Spatial Compound Imaging

Published on 06/02/2015 by admin

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7 Spatial Compound Imaging

In conventional sonography, tissue is insonated from a single direction. Spatial compound imaging combines multiple lines of sight to form a single composite image at real-time frame rates. The ultrasound beam is steered by a different set of predetermined angles, typically within 20 degrees from the perpendicular.

One benefit of the use of spatial compound imaging is the reduction of angle-dependent artifacts (Table 7-1). Speckle is the granular appearance of a sonographic image that results from scattering of the ultrasound beam from small tissue reflectors. This speckle artifact results in the grainy appearance observed on sonograms, representing noise in the image. Improved image quality may be obtained by using spatial compound imaging, which can reduce speckle noise.

Table 7-1 Advantages and Disadvantages of Spatial Compound Imaging

Advantages	Disadvantages
Reduction of angle-dependent artifacts (e.g., posterior acoustic enhancement and speckle)

Frame averaging (persistence or motion blur effect)

Needle tip imaging Limited angle effects (typically <20 degrees) Nerve border definition Fascia contours Imaging around bone Wider field of view with stray lines of sight

There is a central triangular region of overlap within the field of view where all angles mesh together for full compounding. The corners of the image receive only a subset of all the lines of sight; therefore, not all the benefits of spatial compounding are manifest. Some machines allow the stray lines of sight (those off the rectangular field of view) to form a trapezoidal image format. This is sometimes useful to view the approaching needle with in-plane technique.

Spatial compound imaging was first designed to eliminate angle-dependent artifacts. This can be accomplished with a narrow range of beam angles. The larger the range of angles subtended by spatial compounding, the smaller the region within the field of imaging that will receive all the lines of sight (i.e., the region of full compounding).

Ultrasound imaging near bone may be improved by spatial compound imaging. This has relevance to imaging for some blocks (e.g., neuraxial, paravertebral, lumbar plexus, intercostals, sacroiliac joint). Although ultrasound waves cannot penetrate mature bone (even with low-frequency ultrasound), spatial compound imaging allows better definition of the bone surface.

Linear test tool images can be used to reveal the number of lines of sight used in spatial compound imaging. These images are generated with a smooth metal surface, such as that of a paper clip, solid metal stylet, or a U.S. nickel. Metal is used because it is relatively nonattenuating, yet produces an echo. Smooth metal is used so that the test tool does not damage the transducer. For these measurements, high receiver gain and a single focal zone near the surface are used. As long as the test tool contact is less than the receiver aperture, the width of the displayed echoes will not change.

Clinical Pearls

• The use of spatial compound imaging can improve imaging of nerve borders and the block needle tip.

• One potential disadvantage of compound imaging is that needle reverberations occur over a broader range of angles and can prevent imaging of deeper structures.

• Compound imaging is being developed for both linear and curved arrays.

• Sliding the transducer along the known course of the nerve is a well-established technique to improve small nerve imaging. However, frame rate reduction that occurs with spatial compound imaging can cause problems with this technique.

• If compound imaging is not an advantage for a particular imaging situation, it can be turned off.

FIGURE 7-1 Spatial compound imaging. Some forms of ultrasound imaging use multiple lines of sight by electronically steering the beam to different angles. This sonogram was obtained by placing a linear array test tool (the solid metal stylet of a 17-gauge epidural needle) over the active face of the transducer to isolate a single element (A and B). The displayed test tool image consists of the receiver apertures of the transducer. In this case, five lines of sight are used to form a compound image.

FIGURE 7-2 Conceptual illustration of transducer and associated scan lines for recording of three single-angle images.

(Adapted from Jespersen SK, Wilhjelm JE, Sillesen H. In vitro spatial compound scanning for improved visualization of atherosclerosis. Ultrasound Med Biol 2000;26:1357–62.)