At Last, 18F-FDG for Inflammation and Infection!

Quantifying 18F-Fluorodeoxyglucose Uptake in Perianal Fistulas on PET/CT: A Retrospective Analysis

RATIONALE AND OBJECTIVES

The use of 18F-fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET/CT) in assessing inflammatory diseases has shown significant promise. Uptake patterns in perianal fistulas, which may be an incidental finding on PET/CT, have not been purposefully studied. Our aim was to compare FDG uptake of perianal fistulas to that of the liver and anal canal in patients who underwent PET/CT for hematologic/oncologic diagnosis or staging.

MATERIALS AND METHODS

We retrospectively identified patients who underwent FDG-PET/CT imaging between January 2011 and May 2023, where the report described a perianal fistula or abscess. PET/CTs of patients included in the study were retrospectively analyzed to record the maximum standardized uptake value (SUVmax) of the fistula, abscess, anal canal, rectum, and liver. Fistula-to-liver and Fistula-to-anus SUVmax ratios were calculated. We statistically compared FDG activity among the fistula, liver, and anal canal. We also assessed FDG activity in patients with vs. without anorectal cancer, as well as across different St. James fistula grades.

RESULTS

The study included 24 patients with identifiable fistulas. Fistula SUVmax (mean=10.8 ± 5.28) was significantly higher than both the liver (mean=3.09 ± 0.584, p < 0.0001) and the anal canal (mean=5.98 ± 2.63, p = 0.0005). Abscess fistula SUVmax was 15.8 ± 4.91. St. James grade 1 fistulas had significantly lower SUVmax compared to grades 2 and 4 (p = 0.0224 and p = 0.0295, respectively). No significant differences existed in SUVmax ratios between anorectal and non-anorectal cancer groups.

CONCLUSION

Perianal fistulas have increased FDG avidity with fistula SUVmax values that are significantly higher than the anal canal.

Evaluating the Change in 18F-Fluorodeoxyglucose Uptake in Perianal Fistulas on PET/CT over Time: A Serial Retrospective Analysis

RATIONALE AND OBJECTIVES

Perianal fistulas on18F-fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET/CT) can be an incidental site of FDG uptake in patients undergoing PET for other indications. There are no longitudinal studies describing FDG uptake patterns in perianal fistulas. Therefore, we aimed to analyze changes in FDG uptake over time in patients with incidental perianal fistulas.

PATIENTS AND METHODS

Patients who underwent at least two FDG-PET/CTs between January 2011 and May 2023, with incidental perianal fistula, were retrospectively identified. We analyzed all sequential PET/CTs to determine the presence of a perianal fistula and recorded the fistula's maximum standardized uptake value (SUVmax). Statistical analysis compared fistula FDG-avidity in the initial versus final PET/CT examinations and assessed the correlation between initial fistula SUVmax and percent change over time.

RESULTS

The study included 15 fistulas in 14 patients, with an average of 5 PET/CT examinations per patient. The average interval between the first and last PET/CT was 24 months (range: 6-64). The average initial fistula SUVmax (11.28 ± 3.81) was significantly higher than the final fistula SUVmax (7.22 ± 3.99) (p = 0.0067). The fistula SUVmax declined by an average of 32.01 ± 35.33% with no significant correlation between initial fistula SUVmax and percent change over time (r = -0.213, p = 0.443, 95% CI -0.66-0.35).

CONCLUSION

FDG uptake in perianal fistulas shows temporal fluctuations but follows a decreasing SUVmax trend, possibly indicating a relationship with inflammatory activity. Further studies with larger cohorts paired with perianal fistula pelvic MR imaging are needed to validate these observations and their utility in guiding further management.

EXPLORing Arthritis with Total-body Positron Emission Tomography

Arthritis has significant adverse consequences on musculoskeletal tissues and often other organs of the body. Current methods for clinical evaluation of arthritis are suboptimal, and biomarkers that are objective and measurable indicators for monitoring of arthritis disease activity are in critical demand. Recently, total-body positron emission tomography (PET) has been developed that can collect imaging signals synchronously from the entire body at ultra-low doses and reduced scan times. These scanners have increased signal collection efficiency that overcomes several limitations of standard PET scanners in the evaluation of arthritis, and they may potentially provide biomarkers to assess local and systemic impact of the arthritis disease process. This article reviews current results from using total-body PET in the assessment of common arthritic conditions, and it outlines future opportunities and challenges.

