Magnetic resonance imaging (MRI) is an innovative technique that provides images of the body in many different planes and represents an extraordinary addition to our diagnostic armamentarium. The images generated vary according to the tissues examined and reflect their physical and chemical properties. It is non-invasive, appears to be relatively innocuous in clinical application, and involves no exposure to ionising radiation. Even in the short period of its use, it has proved to be unusually rewarding in the detection, localisation, and assessment of extent and character of disease in the central nervous, musculoskeletal, and cardiovascular systems. In the brain, for example, it has a proven capacity to define some tumours and the plaques of multiple sclerosis provided by no other technique. It is a competing imaging method in the evaluation of many other organs. The full potential of MRI has not been reached, and continuing refinement of equipment, contrast agents, and software may be anticipated. As higher magnet strengths and rapid imaging sequences are investigated, further study of the long-term biologic effects of magnetic fields is required.
MRI is a technique that affords anatomic images in multiple planes. The first magnetic resonance image was published in 1973 and since that time, major technological have been accompanied by the development of equipment that is now clinically applicable with potentially great benefits in assessing pathophysiologic states.
The MR images are obtained by placing the patient or area of interest within a powerful, highly uniform, static magnetic field. Magnetised protons (hydrogen nuclei) within the patient align like small magnets in this field. Radiofrequency pulses are then utilised to create an oscillating magnetic field perpendicular to the main field, from which the nuclei absorb energy and move out of alignment with the static field, in a state of excitation. As the nuclei return from excitation to the equilibrium state, a signal induced in the receiver coil of the instrument by the nuclear magnetisation can then be transformed by a series of algorithms into diagnostic images. Images based on different tissue characteristics can be obtained by varying the number and sequence of pulsed radiofrequency fields in order to take advantage of magnetic relaxation properties of the tissues.
Magnetic resonance images differ from those produced by x-rays: the latter are associated with absorption of x-ray energy, while MR images are based on proton density and proton relaxation dynamics. These vary according to the tissue under examination and reflect its physical and chemical properties.
MRI is generally safe. Risks are primarily related to the static and oscillating magnetic fields used in MRI. These fields are capable of producing adverse biologic effects at a sufficiently high exposure, but effects have not been observed at the levels currently employed in clinical practice.
The most important known risk is the projectile effect, which involves the forceful attraction of ferromagnetic objects to the magnet. Caution also must be exercised when there are ferromagnetic objects embedded in the patient, such as shrapnel, or implants such as pacemaker wires. MRI should not be performed on patients with cardiac pacemakers or aneurysm clips.
Biologic effects of static magnetic fields, such as ECG changes in T wave amplitude are transient. In the short-term studies these do not appear to be hazardous at field strengths below 2 tesla. Rapidly changing gradient fields can induce electric currents in conductive tissues. Studies indicate no interference with cardiac function or nerve conduction at 2 to 7 tesla, which are below those that would induce neuromuscular stimulation, are believed to provide a wide margin of safety in this respect. Heating may occur in tissues as a result of resistive losses due to circulating currents from radiofrequency coils. High-field scanners are more likely to cause measurable temperature elevations than low-field devices. Although no adverse effects have been observed at approved absorption rates. Care must be taken with patients whose heat loss mechanisms are impaired and with hyperpyrexic individuals. Pulse sequences should be modified to prevent excessive heat build up, particularly in warm and humid environments.
Caution must be exercised in the MRI examination of infants, patients requiring monitoring and life-support systems, and patients who are pregnant. Although there is no evidence that magnetic and electric fields associated with MRI interfere with human development, in vitro studies and theoretical predictions raise the question of whether exposure might pose risks to the developing embryo and foetus.
MRI provides information that differs from other imaging modalities. Its major technological advantage is that it can characterise and discriminate among tissues using their physical and biochemical properties (water, iron, fat, and extravascular blood and its breakdown products). Blood flow, cerebrospinal fluid flow, and contraction and relaxation of organs, both physiologic and pathologic, can be evaluated. Because calcium emits no signal on spin echo images, tissues surrounded by bone, such as the spine, can be imaged, and beam-hardening artifacts are avoided. MRI produces sectional images of equivalent resolution in any projection without moving the patient. The ability to obtain images in multiple planes adds to its versatility and diagnostic utility and offers special advantages for radiation and/or surgical treatment planning. Excellent delineation of anatomic structures results from inherent high levels of contrast resolution. Contrast agents, which appear to be relatively nontoxic, permit evaluation of the integrity of the blood-brain barrier, and the extracellular space. MR image acquisition does not use ionising radiation, nor does it require iodinated contrast agents. Because it requires little patient preparation and is non-invasive, patient acceptability is high.
The relatively slow scan acquisition time results in artifacts due to biological (physiological) motion, e.g., cardiac, vascular, cerebrospinal fluid pulsation, respiratory excursion, and gastrointestinal peristalsis. Technological advances now evolving, such as cine MRI, improved surface coils, respiratory, cardiac, and peripheral gating, chemical shift imaging, and fast scanning, may resolve many of these problems. Some patients, particularly acutely ill patients, cannot co-operate and movement artefacts result. Patient throughput is slow compared with other imaging modalities.
