Errors introduced by tissue heterogeneity in estimation of local cerebral glucose utilization with current kinetic models of the [18F]fluorodeoxyglucose method

Early metabolic changes in the brain of Alzheimer's disease rats are driven by GLAST+ cells

Glucose metabolic dysfunction is a hallmark of Alzheimer's disease (AD) pathology and is used to diagnose the disease or predict imminent cognitive decline. The main method to measure brain metabolism in vivo is positron emission tomography with 2-Deoxy-2-[18F]fluoroglucose ([18F]FDG-PET). The cellular origin of changes in the [18F]FDG-PET signal in AD is controversial. We addressed this by combining [18F]FDG-PET with subsequent cell-sorting and γ-counting of [18F]FDG-accumulation in sorted cell populations. 7-month-old male TgF344-AD rats and wild-type controls (n = 24/group) received sham or ceftriaxone (200 mg/kg) injection prior to [18F]FDG-PET imaging to increase glutamate uptake and glucose utilisation. The same animals were injected again one week later, and radiolabelled brains were dissected, with hippocampi taken for magnetically-activated cell sorting of radioligand-treated tissues (MACS-RTT). Radioactivity in sorted cell populations was measured to quantify cell-specific [18F]FDG uptake. Transcriptional analyses of metabolic enzymes/transporters were also performed. Hypometabolism in the frontal association cortex of TgF344-AD rats was identified using [18F]FDG-PET, whereas hypermetabolism was identified in the hippocampus using MACS-RTT. Hypermetabolism was primarily driven by GLAST+ cells. This was supported by transcriptional analyses which showed alteration to metabolic apparatus, including upregulation of hexokinase 2 and altered expression of glucose/lactate transporters. See Figure 1 for summary.

EMATA: a toolbox for the automatic extraction and modeling of arterial inputs for tracer kinetic analysis in [18F]FDG brain studies

PURPOSE

PET imaging is a pivotal tool for biomarker research aimed at personalized medicine. Leveraging the quantitative nature of PET requires knowledge of plasma radiotracer concentration. Typically, the arterial input function (AIF) is obtained through arterial cannulation, an invasive and technically demanding procedure. A less invasive alternative, especially for [18F]FDG, is the image-derived input function (IDIF), which, however, often requires correction for partial volume effect (PVE), usually performed via venous blood samples. The aim of this paper is to present EMATA: Extraction and Modeling of Arterial inputs for Tracer kinetic Analysis, an open-source MATLAB toolbox. EMATA automates IDIF extraction from [18F]FDG brain PET images and additionally includes a PVE correction procedure that does not require any blood sampling.

METHODS

To assess the toolbox generalizability and present example outputs, EMATA was applied to brain [18F]FDG dynamic data of 80 subjects, extracted from two distinct datasets (40 healthy controls, 40 glioma patients). Additionally, to compare with the reference standard, quantification using both IDIF and AIF was carried out on a third open-access dataset of 18 healthy individuals.

RESULTS

EMATA consistently performs IDIF extraction across all datasets, despite differences in scanners and acquisition protocols. Remarkably high agreement is observed when comparing Patlak's Ki between IDIF and AIF (R2: 0.98 ± 0.02).

CONCLUSION

EMATA proved adaptability to different datasets characteristics and the ability to provide arterial input functions that can be used for reliable PET quantitative analysis.

In vivo methods for imaging blood-brain barrier function and dysfunction

The blood-brain barrier (BBB) is the interface between the central nervous system and systemic circulation. It tightly regulates what enters and is removed from the brain parenchyma and is fundamental in maintaining brain homeostasis. Increasingly, the BBB is recognised as having a significant role in numerous neurological disorders, ranging from acute disorders (traumatic brain injury, stroke, seizures) to chronic neurodegeneration (Alzheimer's disease, vascular dementia, small vessel disease). Numerous approaches have been developed to study the BBB in vitro, in vivo, and ex vivo. The complex multicellular structure and effects of disease are difficult to recreate accurately in vitro, and functional aspects of the BBB cannot be easily studied ex vivo. As such, the value of in vivo methods to study the intact BBB cannot be overstated. This review discusses the structure and function of the BBB and how these are affected in diseases. It then discusses in depth several established and novel methods for imaging the BBB in vivo, with a focus on MRI, nuclear imaging, and high-resolution intravital fluorescence microscopy.

Hemodynamic and metabolic correspondence of resting-state voxel-based physiological metrics in healthy adults

Voxel-based physiological (VBP) variables derived from blood oxygen level dependent (BOLD) fMRI time-course variations include: amplitude of low frequency fluctuations (ALFF), fractional amplitude of low frequency fluctuations (fALFF) and regional homogeneity (ReHo). Although these BOLD-derived variables can detect between-group (e.g. disease vs control) spatial pattern differences, physiological interpretations are not well established. The primary objective of this study was to quantify spatial correspondences between BOLD VBP variables and PET measurements of cerebral metabolic rate and hemodynamics, being well-validated physiological standards. To this end, quantitative, whole-brain PET images of metabolic rate of glucose (MRGlu; 18FDG) and oxygen (MRO2; 15OO), blood flow (BF; H215O) and blood volume (BV; C15O) were obtained in 16 healthy controls. In the same subjects, BOLD time-courses were obtained for computation of ALFF, fALFF and ReHo images. PET variables were compared pair-wise with BOLD variables. In group-averaged, across-region analyses, ALFF corresponded significantly only with BV (R = 0.64; p < 0.0001). fALFF corresponded most strongly with MRGlu (R = 0.79; p < 0.0001), but also significantly (p < 0.0001) with MRO2 (R = 0.68), BF (R = 0.68) and BV (R=0.68). ReHo performed similarly to fALFF, with significant strong correspondence (p < 0.0001) with MRGlu (R = 0.78), MRO2 (R = 0.54), and, but less strongly with BF (R = 0.50) and BV (R=0.50). Mutual information analyses further clarified these physiological interpretations. When conditioned by BV, ALFF retained no significant MRGlu, MRO2 or BF information. When conditioned by MRGlu, fALFF and ReHo retained no significant MRO2, BF or BV information. Of concern, however, the strength of PET-BOLD correspondences varied markedly by brain region, which calls for future investigation on physiological interpretations at a regional and per-subject basis.

Principles of Tracer Kinetic Analysis in Oncology, Part I: Principles and Overview of Methodology

Learning Objectives: On successful completion of this activity, participants should be able to describe (1) describe principles of PET tracer kinetic analysis for oncologic applications; (2) list methods used for PET kinetic analysis for oncology; and (3) discuss application of kinetic modeling for cancer-specific diagnostic needs.Financial Disclosure: This work was supported by KL2 TR001879, R01 CA211337, R01 CA113941, R33 CA225310, Komen SAC130060, R50 CA211270, and K01 DA040023. Dr. Pantel is a consultant or advisor for Progenics and Blue Earth Diagnostics and is a meeting participant or lecturer for Blue Earth Diagnostics. Dr. Mankoff is on the scientific advisory boards of GE Healthcare, Philips Healthcare, Reflexion, and ImaginAb and is the owner of Trevarx; his wife is the chief executive officer of Trevarx. The authors of this article have indicated no other relevant relationships that could be perceived as a real or apparent conflict of interest.CME Credit: SNMMI is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to sponsor continuing education for physicians. SNMMI designates each JNM continuing education article for a maximum of 2.0 AMA PRA Category 1 Credits. Physicians should claim only credit commensurate with the extent of their participation in the activity. For CE credit, SAM, and other credit types, participants can access this activity through the SNMMI website (http://www.snmmilearningcenter.org) through March 2025PET enables noninvasive imaging of regional in vivo cancer biology. By engineering a radiotracer to target specific biologic processes of relevance to cancer (e.g., cancer metabolism, blood flow, proliferation, and tumor receptor expression or ligand binding), PET can detect cancer spread, characterize the cancer phenotype, and assess its response to treatment. For example, imaging of glucose metabolism using the radiolabeled glucose analog ¹⁸F-FDG has widespread applications to all 3 of these tasks and plays an important role in cancer care. However, the current clinical practice of imaging at a single time point remote from tracer injection (i.e., static imaging) does not use all the information that PET cancer imaging can provide, especially to address questions beyond cancer detection. Reliance on tracer measures obtained only from static imaging may also lead to misleading results. In this 2-part continuing education paper, we describe the principles of tracer kinetic analysis for oncologic PET (part 1), followed by examples of specific implementations of kinetic analysis for cancer PET imaging that highlight the added benefits over static imaging (part 2). This review is designed to introduce nuclear medicine clinicians to basic concepts of kinetic analysis in oncologic imaging, with a goal of illustrating how kinetic analysis can augment our understanding of in vivo cancer biology, improve our approach to clinical decision making, and guide the interpretation of quantitative measures derived from static images.

