18 F-fluoride for bone imaging

Assessment of tissue response in vivo: PET-CT imaging of titanium and biodegradable magnesium implants

To study in vivo the bioactivity of biodegradable magnesium implants and other possible biomaterials, we are proposing a previously unexplored application of PET-CT imaging, using available tracers to follow soft tissue and bone remodelling and immune response in the presence of orthopaedic implants. Female Wistar rats received either implants (Ti6Al7Nb titanium or WE43 magnesium) or corresponding transcortical sham defects into the diaphyseal area of the femurs. Inflammatory response was followed with [18F]FDG and osteogenesis with [18F]NaF, over the period of 1.5 months after surgery. An additional pilot study with [68Ga]NODAGA-RGD tracer specific to αvβ3 integrin expression was performed to follow the angiogenesis for one month. [18F]FDG tracer uptake peaked on day 3 before declining in all groups, with Mg and Ti groups exhibiting overall higher uptake compared to sham. This suggests increased cellular activity and tissue response in the presence of Mg during the initial weeks, with Ti showing a subsequent increase in tracer uptake on day 45, indicating a foreign body reaction. [18F]NaF uptake demonstrated the superior osteogenic potential of Mg compared to Ti, with peak uptake on day 7 for all groups. [68Ga]NODAGA-RGD pilot study revealed differences in tracer uptake trends between groups, particularly the prolonged expression of αvβ3 integrin in the presence of implants. Based on the observed differences in the uptake trends of radiotracers depending on implant material, we suggest that PET-CT is a suitable modality for long-term in vivo assessment of orthopaedic biomaterial biocompatibility and underlying tissue reactions. STATEMENT OF SIGNIFICANCE: The study explores the novel use of positron emission tomography for the assessment of the influence that biomaterials have on the surrounding tissues. Previous related studies have mostly focused on material-related effects such as implant-associated infections or to follow the osseointegration in prosthetics, but the use of PET to evaluate the materials has not been reported before. The approach tests the feasibility of using repeated PET-CT imaging to follow the tissue response over time, potentially improving the methodology for adopting new biomaterials for clinical use.

Liquid solution centrifugation for safe, scalable, and efficient isotope separation

A general method of separating isotopes by centrifuging dissolved chemical compounds in a liquid is introduced. This technique can be applied to almost all elements and leads to large separation factors. The method has been demonstrated in several isotopic systems including Ca, Mo, O, and Li with single-stage selectivities of 1.046 to 1.067 per neutron mass difference (e.g., 1.43 in ⁴⁰Ca/⁴⁸Ca), which are beyond the capabilities of various conventional methods. Equations are derived to model the process, and the results agree with those of the experiments. The scalability of the technique has been demonstrated by a three-stage enrichment of ⁴⁸Ca with a total ⁴⁰Ca/⁴⁸Ca selectivity of 2.43, and the scalability is more broadly supported through analogies to gas centrifuge, whereby countercurrent centrifugation can further multiply the separation factor by 5 to 10 times per stage in a continuous process. Optimal centrifuge conditions and solutions can achieve both high-throughput and highly efficient isotope separation.

[18F]Fluoride uptake in various bone types and soft tissues in rat

BACKGROUND

In the development of new ¹⁸F-labelled tracers, it is important to assess the amount of released [¹⁸F]fluoride taken up in the bones of experimental animals because all ¹⁸F-labelled PET-tracers are prone, to lesser or higher degree, to undergo defluorination, with subsequent release of [¹⁸F]fluoride during scanning. However, the pharmacokinetics of [¹⁸F]fluoride in bones and other organs of healthy rats have not been well documented in a comprehensive manner. We aimed to study pharmacokinetics of [¹⁸F]NaF in rats in order to increase our understanding of the biodistribution of [¹⁸F]fluoride originating from defluorination of ¹⁸F-labelled tracers. We studied [¹⁸F]fluoride uptake in Sprague Dawley rat bones, including the epiphyseal parts of the tibia and radius, the mandible, ilium, lumbar vertebrae, costochondral joints, tibia, radius, and ribs, with 60-min in vivo PET/CT imaging. Kinetic parameters, K₁, K_i, K_i/K₁, and k₃ were calculated with a three-compartment model. In addition, separate groups of male and female rats were studied with ex vivo bone and soft tissue harvesting and gamma counting over a 6-h period.

RESULTS

[¹⁸F]fluoride perfusion and uptake varied among the different bones. [¹⁸F]fluoride uptake was higher in trabecular bones, due to high perfusion and osteoblastic activity, compared to cortical bones. In soft tissues, the organ-to-blood uptake ratios increased over time in the eyes, lungs, brain, testes, and ovaries during the 6 h study period.

CONCLUSION

Understanding the pharmacokinetics of [¹⁸F]fluoride in various bones and soft tissues is highly useful for assessing ¹⁸F-labelled radiotracers that release [¹⁸F]fluoride.

18F-Sodium Fluoride PET/CT in Assessing Valvular Heart and Atherosclerotic Diseases

Aortic valve stenosis is the most common valvular disease in Western countries, while atherosclerotic cardiovascular disease is the foremost cause of death and disability worldwide. Valve degeneration and atherosclerosis are mediated by inflammation and calcification and inevitably progress over time. Computed tomography can visualise the later stages of macroscopic calcification but fails to assess the early stages of microcalcification and cannot differentiate active from burnt out disease states. Molecular imaging has the ability to provide complementary information related to disease activity, which may allow us to detect disease early, to predict disease progression and to monitor preventive or therapeutic strategies for in both aortic stenosis and atherosclerosis. PET/CT is a non-invasive imaging technique that enables visualization of ongoing molecular processes within small structures, such as the coronary arteries or heart valves. ¹⁸F-sodium fluoride (¹⁸F-NaF) binds hydroxyapatite deposits in the extracellular matrix, with preferential binding to newly developing deposits of microcalcification, which provides an assessment of calcification activity. In recent years, ¹⁸F-NaF has attracted the attention of many research groups and has been evaluated in several pathological cardiovascular processes. Histologic validation of the ¹⁸F-NaF PET signal in valvular disease and atherosclerosis has been reported in multiple independent studies. The selective high-affinity binding of ¹⁸F-NaF to microscopic calcified deposits (beyond the resolution of μCT) has been demonstrated ex vivo, as well as its ability to distinguish between areas of macro- and active microcalcification. In addition, prospective clinical studies have shown that baseline ¹⁸F-NaF uptake in patients with aortic stenosis and mitral annular calcification is correlated with subsequent calcium deposition and valvular dysfunction after a follow-up period of 2 years. In patients with surgical bioprosthetic aortic valves but without morphological criteria for prosthetic degeneration, increased ¹⁸F-NaF uptake at baseline was associated with subsequent bioprosthetic degeneration over time. Similar data were obtained in a cohort of patients with transcatheter aortic valve implantation. Furthermore, several studies have confirmed the association of coronary ¹⁸F-NaF uptake with adverse atherosclerotic plaque features, active disease and future disease progression. ¹⁸F-NaF uptake is also associated with future fatal or nonfatal myocardial infarction in patients with established coronary artery disease. The link between ¹⁸F-NaF uptake and active atherosclerotic disease has not only been demonstrated in the coronary arteries, but also in peripheral arterial disease, abdominal aortic aneurysms and carotid atherosclerosis. It can be assumed that ¹⁸F-NaF PET/CT will strengthen the diagnostic toolbox of practitioners in the coming years. Indeed, there is a strong medical need to diagnose degenerative valvular disease and to detect active atherosclerotic disease states. Finally, the use of ¹⁸F-NaF as a biomarker to monitor the efficacy of drug therapies in preventing these pathological processes is attractive. In this review, we consider the role of ¹⁸F-NaF PET/CT imaging in cardiac valvular diseases and atherosclerosis.

