Basic principle of gamma camera

BASIC PRINCIPLE OF
GAMMA CAMERA
Upakar Paudel (B.Sc
MIT) UCMSTH

INTRODUCTION
• Nuclear medicine is a branch of medicine and medical imaging
that uses small amount of radiotracer/radiopharmaceuticals
to diagnose disease and treat disease.
• Nuclear medicine differs from other medical imaging
modalities in the sense that CT and MR scans are anatomically
based but nuclear medicine looks at physiology of all the
organ system. So, we can follow the physiological processes as
they occur in the living humans using these
radiopharmaceuticals through use of appropriate imaging
system.

HISTORY
 1895 : Discovery of x-ray by Roentgen
 1896 : Discovery of radioactivity by Bequerel
 1898 : Production of radium by Curie
 1927 :Use of radon to measure the blood transit
 1945 : Invention of nuclear reactor
 1951 : Rectilinear scanner to acquire images
 1958 : Invention of Anger camera
 1964 : Use of Tc-99m (I-131 only prior to 1964)
Tc-99m: metastable (T1/2 = 6.01 hr) pure γ decay (E = 140
keV), flexible for labeling.
I-131: electrons and 364 keV photons, thyroid disorders
only
 1970 : Derivation of image reconstruction algorithm for
tomography (CT, SPECT, PET)

Activity
• The quantity of radioactive material, expressed as the number
of radioactive atoms undergoing nuclear transformation per
unit time, is called activity (A).
Decay constant
• Number of atoms decaying per unit time is proportional to the
number of unstable atoms
• Constant of proportionality is the decay constant ()
– -dN/dt =  N
– A =  N

Half life
• Useful parameter related to the decay constant; defined as
the time required for the number of radioactive atoms in a
sample to decrease by one half.
• Physical half-life and decay constant are inversely related and
unique for each radionuclide

Nuclear transformation
• When the atomic nucleus undergoes spontaneous
transformation, called radioactive decay, radiation is emitted
If the daughter nucleus is stable, this spontaneous
transformation ends
If the daughter is unstable, the process continues until a
stable nuclide is reached
• Most radionuclides decay in one or more of the following
ways:
(a) alpha decay
(b) Beta decay (+or -)
(c) Gamma emission

energyntransitioHeYX 24
2
4A
2Z
A
Z  

energyβYX -A
1Z
A
Z   
energyβYX A
1-Z
A
Z  

energyYeX A
1-Z
-A
Z  
Alpha Decay
Beta minus decay
Beta plus decay
Electron capture decay

Gamma Emission
 Radionuclides emitting gamma radiation during their decay
are potential imaging agents for nuclear medicine providing
their gamma energies are between 80 and 200 KeV.
 This is the ideal energy range for gamma cameras since lower
energies undergo tissue absorption and higher energies are
not absorbed by thin NaI(Tl) detector used in gamma camera
construction.
 Gamma emission results from any radioactive decay and
radioactive decay followed by isomeric transition of
metastable radionuclide to daughter product.
 Clinical Value :
Gamma radiation within the energies 90 to 200 KeV is
optimal for gamma camera imaging.

Isomeric Transition
energyXX A
Z
Am
Z 
•During radioactive decay, a daughter may be formed in an
excited state.
•Gamma rays are emitted as the daughter nucleus
transitions from the excited state to a lower-energy state.
•Some excited states may have a half-lives ranging up to
more than 600 years.
•Clinical Value :
Those isotopes that decay by isomeric transition
provide a source of activity that is short lived yielding a
pure gamma emitter with a minimal patient radiation
dose.

Gamma Camera
• Developed by Hal Anger at Berkeley in 1957 therefore also
called Anger camera
• An electronic device that detects gamma rays emitted by
radio pharmaceutical (e.g technetium 99m (Tc-99m) that have
been introduced into the body as tracers.
• Once a radiopharmaceutical has been administered, it is
necessary to detect the gamma ray emissions in order to
attain the functional information.
• The position of the source of the radioactivity can be plotted
and displayed on a TV monitor or photographic film.
• Either digital or analog.

