Biomedical subjects

T A Carpenter

Publications and source records attributed to T A Carpenter.

At least 55 records · Page 3Linked to original sources

PubMed1999

Infusion pump performance in an MR environment.

Concerns about life support equipment accompanying the critically ill patient have to date made magnetic resonance imaging (MRI) studies of this patient group the exception. We present here a series of tests performed on an IVAC P3000 infusion pump to investigate its suitability for the magnetic resonance imaging environment. We investigate safety, pump performance and image quality issues. The pump was housed at the end of the patient couch to prevent motion towards the scanner. Gravimetric tests found the pump to work within acceptable parameters at a static field of 10 mT. Image interference issues were addressed.

Artifacts↗

PubMed1999

Redefining the functional organization of working memory processes within human lateral prefrontal cortex.

It is widely held that the frontal cortex plays a critical part in certain aspects of spatial and non-spatial working memory. One unresolved issue is whether there are functionally distinct subdivisions of the lateral frontal cortex that subserve different aspects of working memory. The present study used positron emission tomography (PET) to demonstrate that working memory processes within the human mid-dorsolateral and mid-ventrolateral frontal regions are organized according to the type of processing required rather than according to the nature (i.e. spatial or non-spatial), of the information being processed, as has been widely assumed. Two spatial working memory tasks were used which varied in the extent to which they required different executive processes. During a 'spatial span' task that required the subject to hold a sequence of five previously remembered locations in working memory a significant change in blood-flow was observed in the right mid-ventrolateral frontal cortex, but not in the anatomically and cytoarchitectonically distinct mid-dorsolateral frontal-lobe region. By contrast, during a '2-back' task that required the subject to continually update and manipulate an ongoing sequence of locations within working memory, significant blood flow increases were observed in both mid-ventrolateral and mid-dorsolateral frontal regions. When the two working memory tasks were compared directly, the one that emphasized manipulation of information within working memory yielded significantly greater activity in the right mid-dorsolateral frontal cortex only. This dissociation provides unambiguous evidence that the mid-dorsolateral and mid-ventrolateral frontal cortical areas make distinct functional contributions to spatial working memory and corresponds with a fractionation of working memory processes in psychological terms.

Adult↗

PubMed1999

Geometrical models of left ventricular contraction from MRI of the normal and spontaneously hypertensive rat heart.

This study develops a quantitative analysis and model for the differences in left ventricular dynamics in normal and spontaneously hypertensive rats, as determined using non-invasive magnetic resonance imaging (MRI). We emerge with a characterization of the geometrical changes in the left ventricle resulting from hypertension. In addition, the techniques we have adopted are potentially applicable to the study of other disease models for important human cardiac pathologies. A gradient-echo multislice imaging sequence (echo time 4.3 ms) achieved complete image coverage of the heart at high time resolution (13 ms) through the cardiac cycle. Cardiac anatomy in two age-matched groups of young adult (8 and 12 weeks old) normal Wistar-Kyoto (WKY, n = 8) and spontaneously hypertensive rats (SHR, n = 8) was imaged in synchrony with the electrocardiographic R wave in defined planes both parallel and perpendicular to the principal cardiac axis. The transverse left ventricular image sections were circularly symmetrical; this permitted application of different analytical models for the three-dimensional geometry of the epi- and endocardial borders. An ellipsoidal figure of revolution offered an effective description of the three-dimensional left ventricular geometry throughout the cardiac cycle in both normal WKY and SHR animals. The model successfully characterized both the dynamic changes in the shape of the left ventricle through the cardiac cycle and the pathological alterations resulting from spontaneous hypertension. The elliptical model also formed the basis of a simple stress distribution analysis. Such parametric descriptions thus provided a useful alternative to more complex finite element analyses of cardiac function. The eccentricity of the ventricle was characterized by an ellipticity factor a, where a = 1 for a sphere and a < 1 for a prolate ellipsoid. At end systole, the endocardial surface of the left ventricle gave a = 0.43+/-0.02 and 0.49+/-0.02 for the WKY and SHR animals respectively (probability, P < 0.05). At end diastole, the endocardial surface of the left ventricle gave a = 0.58+/-0.02 and 0.63+/-0.02 for the WKY and SHR animals respectively (P < 0.05). Such a difference in ventricular shape was a potential adaptation to increased blood pressure. Hypertension thus altered the left ventricular ellipticity to give a more spherical geometry compared with the normal rats.

