This paper presents an overview of the application of and risks of exposure to Magnetic Resonance Imaging (MRI) in pregnancy. It reviews the risks to the fetus by considering the hazards in terms of the three main components of an MRI system. These are the static magnetic field, the time-varying magnetic gradient fields and the pulsed radio frequency fields. The hazards discussed are biological effects, miscarriage, heating effects and acoustic noise exposure. This paper also presents a survey of MRI sites within the United Kingdom to ascertain the extent of MRI usage in pregnancy. To validate the situation of MRI in pregnancy a survey was sent to 352 MR units throughout the United Kingdom. The questions were grouped to assess (a) maternal MRI diagnosis (b) fetal MRI and (c) work practices for pregnant MRI staff. The results showed that 91% of sites were imaging pregnant women in need of diagnosis in the second and third trimester. This paper highlights that MRI can add information for fetal central nervous system abnormalities identified by ultrasound screening, however within the UK direct fetal imaging was only performed in 8% of sites. This paper indicates the need for research to be undertaken for specific MRI clinical conditions. It also advises that risk assessment for pregnant staff working in MRI is performed, and that there is a clear need for further research into the effects of MRI in pregnancy as there is a need for clear authoritive advice.

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This work describes the development of a quality control protocol, which can be implemented to assess the accuracy, precision and reproducibility of the apparent diffusion coefficient (ADC) measurement on a clinical magnetic resonance imaging (MRI) system. The precision and accuracy of the ADC measurement are analysed with regard to MRI system noise, signal reproducibility and differences between nominal and effective b values. Two aqueous test-solutions of CuSO4 and sucrose are prepared for the quality control protocol. ADC measurement with the CuSO4 solution is more sensitive to differences between nominal and effective b values, on account of the solution's high ADC. ADC measurement with the sucrose solution is more sensitive to signal reproducibility due to the solution's low baseline signal intensity. The ADC of the test-solutions is measured on an MRI system at our centre with a sequence used for clinical studies using diffusion imaging. Two parameters, Q and R, are defined for the analysis of the quality control ADC values. The Q parameter is the ratio of the standard deviation of the quality control mean ADC values over time to the optimal standard deviation, as derived from the effect of thermal noise on the ADC measurement uncertainty. Analysis with the Q parameter indicates that signal reproducibility errors contribute to ADC variations on our MRI system when imaging with high b values (b > 500 mm s(-2)), whereas differences between nominal and effective b values have a greater impact on the ADC measurement when imaging with low b values (b < 500 mm s(-2)). The R parameter is defined as the ratio of the directional variation of the ADC quality control values to the uncertainty of the ADC measurement. Analysis with the R parameter shows that the effect of directional variation of the ADC measurement on our MRI system is more pronounced when imaging with low b values. The quality control protocol identified a systematic error, which introduced a small system-induced anisotropy in the ADC measurement. This error is currently taken into account in the analysis of clinical studies employing the diffusion imaging sequence used in this quality control protocol.

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The assessment of diagnostic image quality for MRI is considered. The assessment of three key image quality determinants is addressed: signal, noise and contrast. There is a distinction between random noise evaluation, for the calculation of the SNR, and structured noise evaluation for the assessment of image artefacts. Specific methods used are correlation techniques and the Wiener spectrum. Contrast is assessed by comparison of experimental data and theoretical predictions. For each assessment, the theory and method of the evaluation strategy are discussed. The discussion is illustrated with analysis results from commercial MR systems. The choice of analysis method and the subsequent derivation of quality indices are shown to be critical in respect of robustness and accuracy.