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M D Robson

Publications and source records attributed to M D Robson.

11 recordsLinked to original sources
PubMed1999

Quantitative imaging of magnetization transfer using multiple selective pulses.

New spectroscopic and imaging methods have been developed for quantitatively measuring magnetization transfer (MT). These methods use trains of radiofrequency (rf) pulses with pulse separations much longer than 1/k(mf) and pulse durations much shorter than 1/k(mf), where k(mf) is the rate of MT from the immobile (macromolecular) protons to the mobile (free water) protons. Signal sensitivity to MT occurs when these pulses affect the mobile and immobile proton pools to different degrees. The signal from water may be quantitatively related to the macromolecular content of the sample using theory. The method has been used to make quantitative measurements of macromolecular content in cross-linked bovine serum albumin and employed in conjunction with echoplanar imaging to produce maps of the spatial distribution of the macromolecular content.

Algorithms↗
PubMed1998

Measurements of the temporal fMRI response of the human auditory cortex to trains of tones.

Averaged single trials (AST) allowed the functional magnetic resonance imaging (fMRI) response to auditory stimuli to be measured at high temporal (1 s) and spatial (0.1 cm3) resolution. Using this paradigm we investigated the transient signal response to 100-ms tone bursts in trains of between 100 ms and 25.5 s in total duration. We have demonstrated that the fMRI response to such auditory stimuli is approximately linear for trains of 6 s and longer, but that shorter stimuli produce signals that are larger than might be expected from the response to the longer stimuli. This nonlinear behavior can be modeled if an adaptive response to each stimulus is assumed. A study using a novel paradigm was also performed in order to study the influence of scanner noise during fMRI experiments on the auditory system response to tones. This study demonstrated that the temporal response to 700-ms tone stimuli is modified when performed in the presence of scanner gradient noise, the modification being a small but significant increase (P < 0.05) in the magnitude of the response. Finally the ability to measure the onset of functional activation using the AST method was examined. It was found, with the aid of computer simulation that a sampling rate of one image per second is adequate to distinguish temporal responses. Using the data acquired in this study, onset times were calculated for the auditory cortex, and these results are consistent with current models of functional activation.

Adult↗
PubMed1997

Diffusion-weighted multiple shot echo planar imaging of humans without navigation.

A new method is proposed for reducing the artifacts produced in diffusion-weighted imaging. When data are acquired using multiple shot echo planar acquisitions, conventional reconstruction methods produce artifactual images as a consequence of diffusion weighting and small amounts of bulk motion of the subject. If the amount of motion can be determined, it is possible to correct the data before reconstruction, which removes the artifact. A method for estimating the motion from the acquired data has been developed and evaluated. This method assumes that ghost image effects will be minimized when motion has been correctly compensated. By considering the amount of signal in the background of the image, appropriate corrections to the data can be made, and the accuracy of the motion compensation may be estimated. This technique has been evaluated by computer simulation, and its performance has been demonstrated in a phantom and humans with both two- and four-shot echo planar acquisitions and using both "mosaic" and "interleaved" sampling schemes.

Artifacts↗
PubMed1997

Measurement of the point spread function in MRI using constant time imaging.

The point spread function is a fundamental property of magnetic resonance imaging methods that affects image quality and spatial resolution. The point spread function is difficult to measure precisely in magnetic resonance even with the use of carefully designed phantoms, and it is difficult to calculate this function for complex sequences such as echo-planar imaging. This report describes a method that measures the point spread function with high spatial resolution at each pixel in samples of uniform intensity distribution. This method uses additional phase encoding gradients before the echo-planar acquisition that are constant in length but vary in amplitude. The additional gradients are applied to image the contents within each individual voxel. This method has been used to measure the point spread function for echo-planar imaging to demonstrate the effects of limited k-space sampling and transverse relaxation, as well as the effects of object motion. By considering the displacement of the point spread function, local distortions due to susceptibility and chemical shift effects have been quantified and corrected. The method allows rapid assessment of the point spread function in echo-planar imaging, in vivo, and may also be applied to other rapid imaging sequences that can be modified to include these additional phase encoding gradients.

Brain↗
PubMed1996

Three-dimensional strain-rate imaging.

