Basic Principles and Physics
Magnetic resonance imaging (MRI) in clinical use relies on two fundamental principles: the excess of hydrogen molecules in water contained in the tissues of the human body (a) and (b) the phenomenon of magnetic resonance, specifically the magnetlike behavior of these protons that causes them to align and spin at a predictable frequency within an external magnetic field. This alignment occurs when a stationary field is applied by the superconducting magnet as the patient lies within the bore. For each imaging sequence, a smaller secondary set of coils within the bore produce rapid bursts of gradient magnetic fields that cause protons to transiently tilt off-axis. As these protons relax from their high-energy state to their original alignment within the static magnetic field, a radiofrequency signal is emitted from the patient and absorbed by the receiving coils strapped directly to the patient in the anatomic area being imaged (Figure 18-1). The characteristics of this signal are deciphered by the computer using complex algorithms to generate an image.
Patient being placed into MRI scanner with receiving coils placed over the chest (arrows) for MRA of the thoracic aorta.
Although MRI has an excellent signal-to-noise ratio (SNR) allowing for superb tissue differentiation, additional clinical information is commonly obtained by using intravenous contrast agents. While several classes have been developed, only gadolinium-based agents are in clinical use in peripheral arterial imaging. Gadolinium is a toxic element, requiring a chelating agent before it can be employed in medical use. Once injected intravenously, it specifically shortens the T1 relaxation period of protons, resulting in an increased signal seen as a brightened structure on the MR image. Consequently, it has become an indispensable tool for imaging the vascular system. Compared with iodinated x-ray contrast, gadolinium-based agents are relatively benign and adverse reactions are rare. A relatively new manifestation of gadolinium-associated adverse reactions, nephrogenic systemic fibrosis (NSF), has been observed in patients with renal disease who receive contrast for MRI studies. Patients primarily experience thickening and tightening of the skin, but reports also describe involvement of the liver, lungs, muscles, and heart. Thus far, only 57 cases have been reported to the Food and Drug Administration, but these have prompted recommendations to minimize the use of gadolinium contrast agents in patients with advanced kidney disease as much as possible.1,2
While the lack of ionizing radiation in MRI compares favorably to x-ray methods, it does carry its own specific safety concerns. The large, superconducting magnet has potential to attract and/or interfere with all ferromagnetic objects within a sufficient distance. These include equipment within the MRI suite (oxygen canisters, IV poles, etc.) and devices implanted within the patient (pacemakers, defibrillators, TENS units, etc). It has been established that most types of cardiovascular stents, prosthetic heart valves, ...