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.
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.
- 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, nerve blocks, etc)
The curvilinear array or convex probe (Fig. 4-2a) is used for scanning deeper structures. The crystals are aligned along a curved surface and cause a fanning out of the beam, which results in a field of view that is wider than the probe’s footprint. The image generated is sector shaped (Fig. 4-2b). These probes have frequencies ranging between 1 and 8 MHz, which allows for greater penetration, but less resolution. These probes are most often used in abdominal and pelvic applications. They are also useful in certain musculoskeletal evaluations or procedures when deeper anatomy needs to be imaged or in obese patients.
- Abdominal aorta
- Abdominal portion of FAST exam
- Kidney and bladder evaluation
- Transabdominal pelvic evaluation
The endocavitary probe (Fig. 4-3a) also has a curved face, but a much higher frequency (8–13 MHz) than the curvilinear probe. This probe’s elongated shape allows it to be inserted close to the anatomy being evaluated. The curved face creates a wide field of view of almost 180° and its high frequencies provide superior resolution (Fig. 4-3b). This probe is used most commonly for gynecological applications, but can also be used for intraoral evaluation of peritonsillar abscesses.
- Transvaginal ultrasound
- Intraoral (tonsillar) evaluation
Phased array probes (Fig. 4-4a) have crystals that are grouped closely together. The timing of the electrical pulses that are applied to the crystals varies and they are fired in an oscillating manner. The sound waves that are generated originate from a single point and fan outward, creating a sector-type image (Fig. 4-4b). This probe has a smaller and flatter footprint than the curvilinear one, which allows the user to maneuver more easily between the ribs and small spaces. These probes have frequencies between 2 and 8 MHz, but they usually operate at the higher end making them ideal for echocardiography. The phased array probe can also function at lower frequencies, which is useful for viewing the abdomen, pelvis, and for procedural guidance.
- Cardiac imaging
- Imaging between ribs in the flank or right upper quadrant
- May be used for transabdominal imaging if curvilinear probe not available
Although most ultrasound machines share some universal features, there is some variation in design between different manufacturers. It is important to become familiar with the particulars of the specific machine before scanning. The control panel of an ultrasound machine has various buttons and knobs that are used to adjust and record images.
B-mode, also known as basic, gray scale, or two-dimensional imaging, refers to the standard black and white image displayed on the ultrasound monitor.
Most machines have settings that will adjust an image based on the anatomy being scanned. These presets are programmed to optimize images based on certain gain and power settings, focal zones, frame rates, and other settings. For instance, the echocardiography setting will automatically default to a higher frame rate in order to capture better cardiac imaging. The correct preset should be chosen before scanning is initiated.