X-Ray Lectures & Lab

Question 1

Where on the EM radiation spectrum do x-rays lie?

Answer 1

Between UV light and gamma radiation

Question 2

What is the energy range of x-rays?

Wavelength?

Frequency?

Answer 2

100 eV to 1 MeV

10-0.01 nanometers

30x10¹⁵ - 30x10¹⁸ Hz

Question 3

What is the name for light emission evoked by scratching or rubbing?

Answer 3

triboluminescence

Question 4

What is the significance of the rotation in a rotating anode x-ray tube?

Answer 4

rotation allows the large amount of heat generated from the electrons striking the anode to dissipate over the entire surface of the anode

Question 5

How are electrons produced in a cathode tube?

Answer 5

By applying a high voltage to a cathode within a rarified (evacuated) medium, which accelerates electrons across the gap between the electrodes.

Question 6

Draw the diagram of an x-ray cathode.

Answer 6

Question 7

What is Bremsstrahlung?

Answer 7

EM radiation produced by deceleration of a charged particle when it is deflected by another charged particle

Question 8

In Bremsstrahlung, what is the relationship between the degree of direction change of a deflected electron and the energy of the resulting x-ray?

Answer 8

the more the electron is deflected from its original path, the greater the x-ray energy

Question 9

What is a characteristic X-Ray?

How are they formed?

Answer 9

X-rays emitted when an outer-shell electron fills an inner-shell vacancy

1. Incident electron strikes a core electron, removing it
2. This creates a “core hole”
3. Outer shell electrons relax to fill the core hole
4. This relaxation releases energy in the form of x-rays

Question 10

Why are characteristic x-rays called “characteristic”?

Answer 10

Relaxation of a certain element’s outer shell electrons emits a distinct quantum of energy because every element has unique electron energy levels.

Question 11

What special phenomenon can occur when an electron relaxes from an outer shell to fill a core hole?

What does it produce?

Answer 11

• the energy emitted from the relaxation can be transfered to another outer shell electron, causing it to escape the atom
• this creates an Auger electron

Question 12

What is a characteristic x-ray called when the core hole is filled by an electron from the adjacent higher-energy shell?

What about an electron from 2 shells out?

3?

And which has the greatest energy?

Answer 12

Alpha Type X Ray

Beta Type

Gamma Type

etc.

The further out the electron comes from, the higher the energy output of its relaxation.

Question 13

How does a graph of a characteristic x-ray spectrum look?

Answer 13

• background of Bremsstrahlung
• peaks for alpha and beta characteristic x-rays
• beta is smaller wavelength, and thus higher E, because it comes from a further out shell

Question 14

Why can an x-ray spectrum only reveal elemental structure and not molecular?

Answer 14

Because outer shell electrons in molecules are tied up in bonding, so only inner shell electrons participate in characteristic x-radiation.

Question 15

What is the main x-ray effect in diagnostic use of x-rays?

Describe it.

Answer 15

X-Ray Photoeffect

• similar to photo-effect with gamma radiation
• incident x-ray transfers its energy to an inner shell electron
• the electron is emitted in a single step

Question 16

Why would a graph such as this one with curves for several different accelerating voltages only show characteristic x-ray peaks above a certain voltage?

Answer 16

Because lower voltages do not produce x-rays with high enough energy to remove tightly bound k shell electrons and make the electron holes necessary for characteristic x-radiation.

Question 17

What is the Duane Hunt Law?

Answer 17

the minimum wavelength of Bremsstrahlung radiation is inversely proportional to accelerating voltage and can be determined via the equation:

λ_min = k / U

k is a constant … K = h ᐧ c/e ≅ 1230 pmᐧkV

Question 18

What 3 processes contribute to the attenuation of x-rays?

2 of them are more important than the 3rd… why?

Answer 18

1. Compton Scatter - photon strikes outer shell e^-, ejects lower energy Compton e^- and is scattered
2. **Photoeffect **- photon strikes inner shell e^-, transfers most of its energy, ejecting photoelectron (dependent on absorber atomic #)
3. Pair Production - photon strikes near nucleus, creates electron and positron pair

• pair production is irrelevant except in high energy therapeutic x-radiation because softer x-rays don’t have enough energy

Question 19

What is the equation for mass attenuation coefficient for photoeffect?

