Page 271 - Biomedical Engineering and Design Handbook Volume 2, Applications

CHAPTER 9

INSTRUMENTATION DESIGN

FOR ULTRASONIC IMAGING

Kai E.Thomenius
GE Corporate Research and Development
Schenectady, New York

9.1 INTRODUCTION 249 9.6 CURRENT TRENDS IN MEDICAL
9.2 BASIC CONCEPTS 249 ULTRASOUND SCANNERS 263
9.3 TYPICAL SYSTEM BLOCK DIAGRAM 251 9.7 SUMMARY 264
9.4 BEAM FORMATION 254 REFERENCES 264
9.5 SIGNAL PROCESSING AND SCAN
CONVERSION 263

9.1 INTRODUCTION

The purpose of this chapter is to show how piezoelectric transduction, sound wave propagation, and
interaction with scattering targets are taken advantage of in image formation with an ultrasound instru-
ment. These instruments have evolved over the last 40 years from relatively simple hand-moved scan-
ners built around an off-the-shelf oscilloscope to rather sophisticated imaging computers. Much
technology has been perfected during this evolution. For example, transducers have grown from circu-
lar single-element probes to precision arrays with more than 1000 elements. With better front-end elec-
tronics, the operating frequencies have increased as weaker echoes can be handled. As the gate counts
of VLSI ASICs (very large-scale integration application-specific integrated circuits) have increased, the
numbers of processing channels in array-based systems have risen. With the introduction of reasonably
low-cost high-speed (20 to 60 MHz) 8- to 14-bit analog-to-digital (A/D) converters, digital beam for-
mation has become the standard. Further, we are witnessing today the beginning of a shift to completely
software-based beam formation that has the potential of elimination of the very high development costs
of ASICs. Along with these developments is a shift to more and more compact systems. Throughout its
history, medical ultrasound has been a very dynamic field; it appears that this will not change in the near
future. The organization of this chapter is based on the discussion of a block diagram of a generalized
ultrasound system. Each component of the block diagram will be reviewed in considerable detail.
Different design approaches for the various blocks will be reviewed and their advantages and disadvan-
tages discussed. Those areas of the block diagram which are targets of significant current research are
summarized along with the major industry trends.

9.2 BASIC CONCEPTS

9.2.1 Image Formation

Image formation in medical ultrasound is accomplished by a pulse-echo mechanism in which a
thin ultrasound beam is transmitted and echoes generated by the interaction of that beam with
scattering targets are received by a transducer or a set of transducer elements. The transmit and

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