written by: Jason C. Chavis•edited by: Paul Arnold•updated: 2/21/2010
There are a number of different types of MRI machines used to locate different tissue types within the body. While each machine is essentially the same, they can be programmed to conduct different types of tests using magnetic fields and radio frequencies.
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Basic Types of MRI Machines
Basic types of MRI machines use a small variety of scanning procedures to identify pathologic tissue such as tumors and separate the information from regular healthy tissue. Each of these different types uses a particular echo style to gather the information. Namely, the frequency and level of magnetic field adjustments allows technicians to identify the desired tissues in different ways.
The spin-lattice relaxation time scan causes the nuclei in a molecule to vibrate and rotate in a constant motion. This creates what is known as a lattice field, causing the two nuclei to distribute energy between themselves. As the higher-energy nuclei distributes energy to the lower-energy nuclei, the rotation and vibration increases and ultimately dissipates, allowing a technician to create an image from this adjustment.
Inversely, the spin-spin relaxation time scan uses the concept of precession to identify the nuclei. The magnetic resonance scan produces a field that causes the nuclei to change its orientation in accordance with its access. As the nucleus of a molecule spins, the magnetic field forces it to change direction, a condition which can be identified by the scanning technology. Different types of molecular tissue spin at different rates, helping technicians to create the necessary image detailing unwanted tissues such as tumors.
Above right: Modern MRI. (Supplied by Braegel at Wikimedia Commons; GNU Free Documentation License; http://upload.wikimedia.org/wikipedia/commons/b/bd/Modern_3T_MRI.JPG)
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Specialized MRI Techniques
Specialized types of MRI machines are used to conduct different types of scans, usually with the purpose of locating specific types of diseases or conditions. Like the basic style of MRI, each uses some for of magnetic field adjustment to aid in creating an image.
One of the most popular examples of a specialized MRI is the diffusion technique. This is used to measure the motion of water molecules from one location to another, known as diffusion. In the most basic sense, the magnetic resonance scan can identity the water molecules within the body. With the knowledge of how water flows through the body in different sections, the rate and location of diffusion is identified.
Magnetization transfer MRI scans also work with the concept of water, but in a different way. Within the body, there are two types of water molecules, free water found in large collections in the body and hydration water, essentially lone water molecules throughout solid tissues. The rotational frequency of free water is faster than hydration, allowing the magnetic fields to identify the different types.
Other types of MRI machines are used to identify specific tissues using a combination of rotation rates and time gradients. An angiography is used to identify narrowing or expanding arteries, also known as stenosis and aneurysms respectively. Intracranial cerebrospinal fluid finds the flow of fluids through ventricles to identify lesions and problems within the circulatory system. Spectroscopy measures the results of cellular processes known as metabolites. Each type of metabolite is identified by a particular adjustment to the magnetic field. The brain is analyzed by a functional MRI scan. A low resolution with a rapid rate of magnetization show the changes in brain activity depending on the blood-oxygen level.
All of the different types of MRI machines have become highly useful to the medical industry. Today, they provide a cost-effective and safe alternative to other invasive forms of diagnostic imaging.
Above left: MRI of Knee. (Supplied by Test21 at Wikimedia Commons; GNU Free Documentation License; http://upload.wikimedia.org/wikipedia/commons/a/a9/MR_Knee.jpg)