Fundamental study for detection of anisakis by photoacoustic imaging

Anisakis larvae infest fish of several types. Consumption of raw fish products containing anisakis can engender a severe human disease known as anisakidosis. Therefore, anisakis must be removed from fish before consumption. Conventional screening methods such as candling can be done easily to detect surface parasites, but such methods are not accurate or sufficiently sensitive to detect parasites that are embedded deeply in fish muscle. To resolve these shortcomings, we devised a method using photoacoustic imaging. This imaging modality combines high tissue specificity with better penetration depth. As a fundamental study, photoacoustic spectra of anisakis and mackerel were measured. Silica glass cells containing a sample were irradiated with nanosecond pulses of laser light (300–700 nm wavelengths). Photoacoustic signals were measured using a hydrophone. The beam sampler reflected part of the incident light. Then the light intensity was measured. The photoacoustic signal intensity was normalized. The spectrum of anisakis decreases monotonically. The spectrum of mackerel increases moderately in 375–400 nm and 490–540 nm. To examine the ability to distinguish between anisakis and mackerel using photoacoustic spectrum differences, the intensity ratio at photoacoustic signals of 375 nm and 400 nm (I375 nm / I400 nm) and 490 nm and 530 nm (I490 nm / I530 nm) was calculated. The ratio of anisakis is higher than 1.0, although the ratio found for mackerel is less than 1.0. These analyses demonstrated the possibility of detecting anisakis in mackerel using multi-wavelength analysis.