Are MRIs safer than X-rays

Magnetic resonance imaging (Magnetic resonance imaging, MRI, MRI)

Magnetic resonance tomography, or MRI for short, is an imaging process that shows organs and tissue in great detail and assesses any changes. The examination method, also known as magnetic resonance tomography, does not use X-rays, but magnetic fields and high-frequency electromagnetic waves.

Short version:

  • Magnetic resonance tomography, similar to computed tomography, can be used to produce precise cross-sectional images of the inside of the body, without the use of x-rays.
  • As a rule, the duration of the examination by means of MRI is around 10 to 20 minutes, with several examination sequences being carried out.
  • No harmful side effects of an MRI have been proven so far.
  • For an MRI examination, patients need a referral from their attending physician.

Information on this page:

Soft parts such as the brain and internal organs are reproduced on an MRI image with particularly high contrast and differentiated, so that even details of less than a millimeter in size can still be recognized. In addition to this high resolution, one advantage of the method is that the recordings can be made in any desired plane - not just across the body, but also lengthways or diagonally. The computer can then even compute a three-dimensional image of the inside of the body from the sectional images.

Magnetic resonance tomography has been used in medicine for more than 30 years. The pioneers of the method, the American chemist and radiologist Paul Christian Lauterbur and the British physicist Peter Mansfield, were awarded the 2003 Nobel Prize for Medicine for their fundamental work in the 1970s. In many areas, MRI is currently the imaging method that can best make pathological changes visible.

What is the difference between an MRI and a CT?

Similar to computed tomography, MRI can be used to produce precise cross-sectional images of the inside of the body, without the use of X-rays. Therefore, unlike a CT, an MRI can be repeated as often as necessary without any risk to the patient.

How does magnetic resonance imaging work?

The MRI makes use of what is known as nuclear spin. This physical term describes the ability of atomic nuclei to rotate around their own axis like a top. This rotating movement gives the atomic nuclei a magnetic impulse - they become tiny magnets. This also applies to the hydrogen atoms ubiquitous in the human body.

Normally the axes of rotation of the hydrogen atoms point in different directions. With the MRI, however, these are now brought into parallel alignment. This is done by using an extremely strong magnet, the magnetic field of which is tens of thousands of times stronger than that of the earth. Under this influence, the hydrogen atoms align themselves like compass needles.

This order is then deliberately disturbed - by means of short pulses of radio waves of a specified wavelength and strength that the MRI machine sends to the parts of the body to be examined. This additional energy tilts the carefully aligned atoms out of line and causes them to totter, as it were.

As soon as the radio wave pulse is over, they return to their position parallel to the magnetic field. During this so-called relaxation time, the hydrogen atoms themselves emit radio waves that are registered by the MRI machine. A special computer then uses these signals to calculate the sectional images of the human body.

The decisive factor for the signal strength is primarily the amount of hydrogen particles: the higher the content of hydrogen atoms in a tissue, the lighter or darker (depending on the examination parameters) it appears on the MRI image. By filtering the MR signals and using different variations of the magnetic field and radio pulse, different types of tissue can be displayed in great detail and any changes (e.g. caused by inflammation or tumors) can be easily identified.

What is the MRI used for and when is it used?

Magnetic resonance tomography is an indispensable instrument for medicine both to determine or exclude a disease and to control the course of therapy.

However, it also has its limitations: organs with a low water content - such as the bones or the lungs - are difficult to make visible. For most other organs and tissues, however, the MRI provides extremely precise images that allow the eye of a trained doctor to recognize even small changes and to assess their position and extent.

One area of ​​application of the method is certainly the examination of the central nervous system, i.e. the brain and spinal cord. The MRI is also very suitable for assessing:

  • Mammary gland
  • internal organs
  • Blood vessels
  • Muscles
  • Tendons
  • Ribbons
  • Cartilage structures

It is now even possible to assess the movement of the beating heart in "nuclear spin".

MRI for early cancer detection

In recent years, magnetic resonance imaging has also gained in importance in the early detection of certain cancers and the control of cancer treatment.

