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Collimation of portable cadmium telluride detectors for biotelemetry
0020-70xX.x2 121359-05~003.000 C‘opyright 0 19X2 Perymon Press Ltd
1nf. J. Appl. Rudiur. 13ur. Vol. 33. pp. 1359 to 1363. 19X1 Printed in Great Britain. All rights reserved
Collimation of Portable Cadmium Telluride Detectors for Biotelemetry JBRGEN
BOJSEN’,
BENT STABERG’
and KLAUS
K0LENDORF3
‘The Finsen Laboratory and ‘Department of Clinical Physiology. The Finsen Institute. 49 Strandboulevard. “Department of Endocrinology E. Frederiksberg Hospital. Copenhagen.
Copenhagen Denmark
and
The spatial properties of small. portable y-sensitive cadmium telluride (CdTe(C1)) detectors are evaluated for biotelemetry. Various single hole lead collimators are investigated with 99mTc as point and plane source. Isocount curves of the most efficient collimator are calculated. During local clearance measurements a reduced sensitivity to alterations in geometry is obtained, when all detected photons are counted, The contribution of counts in percent from deep layers of a ““Tc volume source is increased, when scattered radiation is counted and a single hole lead collimator with parallel sides is used. compared to results obtained with no collimator. ~-Ray energy spectrometers are not required for single tracer biotelemetry investigations with y9mTc.
Introduction solid-state, y-sensitive detectors like chlorine compensated, cadmium telluride (CdTe(CI)) detectors are valuable for biotelemetry with radionuclide tracers!‘.” The charge collection properties hamper, however, their use as energy spectrometers,‘3,4’ and consequently biotelemetry investigations in the past have only been based on single tracer studies. Contrary to conventional, ijz civo measuring techniques used in nuclear medicine, the biotelemetry methods allow the test person to be ambulatory during measurement. (‘) In order to avoid alterations in geometry during measurement. the small biotelemetry detectors are fixed by adhesive tape to the skin surface of the body,“,“’ and consequently the detector is positioned very close to the radioactive source. This may be a point source injected as a tracer in tissue only a few millimeters from the detector,“.” or a volume source injected as a tracer diluted in the extracellular fluid of the whole body.“’ In the latter case the tissue layers close to the detector mainly contribute to the count rate. Therefore even small changes in geometry may interfere with the measurements introducing erroneous results. It is the aim of this study to elucidate the spatial properties of standard CdTe(C1) detectors used as portable y-detectors in biotelemetry with 99mTc as radioactive source. Furthermore to demonstrate how PORTABLE,
* Radiation Monitoring Devices Inc., Watertown. Massachusetts, U.S.A. (RMD). All correspondance should be addressed to: Jorgen BOJsen, The Finsen Laboratory, The Finsen Institute, 49, Strandboulevard. DK-2100 Copenhagen. Denmark I359
to reduce interference from alterations in geometry during measurement and to increase the contribution of counts in percent from deep tissue layers by means of proper impulse height discrimination and lead collimation. The detector characteristics required for biotelemetry are discussed.
Materials and Methods Detector
A standard, disc-shaped p-type CdTe(C1) detector (74 x 2mm), type A 101 (RMD)*. is mounted close to a preamplifier”) in a small detector house (21 x 37 x 28 mm) for replacement of lead collimators. In front of the the collimator aperture a 2mm thick Perspex base plate is positioned for fixation of the detector housing during measurement. The preamplifier output is connected to a standard threshold impulse height discriminator unit. An impulse height distribution of a y9mTc point source in air is obtained by the detector used in the present study (Fig. I). Including scattered radiation the maximum counting sensitivity (IOOY,) is obtained at a discriminator threshold level “I”, Fig. 1 giving a constant background of 10~20cpm. In some investigations the amplitudes of the detected photons exceeding the threshold levels. indicated by threshold “2” or “Ii”, are giving 287, or 49, of the maximum count rate, respectively.
As the portable detector is fixed to the skin surface. the acceptable weight of the lead collimator is limited. The transmission of 140 keV (9ymTc) through 2 mm
Jorye
Bojsrn et al.
efficient (Pi) of Perspex at 140 keV (“““‘Tc) is 0. I8 I2 cm- ’ .c9)The attenuation in air is neglected. The radiation sources contain 99”‘Tc with a photon energy of 140 keV (‘+ = 6 h). Point (0.84 x I.0 mm) and homogeneous plane sources (400cm’ x 0.5 mm) are used in the investigations.
