Ultrasound 2017 Flashcards
What type of wave is a sound wave?
Mechanical wave, longitudinal
Write down the wave equation (which relates speed c, wavelength λ, and frequency f)
C=fλ
Calculate the wavelength for US at 1540m/s at 1, 5, 10 and 20 MHz
1540/1000000 x 1000=1.54mm, 1540/5000 000 x 1000=0.308mm, 1540/10000000 x 1000=0.154mm, 1540/20000000 x 1000 =0.077mm
Explain the terms compression and rarefaction
A region where the coils are spread apart, thus maximizing the distance between coils, is known as a rarefaction or a point on a medium through which a longitudinal wave is traveling which has the minimum density
What tissue property affects impedance?
Resistance
Why is the matching gel used in Ultrasound?
Reduce the acoustic impendance at air/skin interface to reduce reflection of sound beam and allow it to be transmitted into the body
Give two examples of a tissue impedance mismatch which would result in a high amount of reflection.
Air/soft tissue and bone/soft tissue
How does a US beam scatter when it encounters an interface about one wavelength in size?
Random scattering in all directions with all different energy amounts – Non-specular reflection
What is the role of the matching layer on an US probe?
Matching layer minimises the energy lost due to an impedance mismatch between the transducer and the gel ; The impedance of the matching layer is chosen to be the geometric mean of the transducer impedance and the matching gel impedance.
What effect does a backing layer have in an US probe?
Backing layer minimises the energy reflected off the back of the transducer ; made from a material with an acoustic impedance close or equal to the transducer impedance
Sketch two pulses, one which will have good axial resolution and one which will have poor axial resolution and note the bandwidth of each
Short pulse, wide bandwidth, lower velocity/depth, increased axial resolution;
Long pulse, narrow bandwidth, higher velocity/depth, decreased axial resolution
- a) Sketch the beam shape for each transducer face
b) Indicate where the focal region is for the curved face transducer
2. What is the importance of beam width to image resolution?
and (
Beam width directly impacts lateral resolution – wider the beam the poorer the resolution
What is the role of the matching layer on an US probe?
Matching layer minimises the energy lost due to an impedance mismatch between the transducer and the gel ; The impedance of the matching layer is chosen to be the geometric mean of the transducer impedance and the matching gel impedance.
What effect does a backing layer have in an US probe?
Backing layer minimises the energy reflected off the back of the transducer ; made from a material with an acoustic impedance close or equal to the transducer impedance
Sketch two pulses, one which will have good axial resolution and one which will have poor axial resolution and note the bandwidth of each
Short pulse, wide bandwidth, lower velocity/depth, increased axial resolution;
Long pulse, narrow bandwidth, higher velocity/depth, decreased axial resolution
Describe
Detection
This received echo is collected by the transducer and converted into a small electrical signal
Describe
Pre-amplification
This signal produced by the transducer is very small, and to prepare it for further analysis the signal is amplified
Describe
Time Gain Compensation
TGC - Because the body is non-homogenous, the energy lost travelling though one part may not be the same as the energy lost travelling through another
This means the compensation required for collecting data in one area of the body may not be the same as is required for collecting a set of data
from a different area or patient
Describe
Compression
This compression rate, that is the extent to which the small signals and large signals are made to look close together, is an operator specified
parameter
Describe
Rectification and Envelope Detection
Rectification and Envelope Detection - To present the data so that we have a clear “spike” from each reflecting surface we take the signal which is below
zero and make it positive, Then we draw an envelope around the outside of the signal
Describe
Rejection
Some signals are so small they are likely to be due to noise, These small signals are discarded or rejected
Describe
Processed signal
- This data resulting from this analysis is then interpreted in terms of physiology and anatomy
Briefly describe the phenomenon of aliasing
Sampling rate too low means inadequate representation of initial information
What effect does it have on US data?
Means we can’t accurately represent velocities in a given sample
How can an US machine be adjusted compensate for aliasing?
Increase PRF, shift baseline, reduce angle of incidence
Why is aliasing not a problem in CW US?
No time waiting for echoes to return as 1 element of transducer solely for pulses and 1 solely for echoes and this happens continuously so sampling rate is as high as possible
At what stage could you see this type of endometrium?
Basal layer endo at menstruation
uterus anteverted or retroverted? How can you tell the difference?
fundus is at left of screen, if retroverted, fundus would be at right.
Also scan in trans – scanning superiorly should be fundus and inf should be cervix
