  Volumetric, multimodal imaging with precise spatial and temporal
co-registration can provide valuable and complementary information for
diagnosis and monitoring. Considerable research has sought to combine 3D
photoacoustic (PA) and ultrasound (US) imaging in clinically translatable
configurations. However, technical compromises currently result in poor image
quality either for photoacoustic or ultrasonic modes. This work aims to provide
translatable, high quality, simultaneously co-registered dual-mode PA/US 3D
tomography. Volumetric imaging based on a synthetic aperture approach was
implemented by interlacing PA and US acquisitions during a rotate-translate
scan with a 5-MHz linear array (12 angles and 30-mm translation to image a
21-mm diameter, 19 mm long cylindrical volume within 21 seconds). For
co-registration, an original calibration method using a specifically designed
thread phantom was developed to estimate 6 geometrical parameters and 1
temporal off-set through global optimization of the reconstructed sharpness and
superposition of calibration phantom structures. Phantom design and cost
function metrics were selected based on analysis of a numerical phantom, and
resulted in a high estimation accuracy for the 7 parameters. Experimental
estimations validated the calibration repeatability. The estimated parameters
were used for bimodal reconstruction of additional phantoms with either
identical or distinct spatial distributions of US and PA contrasts.
Superposition distance of the two modes was within < 10% of the acoustic
wavelength and a wavelength-order uniform spatial resolution was obtained. This
dual-mode PA/US tomography should contribute to more sensitive and robust
detection and follow-up of biological changes or the monitoring slower-kinetic
phenomena in living systems such as the accumulation of nanoagents.
