Report About Medical Physics

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15 Νοε 2013 (πριν από 4 χρόνια και 1 μήνα)

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Report About Medical
Physics










Prepared By:


AHMED ABD ALHFEEZ AREF


Under Supervision of:


Dr:















Timetable for Medical Physics


1.

Weeks 1 to 4 :

Radiation physics .

(Quality assurance and calibration)

(Treatment planning)

2.

Weeks 5
to 7
:

R
a
dia
tion protection and safety
.

3.

Week 8 to
9
:


Imaging physics

.

(Diagnostic radiology)

4.

Weeks

10 to 11
:


Imaging physics

.

(nuclear medicine)

5.

Weeks 12 :

R
adiation
b
iolog
y .


















Section One







linear accelerator


What is a linear a
ccelerator?



A linear accelerator is the device most commonly used for external
beam radiation treatments for patient with cancer or tumor. The LINAC,
with a stereo tactic frame can be used in stereo tactic radio surgery
similar to that achieved using t
he gamma knife to targets within the brain
and to treat areas outside of the brain. It delivers a uniform dose of high
-
energy x ray to the region of the patient's tumor. These x rays can destroy
the cancer cells, while sparing the surrounding normal tissue
.

The LINAC uses microwave technology to accelerate electrons in a part
of the accelerator called the wave guide and then allows these electrons to
collide with a heavy metal target. As a result of these collision, high
energy x rays are scattered from the

target.


The patient's radiation oncologist prescribes the appropriate treatment
volume and dosage. The medical radiation physicist and the dosimeters
determine how to deliver the prescribed dose and calculate the amount of
time it will take the accele
rator to deliver that dose. Radiation therapists
operate the LINAC and give patients their daily radiation treatments.





What is the linac consists of:

1
-

Accelerator guides or Wave guides: Depend on the linac type.

2
-

Electron gun.

3
-

Power source: Klystron or
Magnetron.

4
-

Bending Magnets.

5
-

Target.

6
-

Flattener.

7
-

Monitor.

8
-

Collimators.


What are the Accelerator guides or Wave guides do?


Accelerator guides are a special types of wave guides in which electrons
are accelerated. The electron in linac experiences a zero vo
ltage at it
location and positive voltage. This means that the charged particle is
pulled to the accelerated.


Electron Gun:

Consist of cathode and anode, in cathode the electron start from rest in
the filter and gain enough energy to accelerator the elec
tron and travels to
the anode.


Power source:

Klystron:

is an amplifier of high frequency waves created by a
radiofrequency driver, Klystron are required for higher energy machines.



Magnetron:

Generates high frequency power, Magnetrons are used in
low e
nergy accelerators.



Bending Magnets:

In low energy linac, standing wave accelerator guides are short enough to
be aimed at the patient.



X ray beam production:

Once the high energy electron beam has been produced with an
accelerator guide and aimed t
oward the patient, the electrons are available
either to produce x ray or electron therapy. The electrons smash into a
heavy metal target to produce x rays.



X ray beam flattening filters:

The electron interactions in the target produce an x ray beam wit
h an
intensity that falls off rapidly in all directions from the central ray.

The intensity is so non uniform that flattening filter must used to make
the radiation beam useful for clinical purposes.


Photon beam collimation:

The collimation is usually mad
e of a single block of either tungsten,
depleted lead, which allows less than 3% transmission of the radiation
beam.


Electron beam production:

The electron beam has been produced from electron gun with an
accelerator guide.




Electron scattering foils:

It is use for broadening electron beams is the use of scattering foils in the
path of the electron beam. It is use to make the electron beam scatter.
Some foils are uniformly thin sheets of metal (steel or AL).



Electron beam collimation:

After the electr
ons beam is scattering and monitored, the broadened
electron beam is collimated to the appropriate size for treatment using a
combination of the lower and upper jaws in the head and additional
electron collimation known as electron cones.


