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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5255_Библиотеки_им_академика_М_И_Перельмана
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122
Radiation Detection in vivo and gamma camera imaging
corresponding photon energy. The energy calibration (tuning) is done for all the PMTs in the
camera.
The QC of instrumentation should be taken as an integral part of the nuclear medicine
facility. The quality control of instrumentation starts right from its selection. Selection of
the equipment, site selection for their installation and provision for providing and maintaining
adequate power and environmental conditions may be treated as a part of full quality control
programme.
While selecting a particular equipment some of the salient points to be considered are:
• Procedures that need to be carried out with the proposed equipment. For example,
the equipment in a centre for neurological sciences may be dedicated SPECT system
for brain imaging or general SPECT with a special collimator for Brain imaging.
Similarly, in a cardiac centre the camera should basically meet the requirements for
cardiac work. One may also keep the future needs in mind treating camera life to be
10-15 years. But in a general nuclear medicine department the equipment should be
able to do all types of scans. It may also require various types of collimators (high
resolution, general purpose, pinhole and medium energy).
• The essential kits/phantoms and standard radiation sources needed for routine quality
control procedures have to be procured or made available at the time of acceptance
testing and then for subsequent routine QC procedures.
• Another important point that should be kept in mind is to purchase the equipment
with a very sound base of service support. The price is one of the most important
factors in developing countries like ours but should not be taken as a sole and
reliable guide if other salient factors are not met.
• The proper operation and service manuals must accompany each instrument. The
power requirements as stated by the manufacturer should be met without any
compromise. Adequate and stable power supply is most important for these equipment
particularly the computers. In case of unstable power supply, an uninterrupted power
supply (UPS) system with at least 30 minute back up time should be available.
• Environmental conditions also need proper consideration. High temperature, high
humidity and dust can cause severe damage to the systems. Rapid temperature
changes are disastrous for the crystal. The temperature variation at the crystal level
should not be more than 1oC per hour. This may be achieved if the variation in room
temperature remains within 3oC per hour. Proper air conditioning should be provided
to maintain optimum cooling and humidity. Direct sunlight should be avoided to the
camera room.
• Eatables should be strictly prohibited in these rooms as the smell of foodstuff
attracts rats, which may cause severe damage to the wiring. Foodstuffs are also
prohibited from the radiation safety standpoint.
Radiation Detection in vivo and gamma camera imaging
123
After meeting all these requirements the system may be installed and subjected to
acceptance tests. The acceptance tests are done to check whether the system conforms to the
quoted specifications of the manufacturer. These tests are usually done as prescribed by the
manufacturer. If the same protocol is to be followed by the user in future then acceptance
and reference tests are to be the same. But if the users want to follow different protocol for
quality control tests at their center then they (reference tests) can be performed after
acceptance tests. The acceptance and reference test parameters should be properly recorded.
There are limits to which the deviation for each performance parameters can be accepted
both at acceptance and at routine or periodic tests. Any deviation beyond the tolerance limits
may need corrective measures. Gradual deterioration in some parameters may indicate a
necessary correction/repair. After the repair or replacement of some components, acceptance
tests may be repeated.
In clinical quality assurance it should be ensured that the patient is absolutely stationary
during image data acquisition. The collimator should be as close to the patient as possible
both in planar and SPECT acquisition. This avoids the deterioration in spatial resolution.
The acquired data should be checked immediately to confirm that the acquisition has been
done properly (sinogram in SPECT). In pediatric patients, all the precautions should be
taken for good acquisition. The imaging rooms should be kept clean and free from radioactive
contamination.
A good organization with adequate and trained staff is needed for proper implementation
of quality control programme in nuclear medicine.
Quality control tests
As there are a number of circuits/components involved in the formation of image, each of
them need to be at their optimum performance for good performance of the camera. The
manufacturers subject the system to various tests before shipment. The most common test
protocols in use have been developed by the National Electrical Manufacturers’ Association
(NEMA), American Association of Physicist in Medicine (AAPM) and International
Electrotechnical Commission (IEC) and IAEA. The NEMA performance standards are the
most commonly used by the manufacturers and users. The performance parameters of different
cameras can also be compared if they have been measured using a common protocol such as
the NEMA protocol.
