Brown fat: atypical locations and appearances encountered in PET/CT. AJR Am J Roentgenol. 2009;193:359-366. [PMID: 19620432 DOI: 10.2214/ajr.08.2081]

An International Survey Investigating the Incidence and Management of Brown Fat Uptake on 18F-FDG PET/CT at Children's Hospitals and Interventions for Mitigation

Brown fat can present challenges in patients with cancer who undergo 18F-FDG PET scans. Uptake of 18F-FDG by brown fat can obscure or appear similar to active oncologic lesions, causing clinical challenges in PET interpretation. Small, retrospective studies have reported environmental and pharmacologic interventions for suppressing brown fat uptake on PET; however, there is no clear consensus on best practices. We sought to characterize practice patterns for strategies to mitigate brown fat uptake of 18F-FDG during PET scanning. Methods: A survey was developed and distributed via e-mail LISTSERV to members of the Children's Oncology Group diagnostic imaging committee, the Society for Nuclear Medicine and Molecular Imaging pediatric imaging council, and the Society of Chiefs of Radiology at Children's Hospitals between April 2022 and February 2023. Responses were stored anonymously in REDCap, aggregated, and summarized using descriptive statistics. Results: Fifty-five complete responses were submitted: 51 (93%) faculty and fellow-level physicians, 2 (4%) technologists, and 2 (4%) respondents not reporting their rank. There were 43 unique institutions represented, including 5 (12%) outside the United States. Thirty-eight of 41 (93%) institutions that responded on environmental interventions reported using warm blankets in the infusion and scanning rooms. Less than a third (n = 13, 30%) of institutions reported use of a pharmacologic intervention, with propranolol (n = 5, 38%) being most common, followed by fentanyl (n = 4, 31%), diazepam (n = 2, 15%), and diazepam plus propranolol (n = 2, 15%). Selection criteria for pharmacologic intervention varied, with the most common criterion being brown fat uptake on a prior scan (n = 6, 45%). Conclusion: Clinical practices to mitigate brown fat uptake on pediatric 18F-FDG PET vary widely. Simple environmental interventions including warm blankets or increasing the temperature of the injection and scanning rooms were not universally reported. Less than a third of institutions use pharmacologic agents for brown fat mitigation.

Physiologic 18F-FDG Uptake in Brown Adipose Tissue and Lactating Breast in a Patient with Hodgkin Lymphoma

We present the case of a young woman with Hodgkin lymphoma exhibiting physiologic 18F-FDG uptake in brown adipose tissue and lactating breast on consecutive 18F-FDG PET/CT scans. Both entities are common imaging interpretation pitfalls and should be recognized in oncologic 18F-FDG PET/CT practice. We review the imaging features and differential diagnosis of these 2 entities and discuss the radiation safety precautions during breastfeeding.

Hibernoma: a case report of a rare cardiac tumour

Background

A cardiac hibernoma is a rare phenomenon, with just a handful of reports in the literature. They are difficult to characterize with conventional imaging including echocardiography, computed tomography (CT), cardiac magnetic resonance (CMR), or positron emission tomography (PET). Their definitive diagnosis relies primarily on histopathology via either endovascular or surgical biopsy. Previous case reports have entailed surgical excision followed by histopathology; however, surgery may be unfavourable in some patients with increased perioperative risk.

Case summary

We present the case of a 57-year-old woman who was referred to our cardiology service with an interatrial lipomatous mass found incidentally on chest CT for assessment of rib fractures. She had 6 months of unexplained syncope, which was attributed to superior vena cava (SVC) compression demonstrated by chest CT. The mass had benign characteristics on echocardiography, CT, and CMR but was glucose-avid on PET, which indicated a possible malignancy such as liposarcoma. Her comorbid and very significant airways disease precluded her from surgical excision, so instead, endovascular biopsy was performed. Histopathology showed brown fat which was negative for mouse double minute 2 amplification on fluorescence in situ hybridisation testing; hence, a diagnosis was made of hibernoma, a rare benign tumour of brown fat. Given the benign diagnosis and her surgical risk with severe chronic obstructive pulmonary disease, a multidisciplinary recommendation was made favouring conservative management, with careful ongoing follow-up and the consideration of SVC stenting if symptoms progressed.

