Utility of a simplified [18F] sodium fluoride PET imaging method to quantify bone metabolic flux for a wide range of clinical applications.

We review the rationale, methodology, and clinical utility of quantitative [18F] sodium fluoride ([18F]NaF) positron emission tomography-computed tomography (PET-CT) imaging to measure bone metabolic flux (Ki, also known as bone plasma clearance), a measurement indicative of the local rate of bone formation at the chosen region of interest. We review the bone remodelling cycle and explain what aspects of bone remodelling are addressed by [18F]NaF PET-CT. We explain how the technique works, what measurements are involved, and what makes [18F]NaF PET-CT a useful tool for the study of bone remodelling. We discuss how these measurements can be simplified without loss of accuracy to make the technique more accessible. Finally, we briefly review some key clinical applications and discuss the potential for future developments. We hope that the simplified method described here will assist in promoting the wider use of the technique.

Input function and modeling for determining bone metabolic flux using [18 F] sodium fluoride PET imaging: A step-by-step guide.

Studies of skeletal metabolism using measurements of bone metabolic flux (Ki ) obtained with [18 F] sodium fluoride ([18 F]NaF) positron emission tomography (PET) scans have been used in clinical research for the last 30 years. The technique has proven useful as an imaging biomarker in trials of novel drug treatments for osteoporosis and investigating other metabolic bone diseases, including chronic kidney disease mineral and bone disorder. It has also been shown to be valuable in metastatic bone disease in breast cancer patients and may have potential in other cancer types, such as prostate cancer, to assess early bone fracture risk. However, these studies have usually required a 60-min dynamic PET scan and measurement of the arterial input function (AIF), making them difficult to translate into the clinic for diagnostic purposes. We have previously proposed a simplified method that estimates the Ki value at an imaging site from a short (4-min) static scan and venous blood samples. A key advantage of this method is that, by acquiring a series of static scans, values of Ki can be quickly measured at multiple sites using a single injection of the tracer. To date, the widespread use of [18 F]NaF PET has been limited by the need to measure the AIF required for the mathematical modeling of tracer kinetics to derive Ki and other kinetic parameters. In this report, we review different methods of measuring the AIF, including direct arterial sampling, the use of a semi-population input function (SP-AIF), and image-derived input function, the latter two requiring only two or three venous blood samples obtained between 30 and 60 min after injection. We provide an SP-AIF model and a spreadsheet for calculating Ki values using the static scan method that others can use to study bone metabolism in metabolic and metastatic bone diseases without requiring invasive arterial blood sampling. The method shortens scan times, simplifies procedures, and reduces the cost of multicenter trials without losing accuracy or precision.

[18F] Sodium Fluoride PET Kinetic Parameters in Bone Imaging.

This report describes the significance of the kinetic parameters (k-values) obtained from the analysis of dynamic positron emission tomography (PET) scans using the Hawkins model describing the pharmacokinetics of sodium fluoride ([18F]NaF) to understand bone physiology. Dynamic [18F]NaF PET scans may be useful as an imaging biomarker in early phase clinical trials of novel drugs in development by permitting early detection of treatment-response signals that may help avoid late-stage attrition.

A Short Dynamic Scan Method of Measuring Bone Metabolic Flux Using [18F]NaF PET.

[18F]NaF PET measurements of bone metabolic flux (Ki) are conventionally obtained with 60-min dynamic scans analysed using the Hawkins model. However, long scan times make this method expensive and uncomfortable for subjects. Therefore, we evaluated and compared measurements of Ki with shorter scan times analysed with fixed values of the Hawkins model rate constants. The scans were acquired in a trial in 30 postmenopausal women, half treated with teriparatide (TPT) and half untreated. Sixty-minute PET-CT scans of both hips were acquired at baseline and week 12 after injection with 180 MBq [18F]NaF. Scans were analysed using the Hawkins model by fitting bone time-activity curves at seven volumes of interest (VOIs) with a semi-population arterial input function. The model was re-run with fixed rate-constants for dynamic scan times from 0-12 min increasing in 4-min steps up to 0-60 min. Using the Hawkins model with fixed rate-constants, Ki measurements with statistical power equivalent or superior to conventionally analysed 60-min dynamic scans were obtained with scan times as short as 12 min.