Aseptic process validation of [18F]Sodium Fluoride radiopharmaceutical in-house production

Sodium fluoride ([18F]NaF) is a PET radiopharmaceutical for vizualization of the skeletal system and microcalcification. In the originally designed in-house method, [18F]NaF is recovered in aqueous solution after cyclotron irradiation, sterilized by passage through a 0.22 µm sterile filter and dispensed under aseptic conditions. To ensure the microbiological safety of drugs produced under aseptic conditions, validation of aseptic procedures is always recommended. This is essential for radiopharmaceuticals because most of them are released for administration before any sterility test can be completed due to their radioactive nature. This study reports the validation of the aseptic process applied to the internal production of [18F]NaF carried out in two phases: testing the number of viable microorganisms in radiopharmaceutical product prior to sterilization and process simulation studies (media fill tests). We found that all samples were sterile and the endotoxin concentration was well below the maximum acceptable level reported in the Ph Eur. monograph on [18F]NaF. The results confirmed that the entire production process of [18F]NaF can be carried out under strictly aseptic conditions following the validated procedures preserving the sterility of the final product.

The uptake pattern of 18F-sodium fluoride radioligand in brain tissue after cerebral infarction

Positron emission tomography with ¹⁸F-sodium fluoride (NaF) radioligand has been actively investigated in atherosclerosis research because it is known to detect microcalcification activity within atheroma. We studied whether NaF shows any uptake in the brain tissue of patients with acute ischemic stroke. This is a post-hoc analysis of previously reported cerebral atherosclerosis research with positron emission tomography which applied the two radioligands, ¹⁸F-fluorodeoxyglucose and NaF for the detection of culprit atheroma among 20 acute cerebral infarction patients (mean age = 75.1 ± 9.0 years; 10 women). In this study, we measured the maximum and mean standardized uptake value (SUVmax and SUVmean) of NaF uptake level in the cerebral infarct region between lesions with and without diffusion weighted image (DWI) positivity, indicating acute ischemic cell death. Correlation analysis was performed between NaF uptake levels and imaging and clinical variables, including neurological severity. The NaF uptake levels were significantly higher in DWI positive lesions than in negative lesions (SUVmax: 2.0 [0.60-4.2] versus 0.20 [0.10-0.40], p = 0.021 by Mann-Whitney U test). The intensity of NaF uptake (SUVmax) was significantly correlated with the initial neurological severity (Spearman's ρ = 0.579, p= 0.007) and white blood cell count (Spearman's ρ = 0.626, p p 0.003). During ischemic stroke NaF was concentrated in brain tissue undergoing acute cell death and its uptake intensity was correlated with neurological severity, suggesting that NaF could reflect acute ischemic cell death after stroke.

The effect of zoledronic acid and denosumab on the mandible and other bones: a 18F-NaF-PET study

OBJECTIVES

The primary purpose of this study was to determine whether both zoledronic acid (ZA) and denosumab (Dmab) equally suppress bone remodeling of the normal mandible, and the secondary purpose was to determine the influence of ZA and Dmab on other normal bones.

METHODS

¹⁸F-sodium fluoride-positron-emission-tomography (¹⁸F-NaF-PET) was used to perform quantitative analysis of the bone metabolism in various parts. The end points of the study were the mean standardized uptake value (SUV) of each member of the ZA group (n = 9), the Dmab group (n = 16), and the Control group (n = 23).

RESULTS

The SUV at the thoracic vertebrae in the ZA group were significantly lower than those of the Dmab and Control group (p < 0.05) In addition, the mean SUVs of the cervical vertebrae in the ZA group were significantly lower than those in the Control group (p < 0.05). There was no significant difference among ZA, Dmab and Control group in the other sites. There was no significant difference between the Dmab and Control groups at all sites.

CONCLUSIONS

The remodeling of mandible was not suppressed due to the treatment with anti-resorptive agents. Differences in the mechanisms of action between the BP and Dmab caused the specificity of the effect on the metabolism of normal bone.

Molecular Imaging of Aortic Valve Stenosis with Positron Emission Tomography

Aortic valve stenosis (AVS) is an increasingly prevalent disease in our aging population. Although multiple risk factors for AVS have been elucidated, medical therapies capable of slowing down disease progression remain unavailable. Molecular imaging technologies are opening up avenues for the non-invasive assessment of disease progression, allowing the assessment of (early) medical interventions. This review will focus on the role of positron emission tomography of the aortic valve with 18F-fluorodeoxyglucose and 18F-sodium fluoride but will also shed light on novel tracers which have potential in AVS, ranging from the healthy aortic valve to end-stage valvular disease.

Comparison of 18F-NaF Imaging, 99mTc-MDP Scintigraphy, and 18F-FDG for Detecting Bone Metastases

Bone is a common metastasis site in several malignancies, most importantly prostate and breast cancers. Given the significance of the early and accurate diagnosis of bone metastases for preliminary staging, treatment planning and monitoring, restaging, and survival prediction in patients with malignancy, it is critical to compare and contrast the strengths and weaknesses of imaging modalities. Although technetium-99m-labeled diphosphonates [99mTc-MDP] scintigraphy has been used for assessing skeletal involvement, there is a renewed interest in fluorine-18-labeled sodium fluoride [18F-NaF] bone imaging with positron emission tomography or positron emission tomography/computed tomography, since this approach provides essential advantages in bone metastases evaluation. This review study aimed to discuss the basic and technical aspects of 18F-NaF imaging and its mechanism of action, and compare this modality with the 99mTc-MDP bone scan and 18F-fluorodeoxyglucose using current evidence from the pertinent literature and case examples of the center in the study.

18F-fluoride PET/MR in cardiac amyloid: A comparison study with aortic stenosis and age- and sex-matched controls

OBJECTIVES

Cardiac MR is widely used to diagnose cardiac amyloid, but cannot differentiate AL and ATTR subtypes: an important distinction given their differing treatments and prognoses. We used PET/MR imaging to quantify myocardial uptake of 18F-fluoride in ATTR and AL amyloid patients, as well as participants with aortic stenosis and age/sex-matched controls.

METHODS

In this prospective multicenter study, patients were recruited in Edinburgh and New York and underwent 18F-fluoride PET/MR imaging. Standardized volumes of interest were drawn in the septum and areas of late gadolinium enhancement to derive myocardial standardized uptake values (SUV) and tissue-to-background ratio (TBRMEAN) after correction for blood pool activity in the right atrium.

RESULTS

53 patients were scanned: 18 with cardiac amyloid (10 ATTR and 8 AL), 13 controls, and 22 with aortic stenosis. No differences in myocardial TBR values were observed between participants scanned in Edinburgh and New York. Mean myocardial TBRMEAN values in ATTR amyloid (1.13 ± 0.16) were higher than controls (0.84 ± 0.11, P = .0006), aortic stenosis (0.73 ± 0.12, P < .0001), and those with AL amyloid (0.96 ± 0.08, P = .01). TBRMEAN values within areas of late gadolinium enhancement provided discrimination between patients with ATTR (1.36 ± 0.23) and all other groups (e.g., AL [1.06 ± 0.07, P = .003]). A TBRMEAN threshold >1.14 in areas of LGE demonstrated 100% sensitivity (CI 72.25 to 100%) and 100% specificity (CI 67.56 to 100%) for ATTR compared to AL amyloid (AUC 1, P = .0004).

CONCLUSION

Quantitative 18F-fluoride PET/MR imaging can distinguish ATTR amyloid from other similar phenotypes and holds promise in improving the diagnosis of this condition.

