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MEDICAL TEST


Ultrasound Imaging: More Than Skin-Deep


by Tom Lecklider, Senior Technical Editor U


ltrasound generally is considered a very safe type of nonintrusive medical scanning. It doesn’t pro- duce the ionizing radiation associated with X-ray imaging, nor does it require the expense and size of an MRI instal- lation. Portable ultrasound machines are manufactured by many companies, and a number of distinct operating modes have been developed. Because it is safe and relatively low cost, ultrasound scans have become a routine part of many medical procedures.


The B scan mode—B for brightness— is the basic imaging capability. In addi- tion, Doppler functionality in some ma- chines overlays a color-coded indication of blood-fl ow speed through arteries and veins within the region of interest. Except for the continuous Doppler mode, pulses of high-frequency sound waves are applied to the relevant area of the body, and the reflected echoes are used to generate an image. High frequency for ultrasound is from 1 MHz to 15 MHz or higher. Doppler can be pulsed, but it also may be implemented as a continuous wave.


Although basic analog ultrasound im- aging began about 50 years ago, today’s machines are largely digital and provide many more options as well as images with much greater detail and color. Most modern equipment is based on phased array scanning similar to advanced sonar and radar, and it all requires very high performance components.


As an example of the circuitry in- volved, National Semiconductor (now part of Texas Instruments) has devel- oped an eight-channel Ultrasound Tx/ Rx Chipset, the LM965XX series of parts. It comprises a digital transmit beamformer, a high-voltage transmit/ receive (Tx/Rx) switch, a high-voltage


20 • EE • February 2012


ultrasound Tx pulser, and an analog front-end (AFE) IC.


For ultrasound applications, a channel is associated with each of the several ele- ments that together form the transducer a doctor holds against a patient’s body. This chipset provides eight high-power pulse inputs to eight elements and de- velops eight digitized AFE received echo outputs.


The recently introduced United Im- aging Healthcare iuStar100 Ultrasound Machine has 48 channels, which is near the minimum quantity in commercial machines. Transducers may comprise 100 or more elements, and typically, each has its own dedicated Tx/Rx elec- tronics. Although these systems require a lot of specialized circuitry because of their nature, integration by National, Maxim Integrated Products, Samplify Systems, Analog Devices, and others has greatly affected the cost, portability, and performance of modern medical ultrasound machines. The iuStar100 is based on Samplify electronics. The National Ul- trasound Chipset primarily is analog in nature other than the Tx beamformer timing. Indeed, the success of a phased array ultrasound de- sign depends on the accuracy with which the elements can be steered to produce a focused energy beam as well as the noise level that can be achieved for the low-level return signals. Assuming both aspects are


optimized, the major remaining job is to condition and combine the digitized return signals to form an image. As succinctly noted in a Texas Instru- ments (TI) white paper,1 “Ultrasound systems are signal-processing intensive.” To put that into perspective, a recent academic paper proposed using two TI eight-core 1.25-GHz C6678 DSP ICs to compute basic beam steering and beamforming algorithms for a relatively small 64-channel ultrasound system. Nevertheless, this was an important contribution because most solutions use FPGAs and/or ASICs to deal with the amount, speed, and complexity of the signal processing.2


Beam Steering Transmit beam steering refers to the coordination of the individual element pulses so the energy from them simul- taneously arrives at the desired point. Physically, the elements are fixed in their relation to one another. They may be glued together to form a simple linear


Figure 1. Focusing Ultrasound From Transducer Elements Courtesy of Texas Instruments


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