MRI (TMJ) Imaging

  • What is MRI and how is it different from CBCT?
    • Magnetic Resonance Imaging (MRI) uses a strong magnetic field and radio waves to generate 3-D images vs. ionizing radiation of CBCT (note: radiation for CBCT is substantially lower than medical CT imaging).
  • Besides no radiation exposure, what are the benefits of MRI (TMJ)?
    • In addition to no biological risk, MRI offers exceptional soft tissue imaging to determine the extent of Temporomandibular Joint Disorders (TMD).
  • What are the downsides / risks of MRI?
    • Claustrophobia – exam requires patients to lay in a somewhat enclosed space for an extended period (30 minutes) vs. the brevity and standing position of CBCT.
    • Medical devices – metal-containing ones may malfunction or dislodge during the exam. The MRI Imaging Center will pre-screen to ensure safety.
  • Should I have my patient’s braces removed prior to an MRI (TMJ) analysis? And what about their implants?
    • Since braces and implants do not dislodge, there is no safety concern. However, stainless steel brackets may potentially lead to non-diagnostic imaging (due to “magnetic susceptibility artifact”). If ceramic or titanium brackets, simply removing the stainless-steel archwire should suffice (but not needed if Ni-Ti, Co-Cr, or Ti-Mb arch wire).
  • My patient is pregnant. Can she get an MRI? And, what about the use of intravenous contrast agent during pregnancy?
    • There is a large consensus that MRI is safe throughout the entire pregnancy, but MRI Imaging Centers will typically limit non-emergency scans to the first trimester out of an abundance of caution. Note this decision is usually case-by-case after consultation with the patient’s physician.
    • Gadolinium is the contrast agent used in MRI and should be avoided during pregnancy (unless absolutely needed) given the association with stillbirth, neonatal death, inflammatory, rheumatological, and infiltrative conditions.
  • Which types of medical devices are not allowed with MRI?
    • Insulin pumps, ventricular assist devices, LINX Reflux Management System, “Triggerfish” contact lenses, pulmonary artery monitoring catheters, and temporary transvenous pacing leads. The MRI Imaging Center will pre-screen to ensure safety.
MRI Fundamentals
  • Larmor (processional) frequency: Measure of the rate of procession of hydrogen atom magnetic dipole moments around an external magnetic field
  • If the radio wave pulse is at the same value as Larmor frequency, hydrogen atom resonance occurs (larger amplitude) causing a change in angle of procession
  • If magnetic field increases, resonance frequency, signal-to-noise ratio, and T1 increases; however, no change in T2
Spin Echo (SE) Sequence
  • Slice encoding gradient + 90° pulse
  • Phase encoding gradient
  • Slice encoding gradient + 180° pulse
  • Frequency encoding gradient + Echo signal

Determining Factors (TR and TE) for SE Image Contrast

T1 Series

  • Brain looks like it’s supposed to (inner white matter brighter than outer grey matter)
  • Fat is bright
    • Good for detecting Lipomas

T1 (+ Contrast agent) Series

  • Turbinates / Septum / Vessels enhanced
  • Contrast agent
    • Gadolinium (Gd-DTPA) enhances T1 contrast in the arterial phase
    • Disrupts local magnetic field
    • Increases Specificity of low-Sensitivity MRI scans

T2 Series

  • Fluid is bright (Eyes, CSF)
    • Good for detecting Cysts, Myxoma, and Joint Effusion
  • Fat is less bright than T1 (but still bright!)

PD Series

  • Highest Signal-to-Noise Ratio
    • Due to differences in number of magnetized protons per unit volume of tissue
    • Good for detecting Disc perforation
Gradient Recalled Echo (GRE) Sequence
  • Unlike SE, 180° pulse not used to avoid inversion of the longitudinal magnetization vector

Determining factors (TE and Flip Angle) for GRE image contrast

Comparison between SE and GRE

Inversion Recovery Sequence
  • Slice encoding gradient + 180° pulse
  • Slice encoding gradient + 90° pulse
  • Phase encoding gradient
  • Slice encoding gradient + 180° pulse
  • Frequency encoding gradient + Echo signal


  • Inversion recovery deploys a preparatory pulse that affects all protons in the imaging field and relies on the specific longitudinal recovery rates of the targeted tissue to reach the desired signal-nulling effect

STIR Series

  • Inversion recovery set to null fat signal
    • Removes fast relaxation proton signal
  • Good for highlighting effect of contrast agent

FLAIR (T2*) Series

  • Inversion recovery set to null fluid signal
    • Removes slow relaxation proton signal
  • Good for calcifications and hemorrhage (dark “blooming” artifact)
  • Good for detecting Multiple Sclerosis
MRI Random Facts

Voxel Calculation

Example: FOV is 20 cm x 20 cm, 4 mm slice thickness, and 320 x 320 matrix

Slice Thickness

  • Pulse bandwidth (keeping gradient strength constant)
    • Narrow bandwidth, then thinner slice
  • Gradient strength (keeping pulse bandwidth constant)
    • Steeper slope (higher amplitude), then thinner slice
  • Angle of Gradient coil
    • More angle, then thinner slice


  • Drop Signal: Very low signal in the middle of a high signal lesion
  • Signal Void: Same appearance as Drop Signal, but within a vessels
  • Chemical Shift: Curvilinear hyperintensity signal adjacent to low signal at a tissue-tissue interface – due to difference in resonance frequency of fat vs. water protons and occurs in the frequency-encoding gradient
  • Magnetic Susceptibility: Very low signal from metal (due to rapid dephasing) – common cause in the maxillofacial region is metallic restorations, implants, and orthodontic brackets

How to Age Trauma-induced Blood within the Brain Parenchyma

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