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A preliminary study on the use of cadmium telluride detectors in the scintigraphy of thyroid gland
Nuclear Instruments and Methods 189 (1981) 637-639 North-Holland Publishing Company
637
A PRELIMINARY STUDY ON THE USE OF CADMIUM TELLURIDE DETECTORS IN THE SCINTIGRAPHY OF THYROID GLAND A.M. MANCINI, A. QUIRINI, L. VASANELLI Istituto di Fisica, Universit~ e Sezione I.N.F.N., Bari, Italy
C. BACCI, R. BERNABEI, R. PANI, B. RISPOLI Istituto di Fisica, Universit3 e Sezione I.N.F.N., Rome, Italy
P.L. BALLESIO and C. FURETTA 1 ° Cattedra Fisica Medica, Universitit Roma, Rome, Italy
Received 1 December 1980 and in revised form 5 June 1981
A cadmium telluride gamma detector has been used for monitoring the activity of a radioactive tracer in a thyroid gland. Preliminary measurements are reported in comparison with those obtained with a standard NaI(T1) scintillator.
In recent years, gamma-ray spectroscopy has been used more and more in nuclear medicine. Thus diagnostic techniques based on the use of gamma-cameras and on scintigraphic measurements have been developed so as to better investigate physiopathological functions of patients. The gamma-ray detectors most used in this field are certainly the NaI(T1)scintillators, on account of their efficiency. However, semiconductor Ge(Li) detectors are used when a rather good energy resolution is needed. Nevertheless, a wider use of this kind of detectors is discouraged, owing to their operating only at very low temperatures. Thus, efforts have been made in order to develop semiconductor gamma-ray detectors with good efficiency and resolution, capable of working at room temperature. Among these the cadmium telluride (CdTe)detectors prove to be very interesting [1]. In fact, they have: (1) high efficiency (due to high atomic number of Cd and Te); (2) good energy resolution [intermediate between the NaI(T1) and Ge(Li) detector ones]; (3) small dimensions. These features make them especially attractive for biomedical applications. A comprehensive review of the CdTe detector applications to this purpose can be found in refs. 2 and 3. In this paper we report a preliminary study on the use of CdTe detectors, to measure the radiation 0029-554X/81/0000-0000/$02.50 © 1981 North-Holland
emitted by a radiotracer in a thyroid gland. Since this is the first time that this application is investigated, our results have been tested by comparing them with those obtained by means of a standard scintigraphic system, using a NaI(TI) scintillator. The detector was prepared at the Physics Institute of Bari University. The CdTe single crystals were grown by the "travelling heater method" [4], which is considered one of the most reliable techniques to grow detector-grade CdTe crystals. The growth apparatus is described in detail elsewhere [5]; the growth temperature was kept at 850°C while the furnace velocity was 0.33 mm/h. No dopants were added during the growth process. The detector was set up with a single crystal surface area of 25 mm 2 and a thickness of 2 ram. The surfaces were mechanically lapped and polished with diamond paste, but no chemical etching was carried out on the sample. Aluminium contacts were applied by vacuum evaporation. The crystal was of p-type conductivity, with a resistivity of about 107 ~2 • cm. The detector used in this work showed an energy resolution (fwhm) of about 8 keV at a photon energy of 80 keV (la3Ba), when biased at 200 V. A probe of about 5 mm in spatial resolution was obtained by mounting the detector into a brass container with a cylindrical lead collimator. Measure-
638
A.M. Mancini et al. / Scintigraphy o f thyroid gland
ments were taken on a thyroid phantom, built up with a perspex-f'ilter paper-perspex sandwich with a laaBa solution absorbed in the filter paper thus simulating a real thyroid. The total activity was about 150/aCi. 133Ba was used as radiotracer instead of 131I generally used in thyroid scintigraphy, because it emits gamma photons with similar energy and also has a longer half-life. The experimental set-up is shown in fig. 1. Measurements were carried out by moving the probe across the phantom with steps of 5 mm. The experimental data were further processed by means of a CAMAC video autonomous read-out CAVIAR microcomputer [6]. The thyroid phantom radioactivity map, as measured with the CdTe probe, is plotted in fig. 2, through eight shades of grey. A three-dimensional representation of the same data is reported in fig. 3. When the scintigraphy of the same thyroid phantom is carried out with a standard NaI(T1) system, a similar image is obtained. Therefore we can state that a CdTe detector gives the same information of the scintillation system, with the additional advantage of having smaller dimensions. However, since the active volume of a CdTe detector is much smaller than that of the usual NaI(T1) scintillator, it has a negative influence on the coun-
Fig. 1. Experimental set-up.
