Magnetic resonance imaging (MRI) is a new technology for making images of the brain and other parts of the body. The technique depends on detection of a phenomenon called nuclear magnetic resonance, and also sometimes called NMR scanning. The discovery and development of MRI imaging is one of the most spectacular and successful events in the history of medical imaging.
The nuclei of some atoms in the body are composed of numbers of nuclear particles. Such nuclei can be detected by sending weak energy signals through very strong magnetic fields. The MRI machine consists of a set of powerful magnets and a source of energy in the same general range used for broadcasting radio. The radio signal is affected in predictable ways by the number of odd-numbered nuclei in its path (Oldendorf; Boller, Grafman and Robertson).
The MRI Procedure
The MRI contains the massive main magnet, which is always on. The unit structure is approximately six or seven feet high and equally wide. As a patient, you will lie on your back on a special table that slides into the magnet through a two-foot-wide tunnel in the middle of the machine. Whether you go in head or feet first depends on the tissue being imaged.
Be prepares for a loud knocking noise; this is not a silent machine. The loud knocking noise is caused by the gradients (small magnets) expanding against the supporting brackets. The MRI scanner will able to pick out voxels (three-dimensional cubes) maybe only one millimeter on each side. It will make a two-dimensional or three-dimensional map of tissue type. The computer will integrate this information and create two dimensional images (the usual) or three-dimensional models. The whole procedure takes from 30-60 minutes (Moe).
Advantages and Disadvantages
Due to the nature of the magnetic probe used in MRI, this technique possesses several fundamental advantages: 1) tissue can be characterized in a number of ways, 2) any plane can be imaged 3) bone is invisible, so all anatomic regions can be examined, and harper images are produced 4) no contrast medium is required and 5) there is no ionizing radiation, which makes it safe for children and for repeated scanning of the same person 6) the level of detailed exceeds the detail of other imaging techniques.
At the present time, there are also several disadvantages 1) he complexity and high cost 2) the long scan time, 3) the noise isolation experienced by patient during scan and 4) the exclusion of substantial fraction of patients dues to pacemakers, metallic artifacts, and inability to cooperate. Furthermore, magnetic strength can be a dangerous thing. Stories abound the magnet’s power to pull metal objects (such as paper clips, keys, scissors, stethoscopes, IV poles, and even oxygen tanks) toward the patient and into the machine.
Even worse, accidents have occurred with metal inside a patient. After an MRI, a metal worker went blind because the magnet moved microscopic metal particles in his eyes, damaging their surrounding structures. A survivor of and aneurysm died during an MRI because the magnet tore off the metal clips holding together a blood vessel in her brain, causing her to bleed to death.
The patient must stay absolutely motionless during the procedure. (Minor motion does not have as much impact on a CT scan.) Therefore, a sedative is often necessary for a child having an MRI scan. The first three of these are under active development, and improvement can be expected. However, gradient coil noise, pacemakers and metallic artifacts are more fundamental problems for which solutions are not yet apparent (Stergiopoulos).
MRI in association with CT
Magnetic resonance imaging is another method for displaying anatomy in the axial, sagittal, and coronal planes. The slice thickness of the images vary between 1 and 10 mm. MRI is especially good for coronal and sagittal imaging, whereas axial imaging is the forte of CT. One of the main strengths of MRI is its ability to detect small changes (contrast) within soft tissues, and MRI soft tissue contrast is better than that found in CT images and radiographs.
CT and MR imaging modalities are digital-cased technologies that require computers to convert digital information to shades of black, white and gray. The major difference in the two technologies is that in MRI the patient is exposed o external magnetic fields and radio frequency waves, whereas the patient is exposed to x-rays during a CT study. The magnetic fields used in MRI are believed to be harmless. MR scanning can be a problem for people who are prone to develop claustrophobia because they are surrounded by a tunnel-like structure for approximately 30-45 minutes.
The external appearance of an MRI scanner or machine is similar to a CT scanner with the exception that the opening is the MR gantry is more tunnel-like. As in CT, the patient is comfortably positioned supine, prone, or decubitus on a couch. The couch moves only when examining the extremities. The patient hears and feels a jackhammer-like thumping while the study is in progress.
The underlying physics of MRI is complicated and strange-sounding terms proliferate. Let’s keep it simple: MRI is essentially the imaging of protons. The most commonly imaged proton is hydrogen, as it is abundant in the human body and is easily manipulated by a magnetic field. However other nuclei can be imaged. Because the hydrogen proton has a positive charge and is constantly spinning at a fixed frequency, called the spin frequency, a small magnetic field with a north and south pole surrounds the proton. Remember that moving charged particles creates a surrounding magnetic field. Thus, these hydrogen protons act like magnets and align themselves within an external magnetic field or the needle of a compass.
In the MR scanner, or magnet, short bursts of radio frequency waves are broadcast into the patient from radio transmitters. The broadcast radio wave frequency is the same as the spin frequency of the proton being imaged (hydrogen in this case). The hydrogen protons absorb the broadcast radio wave energy and become energized, or resonate. Hence, the term magnetic resonance.
