Brown adipose tissue 18F-FDG uptake in pediatric PET/CT imaging

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.

Incidence and Risk Factors for Brown Adipose Tissue Uptake in PET Imaging in Pediatric Patients

BACKGROUND

Positron emission tomography (PET) scans are used in disease diagnosis and evaluation for pediatric oncology patients. Brown adipose tissue (BAT) 18 F-fluorodeoxyglucose-PET uptake is reported in 35% to 47% of pediatric patients. Several risk factors may be associated with BAT uptake.

OBJECTIVE

The aim was to determine the incidence and risk factors for BAT in pediatric patients using a consensus-based system and a novel grading scale.

METHODS

A total of 285 PET scans in 154 patients were retrospectively reviewed for the presence of BAT from September 2015 through December 2016. A consensus review was done by 2 radiologists, who graded BAT on a 0 to 3 scale and assessed its impact on PET interpretation.

RESULTS

The presence of moderate to severe BAT occurred in 11% of PET scans, and 6% of PETs had limited interpretation. Hodgkin lymphoma (n=53) patients had a 3.62-fold increased odds of moderate or severe BAT and a 6.59-fold increased odds of limited interpretation on PET imaging.

CONCLUSION

The incidence of BAT was low but impacted radiologic interpretation when present. Further studies with a larger group of Hodgkin lymphoma patients are needed to explore the risk factors associated with moderate or severe BAT.

Longitudinal imaging of murine atherosclerosis with 2-deoxy-2-[18F]fluoro-D-glucose and [18F]-sodium fluoride in genetically modified Apolipoprotein E knock-out and wild type mice

In a longitudinal design, four arterial segments in mice were followed by positron emission tomography/computed tomography (PET/CT) imaging. We aimed to determine how the tracers reflected the development of atherosclerosis via the uptake of 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) for imaging inflammation and [18F]-sodium fluoride (Na[18F]F) for imaging active microcalcification in a murine model of atherosclerosis. Apolipoprotein E knock-out (ApoE) mice and C57 BL/6NtaC (B6) mice were divided into four groups. They received either normal chow (N = 7, ApoE mice and N = 6, B6 mice) for 32 weeks or a high-fat diet (N = 6, ApoEHFD mice and N = 9, B6HFD mice) for 32 weeks. The mice were scanned with [18F]FDG and Na[18F]F using a dedicated small animal PET/CT scanner at three timepoints. The tracer uptakes in four aortic segments (abdominal aorta, aortic arch, ascending aorta, and thoracic aorta) were measured and reported as SUVmax values. The uptake of [18F]FDG (SUVmax: 5.7 ± 0.5 vs 1.9 ± 0.2, 230.3%, p =  < 0.0001) and Na[18F]F (SUVmax: 9.6 ± 1.8 vs 4.0 ± 0.3, 175%, p = 0.007) was significantly increased in the abdominal aorta of ApoEHFD mice at Week 32 compared to baseline abdominal aorta values of ApoEHFD mice. [18F]FDG uptake in the aortic arch, ascending aorta and the thoracic aorta of B6HFD mice at Week 32 showed a robust resemblance to the abdominal aorta uptake whereas the Na[18F]F uptake only resembled in the thoracic aorta of B6HFD mice at Week 32 compared to the abdominal aorta. The uptake of both [18F]FDG and Na[18F]F increased as the disease progressed over time, and the abdominal aorta provided a robust measure across mouse strain and diet. Therefore, it seems to be the preferred region for image readout. For [18F]FDG-PET, both B6 and ApoE mice provide valuable information and either mouse strain may be used in preclinical cardiovascular studies, whereas for Na[18F]F -PET, ApoE mice should be preferred.

Posterior Cervical Brown Fat and CXCL14 Levels in the First Year of Life: Sex Differences and Association With Adiposity

CONTEXT

Brown adipose tissue (BAT) is particularly abundant in neonates, but its association with measures of adiposity and metabolic health in early infancy is poorly delineated. Besides sustaining nonshivering thermogenesis, BAT secretes brown adipokines that act on systemic metabolism. The chemokine CXCL14 has been identified as a brown adipokine in experimental studies.

