Photoacoustic Cystography

Summary

Photoacoustic cystography (PAC) has a great potential to map urinary bladders, a radiation sensitive internal organ in pediatric patients, without using any ionizing radiation or toxic contrast agent. Here we demonstrate the use of PAC for mapping urinary bladders with an injection of optical-opaque tracers in rats in vivo.

Abstract

Conventional pediatric cystography, which is based on diagnostic X-ray using a radio-opaque dye, suffers from the use of harmful ionizing radiation. The risk of bladder cancers in children due to radiation exposure is more significant than many other cancers. Here we demonstrate the feasibility of nonionizing and noninvasive photoacoustic (PA) imaging of urinary bladders, referred to as photoacoustic cystography (PAC), using near-infrared (NIR) optical absorbents (i.e. methylene blue, plasmonic gold nanostructures, or single walled carbon nanotubes) as an optical-turbid tracer. We have successfully imaged a rat bladder filled with the optical absorbing agents using a dark-field confocal PAC system. After transurethral injection of the contrast agents, the rat's bladders were photoacoustically visualized by achieving significant PA signal enhancement. The accumulation was validated by spectroscopic PA imaging. Further, by using only a laser pulse energy of less than 1 mJ/cm² (1/20 of the safety limit), our current imaging system could map the methylene-blue-filled-rat-bladder at the depth of beyond 1 cm in biological tissues in vivo. Both in vivo and ex vivo PA imaging results validate that the contrast agents were naturally excreted via urination. Thus, there is no concern regarding long-term toxic agent accumulation, which will facilitate clinical translation.

Introduction

X-ray cystography¹ is an imaging process to identify bladder-related diseases such as bladder cancer, vesicoureteral reflux, blockage of ureters, neurogenic bladder, etc.^2-5 Typically, urines are voided and a radio-opaque agent is injected through a catheter. Then, fluoroscopic X-ray images are acquired to delineate urinary bladders. However, the key safety issue is that harmful ionizing radiation is used in this procedure. The percentage of cumulative cancer risk to age 75 years owing to diagnostic X-rays ranges from 0.6 to 1.8%.⁶ In addition, the carcinogenic threat is significant in pediatric patients. A UK study showed tha....

Protocol

1. Deep Reflection Mode Photoacoustic Cystography (PAC) System

1. System configuration^{17, 18}
   1. A Q-switched Nd:YAG laser (SLII-10; Continuum; 532 nm) pumps a wavelength-tunable laser (Surelite OPO PLUS; Continuum; wavelength tuning range: 680 to 2,500 nm).
   2. The pulse duration of each laser shot is ~5 nsec, and the laser repetition rate is 10 Hz.
   3. The wavelength depends on the optical absorption peak of the used contrast agent. If methylene blue serves as the contrast agent, an optical wavelength of 667 nm is used, where the peak absorption is. Plasmonic gold nanostructures can have tunable localized surface plasmon res....

Results

Figure 1 shows the In vivo nonionizing and noninvasive PAC using optically turbid methylene blue (MB). The control PA image was obtained at 667 nm, at the peak optical absorption for MB (Figure 1A). Although the blood vessels in the FOV are clearly visualized, the bladder is invisible because it is optically transparent at this wavelength. As shown in Figure 1B, the bladder is clearly revealed in the PA image acquired at 0.2 hr post-injection of MB. To confirm t.......

Discussion

In conclusion, we have shown the possibility of nonionizing PAC using nontoxic optical absorbers in a rat model in vivo. We have successfully imaged a rat bladder filled with optical absorbents using our nonionizing and noninvasive PAC system. Two critical safety issues have been resolved in our approach: (1) the use of nonionizing radiation for cystographic applications and (2) no accumulation of contrast agents in the body.

Our clinical interest includes monitoring vesicoureteral re.......

Materials

Name	Company	Catalog Number	Comments
Name of Reagent/Material	Company	Catalog Number	Comments
Q-switched Nd:YAG laser	Continuum	SLII-10	pump laser
OPO laser	Continuum	Surelite OPO PLUS	tunable laser
Prisms	Thorlabs	PS908	light deliver
Ultrasound transducer	Olympus NDT	V308	5 MHz
Ultraoundpulser/receiver	Olympus NDT	5072PR	amplifier
Oscilloscope	Tektronix	TDS5054	data acquisition
Scanning stage	Danaher Dover	XY6060	raster scanning
Methylene blue	Sigma-Aldrich	M9140-25G	contrast agent
Rats	Harlan	Spague-Dawley	animal subject
Isoflourane vaporizer	Euthanex	EZ-155	anesthesia
Ultrasound gel	Sonotech	Clear Image singles	acoustic coupling