How Are Brain Scans Used to Diagnose Mental Illness? (choosingtherapy.com) 

Reviewed by: Benjamin Troy, MD

Brain scans, or neuroimaging, provide information about the structure and function of the brain. Several types of neuroimaging tools can help diagnose and treat mental illness. While these tools are powerful for detecting certain neurological conditions, they are limited in diagnosing mental illnesses like depression. Instead, they’re mainly used to rule out other medical conditions.

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Neuroimaging allows us to see the structure and function of the brain by creating pictures of its anatomy and, in some cases, its activity over time. Using this technology, doctors can observe changes in blood flow, metabolism, and electrical activity at rest or in response to specific challenges. While these tools are widely used in research to help clinicians understand mental illness, they are still limited diagnostically.

Current brain imaging technology options for diagnostic purposes include:

• Electroencephalography (EEG): An EEG recording measures electrical activity in the brain over time from electrodes placed on the scalp. Often recorded concurrently with other physiological measures (e.g., oxygen saturation, respiration, muscle contraction), this technique is a relatively inexpensive and non-invasive method for diagnosing sleep disorders and epilepsy. Polysomnography (PSG) refers to such a recording to precisely evaluate the sleep/wake cycle.
• Evoked Potentials (EPs) and Event-Related Potentials (ERPs): Related to EEG, EPs, and ERPs measure electrical activity in the brain that occurs in response to a stimulus, such as a sound or touch. Clinically, these measures can be useful for diagnosing disorders of the central and peripheral nervous systems, such as multiple sclerosis (MS), in which signal transmission impairments are suspected, or for determining traumatic brain injury and coma prognosis.1, 2
• Magnetoencephalography (MEG): MEG is a non-invasive test that measures the magnetic fields of the brain’s electrical activity through hundreds of small sensors in a helmet. It provides similar information as EEG but with greater spatial resolution.3 The MEG is a diagnostic and pre-surgical planning tool for epilepsy and brain tumors.4
• Magnetic Resonance Imaging (MRI): A MRI scan can non-invasively provide a high-resolution brain image without radiation exposure (in contrast to CT or PET). An MRI uses a strong magnetic field and radio frequency to capture energy emissions from brain tissue and bone. This tool detects structural abnormalities like tumors, cysts, inflammation, changes in gray matter, and deterioration (e.g., as with Alzheimer’s disease). Due to the strong magnetic field, anyone with ferrous metal implants cannot receive an MRI scan.
• Magnetic Resonance Spectroscopy (MRS): MRS is a non-invasive technique performed using an MRI machine. However, instead of anatomical measurements, it measures brain metabolism. It’s used in neurotransmitter research or clinically for determining the composition of abnormal brain tissue (e.g., potential tumors, stroke, epilepsy).
• Diffusion-Weighted Imaging (DWI): DWI is a variant of MRI that measures water diffusion to image brain structure and connectivity. Also referred to as Diffusion-Tensor Imaging (DTI), this technique distinguishes white matter fiber tracts in the brain, which can inform presurgical planning.
• Functional Magnetic Resonance Imaging (fMRI): This non-invasive neuroimaging tool is similar to MRI but shows changes in brain activity over time. By measuring small changes in the brain’s blood flow, fMRI can help doctors assess brain function for diagnosing and monitoring strokes, brain injury, and degenerative diseases, and it can help neurosurgeons in pre-surgical planning (e.g., determining language areas of the brain to avoid during surgery).5
• Positron Emission Tomography (PET): A PET scan shows activity (e.g., blood flow, metabolism) in the brain over time by measuring the presence of radiotracers (administered by injection into a vein) throughout various brain regions.  PET scans are used to diagnose cancer, head trauma, tumors, Alzheimer’s disease, and seizures, but because they are expensive and involve the administration of radiotracers (which may be contraindicated for conditions like kidney disease and allergies), they are ordered sparingly.
• Single Photon Emission Computer Tomography: (SPECT): SPECT is a type of nuclear imaging test that uses a radioactive substance to detect blood flow and neurotransmitter (via DaTscan) changes in the brain. This type of imaging can help diagnose neurodegenerative diseases like Parkinson’s disease and dementia, as well as neurovascular diseases.
• Computer Tomography (CT): A CT scan captures still images of brain structures (including brain tissue, blood vessels, and the skull) using X-rays, but in more detail than a traditional X-ray.  It is particularly useful in detecting structural abnormalities, like brain tumors, swelling, bleeding, and deterioration. For some applications, a contrast dye may be given before the scan to enhance the detail of the scan.
• Near-Infrared Spectroscopy (NIRS): NIRS uses near-infrared light that penetrates the scalp, skull, and cerebral tissue to measure regional cerebral oxygen saturation (rcSO2) changes in brain tissue. It works like an oximeter you may have had on your finger at urgent care or a doctor’s office. It is typically used as a protective monitoring tool to ensure brain oxygenation during cardiac surgery, in pre-term neonates, and victims of stroke and severe brain injury, but it does not provide a structural picture of the brain.6
• Combined Imaging: As different tools have unique strengths and weaknesses, concurrent recordings using multiple techniques can create a more robust picture of brain structure and function. For example, several studies use fMRI and EEG concurrently to achieve a picture of brain activity in high spatial and temporal resolution. Other imaging combinations include SPECT/CT, PET/CT, PET/MRI, and MEG/NIRS.

