Dementia: Discrepancies in PET and SPECT Imaging Results

Q: Dementia: Discrepancies in PET and SPECT Imaging Results

Hello： FDG-PET and ECD-SPECT are both functional imaging tests that provide information about the current metabolic state of the brain. <br>The image quality can vary significantly between different machines, and there may also be differences in interpretation among various physi

Is it dementia?

Hello Doctor: I have been experiencing sleep disturbances since November of last year, along with afternoon dizziness and brain fog.
Despite consulting various doctors, no cause has been identified.
In January of this year, I started taking psychiatric medication, but my condition did not improve.
By late February, my symptoms had transitioned to persistent dizziness and mild brain fog throughout the day.
I underwent an MRI and various tests, but nothing was found.
In May, in addition to dizziness, I began to experience visual disturbances (slowed dynamic focus and delayed visual processing for both near and far objects).
Initially, I suspected the psychiatric medication was the issue, so I discontinued it, but there was no improvement.
By June, in addition to the aforementioned symptoms, I developed severe disorientation and visual-spatial difficulties.
In July, my condition worsened, with severe dizziness and the onset of cognitive issues, including significant memory loss (forgetting what I had for breakfast and struggling to differentiate between events from yesterday and a few days prior), severely impacting my daily life.
In mid-July, I had a lumbar puncture and an FDG PET scan; the doctor reported no inflammation (though protein levels were slightly elevated, around 62), and the PET scan indicated no metabolic abnormalities, leading to my discharge.
However, my symptoms continued to worsen, making daily life unbearable.
Therefore, in early August, I underwent a SPECT ECD brain perfusion scan.
I just reviewed the report, which showed many abnormalities, including findings from the nuclear medicine report indicating cingulate island sign, occipital hypoperfusion, and basal ganglia hyperperfusion, suggesting possible Dementia with Lewy Bodies (DLB).
Upon researching, I found that my symptoms closely resemble DLB.
I would like to ask the doctor how to interpret the discrepancy between the normal glucose PET report from July and the numerous abnormalities found in the August SPECT report.
Although my symptoms have indeed worsened, I was already feeling quite unwell in July.
Is it possible for a condition to appear completely normal one month and then suddenly show significant abnormalities the following month? Or is the SPECT ECD scan inherently more accurate than the PET scan? (The SPECT was performed at Changhua Show Chwan Hospital, which mentioned having analytical software, while the PET was done at China Medical University Hospital in Taichung.) Thank you, Doctor.

Haizi de ba, 30~39 year old female. Ask Date: 2022/08/05

Dr. Hong Weibin reply Neurology

Hello: FDG-PET and ECD-SPECT are both functional imaging tests that provide information about the current metabolic state of the brain.
The image quality can vary significantly between different machines, and there may also be differences in interpretation among various physicians.
Therefore, it is recommended to consider the test results as a reference rather than a definitive basis for conclusions.
These tests should be interpreted in conjunction with your symptoms.
It is advisable to discuss this further with your attending physician.

Reply Date: 2022/08/05

More Info

Understanding the discrepancies between PET (Positron Emission Tomography) and SPECT (Single Photon Emission Computed Tomography) imaging results can be quite complex, especially in the context of dementia and related neurodegenerative disorders. Your situation, characterized by a range of cognitive and perceptual symptoms, alongside the imaging findings, raises important questions about the interpretation of these scans.

Firstly, it’s essential to understand what each imaging modality measures. PET scans typically assess metabolic activity in the brain by using radiolabeled glucose (FDG-PET), which highlights areas of the brain that are more or less active. In contrast, SPECT scans evaluate blood flow and perfusion in the brain, often using different radiotracers. This fundamental difference means that the two modalities can yield different results depending on the underlying pathology.

In your case, the FDG-PET scan performed in July indicated no significant metabolic abnormalities, which could suggest that, at that time, the brain regions typically affected in dementia were still functioning within normal limits. However, the subsequent SPECT scan in August revealed significant abnormalities, including cingulate island sign, occipital hypoperfusion, and basal ganglia hyper-perfusion, which are suggestive of Dementia with Lewy Bodies (DLB). This discrepancy raises several points for consideration.

1. Temporal Changes: It is possible for neurodegenerative processes to evolve rapidly, especially in the context of dementia. The symptoms you experienced, such as severe cognitive decline and visual disturbances, may reflect a progression of the underlying disease that was not yet detectable in the PET scan. Neurodegenerative diseases can have fluctuating presentations, and changes in blood flow (as seen in SPECT) may occur before significant metabolic changes are evident on PET.

2. Sensitivity and Specificity: SPECT may be more sensitive to certain changes in brain perfusion that occur in specific types of dementia, such as DLB. The findings of hypoperfusion in specific regions can be indicative of the disease process, even when metabolic activity appears normal. This does not necessarily mean that SPECT is "more accurate" than PET; rather, they provide complementary information that can help in diagnosing and understanding the disease.

3. Clinical Correlation: The interpretation of imaging results should always be correlated with clinical symptoms. Your reported symptoms of cognitive decline, visual disturbances, and severe disorientation align with the findings from the SPECT scan. This correlation strengthens the case for a diagnosis of DLB, which is characterized by fluctuating cognition, visual hallucinations, and parkinsonism.

4. Further Evaluation: Given the significant discrepancies between the two imaging studies and the worsening of your symptoms, it would be prudent to discuss these findings with your healthcare provider. They may consider further evaluations, including neuropsychological testing, additional imaging, or even a trial of symptomatic treatment, to better understand the nature of your cognitive decline.

In conclusion, while both PET and SPECT are valuable tools in the assessment of dementia, they provide different insights into brain function. The normal PET results do not negate the findings from the SPECT scan, especially in light of your clinical symptoms. It is crucial to maintain an open dialogue with your healthcare team to ensure that all aspects of your condition are thoroughly evaluated and managed appropriately.

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