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D W McRobbie

Publications and source records attributed to D W McRobbie.

14 recordsLinked to original sources
PubMed2002

Effectiveness and relevance of MR acceptance testing: results of an 8 year audit.

The effectiveness and relevance of independent acceptance testing was assessed by means of an audit of acceptance procedures for 17 MRI systems, with field strengths in the range 0.5-1.5 T, acquired over 8 years. Signal-to-noise ratio and geometric linearity were found to be the image quality parameters most likely to fall below acceptable or expected standards. These received confirmed successful corrective action in 69% of instances. Non-uniformity, ghosting and poor fat suppression were the next most common non-compliant parameters, but yielded less satisfactory outcomes. Spatial resolution was not found to be a sensitive parameter in determining acceptability. 49% of all non-compliant parameters received verifiable corrective attention. A schedule of actual acceptance criteria is presented and shown to be reasonable. Parameter failure rates were shown not to have improved with time. A safety audit of 11 of the installations revealed the most common failings to be inadequate suite layout and poor use of signs. The mean number of safety issues per installation identified as requiring attention was 5, from a questionnaire of 100 points. A number of anecdotal errors and omissions are reported. The data support the importance of an appropriate acceptance procedure for new clinical MRI equipment and for the involvement of a suitably qualified safety adviser on the project team from the outset.

Equipment Safety↗
PubMed1997

A three-dimensional volumetric test object for geometry evaluation in magnetic resonance imaging.

The design of a test object for the volumetric evaluation of geometric image quality parameters in magnetic resonance imaging (MRI) is presented. Two-dimensional parameters: linearity, distortion, slice position, slice width, slice oblique angle, spatial resolution, line spread function (LSF) are measurable in all slice planes within a volume corresponding to a typical head examination. Additionally, parameters particular to three-dimensional Fourier transform (3-DFT) imaging are measurable: 3D ghosting and 3D LSF. Parameter measurements are correctable with regard to test object positioning through the determination of the six degrees of freedom of the test object within the scanner's field of view.

Evaluation Studies as Topic↗
PubMed1996

Short communication: The absolute signal-to-noise ratio in MRI acceptance testing.

The absolute signal-to-noise ratio (SNR) measurable from images is defined and related to the theoretical fundamental SNR for MRI. Absolute SNR for quadrature head coils was obtained by both measurement and analysis of published data for MRI systems in the range 0.5-1.5 T. Mean absolute SNR for 22 MRI systems was 1.59 +/- 0.25 x 10(4) Hz1/2 ml-1 T-1 and did not vary significantly with the static field strength or gradient strength and signal bandwidth. This can be related to the fundamental SNR calculated as 2.36 x 10(4) Hz1/2 ml-1 T-1 through an effective noise figure with an average value of 3.4 dB for the 22 systems. Use of this methodology is discussed in relation to acceptance testing and performance assessment.

Electricity↗
PubMed1994

Measurement of image intensifier system modulation transfer function by video signal analysis.

A technique for the measurement of the modulation transfer function (MTF) of image intensifier systems by means of video signal analysis is presented. The MTF can be measured in both horizontal and vertical directions. The method is applicable to routine quality assurance testing in the field. The limitations on accuracy of the technique are investigated by computer simulation. Experimental results reported for several image intensifier systems correlate well with subjective assessments of limiting resolution and other parameters.

Humans↗
PubMed1992

A figure of merit for the assessment of image intensifier systems.

A dimensionless figure of merit (F), which combines key aspects of the performance of image intensifier systems, is derived. This figure is ideally independent of input air kerma rate and field size and relates to the fundamental characteristics of the image reception chain. In practice it is calculated from measurements of the threshold contrast (noise) and limiting resolution using the Leeds test objects and the input air kerma rate. Several practical examples illustrate its usefulness in simple performance assessment and quality assurance procedures. A comparison is made with the use of detection index diagrams.

Fluoroscopy↗
PubMed1988

Quantitative magnetic resonance imaging in assessment of the blood-retinal barrier.

Application of magnetic resonance imaging (MRI) in two-dimensional quantitative assessment of blood-retinal barrier dysfunction was investigated in rabbits using a 0.1 T (4.25 MHz) resistive system. Reliable and consistent measurements of vitreous T1 were obtained repeatedly, in slices of width 5 mm and X-Y resolution of 1.2 mm. Calibration of reduction of T1 in eyes after injection of gadolinium-DTPA (Gd-DTPA) was performed, resulting in a dose-related response of relaxation rate (1/T1) to the dose of Gd-DTPA injected. Follow-up scans of injected eyes demonstrated a gradual spread of the T1 "hot-spot" as the contrast agent diffused through the vitreous. T1 rose gradually to basal levels by 72 hr. No local effect of Gd-DTPA was found by ophthalmoscopy. Xenon arc photocoagulation of rabbit retina reduced T1 from 1638 +/- 54 (n = 6, mean +/- SD) ms to 1408 +/- 118 (n = 4) msec (P less than 0.01) throughout the vitreous 5-7 hr after treatment. In treated rabbits receiving 1.0 mmol/l Gd-DTPA intravenously, T1 adjacent to lesions 90-120 min after injection was further reduced in a 63 microliter voxel to 670 +/- 50 ms (mean +/- SD, n = 5) with a minimum pixel value of 285 +/- 52 ms. It was estimated that this represented leakage into vitreous of 8.3 nmol Gd-DTPA. Plasma Gd-DTPA concentrations declined rapidly, with half-life of 20-40 min. The findings indicate that MRI is a technique with the potential for repeated quantitative three-dimensional assessment of blood-retinal barrier dysfunction.

Animals↗
PubMedDate not supplied

Multi-center trial with protocols and prototype test objects for the assessment of MRI equipment. EEC Concerted Research Project.

A trial was carried out in which the protocols and prototype test objects developed under a European Economic Community concerted research project for the assessment of MR imaging equipment were tested in a series of commercial imaging systems situated throughout Europe. In general, many imperfections were discovered in the imaging performance of the scanners and, in particular, the accuracy and precision with which the relaxation times T1 and T2 could be measured from the images were found to be rather disappointing.

European Union↗
PubMedDate not supplied

Radio frequency eddy current losses for an annular conductor in MRI: theory and applications.

A theoretical derivation of the equivalent loading resistance of an annular volume conductor within a magnetic resonance imaging radio frequency (RF) coil is presented. Theoretical predictions of the magnitude of the load resistance agree well with measurement over a range of frequencies. The loading resistance is proportional to the sample conductivity, the frequency and coil sensitivity squared, and depends strongly upon the sample dimensions. The orientation of the magnetic vector potential for the specific coil geometry is also important. An experimental comparison of loading by annular and cylindrical objects with the human head over the imaging frequency range is made. Cylindrical test objects are adversely affected by the skin effect. The optimum means of simulating RF induced losses for quality assurance and performance evaluation is discussed in the light of these results.

Biophysical Phenomena↗