Discussion

The definitive diagnosis of a cardiac hibernoma is complex and relies heavily on histopathology due to the contradictory findings on chest imaging. Careful consideration of management within a multidisciplinary team setting is essential to achieve a successful outcome.

PET/CT Variants and Pitfalls in Head and Neck Cancers Including Thyroid Cancer

PET/CT imaging is a dual-modality diagnostic technology that merges metabolic and structural imaging. There are several currently available radiotracers, but ¹⁸F-FDG is the most commonly utilized due to its widespread availability. ¹⁸F-FDG PET/CT is a cornerstone of head and neck squamous cell carcinoma imaging. ⁶⁸Ga-DOTA-TOC is another widely used radiotracer. It allows for whole-body imaging of cellular somatostatin receptors, commonly expressed by neuroendocrine tumors and is the standard of reference for the characterization and staging of neuroendocrine tumors. The normal biodistribution of these PET radiotracers as well as the technical aspects of image acquisition and inadequate patient preparation affect the quality of PET/CT imaging. In addition, normal variants, artifacts and incidental findings may impede accurate image interpretation and can potentially lead to misdiagnosis. In order to correctly interpret PET/CT imaging, it is necessary to have a comprehensive knowledge of the normal anatomy of the head and neck and to be cognizant of potential imaging pitfalls. The interpreter must be familiar with benign conditions which may accumulate radiotracer potentially mimicking neoplastic processes and also be aware of malignancies which can demonstrate low radiotracer uptake. Appropriate use of structural imaging with either CT, MR or ultrasound can serve a complimentary role in several head and neck pathologies including local tumor staging, detection of bone marrow involvement or perineural spread, and classification of thyroid nodules. It is important to be aware of the role of these complementary modalities to maximize diagnostic accuracy and patient outcomes. The purpose of this article is to outline the basic principles of PET/CT imaging, with a focus on ¹⁸F-FDG PET/CT and ⁶⁸Ga-DOTA PET/CT. Basic physiology, variant imaging appearances and potential pitfalls of image interpretation are presented within the context of common use cases of PET technology in patients with head and neck cancers and other pathologies, benign and malignant.

[Embolizing cardiac hibernoma and acute respiratory failure]

Not only ae cardiac hibernomas rare, ante-mortem diagnosis is poor, due to the clinical and radiological similarity with other intracardiac masses. Furthermore, cardiac hibernomas can be asymptomatic and thus escape detection with imaging studies. We present a case of an 81-year-old woman who died as a result of pulmonary tumor embolism. This unusual case highlights the necessity for clinicians and pathologists to be familiar with intra-cardiac neoplasms. We discuss its diagnostic difficulty and the complication of pulmonary tumor embolism.

Multimodal imaging approach to monitor browning of adipose tissue in vivo

The discovery that white adipocytes can undergo a browning process to become metabolically active beige cells has attracted significant interest in the fight against obesity. However, the study of adipose browning has been impeded by a lack of imaging tools that allow longitudinal and noninvasive monitoring of this process in vivo. Here, we report a preclinical imaging approach to detect development of beige adipocytes during adrenergic stimulation. In this approach, we expressed near-infrared fluorescent protein, iRFP720, driven under an uncoupling protein-1 (Ucp1) promoter in mice by viral transduction, and used multispectral optoacoustic imaging technology with ultrasound tomography (MSOT-US) to assess adipose beiging during adrenergic stimulation. We observed increased photoacoustic signal at 720 nm, coupled with attenuated lipid signals in stimulated animals. As a proof of concept, we validated our approach against hybrid positron emission tomography combined with magnetic resonance (PET/MR) imaging modality, and quantified the extent of adipose browning by MRI-guided segmentation of 2-deoxy-2-18F-fluoro-d-glucose uptake signals. The browning extent detected by MSOT-US and PET/MR are well correlated with Ucp1 induction. Taken together, these systems offer great opportunities for preclinical screening aimed at identifying compounds that promote adipose browning and translation of these discoveries into clinical studies of humans.

