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.
