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The ultrasound detector is the heart of a photoacoustic imaging system. In photoacoustic imaging of the breast there is a requirement to detect tumors located a few centimeters deep in tissue, where the light is heavily attenuated. Thus a sensitive ultrasound transducer is of crucial importance. As the frequency content of photoacoustic waves are inversely proportional to the dimensions of the absorbing structures, and in tissue can range from hundreds of kHz to tens of MHz, a broadband ultrasound transducer is required centered on an optimum frequency. A single element piezoelectric transducer structurally consists of the active piezoelectric material, front- and back-matching layers and a backing layer. To have both high sensitivity and broad bandwidth, the materials, their acoustic characteristics and their dimensions should be carefully chosen. In this paper, we present design considerations of an ultrasound transducer for imaging the breast such as the detector sensitivity and frequency response, which guides the selection of active material, matching layers and their geometries. We iterate between simulation of detector performance and experimental characterization of functional models to arrive at an optimized implementation. For computer simulation, we use 1D KLM and 3D finite-element based models. The optimized detector has a large-aperture possessing a center frequency of 1 MHz with fractional bandwidth of more than 80%. The measured minimum detectable pressure is 0.5 Pa, which is two orders of magnitude lower than the detector used in the Twente photoacoustic mammoscope. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Photoacoustic imaging is based on detecting laser pulse induced ultrasound transients from absorbing structures in tissue. The technique combines the advantages of a high optical absorption exhibited by tumors for example with the high resolution possible with ultrasound. A conventional photoacoustic imager operating in a CT geometry comprises a light source, an ultrasound detector array and a coupling medium usually water between the subject under investigation and detector. The imaging protocol usually consists of obtaining projections at angles around the subject. Recently we reported a method of measuring the local speed-of-sound variations by the addition of an absorber to the conventional imager, arranged between the light source and subject. This generates an ultrasound signal which interacts with the subject which can then be detected at the far-end by the detector. This allows the measurement of the integrated speed-of-sound (SOS) at projections around the subject leading to the formation of SOS images. In this article we discuss further analysis of the ultrasound obtained at projections to obtain images of the acoustic attenuation as well. In addition conventional photoacoustic images of the subject are also formed. The principle is demonstrated on well-characterized phantoms and we discuss the potential applications of this technique

We present PAM, the Photoacoustic Mammoscope developed at the University of Twente, intended for initial retrospective clinical studies on subjects with breast tumours. A parallel plate geometry has been adopted and the breast will be gently compressed between a glass plate and a flat ultrasound detector matrix. Pulsed light (5 ns) from an Nd:YAG laser will impinge the breast through the glass plate in regions of interest; an appropriate number of the 590 elements of the detector matrix will be activated in succession to record photoacoustic signals. Three-dimensional image reconstruction employs a delay-and-sum beamforming algorithm. We discuss various instrumental aspects and the proposed imaging protocol. Performance studies of the ultrasound detector are presented in terms of sensitivity, frequency response and resolution. Details of the patient–instrument interface are provided. Finally some imaging results on well-characterized breast tissue phantoms with embedded tumour simulating inserts are shown.

A large surface area transducer is preferable to be used to detect extremely weak photoacoustic signals in mammography due to its high sensitivity. The lateral resolution is limited by the small acceptance angle of such a transducer. We introduce an excellent material for an acoustic lens used to enlarge the transducer's acceptance angle. Our acoustic characterizations showed that this material has tissue-like acoustic impedance, large speed of sound and low acoustic attenuation. These acoustic properties ensures an excellent acoustic lens material. We further investigated the acoustic irradiation pattern of a 1 MHz, 5 mm x 5 mm single element transducer. Transducer irradiation pattern with and without acoustic lens made from our proposed material and common used lens material are simulated using the Field II program and also the k-wave package. Good agreement has been achieved comparing the simulation results from two different methods. Both simulations show that the proposed material not only enlarged the acceptance angle of the transducer but also minimized the signal loss compared to the common used lens material. We conclude that the proposed material can be used as an excellent acoustic lens for photoacoustic tomography.