Ultrasound Flashcards
At what frequency are the pulses of ultrasound waves produced by an ultrasound scanner?
1 to 15 MHz
How does an ultrasound scanner work?
The scanner is placed on the surface of the patient’s skin
When an ultrasound wave meets a boundary between two different materials, some of it is reflected and some of it is transmitted. The reflected waves are detected by the ultrasound scanner and are used to generate an image
What is acoustic impedance?
The amount of reflection an ultrasound wave experiences at a boundary depends on the difference in acoustic impedance between the materials
What happens if two materials have a large difference in impedance?
Most of the energy is reflected
What happens if the impedance of the two materials is the same?
There is no reflection
What does it mean when ultrasound waves undergo attenuation when traveling through a material?
This is when the waves are absorbed and scattered, making them harder to detect. This means that the
intensity of the reflected wave will always be lower the further the boundary is from the transducer.
What gives better resolution?
Higher-frequency waves give better resolution
What needs to be used to image tissue deeper within the body?
The higher the frequency of the wave, the more it is attenuated, this means lower-frequency waves have to be used to image tissue within the body
Relationship between acoustic impedance and attenutation
The larger the impedance, the greater the attenuation of the ultrasound moving through the material
What do piezoelectric crystals produce?
A potential difference when they are deformed (squashed or stretched) - the rearrangement in structure displaces the centres of symmetry of their electric charged. This is called the Piezoelectric effect.
Where is ultrasound produced and detected?
On a transducer
How is the crystal deformed?
When you apply a pd across it.
What happens if the pd is alternating?
The crystal vibrates at the same frequency
What can the piezoelectric act as?
- A receiver of ultrasound, converting sound waves into alternating voltages
- A transmitter, converting alternating voltages into sound waves
What crystals do ultrasound devices use?
Lead zicronate titanate (PZT) crystals
Why do ultrasound devices use PZT crystals?
The thickness of the crystal is half the wavelength of the ultrasound. Ultrasound of this frequency will make the crystal resonate and produce a large signal.
Why is the PZT heavily damped?
It is damped using the backing material to produce short pulses and increase the resolution of the device.
Why do you need a coupling medium?
Soft tissue has a very different acoustic impedance from air, so almost all the ultrasound energy is reflected from the surface of the body if there is air between the transducer and the body. This displaces the air and has an impedance much closer to that of body tissue.
What is the use of coupling media an example of?
Impedance matching
What is used as a coupling medium?
Gel or oil smeared onto the skin
What are the two types of ultrasound scan?
The A-scan and the B-scan
What is an A-scan?
The A-scan sends a short pulse of ultrasound into the body simultaneously with an electron beam sweeping across a cathode ray oscilloscope screen. The scanner receives reflected ultrasound pulses that appear as vertical deflections on the CRO screen.
What is time-gain compensation?
When weaker pulses, that have travelled further in the body and arrive later, are amplified more to avoid the loss of valuable data.
What do the horizontal positions of the reflected pulses indicate?
The time the ‘echo’ took to return, and are used to work out distances between structures in the body.
What is a B-scan?
In a B-scan, the electron beam sweeps down the screen rather than across. The amplitude of the reflected pulses is displayed as the brightness of the spot.
What can transducers in a B-scan do?
You can use a linear array of transducers to produce a two-dimensional image.
What are B-scans used for?
- To check if the heart is functioning properly
- Allows the doctor to look at an image of the heart in real time.
Advantages of ultrasound
- No known hazards or side effects
- No exposure to ionising radiation
- Good for imaging soft tissues, since you obtain real time images
- Ultrasound devices are relatively cheap and portable
- Scan is a quick procedure
- Patient can move during the scan
Disadvantages of ultrasound
- Ultrasound doesn’t penetrate bone - can’t be used to detect fractures or examine the brain
- Ultrasound cannot pass through air spaces in the body - can’t produce images from behind the lungs
- Resolution is poor, so you can’t see fine detail
- Ultrasound cannot give information about any solid masses found
