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 that among 9 major internal organs, the average annual radiation dose from diagnostic X-rays was highest in the bladders in female children less than 4 and second highest in male children less than 4.⁷ This indicates that the bladder cancer risk is most significant in pediatric patients. Although pediatric radiologists endeavor to reduce the radiation exposure rate as low as reasonably achievable, ionizing radiation cannot be completely excluded. Therefore, the limitation creates a need for a completely radiation-free, sensitive, cost-effective, and high-resolution imaging modality with nonradioactive contrast agents in cystography.

Recently, photoacoustic tomography (PAT) has become a premier biomedical imaging modality because PAT can provide strong optical absorption contrasts and a high ultrasonic spatial resolution in biological tissues.⁸ The principle of PAT is that ultrasonic waves are induced due to thermoelastic expansion of a target followed by light absorption. By detecting time-resolved acoustic waves travelling via a medium, two- or three-dimensional photoacoustic (PA) images are formed. Because ultrasound (US) is much less scattered in tissues compared to light (typically two or three orders of magnitude), the imaging depth of PAT can reach up to ~8 cm in tissues, while the spatial resolution is maintained to 1/200 of the imaging depth.⁹ The key advantages of PAT for the cystographic application include: (1) PAT is completely free from ionizing radiation. (2) ClinicalUSimaging systems can be easily adapted to supply dual-modal PA and US imaging capabilities. Thus, the dual-modal PA/US imaging system can be relatively portable, cost-effective, and fast, which are key criteria for fast clinical translation. Using both endogenous and exogenous contrasts, PAT has provided high-resolution morphological, functional, and molecular imaging of tissues to study tumor physiopathology, brain hemodynamics, internal organs, ophthalmology, angiography, and etc. ^10-16

In this article, we demonstrate the experimental protocols of nonionizing photoacoustic cystography (PAC) using near-infrared (NIR) optical absorbents (i.e. methylene blue, gold nanocages, or single walled carbon nanotubes) as nontoxic optical-turbid tracers. Rat bladders filled with the contrast agents were photoacoustically and spectroscopically delineated in vivo. No agents persistently accumulated in the bladders and kidneys of the rats. Thus, long-term toxicity which may be caused by agent accumulation can be excluded. This result implies that PAC with combination of the optical absorbers can potentially be a truly harmless cystographic modality for pediatric patients. The system configuration, system alignment, and in vivo / ex vivo imaging procedures are discussed in this article.