First results on kinetic modelling and parametric imaging of dynamic 18F-FDG datasets from a long axial FOV PET scanner in oncological patients

PURPOSE

To investigate the kinetics of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) by positron emission tomography (PET) in multiple organs and test the feasibility of total-body parametric imaging using an image-derived input function (IDIF).

METHODS

Twenty-four oncological patients underwent dynamic ¹⁸F-FDG scans lasting 65 min using a long  axial FOV (LAFOV) PET/CT system. Time activity curves (TAC) were extracted from semi-automated segmentations of multiple organs, cerebral grey and white matter, and from vascular structures. The tissue and tumor lesion TACs were fitted using an irreversible two-tissue compartment (2TC) and a Patlak model. Parametric images were also generated using direct and indirect Patlak methods and their performances were evaluated.

RESULTS

We report estimates of kinetic parameters and metabolic rate of glucose consumption (MR_FDG) for different organs and tumor lesions. In some organs, there were significant differences between MR_FDG values estimated using 2TC and Patlak models. No statistically significant difference was seen between MR_FDG values estimated using 2TC and Patlak methods in tumor lesions (paired t-test, P = 0.65). Parametric imaging showed that net influx (K_i) images generated using direct and indirect Patlak methods had superior tumor-to-background ratio (TBR) to standard uptake value (SUV) images (3.1- and 3.0-fold mean increases in TBR_mean, respectively). Influx images generated using the direct Patlak method had twofold higher contrast-to-noise ratio in tumor lesions compared to images generated using the indirect Patlak method.

CONCLUSION

We performed pharmacokinetic modelling of multiple organs using linear and non-linear models using dynamic total-body ¹⁸F-FDG images. Although parametric images did not reveal more tumors than SUV images, the results confirmed that parametric imaging furnishes improved tumor contrast. We thus demonstrate the feasibility of total-body kinetic modelling and parametric imaging in basic research and oncological studies. LAFOV PET can enhance dynamic imaging capabilities by providing high sensitivity parametric images and allowing total-body pharmacokinetic analysis.

Cancer and the Dopamine D2 Receptor: A Pharmacological Perspective

The dopamine D2 receptor (D2R) family is upregulated in many cancers and tied to stemness. Reduced cancer risk has been correlated with disorders such as schizophrenia and Parkinson's disease, in which dopaminergic drugs are used. D2R antagonists are reported to have anticancer efficacy in cell culture and animal models where they have reduced tumor growth, induced autophagy, affected lipid metabolism, and caused apoptosis, among other effects. This has led to several hypotheses, the most prevalent being that D2R ligands may be a novel approach to cancer chemotherapy. This hypothesis is appealing because of the large number of approved and experimental drugs of this class that could be repurposed. We review the current state of the literature and the evidence for and against this hypothesis. When the existing literature is evaluated from a pharmacological context, one of the striking findings is that the concentrations needed for cytotoxic effects of D2R antagonists are orders of magnitude higher than their affinity for this receptor. Although additional definitive studies will provide further clarity, our hypothesis is that targeting D2-like dopamine receptors may only yield useful ligands for cancer chemotherapy in rare cases.

Inhibition of UGT1A1 by natural and synthetic flavonoids

Flavonoids are widely distributed phytochemicals in vegetables, fruits and medicinal plants. Recent studies demonstrate that some natural flavonoids are potent inhibitors of the human UDP-glucuronosyltransferase 1A1 (UGT1A1), a key enzyme in detoxification of endogenous harmful compounds such as bilirubin. In this study, the inhibitory effects of 56 natural and synthetic flavonoids on UGT1A1 were assayed, while the structure-inhibition relationships of flavonoids as UGT1A1 inhibitors were investigated. The results demonstrated that the C-3 and C-7 hydroxyl groups on the flavone skeleton would enhance UGT1A1 inhibition, while flavonoid glycosides displayed weaker inhibitory effects than their corresponding aglycones. Further investigation on inhibition kinetics of two strong flavonoid-type UGT1A1 inhibitors, acacetin and kaempferol, yielded interesting results. Both flavonoids were competitive inhibitors against UGT1A1-mediated NHPN-O-glucuronidation, but were mixed and competitive inhibitors toward UGT1A1-mediated NCHN-O-glucuronidation, respectively. Furthermore, docking simulations showed that the binding areas of NHPN, kaempferol and acacetin on UGT1A1 were highly overlapping, and convergence with the binding area of bilirubin within UGT1A1. In summary, detailed structure-inhibition relationships of flavonoids as UGT1A1 inhibitors were investigated carefully and the findings shed new light on the interactions between flavonoids and UGT1A1, and will contribute considerably to the development of flavonoid-type drugs without strong UGT1A1 inhibition.

Effects of shortened scanning intervals on calculated regional rates of cerebral protein synthesis determined with the L-[1-11C]leucine PET method

To examine effects of scan duration on estimates of regional rates of cerebral protein synthesis (rCPS), we reanalyzed data from thirty-nine previously reported L-[1-¹¹C]leucine PET studies. Subjects consisted of 12 healthy volunteers studied twice, awake and under propofol sedation, and 15 subjects with fragile X syndrome (FXS) studied once under propofol sedation. All scans were acquired on a high resolution scanner. We used a basis function method for voxelwise estimation of parameters of the kinetic model of L-[1-¹¹C]leucine and rCPS over the interval beginning at the time of tracer injection and ending 30, 45, 60, 75 or 90 min later. For each study and scan interval, regional estimates in nine regions and whole brain were obtained by averaging voxelwise estimates over all voxels in the region. In all three groups rCPS was only slightly affected by scan interval length and was very stable between 60 and 90 min. Furthermore, statistical comparisons of rCPS between awake and sedated healthy volunteers provided almost identical results when they were based on 60 min scan data as when they were based on data from the full 90 min interval. Statistical comparisons between sedated healthy volunteers and sedated subjects with FXS also yielded almost identical results when based on 60 and 90 min scan intervals. We conclude that, under the conditions of our studies, scan duration can be shortened to 60 min without loss of precision.

Spectral Clustering Predicts Tumor Tissue Heterogeneity Using Dynamic 18F-FDG PET: A Complement to the Standard Compartmental Modeling Approach

In this study, we described and validated an unsupervised segmentation algorithm for the assessment of tumor heterogeneity using dynamic ¹⁸F-FDG PET. The aim of our study was to objectively evaluate the proposed method and make comparisons with compartmental modeling parametric maps and SUV segmentations using simulations of clinically relevant tumor tissue types. Methods: An irreversible 2-tissue-compartmental model was implemented to simulate clinical and preclinical ¹⁸F-FDG PET time-activity curves using population-based arterial input functions (80 clinical and 12 preclinical) and the kinetic parameter values of 3 tumor tissue types. The simulated time-activity curves were corrupted with different levels of noise and used to calculate the tissue-type misclassification errors of spectral clustering (SC), parametric maps, and SUV segmentation. The utility of the inverse noise variance- and Laplacian score-derived frame weighting schemes before SC was also investigated. Finally, the SC scheme with the best results was tested on a dynamic ¹⁸F-FDG measurement of a mouse bearing subcutaneous colon cancer and validated using histology. Results: In the preclinical setup, the inverse noise variance-weighted SC exhibited the lowest misclassification errors (8.09%-28.53%) at all noise levels in contrast to the Laplacian score-weighted SC (16.12%-31.23%), unweighted SC (25.73%-40.03%), parametric maps (28.02%-61.45%), and SUV (45.49%-45.63%) segmentation. The classification efficacy of both weighted SC schemes in the clinical case was comparable to the unweighted SC. When applied to the dynamic ¹⁸F-FDG measurement of colon cancer, the proposed algorithm accurately identified densely vascularized regions from the rest of the tumor. In addition, the segmented regions and clusterwise average time-activity curves showed excellent correlation with the tumor histology. Conclusion: The promising results of SC mark its position as a robust tool for quantification of tumor heterogeneity using dynamic PET studies. Because SC tumor segmentation is based on the intrinsic structure of the underlying data, it can be easily applied to other cancer types as well.

