In diffuse optical tomography (DOT), a typical perturbation approach requires two sets of measurements obtained at the lesion breast (lesion or target site) and a contra-lateral location of the normal breast (reference site) for image reconstruction. For patients who have a small amount of breast tissue, the chest-wall underneath the breast tissue at both sites affects the imaging results. In this group of patients, the perturbation, which is the difference between measurements obtained at the lesion and reference sites, may include the information of background mismatch which can generate artifacts or affect the reconstructed quantitative absorption coefficient of the lesion. Also, for patients who have a single breast due to prior surgery, the contra-lateral reference is not available. To improve the DOT performance or overcome its limitation, we introduced a new method based on an exogenous contrast agent and demonstrate its performance using animal models. Co-registered ultrasound was used to guide the lesion localization. The results have shown that artifacts caused by background mismatch can be reduced significantly by using this new method.
We present a photoacoustic tomography-guided diffuse optical tomography approach using a hand-held probe for detection and characterization of deeply-seated targets embedded in a turbid medium. Diffuse optical tomography guided by coregistered ultrasound, MRI, and x ray has demonstrated a great clinical potential to overcome lesion location uncertainty and to improve light quantification accuracy. However, due to the different contrast mechanisms, some lesions may not be detectable by a nonoptical modality but yet have high optical contrast. Photoacoustic tomography utilizes a short-pulsed laser beam to diffusively penetrate into tissue. Upon absorption of the light by the target, photoacoustic waves are generated and used to reconstruct, at ultrasound resolution, the optical absorption distribution that reveals optical contrast. However, the robustness of optical property quantification of targets by photoacoustic tomography is complicated because of the wide range of ultrasound transducer sensitivity, the orientation and shape of the targets relative to the ultrasound array, and the uniformity of the laser beam. We show in this paper that the relative optical absorption map provided by photoacoustic tomography can potentially guide the diffuse optical tomography to accurately reconstruct target absorption maps.
A handheld photoacoustic tomography-guided diffuse optical tomography system for imaging deeply-seated targets in
scattering media is presented. This hybrid imager consists of a probe with an ultrasound transducer in the center and
straddled by two optical fibers for taking photoacoustic images. The diffuse optical tomography component comprises of
9 light-source fibers for delivering light to the imaged tissue, and 14 detector fibers for collecting the light. Single- and
two-phantom targets of high and low optical contrasts were immersed in a scattering intralipid solution to depths of up to
3cm and imaged. The reconstructed absorption coefficients of the targets with guidance from photoacoustic tomography
were compared to those obtained with a-priori depth-only information, and no a-priori information. The reconstructed
absorption maps yielded as much as 2.6-fold improvement in the quantification accuracy compared to the cases with no
guidance from photoacoustic tomography.
Currently, most of the cancers in the ovary are detected when they have already metastasized to other parts of
the body. As a result, ovarian cancer has the highest mortality of all gynecological cancers with a 5-year survival rate of
30% or less [1]. The reason is the lack of reliable symptoms as well as the lack of efficacious screening techniques [2,3].
Thus, there is an urgent need to improve the current diagnostic techniques.
We have investigated the potential role of co-registered photoacoustic and ultrasound imaging in ovarian cancer
detection. In an effort to bring this technique closer to clinical application, we have developed a co-registered ultrasound
and photoacoustic transvaginal probe. A fiber coupling assembly has been developed to deliver the light from around the
transducer for reflection geometry imaging. Co-registered ultrasound and photoacoustic images of swine ovaries through
vagina wall muscle and human ovaries using the aforementioned probe, demonstrate the potential of photoacoustic
imaging to non-invasively detect ovarian cancer in vivo.
The chest wall underneath breast tissue distorts light reflection measurements, especially measurements obtained from distant source-detector pairs. For patients with a chest wall located at a shallower depth, the chest-wall effect needs to be considered in the image reconstruction procedure. Following our previous studies, this work systemically evaluates the performance of a two-layer model-based reconstruction using the finite element method, and compares it with the performance of the semi-infinite model. The results obtained from simulations and phantom experiments show that the two-layer model improves the light quantification of the targets. The improvements are attributed to improved background estimation and more accurate weight matrix calculation using a two-layer model compared to the semi-infinite model. Fitted two-layer background optical properties obtained from a group of ten patients with chest walls located less than 2 cm deep are more representative of breast tissue and chest-wall optical properties.
