Magnetic resonance imaging

Magnetic resonance imaging (MRI) is a tomographic imaging modality that employs electromagnetic fields and radio waves rather than ionizing radiation to visualize structures and processes within the body. Due to the high level of detail it can provide, MRI is especially useful for imaging soft and nervous tissue. MRI operates by generating a strong electromagnetic field that forces the protons of hydrogen atoms in the body to align with that field. These protons are then stimulated by pulsing a radiofrequency current through the patient, causing them to spin out of alignment. Once the radiofrequency current is turned off, the protons realign with the electromagnetic field, releasing radiofrequency energy that is registered by detectors and computed to generate an image. Unlike radiography, MRI does not employ ionizing radiation and, accordingly, has no radiation-associated effects. However, due to their time-consuming and cost-intensive nature, MRI studies may not be the preferred approach compared to radiographical studies, especially in emergency settings. In addition, the strong electromagnetic field produced by MRI interacts with ferromagnetic materials, making it impossible for patients with certain nonremovable metal implants to undergo the procedure.

MRI is particularly suitable for examining soft tissue structures and nervous tissue, e.g.:

• Injury of cartilaginous structures and ligaments (e.g., ankle joint and cruciate ligament injury)
• Tumor evaluation (e.g., breast cancer)
• Evaluation of CNS disease (e.g., encephalitis, demyelination, acoustic neuroma)

The American College of Radiology offers ACR Appropriateness Criteria®, which are evidence-based guidelines intended to help healthcare providers in making clinical decisions regarding imaging for a wide variety of diagnostic and interventional topics. They can be found at https://acsearch.acr.org/list. ^[1]

MRI is indicated especially for the evaluation of soft and nervous tissue, while CT is preferable in the emergency setting and for the evaluation of bone structures!

Absolute contraindication: patients with ferromagnetic medical implants

• Ferromagnetic metals
  • Interfere with the static magnetic field of MRI.
  • Causes injury due to MRI-related heating and movement of implants (e.g., certain cardiac pacemakers and cochlear implants)
  • Examples: iron, nickel, and cobalt
• Nonferromagnetic metals
  • Do not interfere with the static magnetic field of MRI.
  • Examples: titanium, gold, silver, and copper

Always ask patients about medical implants before performing an MRI!

We list the most important contraindications. The selection is not exhaustive.

Times

• Time to echo (TE): The time between the delivery of external radiofrequency energy and measurement of emitted radiofrequency energy on MRI.
• Repetition time (TR): The amount of time between radiofrequency pulses delivered to a slice on MRI.

Signal intensity

MRI images display not only morphological features but also characteristic signal intensities for every type of normal and abnormal tissue.

• Hyperintensity: Tissue has higher signal intensity and appears brighter than surrounding or normal tissue.
• Hypointensity: Tissue has lower signal intensity and appears darker than surrounding or normal tissue.
• Isointensity: Signal intensity and tissue brightness are the same as those of surrounding or normal tissue.

Findings based on MRI sequence

Water appears hypointense in T1 weighted images but hyperintense in T2 weighted images!

WW2: "Water looks white in T2”

MRI with contrast

Gadolinium-based contrast agents are most commonly used for MRI.

• Mode of action: Gadolinium shortens the relaxation time of protons in its vicinity (especially protons in water molecules), thereby increasing the contrast of tissues that carry it.
• Application: usually intravenous
• Complications
  • Nephrotoxicity: Gadolinium-based contrast agents should be avoided in patients with significant renal failure (GFR < 30 mL/min), particularly those receiving dialysis.
    • Possible complication: nephrogenic systemic fibrosis:
      • High mortality rate (20–50%)
      • Fibrosis progresses systemically in most cases, affecting muscles and internal organs in its course
      • Proliferation of dermal fibroblasts and dendritic cells → thickening, subdermal sclerosis, and hyperpigmentation of the skin → joint contractures
    • If gadolinium-based contrast MRI is absolutely necessary in a patient with renal insufficiency, precautions should be taken (although scientific evidence for these recommendations is limited, they are considered “good clinical practice“).
      • In general: Low-risk contrast media based on macrocyclic, nonionic chelating agents should be preferred.
      • GFR < 30 mL/min: single dose with smallest amount possible, and no further MRI examination with gadolinium-based contrast agents within the next 3 months
      • Dialysis patients: Gadolinium-based MRI examinations should be done on the day of dialysis before the dialysis session, followed by an additional dialysis session the day after (close contact to the supervising dialysis center is required for organizational purposes).
  • Neurotoxicity: In a few isolated cases, deposition of gadolinium in CNS structures has been reported. However, it is not clear if this phenomenon is a clinically relevant complication.