Nuclear magnetic resonance can be detected from several isotopes, but current technology uses mainly signals derived from hydrogen atoms because hydrogen is the most abundant element in tissue and yields the strongest magnetic signal as discussed by Horowitz. The image is essentially a map of the hydrogen content of tissue, therefore reflecting largely the water concentration, but influenced also by the physical and chemical environment of the hydrogen atoms. MRI is accomplished by placing the patient within a powerful magnetic field, causing certain endogenous isotopes to be aligned in the longitudinal orientation of the magnetic field. Application of a brief (few milliseconds) radiofrequency (RF) pulse into the field changes the axis of alignment of the atoms. When the RF pulse ceases, the atoms return to their original alignment and the RF energy that was absorbed is then emitted by the isotopes, producing an electric signal that is detected by receiver coils. To create contrasting tissue images from these signals, the RF pulse must be repeated many times (a pulse sequence), the signals being measured after the application of each pulse. The scanner stores the signals as a matrix of data, which is subjected to computer analysis that allows reconstruction of two-dimensional images.
The terms T1and T2-weighting refer to the time constants for proton relaxation; these may be altered to highlight certain features of tissue structure. In T1-weighted images, CSF appears dark and gray matter is hypointense to white matter. In T2-weighted images, CSF appears bright, and gray matter is hyperintense to white matter. Lesions within the white matter, such as the demyelination of multiple sclerosis, are more easily seen on T2-weighted images, appearing hyperintense against normal white matter (Table 2-3).
A high degree of contrast is seen between white and gray matter on both T1and T2-weighted images, allowing the identification of many discrete structures (Fig. 2-3). Lesions near the skull base and within the posterior fossa, in particular, are seen with greater clarity on MRI compared to CT, unmarred by signals from adjacent skeletal structures. The products of disintegrated RBCs—oxyhemoglobin, deoxyhemoglobin, methemoglobin, and hemosiderin—can be recognized, enabling one to approximate the age of hemorrhages and to follow their resolution, as discussed in Chaps. 33 and 34. Gradient-echo (GRE), or susceptibility weighted imaging (SWI), is especially sensitive to blood and its breakdown products that appear hypointense.
As mentioned earlier, MRI of the spine provides clear images of the vertebral bodies, intervertebral discs, spinal cord, and cauda equina (Fig. 2-2D–F). Abnormalities such as syringomyelia, herniated discs, tumors, epidural or subdural hemorrhages, areas of demyelination, and abscesses are well delineated (see Modic).
Additional radiofrequency pulses can be applied to T1and T2-weighted images in order to selectively suppress signal from fluid or fat. The FLAIR (fluid-attenuated inversion recovery) sequence is a T2-weighted sequence in which the bright signal of fluid that is not contained within tissues is suppressed. This is a particularly useful sequence for visualizing lesions located near CSF compartments. Fat suppression, which can be applied to T1 or T2 sequences, can be used to improve the demonstration of inflammation of the optic nerve, visualize pathologic inflammation within the vertebral bodies, and show thrombus within the false lumen of a cervical dissection.