Ultrasound imaging has played an integral role in medical diagnostics for many years. Recent advances in image quality and resolution have now made it possible to examine living tissue ultrasonically at a microscopic level. Indeed, the use of ultrasound biomicroscopy is of growing interest and great importance.1 Advances in computer technology have paved the way for currently available ultrasound units that are portable and cost-effective for office practice.
Indications for diagnostic ultrasound of the foot and ankle include the prevention and treatment of wounds, detecting tendon and ligament pathology, and detecting soft tissue lesions and foreign bodies. Many people believe ultrasound imaging will become an integral part of podiatric practice.
Identifying Skin Lesions
Images using 20 MHz ultrasound reveal detailed anatomical images of the skin. There have been studies that compared ultrasound imaging to histological sections, revealing that ultrasound provides accurate assessment of the skin structure. In fact, ultrasound measurement of skin thickness is likely to provide a more accurate indication of skin thickness than histological measurements since the latter are subject to dimensional changes due to fixation and processing.3
Reflective patterns of the skin enable you to visualize the stratum corneum and differentiate it from the deeper layers of the epidermis. You can also get a clear view of the epidermal-dermal juncture. You can see reflections from acoustic interfaces (e.g. between collagen and hydrated ground substance) in the papillary layer of dermis, and differentiate the denser reticular layer of the dermis. High-frequency ultrasound clearly demonstrates the juncture between the dermis and the adipose hypodermis.4 You can accurately measure variations in skin thickness and identify and quantify lesions within the skin.
You can use high-resolution ultrasound to image skin lesions and other common soft tissue masses within the foot and ankle. Skin cancers, such as basal cell and squamous cell carcinoma and melanoma, can be imaged. This technology allows you to reliably demonstrate the lesion margin and the overall architecture.5 While you cannot make a tissue diagnosis with ultrasound, you can determine valuable information such as depth, subcutaneous satellites and sub-surface size.
Can This Modality Make An Impact In Preventing And Treating Wounds?
Obviously, we see plenty of diabetic wounds, pressure ulcers and venous stasis wounds in our practices. High resolution ultrasound enables you to monitor the depth and width of these wounds, edematous regions, granulation tissue and developing scar tissue. Indeed, we can use this information to prevent and direct treatment of these conditions.6
Pressure ulcers are a very common problem in people with compromised mobility and fragile skin. Many pressure relieving devices are available, but it is not feasible to use them at all times. The heel is one of the most common sites for pressure ulcers and they can lead to amputation without proper care.
One of the great things about using ultrasound is it’s very sensitive in detecting fluid changes within the tissue.7 When we use a high resolution ultrasound, it enables us to detect a pressure ulcer before there are clinical signs. We have seen an increase in fluid 48 to 72 hours prior to clinical signs. This allows us to employ pressure relief measures before the ulcer breaks through the skin.
There are two common patterns that you will see. In superficial ulcers that are caused by friction on rubbing, you will see an area of inflammation (a hypoechoic band) directly beneath the epidermis. You will see this inflammation spread downward into the dermal tissue as the ulcer progresses. The lesion will also break through the epidermis and form a friction ulcer. You’ll see the other common pattern in ulcers caused by prolonged direct pressure. In these ulcers, you will see an area of inflammation beneath the dermis with strips of inflammation protruding upward into the dermis as the tissue damage progresses. As the dermis breaks down further, it will eventually break through the epidermis, forming an overt pressure ulcer. You can obtain ultrasonic images of the initial phases of both of these types of ulcers before seeing any clinical signs.
Ultrasound imaging can be part of a total pressure ulcer prevention program that allows for decreased morbidity and liability. Ultrasound examination of common pressure ulcer sites adds about 10 minutes to a routine nursing assessment.
Ultrasound assessment of chronic wounds can provide important clinical information. You can use it to evaluate for undermining, sinus tract formation, and abscess and depth assessment. Using this technology also enables you to determine the amount of edema beneath the wound, and to follow and record the healing process beneath the surface. This gives you valuable information in determining if the current treatment regimes are effective.
Performing an ultrasound examination through the wound will detect sinus tract formation and the depth of the wound. Sinus tracks are easy to identify as strips of hypoechoic activity in the sub-wound tissue. Tracking down to bone is an indication for further studies to confirm osteomyelitis. You can determine wound depth by comparing the damaged tissue to the surrounding normal tissue. The damaged tissue will be generally less dense, indicating increased fluid volume within the tissue. Undermining is also easy to determine by a hypoechoic band between layers of normal tissue. You can identify abscess formation as a hypoechoic pocket within the tissue.
Tissue edema is one of the main reasons that lower extremity wounds are delayed in healing. Many methods, including compression devices and physical modalities, are used to decrease the tissue edema. Ultrasound imaging allows you to monitor these treatments and make subsequent adjustments, if necessary, to enhance the effectiveness of treatment.8
What About Soft Tissue Lesions?
Using high-frquency ultrasound is also valuable in determining whether soft tissue masses are solid or fluid filled. A solid lesion will have reflections within it whereas a fluid-filled lesion will have very few reflections. It can also help you quantify the depth and size of the lesion. Ganglions appear as fluid-filled masses and separate lobes can be identified. You can also determine if the ganglion is originating from a tendon.
