Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) is a medical device that uses a magnetic field and the natural resonance of atoms in the body to obtain images of human tissues. The basic device was first developed in 1945, and the technology has steadily improved since. With the introduction of high-powered computers, MRI has become an important diagnostic device. It is noninvasive and is capable of taking pictures of both soft and hard tissues, unlike other medical imaging tools. MRI is primarily used to examine the internal organs for abnormalities such as tumors or chemical imbalances.
The development of magnetic resonance imaging (MRI) began with discoveries in nuclear magnetic resonance (NMR) in the early 1900s. At this time, scientists had just started to figure out the structure of the atom and the nature of visible light and ultraviolet radiation emitted by certain substances. The magnetic properties of an atom's nucleus, which is the basis for NMR, were demonstrated by Wolfgang Pauli in 1924.
The first basic NMR device was developed by I. I. Rabi in 1938. This device was able to provide data related to the magnetic properties of certain substances. However, it suffered from two major limitations. Firstly, the device could analyze only gaseous materials, and secondly, it could only provide indirect measurements of these materials. These limitations were overcome in 1945, when two groups of scientists led by Felix Bloch and Edward Purcell independently developed improved NMR devices. These new devices proved useful to many researchers, allowing them to collect data on many different types of systems. After further technological improvements, scientists were able to use this technology to investigate biological tissues in the mid 1960s.
The use of NMR in medicine soon followed. The earliest experiments showed that NMR could distinguish between normal and cancerous tissue. Later experiments showed that many different body tissues could be distinguished by NMR scans. In 1973, an imaging method using NMR data and computer calculations of tomography was developed. It provided the first magnetic resonance image (MRI). This method was consequently used to examine a mouse and, while the testing time required was more than an hour, an image of the internal organs of the mouse resulted. Human imaging followed a few years later. Various technological improvements have been made since to reduce the scanning time required and improve the resolution of the images. Most notable improvements have been made in the three-dimensional application of MRI.
The basic stages of an MRI reading are simple. First the patient is placed in a strong constant magnetic field and is surrounded by several coils. Radiofrequency (RF) radiation is then applied to the system, causing certain atoms within the patient to resonate. When the RF radiation is turned off, the atoms continue to resonate. Eventually, the resonating atoms return to their natural state and, in doing so, emit a radiofrequency radiation that is an NMR signal. The signal is then processed through a computer and converted into a visual image of patient.
The NMR signals that are emitted from the body's cells are primarily produced by the cells' protons. Early MR images were constructed based solely on the concentration of protons within a given tissue. These images, however, did not provide good resolution. MRI became much more useful for constructing an internal image of the body when a phenomena known as relaxation time, the time it takes for the protons to emit their signal, was taken into consideration. In all body tissues, there are two types of relaxation times, T1 and T2, that can be detected. Different types of tissues will exhibit different T1 and T2 values. For example, the gray matter in the brain has a different T1 and T2 value than blood. Using these three variables (proton density, T1, and T2 value), a highly resolved image can be constructed.
MRI is most used for creating images of the human brain. It is particularly useful for this area because it can distinguish between soft tissue and lesions. In addition to structural information, MRI allows brain functional imaging. Functional imaging is possible because when an area of the brain is active, blood flow to that region increases. When the scans are taken with sufficient speed, in fact, blood can be seen moving through organs. Another application for MRI is muscular skeletal imaging. Injuries to ligaments and cartilage in the joints of the knees, wrists, and shoulder can be readily seen with MRI. This eliminates the need for traditional invasive surgeries. A developing use for MRI is tracking chemicals through the body. In these scans NMR signals from molecules such as carbon 13 and phosphorus 31 are received and interpreted.
The primary functioning parts of an MRI system include an external magnet, gradient coils, RF equipment, and a computer. Other components include an RF shield, a power supply, NMR probe, display unit, and a refrigeration unit.
The magnet used to create the constant external magnetic field is the largest piece of any MRI system. To be useful, the magnet must be able to produce a stable magnetic field that penetrates throughout a certain volume, or slice, of the body. There are three different kinds of magnets available. A resistive magnet is made up of thin aluminum bands wrapped in a loop. When electricity is conducted around the loop a magnetic field is created perpendicular to the loop. In an MRI system, four resistive magnets are placed perpendicular to each other to produce a consistent magnetic field. As electricity is conducted around the loop, the resistance of the loop generates heat, which must be dissipated by a cooling system.
Superconducting magnets do not have the same problems and limitations of the resistive type of magnet. Superconducting magnets are ring magnets, made out of a niobium-titanium alloy in a copper matrix, which are supercooled with liquid helium and liquid nitrogen. At these low temperatures, there is almost no resistance, so very low levels of electricity are needed. This magnet is less expensive to run than the resistive type, and larger field strengths can be generated. The other type of magnet used is a permanent magnet. It is constructed out of a ferromagnetic material, is quite large, and does not require electricity to run. It also provides more flexibility in the design of the MRI system. However, the stability of the magnetic field the permanent magnet generates is questionable, and its size and weight may be prohibitive. While each of these different kind of magnets can produce magnetic fields with varying strength, an optimum field strength has not been discovered.
