Synonyms: MRI, nuclear spin tomography
Magnetic resonance tomography, from the Ancient Greek τομή, tomos, “a cut” and γράφειν, graphein, “to write”, is a cross-sectional imaging procedure which is primarily used to visualise the structure and function of body tissues and organs. By using electromagnetic waves and special computers, it is possible to depict the organs in the body as cross-sectional images without using harmful radiation. The radiologist can thus obtain information about the inside of the body and associate any changes with a particular disease. This makes it possible to obtain a precise diagnosis and implement the corresponding treatment.
What it is
An MRI scan is an ultra-modern procedure for examining specific organs using electromagnetic waves and special computers. Cross-sectional images of these organs are created and assessed by the radiologist on a monitor.
How it works
Modern 3 tesla clinical MRI scanner
A magnetic resonance scanner (or MRI scanner) is a large piece of equipment, consisting primarily of a magnet with a central hole measuring 70 cm across. During an MRI scan, a very powerful and regular magnetic field is generated by extreme cooling of the magnet and application of a high-voltage current. This magnetic field also affects the human body, as every molecule and atom has its own small magnetic field. When a person enters the MRI scanner, these small magnetic fields all turn in the same direction and are aligned in one row. Special computers can change the strength and alignment of the large magnetic field, causing the individual’s small magnetic fields to realign too. This process of realignment of all the atoms and molecules only lasts for a few milliseconds. Some body tissues take longer to move to their new position, whereas others move more quickly. This results in small time differences, which the MRI scanner can measure in order to create images of the organs. This process involves not just one, but thousands of measurements; a powerful computer then uses this information to create hundreds or thousands of cross-sectional images of the area under examination. During the examination, certain questions may arise which are assessed very carefully by the radiologist. In order to make the vessels more visible, an additional contrast agent may be injected into the elbow during the procedure, using a synthetic intravenous cannula. Any necessary drugs may also be administered via this cannula, for example in cases where the patient has a known allergy to contrast agents.
MRI scanning can be used to detect a wide variety of disorders, which can then be treated accordingly. Ambiguous results from a preliminary examination (e.g. ultrasound or X-ray) can often be assessed definitively. The aim is to detect any changes, to distinguish between malignant and non-malignant changes and to classify these where possible so that a suitable treatment programme can be initiated if required. Many organs such as the brain, nerve tissue or certain joint components can only be viewed using MRI scans. New rapid recording techniques now permit real-time imaging, enabling certain functional processes to be viewed. This recording technique is currently used in heart imaging to obtain a good view of the beating heart, for example.
Target Patient Group
An MRI scan is performed for various diagnostic tasks, especially if the patient has an allergy to iodine as a contrast agent and a CT scan cannot be performed. Examples1 include:
1. Tumour detection, investigation and operation planning
3. Congenital abnormalities
4. Vascular changes
5. Lymph node changes 6. Soft tissue tumours
You will be looked after by an experienced team consisting of a medical radiology technician (MRI technician) and a radiologist. You will also be monitored from a screen in the switch room. Before the procedure and depending on the diagnostic task in question, the radiologist will insert an intravenous cannula in your forearm, through which the contrast agent, and any necessary medication, can be administered. The procedure itself will be carried out by the radiology technician. The radiologist watches and assesses all the images from the procedure. A written report is then drawn up. This is either given to you or sent on to the doctor who referred you.
1. You can eat a light meal before the procedure. You may drink as much water, soup or juice as you like.
2. You should take your usual daily medication with some liquid in the morning.
3. If you are a diabetic, please ask your doctor how you should take your medication on the day of your procedure. If you think that your blood sugar levels are low, please inform the MRI technician immediately.
4. Before the procedure, please tell the radiologist about all medication you are taking.
5. Please bring up-to-date lab results with you, especially those which relate to kidney function (no more than 7 days old).
Please also inform the doctor without fail if one of the following applies:
1. Pregnancy: This procedure can be performed if you are pregnant, unlike CT and X-ray procedures, as it does not use ionising radiation. However, contrast agents may only be used in an emergency.
2. Allergies to MRI contrast agents: If you have ever had an allergic reaction to MRI contrast agents, there is a high risk associated with administering these substances again. However, if the allergic reaction was only minor, it is possible to minimise the risk of a further allergic reaction by administering an appropriate drug. Please contact the doctor who referred you before the procedure.
3. Kidney disease: If you suffer from renal impairment, you should only be given the contrast agent if your lab results permit. Please discuss this beforehand with the doctor who referred you.
4. Pacemakers / defibrillators: If you have a pacemaker or a defibrillator fitted, it will not be possible to perform an MRI scan. Life-threatening side effects could arise, as the control settings of these devices are affected by the magnetic field.
5. Metal implants / foreign objects / tattoos: As a general rule, patients with metal implants or foreign objects should not have an MRI scan, as the metal can move in the magnetic field and may also become quite hot. However, modern implants such as knee or hip prostheses are usually acceptable for use with MRI scanners as they are made from titanium. The length of the metal and the strength of the equipment’s magnetic field must also be taken into account. The radiologist will decide whether or not the procedure can go ahead based on corresponding guidelines. If you have old X-rays of these implants or foreign objects, please bring them with you for your procedure.