While other areas of medicine have diagnostic tools that can reveal biological mechanisms, mental illness diagnoses are based on subjective reports of clients, family members, and a clinician’s observations. Clinicians then assess the gathered information within the framework of the Diagnostic and Statistical Manual of Mental Disorders (DSM) or International Classification of Diseases (ICD), which provides some degree of consistency, but still, there is often a discrepancy between practitioners’ interpretations of clinical findings, making diagnosis more challenging than in other medical fields.11, 12

Neuroimaging will not replace current methods for diagnosing mental health disorders (at least not anytime soon), but it can be a powerful tool to use with existing diagnostic strategies. In an article entitled “This is Why You’ve Been Suffering,” mental health clinicians expressed a sense that people with mental health disorders might be more compliant with treatment if they could see their mental illness documented on a brain scan, which would enable them to understand their illness better and potentially feel relief that there is a biological explanation for their symptoms.13

Brain imaging can help identify:

• Structural Abnormalities: The most common way brain scans identify the cause of mental health symptoms is by identifying brain areas affected by neurological disease, injury, or a congenital condition or ruling out neurological conditions. MRI and CT are most commonly used to assess overall brain structure.
• Gray Matter Changes: Gray matter refers to the brain tissue that looks folded and contains cell bodies, dendrites, and axon terminals. Brain scan methods like MRI and CT can help measure gray matter volume, thickness, and shape, and abnormalities have been documented with some of the most common mental health disorders like depression, ADHD, and anxiety.14 But at this time it is not the standard of care to order scans for those conditions.
• White Matter Integrity: White matter refers to the axons along which neural signals travel between brain regions. DTI can assess these tracts regarding packing density, myelination, and axon diameter, and abnormalities have been reported across several mental health disorders, most notably schizophrenia and substance use disorder.15, 16, 17
• Enlarged Ventricles: The brain’s ventricles are cavities that produce, store, and secrete cerebrospinal fluid (CSF), which helps cushion the brain, remove waste, distribute nutrients, and maintain temperature and intracranial pressure.  CT and MRI imaging can help diagnose ventricle abnormalities (e.g., hydrocephalus, meningitis) that can give rise to mental health symptoms and have revealed ventricular enlargement in schizophrenia.18
• Functional Changes: Using fMRI to examine activation of brain regions while a person performs a task (e.g., viewing facial expressions) or simply rests can reveal functional disruptions in sensory, cognitive, and affective processing that, in the future might help with diagnosing an array of mental health disorders including depression subtypes.19
