ATTENUATION

Dr. ARCHANA KOSHY

The reduction in the intensity of an Xray beam as it traverses matter either by absorption or deflection of photons from the beam .

The quality of monochromatic radiation does not change as it passes through the absorber .

Mainly deals with low energy photos ( 20-80 kev) ,primary photons that have only one interaction .

A 50% reduction in the number of photos is a 50% reduction in the intensity of the beam .

When the number of transmitted photons and absorber thickness are plotted on a linear graph paper , it results in a curved line .

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MONOCHROMATIC RADIATION

EXPONENTIAL ATTENUATION: When the number of photons in the beam decrease by the same percentage with each increment of the absorber. As seen in monochromatic radiation .

-Plots a straight line on a semi-logarithmic graph .

Measure of the quantity of radiation attenuated by a given thickness of an absorber .

Name is determined by the units used to measure the thickness of the absorber .

(i) LINEAR ATTENUATION COEFFICENT

(ii) MASS ATTENUATION COEFFICIENT

ATTENUATION COEFFICIENTS

Most important for diagnostic radiology .

Quantitative measurement of attenuation per centimeter of the absorber.

Is for monochromatic radiation and is specific for both the energy of the xray beam and the type of absorber .

When the energy of the radiation is increased,the number of Xrays that are attenuated decreases , and so does the linear attenuation co efficient

Linear attenuation coefficient (µ)

Absorber thickness required to reduce the intensity of the original beam by one half .

HVL= 0.693/µ

Common method for expressing the quality of an Xray beam .

A beam with a high HVL is a more penetrating beam than one with a low HVL .

HVL of a typical diagnostic beam is : -30 mm : Tissue -12 mm: Bone -0.15 mm : Lead

HALF VALUE LAYER (HVL)

Quantitates the attenuation of materials independent of their physical state .

Obtained by dividing the linear attenuation coefficient by the density .

MASS ATTENUATION COEFFICIENT : µ/Þ Unit – g/cm²

Mass attenuation coefficient is independent of the density of the absorber

MASS ATTENUATION COEFFICIENT

(I) Energy of the radiation

(II) Density

(III) Atomic number

(IV) Electrons per gram

FACTORS AFFECTING ATTENUATION

Elements with higher atomic number are denser than elements with lower atomic numbers

Few Exceptions : Gold( Z=79 , Þ= 19.3) Lead (Z=82 , Þ= 11.0)

No relationship between atomic number and density when different physical states of matter are involved .

-Water – Effective atomic number – 7.4 (in all three forms of Ice,liquid ,vapour despite its varying density.)

DENSITY AND ATOMIC NUMBER

Density and electrons per gram Density depends on volume ( weight/Volume ) , hence there is no

relationship between density and electrons per gram .

A gram of water has the same number of electrons , irrespective of whether they are compressed together in a 1 cm cube as a liquid or spread out as vapour .

Atomic number and electrons per gram Electrons per gram is a function of the neutrons in an atom .

If there are no neutrons, there will be 6.0 x 10²³ electrons

Hydrogen has twice as many electrons as any other element .

Elements with low atomic numbers have more e/g than those with higher atomic number .

Determines the percentage of each type of basic interaction .

With extremely low energy radiation (20keV) , PHOTOELECTRIC ATTENUATION predominates irrespective of the atomic number of the absorber .

Attenuation is always greater when the photoelectric effect predominates .

As the radiation energy increases , Compton scattering predominates until eventually it replaces photoelectric reaction .

EFFECTS OF ENERGY AND ATOMIC NUMBER

The linear attenuation co efficient is the the collective sum of the contributions from COHERENT,COMPTON and PHOTOELECTRIC reactions .

For water, the Mean attenuation and the linear coefficient are the same because the density of water is 1 g/cm³

Energy has a direct effect on attenuation .

The percentage of transmitted photons increase as the energy of the beam increases .

Binding energy of the K shell electron .

As radiation energy increases, xray transmission increases with decreased attenuation .

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But as a paradox , with higher high atomic number absorbers, transmission may decrease .

Abrupt change in the likelihood of a photoelectric reaction as the radiation energy reaches the binding energy of an inner shell electron .

K – EDGE

ENERGY (keV) TRANSMISSION (%)

50 0.01660 0.4080 6.888

12.0

88 0.026100 0.14150 0.96

When maximum Xray absorption is desired, the K-edge of an absorber should be closely matched to the energy of the Xray beam .

For low energy radiation(30-35 kVp) , xeroradiography emoloys Selenium ( K edge 12.7 ) as the Xray absorber

High energy radiation ,Tungsten with a K edge of 59.5keV is a much better absorber .

Tissue density is one of the most important factors in Xray attenuation .

If the density of a material is doubled, attenuation doubles .

Effects of density

From a clinical radiology point of view, Density determines the number of electrons that will be present in a given thickness and this determines the X-ray attenuation .

The number of e/g can be calculated by No = NZ/A

No=Number of electrons per

N=Avogadro’s number(6.02 x 10²³) Z=Atomic number A=Atomic weight

Effects of electrons /grams

More complex than the attenuation of monochromatic radiation .

Contains a whole spectrum of photons of varying energies .

In general , the mean energy of polychromatic radiation is between one thirs and one half of its peak energy ,

POLYCHROMATIC RADIATION

As polychromatic radiation passes through the absorber

Transmitted photons undergo a change in both quantity and quality

Number of photons decrease because some are deflected and absorbed out of the beam .

Quality also changes because the lower energy photons are readily attenuated than the higher energy photons .

The photons in an xray beam enter a patient with uniform distribution and emerge in a specific pattern of distribution .

Transmitted and attenuated photons are equally important .

Image formation depends on a differential attenuation between tissues .

APPLICATIONS OF DIAGNOSTIC RADIOLOGY

The primary radiation passes through the patient unchanged or is completely removed from the useful beam .

Scatter radiation detracts from film quality and contributes from film quality .

With thick parts such as the abdomen , only 1% of the photons in the initial beam reach the film .

The rest are attenuated, the majority by compton scattering .

SCATTER RADIATION

(1) Kilovoltage

(2) Part thickness

(3) Field size

FACTORS AFFECTING SCATTERING RADIATION

FIELD SIZE Most important factor in the production of scatter radiation .

A narrow beam irradiates only a small volume of tissue , so it generates only a small number of scattered photons .

Most of them miss the film because they have a large angle of escape .

As the Xray field is enlarged , the quantity of scatter radiation increases rapidly at first and then gradually tapers off until it reaches a plateau .

PART THICKNESS

-The total number of photons keeps increasing as the part becomes thicker , but photons originating in the upper layers of the patient do not have sufficient energy to reach the film .

KILOVOLTAGE

• In the low energy range –extremely little scatter radiation is produced .

• As the radiation energy increases-so does the scatter radiation .

• Unlike field size and part thickness, the plateau is not as well defined

• This is due to the increasing beam energy that causes more photons scatter in the forward direction allowing them to penetrate greater thicknesses of tissue to reach the film

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