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102
Radionuclide Dose Calibrator
The vial containing substantial amount of radioactivity is slipped into the black tube
from its mouth or alternatively from the bottom. The black tube is then placed into the well
of the calibrator, if necessary one may take out the lining of the calibrator well. Calibration
factors are calculated and linearity is checked. The instructions are given in their manual for
measuring the linearity. This method is hardly in common use probably due to added cost.
Geometric factors influencing the measurement in a dose calibrator
Sample size
A volume of 0.5 ml or 1 ml of a given radioactive sample may be transferred into a vial
and measured in the dose calibrator. The measured activity is recorded and then the volume
of the sample is increased in steps of 1 ml by simply adding plain water to the sample. One
can increase the volume in this manner up to 10-15 ml and record the change in activity
with increase in volume if any.
Position in the well
Usually the vial/ syringe holders are made in such a way that the activity is accurate and
reproducible if placed at that given position in the ion chamber well. However, one may
check this by making measurements along the well at various positions and record the
position with least/no variation.
Syringe factor
While measuring the syringe activity in the dose calibrator, the position of source activity
is different than the actual position of activity measurement (vial holder) due to attached
needle. In such a situation the measured activity in the syringe may not represent the true
activity. It is therefore necessary to multiply the measured activity in the syringe with
syringe factor. The syringe factor can be estimated by transferring the known activity in the
syringe from a vial. For this one should measure the vial activity first then draw some
activity in the syringe and measure the vial again. The difference of vial activites gives the
true activity in the syringe. The syringe should then be introduced into the well of the dose
calibrator to have the syringe reading. The syringe factor then is calculated as follows.
Syringe factor = (True activity /Measured activity) in the syringe
The measured activity in the syringe has to be multiplied by the syringe factor for
estimation of actual activity.
Background response
The background reading is recorded without any radioactivity in the well and subtracted
from the subsequent measurements. Most of the newer models have the facility for auto
GSPant\Newbook\Final-2007\6-chp\102
Radionuclide Dose Calibrator
103
zeroing after background measurement. Any significant increase in background reading may
be investigated properly for a possible contamination.
Summary
Simple daily/weekly tests with a standard source of long half-life such as
the stability, reliability/reproducibility and accuracy of a dose calibrator. If there is any drift
in the measured value then measurements should be repeated before subjecting the instrument
to various tests or taking corrective measures. Periodic checks of the performance parameters
are essential to have confidence on accuracy and reliability of the instrument.
137
Cs provides
Suggested reading
1. Baldock C and Batchelor S. Analysis of sample volume dependence of I-125 in a radionuclide
calibrator. Nucl Med Communication 1991; 12: 445-450.
2. Calhoun JM, Golas DB and Harris SG. Effects of varying geometry on dose calibrator response
Co-57 and Tc-99m. J Nucl Med 1987; 28: 1478-1483.
3. Chu RY. Accuracy of dose calibrator linearity test. Health Phys 1988; 55: 95.
4. IAEA-TECDOC 602. Radionuclide dose calibrator, In Quality Control of nuclear medicine instruments.
1991; pp 17-34.
GSPant\Newbook\Final-2007\6-chp\103
Scintillation detection (in vitro)
G.S. Pant
Well counter (Gamma ray spectrometer)
As mentioned in the previous chapter, most of the scintillation detectors in nuclear medicine
use NaI(Tl) crystal. To increase the detection efficiency particularly for measuring small
amount of radioactivity, the detector is given the shape of a well. Geometric efficiency of
the well-shaped scintillation detectors (normally called well counters) is 95%. The crystal
is covered with thin aluminum foil from all sides except the bottom that is open to PMT for
transmission of scintillations. The outer cylindrical surface is also coated with some material
that reflects light towards the bottom. The PMT is attached to the crystal by transparent
silica glue. Being hygroscopic the detector is hermetically sealed from all sides. Figure 1
shows the diagram of a simple gamma well counter.
