Dr Takayuki Obata, Department of Medical Imaging, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan. Email: email@example.com
Abstract The aim of this study was to evaluate the usefulness of proton and phosphorus (1H and 31P) magnetic resonance spectroscopy (MRS) for temporal lobe epilepsy (TLE) patients, and to evaluate neural damage and metabolite dysfunction in the TLE patient brain. We performed 1H and 31P MRS of medial temporal lobes (MTL) in the same TLE patients (n = 14) with a relatively wide range of severity from almost seizure-free to intractable, and calculated the ratio of N-acetylasparate to choline-containing compounds and creatine + phosphocreatine (NAA/Cho + Cr) in 1H MRS and inorganic phosphate to all main peaks (%Pi) in 31P MRS. There was no significant correlation between NAA/(Cho + Cr) and %Pi in each side (ipsilateral, r = −0.20; contralateral, r =−0.19). The values of NAA/(Cho + Cr) showed a significant difference between ipsilateral and contralateral MTLs to the focus of TLE patients (P < 0.01, paired t-test). Although %Pi also had a tendency to show the laterality of TLE, there was no significance. Ipsilateral (r = −0.90, P < 0.0001) and contralateral (r = −0.70, P < 0.005) NAA/(Cho + Cr) decreases and contralateral %Pi increase (r = 0.81, P < 0.001) had significant correlation with seizure frequency. 1H MRS provides more important information concerning neuronal dysfunction in MTL of TLE patients than 31P MRS.
Surgical treatment for medically intractable temporal lobe epilepsy (TLE) is an effective procedure in selected patients in whom the epileptogenic focus is localized in a unilateral anterior and medial temporal lobe (MTL).1 Accurate localization of the epileptogenic focus has largely and traditionally been dependent on a scalp-sphenoidal electroencephalogram (EEG). Recently, magnetic resonance imaging (MRI), positron emission tomography (PET) and single photon emission computed tomography (SPECT) have provided important additional information about the epileptogenic focus.2,3 However, subdural EEG electrode implantation is still required when imaging and scalp EEG are inconclusive. Thus, a more sensitive and non-invasive neurological measurement modality is needed that interictally lateralizes the epileptogenic focus and provides useful information about the pathological changes of TLE.
Magnetic resonance spectroscopy (MRS) is widely used in the field of psychiatry and clinical neuroscience.4–17 Proton and phosphorus magnetic resonance spectroscopies (1H and 31P MRS) were non-invasive functional neurological measurement modalities. Proton MRS of the brain includes signals from N-acetylasparate (NAA), choline-containing compounds (Cho) and creatine + phosphocreatine (Cr). There have been several studies that observed a reduction in the NAA signal or its ratio to other metabolic signals in the ipsilateral temporal lobe to the epileptogenic focus in TLE patients, suggesting that NAA concentration reflects neural damage.4–13 The values of inorganic phosphate (Pi) and phosphocreatine (PCr) that can be detected in 31P MRS change according to the condition of oxidative phosphorylation, and they were reported to be good indexes for detecting brain metabolite dysfunction.14–16
Magnetic resonance nuclei have been used in TLE studies, but studies using both nuclei MRS of the same TLE patients have hardly been reported. In the present study, we aimed to evaluate the 1H and 31P MRS signal changes in bilateral temporal lobes of TLE patients with a relatively wide range of severity from almost seizure-free to intractable, and to clarify the relation between neural damage and metabolite dysfunction in the TLE patient brain. Some 1H MRS data in this study was from that already reported by Someya et al.11
This study was approved by the Ethics and Radiation Safety Committee of the National Institute of Radiological Sciences, Chiba, Japan. We studied 14 patients, four males and 10 females, aged 31.5 ± 8.4 years (mean ± SD), who were diagnosed with the amygdala-hippocampus type of TLE on the basis of clinical symptoms and EEG findings.18 They were asked to participate in this study, and written informed consent was obtained. The patients showed unilateral temporal spikes or sharp waves in their repetitive (three or more) scalp EEG and/or stereotactic implanted depth electrode EEG record. The data of the patients are summarized in Table 1. Seven healthy volunteers, five males and two females, aged 27.0 ± 5.4 year (mean ± SD), were also studied as controls. No significant difference was observed in gender (P = 0.06; Pearson's χ2 test) or age (t = 1.37, P = 0.19; t-test) between controls and patients.
