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11C-flumazenil PET, volumetric MRI, and quantitative pathology in mesial temporal lobe epilepsy
M J Koepp1, M P Richardson, C Labbé
1MRC Cyclotron Unit, Hammersmith Hospital, London, UK.
Background:
Using statistical parametric mapping and 11C-flumazenil (FMZ) PET we have previously shown reduction of central benzodiazepine receptor (cBZR) binding restricted to the hippocampus in mesial temporal lobe epilepsy (mTLE) due to hippocampal sclerosis (HS). The limited spatial resolution of PET, however, results in partial-volume averaging that affects quantitative analysis of cBZR density.
Method:
We determined hippocampal volume loss and reduction in cBZR binding using an MRI-based method for partial-volume effect correction of 11C-FMZ volume of distribution (FMZ-Vd) in 17 patients with refractory mTLE and an MRI diagnosis of HS that was subsequently histologically verified in all cases. Quantitative neuropathology was performed with assessment of neuron density in 14 of the 17 patients. Absolute FMZ-Vd and asymmetry indices (FMZ-AI) were compared before and after partial-volume effect correction with MRI-determined hippocampal volumes (HCV), hippocampal T2 measurements, and, if available, neuronal cell densities.
Results:
Compared with 15 age-matched healthy volunteers, significant reductions of absolute hippocampal FMZ-Vd were found before correction for partial-volume effects in 11 of 17 patients (65%) and only abnormal FMZ-AI in the other six patients. After partial-volume effects correction all 17 patients (100%) showed both significant unilateral reduction of absolute FMZ-Vd and abnormal FMZ-AI. There was no correlation between corrected absolute FMZ-Vd and HCV or neuronal cell density. After correction for partial-volume effect we found a mean 38% reduction of FMZ-Vd in the sclerosed hippocampus, over and above the reduction of HCV.
Conclusion:
Correction for partial-volume effect allows absolute quantitation of FMZ-PET and increases its sensitivity for detecting abnormalities in TLE due to HS. The lack of correlation between cBZR binding and neuronal density implies that atrophy with neuron loss is not the sole determinant of reduced cBZR binding in patients with mTLE and hippocampal sclerosis.
Insights
Partial-volume correction enhances PET imaging sensitivity for detecting central benzodiazepine receptor (cBZR) binding reductions in mesial temporal lobe epilepsy (mTLE) with hippocampal sclerosis (HS). This method reveals abnormalities in all patients, independent of neuronal density.
Area of Science:
- Neuroimaging
- Epilepsy Research
- Molecular Imaging
Background:
- Mesial temporal lobe epilepsy (mTLE) with hippocampal sclerosis (HS) is associated with reduced central benzodiazepine receptor (cBZR) binding in the hippocampus.
- Positron Emission Tomography (PET) using 11C-flumazenil (FMZ) is employed to assess cBZR binding.
- Partial-volume averaging in PET limits accurate quantification of cBZR density.
Purpose of the Study:
- To improve the quantitative analysis of cBZR binding in mTLE with HS.
- To assess the impact of MRI-based partial-volume effect correction on FMZ-PET data.
- To investigate the relationship between cBZR binding, hippocampal volume, and neuronal density.
Main Methods:
- Seventeen patients with refractory mTLE and confirmed HS underwent FMZ-PET and MRI.
- An MRI-based method was used to correct for partial-volume effects in the FMZ volume of distribution (FMZ-Vd).
- Absolute FMZ-Vd and asymmetry indices (FMZ-AI) were compared before and after correction, alongside hippocampal volume and neuronal cell density.
Main Results:
- Before correction, only 65% of patients showed reduced FMZ-Vd, and six had abnormal FMZ-AI.
- After partial-volume correction, 100% of patients exhibited significant unilateral reduction in FMZ-Vd and abnormal FMZ-AI.
- A mean 38% reduction in FMZ-Vd was observed in the sclerotic hippocampus post-correction, independent of hippocampal volume or neuronal density.
Conclusions:
- Partial-volume correction significantly enhances the sensitivity of FMZ-PET for detecting cBZR binding abnormalities in mTLE with HS.
- The findings suggest that factors beyond hippocampal atrophy and neuron loss contribute to reduced cBZR binding in this patient group.
- This corrected quantitative approach improves diagnostic accuracy for mTLE.