Animals↗

PubMed1999

Magnetic resonance imaging compatibility testing of intracranial pressure probes. Technical note.

There is increasing recognition that magnetic resonance (MR) imaging and spectroscopy may provide important information in the assessment of patients with acute brain injury. However, optimum care of the acutely head injured patient requires monitoring of intracranial pressure (ICP). Although many monitoring modalities have been integrated into commercially available MR-compatible systems, there have been no reports of commonly used intraparenchymal ICP sensors in an MR environment. The authors describe the use of an ICP micromanometer probe in an MR environment, with a fiberoptic connection that interfaces the probe with a commercially available MR-compatible monitoring system. Phantom studies were performed to demonstrate the safety and compatibility of the modified MR system at 0.5 tesla. The safety of the device was assessed in relation to its interaction with the static, gradient, and radiofrequency fields used in MR imaging. The MR compatibility was documented by demonstrating that its performance was unaffected by the operation of imaging sequences and by showing that there was no degradation of the diagnostic quality of imaging data obtained during ICP monitoring.

Equipment Design↗

PubMed1998

Magnetic resonance imaging analysis of left ventricular function in normal and spontaneously hypertensive rats.

1. We have used magnetic resonance imaging (MRI) to examine acute morphological changes in the left ventricle throughout the cardiac cycle in normal Wistar Kyoto rats (WKY) and also to follow the development of chronic changes in spontaneously hypertensive rats (SHR). This involved the development of MRI and quantitative analysis techniques for characterizing contractile changes during the cardiac cycle. 2. Images of the cardiac anatomy in two age groups (8 and 12 weeks old) of young anaesthetized adult normal WKY and SHR were acquired in planes both parallel and perpendicular to the principal cardiac axis. 3. Complete coverage of the heart by imaging planes was achieved with high time resolution (13 ms), with typically 12 time frames in the cardiac cycle, using a short echo time (5 ms) multislice gradient-echo imaging sequence. Imaging was synchronized to the R wave of the electrocardiogram. 4. The image slices could be reconstructed into complete geometrically and temporally coherent three-dimensional data sets. Left ventricular (LV) volumes were thus reconstructed throughout the cardiac cycle by combining transverse cardiac image sections. This volume analysis revealed structural and functional differences between the normal WKY and SHR in both age groups of 8 and 12 weeks. Measurements from the cardiac images were additionally validated against histological measurements. 5. The SHR showed a raised LV end-systolic volume and a correspondingly poorer ejection fraction as well as LV hypertrophy when compared with the controls. Left ventricular function in the SHR appeared stable between the two age groups. 6. We developed a simple geometrical model of the left ventricle based on a single longitudinal image section and successfully used this to describe some functional parameters of the left ventricle in the WKY and SHR. This geometrical model has the potential to greatly reduce the imaging time needed to study the beating heart in future serial investigations of cardiac physiology in rats. 7. Our experimental and analytical methods together form a powerful set of quantitative techniques which combine both imaging and functional analysis and will be applicable for future studies of chronic physiological changes in animal disease models.

Animals↗

PubMed1997

Design of a biplanar gradient coil using a genetic algorithm.

A biplanar z-gradient coil has been designed using a genetic algorithm, and its efficiency for producing a gradient along the axis of a solenoid magnet compared to that of a conventional Maxwell coil set. Coils of 21.8 cm by 20.9 cm area and 10 cm separation give 0.37 m Tm-1 A-1 with standard and maximum deviations of 2.6 and 13.1% of this value over an optimised cuboid region of 12 by 15 by 1.8 cm. The experimentally useable linear volume extends beyond this to 50% of the separation between the planes. Design data are also given for a transverse gradient set.

Algorithms↗

PubMed1997

A general method for measurement of the time integral of variant magnetic field gradients: application to 2D spiral imaging.

A new method is proposed by which the time integral of time-varying magnetic fields in different voxels can be determined by MRI. A further processing step is employed which enables the integral of the magnetic field gradients experienced by groups of voxels to be deduced. Analysis of the theoretical background and complete simulations have been used to assess the limitations of the technique and the method is applied to the measurement of a 2D spiral k-space trajectory and also of its 1D components. These measured trajectories have been used to reconstruct images from experimental spiral-scan raw data of a phantom acquired using the 2D spiral k-space trajectory. Comparisons of the resulting images are made with the image reconstructed using the corresponding theoretical trajectory. The importance of the repeatability, reliability and accuracy of the technique is demonstrated.