The system is then subjected to quality control tests to check the various performance
parameters. The important parameters that need to be evaluated and checked for an Anger
scintillation camera are as under:
• Spatial resolution
• Uniformity
• Energy resolution
• Spatial linearity/distortion
124
s
R
2 2
( )
R R
Radiation Detection in vivo and gamma camera imaging
• Count rate capability
• System sensitivity
• Detector Head Shield Leakage
• Multiple window spatial registration (MWSR)
• Collimator Evaluation
• Test for Crystal hydration
Spatial resolution
The spatial resolution is the ability of the imaging system to identify distinctly two closely
placed point or line sources of radioactivity. The smaller the distance (separation) between
two distinctly identifiable sources better is the resolution. It is normally expressed in
millimeters (mm). The spatial resolution may be measured without collimator (Intrinsic
resolution Ri) and with collimator (Rc). Although the image data are always acquired with
collimator in place but intrinsic resolution has its significance at the time of acceptance as
well as to evaluate the effect of a particular collimator over a period of time. The system
resolution, Rs, for a source positioned at a given distance from the collimator is expressed
as :
=
i c
Intrinsic resolution
It is the resolution of the crystal and photomultiplier tube assembly. The quantitative
assessment of intrinsic resolution is expressed by full width half maximum (FWHM) of the
line spread (or point spread) function (LSF) of a collimated line source placed on the
surface of the detector. A 30- 40 mm line source of
is collimated to less than 0.5 mm aperture on the crystal surface. Care should be taken to
collimate the source with lead on the detector face. It is advisable to use fine glass capillary
tubes (diameter less than 0.5 mm). The activity enters these tubes by capillary rise, when
one end of these tubes in dipped in the radioactive solution.
At least 10K counts should be measured for the peak channel (Pixel) of the LSF and
there should be 10 data points from the peak on either side. This is possible with computer
matrix of 512 × 512 or preferably 1024× 1024. Software should be available to display the
profile and calculate the FWHM in mm.
Most of the Gamma camera suppliers provide software for calculating the resolution
with a matrix size 256x256. However, if the data/image file format is known then software
can be developed by the user for calculating spatial resolution. The line source(s) may be
moved to various positions in X and Y directions on the crystal to find out the intrinsic
resolution in all four quadrants of the detector.
99m
Tc containing about 37 MBq (1mCi)
Alternatively, some of the suppliers of the camera use a multiple slit phantom (NEMA)
Radiation Detection in vivo and gamma camera imaging
FWHM
W D
S
125
for more accurate and extensive measurement of intrinsic resolution. The slit phantom is
placed on the crystal surface. A point source of, preferably,
99m
Tc (about 400 MBq) is placed
on the central axis of the detector head at a distance of about 1.5 m or more. If the distance
is increased beyond 1.5 m then the activity of the point source may be increased accordingly.
The size of the data acquisition matrix may be increased to 1024 × 1024 for the measurement
of LSF. The measurements are taken with phantom lines first positioned in X and then in Y
direction. By using suitable software, a Gaussian fit is applied to each profile to identify the
peaks. The software also estimates the central (peak) pixel, FWHM and FWTM for each
peak and then calculates the mean FWHM/FWTM, standard deviation, coefficient of variation
for either selected profiles in X and Y direction or for the total set of X, Y measurements.
There are many phantoms available (such as Anger Pie phantom, Quadrant bar phantom,
Orthogonal hole phantom, Multiple slit phantom) to evaluate the intrinsic resolution of the
camera.
