Nils Hüttebräuker

Spatial compounding with tissue harmonic images and monostatic synthetic aperture reconstruction

A high precision mechanical add-on module was realized to implement a transmission tomography system around a commercial analog ultrasound system. The modular nature of the add-on lets it be used, however, more universally. Without enormous effort the module could be programmed to be coupled to a fully digital high end commercial ultrasound system. A Siemens Antares was used for this work, which allows the acquisition of RF data of high quality B-scans and tissue harmonic imaging (THI) over the Axius Direct Ultrasound Research Interface (URI). This possibility was exploited to explore some more intricate processing options for image compounding in addition to the conventional spatial compounding approach. Several compounding schemes were implemented. One of the two new strategies for compounding consists of calculation of a compound image from the THI data, as the latter is known to possess a better contrast and lateral resolution. While the RF data acquired from various transducer positions provides all the essential input for monostatic synthetic aperture focusing (SAFT), the other strategy was to implement SAFT imaging. The effect of inhomogeneous speed of sound was studies carefully and two different schemes were implemented to overcome the problems of image registration arising from it. The algorithms were tested on polypropylene fiber phantoms in water and ethylene glycol. While the monostatic synthetic aperture reconstruction technique was found to be best in enhancing signal to noise ratio, the speckle reduction was notably better for the THI compounding scheme than that of the conventional compounding strategy. The resulting spatial resolution of the compound images was comparable for the latter two cases and the former technique outperformed the other ones in this respect.

Full angle spatial compounding for improved replenishment analyses in contrast perfusion imaging: In vitro studies

For contrast enhanced perfusion imaging semi-quantitative methods (such as the bolus-, replenishment- or depletion-method) are commonly used to analyze the dynamic changes in concentration of contrast agent induced by insonification. In order to apply these methods and to decrease artifacts from tissue nonlinearity, perfusion imaging is conducted using decreased transmit power. However, echo signals from deeper structures are often too weak to be successfully analyzed. Furthermore, shadowing artifacts may occur as a result of high concentration of contrast agent in the beam path. Thus, those semi-quantitative methods often fail or yield ambiguous diagnoses. Imaging an object (e.g., the female breast) from multiple viewing angles (spatial compounding) may overcome these issues. In addition, spatial compounding achieves an isotropic resolution and reduces speckle and further common artifacts. In this paper we present results obtained from a combination of spatial compounding with contrast-enhanced perfusion imaging. Applying the replenishment method, we extracted perfusion-related parameters and compared the conventional parametric images with the compound parametric images. We found that the compounded parametric images outperform the conventional images due to reduced noise and suppression of artifacts.

Ultrasonographic contrast-agent imaging of sub-millimeter vessel structures with spatial compounding: in vitro analyses / Kontrastmittelgestützte Ultraschallabbildung von Sub-Millimeter-Gefäßstrukturen mittels Spatial Compounding: In-vitro Analysen

In clinical diagnostics, ultrasonographic contrast-agent imaging gives access to medical parameters such as perfusion and vascularization. In addition to the artifacts that are typical for ultrasonic imaging, e.g., speckle noise and depth-dependent sensitivity and resolution, contrast-agent imaging shows more pronounced depth dependence and may suffer from shadowing artifacts that arise from high attenuation of the ultrasound waves by the contrast agent at high concentrations. By imaging an object from different viewing angles in one 2D image plane and summing the images obtained (spatial compounding), image quality can be increased and artifacts can be suppressed. In the present study, we combined both techniques to overcome the limitations of contrast-agent imaging. We used a commercially available ultrasound scanner and a custom-made high-precision mechanical system to rotate the ultrasound transducer fully around the object under investigation. Using this set-up, ultrasound data were acquired in reflection mode to generate a 360 degrees compound scan of a flow-mimicking phantom supplied with contrast agent.In clinical diagnostics, ultrasonographic contrast-agent imaging gives access to medical parameters such as perfusion and vascularization. In addition to the artifacts that are typical for ultrasonic imaging, e.g., speckle noise and depth-dependent sensitivity and resolution, contrast-agent imaging shows more pronounced depth dependence and may suffer from shadowing artifacts that arise from high attenuation of the ultrasound waves by the contrast agent at high concentrations. By imaging an object from different viewing angles in one 2D image plane and summing the images obtained (spatial compounding), image quality can be increased and artifacts can be suppressed. In the present study, we combined both techniques to overcome the limitations of contrast-agent imaging. We used a commercially available ultrasound scanner and a custom-made high-precision mechanical system to rotate the ultrasound transducer fully around the object under investigation. Using this set-up, ultrasound data were acquired in reflection mode to generate a 360 degrees compound scan of a flow-mimicking phantom supplied with contrast agent.