• Internal radiation is administered by means of a
pharmaceutical which is labeled with a radioactive isotope /
tracer / radiopharmaceutical, is either injected, ingested, or
inhaled.
• The radioactive isotope decays, resulting in the emission of
gamma rays.
• The Gamma camera collects gamma rays that are emitted
from within the patient, enabling us to reconstruct a picture
of where the gamma rays originated. From this, we can
determine how a particular organ or system is functioning.
• The gamma camera can be used in planar imaging to acquire
2-dimensional images
Gamma Camera

Components of Gamma Camera
1) The collimator
2) Detector crystal
3) Photomultiplier tube array
4) Position logic circuits
5) Gantry

Collimator
 Made of perforated or folded usually lead or tungsten and is
interposed between the patient and the scintillation crystal.
 Is about ½ - 2 inches thick slab.
 Allows the gamma camera to localize accurately the
radionuclide in the patient’s body.
 Performs this function by absorbing and stopping most
radiation except that arriving almost perpendicular to the
detector face.

 Of all the photons emitted by an administered
radiopharmaceutical, more than 99% are wasted and not
recorded by the gamma camera, less than 1% are used to
generate the desired image.
 Thus it is rate limiting step in the imaging chain of gamma
camera technology.
 Consist of single or many holes
 The lead walls between the holes are referred as septa.
Collimator

Types of Collimator
 Pin hole collimator
 Multi hole collimator
-parallel hole collimator
-converging collimator
-diverging collimator

Pin Hole Collimator
 Collimator consists of small pinhole aperture in a piece of
Lead, (most common),Tungsten, platinum aperture is located
at the end of a lead cone , typically 20-25cm from detector.
 Size of pinhole can be varied by using removable inserts.
 Radiation must pass through the pinhole aperture to be
imaged and the image is always inverted on the scintillation
crystal.
 Used primarily for magnification imaging of small objects
 Poor sensitivity.
 Are routinely used for very high resolution images of small
organs , such as thyroid and for certain skeletal regions such
as hips or wrists.

Increasing source-to-detector distance leads to :
1) increasing FOV
2) decreasing image size
3) lower R
4) lower sensitivity
Pin Hole Collimator

Multi Hole Collimator
 Most widely used multihole collimator in nuclear medicine
laboratories.
 Consists of thousands of parallel holes with la long axis
perpendicular to the plane of the scintillation crystal.
 The lead walls between the holes – septa.
 Holes may be round, square, triangular or hexagonal.
 Septa absorbs most gamma rays that do not emanate from
the direction of interest.
 For high energy gamma rays- thicker septa are used than for a
low energy rays.
 Septa are designed in such a way that septal penetration by
unwanted gamma rays does not exceed 10% to 25%.

 Collimators are available with different lengths and different
widths of septa.
 Longer the septa, the better is the resolution but lower is the
count rate for ( Sensitivity) for a given amount of radionuclide.
 The count rate is inversely proportional to the square of the
collimator hole length.
 There is an inherent compromise between the spatial
resolution and the efficiency ( sensitivity) of the collimator.
 If length of the septa decreased, the detected count rate
increases and resolution decreases.
 Size of object = size of image i.e. neither magnification nor
minification gain.

Basic principle of gamma camera

 Most common designs are Low Energy All-Purpose (LEAP),
Low Energy High-Resolution (LEHR) of less than 140 KeV and
Medium of less than 260 KeV and High Energy collimators of
less than 400 KeV.
 LEAP collimators have holes with a large diameter. The
sensitivity is relatively high & resolution is moderate (average
sensitivity500,000 cpm for a 1-uCi source & resolution is
1.0cm at 10cm from the patent side of the collimator)
 LEHR collimators have higher resolution images than the LEAP.
Holes are smaller & deeper. The sensitivity is approx. 185,000
cpm for 1-uCi source, and the resolution is higher 0.65cm at
10cm from the patient side of the collimator.

 Medium Energy Collimators are used for medium energy
photons of nuclides such as Krypton81, Gallium67,
Indium111.
 High Energy Collimators are used for Iodine131 and F-18FDG.
 These collimators have thicker septa than LEAP and LEHR
collimators (mainly used with Technetium 99m) in order to
reduce septal penetration by the higher energy photons.

Converging Collimator
 Converging collimator have an array of tapered holes that
converge at a point (usually 50 cm) in front of collimator
(Focal point)
 This convergence forms a magnified image.
 Resolution(high at surface) and decreases with distance
 Sensitivity- slowly increases as source is moved from
collimator face to focal plane and then decreases.
 Good for imaging smaller objects.

Diverging Collimator
 Has an array of tapered holes that diverge from hypothetical
point behind crystal( 40-50 cm).
 Generally, the use of a diverging collimator increases the
imaged area by about 30% over that obtained with a parallel
hole collimator.
 The image so obtained is minified.
 Both the sensitivity and resolution worsens as the object of
interest moves away from the collimator.
 Used particularly on cameras with small crystal faces to image
large organs such as lungs.

