Nuclear Instruments and Methods in Physics Research A 477 (2002) 509–513
A compact gamma ray imager for oncology R. Pania,*, A. Solurib, R. Scafe" c, R. Pellegrinia, A. Tat"ıc, F. Scopinaroa, G. De Vincentisa, T. Gigliottia, A. Festinesic, F. Garibaldid, A. Del Guerrae a
Department of Experimental Medicine and Pathology, University of Rome, La Sapienza, V.le Regina Elena 329, I-00185 Roma, Italy b Institute of Biomedical Technologies CNR, Rome, Italy c ENEA-INN, CR Casaccia, Rome, Italy d ISS Rome, Italy e Department of Physics, University of Pisa, Italy
Abstract A variety of new techniques based on radiopharmaceuticals are showing a valid support for cancer detection and interventional procedures. Axillary lymph nodes status is the most important prognostic factor for determining breast cancer prognosis. The use of dedicated gamma cameras characterized by low costs and weight, could be easily transferred to detection for bioptical procedures. To this aim this paper presents a new detection system having two heads with 4 and 25 cm2 Field of View (FOV) and 0.8 and 3.6 kg weight, respectively. This novel scintillation camera is based upon a compact Position Sensitive Photo Multiplier Tube (PSPMT) Hamamatsu R5900-C8 as individual or array assembled. The Hamamatsu R5900-C8 is a metal channel dynode PMT with a crossed wire anode. The overall dimensions are 28 28 mm2 and 20 mm height. It was coupled to a CsI(Tl) array of individual 3 3 3 mm3 crystals. The measured intrinsic spatial resolution proved much better than the pixel size. A clinical image obtained from a breast phantom showed a 6 mm sized tumor. r 2002 Elsevier Science B.V. All rights reserved.
1. Introduction A variety of new techniques based on radiopharmaceuticals are showing a valid support for cancer detection and interventional procedures [1–3]. The sentinel lymph node detection represents a promising field of application of nuclear medicine in the management of breast cancer. In fact, axillary lymph node status is the most important prognostic factor taking into account the prognosis and the surgical treatment of patients *Corresponding author. Tel.: +39-6-4991-8277; fax: +39-6495-8426. E-mail address:
[email protected] (R. Pani).
affected by such pathology. Several authors suggest that if the ‘‘sentinel lymph node’’ (first lymph node involved in the lymphatic drainage from the area of breast in which a carcinoma is located) is shown to be not affected by tumoral invasion, all lymph nodes could be assumed as ‘‘free of disease’’ and, as a consequence, lymph nodal dissection could be avoided, with improvement of the quality of life for a significant number of patients. As a consequence, it is mandatory to localize with high precision the ‘‘hot spot’’ corresponding to the sentinel lymph node. Imaging by traditional gamma cameras could prove expensive and time-consuming for a nuclear medicine section. On the contrary applying a dedicated gamma imager [4–7] characterized by
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 7 9 5 - 8
510
R. Pani et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 509–513
the possibility to be easily transferred to the surgical theatre, the sentinel lymph node imaging and localization could be rapidly obtained at the time of intervention. By considering this scenario, it appears really strategic to use an imager characterized by low weight and costs. In this paper, we propose a novel imaging probe based upon the recent generation Position Sensitive Photo Multiplier Tube (PSPMT) Hamamatsu R5900-C8 [8]. Compactness is the peculiarity of this gamma ray imager, in fact due to the small size and small insensitive detection area they can be assembled as a single tube gamma camera or as a 2 2 tube array obtaining 2 2 cm2 Field Of View (FOV) and 5 5 cm2 FOV, respectively [9–11]. The strong volume reduction of the detection head allows a consequent reduction of shielding weight. It allows the imager to be easily handled during clinical investigation or surgical intervention. The basic concept is the same as an Anger Camera but with the substitution of conventional PMTs with PSPMTs. In this way, it is possible to utilize scintillation devices with narrow light distributions, such as crystal arrays, and at the same time to have the position determination in the dead area between neighboring PSPMTs by a simple light guide.
2. Equipment and method The detection head mainly consists of a lead collimator, shielding housing, and a CsI(Tl) crystal array coupled to a PSPMT. The Hamamatsu R5900-C8 [8] is a metal channel dynode PSPMT with a crossed wire anode (4X+4Y) [2] The total active area (22 22 mm2) is obtained by using a grid to focus electrons emitted from the peripheral region of the cathode. The PSPMT consists of ten multiplication stages, nine metal channel dynodes followed by a reflective one. The anode of Y-axis consists of four thin metal strips 18 mm length with rectangular holes, to allow the transmission of the charge to the X-axis anode. Each anode is composed of two 5.50 mm wide inner strips and two 2.75 mm wide outer strips. Anode strips are 0.50 mm apart. The PSPMT has a bialkali photocathode with a 1.5 mm borosilicate glass window.