Comparison of ordered-subset expectation maximization and filtered back projection reconstruction based on quantitative outcome from dynamic [18F]NaF PET images.

[18F]NaF PET imaging is a useful tool for measuring regional bone metabolism. However, due to tracer in urine, [18F]NaF PET images of the hip reconstructed using filtered back projection (FBP) frequently show streaking artifacts in slices through the bladder leading to noisy time-activity curves unsuitable for quantification. This study compares differences between quantitative outcomes at the hip derived from images reconstructed using the FBP and ordered-subset expectation maximization (OSEM) methods. Dynamic [18F]NaF PET data at the hip for four postmenopausal women were reconstructed using FBP and nine variations of the OSEM algorithm (all combinations of 1, 5, 15 iterations and 10, 15, 21 subsets). Seven volumes of interest were placed in the hip. Bone metabolism was measured using standardized uptake values, Patlak analysis (Ki-PAT) and Hawkins model Ki-4k. Percentage differences between the standardized uptake values and Ki values from FBP and OSEM images were assessed. OSEM images appeared visually smoother and without the streaking artifacts seen with FBP. However, due to loss of counts, they failed to recover the quantitative values in VOIs close to the bladder, including the femoral head and femoral neck. This was consistent for all quantification methods. Volumes of interest farther from the bladder or larger and receiving greater counts showed good convergence with 5 iterations and 21 subsets. For VOIs close to the bladder, including the femoral neck and femoral head, 15 iterations and 10, 15 or 21 subsets were not enough to obtain OSEM images suitable for measuring bone metabolism and showed no improvement compared to FBP.

Evaluation of aortic 18F-NaF tracer uptake using PET/CT as a predictor of aortic calcification in postmenopausal women: A longitudinal study.

INTRODUCTION: Aortic calcification as detected by computed tomography is associated with arterial stiffening and is an important predictor of cardiovascular morbidity and mortality. Uptake of 18F-sodium fluoride (18F-NaF) in the aortic wall reflects metabolically active areas of calcification. The aim of this study was to determine if 18F-NaF uptake in the aorta is associated with calcification and progression of calcification as detected by computed tomography. METHODS: Twenty-one postmenopausal women (mean age 62 ± 6 years) underwent assessment of aortic 18F-NaF uptake using positron emission tomography/computer tomography at baseline and a repeat computed tomography scan after a mean follow-up of 3.8 ± 1.3 years. Tracer uptake was quantified by calculating the target-to-background (TBR) ratios at baseline and follow-up. Calcification was assessed at baseline and follow-up using computed tomography. RESULTS: Over the follow-up period, aortic calcium volume increased from 0.46 ± 0.62 to 0.71 ± 0.93 cm3 (P < 0.05). However, the change in calcium volume did not correlate with baseline TBR either unadjusted (r = 0.00, P = 1.00) or adjusted for age and baseline calcium volume (beta coefficient = -0.18, P = 0.42). TBR at baseline did not differ between participants with (n = 16) compared to those without (n = 5) progression in calcium volume (2.43 ± 0.46 vs. 2.31 ± 0.38, P = 0.58). In aortic segments identified to have the highest tracer uptake at baseline, calcium volume did not significantly change over the follow-up period (P = 0.41). CONCLUSION: In a cohort of postmenopausal women, 18F-NaF uptake as measured by TBR in the lumbar aorta did not predict progression of aortic calcification as detected by computed tomography over a four-year follow-up.

The influence of CT and dual-energy X-ray absorptiometry (DXA) bone density on quantitative [18F] sodium fluoride PET.

