Executive Summary:

Introduction

Recent advances in clinical ultrasound require imaging systems capable of producing high-quality images for the early detection and monitoring of various disease states. One of the most critical image quality drivers in today's state-of-the-art ultrasound scanners is a system's focusing or resolving power. A system's ability to provide increasingly subtle information, such as the internal architecture of complex cysts, improves with its ability to resolve smaller and smaller areas. This resolution is largely driven by two parameters: Higher imaging frequencies which, with their shorter wavelengths, improve both axial and lateral resolution; and wider aperture sizes used to form the ultrasound beams, which provide better lateral resolution.

Until recently, several obstacles prevented significant improvements in ultrasound imaging resolution. The GE LOGIQ 700 MR (Maximum Resolution) system has overcome these obstacles, producing anatomical details which previously have been difficult or impossible to visualize. Among the contributors to this improvement is the implementation of wide aperture/low F-number imaging.

The Impact Of Low F-Number Imaging

Basic imaging theory has established that lateral resolution is a function of:

Aperture is the size of the active transmitting and receiving portion of the transducer array. In most systems, it is a function of the number of transducer elements used simultaneously to form an image. Ultrasound scanners must vary their aperture size with depth in order to provide uniform lateral resolution over the entire depth of field. For a fixed frequency, the size of the aperture must increase as the scanning depth increases in order to maintain uniform lateral resolution throughout the image. Most systems select a transmit aperture based on the scan depth setting and continuously vary the aperture size on reception. A common measurement of aperture size is F-number (F#). F-number is defined as the ratio of Depth to Aperture. See figure 1.

Figure 1

Smaller F#s mean larger apertures and provide better lateral resolution. Figure 2 illustrates the effect of changing the aperture size on lateral resolution. Note that with the wider aperture image on the right, the pins in this phantom are much better defined.

Fig. 2 - Phantom comparison of low F# (right) to high F# (left).

Typical ultrasound systems use F#s around 2, attempting to aintain an aperture size that is one half of the distance to the point of focus. One might be prompted to ask, "What prevents ultrasound systems from using smaller F#s in order to achieve dramatic improvements in lateral resolution?" The answer lies in two limitations of modern ultrasound instruments: Channel count and frame rate.

Channel Count

The channel count limit on F-number is straightforward. In order to image with larger apertures, one adds more active elements to the transducer. In most ultrasound systems, each transducer crystal has its own dedicated signal path (channel) through the ultrasound processor, which requires that additional channels be added to the ultrasound system in order to accommodate the additional elements.

For example, a 5 MHz linear transducer with 64 elements will generally have a maximum active aperture size around 2 cm. With an F# of 2, this provides lateral resolution of approximately 0.6 mm at 4 cm depth, 1.2 mm at 8 cm. By doubling the number of elements (F# = 1), one can halve the lateral resolution to 0.3 and 0.6 mm respectively.

Since aperture size is directly proportional to lateral resolution, the larger system channel count can provide for an increase in resolution by a factor of up to 2. This is particularly relevant for mid- to far-field imaging where the physical aperture size becomes the limiting factor in determining resolution. It is possible to obtain larger apertures without increasing channel count, but this results in reductions of frame rate and sensitivity which are often unacceptable. At the same time, increasing the channel count with the same aperture will have no effect on resolution.

Frame Rate

The frame rate limitation on resolution stems from the need to maintain image uniformity over a large scanning area. In other words, as a function of depth, ultrasound images should maintain a relatively constant resolution, which provides uniformity in dot size and speckle texture. Although there is a significant improvement in lateral resolution with wider apertures, there is a corresponding decrease in depth of focus. F#s that are too low often have very good resolution at the focal zone but have poor lateral resolution away from the focal zone. See Figures 4a & 4b. Note that the image focus is excellent at the focal point (arrow) but poor away from the focus. In order to maintain consistency in resolution with low F#s, ultrasound systems have to use multiple focal zones. Figure 3 shows the resolution which results with a single focal zone using conventional narrow aperture focusing techniques. Notice how the narrow aperture produces a "softer" focus which provides a reasonable depth of field and relatively constant resolution. Figure 4 illustrates the impact on resolution and depth of field if a single zone is used with wide aperture focusing. There is a significant increase in resolution at the focal zone with a considerable loss in resolution away from the focal zone, resulting in poor depth of field.

