# Photoacoustic Imaging of Tattoo Inks: Phantom and Clinical Evaluation
## 4 Discussion  
In this report, we used PAI a relatively new optical imaging method for imaging tattoo inks in vitro and for in vivo visualization of a tattoo in the wrist of a healthy volunteer. Given its portability, low cost and high resolution, US is widely used in dermatology [13]. Clinical studies have also demonstrated the usefulness of US for imaging musculoskeletal anatomy [14; 15]. Tattooo inks exhibited strong yet distinct photoacoustic properties that can be exploited for non-invasive visualization using PAI. Our phantom study demonstrated that the PA signal varies greatly among tattoo inks. Lighter colors like white mostly reflect laser light so there is less absorption and low PA signal. Darker tattoo inks, especially the black color generates more robust signals. Dermal tattoo pigmentation was successfully imaged in vivo using PAI. Our study demonstrates the feasibility of PAI to spatially map tattoos in human skin. To the best of our knowledge, this is the first report on the use of PAI for visualization of an intradermal tattoo.  
While tattooing is relatively cheap, tattoo removal is an expensive procedure that involves use of laser light and relies on photolysis of the tattoo pigment. Currently, laser removal of tattoos including the wavelength of the laser light and fluence is based on qualitative, visual examination. As a result, the outcome following tattoo removal procedures is dependent on the experience of the clinician. A non-invasive imaging method that can provide an objective assessment of the extent of the tattoo in all three dimensions could be valuable in guiding tattoo removal. In this regard, PAI could serve as a relatively inexpensive, non-invasive, clinical tool for preoperative evaluation of tattoos and for objective monitoring of laser removal procedures.  
Conceptualization: M.S., A.K.S., G.P.; Data curation: E.R.B., L.J.R., A.K.S., M.S.; Formal analysis: E.R.B., L.J.R., M.S.; Funding acquisition: A.K.S., M.S.; Investigation: E.R.B., L.J.R., A.K.S., M.S.; Methodology: E.R.B., L.J.R., A.K.S., M.S.; Project administration: A.K.S., M.S.; Resources: A.K.S., M.S.; Supervision: A.K.S., M.S.; Validation: E.R.B., L.J.R., G.P., O.R.C., A.K.S., M.S.; Visualization: E.R.B., L.J.R., O.R.C., M.S.; Writing--original draft preparation: E.R.B., L.J.R., M.S.; Writing--review and editing: E.R.B., L.J.R., G.P., O.R.C., A.K.S., M.S. All authors have read and agreed to the published version of the manuscript. This work was supported by grants from the National Cancer Institute 1R01 CA204636, National Center for Research Resources S100 DD010393-01 and the Alliance Foundation of Western New York (to MS), and utilized  
Figure 3: **PAI of an intradermal tattoo**. The panel of images on the top represents digital photograph (left), a B-mode surface rendering (middle), and a reconstructed maximum intensity projection (MIP; right) image (800 nm) of a black tattoo on the wrist of a healthy volunteer. Corresponding longitudinal (left) and transverse axial (right) images of a part of the tattoo (outlined in black) is shown. The images have been pseudo-colorized for enhanced visualization.  
shared resources supported by Roswell Park Cancer Institute Cancer Center Support Grant from the National Cancer Institute P30 CA06156. The funding sponsors had no role in the design of the study, collection, analyses, or interpretation of data, writing of the manuscript, and in the decision to publish the results.  
The authors declare no conflict of interest.