1. Chp 2 : Interaction of radiation with matter Radiation Contrast
DIAGNOSTIC RADIOLOGY
Chp 2 : Interaction of radiation with matter Radiation Contrast
Aim: To become familiar with the basic knowledge of radiation contrast obtained by Intraction of X-ray with tissues.
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2. Contents Primary and secondary ionization
Photo-electric effect and Compton scattering
Beam attenuation and Half value thickness
Principle of radiological image formation

3. Topic 1 : Primary and secondary ionization
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Learning objectives: Upon completion of this lesson, the students will be able to: …
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4. Stopping power Loss of energy along track through collisions
The linear stopping power of the medium
S = E / x [MeV.cm-1]
E: energy loss
x: element of track
for distant collisions : the lower the electron energy, the higher the amount transferred
most Bremsstrahlung photons are of low energy
collisions (hence ionization) are the main source of energy loss
except at high energies or in media of high Z
Stopping power of an absorber is its ability to remove energy from a beam of charged particles. Stopping power is measured as the average energy lost by a charged particle per unit distanced travelled (MeV/cm).

5. Linear Energy Transfer
Biological effectiveness of ionizing radiation
Linear Energy Transfer (LET): amount of energy transferred to the medium per unit of track length of the particle
Unit : e.g. [keV.m-1]
Another measure of energy deposited in an absorber by a charged particle is the Linear Energy Transfer (LET). The LET is the average energy locally deposited in an absorber resulting from a charged particle per unit distance of travel (keV/cm). The LET is therefore a measure of the local concentration of energy per path length resulting from ionization effects. Biological damage from radiation results from ionization; therefore, the LET is used for calculating quality factors in the calculation of dose equivalent.
Stopping power and LET may have the same units but are not equal because, although ionization may occur and removes energy from the beam, not all of that energy gets deposited locally and so does not contribute to LET. In other
words, LET stopping power because some electron ions may interact via Bremsstrahlung or excitation and the resulting photons escape the local area. Materials having higher stopping power values cause the particle to lose its energy over shorter distances.

6. Topic 2 : Photo-electric effect and Compton scattering
Part …: (Add part number and title)
Module…: (Add module number and title)
Lesson …: (Add session number and title)
Learning objectives: Upon completion of this lesson, the students will be able to: …
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7. X-ray interaction with matter
Coherent Scattering
Photoelectric Effect
Compton Scattering
Pair Production
Photodisintegration .

8. Coherent Scattering (الاستيك)
در اين نوع برخورد ، مسير تشعشع عوض مي شود ولي طول موج و انرژي تغيير نمي كند.
دو نوع برخورد الاستيك وجود دارد:
Thomson برخورد با يك الكترون صورت مي پذيرد.
Raleigh برخورد با همه الكترونهاي يك اتم صورت مي پذيرد

9. Photoelectric Effect:
در اين نوع برخورد، اغلب انرژي فوتون صرف جدا كردن الكترون، از لايه ها شده و بقيه انرژي بصورت انرژي جنبشي الكترون آزاد شده بكار مي رود تا الكترون به خارج پرتاب شود.
برخود فتوالكترويك داراي سه محصول مي باشد:
1- Negative ion( فتوالكترون )
2- Positive ion( اتم بدون الكترون )
3- Charactristic Radiation ( X-ray )
( اين نوع برخورد متناسب با مي باشد).
بهمين دليل بيشترين احتمال فتوالكتريك در لايه K اتفاق مي افتد. همچنين بيشترين احتمال فتوالكتريك زماني است كه انرژي فوتون مساوي يا كمي بزرگتر از انرژي اتصال لايه باشد و با افزايش انرژي احتمالش كاهش مي يابد

10. Photoelectric effect Incident photon with energy h
Absorption:  all photon energy absorbed by a tightly bound orbital electron
 ejection of electron from the atom
Kinetic energy of ejected electron : E = h - EB
Condition : h > EB (electron binding energy)
Recoil of the residual atom
Attenuation (or interaction) coefficient
 photoelectric absorption coefficient

11. Factors influencing photoelectric effect
Photon energy (h) > electron binding energy EB
The probability of interaction decreases as h increases
It is the main effect at low photon energies
The probability of interaction increases with Z3 (Z: atomic number)
High-Z materials are strong X-ray absorber

