Imaging optical contrast by means of the photoacoustic effect

The discovery in 1881: Absorption of light induces acoustic signals.

iThera Medical’s optoacoustic imaging (OAI) systems are based on the photoacoustic effect. In 1881, Alexander Graham Bell discovered that light energy absorbed by a material results in an acoustic signal.

While Alexander Graham Bell used sunlight as incident light and a hearing tube as acoustic detector, modern optoacoustic imaging systems use high-energy pulsed lasers and highly sensitive broadband ultrasound detectors. By exciting tissue with a laser pulse and detecting the signals generated by the conversion of light energy into sound waves, optical absorption in tissue can be detected and visualized.

Leveraging the photoacoustic effect, OAI has the unique capability of visualizing optical contrast at high resolution (down to 10 µm) in deep tissue (up to 3 cm) and displaying the image in real time. The frame rate is dependent on the pulse repetition rate of the light source and can reach up to hundreds of Hertz.

Differentiating chromophores via multispectral OAI

Acoustic signals are proportionate to optical absorption. Exciting tissue with a single wavelength will therefore enable a composite result of tissue absorbance. It can however not address the question which tissue chromophores contribute to the signal. In order to differentiate the contribution of different chromophores and thereby analyze their distribution and concentration, it is necessary to illuminate the tissue with multiple wavelengths. The resulting images are then analyzed on a pixel basis regarding their intensity change with a change in wavelength. If chromophores of interest have a characteristic absorption spectrum in the laser wavelength range, their contribution to the overall signal amplitude can be differentiated. Light in the near-infrared (NIR) region can penetrate the deepest into tissue, as this is the region where blood has its lowest absorption. This is therefore the preferred wavelength range for OAI of deep tissue.

Endogenous chromophores in the NIR include oxy-hemoglobin, deoxy-hemoglobin, melanin, lipid and collagen. Extrinsic absorbers include fluorophores, quenchers, various kinds of nanoparticles and other probes used for optical imaging.

Introducing different imaging regimes: the tradeoff between depth penetration and spatial resolution

Bioimaging can be differentiated into three regimes of imaging depth and spatial resolution: microscopy, mesoscopy and macroscopy.

iThera Medical’s product range covers imaging systems for preclinical and clinical macroscopy (MSOT inVision, MSOT Acuity) and mesoscopy (RSOM Explorer P50, RSOM Explorer C50). Besides depth penetration and resolution, major differences between the macroscopy and mesoscopy technologies provided by iThera Medical are the field of view and frame rate. While the macroscopy products use detector arrays enabling frame rates of 10-100 Hz with a field of view of several centimeters, the mesoscopy products use single-element detectors and therefore require raster-scanning to form an image. This limits the field of view to several millimeters and results in acquisition times of approx. 1 minute for a 5 x 5 mm field of view.

From data acquisition to image visualization: the optoacoustic imaging chain

Following data acquisition, the acoustic signals undergo tomographic reconstruction to form images. Given that the acoustic signal is proportionate to incident light, fluence correction is applied during image reconstruction to account for reduction in light fluence at depth. Spectral analysis then allows for separating the contribution of different chromophores – both intrinsic (e.g., hemoglobin, melanin, lipids) and extrinsic (e.g., fluorescent probes, nanoparticles). Finally, the image is filtered to enhance anatomical features. All steps can be performed in real time, i.e. at frame rates in the range of 10-100 Hz.

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