Melanin-targeted [18F]-PFPN PET imaging may shed light for clear cell sarcoma

PURPOSE

Intracytoplasmic melanin pigment is a characteristic of clear cell sarcoma (CCS), which is a particularly deadly type of soft-tissue sarcoma. [18F]-N-(2-(diethylamino)ethyl)-5-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)picolinamide ([18F]-PFPN) is a positron emission tomography (PET) probe characterized by high melanin affinity. Therefore, this study aimed to investigate the feasibility of melanin-targeted [18F]-PFPN PET in patients with CCS.

METHODS

This prospective single-centre study recruited patients with pathologically confirmed CCS. [18F]-FDG PET/computed tomography and [18F]-PFPN PET/magnetic resonance imaging scans were performed within 1 week of each other. The lesion numbers and [18F]-FDG and [18F]-PFPN PET parameters (maximum standardized uptake value [SUVmax], mean standardized uptake value [SUVmean], metabolic/melanotic tumour volume [MTV/MLTV], and total lesion glycolysis/melanin [TLG/TLM]) were collected.

RESULTS

Three patients with CCS were recruited and received PET imaging. A total of 56 lesions were detected on [18F]-PFPN PET, including primary tumour and distant metastases. Identical lesions were not detected on [18F]-PFPN and [18F]-FDG PET. Twelve lesions (12/39, 30.77%) on [18F]-FDG imaging were missed on [18F]-PFPN, and 20 lesions (20/47, 42.55%) on [18F]-PFPN imaging were missed on [18F]-FDG. In quantitative analysis, the [18F]-FDG SUVmean (4.60 ± 3.24) was higher than the [18F]-PFPN SUVmean (3.0 ± 2.63) in all lesions (P = 0.01). No significant correlations were found between the SUVmax, SUVmean, MLTV/MTV, and TLM/TLG values of [18F]-PFPN and [18F]-FDG (P > 0.05).

CONCLUSION

Melanin-targeted [18F]-PFPN PET imaging is feasible for the diagnosis of CCS. Different imaging features were displayed on [18F]-PFPN and [18F]-FDG PET imaging, demonstrating the complementary role of the tracers. Combined use of the two imaging modalities would be preferred in patients with CCS.

CLINICAL TRIAL REGISTRATION

NCT05963035.

Imaging-Selected Host Responses in the Context of Infections

Recently developed molecular imaging approaches can be used to visualize specific host responses and pathology in a quest to image infections where few microbe-specific tracers have been developed and in recognition that host responses contribute to morbidity and mortality in their own right. Here we highlight several recent examples of these imaging approaches adapted for imaging infections. The early successes and new avenues described here encompass diverse imaging modalities and leverage diverse aspects of the host response to infection-including inflammation, tissue injury and healing, and key nutrients during host-pathogen interactions. Clearly, these approaches merit further preclinical and clinical study as they are complementary and orthogonal to the pathogen-focused imaging modalities currently under investigation.

Radiotracer Development for Fungal-Specific Imaging: Past, Present, and Future

Invasive fungal infections have become a major challenge for public health, mainly due to the growing numbers of immunocompromised patients, with high morbidity and mortality. Currently, conventional imaging modalities such as computed tomography and magnetic resonance imaging contribute largely to the noninvasive diagnosis and treatment evaluation of those infections. These techniques, however, often fall short when a fast, noninvasive and specific diagnosis of fungal infection is necessary. Molecular imaging, especially using nuclear medicine-based techniques, aims to develop fungal-specific radiotracers that can be tested in preclinical models and eventually translated to human applications. In the last few decades, multiple radioligands have been developed and tested as potential fungal-specific tracers. These include radiolabeled peptides, antifungal drugs, siderophores, fungal-specific antibodies, and sugars. In this review, we provide an overview of the pros and cons of the available radiotracers. We also address the future prospects of fungal-specific imaging.

FDG PET in Evaluation of Patients With Fever of Unknown Origin: AJR Expert Panel Narrative Review

Fever of unknown origin (FUO) is a diagnostic challenge, with its cause remaining undiagnosed in approximately half of patients. Nuclear medicine tests typically are performed after a negative or inconclusive initial workup. Gallium-67 citrate and labeled leukocytes were previous mainstays of radionuclide imaging for FUO, although they had limited diagnostic performance. FDG PET/CT has subsequently emerged as the nuclear medicine imaging test of choice, supported by a growing volume of evidence. A positive FDG PET/CT result contributes useful information by identifying potential causes of fever, localizing sites for further evaluation, and guiding further management; a negative result contributes useful information by excluding focal disease as the cause of fever and predicts a favorable prognosis. In 2021, CMS rescinded a prior national noncoverage determination for FDG PET for infection and inflammation, leading to increasing national utilization of FDG PET/CT for FUO workup. This article reviews the current status of the role of FDG PET/CT in the evaluation of patients with FUO. The literature reporting the diagnostic performance and yield of FDG PET/CT in FUO workup is summarized, with comparison with historically used nuclear medicine tests included. Attention is also given to the test's clinical impact; protocol, cost, and radiation considerations; and application in children.

