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medical physics Flashcards

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USMLE® Step 1 flashcards
1
Q

Specific Acoustic Impedance

A

Product of density of medium and speed of sound in medium

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2
Q

Sharpness

A

Clarity of resolution of image

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3
Q

Hardness of X-ray beam:

A
  • Penetration of beam.

- Greater hardness means greater penetration

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4
Q

How hardness of X-ray beam is controlled:

A
  • Controlled by changing the anode voltage

- Higher anode voltage, greater penetration

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5
Q

Linear Absorption coefficient:

A
  • Parallel beam in matter.
  • I = I0 exp(-µx)
  • Where I is intensity transmitted, I0 is initial intensity, µ is
    attenuation coefficient of the material, and x is thickness
    of material
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6
Q

Attenuation of an ultrasound wave

A

Loss of intensity in waves as they pass thru medium

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7
Q

Contrast

A

Difference in degree of blackening btw structures

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8
Q

Main principles of the GENERATION of ultrasound waves for medical use:

A
  • Generation of ultrasound is made possible by using
    quartz crystal through piezoelectric effect.
  • When alternating pd is applied across the quartz crystal,
    distortion takes place.
  • Resonance takes place when the applied alternating pd
    has a freq equal to the natural freq of the crystal.
  • The natural freq of crystal is in ultrasound range
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9
Q

Suggest why, when the signal from a pulse is processed, any signal received later at the detector is usually amplified more than that received at earlier time:

A
  • Later signal has passes through greater thickness of
    medium
  • So greater attenuation / absorption
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10
Q

Explain why ultrasound is emitted in pulses

A
  • Allows the reflected signal to be distinguished from the
    emitted signal
  • Detection occurs at time between emitted pulses
  • To determine the depth of boundary
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11
Q

Explain why gel is applied to surface of skin during ultrasound scan:

A
  • Without the gel, most of the ultrasound is reflected with
    no transmission
  • Bcuz value of Z are different
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12
Q

By ref to specific acoustic impedance, explain why there is little transmission of ultrasound waves from air to skin:

A
  • Specific acoustic impedance of skin and air are very
    different.
  • The intensity reflected depends on the diff btw acoustic
    impedance.
  • Since the intensity reflection is nearly 1, there is full
    reflection and 0 transmission
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13
Q

Explain why an aluminum filter may be placed in the X-ray beam when producing X-ray image of patient:

A
  • X-ray beam contains many wavelengths
  • Aluminum filter absorbs long-wavelength X-ray radiation
    that would be absorbed into the body and does not
    contribute to the image
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14
Q

Why is it advantageous to filter out low E photons from X-ray beam

A
  • Low-E photons absorbs much more easily
  • Low-E photons are far less penetrating
  • Low-E photons do not contribute to X-ray image
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15
Q

Main principles behind the USE of ultrasound to obtain diagnostic information about internal body structures:

A
  • Pulse of ultrasound is reflected at the boundaries btw 2
    medium
  • The reflected pulse is detected by ultrasound generator
  • The time delay btw transmission and receipt provides
    info about the depth of boundary.
  • The intensity reflection provides the info about the
    nature of the boundary.
  • Gel is used to minimize the reflection at skin
  • Degree of reflection depends on the impedance of the
    2 medium
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16
Q

Outline the principles of CT scanning.

A
  • X-rays are used, a section of the object is scanned.
  • CT scan takes many 2D images from many directions to
    give an image of the slice.
  • This is repeated for many slices and are combined to
    give a 3D image of the whole structure.
  • The 3D images can be rotated and viewed from any
    angle
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17
Q

1 adv of using high freq ultrasound and explain

A
  • Smaller structures can be observed due to better resolution
18
Q

Explain how the hardness of an X-ray beam is controlled by the accelerating voltage in the X-ray tube:

A
  • Changing voltage changes the energy of electrons.
  • Electrons striking the anode have changed kinetic
    energy,
  • so X-ray photons will have different wavelengths
19
Q

2 cause of lack of sharpness of an X-ray image

A
  • Scattering of X-ray beam
  • Lack of collimation (alignment) of beam
  • Pd of beam is low
20
Q

Describe 2 factors of sharpness of an X-ray image:

A
  • Size of X-ray source

- Scattering of X-ray beam

21
Q

Suggest why exposure to radiation differs btw X-ray and CT scan

A
  • X-ray image involves a single exposure
  • CT scan involves exposure of many images of a SLICE
    from many diff angles, and repeated for different slices
  • CT scan involves a greater exposure to radiation
22
Q

Uses of piezo-electric transducers

A

Ultrasound
Microphone
Pressure sensor
Watch

23
Q

Discuss qualitatively how the relative magnitudes of the two specific acoustic impedances affect the reflected intensity

