Transcript The Components of gamma camera

Chapter-2
The Planar Imaging
Gamma Camera
The Components of gamma camera
• The collimator
• Camera head
• Scintillation crystal
• Light guide
• Photo-multiplier tubes (PMT)
• Electronic (preamplifiers and
amplifiers-digital position logic
circuits, pulse-height analyzer
circuits)
• Computer monitor
Components of a standard nuclear medicine
imaging system
Chapter-2
The Planar Imaging
Camera head
The Photomultiplier tubes (PM tubes-PMTs)
Chapter-2
The Planar Imaging
Camera head
The Photomultiplier tubes (PM tubes-PMTs)
• Convert the light to multiplied electrical
signal
• The PM tube is a vacuum tube with a
photocathode on the end, placed adjacent
to the crystal
• The photocathode is a clear
photosensitive glass surface
• This is couples with a light conductive
transparent gel to the surface of the
crystal (the transparent gel has the same
refractive index as the crystal and the PMT
window
Chapter-2
The Planar Imaging
Camera head
The Photomultiplier tubes (PM tubes-PMTs)
• The light striking the photocathode causes
it to emit electrons, referred as
photoelectrons (on average, four to six
light photons strike the photocathode for
each photoelectron produced)
• The number of electrons produced at the
photocathode is greatly increased by the
multiplying action within the tube.
Chapter-2
The Planar Imaging
Camera head
The Photomultiplier tubes (PM tubes-PMTs)
• As soon as they are produced, the
electrons cascade along the multiplier
portion of the tube successively striking
each of the tube’s dynodes.
• Dynodes are metal electrodes, each held
at a progressively higher positive charge
than the one before it.
• As an electron strikes a dynode, it knocks
out two to four new electrons, each of
which joins the progressively larger pulse
of electrons cascading toward the anode
at the end of the tube
Chapter-2
The Planar Imaging
Gamma Camera
The Components of gamma camera
• The collimator
• Camera head
• Scintillation crystal
• Light guide
• Photo-multiplier tubes (PMT)
• Electronic (preamplifiers and
amplifiers-digital position logic
circuits, pulse-height analyzer
circuits)
• Computer monitor
Components of a standard nuclear medicine
imaging system
Chapter-2
The Planar Imaging
Camera head
Preamplifiers and amplifiers
•
the current from the PMT must be further
amplified before it can be processes and
counted.
• Despite the multiplication within the
photomultiplier tube, the number of
electrons yielded by the chain of events
that begins with the absorption of a single
gamma ray in the crystal is still small and
must be further increased or amplified
PMT and its preamplifier and
amplifier
Chapter-2
The Planar Imaging
Camera head
Preamplifiers and amplifiers
Typically, this amplification is a two-stage
process
• In the first stage, a small preamplifier
located close to the photomultiplier
increase the number of charges
sufficiently to allow the current to be
transmitted through a cable to the ,main
amplifier.
• In the second stage, the current of the
electrical pulse is further increased by the
main amplifier.
PMT and its preamplifier and
amplifier
Chapter-2
The Planar Imaging
Gamma Camera
The Components of gamma camera
• The collimator
• Camera head
• Scintillation crystal
• Light guide
• Photo-multiplier tubes (PMT)
• Electronic (preamplifiers and
amplifiers-digital position logic
circuits, pulse-height analyzer
circuits)
• Computer monitor
Components of a standard nuclear medicine
imaging system
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
• The amount of light received by PMT is related to the proximity of the tube to the site
of interaction of the gamma ray in the crystal  the PMT closet to the site of
interaction receive the greatest number of photons and generates the greatest output
pulse; the tube farthest from the nuclide source receives the fewest light photons and
generates the smallest pulse.
• Although an image can be composed solely of the point corresponding to the PMT
with the highest output at each photon interaction, the number of resolvable points
is then limited to the total number of PMT tubes (up to 128 per cameras)
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
The positioning circuit
improves image
resolution
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
• The positioning circuit improves resolution by factoring in the output from
adjacent tubes.
• The positioning circuit uses a “voltage divider” to weigh the output of each
tube in relation to its position on the face of the crystal (a simple voltage
divider can be constructed from an electrical resister).
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
In a very simple explanation of the positioning circuit
• The output of each amplifier attached to each PMT is connected to four
directional terminals (X+, X-, Y+, Y-).
• The size of the current pulse reaching each of the four terminals from the
amplifier is dependent on the proximity of the PMT to each terminal.
• The voltage divider (resisters) can be used to weight (increase or decrease)
the size of the current pulse reaching the terminals based on the distance
between the PMT and each terminal.
