Comparison of ectopic versus normal gray matter via quantitative magnetic resonance imaging: a cross-sectional study

Introduction

Gray matter heterotopia (GMH) refers to the ectopic migration of cortical neurons into the white matter, but the etiology of GMH is not fully understood. Genetic factors and neuronal migration impairment, often due to infection, during the 6th to 16th week of pregnancy are considered the primary causes (1).

GMH often exhibits extensive connections with the normal visual cortex, including the hippocampus and contralateral hemisphere, potentially resulting in epileptic-like activity (2,3). Epilepsy in patients with GMH is often focal and resistant to drug treatment. Therapy typically involves resection or minimally invasive multitarget ablation (4). Magnetic resonance imaging (MRI) can identify the anatomical location of the gray matter; however, it cannot ascertain whether functional differences exist between GMH and normal gray matter (NGM) within the cerebral cortex (5). Generally, ectopic gray matter is best visualized with the T1-weighted brain volume (T1 BRAVO) imaging sequence (6). Heterotopia may be manifest as large nodular isointense masses on MRI, sometimes simulating tumors. In such cases, proton magnetic resonance spectroscopy can indicate normal peaks of the metabolites or discrete relative N-acetylaspartate reduction (7).

Arterial spin labeling (ASL) and MRI compilation (MAGiC) sequences in MRI can quantitatively reveal subtle differences that are not observable to the naked eye (8,9). In this study, ASL and MAGiC were used to identify the differences in perfusion and structure between GMH and NGM, thereby laying the groundwork for the further determination of epileptogenic gray matter. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-1237/rc).

Methods

Participants

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments, and was approved by the ethics committee of the Second Affiliated Hospital, Jiangxi Medical College, Nanchang University (No. O-2024-259). The requirement for informed consent was waived due to the retrospective nature of the study.

This study enrolled all patients diagnosed with GMH at the Second Affiliated Hospital, Jiangxi Medical College, Nanchang University, from October 2022 to December 2023 via MRI examinations including T1 BRAVO, T2-weighted imaging (T2WI), MAGiC, and ASL sequences [postlabeling delays (PLDs) of 1.5 s and 2.5 s]. The inclusion criteria were as follows: (I) complete imaging data (T1 BRAVO, T2WI, MAGiC, and ASL, with PLDs of 1.5 s and 2.5 s), (II) the presence of at least one cortical-like signal nodule within the normal brain white matter, and (III) a history of at least three epileptic seizures or abnormal findings on electroencephalography (EEG). Meanwhile, the exclusion criteria included the following: (I) incomplete imaging or clinical data, (II) poor image quality or artifacts, and (III) MRI contraindications.

MRI scanning protocol

In this study, the SIGNA Architect 3.0 T MRI system (GE HealthCare, Chicago, IL, USA) was used to conduct the T1 BRAVO, T2WI, MAGiC, and ASL sequences (with PLDs of 1.5 s and 2.5 s). The scanning parameters for T1 BRAVO were a repetition time (TR) of 7,800 ms, an echo time (TE) 31 of ms, a slice thickness of 1 mm, a field of view (FOV) of 256 mm × 230 mm, a matrix size of 256×256, and a flip angle of 8°; for T2WI, they were a TR of 4,500 ms, a TE of 120 ms, a slice thickness 5 mm, a slice gap of 1.5 mm, an FOV 240 mm × 240 mm, a matrix size of 320×256, and a flip angle of 111°; and for ASL, they were a TR of 4,838 ms, a TE of 57.4 ms, a resolution of 512, 8 arms, a number of excitations (NEX) of 3, and a slice thickness of 3 mm. Meanwhile, the MAGiC sequence (GE HealthCare), which is based on 2D fast spin echo technology, employs the multidynamic multiecho principle, using alternating 120° saturation pulses and a multiecho collection strategy. In a single scan, MAGiC facilitates the simultaneous acquisition of 5 quantitative maps and 10 contrast images, enabling the quantitative measurement of relaxation metrics including T1, T2, and proton density (PD) values. In this study, the parameters for the axial MAGiC sequence were an FOV of 24 cm × 19.2 cm, a TE 21.4 of ms, a TR 4,000 of ms, a matrix size of 320×256, a slice thickness of 5 mm, a slice gap of 1.5 mm, an NEX of 1, and a scan time of 4 min.