Figure 1: Schematic diagram of a simple gamma (well) counter
10 4
Scintillation detection (in vitro)
105
The detector along with the PMT is shielded with adequate lead lining. The dimensions
of the well and crystal depend upon the type of radionuclide (gamma energy) to be measured.
As per IAEA technical document for measurement of medium energy gamma rays a standard
well type crystal with 45 mm diameter and 16 mm well diameter is satisfactory. The crystal
thickness may be increased or decreased if the system is to be used specifically for higher or
lower energy photons. As explained in the previous chapter, PMT coverts the scintillations
into voltage signal. The voltage signal is week and needs amplification by pre-amplifier and
a linear amplifier before being sent to the pulse height analyzer (PHA). The PHA is one of
the most important components of any scintillation detector that rejects most of the scatter
and background radiation and allows only those signals, which fall between two pre-selected
energies. The lower threshold energy is called lower level discriminator (LLD) and upper
threshold energy is called upper level discriminator (ULD). Pulses coming from pre-amplifier
or amplifier are allowed only if they fall between these two threshold energies. The PHA is
an anti coincidence circuit in a way as the pulse is allowed to pass when its (energy) height
is more than one threshold (LLD) but less than the other (ULD). The difference of voltage
and energy between LLD and ULD is called window width. An output pulse is generated
with the help of anticoincidence circuit only when the PHA allows it (see figure 2).
Figure 2: Block diagram of a single channel analyzer
A spectrum can be plotted by measuring the pulses (photopeak counts) from low to
highest energy possible for the chosen radionuclide with a given energy window. This is a
single channel analyzer (SCA), which can show one energy spectrum at a time. In multi
channel analyzer (MCA) a large number of voltage/energy channels are created with the
help of analogue to digital converter (ADC) and the entire spectrum can be seen
simultaneously.
Calibration of a spectrometer
As has already been described that a spectrometer rejects the pulses, which do not fall
within the pre-selected energy window. Calibration of a spectrometer refers to determine the
GSPant\Newbook\Final-2007\7-chp\105
106
Scintillation detection (in vitro)
proper operating voltage to PMT dynodes that will convert the arbitrary units of potentiometer
in spectrometer to meaningful energy units. The simple way of this calibration is to set the
window and energy of the radionuclide placed in the detector well and then increase the
PMT voltage from minimum till we get peak counts which drops on further increase in
voltage. The potentiometer reading at the peak should show the voltage say 6.62 volt that
corresponds to 662 keV energy. The other gamma energies such as from
show a peak at 3.64 V and similarly
99m
Tc gamma rays will have a peak at 1.4 V. Once this
131
I would then
is done we can determine the energy of any unknown radionuclide and also find its energy
resolution. It should be noted that increase in PMT voltage will shift the peak towards right
and will correspond to different units of potentiometer. Theoretically the peak can be set at
any potentiometer reading but for the sake of convenience the method described above suits
the user and is commonly used in routine practice.
99m
For
Tc gamma rays a 20 % window width will be 28 keV with LLD at 126 keV
and ULD at 154 keV or in terms of voltage it will be from 1.26 V to 1.54 V. If all the pulses
above a given threshold voltage/energy are to be measured then the ULD is not used and the
analyzer is then called a discriminator.
By varying the photopeak energy from a minimum to a maximum (keeping the same
energy window) as desired one can plot the entire energy spectrum. Look at the entire
spectrum of
137
Cs gamma energy using NaI(Tl) detector. There are small peaks before the
photopeak for 662 keV gamma photons. The first peak appears for Barium X-rays, and then
the backscatter peak caused due to photons scattered at 180° from outside towards the
detector. Then a broad peak appears for Compton scattered photons. Normally a valley is
observed after the Compton peak (Compton valley) before the photopeak for 662 keV photons
appears. Figure 3 shows the pulse height spectrum of
137
Cs as might be recorded by a
NaI(Tl) detector. When photons interact with iodine atoms in the NaI(Tl) crystal followed
by the escape of characteristic iodine K-x-rays (30 keV), a peak appears at about E -30
keV. Where E is the gamma ray energy in keV. Thus iodine escape peak will appear at
about 30 keV below the photopeak and can be commonly seen with low energy gamma
emitters.
