The first 15 years of the new millennium have seen a great increase in research on neuroimaging in children and adolescents who have a psychiatric disorder. In addition, imaging modalities continue to evolve, and are becoming increasingly accessible and informative. The literature is now replete with reports of neurostructural differences between patients and healthy subjects in a variety of common pediatric psychiatric conditions, including anxiety disorders, mood disorders, autism spectrum disorder (ASD), and attention-deficit/hyperactivity disorder (ADHD).
Historically, the clinical utility of neuroimaging was restricted to the identification of structural pathology. Today, accumulating data reveal novel roles for neuroimaging; these revelations are supported by studies demonstrating that treatment response for psychotherapeutic and psychopharmacotherapeutic interventions can be predicted by neurochemical and neurofunctional characteristics assessed by advanced imaging technologies, such as magnetic resonance spectroscopy (MRS) and functional MRI.
However, such advanced techniques are (at least at present) not ready for routine clinical use for this purpose. Instead, neuroimaging in the child and adolescent psychiatric clinic remains largely focused on ruling out neurostructural, neurologic, “nonpsychiatric” causes of our patients’ symptoms.
Understanding the role and limitations of major imaging modalities is key to guiding efficient and appropriate neuroimaging selection for pediatric patients. In this article, we describe and review:
- neuroimaging approaches for children and adolescents with psychiatric disorders
- the role of neuroimaging in (1) the differential diagnosis and workup of common psychiatric disorders and (2) urgent clinical situations
- how to determine what type of imaging to obtain.
Computed tomography
CT, which utilizes ionizing radiation, often is reserved, in the pediatric setting, for (1) emergency evaluation and (2) excluding potentially catastrophic neurologic injury resulting from:
- ischemic or hemorrhagic stroke
- herniation
- intracerebral hemorrhage
- subdural and epidural hematoma
- large intracranial mass with mass effect
- increased intracranial pressure
- acute skull fracture.
Although a CT scan is, typically, quick and has excellent sensitivity for acute bleeding and bony pathology, it exposes the patient to radiation and provides poor resolution compared with MRI.
In pediatrics, there has been practice-changing recognition of the importance of limiting lifetime radiation exposure incurred from medical procedures and imaging. As a result, most providers now agree that use of MRI in lieu of CT is appropriate in many, if not most, non-emergent situations. In an emergent situation, however, CT imaging is appropriate and should not be delayed. Moreover, in an emergent situation, you should not hesitate to use head CT in children, although timely discussion with the radiologist is recommended to review your differential diagnosis to better determine the preferred imaging modality.
Magnetic resonance imaging
Over the past several decades, MRI has been increasingly available in most pediatric health care facilities. The modality offers specific advantages for pediatric patients, including:
- better spatial resolution
- the ability to concurrently assess multiple pathologic processes
- lack of exposure to ionizing radiation.1
A number of MRI sequences, described below, can be used to assess vascular, inflammatory, structural, and metabolic processes.
A look inside. Comprehensive review of the physics that underlies MRI is beyond the scope of this article; several important principles are relevant to clinicians, however. Image contrast is dependent on intrinsic properties of tissue with regard to proton density, longitudinal relaxation time (T1), and transverse relaxation time (T2). Pulse sequences, which describe the strength and timing of the radiofrequency pulse and gradient pulses, define imaging acquisition parameters (eg, repetition time between the radio frequency pulse and echo time).
In turn, the intensity of the signal that is “seen” with various pulse sequences is differentially affected by intrinsic properties of tissue. At most pediatric institutions, the standard MRI-examination protocol includes: a T1-weighted image (Figure 1A); a T2-weighted scan (Figure 1B); fluid attenuated inversion recovery (FLAIR) (Figure 1C); and diffusion-weighted imaging (DWI) (Figure 1D).
Specific MRI sequences
T1 images. T1 sequences, or so-called anatomy sequences, are ideally suited for detailed neuroanatomic evaluations. They are generated in such a way that structures containing fluid are dark (hypo-intense), whereas other structures, with higher fat or protein content, are brighter (iso-intense, even hyper-intense). For this reason, CSF in the intracranial ventricles is dark, and white matter is brighter than the cortex because of lipid in myelin sheaths.
In addition, to view structural abnormalities that are characterized by altered vascular supply or flow, such as tumors and infections (abscesses), contrast imaging can be particularly helpful; such images generally are obtained as T1 sequences.