Quality assurance refers to a system
designed to identify problems, propose solutions and monitor results to achieve
the desired level of performance
Quality control refers to the testing
designed to identify equipment problems.
THE
SCINTILLATION CAMERA OR GAMMA CAMERA
Characteristics of an Ideal
Gamma Camera
1. Field
Uniformity
Field
uniformity is the ability of a scintillation camera to produce a uniform image
when the source provides a uniform distribution of photons over the detector.
This has to be performed on a daily basis.
Two types of
field uniformity testing
a. Intrinsic
testing monitors the condition of the sodium iodide crystal and electronics
associated with the scintillation camera detector and is performed without a
collimator attached to the detector.
b. Extrinsic
testing assesses the instrument as it is used clinically so collimator is
placed in it.
Comparison of
Intrinsic and Extrinsic Scintillation Camera Quality Control
Intrinsic
Extrinsic
Collimator None Low energy
Parallel–hole
Radiation
source Tc99m point source Tc99m liquid flood
Tc99m
liquid flood Co57 solid sheet
Co57
solid sheet
Source
distance point source
placed flood
source placed
4
– 5 crystal diameters directly on
face of
away
from detector collimator
flood
source placed on
crystal
face
Causes of
field non–uniformities
a. Misadjusted
photopeak
b. Damaged
crystal
c. Collimator
defect
d. “Mistuned”
photomultiplier tube
e. Photomultiplier
tube malfunction
f.
Radioactive contamination on
crystal or collimator
g. Scratched,
dirty CRT screen or multiformatter lenses
h. Film
processing artifacts
2. Spatial
resolutions
Spatial
resolution is the ability of a scintillation camera to reproduce small
differences in radionuclide concentration in closely spaced areas. Clinically,
resolution affects the ability to visualize small defects. This can be
performed intrinsically or extrinsically using a bar or multihole phantom.
a. Hine–Duley
Phantom
The Hine–Duley
phantom consists of lead bars and spaces of three different widths; the bars
are embedded in plastic. Since the resolution is not measured equally over all
areas with this phantom, multiple images must be acquired to assess resolution
for the entire detector area.
b. Four
quadrant Bar Phantom
The
four–quadrant bar phantom contains spaces and lead bars of four different
widths. The spaces and bars in each quadrant are of equal width. Four images,
acquired at 90o to one another are necessary to monitor resolution
over the entire crystal.
c. Parallel–line
Equal Space (PLES) Phantom
All lead bars
and spaces are of equal width in the PLES phantom. Two images acquired at 90o
to one another are necessary to monitor resolution over the entire crystal.
d. Orthogonal–hole
phantom
The orthogonal–hole
phantom consists of a lead sheet containing holes of equal diameter arranged at
right angles to one another. Only one image is needed to determine resolution
over all areas of the detector.
Factors
affecting scintillation camera spatial resolution
a. Photopeak
adjustment
b. PHA
window width
c. Source–detector
distance
d. Gamma
ray energy
e. Collimator
type
3. Spatial
Linearity
Spatial linearity
is the ability of a scintillation camera to produce a uniform image with
straight lines corresponding to straight lines in a phantom. Clinically, it is
the accurate portrayal of a true organ shape. Linearity can be assessed along
with resolution by examining the straightness of a set of parallel bars or, in
the case of the orthogonal–hole phantom, the straightness of parallel rows of
holes.
4. Sensitivity
Sensitivity is
the ability to detect ionizing events in a sodium iodide crystal expressed in counts
per second per microcurie (cps/ µCi). It can be measured simultaneously when
uniformity is performed if the activity of the source and the room background
is known. The activity in the source must be carefully measured and corrected
for decay, then the count rate obtained per unit activity can be calculated.
Sensitivity = (counts –
background) ÷ time
activity
of source
= cps / µCi or cpm / µCi
Sensitivity
can be affected by incorrect energy settings, incorrect collimation, if
extrinsic sensitivity is performed, or improper detector–source distance.
