Ultrasound physics Flashcards
1
Q
What two things determine strength of sound?
A
- Pressure amplitude
- Intensity
2
Q
What is/how do you explain a harmonic frequency?
A
- A sinusoidal waveform is characterized by a single waveform (fundamental frequency).
- Any other wave shape contains additonal freuencies that are even or odd multiples of that original frequency.
- as the wave becomes less sinusoidal, the harmonics become stronger
3
Q
How do you calculate beam intensity?
A
I = power (energy) ÷ area
4
Q
How does intensity relate to pressure?
A
I is proportional to P2
(doubling pressure quadruples intensity)
5
Q
Describe the relationship between speed of sound, wavelength and frequency?
Of these variabel, which are affected by the medium?
A
- speed of sound (c) = wavelength x frequency (f)
- frequency is unaffected by the propagation medium, unlike the other two
6
Q
What are the frequencies for:
- infrasound
- audible sound
- ultrasound
- medical ultrasound
A
- infrasound = <15 cyles/s (Hz)
- Audible sound = 15 Hz- 20 kHz
- Ultrasound = >20 kHz
- medical ultrasound = 2-50 MHz
7
Q
What is a period?
A
The time it takes for one cylce to occur
period = 1 ÷ frequency (f)
8
Q
What two factors affect speed propagation and which is more important?
A
- Tissue stiffness and tissue density
- tissue stiffness effects>>tissue density
- propagation speed increases with increased stiffness
9
Q
What is attenuation coefficient?
A
relative intensity loss that occurs with each cm the sound travels (Att coeff = dB/cm)
10
Q
How do you calculate atteunation?
A
a (dB) = u (dB/cm) x L (cm)
increases in attenuation coeff or path length will increas attenuation
11
Q
What is the attenuation coefficient in soft tissue?
A
about 0.5 dB/cm for each MHz of frequency
a (dB) = 0.5 (dB/cm) x f (MHz) ÷ L (cm)
12
Q
How do you calculate relative intensity?
A
Relativ intensity (dB) = 10 log [Iincident/Iecho]
13
Q
What is the relation ship between acoustic impedance (Z), density (p) and propagation speed (c)?
A
Z (kg/m2sec) = p (kg/m3) x c (m/sec)
14
Q
How do you calculate the reflection pressure coefficient? How can the Intensity Reflection Coefficient (IRC) be extrapolated from this? page 5
A
Rp = Pr ÷ Pi = [(Z2 - Z1) ÷ (Z2 + Z1)]
Since intensity (I) is proportional to P2, then:
IRC = Ir ÷ Ii = [(Z2 - Z1) ÷ (Z2 + Z1)]2
15
Q
How does the intensity transmission coefficient (ITC) relate to the intensity reflection coefficient (IRC)?
A
ITC = 1 - IRC
16
Q
What is the critical angle?
A
angle of incidence of the sound beam with a boundry b/w two media that when exceeded will cause total reflection.
Critical angle = SinØc = c1 ÷ c2
17
Q
What is Snell’s law?
A
[sinØi ÷ SinØt] = C1 ÷ C2
If C2 > C1, then the angle of transmission is > the angle of incidence
If C2< C1, then the angle of transmission is < the angle of incidence
18
Q
What determines the resonant frequency of a piezoelectric element?
A
- The thickness of the piezoelectric element (0.2 - 1mm)
- Propagation speed of the element material (4-6 mm/us)
f (MHz) = [Ct (mm/us)] ÷ 2 x thickness (mm)
19
Q
What is the pulse repitition period?
A
Time from the beginning of one pulse to the beginning of the next: PRP = 1 ÷ PRF
20
Q
How does dampening relate to bandwidth?
A
Shortening the pulse broadens the bandwidth
21
Q
What are the effects of dampening and what are some advantages and disadvantages?
A
- Reduces the spatial pulse length
- Reduces the pulse duration
Advantages:
- improves axial resolution in the near field
- allows harmonic imaging
Disadvantages
- reduces ultrasound amplitude, reducing efficiency and sensitivity of the system
22
Q
What is Q factor?
A
Describes the bandwidth of the sound emanating from the transducer:
Q = f0 ÷ bandwidth
23
Q
What is the difference between a High Q transducer and a Low Q transducer?
A
- High Q transducer:
- Narrow bandwidth (little dampening)
- long spatial pulse length and decreased resolution
- Low Q transducer:
- Wide bandwidth (more dampening)
- small spatial pulse length and increased axial resolution
24
Q
What is Thermal Index (TI)?
A
TI = the ratio of acoustic power (W) produced by the transducer to the power required to raise the tissue in the beam area by 1°C
25
What is mechanical Index?
