invasive. These advantages can be helpful not only in research but also in planning neurosurgical procedures to avoid eloquent areas of cerebral function such as those involved with language or vision.

Physiologic information is also available through the use of positron emission tomography (PET) or single-photon emission computed tomography (SPECT). In PET, injection of a radioisotope with a short half-life is followed by imaging of the positrons produced during their decay. The uptake of short-lived radioactive isotopes such as fluorine (18F), carbon (11C), nitrogen (13N), or oxygen (15O) is related to metabolic activity. PET with [18F]-fluoro-2-deoxyglucose has shown increased visual cortex metabolism during ictal visual hallucinations and has detected regions of hypoperfusion in cases associated with visual cortex ischemia. PET is usually combined with CT to enhance resolution. SPECT uses an iodinated radiotracer or technetium-99m as a cerebral perfusion and extraction agent. Because of their biological actions as tracers, these agents reveal patterns of blood flow and cerebral metabolism and thus can be used to study stroke, epilepsy, and dementia. SPECT is more widely available than PET, but it lacks sufficient resolution and specificity. Even after nondiagnostic or normal MRI results, functional imaging may reveal altered regional cerebral blood flow in patients with cerebral visual impairment from various causes.

Sonography

Sonography is an excellent noninvasive technique for imaging the orbit and the carotid arteries. It does not use ionizing radiation and is relatively inexpensive, quick, and office-based; sonography does, however, require expertise in image acquisition and interpretation. The images are based on the reflection of 8–20 MHz ultrasound waves at acoustic interfaces. A particular form of ultrasonography, carotid Doppler is generally accurate at detecting cervical carotid artery stenosis, but it does not provide information about more proximal or distal vessels. The major use of carotid Doppler imaging in neuro-ophthalmology is in detecting cervical carotid artery stenosis in patients with transient monocular blindness suggestive of retinal or optic nerve ischemia. Although carotid Doppler sonography is generally accurate at detecting cervical carotid artery stenosis, it is not accurate at detecting carotid artery dissection. Orbital Doppler sonography is a useful technique for evaluating suspected carotid-cavernous fistula, by detecting a reversal of normally retrograde venous blood flow.

Orbital ultrasound provides useful data concerning the optic nerve and retrobulbar structures, including muscle, optic nerve, and vessels, but it does not provide accurate imaging of the orbital apex. Ultrasound is useful in the globe and may help distinguish disc edema from optic nerve head drusen, which are strongly echogenic (see Chapter 4, Fig 4-13).

Retinal and Nerve Fiber Layer Imaging

Several techniques for noninvasive imaging of the optic nerve head and retinal nerve fiber layer are in clinical use, including Heidelberg retinal tomography (HRT), scanning laser polarimetry (GDx), and optical coherence tomography (OCT). These techniques use laser or short coherence light to image the retina and optic nerve head rapidly and are increasingly being used to evaluate or monitor certain optic nerve diseases such as glaucoma, in addition to being used in research in neuroophthalmic diseases. These techniques are discussed further in BCSC Section 10, Glaucoma.