Magnetic Resonance for Evaluation of the Thorax
Magnetic resonance imaging, in a few short years, has become an important clinical tool for a variety of conditions. Although the physics of MR is beyond the scope of this article, the technique basically consists of inducing transitions between energy states by causing certain atoms to absorb and transfer energy. This is accomplished by directing a radiofrequency pulse at a substance placed within a large magnetic field. Measures of the time required for the material to return to a baseline energy state (relaxation time) can be translated by a complex computer algorithm to a visual image on a television monitor. There are two time constants associated with relaxation, called T1 and T2. Pulse sequences are used to generate MR images. A pulse sequence is a set of radiofrequency magnetic field pulses and time spacings between these pulses. These are used in conjunction with gradient magnetic fields and MR signal reception to produce MR images. The most commonly used pulse sequences are the spin echo and inversion recovery type. Another newer pulse sequence utilized for imaging is the fast field echo technique which uses a gradient echo. Other terminology used to describe the pulse sequences include TR (repetition time) and TE (echo delay time). TR is the period of time between the beginning of a pulse sequence and the beginning of the succeeding pulse sequence. TE is the time in a spin echo pulse sequence between the middle of the initiating 90° radiofrequency pulse and the middle of the spin echo. website
Magnetic resonance imaging has many of the advantages offered by other imaging modalities without some of the associated disadvantages. Similar to ultrasound, MR is non-ionizing and capable of imaging in multiple planes. Unlike ultrasound, MR is not dependent on the operators skill or the habitus of the patient, and MR can penetrate bone without significant attenuation, allowing the underlying tissue to be clearly imaged. Magnetic resonance imaging provides excellent spatial resolution similar to that of computed tomography (CT) and far better soft tissue contrast resolution. Intravenous injection of contrast media is unnecessary with magnetic resonance since flowing blood provides natural contrast between the blood and cardiovascular structures.