4. RADIONUCLIDES
Radionuclides are unstable nuclei
• Having a neutron excess or deficit, are
radioactive → 'decay‘ to become stable nuclei,
with the emission of any combination of alpha,
beta, and gamma radiation.
Production of radionuclides:
• Natural radionuclides: few, sufficiently long
lived, e.g.uranium, radium, and radon.
• Radionuclides in medical imaging → produced
artificially, in the following way
5. Some radionuclides produced by additional
neutron
forced into a stable nucleus:
• Nucleus has a neutron excess → unstable.
• e.g. with Molybdenum (Mo):
98Mo + n → 99Mo
• Same atomic number.
• Mass number increased by 1.
• Radionuclides produced by this process have
same chemical properties and called "an isotope"
.
6. Radionuclides produced by additional proton
forced into a stable nucleus → knocking out a
neutron:
• Nucleus has a neutron deficit → unstable.
e.g. with Boron (B):
11B + p → 11C + n
• Atomic number increased by 1.
• Same mass number.
7. RADIOACTIVE DECAY
Nuclides with a neutron excess:
β - Decay
• Neutron change into a proton & an electron → the
electron is ejected from the nucleus with high energy “-ve
beta particle”
n → p + β -
• For example, Iodine-131 (131I, Z= 53) → Xenon-131
(131Xe, Z= 54).
• Same mass number.Atomic number increased by 1 .
•The daughter nucleus mostly produced with excess energy
relaxes
with immediate loss of energy with the emission of one or
more γ photons.
8. Isomeric transition: in this some radionuclides
the gamma ray is not emitted until an
appreciable time after the emission of beta
particle.
• For ex:99Mo decays by emission of –ve beta
particle, the daughter nucleus Tc remains in
exited stable for variable length of time, it is said
to be metastable and is written as 99mTc.
• Its decay to ground state , 99Tc, is most often
with emission of gamma ray of energy 140kev.
9. Nuclides with a neutron deficit:
β + Decay
• A proton change into a neutron and a positive
electron → the latter is ejected from the nucleus
with high energy "+ve β particle".
p → n + β+
• For example,
Carbon-11 (11C, Z= 6) → Boron-11 (11B, Z= 5).
• Same Mass and charge.The atomic number
decreased by 1 .
•The daughter nucleus, if excited → loses excess
energy by the emission of gamma photons till
reaches the ground state.
10. RADIOPHARMACEUTICALS
Radiopharmaceuticals are the radioactive
substances or radioactive drugs for diagnostic or
therapeutic interventions.
It contains a radioactive isotope that can be
injected safely into the body, and a carrier
molecule which delivers the isotope to the area
to be treated or examined.
• Isotope during its conversion to stable forms
emits radiation.
EG:Tc 99m –sestamibi,pertechnetate,DTPA,
13. COMPONENTS OF GAMMA CAMERA
Collimator – used as mechanical lenses.
Radiation detector crystal - Counts the incident
gamma photons
Photomultiplier tube-amplifies the energy
recevied.
Computer system - Creates 2-D images from
detector data
Gantry system - Supports and moves gamma
camera and patient
14.
15. COLLIMATOR
Made from lead.
Maintains quality of the image-smaller holes and longer
length –increases the resolution .
Spaces between the holes are called septa.-absorbs
scattered photons
Types of collimators-
1.Parallel hole collimators-most common
2.Pin hole collimators-for magnifying small objects
3
. Divering-for minified images
4.Converging-for magnified images
16.
17.
18.
19. SCINTILLATION CRYSTAL
The scintillation crystal in the gamma camera
converts gamma ray photons incident on the
crystal into a number of visible light photons.
The crystals used in gamma cameras are typically
made of NaI with Thallium.
Thicker crystal have higher sensitivity and poorer
resolution.
20.
21.
22. PHOTODETECTOR ARRAY OR PMTs
The photodetector array is comprised of a set of
30–90 PMTs arranged in a hexagonal close packed
arrangement.
This converts light produced in scintillation layer
into electrical signals
Photocathode in PMT when stimulated by light
photons ejects electrons.
It amplifies the electrons produced .
23. The purpose of these is to multiply the small
amount of light detected from the
scintillation crystal to a large signal.
24. PREAMPLIFIER
This converts the current produced at the anode
of the PMT to a voltage pulse.
The amplitude of the voltage pulse is directly
proportional to the charge produced at the
anode
Therefore, the amount of light received by the
PMT, which is proportional to the number of
gamma photons that hit the scintillation crystal.
25.
26. IMAGE FORMATION
Gamma photon interactions with crystal:
Photoelectric – full energy absorbed by crystal
Compton – proportion of energy absorbed by
crystal
The spectrum has a peak (photopeak) that
corresponds to the maximum gamma photon
energy (for 99mTc this is 140 keV).
The Compton band corresponds to photons that
have undergone Compton interactions and,
therefore, have a lower absorbed energy.
27.
28. Gamma photon energies within the Compton
band can be due to:
1.Unscattered photons that have undergone
Compton interactions with the crystal
2.Scattered photons that have undergone Compton
interactions within the patient
Each time a gamma photon that falls within the
acceptable energy window is detected it is
mapped on to its corresponding coordinate
within the image.
