Optical/Acoustic Radiation Imaging (OARI) is a novel imaging modality that is being developed to interrogate the optical and mechanical properties of soft tissues. OARI involves the use of acoustic radiation force (ARF) to generate displacement in soft tissue. The optical images before and after the application of the force are used to generate displacement maps that provide information about the mechanical properties of the tissue under interrogation. Since the images are optical images, they also represent the optical properties of the tissue as well. In this dissertation, we propose the use of acoustic radiation force with optical coherence tomography (OCT) to provide information about both the optical and mechanical properties of soft tissues to assist in the diagnosis and staging of epithelial cancer, and in particular bladder cancer.
To test this new imaging modality, we developed a protocol to produce phantoms that possess mechanical, acoustic, and optical properties similar to those of the urinary bladder wall. The phantom is made up of gelatin, polystyrene and copolymer microspheres, and bovine serum albumin (BSA). The phantom possesses a speed of sound of 1591 m/s, an attenuation coefficient of 0.66 dB/cm/MHz, an optical scattering coefficient of 1.80 mm-1 at 1304 nm, a density of 1.054 g/cm3, and a Young's Modulus of 17.12 kPa. This phantom was designed as a tool to evaluate the feasibility of OARI for detecting the mechanical and optical changes that may be indicative of the onset or development of cancer in the urinary bladder. Using the bladder wall phantom model, we first demonstrated the feasibility of OARI in the urinary bladder using a bench top OARI system. The minimum resolvable OARI displacement of the system was approximately 6 μm. A maximum displacement of 84 μm in bladder wall phantoms was observed using an ISPTA.3 of 112.1 W/cm2. The bench top OARI was suitable for phantoms and excised tissues only, and not for in vivo measurements. To make OARI possible for in vivo applications, we designed and fabricated a prototype probe.
The OARI probe consists of an OCT probe encased in a plastic sheath, a miniaturized transducer glued to a plastic holder, both of which are encased in a 10 cm long stainless steel tube with an inner diameter of 10 mm. The tube is filled with deionized water for acoustic coupling and is covered by a low density polyethylene (LDPE) membrane cap. The characterization of the OARI probe yielded an ISPTA.3 of 17.51 W/cm2 at a transmit voltage of 100 Vp-p. At this acoustic intensity, we obtained displacements of 7.9 μm, 5.93 μm, and 2.15 μm for the 3%, 4%, and 5% bladder wall phantoms respectively. The corresponding theoretical finite element model (FEM) displacement was 5.8 μm, 5.4 μm, and 5.0 μm for the 3%, 4%, and 5% bladder wall phantoms. The FEM displacement and the OARI displacement deviated by 26.6%, 8.9 % and 57% for the 3%, 4%, and 5% phantoms. The results were comparable to what we obtained from the bench top OARI.
Future work will focus on utilizing phase-sensitive optical coherence elastography (OCE) to obtain the resulting OARI displacements, as opposed to the current cross correlation algorithm that was employed in this dissertation. This would improve the resolution of the probe, and enable physicians to adequately characterize soft tissues.
The OARI probe has the ability to characterize phantoms and soft tissue. This could prove useful in early epithelial cancer detection. Because the probe is 10 mm in diameter, it is currently only useful for skin and oral applications. The probe would have to be reduced in size to make it applicable for cancer detection in other internal sites.
|Advisor:||Zara, Jason M.|
|Commitee:||Kay, Matthew, Loew, Murray H., Manyak, Michael J., Sadeghi, Nader, Zderic, Vesna|
|School:||The George Washington University|
|School Location:||United States -- District of Columbia|
|Source:||DAI-B 73/06, Dissertation Abstracts International|
|Subjects:||Biomedical engineering, Electrical engineering, Medical imaging|
|Keywords:||Acoustic radiation force, Epithelial cancer detection, Focused ultrasound, Optical coherence tomography, Phantoms|
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