PET/CT imaging of spinal inflammation and microcalcification in patients with low back pain: A pilot study on the quantification by artificial intelligence-based segmentation

Background

Current imaging modalities are often incapable of identifying nociceptive sources of low back pain (LBP). We aimed to characterize these by means of positron emission tomography/computed tomography (PET/CT) of the lumbar spine region applying tracers ¹⁸F‐fluorodeoxyglucose (FDG) and ¹⁸F‐sodium fluoride (NaF) targeting inflammation and active microcalcification, respectively.

Methods

Using artificial intelligence (AI)‐based quantification, we compared PET findings in two sex‐ and age‐matched groups, a case group of seven males and five females, mean age 45 ± 14 years, with ongoing LBP and a similar control group of 12 pain‐free individuals. PET/CT scans were segmented into three distinct volumes of interest (VOIs): lumbar vertebral bodies, facet joints and intervertebral discs. Maximum, mean and total standardized uptake values (SUVmax, SUVmean and SUVtotal) for FDG and NaF uptake in the 3 VOIs were measured and compared between groups. Holm–Bonferroni correction was applied to adjust for multiple testing.

Results

FDG uptake was slightly higher in most locations of the LBP group including higher SUVmean in the intervertebral discs (0.96 ± 0.34 vs. 0.69 ± 0.15). All NaF uptake values were higher in cases, including higher SUVmax in the intervertebral discs (11.63 ± 3.29 vs. 9.45 ± 1.32) and facet joints (14.98 ± 6.55 vs. 10.60 ± 2.97).

Conclusion

Observed intergroup differences suggest acute inflammation and microcalcification as possible nociceptive causes of LBP. AI‐based quantification of relevant lumbar VOIs in PET/CT scans of LBP patients and controls appears to be feasible. These promising, early findings warrant further investigation and confirmation.

Molecular imaging in atherosclerosis

Purpose

As atherosclerosis is a  prominent cause of morbidity and mortality, early detection of atherosclerotic plaques is vital to prevent complications. Imaging plays a significant role in this goal. Molecular imaging and structural imaging detect different phases of atherosclerotic progression. In this review, we explain the relation between these types of imaging with the physiopathology of plaques, along with their advantages and disadvantages. We also discuss in detail the most commonly used positron emission tomography (PET) radiotracers for atherosclerosis imaging.

Method

A comprehensive search was conducted to extract articles related to imaging of atherosclerosis in PubMed, Google Scholar, and Web of Science. The obtained papers were reviewed regarding precise relation with our topic. Among the search keywords utilized were "atherosclerosis imaging", "atherosclerosis structural imaging", "atherosclerosis CT scan" "positron emission tomography", "PET imaging", "18F-NaF", "18F-FDG", and "atherosclerosis calcification."

Result

Although structural imaging such as computed tomography (CT) offers essential information regarding plaque structure and morphologic features, these modalities can only detect macroscopic alterations that occur later in the disease’s progression, when the changes are frequently irreversible. Molecular imaging modalities like PET, on the other hand, have the advantage of detecting microscopic changes and allow us to treat these plaques before irreversible changes occur. The two most commonly used tracers in PET imaging of atherosclerosis are 18F-sodium fluoride (18F-NaF) and 18F-fluorodeoxyglucose (18F-FDG). While there are limitations in the use of 18F-FDG for the detection of atherosclerosis in coronary arteries due to physiological uptake in myocardium and high luminal blood pool activity of 18F-FDG, 18F-NaF PET is less affected and can be utilized to analyze the coronary arteries in addition to the peripheral vasculature.

Conclusion

Molecular imaging with PET/CT has become a useful tool in the early detection of atherosclerosis. 18F-NaF PET/CT shows promise in the early global assessment of atherosclerosis, but further prospective studies are needed to confirm its role in this area.

Updates in the Impact of Chronic Systemic Inflammation on Vascular Inflammation by Positron Emission Tomography (PET)

PURPOSE OF REVIEW

In this review, we focus on the clinical and epidemiological studies pertaining to systemic and vascular inflammation by positron emission tomography (PET) in patients with chronic inflammatory conditions such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), human immunodeficiency virus (HIV), and psoriasis to highlight the importance of chronic systemic inflammation on vascular inflammation by PET in these disease states.

RECENT FINDINGS

Recent clinical and translation advancements have demonstrated the durable relationship between chronic systemic inflammation and cardiovascular disease (CVD). In chronic inflammatory states, this relationship is robustly evident in the form of increased vascular inflammation, yet traditional risk estimates often underestimate the subclinical cardiovascular risk conferred by chronic inflammation. PET has emerged as a novel, non-invasive imaging modality capable of both quantifying the degree of systemic and vascular inflammation and detecting residual inflammation prior to cardiovascular events. We begin by demonstrating the role of inflammation in the pathogenesis of atherosclerosis, discussing how PET has been utilized to measure systemic and vascular inflammation and their effect on subclinical atherosclerosis, and finally reviewing recent applications of PET in constructing improved risk stratification for patients at high risk for stroke and CVD.

[18F] Sodium Fluoride Dose Reduction Enabled by Digital Photon Counting PET/CT for Evaluation of Osteoblastic Activity

The aim of the study was to assess the quality and reproducibility of reducing the injected [¹⁸F] sodium fluoride ([¹⁸F]NaF) dose while maintaining diagnostic imaging quality in bone imaging in a preclinical skeletal model using digital photon counting PET (dPET) detector technology. Beagles (n = 9) were administered three different [¹⁸F]NaF doses: 111 MBq (n = 5), 20 MBq (n = 5), and 1.9 MBq (n = 9). Imaging started ≃45 min post-injection for ≃30 min total acquisition time. Images were reconstructed using Time-of-Flight, ultra-high definition (voxel size of 1 × 1 × 1 mm³), with 3 iterations and 3 subsets. Point spread function was modeled and Gaussian filtering was applied. Skeleton qualitative and quantitative molecular image assessment was performed. The overall diagnostic quality of all images scored excellent (61%) and acceptable (39%) by all the reviewers. [¹⁸F]NaF SUV_mean showed no statistically significant differences among the three doses in any of the region of interest assessed. This study demonstrated that a 60-fold [¹⁸F]NaF dose reduction was not significantly different from the highest dose, and it had not significant effect on overall image quality and quantitative accuracy. In the future, ultra-low dose [¹⁸F]NaF dPET/CT imaging may significantly decrease PET radiation exposure to preclinical subjects and personnel.

Clinical doses production of pharmaceutical grade Sodium[18F]Fluoride using modified integrated fluidic processor in a SYNTHERA® module

A fully automated large-scale production of sodium [¹⁸F]fluoride ([¹⁸F]NaF) using SYNTHERA module with a modification in integrated fluidic processor (IFP) is reported. This modified IFP module is used to prepare [¹⁸F]NaF with more than 98% non-decay corrected radiochemical yield (RCY) within 5 min with specifications in accordance with United State Pharmacopeia (USP) monograph. The graphical user interface (GUI) is designed to perform the synthesis steps either manually or automatically and give information to the operator during the course of production. The desired clinical results add support to indigenously produced [¹⁸F]NaF as a pharmaceutical grade diagnostic radiopharmaceutical.

Comparison of [18F] NaF PET/CT dynamic analysis methods and a static analysis method including derivation of a semi-population input function for site-specific measurements of bone formation in a population with chronic kidney disease-mineral and bone disorder

PURPOSE

The purpose of this study is to compare dynamic and static whole-body (WB) [¹⁸F]NaF PET/CT scan methods used for analysis of bone plasma clearance in patients with chronic kidney disease-mineral and bone disorder (CKD-MBD).