Monitor chamber:

They must first be measured or monitored, in order to allow delivery of
the prescribed amount of radiation. The device used for this purpose is
called a monitor ionization chamber.



Radiation safety?



Patient safety is very important. During treatmen
t the radiation
therapist continuously watches the patient through a closed circuit
television monitor. There is also a microphone in the treatment room so
that the patient can speak to the therapist if need. Port films are checked
regularly to make sure t
hat the beam position does not vary form the
original plan


The linear accelerator sits in a room with concrete walls and a lead and
plastic door so that the high
-
energy x ray and any neutrons generated do
not escape but are reduced to an acceptable do
se rate. The radiation
therapist must turn on the accelerator form outside the treatment room.
Because the accelerator only gives off radiation when it is actually turned
on, the risk of accidental exposure is extremely low. Before any patient
treated, the

radiation therapist will check the values and results and
contrast this with any patient with treatment plan, it comes from the
medicinal physicist.



What are Wedge Filters?


1
-


There are situations in radiotherapy when it is required to treat a
tumor fro
m one side on the patient only and it is necessary to use
more than one field. As example is the tumor of the middle ear for
which two angled beams make the treatment. This method gives a
very inhomogeneous dose distribution over the region common to
both
fields


This is done by using which filters which turn the isdose curve
through an angle. In normal distributions, the isodose curve usually
cut the central axis of the beam at right angle. The value of angle
depends o the designer of the wedge filter. Th
us, the propose at this
time of beam modification is to change the radiation distribution from
normal which is symmetrical about the central axis of the beam into a
distribution where the dose falls steadily from one side of the beam to
the other side.




This is done by using a filter which is thicker at on end and tapers
to nothing at the other end as shown :



Wedge filters have been used to both kV and MV beams but there
main value is which MV radiation due to there being less scattering.
The
material used for wedge filter is unimportant specially MV rang
since there is no question of Harding of the beam. Weight for weight,
almost all materials attenuates radiation to the same extent. Al, Copper
or lead are almost used but have the advantage of

giving thinner filter.


What is the Block?



Shielding blocks used to be commonly made of lead. The thickness
of lead required to provide adequate protection of the shielded areas
depends on the beam quality and allowed transmission through the
block.

A primary beam transmission of 5 % through the block is
considered acceptable for most clinical situation. The thickness of lead
required for shielding increases substantially. The lead blocks are then
placed above the patient supported in the beam on a t
ransparent plastic
tray, called the shadow tray. Gives the recommended lead shields
thicknesses for various quality beams.


Medium Melting Alloy: (MMA)


On average approximately 20 pounds of 95c
o
(LMPA= Low Melting
Point Alloy). MMA is needed per patient
.

This is toxic and hazardous waste. Do not throw it into the garbage as
details are:


1
-

52 % Bismuth.

2
-

32 % Lead.

3
-

15 % Tin.

4
-

10 % Cadmium





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?




Multi
-
leaf collimator (MLC) for a photon beams consists of
a large number

of collimating blocks or leaves that can be
driven automatically, independent of each other, to generate a
field of any shape.



The thickness of the leaves along the beam direction is sufficient to
provide acceptably low beam transmission. The width o
f each leaf is
usually about 1 cm as projected at the isocenter. The field edges are,
therefore, formed stepwise, 1 cm wide. The MLC device contains 120
leaves on it (60 on each side). With the central 20 cm comprised of 0.5
cm wide leaves and the outer 10

cm for each direction comprised of 1 cm
under leaves.



