Reducing artifacts in photoacoustic imaging by using multi-wavelength excitation and transducer displacement

: The occurrence of artifacts is a major challenge in photoacoustic imaging. The artifacts negatively affect the quality and reliability of the images. An approach using multi-wavelength excitation has previously been reported for in-plane artifact identification. Yet, out-of-plane artifacts cannot be tackled with this method. Here we propose a new method using ultrasound transducer array displacement. By displacing the ultrasound transducer array axially, we can de-correlate out-of-plane artifacts with in-plane image features and thus remove them. Combining this new method with the previous one allows us to remove potentially completely both in-plane and out-of-plane artifacts in photoacoustic imaging. We experimentally demonstrate this with experiments in phantoms as well as in vivo.

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Combin
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Experime
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Phantom
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Phantom
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In vivo
We also asses at fingers whi the imaged fin Figure 14 an ink mark w 14(a). The w experiments. Acquired principle, they and bone lay However, in t the position in OPAs.  cted images. In ayers are rem f2 in Fig. 15 In the proposed method, correcting OPAs can be done without tissue deformation. Lastly, deforming tissue by focusing strong pressure, as is done in LOVIT, might violate US safety.
The proposed method for correcting OPA relies on segmentation. In this work, a simple segmentation approach was used for a low calculation cost and time consumption. However, it might not segment images properly giving inaccurate axial dimension of OPAs. As a result, OPAs might not be correctly removed. Over-segmentation also might happen as pointed out in [14]. A more effective segmentation algorithm should be considered for a better performance.
In our experiments, while the probe was displaced, the fiber bundle remained fixated. The purpose of this was to maintain the signal strength of image features. However, if the light source is displaced with the probe, the laser beam is also repositioned and thus excites different tissue volumes. Acquired images along the displacement might show different structures resulting in miscorrecting.
In clinical applications, the displacement distance, z Δ , might be limited. Depending on the location of out-of-plane absorbers, T z Δ might not be achieved, as discussed in section 3.1. As a result, OPAs are not completely removed, in which case another approach is needed. However, our results show that within 5 mm displacement, OPAs can be completely removed for a large range of locations and axial dimensions.
In this work, out-of-plane absorbers were positioned outside of the imaging plane elevationally. Lateral out-of-plane absorbers can also cause OPAs. The principle of the proposed method still holds for these OPAs. The quantity o x , used to describe out-of-plane absorbers in section 3.1, in this situation will be the lateral distance between the out-of-plane absorber and the imaging plane. Therefore, lateral OPAs can be identified and removed.
Axially displacing the probe in essence is to adjust the distance between the probe and inplane and out-of-plane absorbers. Displacing the probe in other directions might also be able to de-correlate in-plane and out-of-plane image features. In a configuration as shown in phantom 1, if the probe is elevationally displaced in the direction to the out-of-plane absorber, real in-plane features will move down and OPAs will move up. However, in a scenario that there is another out-of-plane absorber in the other side of the imaging plane. OPAs of this out-of-plane absorber will also move down. Therefore, elevationally displacing the probe in both directions is required to identify all OPAs. The amount of work is double compared to using axial displacement. Nevertheless, displacing the probe in other manners and comparing with the proposed method will be investigated in our future work.
In a situation that an OPA appears at the same position with a real in-plane feature as a single feature, the image value is a sum of the OPA and the in-plane features. Displacing the transducer array can separate these two and remove the OPA. However, true image value of the real in-plane feature cannot be recovered. Interpolating or extrapolating image values of the OPA along the displacement might be able to estimate its value at the superposition. True image value of the real in-plane feature can, therefore, be recovered. Additionally, in the proposed method, OPAs are removed by setting their pixel values to 0. This might also remove the background information behind the OPAs. If the image value of the OPAs can be estimated, the background information can be retained while removing OPAs by subtracting the recorded value by the estimated one. This will be investigated in our future work.
In this work, the volunteer had to keep the finger still for ~5 minutes. Slight movements were inevitable resulting in some miscorrection. This is not ideal for clinical applications. However, the long experiment time was due to technical limitations. In particular, the translating stage was slow. It took ~2 minutes to acquire in total 5 PA images along 5 mm of the probe displacement. Using a higher speed translating stage will significantly reduce the experiment time. The acquiring data process with 8 wavelengths took ~2 minutes. This was due to the laser pulse repetition rate of 20 Hz. Using a high repetition rate laser would potentially achieve real-time artifact correction as shown in [14].

Conclusion
We have proposed a new method to remove out-of-plane artifacts exploiting different behaviors of out-of-plane artifacts and in-plane image features by axially displacing the transducer array. Combining this new method with our previous method for in-plane artifacts using multiple wavelengths [14], in-plane and out-of-plane artifacts in photoacoustic imaging can be identified and thus removed. Experiments in phantoms and in vivo were carried out to evaluate the combination of the two methods as a proof of concept. Results show the potential of this combined method for providing true photoacoustic images with no ultrasound images needed. In addition, a handheld probe suitable for clinical applications was used in the experiments bringing this method a step forwards to clinical translation.