29. SCATTER REJECTION
If a gamma photon scatters within the patient’s
body (via Compton scatter) it will change
direction and, therefore, will not hit the
detector at a location corresponding to its
location of origin.
It is important to reject these scattered photons
as they degrade the image contrast and spatial
resolution.
This cannot be done by the collimator and is,
therefore, done electronically by a process
called energy discrimination.
30. SPECT
SINGLE PHOTON-uses single gamma photon
detection that are produced by gamma photon
decay
EMISSION:Radioactivity used to create image is
emitted from patient rather than transmitted
through patient from an outside source as is
done in x-ray imaging
COMPUTED TOMOGRAPHY:
Slices are imaged that can be reconstructed
into 3D data
31.
32. SPECT imaging is performed by using a gamma
camera to acquire multiple 2-D images , from
multiple angles.
A computer is then used to apply a tomographic
reconstruction algorithm to the multiple
projections, yielding a 3-D data set.
This data set may then be manipulated to show
thin slices along any chosen axis of the body
33. SPECT studies use standard radionuclides (eg,
technetium-99m or iodine-123.
These standard radionuclides commonly emit
gamma-ray photons with energies that are
much lower than 511 keV. A typical example is
Tc-99m, which emits 140 keV photons.
• An exception is the widely used myocardial
agent, thallium-201, emits average energy of
about 72 keV.
34. TYPES BASED OF HEADS.
SINGLE HEADED
DOUBLE HEADED
TRIPLE HEADED
35.
36.
37.
38.
39. PET
POSITRON EMMISION TOMOGRAPHY.
The concept of PET is to radiolabel a bio-
compound, inject it into the patient,
Then measure its bio-distribution as a function of
time to determine physiologic quantities
associated with the biocompound.
All PET compounds are radiolabeled with
positron-emitting radionuclides.
40. These radionuclides have decay characteristics
that enable localization in the body.
A positron is emitted from the nucleus, travels a
short distance, and annihilates with its
antiparticle (an electron), which results in two
511-keV photons traveling in opposite directions.
After both photons are detected, the activity is
localized somewhere along the line defined by
the two detectors.
41. WHAT IS ANNHILATION?
1. Positron decay
In positron decay a positron (represented as e+, β+ or e)
is released, which is the antiparticle of the electron (e–).
A positron has the same mass and magnitude of charge
except that the charge is positive.
42. 2. Positron travels through matter
As it travels it collides with atoms losing energy and
causing ionisation
As it collides the positron is deflected and the path
becomes tortuous.
The length of the path depends upon the number of
collisions and the starting energy of the positron
43. Positron released → travels through body →
interacts with electron (annihilation) → release
two gamma photons of 511 keV that travel in
opposite directions
3. Annihilation -TO DESTROY
44. RADIOPHARMACEUTICAL USED?
MC used is a glucose analogue, 2-[F18]fluoro-2-
deoxy-Dglucose(FDG)
WHY FDG?
18F is a small atom.
Its addition to a molecule does not deform it to
the point where it is not recognized by the body
anymore
Has a half-life of 109 minutes.
It is also not long enough to keep the radiation
burden to patient low.
15O,11Ammonia,11Leucine,18Fluorine
45. FDG is uptaken by metabolically active cells
Metabolised and undergoes beta+ decay to
produce neutron and a positron
The positron travels a short distance and
annihilates with an electron.
The annhilation reaction results in the
formation of two high energy photons which
travel in diametrically opposite directions.
48. Detectors are 18-40 rings of crystals forming a
cylindrical field of view about 15cm long that can
acquire many slices of coincidence data
Group of crystals is put together into a block
Four PMT’s to each block of crystal
Localizing the site of impact is achieved by
measuring the light detected in each PMT
Signal is then amplified.
System must be able to determine which signals
come from paired 511keV photons and record the
time of detection
50. Forming an image
As annihilation produces two gamma photons that
travel in opposite directions, this is used to
determine which photons should be used to form
the image.
Two opposite detector elements must
simultaneously detect a gamma photon for those
photons to contribute to the image.
The simultaneous gamma photon by opposite
detector elements is called a coincidence and
the line between the two detector elements is
called the line of response.
55. PET MRI
Hybrid technique utilising the functional uptake
information of pet with the anatomical and soft tissue
detail of mri.
PET-MRI offers a lower ionising radiation dose
56. PET vs CT MRI
PET CT MRI
Shows extent of disease Detects changes in body
structure
Can help in monitoring
treatment and shows it’s
effectiveness
Simply confirms the presence
of a mass
Reveals disease earlier, can
diagnose faster
Can detect whether a mass is
benign or malignant
Can detect abnomalities before
there is an anatomical change
57. SUMMARY
-BOTH FUNCTIONAL AND METABOLIC INFORMATION
SPECT:
1 gamma photon of 140kev.More radiation exposure and
time.
NaI with thallium is scintillator.
Gamma emittors are used.
usually one large crystal based detector
PET:
2 gamma photons of 511 kev .Less radiation exposure and
time.
Annhilation reaction(coincidence imaging).
Positron emittors are used.
has a ring of multiple detectors
