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Magnetic Resonance Imaging - MRI without Contrast

Diagnostic Study - Description & Definition

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. Thusgenerating thin-section images of any part of the human body, from any angle and direction.

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.1

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 field4. 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. 

Normal Study Findings - Images (For abnormal findings images, click on Diagnoses below)
MRI Wrist T1
MRI Wrist T2
Normal Study Findings - Video
Diagnoses Where These Studies May Be Used In Work-Up (with abnormal findings images)
Comments and Pearls
  • MRI uses harmless radio waves, rather than ionizing radiation to produce images of body structures.
  • MRI is a first-line imaging tool for the evaluation of soft-tissue masses, as it can differentiate between tissue types and often provide histologic diagnosis for benign lipomas and vascular malformations.3
  • MRI without intra-articular contrast is not recommended for diagnosing internal derangement of the wrist; standard MRI is highly specific but not sensitive compared with arthroscopy as the gold standard.4
  • In some patients, MRI is not possible owing to safety reasons (eg, pacemaker, shrapnel, other metallic foreign bodies), 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 the bone is better delineated.  Bones with avascular necrosis (AVN) lose their fatty marrow signal and are black. 
  • There are multiple ways to saturate out fat from your images. The basic concept is the signal from fat is nulled out from the image. This can be done on T1 and T2 weighted images.5
  • T2 images are usually done with fat saturation to easily differentiate between fat and fluid signal intensity,therefore the fat is dark and the bone containing fatty marrow is dark. In T2-weighted, fat saturated 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.  Acute injury results in some degree of hemorrhage which demonstrates increased T2 signal on MRI6.
References
  1. A Short History of the Magnetic Resonance Imaging (MRI). 2012. (Accessed August 27, 2015, at http://www.teslasociety.com/mri.htm.)
  2. Greenspan A, Beltran J. Orthopedic Imaging: A Practical Approach. Sixth ed. Philadelphia: Wolters Kluwer; 2015.
  3. 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.
  4. https://radiopaedia.org/articles/resonance-and-radiofrequency?lang=us
  5. https://radiopaedia.org/articles/fat-suppressed-imaging?lang=us
  6. https://radiopaedia.org/articles/bone-contusion?lang=us
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