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C2 Modules CLINICAL > Application of ultrasound > Flashcards

Application of ultrasound Flashcards

1
Q

What is Doppler effect used to measure?

A

Speed of blood

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2
Q

How is the ultrasound Doppler shift measured?

A

Comparing transmitted and received ultrasound signals (usually pulses)

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3
Q

What does ultrasound doppler do?

A

Send short pulses of sound and receiving back via the transducer equipment

Looks at differences in frequencies within that signal

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4
Q

What is Doppler ultrasound at a single location?

A

Spectral Doppler

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5
Q

What is Doppler ultrasound over a region?

A

Doppler Imaging

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6
Q

What do you get with Doppler ultrasound?

A

A regional map of how blood is flowing

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7
Q

What is Doppler shift (kHz) proportional to?

A

Blood speed for a fixed angle between artery (direction of blood flow) and ultrasound beam direction (=ultrasound beam direction)

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8
Q

Why do you need to calibrate the machine?

A

Get a good estimate of speed of blood

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9
Q

How do you calculate the speed in m/s?

A

Calculate the cross-sectional area and multiple the speed by that cross-sectional area
units of m3/second

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10
Q

What is found on the overlay on the B-mode image?

A

Dark region within artery

Haven’t measured blood flow within that region

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11
Q

What is the B-mode image?

A

Grey-level image that is displayed

Provide anatomical information

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12
Q

What is the blood flow colour coded according to?

A

Direction

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13
Q

What is the direction of the blood flow relative to?

A

Ultrasound waves

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14
Q

How are veins displayed on the spectral trace?

A

Shades of blue and green

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15
Q

What does the spectral Doppler trace measure?

A

Speed of blood at a particular point
Measuring within the middle of artery
Between 2 little gates

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16
Q

How is the line of the spectral trace angled?

A

It is in the same direction as the artery which should be close to direction of blood

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17
Q

What can the characteristic of waveform tell you?

A

Which artery is ambiguous

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18
Q

Where is the ultrasound transducer surface placed on?

A

On the skin

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19
Q

What is the ultrasound tracer sending out?

A

Ultrasound pulses that are parallel to the white line

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20
Q

What is the path for the ultrasound pulses?

A

Ultrasound pulses that are parallel to the sides of the green box which looks like a parallelogram

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21
Q

What is the colour Doppler Imaging measuring?

A

Blood flow at somewhere between 1 and 2 cm

Nearly 2cm into the tissue

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22
Q

What is a sample volume?

A

Measuring at a specific speed

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23
Q

What is aligned within the vessel?

A

Angle correction line

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24
Q

What does the envelope of the spectral Doppler trace measure?

