Passive element enriched photoacoustic computed tomography (PER PACT) for simultaneous imaging of acoustic propagation properties and light absorption

Abstract

We present a "hybrid" imaging approach which can image both light absorption properties and acoustic transmission properties of an object in a two-dimensional slice using a computed tomography (CT) photoacoustic imager. The ultrasound transmission measurement method uses a strong optical absorber of small cross-section placed in the path of the light illuminating the sample. This absorber, which we call a passive element acts as a source of ultrasound. The interaction of ultrasound with the sample can be measured in transmission, using the same ultrasound detector used for photoacoustics. Such measurements are made at various angles around the sample in a CT approach. Images of the ultrasound propagation parameters, attenuation and speed of sound, can be reconstructed by inversion of a measurement model. We validate the method on specially designed phantoms and biological specimens. The obtained images are quantitative in terms of the shape, size, location, and acoustic properties of the examined heterogeneities.

Hybrid photoacoustic and ultrasound section imaging with optical ultrasound detection

“…Laser-ultrasound (LUS) has been widely used in non destructive testing of various non-biological samples [13]. In biomedical imaging, LUS has been proposed for speed of sound and acoustic attenuation imaging using acoustic waves generated at a point like absorber and transmitted through a sample [14]. The measured quantity in this application is the time of flight.…”

Multiple passive element enriched photoacoustic computed tomography

“…Recently, we presented a method that in addition to conventional PA imaging, permits a simultaneous imaging of ultrasound transmission parameters [4,5]. These parameters, speed of sound (SOS) and acoustic attenuation (AA), constitute additional tissue characterization [6] providing anatomic information in the context of which the functional information from PA can be visualized. Further, the availability of an SOS map can facilitate a correct reconstruction of the PA image using actual local SOS values using acoustic backprojection [7].…”

“…Acquiring a multitude of such projection views around the object applying the appropriate image reconstruction algorithms [7] permits the development of two-dimensional slices of the SOS and AA distribution in the object. Simultaneously, PA signals from the object are also recorded by the detector array, since most of the light passes on to illuminate the object, allowing PA tomograms of the object to be reconstructed [6].…”

“…The factor ν is the Nyquist frequency related to the lowest density of spacing (a) between the "rays" (M) in each view. For the experimental setup used [6], the number of rays is the number of elements in the detector array, thus M ¼ 32. Further, R ¼ 15 mm and ζ ¼ 40°, which yields a value for N min as 80.…”

Optimizing ultrasound detection for sensitive 3D photoacoustic breast tomography

“…This can be obtained in the clinical version by placing a passive element (absorbing sphere) in front of each fiber bundle output to generate ultrasound pulses [56,58]. The method permits the co-registration of SOS, acoustic attenuation (AA) and optical absorption in a PA imager without the addition of extra active ultrasound generating elements [57].…”