The Effects of Capillary Transit Time Heterogeneity (CTH) on the Cerebral Uptake of Glucose and Glucose Analogs: Application to FDG and Comparison to Oxygen Uptake

Glucose is the brain's principal source of ATP, but the extent to which cerebral glucose consumption (CMR_glc) is coupled with its oxygen consumption (CMRO₂) remains unclear. Measurements of the brain's oxygen-glucose index OGI = CMRO₂/CMR_glc suggest that its oxygen uptake largely suffices for oxidative phosphorylation. Nevertheless, during functional activation and in some disease states, brain tissue seemingly produces lactate although cerebral blood flow (CBF) delivers sufficient oxygen, so-called aerobic glycolysis. OGI measurements, in turn, are method-dependent in that estimates based on glucose analog uptake depend on the so-called lumped constant (LC) to arrive at CMR_glc. Capillary transit time heterogeneity (CTH), which is believed to change during functional activation and in some disease states, affects the extraction efficacy of oxygen from blood. We developed a three-compartment model of glucose extraction to examine whether CTH also affects glucose extraction into brain tissue. We then combined this model with our previous model of oxygen extraction to examine whether differential glucose and oxygen extraction might favor non-oxidative glucose metabolism under certain conditions. Our model predicts that glucose uptake is largely unaffected by changes in its plasma concentration, while changes in CBF and CTH affect glucose and oxygen uptake to different extents. Accordingly, functional hyperemia facilitates glucose uptake more than oxygen uptake, favoring aerobic glycolysis during enhanced energy demands. Applying our model to glucose analogs, we observe that LC depends on physiological state, with a risk of overestimating relative increases in CMR_glc during functional activation by as much as 50%.

Feasibility of 18F-Fluoromisonidazole Kinetic Modeling in Head and Neck Cancer Using Shortened Acquisition Times

(18)F-fluoromisonidazole dynamic PET (dPET) is used to identify tumor hypoxia noninvasively. Its routine clinical implementation, however, has been hampered by the long acquisition times required. We investigated the feasibility of kinetic modeling using shortened acquisition times in (18)F-fluoromisonidazole dPET, with the goal of expediting the clinical implementation of (18)F-fluoromisonidazole dPET protocols.

METHODS

Six patients with squamous cell carcinoma of the head and neck and 10 HT29 colorectal carcinoma-bearing nude rats were studied. In addition to an (18)F-FDG PET scan, each patient underwent a 45-min (18)F-fluoromisonidazole dPET scan, followed by 10-min acquisitions at 96 ± 4 and 163 ± 17 min after injection. Ninety-minute (18)F-fluoromisonidazole dPET scans were acquired in animals. Intratumor voxels were classified into 4 clusters based on their kinetic behavior using k-means clustering. Kinetic modeling was performed using the foregoing full datasets (FD) and repeated for each of 2 shortened datasets corresponding to the first approximately 100 min (SD1; patients only) or the first 45 min (SD2) of dPET data. The kinetic rate constants (KRCs) as calculated with a 2-compartment model for both SD1 and SD2 were compared with those derived from FD by correlation (Pearson), regression (Passing-Bablok), deviation (Bland-Altman), and classification (area-under-the-receiver-operating characteristic curve) analyses. Simulations were performed to assess uncertainties due to statistical noise.

RESULTS

Strong correlation (r ≥ 0.75, P < 0.001) existed between all KRCs deduced from both SD1 and SD2, and from FD. Significant differences between KRCs were found only for FD-SD2 correlations in patient studies. K1 and k3 were reproducible to within approximately 6% and approximately 30% (FD-SD1; patients) and approximately 4% and approximately 75% (FD-SD2; animals). Area-under-the-receiver-operating characteristic curve values for classification of patient clusters as hypoxic, using a tumor-to-blood ratio greater than 1.2, were 0.91 (SD1) and 0.86 (SD2). The percentage SD in estimating K1 and k3 from 45-min shortened datasets due to noise was less than 1% and between 2% and 12%, respectively.

CONCLUSION

Using single-session 45-min shortened (18)F-fluoromisonidazole dPET datasets appears to be adequate for the identification of intratumor regions of hypoxia. However, k3 was significantly overestimated in the clinical cohort. Further studies are necessary to evaluate the clinical significance of differences between the results as calculated from full and shortened datasets.

Generalized whole-body Patlak parametric imaging for enhanced quantification in clinical PET

We recently developed a dynamic multi-bed PET data acquisition framework to translate the quantitative benefits of Patlak voxel-wise analysis to the domain of routine clinical whole-body (WB) imaging. The standard Patlak (sPatlak) linear graphical analysis assumes irreversible PET tracer uptake, ignoring the effect of FDG dephosphorylation, which has been suggested by a number of PET studies. In this work: (i) a non-linear generalized Patlak (gPatlak) model is utilized, including a net efflux rate constant kloss, and (ii) a hybrid (s/g)Patlak (hPatlak) imaging technique is introduced to enhance contrast to noise ratios (CNRs) of uptake rate Ki images. Representative set of kinetic parameter values and the XCAT phantom were employed to generate realistic 4D simulation PET data, and the proposed methods were additionally evaluated on 11 WB dynamic PET patient studies. Quantitative analysis on the simulated Ki images over 2 groups of regions-of-interest (ROIs), with low (ROI A) or high (ROI B) true kloss relative to Ki, suggested superior accuracy for gPatlak. Bias of sPatlak was found to be 16-18% and 20-40% poorer than gPatlak for ROIs A and B, respectively. By contrast, gPatlak exhibited, on average, 10% higher noise than sPatlak. Meanwhile, the bias and noise levels for hPatlak always ranged between the other two methods. In general, hPatlak was seen to outperform all methods in terms of target-to-background ratio (TBR) and CNR for all ROIs. Validation on patient datasets demonstrated clinical feasibility for all Patlak methods, while TBR and CNR evaluations confirmed our simulation findings, and suggested presence of non-negligible kloss reversibility in clinical data. As such, we recommend gPatlak for highly quantitative imaging tasks, while, for tasks emphasizing lesion detectability (e.g. TBR, CNR) over quantification, or for high levels of noise, hPatlak is instead preferred. Finally, gPatlak and hPatlak CNR was systematically higher compared to routine SUV values.

Gaussian process models of dynamic PET for functional volume definition in radiation oncology

In routine oncologic positron emission tomography (PET), dynamic information is discarded by time-averaging the signal to produce static images of the "standardised uptake value" (SUV). Defining functional volumes of interest (VOIs) in terms of SUV is flawed, as values are affected by confounding factors and the chosen time window, and SUV images are not sensitive to functional heterogeneity of pathological tissues. Also, SUV iso-contours are highly affected by the choice of threshold and no threshold, or other SUV-based segmentation method, is universally accepted for a given VOI type. Gaussian Process (GP) time series models describe macro-scale dynamic behavior arising from countless interacting micro-scale processes, as is the case for PET signals from heterogeneous tissue. We use GPs to model time-activity curves (TACs) from dynamic PET and to define functional volumes for PET oncology. Probabilistic methods of tissue discrimination are presented along with novel contouring methods for functional VOI segmentation. We demonstrate the value of GP models for voxel classification and VOI contouring of diseased and metastatic tissues with functional heterogeneity in prostate PET. Classification experiments reveal superior sensitivity and specificity over SUV calculation and a TAC-based method proposed in recent literature. Contouring experiments reveal differences in shape between gold-standard and GP VOIs and correlation with kinetic models shows that the novel VOIs contain extra clinically relevant information compared to SUVs alone. We conclude that the proposed models offer a principled data analysis technique that improves on SUVs for oncologic VOI definition. Continuing research will generalize GP models for different oncology tracers and imaging protocols with the ultimate goal of clinical use including treatment planning.

