Executive Summary:

Introduction

It is intuitive that high resolution images provide more diagnostic information than low resolution images. Since ultrasound resolution is largely dependent on imaging frequency, many manufacturers have introduced high frequency transducers in the quest for sub- millimetric resolution. These transducers are typically labeled with frequencies between 5 and 10 MHz, though the actual imaging frequencies tend to be centered around 7.5 MHz.

Spurred by the availability of high resolution instruments, musculo-skeletal and breast ultrasound procedures have experienced growth rates between 10 and 25%. This makes them among the fastest growing applications of ultrasound. High frequency transducers are ideally suited to imaging superficial organs such as these. As good as these transducers are, however, the diagnostic information provided by the ultrasound instrument could be further enhanced by extending spatial resolution beyond the current state of the art.

High Frequency Imaging Theory

Though ultrasound resolution is affected by several factors1, imaging frequency has the most direct impact. Ultrasound resolution improves in direct proportion to imaging frequency. In a typical 5 - 1O MHz system, the resolution cell measures roughly 0.7 x 0.35 mm. The result is that anatomical structures smaller than 1 mm are likely to be missed.

In digital ultrasound systems, the maximum imaging frequency is limited by the speed of the system's analog-to-digital converter. Conventional systems use A/D converters running at approximately 20 MHz. This limits the maximum imaging frequency to 10 MHz according to the Nyquist sampling theorem. The situation is even more difficult for analog systems since adequate focusing precision becomes more difficult as the frequency increases. Moreover, as the imaging frequencies increase, transducer design and fabrication become increasingly difficult.

Fig. 1 High frequency 12MHz imaging provides exceptional definition of the margins and thin septation in this bi-lobed breast cyst.

Further complicating matters is the well known principle that ultrasound penetration (the ability to image anatomical structures far from the skin surface) decreases as the imaging frequency increases.

Table

Conventional Limitations for High Frequency Ultrasound
Electronics Precision in Analog Systems
A/D Conversion Rates in Digital Systems
Transducer Design and Fabrication
Tissue Penetration at High Frequencies

Advanced Technology

Recent advances in GE Ultrasound's electronics and transducer technologies have enabled an extension of the imaging envelope well above the conventional 5 - 10 MHz range. A high speed 40 MHz A/D converter in GE's digital beamformer has raised the Nyquist frequency, making 20 MHz imaging a possibility. This opens the possibility of spatial resolution as fine as 70 microns.

Long ignored as important imaging characteristics, probe and system bandwidth are becoming increasingly important to high resolution imaging. Spatial resolution improves in direct proportion to bandwidth, as well as frequency. The combination of high frequency electronics (as described above), with improved transducer fabrication, can provide an imaging system with substantially improved performance characteristics. Figure 2 graphically demonstrates the key parameters. Conventional high frequency transducers are of two types: High frequency narrow band transducers, and high frequency broad band transducers. The broad band transducers have a significant advantage over the narrow band transducers in that they provide significantly better spatial resolution.

Note that the broad band transducers labeled as "5 - 10 MHz" often have limited energy at the extremes of the labeled frequency range. Using new materials and assembly methods, one can produce transducers that extend the broad band concept by significantly increasing the imaging frequency. The imaging improvement is illustrated in figures 3 and 4.

Fig. 3 and 4 illustrate the advantage of high frequency broad bandwidth imaging. Note the improvement in spatial resolution in these images of 300 micron wires in a standard RMI 403GS ultrasound phantom.

Clearly, at high frequencies, one should be concerned about the ability to provide adequate ultrasound penetration. Though frequency is certainly an important parameter affecting penetration, it is not the only one. Using new transducer materials and assembly techniques, GE has been able to significantly improve sensitivity. Combining those transducer designs with extended system dynamic range and broad band imaging that has sufficient energy at the low end of the frequency spectrum, one can provide 12 MHz images that provide penetration equal to that of many 5 - 10 MHz systems.

GE's high frequency linear transducer provides a true 12 MHz imaging frequency with more than 80% fractional bandwidth. As illustrated in figure 2, 12 MHz is well within the band of frequencies used, not the upper limit. This permits the system to produce an extremely small resolution cell while maintaining acceptable penetration. Clinical investigators have reported seeing objects smaller than 200 microns using this new transducer. With system capabilities up to 20 MHz, these new transducers are only the beginning of the possibilities for high frequency imaging.

Clincal Benefits

The probe and system combination described above has been implemented in GE's ultrasound systems with Micron Imaging. According to clinicians who have used the system, Micron Imaging provides improved resolution in a wide variety of tissue types and has provided more diagnostic information on which to base their diagnoses. Physicians and sonographers at several large medical centers have demonstrated superior axial, lateral and contrast resolution in routine clinical cases.

Use of high dynamic range, in combination with the high frequency techniques, has provided clear visualization of tendon fibers and muscle layers with resolution similar to that of Magnetic Resonance imaging. (See figures 5, 6, and 7.) Superficial vascular studies have demonstrated improved definition of the vessel walls and atherosclerotic lesions.2

Breast imaging has seen similar benefits, providing better defined margins and clearer visualization of the internal structure of lesions in the breast. The complementary roles of ultrasound and x-ray mammography have been well established. The improvements of Micron Imaging should further improve imaging in this application. For example, clinicians have reported the ability to tract tiny mammary ducts to determine the presence of ductile invasion of breast carcinoma.

These examples are only the beginning of the clinical data being collected. The benefits and potential applications of Micron Imaging are not yet fully explored. Cost containment, ease of use, reduced exam time, and excellent diagnostic results are the keys to patient, physician and administrator acceptance of these new applications.

Fig, 5. 12 MHz broad band imaging combined with wide dynamic range provides excellent contrast resolution in this rotator cuff image. Musculo-skeletal imaging is among the fastest growing applications of high frequency ultrasound

Fig. 6. 12 MHz broad band imaging of the right forearm demonstrates the fibers of the median nerve situated between deep and superficial flexor muscles. Injury to the median nerve is frequently associated with Carpal Tunnel Syndrome.

Fig. 7 12 MHz broad band imaging of this displaced bicep's tendon demonstrates the advantages of high frequency imaging. Note the clear delineation of the margins and internal structure of the tendon and surrounding muscle tissue.

Fig.8. 12MHz broad band imaging of this breast clearly demonstrates the margins of this cyst. Note also the fine definition and contrast resolution of the surrounding breast tissue.

Fig. 9. 12 MHz broad band imaging can provide exceptional detail as illustrated by the clear visualization of TDLUs in this breast scan.

Fig.10. High frequency imaging combined with exceptional contrast resolution clearly demonstrates the extent of this breast carcinoma invading Cooper's ligament.

Fig. 11. This transverse image of a normal thyroid exemplifies the minute detail that can be imaged when employing high frequency ultrasound.

Fig.12. The improvement in axial resolution with 12 MHz imaging is demonstrated in this image of these opposing arterial walls of the paired penetrating arteries of the testis.

Fig. 13. 12MHz broad band imaging combined with high dynamic range demonstrates aggregate blood flow in varicose vein and posterior tibial veins.

Print copies of these technical papers are available from GE. Many of the figures are easier to read on the print copies than they are here. To receive your own copy, send email to GE and reference publication number 96-4531.