Combination of dynamic and integral methods for generating reproducible functional CBF images

Age-Dependent Changes in Regulation of Water Inflow Into the Vitreous Body

Purpose

Water inflow into the vitreous body regulated by retinal aquaporin-4 distributed within Müller cells has been observed in mice; however, the changes in this phenomenon with age remain unknown. This study aimed to evaluate whether intravenously injected H2O also flows into the vitreous body of human subjects and to investigate whether water dynamics in the human posterior eye change with age using [15O]H2O positron emission tomography (PET).

Methods

Forty-six normal adult volunteers underwent [15O]H2O PET, and the standard uptake value (SUV) in the center of the vitreous body after 1000-MBq [15O]H2O administration was assessed. The SUV was fitted to an exponential curve, and y0, the steady state of the SUV, and b, the speed of increase in the SUV, were calculated. The results for patients ranging from in age from 20 to 39, 40 to 59, and 60 to 79 years were compared using analyses of variance followed by Games to Howell tests.

Results

For the parameter y0, statistical analysis revealed no statistically significant differences among the three groups. For parameter b, statistical analysis revealed statistically significant differences between the 20 to 39 and 60 to 79 age groups (P = 0.000), the 40 to 59 and 60 to 79 age groups (P = 0.025), and the 20 to 39 and 40 to 59 age groups (P = 0.037).

Conclusions

The present study revealed that H2O injected into the vein flows into the human vitreous body and that the speed of increase in water flow into the vitreous body decreases with aging. This study suggests that water dynamics in the posterior eye, or the retinal glymphatic pathway, change significantly with aging.

Total-Body PET Multiparametric Imaging of Cancer Using a Voxelwise Strategy of Compartmental Modeling

Quantitative dynamic PET with compartmental modeling has the potential to enable multiparametric imaging and more accurate quantification than static PET imaging. Conventional methods for parametric imaging commonly use a single kinetic model for all image voxels and neglect the heterogeneity of physiologic models, which can work well for single-organ parametric imaging but may significantly compromise total-body parametric imaging on a scanner with a long axial field of view. In this paper, we evaluate the necessity of voxelwise compartmental modeling strategies, including time delay correction (TDC) and model selection, for total-body multiparametric imaging. Methods: Ten subjects (5 patients with metastatic cancer and 5 healthy volunteers) were scanned on a total-body PET/CT system after injection of 370 MBq of 18F-FDG. Dynamic data were acquired for 60 min. Total-body parametric imaging was performed using 2 approaches. One was the conventional method that uses a single irreversible 2-tissue-compartment model with and without TDC. The second approach selects the best kinetic model from 3 candidate models for individual voxels. The differences between the 2 approaches were evaluated for parametric imaging of microkinetic parameters and the 18F-FDG net influx rate, KiResults: TDC had a nonnegligible effect on kinetic quantification of various organs and lesions. The effect was larger in lesions with a higher blood volume. Parametric imaging of Ki with the standard 2-tissue-compartment model introduced vascular-region artifacts, which were overcome by the voxelwise model selection strategy. Conclusion: The time delay and appropriate kinetic model vary in different organs and lesions. Modeling of the time delay of the blood input function and model selection improved total-body multiparametric imaging.

Efficient Delay Correction for Total-Body PET Kinetic Modeling Using Pulse Timing Methods

Quantitative kinetic modeling requires an input function. A noninvasive image-derived input function (IDIF) can be obtained from dynamic PET images. However, a robust IDIF location (e.g., aorta) may be far from a tissue of interest, particularly in total-body PET, introducing a time delay between the IDIF and the tissue. The standard practice of joint estimation (JE) of delay, along with model fitting, is computationally expensive. To improve the efficiency of delay correction for total-body PET parametric imaging, this study investigated the use of pulse timing methods to estimate and correct for delay. Methods: Simulation studies were performed with a range of delay values, frame lengths, and noise levels to test the tolerance of 2 pulse timing methods-leading edge (LE) and constant fraction discrimination and their thresholds. The methods were then applied to data from 21 subjects (14 healthy volunteers, 7 cancer patients) who underwent a 60-min dynamic total-body ¹⁸F-FDG PET acquisition. Region-of-interest kinetic analysis was performed and parametric images were generated to compare LE and JE methods of delay correction, as well as no delay correction. Results: Simulations demonstrated that a 10% LE threshold resulted in biases and SDs at tolerable levels for all noise levels tested, with 2-s frames. Pooled region-of-interest-based results (n = 154) showed strong agreement between LE (10% threshold) and JE methods in estimating delay (Pearson r = 0.96, P < 0.001) and the kinetic parameters v_b (r = 0.96, P < 0.001), K_i (r = 1.00, P < 0.001), and K₁ (r = 0.97, P < 0.001). When tissues with minimal delay were excluded from pooled analyses, there were reductions in v_b (69.4%) and K₁ (4.8%) when delay correction was not performed. Similar results were obtained for parametric images; additionally, lesion K_i contrast was improved overall with LE and JE delay correction compared with no delay correction and Patlak analysis. Conclusion: This study demonstrated the importance of delay correction in total-body PET. LE delay correction can be an efficient surrogate for JE, requiring a fraction of the computational time and allowing for rapid delay correction across more than 10⁶ voxels in total-body PET datasets.

Evaluation of Arterial Spin Labeling MRI-Comparison with 15O-Water PET on an Integrated PET/MR Scanner

Cerebral blood flow (CBF) measurements are of high clinical value and can be acquired non-invasively with no radiation exposure using pseudo-continuous arterial spin labeling (ASL). The aim of this study was to evaluate accordance in resting state CBF between ASL (CBF_ASL) and ¹⁵O-water positron emission tomography (PET) (CBF_PET) acquired simultaneously on an integrated 3T PET/MR system. The data comprised ASL and dynamic ¹⁵O-water PET data with arterial blood sampling of eighteen subjects (eight patients with focal epilepsy and ten healthy controls, age 21 to 61 years). ¹⁵O-water PET parametric CBF images were generated using a basis function implementation of the single tissue compartment model. Cortical and subcortical regions were automatically segmented using Freesurfer. Average CBF_ASL and CBF_PET in grey matter were 60 ± 20 and 75 ± 22 mL/100 g/min respectively, with a relatively high correlation (r = 0.78, p < 0.001). Bland-Altman analysis revealed poor agreement (bias = −15 mL/100 g/min, lower and upper limits of agreements = −16 and 45 mL/100 g/min, respectively) with a negative relationship. Accounting for the negative relationship, the width of the limits of agreement could be narrowed from 61 mL/100 g/min to 35 mL/100 g/min using regression-based limits of agreements. Although a high correlation between CBF_ASL and CBF_PET was found, the agreement in absolute CBF values was not sufficient for ASL to be used interchangeably with ¹⁵O-water PET.