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
A simplified diagram of 20 PMTs, each of
which is connected through a resister to
the X- and X+ terminals
The X-axis positioning circuit divides the PMT
outputs between the X+ and X- terminals
• A photon interaction in the crystal the
output from each PMT is weighted in
proportion to its distance from the Xterminal.
• The output for the X- terminal is the sum of
these weighted outputs from the 20 PMTs
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
A simplified diagram of 20 PMTs, each of
which is connected through a resister to
the X- and X+ terminals
• The first tube is closet to the Xterminal; therefore, the full voltage
output from this rube goes to the Xterminal.
• The next PMT is one fourth (1/4) of the
distance away; so we discard a quarter
of the output and retain the remaining
three fourths.
• For the last tube all of the output is
discarded
The X-axis positioning circuit divides the PMT
outputs between the X+ and X- terminals
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
A simplified diagram of a 30-cm
crystal is attached to 20 PMTs
Locating the position of a source in
the X-axis using the positioning
circuit
Chapter-2
The Planar Imaging
Camera head
Positioning Circuit
A simplified diagram of a 30-cm crystal is attached to 20 PMTs to show the steps involved
in calculating the location of the site on interaction in the X direction as a um of the pulses
reaching the X- and X+ terminals
• When a gamma photon interacts with a crystal at “source” four PMTs receive light
photons
• The upper left of these four tubes receives the greatest number of photons (36% of the
total) since it is closet to the source; the other receive 29%, 21% and 14% respectively.
• In the next step of the process, the signal from each of the four tubes is multiplied by its
weighting factor.
• X- + X+ = X =0.6 - 0.43 = 0.017
• The positioning circuit places the source at
• 0.17x15 cm=2.55 cm, from the center of the crystal in the X+ direction.
• Te output of the Y- and Y+ terminals are processed in the same way
Chapter-2
The Planar Imaging
Gamma Camera
The Components of gamma camera
• The collimator
• Camera head
• Scintillation crystal
• Light guide
• Photo-multiplier tubes (PMT)
• Electronic (preamplifiers and
amplifiers-digital position logic
circuits, pulse-height analyzer
circuits)
• Computer monitor
Components of a standard nuclear medicine
imaging system
Chapter-2
The Planar Imaging
Camera head
Pulse-height analyzer circuit (Pulse-height analyzer)
• The amplifiers are designed to ensure that the amplitude of each pulse is
proportional to the energy absorbed in the crystal from the gamma radiation
• The amplitude of each electrical pulse from the amplifiers is measured in the
electrical circuits of the pulse-height analyzer.
• A tally is kept showing the number of pulses of each height
• A plot of the number of pulses against their height that is their energy is
called the pulse height spectrum
Chapter-2
The Planar Imaging
Camera head
Pulse-height analyzer circuit (Pulse-height analyzer )
Detector Energy spectrum
• The shape of the pulse-height spectrum is dependent on the photon energies and the
characteristics of the crystal detector
Chapter-2
The Planar Imaging
Camera head
Pulse-height analyzer circuit (Pulse-height analyzer)
• The pulse-height analyzer is often used to “select” only pulses
(conventionally called Z-pulses) that correspond to a range of the acceptable
energies. This range is called the energy window.
• A window setting of 20% for the 140 KeV photopeak of 99m-Tc means that Zpulses corresponding to a 28 KeV range centered on the 140KeV will be
accepted and counted
Chapter-2
The Planar Imaging
The Photopeak
Chapter-2
The Planar Imaging
The Photopeak
Chapter-2
The Planar Imaging
Camera head
Computers
• Nuclear medicine computers are used for the acquisition, storage and processing of
the data
• The image data are stored in digital form (digitized) as for each Z-pulse that is
accepted by the pulse-height analyzer, one count is added to the storage location that
corresponds to its x, y location determined by the positioning circuit.
• The data storage can be visualized as a matrix (a kind of 2D checkerboard)
• Each position within the matrix corresponds to a pixel within the image and is
assigned a unique “address” composed of the row and column of its location.
Chapter-2
The Planar Imaging
Camera head
Computers
Chapter-2
The Planar Imaging
Camera head
Computers
Chapter-2
The Planar Imaging
Camera head
Computers
• Matrices are defined by the number of subdivisions along each axis.
• The operator can select from several matrix configuration of successively finer
divisions; 64x64, 128x128,256x256 and 512x512 or more (these numbers refer to the
number of columns and rows in a square matrix.
• The outside dimensions of all matrices are the same size. What varies is the pixel size
and hence the total number of pixel
• Example; A 64x64 matrix has 4096 pixel ; a 128x128 matrix has 16384 pixels.
Chapter-2
The Planar Imaging
Camera head
Computers
• The greater the number of pixels the smaller is each pixel for a given field of view
(FOV) and the better preserved is the resolution of the image.
• The camera and computer system cannot reliably distinguish between two points that
are separated by less than 1 pixel.