Electromagnetic Fields↗

PubMed1997

Optimised diffusion-weighting for measurement of apparent diffusion coefficient (ADC) in human brain.

This work studies the effect of diffusion-weighting on the precision of measurements of the apparent diffusion coefficient (ADC, or D) by diffusion-weighted magnetic resonance imaging. The precision in the value of the ADC was described in terms of a diffusion-to-noise ratio (DNR) which was calculated as the signal-to-noise ratio in the resultant ADC. A theoretical analysis decomposed the DNR into the signal-to-noise ratio in the diffusion-weighted image and the sensitivity of diffusion-weighting, "KD". The latter reflects the effect of the sampling strategy in the diffusion-weighting domain on the DNR. The theoretical analysis demonstrated that optimal two-point diffusion-weighting could be achieved in the vicinity of zeta = D(b2-b1) = 1.1, where zeta is a non-dimensional parameter of diffusion-weighting, and b1 and b2 are the diffusion-weighting factors for the two-point diffusion-weighting. This approach also derived an optimised signal averaging scheme. The limitations and restrictions of the two-point scheme for in vivo ADC measurement were also considered; these included a detailed discussion on partial volume effects. The theory was verified by experiments on phantoms and on the brain of a healthy volunteer using a diffusion-weighted echo-planar imaging protocol. This led to an optimal two-point diffusion-weighting for ADC measurement in human brain using b1 = 300, and b2 = 1550 +/- 100 s/mm2. Such a two-point scheme successfully measured values of the ADC in gray matter, white matter and cerebrospinal fluid in human brain. It thus offers an alternative to the commonly used multiple-point schemes and has the advantage of requiring significantly shorter imaging times.

Adult↗

PubMed1997

An algorithm for numerical calculation of the k-space data-weighting for polarly sampled trajectories: application to spiral imaging.

Image reconstruction of data sampled into a non-uniform k-space grid requires appropriate data weighting and, for spiral-scan raw data, this weighting is usually calculated analytically under the assumption of uniform radial k-space sampling. This paper presents a simple and efficient algorithm for explicit numerical calculation of the data weighting for generalised polarly sampled k-space trajectories. The algorithm has been tested against both simulated data and data from experimental spiral-scan imaging and clearly provides significant improvements in image reconstruction when the radial uniformity of the k-space trajectory is poor.

Algorithms↗

PubMed1997

MR protocols for imaging the guinea pig knee.

Magnetic resonance images of the femorotibial joints of male Dunkin-Hartley guinea pigs were obtained in two and three dimensions at 2.35 T using a wide range of T1- and T2-weighted imaging sequences. The effect of slice position on visualisation of articular cartilage, bone and periarticular tissues in sagittal and coronal sections was investigated along with the resolution and signal/noise ratio achievable. Based on that survey, a two-dimensional spin echo sequence (repetition time = 1500 ms, echo time = 40 ms) was found to give optimum visualisation of the normal joint anatomy with in-plane resolution of 75 x 150 microns and a 1 mm slice thickness in an imaging time of 25 min. This protocol was also found to be highly effective in distinguishing many features of the spontaneous, osteoarthritic-like pathology found in the joints of older animals compared to juveniles and therefore provides a means of monitoring disease progression longitudinally. Three-dimensional spin echo imaging methods demonstrated focal changes in signal intensity in the articular cartilage of the medial tibial plateau in older animals. The resulting imaging times of several hours, however, precludes their routine use in vivo.

Algorithms↗

PubMed1997

A quantitative description of dynamic left ventricular geometry in anaesthetized rats using magnetic resonance imaging.