A short-scan method for k(3) estimation with moderately reversible PET ligands: application of irreversible model to early-phase PET data

Long dynamic scans (60-120 min) are often required for estimating the k(3) value, an index of receptor density, by positron emission tomography (PET). However, the precision of k(3) is usually low in kinetic analyses for reversible PET ligands compared with irreversible ligands. That is largely due to unstable estimation of the dissociation rate constant, k(4). We propose a novel '3P+' method for estimating k(3) of moderately reversible ligands, where a 3-parameter model without k(4) is applied to early-phase PET data to obtain a good model-fit of k(3) estimation. By using [(11)C] Pittsburgh compound B (PIB) (k(4) = 0.018/min) as an example of a moderately reversible ligand, the 3P+ method simulation with a 28 min PET scan yielded less than 3% k(3) relative bias with a +100% k(3) change. In [(11)C]PIB PET scans of 15 normal controls (NC) and nine patients with Alzheimer's disease (AD), the 3P+ method provided a precise k(3) estimate (mean SE of 13.6% in parietal cortex; covariance matrix method). The results revealed linear correlations (r = 0.964) of parietal k(3) values in 24 subjects between 28minute 3P+ method and conventional 90 minute 4-parameter method. A good separation of k(3) between NC and AD groups (P < 0.001; t-test) was replicated in 28 minute 3P+ method. The short-scan 3P+ method may be a practical alternative method for analyzing reversible ligands.

Quantitative assessment of heterogeneity in tumor metabolism using FDG-PET

PURPOSE

[(18)F]-fluorodeoxyglucose-positron emission tomography (FDG-PET) images are usually quantitatively analyzed in "whole-tumor" volumes of interest. Also parameters determined with dynamic PET acquisitions, such as the Patlak glucose metabolic rate (MR(glc)) and pharmacokinetic rate constants of two-tissue compartment modeling, are most often derived per lesion. We propose segmentation of tumors to determine tumor heterogeneity, potentially useful for dose-painting in radiotherapy and elucidating mechanisms of FDG uptake.

METHODS AND MATERIALS

In 41 patients with 104 lesions, dynamic FDG-PET was performed. On MR(glc) images, tumors were segmented in quartiles of background subtracted maximum MR(glc) (0%-25%, 25%-50%, 50%-75%, and 75%-100%). Pharmacokinetic analysis was performed using an irreversible two-tissue compartment model in the three segments with highest MR(glc) to determine the rate constants of FDG metabolism.

RESULTS

From the highest to the lowest quartile, significant decreases of uptake (K(1)), washout (k(2)), and phosphorylation (k(3)) rate constants were seen with significant increases in tissue blood volume fraction (V(b)).

CONCLUSIONS

Tumor regions with highest MR(glc) are characterized by high cellular uptake and phosphorylation rate constants with relatively low blood volume fractions. In regions with less metabolic activity, the blood volume fraction increases and cellular uptake, washout, and phosphorylation rate constants decrease. These results support the hypothesis that regional tumor glucose phosphorylation rate is not dependent on the transport of nutrients (i.e., FDG) to the tumor.

Increased brain metabolism after acute administration of the synthetic cannabinoid HU210: a small animal PET imaging study with 18F-FDG

Cannabis use has been shown to alter brain metabolism in both rat models and humans although the observations between both species are conflicting. In the present study, we examined the short term effects of a single-dose injection of the synthetic cannabinoid agonist HU210 on glucose metabolism in the rat brain using small animal (18)F-2-fluoro-deoxyglucose (FDG) Positron Emission Tomography (PET) 15 min (Day 1) and 24h (Day 2) post-injection of the agonist in the same animal. Young adult male Wistar rats received an intra-peritoneal injection of HU210 (100 μg/kg, n=7) or vehicle (n=5) on Day 1. Approximately 1mCi of (18)F-FDG was injected intravenously into each animal at 15 min (Day 1) and 24h (Day 2) post-injection of HU210. A 5-min Computer Tomography (CT) scan followed by a 20-min PET scan was performed 40 min after each (18)F-FDG injection. Standardised Uptake Values (SUVs) were calculated for 10 brain regions of interest (ROIs). Global increased SUVs in the whole brain, hence global brain metabolism, were observed following HU210 treatment on Day 1 compared to the controls (21%, P<0.0001), but not in individual brain regions. On Day 2, however, no statistically significant differences were observed between the treated and control groups. At the 24h time point (Day 2), SUVs in the HU210 treated group returned to control levels (21-30% decrease compared to Day 1), in all ROIs investigated (P<0.0001). In the control group, SUVs did not differ between the two acquisition days in all brain regions. The present results suggest that high-dose HU210 increases brain glucose metabolism in the rat brain shortly after administration, in line with normalised human in vivo studies, an effect that was no longer apparent 24 h later.

Methodological considerations in quantification of oncological FDG PET studies

Purpose

This review aims to provide insight into the factors that influence quantification of glucose metabolism by FDG PET images in oncology as well as their influence on repeated measures studies (i.e. treatment response assessment), offering improved understanding both for clinical practice and research.

Methods

Structural PubMed searches have been performed for the many factors affecting quantification of glucose metabolism by FDG PET. Review articles and references lists have been used to supplement the search findings.

Results

Biological factors such as fasting blood glucose level, FDG uptake period, FDG distribution and clearance, patient motion (breathing) and patient discomfort (stress) all influence quantification. Acquisition parameters should be adjusted to maximize the signal to noise ratio without exposing the patient to a higher than strictly necessary radiation dose. This is especially challenging in pharmacokinetic analysis, where the temporal resolution is of significant importance. The literature is reviewed on the influence of attenuation correction on parameters for glucose metabolism, the effect of motion, metal artefacts and contrast agents on quantification of CT attenuation-corrected images. Reconstruction settings (analytical versus iterative reconstruction, post-reconstruction filtering and image matrix size) all potentially influence quantification due to artefacts, noise levels and lesion size dependency. Many region of interest definitions are available, but increased complexity does not necessarily result in improved performance. Different methods for the quantification of the tissue of interest can introduce systematic and random inaccuracy.

Conclusions

This review provides an up-to-date overview of the many factors that influence quantification of glucose metabolism by FDG PET.

Voxel-based estimation of kinetic model parameters of the L-[1-(11)C]leucine PET method for determination of regional rates of cerebral protein synthesis: validation and comparison with region-of-interest-based methods

We adapted and validated a basis function method (BFM) to estimate at the voxel level parameters of the kinetic model of the L-[1-(11)C]leucine positron emission tomography (PET) method and regional rates of cerebral protein synthesis (rCPS). In simulation at noise levels typical of voxel data, BFM yielded low-bias estimates of rCPS; in measured data, BFM and nonlinear least-squares parameter estimates were in good agreement. We also examined whether there are advantages to using voxel-level estimates averaged over regions of interest (ROIs) in place of estimates obtained by directly fitting ROI time-activity curves (TACs). In both simulated and measured data, fits of ROI TACs were poor, likely because of tissue heterogeneity not taken into account in the kinetic model. In simulation, rCPS determined from fitting ROI TACs was substantially overestimated and BFM-estimated rCPS averaged over all voxels in an ROI was slightly underestimated. In measured data, rCPS determined by regional averaging of voxel estimates was lower than rCPS determined from ROI TACs, consistent with simulation. In both simulated and measured data, intersubject variability of BFM-estimated rCPS averaged over all voxels in a ROI was low. We conclude that voxelwise estimation is preferable to fitting ROI TACs using a homogeneous tissue model.

Correction for the effect of rising plasma glucose levels on quantification of MR(glc) with FDG-PET

Positron emission tomography (PET) using the tracer [18F]-fluorodeoxyglucose (FDG) is commonly used for measuring metabolic rate of glucose (MR(glc)) in the human brain. Conventional PET methods (e.g., the Patlak method) for quantifying MR(glc) assume the tissue transport and phosphorylation mechanisms to be in steady state during FDG uptake. As FDG and glucose use the same transporters and phosphorylation enzymes, changing blood glucose levels can change the rates of FDG transport and phosphorylation. Compartmental models were used to simulate the effect of rising arterial glucose, from normal to hyperglycemic levels on FDG uptake for a typical PET protocol. The subsequent errors on the values of MR(glc) calculated using the Patlak method were investigated, and a correction scheme based on measured arterial glucose concentration (G(p)) was evaluated. Typically, with a 40% rise in G(p) over the duration of the PET study, the true MR(glc) varied by only 1%; however, the Patlak method overestimated MR(glc) by 15%. The application of the correction reduced this error to approximately 2%. In general, the application of the correction resulted in values of MR(glc) consistently significantly closer to the true steady state calculation of MR(glc) independently of changes to the parameters defining the model.

