One of the leading causes of medical malpractice claims in emergency medicine is the misdiagnosis of the presence of
foreign bodies. Radiolucent foreign bodies are especially difficult to differentiate from surrounding soft tissue, gas, and
bone using existing clinical imaging modalities. Because many radiolucent foreign bodies have sufficient contrast for
imaging in the optical domain, we are exploring the use of
laser-induced optoacoustic imaging for the detection of
foreign bodies, especially in orbital and craniofacial injuries, in which the foreign bodies are likely to lie within the
penetration depth of visible and near infrared wavelengths. In order to evaluate the performance of optoacoustic
imaging for clinical detection and characterization, common foreign bodies have been scanned over a range of visible
and near infrared wavelengths to obtain the spectroscopic properties of the materials commonly associated with these
foreign bodies. The foreign bodies are also being embedded in realistic ex vivo tissue phantoms to evaluate the changes
that may occur in the spectroscopic absorption of the materials due to the interaction with tissue absorbers. Ultimately,
we anticipate that spectroscopic characterization will help identify specific wavelengths to be used for imaging foreign
bodies that will provide useful diagnostic data about the material properties of the object, thereby enabling the
characterization, as well as the location, of the objects. This information will aid the clinician in choosing the optimal
treatment course for the patient.
One of the leading causes of medical malpractice claims in emergency medicine is the misdiagnosis of the presence of
foreign bodies. Radiolucent foreign bodies are especially difficult to differentiate from surrounding soft tissue, gas, and
bone. Current imaging modalities employed for the detection of foreign bodies include: X-ray computed tomography,
magnetic resonance, and ultrasound; however, there is no consensus as to which modality is optimal for diagnosis.
Because many radiolucent foreign bodies have sufficient contrast for imaging in the optical domain, we are exploring the
use of laser-induced optoacoustic imaging for the detection of foreign bodies, especially in craniofacial injuries, in which
the foreign bodies are likely to lie within the penetration depth of visible and near infrared wavelengths. Tissue-simulating
phantoms containing various common foreign bodies have been constructed. Images of these phantoms have
been successfully generated using two laser-based optoacoustic imaging methods with different detection modalities. In
order to enhance the image contrast, common foreign bodies are being scanned over a wide range of wavelengths to obtain the spectroscopic properties of the materials commonly associated with these foreign bodies. This spectroscopic
characterization will help select specific wavelengths to be used for imaging specific objects and provide useful diagnostic data about the material properties of the object.
Detection of non-radio-opaque foreign bodies can be difficult. Current imaging modalities employed for detection of
foreign bodies include: X-ray computed tomography, magnetic resonance, and ultrasound. Successful diagnosis of the
presence of foreign bodies is variable because of the difficulty of differentiating them from soft tissue, gas, and bone.
We are applying laser-induced optoacoustic imaging to the detection of foreign bodies. Tissue-simulating phantoms
containing various common foreign bodies have been constructed. Images of these phantoms were generated by two
laser-based optoacoustic methods utilizing different detection modalities. A pre-commercial imager developed by Seno
Medical Instruments (San Antonio), incorporated an ultrasound transducer to detect induced optoacoustic responses,
while a laboratory-built imaging system utilized an optical probe beam deflection technique (PBDT) to detect the
optoacoustic responses. The laboratory-built unit also included an optical parametric oscillator as the pump, providing
tunable wavelength output to optimize the optoacoustic measurements by probing the foreign bodies at their maximum
optical absorption. Results to date have been encouraging; both methodologies have allowed us to reconstruct
successfully the image of foreign-body containing phantoms. In preliminary work the PBDT approach appeared to
produce higher resolution than did the ultrasound detector, possibly because PBDT is not constrained by the lower
bandwidth limit imposed on the ultrasound transducer necessary to increase imaging depth. During the research in
progress, we will compare the optoacoustic images to those generated by MRI, CT, and ultrasound, and continue to
improve the resolution of the technique by using multiple detection sensors, and to improve image contrast by scanning
foreign bodies over a range of wavelengths.
Bacterial contamination can be detected using a minimally invasive optical method, based on laser-induced
optoacoustic spectroscopy, to probe for specific antigens associated with a specific infectious agent. As a model
system, we have used a surface antigen (Ag), isolated from Chlamydia trachomatis, and a complementary antibody
(Ab). A preparation of 0.2 mg/ml of monoclonal Ab specific to the C. trachomatis surface Ag was conjugated to
gold nanorods using standard commercial reagents, in order to produce a targeted contrast agent with a strong
optoacoustic signal. The C. trachomatis Ag was absorbed in standard plastic microwells, and the binding of the
complementary Ab-nanorod conjugate was tested in an immunoaffinity assay. Optoacoustic signals were elicited
from the bound nanorods, using an optical parametric oscillator (OPO) laser system as the optical pump. The
wavelength tuneability of the OPO optimized the spectroscopic measurement by exciting the nanorods at their
optical absorption maxima. Optoacoustic responses were measured in the microwells using a probe beam deflection
technique. Immunoaffinity assays were performed on several dilutions of purified C. trachomatis antigen ranging
from 50 μg/ml to 1 pg/ml, in order to determine the detection limit for the optoacoustic-based assay. Only when the
antigen was present, and the complementary Ab-NR reagent was introduced into the microwell, was an enhanced
optoacoustic signal obtained, which indicated specific binding of the Ab-NR complex. The limit of detection with
the current system design is between 1 and 5 pg/ml of bacterial Ag.
Carbon dioxide lasers are used in numerous applications that involve human exposure to
the radiation that can produce ocular injury. The objective of this study is to show that
the thermal gradient produced in the eye by the radiation from an 80 ns CO2 laser pulse
can generate a thermoacoustical tensile pressure wave with large enough magnitude to
rupture the epithelial layer of the cornea. A Gaussian-shaped temperature distribution
will be employed. It is assumed that the corneal tissue is inhomogeneous, with the
density and wave velocity varying slowly in space. Under these conditions, the
acoustical wave equation is decoupled into two first-order partial differential equations,
one that propagates energy into the eye from the point of thermoacoustical wave
generation, and the other toward the front of the eye. These equations are solved
numerically using the Lax-Wendroff numerical method. A compressional wave
generated in the epithelial tissue of the cornea due to the thermal gradient of the laser
arrives at the air-tear layer interface with a pressure amplitude of ~6600 Pa. When this
wave is reflected back into the eye, the resulting tensile pressure wave has a tensile
strength of approximately 4.6 x 108 Pa/m just inside of the epithelial layer of the cornea.
This is an order of magnitude larger than what is necessary to produce cellular damage to
the cornea.
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