Magnetic Resonance Imaging Explained

Magnetic resonance imaging (abbreviation: MRI; synonyms: nuclear magnetic resonance imaging, magnetic resonance imaging) is an imaging technique that can be used to accurately image tissue arrangements without the use of X-rays. The procedure, which can produce cross-sectional images of all body structures, 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 pulses that are emitted into the body’s tissue. 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 in turn emit electromagnetic radiation, 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 MRI image can now be calculated via the echo (the returned waves). The tissue to be examined itself has a so-called intrinsic angular momentum (spin), so that it itself has a magnetic effect. A location-dependent magnetic field is generated to determine the exact position of the atomic nuclei, resulting in 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 Nobel Prize in Medicine and Physiology for this in 2003. Lauterburg was supported by Briton Sir Peter Mansfield, who was also awarded the Nobel Prize for co-developing MRI. 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. Moreover, they succeeded in creating a filtered back projection of the object under investigation, through which an image of the object under investigation could be calculated.

The method

The principle of magnetic resonance imaging is the use of protons (hydrogen nuclei) to produce a measurable echo. To ensure this, a huge number of protons are required, which are first distributed in space in a disordered manner and then arranged in parallel to each other by an externally created magnetic field. To create such a strong magnetic field, only an electromagnet is suitable, which itself is 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 strength of the magnetic field determines the image quality, as this leads to a reduction in so-called image noise. In addition to the main magnetic field, there is an additional need for magnetic fields of reduced strength for location coding, which can be generated by conventional electromagnets. The examination time is determined by the switching on of 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 imaging of the skeleton in orthopedics, special procedures are part of the basic diagnostics in the patient. The following MRI systems are to be emphasized here:

  • Magnetic resonance angiography (MRA) – procedure for imaging the human vascular system using MRI methodology. Depending on the procedural technique, it is performed completely noninvasively or with the use of contrast agents. In contrast to conventional angiography, the imaging is three-dimensional, so that an assessment of the vessels can be performed more precisely. Furthermore, no catheter is necessary for vascular imaging.
  • Functional magnetic resonance imaging (fMRI) – through this procedure it is possible to represent active metabolic processes in the tissue and determine their localization. An fMRI is performed in three scanning phases, which differ both in the resolving power and speed of imaging.
  • Perfusion magnetic resonance imaging (perfusion MRI) – MRI procedure to check the perfusion of various organs.
  • Diffusion Magnetic Resonance Imaging (Diffusion MRI) – novel MRI technique that allows an assessment of the diffusive motion of water molecules in body tissues to be both measured and spatially resolved.
  • Magnetic resonance elastography – this diagnostic procedure is based on the principle that tumor tissue often has a higher degree of density than normally differentiated tissue. By using this technique, an attempt is made to achieve imaging of the visco-elastic properties of different tissues. The mode of operation is as follows. The organ can be compressed three-dimensionally by an externally applied pressure wave, while images of the tissue are taken simultaneously. This examination is followed by the creation of an elastogram, which is used to differentiate malignant from benign tumors.

The division of the various types of devices is made by classifying them 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 structure can be easier access to the patient.

In addition to the different design, there is the possibility to arrange the various systems according to their field strength. To be considered the strongest are the superconducting electromagnets. Due to the enormous technical progress in the field of MRI research, especially MR gradient technology and the production of organ-specific contrast agent, it is now possible to image the entire human body in just one examination procedure. However, for whole-body imaging, a magnet with high main field strength is necessary to ensure adequate imaging. Moreover, special requirements must also be placed on the gradient systems:

  • A fast gradient rise rate is required.
  • Moreover, a high amplitude of the gradient is required for display.
  • To reduce image distortion, there must be high gradient linearity over a wide range.

MRI can be used for many different complaints or diseases. The following MRI examinations are commonly performed:

  • Abdominal MRI (imaging of the abdominal cavity and its organs).
  • Angio-MRI (imaging of blood vessels throughout the body).
  • Pelvic MRI (imaging of the pelvis and its organs).
  • Pelvic MRI (imaging of the pelvis and its organs).
  • Extremities MRI (imaging of arms and legs including joints).
  • Cardio-MRI (imaging of the heart and its coronary arteries/coronary vessels).
  • Magnetic resonance cholangiopancreatography (MRCP).
  • Mamma MRI (imaging of breast tissue).
  • Cranial MRI (imaging of the skull, brain and vessels).
  • Thoracic MRI (imaging of the chest and its organs).
  • Spine MRI (imaging of the bones, intervertebral discs, ligaments and spinal cord).

Possible complications

Ferromagnetic metal bodies (including metallic makeup or tattoos) can lead to local heat generation and possibly cause paresthesia-like sensations (tingling). Regarding tattoos in MRI: To the extent that colors in tattoos contain pigments that are ferrous, these can be attracted by strong magnetic fields in MRI, which in turn can cause patients to feel a tug on the tattooed skin or cause the tattoo to heat up. Some patients also reported a “tingling sensation on the skin,” but this disappeared within 24 hours.Note: In the study, patients were excluded if individual tattoos extended more than twenty centimeters on the skin and multiple tattoos covered more than five percent of the body. Allergic reactions (up to and including life-threatening, but very rare anaphylactic shock) can occur as a result of contrast medium administration. Administration of a gadolinium-containing contrast agent can also cause nephrogenic systemic fibrosis (NSF; scleroderma-like condition) in rare cases. The use of a gadolinium-containing contrast agent is considered critical throughout pregnancy. In the first trimester (third trimester), primarily because of its direct teratogenic effects, and in the second and third trimesters, because gadolinium is expected to enter the fetus via the placenta and be excreted into the amniotic fluid via the fetal kidneys.This in turn would mean that it could be absorbed again by the unborn child. It also increases the risk of children being born dead or dying shortly after birth. There was no increased risk of miscarriage in women who had had an MRI in early pregnancy.