Propofol anesthesia does not alter regional rates of cerebral protein synthesis measured with L-[1-(11)C]leucine and PET in healthy male subjects

We report regional rates of cerebral protein synthesis (rCPS) in 10 healthy young males, each studied under two conditions: awake and anesthetized with propofol. We used the quantitative L-[1-(11)C]leucine positron emission tomography (PET) method to measure rCPS. The method accounts for the fraction (lambda) of unlabeled leucine in the precursor pool for protein synthesis that is derived from arterial plasma; the remainder comes from proteolysis of tissue proteins. Across 18 regions and whole brain, mean differences in rCPS between studies ranged from -5% to 5% and were within the variability of rCPS in awake studies (coefficient of variation range: 7% to 14%). Similarly, differences in lambda (range: 1% to 4%) were typically within the variability of lambda (coefficient of variation range: 3% to 6%). Intersubject variances and patterns of regional variation were also similar under both conditions. In propofol-anesthetized subjects, rCPS varied regionally from 0.98+/-0.12 to 2.39+/-0.23 nmol g(-1) min(-1) in the corona radiata and in the cerebellum, respectively. Our data indicate that the values, variances, and patterns of regional variation in rCPS and lambda measured by the L-[1-(11)C]leucine PET method are not significantly altered by anesthesia with propofol.

Wavelet denoising in voxel-based parametric estimation of small animal PET images: a systematic evaluation of spatial constraints and noise reduction algorithms

Voxel-based estimation of PET images, generally referred to as parametric imaging, can provide invaluable information about the heterogeneity of an imaging agent in a given tissue. Due to high level of noise in dynamic images, however, the estimated parametric image is often noisy and unreliable. Several approaches have been developed to address this challenge, including spatial noise reduction techniques, cluster analysis and spatial constrained weighted nonlinear least-square (SCWNLS) methods. In this study, we develop and test several noise reduction techniques combined with SCWNLS using simulated dynamic PET images. Both spatial smoothing filters and wavelet-based noise reduction techniques are investigated. In addition, 12 different parametric imaging methods are compared using simulated data. With the combination of noise reduction techniques and SCWNLS methods, more accurate parameter estimation can be achieved than with either of the two techniques alone. A less than 10% relative root-mean-square error is achieved with the combined approach in the simulation study. The wavelet denoising based approach is less sensitive to noise and provides more accurate parameter estimation at higher noise levels. Further evaluation of the proposed methods is performed using actual small animal PET datasets. We expect that the proposed method would be useful for cardiac, neurological and oncologic applications.

Regional rates of cerebral protein synthesis measured with L-[1-11C]leucine and PET in conscious, young adult men: normal values, variability, and reproducibility

We report regional rates of cerebral protein synthesis (rCPS) measured with the fully quantitative L-[1-(11)C]leucine positron emission tomography (PET) method. The method accounts for the fraction (lambda) of unlabeled amino acids in the precursor pool for protein synthesis derived from arterial plasma; the remainder (1-lambda) comes from tissue proteolysis. We determined rCPS and lambda in 18 regions and whole brain in 10 healthy men (21 to 24 years). Subjects underwent two 90-min dynamic PET studies with arterial blood sampling at least 2 weeks apart. Rates of cerebral protein synthesis varied regionally and ranged from 0.97+/-0.70 to 2.25+/-0.20 nmol/g per min. Values of rCPS were in good agreement between the two PET studies. Mean differences in rCPS between studies ranged from 9% in cortical regions to 15% in white matter. The lambda value was comparatively more uniform across regions, ranging from 0.63+/-0.03 to 0.79+/-0.02. Mean differences in lambda between studies were 2% to 8%. Intersubject variability in rCPS was on average 6% in cortical areas, 9% in subcortical regions, and 12% in white matter; intersubject variability in lambda was 2% to 8%. Our data indicate that in human subjects low variance and highly reproducible measures of rCPS can be made with the L-[1-(11)C]leucine PET method.

Stochastic dynamic model for estimation of rate constants and their variances from noisy and heterogeneous PET measurements

Tissue heterogeneity, radioactive decay and measurement noise are the main error sources in compartmental modeling used to estimate the physiologic rate constants of various radiopharmaceuticals from a dynamic PET study. We introduce a new approach to this problem by modeling the tissue heterogeneity with random rate constants in compartment models. In addition, the Poisson nature of the radioactive decay is included as a Poisson random variable in the measurement equations. The estimation problem will be carried out using the maximum likelihood estimation. With this approach, we do not only get accurate mean estimates for the rate constants, but also estimates for tissue heterogeneity within the region of interest and other possibly unknown model parameters, e.g. instrument noise variance, as well. We also avoid the problem of the optimal weighting of the data related to the conventionally used weighted least-squares method. The new approach was tested with simulated time-activity curves from the conventional three compartment - three rate constants model with normally distributed rate constants and with a noise mixture of Poisson and normally distributed random variables. Our simulation results showed that this new model gave accurate estimates for the mean of the rate constants, the measurement noise parameter and also for the tissue heterogeneity, i.e. for the variance of the rate constants within the region of interest.

Quantification of Cellular Proliferation in Tumor and Normal Tissues of Patients with Breast Cancer by [18F]Fluorothymidine-Positron Emission Tomography Imaging: Evaluation of Analytical Methods

There is an unmet need to develop imaging methods for the early and objective assessment of breast tumors to therapy. 3'-Deoxy-3'-[18F]fluorothymidine ([18F]FLT)-positron emission tomography represents a new approach to imaging thymidine kinase activity, and hence, cellular proliferation. We compared graphical, spectral, and semiquantitative analytic methodologies for quantifying [18F]FLT kinetics in tumor and normal tissue of patients with locally advanced and metastatic breast cancer. The resultant kinetic parameters were correlated with the Ki-67 labeling index from tumor biopsies. [18F]FLT accumulation was detected in primary tumor, nodal disease, and lung metastasis. In large tumors, there was substantial heterogeneity in regional radiotracer uptake, reflecting heterogeneity in cellular proliferation; radiotracer uptake in primary tumors also differed from that of metastases. [18F]FLT was metabolized in patients to a single metabolite [18F]FLT-glucuronide. Unmetabolized [18F]FLT accounted for 71.54 +/- 1.50% of plasma radioactivity by 90 minutes. The rate constant for the metabolite-corrected net irreversible uptake of [18F]FLT (Ki) ranged from 0.6 to 10.4 x 10(-4) and from 0 to 0.6 x 10(-4) mL plasma cleared/s/mL tissue in tumor (29 regions, 15 patients) and normal tissues, respectively. Tumor Ki and fractional retention of radiotracer determined by spectral analysis correlated with Ki-67 labeling index (r = 0.92, P < 0.0001 and r = 0.92, P < 0.0001, respectively). These correlations were superior to those determined by semiquantitative methods. We conclude that [18F]FLT-positron emission tomography is a promising clinical tool for imaging cellular proliferation in breast cancer, and is most predictive when analyzed by graphical and spectral methods.

Resolution, sensitivity and precision with autoradiography and small animal positron emission tomography: implications for functional brain imaging in animal research

Quantitative autoradiographic methods for in vivo measurement of regional rates of cerebral blood flow, glucose metabolism, and protein synthesis contribute significantly to our understanding of phsysiological and biochemical responses of the brain to changes in the environment. A disadvantage of these autoradiographic methods is that experimental animals can be studied only once. With the advent of small animal positron emission tomography (PET) and with increases in the sensitivity and spatial resolution of scanners it is now possible to use adaptations of these methods in experimental animals with PET. These developments allow repeated studies of the same animal, including studies of the same animal under different conditions, and longitudinal studies. In this review we summarize the tradeoffs between the use of autoradiography and small animal PET for functional brain imaging studies in animal research.

