Gamma Cameras Flashcards
What are the four desirable scintillator properties?
- highest possible intensity for a given amount of energy absorbed
- transparent to its own light
- decay time should be short so photocathode can differentiate between 2 sep. events
- must be adequate to cause all the energy of the even to be deposited into the detector
why is transparency important?
if the scintillator is transparent to its own light, photons will reach the PMT with minimum loss of intensity
what is intensity?
it describes the number of light photons
how does radiation interact with scintillators?
Compton, photoelectric effect and pair production
what are the four steps to processing detection?
- interaction
- light production
- photoelectron released
- multiplication of electrons to form pulse
what is the preferred type of interaction?
photoelectric for complete gamma absorption
how does light production occur?
when the excited atoms in the scintillator returns to ground state
light intensity is _______ to the amount of energy absorbed.
proportional
where does the exponential amplification of electrons occur?
in the PMT
what causes the exponential amplification?
a series of charged plates to accelerate the electrons to produce secondary electrons
what are dynodes?
the series of charged plates
how is a pulse created?
by the collection of electrons at the anode
what is a voltage pulse?
a burst of electrons collected at the anode
describe the relationship between voltage pulse height and the number of electrons collected at the anode.
number of electrons collected there is PROPORTIONAL to the voltage pulse height
which pulse does the amplifier manipulate?
the z pulse
what does the z pulse represent?
size of the pulse in volts
aka the result of the energy absorbed within the crystal
what is a collimator?
1/2 inch to 2 inch piece of lead that is meant to attenuate gamma coming towards the crystal from a bad angle
what does absorptive collimation describe?
how the collimator only allows the photons travelling in an appropriate direction to interact with the crystal
larger holes in the collimator = _____ permissive = ____ resolution, _____ sensitivity
larger holes = more permissive = less resolution = high sensitivity
smaller holes in the collimator = _____ permissive = ____ resolution, _____ sensitivity
smaller holes = less permissive = higher resolution = lower sensitivity
what is the physical characteristics on a parallel hole collimator dependent on?
- energy of radionuclide
- required performance (reso vs. sensitivity)
when do you use a DIVERGING collimator?
when you need to increase the FOV of small diameter crystals for large organs
what type of collimators MAGNIFY?
1) pinhole
2) converging
what will pinhole collimators do?
1) magnify
2) reverse the image
what is the slant hole collimator?
a variation of the parallel-hole collimator on a 30 degree slant
fan beam collimators consist of: one axis is a _____ collimator, and the other axis is a _____ collimator
parallel-hole collimator + converging collimator
what would a fan beam collimator be used for?
SPECT studies
what is the role of the console?
to accept and analyze information from the head
what is the role of the camera head?
detection and determination of the position of activity in the organ of interest
more PMT = ______ spatial resolution and linearity
more PMT = better spatial resolution and linearity
what are the three common shapes of PMTs?
- circular
- hexagonal
- square
what type of positioning logic does an analog system use?
coordinate system
what is a coordinate system?
when the origin is (0,0) and it’s the centre of the crystal
what is the role of an amplifier?
shaping and increasing the pulse size
what are 3 parameters that determine collimator spatial resolution?
- hole length
- hole diameter
- distance between source and collimator
_____ length and _____ diameter holes produce better spatial resolution. why?
longer length + smaller diameter
- because they limit the nonperpendicular photon angles accepted
what are the two methods used to determine the centroid?
coordination system + PMT output to be digitized by ADC
what does normalization do?
it keeps the image size constant regardless of the photon energy and helps maintain good spatial resolution
which pulse is analyzed by the PHA?
z pulse
what information does the X and Y signal give?
it will determine the spatial location of the interaction within the crystal
what program control is only used for calibration?
spectrum display
which program control is used for positioning?
persistence scope
is the persistence scope affected by the settings on the console?
it is NOT
which scintillation camera correction is required because of NONUNIFORM RESPONSES OF THE PMTS?
energy correction
what can be found between the detector and the pmt?
a light pipe
what is a light pipe?
transparent material that allows scintillation light to SPREAD AS IT EXITS THE CRYSTAL
what is spatial linearity?
accuracy of the positioning of detected events
why do you need to do a spatial linearity correction?
because of the nonlinearity of PMT responses and the mispositioning of events
what is uniformity?
the ability of the camera to reproduce a uniform radioactive distribution
what does uniformity correction do?
it decreases the variability in count densities to an acceptable level
what causes an energy spectrum to shift?
whether the event is directly under the PMT or between PMTs
what will happen if you keep the photo peak centred?
increase in energy resolution of the system
what are the 4 parameters that affects the collimator’s sensitivity?