1
2
3
CHANNELS THRESHOLD
FIG;. I.
THF “99”‘Tc energy spectrum” or impulse height distribution measured in air by the CdTe(C1) detector used in the investigation. The maximum counting sensitivity ( IOO”,,.threshold “I”) includes maximum scattered radiation. At threshold “2” or “3”. 28”,, or 4”,, respectively of all detected photons. exceeds the discriminator threshold level and are counted.
thick lead is about 2”,,,,,‘“’ and therefore this thickness has been chosen for all sides except the front side of the collimators. In order to obtain the highest possible counting sensitivity the maximum detector surface area is exposed using only single hole collimators. Four different lead collimators are examined with “‘“7~ as source. The size of each collimator is shohn in Fig. 2.
The detector discriminator threshold level is adjusted to count scattered radiation. All measurements are performed with the detector fixed on nearly tissue equivalent Perspex phantoms as scattering and attenuation medium. The total linear attenuation CO-
Using various radionuclides as point and volume source, the counting sensitivities of CdTe(Cl) detectors have been measured and calculated recently.“’ In the comparison of various detector systems with collimators. plane source measurements are commonly used for evaluation of collimator etticienties.“” The plane source (I /Ki-cm ‘) is placed perpendicularly to the collimator axis and close to the Perspex base plate of the detector housing, and measurements with each of the four collimators are performed with the plane source for comparison.
The isocount curves are calculated from the ratio Gp/Gp,,, in percent including attenuation rn Perspex. Gp is the fractional solid angle for a point source at various spatial points P at distances IK 1 from the center of the visible detector area (A’,,) and the angle expressed by 0. ((RI fi’). Gp,,,<, is the fractional solid angle at the point close to and in front 01 the detector, giving the maximum count rate (IOO”,,). In the general case of a source point off the center axis of the detector surface. the fractional solid angle Gp can be expanded in infinite series. Some app~mximate formula for calculation of Gp nils given by JAFFI;Y”” assuming rO IR 1 small (r,) = radius of the exposed detector area (.A,,)). The mathematical clprcssions arc based on the approximation that the visible detector area seen from tbc point source I’. is equivnlent to the area of its prqiectlon on tlic tangent plane of ;I sphere of radius 1R 1. In .laffe)‘s I’OI~IUI~I no influence from a lead collimator is included. The approximate formul;~ of Gp in the present stud! iq dc-
169
1’6’
Cudnhm
telluride detectorsfor
rived from a geometrical model including the most efficient collimator as shown in Fig. 3. The collimator aperture defines the solid angle within which radiation is measured by an ideal CdTe(C1) detector like a y-ray energy spectrometer. It is assumed that the collimator material is opaque to radiation. that no scattering occurs from the collimator and that the point source is mono-energetic at I40 keV. The actual radius r0 of the total detector area (A,) is 0.35 cm. The thickness of the lead collimator b is only numerical 0.2cm. In the following equations quantities are used. Gp=_
Ab.sin 4n.R’
II .’
’
= 0.5 cm) (R 0 11,111
A..sin
0
47r. R’
If 0 < tg-’
from Fig, 3 the following equations R,
= R,sinO
=
R,cos
0 -
R.sinO
tJ cos-
cp= ~x0 2
2 r.
rO (4)
1 2
=+
(6) (7)
Luterul
(1)
II < I’~. then A; 2 A,. and Gp = ~~
x,
1361
Equation (2) in (3) and (4) in (5) and (6) in (7) gives AL (7) in (I) gives Gp. In general the uncertainty in these approximate calculations decreases when R 3 r0 3 b and at points close to the center axis of the collimator aperture.