Total-Body Quantitative Parametric Imaging of Early Kinetics of 18F-FDG

Parametric imaging has been shown to provide better quantitation physiologically than SUV imaging in PET. With the increased sensitivity from a recently developed total-body PET scanner, whole-body scans with higher temporal resolution become possible for dynamic analysis and parametric imaging. In this paper, we focus on deriving the parameter k ₁ using compartmental modeling and on developing a method to acquire whole-body ¹⁸F-FDG PET parametric images using only the first 90 s of the postinjection scan data with the total-body PET system. Methods: Dynamic projections were acquired with a time interval of 1 s for the first 30 s and a time interval of 2 s for the following minute. Image-derived input functions were acquired from the reconstructed dynamic sequences in the ascending aorta. A 1-tissue-compartment model with 4 parameters (k ₁, k ₂, blood fraction, and delay time) was used. A maximum-likelihood-based estimation method was developed with the 1-tissue-compartment model solution. The accuracy of the acquired parameters was compared with the ones estimated using a 2-tissue-compartment irreversible model with 1-h-long data. Results: All 4 parametric images were successfully calculated using data from 2 volunteers. By comparing the time-activity curves acquired from the volumes of interest, we showed that the parameters estimated using our method were able to predict the time-activity curves of the early dynamics of ¹⁸F-FDG in different organs. The delay-time effects for different organs were also clearly visible in the reconstructed delay-time image with delay variations of as large as 40 s. The estimated parameters using both 90-s data and 1-h data agreed well for k ₁ and blood fraction, whereas a large difference in k ₂ was found between the 90-s and 1-h data, suggesting k ₂ cannot be reliably estimated from the 90-s scan. Conclusion: We have shown that with total-body PET and the increased sensitivity, it is possible to estimate parametric images based on the very early dynamics after ¹⁸F-FDG injection. The estimated k ₁ might potentially be used clinically as an indicator for identifying abnormalities.

Semi-quantitative cerebral blood flow parameters derived from non-invasive [15O]H2O PET studies

Quantification of regional cerebral blood flow (CBF) using [¹⁵O]H₂O positron emission tomography (PET) requires the use of an arterial input function. Arterial sampling, however, is not always possible, for example in ill-conditioned or paediatric patients. Therefore, it is of interest to explore the use of non-invasive methods for the quantification of CBF. For validation of non-invasive methods, test-retest normal and hypercapnia data from 15 healthy volunteers were used. For each subject, the data consisted of up to five dynamic [¹⁵O]H₂O brain PET studies of 10 min and including arterial sampling. A measure of CBF was estimated using several non-invasive methods earlier reported in literature. In addition, various parameters were derived from the time-activity curve (TAC). Performance of these methods was assessed by comparison with full kinetic analysis using correlation and agreement analysis. The analysis was repeated with normalization to the whole brain grey matter value, providing relative CBF distributions. A reliable, absolute quantitative estimate of CBF could not be obtained with the reported non-invasive methods. Relative (normalized) CBF was best estimated using the double integration method.

[18F]NaF PET/CT scan as an early marker of heterotopic ossification in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease with a progressive course characterized by episodically local flare-ups, which often but not always leads to heterotopic bone formation (HO). Recently, we showed that [18F]NaF PET/CT may be the first tool to monitor progression of a posttraumatic flare-up leading to new HO, which was demonstrated in a patient with FOP who underwent a maxillofacial surgery. This paper evaluates [18F]NaF PET/CT as a marker of FOP disease activity, comparing its use with other imaging modalities known in literature. In addition, the follow-up of a spontaneous flare-up in a 19-year old patient is presented showing high muscle [18F]NaF uptake in one defined part within the flare-up area after three weeks. During follow-up [18F]NaF PET /CT scan revealed newly formed heterotopic bone but only in this previously active [18F]NaF region. In conclusion, increased muscle [18F]NaF uptake may predict future HO development in FOP patients. At present [18F]NaF PET/CT appears to be a sensitive imaging modality to serve as a noninvasive marker for bone formation and to monitor disease activity during flare-ups in FOP.

Quantification of 18F-Fluoride Kinetics: Evaluation of Simplified Methods

(18)F-fluoride PET is a promising noninvasive method for measuring bone metabolism and bone blood flow. The purpose of this study was to assess the performance of various clinically useful simplified methods by comparing them with full kinetic analysis. In addition, the validity of deriving bone blood flow from K1 of (18)F-fluoride was investigated using (15)O-H2O as a reference.

METHODS

Twenty-two adults (mean age ± SD, 44.8 ± 25.2 y), including 16 patients scheduled for bone surgery and 6 healthy volunteers, were studied. All patients underwent dynamic (15)O-H2O and (18)F-fluoride scans before surgery. Ten of these patients had serial PET measurements before and at 2 time points after local bone surgery. During all PET scans, arterial blood was monitored continuously. (18)F-fluoride data were analyzed using nonlinear regression (NLR) and several simplified methods (Patlak and standardized uptake value [SUV]). SUV was evaluated for different time intervals after injection and after normalizing to body weight, lean body mass, and body surface area, and simplified measurements were compared with NLR results. In addition, changes in SUV and Patlak-derived fluoride influx rate (Ki) after surgery were compared with corresponding changes in NLR-derived Ki. Finally, (18)F-fluoride K1 was compared with bone blood flow derived from (15)O-H2O data, using the standard single-tissue-compartment model.

RESULTS

K1 of (18)F-fluoride correlated with measured blood flow, but the correlation coefficient was relatively low (r = 0.35, P < 0.001). NLR resulted in a mean Ki of 0.0160 ± 0.0122, whereas Patlak analysis, for the interval 10-60 min after injection, resulted in an almost-identical mean Ki of 0.0161 ± 0.0117. The Patlak-derived Ki, for 10-60 min after injection, showed a high correlation with the NLR-derived Ki (r = 0.976). The highest correlation between Ki and lean body mass-normalized SUV was found for the interval 50-60 min (r = 0.958). Finally, changes in SUV correlated significantly with those in Ki (r = 0.97).

CONCLUSION

The present data support the use of both Patlak and SUV for assessing fluoride kinetics in humans. However, (18)F-fluoride PET has only limited accuracy in monitoring bone blood flow.

Imaging of the appearance time of cerebral blood using [15O]H2O PET for the computation of correct CBF

Background

Quantification of cerebral blood flow (CBF) is important for the understanding of normal and pathologic brain physiology. Positron emission tomography (PET) with H₂¹⁵O (or C¹⁵O₂) can quantify CBF and apply kinetic analyses, including autoradiography (ARG) and the basis function methods (BFM). These approaches, however, are sensitive to input function errors such as the appearance time of cerebral blood (ATB), known as the delay time. We estimated brain ATB in an image-based fashion to correct CBF by accounting for differences in computed CBF values using three different analyses: ARG and BFM with and without fixing the partition coefficient.

Methods

Subject groups included those with no significant disorders, those with elevated cerebral blood volume, and those with reduced CBF. All subjects underwent PET examination, and CBF was estimated using the three analyses. The ATB was then computed from the differences of the obtained CBF values, and ATB-corrected CBF values were computed. ATB was also estimated for regions of interest (ROIs) of multiple cortical regions. The feasibility of the present method was tested in a simulation study.

Results

There were no significant differences in the obtained ATB between the image- and ROI-based methods. Significantly later appearance was found in the cerebellum compared to other brain regions for all groups. In cortical regions where CBF was reduced due to occlusive lesions, the ATB was 0.2 ± 1.2 s, which was significantly delayed relative to the contralateral regions. A simulation study showed that the ATB-corrected CBF was less sensitive to errors in input function, and noise on the tissue curve did not enhance the degree of noise on ATB-corrected CBF image.

Conclusions

This study demonstrates the potential utility of visualizing the ATB in the brain, enabling the determination of CBF with less sensitivity to error in input function.

