The ultrasound probe generates inaudible high-frequency sound waves. The waves are reflected off the body structures and are detected by the probe.
Ultrasound waves are:
- Reflected by Solid and Gas: Bright
- Transmitted (not reflected) by Fluid: Dark
This reflection or transmission causes an additional effect distal to it:
- Distal to a bright object: dark acoustic shadowing
- Distal to a dark object: bright acoustic enhancement
Doppler: detects movement relative to probe, by whether reflected sound waves are compressed.
The blood in vessels is mobile, therefore will show up as colour on Doppler. This can be used to find the presence of vessels within an organ or tumour, or confirm lack of flow in a DVT. Doppler can also be used to measure the patent diameter of a vessel lumen. This is useful in carotid atherosclerosis. ‘Duplex’ ultrasound simply refers to the overlay of Doppler colours over the basic grey ultrasound image.
The long axis of the probe “slices” through the body.
There are two main planes in ultrasound – the transverse section (TS), and the longitudinal section (LS).
In transverse section, the left side of the image is the right side of the patient.
In longitudinal section (also known as sagittal section), the left side of the image is in the direction of the patient’s head.
The most common method of ultrasound is transthoracic (for heart i.e. echocardiogram) or transabdominal (for abdominal and pelvic organs) as it is less invasive, quicker and more acceptable.
In certain cases, a more invasive method may be indicated, such as transoesophageal, transrectal or transvaginal ultrasound. This is used particularly when the area of interest is within or adjacent to the wall of the lumen, for example intramural cancers and adjacent lymph nodes. Patient habitus may also necessitate the usage of these methods.
Ultrasound-guided biopsy can be performed on superficial lesions.
Duplex ultrasonography helps with the insertion of central lines and the measurement of ABPI.