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Comparison of ictal SPECT and interictal PET in the presurgical evaluation of temporal lobe epilepsy.

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Comparison of Ictal SPECT and Interictal
PET in the Presurgcal Evaluation of
Temporal Lobe Epilepsy
Susan S. Ho, FRACP, MRCP(UK),* Samuel F. Berkovic, MD, FRACP," Salvatore U. Berlangieri, FRACP,t$
Mark R. Newton, MD, FRACP,"f Gary F. Egan, PhD,I- Henri J. Tochon-Danguy, PhD,I
and W. John McKay, FRACPI-$
~
We retrospectively compared ictal technetium Wm hexarnethylpropyleneamineoxime single-photon emission computed tomography (SPECT) and interictal '*F-fluorodeoxyglucose positron emission tomography (PET) in 35 patients
with well-lateralized temporal lobe epilepsy (TLE). Based on SPECT scans the two observers correctly lateralized
seizure foci with certainty in 89% of patients; interobserver agreement was excellent. Both observers incorrectly
lateralized the seizure focus on two SPECT scans; one error was explained by rapid electroencephalographic spread
to the contralateral side and for the other patient, isotope was injected during a brief aura. Based on PET scans,
observers correctly lateralized the foci with certainty in 63% and with lesser confidence in 83%; four incorrect
lateralizations were made by one observer and none by the other. PET interobserver disagreement was explained by
differences between observers in weighting the relative hypometabolism in medial and lateral temporal regions. The
detection rate for PET was lower in the absence of structural imaging abnormalities (60 vs 87%). PET yielded correct
lateralizations in the 2 patients for whom SPECT interpretation was difficult. We conclude that both ictal SPECT and
interictal PET are sensitive methods for the lateralization of TLE, but SPECT can be interpreted with greater certainty
and is more sensitive when magnetic resonance imaging findings are negative. False lateralization is rare with ictal
SPECT and can be explained when interpreted in conjunction with electroclinical data. Both investigations have
complementary roles when localization is difficult.
Ho SS, Berkovic SF, Berlangieri SU, Newton MR, Egan GF, Tochon-Danguy HJ, McKay WJ.
Comparison of ictal SPECT and interictal PET in the presurgical evaluation
of temporal lobe epilepsy. Ann Neurol 1995;37:738-745
Positron emission tomography (PET) and ictal singlephoton emission computed tomography (SPECT) are
established tools for the presurgical evaluation of
temporal lobe epilepsy (TLE) 11-51. They investigate
different aspects of altered cerebral function in epilepsy r6-91 and some practical differences exist. Technetium 99m hexamethylpropyleneamineoxime (""'TcHMPAO) SPECT provides an indirect measurement
of cerebral blood flow (CBF) changes, and can be
performed in both interictal and ictal states but ictal
studies demand concurrent inpatient videoelectroencephalographic (video-EEG) monitoring. In contrast,
'*F-fluorodeoxyglucose (FDG) PET measures changes
in cerebral glucose metabolism and has a higher spatial
resolution and more reliable quantitation than SPECT,
but the temporal resolution of PET with 18F-FDG is
unfavorable for ictal studies [ 10- 121.
In patients with complex partial seizures, "F-FDG
PET has a sensitivity of 60 to 90% for the detection
of interictal temporal lobe hypometabolism {2- 5 , 13151, with the higher sensitivities in more recent reports
attributed to newer-generation scanners with better
spatial resolution. Ictal '"Tc-HMPAO
SPECT shows
a focal increase in regional CBF in approximately 95%
of patients with TLE 11, 16, 171. In contrast, postictal
and interictal SPECT studies are less helpful and localize the epileptic focus in only 70 and 40% of patients,
respectively {I, 18, 191. Previous studies compared interictal SPECT and PET in the same population of
epileptic patients 115, 20, 21) and showed the superiority of PET.
The sensitivities reported for ictal SPECT and interictal PET are similar but they have not been directly
compared. This is important for two reasons. First, it
is essential to determine whether these two functional
imaging techniques provide complementary or entirely
redundant information for the localization of seizure
foci. Second, these techniques place different eco-
From the 'Department of Neurology, Universiry of Melbourne,
$Centre for Positron Emission Tomography, and $Department of
Nuclear Medicine, Austin Hospital, Melbourne, Victoria, Australia.
