Reversibly switchable photoacoustic imaging systems and methods

Abstract

Reversibly switchable photoacoustic tomography (RS-PAT), a photoacoustic technique with enhanced sensitivity and resolution, is disclosed. RS-PAT utilizes a subtractive process for the formation of a photoacoustic image of a region containing a plurality of switchable photoacoustic probes. In various aspects, the photoacoustic detection in RS-PAT imaging occurs minimally twice: a first image obtained when the photoacoustic probe is in active (absorbing or ON) state and a second image obtained when the photoacoustic probe is in an inactive (less-absorbing or OFF) state. Subtraction of the second image from the first image is used to obtain the RS-PAT image.

What is claimed is: 
     
         1 . A method of obtaining an RS-PA image of a region of interest, the method comprising:
 obtaining a plurality of first PA signals from the region of interest using a first laser pulse wavelength, the region of interest comprising a plurality of reversibly-switchable probes configured in a first state;   switching the plurality of reversibly-switchable probes from the first state to a second state;   obtaining a plurality of second PA signals from the region of interest using the first laser pulse wavelength;   reconstructing a first PA image of the region of interest from the plurality of first PA signals;   reconstructing a second PA image of the region of interest from the plurality of second PA signals;   subtracting the second PA image from the first PA image pixelwise to obtain the RS-PA image of the region of interest.   
     
     
         2 . The method of  claim 1 , further comprising:
 obtaining at least one additional plurality of first PA signals and at least one additional plurality of second PA signals;   reconstructing at least one additional first PA image and at least one additional second PA image from the at least one additional plurality of first PA signals and the at least one additional plurality of second PA signals, respectively;   averaging the first PA image and each of the at least one additional first PA images pixelwise to obtain an averaged first PA image and averaging the second PA image and each of the at least one additional second PA images pixelwise to obtain the RS-PA image of the region of interest; and   subtracting the averaged second PA image from the averaged first PA image pixelwise to obtain the RS-PA image of the region of interest.   
     
     
         3 . The method of  claim 2 , further comprising switching the plurality of reversibly-switchable probes from the second state to the first state prior to obtaining each of the at least one additional pluralities of first PA signals. 
     
     
         4 . The method of  claim 3 , wherein the plurality of reversibly-switchable probes is switched from the second state to the first state by illuminating the plurality of reversibly-switchable probes using a second laser pulse wavelength. 
     
     
         5 . The method of  claim 4 , further comprising illuminating the plurality of reversibly-switchable probes with the second laser pulse wavelength after obtaining each plurality of first PA signals to maintain the plurality of reversibly-switchable probes in the first state. 
     
     
         6 . The method of  claim 5 , wherein the plurality of reversibly-switchable probes is switched from the first state to the second state by illuminating the plurality of reversibly-switchable probes using the first laser pulse wavelength. 
     
     
         7 . The method of  claim 1 , wherein the plurality of reversibly-switchable probes are illuminated using the second laser pulse wavelength after obtaining each first PA signal to maintain the plurality of reversibly-switchable probes in the first state. 
     
     
         8 . The method of  claim 1 , wherein the plurality of reversibly-switchable probes are switched from the first state to the second state by illuminating the plurality of reversibly-switchable probes using a third laser pulse wavelength. 
     
     
         9 . A reversibly-switchable probe for use in RS-PA imaging, the reversibly-switchable probe comprising a bacterial phytochrome, the bacterial phytochrome comprising a photosensory core module, an output effector domain, and a chromophore, the chromophore comprising biliverdin Ixα, wherein the reversibly-switchable probe is either endogenously produced by at least one cell to be imaged or is exogenously introduced into at least a portion of a field of view to be imaged using the RS-PA imaging. 
     
     
         10 . A system for obtaining an image of a region of interest using RS-PA imaging, the system comprising:
 a first laser source to produce at least one first laser pulse at a first pulse wavelength suitable for PA imaging;   a second laser source to produce at least one second laser pulse at a second wavelength suitable for switching a plurality of reversibly-switchable probes from a second state to a first state;   a controller to control a relative timing of the at least one first laser pulse and the at least one second laser pulse according to a predetermined sequence;   an optical combining module to direct the at least one first laser pulse and the at least one second laser pulse into a corresponding optics module of a PA imaging device, wherein the corresponding optics module directs the at least one first laser pulse and the at least one second laser pulse into the region of interest.   
     
     
         11 . The system of  claim 10 , wherein the first laser source and the second laser source are independently selected from one of: a diode laser, a pulsed laser, a dye laser pumped by a pulsed laser, and an OPO laser pumped by a pulsed laser. 
     
     
         12 . The system of  claim 11 , wherein the first laser source is a Ti:Sapphire laser pumped by an Nd:YAG pulsed laser, and the second laser source is an OPO laser pumped by the Nd:YAG pulsed laser. 
     
     
         13 . The system of  claim 11 , wherein the first laser source is a dye laser pumped by a pulsed laser, and the second laser source is a diode laser. 
     
     
         14 . The system of  claim 10 , wherein the optical combining module comprises:
 a mirror operatively coupled to the second laser source; and   a dichroic mirror operatively coupled to the first laser source and to the mirror, wherein:
 the mirror reflects the at least one second laser pulse to the dichroic mirror; and 
 the dichroic mirror transmits the at least one first laser pulse and reflects the at least one second laser pulse along a combined beam path to direct the at least one first laser pulse and the at least one second laser pulse into the corresponding optics module of the PA imaging device. 
   
     
     
         15 . The system of  claim 10 , wherein the PA imaging device comprises any one of: a PAT device and a PAM device. 
     
     
         16 . A system for obtaining an image of a region of interest using RS-PAM imaging, the system comprising:
 a first laser source operatively coupled to an optical combining module, wherein the first laser source produces at least one first laser pulse at a first pulse wavelength suitable for PA imaging;   a second laser source operatively coupled to the optical combining module, wherein the second laser source produces at least one second laser pulse at a second wavelength suitable for switching a plurality of reversibly-switchable probes from a second state to a first state;   a controller operatively coupled to the first and second laser sources, wherein the controller controls the relative timing of the at least one first laser pulse and the at least one second laser pulse according to a predetermined sequence;   the optical combining module to direct the at least one first laser pulse and the at least one second laser pulse into an optics module;   the optics module operatively coupled to the optical combining module, wherein the optics module splits the at least one first laser pulse and the at least one second laser pulse into a first portion and a second portion, and further directs the first portion into the region of interest along a first path and directs the second portion into the region of interest along a second path; and   at least one ultrasound transducer to receive a plurality of PA signals produced by the region of interest in response to illumination by the at least one first laser pulse.   
     
     
         17 . The system of  claim 15 , wherein the first laser source and the second laser source are independently selected from one of: a diode laser, a pulsed laser, a dye laser pumped by a pulsed laser, and an OPO laser pumped by a pulsed laser. 
     
     
         18 . The system of  claim 17 , wherein the first laser source is the dye laser pumped by the pulsed laser, and the second laser source is the diode laser. 
     
     
         19 . The system of  claim 16 , wherein the first path and the second path are coaxial and originate from opposite sides of the region of interest. 
     
     
         20 . The system of  claim 16 , further comprising an optical-acoustic coupling device operatively coupled to the at least one ultrasound transducer, wherein the optical-acoustic coupling device transmits the at least one first laser pulse along the first direction into the region of interest, and directs the plurality of PA signals travelling along the first direction from the region of interest to the at least one ultrasound transducer.