ABSTRACT
Photoacoustic microscopy (PAM) is a noninvasive imaging modality, which hybrids high contrast of optical imaging and deeper imaging depth of ultrasound. PAM images optical absorption contrast based on the photoacoustic effect [1], PAM has a wide range of application in biomedicine, such as subcutaneous microvasculature imaging, brain functional imaging, or early detection of tumors [2-4], In addition, PAM has been used to monitor the blood oxygenation of vessels with high sensitivity and high specificity [2, 5], At present, almost all of the PAMs still utilize the ultrasound transducer to detect the PA signals in contact fashion. Nevertheless, ultrasound waves have a strong attenuation in air. So, in order to attain sensitivity signals, the coupling media, such as water or ultrasound gel, has to been added between the sample and ultrasound transducer. This requirement of the contact detection mode is often difficult for PAM imaging of biomedical samples, which limits the practical application of PAM in ophthalmology and surgical navigation. 200Compared with the use of an ultrasound transducer, the optical detection method can detect ultrasound signal in noncontact fashion [6], thus eliminating the requirement of the coupling media, which is attractive for in vivo imaging. Besides, optical detection also has an important advantage of potentially offering a higher imaging resolution than PAM. Due to imaging of PAM, the broadband ultrasound transducer is required to detect the broadband acoustic signal that excited by pulsed laser. Furthermore, in PAM, the axial resolution is inversely proportional to the bandwidth of the ultrasonic transducer, so its bandwidth is a critical factor to the improvement of imaging quality. However, owing to the limit of piezoelectric material and preparation technology, nearly all of ultrasound transducers usually offer finite detection bandwidth. Although the maximum bandwidth of piezoelectric ultrasonic detector can reach the center frequency of 120%, it is impossible to make a broadband ultrasound transducer that can cover from low frequency to high frequency. Due to photoelectric detector with a broadband characteristic, noncontact optical detection is presented constantly and has a wide application. In general, optical detection of ultrasound is a promising ultrasound detection modality, which can provide high detection sensitivity of ultrasound [6] and broadband width [7], Because of the noncontact character of PAM based on the optical detection method, it could be conveniently used in many special applications such as the disease diagnosis in ophthalmology and imaging of the affected area in surgical operation.
