MRI or Magnetic Resonance Imaging is a diagnostic tool that uses a large magnet and radio waves to take scans of the body that are able to visualise internal organs and structures. It is useful in picking up signs of various diseases or ailments , from torn ligaments to tumours and has proven to be helpful in diagnosing neurological diseases, like conditions of the brain, spinal cord as well as other soft tissues. The advantage of an MRI is that it does not use ionizing radiation and is therefore considered safer than X ray or CT scans, especially in children.
The Principle :
The MRI uses a large , strong magnetic field along with radio frequency pulses to re-align hydrogen atoms within the body. As the hydrogen atoms return to their usual alignment, they emit energy , which varies, depending on the type of body tissue. A scanner captures this energy and converts it into computer generated, two dimensional pictures or cross sectional pictures of the body and organs. These images are analysed by a radiologist to pick up changes in the organ or tissue structure, even picking up minor signs of swelling, bleeding, tumours, cysts etc. These images are viewed on a computer monitor or can be sent electronically, printed or copied to a CD, or even uploaded to a digital cloud server.
The magnetic field produced by the MRI are of different strengths which are measured in a units called “ Teslas (T)”. The strengths are 1.5 T and 3T, where a 3T MRI scan would have a more powerful magnetic field than a 1.5T MRI. The image quality obtained during a scan depends on the magnetic signal and field strength. Therefore, a 3T scan is able to produce clear and vivid images for diagnosis, and can be done faster, decreasing overall scan time. It is most often used for imaging small bones, muscular-skeletal , breast and vascular scans and is especially functional while taking neurological scans.
However, 3T machines also create more noise and heat and because of its ability to catch minute differences in motion, it can sometimes pick up flow artefacts, due to movement of blood and fluid in the body.
The MRI machine is large and is present in a dedicated room. It is composed of a slide scanning table, where the patient is expected to lie down. The table slides into a large tunnel called the scanner, during the procedure and slides out of the scanner after the procedure.
Closed versus Open MRIs:
These are closed MRIs, where the tunnel or capsule completely surrounds the patient lying on the slide scanning table. The MRI scan can take from 30 to 90 mins , depending on the type of scan being done, and patients with claustrophobia as well as young children have difficulty remaining still and often experience claustrophobia or panic attacks in the small enclosed space. The images are not clear if the patient moves and repeat scans may have to be taken, further prolonging the time spent inside the capsule.
Due to patient discomfort, wide bore capsule MRIs and Open MRIs have been developed, which uses magnetic fields generated by magnets located at the top and bottom of the room, but the scanning table is open on all four sides, decreasing patient apprehension. However, the use of open MRIs is limited , as the images produced are often not as clear as those produced in a closed MRI, making it necessary for patients to undergo scans in a closed MRI only, if accurate diagnosis of their conditions requires it. This often prompts sedation of patients while they undergo the procedure and it is especially helpful in case of small children.
Sedation during an MRI:
1.It is understandable that children will be apprehensive to undergo an MRI scan. Therefore it is important to prepare the child beforehand:
- by explaining the procedure,
- taking them to see the machine
- conducting mock scans ( simulations) to see how they perform in the capsule, to screen if the child needs to be sedated or not.
- using techniques to help the child to relax, including distraction through audio visual aids.
2.Usually for children below the age of 5 years, patient sedation is indicated . However, between 5-7 years depending on the child’s ability to stay still and obey instructions, sedation may not be required.
3.Some of the medications that are used to sedate children are : chloral hydrate , midazolam, pentobarbital, fentanyl. These may be given orally or intravenously and usually act for a few hours, although the child may be drowsy or irritable for the whole day, after.
Precautions to be taken before an MRI:
MRI s are considered to be pretty safe as long as precautions are followed, and causes no long term or short term pain or other effects. The magnetic field created by the scanner can attract metal objects, causing them to move with rapid force towards the machine, which puts people or objects that come in its way, at risk of trauma. Therefore, there is great care taken to avoid taking any metallic objects into the scanning room, including cell phones, coins purses , hair pins, buttons or zips. Hence, patients are usually given separate gowns to change into before the scan. If a child has a hearing aid, or metallic implant in the body, medication patches with foil parts, braces etc, appropriate history should be provided to the doctors or the technician performing the scan, as they may be contra-indications.