Jan Laufer

Backward-mode multiwavelength photoacoustic scanner using a planar Fabry-Perot polymer film ultrasound sensor for high-resolution three-dimensional imaging of biological tissues

A multiwavelength backward-mode planar photoacoustic scanner for 3D imaging of soft tissues to depths of several millimeters with a spatial resolution in the tens to hundreds of micrometers range is described. The system comprises a tunable optical parametric oscillator laser system that provides nanosecond laser pulses between 600 and 1200 nm for generating the photoacoustic signals and an optical ultrasound mapping system based upon a Fabry-Perot polymer film sensor for detecting them. The system enables photoacoustic signals to be mapped in 2D over a 50 mm diameter aperture in steps of 10 microm with an optically defined element size of 64 microm. Two sensors were used, one with a 22 microm thick polymer film spacer and the other with a 38 mum thick spacer providing -3 dB acoustic bandwidths of 39 and 22 MHz, respectively. The measured noise equivalent pressure of the 38 microm sensor was 0.21 kPa over a 20 MHz measurement bandwidth. The instrument line-spread function (LSF) was measured as a function of position and the minimum lateral and vertical LSFs found to be 38 and 15 microm, respectively. To demonstrate the ability of the system to provide high-resolution 3D images, a range of absorbing objects were imaged. Among these was a blood vessel phantom that comprised a network of blood filled tubes of diameters ranging from 62 to 300 microm immersed in an optically scattering liquid. In addition, to demonstrate the applicability of the system to spectroscopic imaging, a phantom comprising tubes filled with dyes of different spectral characteristics was imaged at a range of wavelengths. It is considered that this type of instrument may provide a practicable alternative to piezoelectric-based photoacoustic systems for high-resolution structural and functional imaging of the skin microvasculature and other superficial structures.

Quantitative spectroscopic photoacoustic imaging: a review

Obtaining absolute chromophore concentrations from photoacoustic images obtained at multiple wavelengths is a nontrivial aspect of photoacoustic imaging but is essential for accurate functional and molecular imaging. This topic, known as quantitative photoacoustic imaging, is reviewed here. The inverse problems involved are described, their nature (nonlinear and ill-posed) is discussed, proposed solution techniques and their limitations are explained, and the remaining unsolved challenges are introduced.

Quantitative spatially resolved measurement of tissue chromophore concentrations using photoacoustic spectroscopy: application to the measurement of blood oxygenation and haemoglobin concentration

A new approach based on pulsed photoacoustic spectroscopy for non-invasively quantifying tissue chromophore concentrations with high spatial resolution has been developed. The technique is applicable to the quantification of tissue chromophores such as oxyhaemoglobin (HbO(2)) and deoxyhaemoglobin (HHb) for the measurement of physiological parameters such as blood oxygen saturation (SO(2)) and total haemoglobin concentration. It can also be used to quantify the local accumulation of targeted contrast agents used in photoacoustic molecular imaging. The technique employs a model-based inversion scheme to recover the chromophore concentrations from photoacoustic measurements. This comprises a numerical forward model of the detected time-dependent photoacoustic signal that incorporates a multiwavelength diffusion-based finite element light propagation model to describe the light transport and a time-domain acoustic model to describe the generation, propagation and detection of the photoacoustic wave. The forward model is then inverted by iteratively fitting it to measurements of photoacoustic signals acquired at different wavelengths to recover the chromophore concentrations. To validate this approach, photoacoustic signals were generated in a tissue phantom using nanosecond laser pulses between 740 nm and 1040 nm. The tissue phantom comprised a suspension of intralipid, blood and a near-infrared dye in which three tubes were immersed. Blood at physiological haemoglobin concentrations and oxygen saturation levels ranging from 2% to 100% was circulated through the tubes. The signal amplitude from different temporal sections of the detected photoacoustic waveforms was plotted as a function of wavelength and the forward model fitted to these data to recover the concentrations of HbO(2) and HHb, total haemoglobin concentration and SO(2). The performance was found to compare favourably to that of a laboratory CO-oximeter with measurement resolutions of +/-3.8 g l(-1) (+/-58 microM) and +/-4.4 g l(-1) (+/-68 microM) for the HbO(2) and HHb concentrations respectively and +/-4% for SO(2) with an accuracy in the latter in the range -6%-+7%.