Figure 5 shows how the combination of wide aperture focusing, with a large number of focal zones, provides the desired resolution improvement while maintaining the depth of field required to adequately visualize the entire region of interest. Each focal zone requires a separate ultrasound transmission and reception. Consequently, the addition of each focal zone slows down the image frame rate

Until recently, imaging with a high number of focal zones has been prohibitive due to this impact on frame rate. A critical element of the LOGIQ 700 system is a proprietary digital processing technique that solves the frame rate problem. This permits the operator to add between 2 and 4 times the standard number of focal zones while retaining standard frame rates. With the frame rate problem solved, one can use multiple tightly controlled focal zones, providing high resolution throughout the imaging field of view. This increase in the number of focal zones, combined with the increase in aperture size, results in lateral resolution improvement up to 4 times over conventional designs.

Many ultrasound systems use low F-numbers on reception because they are able to continuously vary the receive aperture size. This is much like a baseball player continuously adjusting his field position to catch the ball as the baseball flies toward him. The transmit aperture size, however, cannot be continuously focused. The batter cannot change the destination of the baseball after it has been hit. For this reason, conventional ultrasound systems use fairly high F-numbers on transmit in order to cover a large area. See figure 3. The actual resolution provided by the system is the combination of transmit and receive focus dimensions. Significant improvement in lateral resolution requires low F#s on transmit and receive.

The LOGIQ 700 system combines these new wide aperture techniques with high frequency broad bandwidth transducers. Conventional ultrasound systems generally use transmit F-numbers in the range of 2-3 (aperture size is 1/2 to 1/3 of the depth of focusing). Maximum Resolution Ultrasound, implemented on a 128-channel digital beamformer, is able to produce images using transmit and receive F-numbers below 1. This means that apertures are actually larger than the focusing depths in many situations. The resulting improvements in lateral resolution can be as much as 4 times.

Figure 6

Wide aperture imaging permits multiple tight focal zones. The result is excellent lateral resolution down to 16 cm in this post TIPS liver image.

Figure 7

Wide aperture imaging permits very tight resolution in this image of the distal abdominal aorta. The combination of wide aperture with high dynamic range provides clear visualization of the circumferential thrombus and surrounding tissue interfaces.

Figure 8

Transverse image of a normal thyroid exemplifies the minute detail that can be demonstrated when employing high frequency and wide aperture imaging.

Figure 9

Long axis projection of irregular plaque in the bulb of the carotid artery and an occluded ECA demonstrates the utility of wide aperture imaging in vascular applications.

Figure 10

Excellent detail of breast architecture surrounding a simple cyst. Note sharpness of borders provided by multiple tight focal zones.

Demonstrable In Images

The theorectical physics, however, are only as good as the images that result. Figure 2 illustrates the Maximum Resolution Ultrasound difference in an ultrasound phantom. Such phantoms are generally considered to be an excellent image evaluation tool because they maintain consistent standards not possible with human or animal subjects. Real value, however, is in the quality of everyday clinical images. Clinicians using Maximum Resolution Ultrasound have reported excellent results in a variety of applications. Figures 6-10 are examples of images acquired using these techniques.

Still images cannot convey the high frame rate imaging described above. However, these images do demonstrate the advantages of multiple tightly focused transmit zones made possible with wide aperture imaging. In the abdominal applications, illustrated in figures 6 and 7, note that the high resolution imaging in the near field is maintained throughout the imaging area. In the case of the TIPS case (figure 6), this fine resolution extends to 16 cm, remarkably deep for this level of spatial resolution.

In small parts imaging, illustrated in figures 8-10, the results are similarly remarkable. In these applications, the advantages of wide aperture imaging are further augmented by the ability to use high frequency transducers. Again, multiple tightly focused zones permit excellent spatial resolution throughout the imaging area.

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 96-4533.