Received Oct 27, 1994, and in revised form Jan 18, 1995. Accepted
for publication Jan 19, 1995.
Address correspondence t~ D~~ ~ ~D~~~~~~~~
k ~ of~Neurology,
, ~ ,
Austin Hospital, Heidelberg (Melbourne), Victoria 3084, Australia.
738 Copyright 0 1995 by the American Neurological Association
nomic strains on comprehensive epilepsy units. PET
has a large capital cost, yet interictal PET can be performed as a n outpatient procedure. SPECT is less expensive b u t ictal studies require special organization t o
achieve and may prolong the inpatient hospital stay.
Here, we did not perform an economic analysis but
t h e data do provide insight as to the relative benefits
of the two procedures.
Materials and Methods
We retrospectively studied interictal 18F-FDG PET and ictal
""Tc-HMPAO SPECT scans of all patients with welllateralized TLE who underwent evaluation at the Austin Hospital's Comprehensive Epilepsy Programme between August
1992 and February 1994. Subjects were selected from a
larger population of patients with refractory complex partial
seizures and were excluded if localization of the seizure focus
was undetermined or extratemporal, or if functional imaging
data were unavailable. PET and SPECT studies were available
to the clinician during the decision-making process for temporal lobectomy. These findings were not used in selecting
patients for inclusion in this retrospective analysis. All subjects gave informed consent for functional imaging studies
and the protocol was approved by the Austin Hospital Human Ethics Committee.
Determination of Temporal Lobe Focus
Seizure focus localization for this retrospective analysis was
determined by a congruence of clinical seizure characteristics,
ictal electroencephalography (EEG), magnetic resonance imaging (MRI), and neuropsychological assessment. Five patients had depth electrode studies. A majority of the patients
subsequently underwent temporal lobectomy with the finding of significant pathological lesions in the resected tissues.
Assessment of correct lateralization of the seizure focus was
further confirmed by postoperative outcome, with a followup period of at least 1 year.
"""Tc-HMPAO Single-Photon Emission
Computed Tomography
Patients underwent continuous video-EEG monitoring and
were injected with 550 to 700 MBq (15-20 mCi) of """TcHMPAO during ongoing seizure activity by methods previously described [22). Interictal studies were performed when
patients had been seizure free for at least 24 hours. All were
scanned within 2 hours after the '"Tc-HMPAO injection.
Subjects studied between 1992 and mid-1993 (n = 28) were
scanned with a single-head rotating gamma camera (400 AC
Starcam, General Electric Medical Systems, Milwaukee, WI)
and SPECT data were acquired as previously described [l8].
Subjects studied after mid-1993 (n = 7) were scanned with
a triple-headed, dedicated head scanner (Triad XLT, Trionix
Research Laboratory, OH). With the triple-headed scanner
using Anger logic for localization of events, the entire brain
was imaged at once. The camera was operated in the "stop
and shoot" mode with acquisition at 3-degree intervals, acquiring 40 views at 40 seconds per interval. Low-energy,
high-resolution, fan-beam collimators were used. System resolution was measured at approximately 10-mm full width at
half maximum (FWHM) in all planes at the center of the
field. A 20% symmetrical energy window was used on the
140-keV peak of OO"'Tc.Data were processed by filtered backprojection using a general Metz (multiplier 30) prefilter and
Butterworth (0.5 cycle/cm and order 10) filter. Attenuation
correction was made at a coefficient of 0.11. Pixels of 1.78
mm were generated on a matrix size of 256 x 128.
During reconstruction, the midsagittal image was identified
and axial slices were generated in the plane of a line drawn
from the inferior surface of the frontal lobe to the most
posterior aspect of the occipital pole to achieve optimal display of the temporal lobes 118). Coronal images were reconstructed perpendicular to this plane.