METHODS

Seventeen patients with CKD-MBD underwent a 60-min dynamic scan followed by a 30-min static WB scan. Tracer kinetics in four thoracic vertebrae were analysed using nonlinear regression and Patlak analysis using image-derived arterial input functions. The static WB scan was analysed using a simplified Patlak method requiring only a single data point in combination with a fixed y-intercept value (V₀), both obtained using a semi-population function. The semi-population function was constructed by combining a previously derived population input function in combination with data from venous blood samples. Static WB scan analysis data, obtained from the semi-population input functions, was compared with paired data obtained using dynamic input functions.

RESULTS

Bone plasma clearance (K_i) from Patlak analyses correlated well with nonlinear regression analysis, but K_i results using Patlak analysis were lower than K_i results using nonlinear regression analysis. However, no significant difference was found between K_i obtained by static WB scans and K_i obtained by dynamic scans using nonlinear regression analysis (p = 0.29).

CONCLUSION

Bone plasma clearance measured from static WB scans correlates with clearance data measured by dynamic analysis. Static [18F]NaF PET/CT scans can be applied in future studies to measure K_i in patients with CKD-MBD, but the results should not be compared uncritically with results obtained by dynamic scan analysis.

Cardiac hybrid imaging: novel tracers for novel targets

Non-invasive cardiac imaging has explored enormous advances in the last few decades. In particular, hybrid imaging represents the fusion of information from multiple imaging modalities, allowing to provide a more comprehensive dataset compared to traditional imaging techniques in patients with cardiovascular diseases. The complementary anatomical, functional and molecular information provided by hybrid systems are able to simplify the evaluation procedure of various pathologies in a routine clinical setting. The diagnostic capability of hybrid imaging modalities can be further enhanced by introducing novel and specific imaging biomarkers. The aim of this review is to cover the most recent advancements in radiotracers development for SPECT/CT, PET/CT, and PET/MRI for cardiovascular diseases.

Implementation of iMiDEV™, a new fully automated microfluidic platform for radiopharmaceutical production

iMiDEV™ microfluidic system is a new automated tool for a small-scale production of radiopharmaceuticals. This new radiochemistry module utilizes microfluidic cassettes capable of producing diversified radiopharmaceuticals in liquid phase reactions in an automated synthesizer. The user interface is intuitive and designed to give the operator all the information required and to allow driving the synthesis either manually or fully automatically. In this work, we have demonstrated liquid phase reaction and presented the first results of an efficient fully automated [18F]NaF radiosynthesis on the iMiDEV™ platform. Different parameters such as a type of cyclotron targets, initial activity, concentration and volume of the fluoride-18 targetry have been investigated in order to elaborate the optimised radiolabelling of the ligand. Single and double sodium [18F]fluoride synthesis procedures have been successfully developed using two chambers of the cassette. A single-dose of radiotracer was produced in an average radiochemical yield of 87% (decay corrected) within 8 min and quality control tests were performed as per European Pharmacopoeia.

Prognostic and Theranostic Applications of Positron Emission Tomography for a Personalized Approach to Metastatic Castration-Resistant Prostate Cancer

Metastatic castration-resistant prostate cancer (mCRPC) represents a condition of progressive disease in spite of androgen deprivation therapy (ADT), with a broad spectrum of manifestations ranging from no symptoms to severe debilitation due to bone or visceral metastatization. The management of mCRPC has been profoundly modified by introducing novel therapeutic tools such as antiandrogen drugs (i.e., abiraterone acetate and enzalutamide), immunotherapy through sipuleucel-T, and targeted alpha therapy (TAT). This variety of approaches calls for unmet need of biomarkers suitable for patients’ pre-treatment selection and prognostic stratification. In this scenario, imaging with positron emission computed tomography (PET/CT) presents great and still unexplored potential to detect specific molecular and metabolic signatures, some of whom, such as the prostate specific membrane antigen (PSMA), can also be exploited as therapeutic targets, thus combining diagnosis and therapy in the so-called “theranostic” approach. In this review, we performed a web-based and desktop literature research to investigate the prognostic and theranostic potential of several PET imaging probes, such as ¹⁸F-FDG, ¹⁸F-choline and ⁶⁸Ga-PSMA-11, also covering the emerging tracers still in a pre-clinical phase (e.g., PARP-inhibitors’ analogs and the radioligands binding to gastrin releasing peptide receptors/GRPR), highlighting their potential for defining personalized care pathways in mCRPC.

Novel Musculoskeletal and Orthopedic Applications of 18F-Sodium Fluoride PET

PET imaging with ¹⁸F-sodium fluoride (NaF), combined with computed tomography or magnetic resonance, is a sensitive method of assessing bone turnover. Although NaF-PET is gaining popularity in detecting prostate cancer metastases to bone marrow, osseous changes represent secondary effects of cancer cell growth. PET tracers more appropriate for assessing prostate cancer metastases directly portray malignant activity and include ¹⁸F-fluciclovine and prostatic specific membrane antigen ligands. Recent studies investigating NaF-PET suggest utility in the assessment of benign musculoskeletal disorders. Emerging applications in assessing traumatic injuries, joint disease, back pain, orthopedic complications, and metabolic bone disease are discussed.

A longitudinal 18F-fluorodeoxyglucose (18F-FDG) and 18F-sodium fluoride (18F-NaF) positron emission tomography/computed tomography (PET/CT) study in apolipoprotein E (ApoE) knockout rats fed with a Western diet

Background

Inflammation and vascular calcification are risk factors for cardiovascular disease, but their relationship is still under investigation. This longitudinal in vivo study aimed to monitor inflammation and calcification during the formation of atherosclerotic plaques in apolipoprotein E knockout (ApoE^-/-) rats by ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) and ¹⁸F-sodium fluoride (¹⁸F-NaF) positron emission tomography/computed tomography (PET/CT).

Methods

In the ApoE group, male ApoE^-/- rats were fed a high-fat Western diet from 13 weeks of age, and in the normal group, male SD rats of the same age were fed a normal diet. A longitudinal PET/CT study using ¹⁸F-FDG and ¹⁸F-NaF was performed at 12, 27, and 46 weeks of age. T1-weighted magnetic resonance imaging (MRI) was used as an atlas template, and the uptake of the tracers in the cardiovascular system was analyzed based on atlas 3D geometry volumes-of-interest (VOIs). After the PET/CT study, pathological and immunohistochemical examinations were performed on the corresponding lesions.

Results

The body weight and plasma cholesterol levels of the ApoE^-/- rats increased with time, and at each time point, significantly higher body weight and plasma cholesterol levels were observed in the ApoE^-/- rats than in the normal rats. PET/CT showed that in ApoE^-/- rats, the uptake of ¹⁸F-FDG was found in the aortic arch, while the uptake of ¹⁸F-NaF was found in pulmonary arteries. The uptake of the two tracers in the ApoE group increased with time. Extensive early stage of atherosclerotic plaques, with high expression of CD68 and alizarin red, were observed in pulmonary arteries. However, only a thickened intima with very high expression of hypoxia-inducible factor-1 alpha (HIF-1α) was seen in the aortic arch.

Conclusions

In ApoE^-/- rats fed a high-fat Western diet, early atherosclerotic lesions developed in the pulmonary arteries; however, ¹⁸F-FDG failed to accumulate in these lesions but to accumulate in the aortic arch with only neointimal hyperplasia and significantly high expression of hypoxia.

