X-Rays and Computerized Tomography (CT) Flashcards
(42 cards)
What are X-RAYS?
- Electromagnetic radiation with a short wavelength and high energy, typically referred to as either soft or hard in terms of energy
- Often measured in eV (1eV = 1.602*10-19J).
- Due to their high energy they are ionizing and can penetrate deeper than light into the body
What is a GRAY, Gy?
A unit used to measure the intensity of an x-ray
1Gy = 1j in 1kg of dry air
measures the absorbed dose, D
What is a SIEVERT, Sv?
A unit used to relate the human exposure to radiation
Sv = Gy * Dose
Sv are defined as the dose equivalent, H
What is BACKGROUND RADIATION?
The unseen radiation the exists and provides a benchmark for risk
What are some SOURCES OF BACKGROUNF RADIATION?
Package inspections, artwork analysis, smoke detectors, water, food, stone, air travel and decay of heavy elements
What are SAFE DOSES OF RADIATION?
5-20mSv/year
50mSv/year for occupational
What are PHOTONS?
Small particles that comprises waves of electromagnetic radiation
How are PHOTONS EMITTED?
Atoms emit photons when they collide with other fast moving particles
* the energy in the collisions excite the atoms causing the electrons to jump up energy levels making them unstable
* the electrons revert to their original level releasing the excess energy in the form of a photon
* for each atom there are many energy transitions, each with a specific energy difference - range of transitions is known as an emission spectrum
How are X-rays GENERATED?
- Generated through vacuum tubes (electrons transmitted from cathode to anode gaining kinetic energy)
- impacts at the anode converts the energy into x-rays
What is a BREMSSTRAHLUNG?
- The electromagnetic radiation emitted in the form of photons when a charged particle is decelerating upon striking against another charged particle
- AKA braking or deceleration radiation
- Deceleration in a series of collisions, less velocity = less kinetic energy
- Produces a range of energy values, broad series of possibilities for deceleration ∴ broad spectrum
What is CHARACTERISTIC RADIATION?
Arriving electrons knock the inner electrons of an atom out of orbit, causing them to be replaced by electrons from the outer shell - for higher electrons, where the energy exceeds the k-shell binding energy.
this causes the release of a photon with an energy equal to the energy difference between levels.
It is material specific hence ‘characteristic’
What is a FOCAL SPOT?
In the order of 10s-100s of µm (depending on the resolution) - small focus spots increase the localized heating effect on the anode constraining the x-ray power. This effect can be countered by a rotating anode
What are some APPLICATIONS OF X-RAYS?
Package inspections/Artwork analysis/Radiotherapy/Diagnostic imaging
How does the BEAM ANGLE and FOCAL SPOT SIZE affect BEAM WIDTH?
The angle of the beam and the size of the focal spots determine the width of the beam
* Focal spot size controlled by applying an electric field to counteract electron-electron repulsion
* The smaller the anode angle the smaller the focal spot of the emitted X-rays
How does the DISTANCE FROM THE FOCAL SPOT affect the system?
The intensity decreases with an increased distance from the focal spot at a rate proportional to the square of the distance (inverse square law)
* Impacts the imaging system on two levels:
* Source-to-object distance (radiation safety)
* Source-to-imager distance (image quality)
What are the 3 TYPES OF SCATTERING?
- Rayleigh scattering
- Compton Scattering
- The photo-acoustic effect
What is RAYLEIGH SCATTERING and when is it used?
- The scattering of photons
- For energies <30keV or high atomic number (z) medias
of concern in low-energy mammography and micro-CT
What is COMPTON SCATTERING and when is it used?
- The scattering of relatively low energy photons emitted from an isotope that pass through the tissue and collide with molecules leading to a change in direction and slight energy loss
- For energies >26keV4
- Dependent on density - independent of atomic number (z)
What is the PHOTO-ELECTRIC EFFECT and when is it used?
- When an x-ray is stopped locally and an electron and low energy photon is emitted.
- The probability is proportional to Z^3/E^3
- Generates contrast between bone and soft tissue
- Enables detection
- The basis of several contrast agents
Why is ABSORPTION important for X-rays?
- The mass attenuation co-efficient is given as the sum of the attenuation coefficients for each of the scattering effects
- Only high energy x-rays are capable of penetrating the body and reaching the film, most are absorbed (1-4% of the primary radiation is transmitted to the detector)
How are x-rays SUPRESSED?
The lower energy x-rays can be filtered by a thin sheet of aluminimum
What is the HALF-VALUE LAYER (HVL)?
The thickness of aluminium needed to cut the beams intensity in half
How does ATTENUATION of the material affect the final image when using X-RAYS?
- an x-ray image is based on the attenuation of the material it passes through
- High attenuation: due to a high density material, i.e., bone appear bright as they absorb more of the x-rays
- Low attenuation: equates to low density matter such as air or fluids, they appear darker
What COMPONENTS make up a SIMPLE x-ray system?
- Source
- Detector
- Screens