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.

• 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

• 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

• 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

• 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

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

Artifacts

• 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