A limited number of patients with ovarian cancer are diagnosed at its early stages. The reason is that current medical imaging techniques still present low sensitivity in the detection of this disease. Molecular imaging, such as photoacoustic (PA), can play an important role to improve ovarian cancer detection. For gynecologic applications, a transvaginal (endo-cavity) ultrasound probe is needed. In this study, the time reversal (TR) and delay and sum (DAS) image reconstruction methods were evaluated for PA signals acquired using a transvaginal transducer. PA signals were recorded from two different phantoms. Phantom A consisted of six hair strands immersed in a water tank positioned at different depths. Image reconstruction using TR after interpolating the acquired scan-lines from 128 to 256 (TR256) and to 512 lines (TR512) were also evaluated. Furthermore, simulations using the same configuration of phantom A were conducted and a similarity index (SSIM) was calculated for each reconstruction method. Phantom B was manufactured using the copolymer styrene-ethylene/butylene-styrene in mineral oil. The phantom had a cubic shape with a cavity to place the transducer and three irregular inclusions were added to the phantom. These inclusions were filled with black pigment to improve light absorption. The SSIM values obtained from simulations for DAS, TR, TR256 and TR512 were 0.52, 0.60, 0.68 and 0.72, respectively. Signal to noise ratio (SNR) and contrast to noise ratio (CNR), for phantom B images, were calculated to compare both methods. The SNR values for DAS, TR, TR256 and TR512 were 19.5 dB, 21.1 dB, 22.0 dB and 22.5 dB and the CNR values were 13.4 dB, 14.5 dB, 15.5 dB and 16.0 dB, respectively. Results showed better performance when the TR method was used, including SNR, CNR, and lateral and axial resolution. However, DAS was less time consuming compared to TR, maintaining a reasonable image quality.
Water-based materials are commonly used in phantoms for ultrasound and optical imaging techniques. However, these materials have disadvantages such as easy degradation and low temporal stability. In this study, we propose an oil-based new tissue mimicking material for ultrasound and optical imaging, with the advantage of presenting low temporal degradation. Styrene-Ethylene/Butylene-Styrene (SEBS) copolymer in mineral oil samples were made varying the SEBS concentration between 5─15%, and low-density polyethylene (LDPE) between 0−9%. Acoustic properties such as speed of sound and attenuation coefficient were obtained by the substitution technique with frequencies ranging from 2.25─10 MHz, and were consistent to that of soft tissue. These properties were controlled varying SEBS and LDPE concentration; speed of sound from 1445─1480 m/s, and attenuation from 0.86─11.31 dB/cm were observed. SEBS gels with 0% of LDPE were optically transparent, presenting low optical absorption and scattering coefficients in the visible region of the spectrum. In order to fully characterize the optical properties of the samples, the reflectances of the surfaces were measured, along with the absorption. Scattering and absorption coefficients ranging from 400 nm to 1200 nm were calculated for each compound. The results showed that the presence of LDPE increased absorption and scattering of the phantoms. The results suggest the copolymer gels are promising for ultrasound and optical imaging, what make them also potentially useful for photoacoustic imaging.