Cortical and subcortical networks in human secondarily generalized tonic-clonic seizures

Generalized tonic-clonic seizures are among the most dramatic physiological events in the nervous system. The brain regions involved during partial seizures with secondary generalization have not been thoroughly investigated in humans. We used single photon emission computed tomography (SPECT) to image cerebral blood flow (CBF) changes in 59 secondarily generalized seizures from 53 patients. Images were analysed using statistical parametric mapping to detect cortical and subcortical regions most commonly affected in three different time periods: (i) during the partial seizure phase prior to generalization; (ii) during the generalization period; and (iii) post-ictally. We found that in the pre-generalization period, there were focal CBF increases in the temporal lobe on group analysis, reflecting the most common region of partial seizure onset. During generalization, individual patients had focal CBF increases in variable regions of the cerebral cortex. Group analysis during generalization revealed that the most consistent increase occurred in the superior medial cerebellum, thalamus and basal ganglia. Post-ictally, there was a marked progressive CBF increase in the cerebellum which spread to involve the bilateral lateral cerebellar hemispheres, as well as CBF increases in the midbrain and basal ganglia. CBF decreases were seen in the fronto-parietal association cortex, precuneus and cingulate gyrus during and following seizures, similar to the 'default mode' regions reported previously to show decreased activity in seizures and in normal behavioural tasks. Analysis of patient behaviour during and following seizures showed impaired consciousness at the time of SPECT tracer injections. Correlation analysis across patients demonstrated that cerebellar CBF increases were related to increases in the upper brainstem and thalamus, and to decreases in the fronto-parietal association cortex. These results reveal a network of cortical and subcortical structures that are most consistently involved in secondarily generalized tonic-clonic seizures. Abnormal increased activity in subcortical structures (cerebellum, basal ganglia, brainstem and thalamus), along with decreased activity in the association cortex may be crucial for motor manifestations and for impaired consciousness in tonic-clonic seizures. Understanding the networks involved in generalized tonic-clonic seizures can provide insights into mechanisms of behavioural changes, and may elucidate targets for improved therapies.

Clinical use of ictal SPECT in secondarily generalized tonic-clonic seizures

Partial seizures produce increased cerebral blood flow in the region of seizure onset. These regional cerebral blood flow increases can be detected by single photon emission computed tomography (ictal SPECT), providing a useful clinical tool for seizure localization. However, when partial seizures secondarily generalize, there are often questions of interpretation since propagation of seizures could produce ambiguous results. Ictal SPECT from secondarily generalized seizures has not been thoroughly investigated. We analysed ictal SPECT from 59 secondarily generalized tonic-clonic seizures obtained during epilepsy surgery evaluation in 53 patients. Ictal versus baseline interictal SPECT difference analysis was performed using ISAS (http://spect.yale.edu). SPECT injection times were classified based on video/EEG review as either pre-generalization, during generalization or in the immediate post-ictal period. We found that in the pre-generalization and generalization phases, ictal SPECT showed significantly more regions of cerebral blood flow increases than in partial seizures without secondary generalization. This made identification of a single unambiguous region of seizure onset impossible 50% of the time with ictal SPECT in secondarily generalized seizures. However, cerebral blood flow increases on ictal SPECT correctly identified the hemisphere (left versus right) of seizure onset in 84% of cases. In addition, when a single unambiguous region of cerebral blood flow increase was seen on ictal SPECT, this was the correct localization 80% of the time. In agreement with findings from partial seizures without secondary generalization, cerebral blood flow increases in the post-ictal period and cerebral blood flow decreases during or following seizures were not useful for localizing seizure onset. Interestingly, however, cerebral blood flow hypoperfusion during the generalization phase (but not pre-generalization) was greater on the side opposite to seizure onset in 90% of patients. These findings suggest that, with appropriate cautious interpretation, ictal SPECT in secondarily generalized seizures can help localize the region of seizure onset.

Evaluating the accuracy of perfusion/metabolism (SPET/PET) ratio in seizure localization

The uncoupling between brain perfusion and metabolism was evaluated as a potential tool for seizure localization by creating an interictal SPET divided by interictal PET functional ratio-image and by evaluating its sensitivity and specificity to areas subsequently surgically resected. The uncoupling between brain perfusion and metabolism was evaluated through the creation of a functional SPET/PET ratio-image relying on interictal single-photon emission computed tomography (SPET) and positron emission tomography (PET) scans in epilepsy patients. The uncoupling of these two physiological brain functions has been demonstrated to be a characteristic of epileptogenic tissue in temporal lobe epilepsy and could potentially serve as a diagnostic measure for localization of seizure onset areas in the brain. The accuracy of hemispheric localization, sensitivity, and specificity of perfusion to metabolism ratio-images were evaluated as compared to standard methods of PET reading.