Recent results show that R5900-C8 has an intrinsic charge spread of o1 mm FWHM [9]. In operation, light from a scintillation event in a CsI(Tl) array individual crystal, strikes the PSPMT photocathode, an electron cloud is emitted, amplified and collected on the four Y anode plates and four X anode plates. The typical current amplification is about 5 105 at 800 V applied voltage. The overall PSPMT dimensions are 28 28 mm2 and 20 mm high. More recently Hamamatsu has reduced dimensions further to o26 26 mm2 (see Fig. 1) cutting 1 mm flanges. In this work the 2 2 PSPMT detector was assembled with both R5900C8 versions, i.e. with and without flanges. The two closely packed PSPMT arrays have an insensitive area of width 6 and 4 mm, respectively. CsI(Tl) scintillation arrays of 7 7 and 15 15, and 3 mm thick produced by Scionix were coupled to both PSPMT configurations (single and 2 2 PSPMT), respectively. They have the same pixel size (3 3 mm2) and an overall dimension of 23 23 and 50 50 mm2, respectively. The scintillating array was directly coupled to the single PSPMT by a quartz light guide 3 or 5 mm thick to the PSPMT array. The thickness of the scintillation crystals was chosen on the basis of a compromise between light spread and detection efficiency (60% at 140 keV) [4]. To analyze the imaging performance of the detectors an ultrahigh resolution parallel hole collimator (Von Gahlen) was utilized. It has hexagonal holes 1.3 mm wide, 35 mm length and
Fig. 1. Photograph of 2 2 R5900-C8 tubes without flanges.
511
R. Pani et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 509–513
0.2 mm septa. The camera sensitivity at 99mTc photon energy was 150 cpm/mCi (1 mCi=37 kBq). A 57Co collimated source with 1 mm aperture diameter was used for spot irradiation of the imagers. The single tube readout electronics consists of eight preamplifiers directly connected to each wire anode, respectively A weighted summing circuit was built to produce two signals related to X and Y positions. The second summing circuit produces two signals related to the total charge collected by the X and Y anodes, respectively. In the 2 2 PSPMT array, along a direction each anode wire was directly connected to the anode in the same position of the corresponding PSPMT. In this way the camera base consists of only 16 preamplifiers [11]. The acquisition system is based upon an Analog to Digital Converter (ADC) National Instruments AT-MIO E Series card plugged into a docking system for a Hewlett-Packard PC notebook Pentium II at 166 MHz. It is interfaced to the scintillation imager by a control unit board for event processing. The software performing the acquisition and the image processing was developed using Graphics-Language (LabView by National Instruments). To make the imaging device more competitive and fast during ‘‘hot spot’’ localization, the acquisition system is able to do real time imaging during clinical examination with a sub-second refreshing time. Data and imaging processing methods for both gamma camera configurations are described elsewhere [11]. To test the clinical performance a breast phantom was realized. It consisted of a 13 cm diameter cylinder filled with 3 cm tecnetiated water
with a radioactivity concentration of 100 nCi/cm3 simulating normal breast tissue in scintimammography. Two cylinders 1.0 and 0.6 cm diameter inside the breast, simulated the presence of two small size tumors. Radioactivity concentration was established as the typical background/target ratio (1 : 10) in scintimammography.
3. Results Imaging and spectrometric results obtained from the imagers with different PSPMT assembling are summarized in Table 1. The intrinsic performance of the single R5900-C8 PSPMT is good when directly coupled to the CsI(Tl) array with 3 3 mm2 pixel size. Fig. 2 shows the image obtained from a flood irradiation of the scintillation array. The PSPMT is able to carry out the position of all crystal pixels with an intrinsic spatial resolution of 1.1 mm and with a good position response. In fact, this size of pixel produces a light spread FWHM of about 4 mm when coupled to a 1.5 mm glass photocathode window. It is compatible with the anode strips of the PSPMT. Fig. 3 shows an image obtained coupling the same crystal configuration to a Hamamatsu R2486 PSPMT [12]. The best spatial resolution obtained for each crystal pixel is about 1.5 mm FWHM. This value is compatible with the wider spread of light introduced by the thicker photocathode glass window (3 mm). The intrinsic spread of charge of R2486 PSPMT has little effect on spatial resolution for wide light spread. The comparison of position linearity, uniformity response and energy resolution between the two PSPMT gave very similar results. Characterization
Table 1 Intrinsic spatial resolution and relative energy resolution of different PSPMT assembling irradiated at 122 keVa
SR active area (mm) SR insens. area (mm) ER active area (%) ER insens. area (%) a
Single PSPMT
2 2 PSPMT wf 3 mm quartz
2 2 PSPMT w/of 3 mm quartz
2 2 PSPMT w/f 5 mm quartz
1.1 F 18 F
2.4 3.2 21 35
2 2 20 27
3 5 22 40
SR=intrinsic spatial resolution, ER=Relative energy resolution, wf=with flanges, w/of=flangeless.
