Medical Physics Semester 1

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Questions and Answers

What is the main mechanism behind characteristic X-ray emission?

An electron is accelerated, emitting an X-ray photon in the process.

An atomic electron transitions to a higher energy state, emitting an X-ray photon.

An electron interacts with an atomic electron, ejecting it from its shell and filling the vacancy by emitting an X-ray photon. (correct)

An X-ray photon interacts with an atomic electron, transferring its energy to the electron.

What is the equation for X-ray attenuation?

I = I˳ * e - μx

I = I˳ e - μx (correct)

I = I˳ / e - μx

I = I˳ e + μx

What is the linear attenuation coefficient (μ) a measure of?

The thickness of the attenuator

The probability of photon interaction per unit length (correct)

The energy of the X-rays

The intensity of the X-ray beam

What is the half-value thickness (HVT)?

The thickness of material that reduces the intensity of the beam of radiation to one-half of its value Signup and view all the answers

What is the most common way X-rays interact with matter at diagnostic X-ray ranges?

Photoelectric effect Signup and view all the answers

What is true about the probability of the photoelectric effect?

It is more common in materials with high atomic numbers Signup and view all the answers

What is the relationship between the linear attenuation coefficient (μ) and the half-value thickness (HVT)?

HVT = 0.693 / μ Signup and view all the answers

What is the unit of measurement for the linear attenuation coefficient (μ)?

Per centimeter Signup and view all the answers

What is the wavelength of X-rays in Angstrom units?

1-0.1 A° Signup and view all the answers

What is the relationship between the frequency of radiation and the energy carried by each photon?

E = h υ Signup and view all the answers

What is the main purpose of the cathode in an X-ray tube?

To provide a source of electrons Signup and view all the answers

What percentage of the accelerated electrons' energy is converted to X-ray photons in an X-ray tube?

1% Signup and view all the answers

What is the effect of increasing the atomic number of the anode on the X-ray beam intensity?

The intensity of the X-ray beam increases Signup and view all the answers

What is the unit of Planck's constant?

Joule.second Signup and view all the answers

What is the purpose of the evacuated space in an X-ray tube?

To speed up the electrons Signup and view all the answers

What is the velocity of light in meters per second?

3*10^8 m/sec Signup and view all the answers

What was the purpose of the screens coated with phosphor layer in X-ray film-screen technique?

To convert X-ray photons to visible light photons Signup and view all the answers

What happens to the valence band electrons in the CR phosphor detectors when exposed to X-rays?

They move to the conduction band Signup and view all the answers

What is the purpose of the laser in the computed radiography (CR) process?

To scan the exposed detector and convert the image information to blue light Signup and view all the answers

What is the advantage of digital radiography (DR) detectors made from amorphous selenium (a-Se)?

They avoid multiple conversions of signals, eliminating light scattering problems Signup and view all the answers

What is the term for the blurred edge of an object in an X-ray image?

Penumbra Signup and view all the answers

What is the main problem involved in obtaining a good X-ray image?

Blurring Signup and view all the answers

What happens to the uniform surface charge on the digital radiography (DR) detector when exposed to X-rays?

It becomes partially discharged Signup and view all the answers

What is the purpose of the photomultiplier system in the computed radiography (CR) process?

To convert blue light to electronic signals Signup and view all the answers

What is the primary mechanism of direct radiation interaction with tissue?

Radiation energy is directly transferred to the DNA Signup and view all the answers

What is the result of deterministic effects of radiation?

Relatively immediate damage to tissues Signup and view all the answers

What is the minimum lag period between irradiation and cancer development for stochastic effects?

5 years Signup and view all the answers

What is the primary purpose of fluoroscopy in medical imaging?

To acquire a sequence of X-ray images over time Signup and view all the answers

What type of procedures are fluoroscopy commonly used for?

Invasive therapeutic procedures Signup and view all the answers

What is the principle behind computed tomography (CT) imaging?

Passing X-rays through the body at multiple angles Signup and view all the answers

What is the term 'tomography' referring to in CT imaging?

A picture of a slice Signup and view all the answers

What is the component of a CT scanner that collects transmission projection data?

Detector array Signup and view all the answers

What is the effect of increasing the current in the cathode circuit?

