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Nuclear Magnetic resonance (N.M.R)

Introduction

(You can find some explanations about terms or abbreviations at the end of the page)

NMR has become very significant in the imaging systems for medical diagnosis. Nowadays, diagnosis applications of the NMR provide mainly morphological information: tomographic pictures of the human body can be obtained by high resolution (to 0.1 mm) with an excellent contrast between the different nature of soft tissues. The Magnetic Resonance Imaging (MRI) is based on the measure of nuclear signals that generated by hydrogen nucleus (protons) in response to radio frequencies impulsion (RF). The carrier frequency of those impulsions is linked to the Larmor frequency of protons in order to produce an efficient interaction ("resonance") between radio frequencies and protons' impulsions.

The optimization of the ratio signal/noise and at the same time of the resolution, are the main reasons for which the RMI is targeted at the proton. Protons in water are particularly more detected since water is the main constituent of biological tissues. The protons in water are the protons that produce the most intense signals.
Despite a lower concentration of sodium ions, it has also been proved that RMI of the 23Na has also a potential clinical utility, namely for vascular accidents.
However, the number of groups engaged in this research is not so important.
The intensities among RMI images of proton are determined by the relaxation of protons in water resulting in a balance, after an excitation by radio frequencies waves.
This relaxation behavior depends on the macromolecular environment of protons in water and as a result can vary from tissue to tissue. So the contrast between tissues by RMI is intrinsic. The contrasts can be modulated when varying the timing of the RF impulsions rhythm. Normally, we try to underline the effect of one or more phenomenons that control the relaxation of protons : the effect T1 or "longitudinal" relaxation and the effect T2 or "transversal" relaxation. Images are said to be whether T1, or T2 weighted.

Unfortunately, the obtained signal in Magnetic Resonance lacks of specificity and sometimes we cannot differentiate two structures. Contrast agents have been developed to solve this problem. The distribution of those in the organism is very near to the contrast products used for the X-rays scanner. They are generally injected through the vain and they have not been toxic until now; they are surer than iodized products. The contrast mechanism with RMI contrast products (for example the Gadolinium, a paramagnetic ion that is fixed to a chelating agent such as DTPA or DOTA) is completely different from what happens with iodized products. Indeed, with IRM, it is not the contrast product but action of the product on the relaxation characteristics of protons in water that produces a variation of the signal. So we can speak about an indirect effect that has no linear link with the injected dose

 

Clinical information

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A small lexicon

- CT ou CT scanner : X-rays scanner, also called tomodensitometry or simply scanner. It is a technique that has been invented by Geoffrey Hounsfield (Nobel Prize of Medicine in 1978) in the early 70's, and which allows a body study called tomographic (in slices)
- RMI : resonance magnetic imaging
- NMR : nuclear magnetic resonance: an investigation technique that is at the basis of the RMI and that has been first used in chemical analysis in laboratory (from the early 50's with the works of Bloch and Purcell). This technique is at the basis of RMI and it also includes the spectroscopy by magnetic resonance that is performed in vivo on the high field machine at the Bordet Institute.
- Adenopathy : abnormal glands
- Angiography : imaging of the blood vessels
- Intra-cavitary antenna : small antenna that can be introduced in a body cavity (for example the rectum for certain examinations in high resolution of the prostate)
- Phased array : coordinated network (it connects antennas to optimise the signal)
- Retroperitoneum : anatomic area that is situated at the back of the abdominal cavity.

 

Booklet realized thanks to the support of the Friends of the Bordet Institute.

 

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