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Feasibility study of a computer-assisted radioguided surgery system
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365 www.elsevier.com/locate/nima
Feasibility study of a computer-assisted radioguided surgery system R. Massaria,b, C. Trottaa,b, G. Trincia, R. Salac, A. Bassoc, E. Zappac, F. Scopinaroa,d, A. Soluria,b, a
Institute of Biomedical Engineering, CNR, Via Salaria Km 29.300, C.P. 10 00016 Monterotondo, Rome, Italy b Li-tech srl, Lauzacco Pavia di Udine (UD), Italy c Politecnico di Milano, IV Facolta` di Ingegneria, Dipartimento di Meccanica, Milan, Italy d Department of Radiological Sciences, University ‘‘La Sapienza’’ Rome, Italy Received 27 July 2007; received in revised form 5 September 2007; accepted 17 September 2007 Available online 29 September 2007
Abstract This paper deals with the study of a system prototype that can be used as an auxiliary tool in radioguided surgery methods. The use of new technologies in radioguided surgery concern the exact positioning of the lesion to be exerted. This is possible, in operation theatre, thanks to portable scintigraphics devices or to radiation counters. Due to lack of a coordinate system in the operation field, it is difficult for the surgeon to localize the pathology after removing the detection instrument. The system proposed in this paper is composed mainly of three elements: a handheld, high-resolution gamma camera with a small Field Of View (FOV) based on Hamamatsu R8900-00-C12 Position Sensitive Photomultiplier Tube (PSPMT), a laser scanner for the reconstruction of the body district and a stereoscopic system for contactless surgical tool tracking. Analyzing a set of scintigraphic images, taken from different projections, it is possible to localize the three-dimensional position of the lesion. Thanks to the use of the scanner and image fusion techniques, the pathology is shown on a PC monitor correctly positioned with respect to the body surface. Using a couple of stereoscopic cameras, the surgical tool can be tracked and shown on the same monitor, so that the surgeon can know the instantaneous relative position between the tool and the pathology. Exploiting these systems, a navigation system prototype has been developed that is suitable for radioguided surgical application. r 2007 Elsevier B.V. All rights reserved. PACS: 87.58.Xs; 29.40.Mc; 85.60.Ha Keywords: Gamma Ray Imager; Scintillation Detector; Position Sensitive Photomultiplier Tube; Portable Gamma Camera
1. Introduction The current method for radioisotope driving surgery consists of acquiring images with large Field Of View (FOV) gamma cameras and then searching the radioactivity peak with a radiation counter during operation. Lack of images in the operating theatre may cause misinterpretation and/or be time consuming also for welltrained equips. For example, during Sentinel Lymph-Node Dissection (SLND) [1,2] the standard method that uses Corresponding author. Institute of Biomedical Engineering, CNR, Via Salaria Km 29.300, C.P. 10 00016 Monterotondo, Roma, Italy. Tel.: +39 6 90672923; fax: +39 6 90672692. E-mail address: [email protected] (A. Soluri).