One can even use two line sources in plastic tubing or capillaries and raise them to the
level of the detector with the help of a stand. The gap between the line sources should be
covered with adequate thickness of lead block (1). The image may then be acquired for
99m
Tc source at 20% window and 2 million counts in 256 × 256 matrix in zoom mode or in
512 × 512 matrix. Two images one in X and the other in Y direction should be acquired. The
count profile against each pixel can be plotted on a linear graph paper. A smooth curve can
be drawn through the data points.
For each peak FWHM in pixels (W) is calculated by linear interpolation between adjacent
pixels, using the highest pixel count in the peak as the maximum. The distance between the
peaks (S) in pixels can be measured. The FWHM can then be calculated for each peak in
mm as:
=
Where D is the spacing between the line sources in mm. The average of FWHM for two
peaks may be calculated.
System Resolution
The quantitative measurement of system resolution may be made with the help of parallel
line source(s) placed on the surface of the collimator or at various distances from it with and
without scattering medium. The measurement of FWHM is similar as described in intrinsic
resolution measurement (Figure 8).
126
MTF
( )cos(2 )
LSF x fx dx
Figure 8: Count profile through images of two line sources at 10 cm from collimator face.
FWHM can be calculated for each of them to determine the average value of spatial resolution
Radiation Detection in vivo and gamma camera imaging
Another mathematical parameter for the spatial resolution is the modulation transfer
function (MTF). It is expressed as a frequency function by taking Fourier transform of the
LSF. The Fourier transforms are characterized by magnitude (modulus) and a phase angle
(angle between the complex vector and the real axis). This modulus in medical imaging is
termed as modulation transfer function. The MTF of a line spread function (LSF) may be
expressed mathematically as:
=
LSF x dx
( )
The MTF varies with frequency (f) for a given setup. Lesser the frequency higher is the
value of MTF and better is the resolution.
Qualitative assessment of intrinsic and system resolution
When the software is not available for quantitative assessment of the spatial resolution a
qualitative measurement may be done. A quadrant bar phantom with different spacing of
lead bars in four quadrants is imaged with and without the collimator (Figure 9). These
phantoms with different bar width and spacing are available and one may select the one in
which one quadrant has bar spacing either less or equal to the intrinsic resolution of the
system. A point source of
kcps and placed on the central axis of the detector at a distance 1.5-2m for intrinsic resolution.
The image of the phantom is acquired for 2 million counts. For measuring system resolution,
a flood phantom filled with a uniform source of
quadrant bar phantom for image acquisition. Alternatively, a uniform flood source of 57Co
may be used, if available.
The bar phantom may be imaged in four different orientations by rotating it through 90°
angle or by rotating through 180° and inverting it before acquisition. The measurements
99m
Tc (30-40 MBq) may be used to have a count rate of 20-30
99m
Tc (150-200 MBq) is placed over the
Radiation Detection in vivo and gamma camera imaging
127
should be taken with 20% PHA window centered on the photopeak. By visual inspection one
can determine the smallest bar spacing that can be resolved in X and Y direction. The
FWHM can be estimated as (1)
FWHM= 1.75B
Where B is the width of the smallest bar spacing that the camera can resolve.
Figure 9: Image of a quadrant bar phantom in a circular field of view gamma camera
Uniformity
Uniformity is one of the most important performance parameters of the gamma camera and
is influenced by various factors. Normally an image is acquired by flooding the detector
with a uniform source of commonly used radionuclide (such as
There is an array of PMTs in the system whose gain should ideally be the same. However,
due to either non-linear response of the detector or difference in gain of the PMTs, the
acquired image may be non-uniform. The variation in PMT gain may be observed with
passage of time as each tube ages at its own rate. It is therefore, necessary to adjust the gain
of each PMT periodically or routinely if needed. The composite peak of Z signals represent
the energy resolution for the scintillation camera. The width of composite peak from all
PMTs should be such that the photopeak of each PMT falls within this width. For this
reason a 20% window is normally centered on the 140 keV energy peak of
window covers all the photons with energies from 126 to 154 keV. A lot of scattered photons
either from the detector or from the patient are also included in this window resulting in loss
of contrast and resolution.
Initially the count skimming method was used in gamma camera for uniformity correction.