BACKGROUND: [18F] sodium fluoride PET/CT provides quantitative measures of bone metabolic activity expressed by the parameters standardised uptake value (SUV) and bone plasma clearance (K i) that correlate with measurements of bone formation rate obtained by bone biopsy with double tetracycline labelling. Both SUV and K i relate to the tracer uptake in each millilitre of tissue. In general, the bone region of interest (ROI) includes both mineralised bone {generally with a high concentration of [18F]NaF} and bone marrow (with a much lower concentration), suggesting that correcting SUV and K i for volumetric bone mineral density (vBMD) and measuring them with respect to the tracer uptake in each gram of bone mineral might improve the correlation with the findings of bone biopsy. As a first test of this hypothesis, we looked for positive correlations between SUV and K i values with CT and DXA bone mineral density (BMD) parameters measured in the same ROI. METHODS: A retrospective reanalysis was performed of 63 lumbar spine [18F]NaF PET/CT scans acquired in four earlier studies. The quantitative PET parameters SUV and K i were measured in L1-L4 and Hounsfield units (HU) measured on the CT scans in the same ROI. Spine BMD data was also obtained from DXA scans in the form of areal BMD and used to derive the bone mineral apparent density (BMAD, an estimate of vBMD). Scatter plots were drawn of SUV and K i against HU, BMAD and areal BMD and the Spearman rank correlation coefficients derived for each plot. RESULTS: All correlations were positive and statistically significant. Correlations were highest for HU (SUV: RS =0.513, P<0.0001; K i: RS =0.429, P=0.0005) and lowest for areal BMD (SUV: RS =0.353, P=0.005; K i: RS =0.274, P=0.03). CONCLUSIONS: The results demonstrate significant positive correlations between SUV and K i and vBMD measurements in the form of HU from CT or BMAD and areal BMD from DXA. These findings justify further exploration of the relationship between SUV and K i [18F]NaF PET/CT measurements and CT or DXA measurements of vBMD to examine whether normalization for bone density might improve their correlation with bone metabolic activity as measured by bone biopsy.

Site specific measurements of bone formation using [18F] sodium fluoride PET/CT.

Dynamic positron emission tomography (PET) imaging with fluorine-18 labelled sodium fluoride ([18F]NaF) allows the quantitative assessment of regional bone formation by measuring the plasma clearance of fluoride to bone at any site in the skeleton. Today, hybrid PET and computed tomography (CT) dual-modality systems (PET/CT) are widely available, and [18F]NaF PET/CT offers a convenient non-invasive method of studying bone formation at the important osteoporotic fracture sites at the hip and spine, as well as sites of pure cortical or trabecular bone. The technique complements conventional measurements of bone turnover using biochemical markers or bone biopsy as a tool to investigate new therapies for osteoporosis, and has a potential role as an early biomarker of treatment efficacy in clinical trials. This article reviews methods of acquiring and analyzing dynamic [18F]NaF PET/CT scan data, and outlines a simplified approach combining venous blood sampling with a series of short (3- to 5-minute) static PET/CT scans acquired at different bed positions to estimate [18F]NaF plasma clearance at multiple sites in the skeleton with just a single injection of tracer.

Imaging of site specific bone turnover in osteoporosis using positron emission tomography.

The functional imaging technique of dynamic fluorine-18 labeled sodium fluoride positron emission tomography ((18)F-NaF PET) allows the quantitative assessment of regional bone formation by measuring the plasma clearance of fluoride to bone at any site in the skeleton. (18)F-NaF PET provides a novel and noninvasive method of studying site-specific bone formation at the hip and spine, as well as areas of pure cortical or trabecular bone. The technique complements conventional measurements of bone turnover using biochemical markers and bone biopsy as a tool to investigate new treatments for osteoporosis, and holds promise of a future role as an early biomarker of treatment efficacy in clinical trials. This article reviews methods of acquiring and analyzing (18)F-NaF PET scan data, and outlines a simplified approach that uses 5-minute static PET scan images combined with venous blood samples to estimate (18)F-NaF plasma clearance at multiple sites in the skeleton with a single injection of tracer.

Correcting (18)F-fluoride PET static scan measurements of skeletal plasma clearance for tracer efflux from bone.

