The physicist behind the MRI scanner...
10/6/14

Under the supervision of Evelyne Balteau, MR physicist at the Cyclotron Research Centre of the University of Liege, Elodie André has just completed a study showing that MRI technology can supply an in-depth knowledge of the human brain, the white matter it contains and the diseases that can develop within this matter. She focused on diffusion phenomena and diffusion imaging to examine the way water molecules diffuse in the brain. In particular, she focused on the diffusion tensor imaging (DTI) model, a simplified representation of diffusion in the brain, and its extension, the diffusion kurtosis imaging (DKI) model,  a promising technology.


tractographieIn its own way, MRI has revolutionised the world of medical imaging by offering hitherto unknown possibilities for the exploration of the human body. This technique has become an integral part of the examinations recommended by doctors to diagnose many pathologies, as a complementary source of information to other techniques such as CT (X-ray Computed Tomography) or PET (Positron Emission Tomography). Yet none of us, when undergoing an MR scan, ever imagines that physicists, thanks to their hard work, constantly keep improving the interpretation of the data acquired by these machines!

As an engineer specialized in physics, Elodie André, under the supervision of Evelyne Balteau, MR physicist at the Cyclotron Research Centre of the University of Liege, has just carried out a study (1) – and completed her doctoral thesis (2) - which shows that MRI technology can supply in-depth information about the human brain, its white matter and the diseases that can develop within it... as long as one can (even) better decode the information resulting from the technology. As Evelyne Balteau explains, “this study is part of a research program intended to push the boundaries for the interpretation of MR images, while at the same time automating rigorous data processing for more reliable and reproducible results". It is an ambitious and daunting task...

Water, water everywhere!

It is worth taking a small trip back in time for those who have not been following the development of this technology or who cannot remember what MRI means. This powerful medical diagnostic tool provides three-dimensional images with high anatomical precision. A relatively recent technology, it has developed rapidly since its design in 1973 by Paul Lauterbur and Peter Mansfield (Nobel Prize winner for medicine in 2003). The first images of the human body were acquired in 1977. 

MRI is non-invasive and has no known side-effects. The technology is based on the physical phenomenon of nuclear magnetic resonance discovered by Felix Bloch and Edward Purcell in 1946 (Nobel Prize for physics in 1952). It consists of observing the nuclear magnetic resonance (NMR) of protons of water contained in our body which, as everyone knows, is made up 70 to 80 % water. In practice, MRI examines the response of nuclei subjected to an external magnetic field and electromagnetic excitation. The excited atom is the proton (H+), the principal component of water (H2O). The energy absorbed by the proton during excitation is restored during the relaxation process, taking the system back to the equilibrium and generating the signal recorded by the MRI equipment. The recorded signals are analyzed by computer in order to reconstruct a 3D image of any area chosen beforehand.  

The intensity of the signal emitted by an element of volume (known as a voxel) depends on the concentration of water and Nuclear Magnetic Resonance (NMR) parameters (especially the relaxation time, indicating the return to magnetic equilibrium after excitation) of tissues encountered in the voxel considered and the method of acquisition applied (MRI sequence). The result is a three-dimensional image of the distribution of water in a patient’s body. The higher (lower) the intensity of a signal from a given point of the body, the whiter (darker) the point corresponding to the image is (respectively). The MRI sequence modifies the contrast between tissues. It is chosen according to the type of tissue that needs to be highlighted or the type of pathology that is being detected (such as tumors). 

Nonetheless, the contrast obtained can be insufficient to suitably differentiate the healthy parts of the body from those affected by a disease. A very simple way of influencing a signal in MRI is to increase the contrast, either by increasing the examination time to allow for more acquisitions, or by using a contrast agent (like gadolinium). This agent makes it possible to show the presence of tumors or other such pathologies more clearly. 

(1) "Influence of noise correction on intra -and inter- subject variability of quantative metrics in diffusion Kurtosis imaging", Plos One, http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0094531
(2) Improvement of data quality for Diffusion Kurtosis Imaging and application to clinical neurological research, Elodie André, thèse de doctorat, Université de Liège. 

 

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