History
A 47-year-old woman with a history of breast cancer was treated with a bilateral mastectomy undergoing Fludeoxyglucose F 18 Injection (18F FDG) PET/CT for restaging. In addition to restaging, information regarding the kinetics of the radiotracer was also the aim of imaging. A dynamic, whole-body PET imaging was therefore performed with Siemens Healthineers’ FlowMotion™ Multiparametric PET Suite. An immediate post-injection, single-bed, 6-minute list-mode (LM) acquisition centered over the heart was performed to be used as the bolus portion image-derived input function (a non-invasive alternative to the arterial input function, which requires blood sampling). This information is the prerequisite for a novel 4D reconstruction that incorporates the graphical Patlak compartmental model fitting of the dynamic data, from which the kinetic indices of the radiotracer are derived and saved as parametric images.1,2 A whole-body dynamic scan with continuous bed motion utilizing FlowMotion technology commenced within 1 minute of the cardiac acquisition. A total of 19 whole-body passes were acquired (4 passes at 2 minutes per pass and 15 passes at 5 minutes). Total imaging time was 90 minutes. Images from whole-body passes 14-19 were obtained at 60-90 minutes post-injection and were summed for the standard uptake value (SUV) and Patlak image reconstructions.
Findings
Summed SUV images demonstrate a hypermetabolic lesion in the left pelvis. A co-registered CT localized the lesion to the superior aspect of the left ovary (Figure 1) and normal distribution of the tracer is seen elsewhere in the field of view. On images that reflect the metabolic rate of 18F FDG (MRFDG), the left ovarian lesion demonstrates prominent 18F FDG metabolism with a measurement of increased metabolic activity, a MRFDG value of 1.15 mg/min/100ml. Distribution volume (DV) images, which reflect the ratio of free non-metabolized 18F FDG in the tissue compared to that in blood, measured 84% in the left adnexal lesion. These findings show that the left ovarian lesion is metabolizing glucose at a rate greater than surrounding normal tissue (Figure 2).
Comments
Diagnostic 18F FDG PET/CT has become widely established for the initial staging, treatment monitoring, and disease surveillance in a wide variety of oncological indications. The quantitative nature of this modality makes it uniquely useful in the assessment of disease severity and treatment response, with the SUV reflecting tumor uptake of the radiotracer in a volume of tissue at a given time point following injection. This measure is actually only semiquantitative as the derivation of the SUV does not consider the concentration of the radiotracer in plasma and is dependent upon the patient’s metabolic status and the post-injection time at scanning.2,3 The SUV is also static reflecting tumor metabolism at only one time point. With dynamic PET imaging being seen as more meaningful, kinetic parameters can be measured, including lesion metabolism and lesional blood flow.3,4 Compelling literature over the last three decades have shown that for many cancer types, these indices allow for a more accurate characterization of individual tumor lesions.4,5,6
Furthermore, since a significant degree of heterogeneity can be observed in different lesions in one patient for a given cancer type, this additional quantitative information has the potential to be particularly useful in customizing treatment planning, improving the accuracy of assessing therapeutic response, and even in the development of more targeted oncology pharmaceuticals.7,8,9
The logistical challenges that parametric PET imaging pose make it time-consuming and thus not cost effective in the routine clinical setting. FlowMotion Multiparametric PET Suite overcomes many of these challenges by integrating certain features into its workflow. As opposed to the conventional method of obtaining multiple blood samplings for the input function, an image derived input function is used by acquiring a 6 minute scan centered over the heart using a cardiac-based region of interest (left ventricular cavity or aortic arch). Thereafter, whole-body, multi-pass imaging is performed and produces the image, from which SUV can be generated from the summation of the last 4-6 acquired images. The multi-pass technique is a significantly faster acquisition method than the conventional ‘step and shoot’ technique, which allows for a wholebody pass in one dynamic sweep. MRFDG and DV quantitative values are derived from a Patlak plot that is based on a two-compartmental model of radiotracer kinetics that is widely accepted for the most commonly used tracer 18F FDG as well as several others.10
The clinical application of the data generated from dynamic PET/CT imaging is becoming of increasing interest. Literature has demonstrated that the analysis of tumor kinetics can possibly differentiate normal tissue from malignant disease, differentiate histological subtypes of disease, differentiate malignant tumor from inflammatory processes, and more accurately measure treatment response.8,9,11,12,13,14,15 Although additional research is needed to further define the clinical significance of parametric imaging measures for various malignancies, data thus far demonstrates the potential of dynamic PET imaging to further characterize disease. The current case illustrates the ability of parametric imaging to identify increase lesional glucose metabolism in the face of normal lesional blood flow, which could reflect an inflammatory process, a physiologic process that requires an increase in energy substrate (such as ovulation) or a variety of malignancy that is not characterized by increased blood flow. Supplementary research is needed to further define this, and other combinations of parametric imaging findings for various physiological and pathological processes.
Conclusion
The true quantitative power of parametric imaging has the potential to further characterize malignant disease by evaluating the kinetic patterns of various radiotracers. As literature continues to validate the significance of dynamic PET imaging, interest beyond research
applications will likely continue to grow. The logistical challenges of this technique have largely confined it to the research arena. With FlowMotion Multiparametric PET Suite, this technology can be more broadly applied enabling the possibility of better disease delineation in routine clinical practice.