0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.09.029
drawings on skin and probes with acoustic signaling, can sometimes miss Sentinel Lymph-Node (SLN), or detection can sometimes be long and cumbersome [3]. Similar procedures are applied in other surgical treatments (parathyroid, osteomielitis, etc.). Thus, groups engaged in the development of new detectors studied alternative ways of lesions detection with portable, small-size imaging devices. These devices can be useful in different fields of radioguided surgery [4–6] as Radioguided Occult Lesion Localization (ROLL) [7] technique in which radiolabeled albumin is used to localize nonpalpable breast lesions. Since 1998 some of the authors have studied an alternative method based on portable, high-resolution, new concept gamma cameras [8–13], named Imaging Probe (IP). Successive improvements of IP since 1998 and long-time
ARTICLE IN PRESS R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
experience in using these cameras during operation have brought us to suggest a particular surgical procedure, with the aim of making more precise the SLND and significantly shorter the withdrawal time of SLN. By themselves, nuclear images do not show anatomic reference points: this is sometimes a problem for surgeons, because their location is neither immediate nor instinctive, also when radioisotope and Rx images had previously been fused, because surgical cutting changes the reference points. The aim of this work has been the development of a system prototype used as an auxiliary tool in radioguided surgery. To this purpose, each element of the system cooperates as follows: the Mini Gamma Camera (MGC) allows to determine in an accurate way the 3D positioning of the lesion; the passive dedicated positioning system and the contactless scalpel tracking system allow to determine real-time the relative position between the lesion and the scalpel in a well-known reference system. The use of the laser scanner provides 3D reconstruction of the anatomy in the same reference system. In this way the surgeon has complete knowledge about the pathology and the operator tools, which allows him to determine the operation strategy and to optimize the surgical work flow. In this article, medical aspects will be neglected and attention will be focused on the engineering ones. 2. Equipment and method The proposed system mainly comprises an MGC, a custom mechanical positioning system, a laser scanner and a device dedicated to the guide of the surgical tool. During the measurement the gamma camera must be very close to the skin: this device was put on a robotic mechanical arm provided with encoders in order to guarantee its accurate positioning and to express the measures in a well-known reference system. The 3D body district reconstruction, expressed in a wellknown reference system, performed by using the scanner, allows to know the relative position between the lesion and the body surface. The work flow during surgery consists of the following steps:
laser scanning of the body district of interest; performing more high-resolution scintigraphies in order to accomplish the localization of the point of interest (i.e. lesion, hot spot); determining the location of the pathological process on the part of the nuclear physician by arrangement with the surgeon.
At this point the system is ready for the computerassisted radioguided surgery. The steps above can be repeated if necessary during operation. 2.1. MGC The MGC was manufactured by Li-tech (Lauzacco, UD, Italy). The crystal array has an innovative structure in
361
which every crystal is inserted in a square hole of a parallel collimator module creating an integrated collimator–scintillator structure [13]. Additional collimation modules can be added depending on the desired performances of the final detector. Results reported in this study have been obtained using a detector with a 24 mm collimator added to the 6 mm collimator– scintillator module. Collimator modules are made of pure Tungsten and have 200-mm-thick septa with square holes of the same size of crystals. The scintillator structure consists of 10 10 CsI(Tl) crystals 2.05 2.05 5.0 mm3 (Spectra Physics–Hilger Crystals, UK) coated with a 100 mm white reflective epoxy layer on their five blind surfaces. The FOV of such a structure is 24.5 24.5 mm2. The collimator–scintillator assembly is matched to an 1 in2 R8900-00-C12 (Hamamatsu, Japan) [14] Position Sensitive Photomultiplier Tube (PSPMT), which has external dimensions of 26.2 26.2 27.2 mm3, active area of 23.7 23.7 mm2 and a 0.8-mm-thick glass window; the photocathode is bialkali and 10-stage metal channel dynodes provide a charge amplification of about 106, at 800 V. The PSPMT charge readout is provided by a resistive chain that reduces output signals to 4. Signals are sampled with a proprietary data acquisition board plugged via USB bus to a PC. The acquisition system provides 4 input up to 20 Msamples/s per channel with a 12-bit resolution. The maximum count rate of the readout electronics and acquisition system is about 15 kCount/s. 2.2. Gamma camera positioning system For the gamma camera support, a passive positioning system has been developed (Fig. 1) equipped with a set of high-accuracy transducers, capable of measuring the position of the arm. The arm can reach any position within a sphere of radius of about 60 cm, with the exception of the volume taken by the arm support. The working volume is wide enough to allow the use of the system in all diagnostic procedures listed above, with a positioning uncertainty lower than 1 mm in the worst working condition. The arm has been designed in order to allow medical staff to obtain the gamma camera positioning in the simplest way. This means that the mechanical arm is characterized by a low inertia and a counter-weight system able to null the gamma camera weight effect on the balance. Once the gamma camera has been positioned, it is possible to lock the arm in a very simple way. Encumbrance has been minimized by mounting gamma camera at the end of the mechanical arm; in this way, all encumbrances are far from the operative area. The system has been balanced so that the displacement, in the working volume, occurs without working versus gravity force, but simply by overcoming inertia of the involved masses.
