Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging is a technique that utilizes magnetic fields and radiofrequency pulses to produce highly detailed images of the human anatomy. Magnetic resonance imaging can detect minute changes, reliably delineate lesions, and characterize vascular malformations. Soft tissues, such as abnormalities affecting non-bony structures, can be evaluated using MRI. Images can be obtained in most planes (commonly used are sagittal, coronal, and axial). Contrary to CT, MRI does not expose patients to ionizing radiation. There are some limitations of this imaging modality: MRI is expensive, time consuming, and not readily available in some centers. Additionally, patients with ferromagnetic implants or devices cannot be exposed to the MRI equipment, which has magnets. Contrast studies may result in renal complications; thus, the determination of renal function is necessary before using certain contrast agents.

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Terminology and Technological Aspects

Mechanism

  • The human body is abundant in protons:
    • These protons align with a strong magnetic field.
    • A radiofrequency (RF) current can make protons spin out of equilibrium.
    • When the current is removed, the protons will realign with the magnetic field. Depending on the chemical nature, the alignment will differ in terms of:
      • The time it takes to realign
      • The amount of energy released
    • Sensors (such as those used in MRI) can differentiate between types of tissues based on these properties.
  • MRI images are created by the magnetic manipulation of hydrogen atoms:
    • Achieved by using a magnet with continuous electrical current, creating a permanent magnetic field
    • RF pulses are transmitted, which excite and change the orientation of protons.
    • Once the RF pulses stop, protons:
      • Realign with the magnetic field
      • Release energy in the form of an RF pulse
    • The coils receive these signals from the protons, which are then processed to an image through a computer algorithm.

Image generation

  • Pulse sequences:
    • Predetermined imaging protocols that are specific to the body part being scanned, the parameters of which include:
      • Repetition time (TR): time between 2 RF pulses
      • Echo time (TE): time between the RF pulse and its echo
    • Highlights different tissue characteristics
  • By setting the above parameters, images are created based on the following factors:
    • Proton density (PD): density of hydrogen ions in the tissues
    • T1: 
      • Relaxation time for the protons to align longitudinal/parallel to the magnetic field 
      • Short TR and TE
    • T2: 
      • Relaxation time for the protons to align transverse/perpendicular to the magnetic field
      • Long TR and TE
  • Signal suppression (inversion recovery pulse sequences):
    • Suppression of signals from certain types of tissues: appear dark rather than bright
    • While generating PD-, T1-, or T2-weighted images, signals from other tissues are suppressed:
      • FLAIR (fluid-attenuated inversion recovery): suppresses water
      • STIR (short tau inversion recovery): suppresses fat
    • Allows for better contrast visualization and characterization of specific lesions

MRI

Image: “MRI-Philips” by Jan Ainali. License: CC BY 3.0

Interpretation

  • Structures are described as: 
    • Hyperintense:
      • Indicating that the structure has more signal intensity
      • Brighter than the surrounding structures
    • Hypointense:
      • Indicating that the structure has low signal intensity
      • Darker than the surrounding structures
  • Use of FLAIR: T2-weighted MRI sequence in which CSF is suppressed, so other T2 hyperintensities are easier to see (edema)
  • Contrast
    • Heavy metals
    • Often, the element gadolinium
    • Increases the speed of proton realignment with the magnetic field (the faster the realignment, the brighter the image)
    • Given to a patient before or during MRI
    • Administered through different routes (e.g., intravenously and intra-articularly)
    • Excreted by the kidneys
Table: Interpretation of MRI
TissueT1-weighted imagesT2-weighted images
Fluid (e.g., CSF)DarkBright
White matterLight grayDark gray
Gray matterGrayLight gray
FatBrightBright
InflammationDarkBright

Indications and Contraindications

Central nervous system

  • Cerebrovascular accidents: MRI shows greater sensitivity in detecting acute infarcts using diffusion-weighted imaging.
  • Brain tumors: primary (e.g., glioblastoma multiforme) or secondary
  • Encephalitis/meningitis
  • Dementia (e.g., Alzheimer’s disease)
  • Spinal disc herniation
  • Spinal cord injuries

Musculoskeletal system

  • Knee injuries: meniscal, anterior cruciate ligament (ACL)/posterior cruciate ligament (PCL) tears, fractures, etc.
  • Soft-tissue tumors: evaluation for soft-tissue component to osseous primary tumors
  • Shoulder injuries: rotator cuff tear, adhesive capsulitis, labral tear, etc.

