Basics of x-rays ,x ray
production and uses
What are x rays
• X-rays are electromagnetic waves, they are more energetic so they can
penetrate many materials to varying degrees.
• When the X-rays hit the film, they expose it just as light would. Since
bone, fat, muscle, tumors and other masses all absorb X-rays at
different levels, the image on the film lets you see different (distinct)
structures inside the body because of the different levels of exposure
on the film
• X-rays have a wavelength in the range of 0.01 to 10
nanometers, corresponding to frequencies in the range 30
petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and
energies in the range 100 eV to 100 keV.
• The wavelengths are shorter than those of UV rays and longer
than those of gamma rays
Electromagnetic spectrum
• X-rays are electromagnetic radiation of exactly the same
nature as light but of very much shorter wavelength
• Travel at the speed of light
Wilhelm Conrad Roentgen (1845-1923)
X-rays were first discovered
in 1895 by the German physicist
William Roentgen, when using a
Crookes tube
He called them ‘x’ rays, ‘x’ for
‘unknown’.
Three things can happen
• X-rays can:
• Pass all the way through the body
• Be deflected or scattered
• Be absorbed
Where on this image
have x-rays passed
through the body
to the greatest degree?
X-rays Passing Through Tissue
• Depends on the energy of the x-ray and the atomic
number of the tissue
• Higher energy x-ray - more likely to pass through
• Higher atomic number - more likely to absorb the x-ray
Diagnosis?
Tissue densities
• A radiographic image is composed of a 'map' of X-rays that have either passed
freely through the body or have been variably attenuated (absorbed or
scattered) by anatomical structures.
• The denser the tissue, the more X-rays are attenuated. For example, X-rays are
attenuated more by bone than by lung tissue.
Describing densities
• Contrast within the overall image depends on differences in both the
density of structures in the body and the thickness of those
structures.
• The greater the difference in either density or thickness of two
adjacent structures leads to greater contrast between those
structures within the image.
5 Basic Radiographic Densities
.
• Air
• Fat
• Soft tissue/fluid
• Mineral
• Metal
3.
Name these radiographic densities.
How do x-rays passing through the body create an
image?
• X-rays that pass through the body to the film render the film dark
(black)
• X-rays that are totally blocked do not reach the film and render the
film light (white)
• Air = low atomic # = x-rays get through = image is dark
• Metal = high atomic # = x-rays blocked = image is light (white)
Basics of X-ray Physics
X-ray production
Key points
• X-rays are produced within the X-ray machine, also known as an X-ray
tube. No external radioactive material is involved
• X-rays are produced by interaction of accelerated electrons with
tungsten nuclei within the tube anode
• Two types of radiation are generated: characteristic radiation and
bremsstrahlung (braking) radiation
• Changing the X-ray machine current or voltage settings alters the
properties of the X-ray beam
X ray tube
• A small increase in the filament voltage (1) results in a large increase in
tube current (2), which accelerates high speed electrons from the very
high temperature filament negative cathode (3) within a vacuum,
towards a positive tungsten target anode (4). This anode rotates to
dissipate heat generated. X-rays are generated within the tungsten
anode and an X-ray beam (5) is directed towards the patient.
Characteristic X-ray
generation
When a high energy electron (1) collides with an inner shell electron (2)
both are ejected from the tungsten atom leaving a 'hole' in the inner
layer. This is filled by an outer shell electron (3) with a loss of energy
emitted as an X-ray photon (4)
Bremsstrahlung/Braking X-ray
generation
• When an electron passes near the nucleus it is slowed and its path is deflected.
Energy lost is emitted as a bremsstrahlung X-ray photon.
• Bremsstrahlung = Braking radiation
• Approximately 80% of the population of X-rays within the X-ray beam consists of
X-rays generated in this way.
X ray production
CATHODE --------
MADE OF TUNGSTEN + 1%-3% THORIUM ( better emission of electrons. )
Tungsten
1.
2.
3.
