Phys of Med Imaging Lecture 3

T or F: Path length and penetration depth are consistent and approximately the same for light charged particles

False

T or F Etr is minimal when the photon is back-scattered

False. Etr is equal to KE of ejected electron, this will be maximized with backscatter.

T or F: Polyenergetic x-rays will experience beam hardening, resulting in an decreasing HVL with depth

False. Results in increasing HVL

T or F: heavy charged particles more likely to engage in nuclear reactions than light charged particles.

False. Slide 34 & 36

T or F: Bremsstrahlung energies are characteristic of the nucleus

False. They depend only on the amount of braking and therefore have a continuous energy spectrum

T or F: heavy charged particles deposit more energy per interaction than light charged particles, when they have the same initial KE.

False. greatest momentum transfer happens when target and projectile have similar masses. since collisions happen mostly w orbital electrons, the target mass is similar to light charged particles. HOWEVER, if heavy and light were going the same velocity (rather than having the same energy), the heavy charged particle would deposit more.

What are examples of High LET radiation

(alpha particles, protons, etc.) - more likely to cause DNA doublebond breaks

What are ways to improve contrast in an image?

- Use a filter to reduce measurement of compton scattered photons - use a contrasting agent with high Z - increase kVp

What is the Half Value Layer

0.693/linear attenuation coeff

What is the min incident energy for bremsstrahlung in carbon? in lead?

1MeV, <100keV

Which of the following will result in a decrease of ionizational energy loss? A. Decreasing particle energy B. Increasing Z C. Increasing electron density D. Decreasing Z

A and D

order the following collisional interactions in terms of increasing energy lost by the projectile: A. Ionization B. Excitation C. Delta Rays

B, A, C. bifurcations in the electron path are called delta rays, and result from the electrons having enough KE to travel macroscopically

Which of the following are examples of directly ionizing radiation? A. low energy photons B. electrons C. beta particles D. alpha particles E. neutrons F. high energy photons G. photons H. gamma rays

B, C, D. the rest are indirectly ionizing.

Describe what is happening in a LET vs Depth graph, for incident charged particles (slide 47)

Bragg Peaks

How many ionizational collisions will a 100 keV electron often undergo before coming to a rest? A. 10 B. 100 C. 1000 D. 10000

C

In a 20 cm thick patient, what percentage of x-rays gets to the middle of the patient? a. 0.1% b. 1% c. 10% d. 50% e. 98%

C General rule: 1 TVL = 10 cm of tissue

What does the rate of energy loss and the ionization density by a charged particle NOT depend on? A The type of particle (mass and charge) B The kinetic energy of the particle C The angle of incidence on a material D Thickness of material E The composition and density (g/cm3) of the medium through which it travels

C,D

Energy released in a photon during Bremsstrahlung radiation follows which dependency? A Z^2/m B Z^3/m^3 C Z D Z^2/m^2 E Z/m^2

D

Which shell is of the greatest interest in medical imaging?

K-shell electrons

How does fluorescence yield depend on Z?

Low Z (<40) - more likely to get electrons, Contributes to dose High Z (40+ - 80% yield) - more likely to get x-rays, Used in X-ray tubes

Which photon interaction is primarily important for image contrast? why?

PE, depends on Z

There are 5 primary x-ray interactions :- Photo-electric effect (κ) - Coherent scattering (σc) - Compton (incoherent) scattering (σ) - Pair Production (π) - Triplet Production (πe) Describe the mass attenuation coefficients, processes, and cross section dependencies of these effects. what about particularly for soft tissue?

Photoelectric: occurs for lower energy photons and high Z, and cross section changes like (Z/E)^3. Cross section is also larger for K-shell electrons because K-shell is closer to the nucleus and has a lower binding energy. Photoelectric effect will effectively attenuate photons better than Compton scattering, since the whole photon is absorbed. Photo electric affect also provides best quality for images, as the attenuation is easily interpretable to create an image, and the degree of attenuation depends on the cross section, which depends very strongly on Z. Coherent Scattering: Dominant for light in the visible range. Will cause no loss of energy of the incident photon and is therefore not useful in imaging. Compton Scattering: Occurs for intermediate energies and does not depend on Z. However, it does depend on electron density so for hydrogen (with higher e density) it will be responsible for more attenuation. It can be used to image object with large density differences by increasing kVp to ranges for Compton and filtering out photons of photoelectric range. I'm lazy and don't wanna write the rest.