Chapter 3 The Role of PET/CT in Squamous Cell Carcinoma of the Head and Neck

Head and neck squamous cell carcinoma is an important cause of cancer morbidity worldwide and has been stratified into human papillomavirus-related and human papillomavirus-unrelated subgroups that affect prognosis and now staging. Conventional anatomical imaging methods are suboptimal for the detection of regional and distant metastases that are important prognosticators associated with poor outcomes. Functional imaging with (F18)-fluorodeoxyglucose-positron emission tomography/computed tomography (PET/CT) is a useful tool in the management of head and neck squamous cell carcinoma, providing complementary physiological and anatomical information. In this article, optimal PET/CT technique will be reviewed and the pretreatment and posttreatment applications of PET/CT will be described. A simplified approach to imaging interpretation, including review of pearls and pitfalls will be discussed. An initial framework for follow-up evaluation will be provided.

Fat: friend or foe? A review of fat-containing masses within the head and neck

Fat-containing lesions of the head and neck are commonly encountered in day-to-day practice. Our aim was to review the various imaging presentations of common and some uncommon fat-containing lesions within the head and neck with potential pitfalls and mimics. While most soft-tissue masses have a fairly similar density, the presence of fat in a mass lesion is easy to identify on both CT/MRI and can help narrow the differential. Case-based examples of lipomas, liposarcomas, lipoblastomas, dermoids, teratomas and other fatty lesions will be used to describe imaging features. While fat density can be helpful, differentiating benign from malignant fat-containing lesions can still pose a challenge. Lesions simulating pathology such as brown fat, fatty changes within organs and post-operative flaps are presented. Finally, examples of fatty lesions in atypical locations are shown to illustrate examples that should be kept in mind in any differential. The presence of fat in head and neck masses can aid radiologists in arriving at an accurate diagnosis. Knowledge of the imaging appearance of these fat-containing lesions and their mimics can help avoid unnecessary biopsy or surgery.

Case 214: Adrenal pheochromocytoma with perirenal brown fat stimulation

History A 61-year-old woman with well-controlled diabetes presented with a 10-year history of hypertension, stifling sensation, and flushing. Her body mass index was 19.1 kg/m(2) (normal range, 18.5-25.0 kg/m(2)). She was being followed up for mild hypercalcemia (calcium level, 10.8 mg/dL [2.7 mmol/L]) (normal range, 8.5-10.5 mg/dL [2.12-2.62 mmol/L]) by the endocrinologist (S.J.M.), who decided to perform a technetium 99m sestamibi ((99m)Tc MIBI) parathyroid scan. The test showed an abnormal tracer deposit in the region of the clavicle and sternum; thus, unenhanced thoracic computed tomography (CT) was performed. No mass was seen in the region of abnormality. In light of these findings, the patient underwent contrast material-enhanced (120 mL of iopromide, Ultravist 300; Schering, Berlin, Germany) thoracic abdominopelvic CT. There was no history of underlying malignancy, and the complete blood counts were normal. The axial and appendicular skeleton showed no sign of lesions.

A peptide probe for targeted brown adipose tissue imaging

The presence of brown adipose tissue responsible for thermogenic energy dissipation has been revealed in adult humans and has high clinical importance. Owing to limitations of current methods for brown adipose tissue detection, analysing the abundance and localization of brown adipose tissue in the body has remained challenging. Here we screen a combinatorial peptide library in mice and characterize a peptide (with the sequence CPATAERPC) that selectively binds to the vascular endothelium of brown adipose tissue, but not of intraperitoneal white adipose tissue. We show that in addition to brown adipose tissue, this peptide probe also recognizes the vasculature of brown adipose tissue-like depots of subcutaneous white adipose tissue. Our results indicate that the CPATAERPC peptide localizes to brown adipose tissue even in the absence of sympathetic nervous system stimulation. Finally, we demonstrate that this probe can be used to identify brown adipose tissue depots in mice by whole-body near-infrared fluorescence imaging.

