Annular array for photoacoustic and ultrasonic imaging

Absorption of short laser pulses in a material gives rise to broadband ultrasound waves that are suitable for high resolution imaging. Two imaging modes are possible depending on whether the ultrasound is generated at the object surface or within the object. The former mode is usually called the laser ultrasound technique and yields images due to reflection and scattering of the wave at structures with acoustic mismatch to their surroundings. The latter mode is called photoacoustic (or optoacoustic) imaging and is based on optical contrast because of the proportionality between optical absorption and generated pressure. In many materials the imaging information obtained with the two methods is complementary. The general objective of the proposed project is therefore to combine the two imaging modes in one single device, which will ideally generate two exactly overlapping images of a single object, one showing structures with acoustic and the other one with optical contrast. This will be achieved by constructing a single transducer that is capable of either generating an ultrasound field by laser irradiation of an absorbing target or passing the incident laser pulse directly to the sample where it is absorbed and generates a photoacoustic wave. In both cases the received ultrasound wave is measured with a broadband ultrasound receiver. Both, the target and the detector, are designed following the principles of limited diffraction beam generation for optimal lateral resolution in a large depth range. The former by using a conical shape ("axicon") and the latter by building an array detector consisting of concentric rings. The strategy to reach the objective are to: Design an annular array consisting of concentric, either piezoelectric or optical ring detectors. Investigate signal processing and image reconstruction techniques for simultaneous generation of ultrasound and photoacoustic images from signals acquired with the array. Design, assemble and characterize the dual mode imaging device, including the array detector, an optically absorbing target for laser-induced generation of ultrasound waves, illumination optics for the target and the sample and a scanning stage with computer control. Prove the applicability of this device for large depth of field imaging of phantoms and in biological tissue. The possibility of imaging in a large depth range with approximately constant lateral resolution is expected to have high impact in various medical applications. A prominent example is the visualization of structures like lymph nodes, which plays an important role in the diagnostics of spreading of malignant tumours.

Absorption of short laser pulses in a material gives rise to broadband ultrasound waves that are suitable for high resolution imaging. Two imaging modes are possible depending on whether the ultrasound is generated at the object surface or within the object. The former mode is usually called the laser ultrasound technique and yields images due to reflection of the wave at structures with acoustic mismatch to their surroundings. The latter mode is called photoacoustic imaging and is based on optical contrast because of the proportionality between optical absorption and generated pressure.The general objective of the project was to design an imaging method for simultaneous ultrasound and photoacoustic imaging, using an array detector consisting of concentric rings, and to prove the applicability of this device for large depth of field imaging. Therefore the following steps were performed:Design of a ring detector array using fiber optic interferometers bent to circles.Design of a piezoelectric annual detector array.Develop methods for signal processing and synthetic aperture focusing techniques as well as deconvolution procedures for reduction of imaging artifacts.Design and testing of a scanning microscope based on the detector, ultrasound generation and signal processing technologies investigated in the previous steps.With approximately 18 scientific publications the scientific output and the fruitful cooperation within this project of the partners from Graz and Linz was the cornerstone for a follow-up project, which has already a 10% participation of a company (radiologist) and is just submitted as a Bridge project, category early stage programme, at the FFG (Austrian research promotion agency).