use these results to optimize the sequence timings for future studies. The moving blood fraction (f), diffusion coefficient (D), and pseudo-diffusion coefficient (D*) were measured. The average value of f was 26 +/- 6 % (mean +/- SD), D was 1.7 +/- 0.5 x 10(-3) mm2/sec, and D* was 57 +/- 41 x 10(-3) mm2/sec. For the optimized values of b, the expected percentage uncertainty in the fitted values of f, D, and D* for the placenta were sigmaf/f = 14.9%, sigmaD/D = 14.3%, sigmaD*/D* = 44.9%, for an image signal-to-noise of 20:1, and a total imaging time of 800 sec.

Analysis of Variance↗

PubMed2000

Time-course of the auditory BOLD response to scanner noise.

It is a concern for auditory fMRI studies that acoustic noise generated by the scanner produces an auditory response that can confound stimulus-induced activation. To establish how to minimize this problem, the present study mapped the time-course of the auditory response to a burst of acoustic scanner noise by employing a single-event method. Recorded bursts of scanner noise were interspersed with clustered-volume acquisitions at a range of stimulus-to-imaging delays to map the response with a temporal resolution of 1 sec. There were strong responses (1.5% signal change) to scanner noise in primary and secondary auditory cortex. In both cortical areas, the mean response rose to a peak by 4-5 sec after stimulus onset and decayed after a further 5-8 sec. The time course indicates that noise contamination in auditory fMRI can be substantially reduced by using a 9-12-sec repetition time, thus maximizing the dynamic range available for displaying the response to acoustical stimuli of interest.

Adult↗

PubMed2000

Sound-level measurements and calculations of safe noise dosage during EPI at 3 T.

This paper describes systematic methods for measuring and controlling sound levels within a magnetic resonance scanner. The methods are illustrated by application to the acoustic noise generated by a 3 T scanner during echoplanar imaging (EPI). Across five measurement sessions, sound pressure levels at the center of the head gradient coil ranged from 122 to 131 dB SPL [123 to 132 dB(A)]. For protection against damaging noise exposure, UK and US industrial guidelines stipulate that the maximum permitted daily noise dosage is equivalent to 90 dB(A) for 8 hours, where noise dosage is a function of the level of an acoustic signal and the length of exposure to it. Without hearing protection, this equivalent level would be exceeded by less than 5 seconds of exposure to the measured levels of scanner acoustic noise. These findings highlight the importance of noise reduction and hearing protection for those exposed to the acoustic noise generated during EPI.

Auditory Threshold↗

PubMed2000

Open access birdcage coils for microscopic imaging of plants at 11.7 T.

The use of a U-shaped high-pass birdcage coil for microscopic imaging at 11.7 T has been investigated. The study was motivated by the requirement for a side access coil, permitting higher filling factors for the in-vivo imaging of plant petioles and stems. The performance of a U-shaped coil (with a cross section consisting of a 16 mm diameter semi-circle plus two 12 mm length straight sections) has been experimentally assessed, and compared both in terms of homogeneity and sensitivity to a 16 mm diameter conventional (linear) birdcage and a saddle coil of the same diameter. The U-shaped coil, which offers 12 mm width side access, has a significantly better performance than the saddle coil, whilst providing 57% of the B(1) sensitivity of the bird-cage.

Biophysical Phenomena↗

PubMed2000

Dual resonant birdcage coils for 1H detected 13C microscopic imaging at 11.7 T.

Liquid state, rotating frame cross polarisation experiments are very sensitive to RF field inhomogeneity. In this work, we present an easily fabricated, co-resident high- and low-pass linear birdcage resonator, optimised to perform liquid state rotating frame polarisation transfer at 1H and 13C frequencies. Both the RF fields have been experimentally mapped, and used to validate the spatial signal dependence of a proton detected, 13C image. The predicted performance was then confirmed using PRAWN-based, cyclic J-cross polarisation (CYCLCROP) imaging. A novel variant of a B(1)-field mapping approach is also presented, using the signal enhancement of the CYCLCROP sequence to generate proton detected, 13C field maps.