Value that estimates the likelihood of cavitation by the ultrasound beam
26
What is ISPTA? And what are the current FDA recommendations?
Spatial peak temporal average intensity (ISPTA) -
* It is a good indicator of thermal US effects
* FDA recommendations
* ISPTA diagnostic ultrasound \<100 mW/cm2
* pulsed doppler \<1000 mW/cm2
27
What are methods to decarease "exposure"?
* Keep power low
* Increase gain instead of increasing power
* Choose scanned modes over unscnanned modes (B is less than M)
* Decrease pulse length
* Use appropraite transducer
28
What is I SPPA?
Spatial peak pulse average intensity -
* indicator for potential mechanical bioeffects and cavitation
* required by FDA
29
What are 3 ways harmonic ultrasound imporves image quality?
* harmonic beam is narrower, flatter and has twice the frequency as the fundamental frequency
* results in improved lateral resolution, elevational resolution and axial resolution
* grating lobe artifacts are eliminated
* the haroni beam is generated at a depth beyond where some of artefactual problems occur so the image degredation they cause is reduced or eliminated e.g superficial reverberation)
30
How do you increase axial resolution?
* decrease the spatial pulse length
* using higher frequency → decreases wavelength, thus decreasing SPL
* increase dampening → lower Q, reduces pulse duration, decreasing SPL
31
What does beam width depend on?
* aperture width
* focal distance
* wavelength
32
How do you determine focal spot width?
* For single element transducers - focal spot width ~ 1/2 transducer diameter
* for multiple array - determined by aperture width
33
How do you increase focal length?
* increase apertture width
* increase frequency (decrease wavelength)
34
How can you improve lateral resolution (for a rectangular aperture)?
df = 2 (wavelength x focal length) ÷ aperture
Lateral resolution is improved by decreasing df (focal beam diameter), thus:
* increase aperture size
* decrease focal length
* decrease wavelength
35
Which component of resolution contributes to partial volume artifact?
Elevational resolution (slice thickness) - occurs when the section thickness \> size of the structure
36
How do you narrow the focal zone?
* increase frequency (decrease wavelength)
* increase aperture width (d)
* increase curvature (decrease focal length)
37
What is PRF and how do you calculate it?
* Pulse repetition frequency - number of pulses occuring in one second
* PRF (kHz) = # pulses/sec
* or: PRF (Hz) = n x LPF x FR (Hz)
* n = number of focal zones
* LPF = lines per frame
* FR = frame rate
38
What is pulse duration?
time that it takes for one pulse to occur
39
How do you calculate pulse duration? How can you decrease pulse duration?
* PD (us) = # cycles per pulse x period
* Decrease by:
* decreasing the numbr of cycles in a pulse
* increase the frequency of the pulse (decreases period - inverse relationship)
40
What is Duty factor and how do you calculate it?
* DF = fraction of time that pulsed US is on
* DF = PD(us) / PRP(us)
41
WHat is pulse repetition period?
PRP (ms) = 1/ PRF(kHz)
time from beginning of one pulse to the beginning of the next
42
What is the Doppler shift equation?
fD = 2 f0 v cosØ / c
can rearrange to solve for velocity; v = fD c / 2 f0 cosØ
43
When does maximum Doppler shift occur?
when the Doppler angle = 0, cos 0 = 1
44
What is the ideal range of Doppler angles and why?
* 30-60°
* \<30° results in total reflection at the vessel wall
* \>60° results in unreliable measurement
45
What are limitations of color flow Doppler?
* Angle dependent
* Lower frame rates (decreased PRF)
* Lack of detailed spectral information
* computes mean rather than maximal velocities
46
How does poer Doppler differ from color flow Doppler?
Rather than assigning various hue, saturation and luminanec values to mean Doppler-shift frequency values, this technique *assigns a sum value* of all Doppler shift frequencies and presents a *pixel intensity based on the sum rather than the mean*
47
WHat are advantages of power Doppler?
* Free of aliasing
* Not angle dependent
* More sensitive to slow flow and flow in small vessels
* Increased SNR
48
What is spectral broadening and what are causes?
* Def: vertical thickening of the spectral trace
* Causes:
* broad range of flow velocities - disturbed or turbulent flow
* multiple Doppler angles
* excessive Doppler gain and beam spreading
49
What are advantages and disadvantages of continuous wave (CW) Doppler?
* Advantages
* highly accurate measurement of blood flow velocity in the heart or large arteries
* No aliasing
* Disadvantages
* No depth resolution - signal results from entire volume - no directional info
* Spectral broadening due to lack of directional info
50
In PW Doppler, how is sample volume determined?
