Spatial and Contrast Enhancement in Photoacoustic Imaging

Photoacoustic imaging have advantages in terms of imaging targets inside a biological tissue and differentiating tissue types. In photoacoustic imaging, a solid-state laser delivers a laser pulse on the target object. This type of laser source is expensive, bulky and it consumes significant power. To facilitate photoacoustic equipment for a wider use in clinical diagnostics, a more portable form is required. To overcome this limitation pulse laser diodes (PLDs) will be investigated. PLDs have advantages over the solid-state laser in terms of price, size and pulse repetition frequency (PRF). However, PLDs suffer from low output energy, generating the acoustic signal with a low signal to noise ratio (SNR) and limiting the imaging depth. Researchers have used some techniques to deal with this issue. For instance, multiple PLDs have been used to increase the output optical energy. The averaging technique has also been used to reduce noise. In addition, some types of coded excitations have been used to investigate the high PRF and reduce noise. In this thesis, the required number of PLDs to generate photoacoustic emissions with a specific SNR was estimated. This technique was compared with the Nd:YAG laser based on the generated photoacoustic emission. This calculation limited the applications of PLDs for superficial imaging. In addition, in this thesis, the photoacoustic unipolar golay code was developed to reduce the number of transmitted pulse sequences from four to two with little effects on its performance. This developed unipolar golay code improved the code gain when the noise level was high. Moreover, in this thesis, a weak photoacoustic emission was extracted, despite limited bandwidth, and was recovered by using the miro-Doppler technique. This technique has been performed by using a linear array transducer. Delay and sum (DAS) is one of the most popular beamforming techniques in photoacoustic imaging due to its simplicity. However, the photoacoustic image is significantly affected by side lobes, clutter and phase aberration when the DAS beamfomer is used. All of these reduce the spatial resolution, SNR and contrast of the image. Researchers have used some adaptive beamforming techniques to reduce these effects. In this thesis, the filter delay multiply and sum (FDMAS) beamformer is applied to photoacoustic imaging. This beamformer that depends on the cross correlation between the delayed RF signals provided photoacoustic images with improved resolution and SNR. The FDMAS beamformer was used also in 3D photoacoustic imaging in conjunction with a linear array transducer. This beamformer showed improvement in the elevation resolution out of the focal point. In this thesis, the dynamic filter delay multiply and sum (D-FDMAS) beamformer was produced to reduce the contrast difference between imaging targets and reduce the computation time. This beamformer improves monitoring biopsy needle and sentinel lymph node (SLN) to be co-located. In addition, the D-FDMAS beamfomer was linearized to study the effect of the non-linearity of the D-FDMAS beamformer in multispectral photoacoustic imaging (MIP). This study showed that there was not significant difference between the MIP images when using the linear and non-linear D-FDMAS beamformers.