We report a functional application of magnetic resonance imaging (MRI) for the quantitative description of left ventricular geometry through systole and diastole in normal anaesthetized Wistar rats that might be applicable for the analysis of chronic changes resulting from pathological conditions. Images of cardiac anatomy were acquired through planes both parallel and perpendicular to the principal cardiac axis at times that were synchronized to the R wave of the electrocardiogram. The images of the transverse sections were assembled into three-dimensional representations of left ventricular geometry at consecutive time points through the cardiac cycle. This confirmed the geometrical coherence of the data sets, that each slice showed circular symmetry, and that the images were correctly aligned with the appropriate anatomical axes. Different models for the three-dimensional geometry of the left ventricle were then tested against the epi- and endocardial surfaces reconstructed from images of the transverse sections of the left ventricle in both systole and diastole using least-squares minimizations in three dimensions. In agreement with previous reports in the human heart, an elliptical figure of revolution offered an optimal fit to the epicardial and endocardial geometry for the rat heart in diastole. This was in preference to models that used spherical, quartic or parabolic geometries. However, in contrast to contraction in the human heart, all these geometrical representations broke down during systolic ejection in the rat heart. We therefore introduced a more general hybrid model which described left ventricular geometry in terms of the variation of the radii r(z), independently determined for each slice, with its position z along the principal cardiac axis. The resulting function r(z) could then be described by a simple ellipsoid of revolution not only during diastole, but also throughout ventricular ejection. The findings also ruled out alternative geometrical representations. It was then possible additionally to reconstruct the luminal and total left ventricular volumes, wall thicknesses and ejection fractions through the cardiac cycle and to confirm that the predicted total ventricular wall volume was conserved throughout the cardiac cycle. Our hybrid model of cardiac geometry may thus be useful for non-invasive serial studies of chronic pathological changes that use the rat as a model experimental system.

Animals↗

PubMed1996

The protective effect of MK-801 on infarct development over a period of 24 h as assessed by diffusion-weighted magnetic resonance imaging.

Diffusion-weighted MRI has been used to investigate therapeutic intervention with MK-801 in an animal model of permanent focal cerebral ischaemia. The animals were imaged continuously for 4 h and again at 24 h following occlusion of the middle cerebral artery (MCA) allowing the development of the ischaemic lesion to be monitored continuously in the same animals. An increased DWI signal, seen as a region of hyperintensity, was detected 1 h after MCA-occlusion in the lateral cortex and caudate nucleus in both control and MK-801 (administered at a dose of 3 mg/kg i.p. 5 min post-ischaemia) treated animals. However, the volume of hemispheric and cortical hyperintensity was smaller in the MK-801-treated animals. The area of hyperintensity progressively increased in the control group over the 4 h imaging time and there was also an increase in the area of hyperintensity between 4 and 24 h. At these time points the area of hyperintensity encompassed the dorsolateral cortex and caudate nucleus. MK-801 treated animals also demonstrated some progressive increase in the area of hyperintensity between 1 and 3 h, but no significant increase in the area of hyperintensity was seen after this time. The hyperintense regions at 4 and 24 h were restricted to the so-called 'core areas' of the lesion in MK-801-treated animals. Thus, using DWI the tissue 'at risk' following ischaemia could be identified and the protective effect of therapeutic intervention demonstrated.

Analysis of Variance↗

PubMed1996

Visualization by magnetic resonance imaging of focal cartilage lesions in the excised mini-pig knee.

This study showed that magnetic resonance imaging can be used to visualize partial thickness cartilage lesions, 0.7 x 10 mm in area and 0.5 mm in depth, surgically induced in the femur (femoropatellar compartment) of a mini-pig knee joint. Formalin-fixed joints, intact as well as disarticulated, were studied by high resolution imaging using a 2.35 T, 31 cm horizontal-bore superconducting magnet. The two-dimensional and three-dimensional spatial resolutions achievable were as follows: 0.12 x 0.23 mm (two-dimensional) and 0.35 x 0.35 x 0.35 mm (three-dimensional) for the intact joint, and 0.08 x 0.08 mm (two-dimensional) and 0.14 x 0.14 x 0.27 mm (three-dimensional) for the disarticulated joint. These results demonstrate that magnetic resonance imaging, together with edge detection and volume rendering, can be used to visualize focal cartilage lesions.

Animals↗

PubMed1996

Measurement of pulsatile flow using MRI and a Bayesian technique of probability analysis.

