Whole-body parametric imaging expands clinical opportunities

Aarhus University Hospital in Denmark validates the clinical potential value of routine whole-body parametric imaging with PET/CT.

Photography by Gorm Branderup
Data courtesy of Aarhus University Hospital, Denmark

PET imaging with fludeoxyglucose F 18 (FDG) is an invaluable diagnostic tool that aids clinicians in making accurate diagnoses and evaluating treatment paths. As the current clinical standard, PET imaging utilizes a static image of the standardized uptake value (SUV), a semi-quantitative measurement of FDG uptake in tissue. With advances in PET/computed tomography (CT) technology hardware and software that make dynamic imaging acquisition achievable in routine clinical use, clinicians now have the ability to measure the true metabolic rate of tracer uptake (MRFDG). Parametric whole-body PET/CT imaging is an exciting new frontier in molecular imaging today.

Enabled on Siemens Healthineers Biograph Vision™ PET/CT scanner, whole body parametric PET imaging is being utilized around the globe across a variety of clinical areas. Siemens Healthineers’ customers are discussing and sharing the value it can bring to clinicians and identifying which patients might likely benefit.

Whole-body parametric PET/CT imaging can be utilized by clinicians for routine care because the acquisition parameters can be quickly assimilated into routine protocols with minimal additional scan time and without a steep learning curve. Clinician researchers at Aarhus University Hospital in Denmark are working on several new research projects using parametric imaging.

“This is the first time we can do whole-body parametric imaging” explains Ole Munk, physicist in the Department of Nuclear Medicine and PET-Center in Aarhus. “So, this makes it possible to investigate new areas in oncology, where before dynamic imaging was limited to a single field of view.”

One of the early users of this technology from Siemens Healthineers, the Aarhus team actively provides feedback on its usability and performance. They were able to acquire hundreds of whole-body FDG patient datasets using the new protocol in a relatively short time span using their three Biograph Vision™ 600 PET/CT scanners. To build these datasets, the team added the parametric imaging protocol to the first patient scan of the day on each system over a period of several months.¹

One of the reasons parametric imaging has so much potential for clinical use is due to its ability to differentiate between the actual tissue tracer uptake and “free” tracer in the blood pool. Thus, a typical image acquisition of parametric PET/CT using Biograph Vision scanner will produce three different images: the MRFDG, which represents the real uptake of tracer into the cells, the distribution volume image (DVFDG) that shows the free component of FDG uptake in the background, and the third image is the standard SUV static image. Understanding the different quantifications in all three images is key to understanding how they can be successfully applied in certain oncology cases, for example.

“In certain cases where we may be in doubt as to whether an FDG-avid lesion on a static scan actually represents a true positive,” says Lars Gormsen, MD, nuclear medicine physician, consultant and clinical professor at the Department of Nuclear Medicine and PET-Center in Aarhus, “we may sometimes look into the quantitative MRFDG image and get a better idea. The point is that we’re adding information from the MRFDG image to what we get from the static SUV image. At present, we don’t see MRFDG as something that will replace static PET scans.”

Frequently on SUV, there is tracer uptake in lesions that would look like pathological uptake, but this uptake is actually a false positive finding. “We’ve seen examples in lymphoma cases after treatment,” Gormsen continues, “where there might be a single lymph node still showing uptake, that sometimes will require you do consolidative radiotherapy on residual disease. We actually have an example of a patient where we could say that this is not residual lymphoma that should have consolidated radiotherapy because we could see it was “free” tracer. It was a clear false positive on the static FDG PET that was rooted out by looking at the MRFDG image.”

The Aarhus team reported that one of the things they are evaluating is the ability to measure tumor to background ratios in a different way. In theory, lesions that occur in very vascularized organs, such as the liver, will be much easier to see on the MRFDG image due to the suppression of background signal. Patients with high blood sugar also are known to have altered background ratios because of the amount of glucose that is already in their system. It increases the background signal and reduces the signal in the lesions, which can impair the evaluation of lesions.

“On the MRFDG image, the measured level of tracer uptake in lesions remains the same, even with increased blood sugar as we are looking at the effective tracer internalization” reports André Dias, MD, nuclear medicine physician and staff specialist at Aarhus University Hospital. “This is previously known but nicely validated by the MRFDG images where we can see it very clearly—we’re getting to visualize the uptake in a more precise fashion.”

One of the barriers to whole-body parametric PET imaging in routine clinical use has been a complicated clinical workflow and long acquisition time. The development of new-generation PET/CT scanners like the Biograph Vision with an extended field of view, as well as more sophisticated evaluation software packages that offer automatic retrieval of the image derived arterial input function and automatic calculation of parametric imaging, in combination with dedicated shorter dynamic protocols, removes these barriers and can facilitate the wider use of whole-body parametric PET imaging. The team at Aarhus University also worked on creating a population-based input function to allow for the reduction of scan time but also to create a standard for industry-wide use. In collaboration with the Siemens Healthineers development team, the Aarhus team created a mathematical model to calculate the input function.

“The population-based input function works very well,” explains Munk. “The original acquisition time of 70 minutes is now reduced to just 20 minutes, which is quite comparable to a standard clinical scan,” Munk explains. “We take this population-based input function, which is standard shape, and we scale it. We scale it to the values that we derived from the last four passes [on the scanner]. So, this means that it’s not the exact same input function for every patient, but it’s actually individualized to each patient.”

The Aarhus team has already published their clinical findings and typical MRFDG values for various tissue types.² They plan to continue research in this area, focusing on comparison studies, clinical applications, and validation studies of whole-body parametric PET imaging techniques for evaluating treatment response and kinetic modeling. 

“PET imaging is really just maturing now,” says Gormsen, “so we are refining the technique. We have spent the past 15 years looking at static PET images, and I think that we are ready to look into the finer details, which is really the kinetic analysis.”