Standard radiotherapy involves geometrical beams forming a box
around the tumor, whilst conformal radiotherapy uses three
-
dimensional
planning, and the volume of radiation therapy given to the tu
mor is
irregular and 'conforms' to the correct area. A major component of
modern linear accelerators which assists this technique is the multi
-
leaf
collimator (MLC). The tumor is positioned under the collimator's centre
using fixed laser measurement equipm
ent, and the photon beam is
modulated with series of leaf pairs, according to the tumor's volume and
profile. The leaves travel on moveable carriages and move independently
into the beam focus in order to block off radiation. An MLC also provides
the autom
ated shaping capability needed for treating patients with
dynamic Intensity Modulated Radiation Therapy (IMRT). By virtually
eliminating heavy custom blocks which existed in linear accelerators
prior to MLC, valuable time is saved, dose distribution is imp
roved and
patient throughput is increased with existing equipment.




















The Steps of QA in linear accelerators daily



1.

We are checking all parts of linac.

2.

We are checking the display (Field size, Collimator angle,
Couch and gantry by the hold
er (Manual Control).

3.

Should be change the Couch to up or down to reach 100 cm
between the couch and the source by press the SSD and optical
(Field size 10x10 cm
2
).

4.

Must be sure of the distance between the source and couch by
rangefinder at 100 cm.

5.

The best

place paper on the couch to display SSD and cross
wire on paper and move collimator into left and right to see the
point of the center.

6.

Put the phantom laser on the couch and make (
S
witch on) laser
to see line of the laser corresponding on the line at the

phantom (Right, left, sagittal and overhead).

7.

Lock the water temp, water pressure and water level.

8.

Sure from audiovisual (mike and camera) and the light of the
room.

9.

Inter locks the door and beam off.

10.

Put the detector on the couch to dosimatric measuremen
t.

11.

Dosimatric measurements of:
-

[a] Electron beam with applicator
.

[b] Photon beam without applicator
.



Section Two



Radiation safety officer:



A radiation safety officer (RSO) it must important for
radiation safety, he will have the responsibility

of implementing
the radiation protection program. Radiation exposures can a rise
in two ways:



A great part of the body may be exposed to radiation from an
external source.



A certain organ may be exposed to radiation from radioactive
materials which have b
een injected or inhaled and retained by
body.


Protections thus consist of keeping both the amounts of
exposure to external radiation and quantities of radioactive
material, which may inter the body to safe levels, which do not
cause harmful effects.


Radiation monitoring :



For area monitoring, the portable battery operated survey
meters are used. For recording doses received by radiation
workers, film badges and TLD (Thermoluminscent Dosimeters)
are widely used for this propose. Ion chamber it lik
e survey
meters but it use electronic screen and it is using different unit.


How can we control of contamination?


In planning work with RAI (Radioactive Isotope), it is necessary
to remember the following points:



The minimum quantity of radioactive mat
erial just enough for
the propose should be use.



Preference should be given to radioactive material of short
half
-
lives, low energies and low toxicity.



A max distance should be kept between the radioactive
source and the worker.



The time spent near the rad
ioactive source should be kept to a
minimum.



Shielding should be used where necessary to reduce the dose rates to
safe levels.


How can we disposal the radioactive waste?



Radioactive waste disposal from analytical, here the amount
of radioactivity in

use is small in mCi level Therefore, waste
disposal is sample:



Gaseous waste: release to the atmosphere.



Liquid waste: discharge to sewers.



Solid waste: burial
.


The Physical Units:


What is Rontgen?


Is the unit of expose in the cgs old system, the qu
antity is a
measure of ionization produced I air by photons.


Define the Rontgen?


The rontagen is the amount of X and gamma rays that
produces a given amount of ionization in each unit of air.

1R= 2.58 x 10
-
4

C/kg air


What is Gray?


Is the unit of
absorbed dose in SI system.


Define the Gray?


The Gary is radiation
-
absorbed dose of 1 J/kg.

1Gy=1J/kg

What is rad?


Is the unit of absorbed dose in cgs old system.


Define the rad?


The rad is defined as 100 ergs/g of tissue

1 rad= 99 ergs/g

100
rad =1Gy

What is rem?


Is the unit of dose equivalent in cgs old system.


Define the rem?


A unit that reflects the biologic response and that could be used
to compare effects of different radiation would be extremely
useful to ward this end.

