High-frequency sound waves (ultrasound) are used in the natural world for example by dolphins and bats to locate moving and static objects around them (echolocation). The use of radar in aviation also uses this technique. Medical imaging uses ultrasound to produce real-time 2D and sometimes 3D anatomical images, to look at normal structures and to identify and classify abnormalities. More advanced techniques can then look at other features such as blood flow within and compressibility of structures to aid in diagnosis.
The main advantages of ultrasound over other scanning techniques are a lack of radiation (unlike CT), ease of use (unlike MRI), and a more straightforward ability to “move through” the patient and watch moving structures such as the heart and pulsating blood vessels. It also produces beautiful higher-resolution images in smaller, more accessible parts of the body such as the neck.
Unlike CT and MRI, ultrasound cannot penetrate through dense objects such as bone, assess deeper structures, or be used to look at large areas of the body at the same time.
Ultrasound is therefore useful in:
-Identifying or excluding lumps within or near normal structures
-Classifying lymph nodes as “benign” or “malignant (cancerous)”
-Classifying thyroid nodules
-Looking for blood flow within the lump
Ultrasound classification of thyroid nodules:
The most commonly used classification is the “U1-5” system by the British Thyroid Association where thyroid lumps are graded using ultrasound from U1 (least suspicious) to U5 (most suspicious).
Although this gives an idea as to the nature of the nodule, the most accurate assessment is to get a sample of cells or tissue from the nodule and for the pathologist to look at it under the microscope and give a final diagnosis.
The lack of radiation and ability to move the ultrasound probe to point to various structures makes it ideal to use to guide a biopsy needle into the area of concern to draw back a sample for analysis, sometimes reducing the need for more invasive surgical procedures.
Fine needle aspiration “FNA” – uses small needles to get a small sample of cells for analysis. This is quicker, safer and less invasive than a core biopsy (see below) but can sometimes be non-diagnostic.
Core biopsy – uses a larger “cutting” needle to get a larger sample of tissue. While more likely to get a diagnosis, this is a longer procedure requiring local anaesthetic and is higher risk, hence is only performed in the neck in certain situations.