Ultrasound

Author: Mr Peter Coombs*

What is an ultrasound?

Ultrasound (US) uses high-frequency sound waves to image soft tissues of the body. The frequency of the sound is less than that used in other clinical areas (e.g. physiotherapy), so it is not associated with local heating effects and is completely safe. US is relatively inexpensive, readily available in all imaging practices and in some clinical consulting rooms. Unlike other imaging studies, US is a dynamic study in that patient movement can be assessed at the time of the examination and can be correlated with the clinical presenting symptoms.

Sound waves are reflected back to the transducer on the skin at different velocities determined by the type of soft tissue they encounter. Patients with excessive body fat do not do well with US, as the sound waves are reflected by the subcutaneous fat before reaching the organ of concern. Sound does not pass through bone or dense soft tissue (e.g. scar, calcification), so structures beneath these regions cannot be seen.

What are the generally accepted indications for an ultrasound?

US is the investigation of choice when the organ of clinical concern is assessable for sound wave interrogation.  The closer and more localized the area of clinical concern is to the US transducer, the better the results. This includes assessment for diagnostic purposes or using US guidance for interventional procedures, such as therapeutic injections or drainages.

US can be used in the clinical consulting room for identification of veins for vascular access. Point of care US (POCUS) is becoming more available to the clinician as machines become smaller and cheaper. This is useful to supplement clinical assessment and diagnosis at the bedside, but is not recommended for primary diagnostic purposes.

What are the prerequisites for having an ultrasound done?

If you are referring a patient for an ultrasound examination, it would be valuable to include:

• A concise, relevant clinical history, as this will improve the efficacy of any diagnostic imaging test.
• Relevant recent pathology or imaging results. Ensure that the patient attends the US department with all prior imaging studies for comparison and correlative assessment by the radiologist.

What are the absolute contraindications for an ultrasound?

There are no absolute contraindications for an ultrasound examination.

What are the relative contraindications for an ultrasound?

In obese patients, the thick subcutaneous fat layer limits sound wave penetration. Where deep structures require interrogation; for example, the pancreas, then alternative methods of imaging, such as computed tomography, should be considered.

Where there is skin sepsis or lack of access allowing skin/transducer contact, then US may be difficult or inappropriate. Practitioners should contact their local imaging provider to discuss possible alternative imaging tests.

What are the adverse effects of an ultrasound?

Your patient should not experience any adverse effects from ultrasound examination.

Is there any specific post-procedural care required following an ultrasound?

Post-procedural information and care will only be required if interventional procedures have been carried out using US guidance.

For simple soft tissue and joint injections, the patient may experience pain at the injection site or in the joint for approximately 24–48 hours. Should the pain continue, or the patient develops systemic symptoms suggestive of infection, then the procedural radiologist should be contacted to discuss further management. If this occurs outside working hours, the patient should be referred to the local emergency department for assessment.

For other interventional procedures: the US department should contact the referring clinician regarding post-procedural care and follow up.

Are there alternative imaging tests, interventions or surgical procedures to an ultrasound?

Ultrasound is generally considered a primary diagnostic modality, because it is cheap, easily accessible and very accurate in many settings. Be aware that patients with specific indications may be better referred to computed tomography and magnetic resonance imaging directly.
If you have any doubt about the appropriateness of a diagnostic test, please contact the local diagnostic imaging department, where the consultant radiologist will generally be happy to discuss the best alternative with you.

*The author has no conflict of interest with this topic.

Page last modified on 11/9/2018.