100 rem=1S
v


What is sievert?


Is the unit of dose equivalent in SI system.


Define the sievert?


The equivalent dose is a measure of the absorbed dose in a
tissue, taking into consideration the radiation weighting factor, 1
for x rays, 10 for imaging some neutro
n.


What is x unit?


Is the unit of is a measure of the photon flux and the unit of
expose in the SI system.


Define the x unit?


The x unit defined that quantity of x or gamma radiation that
produces in air.

1 x unit =1 C/kg in air.

1 x unit = 3881
R.

What is Ci?


Is the unit of activity radiation in cgs old system.

1
Ci = 3.7 x 10
16

BqS


What is Bq?


Is the unit of activity radiation in SI system.


Radiation Safety


45
1 Ion Chamber Survey Meter Family


Ion Chambers Instrument



451 P Pressurized
Ion Chamber Survey
Meter



451B Ion Chamber
Survey Meter with
Beta Slide



451 EXL
-
Software
-
Assistant for Excel



470A Ion Chamber
Survey Meter





"Identifier" Multi Channel Analyser

GM & Scintillation
Instruments and Probes



"Identifier" Multi
Channel analyzer
-

Complete radiation
safety protection


Mod 190



190 Digital Survey
Meter with extern
probe



190 Digital survey
Meter with internal
GM
-
detector



190 with Neutron
Probe




Selections of probes






Section three






Production of x rays




X ray discovered by roen
tgen in 1895 while studying cathode rays in a
gas discharge tube. He observed that another type of radiation was
produced which could be detected outside the tube.




The x ray tube:


The tube consists of a glass envelope which has been evacuated to hig
h
vacuum. At one end is a cathode (e
-
) and at the other an anode (e
+
), both
hermetically sealed in the tube. The cathode is a tungsten filament which
when heated emits electrons, a phenomenon known as thermionic
emission. The anode consists of a thick copp
er rod at the end of which is
placed a small piece of tungsten target. When a high voltage is applied
between the anode and the cathode, the electrons emitted from the
filament are accelerated the anode and achieve high velocities before
striking the targe
t.


The anode:


The choice of tungsten as the target material in conventional X ray
tubes is based on the criteria that the target must have high atomic
number and high melting point. The efficiency of x ray production
depends on the atomic number, and f
or that reason, tungsten with Z=74 is
a good target material. Which has a melting point of 3370C
o
. efficient
removal of heat from the target is an important requirement for the anode
design. Some of tubes use thick copper anode to the outside of the tube
w
here it is cooled by water or oil. The heat generated in the rotating
anode is radiated to the oil reservoir surrounding the tube.


The cathode:



The cathode consists of:

1
-

Wire filament.

2
-

Circuit to provide filament current.

3
-

Negatively charged focusing cu
p.



The function of the cathode cup is to direct the electron to the anode so
that they strike the target in the well defined area, the focal spot.

Since size of focal spot depends on filament size.


The glass envelope:


The glass envelope surroundin
g the components of the X ray tube is
evacuated to prevent the electrons form interaction with gas molecules
before they reach the x ray target.


Shield:


Some stationary anodes are hooded by a copper and tungsten shield to
prevent stray electron from st
riking the walls of the tube. For that the
copper shield absorbs the unwanted x ray.





















Computed tomography (CT)


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:
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Computed tomography (CT) scans are completed with the use
of a x
-
ray beam rotating through a 360
o

angle and co
mputer
production of images. These scans allow for cross
-
sectional
views of body organs and tissues.


In it’s basic form, a rotating x ray beam emits ionizing
radiation of a defined thickness which is used to irradiate the
patient from numerous projecti
on. Detectors located on the
opposite side of the patient to the beam, register the amount of
radiation that has penetrated the patient.

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.
.












U
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w
w
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u
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1
1
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0
k
k
V
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a
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2
2
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5
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m
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A
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f
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.





