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Flashcards in Medical Physics Deck (19):

What factors effect acoustic impedance

-Density of medium
-Speed of ultrasound in the medium


Outline the production of ultrasound

-Use a piezo-electric crystal
-Apply a p.d across it and its shape changes
-Alternating p.d. causes oscillations
-Crystal cut to produce resonance at frequency of applied p.d.


Explain why a coupling medium is used

-acoustic impedance of air is very low
-All ultrasound would be reflected at air/skin
-coupling medium has the same acoustic impedance as skin and transducer
-acoustic impedance matching = maximum transmission of ultrasound


Explain how image intensifiers are used to improve the quality of an X-ray image

-Absorption of X-rays by a photographic film
-Use of florescent/ scintillator/ phosphor
-Converts X-ray Photons into visible light electrons (which is accelerated onto a fluorescent screen)
-Lower exposure needed
-Number of electrons increased


Explain how contrast media are used to improve the quality of an X-ray image

-Different soft body tissue produce little difference in contrast/ attenuation
-Contrast media with high atomic number, eg barium or iodine used to get a greater contrast
-liquid injected or swallowed into soft tissue areas
-Scan shows outline/ shape of soft tissue


Explain how Ultrasound is used

-Pulses of ultrasounds sent into the body
-Some ultrasound pulses are reflected at boundary of tissue dependent on the acoustic impedance
-Time of delay used to determine depth/ thickness
-The fraction of reflected signal is used to identify the tissue


Advantages of a CAT scan

-Produces 3D image
image can be rotated
-detail of soft tissue


Describe the use of medical tracers

-Tracer is injected into the body (a radioactive substance is used)
-Tracer is absorbed by organ/ shows blockage
-Beta detector/ gamma camera is used to detect radiation from the body
eg Technetium (-99m) or Iodine (-131)


Describe a PET scan

-Positron, Beta plus emitting tracer is used
-The positron annihilates with an electron
-This produces 2 gamma photons
-The photons travel in opposite directions
-The patient is surrounded by a ring of gamma detectors
-The arrival times of the photons indicates location
-3D image is created by a computer


Describe the differences between ultrasound A-scans and B-scans

A-scan is one directional
B scan involves different directions/ angles
B-scan consists of many A-scans to produce a 2D or 3D image


Advantages of non-invasive technique

-no cutting the patient
-no surgery
-lower risk of infection
-less trauma


What is a collimator (gamma camera)

Gamma rays travel along the axis of thing long lead tubes or allows parallel gamma through
This makes the image sharper


What does a Scintillator do (gamma camera)

Gamma ray photon produces thousands of photons of visible light


What are X-rays

EM transverse waves travelling at the speed of light
they can travel in a vacuum
Can cause Ionisation
wavelength of 10^-10 m


What is the Compton effect

-X-ray photon interacts with an orbital electron
-The scattered photon has a longer wavelength/ lower frequency
-The electron is ejected from the atom at high speeds


What does a Photomultiplier tube do (gamma camera)

Electrical pulse/ signal/ electrons produced by photons of visible lights


What does the computer do (gamma camera)

Electrons from photomultiplier tubes are used to generate an image


Explain how ultrasound is used to determine the speed of blood in an artery in the arm

-Transducer placed at angle in the artery
-Ultrasound pulses are reflected by moving blood cells
-The frequency/ wavelength of ultrasound is changed
-Change in frequency is related to the speed of blood


What is an energy level on an atom

Discrete energy of electrons in an atom