Ovarian cancer has a five-year survival rate of only 30%, which represents the highest mortality of all
gynecologic cancers. The reason for that is that the current imaging techniques are not capable of detecting ovarian
cancer early. Therefore, new imaging techniques, like photoacoustic imaging, that can provide functional and molecular
contrasts are needed for improving the specificity of ovarian cancer detection and characterization. Using a coregistered
photoacoustic and ultrasound imaging system we have studied thirty-one human ovaries ex vivo, including
normal and diseased. In order to compare the photoacoustic imaging results from all the ovaries, a new parameter using
the RF data has been derived. The preliminary results show higher optical absorption for abnormal and malignant
ovaries than for normal postmenopausal ones. To estimate the quantitative optical absorption properties of the ovaries,
additional ultrasound-guided diffuse optical tomography images have been acquired. Good agreement between the two
techniques has been observed. These results demonstrate the potential of a co-registered photoacoustic and ultrasound
imaging system for the diagnosis of ovarian cancer.
The chest wall underneath the breast tissue affects near-infrared (NIR) diffusive waves measured with reflection geometry. With the assistance of a co-registered ultrasound, the depth and the tilting angle of the chest wall can be determined and are used to model the breast as a two-layer medium. Finite element method (FEM) is suitable for modeling complex boundary conditions and is adapted to model the breast tissue and chest wall. Four parameters of bulk absorption and reduced scattering coefficients of these two layers are estimated and used for imaging reconstruction. Using a two-layer model, we have systematically investigated the effect of the chest wall on breast lesion reconstruction. Results have shown that chest-wall depth, titling angle, and difference between optical properties of two layers of lesion and reference sites affect the lesion reconstruction differently. Our analysis will be valuable and informative to researchers who are using reflectance geometry for breast imaging. The analysis can also provide guidelines for imaging operators to minimize image artifacts and to produce the best reconstruction results.
We report experimental investigations of photoacoustic guidance of diffusive optical
tomography for detection and characterization of optical contrast targets. The hybrid
system combined an 8-source, 10-detector frequency domain DOT with a clinical
reflection geometry probe. For the photoacoustic tomography (PAT) functionality, a
high-energy 1×7 optical fiber delivery system illuminated a 2 cm central region for
localization of absorptive targets. Two-dimensional PAT images along one central axis
of the probe defined of regions of interest for a dual-zone mesh DOT imaging algorithm.
PVC Plastisol phantom absorbers, 1 cm on a side, with absorption coefficients ranging
from 0.075 to 0.23 cm-1 were imaged at depths up to 2.5 cm. Pairs of absorbers
simulating a multi-lobed heterogeneous tumor were also investigated. Without PAT
guidance, the absorber location was not clear and lower contrast targets in the twoabsorber
configurations were not distinguishable. With PAT guidance, the two targets
were well resolved and the reconstructed absorption coefficients improved to within 15%
of the true values.
The chest-wall underneath the breast tissue distorts the diffused near infra-red light measured at distant source-detector
pairs. Common image reconstruction method consider the media as homogeneous and applying the semi-infinite model.
In this paper, we have compared the performance of our two-layer model with semi-infinite model by simulation and a
clinical case. The results show that when the chest wall has significant effect on the measurement data, a benign lesion
with low absorption can be misled as a malignant case with high absorption by using semi-infinite model.
We have also shown the influence of mismatch geometry of breast tissue and chest-wall at lesion and reference sides on
the reconstructed image and a correction method has been introduced to reduce these effects. With the assistance of two
orthogonal co-registered ultrasounds, the geometry of the breast tissue and chest wall interface can be determined and
modeled as a two-layer medium with 3D finite element mesh. Since numerical algorithms based on finite element
methods (FEM) are suitable for complex geometry and boundary conditions, this method is adapted to model the chestwall.
Four parameters of bulk absorption and reduced scattering coefficients of the first and second layers are estimated
and used to characterize the optical properties of the medium. We used a finite element model based on modified born
approximation for image reconstruction. A mismatch correction algorithm has been applied to compensate the mismatch
geometry of the breast tissue and chest-wall interface at the reference and the lesion side.
We develop a new tomographic imaging reconstruction algorithm for a two-layer tissue structure. Simulations and phantom experiments show more accurate reconstruction of target optical properties compared with those results obtained from a semi-infinite tissue model for layered structures. This improvement is mainly attributed to the more accurate estimation of background optical properties and more accurate estimation of weight matrix for imaging reconstruction by considering the light propagation effect in the second layer. Clinical results of breast lesions are also presented to demonstrate the utility of this new imaging algorithm.
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