OBJECTIVE: The aim of the study was to examine whether (18)F-fluoride PET ((18)F-PET) static scan measurements of bone plasma clearance (Ki) can be corrected for tracer efflux from bone from the time of injection. MATERIALS AND METHODS: The efflux of tracer from bone mineral to plasma was described by a first-order rate constant kloss. A modified Patlak analysis was applied to 60-min dynamic (18)F-PET scans of the spine and hip acquired during trials on the bone anabolic agent teriparatide to find the best-fit values of kloss at the lumbar spine, total hip and femoral shaft. The resulting values of kloss were used to extrapolate the modified Patlak plots to 120 min after injection and derive a sequence of static scan estimates of Ki at 4-min intervals that were compared with the Patlak Ki values from the 60-min dynamic scans. A comparison was made with the results of the standard static scan analysis, which assumes kloss=0. RESULTS: The best-fit values of kloss for the spine and hip regions of interest averaged 0.006/min and did not change when patients were treated with teriparatide. Static scan values of Ki calculated using the modified analysis with kloss=0.006/min were independent of time between 10 and 120 min after injection and were in close agreement with findings from the dynamic scans. In contrast, by 2 h after injection the static scan Ki values calculated using the standard analysis underestimated the dynamic scan results by 20%. CONCLUSION: Using a modified analysis that corrects for F efflux from bone, estimates of Ki from static PET scans can be corrected for time up to 2 h after injection. This simplified approach may obviate the need to perform dynamic scans and hence shorten the scanning procedure for the patient and reduce the cost of studies. It also enables reliable estimates of Ki to be obtained from multiple skeletal sites with a single injection of tracer.

(18)F-fluoride positron emission tomography measurements of regional bone formation in hemodialysis patients with suspected adynamic bone disease.

(18)F-fluoride positron emission tomography ((18)F-PET) allows the assessment of regional bone formation and could have a role in the diagnosis of adynamic bone disease (ABD) in patients with chronic kidney disease (CKD). The purpose of this study was to examine bone formation at multiple sites of the skeleton in hemodialysis patients (CKD5D) and assess the correlation with bone biopsy. Seven CKD5D patients with suspected ABD and 12 osteoporotic postmenopausal women underwent an (18)F-PET scan, and bone plasma clearance, K i, was measured at ten skeletal regions of interest (ROI). Fifteen subjects had a transiliac bone biopsy following double tetracycline labeling. Two CKD5D patients had ABD confirmed by biopsy. There was significant heterogeneity in K i between skeletal sites, ranging from 0.008 at the forearm to 0.028 mL/min/mL at the spine in the CKD5D group. There were no significant differences in K i between the two study groups or between the two subjects with ABD and the other CKD5D subjects at any skeletal ROI. Five biopsies from the CKD5D patients had single tetracycline labels only, including the two with ABD. Using an imputed value of 0.3 µm/day for mineral apposition rate (MAR) for biopsies with single labels, no significant correlations were observed between lumbar spine K i corrected for BMAD (K i/BMAD) and bone formation rate (BFR/BS), or MAR. When biopsies with single labels were excluded, a significant correlation was observed between K i/BMAD and MAR (r = 0.81, p = 0.008) but not BFR/BS. Further studies are required to establish the sensitivity of (18)F-PET as a diagnostic tool for identifying CKD patients with ABD.

The relationship between inhibitors of the Wnt signalling pathway (Dickkopf-1(DKK1) and sclerostin), bone mineral density, vascular calcification and arterial stiffness in post-menopausal women.