METHODS

Interictal HMPAO-SPET and FDG-PET scans were obtained from 21 patients who then went on to remain seizure free for a minimum of 1 year post surgical resection. Using Statistical Parametric Mapping (SPM2), the SPET and PET scans were spatially registered and spatially normalized to a standard template (geometric warping). A functional image was created by calculating the ratio of perfusion to metabolism. Discrete areas of uncoupling in the ratio-images were selected, quantified, and compared to visually interpreted PET readings as well as the actual site of subsequent surgical resection. Localization was determined by comparing the hemispheric location of these areas to sites of surgical resection. Sensitivity and specificity of ratio-images and PET readings were calculated by dividing the brains into four sections per hemisphere.

RESULTS

When compared to known sites of successful surgical resection, the pre-surgical visually interpreted PET readings had a correct hemispheric localization in 69.6% of cases, while the regions of uncoupling selected in the pre-surgical ratio-images had a correct hemispheric localization of 82.6%. In addition, the regional sensitivity of visually interpreted PET readings was 63.0% with a specificity of 95.7%, while the sensitivity of the ratio-images was 68.0% with a specificity of 96.0%.

CONCLUSION

Compared to the PET readings, the ratio-images yielded similar sensitivity and specificity measures, but had an improved hemispheric localization. Hence, ratio-images may be a valuable diagnostic tool in the hemispheric localization, which could enhance the use of PET readings alone.

Interictal 99mTc-HMPAO SPECT in temporal lobe epilepsy: relation to clinical variables

PURPOSE

Factors affecting blood flow observed by interictal single-photon emission computed tomography (SPECT) images in temporal lobe epilepsy (TLE) have not been systematically studied or consistently demonstrated. We evaluated interictal SPECT results with respect to many clinical variables in a large population of TLE patients, all of whom underwent temporal lobectomy.

METHODS

Interictal 99mTc-HMPAO SPECT scans from 61 TLE patients were obtained before an anterior temporal lobectomy. SPECT was analyzed using a region of interest analysis (ROI) in the cerebellum, anterior temporal lobe, lateral temporal lobe, mesial temporal lobe, whole temporal lobe, and inferior frontal lobe. Asymmetry indices (AIs) were calculated. Correlative analysis of AIs and clinical variables was performed.

RESULTS

The AIs from TLE patients differed significantly from those of controls in the anterior temporal (p < 0.01), lateral temporal (p < 0.001), and whole temporal (p < 0.01) regions. No consistent overall correlation between the AIs and clinical variables existed. In right TLE (RTLE) only, AIs in the lateral and whole temporal lobe were positively correlated with age of onset (r = 0.470, p < 0.05; r = 0.548, p < 0.01, respectively). Similarly, in RTLE only, duration of epilepsy was negatively correlated with the anterior (r = -0.395, p < 0.05) and mesial (r = -0.45, p < 0.05) temporal lobe AI. No correlations were found between clinical variables and AIs in left TLE (LTLE) patients.

CONCLUSIONS

Significant correlation of age at onset and duration of epilepsy with AIs in RTLE but not LTLE suggests physiologic processes may be determined in part by laterality of TLE. Clinical applications are problematic.

Estimating tissue deformation between functional images induced by intracranial electrode implantation using anatomical MRI

This paper examines a solution to the general problem of accurately relating points within functional data acquired before and after subdural intracranial electrode implantation. We develop an approach based on nonrigid registration of high resolution anatomical MRI acquired together with the functional data. This makes use of a free-form B-Spline deformation model and registration is recovered by maximizing the normalized mutual information between the preimplant MRI and the postimplant MRI. We apply the approach to estimate the tissue deformation induced by the presence of intracranial electrodes over 15 patient studies. Maximum tissue displacements of 4 mm or greater were observed in all cases either in the cortex or around the ventricles due to CSF loss. In studies involving larger 4 x 4 grids, local tissue displacement estimates exceeded 10 mm from the preimplant brain shape. The key issue with this approach is whether the deformation estimates are contaminated by the presence of susceptibility-induced imaging artifacts. We therefore evaluate the deformation estimates in recovering alignment of essentially identical SPECT studies of eight patients acquired before and after electrode placement. An ROI-based analysis of the variance of resulting subtraction values between pre- and postimplant SPECT was carried out in regions of tissue below electrode grids. Results indicate for all cases a substantial reduction in residual SPECT subtraction artifacts to a level comparable to that in an equivalent region of undeformed tissue.

Ratio-images calculated from interictal positron emission tomography and single-photon emission computed tomography for quantification of the uncoupling of brain metabolism and perfusion in epilepsy

PURPOSE

Image processing techniques were applied to interictal positron emission tomography (PET) and single-photon emission computed tomography (SPECT) brain images to aid in the localization of epileptogenic foci by calculating a functional image that represents the degree of coupling between perfusion and metabolism. Uncoupling of these two functions has been demonstrated to be a characteristic of epileptogenic tissue in temporal lobe epilepsy and has the potential to serve as a diagnostic measure for localization in other areas as well.

METHODS

Interictal PET ((18)F-FDG) and interictal SPECT ((99m)Tc-HMPAO) scans were acquired from 11 epilepsy patients. The metabolism and perfusion images were three-dimensionally spatially registered, and a functional ratio-image was computed. These functional maps are overlaid onto a three-dimensional rendering of the same patient's magnetic resonance imaging anatomy.

