Magnetic resonance imaging in paediatrics.

Magnetic resonance imaging (MRI) has been available in clinical practice since the early 1980s. From the start MRI was attractive because of the safety of the technique when compared to x ray imaging, as well as the anatomical detail and pathological contrast seen on the images. By the end of the decade MRI had evolved into a useful method for the evaluation of the central nervous and musculoskeletal systems. The number of machines available in the UK was beginning to increase, although the number per head of population has always remained behind that of the USA, Japan, and most European countries. By the end of 1992, the Royal College of Radiologists reported that there were 90 fixed site machines for MRI in the UK.' The college recommended an increase to 250 machines (four per million population) in order to cope with the increased demand for central nervous system and musculoskeletal MRI. While the report of the Royal College of Radiologists acknowledged new indications and methodologies for MRI, it is clear with hindsight the authors underestimated the current renaissance in MRI. The key to this is better hardware for MRI. Magnet technology is now universally good. Important recent technical advances have been in gradient coil power, design, and stability. This has allowed faster and faster switching of stronger and stronger gradient fields. There are many methods to obtain a magnetic resonance signal from a patient. All of these involve the use of radiofrequency pulses which perturb the induced resting magnetisation in the patient's body water within the strong external magnetic field. In the 1980s the workhorse pulse sequences were so-called 'spin echo', which involved the use of both 900 and 1800 radiofrequency pulses. A 90° pulse will rotate the magnetic field through 90's into the transverse plane, while a 1800 pulse will reverse the direction of the magnetic vector. Although robust and informative the spin echo sequences are slow. Today many have been replaced by gradient echo sequences, which can be used to improve anatomic resolution and/or reduce imaging time. In gradient echo sequences the magnetic vector may be rotated through a reduced angle (even 1-2°s is possible) with considerable improvement in image acquisition time. One particular advance has been in the use of gradient echo pulse sequences to rapidly acquire multiple single slices. These can be repeated at the same anatomic level to provide a cine effect, for example the …