= O.l225(in air)
If R.cos
hiotelemetry
are derived: (2)
displucrrmwt
deperldmce
und depth depmderlce
A comparison of isocount curves from various detector systems is not easily accomplished, therefore the quantities used by us for evaluation of biotelemetry collimator-detector systems with point sources are: (1) the “full width at half maximum” (FWHM) measured by the total lateral displacement (cm) of the detector on each side of the count rate maximum are peruntil 509; is reached. The measurements formed successively at various planes perpendicular to and along the center axis of the collimator (R,. Fig. 3). and (2) the depth dependence measured along the center axis as the count rate in percent of the maximum count rate obtained. These quantities are measured, with the most efficient coliimator and without collimator, at various threshold levels (Fig. 1). The experimental and calculated results are curve fitted by the least square fit. The FWHM as a function of the distance R, (Fig. 3) is fitted to linear functions. The depth dependence as a function of the point source to detector distance (R,) is fitted to power functions.
The detector is fixed on the Perspex phantom. the “)“‘Tc plane source is positioned at various depths in Perspex and the count rate measured in relation to
TAWE I. Comparison between collimators (Fig. 2) Perspew is placed behind the plane source as a scatter medium Lead Collimator No.
I
2
3
1670
870
430
I00
52
26
4
“““‘Tc
FIG. 3. The geometrical model (The “ycometric field of VICK”) for the approximate calculation of the fractional solid angle and isocount curves.
plane-source sensitivity Measured Measured In “<, of No. I
16lOcps.cm’-~tCiV 96”,,
CdTe
5 cm 1
FIG. 4. Calculated geometric isocount curves (140 keV) in Perspex. The i FWHM is indicated by the dashed line. the maximum count rate obtained is calculated in percent (threshold level “1”. Fig. 1). Measurements were performed with no collimator and with the most efficient collimator and compared with results calculated for an infinite plane source(‘“,‘3) at various depths (.Y)in perspex.
point and plane sources of ““‘Tc show. that adjustment of the spatial properties of CdTe(C1) detectors is possible by impulse height threshold discrimination and lead collimation. Based on plane source and isocount curve measurcments, collimator No. I is selected because of its high efficiency and pronounced collimation. The collimutor is similar to that in a commercial equipment*. The geometric model presented and the calculated approximate isocount curves agree with the experimental curves obtained and are sufficient for evaluation of the point source response of the collimatordetector system. In biotelemetry with radionuclide tracers it is of great importance that small alterations in geometry. which may appear during measurement. are suppressed in the results. Such alterations may happen when the disappearance from a point source is measured during physical activity. or if the point source depot for some reason changes its distribution in the tissue. In the characteriration of ;‘-ray imaging devices the FWHM is normally measured by a line source. 1’0,‘2’ In biotelemctry. however, the purpose is to measure physiological functions, and therefore we dccidcd to define the FWHM by the detector response to ;I point
Results The isocount curves obtained by the CdTe(Cl) detector without and with four different collimators are shown in Fig. 2. Table I shows plane source sensitivities obtained by measurements with the same collimators. Measured sensitivities are compared to that of the most efficient collimator (No. 1). The approximate calculations of geometric isocount curves are shown in Fig. 4. The lateral displacement dependence (FWHM) and the depth dependence are measured as functions of the detector to point source distance (R,). along the center axis of the collimator. The threshold levels from Fig. I are shown as parameter (Figs 5A and B). The curves include results obtained without collimator as well as calculated geometric curves from the ideal ;‘-ray energy spectrometer (Fig. 4). Curve fitting is shown in Table 2. The count rates measured with the 99mT~ plane source positioned at various depths in Perspex are shown in percent in Fig. 6. Scattered radiation is included in the measurements. Curve A is measured with the collimator (No. I. Fig. 2). Without collimator the curve obtained is identical to theoretical results calculated from attenuation of 99mTc in Perspex (curve B. Fig. 6).
Discussion One of the most frequently used radionuclides for clinical investigations is 99mT~. Investigations with * MEMOLOG@. Pharmacia Denmark.
Electronics Inc., Hillerod,
(RMDI
16t
0
2 b
source
, Geometric 0
4 6 Detector to
2
6
distance:
3 2
4
R,,
,I , I 6
Detector to source distance
cm
:Threshold )
8
: R, ,
cm
FE. 5. The FWHM (A) and the depth dependence in percent of the maximum count rate (B). as a function of the perpendicular distance from source to detector (R,) along the center axis of the collimator, Calculated “geometric” curves. curves fitted from various threshold levels with the most efficient collimator and without any collimator are shown.
