Evaluation of Huffman Sequences based Mismatched filter for Bandwidth limited 3D USCT System Shreyank Gupta 1,2 , Michael Zapf 2 , Herbert Krauß 1 , Nicole V. Ruiter 2 Department of Electrical Engineering and Information Technology 1 Hochschule Darmstadt University of Applied Sciences, Darmstadt, Germany 1 Institute for Data Processing and Electronics, Karlsruhe Institute of Technology, Germany 2 Email: shreyank.gupta@gmail.com Abstract—3D Ultrasound Computer Tomography (USCT) aims at early detection of breast cancer. The system was optimized for Synthetic Aperture Focusing Technique (SAFT) using unfo- cused ultrasound transducers. Due to low SNR of the signals, Coded Excitation (CE) and Matched Filter (MF) are applied. For complex objects the received signals (A-scans) consist of multiple overlaid reflections. The overlaid reflections are difficult to separate due to the system’s limited bandwidth which results in side lobes. Non-separable reflections limit the resolution and contrast of the resulting images. Using Huffman Sequences (HS) as CE, a trade-off can be set between the side lobe suppression and loss of main lobe energy. Higher suppression of side lobes results in reduced energy in the CE, i.e. smaller SNR. So an approach has been implemented which suppresses the side lobes, but preserves the energy of CE by designing a Huffman sequences based mismatched filter. Combination of a CE and different filters were evaluated for single reflection and multiple overlaid reflections. The results show high side lobe suppression within limited bandwidth and preserve the SNR. Keywords: USCT, signal-processing, Huffman sequence, chirps, matched filtering, mismatched filtering. I. I NTRODUCTION At at KIT a 3D Ultrasound Computer Tomography (USCT) II system shown in Fig. 1, was developed [1]. The goal of developing USCT is to detect cancer before it spreads. Fig. 1: (a) Position of a patient during measurement. (b) Container in which breast is submerged for detection. (c) 3D USCT system. System Responsen USCT Container Collected system response Image renconstruction from the collected system response Measurement which is done N times Fig. 2: A simple model of 3D USCT system where ”System responsen” shows a system responses for each iteration. N represents whole number of iterations. Red box shows part of the system during measurement, green box shows off-line mode where imaging from the recieved data is done. Fig. 2 shows a simplified scheme of the system with focus on the signal acquisition and processing. The imaging is done via system excitation, later called input signal. Data acquisition with 3D USCT can be divided in two parts: the first part is the measurement step where all the measurements are done and the second part is the imaging. Image reconstruction of the breast is done off-line, therefore the signals can be processed with sophisticated algorithms. The advanced signal processing should increase the SNR while retaining the bandwidth. A. Current USCT signal processing chain The signal processing chain is shown in Fig. 3. The ex- citation signal used is a coded excitation signal (CE), i.e. a frequency modulated Chirp. Once signals are received, a pulse compression technique is applied to optimize the SNR. This kind of pulse compression technique is called matched filtering, which retains the bandwidth. Any real digital system has limited bandwidth. The bandwidth of 3D USCT is shown in Fig. 4. Limited bandwidth causes filtered output contain side lobes as shown in Fig 4b. Imaging a complex object, the received signal consists of multiple overlaid reflections which is difficult to separate in the presence of side lobes. The disadvantage of this approach is that matched filter and Chirp as CE USCT container System response Filtered output Fig. 3: Chirp and system response are the transmitted and received pulse respectively. This diagram is specific to USCT and shows matched filter process where the output is the cross-correlation of the received signals and the transmitted pulse time-reversed conjugate. Yellow boxes denotes matched filtering process.