Fig. 2. Reconstructed image of the thyroid phantom at the video display of the microcomputer.
ting efficiency. The smaller counting efficiency of the CdTe detector is partially compensated by the gain in the solid angle achievable by putting the detector nearer to the radioactive source. In fact, if the counting rates of CdTe and NaI(T1) detectors in the same
A.M. Mancini et al. / Scintigraphy o f thyroid gland
639
Fig. 3. Three-dimensional representation of the activity of the thyroid phantom.
region of the phanto m are compared, one obtains a value about fifty times smaller for the first one; hence an activity map can be plotted with the CdTe probe in a measurement period fifty times longer. This difference is not a deterrent, considering that some improvements of the performances of the CdTe probe may be possible. First of all, the small dimensions of the CdTe crystals allow the construction of more sophisticated probes, constituted by arrays of detectors, which can measure simultaneously different points of the phantom by means of suitable collimators. Secondly, better performances of a CdTe system could be obtained by improving the average quality of the CdTe crystals. In fact, according to ref. 7, the detector used in this work can be classified as a medium performance detector. Better performances o f CdTe detectors have been previously reported in the literature, but their reproduction is not easy. At present, realistic evaluations can be made only with a medium performance detector. However, improvements can be expected i n the near future. In conclusion, it can be stated that CdTe detectors seem very promising for the construction of an apparatus for thyroid analysis. Their main advantages rest in their small dimensions and in their
working simplicity. Until now, their main limitation lies in their small counting efficiency. Therefore the introduction of this kind of detector in thyroid diagnostics demands the development of more sophisticated systems requiring shorter measurement times. The authors wish to thank Dr. M. Tarantino for revising the English version.
References [1] Proc. 1st Int. Conf. on CdTe, a material for "r-ray detectors, Strasbourg (1971). eds., P. Siffert and A. Cornet. [2] R. AUemand, R. Bouteiller and M. Laval, Rev. Phys. Appl. 12 (1977) 13. [3] F.V. Wald and G. Entine, Nucl. Instr. and Meth. 150 (1978) 13. [4] R.O. Bell, N. Hemmet and F.V. Wald, Phys. Stat. Sol. (a) 1 (1970) 375. [5] A.M. Mancini, A. Quirini, A. Rizzo and L. VasaneUi, Rept. INFN/TC-79/5 (1979). [6] S. Cittolin and B.G. Taylor, Joint Conf. on Microprocessors in automation and communication, University of Kent (1978). [7] C. Scharager, P. Siffert, A. Holtzer and M. Schieber, IEEE Trans. Nucl. Sci. NS-27 (1980) 276.
637
A PRELIMINARY STUDY ON THE USE OF CADMIUM TELLURIDE DETECTORS IN THE SCINTIGRAPHY OF THYROID GLAND A.M. MANCINI, A. QUIRINI, L. VASANELLI Istituto di Fisica, Universit~ e Sezione I.N.F.N., Bari, Italy
C. BACCI, R. BERNABEI, R. PANI, B. RISPOLI Istituto di Fisica, Universit3 e Sezione I.N.F.N., Rome, Italy
P.L. BALLESIO and C. FURETTA 1 ° Cattedra Fisica Medica, Universitit Roma, Rome, Italy
Received 1 December 1980 and in revised form 5 June 1981
A cadmium telluride gamma detector has been used for monitoring the activity of a radioactive tracer in a thyroid gland. Preliminary measurements are reported in comparison with those obtained with a standard NaI(T1) scintillator.