It used to subtract counts from areas of higher PMT gain or add counts to the areas with
lower gain. There was every chance of error at a count density other than used for uniformity
99m
Tc) at 20% PHA window.
99m
Tc. This
128
100
DU
100
Radiation Detection in vivo and gamma camera imaging
measurement so the method was discarded by the manufacturers. In the newer design of
gamma cameras, a microprocessor in the system monitors the variation in the gain of PMTs.
A correction is then applied to each Z signal, if needed, depending upon its specific X and Y
location so that photopeak of all locations exactly coincide. This provides a narrower
composite peak and allows a narrower PHA window in modern cameras.
Some of the cameras contain a pulsed light source fibre-optically fed to each PMT so
that the individual gain can be adjusted every few seconds (1).
Quantitative measurement of uniformity
The uniformity is checked over the field of view of the camera by observing the variation
in pixel counts. An image of a uniform source is acquired with a lead mask to cover the
edges and exposing only the useful field of view (UFOV). If such a mask is not available
then the entire FOV may be exposed to uniform source of activity. The software determines
the maximum (Max) and minimum (Min) counts in the pixels lying within the UFOV and
the central field of view (CFOV). The UFOV and CFOV are 95% and 75% of the total FOV
of the camera respectively. The uniformity index for UFOV and CFOV is defined by integral
and differential uniformity. The integral uniformity (IU) is defined as:
( )
IU =
Max Min
( )
Max Min
For calculating differential uniformity (DU) the software determines the maximum count
difference in any two contiguous pixels. This monitoring is done both in X and Y direction
of the pixel matrix. At a time 5 or 6 contiguous pixels are monitored. The highest count
difference in any two contiguous pixels along the rows or columns over the FOV of the
camera is recorded for the estimation of differential uniformity (DU).
Hi Low
( )
=
( )
Hi Low
The Hi and Low are the highest and lowest counts in any two contiguous pixels along
the row or column of the matrix size chosen for the uniformity test.
Intrinsic uniformity
The intrinsic uniformity may be checked either with a point source of
or a uniform liquid source in a flood phantom. In either case the count rate be kept below 30
kcps and flood image acquired at a 20% PHA window. It is advisable to follow the instructions
given by the manufacturer in the users’ manual for a given camera as they normally specify
the procedure to be followed.
99m
Tc (10-20 MBq)
Radiation Detection in vivo and gamma camera imaging
129
The collimator is removed and the point source is placed at a distance more than 5 times
the FOV of the camera. The source may be suspended in air at a distance of 2 m or more
such that photons fall on the detector perpendicularly. Acquire the flood image as specified
for the system. For acceptance testing one can acquire flood image such that each pixel of
the matrix gets more than 5000 counts.
Figure 10: Intrinsic uniformity. The software calculates all the four uniformity indices, the
maximum, minimum and mean counts in the camera FOV.
The data is analyzed with the available software to calculate the IU and DU for both
UFOV and CFOV (Figure 10). The software may also be developed, if possible, which
calculates the mean counts, standard deviation (s.d.) and coefficient of variation (COV). The
number of pixels lying within 5% and 10% of the mean value should also be calculated if
possible. The routine (daily) check of uniformity may be done for lesser counts e.g. 2-4
million counts depending upon the convenience. It is also advisable to measure the activity
of the point source before being used for uniformity check. The point source may be positioned
at the same location i.e. always at a fixed distance form the detector. This enables to check
the count rate (sensitivity) per unit activity on a routine basis along with the uniformity and
ensures the reproducibility of uniformity (5).
The uniformity should be checked for various count rates at the time of acceptance and
the count rate that provides the optimum uniformity values should be noted. The variation in
uniformity may be reviewed periodically. In some of the systems the uniformity is highly
dependent on count rate and degrades when count rate is increased beyond 30 kcps. Generally
130
Radiation Detection in vivo and gamma camera imaging
the count rate should be maintained between 20-30 kcps. The uniformity may also be
checked for various PHA windows and appropriate window may be used for routine clinical
use. The intrinsic uniformity may also be checked at energies other than 140 keV (
such as 57Co (122keV) and
131
I (364 keV) or
113m
In (392keV) which are also used or likely to
99m
Tc),
be used for diagnostic purposes.
