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

Systemic vasculitidies are autoimmune diseases which cause inflammation of blood vessels and are categorised according to the size of blood vessels they predominantly involve. The main forms of LVV are giant cell arteritis (GCA) and Takayasu arteritis (TA).

Various imaging modalities are available for the diagnosis of large vessel vasculitis (LVV): ultrasound, magnetic resonance angiography (MRA), computed tomography angiography (CTA) and [18F]fluorodeoxyglucose ([18F]FDG) PET/CT.

Whilst [18F]FDG PET/CT is established in the diagnostic work up of LVV, the relatively low spatial resolution, radiation exposure, and long imaging times have limited its use especially for assessing smaller calibre vessels and coronary arteries.

Digital long-axial field of view (LAFOV)-PET/CT systems are expected to significantly improve the diagnosis and management of LVV, as their increased sensitivity allows for improved detection of the involvement of smaller calibre vessels and low-grade inflammation, even at shorter scan times and/or reduced tracer doses.

LAFOV-PET/CT has the potential to perform dynamic whole-body imaging from the time of radiotracer injection for detailed analysis of [18F]FDG uptake. Additionally, there is the opportunity to acquire LAFOV-PET/CT images and combine this approach with CT angiogram imaging of the heart and all large vessels within one sitting. At present, determining the involvement of small vessels currently relies on the treating physician requesting a CT angiogram (CTA) in addition to other imaging investigations at diagnosis, which is not currently common practice. A combined protocol of [18F]FDG LAFOV-PET/CT and CTA performed that could be performed at diagnosis and during follow-up may allow the burden of coronary artery disease in this cohort to be fully established and to better define its natural history on treatment. For follow-up response cases, particularly in TA, the potential for using lower radiation does with LAFOV-PET/CT is important to further develop the use of PET/CT in these cohort for surveillance monitoring.

The current European Association of Nuclear Medicine (EANM) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI) guidelines for assessment of LVV on [18F]FDG PET/CT recommend visual interpretation of static images using a 4-point visual scale. Although visual analysis of static [18F]FDG PET/CT data is currently clinically recommended for LVV cases. this has limitations. The improved spatial resolution of LAFOV-PET/CT could lead to higher vessel identification through the use of semi-quantitative analysis methods.

The improved spatial resolution of LAFOV-PET/CT could lead to higher vessel identification and the potential misdiagnosis of LVV in normal vessels compared to standard PET/CT. We will therefore need to consider the use of different thresholds when using LAFOV-PET/CT.

In addition to collecting data from patients diagnosed with LVV, data from healthy volunteers (HV) will help us to establish normal background vessel activity on LAFOV-PET/CT. The HV cohort shall have MR angiography (MRA) instead of CTA to keep radiation doses as low as possible.

This study will establish a protocol for LAFOV-PET/CT imaging and provide pilot data for a larger study, which may lead to changing the current visual reporting criteria for LVV by establishing new reference ranges for disease activity. LAVA-FLOW aims to establish an [18F]FDG LAFOV-PET/CT protocol utilising both dynamic and static imaging in combination with angiography.