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Multiple previous studies have concluded that a large incidence angle should be avoided during transcranial\u2010focused ultrasound therapy to ensure transmission through the skull. Alternatively, some other studies have shown that longitudinal\u2010to\u2010shear wave mode conversion might improve transmission through the skull when the incidence angle is increased above the critical angle (i.e., 25\u00b0 to 30\u00b0).<\/jats:p><\/jats:sec><jats:sec><jats:title>Purpose<\/jats:title><jats:p>The effect of skull porosity on the transmission of ultrasound through the skull at varying incidence angles was investigated for the first time to elucidate why transmission through the skull at large angles of incidence is decreased in some cases but improved in other cases.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>Transcranial ultrasound transmission at varying incidence angles (0\u00b0\u201350\u00b0) was investigated in phantoms and ex vivo skull samples with varying bone porosity (0% to 28.54%\u00a0\u00b1\u00a03.36%) using both numerical and experimental methods. First, the elastic acoustic wave transmission through the skull was simulated using micro\u2010computed tomography data of ex vivo skull samples. The trans\u2010skull pressure was compared between skull segments having three levels of porosity, that is, low porosity (2.65%\u00a0\u00b1\u00a00.03%), medium porosity (13.41%\u00a0\u00b1\u00a00.12%), and high porosity (26.9%). Next, transmission through two 3D\u2010printed resin skull phantoms (compact vs. porous phantoms) was experimentally measured to test the effect of porous microstructure alone on ultrasound transmission through flat plates. Finally, the effect of skull porosity on ultrasound transmission was investigated experimentally by comparing transmission through two ex vivo human skull segments having similar thicknesses but different porosities (13.78%\u00a0\u00b1\u00a02.05% vs. 28.54%\u00a0\u00b1\u00a03.36%).<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>Numerical simulations indicated that an increase in transmission pressure occurs at large incidence angles for skull segments having low porosities but not for those with high porosity. In experimental studies, a similar phenomenon was observed. Specifically, for the low porosity skull sample (13.78%\u00a0\u00b1\u00a02.05%), the normalized pressure was 0.25 when the incidence angle increased to 35\u00b0. However, for the high porosity sample (28.54%\u00a0\u00b1\u00a03.36%), the pressure was no more than 0.1 at large incidence angles.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusions<\/jats:title><jats:p>These results indicate that the skull porosity has an evident effect on the transmission of ultrasound at large incidence angles. The wave mode conversion at large, oblique incidence angles could enhance the transmission of ultrasound through parts of the skull having lower porosity in the trabecular layer. However, for transcranial ultrasound therapy in the presence of highly porous trabecular bone, transmission at a normal incidence angle is preferable relative to oblique incidence angles due to the higher transmission efficiency.<\/jats:p><\/jats:sec>","DOI":"10.1002\/mp.16318","type":"journal-article","created":{"date-parts":[[2023,2,22]],"date-time":"2023-02-22T12:38:57Z","timestamp":1677069537000},"page":"3092-3102","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":17,"title":["Effect of skull porosity on ultrasound transmission and wave mode conversion at large incidence angles"],"prefix":"10.1002","volume":"50","author":[{"given":"Bowen","family":"Jing","sequence":"first","affiliation":[{"name":"Wallace H Coulter Department of Biomedical Engineering Georgia Institute of Technology and Emory University  Atlanta Georgia USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Stephan","family":"Strassle Rojas","sequence":"additional","affiliation":[{"name":"School of Electrical and Computer Engineering Georgia Institute of Technology  Atlanta Georgia USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Brooks D.","family":"Lindsey","sequence":"additional","affiliation":[{"name":"Wallace H Coulter Department of Biomedical Engineering Georgia Institute of Technology and Emory University  Atlanta Georgia USA"},{"name":"School of Electrical and Computer Engineering Georgia Institute of Technology  Atlanta Georgia USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2023,3]]},"reference":[{"key":"e_1_2_8_2_1","unstructured":"U.S. Food & Drug Administration. 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