A microelectromechanical systems (MEMS)-based photoacoustic imaging system is reported for the first time. In this system, the MEMS-based light scanning subsystem and a ring-shaped polyvinylidene fluoride (PVDF) transducer are integrated into a miniaturized probe that is capable of three-dimensional (3D) photoacoustic imaging. It is demonstrated that the imaging system is able to image small objects embedded in phantom materials and in chicken and to in vivo visualize blood vessels under the skin of a human hand.

Materials for solid photoacoustic breast phantoms, based on poly(vinyl alcohol) hydrogels, are presented. Phantoms intended for use in photoacoustics must possess both optical and acoustic properties of tissue. To realize the optical properties of tissue, one approach was to optimize the number of freezing and thawing cycles of aqueous poly(vinyl alcohol) solutions, a procedure which increases the turbidity of the gel while rigidifying it. The second approach concentrated on forming a clear matrix of the rigid poly(vinyl alcohol) gel without any scattering, so that appropriate amounts of optical scatterers could be added at the time of formation, to tune the optical properties as per requirement. The relevant optical and acoustic properties of such samples were measured to be close to the average properties of human breast tissue. Tumour simulating gel samples of suitable absorption coefficient were created by adding appropriate quantities of dye at the time of formation; the samples were then cut into spheres. A breast phantom embedded with such 'tumours' was developed for studying the applicability of photoacoustics in mammography.

We present a laboratory version of a photoacoustic mammoscope, based on a parallel plate geometry. The instrument is built around a flat high-density ultrasound detector matrix. The light source is a Q-switched Nd:YAG laser with a pulse duration of 5 ns. To test the instrument, a novel photoacoustic phantom is developed using poly(vinyl alcohol) gel, prepared by a simple procedure that imparts optical scattering suggestive of breast tissue to it without the requirement for extraneous scattering particles. Tumor simulating poly(vinyl alcohol) gel spheres appropriately dyed at the time of preparation are characterized for optical absorption coefficients. These are then embedded in the phantom to serve as tumors with absorption contrasts ranging from 2 to 7, with respect to the background. Photoacoustic studies in transmission mode are performed, by acquiring the laser-induced ultrasound signals from regions of interest in the phantom. Image reconstruction is based on a delay-and-sum beamforming algorithm. The results of these studies provide an insight into the capabilities of the prototype. Various recommendations that will guide the evolving of our laboratory prototype into a clinical version are also discussed.

A novel photoacoustic breast phantom was developed using poly(vinyl alcohol) gel prepared by a simple technique that imparts optical scattering to the gel without the neccessity for scattering particles. Tumour simulating gel samples of suitable absorption coefficient were also prepared using a second technique, by adding appropriate quantities of dye at the time of formation; the samples were then cut into spheres. The optical absorption coefficient of the spheres was chosen as between 4 -7 times that of breast tissue. A breast phantom with a thickness of 60 mm, embedded with such 'tumours' was developed for studying the applicability of photoacoustics in mammography. Time resolved photoacoustics, in a transmission mode, was used to image the inhomogeneities. Light excitation was from a liquid-light guide coupled Nd:YAG laser at 1064 nm, with a 5 ns pulse duration. The guide was mechanically scanned across the surface of the phantom, with the time-of-flight signals recorded using a PVDF based detector array. A modified delay and sum beamforming algorithm was used to reconstruct the photoacoustic sources. Results of these experiments are discussed.

Performance studies of a clinical prototype for detecting tumors in the breast based on the photoacoustic effect are presented in terms of sensitivity, frequency response and resolution. Some imaging results on well characterized breast tissue phantoms with embedded tumor simulating inserts are also shown.

A laboratory prototype of a time-resolved photoacoustic mammograph, based on a parallel plate geometry is presented. Light is delivered from a Q-switched Nd:YAG laser using fibre-optic bundles which can be mechanically scanned across the surface of a phantom. The ultrasound signals produced by the photoacoustic effect are measured in a transmission mode, using a large-area ultrasound detector matrix. Signals from the matrix are acquired using fast digitizers. Various performance studies of the system are presented. A breast phantom of dimensions (150x120x60)mm was created based on poly(vinyl alcohol) (PVA) gel, which can be imparted with the average optical scattering properties of breast tissue by a simple process of freezing and thawing of an aqueous poly(vinyl alcohol) solution. The acoustic properties are also found to match those of breast tissue. Such a photoacoustic breast phantom was embedded with several tumour-simulating inhomogeneities. These inserts were also based on poly(vinyl alcohol) gels, appropriately dyed at the time of formation, to possess various optical absorption coefficients, between 2 and 7 times that of the background. Using the signals collected from regions-of-interest (ROT) in the volume of the phantom, three-dimensional images were obtained using a modified delay-and-sum beamforming algorithm. The results indicate that photoacoustics, as embodied in this instrument, has a potential for detecting tumours in the breast.

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