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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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132
Radiation Detection in vivo and gamma camera imaging
the crystal. One may take the images of a small disc source of radiation at many locations
over the surface of the crystal and observe if there is any significant variation in count
density and change in source diameter with position. If any a change is observed then it
shows that there are areas of local mispositioning which are not random but are either in a
given area or direction. If these local non-linearities are mapped over the surface of the
detector, a set of correction factors could be generated which would reposition the events in
their true X and Y locations within the limit of intrinsic resolution of the camera. A
quantitative measurement of spatial linearity or spatial distortion may be obtained during
the measurement of intrinsic/system resolution with slit phantom. These phantoms are made
up of lead plate with multiple strips with a line spacing of 30mm. Phantoms with a line
spacing of 15 mm have also been used for this purpose (8). These phantoms can actually
measure both the spatial resolution and linearity.
The spatial linearity is normally measured without the collimator (intrinsic), as the
linearity phantom needs to be attached to the detector. Data should be collected in both X
and Y directions. For each line source image the correction factors are generated and stored
in the computer memory. The fractional value of the X and Y signal for a particular location
is either added or subtracted during correction depending upon the direction of mispositioning
of the event.
The absolute magnitudes of the differences between fitted and mean positions are
calculated for all line segments in each of the X and Y positions. The mean and maximum
deviation may be obtained in addition to the maximum difference between fitted peak
positions for any line in adjacent profiles (13). The maximum deviation from the fitted peak
is termed as absolute linearity whereas the maximum difference between fitted peak positions
for any line in adjacent profiles is called differential linearity.
The spatial linearity may be assessed by imaging the linearity phantom aligned in both
X and Y direction. It must be noted that the assessment requires the use of highly accurate
phantoms to ensure that any inhomogeneity is due to the camera rather than the phantom.
Qualitative evaluation
Where the software and necessary phantoms are not available, the evaluation of linearity
may be made qualitatively. One method may be to acquire a bar phantom image and check
the linearity of the bar lines in each quadrant. Thus the bar phantom image may be used for
checking both the resolution and linearity. The second method is to image line sources at
various distances from the face of the collimator in X and Y directions as can be seen in
figure 12.
Radiation Detection in vivo and gamma camera imaging
Figure 12: Images of line sources at 0cm, 5 cm, 10 cm, 20 cm from the face of the collimator
both in X and Y direction. The distance between the line sources vary from 0.5 to 5 cm.
133
Count rate performance
The count rate performance of the system can be evaluated both intrinsically and extrinsically.
The intrinsic count rate performance shows the response of the detector with increasing flux
of gamma rays falling on it from a radioactive source placed at a given location. To get
varying flux of radiation, a large number of radioactive sources with different strength are
needed. However, a high activity source can alone serve the purpose provided radiation
absorbers of uniform thickness are used in a sequential manner. The simple and quick
method is to use the calibrated absorbers.
Procedure
The absorbers are first calibrated with respect to their attenuation for
20% PHA window without the collimator. After calibration of 15 absorbers (1), the source
activity is made available that provides a low count rate (1-3 kcps) with absorbers 1-15 in
place over the source in order with absorber 1 on the top. The net count rate corrected for
background and decay (say Co) is recorded. At low count rate the input (Ro) and the
observed (Co) count rate should be equal. Then the absorbers are removed one by one and
the counts for a preset time can be noted. Initially, with all absorbers in place Ro=Co. After
removal of one absorber R1= Co/f1, where f1 is the attenuation factor for absorber 1. The
input count rate R2 = Co/f1f2, R3 and so on can be measured by removing the second, third
(and so on) absorbers one by one sequentially. The same value of attenuation can be used if
all of them have the same thickness. A graph can be plotted between input and output
countrate to show their relationship. From the graph it is easy to determine the maximum
observed count rate and observed count rate at 20% loss (C=0.8R).
The test should be done as an acceptance and reference test and be repeated at yearly or
half-yearly intervals. At the time of acceptance testing the count rate at 20% loss should be
99m
Tc gamma rays at
134
12 1 2
2 ( )
( )
R R R
20%
1 10 0.2231
20% 20%
Radiation Detection in vivo and gamma camera imaging
within ±10% of the specified value whereas at routine tests little more deviation (±15-20%)
may be accepted.
Intrinsic count rate performance (Count rate at 20% count loss)
Resolving time of the detector (Two point source method)
Two point sources (
distance of about 1.5 m from the detector head positioned to face horizontally without
collimator. A 20% PHA window is centered on the photopeak. First source No.1 is placed in
front of the detector and counts are acquired for 100 seconds and the count rate (R1) is
noted. The total counts acquired should not be less than 1000 K (count rate about 10kps).
