Magnetic resonance imaging uses radiation

In magnetic resonance imaging(Abbreviation: MRT; Synonyms: Magnetic Resonance Imaging) it is an imaging process with which tissue arrangements can be precisely mapped without the use of X-rays. The process with which sectional images of all body structures can be created is based on the physical principle of Nuclear magnetic resonance spectroscopy.

The wide range of applications of magnetic resonance imaging is explained by the use of electromagnetic impulses that are emitted into the tissue of the body. Various atomic nuclei, whose function is to act as individual magnets, can be excited by the electromagnetic radiation (resonance function). As a consequence, the atomic nuclei send out electromagnetic radiation again, which is now sent back to the starting point of the electromagnetic waves. Depending on the wave strength, the brightness of the image of the tissue on the MRT image can now be calculated from the echo (the waves sent back). The tissue to be examined itself has what is known as its own angular momentum (spin), so that it itself has a magnetic effect. To determine the exact position of the atomic nuclei, a location-dependent magnetic field is generated, which leads to a highly precise image of the tissue.

The development of the magnetic resonance tomograph is largely based on the research of the American Paul Lauterburg, who received the 2003 Nobel Prize in Medicine and Physiology for this. Lauterburg was supported by the British Sir Peter Mansfield, who was also awarded the Nobel Prize for developing the MRT. The two researchers were the first to be able to create a magnetic gradient field through which a spatial assignment of the existing signals could be achieved. In addition, it was possible to produce a filtered back projection of the object to be examined, by means of which an image of the object to be examined could be calculated.

The procedure

The principle of magnetic resonance tomography is the use of protons (hydrogen nuclei) to generate a measurable echo. In order to guarantee this, a huge number of protons are required, which are initially distributed randomly in space and are arranged parallel to one another by an externally generated magnetic field. To create such a strong magnetic field, only an electromagnet is suitable, which is itself cooled with liquid helium so that it does not overheat due to the high energy input. Furthermore, the magnet cannot be switched off, which means that it permanently generates a strong magnetic field. The image quality results from the strength of the magnetic field, since this leads to a reduction in the so-called image noise. In addition to the main magnetic field, there is an additional need for magnetic fields of reduced strength for spatial coding, which can be generated by conventional electromagnets. The examination time is determined by switching on the additional fields, which is accompanied by a loud noise, since stronger and faster gradient fields not only achieve a higher image resolution, but also accomplish this in a shorter time.

However, MRI is by no means a single system, but rather a collection of diverse methods. Particularly in internal medicine, but also in the representation of the skeleton in orthopedics, special procedures are part of the basic diagnosis for patients.

The following MRI systems should be emphasized here:

  • Magnetic resonance angiography (MRA) - Process for the representation of the human vascular system by means of MRT methodology. Depending on the process technology, it is carried out completely non-invasively or with the use of contrast media. In contrast to conventional angiographies, the representation is three-dimensional, so that an assessment of the vessels can be carried out more precisely. Furthermore, no catheter is necessary for vascular imaging.
  • FunctionalMagnetic resonance imaging (fMRI) - this method makes it possible to display active metabolic processes in the tissue and to determine their localization. An fMRI is carried out in three scanning phases, which differ in terms of both the resolution and the speed of display.
  • Perfusion magnetic resonance imaging (Perfusion MRT) - MRT procedure for checking the perfusion of various organs.
  • Diffusion magnetic resonance imaging (Diffusion MRT) - a new type of MRT method with which an assessment of the diffusion movement of water molecules in body tissue can be measured and spatially resolved.
  • Magnetic resonance elastography - This diagnostic method is based on the principle that tumor tissue often has a higher degree of density than normally differentiated tissue. By using this method, an attempt is made to achieve imaging of the visco-elastic properties of the different tissues. The way it works is as follows: The organ can be compressed three-dimensionally by an externally acting pressure wave, while the tissue is recorded at the same time. This examination is followed by the creation of an elastogram, which is intended to distinguish between malignant and benign tumors.