Novel Diagnostic Imaging Approach for Patients With Spontaneous Coronary Artery Dissection: The Utility of 18F-FDG PET Imaging

ABSTRACT

Spontaneous coronary artery dissection (SCAD) is an underdiagnosed etiology of acute coronary syndrome in women. Accurate diagnosis remains challenging but is imperative for treatment and prevention. We show here the utility of 18F-FDG PET imaging in SCAD diagnosis. We present 1 representative case of 4 women with suspected SCAD on coronary angiography from the EVACS (Evolocumab in Acute Coronary Syndromes) clinical trial. 18F-FDG PET imaging showed acute inflammation in the distribution of the suspected dissected coronary artery identified on angiography. Localized myocardial inflammation identified on 18F-FDG PET imaging can aid in diagnosing SCAD suspected on coronary angiography.

Synthesis and Evaluation of Fluorine-18-Labeled L-Rhamnose Derivatives

The use of radiolabeled glucose for PET imaging resulted in the most commonly used tracer in the clinic, 2-deoxy-2-[¹⁸F]fluoroglucose (FDG). More recently, other radiolabeled sugars have been reported for various applications, including imaging tumors and infections. Therefore, in this study, we developed a series of fluorine-18-labeled L-rhamnose derivatives as potential PET tracers of various fungal and bacterial strains. Acetyl-protected triflate precursors of rhamnose were prepared and radiolabeled with fluorine-18 followed by hydrolysis to produce L-deoxy [¹⁸F]fluororhamnose. The overall radiochemical yield was 7-27% in a 90 min synthesis time with a radiochemical purity of 95%. In vivo biodistribution of the ligands using PET imaging showed that 2-deoxy-2-[¹⁸F]fluoro-L-rhamnose is stable for at least up to 60 min in mice and eliminated via renal clearance. The tracer also exhibited minimal tissue or skeletal uptake in healthy mice resulting in a low background signal.

A Comparative Study of 18F-FAPI-42 and 18F-FDG PET/CT for Evaluating Acute Kidney Injury in Cancer Patients

PURPOSE

Compare the value of imaging using positron ¹⁸F-labeled fibroblast activation protein inhibitor-42 (¹⁸F-FAPI-42) and ¹⁸F-labeled deoxyglucose (¹⁸F-FDG) for assessment of AKI.

PROCEDURES

This study analyzed cancer patients who received ¹⁸F-FAPI-42 and ¹⁸F-FDG PET/CT imaging. Eight patients had AKI with bilateral ureteral obstruction (BUO), eight had BUO (CKD1-2) with no acute kidney disease (AKD), and eight had no ureteral obstruction (UO) with normal renal function. The average standardized uptake value (SUV_ave) of the renal parenchyma (RP-SUV_ave), the blood pool SUV_ave (B- SUV_ave), SUV_ave in the highest region of the renal collective system (RCS-SUV_ave), and the highest serum creatinine level (top SCr) were recorded.

RESULTS

The ¹⁸F-FAPI-42 and ¹⁸F-FDG results showed that radiotracer of renal parenchyma was more concentrated in the AKI group than in the other two groups, whereas the RP-SUV_ave from ¹⁸F-FAPI-42 was higher than that from ¹⁸F-FDG in the AKI group (all P < 0.05). ¹⁸F-FAPI-42 imaging in the AKI group showed uptake by the renal parenchyma with a diffuse increase, but very little radiotracer in the renal collecting system, similar to a "super kidney scan." The renal parenchyma also had an increase of SUV_ave, with accumulation of radiotracer in the renal collecting system. AKI was more severe when a patient had a "super kidney scan" in both kidneys (P < 0.05). The B-SUV_ave level was higher in the AKI group than in the other two groups in ¹⁸F-FAPI-42 (both P < 0.05).

CONCLUSIONS

¹⁸F-FAPI-42 imaging had higher RP-SUV_ave than ¹⁸F-FDG imaging in cancer patients who had BUO with AKI. An increased renal parenchyma uptake in both kidneys and low radiotracer distribution in the collecting system suggest more severe AKI.

