Infection and Inflammation Imaging: Beyond FDG

Imaging of atherosclerosis with [64Cu]Cu-DOTA-TATE in a translational head-to-head comparison study with [18F]FDG, and Na[18F]F in rabbits

Atherosclerosis is a chronic inflammatory disease of the larger arteries that may lead to cardiovascular events. Identification of patients at highest risk of cardiovascular events is challenging, but molecular imaging using positron emission tomography (PET) may prove useful. The aim of this study was to evaluate and compare head-to-head three different PET tracers. Furthermore, tracer uptake is compared to gene expression alterations of the arterial vessel wall. Male New Zealand White rabbits (control group; n = 10, atherosclerotic group; n = 11) were used for the study. Vessel wall uptake was assessed with the three different PET tracers: [¹⁸F]FDG (inflammation), Na[¹⁸F]F (microcalcification), and [⁶⁴Cu]Cu-DOTA-TATE (macrophages), using PET/computed tomography (CT). Tracer uptake was measured as standardized uptake value (SUV), and arteries from both groups were analyzed ex vivo by autoradiography, qPCR, histology, and immunohistochemistry. In rabbits, the atherosclerotic group showed significantly higher uptake of all three tracers compared to the control group [¹⁸F]FDG: SUV_mean 1.50 ± 0.11 versus 1.23 ± 0.09, p = 0.025; Na[¹⁸F]F: SUV_mean 1.54 ± 0.06 versus 1.18 ± 0.10, p = 0.006; and [⁶⁴Cu]Cu-DOTA-TATE: SUV_mean 2.30 ± 0.27 versus 1.65 ± 0.16; p = 0.047. Of the 102 genes analyzed, 52 were differentially expressed in the atherosclerotic group compared to the control group and several genes correlated with tracer uptake. In conclusion, we demonstrated the diagnostic value of [⁶⁴Cu]Cu-DOTA-TATE and Na[¹⁸F]F for identifying atherosclerosis in rabbits. The two PET tracers provided information distinct from that obtained with [¹⁸F]FDG. None of the three tracers correlated significantly to each other, but [⁶⁴Cu]Cu-DOTA-TATE and Na[¹⁸F]F uptake both correlated with markers of inflammation. [⁶⁴Cu]Cu-DOTA-TATE was higher in atherosclerotic rabbits compared to [¹⁸F]FDG and Na[¹⁸F]F.

Vascular Graft Infection Imaging

Vascular graft infection is a rare, life threatening complication of vascular repair with synthetic or native material. The pathogenesis, causative microorganisms and clinical manifestations vary according to graft's location and time duration since surgery. The diagnosis of graft infection is challenging since there is no single "gold standard" test and diagnosis is based on clinical and radiological criteria. Early and accurate diagnosis are essential for patient management and prevention of further complications. The first-choice imaging modality is computed tomography angiography (CTA) that can demonstrate typical signs of graft infection but has limited sensitivity and specificity, especially in early and low-grade infections. Nuclear medicine imaging methods, including labeled white blood cell scintigraphy and FDG PET/CT demonstrate improved diagnostic accuracy and play a pivotal role in the diagnosis of vascular graft infection. The different radiologic and the nuclear medicine imaging techniques, their advantages and limitations, and the recent guidelines detailing their use are reviewed.

Cold Kit Labeling: The Future of 68Ga Radiopharmaceuticals?

Over the last couple of decades, gallium-68 (68Ga) has gained a formidable interest for PET molecular imaging of various conditions, from cancer to infection, through cardiac pathologies or neuropathies. It has gained routine use, with successful radiopharmaceuticals such as somatostatin analogs ([68Ga]Ga-DOTATOC and [68Ga]GaDOTATATE) for neuroendocrine tumors, and PSMA ligands for prostate cancer. It represents a major clinical impact, particularly in the context of theranostics, coupled with their 177Lu-labeled counterparts. Beside those, a bunch of new 68Ga-labeled molecules are in the preclinical and clinical pipelines, with some of them showing great promise for patient care. Increasing clinical demand and regulatory issues have led to the development of automated procedures for the production of 68Ga radiopharmaceuticals. However, the widespread use of these radiopharmaceuticals may rely on simple and efficient radiolabeling methods, undemanding in terms of equipment and infrastructure. To make them technically and economically accessible to the medical community and its patients, it appears mandatory to develop a procedure similar to the well-established kit-based 99mTc chemistry. Already available commercial kits for the production of 68Ga radiopharmaceuticals have demonstrated the feasibility of using such an approach, thus paving the way for more kit-based 68Ga radiopharmaceuticals to be developed. This article discusses the development of 68Ga cold kit radiopharmacy, including technical issues, and regulatory aspects.

