Ultrasound is an interactive sport with the ultrasonographer playing a major role in obtaining quality diagnostic images. Ultrasound probe selection and an understanding of ultrasound machine settings are vital to obtaining quality diagnostic ultrasound images.
High-quality diagnostic ultrasound images start with good probe selection. Most ultrasound machines have a variety of ultrasound probes. The probes vary in frequency of sound they emit, shape of the imaging field of view and shape and size of the probe surface that is in contact with the patient (“footprint”). These qualities of ultrasound probes need to be taken into consideration when choosing which probe to use on a particular patient/particular part of the patient.
Higher frequency probes result in better image resolution at the expense of decreased depth of penetration (i.e., you can get really nice images of the urinary bladder with a high frequency probe but it would not be a good choice for evaluation of a large dog’s liver).
There is a variety of shapes of ultrasound probes used in abdominal imaging. These include microconvex, linear and curvilinear probes.
Microconvex probes have a diverging field of view (narrow in the near field and wide in the far field) and a small footprint. They are generally in the middle to high frequency range.
Linear probes have a rectangular field of view and a long footprint. They are generally high frequency.
Curvilinear probes have a diverging field of view, large footprint and are generally lower frequency.
Rule of thumb: Select the highest frequency probe (to maximize resolution) that allows adequate depth of penetration for organ you are evaluating.
Use of a “pencil grip” is recommended. The probe can be moved in a variety of ways, including distance motion (sliding), non-distance, angular motion (fanning) and non-distance, rotational motion. If possible, keeping the probe perpendicular to the body wall and sliding the probe to scan through the organ of interest in sagittal and transverse orientation will result in the most accurate representation of the anatomy and avoid obtaining oblique images of the organs.
The 5 “knobs” everyone needs to know:
1. Frequency: Most modern ultrasound probes are multifrequency allowing the operator to select different frequencies without switching probes. Use the highest frequency that allows for penetration of the organ of interest.
2. Depth: Adjust the depth of the field of view so that the organ/structure of interest takes up approximately 75% of the field of view.
3. Focal zone: Adjusting the focal zone maximizes lateral resolution (resolution in the horizontal plane). Place the focal zone at the mid to deep portion of the organ/structure of interest. Some machines allow for the operator to select multiple focal zones; however, realize that this will result in a slower frame rate.
4. Gain: Controls the overall brightness of the image. Increasing the gain will increase the brightness of the image across the entire field of view.
5. Time gain compensation: Selectively controls the brightness of the image at various depths. This is usually a collection of multiple slider bars on the ultrasound machine with the top bar representing the near field and the bottom bar representing the far field with various points in between. At the left side of each slide bar representing no gain and the right side representing maximal gain. As sound travels through tissue, various interactions take place that result in less sound waves reaching deeper tissues than superficial tissues. Time gain compensation is a means for evening out image brightness to compensate for the decreased amount of sound reaching deeper tissues. In general, the time gain compensation is set so that there is less gain in the superficial tissues and more gain in the deeper tissues.
Other good knobs to know:
Dynamic range: Controls the contrast of the ultrasound image. A high dynamic range setting allows for many shades of gray and a less contrasty image. A low dynamic range setting allows for less shades of gray (more black and white) and results in a more contrasty image.
Doppler: Being familiar with the Doppler options on your machine is helpful in many situations. Types of Doppler include color, power, pulse wave and continuous wave:
- Color Doppler identifies the direction and relative velocity of blood flow.
- Power Doppler is sensitive to slow flowing blood but does not identify direction of blood flow.
- Pulse wave Doppler allows for evaluation of velocity and direction of flow in a specific vessel.
- Continuous wave Doppler is used in echocardiography and allows for evaluation of high velocity blood flow.
Keep in mind that Doppler signal is dependent on the angle of the ultrasound probe in relation to blood flow. Best Doppler results are obtained when the angle of the probe is small (less than 60 degrees) in comparison to the direction of blood flow.