What are the top brain imaging technologies for ADHD?

Brain imaging has taught us a great deal about ADHD, but it remains a research tool rather than a test

Decades of imaging research have made ADHD one of the better-characterized conditions in neuroscience. Studies using several different technologies have found consistent average differences in brain structure, activity and connectivity between groups of people with ADHD and groups without it, particularly in networks that handle attention, reward and self-control. That body of evidence is one reason ADHD is understood as a neurodevelopmental condition and not a character flaw.

The limitation is just as important as the finding. The differences are averages across groups, with wide overlap between individuals. A given person with ADHD may have a scan indistinguishable from someone without it, and vice versa. The Canadian ADHD Resource Alliance (CADDRA) guidelines do not include imaging in the diagnostic process, and neither do other major international guidelines. This page describes the main technologies and what each contributes; a separate answer covers whether scans or EEG have any role in an actual assessment.

Six technologies account for most ADHD imaging research

MRI-based methods dominate current research because they involve no radiation and can be repeated. PET and SPECT are used sparingly because they require a tracer and are expensive. EEG is inexpensive and portable, which explains why it has attracted the most commercial interest.

Each technology has produced consistent group-level findings

The findings below are drawn from large multi-site collaborations and meta-analyses rather than single studies, which is where imaging evidence is most reliable. They describe what has been observed on average.

• Structural MRI. A landmark study from the US National Institute of Mental Health tracked cortical development in children and found that the cortex in children with ADHD reached peak thickness later than in peers, most markedly in prefrontal regions. Large international pooled analyses have since reported slightly smaller average volumes in several subcortical structures, with the differences most visible in childhood.
• Functional MRI. Task-based studies commonly show reduced activation in frontal and striatal regions during tasks that demand inhibition or sustained attention. Resting-state studies describe altered connectivity in the default mode network, which is thought to relate to mind-wandering and difficulty switching into task focus.
• DTI. White-matter findings point to differences in the tracts linking the prefrontal cortex with deeper structures, consistent with the idea that ADHD involves communication between regions rather than damage to any one of them.
• PET and SPECT. Tracer studies have examined dopamine transporter availability and shown how stimulant and non-stimulant medications occupy their targets, which helped explain their mechanism of action.
• EEG. An elevated theta-to-beta power ratio was reported in many children with ADHD and became the basis for a device cleared in the United States in 2013. Later analyses found the ratio was far less consistent than first thought.

The blog post on brain imaging studies and what they reveal about ADHD goes into the underlying neuroscience in more depth.

No imaging method can diagnose ADHD in one person, for reasons that are unlikely to change soon

A diagnostic test has to separate individuals reliably, not just show that two groups differ on average. ADHD imaging findings fail that bar for several reasons. The average differences are small relative to normal variation between people. Many of the same patterns appear in anxiety, depression, learning disabilities and sleep deprivation. Brains change with age, so a finding in children may not apply to adults. And ADHD is defined by behaviour and impairment across settings, which no scan can observe.

The EEG story is the clearest example. After the theta/beta device was cleared, the American Academy of Neurology reviewed the evidence and issued a practice advisory stating that the ratio should not be used to confirm an ADHD diagnosis outside research, because its accuracy varied widely between studies and it risked both missed and false diagnoses. CADDRA takes the same position for Canadian practice: imaging and EEG may be ordered when a clinician suspects another neurological condition, such as a seizure disorder or the effects of a head injury, but not to establish ADHD.

A Canadian ADHD assessment relies on history, rating scales and clinical judgement

In place of a scan, clinicians use a structured interview covering childhood and current symptoms, validated rating scales such as the Adult ADHD Self-Report Scale and the CADDRA assessment forms, evidence of impairment in at least two settings, and a screen for conditions that mimic or accompany ADHD. The process is designed to answer the questions a scan cannot: when symptoms began, how they affect daily life, and what else could explain them. The page on what happens during an online ADHD assessment describes each step.

Finding Focus provides online ADHD assessment and treatment for adults and teens in several Canadian provinces, using this guideline-based clinical process. After a short online intake you meet a licensed Canadian clinician for one consultation, no referral is needed, and assessments start from $399. If your history suggests a neurological cause that warrants imaging, the clinician will say so and point you toward the right specialist.