OBJECTIVE

To determine the relationships among BAT activity, adiposity, and circulating CXCL14 levels in the first year of life in girls and boys.

METHODS

Indices of fat accretion, circulating endocrine-metabolic parameters and serum CXCL14 levels were assessed longitudinally in a cohort of infants at birth and at 4 and 12 months. BAT activity was estimated using infrared thermography only at age 12 months.The main outcome measures were weight and length Z-scores, total and abdominal fat content (by dual X-ray absorptiometry), BAT activity at the posterior cervical and supraclavicular regions, serum levels of glucose, insulin, insulin-like growth factor-I, high-molecular-weight adiponectin, and CXCL14; CXCL14 transcript levels in neonatal BAT and liver.

RESULTS

Posterior cervical BAT was more active in girls than in boys (P = .02). BAT activity was negatively associated with adiposity parameters only in girls. CXCL14 levels were higher in girls than in boys at age 12 months and correlated positively with the area of active posterior cervical BAT in girls. Neonatal BAT showed high CXCL14 gene expression levels.

CONCLUSION

BAT activity and the levels of CXCL14-a potential surrogate of BAT activity-are sex specific in the first year of life. Posterior cervical BAT activity associates negatively with indices of adiposity only in girls.

Effect of Propranolol on 18F-Fluorodeoxyglucose Uptake in Brown Adipose Tissue in Children and Young Adults with Neoplastic Diseases

PURPOSE

To evaluate the effectiveness of propranolol at mitigating FDG uptake in brown adipose tissue (BAT) of pediatric patients with known or suspected malignancies.

METHODS

PET/CT scans of 3 cohorts of patients treated from 2005 to 2017 were scored for the presence of FDG uptake by BAT at 7 sites: right or left neck/supraclavicular area, right or left axilla, mediastinum, posterior thorax, and abdomen/pelvis. Uptake was scored as follows: 0, none; 1, mild uptake < liver; 2, moderate uptake = liver; and 3, intense uptake > liver. Group 1 consisted of 323 patients (630 scans) who had no specific preparation to mitigate FDG uptake by BAT. Group 2 consisted of 345 patients (705 scans) who underwent only warming in an uptake room with a fixed temperature at 24 °C. Group 3 consisted of 622 patients (1457 scans) who underwent warming. In group 3, patients 8 years and older, 471 patients (1114 scans), were also pre-medicated with oral propranolol 60 min before injection of FDG. Generalized estimation equation, using the logit link method, was used to model the relationship between the incidence of BAT score > 0, in any site, as a function of age, sex, seasonal effect, and body surface area (BSA).

RESULTS

In patients aged 8 years or older, the incidence of BAT uptake was 35-44 % and declined to 15 % with propranolol. BAT was most frequent in the neck (26 %), axilla (18 %), posterior thorax (18 %), mediastinum (14 %), and abdomen/pelvis (8 %); BAT was less common in warm months (p = 0.001). No substantial benefit was shown with pre-injection warming alone. No significant effect was found for age, sex, or BSA separately. When BAT uptake was present, it was usually intense.

CONCLUSION

Propranolol preparation minimizes FDG uptake by BAT and should be considered routine for pediatric FDG PET/CT cancer-related protocols in children, adolescents, and young adults.

Adipose-derived exosomal miR-210/92a cluster inhibits adipose browning via the FGFR-1 signaling pathway in high-altitude hypoxia

Cold and hypoxia are critical drivers of adaptation to high altitudes. Organisms at high altitudes have adapted to maximize the efficiency of oxygen utilization and are less prone to obesity and diabetes than those at low altitudes. Brown adipose tissue (BAT) dissipates energy in the form of heat in both humans and rodents; it also serves to regulate metabolism to curb obesity. However, the role of BAT in high-altitude populations is poorly understood. Serum exosomes can be easily obtained, enabling the study of BAT functions and identification of biomarkers in serum exosomes, both of which contribute to understanding the role of BAT in high-altitude populations. ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography integrated with computed tomography (PET/CT) is the gold standard for studying BAT in human adults. Here, we studied BAT in healthy high-altitude populations via PET/CT and serum exosomal microRNAs (miRNAs). The observations were validated in mouse tissues and demonstrated that high-altitude hypoxia activated BAT through attenuated white adipose tissue (WAT) secreted exosomal miR-210/92a, which enhanced the FGFR-1 expression in BAT.

