Physics of Diagnostic x- ray
CHAPTER 16
Physics of Diagnostic x- ray
By; Dr.Khalid Ghanim Al ghabsha
In the 1895 ,W.C Roentgen was studying cathode
ray in his lab.
The field of radiology has three major branches;
1.Diagnostic radiology.
2.Radiation therapy.
3. Nuclear medicine.
Production of x- ray Beams;
A high speed electron convert some or all of its
energy into an x- ray photon when it strikes an
atom.
The main components of a modern x-ray unite
are;
1. A source of electrons a filament ,or
cathode.
2. An evacuated space in which to speed up
the electrons.
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Physics of Diagnostic x- ray
3. A high positive potential to accelerate the
negative electrons.
4. A target or anode ,which the electrons
strike to produce x- rays.
In modern x-ray tube the number of electrons
accelerated toward the anode depends on the
temperature of the filament ,and the
maximum energy of the x-ray photons
produced is determined by the accelerating
voltage – kilovolt peak (K VP ).
An X-ray tube operating at 80 K Vp will
produce X- rays with a spectrum of energies
up to a maximum of 80 K eV.
The kilo volt peak used for an x-ray study
depends on;
1. The thickness of the patient.
2. The type of study being done.
X-ray studies of the breast (mammography)
are usually done at 25 to 50 (K VP ) ,while
some hospitals use up to 350 (K VP ) for chest
x-rays.
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The intensity of the x-ray beam produced
when the electrons strike the anode is highly
dependent on the anode material.
In general ,the higher the atomic number (Z)
of the target ,the more efficiently x-rays are
produced.
The target material used should also have a
high melting point.
Nearly all x-ray tubes use tungsten targets.
The Z of tungsten is 74 and its melting point
is about 3400 oC.
Since;
P=IV
Where; P is the power at the target of an x-
ray tube in (watt).
I is in Amperes.
V is in Volt.
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Line - Focus Principle;
A technique is used for increasing the area on the
target struck by electrons to avoid overheating
without increasing the blurring of the x-ray image.
The normal rotation rate of the anode is 3600 rpm
and the heat is spread over a large area as the
anode rotates.
When a short exposure is used ,the anode does not
always make a full rotation at 3600 rpm and thus
its full heat capacity is not utilized.
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Physics of Diagnostic x- ray
For this reason , special high speed anodes that
operate at rates of up to 104 rpm where
developed ,since any rotating object can cause
vibration if it not carefully balanced.
A resonance will occur at exactly the operating
frequency of 104 rpm.
Bremsstrahlung ;
When one of the electrons get close enough to the
nucleus of the target atom to be diverted from its
path and emits an x-ray photon. It is also means
braking radiation.
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How x-rays are absorbed;
The attenuation of an x-ray beam is its reduction
due to the absorption and scattering of some of
the photons out of the beam;
I =IO e-µx
IO is un attenuated beam intensity.
I is Intensity after attenuation.
e is equal to 2.718
µ is linear attenuation coefficient.
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Physics of Diagnostic x- ray
x is thickness of the attenuator.
X-ray are not absorbed equally well by all
materials. Heavy elements such as calcium are
much better absorbers of x-ray than light elements
such as carbon ,oxygen ,and hydrogen ,and as a
result ,structures containing heavy elements ,like
the bones ,stand out clearly.
The soft tissues like fat, muscles ,and tumors all
absorb about equally well and are thus difficult to
distinguish from each other on an x-ray image.
Half Value Layer (HVL) ;
It is the thickness of a given material that will
reduce the beam intensity by one half.
HVL =
0.693
µ
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The mass attenuation coefficient ( µm) is used to
remove the effect of density when comparing
attenuation in several materials.
µ is the linear attenuation coefficient which is
dependent on the energy of the x-ray photons; as
the beam becomes harder, it decreases. Since the
lead is used for shielding materials.
µm = ρ
µ
I = IO e− ( µρ ) ρx
I= IO -µ ρ x
e m
Since;
gm
ρ= cm3
gm
ρx = cm2
Mass attenuation coefficient µm =
cm2
gm
Linear attenuation coefficient µ = cm-1
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Physics of Diagnostic x- ray
X- RAY LOSS ENERGY ;
X –ray lose energy in three ways;
1. Photoelectric Effect;
All of the photon energy is given to the
photoelectrons.
2.Compton Effect ;
Some energy is given to an electron and some
goes into a scattered photon.
3. Pair production ;
A high energy photon is converted into an electron
and a positron (β+). The positron(β+) annihilates
to form two photons of 511 KeV each that go in
opposite direction.
Since, a minimum of 1.02 MeV is necessary for
pair production.
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CONTRAST MEDIA ;
Materials of high (Z) are often injected into
different parts of the body by the radiologists.
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1. Compound containing iodine are often injected
into the blood stream to show the arteries.
2. An oily mist containing iodine is sometimes
sprayed into the lungs to make the airways visible.
3. Barium compounds are given by radiologists
orally to see parts of the upper gastrointestinal
tract (upper GI) and barium enemas to view the
other end of the digestive system (low GI ).
4.It is possible to use air a contrast medium.
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The amount of scattered radiation at the
film ;
The amount of scattered radiation at the film depend on;
1. The energy of the x –rays.
2.The thickness of the tissue,
That the x- ray beam passes through is the most important
factor, so the thicker the tissue, making the greater of the
scatter radiation.
3.The larger of the beam;
When it is greater so, the scatter also greater , and thus
one simple way of reducing radiation is by keeping the x-
ray beam as small as possible.
The most significant way of reducing the amount of
scattered radiation striking the film is by using a grid
consisting of a series of lead and plastic strips.
The strips are aligned so the un scattered x-rays from
the source will go through the plastic strips and strike the
film, while most of the scattered radiation will strike the
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Physics of Diagnostic x- ray
lead strips and be absorbed. Since the lead strips do not
produce visible shadows on the image.
This grid is called a focused grid.
It is necessary to increase the exposure ;
1. To obtain an optimum darkness (optical density) of the
film.
2. A higher exposure must be given because the lead
strips absorb some of the un scattered radiation.
Intensifying screens ;
Most x-ray image are made on a special film
sandwiched tightly between two intensifying screens –
cardboards covered with a thin coating of crystal ( Ca WO4
) that absorb x-rays well, and give off visible or (UV) light
(fluoresce) when struck by x- rays.
The film is coated on both sides with a light- sensitive
emulsion, and each side takes a" picture" of the light
from the intensifying screen with which it is in contact.
Intensifying screens are much more efficient for making x-
ray image than film alone.
The screens are mounted in a cassette with a
compressible felt backing that holds the film and the
screens in close contact. To get a better contact, vacuum
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cassettes have been developed.
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