To provide a method for decoding the NMR signal that is received from a sample, magnetic field gradients are used. Typically, three sets of gradient coils are used to provide data in each of the three dimensions. Like the primary magnets, these coils are made of a conducting loop that creates a magnetic field. In the MRI system, they are wrapped around the cylinder that surrounds the patient.
The RF system has various roles in an MRI machine. First, it is responsible for transmitting the RF radiation that induces the atoms to emit a signal. Next, it receives the emitted signal and amplifies it so it can be manipulated by the computer. RF coils are the primary pieces of hardware in the RF
The final link in the MRI system is a computer, which controls the signals sent and processes and stores the signals received. Before the received signal can be analyzed by the computer, it is translated through an analog-digital convertor. When the computer receives signals, it performs various reconstruction algorithms, creating a matrix of numbers that are suitable for storage and building a visual display using a Fourier transformer.
The individual components of an MRI system are typically manufactured separately and then assembled into a large unit. These units are extremely heavy, sometimes weighing over 100 tons (102 metric tons).
- 1 The most frequently used magnets in an MRI system are superconducting electromagnets. These can be made using various materials, but the basic design involves a coil of conductive wire, a cooling system, and a power supply. The coils are made by wrapping wire, constructed from filaments of a niobium titanium alloy embedded in copper, in a large loop. To create the necessary magnetic field, a number of coils are used. In one type of system eight coils are used, six to create the primary magnetic field and two to compensate for the excess field.
- 2 The coils are immersed in a vessel containing liquid helium. This reduces the temperature to a level that makes them superconductive. To help keep the temperature stable, the vessel is surrounded by two more vessels containing other coolants like liquid nitrogen. This construction is then suspended with thin rods in a vacuum-sealed container. A power supply is hooked up to the magnetic coils and is used only when the magnet needs to be energized. The magnet is attached to the patient support, which is a sliding table that brings the patient into the magnetic field.
- 3 The gradient coils are resistant type electromagnets. In an MRI system, there are typically three sets of gradient coils. Each coil is made by winding thin strips of copper or aluminum in a specific pattern. The coils are given strength by introducing an epoxy into their structure. The size of these coils determines the width of the opening into which the patient is placed. Since a smaller coil requires less energy, this width must be large enough to prevent claustrophobia in the patient but small enough to require a reasonable amount of electricity. These gradient coils are typically shielded to prevent interfering eddy currents.
- 4 The electronic components of the RF system may be provided by outside suppliers and assembled by the MRI manufacturer. These components are attached to the RF coils, which are made with varying designs. The transmitter and receiver coils are composed of the same type of materials as the gradient coils. They are also constructed much like the main magnet. However, they are made up of a loop of conducting material, such as copper, that can create an oscillating magnetic field. One type of RF coil is a surface coil, which is shaped in a circle and is applied directly on the patient. Another type is the saddle coil. These can either be fitted right into the magnet bore or shaped into a birdcage coil and placed just inside the gradient coils. Each type of coil is attached to a power source.
- 5 The computer is supplied by computer manufacturers and modified and programmed for use in an MRI system. Attached to it is the user interface, the Fourier transformer, the signal converter, and a preamplifier. A display device and a laser printer are also included.
- 6 Each of the components of the MRI are assembled together and placed into an appropriate frame. Assembly can take place at the plant or on-site, where the system will be used. In either case, the nature of the magnet typically requires special handling precautions, such as transporting it in an air-suspended vehicle.
The quality of each MRI system being manufactured is ensured by making visual and electrical inspections throughout the entire production process. The performance of the MRI is tested to be sure it is functioning properly. These tests are done under different environmental conditions, such as excessive heat and humidity. Most manufacturers set their own quality specifications for the MRI systems that they produce. Standards and performance recommendations have also been proposed by various medical organizations and governmental agencies.
The focus of current MRI research is in areas that include improving the scan resolution, reducing scan time, and improving MRI design. The methods for improving resolution and decreasing scan time involve reducing the signal to noise ratio. In an MRI system, noise is caused by randomly generated signals that interfere with the signal of interest. One method for reducing it is by using a high magnetic field strength. Improved designs for MRI systems will also help reduce this interference and decrease the noise associated with electromagnets. In the future, real time MRI scans should be available.
Where to Learn More
Boer, Jacques and Marinus Vlaardingerbroek. Magnetic Resonance Imaging Theory and Practice. Springer, 1996.
Brown, J. and J. Heiken. Manual of Clinical Magnetic Resonance Imaging. Raven Press, 1991.
Rinck, P. Magnetic Resonance in Medicine. Blackwell Scientific Publications, 1993.
— Perry Romanowski