The various device types are divided into closed and open designs:

  • Closed tunnel system - due to the structure, improved image quality is achieved when using this system.
  • Open tunnel system - as a result of the construction, easier access to the patient can be achieved.

In addition to the different designs, it is possible to arrange the various systems according to their field strength. The most powerful are the superconducting electromagnets.

Due to the enormous technical progress in the field of MRT research, in particular MR gradient technology and the production of organ-specific contrast media, it is now possible to display the entire human body in just one examination process. However, a magnet with a high main field strength is necessary for the full-body display in order to ensure an adequate display. In addition, special requirements must also be placed on the gradient systems:

  • A rapid rate of gradient rise is needed.
  • In addition, a high amplitude of the gradient is necessary for the display.
  • To reduce the image distortion, there must be a high gradient linearity over a wide range.

The MRI can be used for many different ailments or diseases.

The following MRI exams are often done:

  • Abdominal MRI (representation of the abdomen and its organs)
  • Angio MRI (showing blood vessels throughout the body)
  • Pelvic MRI (showing the pelvis and its organs)
  • Pelvic floor MRI (showing the pelvis and its organs)
  • Extremity MRI (representation of arms and legs including the joints)
  • Cardio MRI (representation of the heart and its coronary arteries / coronary vessels)
  • Magnetic resonance cholangiopancreatography (MRCP)
  • Breast MRI (representation of the breast tissue)
  • Skull MRI (showing the skull, brain and blood vessels)
  • Chest MRI (showing the chest and its organs)
  • Spine MRI (showing the bones, discs, ligaments and spinal cord)

Possible complications

Ferromagnetic metal bodies (also metallic make-up or tattoos) can be used for local heat generation and possibly trigger paresthesia-like sensations (tingling).

Wg.Tattoos In the MRI: If the colors in tattoos contain pigments that contain iron, these can be attracted by strong magnetic fields in the MRI, which in turn can lead to the patient feeling a pull on the tattooed skin or the tattoo warming up. Some patients also report "tingling of the skin", which however disappeared within 24 hours [6].
Note: Patients were excluded from the study if individual tattoos stretched more than eight inches on the skin and multiple tattoos covered more than five percent of the body.

By a Administration of contrast medium can allergic reaction (up to life-threatening, but only very rare, anaphylactic shock). The administration of a gadolinium-containing contrast agent can also in rare cases a nephrogenic systemic fibrosis (NSF; scleroderma-like disease).
Note: An MRI with a gadolinium-containing contrast agent is not recommended in patients with grade 4 to 5 renal insufficiency.

The use of a gadolinium-containing contrast agent is during the judged as critical throughout pregnancy. In the first trimester (third of pregnancy) primarily due to its direct teratogenic effects and in the second and third trimesters because it can be assumed that gadolinium reaches the fetus via the placenta and is excreted into the amniotic fluid via the fetal kidneys. This in turn would mean that it could be taken in again by the unborn child. It also increases the risk that children will be born dead or die shortly after birth [5].

Women who had an MRI in early pregnancy do not have an increased risk of miscarriage [5].

L.iterature

  1. Dössel O: Imaging procedures in medicine. From technology to medical application. Springer Verlag 2000
  2. Imhoff A: Advanced training in orthopedics. Magnetic resonance imaging. Steinkopff Verlag 2001
  3. Gaulrapp H, Szeimies U: Diagnostics of the joints and soft tissues. Sonography or MRI. Urban & Fischer Verlag 2008
  4. Goyen M: Real full-body MRI - requirements - indications - perspectives. ABW Wissenschaftsverlag 2006
  5. Ray JG et al .: Association Between MRI Exposure During Pregnancy and Fetal and Childhood Outcomes. JAMA 2016; 316: 952-961. doi: 10.1001 / jama.2016.12126
  6. Callaghan MF et al .: Safety of Tattoos in Persons Undergoing MRI. N Engl J Med 2019; 380: 495-496 doi: 10.1056 / NEJMc1811197
     
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