Quantitative assessment of glucose transport in human skeletal muscle: dynamic positron emission tomography imaging of [O-methyl-11C]3-O-methyl-D-glucose

Insulin-stimulated glucose transport in skeletal muscle is regarded as a key determinant of insulin sensitivity, yet isolation of this step for quantification in human studies is a methodological challenge. One notable approach is physiological modeling of dynamic positron emission tomography (PET) imaging using 2-[18-fluoro]2-deoxyglucose ([(18)F]FDG); however, this has a potential limitation in that deoxyglucose undergoes phosphorylation subsequent to transport, complicating separate estimations of these steps. In the current study we explored the use of dynamic PET imaging of [(11)C]3-O-methylglucose ([(11)C]3-OMG), a glucose analog that is limited to bidirectional glucose transport. Seventeen lean healthy volunteers with normal insulin sensitivity participated; eight had imaging during basal conditions, and nine had imaging during euglycemic insulin infusion at 30 mU/min.m(2). Dynamic PET imaging of calf muscles was conducted for 90 min after the injection of [(11)C]3-OMG. Spectral analysis of tissue activity indicated that a model configuration of two reversible compartments gave the strongest statistical fit to the kinetic pattern. Accordingly, and consistent with the structure of a model previously used for [(18)F]FDG, a two-compartment model was applied. Consistent with prior [(18)F]FDG findings, insulin was found to have minimal effect on the rate constant for movement of [(11)C]3-OMG from plasma to tissue interstitium. However, during insulin infusion, a robust and highly significant increase was observed in the kinetics of inward glucose transport; this and the estimated tissue distribution volume for [(11)C]3-OMG increased 6-fold compared with basal conditions. We conclude that dynamic PET imaging of [(11)C]3-OMG offers a novel quantitative approach that is both chemically specific and tissue specific for in vivo assessment of glucose transport in human skeletal muscle.

Non-invasive assessment of distribution volume ratios and binding potential: tissue heterogeneity and interindividually averaged time-activity curves

Due to the stochastic nature of radioactive decay, any measurement of radioactivity concentration requires spatial averaging. In pharmacokinetic analysis of time-activity curves (TAC), such averaging over heterogeneous tissues may introduce a systematic error (heterogeneity error) but may also improve the accuracy and precision of parameter estimation. In addition to spatial averaging (inevitable due to limited scanner resolution and intended in ROI analysis), interindividual averaging may theoretically be beneficial, too. The aim of this study was to investigate the effect of such averaging on the binding potential ( BP) calculated with Logan's non-invasive graphical analysis and the "simplified reference tissue method" (SRTM) proposed by Lammertsma and Hume, on the basis of simulated and measured positron emission tomography data [[(11)C] d- threo-methylphenidate (dMP) and [(11)C]raclopride (RAC) PET]. dMP was not quantified with SRTM since the low k(2) (washout rate constant from the first tissue compartment) introduced a high noise sensitivity. Even for considerably different shapes of TAC (dMP PET in parkinsonian patients and healthy controls, [(11)C]raclopride in patients with and without haloperidol medication) and a high variance in the rate constants (e.g. simulated standard deviation of K(1)=25%), the BP obtained from average TAC was close to the mean BP (error <5%). However, unfavourably distributed parameters, especially a correlated large variance in two or more parameters, may lead to larger errors. In Monte Carlo simulations, interindividual averaging before quantification reduced the variance from the SRTM (beyond a critical signal to noise ratio) and the bias in Logan's method. Interindividual averaging may further increase accuracy when there is an error term in the reference tissue assumption E= DV(2)- DV' ( DV(2) = distribution volume of the first tissue compartment, DV' = distribution volume of the reference tissue). This can be explained by the fact that the distribution volume ratio ( DVR= DV/DV') obtained from averaged TAC is an approximation for Sigma DV/Sigma DV' rather than for Sigma DVR/ n. We conclude that Logan's non-invasive method and SRTM are suitable for heterogeneous tissues and that discussion of group differences in PET studies generally should include qualitative and quantitative assessment of interindividually averaged TAC.

Cerebral blood flow and glucose metabolism in hypothyroidism: a positron emission tomography study

Hypothyroidism is often associated with defective memory, psychomotor slowing, and depression. However, the relationship between thyroid status and cognitive or psychiatric disturbances remains unclear. Using psychometric scales, 10 patients who had undergone total thyroidectomy for thyroid carcinoma were evaluated for depression, anxiety, and psychomotor slowing; they were examined both when euthyroid and hypothyroid after thyroid hormone withdrawal. Positron emission tomography was used, with oxygen-15-labeled water and fluorine-18F-labeled 2-deoxy-2fluoro-D-glucose as the tracers, to correlate the regional cerebral blood flow and cerebral glucose metabolism with the mental state in patients. Two different image analysis techniques (regions of interest and statistical parametric maps) were applied. In hypothyroidism, there was a generalized decrease in regional cerebral blood flow (23.4%, P < 0.001) and in cerebral glucose metabolism (12.1%, P < 0.001) and there were no specific local defects. Patients were also significantly more depressed (P < 0.001), anxious (P < 0.001) and psychomotor slowed (P < 0.005) in hypo than in euthyroid status. These results indicate that the brain activity was globally reduced in severe hypothyroidism of short duration without the regional modifications usually observed in primary depression.

Simultaneous estimation of physiological parameters and the input function--in vivo PET data

Dynamic imaging with positron emission tomography (PET) is widely used for the in vivo measurement of regional cerebral metabolic rate for glucose (rCMRGlc) with [18F]fluorodeoxy-D-glucose (FDG) and is used for the clinical evaluation of neurological disease. However, in addition to the acquisition of dynamic images, continuous arterial blood sampling is the conventional method to obtain the tracer time-activity curve in blood (or plasma) for the numeric estimation of rCMRGlc in mg glucose/100-g tissue/min. The insertion of arterial lines and the subsequent collection and processing of multiple blood samples are impractical for clinical PET studies because it is invasive, has the remote, but real potential for producing limb ischemia, and it exposes personnel to additional radiation and risks associated with handling blood. In this paper, based on our previously proposed method for extracting kinetic parameters from dynamic PET images, we developed a modified version (post-estimation method) to improve the numerical identifiability of the parameter estimates when we deal with data obtained from clinical studies. We applied both methods to dynamic neurologic FDG PET studies in three adults. We found that the input function and parameter estimates obtained with our noninvasive methods agreed well with those estimated from the gold standard method of arterial blood sampling and that rCMRGlc estimates were highly correlated (r = 0.973). More importantly, no significant difference was found between rCMRGlc estimated by our methods and the gold standard method (P > 0.16). We suggest that our proposed noninvasive methods may offer an advance over existing methods.

Synthesis and in vivo evaluation of [(11)C]CGP62349, a new GABA(B) receptor antagonist

This paper describes the radiosynthesis of [(11)C]CGP62349, a potential ligand to assess GABA(B) receptors in vivo. (11)C was introduced by O-methylation of the corresponding des-methyl precursor, namely CGP67780. The final product was obtained with a reliable method in good yield. The radioligand was tested in monkey, revealing negligible blood-brain barrier penetration and brain uptake, thus prompting us to search for a new target structure with a better lipophilicity.

Dynamic changes in cerebral glucose metabolism in conscious infant monkeys during the first year of life as measured by positron emission tomography

Recently, advances in spatial resolution have provided the opportunity to utilize positron emission tomography (PET) to examine local cerebral metabolic rates for glucose (lCMR(glc)) in large animals noninvasively, thereby allowing repeated lCMR(glc) measurements in the same animal. Previous studies have attempted to describe the ontogeny of cerebral glucose metabolism in anesthetized nonhuman primates using [18F]fluorodeoxyglucose (FDG) and PET. However, the use of sedation during the tracer uptake period may influence lCMR(glc). This study was conducted to describe lCMR(glc) in conscious infant vervet monkeys (Cercopithecus aethiops sabaeus) during the first year of life utilizing FDG-PET. Cross-sectional studies (n=23) displayed lowest and highest lCMR(glc) in all structures at the 2-3 and 8-9 month age groups, respectively. The metabolic pattern suggested an increase in lCMR(glc) values between 2 and 8 months of age with decreased metabolism observed at 10-12 months of age in all regions. Peak lCMR(glc) values at 8 months were an average of 84+/-24% higher than values seen at the youngest age examined quantitatively (2-3 months). The regions of greatest and smallest increases in lCMR(glc) at 8 months were the cerebellar hemispheres (90%) and the thalamus (39%), respectively. Longitudinal analysis in 4 animals supported this developmental pattern, demonstrating the ability to detect changes in cerebral glucose metabolism within animals and the potential for FDG-PET in nonhuman primate models of brain maturation. By determining the normative profile of lCMR(glc) during development in monkeys, future application of FDG-PET will provide the opportunity to longitudinally assess the effects of environmental or pharmacological intervention on the immature brain.

