M. Pérez-Liva

Tissue-Dependent and Spatially-Variant Positron Range Correction in 3D PET

Positron range (PR) is a significant factor that limits PET image resolution, especially with some radionuclides currently used in clinical and preclinical studies such as 82Rb, 124I and 68Ga. The use of an accurate model of the PR in the image reconstruction may minimize its impact on the image quality. Nevertheless, PR distributions are difficult to model, as they may be different at each voxel and direction, depending on the materials that the positron flies through. Several approximated methods have been proposed, considering only one or several propagating media without taking into account boundaries effects. In some regions, like lungs or trachea, these methods may not be accurate enough and yield artifacts. In this work, we present an efficient method to accurately incorporate spatially-variant PR corrections. The method is based on pre-computing voxel-dependent PR kernels using a CT or a manually segmented image, and a model of the dependence of the PR on each material derived from Monte Carlo simulations. The images are convoluted with these kernels in the forward-projection step of the iterative reconstruction algorithm. This implementation of the algorithm adds a modest overhead to the overall reconstruction time and it obtains artifact-free PR-corrected images, even when the activity is concentrated at tissue boundaries with extreme changes of density. We verified the method with the preclinical Argus PET/CT scanner, but it can be also applied to other scanners and improve the image quality in clinical PET studies using isotopes with large PR.

Ultrasound computed tomography for quantitative breast imaging

Ultrasound computer tomography is an emerging medical imaging modality with promising capabilities for breast cancer screening. It allows studying several acoustical properties of the tissues. The transmission modality of USCT provides maps of the speed of sound and the acoustic attenuation of the tissues. The mentioned properties offer high resolution and excellent contrast tissue representation. The levels of contrast in a speed of sound map are comparable to the ones obtained in a mammogram as this property is well correlated with the density in the tissues. On the other hand, the acoustic attenuation is expected to provide enhanced levels of contrast compared to mammography, due to a higher tissue type dependence, improving the detection of malignancies. This work presents preliminary results for acoustic attenuation and speed of sound reconstruction using a ray-tracing algorithm. We investigated experimental data from the acquisition of a tissue-mimicking phantom. The results were compared with an estimation of the X-ray attenuation coefficient (25 keV) of the phantom (information provided by mammography) to demonstrate the improvements in terms of contrast when acoustic attenuation maps are employed. Further tests are required, but these results show the potential of the USCT maps to identify malignant tissues in the breast.

Refraction correction in Full Angle Spatial image Compounding

Currently there is a growing interest on Full Angle Spatial Compounding (FASC) for breast cancer detection. This technique obtains tomograms at coronal planes of a hanging breast immersed in a water tank and surrounded by ring of ultrasonic transducers. The tomograms are obtained by compounding sector images taken from different orientations in 360°. The well known advantages of the technique include improved contrast-to-noise ratio, high and uniform resolution and the possibility of obtaining 3D images by stacking several tomograms. Since FASC is applied in immersion, refraction at the skin due to different propagation velocities in water and tissue deteriorates significantly the image if not taken into account. Image compounding in presence of refraction is more complicated than in the homogeneous case. This work addresses this problem by proposing an algorithm that performs the required interpolation of many scan lines in the compounding region, taking into account the refraction at the interface.

Full-wave attenuation reconstruction in the time domain for ultrasound computed tomography

Acoustical attenuation (AA) maps in Ultrasound Computed Tomography (USCT) provide enhanced contrast between tissues compared to the speed of sound (SS), which is the most common property of tissue studied with this technique. Currently, the full wave inversion (FWI) methods used for their reconstruction are very different: the AA is mainly estimated using frequency domain algorithms, while the SS is more often recovered in the time domain. In this work we present a novel strategy to recover the attenuation maps through a straightforward and simplified procedure in the time domain. A gradient descent method was employed to optimize iteratively the attenuation distribution. The expression for the functional gradient of the norm of the global deviation between experimental and simulated data was obtained using an adjoint method. The optimization code, implemented in C++, employs a CUDA version of the k-Wave software to perform forward and backward wave propagation. Noisy simulated data was used to test the performance of the proposed method. The simplicity of the formulation of this new method may facilitate the reconstruction of AA and SS maps under a common framework in USCT.

Phase space determination from measured dose data for intraoperative electron radiation therapy.

