Quantification in clinical fluorodeoxyglucose positron emission tomography

Simultaneous estimation of a model-derived input function for quantifying cerebral glucose metabolism with [18F]FDG PET

BACKGROUND

Quantification of the cerebral metabolic rate of glucose (CMRGlu) by dynamic [18F]FDG PET requires invasive arterial sampling. Alternatives to using an arterial input function (AIF) include the simultaneous estimation (SIME) approach, which models the image-derived input function (IDIF) by a series of exponentials with coefficients obtained by fitting time activity curves (TACs) from multiple volumes-of-interest. A limitation of SIME is the assumption that the input function can be modelled accurately by a series of exponentials. Alternatively, we propose a SIME approach based on the two-tissue compartment model to extract a high signal-to-noise ratio (SNR) model-derived input function (MDIF) from the whole-brain TAC. The purpose of this study is to present the MDIF approach and its implementation in the analysis of animal and human data.

METHODS

Simulations were performed to assess the accuracy of the MDIF approach. Animal experiments were conducted to compare derived MDIFs to measured AIFs (n = 5). Using dynamic [18F]FDG PET data from neurologically healthy volunteers (n = 18), the MDIF method was compared to the original SIME-IDIF. Lastly, the feasibility of extracting parametric images was investigated by implementing a variational Bayesian parameter estimation approach.

RESULTS

Simulations demonstrated that the MDIF can be accurately extracted from a whole-brain TAC. Good agreement between MDIFs and measured AIFs was found in the animal experiments. Similarly, the MDIF-to-IDIF area-under-the-curve ratio from the human data was 1.02 ± 0.08, resulting in good agreement in grey matter CMRGlu: 24.5 ± 3.6 and 23.9 ± 3.2 mL/100 g/min for MDIF and IDIF, respectively. The MDIF method proved superior in characterizing the first pass of [18F]FDG. Groupwise parametric images obtained with the MDIF showed the expected spatial patterns.

CONCLUSIONS

A model-driven SIME method was proposed to derive high SNR input functions. Its potential was demonstrated by the good agreement between MDIFs and AIFs in animal experiments. In addition, CMRGlu estimates obtained in the human study agreed to literature values. The MDIF approach requires fewer fitting parameters than the original SIME method and has the advantage that it can model the shape of any input function. In turn, the high SNR of the MDIFs has the potential to facilitate the extraction of voxelwise parameters when combined with robust parameter estimation methods such as the variational Bayesian approach.

Comparative assessment of linear least-squares, nonlinear least-squares, and Patlak graphical method for regional and local quantitative tracer kinetic modeling in cerebral dynamic 18 F-FDG PET

PURPOSE

Dynamic ¹⁸ F-FDG PET allows quantitative estimation of cerebral glucose metabolism both at the regional and local (voxel) level. Although sensitive to noise and highly computationally expensive, nonlinear least-squares (NLS) optimization stands as the reference approach for the estimation of the kinetic model parameters. Nevertheless, faster techniques, including linear least-squares (LLS) and Patlak graphical method, have been proposed to deal with high resolution noisy data, representing a more adaptable solution for routine clinical implementation. Former research investigating the relative performance of the available algorithms lack precise evaluation of kinetic parameter estimates under realistic acquisition conditions.

METHODS

The present study aims at the systematic comparison of the feasibility and pertinence of kinetic modeling of dynamic cerebral ¹⁸ F-FDG PET using NLS, LLS, and Patlak method, based on numerical simulations and patient data. Numerical simulations were used to study the bias and variance of K₁ and K_i parameters estimation under representative noise levels. Patient data allowed to assess the concordance between the three methods at the regional and voxel scale, and to evaluate the robustness of the estimations with respect to patient head motion.

RESULTS AND CONCLUSIONS

Our findings indicate that at the regional level NLS and LLS provide kinetic parameter estimates (K₁ and K_i ) with similar bias and variance characteristics (K₁ bias ± relative standard deviation [RSD] 0.0 ± 5.1% and 0.1% ± 4.9% for NLS and LLS respectively, K_i bias ± RSD 0.1% ± 4.5% and -0.7% ± 4.4% for NLS and LLS respectively). NLS estimates appear, however, to be slightly less sensitive to patient motion. At the voxel level, provided that patient motion is negligible or corrected, LLS offers an appealing alternative solution for local K₁ mapping. It yields K1 estimates that are highly correlated, with high correlation with NLS values (Pearson's r = 0.95 on actual data) within computations times less than two orders of magnitude lower. Last, Patlak method appears as the most robust and accurate technique for the estimation of K_i values at the regional and voxel scale, with or without head motion. It provides low bias/low variance K_i quantification (bias ± RSD -1.5 ± 9.5% and -4.1 ± 19.7% for Patlak and NLS respectively) as well as smooth parametric images suitable for visual assessment.

