Figure 5: Data processing in mmPA imaging of MNP-gold hybrid NPs.

A PVA phantom holds three 2-mm-diameter inclusions, in which the one on the left contains gold nanorods with absorption comparable to the 3 nM MNP-gold hybrid NPs placed in the centre inclusion, and the third inclusion on the right contains 3 nM MNPs. A conventional PA image of this phantom is presented in (a) on a logarithmic scale over a 40 dB display range. The gold nanorod inclusion on the left serves as a magnetic reference and mimics strong background tissue signals. The MNP inclusion on the right serves as an optical reference to MNP-gold hybrid NPs. Using a sequence of PA images acquired while a magnetic field was turned on and then off over a 10-s interval, the maximum displacement resulting from the action of the magnetic field is presented in (b) on a pixel–pixel basis over a display range of (0 (dark), 30 (light)) μm. (c) Three representative displacement traces and their fitted curves over the entire time interval of the experiment for pixels in different inclusions. Using these curves, velocity was computed over the full 10-s interval and the maximum positive and negative velocities, presented in (d) and (e) over a (−20, 20) μm s⁻¹ display range, were used to create a weighting image, presented in (f) over a (0, 1) display range, based on the magnitude of difference between peak positive velocity in the first half and peak negative velocity in the second half. (g) mmPA image produced from the product of (a) with (f) over a 40 dB display range demonstrates that the gold nanorod inclusion mimicking a strong background signal is almost completely suppressed.