18F-2-fluoro-2-deoxyglucose uptake in white adipose tissue on pediatric oncologic positron emission tomography (PET)/computed tomography (CT)

BACKGROUND

Altered biodistribution of [F-18]2-fluoro-2-deoxyglucose (FDG) is sometimes encountered in pediatric patients undergoing chemotherapy for lymphoma on post-induction positron emission tomography (PET) imaging. A characteristic pattern of increased FDG uptake in white adipose tissue can be seen, particularly in the buccal regions, body wall and gluteal regions, with a shift of radiotracer away from the blood pool and liver. This altered biodistribution has been attributed to effects of corticosteroids in pediatric and adult patients and is important to recognize because of its potential for limiting the diagnostic quality of the PET scan and interfering with therapeutic response assessment.

OBJECTIVE

In contrast to the well-known metabolically active brown fat seen on up to one-third of pediatric PET scans, white fat is usually non-metabolically active. We sought to determine the incidence of altered distribution of FDG in subcutaneous white adipose tissue in pediatric patients undergoing PET imaging and to assess the association with corticosteroid use.

MATERIALS AND METHODS

We reviewed the medical records and imaging for four children in whom altered biodistribution in white adipose tissue was present on post-induction FDG PET/CT, identified during routine clinical practice. All four were receiving corticosteroids as part of their chemotherapy. We then retrospectively reviewed oncology FDG PET/CT scans over a 2-year period (1,361 scans in 689 patients) to determine the incidence of uptake in white fat by qualitative visual assessment. In the children identified with altered biodistribution, we measured maximum standard uptake value (SUV_max) and mean standard uptake value (SUV_mean) in areas of subcutaneous white fat, the buccal regions, body wall or gluteal soft-tissue regions, liver and blood pool. We reviewed all medical records, including medication lists. We summarize the relevant clinical and imaging findings of 13 pediatric patients, including the 4 index patients.

RESULTS

We determined the incidence of FDG uptake in white fat to be rare, found in 9 of 1,361 (0.6%) PET scans performed for pediatric cancer evaluation. FDG uptake was increased in subcutaneous adipose tissue, particularly in the buccal regions, body wall and gluteal regions, with a shift of radiotracer away from the blood pool and liver. The degree of increased uptake in peripheral white fat varied from marked to mild, and the biodistribution was distinct from that of brown adipose tissue. Children with this altered biodistribution were uniformly receiving corticosteroids as part of induction treatment for their cancer, and these findings were only identified on post-induction PET/CT. Follow-up PET/CT documented resolution of this effect after treatment with corticosteroids ceased.

CONCLUSION

Our findings support the current understanding that characteristic uptake of FDG in white adipose tissue is mediated by corticosteroid effect. Although this altered biodistribution is rare (<1% of PET scans) it could impair the diagnostic quality of the scan, affecting image interpretation, and should be recognized when present.

Brown Adipose Tissue in Human Infants

Adapting to the cold extrauterine environment after birth is a great challenge for the newborn. Due to their high surface area-to-volume ratio, infants tend to lose more heat to the environment as compared to adults. In addition, human newborns lack sufficiently developed skeletal muscle mass to maintain body temperature through shivering thermogenesis, an important source of heat in cold-exposed adults. Evolution has provided humans and other placental mammals with brown adipose tissue (BAT), a tissue that converts chemically stored energy, in the form of fatty acids and glucose, into heat through non-shivering thermogenesis. The thermogenic activity of this tissue is significant for the human infant's ability to maintain a sufficiently high core body temperature. Although BAT has been studied in human infants for more than a century, the literature covering different aspects of the tissue is rather limited. The aim of this review is to summarize the literature and describe what is actually known about the tissue and its importance for early human life.