Cerebral blood flow and glucose metabolism in healthy volunteers measured using a high-resolution PET scanner

Background

Positron emission tomography (PET) allows for the measurement of cerebral blood flow (CBF; based on [¹⁵O]H₂O) and cerebral metabolic rate of glucose utilization (CMR_glu; based on [¹⁸ F]-2-fluoro-2-deoxy-d-glucose ([¹⁸ F]FDG)). By using kinetic modeling, quantitative CBF and CMR_glu values can be obtained. However, hardware limitations led to the development of semiquantitive calculation schemes which are still widely used. In this paper, the analysis of CMR_glu and CBF scans, acquired on a current state-of-the-art PET brain scanner, is presented. In particular, the correspondence between nonlinear as well as linearized methods for the determination of CBF and CMR_glu is investigated. As a further step towards widespread clinical applicability, the use of an image-derived input function (IDIF) is investigated.

Methods

Thirteen healthy male volunteers were included in this study. Each subject had one scanning session in the fasting state, consisting of a dynamic [¹⁵O]H₂O scan and a dynamic [¹⁸ F]FDG PET scan, acquired at a high-resolution research tomograph. Time-activity curves (TACs) were generated for automatically delineated and for manually drawn gray matter (GM) and white matter regions. Input functions were derived using on-line arterial blood sampling (blood sampler derived input function (BSIF)). Additionally, the possibility of using carotid artery IDIFs was investigated. Data were analyzed using nonlinear regression (NLR) of regional TACs and parametric methods.

Results

After quality control, 9 CMR_glu and 11 CBF scans were available for analysis. Average GM CMR_glu values were 0.33 ± 0.04 μmol/cm³ per minute, and average CBF values were 0.43 ± 0.09 mL/cm³ per minute. Good correlation between NLR and parametric CMR_glu measurements was obtained as well as between NLR and parametric CBF values. For CMR_glu Patlak linearization, BSIF and IDIF derived results were similar. The use of an IDIF, however, did not provide reliable CBF estimates.

Conclusion

Nonlinear regression analysis, allowing for the derivation of regional CBF and CMR_glu values, can be applied to data acquired with high-spatial resolution current state-of-the-art PET brain scanners. Linearized models, applied to the voxel level, resulted in comparable values. CMR_glu measurements do not require invasive arterial sampling to define the input function.

Trial registration

ClinicalTrials.gov NCT00626080

Optimization of methods for quantification of rCBF using high-resolution [¹⁵O]H₂O PET images

This study aimed to derive accurate estimates of regional cerebral blood flow (rCBF) from noisy dynamic [¹⁵O]H₂O PET images acquired on the high-resolution research tomograph, while retaining as much as possible the high spatial resolution of this brain scanner (2-3 mm) in parametric maps of rCBF. The PET autoradiographic method and generalized linear least-squares (GLLS), with fixed or extended to include spatially variable estimates of the dispersion of the measured input function, were compared to nonlinear least-squares (NLLS) for rCBF estimation. Six healthy volunteers underwent two [¹⁵O]H₂O PET scans with continuous arterial blood sampling. rCBF estimates were obtained from three image reconstruction methods (one analytic and two iterative, of which one includes a resolution model) to which a range of post-reconstruction filters (3D Gaussian: 2, 4 and 6 mm FWHM) were applied. The optimal injected activity was estimated to be around 11 MBq kg⁻¹ (800 MBq) by extrapolation of patient-specific noise equivalent count rates. Whole-brain rCBF values were found to be relatively insensitive to the method of reconstruction and rCBF quantification. The grey and white matter rCBF for analytic reconstruction and NLLS were 0.44 ± 0.03 and 0.15 ± 0.03 mL min⁻¹ cm⁻³, respectively, in agreement with literature values. Similar values were obtained from the other methods. For generation of parametric images using GLLS or the autoradiographic method, a filter of ≥ 4 mm was required in order to suppress noise in the PET images which otherwise produced large biases in the rCBF estimates.

Day-to-day test-retest variability of CBF, CMRO2, and OEF measurements using dynamic 15O PET studies

Purpose

We assessed test–retest variability of cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO₂), and oxygen extraction fraction (OEF) measurements derived from dynamic ¹⁵O positron emission tomography (PET) scans.

Procedures

In seven healthy volunteers, complete test–retest ¹⁵O PET studies were obtained; test–retest variability and left-to-right ratios of CBF, CBV, OEF, and CMRO₂ in arterial flow territories were calculated.

Results

Whole-brain test–retest coefficients of variation for CBF, CBV, CMRO₂, and OEF were 8.8%, 13.8%, 5.3%, and 9.3%, respectively. Test–retest variability of CBV left-to-right ratios was <7.4% across all territories. Corresponding values for CBF, CMRO₂, and OEF were better, i.e., <4.5%, <4.0%, and <1.4%, respectively.

Conclusions

The test–retest variability of CMRO₂ measurements derived from dynamic ¹⁵O PET scans is comparable to within-session test–retest variability derived from steady-state ¹⁵O PET scans. Excellent regional test–retest variability was observed for CBF, CMRO₂, and OEF. Variability of absolute CBF and OEF measurements is probably affected by physiological day-to-day variability of CBF.

Impaired verbal memory in Parkinson disease: relationship to prefrontal dysfunction and somatosensory discrimination

Objective

To study the neurocognitive profile and its relationship to prefrontal dysfunction in non-demented Parkinson's disease (PD) with deficient haptic perception.

Methods

Twelve right-handed patients with PD and 12 healthy control subjects underwent thorough neuropsychological testing including Rey complex figure, Rey auditory verbal and figural learning test, figural and verbal fluency, and Stroop test. Test scores reflecting significant differences between patients and healthy subjects were correlated with the individual expression coefficients of one principal component, obtained in a principal component analysis of an oxygen-15-labeled water PET study exploring somatosensory discrimination that differentiated between the two groups and involved prefrontal cortices.

Results

We found significantly decreased total scores for the verbal learning trials and verbal delayed free recall in PD patients compared with normal volunteers. Further analysis of these parameters using Spearman's ranking correlation showed a significantly negative correlation of deficient verbal recall with expression coefficients of the principal component whose image showed a subcortical-cortical network, including right dorsolateral-prefrontal cortex, in PD patients.

Conclusion

PD patients with disrupted right dorsolateral prefrontal cortex function and associated diminished somatosensory discrimination are impaired also in verbal memory functions. A negative correlation between delayed verbal free recall and PET activation in a network including the prefrontal cortices suggests that verbal cues and accordingly declarative memory processes may be operative in PD during activities that demand sustained attention such as somatosensory discrimination. Verbal cues may be compensatory in nature and help to non-specifically enhance focused attention in the presence of a functionally disrupted prefrontal cortex.

Reproducibility of tumor perfusion measurements using 15O-labeled water and PET

PET and 15O-labeled water (H215O) can be used to noninvasively monitor tumor perfusion. This allows evaluation of the direct target of antiangiogenic drugs, that is, tumor vasculature. Because these drugs often result in consolidation rather than regression of the tumor mass, a change in perfusion might be a more sensitive way to evaluate response than are indirect size measures on a CT scan. However, to use the technique for serial imaging of individual patients, good reproducibility is essential. The purpose of the present study was to evaluate the reproducibility of quantitative H215O measurements.

METHODS

Nine patients with non-small-cell lung cancer (NSCLC) were scanned twice within 7 d and before any therapy. All H215O scans were followed by an 18F-fluorothymidine scan to allow for adequate volume-of-interest (VOI) definition. VOIs were defined using a 3-dimensional threshold technique. Tumor perfusion and the volume of distribution (VT) were obtained using a 1-tissue-compartment model including an arterial blood volume component and an image-derived input function. The level of agreement between test and retest values was assessed using the intraclass correlation coefficient (ICC) and Bland-Altman analyses. Possible dependency on absolute values and lesion size was assessed by linear regression.