RESULTS

In all patients, an average uniform perfusion-to-metabolism ratio showed approximately constant values throughout most of the whole brain. However, the epileptogenic area (confirmed on surgery) demonstrated an area of elevated perfusion/metabolism in the grey matter.

CONCLUSIONS

Although hypometabolism in the PET image was observed in most of these patients, the calculation of a functional ratio-image demonstrated localized foci that in some cases could not be observed on the PET image alone. The ratio-image also yields a quantitative measure of the uncoupling phenomenon.

Decreased cerebral blood flow during seizures with ictal SPECT injections

Increased regional cerebral blood flow (rCBF) at the epileptogenic site has been consistently reported for single photon emission computed tomography (SPECT) injections made during seizure activity, and the increased rCBF has been shown to remain elevated at the epileptogenic site in some cases, even when SPECT injections are made after seizure termination (postictal). A sustained increase in rCBF after seizure cessation was recently confirmed, but for no more than 100 s from seizure onset [Avery, R.A., Spencer, S.S., Spanaki, M.V., Corsi, M., Seibyl, J.P., Zubal, I.G., 1999. Effect of injection time on postictal SPET perfusion changes in medically refractory epilepsy. Eur. J. Nucl. Med. 26, 830-836]. In the current study, it is examined whether ictal SPECT injections demonstrate a similar change in rCBF around 100 s from seizure onset. Twenty-one patients with medically refractory epilepsy and a known area of seizure onset receiving ictal and interictal 99mTc-Hexamethyl-propyleneamineoxime (HMPAO) SPECT scans were studied. The results of SPECT subtraction analysis which visualize increased and decreased rCBF were compared to seizure duration and HMPAO injection time. Five patients received ictal SPECT injections (during ongoing seizure activity) more than 90 s after seizure onset and demonstrated decreased rCBF. Two of these patients also demonstrated areas of increased rCBF. Decreased rCBF was localized to the epileptogenic lobe in four of the five patients. By examining ictal SPECT injections made 90 s after seizure onset, evidence was found that reduced rCBF may exist during ictus. The change in rCBF around 90 s is also observed in postictal injections, suggesting a common metabolic mechanism may be responsible.

Reproducibility of serial peri-ictal single-photon emission tomography difference images in epilepsy patients undergoing surgical resection

Peri-ictal single-photon emission tomography (SPET) difference images co-registered to magnetic resonance imaging (MRI) visualize regional cerebral blood flow (rCBF) changes and help localize the epileptogenic area in medically refractory epilepsy. Few reports have examined the reproducibility of SPET difference image results. Epilepsy patients having two peri-ictal and at least one interictal SPET scan who later underwent surgical resection were studied. Localization accuracy of peri-ictal SPET difference images results, interictal electroencephalography (EEG), and ictal EEG from the first (seizure 1) and second (seizure 2) seizure, as well as MRI and positron emission tomography (PET) findings, were compared using surgical resection site as the standard. Thirteen patients underwent surgical resection (11 temporal lobe and 2 extratemporal). SPET results from seizure 1 were localized to the surgical site in 12/13 (92%) patients, while SPET results from seizure 2 were localized in 13/13 (100%) patients. All other modalities were less accurate than the SPET results interictal EEG--seizure 1 6/13 (46%); ictal EEG--seizure 1 5/13 (38%); interictal intracranial EEG--seizure 2 4/9 (44%); ictal intracranial EEG--seizure 2 results 8/9 (89%); MRI 6/13 (46%); PET 9/13 (69%)[. SPET results were reproducible in 12/13 (92%) patients. SPET difference images calculated from two independent peri-ictal scans appear to be reproducible and accurately localize the epileptogenic area. While SPET difference images visualize many areas of rCBF change, the quantification of these results along with consideration of injection time improves the diagnostic interpretation of the results.

The role of quantitative ictal SPECT analysis in the evaluation of nonepileptic seizures

Nonepileptic seizures may represent difficult diagnostic problems. Identifying their presence and frequency is critical for determining appropriate treatment. The authors investigated the value of quantitative perfusion changes as measured by ictal single-photon emission tomography (SPECT) difference images in differentiating nonepileptic from epileptic seizures. Eleven patients with a clinical suspicion of nonepileptic events had ictal and interictal technetium-99m hexamethylpropylene amine SPECT scans during continuous audiovisual surface electroencephalogram (EEG) monitoring. The authors analyzed perfusion difference images based on registration, normalization, and subtraction of ictal and interictal SPECT images. The difference images were registered to each patient's magnetic resonance imaging scan to anatomically localize ictal perfusion changes. Three of 11 patients also carried the diagnosis of epilepsy and were taking antiepileptic medication. Five patients were taking antiepileptic drugs, but the diagnosis of epilepsy was not confirmed. In all patients, continuous video EEG monitoring revealed no ictal EEG findings. In nine of these patients, visual interpretation of ictal SPECT was suggestive of localized increased (n = 6) or decreased perfusion (n = 3). In all patients, however, no blood flow changes were noted on quantitative SPECT analysis with injections performed during the seizure-like event, suggesting the diagnosis of pseudoseizures. The authors' results suggest that quantitative ictal SPECT analysis is a useful tool in the diagnosis of nonepileptic seizures.