A

Blood velocity

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25
What is the envelope?
Maximum velocity speed that is being measured
26
What gives the spectral broadening shape?
Spread of speed within a sample of small volume
27
Where is there a proportionality between?
Doppler shift and Speed of blood
28
What is the Doppler shift?
The difference between received frequency of ultrasound and transmitted frequency and measure angle theta
29
What is constant?
Speed of sound around 15-40m/s
30
What is the angle measured by?
The scanner for particular values of theta
31
What is the Doppler velocity measurement theoretically?
Doppler shift = 0 for theta = 90°
32
What are errors in theta and velocity significant for?
theta > 60°
33
What is the best practice for Doppler velocity measurement?
theta < 60° and fixed
34
What are doppler frequencies?
Audible | Audible range - 20kHz
35
How is the signal from vessel walls filtered out?
Using a ''wall filter'' | High amplitude, low Doppler shifts
36
What are the number of sources of error for Doppler velocity measurement?
1. Measurements can be inaccurate | 2. Subject to significant inter-intraobserver variability
37
What are the types of Doppler ultrasound device?
1. Continuous wave (CW) 2. Pulsed Wave (PW) 3. Doppler Imaging
38
What does Continous wave measure?
Velocities over a wide range of depths
39
What are Pulsed wave (PW) gated to measure?
velocity at a particular depth (i.e. within a sample volume)
40
What can Pulse repetition frequency (PRF) of Pulsed wave determine?
Maximum velocity that can be measured | ~2-8kHz
41
What can Pulsed wave Doppler measure?
Velocities just in a small region
42
What is gating?
Changing the depth of the region Screen out other parts of the signal and just retain signal that is particular depth of where you want to measure speed of blood
43
What is colour doppler?
Colour = velocity + direction
44
What is power Doppler?
Colour = strength of Doppler signal (~flow/no flow)
45
Colour Doppler
not accurate enough to be used quantitatively
46
What are examples of Colour Doppler Artefacts?
1. Noise/flash artefacts 2. Brief of random colour all the way across the image 3. Vessel is being obscured by flash artefacts 4. Aliasing
47
What is Aliasing?
• The very high speeds are actually being mapped as low speed in opposite direction
48
What does Doppler give a good indication of?
Surface of plaque
49
What does pixel colour represent?
= amplitude squared | power of received Doppler shift signal
50
What doesnt Power Doppler Imaging provide?
NO velocity information
51
What are the advantages of Power Doppler Imaging?
- Relative independence to vessel orientation - Sensitivity to low blood flow - Useful for visualising complex vascular geometries (e.g. in brain!)
52
What are the clinical applications of Power Doppler Imaging?
1. Extracranial | 2. Intracranial
53
What are the features of Extracranial?
- Assessment of carotid atherosclerotic stenosis - Arterial dissection - Carotid body and head and neck tumours
54
What are the features of Intracranial?
• Assessment of arterial stenosis - Atherosclerosis, acute stroke, sickle cell, and SAH • Evaluation of collateral flow and arterial malformations • Aneurysm detection - Thin walled enlargement of a blood vessel • Investigation of dynamic cerebrovascular response • Embolus detection • Intraoperative monitoring
55
What are the main uses of Doppler Ultrasound?
look for disease in the carotid arteries
56
What does the common carotid artery split into?
1. ICA | 2. ECA
57
What is a common site for artherosclerosis?
Carotid bifurcation
58
Carotid atherosclerosis
~15-20% stroke
59
What are embolic stroke?
Fragment of disease | Blood clot that is becoming dislodged from the plaque and travels upstream
60
What is the pathology of artherosclerosis?
1. Interior of artery is narrowed 2. Thickened region of plaque 3. Lumen of artery is narrowed (stenosed) 4. Plaques become unstable, rupture and produce emboli --> brain infarction
61
What are the clinical importance of assessing carotid stenosis?
1. Risk factor for stroke and TIA | 2. Indication for carotid endarterectomy (surgical removal of plaque)
62
What the problems for assessing carotid stenosis?
1. Several different definitions of stenosis 2. Little standardisation i measurement between centres 3. Accuracy of methods used for clinical measurement is often limited
63
What is the aim of influential carotid surgery RCT?
Try and relate the amount of disease [degree of stenosis] to risk of stroke and benefits of surgery
64
What are the measurements of degree of stenosis?
1. Measuring a diameter within a vessel and compare with a reference diameter
65
What is X REF a diameter of?
1. Distal normal ICA (NASCET) 2. Carotid bulb (ECST) 3. CCA 4. Wall at site of maximal stenosis - not possible with angiography
66
What can measurement of degree of stenosis generalised to?
Cross-sectional area or even volume
67
What are the disadvantages of x-ray/CT/MR angiography?
1. Invasive: a contrast agent is usually required 2. Only the lumen is visualised 3. Ionising radiation dose in case of x-ray or CT
68
What are the disadvantages of 3D diagnostic scanners?
1. Not portable in general 2. Expensive 3. Not suitable for realtime imaging
69
What is the disadvantage for traditional projection angiography?
Vessel diameter measurements are highly dependent on the projection plane
70
What are the advantages of Doppler Ultrasound?
1. Low cost equipment 2. Portable 3. Real-time imaging 4. Visualisation of vessel wall as well as blood 5. Multi-modal (imaging and velocity measurement)
71
What are the two methods to estimate the amount of disease?
1. Direct measurements from transverse images | 2. Measure velocity
72
What are the principle of Doppler-based methods?
- Velocity increases with degree of stenosis (lumen area reduction) - Relate velocity to stenosis (and in turn clinical significance) - As lumen narrows, higher velocity – varies with individual patients
73
Where do you measure velocity?
at site of maximal stenosis | located visually or find location with maximum velocity
74
What are the problems with Doppler-based stenosis estimation?
1. Large variability: peak PSV only loosely related to stenosis 2. Errors in Doppler velocity measurements
75
What is Doppler clinically useful for?
Discriminating moderate to severe stenosis
76
What can variations in blood velocity between patients unrelated to carotid disease be due to?
1. Cardiac output 2. Blood pressure 3. Contralateral disease
77
What is TCD ultrasound?
1. Measurement of cerebral blood velocities 2. Low frequency vs diagnostic ultrasound (~2MHz) 3. Access via temporal or occipital windows or eye 4. Temporal window: MCA --> Circle of Willis 5. Occipital window: Basilar arteries
78
What can TCD ultrasound detect?
1. Stenosis 2. Aneurysm 3. Vasospasm 4. Embolus
79
What is TCD useful for?
Monitoring neurovascular surgical procedures
80
What are the advantages of TCD ultrasound?
1. Low-cost 2. Portable 3. Real-time velocity monitoring (at multiple sites using modern equipment) 4. No contrast agent required
81
What are examples of real-time velocity monitoring?
1. Carbon dioxide reactivity 2. Dynamic autoregulation investigations 3. Cerebral lateralisation 4. Intraoperative and bedside monitoring
82
What are limitations of TCD?
1. Access to limited sites due to limited number of windows 2. Not all patients have an acoustic window 3. The sample gate is positioned blind (without imaging) 4. Higher end equipment combines imaging and Doppler velocity measurements
83
What is vessel determined by?
velocity waveform characteristics
84
Real time velocity measurement during surgery
1. Usually MCA 2. Indicates velocity changes and emboli 3. Used to determine co-lateral blood flow and necessity for shunt insertion 4. Indicates kinking of shunt or poor air seal

C2 Modules CLINICAL (26 decks)

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