12. Es > 80% (h) if h <1 keV
Compton scattering
Interaction between photon and electron
h = Ea + Es (energy is conserved)
Ea: energy transferred to the atom
Es : energy of the scattered photon
momentum is conserved in angular distributions
At low energy, most of initial energy is scattered
Es > 80% (h) if h <1 keV
Increasing Z  increasing probability of interaction. Compton is practically independent of Z in diagnostic range
The probability of interaction decreases as h increases
photon energy is transferred to the electron, the remainder to the scattered photon

13. Compton scattering and tissue density
Variation of Compton effect according to energy:
lower E  Compton scattering process  1/E
Increasing E  decreasing photon deviation angle (Scatter angle)
Mass attenuation coefficient  constant with Z
Compton effect is proportional to the electron density in the medium

14. (Linear Attenuation-Coefficient) ضريب كاهش خطي
ضريب كاهش خطي (m) به معني احتمال كاهش فوتون در يك سانتي متر ضخــامت يك ماده مي باشـد.
در حـد انرژي تشخيصـي m با افزايش انرژي ، كاهش پيدا مي كنـــد ( به استثناي k-edge )
m براي بافت نرم cm تا cm-1 است.

15. ضريب كاهش جرمـي
m بستگي به density مواد نيز دارد ، لذا جهت رفع اين وابستگي، ضريب كاهش جرمـي را تعريف مي كنند.
واحد ضريب جذب جرمي بدينصورت است :
با افزايش ضخامت ديگر m خطي نيست و رابطه نما ئي است :

16. Topic 3 : Beam attenuation and Half value thickness
Part …: (Add part number and title)
Module…: (Add module number and title)
Lesson …: (Add session number and title)
Learning objectives: Upon completion of this lesson, the students will be able to: …
. (Add a list of what the students are expected to learn or be able to do upon completion of the session)
Activity: (Add the method used for presenting or conducting the lesson – lecture, demonstration, exercise, laboratory exercise, case study, simulation, etc.)
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17. Exponential attenuation law of photons (I)
Any interaction  change in photon energy and or direction
Accounts for all effects : Compton, photoelectric,…
dI/I = -  dx
Ix = I0 exp (- x)
I : number of photons per unit area per second
 : the linear attenuation coefficient
 / [m2.kg-1] : mass attenuation coefficient
 [kg.m-3] : material density

18. Attenuation coefficients
Linear attenuation depends on :
characteristics of the medium (& density )
photon beam energy
Mass attenuation coefficient :  / [m2kg-1]
 / same for water and water vapor (different )
 / similar for air and water (different µ)

19. Attenuation of an heterogeneous beam
Various energies  No more exponential attenuation
Progressive elimination of photons through the matter
Lower energies preferentially
This effect is used in the design of filters
 Beam hardening effect

20. Half Value Layer (HVL) HVL: thickness reducing beam intensity by 50%
Definition holds strictly for monoenergetic beams
Heterogeneous beam  hardening effect
I/I0 = 1/2 = exp (-µ HVL) HVL = / µ
HVL depends on material and photon energy
HVL characterizes beam quality
 modification of beam quality through filtration
 HVL (filtered beam)  HVL (beam before filter)

21. (HVL) Half Value Layer:
HVL اندازه گيري (معيار) غير مستقيم انرژي فوتون يا كيفيت تشعشع مي باشد. HVL بمعني ضخـامت ماده مورد نيـــاز جهت كــاهش شدت اشعـــه به نصف مقدار اوليه است.
Homogeneity Coefficient:
از آنجائيكه تشعشع بر مشترالانگ تك انرژي نيست ، مقدار تشعشع كاهش يافته در ضخامت هاي اوليه مثلاً اولين HVL سريعتر از لايه هاي دوم و سوم خواهد بود ولي تشعشع سخت تر مي شود نسبت HVL اول به دوم ضريب يكنواختي نام دارد و پراكندگي انرژي تشعشع را نشان مي دهد

23. Photon interactions with matter
Scattered photon
Compton effect
Secondary
photons
Fluorescence photon
(Characteristic radiation)
Annihilation photon
Incident
photons
Non interacting photons
Recoil electron
Secondary
electrons
Photoelectron
(Photoelectric effect)
Electron pair
E > 1.02 MeV
(simplified representation)

