\begin{table}[htbp]
\centering
\caption{PET scans in various categories. Several different scans correspond to the same patients in different periods of the ADNI's longitudinal study, therefore there are more scans than subjects.}
\label{tab: scan statistics}
\begin{tabular}{lcccc}
\\
\toprule

    \textbf{Group} & AD & MCI & NC & All   \\
    \midrule
    \textbf{No. of scans} & 314 & 714 & 365 & 1393   \\
    \textbf{Proportion of total (\%)}& 22.5 & 51.2 & 26.3 & 100   \\
    
    \textbf{Proportion of scans with fixations (\%)} & 4.2 & 4.2 & 4.2 & 12.6    \\

    
\bottomrule
\end{tabular}
\end{table}