Extrinsic or system uniformity
To check the uniformity with the collimator in place (system uniformity) a flood phantom
containing 100-200 MBq of
57
of
Co of similar activity may be used. The flood phantom/flood source is placed on the
99m
Tc solution in water is required. Alternatively a flood source
face of the collimator with polythene and an absorbent sheet in between to avoid any
possible contamination. Twenty to 120 million counts are acquired for a flood source
depending upon the matrix size (64×64 to 256×256 or more) at 20% PHA window centered
on 140keV for
99m
Tc or 122 keV for 57Co source. As a thumb rule 3000 to 5000 counts
should be acquired per pixel. After completing the acquisition, the measurement should be
repeated with less number of counts (2-5 million counts), which will provide the reference
values of uniformity indices for routine measurements. The data analyses with the available
software may be done to calculate the IU and DU for UFOV and CFOV with mean counts,
s.d., COV and other parameters as is done for intrinsic uniformity. The flood images in a
dual head camera are shown in figure 11. The system uniformity may be checked with all
the collimators available at various count rates and PHA window width.
IU in CFOV
Detector 1 = 2.5%
Detector 2 = 2.91%
Figure 11: Extrinsic uniformity for both the detectors in a dual head camera. Energy resolution
and peak energy is also shown on the right side.
Radiation Detection in vivo and gamma camera imaging
131
There are old gamma cameras available in many centers particularly in the developing
countries without any software for the calculation of uniformity parameters. It is advisable
to use small (rectangular) region of interest (ROIs) over the FOV of uniform image acquired
exactly as described above. The size of ROI may be as small as possible (5-10 pixels). The
whole UFOV/CFOV may be scanned and the ROI may be treated as a pixel for the calculation
of uniformity parameters. However, if there is an access to the compiler of the system, then
software may be developed for such calculations.
Alternatively the flood phantom filled with a uniform solution of radioactivity (
99m
Tc)
may be used for measurement of uniformity. The filling of the phantom may be done very
carefully with radioactive solution.
Energy resolution
The energy resolution may be calculated with the help of multi channel analyzer (MCA).
The peaks of various radionuclides may be checked at 20% window. The channel
corresponding to the peak of the energy spectrum may be noted. A graph is plotted between
energy and the channel numbers. The FWHM and FWTM of the photopeak may then be
found out for the given radionuclide (say
converted into energy units. The energy resolution is expressed as a percentage of gamma
ray energy. The value of energy resolution with collimator in place deteriorates slightly due
to characteristic x-rays (74 keV) from lead of the collimator. The energy resolution for
99m
Tc gamma rays is < 10% and is < 7% for
data points on either side of photopeak and at least 10K counts are registered in the peak
channel (6-7).
99m
Tc) first in terms of channels, which are then
137
Cs. As per NEMA there should be at least 10
Energy correction
The spatial variations in energy response contribute significantly to the non-uniformity in
scintillation cameras. This results in a shift in average photopeak pulse height relative to the
selected energy window. Normally these variations are due to incorrect tuning of PMTs or
deterioration of electronic components with time. Another cause may be the optical design
where efficiency of light collection varies as a function of position (8). The manufacturers
of gamma cameras normally mention their method of energy correction in the manual. Most
of the modern systems incorporate auto correction (auto tuning) for PMT gain during
acquisition itself (9-10). The procedures of energy correction slightly vary from system to
system (11). Users should strictly follow the instructions in the manual for energy correction.
Spatial linearity
One of the major causes of camera non-uniformity is the mispositioning of the events by the
electronics of positioning circuit (12). As a matter of fact camera non-uniformity is more
because of mispositioning of events rather than variation in photon detection efficiency on
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