RESULTS

All primary tumors and more than 90% of clinically suspected locoregional metastases could be delineated. In total, 14 lesions in 9 patients were analyzed. Tumor perfusion showed excellent reproducibility, with an ICC of 0.95 and SD of 9%. The VT was only moderately reproducible, with an ICC of 0.52 and SD of 16%. No dependency was found on absolute values of perfusion (P = 0.14) and VT (P = 0.15). In addition, tumor volume did not influence the reproducibility of perfusion (P = 0.46) and VT (P = 0.25).

CONCLUSION

Quantitative measurements of tumor perfusion using H215O and PET are reproducible in NSCLC. When patients are repeatedly being scanned during therapy, changes of more than 18% in tumor perfusion and 32% in VT (>1.96 x SD) are likely to represent treatment effects.

Bone metabolism after total hip revision surgery with impacted grafting: evaluation using H2 15O and [18F]fluoride PET; a pilot study

Purpose

To evaluate bone blood flow and bone formation in patients after total hip revision surgery with impacted bone grafting using H₂¹⁵O and [¹⁸F]fluoride positron emission tomography (PET).

Procedures

To asses bone blood flow and bone metabolism in bone allograft after impaction grafting, four patients treated with total hip revision surgery were enrolled prospectively in this study. Six patients scheduled for primary hip arthroplasties were included as a control group. The study protocol consisted of three H₂¹⁵O and [¹⁸F]fluoride PET scans in each patient.

Results

Bone blood flow increased significantly compared to the preoperative state in patients treated for primary hip arthroplasty. In patients undergoing revision surgery, bone blood flow was twofold to threefold higher compared to the preoperative state, but did not reach significance. Bone metabolism in patients undergoing revision was threefold higher 2 weeks postoperatively compared to the primary hip group. We found a significant correlation between Ki and bone blood flow.

Conclusions

Allogeneic bone grafts induce a higher rate of local periprosthetic bone formation compared to periprosthetic bone formation after a primary total hip placement. In vivo coupling between bone blood flow and bone metabolism suggests that bone metabolism in allogeneic bone grafts may partly rely on bone blood flow adaptations.

Gray matter blood flow change is unevenly distributed during moderate isocapnic hypoxia in humans

Hypoxia increases cerebral blood flow (CBF), but it is unknown whether this increase is uniform across all brain regions. We used H(2)(15)O positron emission tomography imaging to measure absolute blood flow in 50 regions of interest across the human brain (n = 5) during normoxia and moderate hypoxia. Pco(2) was kept constant ( approximately 44 Torr) throughout the study to avoid decreases in CBF associated with the hypocapnia that normally occurs with hypoxia. Breathing was controlled by mechanical ventilation. During hypoxia (inspired Po(2) = 70 Torr), mean end-tidal Po(2) fell to 45 +/- 6.3 Torr (means +/- SD). Mean global CBF increased from normoxic levels of 0.39 +/- 0.13 to 0.45 +/- 0.13 ml/g during hypoxia. Increases in regional CBF were not uniform and ranged from 9.9 +/- 8.6% in the occipital lobe to 28.9 +/- 10.3% in the nucleus accumbens. Regions of interest that were better perfused during normoxia generally showed a greater regional CBF response. Phylogenetically older regions of the brain tended to show larger vascular responses to hypoxia than evolutionary younger regions, e.g., the putamen, brain stem, thalamus, caudate nucleus, nucleus accumbens, and pallidum received greater than average increases in blood flow, while cortical regions generally received below average increases. The heterogeneous blood flow distribution during hypoxia may serve to protect regions of the brain with essential homeostatic roles. This may be relevant to conditions such as altitude, breath-hold diving, and obstructive sleep apnea, and may have implications for functional brain imaging studies that involve hypoxia.

The neurotransmitter basis of cognition: psychopharmacological activation studies using positron emission tomography

The neuromodulatory effect of manipulating monoaminergic receptor function was assessed by combining a psychological and a pharmacological activation during repeated positron emission tomographic (PET) scans. The effects of buspirone (a 5-HT1A receptor partial agonist) on changes in regional cerebral blood flow (rCBF) associated with free word recall were examined. A factorial design was used to demonstrate a significant interaction (changes in rCBF brought about by psychological activation which depend on drug administration) in the left parahippocampal region. This interaction was an attenuation of increases in local neuronal activity (rCBF) related to memory function. Buspirone-induced decreases in rCBF, independent of the memory effect, were seen in the left prefrontal and parietal cortices. We suggest that combined psychological and pharmacological activation is a way of measuring direct (main) drug effects and modulatory effects on neurotransmission associated with cognitive functions (interaction).

Language activation studies with positron emission tomography

Behavioural tasks produce changes in regional cerebral blood flow (rCBF), the result of increased local neural activity. These changes can be measured with positron emission tomography (PET). Language activation studies by means of PET are being used to relate regional patterns of cerebral activation to information-processing models of speech and reading. Significant activation confined to both superior temporal gyri has been observed when normal subjects hear words played backwards, listen to non-words, and perform category judgements on pairs of heard real words. Prestriate cortex is activated by seeing strings of letter-like symbols, consonant strings, pronounceable non-words and real words, with additional activation in left medial prestriate cortex in response to the non-words and real words. Left posterior superior temporal gyrus (PSTG), left dorsolateral prefrontal cortex (DLPFC) and supplementary motor area (SMA) are engaged when subjects retrieve verbs from memory to match nouns. Finally, primary sensorimotor cortex is activated during articulation. There is particular interest at present in the precise roles of left PSTG and DLPFC in single-word comprehension and generation, and interpretation of the results depends critically on the design of the single-word tasks used for behavioural activation.

The functional anatomy of recovery from brain injury

The functional neuroanatomical basis for recovery from ischaemic brain injury is not known. We have used positron emission tomography (PET) to study changes in the functional organization of the brain in patients recovering from striatocapsular motor strokes. Significant changes in regional cerebral blood flow (rCBF) were found during repetitive sequential opposition movements of the fingers in normal subjects and in patients with recovery from motor deficits. There was a difference in the pattern of cerebral activation when patients performed the motor task with the unaffected hand (when the activation was lateralized to contralateral sensorimotor and premotor cortex and ipsilateral cerebellum) and when the task was performed with the recovered, previously plegic hand (when the activation was bilateral and involved novel areas of cortex, especially area 40). Comparisons of rCBF maps at rest in the patient group and in normal subjects showed areas with significantly decreased rCBF in the patients (contralateral to the plegic hand in the basal ganglia, thalamus, insular cortex, brainstem and ipsilateral cerebellum), which reflected the distribution of dysfunction caused by the ischaemic lesions. A significantly increased activation over and above that in normal subjects was found in patients during movement of the recovered fingers in ipsilateral premotor cortex and bilateral frontal opercular/insular regions and area 40, the ipsilateral basal ganglia (the ischaemic lesion lying contralaterally) and the contralateral cerebellum. We postulate that these findings may be explained by the generation of movements by pathways that are different from those that normal subjects use to perform what are ordinarily fairly simple, automated tasks. We suggest that this is a direct demonstration of cerebral plasticity resulting in the resolution of acquired motor deficits.