Effect of injection time on postictal SPET perfusion changes in medically refractory epilepsy

Single-photon emission tomography (SPET) brain imaging in epilepsy has become an increasingly important noninvasive tool in localizing the epileptogenic site. Ictal SPET demonstrates the highest localization sensitivity as compared with postictal and interictal SPET. While ictal SPET consistently reveals hyperperfusion at the epileptogenic site, postictal SPET reveals either hyper- or hypoperfusion depending on the timing of radiopharmaceutical injection. Much discussion in the literature exists about exactly when the transition from hyper- to hypoperfusion occurs at the epileptogenic site in postictal SPET. The systematic examination of two clinical variables - time of injection from seizure onset and offset - was useful in understanding postictal perfusion changes. Twenty-seven patients with medically refractory epilepsy receiving postictal and interictal SPET scans were studied. Quantitative SPET difference imaging was used to evaluate perfusion changes in relationship to injection time. Perfusion changes were found to reflect the time of injection in relation to seizure onset, but to be somewhat independent of seizure offset. Thus, the majority of patients (8/12, 67%) receiving postictal injections within 100 s after seizure onset demonstrated hyperperfusion, while all patients (15/15, 100%) receiving postictal injections more than 100 s after seizure onset showed hypoperfusion. The explanation of this phenomenon is unknown but the findings appear to parallel known changes in cerebral lactate levels.

Sensitivity and specificity of quantitative difference SPECT analysis in seizure localization

True ictal SPECT can accurately demonstrate perfusion increases in the epileptogenic area but often requires dedicated personnel waiting at the bedside to accomplish the injection. We investigated the value of perfusion changes as measured by ictal or immediate postictal SPECT in localizing the epileptogenic region in refractory partial epilepsy.

METHODS

Quantitative perfusion difference images were calculated by registering, normalizing and subtracting ictal (or immediate postictal) from interictal SPECT for 53 patients with refractory epilepsy. Perfusion difference SPECT results were compared with visually interpreted SPECT, scalp electroencephalography (EEG), MRI, PET and intracranial EEG.

RESULTS

In 43 patients (81%), discrete areas of increased perfusion (with ictal injections) or decreased perfusion (with postictal injections) were noted. Interictal scalp EEG was localizing in 28 patients (53%), ictal scalp EEG was localizing in 35 patients (66%) and intracranial EEG was localizing in 22 patients (85%) (of 26 patients who underwent invasive study). MRI was localizing in 34 patients (64%), PET was localizing in 32 of 45 patients (71%), interictal SPECT was localizing in 26 patients (49%) and peri-ictal SPECT (visual interpretation) was localizing in 30 patients (57%). By comparison with an intracranial EEG standard of localization, SPECT subtraction analysis had 86% sensitivity and 75% specificity.

CONCLUSION

Our data provide evidence that SPECT perfusion difference analysis has higher sensitivity and specificity than any other noninvasive localizing criterion and can localize epileptogenic regions with accuracy comparable with that of intracranial EEG. To obtain these results, one must apply knowledge of the timing of the ictal injection relative to seizure occurrence.

Periictal SPECT localization verified by simultaneous intracranial EEG

PURPOSE

We investigated whether blood-flow changes measured by ictal or immediate postictal single photon emission computed tomography (SPECT) reflect with accuracy the actual location of ictal discharge as measured by simultaneous intracranial EEG. In addition, we evaluated the reliability of ictal SPECT obtained with implanted electrodes by comparing results with those of ictal SPECT performed during scalp EEG monitoring in selected patients.

METHODS

Eleven patients with intractable partial epilepsy who had both ictal and interictal SPECT scans during invasive EEG monitoring were studied. We analyzed perfusion differences based on registration, normalization, and subtraction of periictal and interictal SPECT images. SPECT results were interpreted in relation to location and evolution of ictal EEG change, as reflected by simultaneous intracranial EEG. In five patients, we also compared ictal SPECT results that were obtained during both scalp and intracranial EEG monitoring.

RESULTS

In 10 of 11 patients, localized increases or decreases in blood flow or both were identified in regions of ongoing or prior seizure discharge, respectively, at the time of SPECT brain perfusion. In one patient, SPECT localization could not be verified by the available electrode array.

CONCLUSIONS

Localization of ictal discharge during or before SPECT injection accurately determines increase or decrease in perfusion, respectively, and both are of equal validity in reflecting the region of epileptic discharge. SPECT perfusion changes can be reliably obtained during intracranial monitoring.

