Magnetic Resonance Imaging

 
  • MRI signal is based on the number of free water protons present in tissue. Water protons precess at a set Larmour frequency in a magnetic field. A radiofrequency (RF) pulse is applied to tissue while its free water protons are aligned in a magnetic field, which causes the protons to change their alignment. The energy released during this realignment of protons is used to create the MRI image.
  • Superconducting magnets are generally used in todays MRI's. Field strengs are generally 1 to 4 Tesla (T). 1 Tesla = 5000 Gauss. 1 Gauss = the strength of the earth's magnetic field.
  • Pulse sequences: Spin-echo = SE; gradient echo = GRE; inversion recovery = IR
  • TR = time to recovery = time in milliseconds to complete an entire sequence of RF pulses.
  • TE = time to echo = time in milliseconds interval between the initial RF pulse and when the receiver coil listens for the returning signal
  • T1 = a SE sequence short TR (400-800msec) and short TE (<30msec). Fat = bright; Water = intermediate; muscle = intermediate; calcium = dark; tendons = dark. Best for osseous detail, bone marrow, meniscal tears.
  • T2 = a SE sequence with long TR (>2000msec) and long TE (>70msec). Water = bright. Best for soft tissue.
  • Proton Density Sequence = PD = a SE sequence with long TR and Short TE. Fat = bright; Water = brighter than T1; muscle = intermediate; calcium = dark; tendons = dark. Best for osseous detail, bone marrow, meniscal tears.
  • FSE = fast spin-echo = a faster way to obtain T2 images. Fat is much brighter than normal T2.
  • Gradient Echo = GE = short TE and short TR. Generally muscle has higher signal, fat and bone lower. Allows for 1mm slices in small tissues.
  • STIR = short tau inversion recovery = best for bone marrow abnormalities
  • Fractures: appear dark on T1 and bright T2 images.

Contraindications

  • Pacemakers, defibrillators or other implanted electrical devices, prior neurosurgical clips.

Shoulder MRI

  • Proton density fat-saturation images, T1-weighted and T2-weighted sequences in the oblique coronal plane
  • T1-weighted and gradient-echo sequences in the axial plane
  • T2-weighted sequence in the oblique sagittal plane.
  • Coronal oblique = along the plane of the supraspinatus tendon
  • Sagittal oblique = perpendicular to the plane of the supraspinatus tendon.
  • Rotator cuff tears measured on the oblique coronal T2-weighted non–fat-saturation image (provides the sharpest margins/most accurate measurement). When unavailable use coronal oblique fat-saturation sequence.
  • MRI arthrogram: use T1-weighted fat-saturation sequence for measurements.
  • Tendon failure = high-signal intensity (fluid-filled) gap in the low-signal intensity tendon on fast spin echo, T2-weighted images.  Constast between tears and the nontorn tendon edges is highest on non–fat-suppressed T2-weighted images.
  • T1-weighted images do not provide adequate image contrast to evaluate tendon tears.
  • Sanders TG, AJSM, 2005;33:1088.
  • (Frank JB, AJSM 2008;36;1496)
  • Shoulder MRI Rx: Include T1 sagittal images medial to the scapular spine attachment to glenoid; T2 non fat sat and T2 fat sat coronal images; GRAS axial

Knee MRI

  • Knee: Sanders TG, AJSM, 2005;33:131.