Chapter-2
The Planar Imaging
Camera head
Computers
Effect of matrix configuration on
image resolution
Chapter-2
The Planar Imaging
Camera head
Computers
A matrix cannot resolve
points separated by less
than 1 pixel
Chapter-2
The Planar Imaging
Camera head
Computers
Chapter-2
The Planar Imaging
Camera head
Computers
Chapter-2
The Planar Imaging
Persistence Scope (P-scope) (P-view)
• The image is stored in the computer memory and then using it to update the image
on screen slowly or rapidly.
• This allows the person who is acquiring the scan to adjust the position of the patient
prior to recording the final image on film or computer
Chapter-2
The Planar Imaging
Some useful videos
https://www.youtube.com/watch?v=I0re3ncCKvM (gamma camera animation)
http://www.youtube.com/watch?v=MTZiHqPKSm8 (gamma camera)
https://www.youtube.com/watch?v=ZRvzHi-GY7c (gamma camera)
http://www.youtube.com/watch?v=1spo1YeiU7Q (Nuclear Medicine)
http://www.youtube.com/watch?v=c716Sj1HYVE (Nuclear Medicine)
Chapter-2
The Planar Imaging
Types of Image acquisition
The gamma photons emitted by the patients may be acquired in various forms
1. Static imaging
2. Dynamic imaging
3. Gated imaging
Chapter-2
The Planar Imaging
Types of Image acquisition
1. Static imaging
•
•
•
Is used to collect images of different regions of
the body or differently angled (oblique) views of
a particular region of interest
Example (Bone scan is composed of static
images of 12 different regions of the body)
Usually there is very little changes in the
distribution of nuclide in the organ on interest
while the images are being acquired.
The images were obtained by moving the gantry at
a steady rate 12 cm/min while acquiring the
imaging data
Chapter-2
The Planar Imaging
Types of Image acquisition
2. Dynamic imaging
•
If the distribution of nuclide in the organ is changing rapidly and it is important to
record this change, multiple rapid images of a particular region on interest are
acquired
•
Example: it is used to collect sequential 1-second frame (called frame) of the flow
of nuclide through the kidney
•
Dynamic imaging can be thought of as a type of video recording to “catch”
images of fast action; static images are similar to photograph
http://www.youtube.com/watch?v=1AmtL0OESuk (example of Dynamic imaging in clinic)
Chapter-2
The Planar Imaging
Types of Image acquisition:
3. Gated imaging cardiac gated or respiratory gated
• Gated images area a variation of dynamic images
• Data are divided into frames but different from the dynamic frames
• Continuous images are obtained of a moving organ and data are coordinated with
the rate of heart beat or respiratory cycle
Chapter-2
The Planar Imaging
Types of Image acquisition:
3. Gated imaging cardiac gated or respiratory gated
An illustration of the principle of respiratory motion
synchronization
Chapter-2
The Planar Imaging
Types of Image acquisition:
3. Gated imaging cardiac gated or respiratory gated
https://www.youtube.com/watch?v=n9O1qNiyO6Q (gated imaging)
https://www.youtube.com/watch?v=X-3GOxUwNj8 (respiratory gating)
Chapter-2
The Planar Imaging
Types of gamma
cameras
triple
single
dual
Chapter-2
The Planar Imaging
Single-headed gamma camera
• The most common type of gamma camera
• It consists of a gamma camera detector, mounted on a gantry that allows the
camera head to be positioned in a flexible way over different regions of the
patients body
• Often moving bed is incorporated to permit imaging studies of the whole
body
• The gamma camera head often is mounted on a rotating gantry, allowing it to
take multiple views around the patients. This feature also is necessary for
producing tomographic images or cross-sectional images through the body
Chapter-2
The Planar Imaging
Single-headed gamma camera
Chapter-2
The Planar Imaging
Dual-headed gamma camera
• In these systems two gamma heads are mounted onto the gantry
• Usually, the two heads can be positioned at a variety of locations on the
circular gantry
• An obvious advantage of a dual-headed camera is that two different views of
the patients can be acquired at the same time
• For example, in whole-body imaging, the two detector head can be placed at
180 degrees to each other to provide anterior and posterior views
simultaneously
Chapter-2
The Planar Imaging
Dual-headed gamma camera
Chapter-2
The Planar Imaging
triple-headed gamma camera
• Primarily for tomographic studies
Chapter-2
The Planar Imaging
a range of specialty gamma cameras have been or are being, developed for
specific imaging tasks.
Chapter-2
The Planar Imaging
Important points in chapter 2
• The gamma camera (the basic principles of the gamma camera)
• The types of gamma camera
• The type of image acquisitions
Chapter-2
The Planar Imaging
End of chapter 2