Philips Research - Technologies

MR elastography

MR Elastography (MRE) is a new procedure that allows in-vivo measurement of the elasticoviscous parameters of tissue. Based on Magnetic Resonance Tomography (MRT), it does not use any potentially harmful X-ray radiation. It can be used, amongst other things, to diagnose cancer: manual palpation has always been used to search for and to classify lesions. This procedure thus offers very good prospects for improving the differential diagnosis in breast cancer diagnosis.

It is more complicated to measure the elasticity, for example, than to measure the absorption coefficient in X-ray mammography. In the latter there is a direct correlation between the darkening of the X-ray film and the local integral absorption of the tissue behind. This is not the case with elastography. Here one has to infer indirectly from the local hardness (elasticity) of the tissue because the available non-invasive image-generating procedures are not sensitive to the physical parameter 'elasticity'. This indirect step is carried out by means of the introduction of mechanical sinusoidal waves into the tissue in question and simultaneous measurement of these waves using Magnetic Resonance Tomography (MRT).

The propagation of mechanical waves in complex viscous media (such as tissue, for example) is described physically by means of a partial differential equation, i.e. the local properties of the wave are correlated via material constants (density, compressibility, attenuation and elasticity) with the properties of the wave in the adjoining areas. If one assumes that the first two values in the tissue are constant (i.e. density and compressibility) it is possible, if the wave propagation is known, to determine the attenuation and the elasticity. In other words: the elastic properties of the tissue affect the wave propagation and if the wave propagation is known (or measured) it is possible to calculate the elastic properties. This correlation is well known. Until now, however, it has not been technically possible to measure the wave propagation. This is where MRE uses MRT as a tool to measure the wave propagation.

MRT is based on the linking of the magnetic properties of the atomic cores with external radio and magnetic fields. As a result, it is possible to generate images of the body interior, whereby the image contrast is normally proportional to certain local relaxation times or to the density of the nuclear spin. This contrast can, however, be modified so that it is proportional to the movement of an object. The introduction of the sinusoidal mechanical wave into the tissue leads to a periodic oscillation (forced vibration). It is then possible to synchronise the MRT measurement with the mechanical wave in such a way that the contrast in the MRT image is proportional to the wave. MRT thus serves purely as a 'camera' to generate a 'snapshot' of the mechanical wave in the tissue. A number of snapshots taken at different intervals give rise to a film, which approximates the continuous propagation of the wave in the tissue. This image sequence forms the basis for the subsequent reconstruction of the elastic parameters..

Shortly after the start of the mechanical vibration, extremely complex wave propagation phenomena take place within the tissue. In order to simplify the reconstruction problem, the MRT measurement commences approximately 2 seconds after the vibration has started. This time domain is known as the stationary time domain, and the tissue now oscillates sinusoidally. A sinusoidal three-dimensional oscillation can be characterised by amplitude and phase in the relevant direction in space, i.e. six numerals per point. The above-mentioned film that is made of the wave propagation makes it possible to determine exactly these six numerals. Using MRT it is thus possible for the oscillation of the tissue to be measured locally. 'All' that is then required to calculate the elasticoviscous parameters is for the differential equation system to be inverted. The images generated by means of MRE are thus purely synthetic. They are derived indirectly from the characteristics of the wave propagation, whereby the MRT is used to measure the wave.