The X-ray photons produced in the target are first filtered by: Principally, the target material itself.
The glass envelope. The insulating oil.
The window of the tube housing. The light beam diaphragm mirror
Inherent filtration is measured in Aluminum Equivalents = the thickness of Al that would produce the same degree of attenuation as the inherent filtration.
Typically, inherent filtration = 0.5-1 mm Al equivalent. In few cases, unfiltered radiation is desirable
As filtration ↑↑ the mean energy of an X-ray beam → it ↓↓ tissue contrast Ì With lower energy radiation (< 30 kVp) this loss of contrast affects image quality Ì When inherent filtration must be minimized, a tube with a window of beryllium (Z
= 4) instead of glass is used e.g. Mammography.
ADDED "or Additional" Filtration:
Uniform flat sheet of metal, usually Aluminum placed between the X-ray tube & patient Ideal filter material → the one which absorbs all low energy photons & transmit all high
energy photons (such material doesn't exist).
The predominant attenuation process should be photoelectric absorption, which varies inversely as the cube of the photon energy. The filter will therefore attenuate the lower- energy photons much more than it does the higher-energy photons.
The total filtration is the sum of the added filtration and the inherent filtration. For general diagnostic radiology it should be at least 2.5 mm Al equivalent.
(This will produce a beam with effective energy of HVL = 2.5 mm Al at 70 kV, and 4.0 mm at 120 kV.)
CHOICE OF FILTER MATERIAL
The Atomic Number should be sufficiently high to make the energy-dependent attenuating process, photoelectric absorption, predominate.
It should not be too high, since the whole of the useful X-ray spectrum should lie on the high-energy side of the absorption edge. If not, the filter might soften the beam.
Aluminum (Z= 13) is generally used: has sufficiently high atomic number to be suitable for low energy radiation & most diagnostic X-ray beams (general purpose filter). With the higher kV values, Copper (Z = 29) is used, being a more efficient filter.
Disadv.: Copper filters can't be used alone because photoelectric interaction with the copper emits 9 keV characteristic X-rays → if reaches patient skin, will increase the skin dose → must be absorbed by a 'backing filter' of aluminum on the patient side of the 'compound filter'.
Molybdenum or Palladium filters have absorption edges (20 or 24 keV, respectively) favorable for mammography.
Erbium (58 keV) has been used at moderate kV values, called 'K-edge filter'. FILTER THICKNESS:
The total filtration for diagnostic radiology as recommended by The national council of radiation protection and measurements:
kVp Total filtration Below 50 kVp 50 – 70 kVp Above 70 kVp 0.5 mm Al 1.5 mm Al 2.5 mm Al Increased filtration has definite disadvantage;
Excessive filtration → absorption of high energy photons → the quality of the beam is not altered significantly but the intensity is greatly diminished → needs ↑ exposure time which may ↑ movement blurring.
EFFECTS OF FILTRATION
Figure 1.18 shows the spectrum of X-rays generated at 60 kV after passing through 1, 2, and 3 mm aluminum.
Filters attenuates lower-energy X-rays more in proportion than higher-energy X-rays → ↑↑ the penetrating power (HVL) of the beam but ↓↓ intensity
It is responsible for the low-energy cut-off of the X-ray spectrum. Increasing the filtration has the following effects:
It causes the continuous X-ray spectrum to shrink and move to the right, Fig. 1.18. 1. It selectively reduces the total number of photons "the area of the spectrum" and the
total output of X-rays → removes much more low energy photons than high energy. 2. ↑↑ Minimum & Effective photon energies but not affect maximum photon energy
3. ↑↑ the exit dose/entry dose ratio, or film dose/skin dose ratio. COMPENSATING OR WEDGE FILLER
A wedge-shaped filter may be attached to the tube to make the exposure across the film more uniform and compensate for the large difference in transmission, for example, between the upper and lower thorax, neck and shoulder, or foot and ankle.