Because of the small bore of the magnet, some patients experience claustrophobia and have difficulty in co-operating. Some obese patients cannot be examined. The strong static magnetic field, which interferes with the proper function of the usual life-support equipment, and the small bore of the magnet make it difficult or impossible to examine some critically ill patients. Patients with pacemakers and ferromagnetic appliances cannot be studied. MRI units require careful siting and shielding. While the appearance of calcium as a signal void provides some advantages, it also limits the ability to detect pathological calcification in soft tissues and tumours, and pathological changes in cortical bone are poorly depicted, using routine spin echo techniques. Other imaging sequences may permit visualisation of some of these lesions. MRI equipment is expensive to purchase, maintain, and operate. Hardware and software are still being developed. MRI is a superb method of studying brain tumours because of the excellent contrast resolution, easy multiplanar imaging, and absence of artefacts. MRI and CT are roughly equivalent for detection of most brain tumours. Within the first 24 to 48 hours, acute intracranial haemorrhage is not easily detected with MRI but is more reliably demonstrated on CT. The subacute hematoma (age 10 to 20 days) is readily detected on MRI, while it may be much less conspicuous on CT. Thus, the two modalities have complementary roles in detection of haemorrhage. CT is more sensitive in acute haemorrhage, while MRI is more sensitive in subacute haemorrhage. Unenhanced CT is often the preferred initial study in patients with stroke because of the clinical need to determine the presence of haemorrhage. MRI is exquisitely sensitive to flowing blood and has proven particularly effective in the detection and localisation of vascular malformations. In head trauma, MRI has proven useful in the detection of all types of intracranial haemorrhage. During the first 1 to 3 days after injury, however, CT is preferable not only because examination time is shorter but also because haemorrhage at this time is more reliably demonstrated by CT.
MRI is recognised as the preferred and most sensitive imaging technique for the diagnosis of multiple sclerosis (MS), but MRI alone cannot establish a definite diagnosis of MS in the absence of strong clinical findings. MRI also exhibits greater sensitivity in the detection of radiation injury to the brain than does CT. In the assessment of dementia, either CT or MRI can be used to demonstrate remediable lesions. In the detection, localisation, and treatment planning of head and neck tumours, MRI offers an advantage over CT due to its multiplanar capabilities, tissue characterisation potential, and the absence of bone and teeth artifacts. MRI affords ready distinction of vessels from lymph nodes. MRI also depicts the contents of the orbit. MRI is capable of demonstrating the entire spinal cord and of differentiating solid from cystic intramedullary tumours. An example of this is the use of MRI for the diagnosis and localisation of acute spinal cord compression.
MRI is particularly valuable as a technique for imaging the heart and great vessels because flowing blood produces a unique signal. Therefore, no contrast medium is required to define the cardiac chambers and the lumen and location of the great vessels. Cardiac evaluation requires either ECG gated MRI or cine MRI. MRI, through definition of the cardiac chambers, great vessels, and flow patterns, represents an important non-invasive diagnostic imaging method in congenital heart disease.
MRI demonstrates the articular cartilages as well as adjacent muscles and tendons. Because it is non-invasive, MRI may be preferable to arthroscopy in the study of the knee. MRI reflects changes in the marrow space by primary tumours and infection. The local extent of primary bone tumours can be staged best by MRI. MRI provides important information regarding muscle, nerve, and vessel invasion or entrapment in malignant soft tissue tumours. A postoperative baseline MRI study can be helpful when the possibility of recurrence must subsequently be evaluated. Due to the excellent contrast resolution of soft tissues, MRI demonstrates muscle and ligament tears and hematomas well.
The MRI machines can be categorized according to Magnetic Field Strength. Low Field units are about .6 Tesla and are suitable for brain and spine imaging. Image quality is not as good as the higher field strength units such as the 1.5 Tesla MRI machines such as the General Electric. The 1.5 Tesla machines give the best image quality and can do very high quality brain and spine imaging as well as excellent orthopedic imaging of the joints. Use of the MRI for chest and abdomen imaging is not very good because of problems with respiratory and cardiac motion which degrades image quality.
There are new MRI machines that have been described as "Open Magnets" , and if you suffer from claustrophobia, the OPEN MRI may be what you need. Otherwise, the standard MRI machines do cause claustrophobia because the patient is placed on a table into a large magnetic machine which has a rather small diameter bore for the patient's body.
Once in the machine, you will not be able to get out of the machine by yourself, and this feeling of helplessness adds to the claustrophobia, which some people find disturbing. In addition, most machines make a loud "knocking" noise from the magnetic coils changing pulse direction. Some patients may benefit from mild sedation with Valium prior to the scan and most centres play music and maintain verbal contact with the patient while they are in the scanner to help them cope with it. Scan times are rather long, so you can expect to spend 30 to 60 minutes in the MRI Magnet.
The pictures themselves look like axial slices through the anatomic structure. The MRI Computer can reconstruct or "re-slice" the images at any angle. Sagital and Coronal plane images are also commonly done. The MRI images differ from CAT scan images in that the MR images represent magnetic characteristics of body tissues, while CT images show only xray photon attenuation which is similar to atomic density if the biologic tissues. Above are some MRI images of the normal brain.