Cudmiunl
TAHL~ 2. Curve fitting:
tellwide
FWHM
detectors,for
I363
hiotelemetrp
(linear) and depth dependence (Fig. 5)
(power
function)
No collimator Threshold Linear function r = 0~ + h(cm) i (cm) r
Power function ?‘ = u._uh(Oo); (.x inserted in cm) (I h r
I
I
z
3
I.878 1.500
0.946 1.266
0.932 I .003
0.394I 0.97
0.137 I.166
0.9983
0.9983
0.9996
0.9979
I.000
48. I I - I.527 0.9905
3X.46 - I .605 0.9954
32.52 - I.861 0.9962
30.05 -2.130 0.9920
2 I .90 - 2.256 0.999
Scattered
photons
criminator. region (RMD)
Depth
I” perspex
X, cm
FIG. 6. The q4”’Tc plane source (400 cm’) measured m percent of the maximum count rate response at various depths (_y) in Perspex. with the most efficient collimator, curve A. The curvIes obtained without collimator and calculated from nttenuation in Perspex (140 keV) are identical, curve B.
source. The depth dependence is useful for evaluation of the biotelemetry detector properties, as the distance between detector and radiation source is very short. The FWHM increases and the depth dependence increases with an increased contribution of detected scattered radiation. Thus the discriminator threshold level shall be adjusted to improve the suppression of alterations in geometry during measurement with point sources, and the suppression is even more pronounced when the collimator is removed. The ‘*geometric” curves equivalent to ideal I’-ray energy spectrometer curves show to be the most sensitive to alterations in the geometry. With the threshold level adjusted to count scattered radiation and with a single hole lead collimator with parallel sides applied in front of the CdTe(C1) detector, it is demonstrated that the contribution of counts, in percent, from deep layers of a yymT~ volume source can be increased. JOHNSTON and BRILL~“‘) have demonstrated similar results with a 99mTc plane source in a water phantom. They used the same type of collimator, but a conventional. stationary NaI(TI) scintillation detector with an impulse height window dis-
were
“Geometric”
I in
the
photopeak
subtracted.
Our data show that the portable standard CdTe(Cl) detector is suitable for single tracer biotelemetry investigations with 99mTc, and that y-ray energy spectrometers are not required. A~krtoM,lrdyr,llr,,ts~The authors wish to thank Mr FLEMMING HASLEV and Mr KLAUS CHRISTENSENfor skilful technical assistance. Also Mrs TOVF NORAGER is acknowledged for skilful typing of the manuscript. This work was supported by grants from the Danish Medical Research Council and Danish Council for Scientific and Industrial Research.
References I BOJS~N J. In
A
Hmdhook
on
Biotrlewtry
und
Rudio
(Eds AMLANER C. J. and MACDOXALD D. W.]
Trtrckimg
pp. I 19-l 30 (Pergamon Press. Oxford. 1979). BOJSEN J.. GR~TH S. and ROSSING N. IfIt. J. Appl. Rudiut. lsot. 32, 719-727 (1981). SIFFERT P.. CORNET A.. STUCK R. et ul. IEEE Trms. ,Vuc/. SC;. NS-22, 21 I-225 (1975). WHITED R. C. and SCHI~HFR M. M. Nucl. Iwtrurn Mrrhods 162. I I3- I23 ( 1979). KOLENDORF K. and BOJS~& J. C/in. Physiol. 2, 13-20 ( 1982). BOJSEN J.. STABERC B. and KOLENDORF K. In Biotrlrwtry (Eds SAKSEN W. and KIMMICH H. P.) VI. (Wouders. Leuven) (in press). 7. BOJSE~. J. and ROSSING N. Nucl. lmtrun~. Methods 150, 49953 (1978). 8. B~JSEN J., VADSTRUP S., SIFF~:RT P. et ul. In BiotekrwtrJ (Eds FRYER T. B., MILLER H. A. and SANDLER H.) Vol. III. pp. 247-250 (Academic Press, New York. 1976). 9. EVANS R. D. In Rtrditrtim Dosimrtry (Eds ATTIX F. H. and RO~S~H W. C.) Vol. I. 2 edn. pp. 114-123 (Academic Press. New York. 1978). IO. MA(.INTYRE E. J.. FEUORUK S. 0.. HARRIS C. C. et trl. R’lrr!lrtrrr,l~di_i,l 8. 99- I46 ( 1969). 1I. JAFF~~ A. H. Rev. %i. Imtruru. 25, 3499354 (1954). 12. HINI: G. J.. PARAS P.. WARR C. P. et (I[. In Mea.suremwts
of
thr
Prr~iwrwr~~ce
Purrrmrters
Parts I and II. US Department
qf
Ga~wnu
of Health, Education and Welfare. Rockville. HEW Publ. (FDA) 78-8049 (1977) and HEW Publ. (FDA) 79-8049 (1979). 13. JOHNSTONR. E. and BRILL A. B. In Medictrl Rrrdioiosorope Scitlticqrupltj*. Vol. I. pp. 6177631 (IAEA. Vienna. 1969). Ctrmws.