Scattering medium around the detector, count rate used during measurement and detector
size affects count distribution in the spectrum. High count rate broadens the photopeak due
to pileup between photopeak and lower energy events. The photopeak may also shift towards
lower energy due to baseline shift at high count rates.
GSPant\Newbook\Final-2007\7-chp\106
Scintillation detection (in vitro)
107
Figure 3: Pulse height spectrum as expected for
137
Cs gamma photons
Operational checks and quality control
The analyzer peak setting and background count rate should be checked before any
measurement is done. The counting systems usually depend on the source geometry and
sample volumes that are introduced into the well for measurements. It is therefore necessary
to have all samples in a batch with identical volume and geometry.
The system should be subjected to acceptance tests immediately after installation. Energy
calibration, sensitivity (efficiency), counting precision, linearity of energy response, and
linearity of activity response should be measured and recorded at the time of acceptance.
These parameters should also be checked periodically and compared with the reference
values for any possible deviation. Corrective measures should be taken if the deviation is
beyond the tolerance limit.
Energy calibration
A small activity source of
a given PMT voltage and amplifier gain, increase the setting of relevant control knob (base
line), from low initial setting to a value when counts start appearing. Increase the settings in
steps till the maximum counts are recorded. Any further increase beyond this setting would
result in decreased count rate. The peak counts correspond to absorption peak for 662 keV
gamma photons from
should be enough to acquire at least 2,500 counts at the maximum count region. The PMT
voltage and amplifier gain should be selected properly before conducting the experiment.
137
Cs (~few Ci or even < 1Ci) is placed inside the detector. For
137
Cs. In order to reduce uncertainty in counts, the counting time
GSPant\Newbook\Final-2007\7-chp\107
108
100
100
2
( )
x x
Scintillation detection (in vitro)
Energy resolution
The same source of
different base line values with a narrow window from minimum to maximum count regions
and beyond with a properly selected PMT voltage and amplifier gain. Counts are plotted
against center of window setting in PHA. Determine the full width at half maximum (FWHM)
for the photopeak. Determine the energy resolution as:
137
Cs may be used for this purpose. Counts/count rate is measured for
Energy resolution (%) =
FWHM
Photopeak energy
Sensitivity (Efficiency)
An accurately measured standard source of
well counter. Counts are recorded for such a time duration that the statistical variations are
minimized (at least 10, 000 counts). The count rate increases with activity linearly in a well
counter but only up to a very small activity range. This must be determined before checking
the efficiency. Low activity may be obtained by dilution method for sources like
Determine the counting efficiency by the following equation:
n
s a
where = counting efficiency (%)
n = observed count rate after background subtraction
s = activity of standard source (Bq) at the time measurement
a = fractional abundance (branching fraction)of detected radiation per
disintegration
137
Cs (low activity) is kept inside the calibrated
99m
Tc.
Counting precision (2 test)
After setting the photopeak with a suitable window width for an appropriate PMT voltage
and amplifier gain, a standard source of
137
Cs is placed in the detector. Time of counting
should be such that at least 10 K counts are recorded in each measurement. Preferably 20
but at least 10 measurements should be taken. More the number of measurements better is
the statistical estimation.
The 2 value is estimated using the following formula:
2
=
i
n
GSPant\Newbook\Final-2007\7-chp\108
Scintillation detection (in vitro)
109
Where xi is an individual measurement (counts) and x is the mean of all measurements.
From the 2 table one may estimate the probability (P) for given measurement. ‘P’ is the
probability that random variations observed in a series of n measurements from Poisson
distribution would equal or exceed the calculated 2 value. Therefore 1-P is the probability
that small variation would be observed. For details on 2 refer chapter “Counting Statistics”
in this book.