24. Dependence on Z and photon energy
Z < 10  predominating Compton effect
Higher Z: increase photoelectric effect
Low E: photoelectric effect predominates in bone compared to soft tissue
high Z (Barium 56, Iodine 53)
photoelectric absorption  Best contrast
use of photoelectric absorption in radiation protection. ex : lead (Z = 82) for photons (E > 0.5 MeV)

25. Radiation Exposure :
- مقدار شارژ حاصل از تشعشع يونيزاسون در جرم هوا را Exposure گويند. براساس كولن در كيلوگرم c/kg بيان مي شود.
- واحد قديمـي آن Roentgen مي باشد. يك رونتگن ، مقدار اشعه x يا گامائي است كه تا ايجاد 2.58*10-4 كولن بار در يك كيلوگرم هوا بكند.
- براي تشعشع در زمان محدود مثل راديوگرافي 1mR ايجاد دانسيته فتوگرافي (Photographic Density) حدود 1.0 مي نمايد.

26. Radiation Exposure :
- در تصويربـــرداري پيوسته مثل فلورئوسكپي، اطلاعــات به بيننـده براساس زمان ثبت اطلاعات (Storage Time) مغز انسـان ( زماني كه اطلاعات ثبت شده باقي مي ماند ) دارد كه حدود100 Ms است.
در فلورئوسكپي اطلاعات براساس Exposure Rate (mR/S) است.
لذا براي مقدار معمول Exposure rate در فلوئورسكپي و براساس Storage Time مقدار اکسبوژر عبارت است:
- شــــدت تشعشـع خروجــــي دستگــــاهX-ray همچنين بصورت(mR/ MAS) بيان مي شود.

27. Summary
Electrons and photons have different types of interactions with matter
Two different forms of x-rays, Bremsstrahlung and characteristic radiation contribute to the image formation process.
Photoelectric and Compton effects have a significant influence on the image quality.
Let’s summarize the main subjects we did cover in this session. (List the main subjects covered and stress again the important features of the session)

28. Chp 2 : x-Ray Image Formation and Radiation Contrast
DIAGNOSTIC RADIOLOGY
Chp 2 : x-Ray Image Formation and Radiation Contrast
Aim: To become familiar with the basic knowledge of image formation and radiation contrast
Part …: (Add part number and title)
Module…: (Add module number and title)
Lesson …: (Add session number and title)
Learning objectives: Upon completion of this lesson, the students will be able to: …
. (Add a list of what the students are expected to learn or be able to do upon completion of the session)
Activity: (Add the method used for presenting or conducting the lesson – lecture, demonstration, exercise, laboratory exercise, case study, simulation, etc.)
Duration: (Add presentation time or duration of the session – hrs)
Materials and equipment needed: (List materials and equipment needed to conduct the session, if appropriate)
References: (List the references for the session)

29. Topic 4 : Principle of radiological image formation
Part …: (Add part number and title)
Module…: (Add module number and title)
Lesson …: (Add session number and title)
Learning objectives: Upon completion of this lesson, the students will be able to: …
. (Add a list of what the students are expected to learn or be able to do upon completion of the session)
Activity: (Add the method used for presenting or conducting the lesson – lecture, demonstration, exercise, laboratory exercise, case study, simulation, etc.)
Duration: (Add presentation time or duration of the session – hrs)
Materials and equipment needed: (List materials and equipment needed to conduct the session, if appropriate)
References: (List the references for the session)

31. كنتراست
كنتراست رابطه بين دو اكسپوز، شدت تشعشع و يا نور را نشان مي دهد و تمايز و تشخيص دو بافت مجاور و حد فاصل آنها را امكان پذير مي سازد.
وقتي شدت تشعشع X، وارد بدن انسان مي شود سه حالت اتفاق مي افتد:
1- انرژي بطور مستقيم از بدن عبور كرده و بدون جذب از وسط ساختار بافت ها مي گذرد.
2- انرژي بطور كامل جذب شده و بدليل جذب فتوالكتريك هيچ انرژي عبور نمي كند.
3- بخشي از انرژي جذب شده و بقيه بصورت Scatter پراكنده مي شوند. اين حالت سوم بيشترين نفع را براي كنتراست بافت نرم تصوير دارد.