Once the radio-frequency wave broadcast is discontinued, the protons revert or decay back to their normal or steady state that existed prior to the radio wave broadcast. As the hydrogen protons decay back to their normal state or relax, they continue to resonate and broadcast radio waves that can be detected by a radio wave receiver set to the same frequency as the broadcast waves and the hydrogen proton spin frequency. The intensity of the radio wave signal detected by the receiver coil indicates the numbers and locations of the resonating hydrogen protons.
Although human anatomy is always the same no matter what the imaging modality, the appearances of anatomic structures are very different on MR and CT images. Sometimes it is difficult for the beginner to differentiate between a CT and an MR image. The secret is to look to the fat. If the subcutaneous fat is black, it is a CT image as fat appears black on studies that use x-rays. If the subcutaneous fat is white (high-intensity signal), then it has to be an MR. next, look to the bones.
Bones should have a gray medullary canal and a white cortex on radiographs and CT images. The medullary canal contains bone marrow, and the gray is due to the large amount of fat in bone marrow. On a MR image, nearly all of the bone appears homogenously white as the bone marrow is fat that emits a high-intensity signal and appears white.
Also, on MR the cortex of the bone will appear black (dark or low intensity signal), whereas on CT images the cortex is white. Soft tissues and organs appear as shades of gray on CT and MR. Air appears black on CT and MR. air appears black on CT and has a low-intensity signal (black or dark) on MR (Moe).
At present, MRI is, by far, the most useful imaging modality for visualizing intracerebral tumors. It provides the most clear, detailed, and comprehensive diagnostic information regarding the tumor ad surrounding normal structures. The introduction of MRI and image-guided technology into the operating room thus allows the surgeon to use high-quality, current image data that reflect the surgical reality of brain tissue deformations and shifts that occur after the bone flap has been turned, the dura opened, and the resection begun.
Today’s intraoperative MRI systems can be classified into two main groups: 1) the high field strength systems and 2) the low compact systems. Both types of systems have advantages and disadvantages. The high-field strength systems (0.5-1.5 T) are typically mounted on a stationary gantry and have gradient capabilities sufficient to produce full head images of quality comparable to that of diagnostic MRI.
Magnetic resonance imaging can satisfy these requirements for therapy. It has excellent anatomic resolution for targeting, high sensitivity for localizing tumors, and temperature sensitivity for online treatment monitoring. Several MRI parameters are temperature sensitive; the one based on the proton resonance frequency allows relatively small temperature elevations to be detected prior to any irreversible tissue damage.
Thus, the location of the focus can be detected at relatively low powers, and the accuracy of targeting can be verified. In addition, using calibrated temperature-sensitive MRI sequences, focal temperature elevations and effective thermal doses may be estimated. Such thermal quantification allows for online feedback to ensure that the treatment is safe, by assuring that the focal heating is confined to the target volume and below the level for boiling. Thermal assessment predicts effectiveness by assuring that the temperature history is sufficient to ensure thermal coagulation (Moore and Zouridakis).
Since the first availability of commercial instruments at the beginning of the 1980s, clinical MR has expanded rapidly in terms of both medical applications and the number of units installed. First considered to be expensive method to create images of inferior quality, it has since established itself as a clinical tool for diagnosis in previously inconceivable applications, and the potential of the method is still not exhausted. MRI has led to the first-scale industrial application of superconductivity and has brought about a grater public awareness of a physical effect previously known only to a handful of scientists.
Up to now, the growth and spectrum of applications of MR have exceeded all predictions. The most recent development is that of rendering brain functions visible. Cardiac MR can display coronaries and analyze perfusion of the myocardium and hemodynamics of the heart. Thus, MRI is entering the domain of nuclear medicine.
An interesting new application of MRI is its use as an imaging modality during minimal invasive procedures such as ablation, interstitial laser therapy, or high intensity focused ultrasound. With temperature-sensitive sequences, the development of temperature and tissue damage can be checked during heating and destroying of diseased tissue. The sensitivity of MRI to flow helps the physician to stay away from vessels during an intervention. MRI is also used for image-guided surgery, e.g., resection of tumors in the brain. Special open systems have been designed for such purposes, and dedicated non magnetic surgery tools have already been developed (Erkonen and Smith).
Boller, François, Jordan Grafman, and Ian H. Robertson. Handbook of Neuropsychology. Vol. 9. New York: Elsevier Health Sciences, 2003.
Erkonen, William E., and Wilbur L. Smith. Radiology 101: The Basics and Fundamentals of Imaging. 2nd ed. New York: Lippincott Williams & Wilkins, 2004.
Moe, Barbara A. The Revolution in Medical Imaging. New York: The Rosen Publishing Group, 2003.
Moore, James E., and George Zouridakis. Biomedical Technology and Devices Handbook. New York: CRC Press, 2004.
Oldendorf, William. Basics of Magnetic Resonance Imaging. Boston: Springer, 1988.
Stergiopoulos, Stergios. Advanced Signal Processing Handbook: Theory and Implementation for Radar … New York: CRC Press, 2001.