Magnetic Resonance Imaging

Syllabus Extract - MR Scanner: Basic principles of MR scanner; cross-section of patient scanned using magnetic fields: hydrogen nuclei excited during the scan emit radio frequency (RF) signals as they de-excite: RF signals detected and processed by a computer to produce a visual image.

Candidates will not be asked about the magnetic fields used in an MR scanner, or about de-excitation relaxation times.

Click here for a comparison of MRI and CT scans

Most of the human body is made up of water molecules, which consist of hydrogen and oxygen atoms.The nucleus of a hydrogen atom is a proton. Protons are very sensitive to magnetic fields.

 

MRI scanners use powerful magnets. When the powerful magnets that are used in magnetic resonance imaging (MRI) are switched on, all the protons in your body are pulled so that they spin in the same direction, in the same way that a magnet can pull the needle of a compass.The MRI scanner sends radio signals to certain areas of the body which ‘snap’ the protons out of position. When this happens, each proton transmits a radio signal that provides information about its exact location in the body.

On its own, a single proton will not provide much useful information, in the same way that a single pixel on a computer screen is essentially just a coloured dot. However, just as millions of pixels can create images, so the radio signals of millions of protons can be collected together and combined to create a detailed image of the inside of the body.

 

USES

Magnetic resonance imaging (MRI) can be used to look at almost any part of the body. It is most often used to study:

• the brain and spinal cord,

 

• the heart and blood vessels,
• other internal organs, such as the lungs or liver,
• bones and joints, and
• breasts.

Functional MRI

Functional MRI is a new technique used to study the workings of the brain. Rather than taking a single scan, functional MRI takes repeated scans, usually one a second. These are used to track the movement of blood through the brain.

By studying the movement of blood, it is possible to tell which sections of the brain are particularly active in real time and to see how brain activity responds to outside events and activities.For example, a volunteer may be asked to solve a problem or to remember a short phrase. Functional MRI will then be used to determine which parts of the brain are active during these tasks.Functional MRI is a relatively new technique, but it has been used by a number of specialists to help plan complex brain surgery.

 

Advantages

The main advantages of magnetic resonance imaging (MRI) scans are that:

• they do not involve exposure to ionising radiation, so they can be safely used in people who might be particularly vulnerable to the effects of radiation, such as pregnant women and babies,
• they are particularly useful for showing soft tissue structures, such as ligaments and cartilage, and organs such as the brain, heart, and eyes, and
• they can provide information about how the blood moves through certain organs and blood vessels, allowing problems with blood circulation, such as blockages, to be identified.

Disadvantages

The main disadvantages of magnetic resonance imaging (MRI) scans are listed below.

• MRI scanners are very expensive; a single scanner can cost over a million pounds. This means that the number of scanners that a Primary Care Trust (PCT) can afford to fund is limited. Therefore, if your condition is non-urgent, you may have to wait several months to have a MRI scan.
• The combination of being put in an enclosed space and the loud noises that are made by the magnets can make some people feel claustrophobic while they are having a MRI scan.

 

• MRI scanners can be affected by movement, making them unsuitable for investigating problems such as mouth tumours because coughing, or swallowing, can make the images that are produced less clear.

Having an MRI Scan

Preparing for the scan

On the day of your magnetic resonance imaging (MRI) scan, you should be able to eat and drink and take any medication as usual.One exception is if you are having an MRI scan of your bile ducts, known as magnetic resonance cholangiopancreatography or MRCP. In this case, you will usually be asked not to eat or drink anything for two to three hours before the scan.Due to the strong magnetic fields that are used by the MRI scanner, it is important to remove any metal objects from your body, including:

• watches,
• jewellery, such as rings,
• piercings, such as ear, nipple and nose rings,
• dentures (false teeth),
• hearing aids, and
• wigs (as some wigs contain traces of metal).

Depending on which part of your body is being scanned, you may need to wear a hospital gown during the procedure. Otherwise, wear clothing without metal zips, fasteners, buttons, belts or buckles.For some MRI scans, you will be given an injection of a special dye, known as a contrast agent. This makes certain tissues and blood vessels show up more clearly and with greater detail on the scan.As MRI scans are painless, anaesthetic is not usually required. If you are claustrophobic, you can request a mild sedative during the scan to help you relax. If this is the case, inform the radiographer. If you decide to be sedated, you will need to arrange for a friend or a family member to drive you home after the scan because you will be unable to drive for the first 24 hours (see After the scan, below).A general anaesthetic is often used when babies and young children are given an MRI scan. This is because it is important to stay very still during the scan, which babies and young children are often unable to do if they are awake.During the scanAn MRI scanner is a short tunnel which is open at both ends. During the procedure, you lie on a motorised bed, which is moved inside the scanner.A small receiving device is placed behind or around the part of your body being scanned. You are moved into the scanning tube, either head- or feet-first, depending on which part of your body is being scanned.

A computer is used to operate the MRI scanner. The computer is located in a different room to the scanner to keep it away from the magnetic field generated by the scanner.As the radiographer operates the computer, they will also be in a separate room to you. However, you will be able to talk to them, usually through an intercom, and they will be able to see you at all times on a television monitor.

During your scan, a friend or family member may be allowed to stay in the room with you. Children can usually have a parent with them. Anyone who stays in the scanner room with you will be asked the same questions as you about pacemakers and metal objects in their body, and will have to follow the same guidelines about clothing and removing metallic objects.

To avoid the images being blurred, it is very important that you keep the part of your body being scanned still throughout the procedure. Depending on the size of the area being scanned and how many pictures are taken, a typical scan lasts between 15 and 90 minutes.

At certain times during the procedure, the MRI scanner will make a loud knocking noise. You may be given earplugs or headphones to wear. The noise is caused by the magnets in the machine being turned on and off. After your scan has been completed, you will be moved back out of the scanner.After the scan MRI scans are usually performed as an outpatient procedure, so there is no need for an overnight stay in hospital.

Once the scan is over, most people can resume their normal activities immediately.However, if you have been given a sedative, you will need to be taken home by a friend or relative, and someone should stay with you for the first 24 hours. It is not safe to drive, operate machinery or drink alcohol for 24 hours after you have had a sedative.If you have been given an injection of a contrast agent, it is a good idea to drink a lot of water for the following 24 hours to help flush the dye out of your body.As your MRI scan will need to be studied by the radiologist and possibly a number of other related specialists, it is unlikely that the results of your scan will be known immediately.

The radiologist will report their findings to the doctor who arranged the scan, who will discuss the results with you. Unless they are needed urgently, it usually takes a couple of weeks for the results of an MRI scan to come through.