MRI-Based Correction for Partial-Volume Effect Improves Detectability of Intractable Epileptogenic Foci on ¹²³I-Iomazenil Brain SPECT Images

Abstract

123 I-Iomazenil brain SPECT has been used for the detection of epileptogenic foci, especially when surgical intervention is considered. Although epileptogenic foci exhibit a decrease in 123 Iiomazenil accumulation, normal cerebral cortices often exhibit similar findings because of thin cortical ribbons, gray matter atrophy, or pathologic brain structures. In the present study, we created 123 I-iomazenil SPECT images corrected for gray matter volume using MRI and tested whether the detectability of the epileptogenic foci improved. Methods: Seven patients (1 male patient and 6 female patients; mean age 6 SD, 34 6 17 y) with intractable epilepsy were surgically treated by resecting the cerebral cortex after surface electroencephalography. Histopathologic examination of the resected specimens and a good outcome after surgery indicated that the resected lesions were epileptogenic foci. These patients underwent 123 I-iomazenil SPECT and 3-dimensional T1-weighted MRI examinations before their operations. Each SPECT image was coregistered to the corresponding MR image, and its partial-volume effect (PVE) was corrected on a voxel-by-voxel basis with a smoothed gray matter distribution image. Four nuclear medicine physicians visually evaluated the 123 I-iomazenil SPECT images with and without the PVE correction. The SPECT count ratio of the suspected focus to the contralateral cerebral cortex was evaluated as an asymmetry index (%) based on the volume of interest. Results: The sensitivity, specificity, and accuracy of focus detection by visual assessment were higher after PVE correction (88%, 99%, and 98%, respectively) than before correction (50%, 92%, and 87%, respectively). The mean asymmetry index for the surgically resected lesions was significantly higher on the PVE-corrected SPECT images (22%) than on the PVEuncorrected ones (16%) (P 5 0.006). Conclusion: MRI-based PVE correction for 123 I-iomazenil brain SPECT improves the sensitivity and specificity of the detection of cortical epileptogenic foci in patients with intractable epilepsy.

Distribution of Cortical Benzodiazepine Receptor Binding in Right-Handed Healthy Humans: A Voxel-Based Statistical Analysis of Iodine 123 Iomazenil SPECT with Partial Volume Correction

“…A previous study, however, demonstrated a dissociation between rGMC in epileptogenic foci and the iomazenil BP. 2 In cases of focal cortical dysplasia, for example, increased rGMC in the lesion annuls the decrease in the BP and makes it difficult to detect the focus on PVE-uncorrected SPECT images. Thus, voxel-based statistical analysis with PVE correction and consideration of the normal rGMC changes in the assessment of CBR binding may be essential to obtain accurate diagnostic assessments by I-123 iomazenil SPECT.…”

“…2 PVE correction for the BP images, therefore, was performed by using 3D T1-weighted MR imaging data by using a method that was basically the same as our previously presented method for qualitative I-123 iomazenil SPECT images (Fig 1). 2 The I-123 iomazenil BP images were coregistered with the rGMC maps by using the Linear Image Registration Tool (FLIRT) of Functional MR Imaging of the Brain (http://www.fmrib.ox.ac.uk/). 15 In this procedure, the BP images were simultaneously reformatted to matrices with the same voxel size as the referenced rGMC maps in the native space.…”

“…Binary mask images for the gray matter were created based on the rGMC maps to eliminate voxels with a very low rGMC (ie, voxels located at irrelevant positions away from the gray matter), because such voxels might amplify the noise in the PVE correction. The threshold for determining the boundary of the binary mask images was set even lower (1% of the maximum rGMC, an empirically determined value) than that used in previous studies 2,13 to minimize the error produced by the smoothing of the masked images before the voxel-based statistical estimation. These masked images based on the rGMC were then applied to the coregistered BP images.…”

“…The masked BP images were then divided by using the rGMC map on a pixel-by-pixel basis. The threshold for masking, on the other hand, was set at 35% of the maximum rGMC (the same as the threshold value used in previous studies 2,13 ) when displaying the PVE-corrected BP images (Fig 1).…”

“…We recently demonstrated that PVE correction for I-123 iomazenil SPECT is necessary for the correct detection of epileptogenic foci by using I-123 iomazenil SPECT. 2 The brain volume is thought to change remarkably with disease or normal aging. 3 The purpose of this study was to assess how PVE produced by regional age-related changes in gray matter volume influences I-123 iomazenil SPECT findings and to clarify age-related changes in CBR binding in healthy humans in a manner that was independent of the regional gray matter volume by using an SPM analysis for I-123 iomazenil SPECT with PVE correction.…”