1. Resident Physics Lectures (Year 1)
Christensen, Chapter 5
Attenuation
George David
Associate Professor
Medical College of Georgia
Department of Radiology

2. Diagnostic X-Ray Beam Characteristics
Characteristic Radiation
Discrete energies
Characteristic of target material
Bremsstrahlung
Energy range 0 – kVp selected

3. Beam Characteristics
Quantity
# photons in beam
1, 2, 3, ... ~ ~ ~ ~ ~

4. Beam Characteristics 1 @ 27 keV, 2 @ 32 keV, 2 at 39 keV, ... ~ ~ ~ ~
Quality
energy distribution of photons in beam
27 keV, 32 keV, 2 at 39 keV, ... ~ ~ ~ ~

5. Beam Characteristics ~ ~ ~ ~ ~ ~ ~ ~ Intensity
324 mR
Intensity
weighted combination of photon # & energy
depends on beam
Quantity
Quality
Beam intensity can be quantified by measuring ionizations
Meter reading in milliRoentgens (mR) ~ ~ ~ ~ ~ ~ ~ ~

6. Roentgen?
Unit of measurement for amount of ionizing radiation that produces 2.58 x 10-4 Coulomb/kg of STP
1 C ~ ×1018 electrons
1901

7. Beam Intensity
Can be measured in terms of # of ions created in air by beam
Valid for both mono-energetic & poly-energetic beams
324 mR

8. Mono-energetic Radiation
Radioisotope
Not x-ray beam
all photons in beam have same energy
attenuation results in
Change in beam quantity
no change in beam quality
# of photons & total energy of beam changes by same fraction

9. Mono-energetic radiation beam
Attenuation Formula
N = No e -mx where No = # incident photons OR # photons penetrating absorber of 0 thickness N = # transmitted photons e = base of natural logarithm (2.718…) m = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm) No N x
Mono-energetic radiation beam

10. Mon-energetic radiation beam
If x=0 (no absorber)
N = No e -mx where No = number of incident photons N = number of transmitted photons e = base of natural logarithm (2.718…) m = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm) No N
X=0
Mon-energetic radiation beam

11. Mon-energetic Radiation
Let’s graph the attenuation of a monochromatic x-ray beam vs. attenuator thickness
60% removed
40% remain
Mono-energetic radiation beam

12. Mono-energetic Radiation
Yields straight line on semi-log graph 1 .1
.01
.001
Fraction
(also fraction of energy)
Remaining or Transmitted
N / No = e -mx 1 2 3 4 5
Attenuator Thickness
Mono-energetic radiation beam

13. Poly-energetic Radiation
X-Ray beam contains spectrum of photon energies
highest energy = peak kilovoltage applied to tube
mean energy 1/3 - 1/2 of peak
depends on filtration

14. X-Ray Beam Attenuation
reduction in beam intensity by
absorption (photoelectric)
deflection (scattering)
Attenuation alters beam
quantity
quality
higher fraction of low energy photons removed
Beam Hardening
Higher
Energy
Lower
Energy

15. Mono-energetic radiation beam
Half Value Layer (HVL)
Absorber thickness that reduces beam intensity by exactly 1/2
Units of thickness
value of x which makes N equal to No / 2
N = No e -mx
X1/2 = HVL = .693 / m
N/No= 0.5 = e -mx
Mono-energetic radiation beam

16. Half Value Layer (HVL) Indication of beam quality
Valid for all beam types
Mono-energetic
Poly-energetic
Higher HVL means
more penetrating beam
lower attenuation coefficient

17. Factors Affecting Attenuation
Energy of radiation / beam quality
higher energy
more penetration
less attenuation
Matter
density
atomic number
electrons per gram
higher density, atomic number, or electrons per gram increases attenuation

18. Poly-energetic Attenuation
Curved line on semi-log graph
line straightens with increasing attenuation
slope approaches that of monochromatic beam at peak energy
Mean energy increases with attenuation
beam hardening 1 .1
Poly-energetic
Fraction
Transmitted
.01
Mono-energetic
.001
Attenuator Thickness

19. Applications As photon energy increases
subject (and image) contrast decreases
differential absorption decreases
at 20 keV bone’s linear attenuation coefficient 6 X water’s
at 100 keV bone’s linear attenuation coefficient 1.4 X water’s

20. Scatter Radiation NO benefit 50-90% of photons exiting patient
no useful information on image
detracts from film quality
exposes personnel, public
50-90% of photons exiting patient

21. Scatter Factors An increase in any of above increases scatter.
Factors affecting scatter
field size
thickness of body part
kVp
An increase in any of above increases scatter.

22. Scatter & Field Size
Reducing field size causes significant reduction in scatter radiation II
Tube
X-Ray II
Tube
X-Ray
One of the most effective ways of minimizing operator exposure is to reduce field size through collimation. Even a relatively small reduction in field size can often result in a substantial reduction in operator exposure. This occurs for two reasons. The first is that a smaller beam irradiates a less volume of tissue so that there is less tissue to act as a scatter radiation source. Secondly reducing beam size means that scatter radiation must travel further through the patient before exiting. The increased travel distance means a less intense scatter field for the operator. A fluoroscopist should always collimate the x-ray beam to a size no larger than is required clinically.

23. Field Size & Scatter
Field Size & thickness determine volume of irradiated tissue
Scatter increase with increasing field size
initially large increase in scatter with increasing field size
saturation reached (at ~ 12 X 12 inch field)
further field size increase does not increase scatter reaching film
scatter shielded within patient

24. Thickness & Scatter
Increasing patient thickness leads to increased scatter but
saturation point reached
scatter photons produced far from film
shielded within body

25. Scatter Management Reduce scatter by minimizing field size thickness
within limits of exam
thickness
mammography compression

26. Scatter Control Techniques: Grid
Directional filter for photons
Increases patient dose

27. Grid Structure

28. Purpose Directional filter for photons Ideal grid
passes all primary photons
photons coming from focal spot
blocks all secondary photons
photons not coming from focal spot
Focal
Spot
“Good” photon
Patient
“Bad” photon X
Grid
Film

29. Scatter Control Techniques: Air Gap
Gap intentionally left between patient & image receptor
Grid not used
Grid
Air Gap
Patient
Patient
Air Gap
Grid
Image Receptor