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Magnetic resonance imaging - MRI with contrast

Diagnostic Study - Description & Definition

Background

MRI is a medical diagnostic technique that creates images of internal body structures using the principle of nuclear magnetic resonance (NMR). MRI scans are performed within a strong, rotating magnetic field. Thus, it can generate thin-section images of any part of the human body, from any angle and direction. The diagnostic performance of MRI is often improved by an intra-articular injection of a contrast medium, which makes derangements and lesions more conspicuous by outlining the joint space.1

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 prototype.2

Description

MRI is based on the reemission of an absorbed radio frequency (rf) signal while a patient is in a strong magnetic field. The ability of MRI to image body parts depends on the intrinsic spin of atomic nuclei. When placed within the main magnetic field, these nuclei will align along the direction of that field. The application of rf pulses causes the nuclei to absorb energy. When the rf field is removed, the energy absorbed during the transition from a high- to low-energy state is released, and this is recorded as an electrical signal that provides the data from which digital images are derived. Signal intensity refers to the strength of the radio wave that a tissue emits after excitation, and the strength of this radio wave determines the degree of brightness of the imaged structures. A bright/white area of demonstrates high signal intensity; a dark/black area demonstrates low signal intensity.3

Normal Study Findings - Images (For abnormal findings images, click on Diagnoses below)
Diagnoses Where These Studies May Be Used In Work-Up (with abnormal findings images)
KIENBOCK'S DISEASE
SCAPHOID FRACTURE
Comments and Pearls
  • MRI uses harmless radio waves, rather than ionizing radiation to produce images of body structures.
  • Contrast MRI should be used to improve the accuracy of preoperative diagnosis.4
  • In some patients, MRI is not possible owing to safety reasons (eg, pacemaker), imaging quality (eg, metal implants) or other patient limitations (claustrophobia)
  • In T1-weighted images tissues with high fat content (subcutaneous layer of the skin and normal bone) appear bright and compartments with water content (such as muscle and avascular bone) appears darker.  T1-weighted images are good for identifying the normal anatomic structures. Therefore in T1 images fat is bright and bone is white.  Bones with avascular necrosis (AVN) are black. 
  • In T2-weighted images tissues with high fat content (subcutaneous layer of the skin) appear dark and compartments with water content (such as avascular bone) appears bright.  T-2 images are helpful when images pathologic lesions with high water content such as an avascular lunate. T2 images are usually done with satuaration therefore the fat is dark and the bone is dark.
References
  1. Maizlin ZV, Brown JA, Clement JJ, et al. MR arthrography of the wrist: controversies and concepts. Hand (N Y) 2009;4:66-73. PMID 19048349
  2. A Short History of the Magnetic Resonance Imaging (MRI). 2012. (Accessed August 27, 2015, at http://www.teslasociety.com/mri.htm.)
  3. Greenspan A, Beltran J. Orthopedic Imaging: A Practical Approach. Sixth ed. Philadelphia: Wolters Kluwer; 2015.
  4. Amrami KK, Bishop AT, Berger RA. Radiology corner: Imaging soft-tissue tumors of the hand and wrist: case presentation and discussion. J Am Soc Surg Hand 2005;5:186-92.