Attention-Deficit/Hyperactivity Disorder (ADHD) is a neurodevelopmental condition characterized by persistent patterns of inattention and/or hyperactivity-impulsivity that interfere with daily functioning and development. This condition arises from differences in brain structure and function, which impact executive functions such as planning, working memory, and self-control. While neuroimaging has been invaluable in understanding the underlying biology of ADHD, its current use for individual diagnosis is limited. Brain imaging cannot yet provide a definitive biological test due to the subtle nature of the differences observed.
Current Clinical Diagnosis of ADHD
The diagnosis of ADHD in clinical practice does not rely on any biological test, such as a blood test or a brain scan. Clinicians adhere to a comprehensive, behavior-based assessment protocol guided by the criteria in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). The DSM-5 defines three presentations: predominantly inattentive, predominantly hyperactive-impulsive, or combined presentation. Diagnosis requires a specified number of symptoms to have persisted for at least six months and been present before the age of twelve.
Clinicians conduct interviews with the individual, parents, or teachers to gather historical information and behavioral observations across multiple settings (home, school, and work). Standardized rating scales and questionnaires are used to quantify the severity and frequency of inattentive and hyperactive-impulsive behaviors. This approach focuses on a consistent, observable pattern of impairment.
Types of Brain Scans Used in ADHD Research
Research into the biological underpinnings of ADHD utilizes several advanced neuroimaging techniques, categorized by what they measure: brain structure, brain function, or electrical activity.
Structural scans, such as Magnetic Resonance Imaging (MRI) and Voxel-Based Morphometry (VBM), visualize the brain’s anatomy. These techniques measure the size and volume of specific regions, including gray matter and white matter density, and identify differences in overall brain volume and subcortical structures.
Functional neuroimaging measures brain activity, often during a cognitive task or while resting. Functional MRI (fMRI) measures changes in blood flow and oxygenation, correlating with neural activity. PET scans track metabolic activity or neurotransmitter receptors using a radioactive tracer.
A third category measures the brain’s electrical signals, such as Electroencephalography (EEG) and Quantitative EEG (QEEG). These methods record brain waves, classified by frequency. One EEG-based system has received FDA clearance as an adjunct to clinical evaluation, but it is not a standalone diagnostic test.
Key Structural and Functional Differences Identified
Neuroimaging research has identified differences in the brains of individuals with ADHD compared to neurotypical control groups. These findings confirm a neurobiological basis for the disorder, characterized by delayed maturation and altered connectivity, primarily in areas responsible for attention, impulse control, and executive functioning.
Structural MRI data shows that individuals with ADHD have a slightly smaller total brain volume. Specific subcortical regions involved in motivation, reward processing, and emotion regulation also show reduced volume. These regions include:
- The caudate nucleus
- The putamen
- The nucleus accumbens
- The amygdala
- The hippocampus
These structural variations are most pronounced in children, suggesting a developmental trajectory that may normalize with age.
Functional studies using fMRI often report reduced activation (hypoactivation) in the prefrontal cortex (PFC), a region central to executive functions like working memory and inhibition. This is frequently observed during tasks requiring sustained attention and cognitive control, such as inhibition tasks.
Differences are also reported in functional connectivity—how various brain regions communicate. Researchers have noted altered connectivity within multiple large-scale brain networks, including the Default Mode Network (DMN), the executive control network, and the frontoparietal network. Altered regulation of the DMN, which is active during mind-wandering, is thought to contribute to difficulties with sustained attention. PET scans also suggest irregularities in the dopamine system, particularly in the frontostriatal circuits.
Why Brain Scans Are Not Yet Diagnostic Tools
Brain scans are not used to diagnose ADHD in a clinical setting because the identified differences are not precise enough for individual assessment. Neuroimaging results rely on statistical comparisons between large groups. Although group differences are statistically significant, the overlap between groups is too substantial to reliably distinguish one person from another.
The structural differences observed, such as smaller brain volume, are very small, often only a few percentage points. These small effect sizes are difficult to interpret meaningfully on a single individual’s scan. Furthermore, the functional and structural variations seen in ADHD are shared with other mental health conditions, including anxiety, depression, and autism. This lack of specificity means a scan cannot definitively rule out other diagnoses.
Practical limitations also prevent the routine clinical use of brain imaging for diagnosis. The technologies are expensive, not widely accessible, and require specialized personnel for administration and interpretation. There is also a lack of standardized protocols for how these scans should be performed and interpreted clinically.