Photoacoustic lifetime for direct in vivo tissue oxygen imaging Qi Shao and Shai Ashkenazi

Department Biomedical Engineering, University of Minnesota

PA lifetime oxygen sensing Tissue oxygen monitor

PALI system

Photoacoustic and Ultrasound Lab

Tumor hypoxia imaging

pO2: Oxygen partial pressure, T: Triplet lifetime at a given pO2

T0: Constant, triplet lifetimes at pO2=0, kQ: Quenching rate constant

• Photoacoustic (PA) effect is the generation of sound by absorption of light. • Photoacoustic lifetime imaging (PALI) relies on PA measurement of the

lifetime of a chromophore’s excited-state absorbance. • pO2 determines the lifetime of an excited oxygen-sensitive dye by Stern-

Volmer relationship. • Methylene blue (MB) is utilized as an oxygen-sensing imaging contrast agent. • PA imaging is used to map the pO2-dependent lifetime distribution.

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Lifetime-pO2 relation Photophysics of MB

PA lifetime probing with pump-probe delay approach Pump: the laser excites the dye, Probe: the laser detects the transient (triplet-triplet) absorption after excitation τ: the pump-probe delay, set by FPGA; t=distance between object and transducer/speed of sound. Lifetime (T) is computed by exponential fitting PA amplitude to τ.

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In vivo multimodal imaging system and breathing modulation model • Ultrasound, photoacoustic imaging, and PALI images using the same ultrasound

system and phased array transducer. • The part of animal intended as imaging target is illuminated by two lasers (pump

and probe). • The percentage of oxygen the mouse inhales is controlled by gas mixer.

• PALI is able to reflect the change of oxygen level with respect to normal, oxygen-rich and oxygen-poor breathing conditions

• Combined US/PALI images are shown (pO2 in mmHg color scale, US in 40dB gray scale).

(a-c)The numbers are given as average ± standard deviation for all valid pixel values displayed. (e)Simultaneous recording from the optical oxygen. Periods of gas mixture and PALI images are indicated.

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Multimodal imaging of a tumor-bearing mouse • Hypoxia region was identified within the tumor. (a) PA image of the transient absorption of MB with an 810-nm laser at a pump–probe delay of 0.25 μs, superimposed on the US image. (b) Transient PA amplitudes of two representative pixels.

PALI tracking of pO2 during breathing modulation

PALI tracking of pO2 during acute ischemia • Acute ischemia is induced by blood flow restricting in upper part hindlimb of normal

mice. • Combined US/PALI images are shown before (normal), during flow restriction

(ischemia), immediately after releasing the restriction (reperfusion). • Case I and II on two mice. • Statistics of the pixel values imaged were given by average and standard deviation.

Therapy/imaging system

Hardware PDT PALI Excitation laser √a √

Oxygen-sensitive dye/ Photosensitizerb √ √ Light guide √ √ Probe laser √

Ultrasound array transducer √ Controlling and processing electronics √

Functionality PDT PALI Cell destruction √

Photosensitizer accumulation √ Ultrasound anatomical imaging √

Tissue oxygen (pO2) distribution √ Other functional information (e.g. SO2) √c

Real-time feedback √ PDT, photodynamic therapy aPDT typically use continuous laser, while pulse laser is require for photoacoustic imaging,bphotosensitizer other than Methylene blue is under testing, cNeeds switching laser wavelength by tunable laser

Reference and acknowledgement Q. Shao et al., In vivo photoacoustic lifetime imaging of tumor hypoxia in small animals. Journal of Biomedical Optics 2013 Q. Shao et al., Noninvasive tumor oxygen imaging by photoacoustic lifetime imaging integrated with photodynamic therapy. SPIE Proc 2014 Q. Shao et al., Direct tissue oxygen monitoring by in vivo photoacoustic lifetime imaging (PALI). SPIE Proc 2014 NIH R21CA135027 NIH R21EB016763