Negligible glucose-6-phosphatase activity in cultured astroglia

2-Deoxy[14C]glucose-6-phosphate (2-[14C]DG-6-P) dephosphorylation and glucose-6-phosphatase (G-6-Pase) activity were examined in cultured rat astrocytes under conditions similar to those generally used in assays of glucose utilization. Astrocytes were loaded with 2-[14C]DG-6-P by preincubation for 15 min in medium containing 2 mM glucose and 50 microM 2-deoxy[14C]glucose (2-[14C]DG). The medium was then replaced with identical medium including 2 mM glucose but lacking 2-[14C]DG, and incubation was resumed for 5 min to diminish residual free 2-[14C]DG levels in the cells by either efflux or phosphorylation. The medium was again replaced with fresh 2-[14C]DG-free medium, and the incubation was continued for 5, 15, or 30 min. Intracellular and extracellular 14C contents were measured at each time point, and the distribution of 14C between 2-[14C]DG and 2-[14C]DG-6-P was characterized by paper chromatography. The results showed little if any hydrolysis of 2-[14C]DG-6-P or export of free 2-[14C]DG from cells to medium; there were slightly increasing losses of 2-[14C]DG and 2-[14C]DG-6-P into the medium with increasing incubation time, but they were in the same proportions found in the cells, suggesting they were derived from nonadherent or broken cells. Experiments carried out with medium lacking glucose during the assay for 2-deoxyglucose-6-phosphatase activity yielded similar results. Evidence for G-6-Pase activity was also sought by following the selective detritiation of glucose from the 2-C position when astrocytes were incubated with [2-3H]glucose and [U-14C]glucose in the medium. No change in the 3H/14C ratio was found in incubations for as long as 15 min. These results indicate negligible G-6-Pase activity in cultured astrocytes.

Early evaluation of tumour metabolic response using [18F]fluorodeoxyglucose and positron emission tomography: a pilot study following the phase II chemotherapy schedule for temozolomide in recurrent high-grade gliomas

Quantitation of metabolic changes in tumours may provide an objective measure of clinical and subclinical response to anticancer therapy. This pilot study assesses the value of quantitation of metabolic rate of glucose (MRGlu) measured in mmol min(-1) ml(-1) to assess early subclinical response to therapy in a relatively non-responsive tumour. Nine patients receiving the CRC Phase II study schedule of temozolomide were assessed with [18F]fluorodeoxyglucose ([18F]FDG) dynamic positron emission tomography (PET) scans prior to and 14 days after treatment with temozolomide given as 750-1000 mg m(-2) over 5 days every 28 days. Tumour MRGlu was calculated and compared with objective response at 8 weeks. Pretreatment MRGlu was higher in responders than non-responders. The responding patient group had a greater than 25% reduction in MRGlu in regions of high focal tumour uptake (HFU). Whole tumour changes in MRGlu did not correlate with response. Percentage change in HFU standardized uptake value (SUV) did discriminate the responding from the non-responding patients, but not as well as with MRGlu. Large differences also occurred in the normal brain SUV following treatment. Thus, MRGlu appeared to be a more sensitive discriminator of response than the simplified static SUV analysis. Changes in MRGlu may reflect the degree of cell kill following chemotherapy and so may provide an objective, quantitative subclinical measure of response to therapy.

The effect of sequential lesioning in the basal forebrain on cerebral cortical glucose metabolism in rats. An animal positron emission tomography study

We studied the effect of the cortical projection from the basal forebrain on the cerebral cortical metabolism using positron emission tomography (PET) with [(18)F] fluorodeoxyglucose. Unilateral damage of the nucleus basalis magnocellularis (NBM) did not cause a permanent reduction of cortical metabolism: recovery was observed 4 weeks after the operation. Destruction of the contralateral side after recovery from unilateral damage produced persistent bilateral suppression of glucose metabolism, with partial recovery. We speculate that recovery from the unilateral NBM lesions is partly ascribable to the cholinergic projection from the contralateral NBM, and partly due to non-cholinergic systems, and conclude that bilateral damage might be responsible for persistent cortical glucose metabolism suppression.

Which linear compartmental systems can be analyzed by spectral analysis of PET output data summed over all compartments?

General linear time-invariant compartmental systems were examined to determine which systems meet the conditions necessary for application of the spectral analysis technique to the sum of the concentrations in all compartments. Spectral analysis can be used to characterize the reversible and irreversible components of the system and to estimate the minimum number of compartments, but it applies only to systems in which the measured data can be expressed as a positively weighted sum of convolution integrals of the input function with an exponential function that has real-valued nonpositive decay constants. The conditions are met by compartmental systems that are strongly connected, have exchange of material with the environment confined to a single compartment, and do not contain cycles, i.e., there is no possibility for material to pass from one compartment through two or more compartments back to the initial compartment. Certain noncyclic systems with traps, systems with cycles that obey a specified loop condition, and noninterconnected collections of such systems also meet the conditions. Dynamic positron emission tomographic data obtained after injection of a radiotracer, the kinetics of which can be described by any model in the class of models identified here, can be appropriately analyzed with the spectral analysis technique.

Application of positron emission tomography to determine cerebral glucose utilization in conscious infant monkeys

Cerebral glucose metabolism has been used as a marker of cerebral maturation and neuroplasticity. In studies addressing these issues in young non-human primates, investigators have used positron emission tomography (PET) and [18F]2-fluoro-2-deoxy-D-glucose (FDG) to calculate local cerebral metabolic rates of glucose (1CMRG1c). Unfortunately, these values were influenced by anesthesia. In order to avoid this confounding factor, we have established a method that permits reliable measurements in young conscious vervet monkeys using FDG-PET. Immature animals remained in a conscious, resting state during the initial 42 min of FDG uptake as they were allowed to cling to their anesthetized mothers. After FDG uptake, animals were anesthetized and placed in the PET scanner with data acquisition beginning at 60 min post-FDG injection. FDG image sets consisted of 30 planes separated by 1.69 mm, parameters sufficient to image the entire monkey brain. Our method of region-of-interest (ROI) analysis was assessed within and between raters and demonstrated high reliability (P < 0.001). To illustrate that our method was sensitive to developmental changes in cerebral glucose metabolism, quantitative studies of young conscious monkeys revealed that infant monkeys 6-8 months of age exhibited significantly higher 1CMRG1c values (P < 0.05) in all regions examined, except sensorimotor cortex and thalamus, compared to monkeys younger than 4 months of age. This method provided high resolution images and 1CMRG1c values that were reliable within age group. These results support the application of FDG-PET to investigate questions related to cerebral glucose metabolism in young conscious non-human primates.

Evaluation of compartmental and spectral analysis models of [18F]FDG kinetics for heart and brain studies with PET

Various models have been proposed to quantitate from [18F]-Fluoro-Deoxy-Glucose ([18F]FDG) positron emission tomography (PET) data glucose regional metabolic rate. We evaluate here four models, a three-rate constants (3K) model, a four-rate constants (4K) model, an heterogeneous model (TH) and a spectral analysis (SA) model. The data base consists of [18F]FDG dynamic data obtained in the myocardium and brain gray and white matter. All models were identified by nonlinear weighted least squares with weights chosen optimally. We show that: 1) 3K and 4K models are indistinguishable in terms of parsimony criteria and choice should be made on parameter precision and physiological plausibility; in the gray matter a more complex model than the 3K one is resolvable; 2) the TH model is resolvable in the gray but not in the white matter; 3) the classic SA approach has some unnecessary hypotheses built in and can be in principle misleading; we propose here a new SA model which is more theoretically sound; 4) this new SA approach supports the use of a 3K model in the heart with a 60 min experimental period; it also indicates that heterogeneity in the brain is modest in the white matter; 5) [18F]FDG fractional uptake estimates of the four models are very close in the heart, but not in the brain; 6) a higher than 60 min experimental time is preferable for brain studies.

Evolution of brain glucose metabolism with age in epileptic infants, children and adolescents

During the first years of life, the human brain undergoes repetitive modifications in its anatomical, functional, and synaptic construction to reach the complex functional organization of the adult central nervous system. As an attempt to gain further insight in those maturation processes, the evolution of cerebral metabolic activity was investigated as a function of age in epileptic infants, children and adolescents. The regional cerebral metabolic rates for glucose (rCMRGlc) were measured with positron emission tomography (PET) in 60 patients aged from 6 weeks to 19 years, who were affected by complex partial epilepsy. They were scanned at rest, without premedication, in similar conditions to 20 epileptic adults and in 49 adult controls. The distribution of brain metabolic activity successively extended from sensorimotor areas and thalamus in epileptic newborns to temporo-parietal and frontal cortices and reached the adult pattern after 1 year of age. The measured rCMRGlc in the cerebral cortex, excluding the epileptic lesions, increased from low values in infants to a maximum between 4 and 12 years, before it declined to stabilize at the end of the second decade of life. Similar age-related changes in glucose metabolic rates were not observed in the adult groups. Despite the use of medications, the observed variations of rCMRGlc with age in young epileptic humans confirm those previously described in pediatric subjects. These metabolic changes are in full agreement with the current knowledge of the synaptic density evolution in the human brain.

