Radiographic imaging phosporesence-1.ppt

A phosphorescent is a subdivision for luminescence which play an important role in medical imaging

1. Rad. Imaging
Phosphorescence and its implication
in imaging

2. Mechanism of phosphorescence
Definition: phosphorescence is a special case of
fluorescence where the emission of light
following absorption of radiation is not
instantaneous but a delay occurs
In phosphorescence, the phosphor material
absorbs X or gamma radiation and photo-or
Compton electrons are produced
Many vacancies or holes are created as the
released electrons excite other electrons to the
conduction band

3. Mechanism of phosphorescence
The electrons then fall into the traps in the
forbidden zone and stay for some time
When these electrons acquire sufficient
energy from lattice electron vibrations, they
are excited back to the conduction band
They now fall to the valence or filled band to
recombine with the holes
This recombination is associated with the
emission of visible light

4. Mechanism of phosphorescence
Alternatively, some electrons in the traps
acquire sufficient energy and fall directly to
recombine with the holes in the valency band
& emit visible light
This radiative ion recombination continues
long after the primary decay time of
luminescence of 10
-8
secs because the
electrons are delayed at a trap centre

5. Mechanism of phosphorescence
This type of luminescence is described as
phosphorescence or after glow
Afterglow is strongly dependent on crystal
defect concentration in the host and on
intensity and duration of excitation
It can however be reduced by doping the
phosphor material with appropriate
impurities and at the required concentration

6. Effects of Phosphorescence on
Imaging and Radiation Detection
Devices
Generally, after glow does not have any
positive contribution in imaging & radiation
detection devices but rather a nuisance
After glow in intensifying screen has been
observed to expose or fog subsequent films
loaded into the cassette after the previous
film had been off-loaded
This is tantamount to film fogging & is
undesirable effect

7. Effects of Phosphorescence on
Imaging and Radiation Detection
Devices
Similarly, in a sequential imaging process
associated with dynamic recording,
phosphorescence causes image pile-up due to
phosphor persistence
The sequential images thus get piled up and
superimposed, making viewing of dynamic
images impossible
In a scanning system the presence of afterglow in
the phosphor reduces image sharpness due to
patient motion

8. Effects of Phosphorescence on
Imaging and Radiation Detection
Devices
However, afterglow in intensifying screen can be
utilised to obtain a second radiograph of an
image in place of copying
At high counting rates in radiation detectors
employing scintillators, phosphorescence will
tend to encourage build up of counts due to
multiple overlap from preceding pulse
The result is inefficient counting statistics & thus
the efficiency of the detector is lowered as result

9. Cont.
Thermoluminescence And Its
Applications

10. Mechanism of Thermoluminescence
Thermoluminescence is one of the delayed
luminescence where light emission occurs
long after cessation of excitation by radiation
In fact, emission of light occurs only when the
phosphor is heated
Its mechanism is based on a class of inorganic
crystals, which exhibit high concentrations of
trapping centres within its band gap

11. Mechanism of Thermoluminescence
An incident radiation interacts with the valence or
filled band & electron-hole pairs are created
Electrons are elevated from valence band to the
conduction band & are subsequently captured by the
trapping centres
Because the distance btw the trap energy level & the
conduction band is large, the probability of electron
escaping from the trap at room temp. is very small
Thus, further exposure of the material to radiation
leads to progressive build-up of trapped electrons

12. Mechanism of Thermoluminescence
In a similar process, a hole created by an incident radiation
migrates via the crystal lattice until it reaches a hole trap
whose energy is above the top of the valence band
If the energy difference between this hole & the valence
band is large the hole becomes locked and cannot move
further
Continuous exposure to radiation also results in build-up of
holes
When thermal energy is applied to this exposed crystal,
usually to a temperature of 400
0c
, the trapped electrons
acquire sufficient energy to escape the trap & undergo
transition back to the valence band

13. Mechanism of Thermoluminescence
Alternatively, when the trapped holes acquire thermal
energy, they can migrate to a trapped electron
The ion recombination that takes place from either
process results in emission of a quantum of visible
light, which is described by the equation:
P = Se
-(E/KT)
Where P is the probability of a trap being emptied,
S = constant related to the atomic vibration frequency
of the crystal, E = energy level of the trap, K = the
Boltzman’s constant and T = absolute temperature