In vivo high-resolution 3D photoacoustic imaging of superficial vascular anatomy

The application of a photoacoustic imaging instrument based upon a Fabry-Perot polymer film ultrasound sensor to imaging the superficial vasculature is described. This approach provides a backward mode-sensing configuration that has the potential to overcome the limitations of current piezoelectric based detection systems used in superficial photoacoustic imaging. The system has been evaluated by obtaining non-invasive images of the vasculature in human and mouse skin as well as mouse models of human colorectal tumours. These studies showed that the system can provide high-resolution 3D images of vascular structures to depths of up to 5 mm. It is considered that this type of instrument may find a role in the clinical assessment of conditions characterized by changes in the vasculature such as skin tumours and superficial soft tissue damage due to burns, wounds or ulceration. It may also find application in the characterization of small animal cancer models where it is important to follow the tumour vasculature over time in order to study its development and/or response to therapy.

In vivo preclinical photoacoustic imaging of tumor vasculature development and therapy

The use of a novel all-optical photoacoustic scanner for imaging the development of tumor vasculature and its response to a therapeutic vascular disrupting agent is described. The scanner employs a Fabry-Perot polymer film ultrasound sensor for mapping the photoacoustic waves and an image reconstruction algorithm based upon attenuation-compensated acoustic time reversal. The system was used to noninvasively image human colorectal tumor xenografts implanted subcutaneously in mice. Label-free three-dimensional in vivo images of whole tumors to depths of almost 10 mm with sub-100-micron spatial resolution were acquired in a longitudinal manner. This enabled the development of tumor-related vascular features, such as vessel tortuosity, feeding vessel recruitment, and necrosis to be visualized over time. The system was also used to study the temporal evolution of the response of the tumor vasculature following the administration of a therapeutic vascular disrupting agent (OXi4503). This revealed the well-known destruction and recovery phases associated with this agent. These studies illustrate the broader potential of this technology as an imaging tool for the preclinical and clinical study of tumors and other pathologies characterized by changes in the vasculature.

Effect of temperature on the optical properties of ex vivo human dermis and subdermis

The effect of temperature on the optical properties of human dermis and subdermis as a function of near-infrared wavelength has been studied between 25 degrees C and 40 degrees C. Measurements were performed ex vivo on a total of nine skin samples taken from the abdomen of three individuals. The results show a reproducible effect of temperature on the transport scattering coefficient of dermis and subdermis. The relative change of the transport scattering coefficient showed an increase for dermis ((4.7+/-0.5) x 10(-3) degrees C(-1)) and a decrease for subdermis ((-1.4+/-0.28) x 10(-3) degrees C(-1)). Note that the magnitude of the temperature coefficient of scattering was greater for dermis than subdermis. A reproducible effect of temperature on the absorption coefficient could not be found within experimental errors. System reproducibility in transport scattering coefficient with repeated removal and repositioning of the same tissue sample at the same temperature was excellent at +/-0.35% for all measurements. This reproducibility enabled such small changes in scattering coefficient to be detected.

In vitro measurements of absolute blood oxygen saturation using pulsed near-infrared photoacoustic spectroscopy: accuracy and resolution.

Pulsed photoacoustic spectroscopy was used to measure blood oxygen saturation in vitro. An optical parametric oscillator laser system provided nanosecond excitation pulses over the wavelength range 740-1040 nm which were used to generate photoacoustic signals in a cuvette through which a saline suspension of red blood cells was circulated. The signal amplitude and the effective attenuation coefficient were extracted from the photoacoustic signals as a function of wavelength to provide photoacoustic spectra of the blood. From these, the relative concentrations of oxy- and deoxyhaemoglobin, and therefore blood oxygen saturation (SO2), were determined using forward models of the absorbed energy distribution based on diffusion theory. A standard linear model of the dependence of absorbance on the concentration of chromophores was also used to calculate the blood oxygen saturation from the signal amplitude spectra. The diffusion approximation model was shown to produce the highest accuracy in blood SO2. The photoacoustically determined oxygen saturation was found to have an accuracy of +/-4% SO2 for signal amplitude data and +/-2.5% SO2 for effective attenuation spectra. The smallest change in oxygen saturation that can be measured using this technique was +/-1% SO2.

