By Donald Woodward
MagneticResonance Imaging (MRI) has evolved to become one of the most valuablediagnostic tools ever developed. MRI's ability to provide unprecedentedvisualization of internal organs and structures without using X-rays makes itthe modality of choice for many applications. In addition, MRI can help reducehealthcare costs by eliminating the need for other diagnostic studies that aremore invasive to the patient.
One of the most important elements in delivering this valuable imagingservice involves maintaining a safe environment. For most patients and settings,MRI is completely safe. For others, though, it can have serious complications.
How MRI Works
The human body contains millions of hydrogen protons found mainly in water.These protons act as tiny magnets. During an MRI exam, these tiny magnets areexposed to a large magnetic field (thousands of times stronger than the Earth'smagnetic field), causing them to align with the field. Radio waves, tuned to aspecific frequency, are then pulsed toward the patient, causing protons to tipaway from the magnetic field. Once radio waves are turned off, protons realignwith the magnetic field. As they realign, protons release energy in the form ofanother radio signal. This signal is picked up by the antenna (called a coil)and sent to a computer, which calculates the signal into an image, which is thentransferred to film for interpretation.
MRI began as a study restricted to the brain and spine because of limitedhardware and software capabilities. As time went on, the technology was improvedto allow imaging of the musculoskeletal and abdominal areas. Newer techniqueshave recently been developed to assess acute brain stroke, tumors, and renal andperipheral vasculature. One of the most exciting applications currently beingdeveloped will assess not only heart structure, but also function. Soon refinedtechniques will preclude the battery of tests required for a complete cardiacworkup.
In general, most MRI scans take between 20-60 minutes and patients shouldexperience no discomfort. They will, however, hear a loud knocking or tappingsound when the scanner is actually acquiring information.
Although MRI has become a well accepted imaging modality, it does have somedegree of risk for the patient. There are three main elements of concern when itcomes to MRI safety--protecting the patient, protecting the equipment, andprotecting the image.
The first concern is the protection of the patient. Some metallic implantsare ferromagnetic (attracted to a magnet) and may cause harm if moved ordislodged by the strong magnetic forces used during an MRI exam. In addition,there are also implants that may be electronically affected by the radio wavesand changing magnetic fields used during the scanning procedure. The best methodto protect patients from either of these situations is to review medical andsurgical history thoroughly. This usually involves a dedicated MRI screeningquestionnaire completed by the patient and reviewed by the facility prior toscanning.
The questionnaire must ask about previous surgical procedures, withparticular regard to head or heart surgery, since most absolutecontraindications involve these areas. For example, a pacemaker or ferromagneticaneurysm clip will prohibit a patient from having an MRI exam, but a hip implantusually will not. Common absolute contraindications include, but are not limitedto: pacemakers, ferromagnetic intra-cranial aneurysm clips, metal fragments inthe eye, and cochlear implants.
There has been a substantial amount of research to determine whichcontraindications are absolute and which would be considered relative (mayprohibit a scan or may alter your approach). These include pregnancy (which isbased on risk versus benefit), certain heart valves, bone stimulators, neuro-stimulators,hearing aids, ocular implants, and certain dental implants.
Protecting the equipment helps to ensure the safety of patient. Items thatare ferromagnetic (attracted to a magnet). These may be drawn to the MRI magnetand become projectiles. They can become dangerous, hurtling through the air fromthe point of origination to the center of the magnet. Thousands of dollars havebeen spent replacing the outer housing of the MRI magnet from damage caused by aprojectile.
Therefore, it is very important to control the environment by establishingzones of protection. MRI technical personnel must screen all persons andequipment for ferromagnetic objects prior to entering these zones.
The goal of the MRI screening is to obtain a high-quality image to safely andaccurately diagnose a patient. However, this image may not be achieved ifcertain objects interfere. Some implants or metal objects can cause severeartifacts, making it difficult to interpret the image. In some cases, theartifacts are so severe, the images are considered non-diagnostic. Examples ofobjects that may cause artifacts are hip-implant hardware or Harrington rods.
Recommendations When Preparing Patients for MRI
In order to have an effective MRI screening, the patient must be thoroughlyprepared for the experience. One of the biggest concerns patients report isclaustrophobia. When patients know what to expect--that the MRI screeninginvolves being in a small, enclosed space and holding still during theprocess--they approach the situation in a calmer manner.
The referring physician and nurse also need to provide as much clinicalhistory as possible to the MRI facility performing the exam. Backgroundinformation, such as seizures, pains, and other factors can help the radiologistdesign the most effective study for the patient. If the patient does requiresome medication to assist with anxiety or pain, it is best ordered prior to thestudy, and clearly communicated to the MRI facility.
MRI screening provides patients and doctors with an invaluable diagnosticresource, but as with any tool, it is most effective when used in accordancewith proper guidelines and safety measures. By properly training all personnelwho come into contact with the technology--from the referring doctor to thescheduling nurse and perhaps most importantly, the patient-- MRI can provide theanswers to many vital medical questions.
Donald Woodward is national manager of the Applications and EducationGroup for Alliance Imaging, Inc. in Anaheim, Calif.For a complete list of references click here