Direct Digital Demultiplexing of Analog TDM Signals for Cable Reduction in Ultrasound Imaging Catheters

In real-time catheter-based 3-D ultrasound imaging applications, gathering data from the transducer arrays is difficult, as there is a restriction on cable count due to the diameter of the catheter. Although area and power hungry multiplexing circuits integrated at the catheter tip are used in some applications, these are unsuitable for use in small sized catheters for applications, such as intracardiac imaging. Furthermore, the length requirement for catheters and limited power available to on-chip cable drivers leads to limited signal strength at the receiver end. In this paper, an alternative approach using analog time-division multiplexing (TDM) is presented, which addresses the cable restrictions of ultrasound catheters. A novel digital demultiplexing technique is also described, which allows for a reduction in the number of analog signal processing stages required. The TDM and digital demultiplexing schemes are demonstrated for an intracardiac imaging system that would operate in the 4- to 11-MHz range. A TDM integrated circuit (IC) with an 8:1 multiplexer is interfaced with a fast analog-to-digital converter (ADC) through a microcoaxial catheter cable bundle, and processed with a field-programmable gate array register-transfer level simulation. Input signals to the TDM IC are recovered with -40-dB crosstalk between the channels on the same microcoax, showing the feasibility of this system for ultrasound imaging applications.