Background
Magnetic resonance imaging (MRI) is a medical diagnostic technique that creates images of internal body structures using the principle of nuclear magnetic resonance (NMR). MRI scans use a superconducting magnet to create a magnetic field around the patient, radiofrequency (Rf) coils to transmit pulses and receive signal from a desired region of the body, and gradient coils to localize where the signal is originating from within the selected region in the x, y and z axis. Thus, generating thin-section images of any part of the human body, from any angle and direction. The diagnostic ability of MRI is can be improved by MR arthrography, specifically for ligamentous and labral pathologies.
Description Historical Overview
Nikola Tesla discovered the Rotating Magnetic Field in 1882 in Budapest, Hungary. All MRI machines are calibrated in "Tesla Units,” and the strength of a magnetic field is measured in Tesla or Gauss Units. In 1937, Isidor Rabi, a Professor at Columbia University observed that atomic nuclei can be visualized by absorbing or emitting radio waves when exposed to a sufficiently strong magnetic field; this quantum phenomenon is known as NMR. In 1971, Raymond Damadian, a physician and professor at the Downstate Medical Center, State University of New York (SUNY), reported that tumors and normal tissue can be distinguished in vivo by NMR. Because cancerous tissue contains more water, more water translates to more hydrogen atoms. In 1973, Paul Lauterbur, a chemist at SUNY, Stony Brook, produced the first NMR image. Mike Goldsmith, a graduate student devised a wearable antenna coil to monitor the hydrogen emission detected by the coil. On July 3, 1977, nearly five hours after the start of the first MRI test, the first human scan was made as the first MRI prototype1.
Description
The ability of MRI to image body parts depends on two fundamental principles, odd number of protons or neutrons, and a positive/negative electric charge in an atom. The human body is mostly composed of water and fat which contain an abundant amount of hydrogen atoms. These hydrogen atoms contain one proton making them ideal for MRI imaging.
When placed within a main magnetic field, these protons will align along and against the direction of the main magnetic field5. The application of a single Rf pulse causes the protons to de-phase. These de-phased protons then try to realign along the direction of the main magnetic field, but the time taken for each of these protons to realign varies depending on the composition of the molecule, fat and water (H+) content. We utilize this time difference in re-phasing and obtain images at different time points. These images are primarily T1 or T2 weighted, though a lot of other MRI sequences have now been developed by modifying the types and number of Rf pulses among other parameters to further characterize the soft tissues. A bright/white area demonstrates high signal intensity; a dark/black area demonstrates low signal intensity2.Gradient coils then localize where in the 3D volume of selected tissue the signal is originating from, and then using complex computer algorithms generates an image.
MR arthrography is further divided into direct and indirect arthrography. Direct arthrography is when the joint of interest is directly injected under fluoroscopic guidance3. Typically, gadolinium is diluted up to 200 times before injecting in the joint of interest.4 Iodinated contrast is used to confirm placement of the needle tip in the joint capsule. MRI contrast is injected with a combination of saline (to dilute the mixture), and lidocaine (local anesthetic to relieve pain by joint capsular distention). Direct MR arthrography improves detection rates of ligamentous and labral abnormalities. These include but are not limited to interosseous ligament tears in the wrist, and pathologies of the triangular fibrocartilage complex5.
Indirect arthrography utilizes an MRI technique which produces arthographic images without directly infusing contrast into the joint, but by administering intravenous contrast and obtaining delayed imaging of the joint of interest6. Images are typically obtained after exercising the joint of interest for a duration of around 10 minutes. This technique is not as consistent in degree of intra-articular enhancement or joint distention. It is also limiting in detecting non-anatomic connections with fluid containing spaces which is an imaging feature used by radiologist to suggest capsular, tendon or even ligament tears.