Epidemiological studies have shown an association between bone loss/osteoporosis and vascular calcification (VC). Recent studies have implicated the Wnt signalling pathway in the pathogenesis of VC. We investigated the association between circulating concentrations of Wnt inhibitors; DKK1 and sclerostin with bone mineral density (BMD), abdominal aortic calcification (AAC) and arterial stiffness in post-menopausal women. One hundred and forty six post-menopausal women aged (mean [SD]) 61.5[6.5] years were studied. Sclerostin and DKK1 were measured in serum. BMD was measured at the lumbar spine (LS), femoral neck (FN), total hip (TH). AAC was detected by Vertebral Fracture Assessment (VFA) imaging and quantified using an 8- and 24- point scoring methods. Arterial stiffness was determined by aortic pulse wave velocity (PWV). A significant positive correlation was observed between sclerostin and BMD at the FN (r = 0.166, p = 0.043) and TH (r = 0.165, p = 0.044). The association remained significant at the FN (p = 0.045) and TH (p = 0.026) following adjustment for confounders. No significant correlation was observed between DKK1 and BMD. In contrast, there was a significant negative correlation between log DKK1 and AAC (24-point score: r = -0.25, p = 0.008 and 8-point score: r = -0.21, p = 0.024). Subjects with AAC score of 1 or less had significantly higher DKK1 (p = 0.01). The association between DKK1 and AAC remained significant following correction for age, blood pressure, cholesterol (24-point score: p = 0.017, 8-point score: p = 0.044). In adjusted linear regression analysis, sclerostin was positively associated with AAC (24-point score: p = 0.048, 8-point score: p = 0.031). Subjects with a PWV>9 m/s had significantly higher sclerostin than those with PWV <9 m/s: 23.8[12.3], vs 29.7 [14] pmol/l, p = 0.03). No association was observed between DKK1 and PWV. The opposite association between AAC and the 2 Wnt signaling inhibitors is of interest and merits further investigations. Future longitudinal studies are needed to establish the precise role of sclerostin and DKK1 in the pathogenesis of VC.

Abdominal aortic calcification detection using dual-energy X-ray absorptiometry: validation study in healthy women compared to computed tomography.

Abdominal aortic calcification (AAC) is an independent determinant of cardiovascular events. Computed tomography (CT) is currently the gold standard measure of AAC but is limited by high radiation exposure. Lateral dual-energy X-ray absorptiometry (DXA) has the potential to detect AAC at a fraction of the radiation dose. Our objective was to determine the accuracy of lateral-DXA in detecting AAC compared to CT in healthy women. Women from the TwinsUK registry aged 52-80 years (n = 105) underwent noncontrast CT and lateral-DXA imaging of the abdominal aorta at lumbar vertebrae L1-L4. Presence of calcium on CT was scored using the volume method. Lateral-DXA images were scored using the previously validated semiquantitative 24-point score and simplified 8-point score. Calcification was present in 81 % of women as determined by CT and 49 % with lateral-DXA. The mean volume score and the 24- and 8-point scores of AAC were 0.20 ± 0.41 cm(2), 2.39 ± 3.91 arbitrary units, and 1.47 ± 2.13 arbitrary units, respectively. There was moderate agreement between CT and 24-point lateral-DXA (Spearman's rank correlation coefficient r = 0.58, P < 0.0001). The sensitivity of lateral-DXA for detecting AAC was 56 % and specificity was 80 %. Sensitivity and specificity of lateral-DXA improved to 64 and 84 % when analysis was limited to calcium volumes ≥0.008 cm(3) as detected by CT. Lateral-DXA imaging may provide a useful alternative to CT in detecting AAC with minimal radiation exposure, which may be used with concurrent bone mineral density assessment.

¹8F-fluoride PET as a noninvasive imaging biomarker for determining treatment efficacy of bone active agents at the hip: a prospective, randomized, controlled clinical study.

The functional imaging technique of ¹8F-fluoride positron emission tomography (¹8F-PET) allows the noninvasive quantitative assessment of regional bone formation at any skeletal site, including the spine and hip. The aim of this study was to determine if ¹8F-PET can be used as an early biomarker of treatment efficacy at the hip. Twenty-seven treatment-naive postmenopausal women with osteopenia were randomized to receive teriparatide and calcium and vitamin D (TPT group, n = 13) or calcium and vitamin D only (control group, n = 14). Subjects in the TPT group were treated with 20 µg/day teriparatide for 12 weeks. ¹8F-PET scans of the proximal femur, pelvis, and lumbar spine were performed at baseline and 12 weeks. The plasma clearance of ¹8F-fluoride to bone, K(i), a validated measurement of bone formation, was measured at four regions of the hip, lumbar spine, and pelvis. A significant increase in K(i) was observed at all regions of interest (ROIs), including the total hip (+27%, p = 0.002), femoral neck (+25%, p = 0.040), hip trabecular ROI (+21%, p = 0.017), and hip cortical ROI (+51%, p = 0.001) in the TPT group. Significant increases in K(i) in response to TPT were also observed at the lumbar spine (+18%, p = 0.001) and pelvis (+42%, p = 0.001). No significant changes in K(i) were observed for the control group. Changes in BMD and bone turnover markers were consistent with previous trials of teriparatide. In conclusion, this is the first study to our knowledge to demonstrate that ¹8F-PET can be used as an imaging biomarker for determining treatment efficacy at the hip as early as 12 weeks after initiation of therapy.