Other systems

  • Rectal cancer
  • Prostate cancer
  • Liver masses:
    • Benign masses
    • Malignant masses
  • Screening for breast cancer in BRCA carriers

Contraindications

Contraindications relevant in patients with:

  • Ferromagnetic implants: Movement or possibility of overheating cause injury.
  • Electrical or mechanical devices:
    • Cochlear implants
    • Pacemakers
    • Drug/insulin-infusion pumps
  • Claustrophobia: pretreated with sedatives
  • Allergy to contrast: may manifest as anaphylaxis
  • Abnormal renal function:
    • A relative contraindication, as newer gadolinium-based agents, such as ProHance, no longer require checking GFR prior to administration.
    • For older types of gadolinium:
      • Check renal function (cannot use for GFR < 30).
      • It is important to avoid nephrogenic systemic fibrosis (a fibrotic disorder of the joints and organs in patients with renal failure receiving gadolinium).

Other Imaging Modalities

Comparison of imaging modalities

Table: Comparison of imaging methods
RadiographyCTUltrasoundMRI
Mechanism of acquisitionIonizing radiationIonizing radiationAcoustic energyFerromagnetic pulses
Relative costInexpensiveExpensiveVery inexpensiveVery expensive
PortableYesNoYesNo
Length of examSeconds< 1 minuteSecondsMany minutes up to about 1 hour
ContrastNoMay be neededMay be neededMay be needed

Other imaging modalities by system

  • Imaging of the CNS (brain, spinal cord, and vertebral column): 
    • Radiography is often used to evaluate fractures of the vertebral column.
    • CT is a good choice for head trauma and to exclude intracranial hemorrhage.
    • MRI provides more detailed images of the brain and spinal cord, allowing for the identification of infarcts, tumors, disc herniation, and demyelinating disease.
  • Pulmonary radiology and imaging of the mediastinum: 
    • Radiography is the preferred initial imaging study for viewing lung pathology.
    • CT provides more detailed views of the lung parenchyma, mediastinal structures, and vasculature.
    • MRI is not often used, but may be employed for evaluating malignancies and cardiac disease.
    • Ultrasound can be used for a rapid assessment of bedside trauma and to guide procedures (thoracentesis).
  • Breast imaging: 
    • Mammography is often the initial choice for breast cancer screening.
    • MRI can be used to further evaluate and stage breast cancer.
    • Ultrasound is helpful in evaluating lymph nodes and guiding biopsy.
  • Imaging of the abdomen and renal imaging: 
    • Radiography is often used to evaluate for kidney stones, bowel obstruction, and pneumoperitoneum. In addition, barium may be used to assess swallowing and bowel function.
    • CT and MRI provide more detailed assessments of the abdominal viscera and vasculature.
    • Nuclear medicine can be used to assess gallbladder function, gastric emptying, and GI bleeding.
  • Imaging of the uterus and ovaries: 
    • Ultrasound is the most commonly used modality to evaluate the ovaries and uterus, including assessing pregnancies and the causes of abnormal uterine bleeding.
    • CT and MRI provide more detailed views and are often useful in assessing cysts, malignancies, and benign masses.
  • Imaging of the musculoskeletal system: 
    • Radiography is often used to exclude fractures.
    • CT is more sensitive to bone pathology, including osteomyelitis.
    • MRI is preferred for soft-tissue evaluation, such as assessing for malignancies and myositis.
    • Bone scan can be useful in identifying occult fractures, osteomyelitis, and metabolic bone disease.

References

  1. Guha-Thakurta, N., Ginsberg, L.E. (2011). Chapter 13. Imaging of the spine. In Chen, M.M., Pope, T.L., Ott, D.J.(Eds.). Basic Radiology, 2e. McGraw-Hill.
  2. Zaer, N.F., Amini, B., Elsayes, K.M. (2014). Overview of diagnostic modalities and contrast agents. In Elsayes, K.M., Oldham, S.A.(Eds.). Introduction to Diagnostic Radiology. McGraw-Hill.

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