• Chemical Imbalances: Brain scans using PET, MRS, or SPECT allow clinicians and researchers to evaluate neurotransmitter levels (e.g., dopamine, glutamine, serotonin, norepinephrine), metabolism (e.g., glucose, oxygen, amino acid), blood flow and volume, and even DNA synthesis in the brain. Clinically, these methods are helpful for the diagnosis of neurological conditions (e.g., tumors, dementia, epilepsy, Parkinson’s disease, stroke), but extensive research with these tools is increasingly informing diagnosis and treatment of mental health disorders like substance misuse, mood, and anxiety disorders.20
• Electrical Abnormalities: EEG, EPs, and ERPs are valuable tools in diagnosing neurological conditions (like epilepsy, multiple sclerosis, and sensory disorders) that can give rise to mental health symptoms. While there is a great deal of research using EP/ERPs to understand better mental health disorders (e.g., anxiety and mood disorders), they are not routinely used clinically for diagnosis.
• Abnormal Sleep Patterns: Polysomnography can help diagnose disordered sleep, such as insomnia and sleep apnea, that can give rise to several mental health symptoms (e.g., mood changes, fatigue, confusion, and cognitive deficits). It is not uncommon for sleep medicine specialists and psychiatrists to refer patients to each other for evaluation and diagnosis, considering the frequent co-occurrence between sleep disorders and mental illness.21
• Subtle Diagnostic Distinctions: With some degree of subjectivity in the diagnosis of mental health disorders, neuroimaging may soon help clinicians distinguish between disorders with overlapping symptoms, such as bipolar disorder and schizophrenia, as well as different subtypes of one disorder (e.g., depression), or disorders with ambiguous diagnostic lines (e.g., ADHD). Such a distinction could beespecially important in guiding treatment, particularly regarding medication or what adjunctive therapy would be effective.19, 22
• Effective treatments: Neuroimaging techniques can help identify the brain regions affected by mental illnesses, thereby guiding the development and application of targeted interventions, such as pharmaceuticals or transcranial magnetic stimulation (TMS), to challenge treatment-resistant cases better. Brain scans also may have the future potential to predict a person’s response to certain treatments, thereby guiding treatment planning more efficiently than the current trial-by-error method for the pharmaceutical treatment of mental health disorders like depression.9
• At-Risk Individuals: Similar to how the BRCA genetic test can identify people at risk for breast cancer, neuroimaging measures can predict those at-risk for mental health disorders like schizophrenia (75%-90% accuracy) and depression (84% accuracy). Identifying such conditions before onset or at an early stage can help improve preventative intervention and disease outcomes, minimizing disruption to the individual and their family.23, 14, 24

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We are on the horizon of using brain scans diagnostically for mental illness. Several groundbreaking studies have demonstrated elements of this exciting potential. However, there are limitations that will have to be overcome before neuroimaging becomes a routine diagnostic tool for mental illness.

Some limitations of using brain imaging for mental illness diagnosis include:

