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Physics > Everything > Flashcards

Everything Flashcards

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1
Q

At what kV does compton scatter predominant?

A

70 kVp

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2
Q

Photon energy of common radiophaemaceuticals

A

Tc99m - 140keV
Iodine 131 - 364 gamma keV
Fluorine18 - 511keV gamma
Gallium68 - 511keV gamma

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3
Q

Half life of common radiopharmaeuticals

A

Tc99m - 6 hours
Iodine 131 - 6 days
Fluorine18 - 2 hours
Gallium68 - 1 hour

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4
Q

What is the equation that relates dose to radius in fluoroscopy

A

dose = r^2 (therefore also diameter squared)

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5
Q

Noise (propotional) equation

A

Noise = 1/root(photons)

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6
Q

SNR equation

A

SNR = N/root(N)

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7
Q

CNR equation

A

CNR = (pixel value of lesion - pixel value background)/root(N)

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8
Q

US: equation relating speed of sound (C) to wavelength and frequency
How does speed of sound related to bulk modulus and density?

A

c = wavelength x frequency
c = root (B/p) - the stiffer the medium (higher B), the faster. The lower the density, the faster

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9
Q

US: how is pressure (wave amplitude) related to power (W)?

A

Power (W) is proportional to pressure^2

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10
Q

US: how do we calculate intensity?

A

Intensity is proportional to power(W)/area(cm2)

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11
Q

US: what is dB?

A

Relative intensity
dB = 10log Intensity2/Intensity1
Change in 3 of dB is half/double
Change in 10 of dB is a tenfold change eg. 10dB 1/100, 20dB 1/1000

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12
Q

US: how do we calculate how much is reflected back?

A

R=([Z2-Z1]/[Z2+Z1])^2
Reflection = difference between acoustic impedance / sum of acoustic impendence

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13
Q

US: what is specular reflection?
what is refraction and how does the angle change with speed of sound changes?

A

incidence angle = reflection angle (on smooth surfaces)
refraction occurs when the sound beam penetrates the interface
- slowing of speed = decrease in transmittance angle
- increasing of speed = increase in transmittance angle

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14
Q

US: what is the soft tissue attenuation coefficient?

A

Soft tissue attenuation coefficient ~0.5(dB/cm)/MHz
therefore Soft tissue attenuation = frequency x 0.5(dB/cm)/MHz

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15
Q

US: what is the formula and SI unit of acoustic impedance (Z)?

A

Z = density (p) x speed of sound (c)
SI unit = kg/m2.s or rayls

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16
Q

US: what is the Q factor? How does it relate to dampening blocks?

A

Q factor describes purity/quality of beam
Higher Q - narrow bandwidth, longer spatial pulse frequency (SPF), less dampening
Lower Q - wide bandwidth, shorter SPF, more dampening

17
Q

US: how does diameter and frequency affect the distance of near field and divergence of far field in the US beam?

A

Increased frequency and increased transducer diameter results in:
- increase of near field distance
- decrease of divergence angle

18
Q

what is the equation linking acoustic impedance (Z), speed of sound (c) and density (p)?

A

Z = c x p

19
Q

How would you define MTF?

A

MTF (mean transfer function) is an imaging systems ability to transfer an input signal to an output signal at a given spatial frequency. It is used to measure spatial resolution, with 1 or 100% meaning perfect signal transfer and 0 meaning complete loss of information

20
Q

What is bright and dark in T1?

A

Fat - bright (short T1)
Water, bone - dark (long T1)

21
Q

What is the definition of T1

A

The time at which longitudinal magnetization is 63% of its final value

22
Q

What is the definition of T2

A

12 representing the time at which the signal has decayed to 37% of its original value of transverse magnetization (63% of it has decayed)

23
Q

How do you define T2*

A

T2* = tissue spin interactions + field inhomogeneity
(random interactions + fixed cause)

24
Q

What is the rough duration of TR, TE for T1w images in SES?

A

Medium TR - 400-600ms to catch the middle of the Mz regrow
Short TE - 3-10ms to minimise effects of Mxy

25
Q

What is the rough duration of TR and TE for T2w images in SES?

A

Long TR - 2000-4000ms to minimise effects of Mz regrow
Medium TE - 80-100ms to catch Mxy

26
Q

What is the flip angle, TR and TE for T1w images in GE?

A

Larger flip angle to maximise Mz regrow time
Short TR/TE - not sure why

27
Q

What is the flip angle, TR and TE for T2w images in GE?

A

Small flip angle to minimise T1w effects
Long TR/TE to minimise T1w effects

28
Q

What is the TR/TE time for PD weighted image?

A

Short TE (3-30ms)
Long TR (2000-4000)

29
Q

What is the limits for MI and TI?

A

TI: 0.7 for obs, 1 in general
MI: 0.3 obs, 1.9 in general

30
Q

What is the speed of sound through tissue?

A

1540m/sec

31
Q
A

Physics (3 decks)

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