Answer 19

τ _m = C⋅λ³ ⋅ Z³

Where Z is atomic number and C is a proportionality factor

Question 20

What takes the place of thickness (x) in the radiation attenuation law, when using a mass (μ_m) rather than linear attenuation coefficient (μ)?

Answer 20

Surface Density (X_m)

• the product of thickness x times density ρ
• has a unit of mass/area … g/cm² in this case

Question 21

What is the half value thickness equivalent when using mass attenuation coefficient?

Answer 21

Half Value Mass (D_m)

• mass of material behind 1 cm² surface area that decreases incident intensity by half

D_m = 0.693/μ_m

Question 22

What is the mass attenuation coefficient?

Answer 22

a density-independent radiation attenuation coefficient obtained by dividing linear attenuation coefficient by density of the absorber

unit is cm²/g

Question 23

How can the atomic number of multi-atomic systems be approximated when trying to determine the mass attenuation coefficient for photoeffect?

Answer 23

Effective Atomic Number or Z_eff

Question 24

For the different absorption processes (including Rayleigh scatter), what is the dependence of attenuation coefficients on atomic number (Z)?

And what is their dependence on E of the incoming radiation?

Answer 24

Question 25

What kind of x-ray is used in diagnostic imagine?

Answer 25

Bremsstrahlung

Question 26

What processes use characteristic x-ray?

Answer 26

• bone densitometry
• ID of materials (toxic elements, etc.)
• x-ray therapy
• structural analysis (ex: DNA by Watson-Crick)

Question 27

At the lower photon energies found in diagnostic x and gamma rays, how are atomic number of the absorbing material and the dominant absorption process related?

Answer 27

low atomic number - Compton effect

high atomic number - photoeffect

Question 28

If higher atomic number (Z) means greater attenuation, why are elements with greater Z than lead not used for radiation protection?

Answer 28

because the higher elements tend to be radioactive themselves

Question 29

Why must x-rays be “filtered”?

How are they filtered?

Answer 29

Bremsstrahlung x-rays have many wavelengths.

Lower wavelengths would be too greatly absorbed in the body, increasing risk of damage.

A 1-2 mm layer of aluminum is placed between the x-ray source and the body to absorb the lower wavelength radiation.

Question 30

What is a “summation image”?

Answer 30

• an x-ray image in which all layers between the source and absorber take part in the absorption of x-radiation such that the image indicates the “sum” of the absorption by all layers
• creates the 2D images seen in standard x-ray pictures
  (ex: multiple different organs all absorbing to a different degree)

Question 31

What are the two types of contrast materials, their effects, usage and examples of them?

What if both of them are used?

Answer 31

1. **Positive **- absorbs more than environment, barium sulfate in GI, iodine intravenously
2. Negative - absorbs less than environment, gases often used

• if both positive and negative are used, this is **dual contrast **imaging

Question 32

How can blood vessel lumens be visualized via x-ray?

Answer 32

Digital Subtraction Angiography

1. x-ray is taken without contrast material
2. positive contrast material is injected into blood, to create an x-ray in which the vessesl stand out
3. image 1 is subtracted from image 2 to produce an image of only the vessels

Question 33

What is an x-ray image amplifier use for and how does it work?

Answer 33

• decreases exposure and increase image quality
• radiation passes from source through body
• attenuated radiation hits luminescent screen
• light from screen hits photocathode
• e^- ejected from cathode is accelerated thru electrodes
• accelerated e^- hits 2nd luminescent screen, forming much brighter image

Question 34

Explain traditional tomography.

Answer 34

​ a special x ray method used to negate the shadowing effects of layers other than the intended, observed layer

• angled x-ray source moves on semi-circular path
• detector parallel to body axis moves in opposite direction, always remaining within path of source
• the center of this circular path is somewhere within the body, producing a sharp image of only that layer
• used in pulmonology to remove rib shadowing and dentistry for dental panoramas

Question 35

What is x-ray CT?

How does it differ from traditional tomography?

And spiral CT?

Answer 35

X-Ray Computed Tomography

• the density of different body parts are calculated by a computer
• spiral CT involves slow movement of the patient along the body axis through a rotating x-ray tube

Question 36

What is the Hounsfield-Scale?

Answer 36

a scale which relates the attenuation of a material to that of water via the equation:

Question 37

What is “windowing” in relation to xray?

Answer 37

relating the entire visible grey scale to a smaller part of the Hounsfield scale to exclude more or less attenuative tissues from the image