Appunti dal Congresso ASNR di Vancouver Risonanza Magnetica a 3 Tesla

Abstract

The theme of this year's 40^th Congress of the American Society of Neuroradiology was 3 Tesla magnetic resonance.

For many years, research into the use of magnetic resonance systems with 3 T magnets mainly focused on spectroscopy and functional magnetic resonance. More recent studies also used 3 T magnets for MR diagnosis, trying to optimize sequences for both anatomic imaging and for MR angiography and diffusion on the basis of protocols currently adopted with 1.5 T magnets^1–3.

The use of higher magnetic fields improves the signal/noise ratio and the chemical-shift sensitivity thereby enhancing spatial resolution (supported by the higher signal/noise ratio) and increasing the reliability of spectroscopy and functional MR imaging.

The major technical problems encountered with 3 T systems are the increased number of artefacts due to magnetic sensitivity and chemical shift, the increase in tissue heating potential and the longer T1 longitudinal relaxation times.

The main advantages of high magnetic field resonances are the higher signal/noise ratio, greater spectra dispersion, improved image resolution, faster acquisition times and greater sensitivity to differences in magnetic susceptibility. The drawbacks include a lower signal/noise ratio in relation to artefacts caused by magnetic susceptibility, longer T1 relaxation times, shorter T2 and T2* relaxation times, convergent tissue relaxation times, increased absorption of RF energy and a more inhomogeneous B1 signal.

The advantages and limitations of higher magnetic field magnets have already been encountered at each stage in the development of MR technology. Once again, the best strategies need to be devised for the use of these new systems. Briefly, the increased signal produced by high field magnets (3 T) offers different advantages. In conventional diagnosis, the spatial resolution of the image is improved or acquisition times shortened, or when contrast is poor with current techniques as in functional MR or spectroscopy. Even the increase in artefacts due to magnetic susceptibility and chemical shift can be exploited advantageously. For example, the enhanced magnetic susceptibility produces a greater loss of signal intensity in sequences with gadolinium bolus perfusion, thereby improving assessment of brain haemodynamics and tissue vitality for a rational selection of candidates for stroke therapy and surgery.

Studies were presented from different stroke centres in the United States using protocols comprising sequences for conventional anatomical investigation, weighted diffusion sequences, MR angiography and bolus perfusion.

Image quality was equivalent or superior to that obtained with 1.5 T systems, especially angiographic sequences which were improved and faster with easier identification of the occluded branch. It was also demonstrated that the examination can be performed while the patient is receiving an infusion of thrombolytic drugs with real time monitoring of the pharmaceutical effect on the thrombosis.

A group of researchers then presented their findings studying patients with an 8T MR system^23–25. Exposure to such a high magnetic field was well tolerated and the study focused on evaluation of the small cerebral vessels exploiting sensitivity differences in blood oxygenation as deoxyhaemoglobin is paramagnetic and the effected of magnetic susceptibility exaggerated by such a high field. Using inhomogeneous local fields with gradient-echo images together with the signal/noise ratio of an 8 T system, minute cerebral vessels with a diameter of around 200 micron could be visualized.

What emerged from the Congress presentations was that we can no longer be satisfied with the MR systems in use today when examinations which are currently long and cumbersome can be carried out more quickly, often with real time results and much shorter processing times. The impression was that the 1.5 T MR systems most of us use will soon be as obsolete as the 0.5 T systems and neuroradiologists' work will be increasingly shifted from morphological investigation to functional analysis, entailing new expertise.

My only concern is the exposure of patients to such high magnetic fields, fearing the biological effects, especially when follow-up examinations have to be repeated. I hope that patients' welfare will not be disregarded in the search for ever greater morphological detail and that the new systems will be used when their true worth is of real benefit.

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