The entire procedure, including preparation, lasts approximately 30 minutes. There may be delays before the procedure if emergency patients have to take precedence. Please allow enough time.
MRI body scans of a man, woman, and child
© eNotes (Simon Fraser, Photo Researchers. Reproduced by permission.)
Before the start of the procedure, you will be issued with an information sheet describing the relevant procedure again and asking you questions about any metal implants or foreign objects which you may have, whether you are pregnant, any pre-existing conditions, current symptoms and allergies/intolerances. The procedure is performed with your upper body undressed. Note that all foreign objects such as necklaces, earrings, bras, etc, must be removed before the procedure to guarantee optimum image assessment. Otherwise it will be impossible to carry out the procedure and assess the images. You will be asked to lie down on the examination table. A number of lightweight perforated plates, which are connected to the main computer by a cable, will then be placed on your abdomen. These plates are highly sensitive devices, which can detect the smallest signals from your body. You will also be given headphones so that the loud noises you hear during the procedure are not so uncomfortable. Before the examination table is moved into the MRI scanner, you will also be given a button to hold. If you press this button, the procedure will be halted immediately and the examination table will be removed from the scanner. In the centre of the MRI scanner there is a cylinder, which is approx. 100 cm long and around 70 cm in diameter. This cylinder is brightly and pleasantly lit and open on the opposite side. When the procedure is to be performed on abdominal organs, only this part of your body will be inside the cylinder. Your head and feet are free. You can talk to the operators via a microphone. Please tell the MRI technicians if you are claustrophobic. When the procedure is over, you can return to the waiting room. After no more than 20 minutes, the intravenous cannula will be removed and you will be able to go home. The results of the procedure will be given to you directly or sent to the doctor who referred you.
You will usually be able to go straight home after the procedure. Please tell an MRI technician immediately if you feel dizzy or sick. They will advise you to remain still until the symptoms improve, or for at least 30 minutes. In certain cases you may also be given medication to deal with the nausea. If you have come by car, please check with the medical staff after the procedure whether you have been given any medication which might affect your concentration and prevent you driving home by car.
MRI is a very low-risk procedure. Intolerance reactions to the contrast agent may occur, but only rarely. You may develop a rash and/or itching in such cases. The symptoms are usually only short-lived and disappear of their own accord. If necessary, medication may also be administered in the form of so-called antihistamines. Metal foreign objects, prostheses or tattoos may heat up in rare cases. If you feel an unusual hot sensation or pain during the procedure, report this immediately via the microphone or press the button in your hand. On extremely rare occasions, severe allergic reactions (anaphylactic shock) may occur, accompanied by breathing and circulation problems and/or loss of consciousness. This can be a life-threatening reaction, depending on its severity, and requires immediate medical treatment. If you suffer from chronic renal impairment and are a dialysis patient, serious side effects may develop after administering certain MRI contrast agents, especially on the skin (see renal impairment info).
MRI is usually the last imaging method used to investigate ambiguous changes which have been picked up in preliminary tests such as CT or ultrasound scans or by radiography, but which could not be pinned down in any further detail. For this reason, conservative alternative procedures have often already been exhausted.
Magnetic resonance imaging of the joints (children)
© GH Vienna
How long does a procedure last?
In most cases the procedure will be over after a maximum of 30 minutes. It may be necessary to record additional images for specific diagnostic tasks, which may prolong the procedure.
Why are we given headphones during the procedure?
The many brief changes in the powerful magnetic field lead to very loud banging noises. These noises recur during all additional measurements throughout the procedure.
Can I have an MRI scan if I am highly claustrophobic?
During the procedure, the body lies in a bright cylinder with a diameter of approx. 70 cm. Your head and feet are free when your kidneys are being scanned. Patients who are claustrophobic should make special preparations for this procedure. Possible options include autogenic training, but drugs can also help reduce the claustrophobic sensations. It is often possible to perform the procedure with just a few drops of an anti-anxiety drug. Many patients even find that they doze off for the 30 minutes of the procedure. If, despite all this, you still experience unbearable anxiety, press the button in your hand and the procedure will be halted immediately. You can also talk to the technicians at any time via the microphone close to your head.
Should an MRI scan be performed as a screening test like a blood test, for example (once a year)?
No. This very expensive process is not suitable for use as a preventative measure. It should only be performed if certain conditions are fulfilled and your doctor feels that it is justified.
Do I have to be an in-patient in the hospital to have an MRI scan?
No, the procedure can also be performed if you are an out-patient. You can go home again afterwards as long as there are no other medical reasons why you should not do so.
Which patients should not undergo this procedure?
As a general rule, this procedure is suitable for all patients. Pregnant women and children should have MRI scans rather than any other form of scan, as X-rays and computer tomography can be particularly dangerous for these patients. Use of this method should be considered very carefully before undertaking the procedure in the following cases, and preparatory measures should be taken if applicable: contrast agent allergies or restricted kidney function (renal failure). Please let the radiologist or the MRI technician team know if this applies to you.
What do I need to bring with me?
You should bring up-to-date lab results (no more than 4 weeks old), showing results for the kidneys and thyroid gland in particular.
How much radiation will I be exposed to?
MRI does not lead to any radiation exposure. It uses electromagnetic waves which do not harm the body.
||Team General Hospital Vienna