Basics:
Magnetic Resonance Imaging (MRI) is a method to acquire images of slices of a sample. It is based upon a phenomenon called nuclear magnetic resonance (NMR). NMR describes the behaviour of nuclear spins in a static magnetic field when influenced by a varying electromagnetic radiofrequency field. For human applications magnetic fields with a strength of 0.5 to 3 Tesla are used. This is approximately the 10,000 to 60,000-fold of the earth's magnetic field. The radio waves have a frequency in the range of broadcast FM waves (20 to 130 MHz). In order to locate the origin of an acquired signal, additional weak magnetic fields are used which varies along one direction in space. Because the frequency of the signal depends on the magnetic field strength at its origin the signal can be decomposed into contributions of different frequency to infer the location of the signal's origin. Using this principle, we can calculate an image of a slice of the measured object.
Abb. 1: MR image of a central slice
through the head of a healthy volunteer
The strength of MRI lies in the high contrast and high spatial resolution of images showing soft tissue. In addition, there are numerous possibilities to vary the measurement parameters, leading to different image intensity distributions (contrasts). In this way it is possible to distinguish between different types of tissue (white and gray matter in the brain, e.g.), to detect and delineate pathological tissue alterations which occur for instance in tumours, and to measure many properties of tissue both qualitatively and quantitatively, such as blood perfusion, oxygen concentration, blood throughput of the heart, diffusion, and the concentration of different metabolites. MRI techniques therefore play a key role in medical diagnostics today.
Fig. 2: Three MR image slices
through the head of a healthy human. All images show the same region
("horizontal" slice, forehead at top, back of head at bottom). However,
the images were acquired with different parameters and hence show
different contrast: left "spin density" contrast, showing the water
content of the tissue; middle so-called T1 contrast with enhancement of
the blood vessels which appear very bright; right T2 contrast which is
very useful for showing pathological abnormalities. Grey (G) and white
(W) matter of the brain and cerebrospinal fluid (CSF) (F) can be easily
distinguished, in particular in the T1 and T2 image.
MRI is also a key tool for research in biomedical science. In neuroscience applications, functional and diffusion weighted MR imaging and MR spectroscopy provide new insight into the living human brain since the function of brain parts and their mutual interplay, their connections via nerve fibres, and even the underlying metabolic processes can be investigated. The MR techniques can contribute to a better understanding of structure and function of the brain, in the healtyh as well as in the pathological state as for example in neurological or psychiatric disorders.
