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Ultrasound

How does it work?

The ultrasound probe is placed on the site of interest. Ultrasound jelly is used to ensure a good contact between probe and skin (or whatever the relevant surface is) which is essential for it to work. High frequency sound waves pass from the probe into the patient. Depending on the reflectivity of the tissues that the sound passes through, the sound is reflected and collected by the probe. The time to echo is one of the variables used to generate a 2d image on the screen.

How does Doppler ultrasound work and what is it used for?

This makes use of the Doppler effect. Fluid moving toward the probe increases the frequency of the reflected sound collected by the probe and fluid moving away from the probe decreases the frequency. This information is processed by the machine and is depicted in colour to the operator.

Doppler ultrasound is used for imaging flow, usually in blood vessels and the heart. A gastroenterologist might be interested in imaging portal venous flow, and Doppler is highly relevant in the transplanted liver.

What are the risks of ultrasound?

Ultrasound is relatively free of risks. There is a theoretical risk to the fetus of heating in prolonged use of Doppler in early pregnancy but this is unlikely to be of relevance to gastroenterologists. There is no radiation involved.

What are the advantages and disadvantages of ultrasound?

Ultrasound is free of risks, is acceptable to the patient. It is however very user dependent. In the abdomen, it can be of less value in patients of high BMI, and does not image the retroperitoneum very well. CT is better in these patients.

What are the uses of ultrasound that might be relevant to a gastroenterologist?

Routine abdominal ultrasound is often the first line investigation in a range of presentations, such as jaundice, weight loss, abdominal pain, suspected abdominal mass. The bowel can be imaged and inflammatory processes can be diagnosed. Hernias can also be detected with ultrasound.

What does a standard ultrasound of the abdomen include? Which organs are not well seen?

Kidneys, liver, gallbladder, biliary system, pancreas, spleen, abdominal aorta, urinary bladder.
Ultrasound is the gold standard for diagnosing gallstones, and is better than CT. The gallbladder itself is also better seen than on CT. For liver pathology, CT is generally more sensitive, but both modalities may provide information, and ultrasound can sometimes be used to characterise a lesion which is indeterminate on CT. Sometimes lesions such as liver metastases are difficult to see on a CT scan and may be better visualised at ultrasound.

The GI tract is not generally well seen as air within it is highly reflective. However see below. The adrenals are also not seen. The retroperitoneum is not well visualised and if you suspect pathology there a CT might be better.

If you ask for an US abdomen, the female reproductive organs (uterus,ovaries) are not routinely included on the study.

What are the indications for ultrasound of the bowel?

Bowel ultrasound is a specialised test which is performed by an experienced radiologist. It is a relatively new technique, as the bowel is not routinely included in an abdominal ultrasound examination owing to difficulties in assessment, largely attributable to the reflectivity of intraluminal air.

Ultrasound can be used to detect areas of inflammatory bowel disease, in which the patient will have thick walled bowel loops with increased Doppler flow (vascularity). These findings are non specific, and are shared with other bowel pathologies, but can guide the need for colonoscopy and biopsy to make a histological diagnosis. Ultrasound may have a more important role in assessing the location of active disease in the patient with known inflammatory bowel disease. It has the advantage here of being non invasive, without the risks that colonoscopy carries, particularly in actively inflamed bowel. Ultrasound can also assess for complications of IBD, such as colections.

Ultrasound (for this indication as well as others) is of limited use in obese patients, although those with inflammatory bowel disease are often thin. The colon may be less well visualised than the small bowel. Strictures are also more difficult to detect. Contrast enhanced ultrasound can increase the sensitivity of this technique for active disease.

Edited by Dr Iain Au-Yong


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