Standoffs – What do they deliver?
Whilst ultrasound technology continues to evolve and improvements are made in image resolution and the reduction of artefacts, some anatomic areas remain a challenge to the achievement of diagnostic quality images.
One example is the ultrasound examination of superficial structures. Areas close to the transducer contact point on the body can suffer reduced image quality for a combination of reasons:
- Any air trapped between the transducer and the skin surface will cause reverberation artefact and prevent the visualisation of any structures deep to that area
- The initial echoes created from superficial interfaces can be of a higher energy and, subsequently, they can appear brighter or hyperechoic on the ultrasound screen
- Some ultrasound transducer designs such as convex or microconvex models produce a sector shaped image and superficial structures are slightly distorted within the narrow part of the sector at the top of the image. As these transducer designs have a curved surface, they may also struggle to contact the skin towards the edges of the transducer surface, especially if the region examined is small or less pliable
- Historically, ultrasound transducers had a fixed focal zone. This is the region of the image where the lateral resolution is at its greatest. Modern systems allow for the focal zone to be adjusted or have software that is capable of keeping the entire image within focus. However, there are often still limits as to how close to the body surface the focal zone can be positioned and this may limit the resolution in superficial areas
Some of the factors above can be improved by careful patient preparation, appropriate adjustment of the ultrasound system controls, and the use of higher frequency linear transducers (Figure 1.). They can also be improved through the use of an ultrasound standoff pad.
Figure 1. Two different transducer designs. A microconvex transducer design with a curved surface is shown on the left. On the right is a linear design with a flat, rectangular surface.
Standoff pads are typically a flexible, acoustically inert material that is placed between the transducer surface and the patient’s skin. They cause minimal attenuation of ultrasound and typically appear anechoic or black on the ultrasound image.
The use of a standoff pad alleviates many of the reasons for reduced image quality when examining superficial structures and their use is essential to achieving the diagnostic images of some anatomic areas.
- As most standoff pads are soft and flexible, they should improve contact between the transducer and the patient’s skin. However, they still rely on adequate coupling gel being applied between the transducer and the standoff, and between the standoff and the patient
- By increasing the distance between the transducer and the skin, the area of interest can be moved out of the immediate near field and into an area where the ultrasound beam is more focused for optimal visualisation. This also reduces the impact of some distortion caused by curved transducer designs
Most modern standoff pads are made from silicone or similar polymers and are designed in a variety of shapes from simple sheets or discs, through to models that fit over specific transducer designs (Figure 2.).
Figure 2. A modern polymer standoff designed to fit over a linear transducer.
Standoff pads can also be made using examination gloves with water or coupling gel. These inexpensive designs can be useful where standoff pads are only needed sporadically. However, it can be difficult to avoid bubbles of air when filling the glove, and any air present will cause artefacts.
In veterinary medicine there are many applications for standoff pads;
- Examining the superficial abdominal structures in small patients, such as the ventral bladder wall or the feline spleen
- Performing ophthalmic ultrasound
- Examining superficial structures like the thyroid and parathyroid glands. The most common use of standoff pads is in musculoskeletal imaging, especially in equine patients where standoffs are used routinely in the examination of the superficial ligaments and tendon branches.
A good example is in the examination of the superficial digital flexor tendon (SDFT). By using a standoff, the palmar aspect of the tendon and overlying skin can be more accurately evaluated (Figures 3 and 4).
Figure 3. Two transverse plane images of the proximal palmar metacarpal region in a horse. In Image A, a standoff is not used. In Image B, the use of a standoff allows a more accurate evaluation of the palmar SDFT.
Figure 4. Two longitudinal plane images of the proximal palmar metacarpal region in a horse. In Image A, a standoff is not used. In Image B, the use of a standoff allows a more accurate evaluation of the palmar SDFT and overlying skin.
Despite some of the difficulties of imaging superficial structures, by considering the use of a standoff along with appropriate transducer choice and patient preparation, diagnostic quality ultrasound images can be achieved.