Case Report: Brown Fat Accumulation of Tc-99m Macroaggregated Albumin in a Lung Perfusion Study in a Patient With Multiple Lung Arteriovenous Malformations and Right-to-Left Shunting

An 18-year-old man was preoperatively assessed for a varicocele and found to be hypoxemic. A Tc-99m macroaggregated albumin lung perfusion scan showed right-to-left shunting, evidenced by increased radiotracer uptake in the brain, kidneys, thyroid gland, and bilateral supraclavicular areas, a typical location for brown adipose tissue. Chest computerized tomography angiogram study showed supraclavicular fat density areas and multiple pulmonary arteriovenous malformations.The authors report a rare case of brown fat visualization on a lung perfusion scan in a patient with right-to-left shunting, likely because of increased perfusion to activated brown adipose tissue.

2-deoxy-2-(18F)fluoro-D-glucose positron emission tomography/computed tomography imaging in paediatric oncology

Positron emission tomography (PET) is a minimally invasive technique which has been well validated for the diagnosis, staging, monitoring of response to therapy, and disease surveillance of adult oncology patients. Traditionally the value of PET and PET/computed tomography (CT) hybrid imaging has been less clearly defined for paediatric oncology. However recent evidence has emerged regarding the diagnostic utility of these modalities, and they are becoming increasingly important tools in the evaluation and monitoring of children with known or suspected malignant disease. Important indications for 2-deoxy-2-(¹⁸F)fluoro-D-glucose (FDG) PET in paediatric oncology include lymphoma, brain tumours, sarcoma, neuroblastoma, Langerhans cell histiocytosis, urogenital tumours and neurofibromatosis type I. This article aims to review current evidence for the use of FDG PET and PET/CT in these indications. Attention will also be given to technical and logistical issues, the description of common imaging pitfalls, and dosimetric concerns as they relate to paediatric oncology.

Molecular imaging of brown adipose tissue in health and disease

PURPOSE

Brown adipose tissue (BAT) has transformed from an interfering tissue in oncological (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to an independent imaging research field. This review takes the perspective from the imaging methodology on which human BAT research has come to rely on heavily.

METHODS

This review analyses relevant PubMed-indexed publications that discuss molecular imaging methods of BAT. In addition, reported links between BAT and human diseases such as obesity are discussed, and the possibilities for imaging in these fields are highlighted. Radiopharmaceuticals aiming at several different biological mechanisms of BAT are discussed and evaluated.

RESULTS

Prospective, dedicated studies allow visualization of BAT function in a high percentage of human subjects. BAT dysfunction has been implicated in obesity, linked with diabetes and associated with cachexia and atherosclerosis. Presently, (18)F-FDG PET/CT is the most useful tool for evaluating therapies aiming at BAT activity. In addition to (18)F-FDG, other radiopharmaceuticals such as (99m)Tc-sestamibi, (123)I-metaiodobenzylguanidine (MIBG), (18)F-fluorodopa and (18)F-14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid (FTHA) may have a potential for visualizing other aspects of BAT activity. MRI methods are under continuous development and provide the prospect of functional imaging without ionizing radiation.

CONCLUSION

Molecular imaging of BAT can be used to quantitatively assess different aspects of BAT metabolic activity.

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.

Motion Artifacts in Oncological and Cardiac PET Imaging

Image quality in PET examinations is influenced by several factors. Patient motion during PET data acquisition is a substantial problem that potentially leads to smearing artifacts, resulting in the loss of diagnostic accuracy both in visual and quantitative image analyses. In hybrid imaging, coregistration of functional (PET) and morphologic (CT or MR imaging) data can be hampered by patient movement between the acquisitions, resulting in additional sources of error. This article describes the artifacts due to patient movement.

Human brown fat and obesity: methodological aspects

Much is known about brown adipose tissue (BAT) in rodents. Its function is to generate heat in response to low environmental temperatures and to diet or overfeeding. The knowledge about BAT in humans is still rather limited despite the recent rediscovery of its functionality in adults. This review highlights the information available on the contribution of BAT in increasing human energy expenditure in relation to obesity. Besides that methodological aspects will be discussed that need special attention in order to unravel the heat producing capacity of human BAT, the recruitment of the tissue, and its functionality.