ARTICLE IN PRESS 362
R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
The spatial position and orientation of the gamma camera are measured by encoders placed inside the five arm couplings. 2.3. Surgical instrument tracking The idea, on which the system of guide of the surgical tool is based, is to obtain a system able to provide real-time information about the position and the orientation of the instrument with respect to the pathology, without modifying in any way the traditional procedure adopted by the medical staff. A contactless guide system based on the use of a couple of stereoscopic cameras [15] has been developed. This
Fig. 1. Arm system.
system provides position and orientation of the surgical instrument, thanks to a set of catadioptric markers. The fundamental request of the system is the necessity to insure that the position of the patient does not change from the beginning of scintigraphy. An important feature of the described solution is that the system is able to provide realtime information about the relative position between the tool tip and pathology: the physician can therefore choose the access trajectory with complete freedom, compatibly with anatomic structures. The system used for the optic localization of the markers is composed of a couple of stereoscopic cameras. These cameras are able to find the three-dimensional coordinates of a tracker present in the shared vision field. The tracker is composed by a light aluminum structure equipped with four spheres with catadioptric effect. Lighting up the scene with a monochromatic radiation (in this case near infrared with wavelength 880 nm), coaxial with cameras, and equipping the cameras with an interferometric filter (centered on the same wavelength), the spheres are much brighter than the background, almost independent of scene lighting conditions. The registration of the stereoscopic cameras with the mechanical arm can be achieved simply by mounting a tracker on the arm’s last link and measuring the tracker three-dimensional coordinates both in the arm and in the stereo camera reference systems. Using the transducers mounted on the cinematic chain, it is possible to obtain the transformation matrix (GAT in Fig. 2), which describes the tracker position and orientation in the arm coordinate system. The coordinates of the same tracker are then measured using the stereo cameras and GST can be found. Once the tracker coordinates are known, both in the arm and in the stereo camera datum systems, the transformation between the two systems (GAS in Fig. 2) can be found.
Fig. 2. (a) Main component of the tracker; (b) stereovision guidance system.
ARTICLE IN PRESS R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
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Fig. 3. Armpit laser scanning and reconstruction.
Now scintigraphy can be carried out and the threedimensional coordinates of pathology can be found in arm datum system. Then, thanks to the already performed system registration, the pathology coordinates can be found in stereo camera reference system. At this point measuring the coordinates of the tracker with the stereo cameras it is possible to obtain the position of the surgical tool in the camera datum system. Thus the pathology and the tool tip, both known in the camera datum system, can be shown on a computer monitor, allowing the surgeon to have information about relative position in real time.
2.4. Laser scanner The goal of the laser scanner is to obtain the shape of the surface of the body district involved in the operation. We choose the laser triangulation as, reconstruction technique, because it guarantees in this application a very high reliability, which is the main goal in the medical measures [16–18]. The scanner is conceptually divided in two different parts: the profilometer and the motion system. It would be possible to move the profilometer in different ways: we choose the rotation, because it allows to carry out a more compact device, in fact this feature is very important in this application, because it must be easily used in the operating theatre. The profilometer comprises a camera, a laser source and a support structure. The firewire camera was a grayscale, equipped with a 2/300 CMOS sensor (resolution 640 480 pixels) [19] and a 12 mm focal length optics, it allows to attain the high frame rate of 50 frames per second. In order to acquire a complete profile for every grabbed image, a line generator diode laser module has been used (Fig. 3). For safety reasons we choose 1 mW power laser and also it has been necessary to choose a wavelength comprised in the visible spectrum: it has been opted in particular for l ¼ 670 nm, since in this region the spectral response of the chosen camera is good. In order to integrate the profilometer system its coordinates must be registered with arm coordinates system.
