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Medical Physics > Ultrasound > Flashcards

Ultrasound Flashcards

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1
Q

Longitudinal vs transverse

A

Longitudinal: particles oscillate in direction of wave propagation - but at speed closer to 1m/s, not speed of sound in medium. Have high and low pressure regions
Transverse: shear waves, oscillate at 90 degrees to direction of propagation. Don’t create pressure changes.

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2
Q

Speed of sound in a medium equation

A

c = sqrt (K/rho)
where K is bulk modulus and rho is density

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3
Q

Equations for circular plane transducer

A

(plane waves in near field and convex in far)
L=a^2/λ
θ = arcsin(0.61λ/a)

a is diameter

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4
Q

Beam width at focus

A

w ~= Fλ/a
where F is focal length
Concave wavefronts that become plain then convex

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5
Q

RIL/RPL

A

Relative intensity level and relative pressure level

RIL = 10log_10(I2/I1) dB
RPL = 20log_10(p2/p1) dB

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6
Q

Decibels

A

Reciprocal of a number is negative - if 46dB for ratio of 200 then ratio of 1/200 would be -46.

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7
Q

Acoustic impedance p/u relationship

A

Z= +/- p/u
p is instantaneous acoustic pressure
u is instantaneous particle velocity

p=+Zu for forward travelling and p=-Zu for backwards

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8
Q

Acoustic impedance K, ρ, c

A

Z = sqrt(Kρ_0) = ρ_0c_0
I = pu = p^2/Z

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9
Q

What must happen at boundary

A

Continuity of pressure and particle velocity. If Z2<Z1 then 180 degree phase shift
Frequency won’t change, wavelength will.

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10
Q

Amplitude reflection coeff and amplitude transmission coeff

A

R_a = p_r/p_i = (Z2-Z1)/(Z2+Z1)
T_a = p_t/p_i = 2Z2/(Z2+Z1)

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11
Q

Intensity reflection coeff and intensity transmission coeff

A

R_i = R_a ^2
T_i = (Z1/Z2) T_a ^2

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12
Q

Specular vs diffuse reflection

A

Specular - smooth surface, reflects fully. Diffuse - rough on the wavelength scale, lots of angles of incidence, scattered over many angles. Allows non-perpendicular images to be seen.

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13
Q

Reflection and refraction equations

A

θi = θr for reflection
sinθt/sinθi = c2/c1
If c2>c1, bent away from normal
Eventually reach 90 degrees, can’t see that area, leads to shadows.

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14
Q

Focussing techniques

A

Concave source, focal length F = radius of curvature
Plane source with convex lens - want speed of sound lower in lens than material of body

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15
Q

Equation for calculating R of lens

A

R = F[(c2/c1)-1]

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16
Q

Intensity/pressure attenuation

A

I=I0 exp( - mu x)
p = p0 exp(alpha x)

mu = intensity attenuation coeff.
alpha = amplitude attenuation coeff

mu = 2 alpha

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17
Q

Attenuation parts

A

Absorption and scattering
alpha = alpha_s + alpha_a

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18
Q

Absorption

A

Conversion of acoustic energy to heat as wave propagates through material. Two main mechanisms: ‘classical’ one due to viscosity and one due to molecular relaxations.

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19
Q

Scattering

A

Formed by scattered echoes from small scale inhomogeneities in bulk modulus and density. If average size of scatterers is much smaller than wavelength have Rayleigh scattering, isotropic and scattered intensity proportional to f^4.

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20
Q

Speed of sound in water

A

1480ms-1

21
Q

Curved array details

A

Lower f,15cm, abdominal or obstetric. Fan shaped. 1-6MHz 128 elements.

22
Q

Relationship between attenuation coefficient and frequency

A

Linear: double frequency, double attenuation coeff.

