In an ultrasound investigation reflected sound waves are used to identify differences in tissues. A high resolution ultrasound uses state of the art machines with high frequency probes (10 to 15 Mhz range). Using ultrasound skilled clinicians can detect small subcapsular metastatic melanoma deposits in lymph nodes, before any mass becomes palpable clinically.

The smallest metastasis that can be seen depends on the location of the lymph node in each patient.  The best pictures are obtained when the lymph node is near the surface of the skin.

This is because the higher frequencies in the ultrasound beam are most quickly absorbed by soft tissue. As a result, metastases as small as 2-3mm can be detected in the neck and groin areas, compared to the detection of metastasis above 4mm in the axilla (armpit), where there is a lot of fatty tissue.

High resolution ultrasound is used to examine lymph nodes during follow up, and to check on sentinel nodes that have not been removed surgically. The location of these sentinel nodes is marked on the skin with a tattoo of carbon black, so that an even more accurate, targeted US exam can be performed.

Following surgery node fields can also be checked for early melanoma metastasis. If any abnormal area is seen in a lymph node(as pictured right) it can be subjected to fine needle aspiration biopsy(FNAB) under US guidance.