14. Mechanism of Thermoluminescence
Since the traps occur at different energy levels, the light
emission occurs at specific temperatures corresponding to
a specific trap & results in glow curve
While some traps are emptied at 70
0c
-100
0c
others need to
be raised to temp. of 200
0c
-300
0c
The intensity of light emitted & represented by the area
under the glow curve is proportional to the amount of
radiation energy absorbed by the crystal
Thus, the total light output or the peak of the curve can be
used as radiation dose-meter to estimate integrated dose
However, the main glow peak temp. is affected by the way
the sample is heated & prepared while the useful range of
dose measured is affected by the sample form

15. Mechanism of Thermoluminescence

16. Thermoluminescent Materials
There are many materials found suitable for
thermoluminescence but the commonly used
ones are LiF, CaF
2, CaSo
4, Li
2,B
7, O
7and BeO
Except LiF and a few others which do not require
activators, most contain small concentrations of
impurities
For example CaSo
4utilize manganese for
operation while CaF
2require Dysprosium
The thermoluminescence of LiF depends on
inherent impurities & crystalline defects

17. Thermoluminescent Materials
The characteristics or behaviour of each
thermoluminescence material depends on the
trap depth & atomic number of the material
Those with shallow traps are:
1.More sensitive dosemeters
2.Able to record lower radiation levels but suffer
from considerable fading with time following
irradiation
3.They are unstably even at room temperatures

18. Thermoluminescent Materials
Thermoluminescent dosemeter (TLD) with high atomic
number have poor linearity response to a wide range of
photon energies due to photoelectric interaction
probabilities which are high at lower energies
LiF has been found to be the most popular
thermoluminescent material because:
1.Its deeper trap ensures negligible fading
2.Long term dose storage abilities at room temperature
though at reduced sensitivity of several others
3.In addition, its relatively low atomic number similar to that
of tissue or air gives reasonably constant response to a
wide range of photon energies
4.It has sensitivity from 10μGy -100Gy

19. TLD System
The TLD system consists essentially of the:
1.Thermoluminescent crystal
2.The badge and
3.The TLD reader
The crystal
LiF crystals dominate the TLD system though others like CaF
are often used for special purposes
The sensitivity of LiF to exposure range from 10μGy -100Gy
is fairly constant
As the dose increases beyond 10Gy the response is no
longer linearly related because traps are created by the
irradiation

20. TLD System
The effect is that the dose per unit of exposure is much
higher at this region than at lower doses, a relationship
described as supra linear
The LiF crystal could be in the form of rods, disc or powder
depending on the radiation environment the dosemeter is
to be used
The crystalline rods or discs are usually a few millimetres in
cross section to fit into dedicated badges or holders & then
wrapped in a Teflon plastic materials
The powder on the other hand is made up of crystals
whose sizes range from 0.005 millimetres to 0.2 millimitres
& the quantity ranging from 50 –100mg for each dose
measurement

21. TLD System
For use, each dose measurement is encapsulated in a
plastic sachet
While the solid rods & discs are suitable for badge
holders
The powder is usually employed in monitoring doses
absorbed by the fingers during handling of radio-
pharmaceutical and body entrance dose estimations
LiF obtained naturally contains 7.4% of
6
Li,
consequently, dosemeters made from them are also
sensitive to slow neutrons through the (n, α) reaction

22. TLD System
Badges
TLD badges have the characteristic colours of yellow or green
different from the blue usually associated to the traditional film
badges
A receptacle exists where the LiF crystal is positioned or fitted
For radio-diagnosis, where only x-rays or gamma rays are involved,
no filters are employed
However, in beta particle environments, comprehensive monitoring
is achieved by using 2 or 3 TLD chips with varying thickness of filter
to differentiate btw deep & shallow doses
The powder chips enclosed in their sachets are usually strapped to
the fingers as rings or any other shapes in a way that does not
affect manual dexterity
When it is employed to the monitor, absorbed doses through body
parts are also secured by strapping over the interested part

23. TLD System
TLD Reader
The TLD reader functions to provide the
thermal energy required to cause an
irradiated TL crystal to emit light
It also measures the intensity of this light as
an estimated absorbed dose
It consists of heating system, the light
collecting system, and the electronic system

24. TLD System
The heating system
Housed in a light tight chamber & made up of the
heating element electrically supplied
There is a stainless steel tray to hold the TLD
sample and an optical filter for infrared radiation
to exclude thermal excitations on the photo-
multiplier tube
A thermocouple monitors the temperature of the
system, which is raised in a reproducible way
approx. 300
0c