1nf. J. Appl. Rudiur. 13ur. Vol. 33. pp. 1359 to 1363. 19X1 Printed in Great Britain. All rights reserved
Collimation of Portable Cadmium Telluride Detectors for Biotelemetry JBRGEN
BOJSEN’,
BENT STABERG’
and KLAUS
K0LENDORF3
‘The Finsen Laboratory and ‘Department of Clinical Physiology. The Finsen Institute. 49 Strandboulevard. “Department of Endocrinology E. Frederiksberg Hospital. Copenhagen.
Copenhagen Denmark
and
The spatial properties of small. portable y-sensitive cadmium telluride (CdTe(C1)) detectors are evaluated for biotelemetry. Various single hole lead collimators are investigated with 99mTc as point and plane source. Isocount curves of the most efficient collimator are calculated. During local clearance measurements a reduced sensitivity to alterations in geometry is obtained, when all detected photons are counted, The contribution of counts in percent from deep layers of a ““Tc volume source is increased, when scattered radiation is counted and a single hole lead collimator with parallel sides is used. compared to results obtained with no collimator. ~-Ray energy spectrometers are not required for single tracer biotelemetry investigations with y9mTc.
Introduction solid-state, y-sensitive detectors like chlorine compensated, cadmium telluride (CdTe(CI)) detectors are valuable for biotelemetry with radionuclide tracers!‘.” The charge collection properties hamper, however, their use as energy spectrometers,‘3,4’ and consequently biotelemetry investigations in the past have only been based on single tracer studies. Contrary to conventional, ijz civo measuring techniques used in nuclear medicine, the biotelemetry methods allow the test person to be ambulatory during measurement. (‘) In order to avoid alterations in geometry during measurement. the small biotelemetry detectors are fixed by adhesive tape to the skin surface of the body,“,“’ and consequently the detector is positioned very close to the radioactive source. This may be a point source injected as a tracer in tissue only a few millimeters from the detector,“.” or a volume source injected as a tracer diluted in the extracellular fluid of the whole body.“’ In the latter case the tissue layers close to the detector mainly contribute to the count rate. Therefore even small changes in geometry may interfere with the measurements introducing erroneous results. It is the aim of this study to elucidate the spatial properties of standard CdTe(C1) detectors used as portable y-detectors in biotelemetry with 99mTc as radioactive source. Furthermore to demonstrate how PORTABLE,
* Radiation Monitoring Devices Inc., Watertown. Massachusetts, U.S.A. (RMD). All correspondance should be addressed to: Jorgen BOJsen, The Finsen Laboratory, The Finsen Institute, 49, Strandboulevard. DK-2100 Copenhagen. Denmark I359
to reduce interference from alterations in geometry during measurement and to increase the contribution of counts in percent from deep tissue layers by means of proper impulse height discrimination and lead collimation. The detector characteristics required for biotelemetry are discussed.