Biophysical Phenomena↗

PubMed2000

Modulation and task effects in auditory processing measured using fMRI.

Active listening has been reported to elicit a different sensory response from passive listening and is generally observed as an increase in the magnitude of activation. Sensory activation differences may therefore be masked by the effect of attention. The present study measured activation induced by static and modulated tones, while controlling attention by using target-discrimination and passive listening tasks. The factorial design enabled us to determine whether the stimulus-induced activation in auditory cortex was independent of the information-processing demands of the task. Contrasted against a silent baseline, listening to the tones induced widespread activation in the temporal cortex, including Heschl's gyrus (HG), planum temporale, superior temporal gyrus (STG), and superior temporal sulcus. No additional auditory areas were recruited in the response to modulated tones compared to static tones, but there was an increase in the response in the STG, anterior to HG. Relative to passive listening, the active task increased the response in the STG, posterior to HG. The active task also recruited regions in the frontal and parietal cortex and subcortical areas. These findings indicate that preferential responses to the changing spectro-temporal properties of the stimuli and to the target-discrimination task involve distinct, non-overlapping areas of the secondary auditory cortex. Thus, in the present study, differences in sensory activation were not masked by the effects of attention.

Acoustic Stimulation↗

PubMed1999

Functional magnetic resonance imaging: imaging techniques and contrast mechanisms.

Functional magnetic resonance imaging (fMRI) is a widely used technique for generating images or maps of human brain activity. The applications of the technique are widespread in cognitive neuroscience and it is hoped they will eventually extend into clinical practice. The activation signal measured with fMRI is predicated on indirectly measuring changes in the concentration of deoxyhaemoglobin which arise from an increase in blood oxygenation in the vicinity of neuronal firing. The exact mechanisms of this blood oxygenation level dependent (BOLD) contrast are highly complex. The signal measured is dependent on both the underlying physiological events and the imaging physics. BOLD contrast, although sensitive, is not a quantifiable measure of neuronal activity. A number of different imaging techniques and parameters can be used for fMRI, the choice of which depends on the particular requirements of each functional imaging experiment. The high-speed MRI technique, echo-planar imaging provides the basis for most fMRI experiments. The problems inherent to this method and the ways in which these may be overcome are particularly important in the move towards performing functional studies on higher field MRI systems. Future developments in techniques and hardware are also likely to enhance the measurement of brain activity using MRI.

Brain↗

PubMed1999

"Sparse" temporal sampling in auditory fMRI.

The use of functional magnetic resonance imaging (fMRI) to explore central auditory function may be compromised by the intense bursts of stray acoustic noise produced by the scanner whenever the magnetic resonance signal is read out. We present results evaluating the use of one method to reduce the effect of the scanner noise: "sparse" temporal sampling. Using this technique, single volumes of brain images are acquired at the end of stimulus and baseline conditions. To optimize detection of the activation, images are taken near to the maxima and minima of the hemodynamic response during the experimental cycle. Thus, the effective auditory stimulus for the activation is not masked by the scanner noise. In experiment 1, the course of the hemodynamic response to auditory stimulation was mapped during continuous task performance. The mean peak of the response was at 10.5 sec after stimulus onset, with little further change until stimulus offset. In experiment 2, sparse imaging was used to acquire activation images. Despite the fewer samples with sparse imaging, this method successfully delimited broadly the same regions of activation as conventional continuous imaging. However, the mean percentage MR signal change within the region of interest was greater using sparse imaging. Auditory experiments that use continuous imaging methods may measure activation that is a result of an interaction between the stimulus and task factors (e.g., attentive effort) induced by the intense background noise. We suggest that sparse imaging is advantageous in auditory experiments as it ensures that the obtained activation depends on the stimulus alone.

Acoustic Stimulation↗

PubMed1999

Continuous saturation EPI with diffusion weighting at 3.0 T.

