Applications of Ultrasound (AQA A Level Physics)

• The two main types of ultrasound used to obtain diagnostic information are:
  • A-scan, or amplitude scan
  • B-scan, or brightness scan

A-scan

• An A-scan, or amplitude scan, uses a single transducer to emit a signal and then later receive the reflected signal back
• It is defined as:

A one-dimensional ultrasound scan used to determine the distance or depth of an internal structure

• This is achieved by:
  • Measuring the time delay between generating and receiving the signal
  • Using the speed of sound in the media to calculate the distance travelled by the signal
• This type of scan is used for:
  • Determining distances from the ultrasound device to the point of reflection (usually the boundary between two media)
  • For example, the length of an eye needs to be determined in planning surgeries or assessing the presence of abnormalities, such as tumours
• This type of scan gives measurements only and does not produce an image

B-scan

• A B-scan, or brightness scan, is a more complex scan that produces a 2D or 3D image of internal structures in the body
• It is defined as:

An ultrasound scan used to build up a two or three-dimensional image of an internal structure using a number of sensors or one sensor in different positions

• This is achieved by:
  • Using pulsed ultrasound waves in different positions to produce several measurements of time intervals between generating and receiving pulses
  • Moving the transducer over the patient’s skin, or using several transducers, to produce a series of A-scans that are combined to form an image

• This type of scan is used for:
  • Creating images of internal structures for diagnostic purposes
  • For example, bones, muscles and organs or checking on the progress of an unborn child
• To achieve the clearest images:
  • Pulsed ultrasound waves are used to allow time for the reflected waves to be received and not interfere with transmitted waves
  • Smaller wavelengths are used to give more detailed images as they will allow the sound waves to diffract around finer points of detail on the internal structure being studied

• Ultrasound is an important medical tool when a quick, safe and non-invasive method is required to image an internal structure, this is particularly useful for
  • Examining a developing foetus
  • Getting an initial prognosis of a medical issue before deciding whether a riskier scan (e.g. CT, PET) or a more invasive method (e.g. endoscope) is required

• However, as with all imaging methods, it has its advantages and disadvantages

Advantages of Ultrasound

• Ultrasound is non-invasive (compared to the insertion of an endoscope or injection of radioactive nuclide in PET scans)
• Ultrasound involves no ionising radiation
• There are no side effects to an ultrasound scan
• It can image soft tissue (organ structure, muscles) as well as bone
• Can produce real-time images and videos to show moving systems
• Patients do not have to remain completely still
• Cheaper, faster and more portable than magnetic resonance (MR) or computerised tomography (CT) scans 

Disadvantages of Ultrasound

• Ultrasound produces lower resolution images compared to MR or X-ray scans
• Cannot penetrate bone or gas as these reflect or dampen the sound waves (hence, can't image the brain or lungs)
• The depth of the scan is limited, causing issues for patients with higher amounts of body fat
• To increase the penetration of the scan, the resolution must be reduced further
• The transducer must be held normally to the surface and there must be no air bubbles in the coupling gel - this requires a skilled operator to carry out the examination and interpret the image
• Cannot distinguish between benign and malignant tumours (both solids with different acoustic impedances) unlike PET scans