One may adjust the acquisition time accordingly. The second source is placed by the side of
first one in such a way that one does not interfere the detector’s view of the other. The
counts are acquired for the same time and the count rate (R12) is recorded. The first source
is removed the counts are registered for the same time and the count rate (R2) is recorded.
The second source is then removed to record the background counts for the same time and
the count rate is obtained. The background count rate is subtracted to get net count rate R1,
R2, and R12.
Now the measurements are repeated exactly in the same manner by reversing the order
of the sources and the count rate for each step is recorded. The count rate in both the
measurements should be recorded with background subtracted. The resolving time () for
both measurement can be calculated as:
99m
Tc) each of about 2 MBq in a suitable container are placed at a
=
( )
The average value of resolving time may be determined and input count rate (R-
ln
2
RR R
121 2
20%
may be determined by the following formula:
R
The observed count rate at 20% loss
=
ln
8
0.8C R
This test should be performed as an acceptance and reference test and then at yearly or
half yearly interval. The measurements should be done following the instructions of the
manufacturer. If the manufacturer does not mention about the count rate then the count rate
should be maintained at about 10 kcps with 20% PHA window for
99m
Tc sources. It must be
noted that the two-source method is no longer applicable for digital cameras. If used, this
method may give resolving time much higher than the actual value. The suggested method is
to use 4-5 small sources (prepared by dilution) of
99m
Tc and count rate is measured to plot a
graph between observed and expected count rate. From the linear portion one can determine
)
Radiation Detection in vivo and gamma camera imaging
135
the expected count rate. The count rate at 20% count loss can be determined from the graph
and resolving time may be estimated (Figure 13).
Figure 13: Measurement of count rate at 20% count loss (R
–20%
)
The values may be compared with those specified by the manufacturers and routine/
periodic test values should be compared with the reference values of the user. The values
within ± 20% of reference value may be accepted. Otherwise the test is repeated. If the
values are still outside the tolerance limits, corrective measures should follow.
System count rate performance
The procedure of this test is exactly same as has been mentioned in the determination of the
resolving time above without collimator. The only difference here is that the two-source
scatter phantom is used and each point source strength should be around 70-260MBq (27mCi) of
The input and observed count rate for 20% count loss is calculated in a manner that has
already been described above. In modern digital camera multiple source method as mentioned
above should be used.
container covered with a plastic sheet (to prevent possible contamination of the collimator),
and viewed by the detector head with collimator in place. Counts for 30 seconds may be
acquired every half an hour for about 60 h or more. Time of the day at each measurement is
noted for decay correction. A graph is then plotted with last measurement as the first data
point and the first measurement as the last data point. The first few points which are strictly
in the straight line portion enable one to determine the input counts. With known time and
activity one can determine the input counts for other data points which do not fall on the
99m
Tc in a volume of 5ml in containers, which fit well in the phantom to be used.
Some centers use very high activity dissolved in water in a circular/rectangular plastic
136
Radiation Detection in vivo and gamma camera imaging
straight portion of the curve. The data points can be fitted to a suitable curve and counts at
20% loss can easily be determined.
Maximum count rate (intrinsic)
The collimator is removed and the detector head is rotated to face horizontally towards a
point source suspended in air at a distance of more than 1.5 m. The source strength should
be about 4 MBq (
source is then moved towards the detector slowly and count rate is recorded. The count rate
will first increase, reaches a maximum value and then starts falling, as the source keeps
moving towards the face of the crystal. The maximum count rate so observed is recorded.
Alternatively, one may keep the source on the floor (without scatter) and move the
detector, facing vertically downwards, towards the source and note the maximum count rate.
This value is compared, at the time of acceptance, with that specified by the manufacturer.
The value is normally accepted if falls within tolerance limits (±10%). This value is recorded
as an acceptance and reference value for future periodic tests at quarterly/half yearly intervals.
The tolerance limits are within ±10% of the specified value by the manufacturer at the
time of acceptance and ±20% at periodic/routine testing.
99m
Tc) and the PHA window of 20% be centered on the photopeak. The
System sensitivity
This is one of the most important tests in the count rate performance of the system. This test
is to see the response of the scintillation camera (with a given collimator) to a known
activity of a radionuclide in common use.
A plastic petri-dish or container in any other form with a diameter of more than 10 cm
(area more than 50 sq cm) and containing about 40 MBq (1 mCi) solution of
at 10 cm (NEMA) with collimator facing vertically upwards. At least 10K counts are acquired
with 20% PHA window centered on the photopeak. The count rate and the exact time of the
day corresponding to the mid-point of the measurement should be recorded.