This work shows that complete spatial information of periodic pulsatile fluid flows can be rapidly obtained by Bayesian probability analysis of flow encoded magnetic resonance imaging data. These data were acquired as a set of two-dimensional images (complete two-dimensional sampling of k-space or reciprocal position space) but with a sparse (six point) and nonuniform sampling of q-space or reciprocal displacement space. This approach enables more precise calculation of fluid velocity to be achieved than by conventional two q-sample phase encoding of velocities, without the significant time disadvantage associated with the complete flow measurement required for Fourier velocity imaging. For experimental comparison with the Bayesian analysis applied to nonuniformly sampled q-space data, a Fourier velocity imaging technique was used with one-dimensional spatial encoding within a selected slice and a uniform sampling of q-space using 64 values of the pulsed gradients to encode fluid flow. Because the pulsatile flows were axially symmetric within the resolution of the experiment, the radial variation of fluid velocity, in the direction of the pulsed gradients, was reconstructed from one-dimensional spatial projections of the velocity by exploiting the central slice theorem. Data were analysed for internal consistency using linearised flow theories. The results show that nonuniform q-space sampling followed by Bayesian probability analysis is at least as accurate as the combined uniform q-space sampling with Fourier velocity imaging and projection reconstruction method. Both techniques give smaller errors than a two-point sampling of q-space (the conventional flow encoding experiment).

Bayes Theorem↗

PubMed1996

Short echo time MRI enables visualisation of the natural state of human stratum corneum water in vivo.

High resolution magnetic resonance imaging studies on skin have been limited by their inability to detect the low-moisture-content outer layers near the surface of the stratum corneum. The hydration of these outer layers is especially important physiologically. The present study shows that by using a short echo time of 5 ms it becomes possible to observe these layers in all but a few individuals with exceptionally dry skin. The quality of the images can be further improved by the use of separate transmit and receive coils, which reduces the rapid fall-off in signal intensity encountered with a single surface coil used both to transmit and receive.

Body Water↗

PubMed1996

Studies of soil-water transport by MRI.

Sequential spin-echo spin-warp MRI pulse sequences have been used to study soil-water transport processes including infiltration, redistribution, and drainage of water in soil columns. Those images provide a means for monitoring and quantifying spatial and temporal changes of soil-water distributions and the movement of wetting fronts. In addition, temporal-geometric changes of unstable wetting fronts during water redistribution were estimated from 2D images and the temporal development of the longest length of finger was described by a fractal relation t approximately L1.38. Bulk dispersion-time-dependent displacement and velocity spectra, as well as 2D maps of flow velocities and dispersion coefficients in soil macropores during saturated steady-state flow, were reconstructed from data obtained using the alternating-pulsed-field-gradient (APFG) pulse sequences.

Fractals↗

PubMed1996

Cartilage swelling and loss in a spontaneous model of osteoarthritis visualized by magnetic resonance imaging.

The objective of this study was to investigate whether the rate of change in cartilage pathology could be effectively monitored by magnetic resonance imaging (MRI) as part of a longitudinal investigation of an osteoarthritis model in vivo, and to define the minimum requirements necessary to establish disease progression. Magnetic resonance images of the knee of eight male Dunkin-Hartley guinea-pigs were obtained at 8, 12, 18, 24, 30, 36 and 52 weeks of age using a two-dimension spin-echo sequence with a TR of 1500 ms and TE of 40 ms. The total thickness of the femoral and tibial cartilage was measured from those images. Over the same time course, sets of spin-spin relaxation-weighted images were acquired from two additional animals of the same age, from which the T2 relaxation times of water in the articular cartilage were estimated and compared with those of muscle and adipose tissue within the same joint. The cartilage thickness of all the animals increased during the first 6 months, then either stayed thicker (4/8) or became progressively thinner (4/8). Up to 18 weeks of age, the cartilage T2 value was between 23-24 ms but became elevated by 30 weeks and the mean value was more than 40 ms at the end of the study, T2 values for the muscle and adipose tissue remained within the range 30-33 ms, or 47 ms, respectively. We concluded that in this model, cartilage thickness measurements from an magnetic resonance image would not provide a reliable marker to stage osteoarthritis progression partly because the cartilage was so thin in a joint of this size, but also because the changes with time were not linear but biphasic. However, quantitation of the T2 relaxation values may provide a more predictable indicator of cartilage pathology for longitudinal studies because the changes were monotonic and independent of cartilage thickness.

Aging↗

PubMed1995

Interpretation of magnetization transfer and proton cross-relaxation spectra of biological tissues.

Magnetization-transfer (MT) experiments have been performed at 300 MHz on agar gels, solutions of sodium alginate, bovine nasal cartilage, and postmortem porcine muscle. The experimental results elucidate MT mechanisms between mobile macromolecules (correlation time TC on the order of 10(-8) s) and water, and demonstrate the need to incorporate their effects in the characterization of biological samples. In addition, the results obtained confirm a recently published three-spin-bath theoretical treatment for proton magnetization transfer.

Agar↗