Diagnostic Ultrasound
Ultrasound is great for soft tissue musculoskeletal injuries! It is a tool for fast and accurate visualization of soft tissue for evaluation that is accessible, easily repeated, and cost effective. Diagnostic ultrasound uses include identifying inflammation, tears and ruptures, lesions, fluid collections, soft tissue masses, measuring changes in muscle thickness due to contraction or atrophy, biofeedback for re-education of muscular activation, and documenting the healing process.
Structures are visualized by creating an image from reflected sound waves. The transducer both generates the sound waves, sending them into the body, and detects the reflected sound waves that have bounced off some structure in the body. Different tissues and materials will have different echo levels that can be catagorized as hyperechoic, hypoechoic, anachoic, or artifact. This is because tissues conduct sound differently so that when sound passes through tissues through tissues of differing acoustic properities, there will be differences in the waveforms reflected back to the transducer. This acoustic impedance, or z-value, generates the tissue echo and is mainely determined by the tissue density since ultrasound volocity in the tissue is relatively constant. Therefore, bones will be hyperechoic and appear white on the image while fluid will by hypoechoic and appear black on the image.
While ultrasound is generally very safe, it is important to determine if it is contraindicated for your patient for any reason before using ultrasound.
Transducers have different diagnostic units that are typically 3.5 MHz, 5 MHz, or 7.5 MHz. So how do you know which one to choose? You have to consider how deep the structure that you are trying to visualize is! As frequency increases, the wavelength decreases so it will not be able to penetrate as far to visualize deeper structures. Frequency must be sacrificed to improve the depth of penetration for deeper tissues. Because of this, lower MHz transducers are used for deeper tissues but have a poor resolution while higher MHz tranducers are used for more superficial applications and have a much clearer picture.
Procedures
Transversus Abdominis
The transversus abdominis (TrA) is a superficial structure for which the linear 7.5 MHz (or higher!) transducer can be utilized. However, if the patient is carrying some extra weight around the middle, the transducer may need to be changed to a lower frequency in order to visualize past the built up adipose tissue.
Have the patient lie in supine. Explain to the patient what they should expect and always ask the patient before exposing any skin. A cloth or paper towel can be tucked into the patient's clothing to protect their clothing from getting ultrasound gel on it. Put gel on the ultrasound head. This gel is used to reduce attenuation and achieve a better image. Orient yourself with the transducer. Wiggle a finger on one end of the transducer to determine which end is up! The side of the transducer the correlates with the blue dot on the ultrasound screen should be pointed up towards the ceiling. Palpate the patient's iliac crest and the inferior angle of the 12th rib. Place the transducer between these two landmarks to get a transverse image of the abdominal wall. From top to bottom, you should be able to visualize the subcutaneous tissue, external abdominal oblique muscle (EAO), internal abdominal oblique muscle (IAO), transversus abdominis muscle, and the underlying viscera.
Once you have a clear picture of the TrA, you can freeze the image to take a measurement. Use the caliper tool on the machine to draw a line from the bottom of the TrA plane to the fascial plane between the TrA and the IAO. This is not a standardized measure as it will be different on everybody. You can compare bilateral measurements to determine if this is the correct muscle thickness for the patient. Having this baseline measure will also help you determine if the muscle is atrophying or hypertrophying.
To utilize the biofeedback benefits of diagnostic ultrasound, have the patient contract their TrA. You can help them with this by giving the verbal cue to "pull your belly button back to the table" or "imagine you are trying to stop peeing mid-stream". You will be able to see the muscles move and contract on the screen. This movement may cause the transducer head to move around. Rest your arm on the patient to help you stablize the transducer head and maintain the image. A measurement can be taken of the contracted muscle to determine if contraction strength is improving with treatment.
Multifidus
The multifidus is a deeper structure for which a lower freqeuncy 5 MHz, curvilinear transducer should be utilized.
Have the patient laying prone with a pillow or two under their belly to reduce the lordotic curve of the lumber spine. Explain to the patient what they should expect and always ask the patient before exposing any skin. A cloth or paper towel can be tucked into the patient's clothing to protect their clothing from getting ultrasound gel on it. Put gel on the ultrasound head. This gel is used to reduce attenuation and achieve a better image. Orient yourself to the patient. Palpate the sacrum, iliac crest, and the spinous processes of L2-L5.
When obtaining a transverse view of the multifidus, determine which segment you would like to visualize. The top of the iliac crest lines up with the L4 spinous process. You can place the tranducer transversely over this point or count the spinous processes up or down to analyze the multifidus at a different level. On the image, the spinous process should be in the middle of the screen and will appear black as an artifact. The multifidi can be visualized on either side of the spinous process as owl eyes. To be sure that you are visualizing the multifidus, you can have the patient lightly left their contralateral leg off the table to see the muscle contract. Drop off to one side of the spinous process to better visualize the multifidus. The multifidus is easily defined medially by the spinous process and inferiorly by the lamina. Continue to have the patient contract and relax the muscle with the contralateral leg lift if you need help visualizing the boundaries of the rest of the muscle. Freeze the image to get a measurement of the multifidus. Press the caliper button and select "manual" to trace the circumference of the multifidus. Use the ultrasound for biofeedback to let the patient know they have correctly activated the muscle. Start with isometric contractions then progress to knee bending and prone leg lifts.
When obtaining a longitudinal view, palpate the sacrum and place the transducer longitudinally along the sacrum and spinous processes. The sacrum will have a signficant oblong appearance compared to the lumbar spinous processes. Use this as your starting point and count up to to whichever level you would like to visualize. Freeze the image and take a thickness measurement of the multifidus. Use the caliper tool to draw a line from the top of the zygopophyseal joint to the subcutaneous fascia directly above.
If you image is not coming in clearly, changing the gain and/or depth are most effective to clarify the image. Adjusting the gain to increase the intesnity of the acoustic pulse will result in a stronger echo and brighter image. Adjecting the depth is essentially the "zoom in" and "zoom out" of the ultrasound machine.