The background count rate is also determined the same way. The test should be done for
all the collimators in use. Similar but a separate petridish may be filled with
solution for high energy (Iodine) collimator. The count rate is recorded at 20% PHA window
or as specified by the manufacturer.
The count rate is corrected for background and also for the radioactive decay and then
the system sensitivity for a given collimator is expressed as:
Counts/Sec/MBq or counts/min/Ci
If the values are beyond ±10% of those specified by the manufacturer then measurements
may be repeated. In case of unacceptable deviation one must check the accuracy of the dose
calibrator and also the collimator for any damage and take appropriate action accordingly.
99m
Tc is placed
131
I source in
Radiation Detection in vivo and gamma camera imaging
137
The ratio of system sensitivities may also be evaluated relative to a most frequently used
collimator.
Detector head shielding leakage
The detector head of the gamma camera is shielded with lead lining of adequate thickness
from all sides except the crystal face, which is covered by the multi parallel hole collimator
to allow the photons to pass through and interact with the crystal.
A point source of about 4 MBq (100 Ci) of a given radionuclide in small volume kept
in a suitable container is used for this measurement. The detector head is rotated vertically
upwards (with collimator in position) and the source is positioned at various different locations
including the one on the face of the collimator (Figure 14). Counts are recorded for a given
time length (100 sec or so). The background counts are also recorded for the same time
duration. The sites of joints in shielding, points of cable/wire outlet should also be checked (1).
Figure 14: Test of detector head shieldin6g leakage. Except from the collimator side (No.13),
the detector is shielded from all the sides.
The standard deviation of background count (B) is calculated and it is checked if the
counts at any site other than those on the collimator exceed the background by more than
three standard deviations (B + 3B). This test is done as an acceptance test. If the deviation
is more than the tolerance limit, investigate for possible contamination in the room/collimator
or any radioactive source lying in the vicinity.
Multiple window spatial registration (MWSR)
There are situations in nuclear medicine investigations, where the radionuclide emits more
than one type of photons and some or all of them become equally important for imaging due
to their branching efficiency such as 67Ga and
photons passing through different PHA window should coincide. The electronics of positioning
201
Tl. The image of an event formed by
138
Radiation Detection in vivo and gamma camera imaging
signals (X and Y) adjusts the gain in such a way that the images acquired at different photon
energies superimpose when more than one PHA window are simultaneously used. This is
actually needed when we use 67Ga and
201
T1 radionuclides for imaging. These radionuclides
are used with three PHA windows in additive mode.
A point source of 67Ga (40MBq) in a small vial, inside a lead shield 6 mm thick, having
a circular aperture of less than 3 mm at the opening, is made available for placing at various
locations on the detector face. The collimator may be removed and placed on the imaging
couch with a polythene sheet over it for placing the point sources. It becomes convenient if
9 such sources are available for positioning one at the center, four at 50% and four at 75%
of the distance from center to the edge (on X+, X-, Y+, Y-). The sources may be placed on the
collimator and then the detector is moved slowly to make the sources as close to it as
possible. The image through each of the PHA channel possible for that radionuclide (e.g.
296 keV, 184 keV and 93 keV for 67Ga) is acquired independently and then simultaneously
through all channels at 20% window centered on the photopeak.
The coordinates of all the 9 images may be determined for each PHA channel and also
for all channels enabled together. The coordinates of the images at a given location should
be same for each independent channel as well as for all channels together. If the provision
for 9 images is not possible then at least 5 images one at the center and four at 75% distance
from the center to the edge should be taken through all channels independently and then
simultaneously. The image at all five locations should coincide. One can even try with a
single source and image it one by one at all suggested locations. The correct registration and
misregistration will appear as shown in figure15.
Figure 15: Image of a point source of 67Ga at two locations. When photons from all three
energy windows register the event at one location (good registration) the image of two point
sources will appear as two dots (a) but when they do not register at identical location, three
points for each energy channel will appear (b).
The displacement of image position through any of the channels should be as small as
possible at the time of acceptance (<10%) and at routine/periodic test it should not exceed
20% of the reference value.
Radiation Detection in vivo and gamma camera imaging
139
Evaluation of collimators
There are various books and manuals on the quality control of various components of
gamma camera-computer system but the collimator check has not attracted much attention
till recently. The collimator is one of the vital parts of the imaging system and it is particularly
important in emission tomography studies. Few papers have been published for collimator
evaluation (14-18). The following four simple tests can be carried out for parallel hole
collimators that are very commonly used.
1. Image of a point source with collimator in place
2. Image of parallel line sources at different distances from collimator face
3. Image of at least five point sources at minimum and maximum distance from the
collimator
4. Centre of rotation (COR) offset at minimum and maximum possible radius of rotation
Image of a point source
A vial containing high quantity (200-300 MBq) of radioactivity is kept in a lead container
and placed at least 3 meters away from the face of the collimator so that the photons fall on
it perpendicularly. The image of the point source will look circular (Figure 16) if there is no
defect in the collimator. This test can only check for any gross defect in the collimator.
