Ultrasonic　tomography　(UT)　is　a　non-invasive　multiphase　flow　detection　technique.　In　ultrasonic　transmission　tomography,　reconstruction　images　are　derived　from　a　direct　wave　signal　transmitted　through　the　materials　in　the　sensing　area.　The　maximum　values　of　the　received　direct　wave　from　the　reference　pattern　and　the　measured　pattern　are　normally　used　to　reconstruct　the　images.　The　speed　of　sound　in　the　pipe　wall　and　the　materials　in　the　sensing　area　affect　the　received　signal.　This　in　turn　affects　image　quality-especially　when　the　arrival　time　of　the　direct　wave　propagated　through　the　sensing　area　is　close　to　the　arrival　time　of　the　direct　wave　propagated　through　the　pipe　wall.　Here,　we　describe　a　novel　shielding　structure　for　ultrasonic　transmission　tomography.　The　relative　variation　ratio　(RVR)　is　defined　and　used　to　evaluate　the　validity　of　the　shielding　structure.　Larger　RVR　values　imply　better　signal　resolution　and　better　signal　suppression.　A　simplified　mode　is　discussed　including　the　influence　of　the　shielding　structure＇s　design　parameters　on　the　received　direct　wave.　The　optimized　shielding　structure　size　was　determined.　A　comparison　of　the　results　between　the　normal　pipe　and　the　shielding　pipe　indicated　that　the　RVR　increased　from　57.25　to　91.27%.　This　proves　that　the　shielding　structure　can　suppress　the　effect　of　the　signal　from　the　pipe　wall　on　received　direct　wave　signals.