High-resolution sonography provides non-invasive views beneath skin

January 1, 2005

Backed by a long and successful history as a tool to examine human anatomy such as the heart, breast, thyroid and prostate, sonography is now proving to be equally valuable in analyzing the skin.

Backed by a long and successful history as a tool to examine human anatomy such as the heart, breast, thyroid and prostate, sonography is now proving to be equally valuable in analyzing the skin.

Thanks to the development of ultrasound units above 20 MHz, dermatologists are now able to view the architecture of the skin in a more precise, non-invasive way.

More precise High-resolution frequency (100 MHz) sonography is much more precise than palpation of the skin with the fingers, according to Dr. El Gammal, who is also president of the International Society for Skin Imaging.

"It provides an accurate view of normal skin structures such as skin layers, hair follicles, sweat gland ducts and pathological structures such as inflammatory skin diseases like sclero-derma, psoriasis and eczema," he tells Dermatology Times.

High-resolution sonography also offers a safe, accurate evaluation of in-depth penetration and lateral extension of skin tumors such as nevocellular nevus, seborrhoic keratosis, malignant melanoma, basal cell carcinoma, squamous cell carcinoma and lymphoma.

"One great advantage is that skin tumors can be easily demarcated and measured, as long as they are confined to the dermis," says Dr. El Gammal. "Nearly all skin tumors are visualized as echo-poor structures. 100 MHz often allows for a more clear demarcation of tumor masses from the surrounding healthy skin than 20 MHz."

100 MHz beneath the skin Until about 30 years ago, the low-resolution, 10 MHz ultrasound or sonography technique used to view internal organs was not suitable to visualize skin structures. Then in the late 1970s, a 15- MHz ultrasound machine was developed which could measure one-dimensional skin thickness.

"This was the first successful application of sonography to the skin," he says.

Today, most commercially available high-resolution ultrasound machines work with 20 and 50 MHz. Prototypes of 100 MHz applicators are available for experimental research.

How it works Similar to the perceptive functioning of a bat, which waits after a scream for an echo to evaluate the distance to an obstacle, sonography visualizes anatomical structures by returning echoes from the structure borders.

While the event seems simplistic, different ultrasound incidences occur while the acoustic wave propagates through the medium or soft tissue. Part of the sound wave energy is either reflected at tissue borders with different acoustic impedance, transmitted into the next tissue medium, absorbed due to inner friction in the tissue, or diffusely scattered.

"The sonographic pictures we see are therefore a complex overlap of these primary ultrasound phenomena," Dr. El Gammal says.

Greatest importance And according to Dr. El Gammal, reflection has the greatest importance for clinical sonography.

"As long as the sound wave propagates in a homogenous medium, such as water," he says, "it is solely influenced by the absorption characteristics of the medium."

When it hits another medium however, part of the wave or energy is reflected and the rest is transmitted into the new medium. Only these echo reflexes - which return to the ultrasound transducer or applicator - participate as echo-rich structures in forming the sonographic picture.

The information gleaned from high-resolution sonography comes as no surprise to Dr. El Gammal.