Increase in the energy of photons Signup and view all the answers

What is the significance of the anode material having a high melting point?

To overcome overheating problems Signup and view all the answers

What is the purpose of equipping the X-ray tube with two filaments?

To produce large or small focal spots interchangeably Signup and view all the answers

What is the advantage of using a rotating anode X-ray tube?

To overcome overheating problems Signup and view all the answers

What type of X-ray spectrum is produced when accelerated electrons come near an atomic nucleus?

Bremsstrahlung Spectrum Signup and view all the answers

What determines the amount of Bremsstrahlung radiation?

The atomic number (Z) of the target Signup and view all the answers

What is the effect of increasing the kV peak on the Bremsstrahlung radiation?

Increase in the amount of Bremsstrahlung radiation Signup and view all the answers

Study Notes

X-Ray Production

X-rays are produced when highly energetic electrons interact with matter, converting some of their kinetic energy into electromagnetic radiation.
The main components of the X-ray tube are:
- A source of electrons (the cathode, filament)
- An evacuated space in which the electrons are accelerated (glass envelope)
- A high positive potential to accelerate the negative electrons (kV)
- A target which the electrons strike (the anode)
In the X-ray tube, up to 99% of accelerated electrons energy is converted to heat and approximately 1% is converted to X-ray photons.

X-Ray Characteristics

X-rays are electromagnetic radiation (EMR) of very short wavelength (λ 1-0.1 A°) and very high penetrating power.
The amount of energy carried by each photon depends on the frequency of radiation: E = h υ = h c /λ
The higher the atomic number (Z) of the anode, the more intense X-ray beam is produced (e.g., Z of tungsten = 74)

Types of X-Rays

Bremsstrahlung Spectrum (Continuous X-ray):
- Produced when accelerated electrons interact with the target and are decelerated, emitting X-ray photons of equal energy
- The amount of bremsstrahlung depends on the Z number of the target and the kV peak
Characteristic X-ray:
- Produced when an electron interacts with an atomic electron within the anode and ejects it from its shell, emitting X-ray photons of equal energy
- The energy of the X-ray photon is equal to the binding energy difference between the two shells

X-Ray Absorption by Tissue

X-ray attenuation is the reduction of the X-ray beam due to absorption and scattering.
The attenuation of an X-ray beam can be measured using the equation: I = I˳ e –μx
Linear attenuation Coefficient (μ) measures the probability that a photon interacts (absorbed or scattered) per unit length it travels in a specified material.
μ depends on:
- Energy of X-rays
- Atomic number (Z) of the material
- Density (ρ) of the material

Methods of X-Ray Interaction with Matter

Photoelectric effect:
- Occurs when low-energy X-ray photons transfer all of their energy to an atomic electron (more likely inner shell electron)
- More common in materials with high Z number than those with low Z number

X-Ray Image Formation

Film-screen technique:
- X-ray film accommodated inside a special cassette is used as an image receptor
- Screens (cardboard coated with phosphor layer) are used to convert X-ray photons to visible light photons, which form the image on the X-ray film
Computed radiography (CR):
- Phosphor detectors are used inside special cassettes as an image receptor
- Exposing detector to X-rays results in an excitation process, forming a latent image
- The exposed detector is scanned with a laser in the reader, converting the image information to electronic signals
Digital radiography (DR):
- Digital detectors made from amorphous selenium (a-Se) are used to detect X-rays
- The uniform surface charge pattern is partially discharged when X-rays are absorbed, forming a latent image as an electronic signal

Radiographic Image Quality

The main problem in obtaining good X-ray images is blurring.
The blurred edge of an object in the X-ray image is called penumbra.
Radiation interactions with tissue can be either direct or indirect.

Radiation Risk of X-Ray Examinations

Adverse health effects of radiation can be classified into two groups:
- Deterministic effects: high radiation doses result in relatively immediate damage
- Stochastic effects: low radiation doses may result in cancer development, with a lag period of at least 5 years and up to 10 or 20 years

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Description

This quiz covers the basic components of an X-ray unit, including the cathode circuit and anode, and techniques to overcome overheating problems in the anode of the X-ray tube.