What is the efficacy and function of a Brain SPECT (Single Photon Emission Computed Tomography) scan?

Efficacy and Function of Brain SPECT Scan

Brain SPECT imaging has limited efficacy compared to other neuroimaging modalities and should be considered a complementary tool rather than first-line imaging for most neurological and psychiatric conditions. 1

Function and Technical Aspects

Brain SPECT (Single Photon Emission Computed Tomography) is a functional neuroimaging technique that measures regional cerebral blood flow (rCBF) and metabolism, providing information about brain function rather than structure. The procedure involves:

• Uses radiopharmaceuticals such as Tc-99m-hexamethylpropyleneamine oxime (HMPAO) or Tc-99m-ethyl cysteinate dimer (ECD) 2
• These tracers cross the intact blood-brain barrier and are metabolized by neurons and glia
• Uptake occurs during first passage in proportion to cerebral blood flow at the time of injection
• Radiopharmaceutical localizes in brain tissues within 1 minute of injection
• Scanning is typically performed within a few hours of injection 2

Clinical Applications

Dementia Evaluation

• Brain perfusion SPECT has lower sensitivity than clinical criteria for diagnosing Alzheimer's disease 1
• FDG-PET/CT has greater accuracy than SPECT for differentiating Alzheimer's disease from non-AD dementias 1
• Canadian guidelines recommend SPECT only when FDG-PET is unavailable for differential diagnosis of dementia 1
• SPECT can help distinguish frontotemporal dementia from other dementias by identifying bilateral anterior brain hypoperfusion 1
• For Lewy Body Dementia, striatal SPECT (DaTscan) can be useful when the diagnosis remains unconfirmed after specialist evaluation 1

Epilepsy

• Ictal SPECT has high clinical value in pre-surgical evaluation of epilepsy with sensitivity of 73% and specificity of 75% for seizure focus localization 2
• Subtraction of ictal and interictal SPECT co-registered to MRI (SISCOM) improves sensitivity to >90% for temporal lobe seizures 2
• Ictal SPECT is a strong predictor of surgical success in non-lesional epilepsy 2

Traumatic Brain Injury (TBI)

• Early subacute SPECT can predict unfavorable outcomes at 3 months in mild to moderate TBI 1, 2
• Normal initial SPECT has high negative predictive value for persistent clinical deficits at 12 months in TBI patients 1, 2
• Despite these findings, there is insufficient evidence to support routine clinical use of SPECT at the individual patient level for TBI 1

Cerebrovascular Disease

• Can detect cerebral blood flow abnormalities in acute stroke with sensitivity of 61-74% and specificity of 88-98% 2
• May identify perfusion deficits before structural changes become apparent on CT or MRI 3

Limitations and Considerations

• SPECT has lower spatial resolution compared to other imaging modalities
• The American College of Radiology does not recommend SPECT as initial imaging for new-onset seizure unrelated to trauma, initial evaluation of movement disorders, or routine clinical use in TBI 2
• For dementia evaluation, MRI without IV contrast is preferred for initial imaging to demonstrate regional atrophy patterns 1
• FDG-PET/CT is preferred over SPECT for differential diagnosis of dementia due to greater accuracy 1
• Cost, availability, and radiation exposure are additional considerations

When to Consider SPECT

SPECT should be considered in specific clinical scenarios:

1. Pre-surgical planning for drug-resistant epilepsy
2. Differentiating dementia types when clinical presentation is atypical and FDG-PET is unavailable
3. Evaluating cerebrovascular disorders when structural imaging is normal
4. Assessing functional deficits in TBI with normal structural imaging but persistent symptoms
5. Using striatal SPECT (DaTscan) for suspected Lewy Body Disease when diagnosis remains unclear after specialist evaluation

Conclusion

Brain SPECT imaging provides functional information about cerebral blood flow and metabolism that can complement structural imaging in specific clinical scenarios. However, its clinical utility is limited compared to newer techniques like FDG-PET, and it should not be used as a standalone diagnostic tool. The most established applications are in pre-surgical epilepsy evaluation, atypical dementia cases when FDG-PET is unavailable, and specific cases of TBI or cerebrovascular disease with normal structural imaging but persistent symptoms.