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Neuroimaging in Psychiatry


Key Points

  • When structural neuroimaging is clinically indicated, magnetic resonance imaging (MRI) is usually the modality of choice. Computed tomography (CT) is typically recommended instead of MRI if an acute bleed is suspected.

  • Guidelines for obtaining structural neuroimaging studies in patients with neuropsychiatric symptoms include patients with acute changes in mental status (including changes in affect, behavior, or personality) plus one of the following criteria: age greater than 50 years; an abnormal neurological examination (especially with focal abnormalities); a history of significant head trauma (i.e., with extended loss of consciousness, neurological sequelae, or a temporal relationship to the mental status change in question); new-onset psychosis; or new-onset delirium or dementia of an unknown cause.

  • Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are functional neuroimaging modalities. They measure cerebral blood flow and cerebral glucose metabolism, both of which are tightly coupled to neuronal activity.

  • In the neuropsychiatric setting, PET and SPECT are potentially useful in the evaluation of dementia and seizures.

  • Many neuroimaging modalities (e.g., functional magnetic resonance imaging [fMRI] and magnetic resonance spectroscopy [MRS]) have limited clinical utility at present, but they should become more clinically useful as the field matures.

Overview

In this chapter we will review neuroimaging modalities that can be used in clinical psychiatry, including the structural neuroimaging modalities computed tomography (CT) and magnetic resonance imaging (MRI), as well as the functional neuroimaging modalities (e.g., positron emission tomography [PET] and single-photon emission computed tomography [SPECT]). We will briefly review the underlying technology for each imaging modality and then will discuss the clinical utility of each. We will conclude with a brief review regarding the use of these, and other, neuroimaging modalities in the context of research. This chapter is an extension of prior reviews we have written on these topics.

Structural Neuroimaging Modalities

Computed Tomography

Technology

Computed tomography (CT) uses multiple serially-acquired x-rays that are attenuated to varying degrees depending on the material through which they pass. For example, low-attenuation materials (such as air or fluid) appear dark on a CT image, whereas high-attenuation materials (such as bone) appear white. Gradations within the spectrum of attenuation allow for the visual differentiation of brain tissue ( Figure 75-1 ). The serially-acquired x-rays are obtained in a rotating manner and these data are then reconstructed using computerized algorithms. The spatial resolution of CT has improved over the years, reaching 1 mm or better in-plane.

Figure 75-1, CT: normal brain.

Contrast

In some clinical situations CT contrast can be used. CT contrast is typically (but not always) ionic contrast that is radiopaque (i.e., very high x-ray attenuation, meaning that it appears white on the CT image). The CT contrast agent is introduced intravenously. Therefore, CT contrast is especially useful for visualization of lesions that compromise the integrity of the blood–brain barrier (e.g., cerebrovascular accidents, tumors, inflammation). Non-ionic contrast is also available, but it is more expensive than ionic contrast. However, ionic contrast is associated with a greater risk of side effects. With ionic contrast, idiosyncratic reactions (including nausea, flushing, hypotension, urticaria, and sometimes frank anaphylaxis) occur in approximately 5% of cases. Those at highest risk for idiosyncratic reactions include the young and the old (<1 year of age or >60 years of age) and those with a history of cerebrovascular disease, asthma, allergies, and, of course, prior contrast reactions. Ionic contrast is also associated with chemotoxic reactions that can occur in the kidney and the brain. Chemotoxic reactions in the kidney include impaired renal function and renal failure. The main risk factor for renal chemotoxic reactions to ionic contrast is pre-existing renal insufficiency. Chemotoxic reactions in the brain typically manifest as seizures. These occur in 1 in every 10,000 cases, unless there is a gross disruption of the blood–brain barrier ; this increases the complication rate to 1%–5%.

Clinical Utility

CT is particularly useful for the detection of acute bleeding (<24 to 72 hours old) or acute trauma, but (as reviewed by Park and Gonzalez ) it is not the modality of choice for subacute bleeding (>72 hours old) or for patients who are markedly anemic (i.e., with a hemoglobin less than 10 g/dl). MRI (described later) is superior to CT for most other clinical situations. It should also be noted that CT uses ionizing radiation; thus, it is strongly contraindicated in pregnancy and it is relatively contraindicated in children. However, CT is appropriate when MRI is contraindicated (e.g., with paramagnetic prostheses).

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