Medical Physics Flashcards
describe the principles of generation and detection of ultrasound.
-A potential difference is applied to the quartz/piezo-electric crystal.
-This potential difference causes it to change shape.
-The alternating p.d. Causes the crystal to vibrate.
-The crystal is specifically to produce resonance.
… these vibrations produce ultrasound.
-When ultrasound is received it makes the crystal vibrate.
-This vibration produces an alternating potential difference across the crystal.
Principles behind the use of ultrasound to obtain diagnostic information about internal structures
-A pulse of ultrasound produced by the quartz crystal
… gets reflected from boundaries.
-That reflected pulse is detected by the ultrasound transmitter.
-The transmitter processes the signal and the displayed intensity of the reflected pulse gives information about the boundary.
-The time delay gives information about the depth.
Define specific acoustic impedance
Product of medium density and the speed of sound in the medium.
The intensity reflection coefficient in ultrasounds
he intensity reflection coefficient is the ratio of the reflected intensity to incident intensity.
Describe the effect of frequency on ultrasound.
-Higher frequency ultrasounds will have shorter wavelengths.
-Therefore smaller structures can be detected.
Expalin how are x-rays produced for use in medical diagnosis.
-In the cathode of the x-ray tube electrons are released through thermionic emission.
-This electrons are accelerated by a p.d.
-The electrons collide with a metal target.
-The sudden deceleration of the electrons produces an X-ray. As energy lost by the electrons is emitted by x rays.
(extra creo)
-The bombarding electrons knock an electron out of an inner shell of the target metal atoms.
-The electrons from higher states drop down to fill the vacancy emitting x-ray photons.
In CT scanning why are multiple x-ray images taken of multiple sections of the body?
-Images of the multiple sections are combined to create a 3D image.
Define the term contrast in x-ray imaging.
The difference in degrees of darkening.
Define the term hardness in x-ray imaging.
The measure of the penetration of the beam.
Define the term sharpness in x-ray imaging.
The ease at which edges can be distinguished.
Principles of CT scanning.
-X-ray images of one slice are taken from different angles, all in the same plane.
-Combined to produce a 2D image of one slice.
-Repeated for many slices and images combined.
-To build up a 3D image of the whole body structure using the computer that can be rotated and viewed from different angles.
Advantage of CT scanning
Produces a 3D image
Disadvantage of CT scanning
Much greater exposure to radiation.
Define a tracer
Is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied.
Explain how the decay of the tracer results in the gamma-ray photons that are detected during a PET scan.
-Positrons and electrons annihilate each other.
-Mass of particles becomes energy of gamma photons.