DOSE
CALIBRATOR
Quality control tests for dose
calibrator
1. Constancy
Constancy
testing determines the reproducibility of measurements of a source of known
activity from day to day. A long lived standard is measured at each of the
commonly used radionuclide settings. Each measurement is compared to the
calculated (decay–corrected) value for the standard at that setting on that
day.
a. Count
a long–lived standard, such as Co57 or Cs 137 of known activity on the
appropriate dose calibrator setting
b. Obtain a background at the same setting to determine
net activity
c. Correct
the known activity for decay and determine the ± 10% range of this value
d. The
measured value should be within 10% of the decay–corrected activity
e. Using
the same reference source, repeat the measurement for all commonly used
radionuclide settings.
f.
Again correcting for background
and decay, determine the ± 10% range for each calculated value. Compare each
measured value to the range of calculated values for that radionuclide setting.
g. Each
measured value should be within ± 10% of the decay–corrected value at each
setting.
CO 57 has a half life 271 days
Cs 137 has a half life 30 years
According to
the NRC, if the constancy error is greater than 10%, the instrument should be
repaired or replaced.
2. Activity
Linearity
A linearity
test ascertains how accurately a dose calibrator measures activities over a
wide range, from millicurie to microcurie amounts. The test involves assaying a
source with a relatively short half–life over a period of several days and
comparing the predicted and measured results.
a. Assay
an aliquot of Tc99m 2–3 hour interval over 3 more days. According to the NRC, the
starting activity should approximate the highest activity that will be
administered to a patient. The starting activity must be assayed until it
reaches 30 µCi
b. Correct
all measurements for background
c. Calculate
the predicted activity for each time the sample was measured.
d. Plot
both the calculated and measured activities on semi–log paper (activity versus
time)
e. Measured
activities should be within ± 10% of calculated activities
According to
NRC, if linearity errors exceed 10%, mathematically corrected dose calibrator
readings should be calculated for activities greater than 10 µCi.
An alternate
method of performing activity linearity test is to use the commercially
available system “Calicheck” or “Lineator.” These systems consist of a series
of lead tubes that absorb Tc99m photons by a known amount. After the source is
assayed initially, it is measured several more times after each of the lead
tubes is placed in sequence around the source. Measurements of the shielded
source simulate radioactive decay. This method saves time and decreases
personnel exposure. The source used to perform the test may also be used in
patients after linearity data collection.
3. Accuracy
Accuracy
testing assesses the ability of a dose calibrator to accurately measure the
activity of radionuclides of different gamma energies. According to NRC, at
least two different standards, one of which must have photon energy between 100
and 500 keV, must be assayed.
Accuracy
testing is performed as follows:
a. Assay
each standard in the dose calibrator three times at the appropriate
radionuclide setting. Measure the background at each setting.
b. For
each standard, average the three readings and subtract the background to obtain
the net activity.
c. Correct
the known reference activity for decay and determine the ± 10% range of the
decay–corrected value.
According to
NRC, if the accuracy error is greater than 10%, the dose calibrator should be
repaired or replaced.
4. Geometric
variations
Geometric
variation determines the effect of sample volume and configuration on the
measurement of sample’s activity.
For example,
20 mCi contained in a 1–ml volume syringe and 20 mCi contained in a 10 ml
volume glass vial may yield significantly different measurements when assayed
in the same dose calibration
a. Assay
1–2 mCi Tc99m in a minimal volume (1 – 2 drops) in a 20 – 30 ml vial. Obtain
the background reading.
b. Increase
the volume in the vial stepwise by adding water. The volumes should be those
commonly assayed.
c. Assay
the vial after each addition of water and obtain a background reading.
d. Correct
each measurement for background and calculate a correction factor for each
volume.
% correction
factor = true activity
measured
activity
e. Plot
the correction factors against the volumes on linear graph paper
f.
After assaying a vial of
activity in the dose calibrator, calculate the true activity using appropriate
correction factor
True activity = measured
activity x correction factor
g. A
similar procedure can be used for various size syringes
According to
the NRC, correction factors should be calculated for any geometric variations
causing measurements to vary by more than 10% from the true activity.