3. Results and discussion Fig. 4 shows a 57Co raw flood field irradiation image of MGC. As visible, all the 10 10 scintillating elements are clearly identified, even the corner ones. The mean width of the central elements is (0.3770.09) mm, for the border zone it reaches (0.6270.22) mm while the cornes ones have a width of (2.270.4) mm. The mean peak to valley ratio has a value of 273737 in the central zone, 104717 in the border elements and 1.770.6 for the corner pixel. To assess the accuracy of the scintigraphic system (MGC and position system), a plexiglass phantom (shown in Fig. 5) was used. A 1 mm diameter 57Co (185 MBq) source was placed at different depth (70, 50, 30, 10 mm) from the external surface. We used triangulation techniques to localize the source. The overall uncertainty of the described system is due to a number of different sources:
the overall accuracy of the system composed of the MGC and the mechanical arm was experimentally estimated to be about 2.5 mm; the stereo cameras have an uncertainty of 1/1000 of the FOV. The FOV is 250 250 250 mm3, so the uncertainty is 0.25 mm; the uncertainty on the surgical tool position and orientation is composed of the uncertainty due to stereo cameras and the uncertainty due to tool–tracker relation. It is important to underline that the tracker has to be mounted on the instrument as near as possible to the tip, in order to minimize the effect of the tracker orientation uncertainty on the tool tip localization uncertainty; the laser scanner has a working volume of 180 180 180 mm3, where the resolution is lower than 0.5 mm; it is characterized by high values of reliability and repeatability. The scanner metrological qualification, carried out on skin-compatible known-geometry objects, has shown that the worst results are obtained with measurands characterized by very high values of curvature. The extended uncertainty (99% confidence level) [20] for a cone reconstruction is less than 1 mm,
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R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
Fig. 4. (a) Scintillating array close up; (b) raw flood field irradiation image of the MGC; (c) a cross section of the image above.
for a cylinder or a plane is less than 0.5 mm. These data show that the uncertainty introduced by the laser scanner is lower than the gamma camera one. In conclusion the overall uncertainty is approximately equal to the MGC uncertainty, estimated to be about 2.5 mm. 4. Conclusions
Fig. 5. Plexiglass phantom used to assess scintigraphic system accuracy (measures in mm).
An auxiliary system prototype, dedicated to surgical navigation, has been proposed. Single technologies, i.e. scintigraphic device, positioning system, tool tracking and laser scanning, have been focused and tested together to achieve development of an integrated navigation system. The overall uncertainty, as discussed in the previous section, is mainly due to the scintigraphic system (slightly greater than the scintillating element size in the usual working condition). However, it is compatible with the foremost surgical application.
ARTICLE IN PRESS R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
The described system shows some advantages if compared to the classic techniques:
achievement of a functional diagnostics instead of a morphologic one, thanks to the use of images that come from the MGC with high resolution; definition of a semi-automatic procedure for stereotaxis localization of the pathology with many images taken with a known position and orientation of the MGC; introduction of an optic system for the guide of the surgical tool, which follows the localization of the pathology. The proposed system is able to give real-time information to the surgeon about tool tip coordinates with respect to the lesion; surgeon has to learn a simple procedure, which allows him to have real-time visualization of the pathology, the body district shape and surgical instrument positions, without changing the traditional way to operate; great versatility that allows to use the same system and the same procedure in many kinds of tests for cancer detection.
To sum up, the system will guarantee a clear improvement in surgical technique accuracy. In the future we will develop a computer-assisted radioguided surgery system like the one described in this paper, with a large-area gamma camera and a better spatial resolution. This new system will be suitable for scintigraphics techniques on neurosurgery and cardiac surgery. References [1] U. Veronesi, G. Paganelli, V. Galimberti, Lancet 2 (1997) 335. [2] U. Veronesi, G. Paganelli, G. Viale, et al., J. Natl. Cancer Inst. 91 (1999) 368.
