The challenge of ultrasound tomography in the presence of high impedance contrast is well known. We have successfully used full 3D transmission inverse scattering and refraction corrected reflection tomography to create 3D high-resolution images of the human breast. However, these tissues do not encompass the high contrast that occurs in orthopaedics scenarios, such as the human knee, where cranial and trabecular bone are present. Even though the high contrast of the bone is problematic for model based iterative reconstruction methods, we successfully image the tissue near, and in, the Femur-Tibia (F-T) space using an adapted QT Ultrasound Scanner and adapted inverse scattering algorithm.
We show preliminary reconstructions of a cadaver knee that indicates that we can quantitatively and accurately image proximal soft tissue structures. We give correlations between MR images and QT Ultrasound transmission images that show correlation with known structures: besides the femur, tibia, and fibula, we see the condyle structures (medial and lateral), medial and lateral menisci internal to the F-T space, collateral ligaments, infrapatellar fat pad (Hoffa’s pad), patellar ligament, and various ligaments, tendons and musculature in the leg above and below the knee.
We establish that a substantially different reconstruction protocol (than that of the breast) for 3D inverse scattering is required to obtain these images and we discuss the implications of these changes. These preliminary results show that high resolution of clinically relevant tissue is feasible with ultrasound tomography even within the F-T space.
There is a need to provide better imaging methods for infants as there are few good options. CT can provide reasonable image quality with limited soft tissue contrast at a cost of large radiation dose. MRI can provide better soft tissue contrast, but the small size of an infant produces poor signal to noise and thus long scan times. Both types require anesthesia, which carries a substantial mortality risk for young patients and especially sick ones. Ultrasound imaging has been principally relegated to relatively simple applications in in orthopedics and diagnostics due to the inability to achieve high resolution at depth in complex structures. Quantitative Transmission (QT) Ultrasound relies on low frequency information which has greater penetrating power and 3D Inverse Scattering to produce high resolution and contrast at substantial depth. We built a prototype device for imaging small animals and tested the performance on 7-10lb piglets to simulate the conditions necessary to scan a newborn infant human. Image acquisition was entirely conventional with the currently available QT ultrasound breast imagers, but reconstruction required significant modification to deal with the additional complexity. We report on the changes in methods as well as the preliminary performance of the system in this configuration.
There has been a great deal of research into ultrasound tomography for breast imaging over the past 35 years. Few successful attempts have been made to reconstruct high-resolution images using transmission ultrasound. To this end, advances have been made in 2D and 3D algorithms that utilize either time of arrival or full wave data to reconstruct images with high spatial and contrast resolution suitable for clinical interpretation. The highest resolution and quantitative accuracy result from inverse scattering applied to full wave data in 3D. However, this has been prohibitively computationally expensive, meaning that full inverse scattering ultrasound tomography has not been considered clinically viable. Here we show the results of applying a nonlinear inverse scattering algorithm to 3D data in a clinically useful time frame. This method yields Quantitative Transmission (QT) ultrasound images with high spatial and contrast resolution. We reconstruct sound speeds for various 2D and 3D phantoms and verify these values with independent measurements. The data are fully 3D as is the reconstruction algorithm, with no 2D approximations. We show that 2D reconstruction algorithms can introduce artifacts into the QT breast image which are avoided by using a full 3D algorithm and data. We show high resolution gross and microscopic anatomic correlations comparing cadaveric breast QT images with MRI to establish imaging capability and accuracy. Finally, we show reconstructions of data from volunteers, as well as an objective visual grading analysis to confirm clinical imaging capability and accuracy.