Specialized Collimator
 Fan beam collimator- hybrid of parallel and converging
collimator ,used in SPECT
 Multiple pinhole collimator -50% more sensitivity than
parallel collimator at same spatial resolution.
 Rotating slant hole collimator- variation of the Parallel hole
,has all tunnels slanted at a specific angle , generates an
oblique view for better visualization of an organ, which is
blocked by other parts of body & can be positioned close to
the body for the maximum gain in resolution.

Scintillation Detector
 Uses Sodium iodide crystals activated with thallium (0.1-0.4
mole %) coupled to PMT as detector .
 The crystal surface may be circular and up to about 22 inches(
10- 21.5 inches) in diameter or it may be square or
rectangular.
 Crystals are usually ¼ - 5/8 thick( usually 3/8 inch)
 The crystal has an aluminum housing that protects it from
moisture, extraneous light, and minor physical damage.
 Larger the crystal surface diameter, the large is the field of
view.
 Thicker the crystal, worse is the spatial resolution.
 With thinner crystal , the overall sensitivity (count) decreases
by 10 % but approx. 30% increase in spatial resolution.

Properties of scintillation crystal
 High efficiency for stopping gamma rays.
 Stopping should be without scatter.
 High conversion of gamma rays into light.
 Wave length of light should match response of PMT’s.
 Crystal should be transparent to the emitted light.
 Crystals should be mechanically robust.
 Length of scintillation should be short.
 High iodine content (Z=53) & high density
(3.67g/cm3)provides high QDE
 Is sensitive to photons of energy 50 to 300 keV

 High conversion efficiency , 13 % of deposited energy is
converted to light – best energy resolution.
 Sensitivity>85% at140 keV
 Transparent to its own scintillation emission
 Emits light very promptly, decay constant – 230 nsec
 Can be grown relatively inexpensively in large plates
 Spectral matching between the emitted light and the
sensitivity of the PMT
 However they are fragile and crack easily if subjected to rapid
temperature change
Properties of scintillation crystal

What happens within crystals..
 Unfortunately, only a small fraction of the energy lost by a
gamma ray is converted into light, typically 10%.
 Interaction of the gamma ray with the crystal may result in
ejection of an orbital electron (photoelectric absorption),
producing a pulse of fluorescent light (scintillation event)
proportional in intensity to the energy of the gamma ray.
 Some photons may pass through the crystal without causing
light event.
 Some scatter out and deposit some energy ( Compton
Scattering)
 In most thallium activated sodium iodide crystals, about 20-30
light photons are released for each KeV of energy absobed.

 About 30% of the light from each event reaches the PMTs so
amplification is necessary.
 Low-energy radionuclides do not show much difference in
sensitivity between the two. As the energy of the radioisotope
increases, the difference in sensitivity increases
 Modern camera have rectangular crystal (60x40 cm) which
provides increased FOV
 Is surrounded by highly reflective material TiO2 or Mag.Oxide
to maximize light output
 At higher photon energies (>=300 KeV) , needs large volume
of NaI(Tl) for adequate detection efficiency
What happens within crystals..

Light Guides
 While the front face and sides of the crystal are canned,
usually with aluminum sufficiently thin so as not to attenuate
the incoming gamma rays unduly, the rear crystal surface
needs a transparent interface between the crystal and PMTs.
 This is usually provided by a Pyrex optical plate or light guide
a few centimeters in thickness.
 Array of PMT is coupled optically to the back face of the
crystal with a silicon based adhesive or grease.
 Light Guides(Lucite light pipe)- employed between the
detector crystal and PM tubes.
 It increase light collection efficiency , improve the uniformity
of light collection as a function of position.

