What is magnetic resonance imaging?
What is an MRI? Many patients ask this question when the doctor orders such an examination. The abbreviation MRI stands for magnetic resonance imaging, also known as magnetic resonance tomography (MRI) or, colloquially, nuclear spin. It is a frequently used imaging procedure that is used to create precise, high-resolution cross-sectional images of the body. The doctor can use these images to assess organ structures and functions. If the entire body is examined using magnetic resonance imaging, this is referred to as a whole-body MRI. However, only individual parts of the body or organs can also be examined. Examples
- Small intestine MRI (Sellink, hydro MRI)
- Abdominal MRI (abdomen)
- Coronary arteries (cardiac MRI, sometimes also under stress as stress MRI)
- Cranial (cranial) MRI (cMRI)
- Joints (for example MRI shoulder or knee joint)
Further information: MRI – Head
Further information: MRI – Knee
You can find out which clinical pictures and injuries in the knee joint can be detected using magnetic resonance imaging in the article MRI: Knee.
Further information: MRI – CERVICAL SPINE
You can find out how MRI of the cervical spine works and when it is carried out in the article MRI: Cervical spine.
MRI: How it works and physical principles
MRI makes use of the fact that atomic nuclei rotate around their own axis. This rotation is called nuclear spin and generates a small magnetic field around each nucleus. The hydrogen atoms found throughout the human body also exhibit this nuclear spin. Normally, their axes of rotation point in different directions. However, this changes during magnetic resonance imaging:
MRI sequences
Radiologists refer to the electromagnetic pulses emitted by the MRI device as sequences. The different sequences depict tissue differently. Frequently used sequences in magnetic resonance imaging are, for example
- Spin-echo sequence (SE)
- Gradient echo sequence (GRE) (for calcifications or hemorrhages)
- Fluid attenuated inversion recovery (FLAIR-MRI for inflammatory diseases such as multiple sclerosis)
- Spin-echo fat saturation (SE fs)
MRI: T1/T2 weighting
As described, the return of the atoms to their initial position is referred to as relaxation. The computer uses this to calculate the sectional images. Depending on whether it is based on the longitudinal or transverse orientation of the atoms, this is referred to as T1 or T2 weighting. With T1 weighting, fatty tissue appears lighter than its surroundings, while with T2 weighting, liquids are displayed.
Magnetic resonance imaging with contrast medium
Further information: MRI – Contrast media
You can read everything you need to know about the use of contrast agents in MRI in the article MRI contrast agents.
Difference: CT – MRI
An important difference (MRI / CT) concerns the radiation exposure: Computed tomography (CT) works with X-rays, which means radiation exposure for the patient. Magnetic resonance imaging, on the other hand, uses magnetic fields and radio waves, which do not expose the patient to radiation.
The disadvantage of MRI is that it takes longer: the examination lasts between 30 and 45 minutes. Computed tomography, on the other hand, is significantly faster with an average duration of 10 minutes and is therefore also the method of choice in emergency cases where the doctor needs a cross-sectional image of the body as quickly as possible. The decision as to whether a patient is more likely to benefit from MRI or CT must therefore always be made by the doctor depending on the suspected diagnosis.
In contrast to CT, which is particularly good at imaging structures with low water content such as bones, magnetic resonance imaging is the method of choice when soft tissue needs to be examined more closely. It is therefore often used in cancer diagnostics, for example to assess the progression of a tumor or to detect metastases. The attending physician also often orders an MRI in the following cases:
- MS (multiple sclerosis)
- Inflammatory diseases of the bone
- Inflammatory diseases of the organs (pancreas, gall bladder, etc.)
- Abscesses and fistulas
- Malformations and protrusions of the vessels (such as aneurysms)
- Joint damage (arthrosis, injuries to tendons, cartilage and ligaments)
What is done during an MRI scan?
The doctor will explain the aim of the examination, the procedure and possible MRI side effects to you beforehand. You will also find out whether you need to fast for the examination (e.g. for small bowel MRI).
If you have a pacemaker or other implanted device, you should inform your doctor of this before the MRI scan. As the magnetic resonance imaging could interfere with the function of the sensitive device, the doctor must decide whether you can undergo the examination at all. If in doubt, he or she must ask the manufacturer beforehand.
In addition, metal parts in the body can shift during an MRI scan or heat up so much that burns can occur. Special care should therefore be taken with:
- Prostheses with metal parts
- nails, plates or screws in the body (e.g. inserted after bone fractures)
- contraceptive coils
- stents
- Metal splinters that remain in the body after accidents or gunshot wounds
For the examination, you have to lie down on the mobile, narrow couch in front of the MRI machine. You will then be pushed into the tube. You should lie as still as possible for the duration of the examination so that sharp images can be taken. You may also have to hold your breath for a short time – you will be instructed to do so via a loudspeaker.
The MRI examination is accompanied by loud knocking noises caused by the switching on and off of the magnetic coils. You will therefore be given hearing protection or soundproof headphones with music in advance.
MRI: claustrophobia in the tube
Open MRI
An open MRI is a good alternative for patients who suffer from claustrophobia. Patients who are very overweight and who are difficult to examine in a conventional MRI scanner simply because of the lack of space also benefit from open MRI.
Another particularly important advantage is that the doctor has access to the patient at all times through the open tube. For example, he can take samples from suspected cancerous lumps or administer locally effective medication under image control.
Not all radiology practices and clinics have an open MRI scanner. If you prefer to be examined in an open system, talk to your doctor about this. They may be able to recommend a suitable practice. Alternatively, you can search the Internet yourself to find out which radiologists offer open MRI.
Magnetic resonance imaging: special procedures
For certain questions, the doctor also uses combined procedures, for example PET/MRI, in which metabolic processes are also visualized. PET stands for positron emission tomography.
What are the risks of an MRI scan?
Magnetic resonance imaging is a very safe, painless diagnostic tool. Only pregnant women in their first trimester and patients with sensitive implants or metal parts in their body are only given an MRI scan if absolutely necessary.
Side effects that may result from the contrast medium are
- feeling of heat
- headache
- Tingling or numbness
- Renal dysfunction
- Intolerance reactions
As long as all metal-containing and magnetizable objects have been removed before the MRI, no dangers (such as burns) are to be expected from this side.
MRI & pregnancy
What should I be aware of after an MRI scan?
If you have been given a sedative for the MRI scan, you should not drive for at least 24 hours. If the MRI scan takes place on an outpatient basis, it is best to organize someone to pick you up beforehand.
The MRI images are available immediately after the examination. However, the doctor must first assess them and prepare a report. You will then usually receive the MRI report by post within a few days, although sometimes you may have to collect it from the radiology practice yourself. You will also receive a CD on which the cross-sectional images are stored. Bring the findings and the MRI CD with you to your next appointment with your doctor.