Quantitative Analysis of Metabolic Abnormality Associated with Brain Developmental Venous Anomalies

Background and Purpose: Abnormal hypometabolism is common in the brain parenchyma surrounding developmental venous anomalies (DVAs), although the degree of DVA-associated hypometabolism (DVAAh) has not been quantitatively analyzed. In this study, we demonstrate a simple method for the measurement of DVAAh and test the hypothesis that DVAs are associated with a quantifiable decrement in metabolic activity.

Materials and Methods: Measurements of DVAAh using ratios of standardized uptake values (SUVs) and comparison to a normal database were performed on a cohort of 25 patients (12 male, 13 female), 14 to 76 years old, with a total of 28 DVAs (20 with DVAAh, seven with isometabolic activity, and one with hypermetabolic activity).

Results: Qualitative classification of none, mild, moderate, and severe DVAAh corresponded to quantitative measurements of DVAAh of 1 ± 3%, 12 ± 7%, 18 ± 6%, and 37 ± 6%, respectively. A statistically significant linear correlation between DVAAh and age was observed (P = 0.003), with a 3% reduction in metabolic activity per decade. A statistically significant linear correlation between DVAAh and DVA size was observed (P = 0.01), with a 4% reduction in metabolic activity per each 1 cm in the longest dimension. The SUV_DVA-based measures of DVAAh correlated (P = 0.001) with measures derived from comparison with a standardized database.

Conclusion: We present a simple method for the quantitative measurement of DVAAh using ratios of SUVs, and find that this quantitative analysis is consistent with a qualitative classification. We find that 54% (15 of 28) of DVAs are associated with a greater than 10% decrease in metabolic activity.

Pattern of 18F-FDG uptake in the spinal cord in patients with non-central nervous system malignancy

STUDY DESIGN

Retrospective review.

OBJECTIVE

To (1) propose a standard method to quantitate 2-deoxy-2-[18F]-fluoro-D-glucose (18F-FDG) uptake in the spinal cord and (2) use this methodology to retrospectively characterize the pattern of uptake within the entire spinal cord using whole-body positron emission tomography/computed tomography (PET/CT) imaging.

SUMMARY OF BACKGROUND DATA

A physiologic understanding of glucose metabolism within the spinal cord may provide insight regarding infectious, inflammatory, vascular, and neoplastic spinal cord diseases.

METHODS

Institutional review board approval was obtained. A total of 131 consecutive whole-body PET/CT studies from July to August 2004 were reviewed, and using exclusionary criteria of: (1) severe spinal arthropathy or curvature, (2) motion artifact, (3) canal hardware, (4) spinal tumor, and (5) marrow hyperplasia, 92 studies of neurologically intact patients (49 men and 43 women) were selected for a retrospective review of spinal cord 18F-FDG activity. The transaxial CT was used to define the canal and circular regions of interests were placed within the canal at the level of the vertebral body midpoint from C1 to L3. Region of interest total count, area, and maximum standardized uptake value (SUVmax) were recorded. Measurements at L5 served as an internal control. For comparative analysis, the cord-to-background (CTB) ratio was defined as spinal cord SUVmax to L5 SUVmax.

RESULTS

Mean CTB decreased along each spinal level from cranial to caudal (P < 0.001). Significant relative increases were observed at the T11-T12 vertebral body levels (P < 0.001). Although insignificant, a relative increase was observed at C4. No significant interactions of age or sex on CTB were observed.

CONCLUSION

The pattern of 18F-FDG uptake within the spinal cord, observed in patients with non-central nervous system malignancy, may be helpful in understanding glucose physiology of spinal cord diseases and warrants further research.

Increased glucose uptake in visceral versus subcutaneous adipose tissue revealed by PET imaging

OBJECTIVES

The current study tested the hypothesis that glucose utilization differs between visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT), and investigated potential mechanisms for such a finding.

BACKGROUND

VAT burden correlates better with cardiovascular risk than does SAT burden. Beyond volumetric measurement, glucose uptake in adipose tissue (AT) might reflect metabolic activity and provide pathophysiologic insight and aid risk stratification.

METHODS

We retrospectively studied tissue-specific glucose uptake in vivo in clinically obtained whole-body fluorodeoxyglucose positron emission tomography (FDG-PET) scans in humans. We also assessed glucose uptake in vitro, using stromal vascular cells isolated from SAT and VAT of diet-induced obese C57BL/6 mice. Quantitative polymerase chain reaction (PCR) evaluated the expression of multiple genes involved in cellular glucose metabolism, including glucose transporters (GLUT-1, -3, and -4) and hexokinases (HK-1 and -2) in SAT and VAT of obese C57BL/6 mice.