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[3] P.J. Borgstein, R. Pijpers, E.F. Comans, P.J. van Diest, R.P. Boom, S. Meijer, J. Am. Coll. Surg. 186 (1998) 275. [4] K. Motomura, A. Noguchi, T. Hashizume, Y. Hasegawa, Y. Komoike, H. Inaji, T. Saida, H. Koyama, J. Surg. Oncol. 89 (2005) 12. [5] W. Kitugawa, K. Shimizu, S. Kumita, J. Surg. Oncol. 80 (2002) 173. [6] E.J. Hoffman, M.P. Tornai, M. Janeek, B.E. Patt, J.S. Iwancyczk, Eur. J. Nucl. Med. 26 (1999) 913. [7] G. Paganelli, U. Veronesi, Nucl. Med. Commun. 23 (2002) 625. [8] F. Scopinaro, E. Di Luzio, R. Pani, G. DeVincentis, M.P. Lamberini, L. Ballesio, V. Tombolini, A. De Cesare, Radionuclides for Limph Nodes, Mediterra Publishers, Athens, 1999. [9] F. Scopinaro, R. Pani, A. Soluri, R. Pellegrini, R. Scafe`, G. De Vincentis, F. Capoccetti, V. David, S. Chiarini, S. Stella, Tumori 86 (2000) 329. [10] O. Schillaci, G. D’Errico, R. Scafe`, A. Soluri, N. Burgio, A. Santagata, A. Spanu, A.M. Mangano, V. David, A. Schiaratura, F. Scopinaro, Tumori 88 (2002) 32. [11] G. D’Errico, R. Scafe`, A. Soluri, A. Schiaratura, A.M. Mangano, V. David, F. Scopinaro, Nucl. Instr. and Meth. A 497 (2003) 105. [12] G. D’Errico, M.A. Rosa, A. Soluri, R. Scafe`, M. Galli, S. Chiarini, N. Burgio, A. Schiaratura, R. Massa, F. Scopinaro, Tumori 88 (2002) 30. [13] A. Soluri, R. Massari, C. Trotta, L. Montani, G. Iurlaro, A.M. Mangano, F. Scopinaro, R. Scafe, Nucl. Instr. and Meth. A 554 (2005) 331. [14] Y. Kawasaki, M.E. Bertaina, N. Sakaki, H.M. Shimizu, N. Inoue, S. Hasegawa, I. Ohtsu, T. Adachi, T. Ebisuzaki, K. Hirota, K. Ikeda, F. Kajino, T. Morishima, M. Nagano, M. Sato, T. Sawabe, T. Shibata, T. Shinohara, M. Takeda, Y. Takizawa, Y. Uchihori, Y. Wada, Nucl. Instr. and Meth. A 564 (2006) 378–394. [15] R. Hartley, A. Zisserman, Multiple View Geometry in Computer Vision, Cambridge University Press, Cambridge, 2000. [16] C. Alfano, P. Mezzana, N. Scuderi, Ind. J. Plast. Surg. 38 (2005) 22. [17] F. Darboux, C.H. Huang, Soil Sci. Soc. Am. J. 67 (2003) 92. [18] E. Trucco, A. Verri, Introductory Techniques for 3D Computer Vision, Prentice-Hall, Englewood Cliffs, NJ, 1998. [19] O. Yadid-Pecht, R. Etienne-Cummings, CMOS Imagers: From Phototransduction to Image Processing, Kluwer Academic Publishers, Dordrecht, 2004. [20] GUM ‘‘Guide to the Expression of Uncertainty in Measurement’’ ISO, 1995.
Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365 www.elsevier.com/locate/nima
Feasibility study of a computer-assisted radioguided surgery system R. Massaria,b, C. Trottaa,b, G. Trincia, R. Salac, A. Bassoc, E. Zappac, F. Scopinaroa,d, A. Soluria,b, a
Institute of Biomedical Engineering, CNR, Via Salaria Km 29.300, C.P. 10 00016 Monterotondo, Rome, Italy b Li-tech srl, Lauzacco Pavia di Udine (UD), Italy c Politecnico di Milano, IV Facolta` di Ingegneria, Dipartimento di Meccanica, Milan, Italy d Department of Radiological Sciences, University ‘‘La Sapienza’’ Rome, Italy Received 27 July 2007; received in revised form 5 September 2007; accepted 17 September 2007 Available online 29 September 2007
Abstract This paper deals with the study of a system prototype that can be used as an auxiliary tool in radioguided surgery methods. The use of new technologies in radioguided surgery concern the exact positioning of the lesion to be exerted. This is possible, in operation theatre, thanks to portable scintigraphics devices or to radiation counters. Due to lack of a coordinate system in the operation field, it is difficult for the surgeon to localize the pathology after removing the detection instrument. The system proposed in this paper is composed mainly of three elements: a handheld, high-resolution gamma camera with a small Field Of View (FOV) based on Hamamatsu R8900-00-C12 Position Sensitive Photomultiplier Tube (PSPMT), a laser scanner for the reconstruction of the body district and a stereoscopic system for contactless surgical tool tracking. Analyzing a set of scintigraphic images, taken from different projections, it is possible to localize the three-dimensional position of the lesion. Thanks to the use of the scanner and image fusion techniques, the pathology is shown on a PC monitor correctly positioned with respect to the body surface. Using a couple of stereoscopic cameras, the surgical tool can be tracked and shown on the same monitor, so that the surgeon can know the instantaneous relative position between the tool and the pathology. Exploiting these systems, a navigation system prototype has been developed that is suitable for radioguided surgical application. r 2007 Elsevier B.V. All rights reserved. PACS: 87.58.Xs; 29.40.Mc; 85.60.Ha Keywords: Gamma Ray Imager; Scintillation Detector; Position Sensitive Photomultiplier Tube; Portable Gamma Camera
1. Introduction The current method for radioisotope driving surgery consists of acquiring images with large Field Of View (FOV) gamma cameras and then searching the radioactivity peak with a radiation counter during operation. Lack of images in the operating theatre may cause misinterpretation and/or be time consuming also for welltrained equips. For example, during Sentinel Lymph-Node Dissection (SLND) [1,2] the standard method that uses Corresponding author. Institute of Biomedical Engineering, CNR, Via Salaria Km 29.300, C.P. 10 00016 Monterotondo, Roma, Italy. Tel.: +39 6 90672923; fax: +39 6 90672692. E-mail address: [email protected] (A. Soluri).
0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.09.029
drawings on skin and probes with acoustic signaling, can sometimes miss Sentinel Lymph-Node (SLN), or detection can sometimes be long and cumbersome [3]. Similar procedures are applied in other surgical treatments (parathyroid, osteomielitis, etc.). Thus, groups engaged in the development of new detectors studied alternative ways of lesions detection with portable, small-size imaging devices. These devices can be useful in different fields of radioguided surgery [4–6] as Radioguided Occult Lesion Localization (ROLL) [7] technique in which radiolabeled albumin is used to localize nonpalpable breast lesions. Since 1998 some of the authors have studied an alternative method based on portable, high-resolution, new concept gamma cameras [8–13], named Imaging Probe (IP). Successive improvements of IP since 1998 and long-time
ARTICLE IN PRESS R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
