Sonographers generally think of diagnostic ultrasound in the frequency range of approximately 2–22 MHz, but with high-frequency ultrasound, there is the capability to image up to 70 MHz. High-frequency ultrasound has great resolution but the main limitation is the lack of imaging depth. Today, it can be used in preclinical applications, such as nanoparticle and animal work, and in clinical applications, such as dermatologic, musculoskeletal, vascular, and rheumatologic settings.

In small animal protocols, multiple novel ultrasound imaging modes can be applied, including color and power Doppler, contrast-enhanced ultrasound, photoacoustic imaging, and more.

What is photoacoustic ultrasound?

To put it simply, light goes in, sound waves come out. Photoacoustic imaging uses light energy that is absorbed into the tissue, produces a thermoelastic expansion, and creates pressure, ie a soundwave. This sound wave is detected by an ultrasound transducer and subsequently produces an ultrasound image. An example of a pre-clinical application of photoacoustic imaging is to evaluate the oxygenation levels in tumors using the oxygen saturation calculation.

Preparing for Photoacoustic Ultrasound Imaging of Small Animals

A common use of photoacoustic imaging is to examine small animals, as the depth needed is minimal and the images can have a very high resolution.

In any small animal study, before sedating any of the animals for photoacoustic imaging, there are a few key steps to take:

• Study the anatomy of the kind of animal you will be scanning.
• As needed, remove hair from the area to be scanned.
• Allow time for the laser to warm up and optimize prior to photoacoustic imaging.
• Adjust ultrasound transducers and machine presets.
• Set up your equipment to be as ergonomic as possible; animal scanning often takes a long time due to the high volume and may include a lot of small, fine movements.
• Gather supplies.

It is also extremely important to align the stage and transducer before sedating the animal because animals are often compromised under anesthesia. So, keep in mind how many modalities will be needed and that the animals should remain under the anesthesia for a limited amount of time.

In addition to preparing the scanning area for the animals, prepare your own area to ensure you have good ergonomics. In many small animal studies, the long duration of high-volume, fine movement scanning without proper ergonomics could become very difficult.

Once all of the preparations have been completed, sedate the animal and secure it to the stage to ensure the animal will not move during imaging.

Now it is time to scan!

To learn more, see the American Institute of Ultrasound in Medicine’s (AIUM’s) on-demand webinar with speaker Corrine Wessner, “High-Frequency Ultrasound & Photoacoustic Scanning: Perspective From a Research Sonographer”, from which this post was adapted. AIUM members can access the webinar for free.

Interested in learning more about high-frequency ultrasound and photoacoustic imaging? Check out the following resources from the American Institute of Ultrasound in Medicine (AIUM):

• Nam K, et al. Three-Dimensional Subharmonic Aided Pressure Estimation for Assessing Arterial Plaques in a Rabbit Model. J Ultrasound Med 2019; 38:1865–1873. https://doi.org/10.1002/jum.14884.
• Delaney LJ, et al. Breast Cancer Brain Metastasis Response to Radiation After Microbubble Oxygen Delivery in a Murine Model. J Ultrasound Med 2019: 38: 3221–3228. https://doi.org/10.1002/jum.15031.
• The Scan blog post, “What Rheumatologists Really Need for Ultrasound Is…” by Arthur M. Mandelin II, MD, PhD, RMSK, RhMSUS