512
R. Pani et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 509–513
Fig. 2. Flood field irradiation of CsI(Tl) scintillating array coupled to R5900-C8 PSPMT.
Fig. 4. Breast phantom image obtained with 2 2 PSPMT imager with flanges coupled to CsI(Tl) array with 5 mm quartz thickness of light guide. The tumors are 2.5 cm apart and the smallest one has 6 mm diameter.
configuration with PSPMTs flangeless with the scintillation crystal coupled to a 3 mm quartz thickness. In fact, the two neighboring crystal pixels are well separated for a spot irradiation. The best uniformity response was obtained for 2 2 PSPMT with flanges configuration with a 5 mm light guide, mainly due to the selection of tubes over a set of 13 PSPMTs. Tubes with the same gain in X and Y directions, respectively, avoid position non-linearity and strong image deformations. As a consequence, for clinical images the imager utilizing the 2 2 PSPMT with 5 mm quartz light guide was preferable for uniformity response. Fig. 4 shows the image obtained from breast phantom, where the smallest size tumor is clearly visible demonstrating a good performance for clinical application. Fig. 3. Flood field irradiation of CsI(Tl) scintillating array coupled to 3 in. PSPMT.
of the 2 2 PSPMT gamma ray imager mainly led to improve the values of intrinsic spatial resolution and of uniformity response in the insensitive area between PSPMTs. As shown in Table 1 the best intrinsic spatial resolution was obtained from the
4. Conclusion A single tube imager seems very attractive because it is ready for clinical tests in the field of application where simple counters are currently used. A 2 2 PSPMT imager fits a larger application field in medicine because it can be
R. Pani et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 509–513
considered as a small FOV gamma camera with very light weight and cost. Also, in this case it can be considered ready for clinical tests. However, it needs further improvements particularly regarding the tube selection, to obtain a good uniformity response. Acknowledgements This work was partially done under MURST Cofinanziamento 1998 and partially supported by AIRC grant. References [1] I. Khalkhali, J.A. Cutrone, I.G. Mena, et al., Radiology 196 (2) (1995) 421. [2] G. De Vincentis, W. Gianni, R. Pani, M. Cacciafesta, R. Pellegrini, A. Soluri, G. Troisi, V. Marigliano, F. Scopinaro, Breast Cancer Res. Treat. 48 (1998) 159.
513
[3] G. De Vincentis, F. Scopinaro, R. Pani, R. Pellegrini, A. Soluri, M. Ierardi, L. Ballesio, I.N. Weinberg, A. Pergola, Anticancer Res. 17 (1997) 1627. [4] R. Pani, R. Pellegrini, F. Scopinaro, et al., IEEE Nuclear Science Symposium and Medical Imaging, Conference Record, 1996, pp. 1170–1174. [5] F. Scopinaro, R. Pani, G. De Vincentis, A. Soluri, R. Pellegrini, L.M. Porfiri, Eur. J. Nucl. Med. 26 (1999) 1279. [6] N.J. Yasillo, R.A. Mintzer, J.N. Aarsvold, et al., IEEE Nuclear Science Symposium and Medical Imaging, Conference Record, Vol. 2, 1994, pp. 1073–1076. [7] M.B. Williams, A.R. Goode, S. Majewski, et al., Proc. SPIE 3115 (1997) 226. [8] Hamamatsu Photonics K. K., Position Sensitive Photomultiplier Tube R5900U-00-C8, Preliminary Data Sheet, Cat. No. TPMH1139E01 , Japan, 1995. [9] R. Pani, A. Pergola, R. Pellegrini, et al., Nucl Instr and Meth 392(1/3) (1997) 319. [10] K. Inoue, Y. Nagai, H. Saito, et al., Nucl. Instr. and Meth. A 423 (1999) 364. [11] R. Pani, et al., IEEE Trans. Nucl. Sci. 46 (3) (1999) 702. [12] Hamamatsu Technical Data Sheets August 1989 review, supersedes October CR 2000, Japan, 1987.