Brain scans can detect tumors, and strokes. The
introduction of CT scanning, especially spiral CT, has helped
reduce the n
eed for more invasive procedures.


Body scans. CT scans of the body will often be used to
observe abdominal organs, such as the liver, kidneys, adrenal
glands, spleen, and lymph nodes


Aorta scans. CT scans can focus on the thoracic or
abdominal ao
rta to locate aneurysms and other possible aortic
diseases.


Chest scans. CT scans of the chest are useful in
distinguishing tumors and in detailing accumulation of fluid
in chest infections.

R
R
a
a
d
d
i
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a
t
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o
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s
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a
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f
f
e
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t
y
y
:
:



Pregnant women or those who could pos
sibly be pregnant
should not have a CT scan unless the risk of non diagnosis
exceeds the radiation risk. Pregnant patients should particularly
avoid full body or abdominal scans. For many CT examinations
patient may be asked to sign contrast agent (orally,

rectally or
via injection).Intravenous, oral and rectally are pharmaceutical
agents liquids and are sometime referred to as dyes or pigment
contrast is used to make specific organs.

There are four types of contrast agent used in CT:



Via intravenous injec
tion.



Orally



Rectally



A much less common type of contrast used in CT is for
special lung and brain imaging.

Sometimes it is necessary to not be drinking anything for hour
to several hour exam (fasting).The preparation times varies
depending on the actual e
xam as well imaging center’s
requirement.


If a contrast medium is administered, the patient may be asked
to fast from about four to six hours prior to the procedure.
Patients will usually be given a gown (like a typical hospital
gown) to be worn during t
he procedure. All metal and jewelry
should be removed to avoid artifacts on the film.

C
C
T
T


e
e
q
q
u
u
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p
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t
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:
:





A CT scan may be performed in a hospital or outpatient
imaging center
. Although the equipment looks large. The patient
is asked to lie narrow table, tha
t slides into the center of the
scanner. The scanner looks like a ring and is round in the
middle, which allows the x
-
ray beam to rotate 360
o

around the
patient. The scanner section may also be tilted slightly to allow
for certain cross
-
sectional angles.

C
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T
T


p
p
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:
:





It is most important to tell the patient that “do not move”
while the CT is scanning so the scanning picture wont be
corrupted. In some studies, such as abdomen CTs, the patient
will be asked to hold his or her breath during image capture.

Following the procedure, films of the images are usually printed
for the radiologist and referring physician to review. A
radiologist can also interpret CT exams on a special computer
screen. The procedure time will vary in length depending on the
area be
ing imaged. Average study times are from 30 to 60
minutes.



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a
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:
:





Contrast agents are often used in CT exams and in other
radiology procedures to illuminate certain details of anatomy
which may not be easily seen. Some contrasts are natura
l, such
as air or water. Other times, a water contrast agent is
administered for specific diagnostic purposes. The patient may
drink this contrast. Oral and rectal contrast are usually given
when examining the abdomen or cells, and not given when
scanning
the brain or chest. Iodine is the most widely used
intravenous contrast agent and is given through an intravenous
needle.

R
R
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u
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l
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:
:





Normal findings on a CT exam show bone, the most dense
tissue, as white areas. Tissues and fat will show as various
shade
s of gray, and fluids will be gray or black. Air will also
look black. Intravenous, oral, and rectal contrast appears as
white areas. The radiologist can determine if tissues and organs
appear normal by the sensitivity of the gray shadows. In CT, the
image
s that can cut through a section of tissue or organ provide
three
-
dimensional viewing for the radiologist and referring
physician.

What is observed?


In the chest for example, the patient will be asked to hold his
or her breath during image capture. It
is very important to tell
the patient that "do not move" while the CT is scanning and
injecting the patient through the VEINS during the test for take
clear image to see an organ.



