Metabolic and Functional Imaging Modalities
Magnetic resonance spectroscopy (MRS) is used for diagnostic in vivo biochemistry and can provide information about the integrity and metabolism of neural tissue. MRS can help distinguish edema from a tumor. Functional MRI (fMRI) depends on changes in regional blood flow in the brain in response to performing specific cognitive, sensory, or motor tasks. The biochemical basis of most fMRI techniques is the measurement of differences in blood oxygenation level–dependent (BOLD) responses during performance of these specific tasks. fMRI is useful for intraoperative monitoring and in the evaluation of patients with cortical vision loss. Positron emission tomography (PET) is performed with the systemic administration of a short–half-life radioisotope, such as fluorodeoxyglucose (FDG), followed by imaging of the positrons produced during their decay. PET shows metabolic perfusion and can detect hypometabolism in patients with visual cortical dysfunction. Single-photon emission computed tomography (SPECT) uses an iodinated radiotracer or technetium-99m as a cerebral perfusion and extraction agent. SPECT is more widely available than PET, but it has poorer resolution and specificity. Functional imaging may reveal altered regional cerebral metabolism in patients with cerebral vision impairment from various causes in patients who have a normal MRI.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.