Updated: October 28, 2019
MRI stands for magnetic resonance imaging is a technique where the MRI scaner uses magnetic and radio waves to create pictures of tissues, organs and other structures within the body, which can then be viewed on a computer. MRI scanning is painless and there is no exposure to X-rays or any other damaging forms of radiation.
It is possible to make images of almost all the tissue in the body using an MRI scanner. The tissue that has the least hydrogen atoms such as bones turns out dark, while the tissue that has many hydrogen atoms such as fatty tissue looks much brighter. It is also possible to gain information about the different types of tissues that are present by changing the timing of the radio wave pulses.
Even though the brain and spinal cord are surrounded by bone tissue, a MRI can provide clear pictures of these structures. Changes within the tissues of the brain, whether subtle or gross, can help with a diagnosis and treatment plan. For example, an MRI of the brain can be done to look for the changes associated with bleeding or when the brain has been starved of oxygen after a stroke. A traumatic brain injury can also be investigated using MRI of head. It will also help diagnose developmental abnormalities.
It can be used to determine whether a tumor is benign or malignant abnormal growths and can monitor their progression. For example if they have changed in size and whether it has been spread to nearby tissues.
MRI scan can give very detailed imaging of the thickness and size of the chamber walls with in the heart. Damage occurring to the heart tissue after a heart attack or in association with cardiovascular disease can also be assessed.
Other structures, such as the large blood vessels within the surrounding tissue can also be examined. For example to check for the build up of plaques, which is the main cause of a heart attack in the future.
MRI can also assess congenital cardiovascular conditions more accurately. The associated changes to the heart which have occurred as a consequence of these conditions can also get detected in MRI.
For assessing joint problems it can be used as MRI is very effective at giving clear images of individual tissues such as muscle, bone and ligaments which are most important part of a joint. Sometimes MRI can also be done to get an image of other parts of your body such as the liver, kidneys, spleen and breasts.
You will have to fast for four to six hours before your MRI scan, if you are going to have a contrast injection such as gadolinium. This is because the injection may cause stomach upset. An IV is inserted into the arm just prior to the scan to receive the contrast injection. The contrast then enters the body through the IV. Just before the procedure, you will be asked to change into a hospital gown and to remove any metal objects as these can interfere with the MRI results. These items include jewelry, glasses, and dentures.
An MRI scan is not suitable for the patient if they have electrical appliances, such an a ear implant, implantable cardioverter defibrillator or pacemaker, or have any metal in their body such as surgical clips. But orthopaedic metal ware, such as artificial hips or bone screws, are not normally a problem. Occasionally, patients will be given a sedative medication to decrease anxiety and relax the patient during the MRI scan. Patients lie within a closed environment inside the magnetic machine. Relaxation is important during the procedure and patients are asked to breathe normally and lie still for best accuracy. Some patients can experience a claustrophobic sensation during the procedure. Therefore, patients with any history of claustrophobia should inform the radiology staff before the test. A mild sedative can be given prior to the MRI scan to help alleviate this feeling. You should also inform the radiology staff if you are pregnant as there is a small theoretical risk to the fetus in the first 12 weeks of pregnancy, and therefore scans are not performed on pregnant women during this time.
Interaction with the MRI technologist is maintained throughout the test. During scan, you may hear clicking, buzzing, and whirring noises. The table will move a few millimeters at a time until the exam is finished. Occasionally, patients require injections of liquid intravenously to enhance the images which are obtained. The MRI scanning time depends on the exact area of the body studied, but ranges from half an hour to an hour and a half.
There is no ionizing radiation (X-rays) involved in producing an MRI scan. MRI scans are generally more detailed compared to other imaging techniques such as CT scan as they are more capable of illustrating all soft tissues and higher density tissues, such as bone.
The patient lies inside a large, cylinder-shaped magnet. 99 per cent of the average human body is made up of Hydrogen, together with oxygen and carbon. Protons are found naturally within the body, mostly in hydrogen atoms. When radio waves 10,000 to 30,000 times stronger than the magnetic field of the earth are then sent through the body, it causes the alignment of photons. This allows detailed MRI images to be created. Once the magnetic field is switched off, the protons begin to lose their alignment, and go back to the position they were in before the magnetic field was applied. They send out radio waves of their own after reaching their original place. The scanner picks up these signals and a computer turns them into an image which are based on the location and strength of the incoming signals.
Depending on the location of proton in particular tissue, different protons send out different signals. For example, a proton found in bone will emit a very different radio wave signal when compared to a proton found in blood. These signal differences allows an image to be created, and allow different tissues or structures to be distinguished from one another. The image and resolution produced by MRI is detailed and can detect tiny changes of structures within the body. In some instances, contrast agents, such as gadolinium, are used to increase the accuracy of the images.
The computer generates visual images of the area of the body that was scanned after the MRI scanning is completed. These images then can be transferred to film or hard copy. A radiologist is a physician who is specially trained to interpret images of the body. The interpretation is transmitted in the form of a report which can be discussed with the patient.