Photoacoustic imaging to guide needle injections

Metal needles are commonly used for drug delivery or biopsy collection in clinical settings. Needle deflection and deformation can occur when inserting needles into soft, non-homogeneous tissues which can affect the location accuracy of insertion. Therefore, the ability to visualize both anatomical surrounding structures and the advancing needle is required. Ultrasound is commonly used for image-guidance of needles; however, specular reflections from the metal surface can deflect the acoustic beam away from the transducer when the needle is even slightly angled from the US transducer thereby rendering the needle invisible in the image. Photoacoustic imaging has been proposed for guidance of metal needles and other metal objects in-vivo. The high optical absorption coefficient of stainless steel can provide high photoacoustic imaging contrast. The photoacoustic signal is produced omni-directionally from the metal surface allowing for greater detection of needles at increasing injection angles compared to ultrasound imaging. In the current work, needles were inserted into excised tissue and imaged using an ultrasound array transducer and a pulsed 800 nm laser. The results showed that at a shallow 10° insertion angle, the photoacoustic ratio of needle signal to background was four-times higher compared to ultrasound. Furthermore, the surrounding tissue composition was observed to have an effect on photoacoustic signal enhancement which correlated with the change of the Grüneisen coefficient of the surrounding tissue environment, suggesting that the photoacoustic signal amplitude could be used to ascertain surrounding tissue composition. Photoacoustic imaging provides sufficient depth penetration for this application and offers excellent image contrast.