## 2 Materials and Methods
### Imaging Acquisition and Reconstruction Parameters  
All internal PET-CT studies were performed as part of routine clinical care using standardised departmental protocols. Imaging was acquired from skull base to upper thighs. No iodinated contrast media was administered. Serum blood glucose was routinely measured prior to imaging and if \(>\)10 mmol/L scanning was not performed. Patients fasted for six hours prior to intravenous injection of 4 MBq/kg of fluorine-18 [18F]FDG.  
Internal imaging prior to June 2010 was performed using a 16-slice Discovery STE PET/CT scanner (General Electric (GE) Healthcare, Chicago, IL, USA) and from June 2010 to October 2015 on a 64-slice Philips Gemini TF64 scanner (Philips Healthcare, Best, Netherlands) or a Discovery 690 scanner (GE Healthcare, Chicago, IL, USA). After October 2015 a Discovery 710 scanner (GE Healthcare, Chicago, IL, USA) replaced the Philips scanner. All machines used iterative reconstruction, computed tomography (CT) for attenuation correction, applied scatter and randoms correction. Image reconstruction and acquisition parameters are outlined in Table 1.  
To ensure alignment between PET and CT images, standard clinical practices at our institution employed careful patient positioning and instructions to breath hold to minimise motion artefact. Whilst mis-registration is not infrequently encountered in some body parts such as within the head and neck or lower thoracic regions; we rarely experienced significant mis-registration between the PET and CT components of the abdominal aorta being analysed. However, if this was an issue, the data were excluded from further analysis to avoid inaccuracies. Furthermore, all examinations were reviewed by radiologists with a minimum of 3 years of experience under the supervision of a dual-certified radiologist/nuclear medicine physician with over 15 years of experience in PET-CT.  
Average AAA growth rates were calculated from the first and last antero-posterior ultrasound measurements obtained from the AAA surveillance programme. Scan intervals  
\begin{table}
\begin{tabular}{l l c c c c} \hline \hline \multicolumn{1}{c}{**Dates**} & \multicolumn{1}{c}{**Scanner**} & \multicolumn{1}{c}{**Reconstruction**} & \multicolumn{1}{c}{**Scatter**} & \multicolumn{1}{c}{**Randoms**} & \multicolumn{1}{c}{**Randoms**} & \multicolumn{1}{c}{**Matrix**} & \multicolumn{1}{c}{**Voxel Size (x, y, z)**} \\ \hline January 2009-May 2010 & GE Healthcare STE & OSEM 1 & Convolution subtraction & Singles & 128 & 4.6875 \(\times\) 4.6875 \(\times\) 3.27 \\ June 2010-October 2015 & Philips Gemini TF64 & BLOB-OS-TF 2 & SS-SIMUL 4 & DIXD 5 & 144 or 169 & 4 \(\times\) 4 \(\times\) 4 \\ June 2010-March 2020 & GE Healthcare & VPFX 3 & Model based & Singles & 192 & 3.65 \(\times\) 3.65 \(\times\) 3.27 \\ May 2015-March 2020 & GE Healthcare & VPFX 710 & Model based & Singles & 192 & 3.65 \(\times\) 3.65 \(\times\) 3.27 \\ \hline \hline \end{tabular}  
* OSEM: Ordered Subset Expectation Maximisation, 2 BLOB-OS-TF: Spherical Symmetric Basis Functions Ordered Subject Time of Flight, 3 VPFX: Time of Flight Ordered Subset Expectation Maximisation, 4 SS-SIMUL: CT-based attenuation correction and scatter correction, and 5 DLYD: delayed event subtraction.  
\end{table}
Table 1: Breakdown of PET-CT Scanners Used and Associated Timeframes.  
were dictated by AAA size as per national guidelines [10]. All scans were performed by experienced sonographers or radiologists.