Week 1: B-mode imaging Flashcards
What is B-mode imaging?
Cross sectional image of organ & tissue boundaries, constructed from echoes, generated by reflection of US at tissue boundaries & scattering from irregularities within tissues
What are the B-Mode imaging variables
- Range of the target
2. Direction of the target from active area of transducer (position and orientation)
What is US?
- high frequency (f) sounds waves (MHz)
- mechanical vibration through a medium
- internal structure, function & amplitude; motion (BF)
How is an US image produced?
- US pulses are transmitted into the body
- US is reflected or scattered back by surfaces in the body (structures) –> US waves returning detecter by the probe.
- Time taken for pulse to go & return = depth of target ( assuming C is constant)
- Brightness of image –> relates to amplitude (Z) or returning US
Transducer f changes (dependent on?)
- depth
* properties I.e. density and stiffness
What’s the relationship in f and ATTENUATION
- Lower f US waves penetrate further due to attenuation
* Higher f waves attenuated more quickly than low f
What’s the relationship with ATTENUATION and AMPLITUDE. And what ways is it presented
Attenuation- a reduction in amplitude if signal
ABSORPTION –> as waves pass through, some is converted to heat
SCATTERING –> when waves come into contact w/ a smaller target - waves become scattered over large angle e.g. blood
What is Attenunation?
US is attenuated when passing through a medium w/ several mechanisms, a reduction in amplitude due to distance/depth
How is attenuation measured?
dBcm-1 MHz-1
When echoes from small scatters with sample volume come together to crease random fluctuation in the image brightness. What is this..?
Speckle
Constructive interference
2 seperate waves in phase meeting at the crest, creating a larger resultant wave w/ > amplitude
Destructive interference
when the 2 waves are out of phase i.e. the crest of 1 meets with trough of the other. Producing reduced amplitude or cancel each other out.
What does brightness at certain points relate to?
- boundaries & amplitude; types of material
- amplitude of the returning echo
What is intensity of a reflected wave dependent on?
- (@boundary) acoustic impedance (z) of material p/side
What is acoustic impedance (Z)?
- property of the tissue (dependent on C)
- number of resistance on US beam encounters w/ passing through a medium or tissue
What is Z dependent on?
- density of tissue (kg. m) K.
- C (m/s)
What does returning echo amplitude dependent on?
- change in Z at interface
What US controls govern the brightness of image point
- Gain
- TGC
- output power
- DR/compression
- Grey scale map
What is Pulse repetition frequency (PRF)?
of pulses transmitted p/sec
Why do we only want 1 pulse in flight at a time for PRF
To avoid range ambiguity
PRF equation
PRF max = C/ 2R
C = Speed of sound
( R = depth)
What is Frame rate (FR)?
of images created p/sec
Max FR given by:
FR = C/ 2RN PRF/N
```
N = # of scan lines
(R = depth)
~~~
What happens if theres double scan lines
Reduced FR (halved)
When looking at moving structure. What do you need to do to FR?
Increase it.
Resolution
Identify 2 objects close to eachother - axial and lateral
Contrast Res
Able to differentiate 2 objects w/ similar reflective properties
Temporal Res
Identify 2 events at a time
What does sensitivity relate to?
Penetration
How can the user optimise these features?
- compromise between axial res and penetration - selected f
- compromise between lateral res and FR - # scan lines
- selection of appropriate gain & amplitude compression
Describe this picture….
- Transmitted US is back scattered from moving blood cells
* The recieved US is unchanged due to velocity of blood cells
Intensity (I) or reflected wave at a boundary depends on…… each side of the boundary
Acoustic impedance (z) of the material
What is the attenuation coefficient for muscle?
0.57 dB cm-1 MHz-1
What property has the greatest attentunation coefficient?
Bone (22dBcm-1 MHz -1)
Describe the image….
- US transmitted is back scattered from moving blood cells
* Recieved US is however, changed due to the velocity of the blood cells.
Why is resolution important?
For high quality image production
What information does Colour flow imaging (CFI) show?
- BF direction, mean velocity in relation to US beam
In CFI. What does red and blue colours indicate?
RED/YELLOW -blood travelling towards the transducer
BLUE - blood is travelling away
Detected Doppler Shift Frequency (Fd) = …… and received frequency (Fr)
transmitted frequency (Ft)
What is the Doppler Shift Frequency dependent of?
- frequency (Ft)
- C (as it passes through tissue)
- Velocity of blood
- angle of insonation (variation of orientation of vessel or probe)
Explain this picture…
- Scan lines are sampled several times (sent and recieved)
- Recieved echoes are displayed at increased depth - moving further away from the transducer.
- Vertical line corresponds with fixed depth
- Amplitude changes when crossing the vertical line
- The change in amplitude of recieved echoes contains the doppler shift and is extracted by demodulation
3-16 pulses
number of pulses required with CFI
What does the displayed colour depend on?
angle of insonation
Alternation angle of insonation. Can be altered by adjusting the orientation of the probe to get highest Doppler Shift Frequecny.
How would you get angle of insonation on higher or lower body limbs that are parallel to skin
When would you get the highest Doppler shift f?
When vessel and beam are aligned.
Whay governs for velocity?
Angle of insonation between beam path and BF direction
What’s the true and no velocities values?
True = 0 degree angle
No velocity detected = 90 degree angle
Angle of insonation of US beam relative to ROI, are greatest when echoes produced with ….. approaching ….
Angle of incidence
Angle of reflection
What happens if angle of insonation is more than 60 degree angle?
More chance for velocity error measurements
What does Angle correction cursor do in relation to velocity measures?
The correct placement is important enabling measurements of angle of insonation so velocity measurements can be obtained
At what percentage on the graph are velocity error measurements likely to occur?
5% error in velocity measurement ( ~10-20) angle of insonation
When would the transducer f change?
with;
1) depth
2) tissue properties e.g. density and stiffness
In CFI with the principle of angle of insonation, when are echoes greater relave to the tissues and organs?
When angle of incidence approaches the angle of reflection
What unit is Z presented?
kg m−2 s−1 or Rayl