Influence of technetium-99m-hexamethylpropylene amine oxime injection time on single-photon emission tomography perfusion changes in epilepsy

By digitally computing perfusion changes from ictal or postictal (peri-ictal) injections referenced to those acquired interictally, an enhanced method for localizing the epileptogenic area is reported. Computer-based image processing methods for quantifying regional percent change in the brain are applied to a group of 19 epilepsy patients after the injection of technetium-99m hexamethylpropylene amine oxime (HMPAO) and after acquiring single-photon emission tomography (SPET) data. Each patient's region of epileptogenesis was independently localized through pathology and/or successful surgery. The positive and negative quantitative perfusion changes were plotted as a function of the time of the 99mTc-HMPAO ictal injection. This time scale was normalized relative to the seizure duration and is referenced to the time of seizure termination. Eight patients, injected ictally, demonstrated perfusion increases of 25%-100% in the area of known epileptogenesis. Five patients, injected immediately after seizure cessation, demonstrated excessive perfusion decreases of 30%-92% associated with the region of seizure onset. Six patients, injected well after seizure termination, demonstrated hypoperfusion changes less than 30% at the epileptogenic area. Observations on perfusion changes calculated from 99mTc-HMPAO SPET scans, as a function of normalized time, support a progression from ictal hyper- to excessive hypo-, then finally to persistent interictal hypoperfusion. By applying this perfusion pattern model and by noting the time of injection for peri-ictal images, an improved method for localizing the epileptogenic area is demonstrated.

Significance of nonuniform attenuation correction in quantitative brain SPECT imaging

The purposes of this study were to develop a method for nonuniform attenuation correction of 123I emission brain images based on transmission imaging with a longer-lived isotope (i.e., 57Co) and to evaluate the relative improvement in quantitative SPECT images achieved with nonuniform attenuation correction.

METHODS

Emission and transmission SPECT scans were acquired on three different sets of studies: a heterogeneous brain phantom filled with 1231 to simulate the distribution of dopamine transporters labeled with 2beta-carbomethoxy-3beta-(4-123I-iodophenyl)tropane (123I-beta-CIT); nine healthy human control subjects who underwent transmission scanning using two separate line sources (57Co and 123I); and a set of eight patients with Parkinson's disease and five healthy control subjects who received both emission and transmission scans after injection of 123I-beta-CIT. Attenuation maps were reconstructed using a Bayesian transmission reconstruction algorithm, and attenuation correction was performed using Chang's postprocessing method. The spatial distribution of errors within the brain was obtained from attenuation correction factors computed from uniform and nonuniform attenuation maps and was visualized on a pixel-by-pixel basis as an error image.

RESULTS

For the heterogeneous brain phantom, the uniform attenuation correction had errors of 2%-6.5% for regions corresponding to striatum and background, whereas nonuniform attenuation correction was within 1%. Analysis of 123I transmission images of the nine healthy human control subjects showed differences between uniform and nonuniform attenuation correction to be in the range of 6.4%-16.0% for brain regions of interest (ROIs). The human control subjects who received transmission scans only were used to generate a curvilinear function to convert 57Co attenuation values into those for 123I, based on a pixel-by-pixel comparison of two coregistered transmission images for each subject. These values were applied to the group of patients and healthy control subjects who received transmission 57Co scans and emission 123I scans after injection of 123I-beta-CIT. In comparison to nonuniform attenuation correction as the gold standard, uniform attenuation with the ellipse drawn around the transmission image caused an approximately 5% error, whereas placement of the ellipse around the emission image caused a 15% error.

CONCLUSION

Nonuniform attenuation correction allowed a moderate improvement in the measurement of absolute activity in individual brain ROIs. When images were analyzed as target-to-background activity ratios, as is commonly performed with 123I-beta-CIT, these outcome measures showed only small differences when Parkinson's disease patients and healthy control subjects were compared using nonuniform, uniform or even no attenuation correction.

Extracardiac activity complicates quantitative cardiac SPECT imaging using a simultaneous transmission-emission approach

Increased extracardiac activity confounds conventional cardiac SPECT image reconstruction using a filtered backprojection method. Others have proposed that simultaneously acquired transmission-emission (STE) images that are reconstructed with a maximum likelihood (ML) method incorporating a nonuniform attenuation correction would less likely be affected by the presence of extracardiac activity. However, this approach corrects only for decreased myocardial counts from attenuation and not for increased myocardial counts from extracardiac activity. Therefore, STE with nonuniform attenuation correction may also result in reconstruction artifacts when extracardiac activity is present.

METHODS

Acquisitions of phantoms with nonuniform and uniform attenuation were performed using STE and conventional approaches, in the absence and presence of extracardiac activity. All acquisitions used a triple-headed SPECT camera. STE acquisitions used fanbeam collimation and a 153Gd transmission source. STE images were reconstructed using ML, with and without nonuniform attenuation correction. Reconstructed short-axis images were quantitated, and percentage variability for each count profile was calculated.

RESULTS

In a nonuniform phantom configuration, STE reconstruction with nonuniform attenuation correction significantly improved image uniformity. This improvement in image uniformity was diminished with the addition of increasing extracardiac activity. In a uniform phantom, STE reconstruction with nonuniform attenuation correction significantly improved uniformity only in the presence of extracardiac activity.