Semiautomatic region-of-interest validation at the femur in (18)F-fluoride PET/CT.

UNLABELLED: The assessment of regional skeletal metabolism using (18)F-fluoride PET ((18)F-PET) requires segmentation of the tissue region of interest (ROI). The aim of this study was to validate a novel approach to define multiple ROIs at the proximal femur similar to those used in dual x-ray absorptiometry. Regions were first drawn on low-dose CT images acquired as a routine part of the PET/CT study and transferred to the (18)F-PET images for the quantitative analysis of bone turnover. METHODS: Four healthy postmenopausal women with a mean age of 65.1 y (range, 61.8-70.0 y), and with no history of metabolic bone disorder and not currently being administered treatment affecting skeletal metabolism, underwent dynamic (18)F-PET/CT at the hip with an injected activity of 180 MBq. The ROIs at the proximal femur included femoral shaft, femoral neck, and total hip and were segmented using both a semiautomatic method and manually by 8 experts at manual ROI delineation. The mean of the 8 manually drawn ROIs was considered the gold standard against which the performances of the semiautomatic and manual methods were compared in terms of percentage overlap and percentage difference. The time to draw the ROIs was also compared. RESULTS: The percentage overlaps between the gold standard and the semiautomatic ROIs for total hip, femoral neck, and femoral shaft were 86.1%, 37.8%, and 96.1%, respectively, and the percentage differences were 14.5%, 89.7%, and 4.7%, respectively. In the same order, the percentage overlap between the gold standard and the manual ROIs were 85.2%, 39.1%, and 95.2%, respectively, and the percentage differences were 19.9%, 91.6%, and 12.2%, respectively. The semiautomatic method was approximately 9.5, 2.5, and 67 times faster than the manual method for segmenting total-hip, femoral-neck, and femoral-shaft ROIs, respectively. CONCLUSION: We have developed and validated a semiautomatic procedure whereby ROIs at the hip are defined using the CT component of an (18)F-PET/CT scan. The percentage overlap and percentage difference results between the semiautomatic method and the manual method for ROI delineation were similar. Two advantages of the semiautomatic method are that it is significantly quicker and eliminates some of the variability associated with operator or reader input. The tube current used for the CT scan was associated with an effective dose 8 times lower than that associated with a typical diagnostic CT scan. These results suggest that it is possible to segment bone ROIs from low-dose CT for later transfer to PET in a single PET/CT procedure without the need for an additional high-resolution CT scan.

Comparison of six quantitative methods for the measurement of bone turnover at the hip and lumbar spine using 18F-fluoride PET-CT.

