• Clinical science

Magnetic resonance imaging

Summary

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, so it is not associated with associated adverse 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.

Indications

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

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!

Contraindications

Absolute contraindication: patients with ferromagnetic medical implants

Always ask patients about medical implants before performing an MRI!

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

Interpretation/findings

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

MRI Sequences Description Hyperintense Hypointense
T1-weighted
  • Pulse sequence that uses short time to echo (TE) and repetition time (TR)
  • Contrast and brightness are primarily determined by the rate at which excited protons return to equilibrium (i.e., to the main longitudinal magnetic vector from the transverse plane).
T2-weighted
  • Pulse sequence produced by using long TE and TR.
  • Image determined by the rate of transverse relaxation (spin-spin relaxation), i.e., differences in decay of proton spin from the transverse plane (toward the main longitudinal magnetic vector)

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:
      • 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.

last updated 07/10/2020
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