CONCLUSION

The addition of attenuation correction in the presence of extracardiac activity can have complex effects on ML reconstruction with nonuniform attenuation correction, which depends on the amount of extracardiac activity and pattern of attenuation.

Compartmental analysis of the complete dynamic scan data for scintigraphic determination of effective renal plasma flow

We have developed an image-based compartmental analysis for estimating effective renal plasma flow (ERPF in units of milliliters per minute) from the full time-activity curves of regions of interest (ROI) placed over the heart, kidneys and bladder.

METHODS

Kidney or time-activity curves are corrected for physical attenuation using estimates of kidney depth derived from patient height and weight. Estimates of the calibration factors, Kp and Kb (mCl/counts/sec), for the plasma and bladder time-activity curves are determined by applying the following ROI analysis to each frame of the dynamic scan: (Kp)Pc(t) + (Kb)Bc(t) = Di - Rq(t), where P c(t) and Bc(t) represent the counting rates measured in ROI placed over the left ventricle blood pool and bladder at time t; Di is the known total injected dose, and Rq(t) represents the millicurie of tracer in the kidneys at time t. Once Kp and Kb have been determined by regression, the calibrated time activity curves are used to solve for the physiological parameter fERPF (min-1), which represents the fraction of the total body plasma cleared of mertiatide per min. The ERPF calculated by the product of fERPF and plasma volume, determined from patient weight, was compared to the ERPF as calculated by blood samples and the Schlegel and renal uptake plasma volume product scintigraphic techniques.

RESULTS

Twenty-five adult patients with a wide range of ages and renal function were studied. The results of this image-based method for calculating ERPF correlated well with the values obtained from blood samples (linear regression slope = 1.06; y-int = -34.68 ml/min, r = 0.905) and offered a significant improvement over both the Schlegel and renal uptake plasma volume product estimates (p < 0.05).

CONCLUSION

A scintigraphic estimation of ERPF without blood samples using time-activity data from the heart, kidneys and bladder acquired over the entire renogram is feasible and correlates well with more invasive techniques requiring blood samples.

Assessment of the rate of uptake-plasma volume product to calculate glomerular filtration rate [corrected]

To further validate the rate of renal uptake of the 99mTc-DTPA-plasma volume product (RUPV) method to estimate glomerular filtration rate (GFR), 104 determinations were performed and compared to blood sample of GFR assays. The interassay consistency was also studied in 42 patients.

METHODS

The studies were performed with 370-550 MBq (10-15 mCi) of 99mTc-DTPA and a gamma camera. The 3-min cumulative renal uptake was calculated from the renogram curves and expressed as the rate of renal uptake in min-1. The plasma volume, in milliliters, was estimated from the patient's body weight. The GFR (ml/min) was calculated from [RU] x [PV] and by using two blood samples. To study interassay consistency, two determinations of GFR were performed on separate days.

RESULTS

The regression equation relating the rate of renal uptake (RU) in the abscissa and the GFR obtained from plasma samples in the ordinate is: y = 3.13 + 10.5x (n = 104; r = 0.90). The regression equation of RUPV estimated GFR (x) compared to the GFR calculated from blood samples (y) is: y = 6.9 + 0.91x (n = 104; r = 0.94). The interassay consistency study showed no statistically significant difference between measurements obtained on Days 1 and 2. The mean +/- s.e.m. GFR for each determination were 84.3 +/- 6.12 and 81.9 +/- 6.21. For the blood sample method, the mean s.e.m. for each day were 87.26 +/- 6.69 and 96.86 +/- 6.58 (p < 0.05). The percent variation coefficient for the RUPV method was: CV% = 6.8 +/- 2.7 and 12.1 +/- 3.3 (p < 0.03) for the blood sample method.

CONCLUSION

The observed accuracy of the determination is comparable to that in our previous study of a separate patient population at another hospital. This method would be suitable for interinstitutional comparison and for longitudinal patient studies.

Difference images calculated from ictal and interictal technetium-99m-HMPAO SPECT scans of epilepsy

Image processing techniques were applied to SPECT brain images to aid in the localization of epileptic foci.

METHODS

Ictal and interictal cerebral perfusion SPECT images were acquired from 12 epilepsy patients (6 temporal, 6 extratemporal) after injection of 20 mCi 99mTc-HMPAO. Each ictal scan was registered to the same patient's interictal scan. Normalization of the three-dimensional data was applied to account for global percent brain uptake and total injected activity. After registration, normalization and subtraction of the SPECT images and functional difference images were computed. Difference images were calculated, which give a quantitative measure of perfusion alterations during ictus. The resulting difference images were also registered with each patient's MRI scan which permits localization of perfusion changes onto anatomical structures.

RESULTS

Areas in the brain where significant perfusion increases occur correlate with areas confirmed to be seizure foci. Four of the six patients with known temporal lobe seizure foci exhibited significant perfusion increases on the difference images. These areas demonstrate a percent increase of perfusion larger than 40%. For the extratemporal seizure patients, four of the four confirmed seizure sites were diagnosed with difference images. Results on the remaining two patients were inconclusive.