SCINTILLATION
SPECTROMETER
A scintillation spectrometer is an
instrument, such as an uptake probe or scintillation well counting system that
uses pulse–height analysis to determine which pulses will be counted.
1. Calibration
of a scintillation spectrometer involves determining the appropriate operating
high voltage and amplifier gain.
a. Operating
voltage is the high voltage applied to the dynodes of the photomultiplier tube
(PMT). The high voltage can be adjusted so that arbitrary units on the pulse–height
analyzer dials, upper and lower level discriminators (ULD and LLD), corresponds
to meaningful energy (voltage) units. The operating voltage is determined in
the following manner
(1) Use
a monoenergetic gamma source of known energy like Cs137
(2) Set
a narrow window around the photopeak energy by adjusting the ULD and LLD of the
pulse–height analyzer. For Cs137, the LLD is 652 and ULD is 672 in 3% window.
(3) Beginning
with the lowest voltage setting, count the source for 1 minute at each voltage
setting, increasing the voltage until the maximum count rate is observed.
(4) Plot
counts per minute versus energy (voltage) on linear graph paper. The voltage
setting at which the highest count rate is obtained is the operating voltage.
The units on the pulse–height analyzer dials are now calibrated for 0–1000 keV.
The window can be moved up or down to accept pulses from the photopeaks of
other radionuclides.
(5) Note
gain settings. Calibration results are valid only on that gain settings.
Increasing the
voltage moves the entire gamma spectrum to the right. When the photopeak moves
into the preset window, the maximum counting rate is observed.
b. Amplifier
Gain
Adjusting the
amplifier gain will permit counting of very high or very low radionuclide
energies. When the amplifier gain is used to alter pulse height and shift the
gamma spectrum, one unit on the ULD and LLD dials is no longer equivalent to
1keV.
Types of Gain
Control
(1) True
Gain Control – an increase in gain produces an increase in pulse– height with a
corresponding decrease in the energy range.
(2) Inverse
Gain Control – an increase in gain produces a corresponding decrease in pulse–height
but a corresponding increase in the energy range.
2. Energy
Resolution – is the conversion from light to electrical energy and is indicated
by the photopeak. This can be checked using Cs137.
a. Using
the proper operating voltage and a narrow window, measure the counts for a
preset time at 10 keV increments around the photopeak.
b. Plot
counts per 10 keV interval versus the midpoint of the counting interval.
c. Calculate
the full width at half maximum (FWHM) as follows:
(1) Divide
the maximum photopeak count by 2
(2) Draw
a line across the photopeak at 50% count level determined by Step a.
(3) Drop
perpendicular lines to the x – axis where the line in step b intersects the two
edges of the photopeak. The distance between the 2 perpendicular lines (in
energy units) is the FWHM
(4) Calculate
the % energy resolution as follows:
% Energy
resolution = FWHM
(keV) x100
Midpoint of photopeak
(keV)
A typical
range for Cs137 is 8% – 12%. The energy resolution will degrade, that is, the
value will become larger as the crystal ages or if a problem develops with the
amplifier or photomultiplier tube.
3. Chi
Square Test
The Chi square
test is a method to check for random errors greater than those that would be
predicted. Random errors affect the reproducibility (or precision) or
measurements. These are always present in radiation counting because of the
random nature of radioactive decay.
a. Using
a long lived standard (Cs137 or Co57), collect a series of measurements (20 –
50), each present time.
b. Compute
the mean average value.
c. Calculate
the chi square value using the formula:
X2 = Σ(Ni
– N)2
N
d. Using
the x2 table, find the probability of p. The p value denotes the
probability that random variations observed in a series would equal or exceed
the calculated x2 value.
Acceptable p
value is between 0.1 and 0.9, values greater than 0.9 or less than 0.1 indicate
that the variations in the measurements do not match the expected and the
instrument should be checked.
SURVEY METER
Survey meter Quality Control
1. Reference
Check are performed by measuring a long lived source, using constant geometry
2. Calibration
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