* spatial pulse length
* gate length = gate depth
* gate width
51
What is the Nyquist limit?
* NL = the maximum Doppler shift (fmax)
* determined by the PRF
* if greater than the NL, then aliasing occurs
* PRF must be set to at least 2x the fmax
52
How do you correct aliasing?
* Increase PRF
* Baseline shift downward
* Reduce depth of the range gate
* Decrease transducer frequency (will decr Doopler shift frequencies) - reduced resolution
* Switch to CW Doppler
* Increase Doppler angle - BAD CHOICE
53
What is resistive index and how is it calculated?
* RI = (Vsytolic - Vdiastolic)/ Vsytolic
* Expresses the resistance to blood flow within an arteriole that can be obtained by PW Doppler
54
What is Pulsatility Index and how is it calculated?
* PI = (Vsytolic - Vdiastolic) / mean
* Can be used as an indicator of distal impedance
55
What is the advantage of pulsatility index?
Takes into account the HR and BP alterations that alter the RI
56
What is the normal RI in canine kidneys?
0.62 with a range 0.56-0.67
57
In kidneys, an increased RI (over 0.70) is an indication of what?
active tubulointerstitial or vascular disease
58
For plug flow, what does the spectral image look like and what type of vessel does it occur in?
* Spectral: thin line in sytole that outlines a clear space called the spectral window
* due to **narrow range of velocities**
* Occurs in large arteries (e.g aorta)
59
Regarding parabolic flow, what type of vessel does it occur in, what does the spectral image look like?
* Occurs in small arteries (i.e renal arteries)
* Spectral display: spectral window not seen during sytole due to the **wide range of flow velocities**
60
Regarding blunted parabolic flow, what type of vessel does this occur in and what does the spectral image look like?
* Middle sized arteries (e.g. celiac artery)
* Spectral display: **Intermediate range of velocites** - small range of velocities are represented in peak systole resulting in a smaller spectral window than in an artery with plug flow
61
What is the Reynold's number and what factors influence it?
* Predicts the onset of turbulent flow: Critical Re = 2000
* Factors:
* increased flow speed
* increased viscosity
* increased vessel diameter
* increased density
62
What is the Bernoulli effect?
describes the decreased pressure in regions of high flow speed → as flow energy increases, pressure energy decreases
63
What is the modified Bernoulli equation?
P1-P2 = 4 (V22 - V12) ⇒ Delta P = 4(V2)2
V2 = the flow speed in the jet
It is most commonly used to calculate the pressure drop at stenotic heart valve.
64
In the Bernoulli equation, why is V1 most often ignored?
In most clinical cases, V2\>\>V1 and V1 = ~1 m/s
65
In AV valve flow, what is the normal E/A ratio and what influences it?
* normally E/A \>1
* influences:
* No A wave with A-fib
* E &A begin to merge with increased heart rate because diastole is shortened
* ventricular compliance and relaxation influence E/A ratio
66
What is the normal velocity range for the tricuspid valve?
60-70 cm/s (E:86, A:60)
67
What is the normal RV/TV systolic pressure gradient?
Delta P= 15-20 mmHg
68
A jet velocity greater than what would indicate RV out flow obstruction (PS, PH, possibly ASD)?
\>275 cm/s
69
What is the normal velocity range for the mitral valve?
75-95 cm/s (E:90, A:63)
70
What is the normal systolic pressure gradient at the mitral valve?
Delta P = 100 mmHg
71
At the mitral valve, a regurgitant velocity \>6 m/s would indicate what?
\>6 m/s suspect systemic hypertension
72
What is the normal peak systolic velocity measured at the aorta and the normal pressure difference?
Ao = 1-2 m/s
delta P = 4-16 mmHg b/w the LV and Ao)
73
What is the normal Ao velocity range? and the normal diastolic gradient?
100-150 cm/s
50-60 mmHg (\<3.5 m/s maximum regurgitant jet in a normal dog)
74
What is the normal PV velocity? Diastolic gradient?
* velocity: 85-120 cm/s
* diastolic gradient: 10-15 cm/s (regurgitant jet \<2 m/s)
75
What is the cut offs for diagnosing pulmonary hypertension?
PH \>30 mmHg (either at PV or TV)
Mild: 3.5-4.5 m/s (delta P 50-80 mmHg)
Severe \> 4.5 m/s (delta P \> 80 mmHg)
76
How do you calculate fractional shortening? What is normal?
FS = (LVIDd - LVIDs) ÷ LVIDd x 100%
Dog: 35-45%, Cat: 45-55%
77
How do you calculate ejection fraction? What is normal?
EF = SV ÷ EDV x 100%
78