"F-Fluorodeoxyghcose Positron Emission Tomography
Interictal FDG PET studies were performed with the patients
maintained on their usual antiepileptic drugs and when they
had been seizure free for at least 24 hours. Subjects received
intravenous injections of 260 to 370 MBq (7-10 mCi) of
FDG, and during the 30-minute uptake period, were kept
in a quiet, dimly lit room with the eyes covered but the
ears not plugged. Scanning was performed with the ECAT
positron tomograph (95 1/3lR, CTI Siemens, Knoxville,
TN). Spatial resolution was 6.5 mm at FWHM. Scanning was
initiated at approximately the orbitomeatal line and 3 1 slices
of 3.375-mm thickness parallel to the orbitomeatal plane
were obtained for each patient. A Hanning filter with a 0.4
cycle/pixel cutoff and scatter correction was used. Pixels of
2.325 mm were generated on a matrix size of 128 x 128
x 31. After reconstruction, axial slices were generated in
the plane of a line drawn from the inferior surface of the
frontal lobe to the posterior aspect of the occipital pole to
achieve optimal display of the temporal lobes as previously
described for SPECT reconstruction. Coronal images at 90
degrees to this plane were also reformatted for analysis.
Data Analysis
SPECT and PET scans were visually analyzed by two pairs
of independent blinded observers who were selected on
the basis of their experience in the techniques (S. F. B. and
M. R. N. for SPECT; S. U. B. and W. J. M. for PET). Scans
were presented in random order to the observers who were
blinded to the patients' identity and any localizing information. The observers analyzed the scans independently and
were required to lateralize the seizure foci (left temporal or
right temporal). For confidence of seizure focus lateralization,
the observers graded the scans on a 5-point scale: 5 = definite Iateralization; 4 = probable lateralization; 3 = subtle
lateralization; 2 = probably normal; 1 = definitely normal.
Individual readings were scored as true positive, false positive, and false negative according to two scoring criteria. In
the first (strict) criterion, scans were considered positive only
if they were lateralized correctly and graded with a confidence of 4 or higher. In the second (less strict) criterion,
scans were considered positive if they were lateralized correctly and graded with a confidence of 3 or higher. W e used
the two scoring criteria to determine the sensitivities (percentage of true positives) for individual observers and for
"combined readings" of both observers for each technique.
When the two observers disagreed on the lateralization of
Ho et al: Ictal SPECT and Interictal PET in TLE
739
the seizure focus on a study, the “combined reading” was
considered negative or nonlateralizing. The investigators also
retrospectively reviewed all the discordant readings to identify reasons for the disagreement.
Quantitative analysis was not included in the present study
because the aim of the study was to test the clinical value
of ictal SPECT and interictal PET. From our own previous
experience of ictal SPECT 1231 and those of others in interictal PET [24],identification of areas of increased blood flow
or hypometabolism by simple visual inspection of scans appears to be of most practical value and quantitative measurements are not routinely used in clinical practice.
reviewed again in conjunction with the interictal study,
both observers lateralized the seizure focus correctly
because the equivocal hyperperfusion abnormality became more obvious. One scan was considered subtle
but the seizure focus was lateralized correctly by both
observers. Interobserver agreement for ictal SPECT
was excellent (K = 0.83 and 1.0 for the strict and less
strict criteria, respectively). Interobserver disagreement occurred in only 1 patient for whom the seizure
focus was correctly lateralized by both observers but
reported with different degrees of confidence.
Statistical Analysis
Interictal Positron Emission Tomography
Focal hypometabolism was correctly lateralized on interictal PET scans of 63% of patients, when the strict
criterion was used (see Table 2). With the less strict
criterion, foci were correctly lateralized in 83% of patients. Observer 2 had no false-positive interpretations
but Observer 1 incorrectly lateralized the foci on four
studies. Although the observers agreed on the majority
of true-positive findings, discordance in false-negative
and false-positive readings resulted in low K values
(K = 0.3 for strict criterion and K = 0.02 for less strict
criterion). This suggested that there were differences
between observers in the identification of abnormalities on the PET scans. Retrospective analysis showed
that the 4 patients for whom the seizure focus was
lateralized incorrectly by Observer 1 had bilateral temporal asymmetries (Fig 3). In these patients, Observer
2 had weighted medial temporal hypometabolism in
favor of lateral temporal changes whereas Observer 1
had placed more emphasis on the changes in the lateral
temporal cortex. This explanation accounted for the
four discordant lateralizations, and in all 4 patients mesial hypometabolism accorded with the correct lateralization.
