FDG PET and PET/CT are now widely used in oncological imaging for tumor characterization, staging, restaging, and response evaluation. However, numerous benign etiologies may cause increased FDG uptake indistinguishable from that of malignancy. Multiple studies have shown that dual time-point imaging (DTPI) of FDG PET may be helpful in differentiating malignancy from benign processes. However, exceptions exist, and some studies have demonstrated significant overlap of FDG uptake patterns between benign and malignant lesions on delayed time-point images. In this review, we summarize our experience and opinions on the value of DTPI and delayed time-point imaging in oncology, with a review of the relevant literature. We believe that the major value of DTPI and delayed time-point imaging is the increased sensitivity due to continued clearance of background activity and continued FDG accumulation in malignant lesions, if the same diagnostic criteria (as in the initial standard single time-point imaging) are used. The specificity of DTPI and delayed time-point imaging depends on multiple factors, including the prevalence of malignancies, the patient population, and the cut-off values (either SUV or retention index) used to define a malignancy. Thus, DTPI and delayed time-point imaging would be more useful if performed for evaluation of lesions in regions with significant background activity clearance over time (such as the liver, the spleen, the mediastinum), and if used in the evaluation of the extent of tumor involvement rather than in the characterization of the nature of any specific lesion. Acute infectious and non-infectious inflammatory lesions remain as the major culprit for diminished diagnostic performance of these approaches (especially in tuberculosis-endemic regions). Tumor heterogeneity may also contribute to inconsistent performance of DTPI. The authors believe that selective use of DTPI and delayed time-point imaging will improve diagnostic accuracy and interpretation confidence in FDG PET imaging.

When should we recommend use of dual time-point and delayed time-point imaging techniques in FDG PET?

Paget disease of bone

Traumatic brain injury (TBI) affects an estimated 1.7 million people in the United States and is a contributing factor to one third of all injury related deaths annually. According to the CDC, approximately 75% of all reported TBIs are concussions or considered mild in form, although the number of unreported mild TBIs and patients not seeking medical attention is unknown. Currently, classification of mild TBI (mTBI) or concussion is a clinical assessment since diagnostic imaging is typically inconclusive due to subtle, obscure, or absent changes in anatomical or physiological parameters measured using standard magnetic resonance (MR) or computed tomography (CT) imaging protocols. Molecular imaging techniques that examine functional processes within the brain, such as measurement of glucose uptake and metabolism using [18F]fluorodeoxyglucose and positron emission tomography (FDG-PET), have the ability to detect changes after mild TBI. Recent technological improvements in the resolution of PET systems, the integration of PET with MRI, and the availability of normal healthy human databases and commercial image analysis software contribute to the growing use of molecular imaging in basic science research and advances in clinical imaging. This review will discuss the technological considerations and limitations of FDG-PET, including differentiation between glucose uptake and glucose metabolism and the significance of these measurements. In addition, the current state of FDG-PET imaging in assessing mild TBI in clinical and preclinical research will be considered. Finally, this review will provide insight into potential critical data elements and recommended standardization to improve the application of FDG-PET to mild TBI research and clinical practice.

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FDG-PET imaging in mild traumatic brain injury: a critical review

Comparison of whole-body PET/CT and PET/MRI in breast cancer patients: lesion detection and quantitation of 18F-deoxyglucose uptake in lesions and in normal organ tissues.

Computational approaches for analyzing the mechanics of atherosclerotic plaques: a review

This study aims to evaluate dynamic 2-deoxy-2-[18F]fluoro-d-glucose (FDG) uptake in normal tissues. Thirty male patients underwent FDG positron emission tomography (PET)/computed tomography imaging at 1, 2, and 3 h after tracer injection. Standardized uptake values (SUV) were obtained in regions of interest of normal tissues. The aorta (blood pool), liver, and spleen FDG activity demonstrated significantly and continuously decreased activity from 1 to 2 and 2 to 3 h, while FDG uptake in the lungs, pancreas, lymph nodes, and skeletal muscle decreased from 1 to 2 h only. In contrast, the left ventricular myocardium demonstrated two patterns of dynamic changes: myocardium with higher FDG uptake (SUVmax ≥ 3.25) on the initial images had more remarkable increased activity on the delayed images, while myocardium with lower FDG uptake (SUVmax < 3.25) on the initial imaging had no increased uptake on delayed imaging. Increased FDG uptake was also observed in the bones on the delayed images. No significant changes of FDG uptake were noted in the parotid gland, thyroid gland, and prostate gland. These findings may help nuclear medicine physicians when comparing images performed at different time points, when using FDG uptake in internal reference regions as a relative indicator of FDG uptake in a specific lesion, and when reading a delayed FDG PET imaging.

Dynamic Changes of FDG Uptake and Clearance in Normal Tissues

OBJECTIVE The aim of this study was to determine the ideal circulation time of fluorine-18 fluorodeoxyglucose (F-FDG) in order to detect and quantify atherosclerotic plaque inflammation with PET computed tomography (CT) imaging. METHODS Fifteen patients underwent multiple time-point imaging at ∼60, 120, and 180 min after F-FDG administration. For each time point, global assessment of aortic and carotid F-FDG uptake was determined qualitatively by visual assessment and semiquantitatively by calculation of the mean and maximum standardized uptake values (SUV) and the corresponding target-to-background ratio (TBR). RESULTS Delayed imaging achieved significant improvement in visualization of atherosclerotic plaque inflammation [Friedman's χ statistic (d.f.=2, n=15)=24.13, P<0.001, Kendall's W=0.80]. This observation was confirmed by semiquantitative image analysis. At 1 h, the aortic and carotid SUVmean-calculated TBR was 1.05 [95% confidence interval (CI)=0.98, 1.11] and 0.88 (95% CI=0.81, 0.96), respectively. At 3 h, the TBR significantly increased to 1.57 (95% CI=1.28, 1.86; P=0.001) for the aorta and to 1.61 (95% CI=1.36, 1.87; P<0.001) for the carotid arteries. SUVmax-calculated TBRs showed a similar increase over time. CONCLUSION One- and 2-h F-FDG PET CT imaging is suboptimal for global assessment of atherosclerotic plaque inflammation compared with imaging at 3 h. Our data support the utilization of 3-h delayed imaging to obtain optimal data for the detection and quantification of atherosclerotic plaque inflammation in human arteries.

Delayed time-point 18F-FDG PET CT imaging enhances assessment of atherosclerotic plaque inflammation.

A 69-year-old man with a lung mass underwent multiple-time-point FDG PET/CT imaging for diagnostic evaluation. The initial PET imaging (performed at 1 hour after tracer injection) revealed equivocal bone marrow uptake in the right iliac bone and proximal femurs in addition to lung and mediastinal lesions. The 3-hour delayed PET imaging, however, demonstrated widespread bone marrow metastases. Biopsies of the right lung mass and right iliac bone marrow were later performed and revealed a poorly differentiated squamous cell carcinoma in both sites. This case indicates the value of delayed FDG PET in detecting superscan-like hypermetabolic bone marrow lesions in patients with lung cancer.

Esther Lim

Papers

Dynamic Changes of FDG Uptake and Clearance in Normal Tissues

Delayed time-point 18F-FDG PET CT imaging enhances assessment of atherosclerotic plaque inflammation.

Superscan-like hypermetabolic lesions on delayed FDG PET/CT imaging in a patient with lung cancer.

Monostotic Paget's disease involving the calcaneus encountered incidentally on bone scintigraphy.