Physics Questions

Question 1

What is the approximate minimum photon energy required to cause ionization?

Answer 1

On average, about 25 eV is required to create an ion pair in water, although the minimum energy needed to eject an electron is only 12.6 eV

Question 2

Which has a higher electron density, bone or lead?

Answer 2

Lead (electron density is proportional to mass density (kg/m^3))

Electron Density of Common Materials

Material Z (eff) Electrons/kg

Hydrogen 1.0 6.00 x 10^26

Fat 5.92 3.48 x 10^26

Water 7.42 3.34 x 10^26

Air 7.64 3.01 x 10^26

Oxygen 8.0 3.01 x 10^26

Bone 13.8 3.00 x 10^26

Calcium 20.0 3.00 x 10^26

Copper 29.0 2.75 x 10^26

Iodine 53.0 2.51 x 10^26

Barium 56.0 2.45 x 10^26

Lead 82.0 2.38 x 10^26

Question 3

Which has a higher electron density, breast tissue or bone?

Answer 3

Breast (electron density is proportional to mass density (kg/m^3)

Electron Density of Common Materials

Material Z (eff) Electrons/kg

Hydrogen 1.0 6.00 x 10^26

Fat 5.92 3.48 x 10^26

Water 7.42 3.34 x 10^26

Air 7.64 3.01 x 10^26

Oxygen 8.0 3.01 x 10^26

Bone 13.8 3.00 x 10^26

Calcium 20.0 3.00 x 10^26

Copper 29.0 2.75 x 10^26

Iodine 53.0 2.51 x 10^26

Barium 56.0 2.45 x 10^26

Lead 82.0 2.38 x 10^26

Question 4

What is the relationship between wavelength, frequency (v) in Hz, speed of light (c):

Answer 4

Speed of light (c) = wavelength x frequency (v)

or Wavelength = speed of light/frequency

  or     Frequency (v) = speed of light (c)/ wavelength

Example: What is the frequency of a photon with a wavelength 15 x 10^-11m
Frequency (v) = 3 x 10^8 m/s / 15 x 10^-11 m = 2 x 10^18 Hz

Question 5

For 100 keV photons, what is the dose delivered to muscle if the exposure to that muscle is 100R (f_med for muscle @ 100 keV is 0.956 cGy/R)

Answer 5

Dose_med = Exposure x f_med

= 100R x 0.956cGy/R (water)

= 95.6 cGy

Question 6

Give examples of High LET and Low LET radiation

Answer 6

High LET (linear energy transfer) radiations include particles such as 290 MeV carbon ions, α-particles and neutrons (all are densely ionizing radiation). 250 kVp X-rays, 200 MeV protons and 1.1 MV γ-rays are all low LET (sparsely ionizing radiation)

As a side note…

High LET radiations, such as iron ions or carbon ions, are components of cosmic rays, while solar flares are composed largely of energetic protons (which are low LET).

Question 70

What is the effect of a change in kVp or mAs on image contrast?

Answer 70

As kVp increases, image contrast decreases. (kVp is tube voltage)

A change in mAs has no effect on image contrast. (mA = tube current; s = exposure time; combine for mAs)

(Increasing the kV increases the kinetic energy of the electrons colliding with the target, leading to greater x-ray production as well as increased heat load on the target. Contrast is reduced @ higher kV settings, as is quantum noise (both are inversely related to the number of photons produced))

Increasing the mA increases the dose rate proportionally.

Question 71

At what energies are PE and Compton effects equal?

Answer 71

For soft tissue, the PE and Compton effects are equal at 25 keV.

For bone, the PE and Compon effects are equal at 40 keV.

Question 72

What are the layers of a Thoraeus Filter

Answer 72

Farthest from patient –> Closest to patient

Tin –> Copper –> Aluminum

Question 73

Beam hardening is mainly due to?

Answer 73

Photoelectric Effect

Question 74

What is the % uncertainty if a photon counter detects 10,000 counts?

Answer 74

Uncertainty is given by the square root of the # of photons detected.

Square root of 10,000 = 100

Percent uncertainty = (100/10,000) x 100 = 1%

Question 75

If a superficial x-ray unit has an SSD of 20cm to the end of the applicator and an additional air gap of 2cm is accidentally left without correction, this would cause the dose at d_max to be:

Answer 75

17% low

(remember the inverse square law for this question)

(20/22)² = .826

.826 x 100 = 82.6% of the desired dose

Question 76

How do you calculate beam divergence?

Answer 76

Find out!!!!

Question 77

Compare dose to water and dose to muscle

Answer 77

100 cGy to water = 99 cGy to muscle (takes 1% more to give same dose to muscle!!)

Question 78

What material is typically used in the treatment door for neutron shielding?

Answer 78

Borylated polyethylene (the polyethylene moderates the neutrons to thermal energies, and they are then captured by boron. The outside of the door must also contain lead or steel to attenuate the gammas produced in this neutron capture event!!

Question 79

What is the effect of scatter on signal, noise, signal to noise ratio (SNR), and contrast to noise ratio (CNR)

Answer 79

Signal is increased by scatter

Noise is increased by scatter (noise = the square root of signal)

Signal to noise ratio (SNR) is increased by scatter

contrast to noise ratio (CNR): decreased when scatter increases

Question 80

How do you calculate Hounsfield units?

Answer 80

The CT number (in Hounsfield units) = 1000 * [(u_material - u_water)/u_water

(where u = the linear attenuation coefficient)

Question 81

If the dose rate is 250cGy/min @ 100cm, at what distance will the dose rate by 100cGy/min?

Answer 81

Dose rate₁/ Dose rate₂ = (distance₂)² / (distance₁)²

250cGy/100cGy = (?)² / (100)²

√2.5 = √((?)^₂/(100)²)

1.58 = ?/100

? = 158

Question 82

If an air gap between an electron cutout and the skin increases from 5cm to 10cm, the maximum dose in tissue (for the same MU) decreases by approximately _____ %

Answer 82

At 100 SSD, the typical cone is 95cm from the source (gap of 5cm), so adding 5 additonal cm causes a loss of ~2%/cm (~10% loss for an increase in 5cm of gap)

This is based on the Inverse Square Law:

I₁/I₂ = D₂² /D₁²

Thus… I₂ = I₁ * D₁² / D₂²

I₁ = Intensity @ Distance 1

I₂ = Intensity @ Distance 2

D₁ = Distance 1

D₂ = Distance 2

Question 83

A cutout material is 1.27 thick and has a half value layer (HVL) of 1.6cm for 6 MV photons. What percentage of an incident 6MV beam is transmitted through the cutout material?

Answer 83

1.27/1.6 = 0.79

(1/2)^0.79 = .57 or 57% transmission