Photo Multiplier Tube
 Is an electronic vacuum tube containing a light sensitivity
photocathode, 10 to 12 electrodes called dynodes and an
anode
 While PMTs with a photocathode diameter of 3 inches are
used mainly, it is also necessary to use some 2 inch diameter
tubes.
 It performs two functions-conversion of light photon to
electrical signal & Signal amplification
 PMT is attached to the back of the crystal directly on the
crystal, connected to the crystal by light pipes or optically
coupled to the crystal with silicon like material.
 PMT detects and amplifies the signal.(106 -108)

 Gamma camera consists of array of PMT(1st gamma camera –
7 PMT)
 Cameras with 37,55,61,75 or 91 tube are common ( 40-100 ).
 The greater the no. of PMTs, the greater is the resolution.
 Number is determined by the size & shape of both crystal &
each individual PMT.
 More the number better is the spatial resolution & linearity
 With thicker crystal, PMTs are farther away from the
scintillation point and are unable to determine the
coordinates as accurately, thus reduced spatial resolution.
 Current tubes have hexagonal cross section to cover more
area for efficient detection of scintillation photons
Photo Multiplier Tube

 When a photon is absorbed by the crystal, a no of PMT
around the specific point will see the light & produce
electrical pulse .
 Amplitude of pulse in a given PMT is directly proportional to
amount of light received by photocathode
 PMT closest to scintillation event will give largest output pulse
 Localization of the event in the final image depends on the
amount of light sensed by each PMT and thus on the pattern
of PMT voltage output.
Position and Summing Circuit

 The summation signal for each scintillation event is then
formed by weighing the output of each tube.
 This signal has three components: spatial coordinates on x-
and y- axes as well as a signal (z) related to the intensity
(energy).
 The x- and y- coordinates may go directly to instrumentation
for display on the CRT or may be recorded in the computer.
 Z (energy) pulse is obtained by adding the signal from all PMTs
& is proportional to total energy deposited in crystal.
 The signal intensity is processed by the PHA.
Position and Summing Circuit

 The basic principle of the PHA is to discard signals from
background and scattered radiation or radiation from
interfering isotopes so that only photons known to come from
the photopeak of the isotope being imaged are recorded.
 The PHA discriminates between events occuring in the crystal
that will be displayed or stored in the computer and events
that will be rejected.
 The PHA can make this discrimination because the energy
deposited by a scintillation event in the crystal bears a linear
relation to the voltage signal emerging from the PMTs.
 The pulse height analyzer allows the operator to select only
the signals from those gammas in which the height of the Z
signal, that is, gamma ray energy, has a certain value or range
of values.
Pulse Height Analyzer (PHA)

 If many useful gammas are not to be excluded from the
image, a range of energies must be allowed through the PHA,
and typically a window equal to 20% of the peak energy value
is used
 For 99mTc with a gamma ray of 140 KeV those signals with
energies between 126 and 154 keV are judged to be
acceptable.
 When using radionuclides ( gallium 67) that emit gamma rays
at different energies, multiple window analyzers need to be
employed.
 Typically a maximum of three sets of windows is available.
 On newer cameras, the signal processing circuitry such as
preamplifiers and PHAs is located on the base of each PMY so
that there is little signal distortion between the camera head
and the console.
Pulse Height Analyzer (PHA)

 Most gamma camera allow for a fine adjustment known as
automatic peaking of the isotope.
 Occasionally, an asymmetric window is used to improve
resolution by eliminating some of the Compton scatter.
 Image exposure time is selected by console control and
usually compromises :
• a preset count, a preset time and a preset information density
for the image a accumulation.
Console Controls

Display
• The final x and y signals generated by the positional circuitry
are accepted by either an analog film formatter or digitized to
take part in computer display system.
• The analog film formatter uses a cathode ray tube that has an
extreamly fine dot dimension.
• The light from the dot is recorded on the flm directly on the
film to produce an image.
• Single dot represents a single gamma photon within the
chosen energy window.
• A good quality image requires at least 1 million of these dots
equivalent to 1 million of the accepted gamma events.

Summary
• A collimator accepts orthogonal gamma events from the
patient.
• A large NaI crystals is the scintillation detector.
• PM tubes forms a hexagonal or rectangular array on its
surface.
• The signals from those goes to charge amplifiers.
• The signals are logarithmically amplified and accepted by the
positional circuitary which computes x and y axes of gamma
events.
• PHA enables this signal if x and y signal is a photo peak event.
• The x and y signals forms the display.

References…
• Essentials of Nuclear Medicine Imaging, 5th
Edition, Fred A. Mettler & Milton J. Guiberteau.
• The Essential Physics of Medical Imaging, 2nd
Edition, Bushberg.
• The Physics of Diagnostic Imaging, 2nd Edition,
Dowsett, Kenny & Johnston
• Various websites.

Basic principle of gamma camera

More Related Content

What's hot (20)

Similar to Basic principle of gamma camera (20)

More from Upakar Paudel (14)

Recently uploaded (20)

Basic principle of gamma camera