RESULTS

We analyzed whole-body FDG-PET scans from 31 obese and 26 lean patients. VAT exhibited higher FDG uptake compared with SAT (p < 0.0001) independent of age, sex, body mass index, comorbidities, and medications. To investigate mechanisms underlying this observation, we studied glucose uptake in the stromal vascular cell fraction of AT, which is rich in inflammatory cells. Stromal vascular cells from VAT of diet-induced obese C57BL/6 mice exhibited higher glucose uptake than those from SAT (p = 0.01). Evaluation of expression of glucose transporters (GLUT-1, -3, and -4) and hexokinases (HK-1 and -2), revealed increased expression of HK-1 in VAT-derived compared with SAT-derived stromal vascular cells, and also in visceral versus subcutaneous unfractionated AT.

CONCLUSIONS

In humans in vivo, VAT has increased glucose uptake compared with SAT, as determined noninvasively with FDG PET imaging. Differential stromal metabolic activity may be 1 mechanism underlying differences in metabolic activity of visceral and subcutaneous AT.

Comparison of Methods for Magnetic Resonance-Guided [18-F]Fluorodeoxyglucose Positron Emission Tomography in Human Carotid Arteries

Background and Purpose— Inflammation is a major risk factor for atherosclerotic plaque rupture and clinical events. Previous studies have shown that plaque [ 18 F]fluorodeoxyglucose (FDG) uptake correlates with macrophage content. In this study we examined the reproducibility of 3 methods of quantifying plaque FDG uptake in the carotid arteries using positron emission tomography (PET). The correlation between 2 simplified uptake parameters (standardized uptake value [SUV], vessel wall-to-blood ratio [VBR]) and a gold standard technique (influx rate [K i ]) was also determined. We used MRI to correct carotid plaque FDG uptake for partial volume error.

Methods— Seven patients with a recent carotid territory transient ischemic attack underwent imaging twice within 8 days using MR and FDG-PET. MR coregistered to PET was used to delineate regions of interest, and to facilitate partial volume correction (PVC).

Results— SUV was the most reproducible parameter irrespective of whether it was normalized by body surface area (BSA), lean body mass, or weight (intraclass correlation coefficient=0.85, 0.88, and 0.90, respectively). VBR correlated better to K i than SUV ( r =0.58 VBR, r =0.46 SUV BSA ). PVC improved these correlations to r =0.81 VBR and r =0.76 SUV BSA , and only slightly degraded the reproducibility of SUV (intraclass correlation coefficient=0.83–0.85).

Conclusions— MR-guided FDG-PET is a highly reproducible technique in the carotid artery and the excellent anatomic detail provided by MR facilitates PVC. Of the methods examined, SUV BSA PVC appears to represent the best compromise between reproducible and accurate determination of FDG metabolism in carotid artery vessel wall.

Long-term precision of 18F-fluoride PET skeletal kinetic studies in the assessment of bone metabolism

(18)F-Fluoride PET allows noninvasive evaluation of regional bone metabolism and has the potential to become a useful tool for assessing patients with metabolic bone disease and evaluating novel drugs being developed for these diseases. The main PET parameter of interest, termed K(i), reflects regional bone metabolism. The aim of this study was to compare the long-term precision of (18)F-fluoride PET with that of biochemical markers of bone turnover assessed over 6 mo.

METHODS

Sixteen postmenopausal women with osteoporosis or significant osteopenia and a mean age of 64 y underwent (18)F-fluoride PET of the lumbar spine and measurements of biochemical markers of bone formation (bone-specific alkaline phosphatase and osteocalcin) and bone resorption (urinary deoxypyridinoline) at baseline and 6 mo later. Four different methods for analyzing the (18)F-fluoride PET data were compared: a 4k 3-compartmental model using nonlinear regression analysis (K(i-4k)), a 3k 3-compartmental model using nonlinear regression analysis (K(i-3k)), Patlak analysis (K(i-PAT)), and standardized uptake values.