P3G-10 A Quantitative Study of the Time Delay Correction for Full Angle Spatial Compounding

Full angle spatial compounding is of interest for imaging small organs (breast, testicles) and small animals. Besides phase aberration and out of plane refraction the variations in speed of sound, usually assumed to be constant, also cause registration error. This error may be negligible regarding the visual appraisal of B-mode images; its effect on spatial compounding of the individual scans may, however, be detrimental to the spatial and contrast resolution depending on the geometry and the variations of the acoustic speed. This effect was studied on the basis of analytical modeling as well as measured data. An exact compensation of the refraction artifacts is achieved via ray tracing for known speed of sound distribution. An alternative approach is to carry out axial correction disregarding the bending due to refraction. The purpose of this paper is to study the characteristics of the axial correction and the extent to which the correction is sufficiently accurate. The effect of the speed of sound variation in spatial compounding was studied with the help of the point spread function (PSF). A geometrical acoustic model was derived with eikonal approximation to calculate the compounding PSF with axial correction as a function of the speed of sound variations and the location of the point scatterer. The effect of the speed of sound variation and the scatterer location on the spatial compounding PSF with axial correction was studied and the results were compared to those without correction. Axial correction was also applied to data measured on a high precision setup on phantoms corresponding to the modeled objects, in order to validate the efficacy of the simulation techniques. The results were found to conform to those predicted by the model. The object was chosen to be of cylindrical shape having a constant velocity either above or below the velocity of the surrounding medium and containing only one scatterer. The PSF of the compounding was shown to be generally of elliptical shape whose extent along the radial direction remains constant for a given speed of sound distribution and whose azimuthal extent decreases with increasing radial distance from the center of the cylinder. For a speed of sound 1550 m/s inside the cylinder of radius 3 cm immersed in water at 22degC, the axial and azimuthal extents of the compounding PSF of a point located 2 cm from the center without correction was 2.32 mm and 1.76 mm respectively considering an ideal B-mode PSF. The axial correction results in PSF radial and azimuthal widths of 1.18 mm and 0.88 mm respectively. It may be concluded that refraction tends to produce circular and elliptical structures with decreasing azimuthal extents for higher radial coordinates. The axial time delay compensation works at the best in the vicinity of the center of rotation and may even add to azimuthal spreading in the worst case for increasing radial coordinates

An Automated System for Full Angle Spatial Compounding in Ultrasound Breast Imaging

For the detection of breast cancer, ultrasound is conventionally used in addition to mammography. However, ultrasound is highly operator-dependent since speckle, depth dependency and artifacts (e.g. shadowing) affect image quality. Full Angle Spatial Compounding (FASC) may overcome those limitations by superimposing ultrasound data acquired in one cross-sectional plane from aspect angles all around an object. Furthermore, we showed in vitro that FASC also improves contrast perfusion imaging. To apply FASC to breast imaging, we developed an add-on system to a conventional ultrasound scanner, integrated in a custom-made examination couch. Here, we present first in-vivo results from test persons and patients with breast lesions.

Ultrasound breast imaging using Full Angle Spatial Compounding: In-vivo results

For the detection of breast cancer, ultrasound is conventionally used in addition to mammography. However, ultrasound is highly operator-dependent since speckle, depth dependency and other artifacts (e.g. shadowing) affect image quality. Full Angle Spatial Compounding (FASC) may overcome those limitations by superimposing ultrasound data acquired in one cross-sectional plane from aspect angles all around an object. Furthermore, we showed in vitro that FASC also improves contrast perfusion imaging. To apply FASC to breast imaging, we developed an add-on system to a conventional ultrasound scanner, integrated in a custom-made examination couch. Here, we present first in-vivo results from test persons and patients with breast lesions.

