2. CONTENTS:
• INTRODUCTION
• GAMMA CAMERA
• WORKING PRINCIPLE OF GAMMA CAMERA
• TYPES OF GAMMA CAMERA
• COMPONENTS
• DATA ANALYSIS
• CONTROL CONSOLE
• GANTRY
• APPLICATION
3. INTRODUCTION
• Developed by Hal Anger at Berkeley in 1957,
Therefore also called Anger Camera.
• Radionuclide imaging is one of the most important
application of radioactivity in nuclear medicine.
• The most widely used imaging device is the gamma
camera, also known as anger camera or scintillation
camera.
4. WHAT IS GAMMA 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 tracer.
• 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 analogy.
5. CONT.…..
• Internal radiation is administered by means of a pharmaceutical
which is labelled 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 –
dimensional images.
6. WORKING PRINCIPLE OF ANGER CAMERA
Radioisotopes are injected into the body (TC99)
Radionuclides emits gamma rays/ radiation
Gamma rays are absorbed by crystal elements (NaI,Tl)
These crystal elements convert gamma rays into light.
The light is captured by PM- tube
7. CONT.……
PM tube converts light into electrical signal
These electrical signals are amplified by amplifier.
Display into computer
15. 1) SCINTILLATION DETECTORS -
NAI (TL) CRYSTAL.
• Uses sodium iodide crystals activated with thallium (0.1-0.4 mol %) 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 aluminium 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% increases in spatial resolution.
16. LIGHT GUIDE
• 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.
17. PHOTOMULTIPLIER TUBE (PMT)
• PMT 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)
18. CONT.……
• 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
19.
20. PRE-AMPLIFIER
• Amplify the signals produced by detector (small).
• Shade the signal pulse for optimum signal processing by subsequent
components.
• It is placed as close to the detector as possible to maximum the
performance (maximum SNR).
21. AMPLIFIER
• Amplify the relatively small pulses from pre-
amplifier (mv to v).
• Reshape the slow decaying pulse from the pre-
amplifier into narrow one to avoid the pulse-pile
up at higher counting rates.
22. PULSE HEIGHT ANALYZER (PHA)
• 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 occurring 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.
23. CONT.….
• 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 99m Tc 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.
24. 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.
25. CONT.…..
• 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.
• Historically holes have been circular, square and hexagonal.
26. SEPTAL THICKNESS
• Septal thickness effective length is the distance travels by photons
through the septa (for photon striking the septa at an angle ).
• For higher energy photons, septal thickness must increase.
28. 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.
29.
30. MULTI HOLE COLLIMATOR
• Most widely used multi hole 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 unwanted
gamma rays does not exceed 10% to 25%.a low energy rays.
31.
32. CONT.…
• 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.
33. 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.
34.
35. 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.
36.
37. 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.
38.
39. DATA ANALYSIS COMPUTER
• Finally, a processing computer is used to deal with the incoming
projection data and processes it into a readable image of the 3D spatial
distribution of activity with in the patient.
• The computer may use various methods to reconstruct an image such as
filtered back projection or iterative method.
41. CONSOLE CONTROLS
• 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.
42. 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
extremely fine dot dimension.
• The light from the dot is recorded on the film 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.
44. GANTRY
• A gamma camera system attached with gantry.
• All circuits and motors related to movement ( longitudinal, rotational up
and down ) of gamma camera placed in gantry.
45.
46. APPLICATION OF GAMMA CAMERA
• used to locate cancerous tumor's, minor bone fractures, abnormal functioning of
organs and other medical problems .
• Iodine-131 is used to detect thyroid (a gland that absorbs Iodine) problems.
• Technetium-99 is used to find tumors' in the body.
• Gamma camera give structural and functional image of body organs.
• Myocardial Perfusion Lungs scan.
• Kidney function.
• Thyroid uptake Whole body scan