We used Fisher’s exact test to compare categorical data. The
significance level was set a t p < 0.05. Interobserver reliability
for each test was compared using Cohen’s kappa (K) [25J
Poor agreement is denoted by K < 0.4; good reproducibility,
by 0.4 5 K 5 0.75; and excellent agreement, by K > 0.75.
Results
Thirty-five patients (20 men, 15 women) with a mean
age of 31.9 years (range, 10-52 years) were studied.
Three had mild intellectual disability and none had focal neurological signs. Localization of the epileptic foci
in the 35 patients was determined by a combination of
electroclinical data and specific focal structural abnormalities in the temporal lobes as summarized in Table
1. Nineteen of the 35 subjects had a well-lateralized
ictal surface EEG focus; in 5, lateralization was made by
depth electrodes, and 30 had structural abnormalities
detected on MRI. Thirty patients underwent temporal
lobectomy; in 28, specific lesions were found at pathology but in 2, no specific histological lesions were found
in the resected temporal tissues.
fctal Single-Photon Emission Computed Tomography
’‘mTc-HMPAO was injected during ongoing seizure
activity in all patients. The mean duration of seizures
studied was 90.8 seconds (range, 35-312 seconds); isotope was injected 30 to 129 seconds after seizure onset
and 2 to 234 seconds before seizure termination.
When the stricter criterion (grade 2 4)was applied,
ictal SPECT focal hyperperfusion was correctly lateralized by both observers in 89% of patients (Table 2).
When the less strict criterion (grade 2 3) was applied,
the sensitivity increased to 94% (Fig 1). In 2 patients
(Patients 16 and 22), the seizure foci were incorrectly
lateralized by both observers. Patient 16 had rapidly
spreading EEG abnormalities on depth electrode studies and video-EEG analysis revealed that at the time of
isotope injection, seizure activity had shifted to the
contralateral temporal lobe, with SPECT lateralization
reflecting this finding (Figs 2A, 2B). In Patient 22, isotope was injected during a brief aura while in all other
patients, isotope was injected during complex partial
seizures. When the ictal SPECT scan of Patient 22 was
740 Annals of Neurology Vol 37 No 6 June 1995
Comparison of Single-Photon Emission Computed
Tomography and Positron Emission Tomography
Ictal SPECT was significantly more sensitive than interictal PET in the whole population of 35 patients
when the strict criterion for confidence of lateralization
was applied ( p = 0.02). However, at a lower degree
of confidence, there was no significant difference between the sensitivities of the two tests. There were
more “subtle” readings with PET than SPECT and interobserver agreement was poor for PET. These findings suggest that ictal SPECT can be interpreted with
greater certainty than interictal PET. Interestingly,
however, PET yielded correct lateralization in the 2
patients for whom SPECT interpretation was difficult.
Correlation of Tomographic Findings with
Magnetic Resonance Imaging
Thirty patients had focal lesions detected on MRIs.
The sensitivities of SPECT and PET were compared
Table 1. Clinical Features, Ictal Electroencephalography (EEG), Magnetic Resonance Imaging (MRI), and Pathological Findings
Patient Age (yr)l Ictal Surface
No.