Separation of input function for rapid measurement of quantitative CMRO2and CBF in a single PET scan with a dual tracer administration method

Cerebral metabolic rate of oxygen (CMRO(2)), oxygen extraction fraction (OEF) and cerebral blood flow (CBF) images can be quantified using positron emission tomography (PET) by administrating (15)O-labelled water (H(15)(2)O) and oxygen ((15)O(2)). Conventionally, those images are measured with separate scans for three tracers C(15)O for CBV, H(15)(2)O for CBF and (15)O(2) for CMRO(2), and there are additional waiting times between the scans in order to minimize the influence of the radioactivity from the previous tracers, which results in a relatively long study period. We have proposed a dual tracer autoradiographic (DARG) approach (Kudomi et al 2005), which enabled us to measure CBF, OEF and CMRO(2) rapidly by sequentially administrating H(15)(2)O and (15)O(2) within a short time. Because quantitative CBF and CMRO(2) values are sensitive to arterial input function, it is necessary to obtain accurate input function and a drawback of this approach is to require separation of the measured arterial blood time-activity curve (TAC) into pure water and oxygen input functions under the existence of residual radioactivity from the first injected tracer. For this separation, frequent manual sampling was required. The present paper describes two calculation methods: namely a linear and a model-based method, to separate the measured arterial TAC into its water and oxygen components. In order to validate these methods, we first generated a blood TAC for the DARG approach by combining the water and oxygen input functions obtained in a series of PET studies on normal human subjects. The combined data were then separated into water and oxygen components by the present methods. CBF and CMRO(2) were calculated using those separated input functions and tissue TAC. The quantitative accuracy in the CBF and CMRO(2) values by the DARG approach did not exceed the acceptable range, i.e., errors in those values were within 5%, when the area under the curve in the input function of the second tracer was larger than half of the first one. Bias and deviation in those values were also compatible to that of the conventional method, when noise was imposed on the arterial TAC. We concluded that the present calculation based methods could be of use for quantitatively calculating CBF and CMRO(2) with the DARG approach.

Medial frontal cortex perfusion abnormalities as evaluated by positron emission tomography in women with climacteric symptoms

OBJECTIVE

To identify functional changes in the brains of women with climacteric symptoms. Images of regional cerebral blood flow (rCBF) were compared statistically between women with and women without symptoms to identify changes in rCBF. Results may provide a better understanding of the neural basis of the symptoms, which are divided into three symptom clusters: vasomotor, psychological, and somatic.

DESIGN

The study participants consisted of 12 women with moderate to severe climacteric symptoms (age 47.5 +/- 5.9 years, mean +/- SD) and 7 women with no symptoms (control group; age 49.6 +/- 4.2 years, mean +/- SD). The study participants were patients at a menopause clinic, and the latter were healthy volunteer nurses and hospital staff. Climacteric symptoms were evaluated by an assessment of the severity of 17 symptoms immediately before positron emission tomography examination of rCBF. The symptoms had been used previously to generate the Kupperman Kohnenki Shogai Index, a modified Kupperman Menopausal Index adapted to Japanese women. rCBF was measured by positron emission tomography with the CO2 dynamic inhalation method.

RESULTS

Reductions in relative rCBF in the patient group were observed in the bilateral rectal gyrus and in the left subcallosal gyrus on a voxel-by-voxel basis as compared with the control group.

CONCLUSIONS

The present study revealed reductions in relative rCBF of the prefrontal cortex of Japanese women with moderate to severe climacteric symptoms. This area is close to that previously addressed in studies of familial bipolar depression and familial unipolar depression, although our participants did not satisfy criteria for depression. This reduction of rCBF may be related to the three climacteric symptom clusters, but further studies are needed for evaluation of its significance. Our results should stimulate investigations into the positron emission tomography rCBF change of these women as to the integration of multiple entities in climacteric symptoms.

Effect of Benzodiazepine Hypnotic Triazolam on Relationship of Blood Pressure and Paco2 to Cerebral Blood Flow During Human Non-Rapid Eye Movement Sleep

We sought to clarify the effect of short-acting benzodiazepine hypnotic on the relationship of arterial blood pressure and arterial partial pressure of carbon dioxide (Paco2) to regional cerebral blood flow (rCBF) during human non-rapid-eye-movement (non-REM) sleep. Nine young normal volunteers were treated in a randomized, crossover design with triazolam or placebo and underwent positron emission tomography at night. During wakefulness and stage 2 and slow wave (stages 3 and 4) sleep, we measured mean arterial blood pressure (MAP), Paco2, and absolute CBF. With triazolam compared to placebo, MAP reduced gradually. During stage 2 sleep, Paco2 increased and whole-brain mean CBF decreased. With triazolam, relative rCBF of the left orbital basal forebrain decreased more during stage 2 than slow wave sleep, whereas absolute CBF of the occipital cortex and cerebral white matter remained constant. During triazolam-induced stage 2 sleep, absolute CBF of the cerebral white matter correlated more strongly to both MAP and Paco2 than during placebo sleep and also correlated more strongly to both MAP and Paco2 than absolute CBF of the occipital cortex. In the frontal white matter, during triazolam-induced stage 2 sleep compared to wakefulness, absolute CBF was significantly better correlated to MAP, but not to Paco2. During triazolam-induced stage 2, the cerebral white matter may receive a modulated CBF regulation having the strengthened relationship of Paco2 to CBF and, more locally, the frontal white matter may depend precariously on CBF regulation.

Rapid quantitative measurement of CMRO(2) and CBF by dual administration of (15)O-labeled oxygen and water during a single PET scan-a validation study and error analysis in anesthetized monkeys

Cerebral blood flow (CBF) and rate of oxygen metabolism (CMRO(2)) may be quantified using positron emission tomography (PET) with (15)O-tracers, but the conventional three-step technique requires a relatively long study period, attributed to the need for separate acquisition for each of (15)O(2), H(2)(15)O, and C(15)O tracers, which makes the multiple measurements at different physiologic conditions difficult. In this study, we present a novel, faster technique that provides a pixel-by-pixel calculation of CBF and CMRO(2) from a single PET acquisition with a sequential administration of (15)O(2) and H(2)(15)O. Experiments were performed on six anesthetized monkeys to validate this technique. The global CBF, oxygen extraction fraction (OEF), and CMRO(2) obtained by the present technique at rest were not significantly different from those obtained with three-step method. The global OEF (gOEF) also agreed with that determined by simultaneous arterio-sinus blood sampling (gOEF(A-V)) for a physiologically wide range when changing the arterial PaCO(2) (gOEF=1.03gOEF(A-V)+0.01, P<0.001). The regional values, as well as the image quality were identical between the present technique and three-step method for CBF, OEF, and CMRO(2). In addition, a simulation study showed that error sensitivity of the present technique to delay or dispersion of the input function, and the error in the partition coefficient was equivalent to that observed for three-step method. Error sensitivity to cerebral blood volume (CBV) was also identical to that in the three-step and reasonably small, suggesting that a single CBV assessment is sufficient for repeated measures of CBF/CMRO(2). These results show that this fast technique has an ability for accurate assessment of CBF/CMRO(2) and also allows multiple assessment at different physiologic conditions.

Evaluation of Basis Function and Linear Least Squares Methods for Generating Parametric Blood Flow Images Using 15O-Water and Positron Emission Tomography

PURPOSE

Parametric analysis of (15)O-water positron emission tomography (PET) studies allows determination of blood flow (BF), perfusable tissue fraction (PTF), and volume of distribution (V (d)) with high spatial resolution. In this paper the performance of basis function and linear least squares methods for generating parametric flow data were evaluated.