4.
5.
Thin wire
Strong
High melting point
Less tendency to vaporize
Long life expectancy
Z # 74
MELTING POINT- 3,370 DEG. CELSIUS
Thermionic emission
Emission of electrons resulting from the absorption of
thermal energy – thermionic emission
(Tungeston heated >22000C)
Electron cloud surrounding the filament produced by
thermionic emission is termed “Edison effect”
Anode +++ parts
1. Anode disk –tungsten
•3600rpm
•Beveled edge – line focus
•Target area increased but effective focal size
remains the same.
2.
3.
4.
5.
Stator
Rotor
Bearings - metallic lubricants (silver )
Stem - molybdenum
90%tungsten W and 10 % rhenium Re- ↑resistance to surface
roughening - ↑thermal capacity
Key points
• X-rays are produced de novo as needed
• No radioactive substance is used
• X-rays travel in straight lines and are attenuated according to density
and thickness of body tissues
• X-rays are potentially hazardous
• Hospital staff have a duty to use X-rays responsibly
Uses
• Medicine: X-rays are used in medicine for medical analysis. Dentists use
them to find complications, cavities and impacted teeth. Soft body tissue
are transparent to the waves. Bones also block the rays.
• X rays are useful in the detection of pathology of skeletal system as well as
for detecting some disease processes in soft tissue
• Xray may also be used to detect pathology such as gall stones or kidney
stones
• Use of xrays as treatment –radiation therapy for management of cancer
Radiation hazards
INTRODUCTION
• Radiation is a double edged tool-analogous to fire
• Benefit and hazards
• Hazards were realized in the beginning of 20th century
BIOLOGIAL EFFECTS
• Radiation deposits energy in tissues randomly and rapidly via excitation, ionization
& thermal heating
 Production of moving electrons
• Electrons interaction with atoms & molecules
• Chemical & molecular changes
BIOLOGIAL EFFECTS
Biologic effects:
(i).Deterministic effect
(ii).Stochastic effects
(a).Somatic Effects
(b).Genetic effects
DETERMINISTIC EFFECTS
• A deterministic effect is one “which increases in severity with increasing
absorbed dose in affected individuals”
• Appear at higher doses (>0.5 Gy)
• Soon after the dose is received
DETERMINISTIC EFFECTS
• Skin erythema,
• epillation,
• organ atrophy,
• fibrosis,
• cataract,
• blood changes,
• reduction in sperm count etc.,
STOCHASTIC EFFECT
• A stochastic effect is one in which “the probability of occurrence increases
with increasing absorbed dose rather than its severity”
• Important at very low levels:<0.5Gy
• Chance of occurrence increases with dose
STOCHASTIC EFFECT
• Seen only at a later period (10-30 years)
• Independent of sex and age
• E.g., carcinogenesis and genetic effects
SAFETY OBJECTIVE
• Prevent the occurrence of deterministic effects
• Minimize the occurrence of stochastic effects
• Keep exposures as low as reasonably achievable (ALARA)
Personnel protection in
Diagnostic Radiology
LEAD APRON
LEAD APRON
• Lead equivalent thickness :0.25-0.5 mm
• Rubber material /flexibility and handling
• Protect the torso of the body
• 0.25 mm: >90% scattered radiation is attenuated
• 0.5 mm: 95-99% attenuation
OTHER SHIELDS
• Thyroid shield: wrap around the neck
• Protective gloves:0.5 mm
thick lead
Personnel monitoring
INTRODUCTION
• Radiation exposure must be monitored for both safety &
regulatory purposes
• This assessment need to be made over a period of several
months (3 months)
• Personnel Monitoring
• Area monitoring
PERSONNEL MONITORING
• Three types of radiation recording devices
1. Film badge
2. Thermo luminescent dosimeter (TLD)
3. Pocket dosimeter
AIM
• Monitor & control individual doses regularly
• Report & investigate over exposures
• Maintain life time cumulative dose records of the users of the
service