Sex
EEG Focus
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Final Focus
Lateralization Pathology
MRI
38lM
42lM
20lF
41lF
29lM
39lF
45lM
49lF
Nonlateralizing
L temporal
R temporal
R temporal
L temporal
L temporal
L temporal
Nonlateralizing
R hippocampal atrophy Right
38lF
45lM
21lM
41lM
191M
41lF
281F
52lM
25lM
21lF
33lM
341F
191M
33lM
35lF
43IF
32lM
34lM
171M
27/M
27lF
33lF
101M
111M
40lF
33lM
20lF
R temporal
Nonlateralizing
L temporal
R temporal
Nonlateralizing
Nonlateralizing
Nonlateralizing
Nonlateralizing'
L temporal
L temporal
Nonlateralizing
Nonlateralizing'
R temporal
Nonlateralizing'
L temporal
Nonlateralizing
L temporal
L temporal
L temporal
Nonlateralizing
Nonlateralizing'
Nonlateralizing'
Nonlateralizing
R temporal
Nonlateralizing
L temporal
L temporal
R hippocampal atrophy Right
L hippocampal atrophy Left
L hippocampal atrophy Left
Left
Right
Right
Left
Left
Left
Left
L hippocampal atrophy
Normal
R temporal lesion
L hippocampal atrophy
L hippocampal atrophy
Normal
L temporal lesion
Normal
L hippocampal atrophy
R hippocampal atrophy
L temporal lesion
L hippocampal atrophy
L temporal lesion
L hippocampal atrophy
R temporal lesion
R hippocampal atrophy
R hippocampal atrophy
L hippocampal atrophy
L hippocampal atrophy
R hippocampal atrophy
Normal
Normal
L hippocampal atrophy
L hippocampal atrophy
L hippocampal atrophy
L hippocampal atrophy
R temporal lesion
R temporal lesion
R hippocampal atrophy
L hippocampal atrophy
L hippocampal atrophy
Right
Left
Right
Left
Left
Left
Left
Right
Right
Right
Left
Left
Right
Left
Left
Left
Left
Left
Left
Right
hght
Right
Left
Left
Hippocampal sclerosis
Hippocampal sclerosis
N o surgery
Cavernous angiorna
Hippocampal sclerosis
Hippocampal sclerosis
N o surgery
Dysembryoplastic neuroepithelial
tumor
Hippocampal sclerosis
Hippocampal sclerosis
Hippocampal sclerosis
Normal
Hippocampal sclerosis
Surgical
Outcomea
IA
h
IA
b
h
IA
IA
h
IA
b
h
Hippocampal sclerosis
Ganglioglioma
Hippocampal sclerosis
IA
Astrocytomal hamartoma
N o surgery
Tuber
N o surgery
Hippocampal sclerosis
IA
Hippocampal sclerosis
Hippocampal sclerosis
Hippocampal sclerosis
Normal
N o surgery
Hippocampal sclerosis
Hippocampal sclerosis
IB
b
IID
b
IA
b
IA
h
h
h
Hippocampal sclerosis
Hippocampal sclerosis
Ganglioglioma
IB
IA
Ganglioglioma
Hippocampal sclerosis
Hippocampal sclerosis
Hippocampal sclerosis
IA
IA
IA
b
h
"Outcome classified according to Engel [29] at a minimum of 12 months' follow-up
bPostoperative follow-up less than 12 months.
'Lateralization made by depth electrodes.
IA
=
seizure free since surgery; IB
=
aura only since surgery; IID
in the presence and absence of structural imaging abnormality (Tables 3, 4). For this analysis, we used the
less stringent criterion for scoring the SPECT and PET
studies because on this criterion, there were fewer discordant readings between the observers; when the observers disagreed on the lateralization of the foci on a
particular study, we considered the study negative or
nonlateralizing. We found that PET had a detection
rate of 87% in the presence of an MRI abnormality
compared to a detection rate of 60% in its absence
( p = 0.16). There was no significant difference in
=
nocturnal seizures only, which cause no disability.
SPECT sensitivities in the presence or absence of MRI
abnormalities.
Cowelation of Tomographic Findings with
Electroencepbalograpby
Nineteen of the patients had well-lateralized ictal surface EEG foci, and in 5 patients, the foci were lateralized by depth electrode studies. For the comparative
analyses of SPECT and PET with EEG, we also used
the less stringent criterion for scoring the scans. For
both SPECT and PET, there were no significant differ-
H o et al: Ictal SPECT and Interictal PET in TLE
741
Table 2. Blinded Anahsis of lctal Single-Photon Emission
Computed Tomography (SFECT) and lnterictal Positron
Emission Tomography PET)
ences in the sensitivities in the presence or absence of
surface EEG foci.