Strain-rate imaging uses large velocity encoding gradients to obtain measurements of velocity that are extremely insensitive to the effects of random noise. The spatial differential of velocity yields the velocity gradient from which the strain-rate and twist-rate tensors can be determined. These tensors represent the distortion of the material and are of interest in the analysis of the dynamic behavior of living tissue (e.g., that of the myocardium). This work presents a new technique that uses the magnitude of the signal in the velocity encoded data to measure through-plane velocity variations at the resolution of the voxel size. The magnitude of the MR signal contains information about the range of phases present within a voxel. When the phase is dependent on the velocity (as in phase velocity imaging), the magnitude contains information about the range of velocities within a voxel. The method presented in this work uses unbalanced slice-refocusing gradients to sample the magnitude variation introduced by the interaction of velocity encoding gradients with spatially dependent velocities. The previously developed in-plane velocity gradient methods can be easily integrated with this new through-plane measurement to characterize the deformation of the myocardium in three spatial dimensions with high accuracy. The applicability of these methods is demonstrated theoretically, in phantoms and in vivo.

Echo-Planar Imaging↗
PubMed1995

A combined analysis and magnetic resonance imaging technique for computerised automatic measurement of cartilage thickness in the distal interphalangeal joint.

It is well known that magnetic resonance imaging (MRI) contrast can be controlled, albeit sometimes at the expense of image resolution and signal-to-noise ratio, and most studies of articular joints have used a single MRI protocol, which is optimised for subjective image analysis. Inevitably that single protocol frequently compromises the detection of one or another of the boundaries between which any measurement must be made. This paper describes an alternative approach in which the criteria for computerised edge detection necessary for fully automated measurement of cartilage thickness are used to define the MRI acquisition parameters. This necessitates the combined use of two MRI sequences, one optimised for the cartilage-bone boundary, and the other for cartilage-synovial fluid. This provides a highly effective combination and its efficacy is demonstrated for the distal interphalangeal joint of a range of asymptomatic adults.

Adult↗
PubMed1987

Effect of first and subsequent use of hemodialyzers on patient well-being: the rise and fall of a syndrome associated with new dialyzer use.

In a single large dialysis unit in which dialyzers are routinely subjected to multiple use, the incidence rates of intradialytic symptoms during first use and reuse were compared. Dialyses administered during two periods were analyzed: During the first (26,592 treatments), dialyzers were processed by a manual method before both first and subsequent use. During the second (12,395 treatments), dialyzers were processed by an automated machine method before first and subsequent use. During the first (manual processing) period, 12 symptoms were found to occur more frequently during first use than during reuse. The most striking findings related to chest pain (2.8 times more frequent with first use), back pain (6 times), and concurrent chest and back pain (42 times). Thus, a "first-use syndrome" characterized by chest and back pain was clearly evident. During the second (machine processing) period, the previously noted increased symptom incidence during first use was no longer present. In particular, the incidence of chest pain and back pain was no longer greater during first use than during reuse. Our results suggest that subjecting dialyzers to an automated reuse processing system before first use can markedly diminish the incidence of first-use syndrome.

Back Pain↗
PubMed1986

Effect of first and subsequent use of hemodialyzers on patient well-being. Analysis of the incidence of symptoms and events and description of a syndrome associated with new dialyzer use.

To determine the effect of multiple dialyzer use on intradialytic symptoms, data from 26,592 successive dialyses on 147 patients were analyzed. Over the 26-month period of study 4,933 new dialyzers were used. All symptoms, considered together, occurred 1.3 times more frequently during the initial than during the subsequent use of the dialyzer. No symptom occurred more frequently in the second or subsequent use of the dialyzer. Concurrent chest and back pain were 41 times more frequent when the dialyzer was used for the first time. A syndrome associated with the first use of the dialyzer is described.

Back Pain↗
PubMed1978

Comparison of intermittent with continuous peritoneal dialysis.

Our experience with 41 patients on CAPD is presented (127 patient months). Thirty-three patients were previously on intermittent peritoneal dialysis. We used 4 exchanges of 2 L each per 24 hours (8, 6, 6 and 4 hours dwell times). There was a dramatic fall in serum creatinine of 27%, BUN fell 22%, total CO2 rose 15%. Haemoglobin rose 10% and serum albumin fell by 5%. The incidence of peritonitis was one episode per 7.1 patient months. All patients noted an increase in well being. There were almost no dietary restrictions and patients gained real body weight. In most instances, their antihypertensive medication could be discontinued. This technique is superior to all the other forms of peritoneal dialysis.

Adult↗