Ultrasound computed tomography (USCT) is an emerging imaging technique that holds great promise for breast cancer diagnosis and screening. Full-waveform inversion (FWI)-based image reconstruction methods can produce high spatial resolution images that depict the acoustic properties of soft tissues. However, FWI is computationally demanding, especially when wave physics is modeled in three dimensions (3D). A common USCT design employs a circular ring-array comprised of elevation-focused ultrasonic transducers, where the array is translated orthogonally to achieve volumetric (3D) imaging. This design allows efficient two-dimensional (2D) slice-byslice reconstruction methods to estimate a 3D volume by stacking reconstructed cross-sectional images at each ring-array position. However, these 2D methods do not account for the 3D wave propagation physics and the focusing properties of the transducers and thus can result in out-of-plane scattering-based artifacts and inaccuracies. Previous work has investigated a learning-based method, in which a deep neural network was used to map the 3D ring-array USCT data to idealized 2D USCT measurements, from which acoustic properties can be estimated by use of 2D FWI. The presented work extends that previous study in two ways. First, sophisticated anatomically realistic 3D numerical breast phantoms are used to construct clinically relevant training and testing sets. Second, a high-fidelity 3D wave propagation forward imaging model incorporating elevation focusing effects is used to virtually image the phantoms. The results show promise in improving image accuracy compared to both 3D FWI and conventional 2D FWI, achieving both mitigation of out-of-plane scattering and 3D-2D model mismatches and significant reduction of computational cost compared to 3D FWI. Additionally, the use of multiple ring-array measurements from adjacent elevations are explored as concurrent neural network inputs, showing improved accuracy compared to only using single-ring data.
SignificanceWhen developing a new quantitative optoacoustic computed tomography (OAT) system for diagnostic imaging of breast cancer, objective assessments of various system designs through human trials are infeasible due to cost and ethical concerns. In prototype stages, however, different system designs can be cost-efficiently assessed via virtual imaging trials (VITs) employing ensembles of digital breast phantoms, i.e., numerical breast phantoms (NBPs), that convey clinically relevant variability in anatomy and optoacoustic tissue properties.AimThe aim is to develop a framework for generating ensembles of realistic three-dimensional (3D) anatomical, functional, optical, and acoustic NBPs and numerical lesion phantoms (NLPs) for use in VITs of OAT applications in the diagnostic imaging of breast cancer.ApproachThe generation of the anatomical NBPs was accomplished by extending existing NBPs developed by the U.S. Food and Drug Administration. As these were designed for use in mammography applications, substantial modifications were made to improve blood vasculature modeling for use in OAT. The NLPs were modeled to include viable tumor cells only or a combination of viable tumor cells, necrotic core, and peripheral angiogenesis region. Realistic optoacoustic tissue properties were stochastically assigned in the NBPs and NLPs.ResultsTo advance optoacoustic and optical imaging research, 84 datasets have been released; these consist of anatomical, functional, optical, and acoustic NBPs and the corresponding simulated multi-wavelength optical fluence, initial pressure, and OAT measurements. The generated NBPs were compared with clinical data with respect to the volume of breast blood vessels and spatially averaged effective optical attenuation. The usefulness of the proposed framework was demonstrated through a case study to investigate the impact of acoustic heterogeneity on OAT images of the breast.ConclusionsThe proposed framework will enhance the authenticity of virtual OAT studies and can be widely employed for the investigation and development of advanced image reconstruction and machine learning-based methods, as well as the objective evaluation and optimization of the OAT system designs.
Ultrasound computed tomography (USCT) has the potential to detect breast cancer by measuring tissue acoustic properties such as speed-of-sound (SOS). Current USCT image reconstruction methods for SOS fall into two categories, each with its own limitations. Ray-based methods are computationally efficient but suffer from low spatial resolution due to neglecting scattering effects, while full-waveform inversion (FWI) methods offer higher spatial resolution but are computationally intensive, limiting their widespread application. To address these issues, a deep learning (DL)-based method is proposed for USCT breast imaging that achieves SOS reconstruction quality comparable to FWI while remaining computationally efficient. This method leverages the computational efficiency and high-quality image reconstruction capabilities of DL-based methods, which have shown promise in various medical image reconstruction problems. Specifically, low-resolution SOS images estimated by ray-based traveltime tomography and reflectivity images from reflection tomography are employed as inputs to a U-Net-based image reconstruction method. These complementary images provide direct SOS information (via traveltime tomography) and tissue boundary information (via reflectivity tomography). The U-Net is trained in a supervised manner to map the two input images into a single, high-resolution image of the SOS map. Numerical studies using realistic numerical breast phantoms show promise for improving image quality compared to naïve, single-input U-Net-based approaches, using either traveltime or reflection tomography images as inputs. The proposed DL-based method is computationally efficient and may offer a practical solution for enhancing SOS reconstruction quality, which could potentially improve diagnostic accuracy.
