HEALTH AND MEDICAL PRODUCTS:MAGNETIC RESONANCE IMAGING.

MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging (MRI) is a medical technique used to obtain images of body tissue. Physicians use the procedure to examine a wide range of medical problems, including diagnosing diseases (multiple sclerosis), identifying the presence of tumors, identifying infections in the body, visualizing torn ligaments, diagnosing tendonitis, and even imaging the flow of blood in virtually any part of the body. The advantage of MRI is that it allows for the identification of abnormal tissue, caused by injury or disease, without invasive surgery or exposure to radiation (x-ray diagnostic test).

An MRI instrument basically consists of three major components: a large cylindrical magnet, devices to transmit and receive radio waves, and an imaging device (for computer analysis). Although newer models are decreasing in size, the MRI instrument itself is a large machine, typically having dimensions of approximately seven feet high by seven feet wide by ten feet long with a large tube (the Bore) running through the mag- net. The magnet is very strong, usually in the range of 0.5 to 2.0 Tesla (the international unit for magnetic flux named for the inventor Nikola Tesla). One Tesla is equal to 10,000 G (Gauss). The earth’s magnetic field is 0.5 to 1 G, so the magnet in an MRI machine can be 20,000 times the strength of the earth’s magnetic field. The main magnet immerses the patient in a stable intense magnetic field. This cause the atoms of the patient’s body to line up in the direction of the magnetic field. MRI specifically targets the hydrogen atoms because of their ideal characteristics. If a patient is lying on his or her back in the magnetic field that runs down the center of the tube of the machine, the hydrogen atoms will line up in the direction of either the feet or the head. Although most hydrogen atom alignments cancel each other out, the large number of hydrogen atoms in the body allows for the analysis and development of detailed images. The next step in the process is to apply a radio frequency pulse (a low-energy pulse that is specific to hydrogen). This pulse is directed to the area to be analyzed. The atoms absorb the energy from the pulse if it is at the same frequency as the radio wave. This causes the hydrogen atoms to precess, or spin, in a different direction. This is the “resonance” step of the MRI process. The exact frequency of the resonance is called the Larmour frequency and is calculated based of the type of tissue being examined and the main MRI magnet. Thus, the physician can tune the machine to examine specific parts of the body. In addition, the MRI system has another type of magnet called a gradient magnet (three, typically). These magnets are very low in strength compared with the main magnet. The gradient magnets are arranged so that when they are turned on and off rapidly they alter the main magnet field at a specific location. In essence, this process allows for exact focusing on a specific point on the body. MRI is capable of examining a precise spot within the body as thin as a few millimeters.

The “imaging” of MRI occurs when the radio frequency pulse is turned off. The hydrogen atoms begin to return to their natural alignment within the field and release the excess stored energy acquired from the radio pulse. This energy is given off as a weak radio signal, which is captured by the coils and sent to the computer system for mathematical analysis (Fourier transform). The strength and length of the energy signal captured depend on various properties of the body tissue; thus, a de- tailed diagnostic image can be produced.