Quantitative image measurements

The data were manually measured by two radiologists with 2 years of work experience. GMH was measured as the mean of 1–3 region of interest (ROI) measurements while NGM was measured as the mean of 1–3 ROI measurements in the ipsilateral hemisphere. Normal white matter (NWM) was measured as the mean of 1–3 ROI measurements in the frontal or occipital lobes of the ipsilateral hemisphere. If a patient had more than one GMH, each was recorded separately during quantitative imaging measurements but treated as one patient for clinical data statistics. T1, T2, and PD values of GMH and ipsilateral NGM were measured separately for each patient. The raw cerebral blood flow (CBF) maps from ASL were imported into the Advantage Workstation 4.7 (GE HealthCare), and the GMH and NGM values were measured under PLDs of 1.5 s and 2.5 s, respectively. The quantitative values obtained from measurements of GMH and NGM were normalized against the contralateral white matter visually identified as normal. The resulting relative quantitative values were recorded as relative GMH (rGMH) and relative NGM (rNGM), respectively.

Statistical methods

The statistical analysis of the data was conducted with SPSS 26.0 (IBM Corp., Armonk, NY, USA), OriginPro 2023 (OriginLab, Northampton, MA, USA) with a significance level set at P<0.05. Component concordance between two radiologists was evaluated via Bland-Altman plots. For continuous variables, the Shapiro-Wilk test was used to assess the normality of their distribution. If the data followed a normal distribution, they were recorded as the mean ± standard deviation (x¯±s). To analyze the differences between the quantitative values of GMH and NGM on the same side before and after standardization, a paired-samples t-test was used. This study evaluated the differences in the quantitative values of GMH before and after standardization as compared to those of NGM on the same side.

Results

Between October 2022 and December 2023, nine patients (32 GMHs) with epilepsy were enrolled according to the inclusion criteria and exclusion criteria (Figure 1).

Figure 1 Patient selection flowchart. MAGiC, magnetic resonance image compilation; MRI, magnetic resonance imaging; T1 BRAVO, T1-weighted brain volume imaging.

Nine patients, including six males and three females with a median age of 21 years, were enrolled in this study. After EEG testing, aberrant EEG waves were found in six patients. There are also some patients with psychomotor development and other central nervous system malformations. The general condition of the patients is shown in Table 1.

Five patients exhibited bilateral GMH. Consequently, a total of 32 GMH nodules were assessed along with the associated normal gray and white matter. Subsequently, each nodule was statistically evaluated for CBF, T1, T2, and PD via color maps (Figure 2).

Figure 2 Example plot of a patient with gray matter heterotopia. (A) T1-weighted brain volume imaging. (B,C) Quantitative color maps of arterial spin labeling (postlabeling delays of 1.5 s and 2.5 s). (D-F) Quantitative color maps of T1, T2, and proton density. The red arrow indicates an ectopic gray matter nodule.

The two radiologists measured the CBF, T1, T2, and PD values of GMH and NGM, with high concordance, as illustrated in the Bland-Altman plot shown in Figure S1.

The CBF values of NGM, GMH, with PLDs of 1.5 s and 2.5 s were all normally distributed among patients, as illustrated in Figure 3.

Figure 3 The CBF values of gray matter heterotopia and normal gray matter. CBF, cerebral blood flow; GMH (1.5), gray matter heterotopia with a postlabeling delay of 1.5 s; GMH (2.5), gray matter heterotopia with a postlabeling delay of 2.5 s; NGM (1.5), normal gray matter with a postlabeling delay of 1.5 s; NGM (2.5), normal gray matter with a postlabeling delay of 2.5 s.

The mean CBF values of GMH with dual PLDs (31.96 and 35.13 mL/100 g/min) were both lower than those of NGM (52.69 and 56.93 mL/100 g/min). The paired-sample t-test (Table 2) indicated that the CBF values of GMH with dual PLDs were statistically different from those of NGM (P<0.001). This result remained significant after standardization (P<0.001). The differences in CBF values between GMH and NGM were calculated separately for each PLD time, with no significant difference attributable to PLD time being observed (P=0.500).

The T1, T2, and PD values of GMH and NGM all followed a normal distribution, as shown in Figure 4. Paired-sample t-tests were conducted to evaluate the differences between GMH and NGM before and after standardization (Table 3). The T1 values before and after standardization exhibited statistically significant differences (P<0.05) in both GMH and NGM. Additionally, the T1 values of GMH were all lower than those of NGM.

Figure 4 The T1, T2, and PD values of GMH and NGM. GMH, gray matter heterotopia; NGM, normal gray matter; PD, proton density; T1, T1 relaxation time; T2, T2 relaxation time.