AIM: The aim of this study was to evaluate the relationship between different quantification methods used for the measurement of bone plasma clearance (K(i)) using F-PET at the hip and lumbar spine. METHODS: Twelve healthy postmenopausal women aged 52-71 years were recruited. Each participant underwent 60-min dynamic F-PET scans at the lumbar spine and hip on two separate occasions with an injected activity of 90 and 180 MBq, respectively. Image-derived input functions were obtained at the aorta from the lumbar spine scans. K(i) was evaluated using a three-compartment four-parameter model (K(i-4k)), three-compartment three-parameter model (K(i-3k)), Patlak analysis (K(i-Pat)), spectral analysis (K(i-Spec)) and deconvolution (K(i-Decon)). Standardized uptake values (SUVs) were also measured. RESULTS: The Pearson correlation between K(i-4k) and K(i-3k), K(i-Pat), K(i-Spec), K(i-Decon) and SUV were 0.91, 0.97, 0.94, 0.95 and 0.93, respectively, with a significance of P less than 0.0001. The differences between the correlations measured using Fisher's Z-test were not significant (P>0.05). Bland-Altman analysis showed that the limits of agreement for K(i) measured as the SD of the differences were 0.0082 (25.9%), 0.0062 (11.7%), 0.0098 (20.1%) and 0.0056 (25.5%) ml/min/ml, respectively, and the biases were -0.0081 (-23.8%), -0.0075 (-23.7%), -0.0107 (-29.5%) and -0.0015 (0.8%) ml/min/ml, respectively. CONCLUSION: All five methods of quantification (K(i-3k), K(i-Pat), K(i-Spec), K(i-Decon) and SUV) strongly correlated with K(i-4k). Although systematic differences of up to 29% were found between K(i-4k) and the other methods (K(i-3k), K(i-Pat), K(i-Spec) and K(i-Decon)), these should not affect the conclusions of clinical studies, provided the methods are applied consistently. However, care should be taken when comparing reports that use different methods of quantification.

Quantitative PET Imaging Using (18)F Sodium Fluoride in the Assessment of Metabolic Bone Diseases and the Monitoring of Their Response to Therapy.

Studies of bone remodeling using bone biopsy and biochemical markers of bone turnover measured in serum and urine are important for investigating how new treatments for osteoporosis affect bone metabolism. Positron emission tomography with (18)F sodium fluoride ((18)F NaF PET) for studying bone metabolism complements these conventional methods. Unlike biochemical markers, which measure the integrated response to treatment across the whole skeleton, (18)F NaF PET can distinguish changes occurring at sites of clinically important osteoporotic fractures. Future studies using (18)F NaF PET may illuminate current clinical problems, such as the possible association between long-term treatment with bisphosphonates and atypical fractures of the femur.

Estimation of regional bone metabolism from whole-body 18F-fluoride PET static images.

PURPOSE: We evaluate a new quantitative method of acquiring and analysing (18)F positron emission tomography (PET) studies that enables regional bone plasma clearance (K ( i )) to be estimated from static scans acquired at multiple sites in the skeleton following a single injection of tracer. METHODS: Dynamic lumbar spine (18)F PET data from two clinical trials were used to simulate a series of static scans acquired 30-60 min after injection. Venous blood samples were taken at 30, 40, 50 and 60 min and K ( i ) evaluated by Patlak analysis and the static scan method. The data were used to evaluate the precision errors of the Patlak and static scan methods expressed as the percentage coefficient of variation (%CV) and compare their response to 6 months of treatment with the bone anabolic agent teriparatide. RESULTS: Static scan K ( i ) measurements 30-60 min after injection were highly correlated with the Patlak results (r > 0.99). The %CV for the static scan method was 17.5% 30 min after injection, decreasing to 14.5% at 60 min, compared with 13.0% for Patlak analysis. Response to teriparatide treatment was +25.2% for the static scan method compared with +24.3% for Patlak analysis. The mean ratio (SD) of the static scan and Patlak K ( i ) results was 1.006 (0.015) at 30 min after injection decreasing to 0.965 (0.015) at 60 min. CONCLUSION: (18)F-Fluoride bone plasma clearance can be estimated from a static scan and venous blood samples acquired 30-60 min after injection. The method enables K ( i ) to be estimated at multiple skeletal sites with a single injection of tracer.

The precision and sensitivity of (18)F-fluoride PET for measuring regional bone metabolism: a comparison of quantification methods.