Ultrasound is also valuable for needle aspiration of lesions. You can image Morton’s neuromas from either the dorsal or plantar aspect. They appear as hypoechoic masses along the interdigital nerve. It has been reported that ultrasound has 95 percent accuracy in the detection of Morton’s neuroma.8 You’ll find that using ultrasound can also be valuable in monitoring the use of sclerosing agents for neuromas, for both identifying the location of the neuroma and in documenting the effectiveness of treatment.
Can This Imaging Aid In Treating Tendonitis And Tendon Tears?
Tendons image on longitudinal scans as a band of hyperechoic lines. These lines are closely packed and run parallel to each other. You can see the synovial sheath as a hypoechoic layer above the tendon. On the Achilles, you can see the peritenon as a hypoechoic strip above the tendon. Transverse scans show a bright oval-shaped structure with a hypoechoic halo representing the synovial sheath. Be sure to image the tendons with the transducer directly perpendicular to the tendon. If you image the tendon in a non-perendicular manner, it will produce a hypoechoic effect and you won’t be able to see the tendon fibers.9 This phenomenon is called anisotropy.
Ultrasound imaging can have distinct advantages over other modalities. It gives you immediate feedback and information regarding the function of the tendon as well as anatomy and pathology. When assessing any pathologic condition, it is recommended that you image the normal contralateral side for comparison. You should hold the probe directly perpendicular to the tendon and manipulate the foot into various positions in order to ensure a complete examination.
You can see tendonitis on both longitudinal and transverse images as areas of less reflection within the tendon and a spreading of the hyperechoic lines. You can distinguish peri-tendon inflammation from tendonitis as areas of hyoechoic activity around the tendon. You can also image bursas and distinguish them from tendonitis. This can be very valuable in assessing Achilles tendon pathology.
In longitudinal scans, partial tears of tendon will show a spreading of the longitudinal lines and a thickening of the tendon. On transverse imaging, there will be a hypoechoic area surrounding the tendon indicative of inflammation. The linear bands of the tendon will be more diffuse and the circumference of the tendon will be increased. Complete tear of the tendon on longitudinal scans will demonstrate a total discontinuity of the tendon with a hypoechoic area replacing the normal hyperechoic banding.9
Following tendon repair, you can employ ultrasound imaging to help determine when to discontinue immobilization, when to weightbear and when to allow the patient to return to full activity. The current standard for fracture care is to take serial X-ray images to monitor healing. Ultrasound imaging allows you to use the same standard for tendon and ligament care. Serial imaging of tendon healing can facilitate better decision making.
Looking At Ligaments, Blood Vessels And Nerves
Ligaments appear as hyperechoic bands. The lines within the ligament are not as organized as tendons. Ligaments are best visualized in areas where there is a better contrast of density with the surrounding structures. You can image the intrinsic ligaments of the ankle as denser and thicker areas of the ankle joint capsule. A tear will appear as a hypoechoic area within the ligament.9
Blood vessels on a longitudinal section appear as strips of hypoechoic areas, representing the lumen with hyperechoic areas on either side of the lumen representing the vessel wall. On a transverse section, the vessels appear as round to oval areas of hypoechoic patterns surrounded by a hyperechoic ring. You can differentiate arteries and veins by increasing the downward pressure of the probe. Veins will compress easier than arteries and the artery wall is thicker. Diseased arteries will show smaller lumen size and increased reflections within the walls.
Nerves will image on a longitudinal scan as hyperechoic bands of fibers. In differentiating them from tendon, keep in mind that they will not become hypoechoic if the probe is non-perpendicular. On a transverse section, nerves appear as a discrete echoic round to oval structure. Smaller nerves are hard to differentiate from the surrounding tissue. Entrapment syndromes or nerve compression can be seen as a narrowing of the involved nerve. Comparison with the opposite side is very helpful.
Better Detection Of Foreign Bodies?
Recovering a foreign body can be one of the most frustrating experiences in podiatric practice. Radiographs and fluoroscopy are valuable for many foreign bodies, however ultrasound imaging has some distinct advantages.
Using an ultrasound allows you to image many substances that cannot be seen on X-rays. These substances include non-leaded glass, wood and organic materials. Foreign bodies generally appear as hyperechoic bodies with distal shadowing and an area of surrounding fluid.9 With high resolution imaging, it is possible to see the exact tissue plane involved and measure the exact depth. Once you’ve located the foreign body with the probe, you can precisely mark the area and plan a precise surgical excision.
Ultrasound biomicroscopy or high resolution ultrasound gives podiatrists an added tool in diagnosing and treating many foot and ankle pathologies. It provides a method of seeing under the skin for some of the most common problems we see in our practices. However, there is a need for more studies and literature to further validate its use and document its effectiveness. That said, I do believe this technology will continue to improve in quality, cost and portability, making it even more attractive. The future for high resolution ultrasound in the podiatric practice is bright and its use can be very rewarding for podiatrists and their patients.
Dr. Quintavalle is the Residency Director at Virtua Health System in Camden, NJ and is a Director of Longport Inc.
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8. Sobiesk GA, Werthheimer SJ, et. al.: Sonographic evaluation in interdigital neuromas. J Foot Ankle Surg 36:364-366, 1997.
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