Match the following terms: 1. Energy Transfer 2. Energy Absorbed 3. Energy Imparted A. The energy that is left in a medium by both charged and uncharged particles B. The energy that ultimately is carried by low energy electrons and transmuted into mechanical (heat) and chemical energy (chemical alterations) C. The energy of the x-ray beam that is transferred to high-energy electrons in the first interaction. The difference between the energy in the beam and the energy transferred is the energy carried away in scattered x-rays.

The difference between the energy transferred and the energy absorbed is the energy radiated from the medium by bremsstrahlung and characteristic x-rays 1C 2B 3A

T or F: The vast majority* of e- - e- interactions results in excitation, where there is a transfer of a few 10's of eV of energy and the sole result is "heat"

True

T or F: radiative interactions and characteristic x-rays produce visible and UV light only, excitation (10s eV)/ionization of valence (30-100eV) interactions produce IR/heat only

True.

T or F: At diagnostic energies and for low-Z materials, the probability of the scattered electron undergoing a radiative loss is low. Thus Eab ~ Etr

True

T or F: Auger electrons can be emitted at any angle

True

T or F: Compton Effect is used to image objects which differ in density

True

T or F: Compton interactions are the single largest contributor of radiation dose in diagnostic imaging

True

T or F: For heavy particles, the path length is essentially equal to their range.

True

T or F: Heavy charged particles follow a relatively straight path in a medium

True

T or F: LET is proportional to particle charge squared and inversely proportional to particle speed (kinetic energy)

True. LET defined over small energy ranges and is typically specified in keV/μm.

What is the bremsstrahlung efficiency *at diagnostic energies?*

Z*E/820 where E is the electrons KE Tungsten anode (Z=74) and E=110 keV, 1.0% of electrons energy produces x-rays. 99.0% of energy produces heat in the anode.

During a characteristic interaction, an M-shell electron fills the resultant K-shell vacancy. The resultant interaction ejects a different M-shell electron. Let Ekand Emrepresent the binding energies of K- and M-shell electrons, respectively. What is the kinetic energy of the ejected M-shell electron? a. Ek- 2Em b. Ek+ 2Em c. Em-2Ek d. Em+ 2Ek e. 2Ek-Em

a. Ek- 2Em

Which particles cause the majority of the damage from x-rays? a. Electrons b. Protons c. Muons d. Neutrons e. Alpha particles

a. Electrons X-rays are not directly ionizing. They eject high energy electrons which travel through the body ionizing potentially thousands of atoms.

What interaction is most responsible for the generation of x-rays? a. Compton Scatter b. Ionization c. Excitation d. Bremstrahlung e. Photoelectric Effect

d.

Which anode gives you the highest energy characteristic x-rays with a kVp of 35? a. Tungsten (Z = 74) b. Carbon c. Oxygen d. Molybdenum (Z = 42) e. Lead (Z = 80)

d. Molybdenum As we will learn more in mammography, molybdenum is one of the favored x-ray target materials because of the low energy (17.5-19.7 keV) characteristic radiation.

The most effective shielding from the exposure of beta particles is: a. Lead b. Tungsten c. Titanium d. Steel e. plastic

e. Beta particles are electrons. Electrons are more likely to produce x-rays from high-Z materials. Therefore, it is best to shield electrons with low-Z materials. Although you need a thicker absorber, they will not produce hard to shield x-rays.

What are examples of Low LET radiation

electrons, positrons, and EM radiation - x-rays and gamma rays

Describe graph on slide 34

https://canvas.upenn.edu/courses/1679367/files/folder/Lecture%20Slides/Lectures%20-%20Exam%201?preview=113462282 - no idea if that'll work

What are the two ways an electron can lose energy in a medium?

ionization and radiative

Explain electron straggling

large variation of collision histories therefore a larger variance in electron path length. Why practical range is extrapolated from the depth that attenuates 50% of electrons

For compton scattering, binding energy effects are most evident at ___ energies and for ___Z materials

low, high

Order the following types of radiation in their ability to penetrate a medium: gamma, alpha, beta

named in order of penetrability, alpha the least, then beta then gamme.