PROCEDURES

Monte Carlo simulations were performed using typical perfusion values for brain, tumor, and heart. Clinical evaluation was performed using seven cerebral and 10 myocardial (15)O-water PET studies. Basis function (BFM), linear least squares (LLS), and generalized linear least squares (GLLS) methods were used to calculate BF, PTF, or V(d).

RESULTS

Monte Carlo simulations and human studies showed that, for low BF values (<1 ml/min(-1)ml(-1), BF, PTF, and V(d) were calculated with accuracies better than 5% for all methods tested. For high BF (>2 ml/min(-1)ml(-1)), use of BFM provided more accurate V(d) compared with (G)LLS.

CONCLUSIONS

In general, BFM provided the most accurate estimates of BF, PTF, and V(d).

Cerebral white matter blood flow is constant during human non-rapid eye movement sleep: a positron emission tomographic study

This study aimed to identify brain regions with the least decreased cerebral blood flow (CBF) and their relationship to physiological parameters during human non-rapid eye movement (NREM) sleep. Using [15O]H2O positron emission tomography, CBF was measured for nine normal young adults during nighttime. As NREM sleep progressed, mean arterial blood pressure and whole brain mean CBF decreased significantly; arterial partial pressure of CO2 and, selectively, relative CBF of the cerebral white matter increased significantly. Absolute CBF remained constant in the cerebral white matter, registering 25.9 ± 3.8 during wakefulness, 25.8 ± 3.3 during light NREM sleep, and 26.9 ± 3.0 (ml·100 g−1·min−1) during deep NREM sleep ( P = 0.592), and in the occipital cortex ( P = 0.611). The regression slope of the absolute CBF significantly differed with respect to arterial partial pressure of CO2 between the cerebral white matter (slope 0.054, R = − 0.04) and frontoparietal association cortex (slope − 0.776, R = − 0.31) ( P = 0.005) or thalamus (slope − 1.933, R = − 0.47) ( P = 0.004) and between the occipital cortex (slope 0.084, R = 0.06) and frontoparietal association cortex ( P = 0.021) or thalamus ( P < 0.001), and, with respect to mean arterial blood pressure, between the cerebral white matter (slope − 0.067, R = − 0.10) and thalamus (slope 0.637, R = 0.31) ( P = 0.044). The cerebral white matter CBF keeps constant during NREM sleep as well as the occipital cortical CBF, and may be specifically regulated by both CO2 vasoreactivity and pressure autoregulation.

Quantification in positron emission tomography for research in pharmacology and drug development

Positron emission tomography (PET) is a quantitative in vivo tracer technique, enabling images of the distribution of biochemical, physiological and pharmacological functions in living tissue, at a resolution of a few millimetres. Applications include the imaging of blood flow rate, metabolic rate and neuroreceptor distribution and function. These applications are playing an increasing role in drug development. This brief article seeks to emphasize how these applications of PET need to rest on a solid quantitative foundation.

Integrated image analysis solutions for PET datasets in damaged brain

OBJECTIVE

To identify and discuss the problems inherent in the processing of multiparametric functional imaging datasets from patients with acute brain injury, using "triple oxygen" positron emission tomography (PET) as an example. To present an integrated imaging solution for analysis of such datasets and report on its use in practice.

METHODS

Structured analysis of the steps in image analysis for triple oxygen studies in patients with acute brain injury, subarachnoid haemorrhage and carotid artery disease was undertaken. Systematic identification of the drawbacks of conventional manual processing techniques, which make use of modules from different commercially available software packages was carried out. This analysis informed the construction of an image analysis gateway that addressed these issues.

RESULTS

We describe an integrated image analysis gateway (PETAn) that provides for the automated integration of all processing steps, along with outputs that detail analysis stages, error logs and results. Implementation of PETAn allowed batch processing of clinical PET data sets, reduced the level of technical expertise required for analysis, improved the quality of results and achieved significant reductions in operator analysis time.

CONCLUSIONS

While modules from several imaging software suites can be usefully combined to allow analysis of multiparametric PET data sets from patients with acute brain injury, sequential use of these modules for various steps in image analysis is fraught with difficulty. Careful automation and integration of these steps greatly facilitates the interrogation of information-rich data sets and increases research productivity.

Positron emission tomographic imaging of angiogenesis and vascular function

Surrogate markers of clinical outcome are important in anticancer drug research, since clinical criteria of response develop only slowly and may be confounded by other processes than drug effect. The need for surrogate outcome markers is especially great with newer agents that may act by tumour stabilization as opposed to shrinkage. Neoplastic angiogenesis is associated with a number of detectable changes at molecular and microcirculatory levels. Therefore, direct study of angiogenic molecular biology and tumour circulation before during and after treatment may offer useful surrogate markers for vascular-targeted therapies. The main advantage of radiotracer imaging with positron emission tomography (PET) is its functional specificity. This article will review two main areas: (a) the methodology behind PET imaging of tumour blood supply with 15O-oxygen labelled compounds; and (b) newer tracers in development as markers of angiogenetic biology.

Assessment of pharmacodynamic vascular response in a phase I trial of combretastatin A4 phosphate

PURPOSE

Clinical evaluation of novel agents that target tumor blood vessels requires pharmacodynamic end points that measure vascular damage. Positron emission tomography (PET) was used to measure the effects of the vascular targeting agent combretastatin A4 phosphate (CA4P) on tumor and normal tissue perfusion and blood volume.

PATIENTS AND METHODS

Patients with advanced solid tumors were enrolled onto part of a phase I, accelerated-titration, dose-escalation study. The effects of 5 to 114 mg/m2 CA4P on tumor, spleen, and kidney were investigated. Tissue perfusion was measured using oxygen-15 (15O)-labeled water and blood volume was measured using 15O-labeled carbon monoxide (C15O). Scans were performed immediately before, and 30 minutes and 24 hours after the first infusion of each dose level of CA4P. All statistical tests were two sided.

RESULTS

PET data were obtained for 13 patients with intrapatient dose escalation. Significant dose-dependent reductions were seen in tumor perfusion 30 minutes after CA4P administration (mean change, -49% at >or= 52 mg/m2; P =.0010). Significant reductions were also seen in tumor blood volume (mean change, -15% at >or= 52 mg/m2; P =.0070). Although by 24 hours there was tumor vascular recovery, for doses >or= 52 mg/m2 the reduction in perfusion remained significant (P =.013). Thirty minutes after CA4P administration borderline significant changes were seen in spleen perfusion (mean change, -35%; P =.018), spleen blood volume (mean change, -18%; P =.022), kidney perfusion (mean change, -6%; P =.026), and kidney blood volume (mean change, -6%; P =.014). No significant changes were seen at 24 hours in spleen or kidney.

CONCLUSION

CA4P produces rapid changes in the vasculature of human tumors that can be assessed using PET measurements of tumor perfusion.

Accurate determination of metabolic rates from dynamic positron emission tomography data with very-low temporal resolution

PURPOSE

The graphical approach is widely used for the pixelwise determination of local metabolic rate of glucose from dynamic positron emission tomography (PET) data. In its conventional implementation, measured integrals over time frames are used to approximate instantaneous tracer concentrations at midframe times ("midframe approach"). This is justified in case of high temporal resolution of the PET measurement; that is, if scan protocols with a large number of short frames are used. This requires fast data handling and large amounts of memory. Cardiac gating and three-dimensional (3D) acquisition of dynamic studies is hardly possible with this approach. Therefore, a new variant of the graphical method is proposed which can be used with a very low number of rather long frames.