A procedure to characterize beams of a medical linear accelerator for their use in Monte Carlo (MC) dose calculations for intraoperative electron radiation therapy (IOERT) is presented. The procedure relies on dose measurements in homogeneous media as input, avoiding the need for detailed simulations of the accelerator head. An iterative algorithm (EM-ML) has been employed to extract the relevant details of the phase space (PHSP) of the particles coming from the accelerator, such as energy spectra, spatial distribution and angle of emission of particles. The algorithm can use pre-computed dose volumes in water and/or air, so that the machine-specific tuning with actual data can be performed in a few minutes. To test the procedure, MC simulations of a linear accelerator with typical IOERT applicators and energies, have been performed and taken as reference. A solution PHSP derived from the dose produced by the simulated accelerator has been compared to the reference PHSP. Further, dose delivered by the simulated accelerator for setups not included in the fit of the PHSP were compared to the ones derived from the solution PHSP. The results show that it is possible to derive from dose measurements PHSP accurate for IOERT MC dose estimations.

USCT reference data base: conclusions from the first SPIE USCT data challenge and future directions

Ultrasound Computer Tomography is an exciting new technology mostly aimed at breast cancer imaging. Due to the complex interaction of ultrasound with human tissue, the large amount of raw data, and the large volumes of interest, both image acquisition and image reconstruction are challenging. Following the idea of open science, the long term goal of the USCT reference database is establishing open and easy to use data and code interfaces and stimulating the exchange of available reconstruction algorithms and raw data sets of different USCT devices. The database was established with freely available and open licensed USCT data for comparison of reconstruction algorithms, and will be maintained and updated. Additionally, the feedback about data and system architecture of the scientists working on reconstruction methods will be published to help to drive further development of the various measurement setups.

Improved misfit function for attenuation and speed reconstruction in ultrasound computed tomography

The reconstruction of acoustic attenuation maps for transmission Ultrasound Computed Tomography (USCT) based on the standard least-squares full wave inversion method requires the accurate knowledge of the sound speed map in the region under study. Any deviation in the reconstructed speed maps creates a very significant bias in the attenuation map, as the standard least-squares misfit function is more sensitive to time misalignments than to amplitude differences of the signals. In this work, we propose a generalized misfit function which includes an additional term that accounts for the amplitude differences between the measured and the estimated signals. The functional gradients used to minimize the proposed misfit function were obtained using an adjoint field formulation and the fractional Laplacian wave equation. The forward and backward wave propagation was obtained with the parallelized GPU version of the software k-Wave and the optimization was performed with a line search method. A numerical phantom simulating breast tissue and synthetic noisy data were used to test the performance of the proposed misfit function. The attenuation was reconstructed based on a converged speed map. An edge-preserving regularization method based on total variation was also implemented. To quantify the quality of the results, the mean values and their standard deviations in several regions of interest were analyzed and compared to the reference values. The proposed generalized misfit function decreases considerably the bias in the attenuation map caused by the deviations in the speed map in all the regions of interest analyzed.

Non contact elastographic techniques

This work presents a review of elastographic techniques commonly used to detect stiffness changes in biological tissues. These changes are frequently related with diseases like cancer. Upon implementing the most widely used techniques like static elastography, we have developed a non-contact technique based on Acoustic Radiation Force Impulse (ARFI). The new technique improves the image quality with regard to that of conventional methods.

Erratum to “Tissue-Dependent and Spatially-Variant Positron Range Correction in 3D PET”

In the above paper (ibid., vol. 34, no. 11, pp. 2394-2403, Nov. 2015), the first footnote should have appeared as presented here.

Regularization of image reconstruction in ultrasound computed tomography

We propose two regularizations techniques for a bent-ray (BR) tracing algorithm to reconstruct the speed of sound maps of breast tissues in an Ultrasound Computed Tomography (USCT) system. When high frequencies are employed, the use of BR is a good approximation to describe the propagation of the front of the pressure wave. The quantitative accuracy of the images reconstructed with the BR algorithm was evaluated without any kind of regularization, and with two regularization methods. The regularizations were based on some available a priori information, namely the known higher and lower values of the speed of sound expected in the breast tissues, and the maps of the internal structures obtained from the standard reflection ultrasound (US) imaging. The use of the proposed regularizations in the implemented algorithm improves the convergence and quality of the resulting images, although further improvements are still possible. These methods will help obtaining quantitative US images in a reasonable amount of time, expanding the possibilities and applications of this technique.