Discussion

ASL is a noninvasive technique that labels proton spins in arterial blood and uses them as an endogenous tracer, allowing for quantitative depiction of CBF perfusion (10). ASL has a wide range of applications in the central nervous system, with diverse purposes including the identification of perfusion defects in ischemic strokes, the evaluation of CBF perfusion in brain tumors, and the assessment of perfusion during epileptic seizures (11-13). This study found there to be differences in CBF values between GMH and NGM at PLDs of 1.5 s and 2.5 s, which persisted after standardization. The mean CBF values of NGM were higher than those of GMH. Within the heterotopic nodules, there is a decrease in α-CaMKII and N-methyl-D-aspartate receptor NR2A/B subunits, along with immature GABAergic neurons. This collectively contributes to an imbalance in excitability of the GMH (14). All investigations took place during the interictal phase of the patients’ epilepsy, during which GMH typically exhibits hypoexcitability and reduced perfusion. The lack of statistical significance in the CBF differences between NGM and GMH at PLDs of 1.5 and 2.5 s suggests that NGM and GMH exhibit similar perfusion characteristics in both the early and late phases. However, GMH located within the white matter receives less perfusion as compared to the gray matter on the cortical surface.

The MAGiC sequence allows for the quantitative assessment of PD, T1, and T2 values in a single scan and is thus widely used for scans of the central nervous system. Kern et al. found that quantitative T2 values are meaningful for grading brain gliomas and identifying mutations in the isocitrate dehydrogenase gene (15). Additionally, the time of stroke onset is associated with changes in T2 fluid-attenuated inversion recovery (FLAIR) signal and T2 values in the stroke region (16). Quantitative imaging can also aid in the diagnosis of multiple sclerosis and epilepsy (17). Patients with epilepsy often display pathological microstructural tissue remodeling and astrocyte proliferation, which is potentially linked to sustained changes in neuronal activity (18). Subtle gray matter damage has been found in the cortex and thalamus of patients with multiple sclerosis (17). Quantitative PD values often serve as surrogate markers of tissue atrophy, with an increase in PD values indicating enlargement of interstitial spaces and thus a decrease in local tissue volume fraction (19,20). Quantitative T1 values are sensitive to changes in tissue water content, myelin content, and tissue iron deposition (21), typically reflecting reduced interstitial spaces, increased proliferation, and decreased water content, which result in decreased T1 values (22). Quantitative T2 values are often associated with gliosis and disruption of the blood-brain barrier. Changes in gliotic tissue can lead to an increase in tissue T2 values or affect the measurement of T2 values (23-25). Ectopic gray matter arises due to inadequate blood supply, leading to neuronal loss and tissue structure atrophy. The resultant damage from the abnormal discharge of ectopic gray matter may further induce pathological alterations, such as the demyelination of ectopic gray matter neurons and iron deposition in surrounding tissues. Therefore, in this study, we examined the differences between GMH and NGM before and after the normalization of quantitative T1 values. However, due to the limited sample size and minimal changes in PD and T2 values observed in this study, no significant differences were detected between GMH and NGM.

The study, for the first time, used quantitative MRI (ASL and MAGiC) to examine the differences in perfusion and signals between GMH and NGM. Our findings lay a theoretical foundation for the further determination of the true epileptogenic gray matter by studies employing a larger sample size. However, several limitations to this study should be noted. First, we employed a single-center design with a small sample size, which might have introduced potential biases. Second, although differences in perfusion and function between GMH and NGM were identified, we did not determine whether there are functional differences across GMH nodules in different locations. Future studies could analyze whether GMH nodules in different locations differ in their impact on the occurrence of epilepsy. Third, the literature suggests (26) that the occurrence of epilepsy may be associated with multiple GMH nodules or single GMH. In this study, there were five patients with bilateral GMH, but it was not determined which side of the heterotopic gray matter was the true epileptogenic focus.

Conclusions

NGM and GMH demonstrate quantitative differences on ASL and MAGiC sequences. This study offers quantitative data for differentiating GMH from NGM, establishing groundwork for future research in this field.

ASL, which assesses brain perfusion without intravenous injection of any contrast material, might be useful in the evaluation of neuronal activity of subcortical band heterotopia and thus in the detection of the epileptogenic area (27).

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-24-1237/rc

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-1237/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments, and was approved by the ethics committee of the Second Affiliated Hospital, Jiangxi Medical College, Nanchang University (No. O-2024-259). The requirement for informed consent was waived due to the retrospective nature of the study.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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