Criterion:
Grade 4 or
Greater = Positive
Discussion
Previous studies directly comparing interictal SPECT
and PET in patients with TLE suggested a better diagnostic value of PET compared with interictal SPECT
115, 20, 211. Our previous experience also suggested
that interictal SPECT has a low sensitivity and is of
limited value in the preoperative evaluation of patients
with intractable complex partial seizures [l, 17, 231.
In contrast, ictal ""'Tc-HMPAO SPECT was shown to
have a sensitivity of approximately 95% for the lateralization of temporal lobe seizure foci 11, 171. Until recently, ictal SPECT was considered a logistically difficult procedure compared to interictal studies which
were less demanding and can be performed in all patients. With the recent development of a method for
rapid preparation and injection of isotope during seizures 1221, ictal SPECT is now a practical and reliable
method of seizure localization in patients undergoing
evaluation for temporal lobectomy. A rational functional neuroimaging strategy that includes the combined deployment of ictal SPECT and interictal PET
has not been formulated previously. We therefore
compared the sensitivities and clinical value of both
ictal SPECT and interictal PET in the same population
of patients with TL.E. This was a retrospective analysis
and functional imaging data had been used unblinded
for clinical decision making; thus absolute sensitivities
cannot be extrapolated to the whole population of patients with TLE.
The present study confirmed that both ictal SPECT
and interictal PET are reliable methods for the lateralization of temporal lobe seizure foci. However, our data
TP
PET"
Observer 1
Observer2
Combinedb
SPECT
Observer 1
Observer2
Combined'
Criterion:
Grade 3 or
Greater = Positive
FN FP TP
FN FP
24 (69%) 9
29 (83%) 6
63%
2
0
31 (89%) 0
33 (94%) 2
83%
4
32 (915%) 1
31 (89%) 2
2
2
33 (94%) 0
33 (94%) 0
94 5%
2
2
89%
0
an = 35 for each technique.
bCombined readings indicate true-positive cases where both observers' findings were concordant.
TP
=
true positive; FN
=
false negative; FP
=
false positive.
Fig I . lnterictal "F$uorodeoxyghmse posityon emission tomography ("F-FDG PET) and ictal technetium 99m hexamethylpropyleneamineoximesingle-photon emission computed tomography ("""Tc-HMPAO SPECT) studies of a patient with l&t
temporal lobe epilepsy (Patient 28) interpreted as definite4 abn o m l (grade 5 ) by all blinded observers. Arrowheads denotefocal hypometabolism on intevictal PET and focal byperperfuion
on ictal SPECT.
742 Annals of Neurology Vol 37 No 6 June 1995
Fig 2. Patient 16. (A) False lateralizing ictal single-photon
emission computed tomography (SPECT) scan. This patient had
lejit temporal lobe epilepsy. During seizures he had rapid electroencephalographicspread to the right temporal lobe and ictal
SPECT shows right temporal hyperperfirsion (white arrow), reflecting the electroencephalographicchange. Interictal positron
emission tomography (PET) shows correct latevalization with focal hypometabolism in the lejit temporal lobe (black arrow). (B)
Seizure recorded through bilateral hippocampal electrodes and
studied with ictal SPECT. The seizure started in the lgt hippocampus (LAT,) and within 7 seconds spread to the right hippocampus (RAT,) with rhythmic recruitment in the LATI-, electrodes. Isotope was injected at 41 seconds from seizure onset.
During this time, ictal activity was present only in the right
hippocampal electrodes and slow waves were present in the ldt
temporal region. The seizure terminated at 79 seconds from onset.
suggest that ictal SPECT is more sensitive and can be
interpreted with a greater degree of certainty than interictal PET. Ictal SPECT is superior to interictal PET,
particularly in patients not showing MRT abnormalities.