- hole length
- hole diameter
- hole geometry
- septal thickness
_____ hole length + _____ hole diameter = greater sensitivity
shorter hole length + larger hole diameter = greater sensitivity
does changing the distance between the source and the collimator have an effect on sensitivity?
nope.
what causes the “pincushion effect”?
malpositioning of the events (when X and Y aren’t normalized)
what does the pincushion effect cause?
poor field uniformity
what is the spatial linearity correction?
usage of a table of position-based correction factors
who is responsible for energy/linearity corrections?
factory or service personnel
who is responsible for camera maintenance?
service personnel
what is responsible for autotuning?
gamma cameras
who is responsible for the uniformity correction map?
technologist
what is the range of temperature that imaging rooms are recommended to be at?
20-21 degrees
what are some other special considerations for imaging rooms?
- air filters should be cleaned regularly
- humidistat to monitor moisture
- having a battery backup
- location of the gamma camera
sudden changes in temperature can damage the crystal. what is the temperature range stated in Prekeges?
more than 12 degrees celsius per hour will damage the crystal
is it analog or digital signals that are able to be manipulated, analyzed and eventually stored?
digital signals
is it analog or digital signals that are dependent on continuously variable voltages?
analog signals
is it analog or digital signals that are dependent on discrete voltages?
digital signals
is it analog or digital signals that are based on binary information?
digital signals
where does the analogy to digital energy conversion occur?
right after the PMT
what are signal weights?
signals created based on responses from PMTs closest to the detection event
what are position calculations?
when the PMT signals are modified by their position, the location of the event is determined with spatial resolution accuracy of about 3.5 mm
what is energy summation?
adding of the digitized signals from each PMT in the area of interaction
= determines the energy of that event
what are the cons of analog signals?
- increased dead time
- decreased efficiency
- affected by electronic noise
- no image manipulation
what are the pro(s) of analogy signals?
it preserves all data
what are the cons of digital signals?
- lose some data in conversion
- down time increases if problem occurs
what are the pros of digital signals?
+ decreased dead time
+ increased efficiency – quicker access and transfer of data
what is the difference between “binary” and “decimal”?
binary = system that has 2 possible digits (0,1)
decimal = system that has 10 possible digits (0-9)
you read binary left to right. t/f
false.
right to left
what is a “BIT”?
single position of binary number representation
What is a “BYTE”?
1 byte = 8 bits
what is a “WORD”?
group of bits that a computer can process at one time
how many bits make up a word? bytes?
16 bits
2 bytes (1 byte = 8 bits)
the diameter of the crystal is equal to the ______ of X and Y signals.
maximum range of X + Y = diameter of crystal
specific area on crystal = specific (A).
each (A) is assigned to a specific (B) in computer memory.
(A) = pixel
(B) = location
is it analog or digital that consists of a grid/matrix of pixels?
digital image
what is a pixel value?
it gives the location information dependent on the number of counts or other quantity associated with that pixel
X (+/-) & Y (+/-) position signals are analyzed using ADCs.
X- and Y-
what is the flow in a ramp conversion?
input pulse —> charge deposited in capacitor (ADC input) —> discharges through resistors + during discharge, gate pulse activates clock oscillator —> production of a train of pulses counted
the number of clock pulses is dependent on?
the initial amount of charge deposited – amplitude of input pulse
is the digital signal or analog signal expressed in binary?
digital signal
what does successive approximation describing?
a trial and error technique that the ADC compares the signal to preset values in succession and transmits the results in digital form
when does the successive approximation process start?
by turning on the most significant bit
when will a bit be turned off during the successive approximation process?
if the value represented is GREATER than the input value
What is the flash/parallel approximation process?
comparing signals to preset values at the same time and transmitting the results in digital form
what is the unit of division in the ADC?
1 bit therefore, 1-bit = dividing into two equal parts
2 bits = dividing into four equal parts
more bits an ADC = better the _______.
accuracy
What is the LUT? (Lookup table)
single digital array consisting of shades of grey with values ranging from zero to high values
what shade is zero on the LUT?
white
what shade is the highest value on the LUT?
black
what is the LUT used for?
to correlate image density to counts per pixel
more bits to an ADC = _____ to digitize a signal
more bits = more time
faster sampling rates = more _____
faster sampling rate = more accurate but need more storage space
loss of information could occur due to slower sampling rate. t/f
true