How brown is brown fat that we can see?

There are many unanswered questions related to the heterogeneity of adipose tissue depots and the paucity of their function, development, and organization at the cellular level. Much effort has been directed at studying white adipose tissue (WAT), the driver of obesity and the associated metabolic disease. In recent years, the importance of brown adipose tissue (BAT) has also been appreciated. While BAT depots are prominent in many small mammal species, their detection in adult humans has been technically challenging and the identity of brown human adipocytes found within depots of WAT has remained controversial. We recently reported a peptide probe that binds to BAT vasculature and, when coupled with a near-infrared fluorophore, can be used to detect BAT in whole body imaging. This probe reliably discriminates between endothelium associated with brown or brown-like (beige/brite) adipocytes and endothelium of visceral WAT. Improved probes based on this approach could aid in assessing human adipose tissue body distribution and remodeling, which is a process underlying various pathologies. This commentary aims at discussing open questions that need to be addressed before full clinical advantage can be taken from adipose tissue imaging, as well as its metabolic activation strategies.

Fluorodeoxyglucose uptake in absence of CT abnormality on PET-CT: What is it?

The purpose of this article is to provide a pictorial review of the findings and interpretative pitfalls about focal fluorodeoxyglucose (FDG) uptake in the absence of corresponding computer tomography (CT) lesion or abnormality on an integrated positron emission tomography (PET)-CT. The integrated CT images in the PET-CT scanner allow correct co-registration and fused imaging of anatomical and functional data. On FDG PET-CT imaging, a real pathologic process often demonstrates abnormal uptake associated with a visible corresponding CT lesion or abnormality. When focal uptake is seen on PET imaging but no corresponding anatomic abnormality is visualized on the integrated CT, one should always be aware of possible mis-registration or mismatch of the PET and CT images due to the patient’s respiratory or body motion. While most of the hot spots in the absence of corresponding anatomic abnormalities are artefactual or secondary to benign etiologies, some may represent small sized or early staged neoplasm or metastases, especially in the gastrointestinal tract and skeletons. Caution should be exercised to simply diagnose a pathology based on the presence of the uptake only, or exclude the disease based on the absence of anatomic abnormality.

Thorax: normal and benign pathologic patterns in FDG-PET/CT imaging

This article describes the normal patterns of thoracic (18)F-fluorodeoxyglucose (FDG) biodistribution, and expands on the role of FDG-PET/computed tomography (CT) for the evaluation of patients suffering from a spectrum of benign pathologic conditions that affect the chest. The discussion addresses the applications of FDG-PET/CT imaging in a wide variety of chest-related disorders. Familiarity with the normal thoracic biodistribution of FDG, coupled with knowledge of the potential nonmalignant causes of increased FDG uptake in the chest, is essential to minimize the incidence of incorrect interpretation of FDG-PET images in daily clinical practice.

Computer-aided staging of lymphoma patients with FDG PET/CT imaging based on textural information

We have designed a computer-aided diagnosis system to discriminate between hypermetabolic cancer lesions and hypermetabolic inflammatory or physiological but noncancerous processes in FDG PET/CT exams of lymphoma patients. Detection performance of the support vector machine (SVM) classifier was assessed based on feature sets including 105 positron emission tomography (PET) and Computed tomography (CT) characteristics derived from the clinical practice and from more sophisticated texture analysis. An original feature selection method based on combining different filter methods was proposed. The evaluation database consisted of 156 lymphomatous and 32 suspicious but nonlymphomatous regions of interest. Different types of training databases including either the PET and CT features or the PET features only, with or without feature selection, were evaluated to assess the added value of multimodality and texture information on classification performance. An optimization study was conducted for each classifier separately to select the best combination of parameters. Promising classification performance was achieved by the SVM classifier combined with the 12 most discriminant PET and CT features with a value of the area under the receiver operating curve of 0.91.

An automated algorithm to identify and quantify brown adipose tissue in human 18F-FDG-PET/CT scans

OBJECTIVE

To develop an algorithm to identify and quantify BAT from PET/CT scans without radiologist interpretation.