32. جـهت محـاسبه كنتراست، اگر توموري به ضخامتx D و ضريب جذب m در روي بافتي به ضخامت x و ضريب جذب m داشته باشيم. كنتراست حاصل از تومور:
1- كنتراست منفي است يعني اضافه كردن بافت تومور باعث كاهش تشعشع خروجي مي شود. 2- كنتراست بستگي به ضريب جذب خطي ( m ) مواد دارد كنتراست فقط بستگي به ضخامت تومور دارد نه ضخامت بيمار يا بقيه بافت، البته اين فقط زماني صحيح است كه تشعشع فقط اوليه باشد و اسكتر نداشته باشد.

33. 4- براي فضاي خالي (Cavity) به ضخامت Dx كنتراست:
Cr = +m Dx
5- در صورتي كه ضريب جذب بافت تومور ( كه در روي بافت ديگر قرار گرفته ) m2 باشد در نتيجه:
6- براي اضــافه كردن تومور يا مـــواد با ضريب جــذب در داخــــل بافت با ضريب جـــذب m1:

34. فاكتورهائي كه در كنتراست مؤثر هستند :
1- ضريب جذب خطي استخوان بيشتر از بافت نرم است. بنابراين كنتراست جزئيات داخل استخوان بهتر از كنتراست جزئيات داخل بافت نرم است.
2- كنتراست هر دو استخوان و بافت نرم با افزايش انرژي بالا كاهش مي يابد (بدليل رابطه لگاريتمي كنتراست).
3- اختلاف ضريب جذب بين استخوان و بافت نرم با افزايش انرژي، كاهش مي يابد (كنتراست بين آندو كاهش مي یابد).
4- وقتي كنتراست بالا بين بافتهـــاي نرم نياز است از مواد كنتراست زا مثل باريم، تزريق هوا و تزريق يد استفاده مي شود.

35. تشعشع اسكتر
5- اسكتر بصورت تشعشع يكنواخت سرتاسر فيلم را اثر مي گـذارد و كنتراست را كاهش مي دهد در حضور اسکتر فرمول كنتراست:
بايد :
1- اندازه ميدان تا حد امكان كاهش يابد ( با استفاده از ديافراگم)
2- ضخامت بيمار تا حد امكان مثلاً توسط كميرسور كاهش يابد.
3- gap يا فضاي هوا بين بيمار و فيلم قرار گيرد ( بدليل كاهش تشعشع هاي زاويه دار )
4- kvp تا حد ممكن و ضروري كاهش يابد.
5- از وسيله اي كه تشعشعات زاويه دار را جذب ميكند مثل گريد ( بوكي ) استفاده شود.

36. X-Ray penetration in human tissues
Attenuation of an X-ray beam :
air : negligible
bone : significant due to relatively high density (atom mass number of Ca)
soft tissue (e.g. muscle,.. ) : similar to water
fat tissue : less important than water
lungs : weak due to low density
bones can allow to visualize lung structures with higher kVp (reducing photoelectric effect)
body cavities are made visible by means of contrast products (iodine, barium).

37. X-Ray penetration in human tissues

38. Purpose of using contrast media
To make visible soft tissues normally transparent to X-rays
To enhance the contrast within a specific organ
To improve the image quality
Main used substances
Barium : abdominal parts
Iodine : urography, angiography, etc.

39. X-ray absorption characteristics of iodine, barium and body soft tissue
100
Iodine 10
Barium
X-Ray ATTENUATION COEFFICIENT (cm2 g-1)
Soft Tissue
Iodine which attenuates X-rays strongly because of its K-absorption edge of 33 kEv is ideal as contrast agent, as is barium. They allow to enhance the surrounding tissues which could not be seen otherwise. 1
(keV)
0.1

40. Photoelectric absorption and radiological image
In soft or fat tissues (close to water), at low energies (E< keV)
The photoelectric effect predominates
 main contributor to image formation on the radiographic film

41. Contribution of photoelectric and Compton interactions to attenuation of X-rays in bone10
1.0
Total
X-Ray ATTENUATION COEFFICIENT (cm2 g-1)
0.1
Compton + Coherent
Photoelectric
(keV)
0.01