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At Forschungszentrum Karlsruhe a 3D ultrasound computer tomograph (USCT) for breast cancer diagnosis is currently under development. For many applications, i.e. reconstruction of speed of sound maps, signal denoising,signal compression and calibration, it is necessary to detect pulses and their parameters (center frequency, bandwidth factor, time of arrival, phase and amplitude) accurately and robustly. The pulse detection is demanding due to low signal-to-noise ratios (SNR) caused by unfocused pulses from single emitters. Besides that angle dependent pulse shapes result in ultrasonic echoes which are similar in their center frequency but vary strongly in their bandwidth factor. Additionally, in sum 3.5 million A-scans (20 GB) are acquired, so that the pulse detection has to be fast and efficient. This is achieved by using a fast pre-classifler in order to separate the ultrasonic echoes from the non-white system noise of our system. The detected echoes are then passed individually to a parameter estimation method to determine pulse parameters accurately. An analysis of several classifiers resulted in an alternating decision tree which is both fast and accurate. Classification performance of 95% could be achieved as well as robust parameter estimation with non-white system noise if the SNR is larger than 3 dB. A comparative image reconstruction resulted in significantly sharper images.

Our experimental setup for ultrasound computer tomography (USCT) has a cylindrical aperture with a diameter of 18 cm and height of 15 cm. In this setup ultrasound can travel over long distances up to 50 cm. Phase aberration due to speed of sound variations cause many pulses not to overlap in the reflectivity reconstruction. 3D speed of sound (SOS) maps were calculated using the recorded data and applied for correction. Due to a sparse sampling of the received transmission signals, the resulting speed of sound maps are noisy and of low detail. But the accuracy of the average speed of sound is sufficient for phase aberration correction of the reflection images down to object size of approx. 1 mm and increases the image quality significantly. Our work shows that high resolution USCT is not possible without phase aberration correction using SOS information.

For our current 3D demonstrator system for ultrasound computer tomography (USCT) a synthetic aperture based image reconstruction is implemented whose calculation time is in the order of days. Faster approximations might lead to large errors for our USCT setup which uses many multistatic emitter-receiver combinations. The aim of this work is the analysis of non-linear signal to image mapping for a cylindrical aperture and quantify the errors of computational less complex approximations. Four approaches with increasing computational complexity were implemented, tested with the cylindrical setup and compared to a commonly used linear transducer setup. For the linear aperture the second approximation leads to satisfying results. For the more diverging USCT setup a more accurate approximation is needed: the third approximation, which is still less computational expensive as the ground-truth implementation, gives satisfying results.