CONCLUSION

When compared to side-by-side visual interpretation of the ictal and interictal SPECT images, registration of SPECT and MR images together with calculated difference maps greatly enhances the ability to localize epileptic seizure foci. This offers the potential to locate epileptic seizure foci using a noninvasive, inexpensive imaging procedure and data processing algorithm.

Opiate dependence and withdrawal: preliminary assessment using single photon emission computerized tomography (SPECT)

Naloxone (0.8 mg, s.c.) effects on opiate withdrawal signs and symptoms and regional brain function were assessed in 10 methadone-maintained patients and 10 healthy subjects in a double-blind, placebo-controlled study. Regional brain function was assessed using single photon emission computerized tomography (SPECT) by evaluating the uptake of [99mTc]d,l-hexamethylpropyleneamine oxime (HMPAO) in the brain, a process related to regional cerebral perfusion. Comparisons of patients and healthy subjects after saline infusion suggested that chronic opiate dependence was associated with lower corrected activity ratios (regional count density/whole brain count density) in frontal and parietal cortices and greater activity ratios in the thalamus. Opiate-dependent patients, but not healthy subjects, developed opiate withdrawal signs and symptoms after naloxone administration. Following naloxone administration, patients undergoing opiate withdrawal exhibited lower whole brain count density than healthy subjects. They also had lower activity ratios in frontal and parietal cortices and increased thalamic activity ratios relative to healthy subjects receiving naloxone. Naloxone administration in healthy subjects, but not opiate withdrawal in patients, was associated with decreased right parietal cortex and increased right temporal cortex and left basal ganglia activity ratios. Relative to naloxone effects in healthy subjects, opiate withdrawal was associated with decreased whole brain count density and a reduced right temporal cortex activity ratio. This preliminary study reports an initial evaluation of HMPAO-SPECT imaging for assessing regional alterations in brain function during opiate dependence and withdrawal. While group differences were reported, the small magnitude of regional alterations in patients undergoing opiate withdrawal raised concern that HMPAO-SPECT methods employed were inadequate for assessing human regional brain function during phases of opiate addiction. Other emerging functional brain imaging technologies should be evaluated relative to improved HMPAO-SPECT methods for this purpose.

Human biodistribution and dosimetry of the SPECT benzodiazepine receptor radioligand iodine-123-iomazenil

SPECT imaging of the brain with [123I]iomazenil has shown avid uptake of the radioligand in a distribution consistent with benzodiazepine receptor binding. The purposes of this study were to measure the whole-body distribution of activity following i.v. administration of [123I]iomazenil and to evaluate the resulting organ radiation burdens.

METHODS

Serial total body scans were obtained in healthy volunteers after thyroid blockade and demonstrated avid brain uptake of radioligand.

RESULTS

Abdominal imaging showed significant activity retention within the urinary and gastrointestinal tracts consistent with excretion via these routes. Absorbed dose to the urinary bladder was calculated to be 0.19 mGy/MBq, to the lower large intestine 0.079 mGy/MBq, to the upper large intestine 0.066 mGy/MBq, and to the thyroid 0.063 mGy/MBq.

CONCLUSION

Thyroid uptake may in part have represented binding to benzodiazepine receptors, since radioligand binding to tissue homogenates prepared from human thyroid showed the presence of benzodiazepine binding sites.

Computerized three-dimensional segmented human anatomy

Manual segmentation of 129 x-ray CT transverse slices of a living male human has been done and a computerized 3-dimensional volume array modeling all major internal structures of the body has been created. Each voxel of the volume contains a index number designating it as belonging to a given organ or internal structure. The original x-ray CT images were reconstructed in a 512 x 512 matrix with a resolution of 1 mm in the x,y plane. The z-axis resolution is 1 cm from neck to midthigh and 0.5 cm from neck to crown of the head. This volume array represents a high resolution model of the human anatomy and can serve as a voxel-based anthropomorphic phantom suitable for many computer-based modeling and simulation calculations.

Quantitative analysis of the technetium-99m-DTPA captopril renogram: contribution of washout parameters to the diagnosis of renal artery stenosis

Retrospective analysis of precaptopril and postcaptopril DTPA renograms from 88 hypertensive patients was performed to refine the quantitative criteria used to diagnose renal artery stenosis (RAS). Of the 88 patients, 45 had RAS and 43 had normal renal arteries at angiography. Using time-activity curves from the essential hypertensive group, diagnostic washout criteria for a positive DTPA renogram were developed. These were based on the 20 and 30 min/peak activity ratios in each kidney. When the washout criteria were retrospectively applied to patient data as a whole, sensitivity and specificity for RAS were 67% and 79%, respectively. When previously described uptake criteria, based on the time to peak activity in each kidney and the GFR ratio between the kidneys, were applied to the same data, sensitivity and specificity for RAS were 89% and 84%, respectively. Quantitative analysis of the DTPA renogram using the time to peak and GFR ratio was both sensitive and specific for RAS. Measurement of 20 and 30 min/peak renal activity ratios did not improve the accuracy of the test.