Advances in Quantitative Analysis of 18F-Sodium Fluoride Coronary Imaging

¹⁸F-sodium fluoride (¹⁸F-NaF) positron emission tomography (PET) has emerged as a promising noninvasive imaging tool for the assessment of active calcification processes in coronary artery disease. ¹⁸F-NaF uptake colocalizes to high-risk and ruptured atherosclerotic plaques. Most recently, ¹⁸F-NaF coronary uptake was shown to be a robust and independent predictor of myocardial infarction in patients with advanced coronary artery disease. In this review, we provide an overview of the advances in coronary ¹⁸F-NaF imaging. In particular, we discuss the recently developed and validated motion correction techniques which address heart contractions, tidal breathing, and patient repositioning during the prolonged PET acquisitions. Additionally, we discuss a novel quantification approach—the coronary microcalcification activity (which has been inspired by the widely employed method in oncology total active tumor volume measurement). This new method provides a single number encompassing ¹⁸F-NaF activity within the entire coronary vasculature rather than just information regarding a single area of most intense tracer uptake.

18F- based Quantification of the Osteogenic Potential of hMSCs

In bone tissue engineering, there is a constant need to design new methods for promoting in vitro osteogenic differentiation. Consequently, there is a strong demand for fast, effective and reliable methods to track and quantify osteogenesis in vitro. In this study, we used the radiopharmacon fluorine-18 (¹⁸F) to evaluate the amount of hydroxylapatite produced by mesenchymal stem cells (MSCs) in a monolayer cell culture in vitro. The hydroxylapatite bound tracer was evaluated using µ-positron emission tomography (µ-PET) scanning and activimeter analysis. It was therefore possible to determine the amount of synthesized mineral and thus to conclude the osteogenic potential of the cells. A Student's t-test revealed a highly significant difference regarding tracer uptake between the osteogenic group and the corresponding control group (µ-PET p = 0.043; activimeter analysis p = 0.012). This tracer uptake showed a highly significant correlation with the gold standard of quantitative Alizarin Red staining (ARS) (r² = 0.86) as well as with the absolute calcium content detected by inductively coupled plasma mass spectrometry (r² = 0.81). The results showed that ¹⁸F labeling is a novel method to prove and quantify hydroxyapatite content in MSC monolayer cultures. The mineral layer remains intact for further analysis. This non-destructive in vitro method can be used to rapidly investigate bone tissue engineering strategies in terms of hydroxylapatite production, and could therefore accelerate the process of implementing new strategies in clinical practice.

An Update on [18F]Fluoride PET Imaging for Atherosclerotic Disease

Atherosclerosis is the leading cause of life-threatening morbidity and mortality, as the rupture of atherosclerotic plaques leads to critical atherothrombotic events such as myocardial infarction and ischemic stroke, which are the 2 most common causes of death worldwide. Vascular calcification is a complicated pathological process involved in atherosclerosis, and microcalcifications are presumed to increase the likelihood of plaque rupture. Despite many efforts to develop novel non-invasive diagnostic modalities, diagnostic techniques are still limited, especially before symptomatic presentation. From this point of view, vulnerable plaques are a direct target of atherosclerosis imaging. Anatomic imaging modalities have the limitation of only visualizing macroscopic structural changes, which occurs in later stages of disease, while molecular imaging modalities are able to detect microscopic processes and microcalcifications, which occur early in the disease process. Na[¹⁸F]-fluoride positron emission tomography/computed tomography could allow the early detection of plaque instability, which is deemed to be a primary goal in the prevention of cardiac or brain ischemic events, by quantifying the microcalcifications within vulnerable plaques and evaluating the atherosclerotic disease burden.

Can Na18F PET/CT bone scans help when deciding if early intervention is needed in patients being treated with a TSF attached to the tibia: insights from 41 patients

Purpose

To demonstrate the usefulness of positron emission tomography (PET)/computed tomography (CT) bone scans for gaining insight into healing bone status earlier than CT or X-ray alone.

Methods

Forty-one prospective patients being treated with a Taylor Spatial Frame were recruited. We registered data obtained from successive static CT scans for each patient, to align the broken bone. Radionuclide uptake was calculated over a spherical volume of interest (VOI). For all voxels in the VOI, histograms and cumulative distribution functions of the CT and PET data were used to assess the type and progress of new bone growth and radionuclide uptake. The radionuclide uptake difference per day between the PET/CT scans was displayed in a scatter plot. Superimposing CT and PET slice data and observing the spatiotemporal uptake of ¹⁸F⁻ in the region of healing bone by a time-sequenced movie allowed qualitative evaluation.

Results

Numerical evaluation, particularly the shape and distribution of Hounsfield Units and radionuclide uptake in the graphs, combined with visual evaluation and the movies enabled the identification of six patients needing intervention as well as those not requiring intervention. Every revised patient proceeded to a successful treatment conclusion.

Conclusion

Numerical and visual evaluation based on all the voxels in the VOI may aid the orthopedic surgeon to assess a patient’s progression to recovery. By identifying slow or insufficient progress at an early stage and observing the uptake of ¹⁸F⁻ in specific regions of bone, it might be possible to shorten the recovery time and avoid unnecessary late complications.

Electronic supplementary material

The online version of this article (10.1007/s00590-020-02776-2) contains supplementary material, which is available to authorized users.

Quantitative thoracic aorta calcification assessment by 18F-NaF PET/CT and its correlation with atherosclerotic cardiovascular disorders and increasing age

OBJECTIVES

We aimed to assess the correlation between age and cardiovascular risk factors with NaF-PET/CT imaging in the thoracic aorta (TA).

METHODS

In this prospective study, 80 healthy controls and 44 patients with chest pain underwent NaF-PET/CT imaging, and three segments of the aorta (ascending, arch, and descending) were examined. Average SUVmax, SUVmean, and Alavi-Carlsen Score (ACS) were calculated in each segment and the entire vessel. The degree of NaF uptake in controls and patients and its correlation with age were determined. Multivariate linear regression and logistic regression models were employed to determine the predictabilities of Framingham Risk Score (FRS) and unfavorable cardiovascular disease (CVD) risk profile by these measurements.

RESULTS

Average SUVmax, average SUVmean, and ACS were significantly higher in patients than in controls, and all correlated well with age. The correlation of average SUVmean with age was significant in both controls (r = 0.32, p = 0.04) and patients (r = 0.64, p < 0.001). ACS of the entire TA was a stronger predictor of FRS compared with average SUVmax and average SUVmean (adjusted R² = 0.38, standardized β = 0.58, p < 0.001). ACS was a significant predictor of unfavorable CVD risk profile as compared with other values (odds ratio = 1.006, 95% CI = 1.000-1.013, p = 0.05).

CONCLUSIONS

Active calcification in TA correlates with age, and its correlation is higher among subjects with CVD risk factors. Global assessment (ACS) can predict unfavorable CVD risk profile. These data provide evidence for the potential role of NaF in assessing micro-calcification in arteries and its relations to cardiovascular events.

KEY POINTS

• Global micro-calcification in the thoracic aorta as measured by NaF-PET/CT imaging correlates with increasing age. • The extent of the correlation was higher among patients with cardiovascular disease (CVD) risk factors. • These data provide evidence for the potential role of NaF in assessing active calcification in arteries and its relations to cardiovascular events.

18F-Sodium Fluoride (18F-NaF) for Imaging Microcalcification Activity in the Cardiovascular System

Accumulating preclinical and clinical evidence suggests that calcification is one of the body's primary responses to injury and a key pathological feature of cardiovascular disease. Calcification activity can now be imaged using 18F-sodium fluoride (18F-NaF) positron emission tomography (PET) in combination with either computed tomography or magnetic resonance. These techniques allow visualization of calcification activity and, therefore, provide different information to the established macroscopic calcium imaged with computed tomography. Indeed, 18F-NaF PET has been used to investigate a wide range of valvular conditions, including aortic stenosis, mitral annular calcification, and bioprosthetic valve disease, as well as vascular conditions, including abdominal aortic aneurysm disease, coronary, and carotid atherosclerosis, peripheral vascular disease, and erectile dysfunction. In this brief review, we will focus on how 18F-NaF PET has improved our pathophysiological understanding of cardiovascular calcification and how it can be used as a marker of vascular calcification, providing a useful tool that can be utilized in clinical trials investigating the prediction of both disease progression and clinical events. Finally, we will discuss how 18F-NaF might be employed clinically to improve patient assessment and to guide decision-making.