• Symptom Variability: Mental health symptoms are highly fluid and can fluctuate in severity, frequency, and presentation over time and between people. Unlike genetic tests, which are relatively stable over time, many functional brain scan measures are inconsistent, making it difficult to identify stable patterns of brain activity associated with specific disorders. Structural brain scan measures may be more stable for clinicians to incorporate into their diagnostic toolbox, but they may need to be more potent in detecting functional deficits that give rise to maladaptive thoughts and behavior.
• Claustrophobia: The confined space of many imaging machines, like PET and MRI, may be problematic for individuals with claustrophobia. In the United States (as of 2015), approximately 25% of MRI machines were open and better tolerated by individuals who have anxiety about confined spaces.25
• Neuroimaging Is Expensive: Many imaging machines require highly-skilled interdisciplinary teams of psychiatrists, psychologists, physicists, biochemists, mathematicians, computer scientists, psychoradiologists, and other related medical practitioners. In addition, the costs of obtaining (e.g., a fMRI machine is over $1 million) and storing temperature-specific chemical compounds needed for certain imaging techniques (e.g., radioactive compounds, liquid helium for MRI) can be expensive too.25
• Accessibility to Machines: Although the availability of neuroimaging machinery has skyrocketed over the past few decades (e.g., 12 MRI machines in the 1980s to over 50,000 machines in 2018 worldwide), many are located in large institutions, like hospitals, and are shared between clinical and research groups. As the diagnostic capability of neuroimaging extends beyond the neurological realm of mental health disorders, there will be an increasing need for additional machines serving more people and regions.25
• Sample Size of Neuroimaging Studies: As neuroimaging tools are expensive to operate and limited in number, research studies often have small sample sizes, which can lead to errors such as missing important information and asserting findings that are not reproducible. This criticism was highlighted by a recent article in the journal Nature, calling for the sharing of neuroimaging datasets across institutions and research groups as a way to increase the sample size.26
• Bioethical Considerations: Using neuroimaging as a predictive tool to identify future illnesses may affect insurance coverage or cause anxiety or depression for something that may never manifest.
• Health Risks: While some neuroimaging tools are non-invasive and come with negligible risk (e.g., EEG, MRI), other tools (PET, CT) involve ionizing radiation or injection of a contrast dye, which can pose health risks.   
• Mental Health Disorders Are Complex: Unlike other neurological diseases that affect a specific brain area (e.g., tumor), mental health disorders like schizophrenia, depression, addiction, and bipolar disorder, seem to have widespread biological underpinnings, often with the impairment involving the functional connectivity between brain regions. There remainsa significant overlap between the underlying neurobiology of different mental illnesses so they may not be immediately unique on an image. Finally, It is still believed that there are other factors that play a role in the development of mental illness besides the biological ones alone that would be evident on an imaging study.

If you are interested in getting a brain scan, but your medical provider doesn’t feel it is necessary for diagnosis or treatment, you may still be able to receive a brain scan by participating in a research study. At the U.S. National Library of Medicine, you can search online for research studies to participate in. Research groups are generally in search of both healthy control subjects as well as people who are experiencing a specific set of symptoms or conditions.  You can narrow your search to a specific region, pharmaceutical, condition, or disease that might apply to you.

Mental health practitioners (e.g., psychiatrists, psychologists) often will be involved in research in addition to their clinical practice or abreast of current research happenings through institutes or associations they are affiliated with. If you are interested in participating in a brain scan study because of mental health issues you are experiencing, you may want to consult with a therapist first. If you need help finding a therapist, you can try searching an online therapist directory or asking your primary care physician for a referral.

At present, the most significant and reliable utility for brain scans in mental health resides in their ability to help rule out various neurological, vascular, and general medical conditions that can cause symptoms similar to those of various psychiatric disorders.  While brain scans are not currently reliable enough to diagnose mental health disorders on their own, they have the potential to help clinicians diagnose psychiatric disorders with greater accuracy and to treat those disorders with more targeted and effective interventions. It is important to remember, however, that brain scans are just one tool in the diagnostic process, as a comprehensive evaluation also includes a detailed medical history, clinical interview and other assessments for accurate diagnosis and effective treatment of mental illness.

  How Are Brain Scans Used to Diagnose Mental Illness? (choosingtherapy.com) 

There are several medical conditions that can give rise to symptoms commonly associated with mental illness (e.g., confusion, agitation, paranoia, apathy). A psychiatrist or neurologist may request neuroimaging tests to rule out neurological conditions (e.g., tumor, Alzheimer’s disease) before diagnosing a mental health disorder.Brain scans help identify conditions that can affect mental health and cognitive functioning, such as:

• Lesions
• Tumors
• Cysts
• Cancer
• Skull fracture
• Aneurysms
• Stroke (ischemic, hemorrhagic)
• Moyamoya
• Seizure disorders
• Dementia
• Alzheimer’s disease
• Parkinson’s disease
• Huntington’s disease
• Enlargement of ventricles
• Encephalitis
• Infection
• Multiple Sclerosis (MS)