Long-Axial Field of View (LAFOV) [18F]FDG PET/CT Imaging in Large Vessel Vasculitis (LVV): Protocol Optimisation Study.

Large vessel vasculitis (LVV) is an autoimmune inflammatory disorder affecting the major arteries of the body. The diagnosis and monitoring this condition can be challenging, as patients often present with symptoms that are unclear, and the diagnostic criteria currently used are varied. Accurate diagnosis is essential because it helps to tailor the treatment to each patient. Some treatments, such as steroids and immunosuppressive medications, can have significant side effects, so they need to be used carefully and only when truly needed.

An \[18F\]FDG Positron Emission Tomography/Computed Tomography (PET/CT) involves the injection of a small amount of radiolabelled sugar, which assesses glucose metabolism within the body. \[18F\]FDG PET/CT is already used by the NHS to detect inflammation within the vessel walls and diagnose LVV. Current diagnostic criteria for LVV largely rely on visual assessment by a radiologist. This approach therefore has limitations and may not fully capture changes in the levels of inflammation over time.

A new generation of scanners, known as Long Axial Field-of-View (LAFOV)-PET/CT or Total Body PET, are currently transforming what is possible in the field of medical imaging. These LAFOV-PET/CT scanners have new digital detectors that are more sensitive, produce sharper images, and can scan the entire body rapidly. This offers several potential advantages for patients with LVV, including the clearer detection of vessel wall inflammation, the ability to administer lower doses of the radiolabelled sugar (\[18F\]FDG), and the ability to take repeated pictures over time to measure subtle changes in blood flow and inflammation. Patients receiving a routine NHS \[18F\]FDG PET/CT scan for LVV are typically scanned 60 minutes after injection of the radiotracer using a standard PET/CT. Another benefit of LAFOV-PET/CT scanners is their ability to assess the amount of the radiolabelled sugar taken up by the whole body almost as soon as it is injected which could give important additional information.

As part of the LAVA-FLOW study we are planning to combine LAFOV-PET/CT with a CT Angiogram (CTA) in patients with LVV. A CTA is a scan that shows doctors what your blood vessels look like. It involves the injection of a dye into a vein that makes your blood vessels visible, highlighting vessel wall inflammation and vessel wall narrowing. This combination of LAFOV-PET/CT and CTA therefore has the potential to give a much more detailed picture of LVV than is achievable using current methods.

The LAVA-FLOW study therefore aims to develop a standardised protocol to be used in different hospital centres across the UK for the imaging of LVV using LAFOV-PET/CT. We also hope that the results of this particular study could lead to further larger studies with the potential to update the diagnostic criteria and improve the monitoring of this condition.