Analytical quantification of aortic valve 18F-sodium fluoride PET uptake

BACKGROUND

Challenges to cardiac PET-CT include patient motion, prolonged image acquisition and a reduction of counts due to gating. We compared two analytical tools, FusionQuant and OsiriX, for quantification of gated cardiac 18F-sodium fluoride (18F-fluoride) PET-CT imaging.

METHODS

Twenty-seven patients with aortic stenosis were included, 15 of whom underwent repeated imaging 4 weeks apart. Agreement between analytical tools and scan-rescan reproducibility was determined using the Bland-Altman method and Lin's concordance correlation coefficients (CCC).

RESULTS

Image analysis was faster with FusionQuant [median time (IQR) 7:10 (6:40-8:20) minutes] compared with OsiriX [8:30 (8:00-10:10) minutes, p = .002]. Agreement of uptake measurements between programs was excellent, CCC = 0.972 (95% CI 0.949-0.995) for mean tissue-to-background ratio (TBRmean) and 0.981 (95% CI 0.965-0.997) for maximum tissue-to-background ratio (TBRmax). Mean noise decreased from 11.7% in the diastolic gate to 6.7% in motion-corrected images (p = .002); SNR increased from 25.41 to 41.13 (p = .0001). Aortic valve scan-rescan reproducibility for TBRmax was improved with FusionQuant using motion correction compared to OsiriX (error ± 36% vs ± 13%, p < .001) while reproducibility for TBRmean was similar (± 10% vs ± 8% p = .252).

CONCLUSION

18F-fluoride PET quantification with FusionQuant and OsiriX is comparable. FusionQuant with motion correction offers advantages with respect to analysis time and reproducibility of TBRmax values.

Influence of Hydroxyapatite Coating for the Prevention of Bone Mineral Density Loss and Bone Metabolism after Total Hip Arthroplasty: Assessment Using 18F-Fluoride Positron Emission Tomography and Dual-Energy X-Ray Absorptiometry by Randomized Controlled Trial

Background

Hydroxyapatite- (HA-) coated implants tend to achieve good osteoinductivity and stable clinical results; however, the influence of the coating on the prevention of bone mineral density (BMD) loss around the implant is unclear. The purpose of this randomized controlled trial was to evaluate the effectiveness of HA-coated implants for preventing BMD loss and to determine the status of bone remodeling after total hip arthroplasty (THA), making comparisons with non-HA-coated implants.

Methods

A total of 52 patients who underwent primary THA were randomly allocated to HA and non-HA groups. BMD was measured by dual-energy X-ray absorptiometry (DEXA) at 1 week postoperation to form a baseline measurement, and then 24 weeks and 48 weeks after surgery. The relative change in BMD was evaluated for regions of interest (ROIs) based on the Gruen zone classifications. ¹⁸F-fluoride positron emission tomography (PET) was performed at 24 weeks postsurgery, and the maximum standardized uptake values (SUV_max) were evaluated in the proximal (HA-coated) and distal (non-HA-coated) areas in both groups.

Results

There were significant differences in BMD loss in ROIs 3 and 6 (p = 0.03), while no significant difference was observed in ROI 7 at either 24 or 48 weeks postsurgery. There was no significant correlation between PET uptake and BMD (24 or 48 weeks) in either group.

Conclusion

The influence of a HA coating in terms of BMD preservation is limited. No significant correlation was found between BMD and SUV_max measured by PET, either with or without the use of a HA coating.

Preclinical validation of [18F]2FNQ1P as a specific PET radiotracer of 5-HT6 receptors in rat, pig, non-human primate and human brain tissue

INTRODUCTION

The aim of this study was to perform in-vitro and in-vivo radiopharmacological characterizations of [¹⁸F]2FNQ1P, a new PET radiotracer of 5-HT₆ receptors, in rat, pig, non-human primate and human tissues. The 5-HT₆ receptor is one of the more recently identified serotonin receptors in central nervous system and, because of its role in memory and cognitive processes, is considered as a promising therapeutic target.

METHODS

In-vitro autoradiography and saturation binding assays were performed in postmortem brain tissues from rat, pig, non-human primate and human caudate nucleus, completed by serum stability assessment in all species and cerebral radiometabolite and biodistribution studies in rat.

RESULTS

In all species, autoradiography data revealed high binding levels of [¹⁸F]2FNQ1P in cerebral regions with high 5-HT₆ receptor density. Binding was blocked by addition of SB258585 as a specific antagonist. Binding assays provided K_D and B_max values of respectively 1.34 nM and 0.03 pmol·mg^-1 in rat, 0.60 nM and 0.04 pmol·mg^-1 in pig, 1.38 nM and 0.07 pmol·mg^-1 in non-human primate, and 1.39 nM and 0.15 pmol·mg^-1 in human caudate nucleus. In rat brain, the proportion of unmetabolized [¹⁸F]2FNQ1P was >99% 5 min after iv injection and 89% at 40 min. The biodistribution studies found maximal radioactivity in lungs and kidneys (3.5 ± 1.2% ID/g and 2.0 ± 0.7% ID/g, respectively, 15 min post-injection).

CONCLUSION

These radiopharmacological data confirm that [¹⁸F]2FNQ1P is a specific radiotracer for molecular imaging of 5-HT₆ receptors and suggest that it could be used as a radiopharmaceutical in humans.

Vulnerable plaque imaging using 18F-sodium fluoride positron emission tomography

Positron emission tomography (PET) with ¹⁸F-sodium fluoride (¹⁸F-NaF) has emerged as a promising non-invasive imaging modality to identify high-risk and ruptured atherosclerotic plaques. By visualizing microcalcification, ¹⁸F-NaF PET holds clinical promise in refining how we evaluate coronary artery disease, shifting our focus from assessing disease burden to atherosclerosis activity. In this review, we provide an overview of studies that have utilized ¹⁸F-NaF PET for imaging atherosclerosis. We discuss the associations between traditional coronary artery disease measures (risk factors) and ¹⁸F-NaF plaque activity. We also present the data on the histological validation as well as show how ¹⁸F-NaF uptake is associated with plaque morphology on intravascular and CT imaging. Finally, we discuss the technical challenges associated with ¹⁸F-NaF coronary PET highlighting recent advances in this area.

Disease Activity in Mitral Annular Calcification

BACKGROUND

Mitral annular calcification (MAC) is associated with cardiovascular events and mitral valve dysfunction. However, the underlying pathophysiology remains incompletely understood. In this prospective longitudinal study, we used a multimodality approach including positron emission tomography, computed tomography, and echocardiography to investigate the pathophysiology of MAC and assess factors associated with disease activity and progression.

METHODS

A total of 104 patients (age 72±8 years, 30% women) with calcific aortic valve disease, therefore predisposed to MAC, underwent ¹⁸F-sodium fluoride (calcification activity) and ¹⁸F-Fluorodeoxyglucose (inflammation activity) positron emission tomography, computed tomography calcium scoring, and echocardiography. Sixty patients underwent repeat computed tomography and echocardiography after 2 years.