METHODS

An operational equation of the graphical method was derived which uses measured time integrals only and, thus, avoids the systematic errors of the midframe approximation. This "integral approach" was evaluated in computer simulations based on experimental data.

RESULTS

The integral approach enables the use of protocols with 3 frames only without compromising accuracy of the derived metabolic rates whereas the midframe approach leads to bias of about 10% to 20% for these protocols. Furthermore, test-retest stability can significantly be improved when using the integral approach.

CONCLUSION

The integral approach to the graphical evaluation of dynamic PET data yields accurate and precise results using scan protocols with down to only 3 frames. This can be relevant to gating and/or 3D acquisition of dynamic studies. The integral approach is applied most naturally whenever the input function is derived from the dynamic PET data.

The effect of apomorphine and buspirone on regional cerebral blood flow during the performance of a cognitive task-measuring neuromodulatory effects of psychotropic drugs in man

Psychopharmacological activation, in conjunction with positron emission tomographic measurements of regional cerebral blood flow (rCBF), was used to investigate the neurotransmitter basis of a specific cognitive function in man. Monoaminergic neurotransmission was pharmacologically manipulated during performance of auditory - verbal memory tasks. Statistical parametric mapping was used to identify the brain sites of interaction between memory-induced increases in rCBF and active drugs. Memory task-induced increases in rCBF in the left prefrontal cortex were attenuated by apomorphine, a non-selective dopamine agonist, whilst buspirone, a serotonin1A partial agonist, augmented rCBF increases in this area. In addition, apomorphine and buspirone augmented memory-induced increases in rCBF centred in the posterior cingulate cortex, whilst buspirone alone attenuated rCBF increases in the retrosplenial cortex and posterior parahippocampal gyrus. These regionally selective interactions may represent neuromodulatory effects of monoaminergic neurotransmission on a specific cognitive function in man.

Central neural contribution to the perception of chest pain in cardiac syndrome X

OBJECTIVE

To investigate the central neural contribution to chest pain perception in cardiac syndrome X (angina-like pain, ECG changes during stress, angiographically normal coronary arteriogram).

SUBJECTS

Eight syndrome X patients and eight healthy volunteers.

METHODS

Dobutamine stress using echocardiography to assess myocardial function, and positron emission tomography to measure changes in regional cerebral blood flow, as an index of neuronal activity.

RESULTS

During similar doses of dobutamine, syndrome X patients and controls showed comparable regional cerebral blood flow changes in the hypothalamus, thalami, right orbito-frontal cortex, and anterior temporal poles, associated with the sensation of a fast or powerful heart beat. In patients, but not controls, the stress also generated severe chest pain associated with increased activity in the right anterior insula/frontal operculum junction. There were ischaemia-like ECG changes in the syndrome X patients, but no left ventricular dysfunction on echocardiography. Activation of the right insula during chest pain clearly distinguished the syndrome X patients from a group of patients with known coronary disease.

CONCLUSIONS

Chest pain and ECG changes were not accompanied by demonstrable myocardial dysfunction in syndrome X patients, but altered central neural handling of afferent signals may contribute to the abnormal pain perception in these patients.

Self-administration of cocaine and the cocaine analog RTI-113: relationship to dopamine transporter occupancy determined by PET neuroimaging in rhesus monkeys

Dopaminergic mechanisms are thought to play a central role in the reinforcing effects of cocaine. The present study examined the reinforcing effects of 3beta-(4-chlorophenyl)tropane-2beta-carboxylic acid phenyl ester (RTI-113), a long-acting, selective, high-affinity dopamine uptake inhibitor. Additionally, the effects of RTI-113 pretreatment on cocaine self-administration were determined. Monkeys were trained to respond under a second-order schedule for intravenous cocaine administration (0.10 or 0.17 mg/kg/infusion). When responding was stable, cocaine (0.0030-1.0 mg/kg/infusion) and RTI-113 (0.010-0.30 mg/kg/infusion) were substituted for the cocaine training dose. Cocaine and RTI-113 were equipotent for their reinforcing effects. However, cocaine maintained higher response rates in two of the three monkeys tested. When administered as a pretreatment, RTI-113 (0.10-0.30 mg/kg) dose-dependently reduced responding maintained by two doses of cocaine. Drug effects on behavior were related to dopamine transporter (DAT) occupancy in monkey striatum during neuroimaging with positron emission tomography. DAT occupancy was determined by displacement of 8-(2-[(18)F]fluroethyl)2beta-carbomethoxy-3beta-(4-chlorophenyl)nortropane (FECNT). DAT occupancy was between 65-76% and 94-99% for doses of cocaine and RTI-113 that maintained maximum response rates, respectively. DAT occupancy did not differ markedly across RTI-113 pretreatment doses and ranged between 72-84%. The results suggest that the pharmacokinetic profile of RTI-113 (i.e., long-acting) may influence its ability to maintain self-administration, and therefore its abuse liability. Additionally, high DAT occupancy is required for RTI-113 to reduce cocaine-maintained responding.

A computationally efficient algorithm for determining regional cerebral blood flow in heterogeneous tissues by positron emission tomography

Inclusion of brain tissues with different rates of blood flow and metabolism within a voxel or region of interest is an unavoidable problem with positron emission tomography due to its limited spatial resolution. Because regional cerebral blood flow (rCBF) is higher in gray matter than in white matter, the partial volume effect leads to underestimation of rCBF in gray matter when rCBF in the region as a whole is determined. Furthermore, weighted-average rCBF itself is underestimated if the kinetic model used in the analysis fails to account for the tissue heterogeneity. We have derived a computationally efficient method for estimating both gray matter and weighted-average rCBF in heterogeneous tissues and validated the method in simulation studies. The method is based on a model that represents a heterogeneous tissue as a weighted mixture of two homogeneous tissues. A linear least squares algorithm is used to estimate the model parameters.

PET/SPECT: functional imaging beyond flow

In this review, first principles of positron emission tomography (PET) and single photon emission tomography (SPECT) are discussed together with the relative strengths and weaknesses of both techniques. With both modalities it is possible to image and, especially with PET, to measure regional tissue function, the particular function being interrogated depending on the actual tracer being used. In the second part, the use of PET for neuroactivation studies is presented, illustrated with some key examples from the literature using both perfusion and metabolism tracers. It is argued that the future of PET (and SPECT) neuroactivation studies lies in the use of ligands. Possible approaches for performing ligand activation studies are discussed.

The neural correlates of updating information in verbal working memory

The aim of the present study was to re-examine cerebral areas subserving the updating function of the central executive with a running span task requiring subjects to watch strings of consonants of unknown length and then to recall serially a specific number of recent items. In order to dissociate more precisely the updating process from the storage function, a four-item instead of a six-item memory load was used, contrary to our previous study (Salmon et al., 1996). In addition, a serial recall procedure was preferred to a recognition procedure in order to suppress the use of visuospatial strategies. The most significant increase of rCBF occurred in the left frontopolar cortex (Brodmann's area 10), spreading to the left middle frontal (Brodmann's area 46). Results suggest that frontopolar activation underlies an updating process in working memory.