Linearity of energy response
In this test several gamma emitting radionuclides such as
99m
completing the energy calibration, position one of the sources in the detector and take the
counts for a suitable preset counting time. Increase the base line as mentioned earlier till we
get the maximum counts corresponding to the photopeak for second radioisotope. The
experiment is repeated for all available radionuclides. The center of photopeak is plotted
against corresponding gamma energy of radionuclides. The line may be extrapolated to the
zero energy. Ideally the extrapolated line should pass through origin. In some of the
instruments it may have an intercept with Y-axis. Any unknown radionuclide can be checked
with the help of this graph if the center of its photopeak is known.
Tc,
131
137
I,
Cs are needed. After
Linearity of activity response
This can preferably be done with a decaying source method but can also be performed with
graded volume method using an auto pipette. It should be kept in mind that the well counters
get saturated with few microcuries of a source due to high sensitivity.
Background count rate
The background count rate has to be subtracted from the gross count rate in all measurements.
It is therefore necessary to record it before all the measurements. The window is symmetrically
placed over the photopeak of a given radionuclide and count rate is measured without source
for a suitably preset counting time. For measuring the integrated count rate the instrument
has to be put in integral mode with threshold/baseline control at low threshold energy (say
20 keV). Count rate is then recorded for a suitable present time.
Use of well counter for activity measurements
Well counters are quite suitable for measuring small quantity of radionuclides provided they
are calibrated for this purpose prior to use. They can measure the activity only up to a point
till the activity follows linear relationship (nCi-Ci range) with measured count or count
rate. In all the measurements one should measure as many counts as possible to minimize
the statistical uncertainty.
GSPant\Newbook\Final-2007\7-chp\109
110
Scintillation detection (in vitro)
Beta counters (Liquid scintillation counters)
The performance evaluation and checks of LS counters are done using standard sources of
3
H and 14C that usually accompany the counters. The count rate is measured under standard
settings of the instrument. The efficiency can also be checked, in modern systems, with the
help of a light source located at the center of the measuring chamber (between the PMTs).
The number of light flashes per unit time is known and should correspond to the counts
measured. The LS counter has been described in a chapter on ‘radiation detectors’ in this
book.
Automatic gamma counters
Most of the counting systems use automatic gamma counters. The QC has become much
simpler with automatic counters. The PHA peak for a given radionuclide (placed in the well)
is automatically set and displayed on the monitor. Similarly the efficiency can be checked
with the help of a standard source (
activity. The efficiency with standard deviation after decay correction is normally displayed
in modern systems. In almost all the automatic counters the QC programme is inbuilt that
makes the performance of routine tests easy. Deviation from the normal, if any, can be
detected and corrective measures are initiated. National inter-comparison of counters, if
possible, could be a welcome step to add confidence in the in vitro measurements.
137
Cs) or any other source with accurately measured
Refrences
1. Principles and Practice of nuclear medicine, eds. Early PJ and Sodee DB, Mosby publications, 1995.
2. Physics in nuclear medicine. Eds. Cherry SR, Sorenson JA and Phelps ME, Saunders, 2003.
3. Quality control of nuclear medicine instruments. IAEA-TECDOC-602, IAEA, Vienna. 1991.
4. Radiation Detection and Measurement. Knoll GF, 3rd Ed, John Wiley and Sons, New York 2000.
GSPant\Newbook\Final-2007\7-chp\110
Radiation Detection
in vivo
and
Gamma Camera Imaging
G.S. Pant
Scintillation detection in vivo (Non imaging)
Thyroid uptake probe
The thyroid uptake probe is a counting device that is used to measure the percentage uptake
131
of
I in the thyroid gland after oral administration of
scintillations in NaI(Tl) crystal, which are converted into an electrical pulses by the
photomultiplier tube (PMT) to which the crystal is optically coupled. The intensity of light
produced is proportional to the energy of the gamma ray absorbed. The associated electronic
components in the circuit such as preamplifier, amplifier, pulse height analyzer and the
counter/scalar are shown in the block diagram (Figure 1).
131
I. The gamma rays produce
Figure 1: Block diagram of thyroid probe
11 1
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