RESULTS

With the exception of a small but significant decrease in K(i-3k) at 6 mo, there were no significant differences between the baseline and 6-mo values for the PET parameters or biochemical markers. The long-term precision, expressed as the coefficient of variation (with 95% confidence interval in parentheses), was 12.2% (9%-19%), 13.8% (10%-22%), 14.4% (11%-22%), and 26.6% (19%-40%) for K(i-3k), K(i-PAT), mean standardized uptake value, and K(i-4k), respectively. For comparison, the precision of the biochemical markers was 10% (7%-15%), 18% (13%-27%), and 14% (10%-21%) for bone-specific alkaline phosphatase, osteocalcin, and urinary deoxypyridinoline, respectively. Intraclass correlation between the baseline and 6-mo values ranged from 0.44 for K(i-4k) to 0.85 for K(i-3k). No significant correlation was found between the repeated mean standardized uptake value measurements.

CONCLUSION

The precision and intraclass correlation observed for K(i-3k) and K(i-PAT) was equivalent to that observed for biochemical markers. This study provided initial data on the long-term precision of (18)F-fluoride PET measured at the lumbar spine, which will aid in the accurate interpretation of changes in regional bone metabolism in response to treatment.

Improvement of semi-quantitative small-animal PET data with recovery coefficients: A phantom and rat study

AIM

To evaluate the accuracy of semi-quantitative small-animal PET data, uncorrected for attenuation, and then of the same semi-quantitative data corrected by means of recovery coefficients (RCs) based on phantom studies.

MATERIALS AND METHODS

A phantom containing six fillable spheres (diameter range: 4.4-14 mm) was filled with an 18F-FDG solution (spheres/background activity=10.1, 5.1 and 2.5). RCs, defined as measured activity/expected activity, were calculated. Nude rats harbouring tumours (n=50) were imaged after injection of 18F-FDG and sacrificed. The standardized uptake value (SUV) in tumours was determined with small-animal PET and compared to ex-vivo counting (ex-vivo SUV). Small-animal PET SUVs were corrected with RCs based on the greatest tumour diameter. Tumour proliferation was assessed with cyclin A immunostaining and correlated to the SUV.

RESULTS

RCs ranged from 0.33 for the smallest sphere to 0.72 for the largest. A sigmoidal correlation was found between RCs and sphere diameters (r(2)=0.99). Small-animal PET SUVs were well correlated with ex-vivo SUVs (y=0.48x-0.2; r(2)=0.71) and the use of RCs based on the greatest tumour diameter significantly improved regression (y=0.84x-0.81; r(2)=0.77), except for tumours with important necrosis. Similar results were obtained without sacrificing animals, by using PET images to estimate tumour dimensions. RC-based corrections improved correlation between small-animal PET SUVs and tumour proliferation (uncorrected data: Rho=0.79; corrected data: Rho=0.83).

CONCLUSION

Recovery correction significantly improves both accuracy of small-animal PET semi-quantitative data in rat studies and their correlation with tumour proliferation, except for largely necrotic tumours.

Balancing bias, reliability, noise properties and the need for parametric maps in quantitative ligand PET: [(11)C]diprenorphine test-retest data

[(11)C]diprenorphine (DPN) is a non-subtype selective opioid receptor PET ligand with slow kinetics and no region devoid of specific binding. Parametric maps are desirable but have to overcome high noise at the voxel level. We obtained parameter values, parametric map image quality, test-retest reproducibility and reliability (using intraclass correlation coefficients (ICCs)) for conventional spectral analysis and a derived method (rank shaping), compared them with values obtained through sampling of volumes of interest (VOIs) on the dynamic data sets and tested whether smaller amounts of radioactivity injected maintained reliability. Ten subjects were injected twice with either approximately 185 MBq or approximately 135 MBq of [(11)C]DPN, followed by dynamic PET for 90 min. Data were movement corrected with a frame-to-frame co-registration method. Arterial plasma input functions corrected for radiolabelled metabolites were created. There was no overall effect of movement correction except for one subject with substantial movement whose test-retest differences decreased by approximately 50%. Actual parametric values depended heavily on the cutoff for slow frequencies (between 0.0008 s(-1) and 0.00063 s(-1)). Image quality was satisfactory for restricted base ranges when using conventional spectral analysis. The rank shaping method allowed maximising of this range but had similar bias. VOI-based methods had the widest dynamic range between regions. Average percentage test-retest differences were smallest for the parametric maps with restricted base ranges; similarly ICCs were highest for these (up to 0.86) but unacceptably low for VOI-derived VD estimates at the low doses of injected radioactivity (0.24/0.04). Our data can inform the choice of methodology for a given biological problem.

Technology related parameters affecting quantification in positron emission tomography imaging

Some of the issues associated with positron emission tomography (PET) technology which still pose challenges for the recovery of quantitative images are discussed. Through these issues reference to what is today considered as the 'gold standard' in quantitative PET imaging is also presented. A brief comparison of 2-D and 3-D PET is given, together with a short discussion of combined PET/CT imaging devices.