Materials and Methods Detector
A standard, disc-shaped p-type CdTe(C1) detector (74 x 2mm), type A 101 (RMD)*. is mounted close to a preamplifier”) in a small detector house (21 x 37 x 28 mm) for replacement of lead collimators. In front of the the collimator aperture a 2mm thick Perspex base plate is positioned for fixation of the detector housing during measurement. The preamplifier output is connected to a standard threshold impulse height discriminator unit. An impulse height distribution of a y9mTc point source in air is obtained by the detector used in the present study (Fig. I). Including scattered radiation the maximum counting sensitivity (IOOY,) is obtained at a discriminator threshold level “I”, Fig. 1 giving a constant background of 10~20cpm. In some investigations the amplitudes of the detected photons exceeding the threshold levels. indicated by threshold “2” or “Ii”, are giving 287, or 49, of the maximum count rate, respectively.
As the portable detector is fixed to the skin surface. the acceptable weight of the lead collimator is limited. The transmission of 140 keV (9ymTc) through 2 mm
Jorye
Bojsrn et al.
efficient (Pi) of Perspex at 140 keV (“““‘Tc) is 0. I8 I2 cm- ’ .c9)The attenuation in air is neglected. The radiation sources contain 99”‘Tc with a photon energy of 140 keV (‘+ = 6 h). Point (0.84 x I.0 mm) and homogeneous plane sources (400cm’ x 0.5 mm) are used in the investigations.
1
2
3
CHANNELS THRESHOLD
FIG;. I.
THF “99”‘Tc energy spectrum” or impulse height distribution measured in air by the CdTe(C1) detector used in the investigation. The maximum counting sensitivity ( IOO”,,.threshold “I”) includes maximum scattered radiation. At threshold “2” or “3”. 28”,, or 4”,, respectively of all detected photons. exceeds the discriminator threshold level and are counted.
thick lead is about 2”,,,,,‘“’ and therefore this thickness has been chosen for all sides except the front side of the collimators. In order to obtain the highest possible counting sensitivity the maximum detector surface area is exposed using only single hole collimators. Four different lead collimators are examined with “‘“7~ as source. The size of each collimator is shohn in Fig. 2.
The detector discriminator threshold level is adjusted to count scattered radiation. All measurements are performed with the detector fixed on nearly tissue equivalent Perspex phantoms as scattering and attenuation medium. The total linear attenuation CO-
Using various radionuclides as point and volume source, the counting sensitivities of CdTe(Cl) detectors have been measured and calculated recently.“’ In the comparison of various detector systems with collimators. plane source measurements are commonly used for evaluation of collimator etticienties.“” The plane source (I /Ki-cm ‘) is placed perpendicularly to the collimator axis and close to the Perspex base plate of the detector housing, and measurements with each of the four collimators are performed with the plane source for comparison.
The isocount curves are calculated from the ratio Gp/Gp,,, in percent including attenuation rn Perspex. Gp is the fractional solid angle for a point source at various spatial points P at distances IK 1 from the center of the visible detector area (A’,,) and the angle expressed by 0. ((RI fi’). Gp,,,<, is the fractional solid angle at the point close to and in front 01 the detector, giving the maximum count rate (IOO”,,). In the general case of a source point off the center axis of the detector surface. the fractional solid angle Gp can be expanded in infinite series. Some app~mximate formula for calculation of Gp nils given by JAFFI;Y”” assuming rO IR 1 small (r,) = radius of the exposed detector area (.A,,)). The mathematical clprcssions arc based on the approximation that the visible detector area seen from tbc point source I’. is equivnlent to the area of its prqiectlon on tlic tangent plane of ;I sphere of radius 1R 1. In .laffe)‘s I’OI~IUI~I no influence from a lead collimator is included. The approximate formul;~ of Gp in the present stud! iq dc-
169
1’6’
Cudnhm
telluride detectorsfor
rived from a geometrical model including the most efficient collimator as shown in Fig. 3. The collimator aperture defines the solid angle within which radiation is measured by an ideal CdTe(C1) detector like a y-ray energy spectrometer. It is assumed that the collimator material is opaque to radiation. that no scattering occurs from the collimator and that the point source is mono-energetic at I40 keV. The actual radius r0 of the total detector area (A,) is 0.35 cm. The thickness of the lead collimator b is only numerical 0.2cm. In the following equations quantities are used. Gp=_
Ab.sin 4n.R’
II .’