KV use:


The set kV depends on the patient age, normally using 120
-
140
KV

for old men or adult's patient and 80 for babies. Higher KV
higher dose to patient and more scatter which may reduce
differences in tissue visible.







































Section Four


Nuclear Medicine



I
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Nuclear Medicine

is used mainly to allow visualization of
organs and regions within organs that can not be seen on
conventional x ray images. Especially tumors may stand out on
nuclear medicine images. It is a different process from x ray, CT
and MRI all of which are look
ing at the anatomy of the body.

G
G
a
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The gamma camera has a large crystal detector and used to
detect the gamma rays from the radiopharmaceutical and to
build them up into image there are two heads of gamma camera.
Each head has a metal colli
mator roughly analogous to lens
followed by a NaI (TL) crystal. The crystal detects the emitted
radiation signal and converts that signal into faint light or pulses
of light. These pulses are amplified by photo multiplier tubes
and passed to a computer tha
t works out where the original
gamma ray comes from. It can also rotate around the patient to
produce series of images.

Gamma Camera Image:


The nuclear medicine image can either be in gray scale (black
& white) for instance in a bone scan or they can be

color coded
to clearly show functional activity like in a cardiac study.


What we are do morning daily in
nuclear medicine?

1
-

Elution:


First, we taking 2 tubes (vacuum tube, cerium tube or
pharmaceutical) and put both of them in generator. The
generator
is using Radio isotope it is Tc
99
, when the saline
passes through in to the vacuum tube we get the Tc
99m

radioactivity.





2
2


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The main radioisotope we use is Technetium
-
99m (Tc99m).
This emits a gamma ray of energy 140 KeV with a ha
lf life of 6
hours. It is ideal for our purposes as it combines good imaging
characteristics with a low radiation dose to the patient. It is
produced from a generator containing its parent isotope
Molybdenum 99 (Mo99).


3

Dose
c
alibrator:




It is ma
chine is use to measure the radioactive isotope for
example , for normal patient 900 MBq
-

1000 MBq, for
children 500 MBq
-

600 MBq. But when we want to scan the
thyroid we get 185 MBq


250 MBq .

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A large number of radiopharma
ceuticals are used in Nuclear
Medicine. Most are known by abbreviations. The most
common are MDP for bone scans, MAA for lung scans, PYP for
heart scans and DTPA for kidney scans. All are prepared in the
department's radio pharmacy.





Section Five



all subjects as follow:
this section I already toke

In


1
-

Introduction to biology part1:


Cell structure ,
cell cycle and cell division
.


2
-

Introduction to biology part 2:


DNA RNA protein


3
-

The physics
and chemistry of radiation absorption
.


4
-

Cell survival curves.


5
-

DNA strand brea
ks and chromosomal aberrations .


6
-

Repair of radiation damage and the dose
-
rate effect.


7
-

The oxygen effect and reoxygenation.


8
-

Linear energy transfer and
relative
biological
effectiveness.












X ray interaction with matter?


1
-
Coherent scattering:


In coherent scattering, incoming photon are absorbed by the atomic
electron. Usually the electron in the outer most shell. When then radiates
the energy of the ph
oton in a slightly different direction. Since the
electron of the atoms are not excited to higher level Z and not ionized.



2
-
Photoelectric effect:


Result in transfer of the total energy of photon to inner electron of an
atom of absorbed medium. The e
lectron ejected from the atom with
kinetic energy


E
k
=hv
-
E
b

W
h
ere:


h
v=the energy of photon.


E
b
= the binding energy of ejected electron.


For example:



Incoming high energy photon interacts with inner electron k shell and
transfers all energ
y in the electron and ejected from the atom and left
positive charge due to hole. In this case the outer electron fills in the hole
or electron L shell fill to k shell, M shell fill to L shell.

The probability of photoelectric interaction depends on the at
omic
number of the absorbed material as well as on the energy of the x ray.




3
-
Compton Effect:

In the Compton which energy is both absorbed and scattered. Incoming
photon interact or knock the electron out of atom, left the positive charge
ion and the

energy of the photon is transferred to free electron in this
medium. Incoming photon knock the electron given us two directions:


1
-
compton electron.

