MRI – Basics and Applications
July, 18 2001 Daniel Logan MS IV H&P Rotation
Multi-planar Allows visualization of complex anatomy Soft tissue contrast Non-invasive Lack of ionizing radiation
Modality of choice for soft tissue masses, soft tissue trauma, detecting AVN Particularly useful for pre-op planning – is accurate in detecting osteomyelitis and abcesses Long standing unexplained pain in the presence of normal radiographs
Patient is placed in a strong magnetic field which causes H protons to align with respect to the field. RF pulses are applied to this steady state, exciting the protons in the field. As the protons relax & return to equilibrium, an emitted RF signal is received which helps create the image.
Types of MRI Scans
By varying the time of applying RF pulses (TR), and the timing of acquisition of the returning signal, a sequence can accentuate T1 or T2 characteristics. T1 – Fat weighted T2 – Water weighted STIR – Fat Suppression
T1 – Fat weighted image
T2 – Water Weighted Image
STIR – Deltoid & ATFL
Metallic Implants & Foreign body contraindications
Aneurysm clips, Carotid artery vascular clamps Certain dental devices Heart valves Intravascular coils, stents, & filters Ocular Implants, Otologic Implants Pellets, bullets, shrapnel, Certain penile implants *The key is to determine if the material is ferromagnetic
Most heart valves actually can be scanned safely Relative contraindications are 1st & 2nd trimester of pregnancy (Medicolegal), middle ear prosthesis, and penile prosthesis. Internal hardware are safe to scan (ferrous artifact) Metal external fixation devices should not be scanned
Those with internal cardiac pacemakers, implantable cardiac defibrillators, cochlear implants, neurostimulators, bone-growth stimulators, implantable drug infusion pumps, or any device influenced by electromagnetic fields should not be evaluated by MRI.
The Magic Angle Effect
Occurs when fibers of the tendon are oriented approximately 55 degrees to the central plane of the magnet. This leads to an apparent shortening of the T1 time, resulting in an increased signal intensity in the tendon, simulating the appearance of a tear or tendinitis.
Applications – Soft tissue tumors
MRI is an excellent imaging modality In some cases the signal intensity may provide information regarding the mass. Most signals have low signal intensity on T1 and increased signal on T2. Mass margins not a reliable indicator of malignancy
MRI often used to evaluate the Achilles, posterior tibial, and peroneal tendons. Chronic tendinitis demonstrates low signal on all sequences. When partially torn, interspersed areas of edema or hemorrhage may brighten on T2. With complete rupture, tendon discontinuity may be seen.
Achilles tendon disruption
Beltran J: Sinus tarsi syndrome. Magn Reson Imaging Clin North Am 1994; 2:59-65. Erickson SJ, Smith JW, Ruiz ME, et al: MR Imaging of the lateral collateral ligament of the ankle. AJR Am J Roentgenol 1991; 156:131-136. Kirsch MD, Erickson SJ: Normal magnetic resonance imaging of the ankle and foot. Magn Reson Imaging Clin North Am 1994; 2:1-22 Link, SC, Erickson SJ, Timins, ME: MR Imaging of the ankle and foot: normal structures and anatomic variants that may simulate disease. AJR AM J Roentgenol 1993; 161:607-612. Zanetti M, Ledermann T, Zollinger H, Hodler J:Efficacy of MR imaging in patients suspected of having Morton’s neuroma. AJR AM J Roentgenol 1997; 168:529-532
T1 – Short TE (15-30 msec), Short TR (200-600 msec) T2 – Long TE (60-80 msec), Long TR (2000 msec)