This paper presents a steady-state method of arterial spin labelling using continuous saturation in conjunction with echo-planar imaging (EPI), which has been implemented at 3 T. The continuous saturation technique has the advantage of having high sensitivity compared to transient labelling techniques, when long repetition times are used. It is also easy to implement and requires minimal data to be acquired for quantitation. Like other arterial spin labelling techniques, continuous saturation is potentially prone to overestimation of perfusion rates due to the effect of tagged blood in vessels within the image slice. Using a simple model of the vasculature, the degree of diffusion weighting required to suppress the arterial signal has been determined, with the results indicating that a value of 2 s/mm2 is adequate. Histogram analysis of the experimental data has been used to evaluate the effect of diffusion weighting. Using a b-value of 2 s/mm2, the mean perfusion-related signal change in grey matter on continuous saturation was found to be 1.5 +/- 0.2%, yielding a mean perfusion rate of 87 +/- 9 ml/100 g/min. Brain activation studies using the diffusion weighted continuous saturation technique gave a mean increase in perfusion of 36 +/- 12% in activated motor cortex.

Arteries↗

PubMed1999

The effect of scanner sound in visual, motor, and auditory functional MRI.

The potentially important effect of gradient switching sound on brain function during functional magnetic resonance imaging (fMRI) was studied by comparing experiments with low and high scanner sound levels. To provide a low sound level experiment, a sparse scanning method was used, characterized by long, 9 sec, periods of scanner silence interspersed with 1 sec echoplanar imaging (EPI) bursts. For the condition with high sound levels, extra EPI gradient modules were inserted in the 9 sec inter-image intervals. Visual, motor, or auditory stimuli were presented in the interval between imaging. It was found that with the addition of gradient sounds, auditory activation was significantly decreased while motor and visual activation were not significantly altered. Other general factors relating to fMRI were also examined, such as experimental duration and fatigue. For example, motion of the subjects during the experiments was found to be related to the time spent in the scanner, rather than to the ambient sound level.

Acoustic Stimulation↗

PubMed1998

In vivo perfusion measurements in the human placenta using echo planar imaging at 0.5 T.

This paper presents the first in vivo measurements of perfusion in the human placenta from 20 weeks gestational age until term, using the non-selective/selective inversion recovery echo-planar imaging sequence, in which data is alternately acquired following a selective and non-selective inversion pulse. Twenty pairs of images were collected, two each at the following inversion times: 20, 310, 610, 910, 1110, 1410, 1910, 2810, 3310, and 4510 ms with the sequence being repeated with a repetition time (TR) of 10 s. The results of these measurements were used to suggest the optimum sequence for future work in terms of the signal to noise ratio in the measured perfusion rate in a given measurement time. The sequence was also analyzed to determine the expected variability in the measurements. In normal pregnancies the average value of perfusion rate was found to be 176 (standard error = +/-24) ml/100 mg/min. (n = 16, standard deviation = 96 ml/100 mg/min). The expected variability in the measured parameters due to signal to noise ratio considerations alone was calculated to be 71%. For a maximum scanning time of 400 s, the optimum sequence for measuring placental perfusion was found to require 8 repetitions at each of 10 inversion times which were geometrically spaced (given by a(o), a(o)r, a(o)r2, a(o)r3, . . .), with a(o) = 850 ms, r = 1.073 and TR = 5 s, giving a pixel variability of 38%. Other timing schemes are recommended for measuring perfusion in other anatomical regions with different values of perfusion rate and longitudinal relaxation time.

Adult↗

PubMed1998

In vivo relaxation time measurements in the human placenta using echo planar imaging at 0.5 T.