2
-
compton scattering.



The scattering photon has reduced energy that is directly related to its
angle

of scatter
φ
.


The Compton electron is set into motion with kinetic energy equal to
the energy transferred by the incident photon. Less any binding energy
the most be overcome in ejecting the electron from its atom. The
direction of the Compton electron angle
θ
.

Bo
th
φ

and
θ

tend to decrease with increasing energy of the incident
photon.


Δλ=2.4x10
-
12

(1
-
cos

θ)

λ
'
=λ+Δλ

Where:


Δλ=Wavelength in nanometer o the x ray.


λ= The incident photon.


λ
'
= The scattered photon.

4
-
Pair production:


If the energy
of the photon is greater than 1.02Mev or 10Mev the
photon may intact with matter through the mechanism of pair production.
The incoming photon interacts with the electron field of the nucleus .the
photon interacts strongly with the electron magnetic field
of atomic
nucleus. And give all energy in the process of creating a pair consisting:


1
-
Negative electron (e
-
).

2
-
Posstive electron (e
+
).



These two particles go to difference directions. Because two particles
created have rest mass energies 1.02Mev tha
t means 0.511 Mev for each
particle. Thus, the threshold energy for the pair production process is
1.02mev. The photon energy increase of this threshold is shared between
the particles as kinetic energy.



Annihilation Radiation:


When the positron come
s to rest because losing its energy, it combines
with free electron .the two particles are thus annihilated, producing two
photon, each having 0.511 Mev energy and the two photon ejected in
opposite direction.




5
-
Photodisgration:



Photodisgration occ
ur when photon have enough energy to eject
nuclear particle when they are absorbed by nuclear.Photodisgartion I
interactions also can be used to measure the energy photon in high energy
X ray beam.


Electron interaction with mater?


1
-

Characteristic Radia
tion:


Electron may interaction with many different types of particles as an
electron travels it makes many such collisions every time it interact. It
either given its energy to another electron in an elastic interaction and
inelastic collision sometime a
long amount of energy is transferred in a
collision. In fact, in some interaction enough energy is given to change
the energy state of the electron that way hit. In this case an electron in
lower shell can be excited to higher shell level in atom and loose
ly bound
to nuclear and has higher potential energy. This process called Excitation.




Sometimes enough energy is given to the electron to remove it totally
from the atom. This called Ionization.








Electron incident on the target also produce cha
racteristic x rays.



Electron incident on the target also produce characteristic x rays. An
electron , with kinetic energy E
o

may interact with the atoms of the target
by ejecting an orbital electron, such as a K,L or M electron, leaving the
atom ioniz
ed. The electron will recede from the collision with energy

E
o
-
ΔE

Where:


ΔE= The energy given to the orbital electron.


E
o
= Kinetic energy.


ΔE is spent in overcoming the binding energy of the electron and the
rest is carried by the ejected electron. When a vacancy is created in an
orbit, an outer orbital elect
ron will fall down to fill that vacancy. The
energy is radiated in the form of electromagnetic radiation. This is called
characteristic radiation. With high Z targets and transition involving inner
shells such as K,L and M the Characteristic radiations emi
tted are of high
enough energies to be considered in the X ray of electromagnetic
spectrum.


2
-
Bremsstrahlung:


Bremsstrahlung is the result of collision between a high speed electron
and nucleus. The electron while passing near a nucleus may be deflected

from its path by the action of coulomb forces of attraction and lose
energy as bremsstrahlung. Phenomenon predicted by Maxwell’s general
theory of electromagnetic radiation.


In bremsstrahlung production the maximum energy of the X ray equals
the kinetic

energy of the incoming electron. As the kinetic energy of the
electron increase. T
h
e direction of x ray emissions becomes increasingly
forward. The energy loss per atom by electrons depends on the square of
the atomic number Z
2
.