The MR system used in this study was a Gyroscan ACS2 operated at 1.5T (Philips Medical Systems Ltd, Best, The Netherlands). 1H and 31P MRS were performed with a birdcage-type coil. The volumes of interest (VOIs) were set at the bilateral MTL under the guidance of proton scout images, with their size being 6 × 3 × 3 cm so as to include all parts of the hippocampus: the head, body, tail, gray and white matter, and amygdala. For 1H MRS, volume selection was done by spin echo sequence for each side. Repetition time (TR) was 1500 msec. The echo time (TE) used was 136 msec because lactate peaks are visible at this TE. There were 1024 data points, and the bandwidth was 2000 Hz. Scan averages were 256 times and scan time was 6.4 min for each side. For 31P MRS, the volume selection was done by two-VOI image-selected in vivo spectroscopy (2-VOI ISIS) sequence. TR was 3000 msec. Data points were 1024 and the bandwidth was 2000 Hz. The scan time was 20 min (384 scans).
To evaluate the structural brain changes, approximately 90 coronal MR images, covering the entire brain, were taken for each subject. An inversion recovery pulse sequence (TR, 2500 msec; TE, 20 msec; inversion time, 300 msec) was used. Matrix size was 205 × 256, and the FOV was 230 mm. Slice thickness was 2 mm without slice gaps. Total scan time was 21 min and 25 s. The coronal slice images were parallel to the baseline of the fourth ventricle. Atrophy of the MTL was judged by two experienced specialists, a radiolologist and a neuropsychiatrist, while were blind to the EEG lateralization.
Data processing and analysis
The spectra in 31P MRS were processed with a 15-Hz exponential filter in the time domain. No filter was used in 1H MRS. To measure the area under each peak, a semi-automatic line-fitting routine assuming a purely Lorentzian line shape was used. NAA intensities were evaluated as relative ratio to the total ratio of Cho and Cr. The Pi value was shown as relative value to the total value of all the peaks (phosphoesters, Pi, phosphocreatine, and adenosine triphosphate).
The paired t-test was used for comparisons of ipsilateral and contralateral MTL to the EEG focus, and the unpaired t-test was used for comparisons of control and patient MTL. Logarithmic transformation was used to normalize the data of seizure frequency. Pearson product-moment correlation analysis was used to evaluate the correlation between each set of MRS data and seizure frequency. Differences were considered to be significant at P < 0.05.
There was no significant correlation between NAA/(Cho + Cr) and %Pi on each side (ipsilateral r = −0.20; contralateral r = −0.19). Atrophy of MTL was detected by MRI in four patients (Table 1). Their lateralities were consistent with the EEG and MRS lateralities except for one case that had lower %Pi in the ipsilateral MTL.
Proton MRS lateralization based on a reduced NAA/(Cho + Cr) ratio was in agreement with the clinical EEG lateralization in 12 of 14 patients, and a significant difference between the ipsilateral and contralateral MTL (P < 0.01, paired t-test) (Table 2) was obtained. The NAA/(Cho + Cr) ratio in the ipsilateral or contralateral MTL was not significantly different from that in MTL of control subjects. Two cases with disagreement between the results of 1H MRS and clinical data were in the low seizure-frequency group (< 1/month).
Table 2. Relative signal intensity of proton and phosphorus MRS
NAA/(Cho + Cr) in the ipsilateral and contralateral MTL decreased as the seizure frequency increased. Both ipsilateral and contralateral NAA/(Cho + Cr) were significantly correlated with seizure frequency (r = −0.90, P < 0.0001; r = − 0.70, P < 0.005, respectively; Fig. 1a).
Phosphorus MRS lateralization based on an increase in %Pi ratio was consistent with the clinical EEG lateralization in 10 of 14 patients, but there was no significant difference (Table 2). The %Pi value in the ipsilateral or contralateral MTL was not significantly different from that in MTL of control subjects. In 10 of 14 patients, the %Pi values in the ipsilateral MTL were higher than the mean value of normal control. They were very scattered, and had no significant correlation with the seizure frequency (r = 0.09, Fig. 1b). Four cases with disagreement between the results from 31P MRS and clinical data were in the high seizure-frequency group (> 1/month). The %Pi values in the contralateral MTL of all the low seizure-frequency group patients were lower than the mean value of the normal control, while those in six of eight patients in the high seizure-frequency group were higher than the mean value of normal control, resulting in significant correlation (r = 0.81; P < 0.001, Fig. 1b).