RESULTS

MAC (mitral annular calcium score >0) was present in 35 (33.7%) patients who had increased ¹⁸F-fluoride (tissue-to-background ratio, 2.32 [95% CI, 1.81-3.27] versus 1.30 [1.22-1.49]; P<0.001) and ¹⁸F-Fluorodeoxyglucose activity (tissue-to-background ratio, 1.44 [1.37-1.58] versus 1.17 [1.12-1.24]; P<0.001) compared with patients without MAC. MAC activity (¹⁸F-fluoride uptake) was closely associated with the local calcium score and ¹⁸F-Fluorodeoxyglucose uptake, as well as female sex and renal function. Similarly, MAC progression was closely associated with local factors, in particular, baseline MAC. Traditional cardiovascular risk factors and calcification activity in bone or remote atherosclerotic areas were not associated with disease activity nor progression.

CONCLUSIONS

MAC is characterized by increased local calcification activity and inflammation. Baseline MAC burden was associated with disease activity and the rate of subsequent progression. This suggests a self-perpetuating cycle of calcification and inflammation that may be the target of future therapeutic interventions.

The Role of Vascular Smooth Muscle Cells in Arterial Remodeling: Focus on Calcification-Related Processes

Arterial remodeling refers to the structural and functional changes of the vessel wall that occur in response to disease, injury, or aging. Vascular smooth muscle cells (VSMC) play a pivotal role in regulating the remodeling processes of the vessel wall. Phenotypic switching of VSMC involves oxidative stress-induced extracellular vesicle release, driving calcification processes. The VSMC phenotype is relevant to plaque initiation, development and stability, whereas, in the media, the VSMC phenotype is important in maintaining tissue elasticity, wall stress homeostasis and vessel stiffness. Clinically, assessment of arterial remodeling is a challenge; particularly distinguishing intimal and medial involvement, and their contributions to vessel wall remodeling. The limitations pertain to imaging resolution and sensitivity, so methodological development is focused on improving those. Moreover, the integration of data across the microscopic (i.e., cell-tissue) and macroscopic (i.e., vessel-system) scale for correct interpretation is innately challenging, because of the multiple biophysical and biochemical factors involved. In the present review, we describe the arterial remodeling processes that govern arterial stiffening, atherosclerosis and calcification, with a particular focus on VSMC phenotypic switching. Additionally, we review clinically applicable methodologies to assess arterial remodeling and the latest developments in these, seeking to unravel the ubiquitous corroborator of vascular pathology that calcification appears to be.

18F-Fluoride Signal Amplification Identifies Microcalcifications Associated With Atherosclerotic Plaque Instability in Positron Emission Tomography/Computed Tomography Images

BACKGROUND

Microcalcifications in atherosclerotic plaques are destabilizing, predict adverse cardiovascular events, and are associated with increased morbidity and mortality.¹⁸F-fluoride positron emission tomography (PET)/computed tomography (CT) imaging has demonstrated promise as a useful clinical diagnostic tool in identifying high-risk plaques; however, there is confusion as to the underlying mechanism of signal amplification seen in PET-positive, CT-negative image regions. This study tested the hypothesis that ¹⁸F-fluoride PET/CT can identify early microcalcifications.

METHODS

¹⁸F-fluoride signal amplification derived from microcalcifications was validated against near-infrared fluorescence molecular imaging and histology using an in vitro 3-dimensional hydrogel collagen platform, ex vivo human specimens, and a mouse model of atherosclerosis.

RESULTS

Microcalcification size correlated inversely with collagen concentration. The ¹⁸F-fluoride ligand bound to microcalcifications formed by calcifying vascular smooth muscle cell derived extracellular vesicles in the in vitro 3-dimensional collagen system and exhibited an increasing signal with an increase in collagen concentration (0.25 mg/mL collagen -33.8×10²±12.4×10² counts per minute; 0.5 mg/mL collagen -67.7×10²±37.4×10² counts per minute; P=0.0014), suggesting amplification of the PET signal by smaller microcalcifications. We further incubated human atherosclerotic endarterectomy specimens with clinically relevant concentrations of ¹⁸F-fluoride. The ¹⁸F-fluoride ligand labeled microcalcifications in PET-positive, CT-negative regions of explanted human specimens as evidenced by ¹⁸F-fluoride PET/CT imaging, near-infrared fluorescence, and histological analysis. Additionally, the ¹⁸F-fluoride ligand identified micro and macrocalcifications in atherosclerotic aortas obtained from low-density lipoprotein receptor-deficient mice.

CONCLUSIONS

Our results suggest that ¹⁸F-fluoride PET signal in PET-positive, CT-negative regions of human atherosclerotic plaques is the result of developing microcalcifications, and high surface area in regions of small microcalcifications may amplify PET signal.

NaF PET/CT for response assessment of prostate cancer bone metastases treated with single fraction stereotactic ablative body radiotherapy

Introduction

In prostate cancer patients, imaging of bone metastases is enhanced through the use of sodium fluoride positron emission tomography (¹⁸F-NaF PET/CT). This imaging technique shows areas of enhanced osteoblastic activity and blood flow. In this work, ¹⁸F-NaF PET/CT was investigated for response assessment to single fraction stereotactic ablative body radiotherapy (SABR) to bone metastases in prostate cancer patients.

Methods

Patients with bone metastases in a prospective trial treated with single fraction SABR received a ¹⁸F-NaF PET/CT scan prior to and 6 months post-SABR. The SUV_max in the tumour was determined and the difference between before and after SABR determined. The change in uptake in the non-tumour bone was also measured as a function of the received SABR dose.

Results

Reduction in SUV_max was observed in 29 of 33 lesions 6 months after SABR (mean absolute decrease in SUV_max 17.7, 95% CI 25.8 to − 9.4, p = 0.0001). Of the three lesions with increased SUV_max post-SABR, two were from the same patient and located in the vertebral column. Both were determined to be local progression in addition to one fracture. The third lesion (in a rib) was shown to be controlled locally but suffered from a fracture at 24 months. Progression adjacent to the treated volume was observed in two patients. The non-tumour bone irradiated showed increased loss in uptake with increasing dose, with a median loss in uptake of 23.3% for bone receiving 24 Gy.

Conclusion

¹⁸F-NaF PET/CT for response assessment of bone metastases to single fraction SABR indicates high rates of reduction of osteoblastic activity in the tumour and non-tumour bone receiving high doses. The occurrence of marginal recurrence indicates use of larger clinical target volumes may be warranted in treatment of bone metastases.

Trial registration

POPSTAR, ‘Pilot Study of patients with Oligometastases from Prostate cancer treated with STereotactic Ablative Radiotherapy’, Universal Trial Number U1111-1140-7563, Registered 17th April 2013.

Electronic supplementary material

The online version of this article (10.1186/s13014-019-1359-0) contains supplementary material, which is available to authorized users.

Kinetic [18F]-Fluoride of the Knee in Normal Volunteers

PURPOSE

[F]-sodium fluoride ([F]NaF) is a well-established bone-seeking agent that has shown promise to assess bone turnover in a variety of disorders, but its distribution in healthy knee joints has not been explored. This study aimed to investigate parametric values for [F]NaF uptake in various bone tissues types of the knee and their spatial distributions.

METHODS

Twelve healthy subjects were hand-injected with 92.5 MBq of [F]NaF and scanned on a 3-T PET/MRI system. Listmode PET data for both knees were acquired for 50 minutes from injection simultaneously with MRI Dixon and angiography data. The image-derived input function was determined from the popliteal artery. Using the Hawkins model, Patlak analysis was performed to obtain Ki (Ki) values and nonlinear regression analysis to obtain Ki, K1, k3/(k2 + k3), and blood volume. Comparisons for the measured kinetic parameters, SUV, and SUVmax were made between tissue types (subchondral, cortical, and trabecular bone) and between regional subsections of subchondral bone.