Bayesian reconstruction of functional images using anatomical information as priors

Proposes a Bayesian method whereby maximum a posteriori (MAP) estimates of functional (PET and SPECT) images may be reconstructed with the aid of prior information derived from registered anatomical MR images of the same slice. The prior information consists of significant anatomical boundaries that are likely to correspond to discontinuities in an otherwise spatially smooth radionuclide distribution. The authors' algorithm, like others proposed recently, seeks smooth solutions with occasional discontinuities; the contribution here is the inclusion of a coupling term that influences the creation of discontinuities in the vicinity of the significant anatomical boundaries. Simulations on anatomically derived mathematical phantoms are presented. Although computationally intense in its current implication, the reconstructions are improved (ROI-RMS error) relative to filtered backprojection and EM-ML reconstructions. The simulations show that the inclusion of position-dependent anatomical prior Information leads to further improvement relative to Bayesian reconstructions without the anatomical prior. The algorithm exhibits a certain degree of robustness with respect to errors in the location of anatomical boundaries.

The technetium-99m-DTPA renal uptake-plasma volume product: a quantitative estimation of glomerular filtration rate

An image-based method for estimating quantitative renal glomerular filtration rates (GFR) by calculating the product of the renal uptake rate and plasma volume is presented. By using the relationship GFR = F.PV/t, F represents renal 99mTc-DTPA uptake after bolus injection, PV is the plasma volume and t is time. This GFR evaluation was carried out on 96 patients and compared to GFR values determined in the same patients using radiotracer blood clearance techniques relying on two venous blood samples. When estimating patient plasma volumes using patient's weight and measured hematocrit values, the image-based method for calculating GFR accurately approximates the values obtained from blood samples (linear regression slope = 1.03; y-intercept = -2.81 ml/min). The two techniques correlate with a value of r = 0.89.

Visual stimulation increases technetium-99m-HMPAO distribution in human visual cortex

The ability of changes in the distribution of technetium-99m-hexamethylpropylene amine oxime (99mTc-HMPAO) to reflect physiologic changes in regional cerebral blood flow (rCBF) was evaluated using photic stimulation, a procedure known to increase rCBF in the striate cortex. Seven healthy subjects were injected with 740 MBq 99mTc-HMPAO on two separate days. On one day, the injection was performed following closure of the eyes and patching for 5 min. On the other day, subjects were exposed to a stroboscopic light to produce photic simulation. Images of distribution of 99mTc-HMPAO were obtained using a Strichman 810X single-photon emission computed tomogram (SPECT) brain scanner. Comparison of images obtained during light occluded versus stimulation conditions revealed a significant increase in distribution of radiopharmaceutical in visual cortex relative to whole brain (peak increase corrected for radiopharmaceutical backdiffusion 36.7% +/- 6.6%). HMPAO appears to provide a useful method for detecting relative rCBF increases with SPECT.

Patients at high risk for renal artery stenosis: a simple method of renal scintigraphic analysis with Tc-99m DTPA and captopril

Fifty patients with suspected renal artery stenosis (RAS) were studied with renal scintigraphy before and after administration of captopril. Twenty-three patients had RAS (greater than or equal to 75% RAS or greater than or equal to 50% RAS with poststenotic dilatation) and 27 had normal renal arteries at angiography. Angiotensin-converting enzyme inhibitors were discontinued 24 hours prior to renal scintigraphy; all other medications were continued. Each patient was evaluated with a simplified captopril renal scintigraphic protocol: renal imaging after administration of 12 mCi (444 MBq) of technetium-99m diethylenetriaminepentaacetic acid (DTPA), a 3-hour wait, oral administration of 50 mg of captopril, a 1-hour wait, and another scintigram obtained after administration of 12 mCi (444 MBq) of Tc-99m DTPA. Times of peak renal activity (Tmax) were determined from renal time-activity curves, and glomerular filtration rates (GFRs) were calculated with the Gates technique. A Tmax greater than or equal to 11 minutes after injection or a GFR ratio (larger GFR/smaller GFR) greater than 1.5 enabled detection of RAS with 91% sensitivity, 93% specificity, and 92% accuracy. Renal scintigraphy without captopril had only 43%-68% sensitivity in detecting RAS, depending on the criteria used.

Measurement of lung volume with 81Krm in a dynamic scintigram

The meaning of 81Krm counts obtained in a dynamic series of gated lung ventilation scans was evaluated in terms of flow rate, lung volume, or the flow/volume ratio. Flow and volume signals were recorded together with scintigraphic events in 29 subjects breathing 81Krm and after its decay, 127Xe as a tracer of lung volume. Gated ventilation scans and respiratory signals were reconstructed. Statistical analyses were carried out for linear regressions between total normalized counts detected by the gamma camera and (1) flow rate, (2) lung volume and (3) flow/volume ratio. Inspiration and expiration were analysed separately. For both isotopes, the best correlation was obtained between counts and lung volume (r greater than 0.93). No correlation was obtained between counts and flow rate or flow/volume ratio. Thus, we conclude that 81Krm count variations in gated scans correlate well with tidal volume.