A
  • If Z1 = Z2,, intensity reflection is nearly zero, full
    transmission and zero reflection
  • If Z1»Z2 or vice versa,, intensity reflection is nearly 1,
    full reflection and zero transmission
24
Q

Explain the principles behind the USE of X-rays for imaging internal body structures

A
  • X-ray beam is directed through body onto detector
    plate
  • Different tissues absorb beam by different amounts,
    giving shadow image of structures
  • Sharpness of X-ray image may be improved by putting
    detection plate as close as possible to patient, while
    moving X-ray source far away.
25
Q

Describe images produced from CT scan vs X-ray

A
  • X-ray images are 2 dimensional regardless of the depth
    of object
  • CT scan takes many 2D images of a slice at different
    angles to give an image of the slice
  • Many sections of images are combined to give a 3D
    image of the whole structure
  • The 3D image can be rotated and viewed from any
    angle
26
Q

Explain why acoustic impedance is important when considering reflection of ultrasound at boundary

A

Difference of acoustic impedance determines the fraction of incident intensity that is reflected

27
Q

Explain why a continuous spectrum of wavelengths is produced during e- bombardment of a metal target

A
  • EM radiation/photons is produced whenever a charged particle
    is accelerated
  • Wavelengths depend on the magnitude of acceleration
  • e- have a distribution of acceleration, hence continuous
    spectrum produced
28
Q

Explain why there is a sharp cut-off at short wavelength.

A
  • Wavelength is shorter for greater acceleration

- e- losses all its’s E in one collision and stops

29
Q

State, with ref to the production of X-rays, what is meant by 80keV X-rays.

A
  • X-rays produced when high speed e- hits the anode

- Electrons have KE of 80keV

30
Q

State 2 reasons why intensity of X-ray beam at B is not as great as A (B placed after block + further from source)

A
  • X-ray Beam is divergent at B
  • Absorption of beam in the block
  • Scattering in block
  • Reflection at boundaries
31
Q

Explain the principles of DETECTION of ultrasound waves for medical use

A
  • Pulso of ultrasound is generated by using quartz crystal
    through piezoelectric effect.
  • The ultrasound is incident on the quartz crystal and
    causes the crystal to oscillate
  • Oscillation of quartz crystal generates emf across the
    crystal.
32
Q

explain why there is a continuous distribution of wavelengths

A
  • X-ray photons are produced when electrons are decelerated
  • Photon E depend on the magnitude of deceleration of e-
  • e- have a distribution of acceleration, hence continuous spectrum of wavelengths
33
Q

explain why at certain wavelengths, there are narrow peaks of increased intensity

A
  • e- in the inner shell of the target atom is excited from
    ↓ to ↑ E levels during collision
  • they e- de-excite, causing emission of photons
  • discrete energy levels so discrete photon wavelengths
34
Q

Why CT scanning was not possible b4 fast computers with large memories were available.

A
  • Combing of images involves very large numbers of

calculations

35
Q

By ref to the intensities, suggest why the X-ray image has a good contrast

A
  • large difference in intensities so good contrast
36
Q

State the relationship btw electric potential and electric field strength at a point

A

electric field strength = - potential gradient

E= - V/d

37
Q

Initially, the helium nucleus is at rest on plate A where x=0
i) The helium nucleus is free to move btw the plates. By considering E changes of the helium nucleus, explain why the speed at which it reaches plate B is independent of the separation of the plates

A
  • eV = 1/2mv^2
  • Loss in Ep is equal to the gain in KE
  • Therefore, the v does not depend on the separation of
    the plates
38
Q

Explain why, after the decay, the nucleus is no longer stationary

A
  • Total momentum is conserved.
  • Therefore, the nucleus must have momentum in opp
    direction to the photon
39
Q

How the hardness of an X-ray beam from X-ray tube is ↑

A
  • greater accelerating pd
    or
  • greater pd btw anode and cathode
40
Q

In an X-ray image, white regions show greater absorption of X-rays than dark regions

State and explain the difference btw the X-ray image of the bone compared to that of muscle

A
  • Bone is seen lighter
  • bcuz bone has a large attenuation coefficient compared
    to that of muscle
41
Q

State and explain 1 situation where resonance is useful

A
  • During vibration of quartz crystal thru piezo-electric effect
  • For accurate timing as resonance will take place when alternating pd has freq equal to natural freq of crystal.
42
Q

In some situations, resonance should be avoided.

–> State 1 situation and suggest how the effects of resonance are reduced.

A
  • Vibration in metal panels

- can be reduced by placing strengthening struts across panels

phy : medical imaging (1 decks)

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