Jens Horstmann

Full-field speckle interferometry for non-contact photoacoustic tomography

A full-field speckle interferometry method for non-contact and prospectively high speed Photoacoustic Tomography is introduced and evaluated as proof of concept. Thermoelastic pressure induced changes of the objects topography are acquired in a repetitive mode without any physical contact to the object. In order to obtain high acquisition speed, the object surface is illuminated by laser pulses and imaged onto a high speed camera chip. In a repetitive triple pulse mode, surface displacements can be acquired with nanometre sensitivity and an adjustable sampling rate of e.g. 20 MHz with a total acquisition time far below one second using kHz repetition rate lasers. Due to recurring interferometric referencing, the method is insensitive to thermal drift of the object due to previous pulses or other motion. The size of the investigated area and the spatial and temporal resolution of the detection are scalable. In this study, the approach is validated by measuring a silicone phantom and a porcine skin phantom with embedded silicone absorbers. The reconstruction of the absorbers is presented in 2D and 3D. The sensitivity of the measurement with respect to the photoacoustic detection is discussed. Potentially, Photoacoustic Imaging can be brought a step closer towards non-anaesthetized in vivo imaging and new medical applications not allowing acoustic contact, such as neurosurgical monitoring or burnt skin investigation.

Noncontact holographic detection for photoacoustic tomography

Abstract. A holographic method for high-speed, noncontact photoacoustic tomography is introduced and evaluated. Relative changes of the object’s topography, induced by the impact of thermoelastic pressure waves, were determined at nanometer sensitivity without physical contact. The object’s surface was illuminated with nanosecond laser pulses and imaged with a high-speed CMOS camera. From two interferograms measured before and after excitation of the acoustic wave, surface displacement was calculated and then used as the basis for a tomographic reconstruction of the initial pressure caused by optical absorption. The holographic detection scheme enables variable sampling rates of the photoacoustic signal of up to 50 MHz. The total acquisition times for complete volumes with 230 MVoxel is far below 1 s. Measurements of silicone and porcine skin tissue phantoms with embedded artificial absorbers, which served as a model for human subcutaneous vascular networks, were possible. Three-dimensional reconstructions of the absorbing structures show details with a diameter of 310  μm up to a depth of 2.5 mm. Theoretical limitations and the experimental sensitivity, as well as the potential for in vivo imaging depending on the detection repetition rate, are analyzed and discussed.

OCT verstehen – Teil 2: Praktische Aspekte und Anwendung

OCT ver be rr ec ht lic h ge sc hü tz t. Den Schwerpunkt dieses Artikels bildet die Vermittlung praktischen Wissens rund um die optische Kohärenztomografie (OCT). Während in Teil 1 [1] die physikalischen Grundlagen vorgestellt wurden, werden hier praktische Aspekte für den Anwender erläutert, unabhängig vom verwendeten Gerätetyp oder Hersteller. Grundsätzliche Parameter und Einstellmöglichkeiten werden vorgestellt. Es wird hinterfragt, was ein OCT‐Bild eigentlich zeigt und wann bei der Interpretation und Auswertung Vorsicht geboten ist. Typische Artefakte sowie deren Ursachen und Möglichkeiten der Vermeidung werden diskutiert. Zuletzt wird eine Übersicht über aktuelle Anwendungsfelder der OCT in der Augenheilkunde gegeben.

Non-contact Photoacoustic Tomography using holographic full field detection

An innovative very fast non-contact imaging technique for Photoacoustic Tomography is introduced. It is based on holographic optical speckle detection of a transiently altering surface topography for the reconstruction of absorbing targets. The surface movement is obtained by parallel recording of speckle phase changes known as Electronic Speckle Pattern Interferometry. Due to parallelized 2-D camera detection and repetitive excitation with variable delay with respect to the image acquisition, data recording of whole volumes for Photoacoustic Imaging can be completed in times far below one second. The size of the detected area is scalable by optical magnification. As a proof of concept, an interferometric setup is realized, capable of surface displacement detection with an axial resolution of less than 3 nm. The potential of the proposed method for in vivo Photoacoustic Imaging is discussed.

Speckle-based off-axis holographic detection for non-contact photoacoustic tomography

Abstract A very fast innovative holographic off-axis non-contact detection method for Photoacoustic Tomography (PAT) is introduced. It overcomes the main problems of most state-of-the-art photoacoustic imaging approaches that are long acquisition times and the requirement of acoustic contact. In order to increase the acquisition speed significantly, the surface displacements of the object, caused by the photoacoustic pressure waves, are measured interferometrically in two dimensions. Phase alterations in the observed speckle field are used to identify changes in the object’s topography. A sampling rate of up to 80 MHz is feasible, which reduces the occurrence of motion artefacts. This approach was validated with a silicone phantom with cylindrical absorbers that are similar to the shape of blood vessels. A tomographic reconstruction leads to the three dimensional location of the absorbers. A reliable reconstruction proves the ability of the method.

Optical full-field holographic detection system for non-contact photoacoustic tomography

We introduce an innovative detection approach for photoacoustic tomography. The pressure induced surface displacement is obtained in 2D by Electronic Speckle Pattern Interferometry (ESPI) in a repetitive measurement with a variable time delay between excitation- and detection pulses. The detection approach works without any physical contact to the object surface and is very versatile in terms of an adjustable object surface area and an adjustable temporal sampling rate. Furthermore, the approach is potentially very fast by the use of a high speed camera and a high repetition laser excitation and detection. In a proof of concept measurement, transparent silicone cubes with black silicone sphere absorbers are measured. In order to validate the acquired displacement data, the pressure is measured using a lipstick needle hydrophone and correlated to the measured displacement.

Photoacoustic blood vessel detection during surgical laser interventions

This paper presents a discussion about the potential of photoacoustics with regard to its application in surgical assistance during minimally invasive, laser assisted interventions. Aim of the work is the detection of obscured large blood vessels in order to prevent unintentional dissection. Based on spectroscopic investigations of the target tissue (liver), a wavelength for the photoacoustic excitation laser was chosen with respect to a high absorption contrast between the vessel and the surrounding liver tissue. An experimental setup featuring a simple liver model is created. Preliminary results show, that vessels with a diameter of 2 mm can be detected up to a distance of 1 mm from the treatment fibre. It is shown, that detection of acoustic waves induced inside liver is feasible over distances higher than 10 cm.