UNLABELLED: The planning of research studies requires an understanding of the minimum number of subjects required. The aim of this study was to evaluate different methods of analyzing (18)F-fluoride PET ((18)F(-) PET) dynamic spine scans to find the approach that requires the smallest sample size to detect a statistically significant response to treatment. METHODS: Eight different approaches to (18)F(-) PET analysis (3 variants of the Hawkins 3-tissue compartmental model, 3 variants of spectral analysis, deconvolution, and Patlak analysis) were used to evaluate the fluoride plasma clearance to bone mineral (K(i)). Standardized uptake values (SUVs) were also studied. Data for 20 women who had (18)F(-) PET spine scans at 0, 6, and 12 mo after stopping long-term bisphosphonate treatment were used to compare precision errors. Data for 18 women who had scans at baseline and 6 mo after starting teriparatide treatment were used to compare response to treatment. RESULTS: The 4 approaches that fitted the rate constant k(4) describing the reverse flow of (18)F from bone as a free variable showed close agreement in K(i) values, with correlation coefficients greater than 0.97. Their %CVs were 14.4%-14.8%, and treatment response to teriparatide was 23.2%-23.8%. The 3 methods that assumed k(4) = 0 gave K(i) values 20%-25% lower than the other methods, with correlation coefficients of 0.83-0.94, percentage coefficients of variation (%CVs) of 12.9%-13.3%, and treatment response of 25.2%-28.3%. A Hawkins model with k(4) = 0.01 min(-1) did not perform any better (%CV, 14.2%; treatment response, 26.1%). Correlation coefficients between SUV and the different K(i) methods varied between 0.60 and 0.65. Although SUV gave the best precision (%CV, 10.1%), the treatment response (3.1%) was not statistically significant. CONCLUSION: Methods that calculated K(i) assuming k(4) = 0 required fewer subjects to demonstrate a statistically significant response to treatment than methods that fitted k(4) as a free variable. Although SUV gave the smallest precision error, the absence of any significant changes make it unsuitable for examining response to treatment in this study.

Validation of image-derived arterial input functions at the femoral artery using 18F-fluoride positron emission tomography.

INTRODUCTION: The use of image-derived arterial input functions (IDAIF) for the dynamic quantification of bone metabolism using 18F-fluoride positron emission tomography 18F-PET is an attractive alternative to direct arterial blood sampling. PURPOSES: (a) To validate a method for obtaining the IDAIF by imaging the femoral artery against a method for deriving the IDAIF at the aorta that was previously validated against direct arterial sampling. (b) To compare the accuracy of bone plasma clearance measurements (Ki) at the total hip site obtained using the femoral artery IDAIF against Ki values at the same site obtained using the aorta IDAIF. METHODS: Twelve healthy postmenopausal women with a mean age of 62.6 years (range, 52.3-70.6 years) had 60-min dynamic 18F-PET scans of the lumbar spine and proximal femur 2 weeks apart. The femoral artery IDAIF was obtained from the proximal femur scan using four different algorithms: (a) fixed partial volume correction (PVC) method; (b) variable PVC method; (c) Chen method; and (d) Cook-Lodge method. The aorta IDAIF was obtained from the lumbar spine scan using a previously validated method and the respective Ki values in the hip were used to assess the performance of each of the femoral artery algorithms. RESULTS: When the femoral artery IDAIF methods were compared with the aorta IDAIF in terms of the area under the curve AUC values calculated in 4-min time intervals over 0-60 min, the absolute root mean square errors were: (a) fixed PVC, 0.52; (b) variable PVC, 0.54; (c) Chen, 0.72; and (d) Cook-Lodge, 0.49 in MBq s/ml. There were small, but statistically significant differences, in the Ki values found by all four femoral artery IDAIF methods when compared with the figures obtained using the aorta IDAIF. Bland-Altman plots of Ki values showed the best agreement for the fixed PVC method with a standard deviation of 0.0020 ml/min/ml, followed by variable PVC, Cook-Lodge and Chen method with standard deviations of 0.0022, 0.0024 and 0.0042 ml/min/ml, respectively. CONCLUSION: We have demonstrated that it is possible to measure regional bone turnover at the hip without the need to perform direct arterial sampling to acquire the arterial input function (AIF). The differences in the Ki values obtained at the hip by using aorta IDAIF and any of the four image-based AIF methods at the femoral artery were small and clinically insignificant. The performance of fixed PVC, variable PVC and Cook-Lodge method was similar although the latter was less robust than the other two methods.