medical physics Flashcards
Specific Acoustic Impedance
Product of density of medium and speed of sound in medium
Sharpness
Clarity of resolution of image
Hardness of X-ray beam:
- Penetration of beam.
- Greater hardness means greater penetration
How hardness of X-ray beam is controlled:
- Controlled by changing the anode voltage
- Higher anode voltage, greater penetration
Linear Absorption coefficient:
- 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
Attenuation of an ultrasound wave
Loss of intensity in waves as they pass thru medium
Contrast
Difference in degree of blackening btw structures
Main principles of the GENERATION of ultrasound waves for medical use:
- 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
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:
- Later signal has passes through greater thickness of
medium - So greater attenuation / absorption
Explain why ultrasound is emitted in pulses
- Allows the reflected signal to be distinguished from the
emitted signal - Detection occurs at time between emitted pulses
- To determine the depth of boundary
Explain why gel is applied to surface of skin during ultrasound scan:
- Without the gel, most of the ultrasound is reflected with
no transmission - Bcuz value of Z are different
By ref to specific acoustic impedance, explain why there is little transmission of ultrasound waves from air to skin:
- 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
Explain why an aluminum filter may be placed in the X-ray beam when producing X-ray image of patient:
- 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
Why is it advantageous to filter out low E photons from X-ray beam
- Low-E photons absorbs much more easily
- Low-E photons are far less penetrating
- Low-E photons do not contribute to X-ray image
Main principles behind the USE of ultrasound to obtain diagnostic information about internal body structures:
- 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
Outline the principles of CT scanning.
- 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
1 adv of using high freq ultrasound and explain
- Smaller structures can be observed due to better resolution
Explain how the hardness of an X-ray beam is controlled by the accelerating voltage in the X-ray tube:
- Changing voltage changes the energy of electrons.
- Electrons striking the anode have changed kinetic
energy, - so X-ray photons will have different wavelengths
2 cause of lack of sharpness of an X-ray image
- Scattering of X-ray beam
- Lack of collimation (alignment) of beam
- Pd of beam is low
Describe 2 factors of sharpness of an X-ray image:
- Size of X-ray source
- Scattering of X-ray beam
Suggest why exposure to radiation differs btw X-ray and CT scan
- 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
Uses of piezo-electric transducers
Ultrasound
Microphone
Pressure sensor
Watch
Discuss qualitatively how the relative magnitudes of the two specific acoustic impedances affect the reflected intensity
- 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
Explain the principles behind the USE of X-rays for imaging internal body structures
- 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.
Describe images produced from CT scan vs X-ray
- 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
Explain why acoustic impedance is important when considering reflection of ultrasound at boundary
Difference of acoustic impedance determines the fraction of incident intensity that is reflected
Explain why a continuous spectrum of wavelengths is produced during e- bombardment of a metal target
- 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
Explain why there is a sharp cut-off at short wavelength.
- Wavelength is shorter for greater acceleration
- e- losses all its’s E in one collision and stops
State, with ref to the production of X-rays, what is meant by 80keV X-rays.
- X-rays produced when high speed e- hits the anode
- Electrons have KE of 80keV
State 2 reasons why intensity of X-ray beam at B is not as great as A (B placed after block + further from source)
- X-ray Beam is divergent at B
- Absorption of beam in the block
- Scattering in block
- Reflection at boundaries
Explain the principles of DETECTION of ultrasound waves for medical use
- 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.
explain why there is a continuous distribution of wavelengths
- 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
explain why at certain wavelengths, there are narrow peaks of increased intensity
- 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
Why CT scanning was not possible b4 fast computers with large memories were available.
- Combing of images involves very large numbers of
calculations
By ref to the intensities, suggest why the X-ray image has a good contrast
- large difference in intensities so good contrast
State the relationship btw electric potential and electric field strength at a point
electric field strength = - potential gradient
E= - V/d
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
- 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
Explain why, after the decay, the nucleus is no longer stationary
- Total momentum is conserved.
- Therefore, the nucleus must have momentum in opp
direction to the photon
How the hardness of an X-ray beam from X-ray tube is ↑
- greater accelerating pd
or - greater pd btw anode and cathode
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
- Bone is seen lighter
- bcuz bone has a large attenuation coefficient compared
to that of muscle
State and explain 1 situation where resonance is useful
- 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.
In some situations, resonance should be avoided.
–> State 1 situation and suggest how the effects of resonance are reduced.
- Vibration in metal panels
- can be reduced by placing strengthening struts across panels
