Magnetic Resonance Imaging
MRI is a noninvasive imaging technique that does not employ ionizing radiation and has no known adverse biological effects (Fig 2-6). MRI is based on the interaction of 3 physical components: atomic nuclei possessing an electrical charge, radiofrequency (RF) waves, and a powerful magnetic field.
Figure 2-5 Hemifacial microsomia. A, Photograph of patient. B, Three-dimensional CT reconstruction of same patient.
(Courtesy of Jill Foster, MD.)
Figure 2-6 Gadolinium-enhanced T1-weighted magnetic resonance (MR) images of the orbit, with fat suppression. A, Axial view. B, Coronal view.
(Courtesy of Thomas Y. Hwang, MD, PhD, and Timothy J. McCulley, MD.)
When tissue that contains hydrogen atoms is placed in the magnetic field, individual nuclei align themselves in the direction of the magnetic field. These aligned nuclei can be excited by an RF pulse emitted from a coil lying within the magnetic field. Excited nuclei align themselves against the static magnetic field; as the RF pulse is terminated, the nuclei flip back to their original magnetized position. The time that this realignment takes (the relaxation time) can be measured.
Each orbital tissue has specific magnetic resonance (MR) parameters that provide the information used to generate an image. These parameters include tissue proton density and relaxation times. Proton density is determined by the number of protons per unit volume of tissue. Fat has greater proton density per unit volume than bone and, therefore, greater signal intensity. T1, or longitudinal relaxation time, is the time required for the net bulk magnetization to realign itself along the original axis. T2, or transverse relaxation time, is the mean relaxation time based on the interaction of hydrogen nuclei within a given tissue, an indirect measure of the effect the nuclei have on one another. Each tissue has different proton density and T1 and T2 characteristics, providing the image contrast necessary to differentiate tissues. Healthy tissue can have imaging characteristics different from those of diseased tissue, a good example being the bright signal associated with tissue edema seen on T2-weighted scans.
MRI is usually performed with images created from both T1 and T2 parameters. T1-weighted images generally offer the best anatomic detail of the orbit. T2-weighted images have the advantage of showing methemoglobin brighter than melanin, whereas these substances have the same signal intensity on T1-weighted images. The difference in brightness seen on T2-weighted images can be helpful in differentiating melanotic lesions from hemorrhagic processes. Gadolinium, a paramagnetic contrast agent given intravenously, allows enhancement of vascularized lesions so that they exhibit the same density as fat. It also enhances lesions that have abnormal vascular permeability. Special MR sequences have been developed to suppress the normal bright signal of fat on T1-weighted images (fat suppression; see Fig 2-6) and the bright signal of cerebrospinal fluid on T2-weighted images (fluid-attenuated inversion recovery, or FLAIR). Gradient echo sequences may reveal hemorrhage in vascular malformations that might be missed on T1- and T2-weighted images.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.