The effect of the nitric oxide synthase inhibitor L-NMMA on basal CBF and vasoneuronal coupling in man: a PET study

Nitric oxide (NO) regulates basal CBF. In a number of animal models NO has been implicated in the mediation of the regional changes in CBF (rCBF) that accompany neuronal activation (vasoneuronal coupling). However, some results in animal models have failed to confirm this finding, and the validity of extrapolation to man from animal data is uncertain. To determine the contribution of NO to basal global CBF and activation-induced changes in rCBF, the authors have performed quantitative H2(15)O positron emission tomography (PET) studies before and after administration of the non-isoform-specific NO synthase inhibitor, N(G)-monomethyl-L-arginine (L-NMMA), in 10 healthy male volunteers. Learning a novel sequence of finger movements was used as a paradigm to induce regional frontal cortex activation. The effect of NO synthase inhibition on the magnitude and pattern of activation was determined. Resting global CBF fell from 33.3 +/- 5.3 mL x 100 g(-1) x min(-1) at rest before L-NMMA, to 26.5 +/- 7.7 mL x 100 g(-1) x min(-1) after L-NMMA (P = 0.001). This fall was reversed by L-arginine administration. Learning sequential finger movements induced increases in rCBF in the left motor, right prefrontal, and bilateral premotor cortices. After NO synthase inhibition with L-NMMA, there was no change in this pattern of activation and no reduction in the magnitude of rCBF responses at the foci of maximal stimulation before and after L-NMMA. These findings confirm that NO production contributes to basal CBF regulation in man, but show that systemic NO synthase inhibition with L-NMMA does not impair regional vasoneuronal coupling.

Regional brain activity during tasks devoted to the central executive of working memory

Most previous PET studies investigating the central executive (CE) component of working memory found activation in the prefrontal cortex. However, the tasks used did not always permit to distinguish precisely the functions of the CE from the storage function of the slave systems. The aim of the present study was to isolate brain areas that subserve manipulation of information by the CE when the influence of storage function was removed. A PET activation study was performed with four cognitive tasks, crossing conditions of temporary storage and manipulation of information. The manipulation of information induced an activation in the right (BA 10/46) and left (BA 9/6) middle frontal gyrus and in the left parietal area (BA7). The interaction between the storage and manipulation conditions did not reveal any significant changes in activation. These results are in agreement with the hypothesis that CE functions are distributed between anterior and posterior brain areas, but could also reflect a simultaneous involvement of controlled (frontal) and automatic (parietal) attentional systems. In the other hand, the absence of interaction between the storage and manipulation conditions demonstrates that the CE is not necessarily related to the presence of a memory load.

Increased activation of frontal areas during arm movement in idiopathic torsion dystonia

Most positron emission tomography (PET) studies of regional cerebral function in idiopathic torsion dystonia (ITD) have failed to show abnormalities, but there have been few studies of the changes in regional cerebral blood flow (rCBF) that occur during movement in dystonia. Using PET, we have studied six patients with familial generalized ITD both at rest and while moving a joystick with the right hand. The patterns of CBF change obtained were compared with those in six age-matched control subjects. In the dystonia group, free selection of movement was associated with relative increases in rCBF above that observed in control subjects in the left premotor area, the supplementary motor area (SMA), the anterior cingulate cortex, and the left dorsolateral prefrontal area. Subcortical increases were observed within the cerebellum and the putamen. There was a relative decrease in flow through the contralateral primary sensorimotor cortex. These findings contrast with those reported in patients with Parkinson's disease undertaking the same task in which the activity in the SMA and putamen was decreased. We suggest that arm dystonia in ITD is associated with overactivity of the premotor areas, including the SMA, and that this results from release of the thalamus from the normal inhibitory influence of the globus pallidus internal segment. Other abnormalities of basal ganglia control of brain stem centers may be involved in axial dystonia.

Where the imaginal appears real: a positron emission tomography study of auditory hallucinations

An auditory hallucination shares with imaginal hearing the property of being self-generated and with real hearing the experience of the stimulus being an external one. To investigate where in the brain an auditory event is "tagged" as originating from the external world, we used positron emission tomography to identify neural sites activated by both real hearing and hallucinations but not by imaginal hearing. Regional cerebral blood flow was measured during hearing, imagining, and hallucinating in eight healthy, highly hypnotizable male subjects prescreened for their ability to hallucinate under hypnosis (hallucinators). Control subjects were six highly hypnotizable male volunteers who lacked the ability to hallucinate under hypnosis (nonhallucinators). A region in the right anterior cingulate (Brodmann area 32) was activated in the group of hallucinators when they heard an auditory stimulus and when they hallucinated hearing it but not when they merely imagined hearing it. The same experimental conditions did not yield this activation in the group of nonhallucinators. Inappropriate activation of the right anterior cingulate may lead self-generated thoughts to be experienced as external, producing spontaneous auditory hallucinations.

Regional differences in cerebral blood flow and glucose utilization in diabetic man: the effect of insulin

To determine the effect of insulin on regional cerebral blood flow (rCBF) and glucose metabolism (CMRglu), we performed quantitative dynamic PET scanning of labeled water (H215O) and deoxyglucose (18FDG) using two protocols in 10 diabetic men. In protocol A, to test reproducibility of the technique, insulin was infused at 1.5 mU.kg-1.min-1 twice (n = 5). In protocol B, low (0.3 mU.kg-1.min-1) and high (3 mU.kg-1.min-1) dose insulin was given on separate occasions (n = 5). Euglycemia (5 mmol/L) was maintained by glucose infusion. In protocol A, CMRglu was 6% higher during the first infusion, and catecholamines were also increased, indicating stress. Blood flow was not different. Changing free insulin levels from 20.5 +/- 4.8 to 191 +/- 44.5 mU/L (P < 0.001, low versus high dose, protocol B) did not alter total or regional CMRglu (whole brain 36.6 +/- 4.0 versus 32.8 +/- 6.2 mumol.100 g-1.min-1, P = 0.32) or CBF (41.7 +/- 5.1 and 45.6 +/- 9.7 mL.100 g-1.min-1, P = 0.4) or rCBF. In cerebellum, CMRglu was lower than in cortex and the ratio between rate constants for glucose uptake and phosphorylation (K1 and k3) was reversed. There are regional differences in cerebral metabolic capacity that may explain why cerebral cortex is more sensitive to hypoglycemia than cerebellum. Brain glucose metabolism is not sensitive to insulin concentration within the physiologic range. This suggests that intracerebral insulin receptors have a different role from those in the periphery.

Reduced transit-time sensitivity in noninvasive magnetic resonance imaging of human cerebral blood flow

Herein, we present a theoretical framework and experimental methods to more accurately account for transit effects in quantitative human perfusion imaging using endogenous magnetic resonance imaging (MRI) contrast. The theoretical transit time sensitivities of both continuous and pulsed inversion spin tagging experiments are demonstrated. We propose introducing a delay following continuous labeling, and demonstrate theoretically that introduction of a delay dramatically reduces the transit time sensitivity of perfusion imaging. The effects of magnetization transfer saturation on this modified continuous labeling experiment are also derived, and the assumption that the perfusion signal resides entirely within tissue rather than the arterial microvasculature is examined. We present results demonstrating the implementation of the continuous tagging experiment with delay on an echoplanar scanner for measuring cerebral blood flow (CBF) in normal volunteers. By varying the delay, we estimate transit times in the arterial system, values that are necessary for assessing the accuracy of our quantification. The effect of uncertainties in the transit time from the tagging plane to the arterial microvasculature and the transit time to the tissue itself on the accuracy of perfusion quantification is discussed and found to be small in gray matter but still potentially significant in white matter. A novel method for measuring T1, which is fast, insensitive to contamination by cerebrospinal fluid, and compatible with the application of magnetization transfer saturation, is also presented. The methods are combined to produce quantitative maps of resting and hypercarbic CBF.