In recent years, gamma-ray spectroscopy has been used more and more in nuclear medicine. Thus diagnostic techniques based on the use of gamma-cameras and on scintigraphic measurements have been developed so as to better investigate physiopathological functions of patients. The gamma-ray detectors most used in this field are certainly the NaI(T1)scintillators, on account of their efficiency. However, semiconductor Ge(Li) detectors are used when a rather good energy resolution is needed. Nevertheless, a wider use of this kind of detectors is discouraged, owing to their operating only at very low temperatures. Thus, efforts have been made in order to develop semiconductor gamma-ray detectors with good efficiency and resolution, capable of working at room temperature. Among these the cadmium telluride (CdTe)detectors prove to be very interesting [1]. In fact, they have: (1) high efficiency (due to high atomic number of Cd and Te); (2) good energy resolution [intermediate between the NaI(T1) and Ge(Li) detector ones]; (3) small dimensions. These features make them especially attractive for biomedical applications. A comprehensive review of the CdTe detector applications to this purpose can be found in refs. 2 and 3. In this paper we report a preliminary study on the use of CdTe detectors, to measure the radiation 0029-554X/81/0000-0000/$02.50 © 1981 North-Holland
emitted by a radiotracer in a thyroid gland. Since this is the first time that this application is investigated, our results have been tested by comparing them with those obtained by means of a standard scintigraphic system, using a NaI(TI) scintillator. The detector was prepared at the Physics Institute of Bari University. The CdTe single crystals were grown by the "travelling heater method" [4], which is considered one of the most reliable techniques to grow detector-grade CdTe crystals. The growth apparatus is described in detail elsewhere [5]; the growth temperature was kept at 850°C while the furnace velocity was 0.33 mm/h. No dopants were added during the growth process. The detector was set up with a single crystal surface area of 25 mm 2 and a thickness of 2 ram. The surfaces were mechanically lapped and polished with diamond paste, but no chemical etching was carried out on the sample. Aluminium contacts were applied by vacuum evaporation. The crystal was of p-type conductivity, with a resistivity of about 107 ~2 • cm. The detector used in this work showed an energy resolution (fwhm) of about 8 keV at a photon energy of 80 keV (la3Ba), when biased at 200 V. A probe of about 5 mm in spatial resolution was obtained by mounting the detector into a brass container with a cylindrical lead collimator. Measure-
638
A.M. Mancini et al. / Scintigraphy o f thyroid gland
ments were taken on a thyroid phantom, built up with a perspex-f'ilter paper-perspex sandwich with a laaBa solution absorbed in the filter paper thus simulating a real thyroid. The total activity was about 150/aCi. 133Ba was used as radiotracer instead of 131I generally used in thyroid scintigraphy, because it emits gamma photons with similar energy and also has a longer half-life. The experimental set-up is shown in fig. 1. Measurements were carried out by moving the probe across the phantom with steps of 5 mm. The experimental data were further processed by means of a CAMAC video autonomous read-out CAVIAR microcomputer [6]. The thyroid phantom radioactivity map, as measured with the CdTe probe, is plotted in fig. 2, through eight shades of grey. A three-dimensional representation of the same data is reported in fig. 3. When the scintigraphy of the same thyroid phantom is carried out with a standard NaI(T1) system, a similar image is obtained. Therefore we can state that a CdTe detector gives the same information of the scintillation system, with the additional advantage of having smaller dimensions. However, since the active volume of a CdTe detector is much smaller than that of the usual NaI(T1) scintillator, it has a negative influence on the coun-
Fig. 1. Experimental set-up.
Fig. 2. Reconstructed image of the thyroid phantom at the video display of the microcomputer.
ting efficiency. The smaller counting efficiency of the CdTe detector is partially compensated by the gain in the solid angle achievable by putting the detector nearer to the radioactive source. In fact, if the counting rates of CdTe and NaI(T1) detectors in the same
A.M. Mancini et al. / Scintigraphy o f thyroid gland
639
Fig. 3. Three-dimensional representation of the activity of the thyroid phantom.
region of the phanto m are compared, one obtains a value about fifty times smaller for the first one; hence an activity map can be plotted with the CdTe probe in a measurement period fifty times longer. This difference is not a deterrent, considering that some improvements of the performances of the CdTe probe may be possible. First of all, the small dimensions of the CdTe crystals allow the construction of more sophisticated probes, constituted by arrays of detectors, which can measure simultaneously different points of the phantom by means of suitable collimators. Secondly, better performances of a CdTe system could be obtained by improving the average quality of the CdTe crystals. In fact, according to ref. 7, the detector used in this work can be classified as a medium performance detector. Better performances o f CdTe detectors have been previously reported in the literature, but their reproduction is not easy. At present, realistic evaluations can be made only with a medium performance detector. However, improvements can be expected i n the near future. In conclusion, it can be stated that CdTe detectors seem very promising for the construction of an apparatus for thyroid analysis. Their main advantages rest in their small dimensions and in their
working simplicity. Until now, their main limitation lies in their small counting efficiency. Therefore the introduction of this kind of detector in thyroid diagnostics demands the development of more sophisticated systems requiring shorter measurement times. The authors wish to thank Dr. M. Tarantino for revising the English version.
References [1] Proc. 1st Int. Conf. on CdTe, a material for "r-ray detectors, Strasbourg (1971). eds., P. Siffert and A. Cornet. [2] R. AUemand, R. Bouteiller and M. Laval, Rev. Phys. Appl. 12 (1977) 13. [3] F.V. Wald and G. Entine, Nucl. Instr. and Meth. 150 (1978) 13. [4] R.O. Bell, N. Hemmet and F.V. Wald, Phys. Stat. Sol. (a) 1 (1970) 375. [5] A.M. Mancini, A. Quirini, A. Rizzo and L. VasaneUi, Rept. INFN/TC-79/5 (1979). [6] S. Cittolin and B.G. Taylor, Joint Conf. on Microprocessors in automation and communication, University of Kent (1978). [7] C. Scharager, P. Siffert, A. Holtzer and M. Schieber, IEEE Trans. Nucl. Sci. NS-27 (1980) 276.