Ultrasound computed tomography (USCT) is an emerging medical imaging modality that holds great promise for breast cancer diagnosis. Full-waveform inversion (FWI)-based image reconstruction methods for USCT can produce high spatial resolution and accurate images of the acoustic properties of soft tissues. A common USCT design employs a circular ring-array comprised of elevation-focused ultrasonic transducers. Volumetric imaging can be achieved by translating the ring-array orthogonally to the imaging plane. Slice-by-slice two-dimensional (2D) reconstruction methods have been implemented to form a composite three-dimensional (3D) volumes by stacking together reconstructed cross-sectional images at each ring-array position. However, this 2D approach does not account for the 3D wave propagation physics and the focusing properties of the transducers, and can result in out-of-plane scattering-based artifacts and inaccuracies. To overcome this, a new 3D time-domain FWI method is proposed for ring-array-based USCT that concurrently utilizes measurement data acquired from multiple positions of the ring-array. A virtual imaging study of ring-array-based USCT that employs a realistic 3D numerical breast phantom was conducted to assess the impact of the number of ring-array measurements on image quality.
In the medical imaging field, task-based metrics of image quality have been advocated as a mean to evaluate the performance of imaging systems and/or reconstruction algorithms. One such way of obtaining these metrics is through a numerical observer. Although the Bayesian ideal observer is optimal by definition, it is frequently intractable and nonlinear. Therefore, linear approximations to the IO are sometimes used to obtain task-based statistics. The optimal linear observer for maximizing the signal-to-noise ratio (SNR) of the test statistic is the Hotelling Observer (HO). However, the computational cost for obtaining the HO increases with image size and becomes intractable for large scale images. In multimodal data, this further becomes an issue because each additional modality dramatically increases the size of the composite image. An alternative to obtaining the HO is approximating the test statistic using a feed-forward neural network (FFNN). However, these methods of learning the HO have not been evaluated on multi-modal data. In this work, a tractable learned multi-modal observer is implemented. The considered task is a signal-known-statistically/background known statistically binary signal detection task. A stylized operator representing an ultrasound computed tomography imaging system and numerical breast phantoms with speed of sound and attenuation modalities are considered. The considered signal is a microcalcification cluster with a random amplitude. It is demonstrated that the learned HO can closely approximate the HO for the considered task.
Ultrasound computed tomography (USCT) is an emerging imaging modality that holds great promise for breast cancer diagnosis. Full-waveform inversion (FWI)-based image reconstruction methods can produce high spatial resolution images of the acoustic properties of the breast tissues. A practical design of breast USCT systems employs a circular elevation-focused ring-array of ultrasonic transducers, where the data is acquired by translating the ring-array orthogonally to the imaging plane. This design allows a fast slice-by-slice (SBS) reconstruction approach by stacking together two-dimensional (2D) images reconstructed for each position of the transducer array. However, the SBS approach assumes 2D propagation physics and usually employs simplified transducer models. Because of this, the reconstructed images can sometimes contain significant artifacts. Thus, threedimensional (3D) imaging models that incorporate the focusing effects of transducers are needed to improve image quality in ring-array-based USCT. To address this, a 3D elevation-focused transducers model for use within a 3D time-domain pseudospectral wave propagation method was developed. The proposed method uses a stylized (line source) geometry of the transducer and achieves elevational focusing by applying a spatially varying delay to the ultrasound pulse (emitter mode) and recorded signal (receiver mode). The proposed transducer model was validated against a semi-analytical method based on the Rayleigh-Sommerfield diffraction integral solution. In addition, a virtual imaging study that utilizes a 3D anatomical numerical breast phantom and the proposed transducer model is presented. The results demonstrate that 3D FWI-based reconstruction methods that incorporate the proposed transducer model hold promise for improving image quality in ring-array-based USCT.
In silico studies for ultrasound computed tomography (USCT) can allow to explore imaging system parameters and reconstruction methods, without the economic burden and ethical concerns of clinical trials. A framework is proposed for virtual imaging trials of USCT. First, an ensemble of three-dimensional numerical breast phantoms consisting of anatomically realistic tissue structures and lesions is created. Next, acoustic properties are assigned to each tissue-type within physiological ranges. Finally, USCT measurement data are computed by simulating acoustic wave propagation. The proposed framework will establish a standard pipeline for USCT virtual imaging trials and provide publicly available large-scale datasets
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