RESULTS

Cortical bone had the highest [F]NaF uptake differing significantly in all measured parameters when compared with trabecular bone and significantly higher SUVmax and K1 than subchondral bone. Subchondral bone also had significantly higher SUV, SUVmax, and Ki than trabecular bone tissue. Regional differences were observed in K1 and k3/(k2 + k3) values.

CONCLUSIONS

Quantitative [F]NaF PET is sensitive to variations in bone vascularization and metabolism in the knee joint.

Radionuclide Imaging of Atherothrombotic Diseases

Purpose of Review

A variety of approaches and molecular targets have emerged in recent years for radionuclide-based imaging of atherosclerosis and vulnerable plaque using single photon emission computed tomography (SPECT) and positron emission tomography (PET), with numerous methods focused on characterizing the mechanisms underlying plaque progression and rupture. This review highlights the ongoing developments in both the preclinical and clinical environment for radionuclide imaging of atherosclerosis and atherothrombosis.

Recent Findings

Numerous physiological processes responsible for the evolution of high-risk atherosclerotic plaque, such as inflammation, thrombosis, angiogenesis, and microcalcification, have been shown to be feasible targets for SPECT and PET imaging. For each physiological process, specific molecular markers have been identified that allow for sensitive non-invasive detection and characterization of atherosclerotic plaque.

Summary

The capabilities of SPECT and PET imaging continue to evolve for physiological evaluation of atherosclerosis. This review summarizes the latest developments related to radionuclide imaging of atherothrombotic diseases.

Applications of PET-Computed Tomography-Magnetic Resonance in the Management of Benign Musculoskeletal Disorders

Although computed tomography (CT) and MR imaging alone have been used extensively to evaluate various musculoskeletal disorders, hybrid imaging modalities of PET-CT and PET-MR imaging were recently developed, combining the advantages of each method: molecular information from PET and anatomical information from CT or MR imaging. Furthermore, different radiotracers can be used in PET to uncover different disease mechanisms. In this article, potential applications of PET-CT and PET-MR imaging for benign musculoskeletal disorders are organized by benign cell proliferation/dysplasia, diabetic foot complications, joint prostheses, degeneration, inflammation, and trauma, metabolic bone disorders, and pain (acute and chronic) and peripheral nerve imaging.

PET-MRI for the Study of Metabolic Bone Disease

PURPOSE OF REVIEW

This review article attempts to summarize the current state and applications of the hybrid imaging modality of PET-MRI to metabolic bone diseases. The advances of PET and MRI are also discussed for metabolic bone diseases as potentially applied via PET-MRI.

RECENT FINDINGS

Etiologies and mechanisms of metabolic bone disease can be complex where molecular changes precede structural changes. Although PET-MRI has yet to be applied directly to metabolic bone disease, possible applications exist since PET, specifically ¹⁸F-NaF PET, can quantitatively track changes in bone metabolism and is useful for assessing treatment, while MRI can give detailed information on bone water concentration, porosity, and architecture through novel techniques such as UTE and ZTE MRI. Earlier detection and further understanding of metabolic bone disease via PET and MRI could lead to better treatment and prevention. More research using this modality is needed to further understand how it can be implemented in this realm.

Target identification for the diagnosis and intervention of vulnerable atherosclerotic plaques beyond 18F-fluorodeoxyglucose positron emission tomography imaging: promising tracers on the horizon

Cardiovascular disease is the major cause of morbidity and mortality in developed countries and atherosclerosis is the major cause of cardiovascular disease. Atherosclerotic lesions obstruct blood flow in the arterial vessel wall and can rupture leading to the formation of occlusive thrombi. Conventional diagnostic tools are still of limited value for identifying the vulnerable arterial plaque and for predicting its risk of rupture and of releasing thromboembolic material. Knowledge of the molecular and biological processes implicated in the process of atherosclerosis will advance the development of imaging probes to differentiate the vulnerable plaque. The development of imaging probes with high sensitivity and specificity in identifying high-risk atherosclerotic vessel wall changes and plaques is crucial for improving knowledge-based decisions and tailored individual interventions. Arterial PET imaging with ¹⁸F-FDG has shown promising results in identifying inflammatory vessel wall changes in numerous studies and clinical trials. However, due to its limited specificity in general and its intense physiological uptake in the left ventricular myocardium that impair imaging of the coronary arteries, different PET tracers for the molecular imaging of atherosclerosis have been evaluated. This review describes biological, chemical and medical expertise supporting a translational approach that will enable the development of new or the evaluation of existing PET tracers for the identification of vulnerable atherosclerotic plaques for better risk prediction and benefit to patients.

Three-Hour Delayed Imaging Improves Assessment of Coronary 18F-Sodium Fluoride PET

Coronary ¹⁸F-sodium fluoride (¹⁸F-NaF) PET identifies ruptured plaques in patients with recent myocardial infarction and localizes to atherosclerotic lesions with active calcification. Most studies to date have performed the PET acquisition 1 h after injection. Although qualitative and semiquantitative analysis is feasible with 1-h images, residual blood-pool activity often makes it difficult to discriminate plaques with ¹⁸F-NaF uptake from noise. We aimed to assess whether delayed PET performed 3 h after injection improves image quality and uptake measurements. Methods: Twenty patients (67 ± 7 y old, 55% male) with stable coronary artery disease underwent coronary CT angiography (CTA) and PET/CT both 1 h and 3 h after the injection of 266.2 ± 13.3 MBq of ¹⁸F-NaF. We compared the visual pattern of coronary uptake, maximal background (blood pool) activity, noise, SUV_max, corrected SUV_max (cSUV_max), and target-to-background (TBR) ratio in lesions defined by CTA on 1-h versus 3-h ¹⁸F-NaF PET. Results: On 1-h PET, 26 CTA lesions with ¹⁸F-NaF PET uptake were identified in 12 (60%) patients. On 3-h PET, we detected ¹⁸F-NaF PET uptake in 7 lesions that were not identified on 1-h PET. The median cSUV_max and TBRs of these lesions were 0.48 (interquartile range [IQR], 0.44-0.51) and 1.45 (IQR, 1.39-1.52), respectively, compared with -0.01 (IQR, -0.03-0.001) and 0.95 (IQR, 0.90-0.98), respectively, on 1-h PET (both P < 0.001). Across the entire cohort, 3-h PET SUV_max was similar to 1-h PET measurements (1.63 [IQR, 1.37-1.98] vs. 1.55 [IQR, 1.43-1.89], P = 0.30), and the background activity was lower (0.71 [IQR, 0.65-0.81] vs. 1.24 [IQR, 1.05-1.31], P < 0.001). On 3-h PET, TBR, cSUV_max, and noise were significantly higher (respectively: 2.30 [IQR, 1.70-2.68] vs. 1.28 [IQR, 0.98-1.56], P < 0.001; 0.38 [IQR, 0.27-0.70] vs. 0.90 [IQR, 0.64-1.17], P < 0.001; and 0.10 [IQR, 0.09-0.12] vs. 0.07 [IQR, 0.06-0.09], P = 0.02). Median cSUV_max and TBR increased by 92% (range, 33%-225%) and 80% (range, 20%-177%), respectively. Conclusion: Blood-pool activity decreases on delayed imaging, facilitating the assessment of ¹⁸F-NaF uptake in coronary plaques. Median TBR increases by 80%, leading to the detection of more plaques with significant uptake than are detected using the standard 1-h protocol. A greater than 1-h delay may improve the detection of ¹⁸F-NaF uptake in coronary artery plaques.