*Article* **Photoacoustic Tomography with a Ring Ultrasound Transducer: A Comparison of Di** ff**erent Illumination Strategies**

#### **Naser Alijabbari 1,**†**, Suhail S. Alshahrani 1,**†**, Alexander Pattyn 1 and Mohammad Mehrmohammadi 1,2,3,\***


Received: 3 June 2019; Accepted: 27 July 2019; Published: 31 July 2019

#### **Featured Application: Biological diagnostic applications based on endogenous or exogenous chromophores and early breast cancer detection in dense tissue.**

**Abstract:** Photoacoustic (PA) imaging is a methodology that uses the absorption of short laser pulses by endogenous or exogenous chromophores within human tissue, and the subsequent generation of acoustic waves acquired by an ultrasound (US) transducer, to form an image that can provide functional and molecular information. Amongst the various types of PA imaging, PA tomography (PAT) has been proposed for imaging pathologies such as breast cancer. However, the main challenge for PAT imaging is the deliverance of su fficient light energy horizontally through an imaging cross-section as well as vertically. In this study, three di fferent illumination methods are compared for a full-ring ultrasound (US) PAT system. The three distinct illumination setups are full-ring, di ffused-beam, and point source illumination. The full-ring system utilizes a cone mirror and parabolic reflector to create the ringed-shaped beam for PAT, while the di ffuse scheme uses a light di ffuser to expand the beam, which illuminates tissue-mimicking phantoms. The results indicate that the full-ring illumination is capable of providing a more uniform fluence irrespective of the vertical depth of the imaged cross-section, while the point source and di ffused illumination methods provide a higher fluence at regions closer to the point of entry, which diminishes with depth. In addition, a set of experiments was conducted to determine the optimum position of ring-illumination with respect to the position of the acoustic detectors to achieve the highest signal-to-noise ratio.

**Keywords:** full-ring illumination; di ffused-beam illumination; point source illumination; ultrasound tomography (UST); photoacoustic tomography (PAT)