9C-2 Reconstruction of Speed of Sound for a Correction of Transit Time in Full Angle Spatial Compounding

For full angle spatial compounding, an object is imaged in one plane from multiple aspect angles and the obtained images are superimposed in correct geometric orientation. Thus, information about the distribution of speed of sound inside the object is required for a correction of individual images with respect to transit time and refraction of the ultrasonic waves. In this paper, we present a method to reconstruct such a distribution by a filtered backprojection of echo data from a reflector positioned behind the object. Using the reconstructed distribution, individual ultrasound images were corrected by a ray tracing algorithm before spatial compounding. Here, refraction was only accounted for at the outer boundary of the object, while inside only transit time was corrected. In vitro analyses are presented that prove the applicability of this approach.

2E-6 Contrast Enhanced Perfusion Imaging by means of Spatial Compounding

In order to decrease artifacts from tissue, contrast enhanced perfusion imaging is usually conducted using low transmit power (low MI). However, with low MI echo signals from deeper structures are weak and often miss higher harmonics due to attenuation. Furthermore, shadowing artifacts may occur as a result of high concentration of contrast agent. Imaging an object (e.g. female breast) from multiple angles of view (spatial compounding) may overcome this issue. In addition, spatial compounding achieves an isotropic resolution and reduces speckle and further common artifacts. Here, first in-vitro results of contrast enhanced spatial compounding are presented

Ultrasound Computed Tomography in Breast Imaging: First Clinical Results of a Custom-Made Scanner

PURPOSE To test a system using ultrasound computed tomography (USCT) that superimposes ultrasound data acquired in one cross-sectional plane from multiple angles around the breast (Full Angle Spatial Compounding, FASC) and to reconstruct the distribution of the speed of sound in the breast (SoS reconstruction). MATERIALS AND METHODS We developed a system combining a conventional ultrasound scanner with a PC-controlled mechanical setup integrated in a custom-made examination couch. In a feasibility study, 3 volunteers (age 26 - 74 years) and one patient with breast cancer were studied. Subjects were placed in the prone position on this couch, with the breast hanging in a water tank. The ultrasound probe was moved in several planes around the breast. A curved reflector that followed the movement of the probe behind the breast was used to calculate the SoS within the breast tissue. Echo-data was processed offline by custom-made software to calculate both FASC and SoS images. RESULTS In FASC images a reduction of artifacts (i. e. shadowing of Coopers ligaments and irregular edges of inhomogeneous lesions) and speckles as well as clear visualization of the inner architecture of the breast was achieved. SoS images delivered further diagnostic information and helped to compensate for geometric distortions in the computed images. Difficulties in the visualization of lesions near the thoracic wall and/or the axillary are limitations of this technique. CONCLUSION The first clinical results of USCT imaging have proven its feasibility as an automated and standardized technique for breast imaging.

Three-dimensional Reconstruction of Fine Vascularity in Ultrasound Breast Imaging Using Contrast-enhanced Spatial Compounding : In Vitro Analyses

RATIONALE AND OBJECTIVES Ultrasound image quality can be improved by imaging an object (here: the female breast) from different viewing angles in one image plane. With this technique, which is commonly referred to as spatial compounding, a more isotropic resolution is achieved while speckle noise and further artifacts are reduced. We present results obtained from a combination of spatial compounding with contrast-enhanced ultrasound imaging in three dimensions to reduce contrast specific artifacts (depth dependency, shadowing, speckle) and reconstruct vascular structures. MATERIALS AND METHODS We used a conventional ultrasound scanner and a custom made mechanical system to rotate an ultrasound curved array probe around an object (360 degrees, 36 transducer positions). For 10 parallel image planes, ultrasound compound images were generated of a flow-mimicking phantom consecutively supplied with water and contrast agent. These compound images were combined to form a volume dataset and postprocessed to obtain a sonographic subtraction angiography. RESULTS Image quality was significantly improved by spatial compounding for the native (ie, without contrast agent), and, in particular, for the contrast-enhanced case. After subtracting the native images from the contrast-enhanced ones, only structures supplied with contrast agent remain. This technique yields much better results for compound images than for conventional ultrasound images because speckle noise and an anisotropic resolution affect the latter. CONCLUSIONS With the presented approach contrast specific artifacts can be eliminated efficiently, and a subtraction angiography can be computed. A speckle reduced three-dimensional reconstruction of submillimeter vessel structures was achieved for the first time. In the future, this technique can be applied in vivo to image the vascularity of cancer in the female breast.