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 resonance at the NIR spectral region, based on their physical and chemical properties. Further, the broad absorption spectrum of single walled carbon nanotubes can provide a wide option for wavelength selection.
   4. Light coming out of the tunable laser is delivered to a spherical conical lens through right angle prisms (PS908, Thorlabs).
   5. A donut-shaped light beam pattern is generated after the light passes through the spherical conical lens. The home-made conical lens is made from a BK7 lens and the cone angle is 152°. The diameter of the lens is 2.5 cm.
   6. The diverging donut-shaped light beam is redirected via an optical condenser, made of a transparent acrylic sheet. The diameters of the top and bottom surfaces are 6.1 and 4.8 cm, respectively. The thickness of the condenser is 2.5 cm. The redirected ring-shaped light beam forms a donut-shaped with a dark center on the tissue surface.
   7. Utilize a small water container to boost acoustic coupling. The water container has a bottom opening wrapped with a clear thin polyethylene film which is optically and acoustically transparent. Small animals are positioned under the water container.
   8. The generated PA waves are detected by a spherically focused ultrasound transducer (V308; Olympus NDT; 5 MHz central frequency), which is mounted in the middle of the optical condenser. The element diameter and focal length of the transducer are 1.9 and 2.5 cm, respectively. Thus, the f-number of the transducer is ~1.3.
   9. The transverse and axial resolutions are 590 and 144 μm, respectively.
   10. The detected PA waves are first amplified by a broadband ultrasonic pulser/receiver (5072PR; Olympus NDT; 35 MHz bandwidth and 59 dB rf gain), and then acquired by an oscilloscope (TDS5054; Tektronix).
   11. Obtain one-dimensional time-resolved images (called as A-line) by measuring the times of arrival of the PA waves. Currently, the speed of PA waves is assumed to be at 1,480 m/sec over the entire image acquisition. Two- (called as B-scans) and three-dimensional PA images can be acquired by mechanically moving the linear raster scanning stage (XY6060; Danaher Dover).
   12. The imaging time is ~25 min for one volumetric single-wavelength PA image with a field of view (FOV) of 2.5 x 2.4x 1.5 cm³ in the x-y-z planes. We acquired 125 samples along the x direction with a step size of 0.2 mm, 60 samples along the y direction with a step size of 0.4 mm. 500 data points with a 50-MHz sampling rate were obtained along the z direction. The mechanical scanning is controlled by a home-made NI LabVIEW software system.
   13. The volumetric data is represented by maximum amplitude projection (MAP) using a MathWorks MATLAB software system.
2. System alignment
   1. After the spherical conical lens, make sure that the beam pattern is a perfect ring shape. If the donut-shaped beam pattern is not properly generated, the PA signals originated from the skin surface are dominant. Thus, it is difficult to achieve deep tissue imaging.
   2. The line-shaped light focus in water should be coaxially aligned with the ultrasound focal zone. If those are not coaxially aligned, the system suffers from low signal-to-noise ratio.
   3. The irradiated laser pulse energy on the skin surface is varied from ~1 - 2 mJ/cm² when the wavelength is tuned from 680 to 1,000 nm, respectively. These laser pulse energies are much lower than the American National Standards Institute safety limit, varying from 20 to 80 mJ/cm² over the spectral region, respectively.

2. In vivo and Ex vivo Imaging Procedures

1. Animal preparation
   1. Use female Spraque-Dawley rats with a weight of 200 - 250 g in all PA imaging experiments.
   2. To begin with, anesthetize the rat by intraperitoneal injection of a mixture of ketamine (85 mg/kg of body weight) and xylazine (15 mg/kg).
   3. Depilate the hairs in the abdominal area.
   4. Position the rat atop of a custom-made animal holder.
   5. Coat a 22-gauge catheter with lubricant to improve the catheter insertion.
   6. Hold the catheter vertically over the urethral opening. Next, insert the distal end of the catheter, horizontally, into the urethra until the hub of the catheter finally reaches the opening.
   7. Urine in the bladder will void via the catheter.
2. In vivo PAC imaging
   1. Position the rat, located on top of the animal holder, below the water container in the PAC system.
   2. Apply ultrasound gel (Sonotech) between the animal skin surface and plastic membrane to improve acoustic coupling.
   3. Fully anesthetize the rat using vaporized isoflurane (1 L/min of oxygen and 0.75% isoflurane) during the in vivo PA imaging experiments.
   4. Obtain a control PA image prior to injection of contrast agents.
   5. Introduce an aqueous solution of methylene blue (0.8 μl/g body weight with a concentration of 30 mM), gold nanocages (1.2 μl/g body weight with a concentration of 2 nM), or single walled carbon nanotubes (0.8 μl/g body weight with a concentration of 0.3 μM) to the bladder via the catheter. Use a 1-ml-syringe with a 22-gauge catheter.
   6. Acquire a series of PA images.
3. Ex vivo PAC imaging
   1. Sacrifice the rat after in vivo PAC imaging, by injecting an overdose of pentobarbital.
   2. Remove the two major organs, bladders and kidneys, to investigate biodistribution, and on a glass plate.
   3. Position the glass plate below the water container in the PAC system.
   4. Apply ultrasound gel (Sonotech) between the excised organs and plastic membrane to improve acoustic coupling.
   5. Acquire PA images.
   6. Photoacoustically image the organs removed from a healthy rat as a control.

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

List of materials used in this article

Name	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