Structural and Functional Neuroimaging

Neuroimaging is the set of techniques that picture the living brain. Structural imaging shows anatomy — the shape, size and tissue composition of brain regions — while functional imaging infers activity, most often from the blood-flow and blood-oxygenation changes that accompany neural processing. Together they let brain structure and function be studied non-invasively.

Definition

Structural neuroimaging visualises the anatomy and tissue properties of the brain, while functional neuroimaging measures correlates of neural activity such as blood flow and oxygenation, allowing both structure and function to be assessed in vivo.

Scope

This topic covers the principal structural modalities (computed tomography and structural MRI, including diffusion imaging and morphometry) and functional modalities (functional MRI based on blood-oxygen-level-dependent contrast, and the analysis of functional and effective connectivity). It describes what each method measures as reference methodology, not as clinical guidance.

Core questions

• What does structural imaging measure, and how does it differ from functional imaging?
• How does functional MRI infer neural activity from blood-oxygenation signals?
• How are images turned into quantitative measures of brain structure and connectivity?

Key concepts

• Computed tomography and structural MRI
• Diffusion-weighted imaging
• Voxel- and surface-based morphometry
• Blood-oxygen-level-dependent (BOLD) contrast
• Functional and effective connectivity
• Resting-state imaging and the default mode
• Image segmentation and quantification

Mechanisms

Structural MRI distinguishes tissues by their magnetic-resonance properties, and diffusion imaging exploits the movement of water to probe microstructure and fibre orientation (Le Bihan et al., 1986). Functional MRI relies on the discovery that deoxygenated and oxygenated blood differ in magnetic susceptibility, so changes in local blood oxygenation that follow neural activity produce a measurable blood-oxygen-level-dependent signal (Ogawa et al., 1990). Analysing how these signals covary across regions yields measures of functional and effective connectivity (Friston, 1994), including patterns seen at rest in the absence of an explicit task (Gusnard & Raichle, 2001). Automated pipelines segment and label images to convert them into quantitative anatomical measurements (Fischl, 2012).

Clinical relevance

Neuroimaging methods underpin how brain structure and function are observed and measured in research and practice, and understanding what each modality measures supports careful interpretation. This entry is methodological reference material and does not provide diagnostic criteria or treatment advice.

History

X-ray computed tomography first let brain anatomy be seen in living people, and magnetic resonance imaging then offered superior soft-tissue contrast and diffusion-based microstructural imaging (Le Bihan et al., 1986). The discovery of blood-oxygen-level-dependent contrast (Ogawa et al., 1990) launched functional MRI, after which connectivity analysis (Friston, 1994) and the study of resting-state activity (Gusnard & Raichle, 2001) broadened the field, supported by automated image-analysis tools (Fischl, 2012).

Debates

What does the BOLD signal actually reflect?

Functional MRI measures a blood-oxygenation signal that is an indirect, vascular proxy for neural activity, so the precise relationship between the BOLD response and underlying neuronal events remains an interpretive caveat.

Key figures

• Seiji Ogawa
• Denis Le Bihan
• Karl Friston
• Marcus Raichle
• Bruce Fischl

Seminal works

• ogawa-1990
• le-bihan-1986
• friston-1994

Frequently asked questions

What is the difference between structural and functional neuroimaging?

Structural neuroimaging pictures the anatomy and tissue of the brain, whereas functional neuroimaging measures correlates of activity, such as the blood-oxygenation changes that accompany neural processing.

Does functional MRI measure neurons directly?

No. It measures a blood-oxygen-level-dependent signal, an indirect vascular proxy that follows neural activity rather than recording the activity of neurons directly.