Figure16: Image of a point source centrally placed at >3 m meter (>10ft) from the collimator
face. Collimators without any damage/defect show annular rings as can be seen in this image
but the defective collimators show irregular pattern.
Image of parallel line sources
The fine capillaries or catheters can be filled with radioactivity and fixed on a perspex/
plastic sheet and placed on the face of the collimator. An absorbent sheet and polythene
sheet may be placed between the collimator and the perspex sheet to avoid any possible
140
Radiation Detection in vivo and gamma camera imaging
contamination. The images are acquired at various distances from the collimator face first in
X direction and then in Y direction (Figure 12). The linearity of the image should not
change with distance. Any distortion such as formation of curvature in the line image will
reflect the angulation defect of collimator holes.
Image of point sources at two positions
Five point sources are placed on a plastic sheet, one at the center and four placed at nearly
equal distances (along X+,X-,Y+, Y-) from the centre. These sources are imaged at two
extreme positions (near & far) perpendicular to the face of the collimator. The images at
these two positions should coincide if the holes are exactly orthogonal. Busemann-Sokole
(16) demonstrated the hole angulation with this method by imaging a point source at two
positions separated by a distance D.
COR offset for two radii of rotation
Though all the tests of collimator evaluation are important for SPECT acquisition but this
test has its significance. The X and Y offset for COR with two different radii of rotation one
possibly 15 cm and second as large as possible may be measured. The X offset may vary
only in amplitude but its difference from pure sine wave should produce nearly a straight
line. The Y offset should always be represented by a flat line.
Crystal hydration
As has been mentioned earlier the Nal(T1) crystal is hermetically sealed to protect it from
getting hydrated. The air tightness of the scintillator casing may deteriorate over a period of
time particularly when it starts aging. The casing may get damaged due to careless handling
allowing the crystal to absorb moisture. The hydrated crystal, instead of being transparent,
becomes yellowish. The affected portion of the crystal can transmit only a small fraction of
the scintillation produced, due to change in refractive index. The spots may be small initially
and grow bigger in size with time and later may affect the whole crystal. The image of a
homogeneous flood source will appear as non-homogeneous. The hydration effect can be
seen as black spots when window is centrally placed over the photopeak in the flood image
but turn into white spots by shifting the peak towards lower energy end. The off peak
imaging technique confirms the crystal hydration (19). In refurbished or old cameras the
hydration test should be done more frequently (20). Figure 17 shows crystal hydration
which is confirmed by off peak imaging.
Radiation Detection in vivo and gamma camera imaging
141
(a) (b)
Figure 17: Crystal hydration. Image of a uniform flood source of
(a) the black spots turn white from black after shifting the photopeak to 130 keV energy
(b) Turning of black spots to white by shifting the photopeak towards lower energy confirms
the crystal hydration.
99m
Tc at 140 keV photopeak
Associated computer system
The gamma camera computer systems are used for acquisition, storing and processing of
image data in a desired manner. The computer system has made the radionuclide imaging
extremely convenient and simple for the operator and useful for the physician. Furthermore,
there is tremendous scope for manipulation of the acquired data to improve image quality.
The gamma camera computer system has an important hardware for converting the analogue
signals to the digital numbers. This component is termed as analogue to digital converter
(ADC). It is with the help of ADCs that we are able to manipulate the (digital) data. The
image data are stored in a computer memory as an array or matrix of counts. Usually the
arrays are square matrices with a dimension of 32×32, 64×64, 128×128, 256×256, 512×512
and 1024×1024. Each element of the matrix is termed as “pixel”. The pixel dimensions
depend upon the matrix size. For example the pixel area in a 64×64 matrix will be four
times than that in 128×128 matrix. The spatial resolution, which is determined by FWHM of
the line spread function, improves with decrease in pixel size or increase in matrix size.
The number of counts that can be stored in a pixel depends upon the number of bits that
are allocated to it. In an 8 bit computer storage each pixel can accommodate 255 (28-1)
counts only. The 16 bit storage system can accommodate counts up to 65536 (216-1) per
pixel. Some manufacturer use 10 bit storage element (210-1) to store 1023 counts in a pixel.
It should be remembered that acquisition in zoom mode does not change the pixel size but it
decreases the field of view. As the spatial sampling of the image is determined by pixel size,
increasing the acquisition zoom is one way of achieving sufficient sampling without additional
storage space as required in larger matrix size. However, zooming acquisition is possible
only for imaging small organs.
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