Significant changes in the NAA/(Cho + Cr) ratio of 1H MRS and the %Pi ratio of 31P MRS in the medial temporal lobes of TLE patients were observed. Both seem useful for the detection of TLE lateralization, but they have different tendencies.
The values of NAA/(Cho + Cr) in 1H MRS showed significant differences between ipsilateral and contralateral MTL of TLE patients. Although %Pi also had a tendency to show the laterality of TLE, there was no significance. This indicates that 1H MRS may be more useful for generally assessing the lateralization of the TLE focus. Most previous studies have shown that TLE patients have lower NAA values in their ipsilateral MTL in 1H MRS. As N-acetylasparate is mainly located in neural cells, a reduction in NAA value is believed to reflect some neural damage.3–13 In contrast, the concentrations of Cho and Cr are much higher in astrocytes and oligodendrocytes than in neurons,19 meaning that changes in their values may reflect reactive astrocytosis. Thus, the decrease in NAA/(Cho + Cr) ratios in ipsilateral MTL may reflect neuronal loss and reactive gliosis as observed in medial temporal sclerosis in TLE patients. We also found a significant correlation between NAA/(Cho + Cr) and the seizure frequency of TLE patients, suggesting that 1H MRS is useful not only for assessment of TLE lateralization but also for evaluation of TLE severity. Significant correlation between contralateral NAA/(Cho + Cr) and the seizure frequency was also detected, suggesting that neural damage due to TLE extended to the contralateral MTL in the high-seizure frequency group.
Although the mean value of %Pi in ipsilateral MTL was higher than in normal controls, the values were widely scattered, and no significant correlation with the seizure frequency was detected. The mean value of %Pi in the contralateral MTL in the low seizure-frequency group (< 1/month) was lower than that of normal controls. The %Pi increase is believed to reflect dysfunction of energy metabolism.16 These results may indicate a hyper-energy status in the contralateral MTL as compensation for dysfunction in the ipsilateral MTL.15 In contrast, the mean value of %Pi in the contralateral MTL in the high seizure-frequency group (> 1/month) was higher than that of normal controls. This seems to indicate a spreading of neural dysfunction to the contralateral MTL. These quite different conditions under different seizure frequencies in contralateral %Pi produced significant correlation with seizure frequency.
The NAA/(Cho + Cr) ratio in the ipsilateral MTL has the largest correlation coefficient with seizure frequency among the four spectroscopy data sets, suggesting that the NAA/(Cho + Cr) ratio in the ipsilateral MTL is the best index for showing TLE severity. The lateralization of the TLE focus is of major importance, especially for presurgical decisions for intractable TLE. In the present study, the lateralization of TLE in the low seizure-frequency group (< 1/month) was detected more clearly using 31P MRS, while 1H MRS showed highly significant lateralization detectability in the high seizure-frequency (> 1/month) group. Two cases with disagreement between the 1H MRS results and clinical data were in the low seizure-frequency group, but four cases with disagreement between the 31P MRS results and clinical data were in the high seizure-frequency group. In total, 1H MRS seems more useful for clinical application.
Although only four patients showed atrophies, the MTL atrophies detected by MRI were located on the ipsilateral side to the focus, and all of the four had smaller NAA/(Cho + Cr) ratios in ipsilateral MTL than in contralateral MTL. Some investigations have reported that morphological MRI study of TLE patients is very useful for evaluating the severity and laterality of TLE.20,21 The reason for the low atrophy detectability by MRI in the present study may lie in the fact that we did not use a pulse sequence with specificity for evaluating MTL atrophy.
In conclusion, we performed 1H and 31P MRS for detecting the lateralization of the TLE focus. There was no significant correlation between NAA/(Cho + Cr) and %Pi. The values of NAA/(Cho + Cr) in 1H MRS showed significant differences between ipsilateral and contralateral MTL of TLE patients. The decrease in ipsilateral NAA/(Cho + Cr) had the largest correlation coefficient with seizure frequency. These results suggest that 1H MRS provides more important information than 31P MRS concerning neuronal dysfunction in MTL of TLE patients.
This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan, by funds for the Research Project with Heavy Ions at the National Institute of Radiological Sciences – Heavy-ion Medical Accelerator in Chiba (NIRS-HIMAC), by the fund for the Neuroscience Project at the National Institute of Radiological Sciences, and by a Postdoctoral Fellowship from the Japan Science and Technology Corporation.