Photoacoustic discrimination of vascular and pigmented lesions using classical and Bayesian methods

Abstract

Discrimination of pigmented and vascular lesions in skin can be difficult due to factors such as size, subungual location, and the nature of lesions containing both melanin and vascularity. Misdiagnosis may lead to precancerous or cancerous lesions not receiving proper medical care. To aid in the rapid and accurate diagnosis of such pathologies, we develop a photoacoustic system to determine the nature of skin lesions in vivo. By irradiating skin with two laser wavelengths, 422 and 530 nm, we induce photoacoustic responses, and the relative response at these two wavelengths indicates whether the lesion is pigmented or vascular. This response is due to the distinct absorption spectrum of melanin and hemoglobin. In particular, pigmented lesions have ratios of photoacoustic amplitudes of approximately 1.4 to 1 at the two wavelengths, while vascular lesions have ratios of about 4.0 to 1. Furthermore, we consider two statistical methods for conducting classification of lesions: standard multivariate analysis classification techniques and a Bayesian-model-based approach. We study 15 human subjects with eight vascular and seven pigmented lesions. Using the classical method, we achieve a perfect classification rate, while the Bayesian approach has an error rate of 20%.

Figure 1

Apparatus for photoacoustic depth determination of melanin content. The frequency-doubled 532-nm Nd:YAG laser had a pulse duration of 5 ns. Laser light was delivered to the probe via 1000-μm-diam optical fiber. A PVDF element within the probe detected photoacoustic waves.

Figure 2

Photoacoustic probe. The optical fiber delivered 532-nm laser light with a pulse duration of 5 ns. Acoustic detection was obtained by a PVDF element within the probe cavity. The PVDF was recessed approximately 3 mm from the bottom of the probe, thus initial photoacoustic waves were delayed approximately 2 μs from the laser pulse.

Figure 3

Photographs and histology of representative (upper) pigmented and (lower) vascular lesions.

Figure 4

Representative photoacoustic signals from (left) pigmented and (right) vascular lesions at 422 and 530 nm. Note that these signals were averaged over 128 pulses.

Figure 5

Montage of histograms for the 15 subjects showing the posterior distribution of the estimated probability of belonging to the vascular lesion group. Subjects with posterior mean greater than 0.5 are classified to the vascular lesion group, whereas the subjects with posterior mean less than 0.5 are classified to the pigmented lesion group. The title of each subject shows a V or P indicating vascular or pigmented according to histological analysis. Additionally, for completeness, the posterior mean and 95% credible interval (in parentheses) are presented.