This paper presents the first in vivo measurements of the nuclear magnetic resonance relaxation times T1 and T2 at 0.5 T in the human placenta from 20 weeks gestational age until term, in both normal and compromised pregnancies. T1 measurements were performed by using both an inversion recovery sequence and the Look-Locher echo planar imaging (EPI) sequence on a total of 41 women with normal pregnancies and 11 women with compromised pregnancies. T2 measurements were performed by using a spin-echo EPI sequence on 36 women with normal pregnancies and 14 women with compromised pregnancies. In normal pregnancies, both the T1 values measured with the inversion recovery sequence and the T2 values were found to decrease with gestational age, the linear regression results gave T1 = -9.1t + 1538 r2 = 0.23 p = 0.03. T2 = -4.0t + 338 r2=0.47 p =410(-6) where t is the gestational age in weeks, and T1 and T2 are the relaxation times in milliseconds. T1 values measured very rapidly with the Look-Locher EPI sequence, but, therefore, with a much lower signal-to-noise ratio, showed no significant trends. The T1 values measured in the abnormal group were significantly lower than those measured in the normal group. Four out of eight patients with compromised pregnancies had placental T1 values lying outside the 90% confidence limits for the normal population based about the regression line, significantly more than expected by chance (p = 0.005). Ten out of fourteen of the T2 measurements in the abnormal group were below the regression line established for the normal group, with 4 lying below the 90% confidence interval, although these trends were only just significant (p = 0.06 and p = 0.03).

Adult↗

PubMed1996

Echo-planar microscopy of porous rocks.

A modified version of the echo-planar imaging sequence incorporating multiple 180 degrees RF pulses (PEPI) has been implemented at 11.7 T and used to generate high resolution images of porous solids. By preceding the PEPI sequence with an inversion recovery or flow-encoding sequence, rapid T1, and velocity mapping is possible in the microscopic domain.

Echo-Planar Imaging↗

PubMed1996

Structure and behavior in hydrophilic matrix sustained release dosage forms: 4. Studies of water mobility and diffusion coefficients in the gel layer of HPMC tablets using NMR imaging.

PURPOSE: The purpose of this study was to characterise the water mobility in the gel layer of hydrating HPMC tablets. Water mobility in the gel layer of different HPMCs was studied. METHODS: NMR imaging, a non-invasive technique, has been used to measure the spatial distribution of self-diffusion coefficient (SDC) and T2 relaxation times across the gel layer. RESULTS: It has been shown that there is a water mobility gradient across the gel layer of HPMC tablets. Although SDC and T2 relaxation times in the outer parts of the gel layer approached that of free water, in the inner parts they decreased progressively. Water mobility and SDC in the gel layer of different HPMCs appeared to vary with degree of substitution of the polymer and the lowest values were obtained across the gel layer of K4M tablets. CONCLUSIONS: Water mobility varies across the gel layer of hydrating HPMC tablets and it is dependent on the degree of substitution of the polymer.

Chemical Phenomena↗

PubMed1995

NMR microscopy of single neurons using spin echo and line narrowed 2DFT imaging.

NMR microimages of single neural cells were acquired at 500 MHz using a conventional spin echo pulse sequence and a line-narrowing sequence that eliminates susceptibility effects. The data show that any contribution to the measured T2 relaxation rate arising from diffusion in local field inhomogeneities using spin echo sequences at high fields and high spatial resolution is relatively small. We conclude that the measured T2 difference between the nucleus and cytoplasm in these cells represents primarily a true T2 relaxation effect arising from the interactions of water with macromolecules in the two compartments and does not result from microsusceptibility differences. These observations have implications regarding water compartmentation in single cells and the interpretation of the MR characteristics of tissues in vivo.

Animals↗

PubMed1995

A robust single-shot partial sampling scheme.

A method of reducing the amount of data required to reconstruct an image is described. In this scheme, fully sampled low spatial frequency data are acquired up to a given cutoff frequency and above this point, only alternate lines are sampled. Two images are produced, one of low definition and one of high definition but aliased. The proposed algorithm unwraps the aliased data, which are then used to enhance the low pass image, yielding a best estimate of the true image. The reduced sampling technique is shown to afford biological images that are almost indistinguishable from those obtained from a complete data set.

Algorithms↗