23
Q

Size of transducer/elements

A

Array length often 38mm, giving pitch of 0.2 or 0.15mm (192 or 256 elements)
1cm other direction

24
Q

Why do we use multiple elements at one time

A

So beam is collimated - width of one element usually smaller than wavelength

25
Q

Rank the components of spatial resolution from best to worst

A

Axial best, then lateral then slice thickness

26
Q

What determines each spatial resolution

A

Axial determined by pulse duration
Lateral by beam width in image plane
Slice thickness by beam width in elevation direction

27
Q

Limit for axial resolution of transucer

A

c_0T/2
T is pulse duration

28
Q

Time taken to acquire a line

A

t=2D/c

29
Q

Frame rate

A

Time resolution: if image is deeper, if there are more lines, if the sector is wider then it will take longer and frame rate will be lower.
Time to get image is Nt (N is number of lines and t is time to acquire a line)
FR = 1/Nt

30
Q

How does B-mode work

A

Pulse echo technique - short pulse generated by transducer and same transducer picks up echo.
Sweep beam across view - 20-40 elements at a time, wait for echoes then move active group by one.

31
Q

Time until echo

A

t = 2d/c_0

32
Q

M-mode imaging

A

Motion mode - used to track features in body over time, acquire A-mode lines one after another of oen location and plot them side by side, mostly used in echocardiography.

33
Q

A-mode imaging

A

Amplitude mode - showed trace. Echoes displayed at delays proportional to their depth. Still used in opthamology, often with high f dedicated non-imaging probe

34
Q

Time gain compensation

A

Ultrasound attenuated through tissue - echoes from deeper become weaker. Apply gain to compensate for this - increases exponentially.

35
Q

Demodulation

A

Gives grey scale images. First get rid of negatives with rectification and then smooth to get rid of high f components.

36
Q

Compression

A

Dynamic range of demodulated signal is very large, 80-100dB. To overcome this we log-compress the data, comes down to 48dB. Most displays have 256 grey levels which matches this.

37
Q

Effect of frequency and damping on beam

A

High frequency: good for resolution but bad penetration
Good damping: good for resolution but less sensitive (amplitude achievable is lower)

38
Q

Bandwidth

A

Range of frequencies - bandwidth of a pulse is indirectly proportional to its length (short B-mode doppler pulse has wide bandwidth but PW doppler toneburst is very narrow)

If we say a 3MHz pulse, that’s the central frequency

39
Q

Bandwidth of a CW sinusoidal signal

A

Zero

40
Q

How is bandwidth of a transducer determined

A

Go down by -3dB and see where that is
Bandwidth of pulse needs to lie inside bandwidth of transducer

41
Q

Relative bandwidth

A

BW/central frequency

42
Q

Attenuation loss

A

a f^b z
(a is attenuation f, frequency, b is constant (2 for water, 1-1.2 for tissues), z is depth)

43
Q

Why is the limit for MI/TI different for the eye?

A

Parts of the eye are more sensitive to potential damage.
Cornea, lens, vitreous body are unperfused tissues - only dissipate heat through thermal conduction
Lack of blood perfusion limits ability to repair damage from excess exposure.

44
Q

What scanner settings do you use during the wire tool test

A

Max power output
Plane and linear scan mode
No compound mode
No special post processing
No beam steering
Should see wire reverberation pattern

45
Q

Describe wavefronts from circular, focused transducer

A

Start out concave, become plane and then convex.
Converge and then diverge after focus
Diameter gets smaller as it converges and is minimum at focus.

46
Q

Linear array details

A

Higher f, 8cm, vascular or MSK, breast. Rectangular. 128-256 elements (same no. scan lines) 4-12MHz (vascular)

47
Q

Phased array details

A

Lowest f, 20cm, echocardiography. Sector. 1-5MHz (3-8 for children) 80 elements. Beam is steered - worse image quality.

48
Q

Radial array details

A

transrectal/transvaginal - transvaginal used in antenatal, 4-10MHz.

Medical Physics (16 decks)

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