Читать книгу Salivary Gland Pathology - Группа авторов - Страница 61

Although the physics and instrumentation of MRI are beyond the scope of this text, a fundamental understanding of the variety of different imaging sequences and techniques should be understood by clinicians to facilitate reciprocal communication of the clinical problem, and understanding of imaging reports.

In contrast to CT, which is based on the use of ionizing radiation, MRI utilizes a high magnetic field and pulsed radiofrequency waves to create an image or obtain spectroscopic data. MRI is based on the proton (hydrogen ion) distribution throughout the body. The basic concept is that protons are normally oriented in a random state. However, once placed in the imaging magnet, a high magnetic field, a large proportion of protons align with the magnetic field. The protons remain aligned and precess (spin) in the magnetic field until an external force acts upon them and forces them out of alignment. This force is an applied radiofrequency pulse, applied for a specified time and specified frequency by an antenna called a transmit coil. As the protons return to the aligned state, they give off energy in the form of their own radiofrequency pulse, determined by their local chemical state and tissue structure. The radiofrequency pulse given off is captured by an antenna, called a receive coil. The energy of the pulse and location are recorded and the process repeated multiple times and averaged, as the signal is weak. The recorded signal is used to form the image. Several different types of applied pulse sequences of radio waves result in different types of images.

The impact of MRI is in the soft‐tissue contrast that can be obtained, noninvasively. The relaxation times of tissues can be manipulated to bring out soft‐tissue detail. The routine sequences used in clinical scanning are spin‐echo (SE), gradient echo (GRE), and echo‐planar (EPI). Typical pulse sequences for head and neck and brain imaging include spin‐echo T1, spin‐echo T2, proton density (PD), fluid attenuation inversion recovery (FLAIR), diffusion weighted images (DWI), post‐contrast T1 and STIR. A variant of the spin‐echo, the fast spin‐echo sequence (FSE) allows for a more rapid acquisition of spin‐echo images. Any one of these can be obtained in the three standard orientations of axial, coronal, and sagittal planes. Oblique planes may be obtained in special circumstances.