INTRODUCTION TO MAGNETIC RESONANCE IMAGING
The crux of medical imaging is what makes black things black and white things white. Plain radiographs are easy to understand: blackness or whiteness is determined by the number of x rays recorded by the image receptor (film/screen). The thickness, density and atomic number of tissues along the x-ray path determine the total attenuation along that path. The smaller the number of transmitted x-rays reaching the image receptor, the less dense (whiter) the x-ray image. In computed tomography (CT). the image contrast as depicted by measured CT numbers depends solely on electron densities: the higher the electron density in a given volume element (voxel). the higher the CT number and the brighter (whiter) the image.
In both plain film radiography and CT the physics behind image interpretation of these modalities remains relatively simple. What makes magnetic resonance (MR) imaging initially difficult to comprehend is that the rules for explaining the whiteness or blackness of an image are not so simple. The same abnormality can appear dark on some MR images and bright on others. The visual vocabulary of blackness and whiteness in MR imaging is derived not only from inherent tissue properties but also from the specific imaging techniques (i.e., the pulse sequence and timing parameters) selected. Furthermore, inherent tissue properties can change with equipment variables such as magnetic field strength.
Understanding how MR works depends on understanding a number of physical principles including the magnetic properties of nuclei, the collective behavior of nuclei when excited by radio waves, the relaxation properties of nuclei due to their macromolecular environments, and the equipment and imaging techniques used to differentiate tissues by maximising differences in image contrast.
BIOLOGICAL EFFECTS OF MAGNETIC FIELD
Interest in the biological effects of magnetic fields dates back to the early Greeks and Chinese (Stoner. 1972 : Needham. 1962 : Carlson. 1975 : Malmstrom. 1976). Magnetic ores were mined from the Greek province of Magnesia in Asia Minor and hence given the name magnet or magnetite. Early investigators believed that magnets were "alive" and processed animating or magical powers. In the early 1770s some attempts were made to use magnets or lodestones to treat human disease (Darnton. 1970) but were soon dismissed as fraudulent.
More recently magnetic properties and spectroscopy of biological materials have been an active area of study. Specifically for NMR methods, fields on the order of 0.05 to 4.0 tesla and gradients averaging 0.01 T/m are commonly used. Rapid switching of gradients can involve changes in magnetic fields of as much as 2 T/sec.
Most of the human body is diamagnetic although areas of paramagnetic character exist, usually due to the presence of iron in the specific tissues or blood. In general, however, the relative magnetic premeability (m r) of the body is close to unity. Thus the values of the magnetic field (H) and the magnetic induction (B) are related by the following expression
B = m r m o H = (1 + Xm )H
where Xm is the relative magnetic susceptibility. In regions of tissue where the m r or Xm is higher, the magnetic field induction (B) will become higher. Magnetic field variation in the local environments of the cell may play some role in the metabolic rate of normal cells. Although the data in this area are sometimes contradictory, often chemical reactions are accelerated in magnetic fields by higher susceptibility and decrease with diamagnetic susceptibility.
Humans spend their entire lives in a "sea" of weak magnetic fields. Depending on location, the Earth’s magnetic fields ranging from 0.2 to 0.5 gauss. Increase in field due to heavy iron-containing building materials, even our automobiles increase our static field local exposure for to many times this value. It is difficult therefore to study the effects of static field under completely controlled condition in humans.
The first serious study to look at static magnetic field effects involved industrial workers whose hands and heads were exposed to fields of 0.035 to 0.35 T and 0.015 to 0.15 T. respectively. These individuals reported symptoms of headaches, pain near heart, dizziness, distorted vision, fatigue, insomnia, loss of appetite, increased perspiration and itching of the hands and wrist (Vyalo. 1971). A small decrease in blood pressure in roughly a third of the workers was reported.