However, in the group with structural lesions seen radiologically, the two techniques had comparable detection rates. The lower detection rate of PET in the absence of structural lesions was similarly reported by
Ryvlin and colleagues 1151 in their series of 20 patients
with TLE, and also in studies of extratemporal lesions
113, 26-28). This finding suggests that irreversible
functional changes related to structural damage at least
partially contribute to the zones of hypometabolism
seen on FDG PET. Engel and coworkers 1241 found
that the degree of relative hypometabolism measured
by PET correlated well with the severity of the parhological lesion, but the size of the hypometabolic zone
was generally much larger than the area of pathological
involvement. They suggested that this discrepancy was
most likely to represent either structural abnormalities
below the resolution of routine histopathological stud-
Ho et al: Ictal SPECT and Interictal PET in TLE
743
Fig 3. An example of discordant positron emission tomography
(PET) interpretation (Patient 30). Observer 1 placed more emphasis on the right lateral temporal hypometabolism (two black
arrowheads, reader's left) whereas Observer 2 placed more emphasis on the left medial temporal hypometabolism (single black
arrowhead). Ictal single-photon emission computed tomography
(SPECT) shows cowect lateralization with focal hyperperfusion
in the left temporal lobe (white arrowhead).
ies (e.g., loss of synapses), or functional inactivation of
neuronal elements associated with the epileptogenic
lesion (e.g., functional suppression of glucose utilization either due to inhibition or inactivation of relatively
healthy neuronal elements or due to variance in vascular perfusion). Furthermore, these changes could explain the situation where interictal PET was diagnostically helpful while tests reflecting dynamic changes
were misleading, as in the patient with rapidly shifting
EEG foci during seizures (Patient 16).
Table 3. Cowelation of Positron Emission Tomography (PET)
and Magnetic Resonance Imaging (MRI) Findings a
PET +
PETTotal
MRI+
MRI-
Total
26
3
29
4
2
30
5
6
35
"Fisher's exact test ( p ) = 0.16.
Table 4. Cowelation of Single-Photon Emission
Computed Tomography (SPECT) and Magnetic
Resonance Imaging (MRI) Findingsa
MRI
SPECT+
SPECTTotal
+
28
2
30
MRI -
Total
5
33
0
5
35
False lateralizations occurred uncommonly with the
two types of functional neuroimaging. False lateraliza-
tions for ictal SPECT were explained when scans were
interpreted in conjunction with electroclinical data.
Hence, the specificity of ictal SPECT is likely to be
related to the constancy and time sequence of focal
ictal onset and its propagation. False lateralizations for
interictal PET in the present study were explained by
interobserver differences in scan interpretation. We
found that PET scans can be visually analyzed more
accurately by the identification of medial rather than
lateral temporal hypometabolic abnormalities. The reason for the greater reliability of medial temporal hypometabolism for lateralization is unclear but it may reff ect the high representation of mesial temporal
pathology (hippocampal sclerosis) in the present study.
Engel and associates [ 2 ] also found a very low frequency of false lateralizations with FDG PET in their
larger series of 153 patients and the false lateralizations
were attributed to artifacts induced by depth electrodes
in place at the time of the FDG PET studies. False
lateralizations made by Observer 1 cannot be attributed to this problem.
The results of this study have the following implications for the presurgical evaluation for partial epilepsy.
First, both interictal PET and ictal SPECT are reliable
confirmatory tests for the laterahation of TLE, and
there is no evidence that patients need both functional
neuroimaging tests when one test is congruent with
scalp/sphenoidal EEG and MRI. However, both ictal
SPECT and interictal PET have complementary roles
where localization is difficult. Particularly, ictal SPECT
is helpful in patients not showing MRI abnormalities
and interictal PET is helpful in patients showing rapid
EEG contralateral spread of seizure activity.
2
"Fisher's exact test ( p ) = 0.73.
744 Annals of Neurology Vol 37 No 6 June 1995
Dr H o is the recipient of a Ciba-Geigy (Australia) Epilepsy Fellowship and a National Health and Medical Research Council Postgraduate Medical Scholarship.
We are grateful to our colleagues at the Austin Hospital in the
Departments of Neurology (Dr Peter F. Bladin), Neurosurgery (Mr
Gavin Fabinyi and Mr Graeme Brazenor), and Anatomical Pathology
(Dr Renate Kalnins) and the technical staff of the Department of
Nuclear Medicine and the PET center for their assistance.
15.
16.
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Ho et al: Ictal SPECT and Interictal PET in TLE 745
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