Multimodal photoacoustic and optical coherence tomography scanner using an all optical detection scheme for 3D morphological skin imaging

A noninvasive, multimodal photoacoustic and optical coherence tomography (PAT/OCT) scanner for three-dimensional in vivo (3D) skin imaging is described. The system employs an integrated, all optical detection scheme for both modalities in backward mode utilizing a shared 2D optical scanner with a field-of-view of ~13 × 13 mm2. The photoacoustic waves were detected using a Fabry Perot polymer film ultrasound sensor placed on the surface of the skin. The sensor is transparent in the spectral range 590-1200 nm. This permits the photoacoustic excitation beam (670-680 nm) and the OCT probe beam (1050 nm) to be transmitted through the sensor head and into the underlying tissue thus providing a backward mode imaging configuration. The respective OCT and PAT axial resolutions were 8 and 20 µm and the lateral resolutions were 18 and 50-100 µm. The system provides greater penetration depth than previous combined PA/OCT devices due to the longer wavelength of the OCT beam (1050 nm rather than 829-870 nm) and by operating in the tomographic rather than the optical resolution mode of photoacoustic imaging. Three-dimensional in vivo images of the vasculature and the surrounding tissue micro-morphology in murine and human skin were acquired. These studies demonstrated the complementary contrast and tissue information provided by each modality for high-resolution 3D imaging of vascular structures to depths of up to 5 mm. Potential applications include characterizing skin conditions such as tumors, vascular lesions, soft tissue damage such as burns and wounds, inflammatory conditions such as dermatitis and other superficial tissue abnormalities.

Three-dimensional noninvasive imaging of the vasculature in the mouse brain using a high resolution photoacoustic scanner.

The application of a novel photoacoustic imaging instrument based on a Fabry-Perot polymer film sensing interferometer to imaging the small animal brain is described. This approach provides a convenient backward mode sensing configuration that offers the prospect of overcoming the limitations of existing piezoelectric based detection schemes for small animal brain imaging. Noninvasive images of the vasculature in the mouse brain were obtained at different wavelengths between 590 and 889 nm, showing that the cerebral vascular anatomy can be visualized with high contrast and spatial resolution to depths up to 3.7 mm. It is considered that the instrument has a role to play in characterizing small animal models of human disease and injury processes such as stroke, epilepsy, and traumatic brain injury.

Quantitative determination of chromophore concentrations from 2D photoacoustic images using a nonlinear model-based inversion scheme

A model-based inversion scheme was used to determine absolute chromophore concentrations from multiwavelength photoacoustic images. The inversion scheme incorporated a forward model, which predicted 2D images of the initial pressure distribution as a function of the spatial distribution of the chromophore concentrations. It comprised a multiwavelength diffusion based model of the light transport, a model of acoustic propagation and detection, and an image reconstruction algorithm. The model was inverted by fitting its output to measured photoacoustic images to determine the chromophore concentrations. The scheme was validated using images acquired in a tissue phantom at wavelengths between 590 nm and 980 nm. The phantom comprised a scattering emulsion in which up to four tubes, filled with absorbing solutions of copper and nickel chloride at different concentration ratios, were submerged. Photoacoustic signals were detected along a line perpendicular to the tubes from which images of the initial pressure distribution were reconstructed. By varying the excitation wavelength, sets of multiwavelength photoacoustic images were obtained. The majority of the determined chromophore concentrations were within +/-15% of the true value, while the concentration ratios were determined with an average accuracy of -1.2%.