A basic understanding of physics and orientation is essential for understanding ultrasound. However, when standing in front of the ultrasound machine, you need to make some concrete decisions about what probe to use, and what buttons to press, in order to obtain a good image—sometimes known as knobology.
This chapter covers the basics of probes and probe selection as well as the buttons found on typical ultrasound machines. While these controls are fairly standard, there are variations from machine to machine and the users will need to spend some time familiarizing themselves with the actual equipment they have—preferably before entering the patient's room. Some additional equipment and logistical concerns have also been discussed.
Probes are generally described by the size and shape of their face ("footprint"). Selecting the right probe for the situation is essential to get good images, although there may be times where more than one probe may be appropriate for a given exam. There are three basic types of probe used in emergency and critical care point-of-care ultrasound: linear, curvilinear, and phased array. Linear (also sometimes called vascular) probes are generally high frequency, better for imaging superficial structures and vessels, and are also often called a vascular probe. Curvilinear probes may have a wider footprint and lower frequency for transabdominal imaging, or in a tighter array (wider field of view) and higher frequency for endocavitary imaging. A phased array probe generates an image from an electronically steered beam in a close array, generating an image that comes from a point and is good for getting between ribs such as in cardiac ultrasound.
Both curvilinear and phased array probes generate sector or "pie-shaped" images, narrower in the near field and wider in the far field, while linear probes typically generate rectangular images on the screen.
Straight Linear Array Probe
The straight linear array probe (Fig. 4-1a) is designed for superficial imaging. The crystals are aligned in a linear fashion within a flat head and produce sound waves in a straight line. The image produced is rectangular in shape (Fig. 4-1b). This probe has higher frequencies (5–13 MHz), which provides better resolution and less penetration. Therefore, this probe is ideal for imaging superficial structures and in ultrasound-guided procedures.
Straight linear array probe (a). Lung imaging using the straight linear array probe (b). Note the rectangular shape produced by the probe. R: rib, P: pleural line, S: rib shadow.
- Vascular access (central and peripheral)
- Evaluate for deep venous thrombosis
- Skin and soft tissue for abscess, foreign body
- Musculoskeletal—tendons, bones, muscles
- Appendicitis in thin patients
- Evaluation of the pleural line for pneumothorax, interstitial fluid
- Ocular ultrasound
- Other procedures (arthrocentesis, paracentesis, thoracentesis, ...