Dichroism-sensitive photoacoustic computed tomography

Abstract

Photoacoustic computed tomography (PACT), a fast-developing modality for deep tissue imaging, images the spatial distribution of optical absorption. PACT usually treats the absorption coefficient as a scalar. However, the absorption coefficients of many biological tissues exhibit an anisotropic property, known as dichroism or diattenuation, which depends on molecular conformation and structural alignment. Here, we present a novel imaging method called dichroism-sensitive PACT (DS-PACT), which measures both the amplitude of tissue's dichroism and the orientation of the optic axis of uniaxial dichroic tissue. By modulating the polarization of linearly polarized light and measuring the alternating signals through lock-in detection, DS-PACT can boost dichroic signals from biological tissues. To validate the proposed approach, we experimentally demonstrated the performance of DS-PACT by imaging plastic polarizers and ex vivo bovine tendons deep inside scattering media. We successfully detected the orientation of the optic axis of uniaxial dichroic materials, even at a depth of 4.5 transport mean free paths. We anticipate that the proposed method will extend the capability of PACT to imaging tissue absorption anisotropy.

Photoacoustic imaging in the second near-infrared window: a review

“…By combining optical contrast and US resolution, PAI offers several advantages: (i) it is noninvasive and uses nonionizing radiation, hence it can be repeatedly used in vivo by keeping the excitation energy below the safety limit; (ii) it provides label-free imaging; (iii) it is speckle-free; (iv) it has higher penetration depth, up to ∼4 cm in vivo 36 and ∼12 cm in vitro; 37 (v) it is faster and less expensive compared to MRI, PET, x-ray CT; 38 (vi) it provides multiscale imaging, allows imaging organelles to organs with consistent contrast while keeping the same depth-toresolution ratio; 38,39 (vii) it provides direct imaging of optical absorption with 100% relative sensitivity-the sensitivity of PAI is 100 times higher than that of OCT and confocal microscopy; and (viii) it provides anatomical, functional, molecular, and kinetic information. 40 PAI was successfully implemented as photoacoustic nanoscopy, [41][42][43][44][45][46] photoacoustic microscopy (PAM), [47][48][49][50][51][52] photoacoustic endoscopy, [53][54][55][56] preclinical photoacoustic computed tomography/photoacoustic tomography *Address all correspondence to Manojit Pramanik, E-mail: manojit@ntu.edu.sg (PACT/PAT), [57][58][59][60][61][62] and clinical PACT. [63][64][65][66][67][68] For deep-tissue imaging, PACT/PAT systems are suitable and hence hold better chance to find preclinical/clinical applications.…”

Photoacoustic computed tomography guided microrobots for targeted navigation in intestines in vivo

“…Leveraging the negligible acoustic scattering in soft tissue, PAT has achieved superb spatial resolution at depths, with a depth-to-resolution ratio of ~200 [12,17], at high imaging rates. As a major incarnation of PAT, photoacoustic computed tomography (PACT) has attained high spatiotemporal resolution, deep penetration, and anatomical and molecular contrasts [18][19][20][21][22]. With all these benefits, PACT shows great promise for real-time navigation of micromotors in vivo for broad applications, particularly, drug delivery.…”

In vivo super-resolution photoacoustic computed tomography by localization of single dyed droplets

“…Photoacoustic computed tomography (PACT) detects acoustic waves generated by both ballistic and diffused photons and retrieves the optical absorption distribution through an inverse algorithm, allowing high-resolution imaging at a depth up to several centimeters [13][14][15]. PACT has been used for imaging small-animal whole-body dynamics, functional whole mouse brain hemodynamics, and human breast tumors at high spatiotemporal resolutions [16][17][18][19][20].…”