1.7 PROPERTIES OF X-AND GAMMA RAYS
The excitations and ionizations produced by the secondary electrons which account for the various properties of X- and gamma rays:
1. The ionization of air and other gases → makes them electrically conducting: used in the measurement of X- and gamma rays.
2. The ionization of atoms in the constituents of living cells cause biological damage & the hazards of radiation exposure.
3. The excitation of atoms of certain materials (phosphors) → makes them emit light (luminescence, scintillation, or fluorescence): used in the measurement of X- and gamma rays and as a basis of radiological imaging.
4. The effect on the atoms of silver and bromine in a photographic film → leads to blackening (photographic effect): used in the measurement of X- and gamma rays and as a basis of radiography.
LUMINESCENCE
1. When a phosphor absorbs X-rays, the secondary electrons set in motion raise valence electrons to a higher energy level.
2. The electrons stay in energy 'traps' and the absorbed energy is stored in the phosphor until the electrons return to the valence shells, with the emission of photons of light. 1. This may happen spontaneously, either:
9 Instantaneously → fluorescence
9 After a noticeable interval of time → phosphorescence.
The latter is called afterglow or lag, and is generally to be avoided in imaging. OR
2. The emission of the light may require stimulation: 9 By heat → thermoluminescence.
9 By intense light from a laser → photostimulation.
Other ionizing radiations
Some ultraviolet radiation has a sufficiently high photon energy to ionize air.
Beta particles, emitted by many radioactive substances and other moving electrons (in a television monitor, for example) also possess the above properties.
Alpha rays (helium nuclei 4He), (which are particularly stable combinations of two neutrons and two protons) are also emitted by some radioactive substances.
Both alpha and beta rays are charged particles and are directly ionizing.
X- and gamma rays are indirectly ionizing, through their secondary electrons; the 'secondary' ions produced along the track of a secondary electron being many times more than the single 'primary' ionization caused by the initial Compton or photoelectric interaction.
1.8 ABSORBED DOSE
The SI unit of absorbed dose is GRAY (Gy); 1 Gy = 1 J \ kg.
The absorbed dose is the energy absorbed as ionization or excitation per unit mass of the material irradiated (in joules per kilogram).
Dose rate is measured in Grays per Second.
The concept of absorbed dose applies to all kinds of direct and indirect ionizing radiations and to any material.
Before 1980 the international unit of absorbed dose was the RAD, 1 Gy = 100 rad & 1 rad = 1 cGy = 10 mGy. KERMA
Kerma is the kinetic energy (of the secondary electrons) released per unit mass of irradiated material.
Absorbed Dose is energy deposited (as ionization & excitation) by those 2ry electrons Kerma is measured in GRAY & is synonymous with absorbed dose.
In most radiodiagnostic situations they are equal and can be used interchangeably. MEASUREMENT OF X- AND GAMMA RAY DOSE
It is extremely difficult to measure dose in solids or liquids directly.
First we measure the dose delivered to air ('air kerma') under same conditions & then multiply it by a conversion factor to obtain the dose in the material.
The conversion factor:
Dose in stated material Dose in air
∴Depends on the relative amounts of energy absorbed in air and the material. Like the mass absorption coefficients, the factor depends on:
1. The effective atomic number of the material. 2. The effective energy of the X- or gamma rays.
For X-rays used in radiology, approximate values of the conversion factors are: 1. For muscle,
Muscle atomic number nearly equal that of air & Compton process predominates ∴ The ratio is close to unity and only varies between 1.0 - 1.1 over the kilovoltage range 2. For compact bone,
With higher atomic number and Photoelectric absorption is important. The ratio varies from about 4.5 at low keV energies to 1.2 at high keV. 3. For fat,
With lower atomic number → ∴ the ratio varies in the opposite direction, from about 0.6 at low keV energies to 1.1 at high keV energies.
4. For the soft tissue elements in cavities within bone