’
= 0.5 cm) (R 0 11,111
A..sin
0
47r. R’
If 0 < tg-’
from Fig, 3 the following equations R,
= R,sinO
=
R,cos
0 -
R.sinO
tJ cos-
cp= ~x0 2
2 r.
rO (4)
1 2
=+
(6) (7)
Luterul
(1)
II < I’~. then A; 2 A,. and Gp = ~~
x,
1361
Equation (2) in (3) and (4) in (5) and (6) in (7) gives AL (7) in (I) gives Gp. In general the uncertainty in these approximate calculations decreases when R 3 r0 3 b and at points close to the center axis of the collimator aperture.
= O.l225(in air)
If R.cos
hiotelemetry
are derived: (2)
displucrrmwt
deperldmce
und depth depmderlce
A comparison of isocount curves from various detector systems is not easily accomplished, therefore the quantities used by us for evaluation of biotelemetry collimator-detector systems with point sources are: (1) the “full width at half maximum” (FWHM) measured by the total lateral displacement (cm) of the detector on each side of the count rate maximum are peruntil 509; is reached. The measurements formed successively at various planes perpendicular to and along the center axis of the collimator (R,. Fig. 3). and (2) the depth dependence measured along the center axis as the count rate in percent of the maximum count rate obtained. These quantities are measured, with the most efficient coliimator and without collimator, at various threshold levels (Fig. 1). The experimental and calculated results are curve fitted by the least square fit. The FWHM as a function of the distance R, (Fig. 3) is fitted to linear functions. The depth dependence as a function of the point source to detector distance (R,) is fitted to power functions.
The detector is fixed on the Perspex phantom. the “)“‘Tc plane source is positioned at various depths in Perspex and the count rate measured in relation to
TAWE I. Comparison between collimators (Fig. 2) Perspew is placed behind the plane source as a scatter medium Lead Collimator No.
I
2
3
1670
870
430
I00
52
26
4
“““‘Tc
FIG. 3. The geometrical model (The “ycometric field of VICK”) for the approximate calculation of the fractional solid angle and isocount curves.
plane-source sensitivity Measured Measured In “<, of No. I
16lOcps.cm’-~tCiV 96”,,
CdTe
5 cm 1
FIG. 4. Calculated geometric isocount curves (140 keV) in Perspex. The i FWHM is indicated by the dashed line. the maximum count rate obtained is calculated in percent (threshold level “1”. Fig. 1). Measurements were performed with no collimator and with the most efficient collimator and compared with results calculated for an infinite plane source(‘“,‘3) at various depths (.Y)in perspex.
point and plane sources of ““‘Tc show. that adjustment of the spatial properties of CdTe(C1) detectors is possible by impulse height threshold discrimination and lead collimation. Based on plane source and isocount curve measurcments, collimator No. I is selected because of its high efficiency and pronounced collimation. The collimutor is similar to that in a commercial equipment*. The geometric model presented and the calculated approximate isocount curves agree with the experimental curves obtained and are sufficient for evaluation of the point source response of the collimatordetector system. In biotelemetry with radionuclide tracers it is of great importance that small alterations in geometry. which may appear during measurement. are suppressed in the results. Such alterations may happen when the disappearance from a point source is measured during physical activity. or if the point source depot for some reason changes its distribution in the tissue. In the characteriration of ;‘-ray imaging devices the FWHM is normally measured by a line source. 1’0,‘2’ In biotelemctry. however, the purpose is to measure physiological functions, and therefore we dccidcd to define the FWHM by the detector response to ;I point
Results The isocount curves obtained by the CdTe(Cl) detector without and with four different collimators are shown in Fig. 2. Table I shows plane source sensitivities obtained by measurements with the same collimators. Measured sensitivities are compared to that of the most efficient collimator (No. 1). The approximate calculations of geometric isocount curves are shown in Fig. 4. The lateral displacement dependence (FWHM) and the depth dependence are measured as functions of the detector to point source distance (R,). along the center axis of the collimator. The threshold levels from Fig. I are shown as parameter (Figs 5A and B). The curves include results obtained without collimator as well as calculated geometric curves from the ideal ;‘-ray energy spectrometer (Fig. 4). Curve fitting is shown in Table 2. The count rates measured with the 99mT~ plane source positioned at various depths in Perspex are shown in percent in Fig. 6. Scattered radiation is included in the measurements. Curve A is measured with the collimator (No. I. Fig. 2). Without collimator the curve obtained is identical to theoretical results calculated from attenuation of 99mTc in Perspex (curve B. Fig. 6).