Is Imaging Bacteria with PET a Realistic Option or an Illusion?

The application of [¹⁸F]-fluorodeoxyglucose ([¹⁸F]FDG) as a radiotracer to detect sites of inflammation (either due to bacterial infection or primary inflammation) has led to exploring the role of PET in visualizing bacteria directly at sites of infection. However, the results from such efforts are controversial and inconclusive so far. We aimed to assess the limitations of PET as an effective modality in the diagnosis of bacterial infections. Inflammation due to bacterial infections can be visualized by using [¹⁸F]FDG-PET. However, the non-specificity of [¹⁸F]FDG makes it undesirable to visualize bacteria as the underlying cause of inflammation. Hence, more specific radiotracers that possibly bind to or accumulate in bacteria-specific receptors or enzymes are being explored. Several radiotracers, including 2-deoxy-2-[¹⁸F]fluorosorbitol ([¹⁸F]FDS), 6-[¹⁸F]-fluoromaltose, [¹¹C]para-aminobenzoic acid ([¹¹C]PABA), radiolabeled trimethoprim (¹¹C-TMP) and its analog fluoropropyl-trimethoprim (¹⁸F-FPTMP), other radiolabeled sugars, and antimicrobial drugs have been used to image microorganisms. Unfortunately, no progress has been made in translating the results to routine human use; feasibility and other factors have constrained their success in clinical settings. In the current article, we discuss the limitations of direct bacterial visualization with PET tracers, but emphasize the important role of [¹⁸F]FDG-PET as the only option for detecting evidence of infection.

Advanced molecular imaging in large-vessel vasculitis: Adopting FDG-PET into a clinical workflow

The use of fluorodeoxyglucose-positron emission tomography (FDG-PET) imaging to detect vascular inflammation is increasingly common in the clinical management of patients with large-vessel vasculitis (LVV). In this review, the role of FDG-PET imaging to diagnose and monitor vascular disease activity will be detailed. Suggestions on incorporation of FDG-PET imaging into a clinical workflow will be provided with emphasis on patient preparation, image acquisition, and image interpretation. If FDG-PET imaging is obtained, multimodal imaging assessment, whereby FDG-PET imaging and non-invasive angiography are obtained concurrently, and correlation of imaging findings with clinical assessment is generally advisable. Considering the clinical scenario and treatment status of the patient is important when interpreting vascular FDG-PET image findings.

Regulatory Agencies and PET/CT Imaging in the Clinic

Purpose of Review

The regulatory steps necessary to bring new PET radiopharmaceuticals to the clinic will be reviewed. The US Food and Drug Administration (FDA) provides approval to manufacture and use diagnostic radiopharmaceuticals, including those for cardiovascular PET/CT. Medicare not only provides insurance reimbursement for imaging procedures for its beneficiaries but also sets an example for third-party insurers to cover these procedures.

Recent Findings

FDA provides extensive guidance for performing studies to obtain the safety and efficacy data needed to approve PET radiopharmaceuticals, and the pace of approval has recently increased. There also has been considerable progress in insurance coverage for PET by Medicare. Several promising agents for cardiovascular PET imaging are in the development pipeline. Challenges remain, however, including low levels of reimbursement and the application of appropriate use criteria for imaging procedures.

Summary

It is important for cardiologists to understand the regulatory steps involved in translating PET radiopharmaceuticals to the clinic. Recent progress in both FDA approvals and Medicare coverage should facilitate the clinical use of new PET agents for molecular imaging of the heart.

Optimal use of the FDG-PET/CT in the diagnostic process of fever of unknown origin (FUO): a comprehensive review

Numerous studies have clarified the usefulness of ¹⁸F-fluorodeoxyglucose (FDG)-PET/CT (positron emission tomography) for diagnosing the cause of fever of unknown origin (FUO). Various types of disease can cause FUO, but the cause remains unknown in a certain proportion of FUO, even when the advanced diagnostic methodologies are used. FDG-PET/CT is regarded as a second-line modality in the diagnostic process of FUO, and its potential to identify the cause of FUO will be maximized when the appropriate clinical considerations are understood. Accordingly, this review presents basic knowledge regarding FUO, and reports the current status of FDG-PET/CT applied to diagnosing the cause of FUO, including diagnostic performance, test protocols, possible factors influencing the diagnostic result, outcomes, and cost-effectiveness. This knowledge will enable effective future use of FDG-PET/CT to improve outcomes in patients with FUO.