DESIGN AND METHODS

Cases (n = 17) were randomly selected from PET/CT scans with documented "brown fat" by the reviewing radiologist. Controls (n = 18) had no documented "brown fat" and were matched with cases for age (49.7 [31.0-63.0] vs. 52.4 [24.0-70.0] yrs), outdoor temperature at scan date (51.8 [38.9-77.0] vs. 54.9 [35.2-74.6] °F), sex (F/M: 15/2 cases; 16/2 controls) and BMI (28.2 [20.0-45.7] vs. 26.8 [21.4-37.1] kg/m(2) ]). PET/CT scans and algorithm-generated images were read by the same radiologist blinded to scan identity. Regions examined included neck, mediastinum, supraclavicular fossae, axilla and paraspinal soft tissues. BAT was scored 0 for no BAT; 1 for faint uptake possibly compatible with BAT or unknown; and 2 for BAT positive.

RESULTS

Agreement between the algorithm and PET/CT scan readings was 85.7% across all regions. The algorithm had a low false negative (1.6%) and higher false positive rate (12.7%). The false positive rate was greater in mediastinum, axilla and neck regions.

CONCLUSION

The algorithm's low false negative rate combined with further refinement will yield a useful tool for efficient BAT identification in a rapidly growing field particularly as it applies to obesity.

Dual-time-point FDG PET/CT for the evaluation of pediatric tumors

OBJECTIVE

The utility of dual-time-point (18)F-FDG PET/CT in differentiating benign from malignant processes in pediatric patients was assessed.

SUBJECTS AND METHODS

Twenty-one patients (13 girls and eight boys; age range, 1-17 years) with suspected malignancy underwent dual-time-point FDG PET/CT. Scan 1 was performed at approximately 60 minutes after i.v. injection of 5.18 MBq/kg of FDG, and scan 2 was performed at 121 ± 43 minutes after the first scan. Regions of interest were overlaid onto each non-attenuated-corrected image, and semiquantitative analysis was performed using the standardized uptake value (SUV) obtained from early and delayed images. A retention index was calculated according to the following equation: [(delayed SUV - early SUV) / early SUV] × 100. Results were compared prospectively in relation to pathologic examination or other conventional radiologic imaging or clinical follow-up. A retention index of 10% or higher was chosen as a cutoff for differentiating malignant from benign entities.

RESULTS

For patients with malignant disease, the average SUV increased from 7.3 ± 1.2 to 10.9 ± 2.7 between the two time points, whereas the SUV changed from 4.5 ± 0.8 to 4.2 ± 1.0 for patients with benign lesions. The average retention index was 37.1% ± 10.8% for patients with malignant lesions versus -9.9% ± 7.1% for benign lesions (p < 0.01). With a cutoff value of 10% or higher for the retention index, the sensitivity and specificity of dual-time-point FDG PET/CT were 77% and 80%, respectively.

CONCLUSION

These data show that dual-time-point FDG PET/CT is useful in distinguishing malignant from benign processes in pediatric patients.

Intense FDG activity in the brown adipose tissue in omental and mesenteric regions in a patient with malignant pheochromocytoma

A 39-year-old man who had a history of surgical excision of left adrenal pheochromocytoma 3 years ago underwent an FDG PET/CT scan to evaluate possible metastatic/recurrent disease. In addition to multiple FDG-avid lesions typical of hypermetabolic malignant disease and the FDG uptake in regions rich in brown adipose tissue, there was also intense FDG activity in the omental and mesenteric regions, which are not common locations of elevated FDG activity. On the repeat FDG PET/CT scan 3 days later after the patient was prepared with propranolol, the omental and mesenteric FDG activity was diminished.

