Interchannel Crosstalk Evaluation in a Prototype Phased Array Ultrasound System Designed for Hyperthermic Cancer Therapy
D.A. Chorman1, R.J. McGough1, T. Hall2, and C. A. Cain2
1Biomedical Ultrasonics and Electromagnetics Laboratory
2Biomedical Ultrasonics Laboratory
Abstract: Ultrasound phased array prototypes designed for hyperthermia breast cancer therapy have been constructed and are being evaluated. Preliminary designs for power amplification and electronic steering systems used for driving and phasing the array prototypes have been developed and interfaced with a computer for PC control. Field programmable gate arrays (FPGA) digitally generate continuous-wave or pulsed control signals, which are amplified and transmitted to the ultrasound transducers. The high-power square-wave outputs of the power amplifiers are filtered using high gain resonant circuits, and the fundamental sinusoidal component powers each transducer. The phased array system is tested in a water tank using a two-dimensional positioning system and a hydrophone. Focal pattern mapping is automated with a 2-D hydrophone positioning system that is computer-controlled through a MATLAB interface.
Array steering is improved with shape calibration, which is a triangulation procedure that determines the spatial orientation of the individual array elements with respect to the coordinate axes defined by the positioning system. Shape calibration results are degraded by crosstalk between the output power amplifier channels. Crosstalk excites multiple elements instead of a single transducer, which produces triangulation errors during the shape calibration. Previous experiments have shown that the crosstalk voltage between adjacent channels should be at or below a maximum value of -30dB for satisfactory shape calibration results.
Crosstalk measurements were performed by driving a single channel and measuring the voltage induced on adjacent channels. The voltage measurements were extracted from the outputs of the power amplifier filters. The driving signal consisted of two cycles of a 971.5 KHz square-wave, which was filtered after amplification. The crosstalk lasted for 30µs immediately after the initiation of the pulse sequence, and the peak voltage value was reached in less than 5µs. The crosstalk values were extracted from peak-to-peak voltages occurring in the 0-30µS time frame. For one prototype array, the crosstalk values range from -30.59dB to -38.74dB. Thus, the measured crosstalk values for this prototype ultrasound phased array are in an acceptable range for shape calibration.
Prototype Ultrasound System Overview
The block diagram below shows the components of the prototype ultrasound system. The components are discussed in more detail below.
Digital Signal Generator:
A digital signal generator synthesizes the low-level signals that are applied to the individual channels of the ultrasound system. The signal generator was constructed using field programmable gate arrays (FPGA’s). The frequency, phase, and mode (Pulsed or CW) of operation is programmable through a USB connected PC, using a MATLAB® command line interface. The Digital Signal Generator boards are shown below in Figure 1.
Figure 1: Digital Signal Generator Boards
Class D Power Amplifier:
The low-level signals produced by the digital signal generator are amplified by a Class-D power amplifier. This amplifier can output a maximum of 10 watts in continuous mode and can operate up to a maximum frequency of 1MHz. More powerful amplifier designs that will output 2-3X the power and operate at a frequency maximum of 3MHz are also being investigated.
Figure 2: Class D Power Amplifier Board
The square-wave output of the power amplifier is filtered using a resonant low pass filter. The filter: 1. removes the DC offset component from the output signal; 2. filters out all of the higher harmonics contained in the square-wave, passing only the fundamental sinusoidal component; and 3. resonates at the fundamental frequency resulting in a voltage output amplitude that is 5-10X the input voltage.
Figure 3: Filter Board
The interconnect is greatly simplified with a flex circuit interface, resulting in a robust and reliable electrical connection. The geometry in this prototype system is a 1D linear phased array.
Figure 4: Ultrasound Array with a Flexible Interconnect