\begin{figure}
\begin{minipage}[t]{0.5\linewidth}
\centering
\includegraphics[width=2.2in]{figure/angled_k_space1}
\caption{fig1}
\label{a}
\end{minipage}
\begin{minipage}[t]{0.5\linewidth}
\centering
\includegraphics[width=2.2in]{figure/angled_k_space2}
\caption{fig2}
\label{b}
\end{minipage}
\end{figure}

\begin{figure}[htbp]
\centering
\subfloat[456]{
\label{fig:improved_subfig_a}
\begin{minipage}[t]{0.5\linewidth}
\centering
\includegraphics[width=2.2in]{figure/angled_k_space1}
\end{minipage}
}
\subfloat[123]{
\label{fig:improved_subfig_b}
\begin{minipage}[t]{0.5\linewidth}
\centering
\includegraphics[width=2.2in]{figure/angled_k_space1}
\end{minipage}
}
\caption{}
\end{figure}