Fundamentals of magneto-resonance tomography

Magneto-resonance tomography is one of the youngest methods of radiation diagnosis. It is based on the phenomenon of nucleic magnetic resonance known since 1946, when F. Bloch and Е. Purcell demonstrated that under the effect of radio frequency pulses some nuclei in the magnetic field induce electromagnetic signal. In 1952, for the discovery of a nucleic magnetic resonance they were awarded the Nobel price.

Then in 2003 the Nobel price in medicine was awarded to the British scientist Peter Mansfield and his American colleague Paul Lauterbur for investigations in the field of magneto-resonance tomography. In the early 1970s, Paul Lauterbur discovered the possibility of obtaining a two-dimensional image due to the creation of gradient in the magnetic field. While analyzing characteristics of the emitted radio-waves, he managed to determine their origin. This made it possible to produce two-dimensional images impossible to be produced otherwise.

Dr. Mansfield has developed the studies by Lauterbur and found how to analyze signals that a human body generates in the magnetic field. He has created a mathematic apparatus capable to transform these signals into a two-dimensional image within the shortest time.

Lots of disputes around the priority of discovering of magneto-resonance tomography have arisen. The American physicist Raymond Damadian has stated that it was him who invented magneto-resonance tomography in reality and created the first tomograph, while the price was awarded to quite other people.

From the other point of view, the principles of plotting the magneto-resonance images of a human body were developed long before Raymond Damadian by Vladislav Ivanov (currently Chief of the Chair of Measuring Technologies and Computer Tomography at the Department of Precise Mechanics and Optics with the St. Petersburg State University). But the main thing is that the investigations seeming purely theoretical at that time have found clinical application in decades (since 1980s).

To obtain MR-signal and subsequent image, a permanent homogenous magnetic field is used and the radiofrequency signal that changes the magnetic field.

Therefore the basic components of any MR-tomograph are:

• Magnet to produce an external permanent magnetic field magnetic induction vector Во; in the CI system the unit of measuring magnetic induction is 1Tl (Tesla); for comparison, the Earth magnetic field accounts for about 5x10^-5Тl; one of the main requirements to be met by magnetic field is its homogeneity in the center of a tunnel;
• Gradient coils to produce a weak magnetic field in three directions in the center of a magnet and allow to select the area of investigations;
• Radiofrequency coils to be used for producing electromagnetic excitation of protons in a patient's body (transmitting coils) and for registration of the generated excitation response (receiving coils). Sometimes the transmitting and receiving coils are combined into a single one to examine various parts of the body, e.g. the head.

While performing MRI, the following chain of events takes place.

1. An object under study is placed into a strong magnetic field.
2. Radiofrequency pulse is generated, and the internal magnetization changes with its gradual returning to the initial level.
3. These changes are multiply read out for each point of an object under study.