25. TLD System
The light collecting system
Consists essentially of PMT optically coupled to the emission from
the TL crystal
The PMT collects the emitted light, converts it to a stream of
electrons & in the process also amplifies the signal
When very low doses are being measured, it is important that in
addition to the infrared filter the PMT is further prevented from
thermal excitations by cooling with liquid nitrogen
The output from the PMT in terms of electronic signal is thus
proportional to the intensity of light incident on its input
Care is taken to exclude extraneous light by ensuring that the light-
tight chamber extends to the PMT

26. TLD System
The Electronic System
Further amplification of the electronic signal takes place in
the electronic system to improve transfer xtics without
signal degradation
This can be displayed in an analogue form but most devices
intercept the signal at this point wih an ADC & display the
output digitally
The magnitude of the signal displayed is proportional to
intensity of light
It is an estimate of the dose the TL sample was exposed to
Due to variability btw samples & methods of heating, there
is need for the calibration of the system to accurately relate
signal output to dose

27. TLD System
In practice, the variations, are considered not to be
very large & only checks are made to standardize the
system
When the TL sample is used for actual measurement,
the sample is then placed in TLD reader
The resultant glow curve obtained is compared with
the parallel sample glow curve & the dose extrapolated
However, after irradiation, the samples should be kept
for 24 hours or held at 100
0c
for 5 minutes to allow
very shallow traps to be emptied before reading

28. TLD System
Readings taking without this procedure will affect
the shape of the glow curve & consequently the
accuracy of the dose extrapolated
If the TL crystal is exposed to high doses of
radiation, a second read-out may be necessary
because an initial read-out cannot empty all the
traps
The dose estimation from this second read-out
however is usually inaccurate & imprecise

29. APPLICATIONS OF TLD
The TLD is very useful in many environments
requiring radiation monitoring due to its
peculiar xtics
In clinical environmentTLD is useful:
for health personnel dose monitoring
Absorbed dose estimation during radio-
diagnostic & radio-therapeutic procedures
In determination of exposure to the hands in
‘hot’ nuclear medicine laboratory

30. APPLICATIONS OF TLD
for personnel radiation dosimetry, LiF crytals are
enclosed in the appropriate badge
Is won behind any protective clothing (e.g. lead
aprons) & at specified parts of the body where
maximum irradiation is likely to occur in the
course of work
The chest or gonad areas common site for
wearing them
Because LiF can retain trapped electrons for long
period, the badges can be won for 3 months
before the TLD material is removed for reading

31. APPLICATIONS OF TLD
However, for accurate absorbed dose estimation to the
wearer, the following guidelines should be observed:
1.The badge must be won behind any protective clothing
like lead apron so that only absorbed dose by the
personnel is recorded
2.It must not be left inside the car & exposed to the heat
of the sun
3.It must not be left in a diagnostic room where the
likelihood of exposure to radiation can record doses
not absorbed by the personnel
4.Badges must not be exchange btw personnel before
reading

32. ADVANTAGES AND LIMITATIONS OF
THE USE OF TLD
TLD has almost replaced photographic film badges as personnel
monitoring device because of certain desirable features it
possesses:
1.The tissue equivalence of LiF makes dose extrapolation in biological
tissue more accurate
2.Its linear response within the exposure dose range encountered in
dosimetry improves accuracy of absorbed dose estimation
3.Because TLD can retain dose for a relatively longer time, its use is
suited for clinical environment where there is usually time lag btw
exposure & reading for dose extrapolation
4.There is a positive cost containment from the ability to re-use the
crystals after obtaining the dose

33. ADVANTAGES AND LIMITATIONS OF
THE USE OF TLD
5. Its sensitivity is fairly constant over wide range of
photon energies
6. LiF can easily be fabricated into thin layers & its size,
which is relatively small, reduces the directional
dependence associated with other dose-meters
7. The TLD system including the read out system is quick
& very suitable for automation &computerisation,
which facilitate information management of
personnel radiation monitoring
8. 7
Li or
6
L content of the LiF can be manipulated to
enable it respond to neutron by the Li (n,α)
1
H
reaction

34. ADVANTAGES AND LIMITATIONS OF
THE USE OF TLD
One of the greatest limitations of the TLD system is that the
process of reading destroys the record of the dose & no
permanent information is kept
However, storage of the individual glow curves obviates this
loss
The glow curve can be retrieved for re-examination if the
need arises
At very high doses exceeding 10Gy, there is a supra linear
relationship btw exposure & dose measured
A second read out may also be necessary at very high
exposures
All these make dose extrapolation inaccurate & imprecise