Ch27: Medical Imaging Flashcards
What is the approximate range of wavelengths for X-Rays?
10^-8 to 10^-13 m
Approximately how many times more energy do X-ray photons have than visible light?
10-10 000 times more
How are X-ray photons produced?
Very fast moving electrons fired from a cathode and are slowed down by interaction with the atoms of a metal anode such as Tungsten. The kinetic energy of the electrons becomes high energy photons.
What is the energy gained by an electron when accelerated through p.d: V?
eV, where e is the elementary charge
How can we calculate the maximum energy of an X-ray Photon?
max X-ray energy = kinetic energy of electron
What is the equation for wavelength of an X-ray photon in relation to the electron that produced it?
λ = hc/eV
Name the 4 ways an X-ray photon can interact with matter.
- Simple scatter
- Photoelectric effect
- Compton scattering
- Pair production
Describe the effect of Simple scatter? What is its photon energy range?
Electrons in an atom scatter the X-ray to a different direction as the photon does not have enough energy to remove the electron.
< 20keV
Describe the Photoelectric effect?
A photon is fully absorbed by an electron and removes it from the atom with no left over photon.
<100keV
What is the typical voltage range for hospital X-ray machines?
30-100kV
Describe the effect of Compton scattering? What is the photon energy range?
A photon provides enough energy to remove an electron from an atom, with enough energy left over to allow a low energy photon to escape as well.
0.5-5MeV
Describe the effect of pair production? What is the photon energy range?
A photon interacts with a nucleus, the electromagnetic energy of the photon creates an electron and a positron which are ejected in opposite directions.
> 1.02MeV
What is an attenuation coefficient?
A value which describes how well a substance absorbs X-rays.
What is the equation for transmitted intensity of X-rays?
I = Io(e^-μx)
Why are contrast mediums used?
to increase absorption coefficients of soft tissues so their structure is visible in an X-ray.
What are the two most common contrast mediums for X-rays and how are they used?
Iodine: typically injected
Barium: typically eaten
What is the attenuation coefficient proportional to?
The attenuation coefficient is proportional to the cube of the atomic number (Z).
What does C.A.T stand for?
Computerised axial tomography
What are the advantages and disadvantages of C.A.T scans?
A: produces a 3D image
D: High radiation exposure and long time to do (10-30 min)
How are CAT scans done?
An X-ray tube produces a fan of x-rays which move 360 degrees around a patient while the patient also moves through the machine; producing slices which a computer puts together to form a 3D image.
What are the key parts of a CAT scan machine?
Rotating gantry, X-ray tube, detector, patient bed, display, and computer.
What are the parts of a gammas camera in order?
collimator, scintillator, light guide, photomultiplier tubes, circuits, computer, and display
What is the most common medical tracer used with the gamma camera?
NaTcO4 (Tc-99)
What is the function of a collimator in a gamma camera?
Only allows through photons travelling straight at the camera.
