1. Khan, The Physics of Radiation Therapy, Ch7
Quality of X-ray Beams
Khan, The Physics of Radiation Therapy, Ch7

2. Outlines What’s the quality of x-ray beam Half-value layer Filter
Effective energy
Megavoltage Beam Energy

3. The quality of the radiation
The penetrating ability of the radiation is often
described as the quality of the radiation
Ideal : spectral spectrum
Practical : HVL (half-value layer)

4. Spectral Distribution of x-rays

5. Half-value layer (HVL)
The thickness of an absorber of specified composition required to attenuate the intensity of the beam to half its original value

6. Half-value layer (HVL)

7. Photon beam attenuation
I(x) = I0e-μx
μ : linear attenuation coefficient (cm-1)
μ / ρ : mass attenuation coefficient (cm2/g)

8. Half-value layer (HVL)
I (x) = I0e-μx
X = HVL , I / I0 = ½
μx = ln ½
X= ln ½ / - μ
= / μ
HVL = / μ

9. Half-value layer (HVL)

10. The quality description of photon beams
X-ray
(1) low energy x-ray (≤ MV)
peak energy or HVL (or both)
(2) high energy x-ray (≥ MV)
peak energy
r-ray
energy or nuclide,
60Co source  1.17 MeV and 1.33 MeV
(average= 1.25 MeV)
 cobalt -60 beam

11. Filters of x-ray beams Diagnosis & superficial x-ray -- 50~150kV
-- 1~6 mm Al filter
Orthovoltage x-ray
-- 150~500 kV
-- combination filter
Megavoltage x-ray
-- no additional filtration is required to improve
the beam quality
-- Flattening filter

12. Schematic graph showing changes in spectral distribution
of 200 kVp x-ray beam with various filters.
Curve A : for Al
Curve B : for Sn + Al (combination filter)
Curve C : for Sn + Cu + Al (combination filter)

13. Inherent filtration of x-ray tube
Including : the glass envelop of the x-ray tube, the surrounding oil, and
the exit window of the tube housing .
This Inherent filtration is equivalent to approximately 1 mm Al
in most x-ray tube.

14. Flattening filter is to make the beam intensity uniform in cross-section .

15. Effective Energy The effective (or equivalent) energy of an x-ray beam
the energy of photons in a monoenergetic beam which is attenuated at the same rate as the radiation in question.

18. Megavoltage Beam Energy
The most practical method is by measuring
percent depth dose (PDD) distribution
tissue air ratio (TAR)
tissue-maximum ratios (TMR)

19. PDD curve

20. THANK YOU