Glucose metabolism of the thyroid in Graves' disease measured by F-18-fluoro-deoxyglucose positron emission tomography

The radiolabeled glucose surrogate F-18-fluoro-deoxyglucose (F-18-FDG) and positron emission tomography (PET) were used to measure glucose metabolism of the thyroid in vivo. We evaluated patients with Graves' disease before therapy with radioiodine in comparison to patients with normal thyroids. Thirty-six patients with Graves' disease underwent scanning the day before radioiodine therapy. Twenty patients with head tumors and normal thyroids were the controls. Overall F-18-FDG uptake was determined for all thyroids and modeling of glucose metabolism was performed in order to differentiate between glucose concentration in the fractional blood volume, glucose transport, and glucose utilization. F-18-FDG uptake was significantly higher in Graves' disease patients compared with controls. In these patients F-18-FDG uptake increased with increasing antithyroid antibodies and shorter radioiodine half-life. Modeling of glucose metabolism revealed substantial differences in thyroid F-18-FDG utilization constants (k3 values) corresponding to enhanced local metabolic rates in Graves' disease. No significant differences in the remaining rate constants and the fractional blood volume were detected. These results indicate that glucose metabolism is enhanced in the thyroid of Graves' disease patients not only due to enhanced fractional blood volume but to enhanced utilization. Whether a lymphocytic infiltration or thyroid epithelial cells utilize this surplus of glucose cannot be determined using in vivo PET measurements in humans. Still, the correlation of radioiodine half-life and glucose hypermetabolism suggests direct or nondirect connections of glucose metabolism and hormone synthesis in thyroid cells.

Does fluorine-18 fluorodeoxyglucose metabolic imaging of tumours benefit oncology?

Fluoro-deoxyglucose (FDG) is a metabolic marker, which follows the same route into cells as that of glucose, and it can be radiolabelled with fluorine-18, 18F-FDG making it suitable for imaging with positron emission tomography (PET). The fact that rapidly proliferating cells such as tumour cells accumulate 18F-FDG more avidly than those with a normal turnover rate has given rise to its potential in oncology. The rationale and previous published uses of 18F-FDG in oncology are reviewed, together with the various analysis techniques and associated methodological difficulties.

Brain energy metabolism in early blind subjects: neural activity in the visual cortex

As an attempt to better understand the metabolic basis for the previously reported increases in glucose metabolism in the visual cortex of congenitally blind subjects, cerebral blood flow, oxygen consumption and glucose utilization were investigated with multitracer positron emission tomography. Measurements were carried out in three subjects who became blind early in life and in three age-matched blindfolded controls. Regional analysis of cerebral blood flow, metabolic rates for oxygen and glucose utilization revealed that these parameters were relatively higher in the visual cortex in case of early blindness (109.7 +/- 2.4%; 114.3 +/- 1.5%; 118.0 +/- 5.5%, respectively) than in controls (98.1 +/- 3.9%; 108.6 +/- 3.6%; 105.2 +/- 4.8%). There were slight differences, albeit statistically not significant, between early blind and control subjects in terms of oxygen-to-glucose metabolic ratios. The relatively preserved stoichiometry in the visual areas of blind subjects points to the lack of variation in the yield of glucose oxidation in this cortex. Those observations suggest that the high level of energy metabolism disclosed in early blind visual cortex is related to neural activity.

Transport of D-glucose and 2-fluorodeoxyglucose across the blood-brain barrier in humans

The deoxyglucose method for calculation of regional cerebral glucose metabolism by PET using 18F-2-fluoro-2-deoxy-d-glucose (FDG) requires knowledge of the lumped constant, which corrects for differences in the blood-brain barrier (BBB) transport and phosphorylation of FDG and glucose. The BBB transport rates of FDG and glucose have not previously been determined in humans. In the present study these transport rates were measured with the intravenous double-indicator method in 24 healthy subjects during normoglycemia (5.2 +/- 0.7 mM). Nine subjects were restudied during moderate hypoglycemia (3.4 +/- 0.4 mM) and five subjects were studied once during hyperglycemia (15.0 +/- 0.7 mM). The global ratio between the unidirectional clearances of FDG and glucose (K1*/K1) was similar in normoglycemia (1.48 +/- 0.22), moderate hypoglycemia (1.41 +/- 0.23), and hyperglycemia (1.44 +/- 0.20). This ratio is comparable to what has been obtained in rats. We argue that the global ratio is constant throughout the brain and may be applied for the regional determination of LC. We also determined the transport parameters of the two hexoses from brain back to blood and, assuming symmetrical transport across the BBB, we found evidence of a larger initial distribution volume of FDG in brain (0.329 +/- 0.236) as compared with that of glucose (0.162 +/- 0.098, p < 0.005). The difference can be explained by the very short experimental time, in which FDG may distribute both intra- and extracellularly, whereas glucose remains in a volume comparable to the interstitial fluid of the brain.

Effect of tissue heterogeneity on quantification in positron emission tomography

As a result of the limited spatial resolution of positron emission tomographic scanners, the measurements of physiological parameters are compromised by tissue heterogeneity. The effect of tissue heterogeneity on a number of parameters was studied by simulation and an analytical method. Five common tracer models were assessed. The input and tissue response functions were assumed to be free from noise and systematic errors. The kinetic model was assumed to be perfect. Two components with different kinetics were mixed in different proportions and contrast with respect to the model parameters. Different experimental protocols were investigated. Of three methods investigated for the measurement of cerebral blood flow (CBF) (steady state, dynamic, integral), the second one was least sensitive to errors caused by tissue heterogeneity and the main effect was an underestimation of the distribution volume. With the steady state method, errors in oxygen extraction fraction caused by tissue heterogeneity were always found to be less than the corresponding errors in CBF. For myocardial blood flow the steady state method was found to perform better than the bolus method. The net accumulation of substrate (i.e. rCMRglc in the case of glucose analogs) was found to be comparatively insensitive to tissue heterogeneity. Individual rate constants such as k2 and k3 for efflux and metabolism of the substrate in the pool of unmetabolized substrate in the tissue, respectively, were found to be more sensitive. In studies of radioligand binding, using only tracer doses, the effect of tissue heterogeneity on the parameter kon.Bmax could be considerable. In studies of radioligand binding using a protocol with two experiments, one with high and one with low specific activity, Bmax was found to be insensitive while Kd was very sensitive to tissue heterogeneity.

Blood-brain barrier permeability of glucose and ketone bodies during short-term starvation in humans

The blood-brain barrier (BBB) permeability for glucose and beta-hydroxybutyrate (beta-OHB) was studied by the intravenous double-indicator method in nine healthy subjects before and after 3.5 days of starvation. In fasting, mean arterial plasma glucose decreased and arterial concentration of beta-OHB increased, whereas cerebral blood flow remained unchanged. The permeability-surface area product for BBB glucose transport from blood to brain (PS1) increased by 55 +/- 31%, whereas no significant change in the permeability from brain back to blood (PS2) was found. PS1 for beta-OHB remained constant during starvation. The expected increase in PS1 due to the lower plasma glucose concentration was calculated to be 22% using previous estimates of maximal transport velocity and Michaelis-Menten affinity constant for glucose transport. The determined increase was thus 33% higher than the expected increase and can only be partially explained by the decrease in plasma glucose. It is concluded that a modest upregulation of glucose transport across the BBB takes place after starvation. Brain transport of beta-OHB did not decrease as expected from the largely increased beta-OHB arterial level. This might be interpreted as an increase in brain transport of beta-OHB, which could be caused by induction mechanisms, but the large nonsaturable component of beta-OHB transport makes such a conclusion difficult. However, beta-OHB blood concentration and beta-OHB influx into the brain increased by > 10 times. This implies that the influx of ketone bodies into the brain is largely determined by the amount of ketones present in the blood, and any condition in which ketonemia occurs will lead to an increased ketone influx.