MRI (magnetic resonance imaging) is a diagnostic technique used to create images of the body like a CT. It is non-invasive (It does not enter the body cavity) and requires no radiation, instead it is based on the magnetic fields of the hydrogen atoms in the body. By scanning the body, MRI is able to provide computer-generated images of the body’s internal tissues and organs. MRI usually scans the body in an axial plane (ie. cutting the body into slices from front to back). Usually the images are 2-dimensional, where the MRI images are usually presented in slices from top to bottom. However, using sophisticated computer calculation, these 2-dimensional slices can be joined together to produce a 3-dimensional model of the area of interest being scanned. This is called 3D MRI.
MRI is used in a wide variety of situations, including the investigation of:
- nerve conditions (including the brain)
- heart and vessel function
- diseases of the liver and bile ducts
- chest imaging
- orthopaedic conditions, such as shoulder injury, hip injury.
3D MRI can provide clearer information for certain conditions, such as hepatobiliary pathology, cardiovascular pathology and even orthopaedic injury. Sometimes 3D reconstruction of the nervous system (particularly the brain) is done.
In most cases, there is no special preparation for an MRI scan. The patient can eat or drink normally on the day of the scan. However because going into the machine can be a new experience for the patient, it is best to avoid food or beverages that can cause anxiety, such as large amounts of coffee. Before the scan, the radiographer will make sure there are no metallic items (either worn or inside the patient) to be brought into the scan room. Items such as keys, credit cards, phone cards, coins, watches, and other metal or magnetic related items will be removed because all these are dangerous due to the strong magnet of the MRI machine. Also, other important implants that contain metals (such as heart valve replacement implant, pacemaker, etc) will not be allowed for an MRI scan.
During the MRI, the patient will be required to lie on the back (or rarely, lie on the tummy) on a table. The table will then be pushed into the bore of the magnet. The patient needs to keep still as the scan is very sensitive to movement. The procedure lasts an average of 30 to 45 minutes.
3D MRI is a very safe procedure, as it is non-invasive and requires no radiation. Although the strong magnetic field produces no harmful effects to the human body, it can cause certain types of metal to move, which can potentially cause injury to the patient.
- hip/knee replacement: these patients can have MRI 6 weeks after surgery
- heart pacemaker/nerve stimulators/mechanical pump: these patients can never go for an MRI scan because they may malfunction or be damaged.
Aside of implanted metals, some MRI scans will require injection of MRI dye into the body for better images. Allergic reactions can occur in such agents, but it is rarer than the reactions towards the contrast agents used in CT scan. Because the patient lies flat on the back on a floating table, which is then pushed into the huge magnetic bore, the space for the patient can be very tight. This can induce claustrophobia in 2-3% of the cases. This is usually prevented or treated by asking the patient to lie in a prone position (lie on the tummy) and playing some music or using an earplug because the noise from the machine can be high.
There are few conditions that disallow MRI (or 3D MRI) scans. These include:
- implanted devices and foreign bodies
- unstable patients (or very sick patients)
- pregnancy (unless the scan is absolutely needed)
- Royal Adelaide Hospital MRI Unit [online]. 1998. [Cited 2005 October 3rd]. Available from: URL: http://www.users.on.net/~vision/misc/MRI-scan-information.htm
- Up To Date: Principles of magnetic resonance imaging [online]. 2005. [Cited 2005 October 3rd]. Available from: URL: http://www.utdol.com/application/topic.asp?file=pulm_img/5798&type=A&selectedTitle=1~483
- Virtual Medical Centre’s information on an MRI scan.