Endocavity ultrasound and photoacoustic system for fetal and maternal imaging: design, implementation, and ex-vivo validation

Abstract

<b>Purpose:</b> Transvaginal ultrasound (TVUS) is a widely used real-time and non-invasive imaging technique for fetal and maternal care. It can provide structural and functional measurements about the fetal brain, such as blood vessel diameter and blood flow. However, it lacks certain biochemical estimations, such as hemoglobin oxygen saturation ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>SO</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> ), which limits its ability to indicate a fetus at risk of birth asphyxia. Photoacoustic (PA) imaging has been steadily growing in recognition as a complement to ultrasound (US). Studies have shown PA imaging is capable of providing such biochemical estimations as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>SO</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> at relatively high penetration depth (up to 30 mm). <b>Approach:</b> In this study, we have designed and developed a multi-modal (US, PA, and Doppler) endocavity imaging system (ECUSPA) around a commercialized TVUS probe (Philips ATL C9-5). <b>Results:</b> The integrated system was evaluated through a set of <i>in-vitro</i>, <i>ex-vivo</i>, and <i>in-vivo</i> studies. Imaging of excised sheep brain tissue demonstrated the system's utility and penetration depth in transfontanelle imaging conditions. The accuracy of using the spectroscopic PA imaging (sPA) method to estimate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>SO</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> was validated by comparing sPA oximetry results with the gold standard measurements indicated by a blood gas analyzer. The ability of US and Doppler to measure moving blood volume was evaluated <i>in-vivo</i>. Spectral unmixing capabilities were tested using fluorophores within sheep brains. <b>Conclusion:</b> The developed system is a high resolution (about <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>200</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>μ</mml:mi> <mml:mi>m</mml:mi></mml:mrow> </mml:math> at 30 mm depth), real-time (at 30 Hz), and quantitative ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>SO</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> estimation error <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo><</mml:mo> <mml:mn>10</mml:mn> <mml:mo>%</mml:mo></mml:mrow> </mml:math> ) imaging tool with a total diameter less than 30 mm, making it suitable for intrapartum applications such as fetal and maternal diagnostics.