In Vivo Targeting of CXCR4-New Horizons

Given its pre-eminent role in the context of tumor cell growth as well as metastasis, the C-X-C motif chemokine receptor 4 (CXCR4) has attracted a lot of interest in the field of nuclear oncology, and clinical evidence on the high potential of CXCR4-targeted theranostics is constantly accumulating. Additionally, since CXCR4 also represents a key player in the orchestration of inflammatory responses to inflammatory stimuli, based on its expression on a variety of pro- and anti-inflammatory immune cells (e.g., macrophages and T-cells), CXCR4-targeted inflammation imaging has recently gained considerable attention. Therefore, after briefly summarizing the current clinical status quo of CXCR4-targeted theranostics in cancer, this review primarily focuses on imaging of a broad spectrum of inflammatory diseases via the quantification of tissue infiltration with CXCR4-expressing immune cells. An up-to-date overview of the ongoing preclinical and clinical efforts to visualize inflammation and its resolution over time is provided, and the predictive value of the CXCR4-associated imaging signal for disease outcome is discussed. Since the sensitivity and specificity of CXCR4-targeted immune cell imaging greatly relies on the availability of suitable, tailored imaging probes, recent developments in the field of CXCR4-targeted imaging agents for various applications are also addressed.

Targeting fibroblast activation protein in newly diagnosed squamous cell carcinoma of the oral cavity - initial experience and comparison to [18F]FDG PET/CT and MRI

Purpose

While [¹⁸F]-fluorodeoxyglucose ([¹⁸F]FDG) is the standard for positron emission tomography/computed tomography (PET/CT) imaging of oral squamous cell carcinoma (OSCC), diagnostic specificity is hampered by uptake in inflammatory cells such as neutrophils or macrophages. Recently, molecular imaging probes targeting fibroblast activation protein α (FAP), which is overexpressed in a variety of cancer-associated fibroblasts, have become available and might constitute a feasible alternative to FDG PET/CT.

Methods

Ten consecutive, treatment-naïve patients (8 males, 2 females; mean age, 62 ± 9 years) with biopsy-proven OSCC underwent both whole-body [¹⁸F]FDG and [⁶⁸Ga]FAPI-04 (FAP-directed) PET/CT for primary staging prior to tumor resection and cervical lymph node dissection. Detection of the primary tumor, as well as the presence and number of lymph node and distant metastases was analysed. Intensity of tracer accumulation was assessed by means of maximum (SUV_max) and peak (SUV_peak) standardized uptake values. Histological work-up including immunohistochemical staining for FAP served as standard of reference.

Results

[¹⁸F]FDG and FAP-directed PET/CT detected all primary tumors with a SUV_max of 25.5 ± 13.2 (FDG) and 20.5 ± 6.4 (FAP-directed) and a SUV_peak of 16.1 ± 10.3 ([¹⁸F]FDG) and 13.8 ± 3.9 (FAP-directed), respectively. Regarding cervical lymph node metastases, FAP-directed PET/CT demonstrated comparable sensitivity (81.3% vs. 87.5%; P = 0.32) and specificity (93.3% vs. 81.3%; P = 0.16) to [¹⁸F]FDG PET/CT. FAP expression on the cell surface of cancer-associated fibroblasts in both primary lesions as well as lymph nodes metastases was confirmed in all samples.

Conclusion

FAP-directed PET/CT in OSCC seems feasible. Future research to investigate its potential to improve patient staging is highly warranted.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05422-z.

Imaging Inflammation with Positron Emission Tomography

The impact of inflammation on the outcome of many medical conditions such as cardiovascular diseases, neurological disorders, infections, cancer, and autoimmune diseases has been widely acknowledged. However, in contrast to neurological, oncologic, and cardiovascular disorders, imaging plays a minor role in research and management of inflammation. Imaging can provide insights into individual and temporospatial biology and grade of inflammation which can be of diagnostic, therapeutic, and prognostic value. There is therefore an urgent need to evaluate and understand current approaches and potential applications for imaging of inflammation. This review discusses radiotracers for positron emission tomography (PET) that have been used to image inflammation in cardiovascular diseases and other inflammatory conditions with a special emphasis on radiotracers that have already been successfully applied in clinical settings.

18F-FDG brain PET hypometabolism in post-SARS-CoV-2 infection: substrate for persistent/delayed disorders?

PURPOSE

Several brain complications of SARS-CoV-2 infection have been reported. It has been moreover speculated that this neurotropism could potentially cause a delayed outbreak of neuropsychiatric and neurodegenerative diseases of neuroinflammatory origin. A propagation mechanism has been proposed across the cribriform plate of the ethmoid bone, from the nose to the olfactory epithelium, and possibly afterward to other limbic structures, and deeper parts of the brain including the brainstem.

METHODS

Review of clinical examination, and whole-brain voxel-based analysis of ¹⁸F-FDG PET metabolism in comparison with healthy subjects (p voxel < 0.001, p-cluster < 0.05, uncorrected), of two patients with confirmed diagnosis of SARS-CoV-2 explored at the post-viral stage of the disease.

RESULTS

Hypometabolism of the olfactory/rectus gyrus was found on the two patients, especially one with 4-week prolonged anosmia. Additional hypometabolisms were found within amygdala, hippocampus, parahippocampus, cingulate cortex, pre-/post-central gyrus, thalamus/hypothalamus, cerebellum, pons, and medulla in the other patient who complained of delayed onset of a painful syndrome.

CONCLUSION

These preliminary findings reinforce the hypotheses of SARS-CoV-2 neurotropism through the olfactory bulb and the possible extension of this impairment to other brain structures. ¹⁸F-FDG PET hypometabolism could constitute a cerebral quantitative biomarker of this involvement. Post-viral cohort studies are required to specify the exact relationship between such hypometabolisms and the possible persistent disorders, especially involving cognitive or emotion disturbances, residual respiratory symptoms, or painful complaints.