This is not least due to a significant advantage that the method has over classic X-ray examinations, but also over computer tomography. In addition to the advantages already mentioned, namely that the MRI does not work with potentially damaging X-rays, it shows one essential aspect higher contrast resolution than any other radiological method.

How is the examination performed?

There are also "open" MRI versions, but most MRI scanners are still tubes that the patient is inserted into on a couch. The area of ​​the body to be examined is placed in an annular magnet tunnel. It is important to lie as quietly as possible during the examination. Because even the smallest movements can severely impair the quality of the MRI images.

In the case of certain questions, it may be necessary for the patient to be given a contrast medium - which is generally well tolerated - into the vein before or during the examination.

An MRI is completely painless. However, the device can sometimes generate quite loud knocking noises, but these are muffled to a tolerable level by earplugs or soundproof headphones. The examination is controlled from an operator station outside the MRT room. The X-ray assistant sees exactly what the patient is doing through a window and can give instructions over loudspeakers or headphones. The patient is continuously monitored by an additional camera system.

If the need arises, the patient can ring the bell at any time during the scan.

How long does an MRI scan take?

How long the actual examination takes depends on the question and the area of ​​the body to be assessed. As a rule, the duration is about 10 to 20 minutes, whereby several examination sequences are carried out.

What must be considered in advance of the investigation?

As mentioned, an MRI works with extremely strong magnetic fields. Therefore, jewelry and other metallic objects such as glasses, braces and dentures, hair clips and hearing aids must be removed before the examination - to protect the patient, but also the extremely expensive MRI machine. By the way, chip and credit cards should stay outside the MRI room as they can be damaged by the magnetic field.

However, it is far more important that patients who have metal parts in their bodies report them to the medical staff in advance. Which includes:

  • artificial joint replacements
  • Screws into the bone after a fracture repair
  • surgical clips
  • artificial heart valves
  • Ear implants

Newer metal implants are often made of non-magnetic material. After prior clarification, patients with such implants can usually be examined with an MRI.

Are there any possible complications?

Since no X-rays, but only magnetic fields and radio pulses, are used, MRI is a very reliable diagnostic method. Harmful side effects have not yet been proven.

If the examination is carried out with contrast agent, allergic reactions can occur. However, statistics show that these rarely lead to life-threatening complications. If hypersensitivity to contrast media is known, this must be stated in any case. It is also important that the doctor knows about any kidney function disorders (renal insufficiency) that may be present before using the contrast medium.

What alternative examination methods are there?

When it comes to examining bony structures or the lungs, classic x-rays and computed tomography (CT) are usually better suited than MRI. Otherwise, the procedure itself is often used as an alternative or as a supplement to other diagnostic methods when they reach their limits.

This means that magnetic resonance tomography is usually only carried out when other techniques such as ultrasound, X-ray and sometimes also CT do not provide any or at least insufficiently reliable information. Accordingly, there is no alternative for many questions.

Where is an MRI done?

An MRI is usually done in hospitals and x-ray institutes. For an MRI examination, patients need a referral from their attending physician. In order for the health insurance to cover the costs, the examination must also be approved by the chief physician.

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Ulrich Kraft (doctor)
Medical review:
Prim. Univ.Prof. Dr. Bernhard Schwaighofer
Editorial editing:
Philip Pfleger, Tanja Unterberger, Bakk. phil. (2019)

Status of medical information:

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Dominik Weishaupt, Victor D. Köchli, Borut Marincek: How does MRI work? An introduction to the physics and how magnetic resonance imaging works. Springer, Berlin 2006.

Maximillian Reiser, Fritz-Peter Kuhn, Jürgen Debus: Radiology. Thieme-Verlag, Stuttgart 2006.
Fritz SchickMRI sequences. In: The Radiologist. Volume 9. Springer, 2006

Benedikt Michael Schaarschmidt, Harald H. Quick, Ken Herrmann, Lale Umutlu: PET / MR: current uses in oncological diagnostics. Radiology up2date 2018; 18 (3): 203-218