Discussion One of the most frequently used radionuclides for clinical investigations is 99mT~. Investigations with * MEMOLOG@. Pharmacia Denmark.
Electronics Inc., Hillerod,
(RMDI
16t
0
2 b
source
, Geometric 0
4 6 Detector to
2
6
distance:
3 2
4
R,,
,I , I 6
Detector to source distance
cm
:Threshold )
8
: R, ,
cm
FE. 5. The FWHM (A) and the depth dependence in percent of the maximum count rate (B). as a function of the perpendicular distance from source to detector (R,) along the center axis of the collimator, Calculated “geometric” curves. curves fitted from various threshold levels with the most efficient collimator and without any collimator are shown.
Cudmiunl
TAHL~ 2. Curve fitting:
tellwide
FWHM
detectors,for
I363
hiotelemetrp
(linear) and depth dependence (Fig. 5)
(power
function)
No collimator Threshold Linear function r = 0~ + h(cm) i (cm) r
Power function ?‘ = u._uh(Oo); (.x inserted in cm) (I h r
I
I
z
3
I.878 1.500
0.946 1.266
0.932 I .003
0.394I 0.97
0.137 I.166
0.9983
0.9983
0.9996
0.9979
I.000
48. I I - I.527 0.9905
3X.46 - I .605 0.9954
32.52 - I.861 0.9962
30.05 -2.130 0.9920
2 I .90 - 2.256 0.999
Scattered
photons
criminator. region (RMD)
Depth
I” perspex
X, cm
FIG. 6. The q4”’Tc plane source (400 cm’) measured m percent of the maximum count rate response at various depths (_y) in Perspex. with the most efficient collimator, curve A. The curvIes obtained without collimator and calculated from nttenuation in Perspex (140 keV) are identical, curve B.
source. The depth dependence is useful for evaluation of the biotelemetry detector properties, as the distance between detector and radiation source is very short. The FWHM increases and the depth dependence increases with an increased contribution of detected scattered radiation. Thus the discriminator threshold level shall be adjusted to improve the suppression of alterations in geometry during measurement with point sources, and the suppression is even more pronounced when the collimator is removed. The ‘*geometric” curves equivalent to ideal I’-ray energy spectrometer curves show to be the most sensitive to alterations in the geometry. With the threshold level adjusted to count scattered radiation and with a single hole lead collimator with parallel sides applied in front of the CdTe(C1) detector, it is demonstrated that the contribution of counts, in percent, from deep layers of a yymT~ volume source can be increased. JOHNSTON and BRILL~“‘) have demonstrated similar results with a 99mTc plane source in a water phantom. They used the same type of collimator, but a conventional. stationary NaI(TI) scintillation detector with an impulse height window dis-
were
“Geometric”
I in
the
photopeak
subtracted.
Our data show that the portable standard CdTe(Cl) detector is suitable for single tracer biotelemetry investigations with 99mTc, and that y-ray energy spectrometers are not required. A~krtoM,lrdyr,llr,,ts~The authors wish to thank Mr FLEMMING HASLEV and Mr KLAUS CHRISTENSENfor skilful technical assistance. Also Mrs TOVF NORAGER is acknowledged for skilful typing of the manuscript. This work was supported by grants from the Danish Medical Research Council and Danish Council for Scientific and Industrial Research.
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A
Hmdhook
on
Biotrlewtry
und
Rudio
(Eds AMLANER C. J. and MACDOXALD D. W.]
Trtrckimg
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of
thr
Prr~iwrwr~~ce
Purrrmrters
Parts I and II. US Department
qf
Ga~wnu
of Health, Education and Welfare. Rockville. HEW Publ. (FDA) 78-8049 (1977) and HEW Publ. (FDA) 79-8049 (1979). 13. JOHNSTONR. E. and BRILL A. B. In Medictrl Rrrdioiosorope Scitlticqrupltj*. Vol. I. pp. 6177631 (IAEA. Vienna. 1969). Ctrmws.