Best practices: consensus on performing positron emission tomography-computed tomography for radiation therapy planning and for therapy response assessment

The incorporation of positron emission tomography-computed tomography (PET-CT) into oncological imaging has expanded rapidly since the hybrid scanners were introduced approximately 10 years ago. PET-CT is becoming the standard of practice for the imaging diagnosis and staging of most cancers. Since its introduction, hardware-registered PET and CT images produced by a PET-CT scan were recognized as valuable not only for detection, staging and restaging applications but also for optimizing radiation treatment planning. Even before the introduction of PET-CT, the value of metabolic imaging with the use of FDG PET was recognized as a potentially powerful means of assessing response to various therapies, particularly chemotherapy regimens. To better understand the optimal use of PET-CT in radiation therapy planning and the role of PET-CT in assessing response to therapy, we invited experts from various disciplines to participate in focus group meetings that took place in 2009 and 2010. The Symposia focused on the use of PET-CT imaging in radiation therapy planning (2009) and the use of PET-CT in therapy response assessment (2010). This article will summarize areas of consensus reached by the group regarding many of the discussion topics. The consensus summaries covered in this article are meant to provide direction for future discussions on how to improve the application of this hybrid modality to optimize patient care.

False-positive uptake on 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (FDG) positron-emission tomography/computed tomography (PET/CT) in oncological imaging

With the increasing utilization of integrated positron-emission tomography/computed tomography (PET/CT) using the glucose analogue 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (FDG) in oncological imaging, it is important for radiologists and nuclear medicine physicians to be aware that FDG uptake is not specific for malignancy, as many different physiological variants and benign pathological conditions can also exhibit increased glucose metabolism. Such false-positive FDG uptake often arises outside the area of primary interest and may mimic malignant disease, thereby confounding accurate interpretation of PET/CT studies. With the use of illustrative clinical cases, this article will provide a systematic overview of potential interpretative pitfalls and illustrate how such unexpected findings can be appropriately evaluated.

Physiology and pathophysiology of incidental findings detected on FDG-PET scintigraphy

A routine feature of positron emission tomography/computed tomography (PET/CT) imaging is whole-body acquisition that results in many unexpected findings identified outside of the primary region of abnormality. Furthermore, (18)F-fluorodeoxyglucose (FDG) is a marker of glycolysis and does not specifically accumulate in malignancy. Understanding the physiology and pathophysiology of normal FDG distribution and common incidental findings is therefore essential to the physician interpreting whole-body FDG-PET/CT studies. Whereas many incidental findings are benign and of limited clinical significance, others represent uncommon manifestations of the primary malignancy, second malignancies, or various clinically significant pathologic processes. Patients with a single malignancy are at greater risk of developing synchronous or metachronous second malignancies, possibly related to exposure to shared carcinogenic agents or presence of prooncogenic mutations. The decision of how to pursue an intervention on the basis of an incidental finding is generally left to clinical judgment.

Seasonal variation in the effect of constant ambient temperature of 24 degrees C in reducing FDG uptake by brown adipose tissue in children

PURPOSE

It has been shown that warming patients prior to and during (18)F-FDG uptake by controlling the room temperature can decrease uptake by brown adipose tissue (BAT). The aim of this study is to determine if this effect is subject to seasonal variation.

METHODS

A retrospective review was conducted of all patients referred for whole-body (18)F-FDG PET between December 2006 and December 2008. After December 2007, all patients were kept in the PET injection room at a constant 24 degrees C for 30 min before and until 1 h following FDG administration. Patients over 22 years of age and those who received pre-medication known to reduce FDG uptake by BAT were excluded. One hundred and three patients were warmed to 24 degrees C prior to scanning. The number of patients showing uptake by BAT in this group was compared to a control group of 99 patients who underwent PET prior to December 2007 when the injection room temperature was 21 degrees C.

RESULTS

Uptake by BAT occurred in 9% of studies performed after patient warming (24 degrees C), compared to 27% of studies performed on the control group (21 degrees C) (p < 0.00001). The effect of warming on decreasing FDG accumulation in BAT was statistically significant in the winter (p < 0.005) and summer (p < 0.001). However, in the spring and autumn, though the effect of warming on decreasing FDG accumulation in BAT was evident, it was not statistically significant (p > 0.05).

CONCLUSION

Maintaining room temperature at a constant 24 degrees C for 30 min prior to and 1 h after IV tracer administration significantly decreases FDG uptake by BAT in children. This effect is greatest in the summer and winter.