experience in using these cameras during operation have brought us to suggest a particular surgical procedure, with the aim of making more precise the SLND and significantly shorter the withdrawal time of SLN. By themselves, nuclear images do not show anatomic reference points: this is sometimes a problem for surgeons, because their location is neither immediate nor instinctive, also when radioisotope and Rx images had previously been fused, because surgical cutting changes the reference points. The aim of this work has been the development of a system prototype used as an auxiliary tool in radioguided surgery. To this purpose, each element of the system cooperates as follows: the Mini Gamma Camera (MGC) allows to determine in an accurate way the 3D positioning of the lesion; the passive dedicated positioning system and the contactless scalpel tracking system allow to determine real-time the relative position between the lesion and the scalpel in a well-known reference system. The use of the laser scanner provides 3D reconstruction of the anatomy in the same reference system. In this way the surgeon has complete knowledge about the pathology and the operator tools, which allows him to determine the operation strategy and to optimize the surgical work flow. In this article, medical aspects will be neglected and attention will be focused on the engineering ones. 2. Equipment and method The proposed system mainly comprises an MGC, a custom mechanical positioning system, a laser scanner and a device dedicated to the guide of the surgical tool. During the measurement the gamma camera must be very close to the skin: this device was put on a robotic mechanical arm provided with encoders in order to guarantee its accurate positioning and to express the measures in a well-known reference system. The 3D body district reconstruction, expressed in a wellknown reference system, performed by using the scanner, allows to know the relative position between the lesion and the body surface. The work flow during surgery consists of the following steps:
laser scanning of the body district of interest; performing more high-resolution scintigraphies in order to accomplish the localization of the point of interest (i.e. lesion, hot spot); determining the location of the pathological process on the part of the nuclear physician by arrangement with the surgeon.
At this point the system is ready for the computerassisted radioguided surgery. The steps above can be repeated if necessary during operation. 2.1. MGC The MGC was manufactured by Li-tech (Lauzacco, UD, Italy). The crystal array has an innovative structure in
361
which every crystal is inserted in a square hole of a parallel collimator module creating an integrated collimator–scintillator structure [13]. Additional collimation modules can be added depending on the desired performances of the final detector. Results reported in this study have been obtained using a detector with a 24 mm collimator added to the 6 mm collimator– scintillator module. Collimator modules are made of pure Tungsten and have 200-mm-thick septa with square holes of the same size of crystals. The scintillator structure consists of 10 10 CsI(Tl) crystals 2.05 2.05 5.0 mm3 (Spectra Physics–Hilger Crystals, UK) coated with a 100 mm white reflective epoxy layer on their five blind surfaces. The FOV of such a structure is 24.5 24.5 mm2. The collimator–scintillator assembly is matched to an 1 in2 R8900-00-C12 (Hamamatsu, Japan) [14] Position Sensitive Photomultiplier Tube (PSPMT), which has external dimensions of 26.2 26.2 27.2 mm3, active area of 23.7 23.7 mm2 and a 0.8-mm-thick glass window; the photocathode is bialkali and 10-stage metal channel dynodes provide a charge amplification of about 106, at 800 V. The PSPMT charge readout is provided by a resistive chain that reduces output signals to 4. Signals are sampled with a proprietary data acquisition board plugged via USB bus to a PC. The acquisition system provides 4 input up to 20 Msamples/s per channel with a 12-bit resolution. The maximum count rate of the readout electronics and acquisition system is about 15 kCount/s. 2.2. Gamma camera positioning system For the gamma camera support, a passive positioning system has been developed (Fig. 1) equipped with a set of high-accuracy transducers, capable of measuring the position of the arm. The arm can reach any position within a sphere of radius of about 60 cm, with the exception of the volume taken by the arm support. The working volume is wide enough to allow the use of the system in all diagnostic procedures listed above, with a positioning uncertainty lower than 1 mm in the worst working condition. The arm has been designed in order to allow medical staff to obtain the gamma camera positioning in the simplest way. This means that the mechanical arm is characterized by a low inertia and a counter-weight system able to null the gamma camera weight effect on the balance. Once the gamma camera has been positioned, it is possible to lock the arm in a very simple way. Encumbrance has been minimized by mounting gamma camera at the end of the mechanical arm; in this way, all encumbrances are far from the operative area. The system has been balanced so that the displacement, in the working volume, occurs without working versus gravity force, but simply by overcoming inertia of the involved masses.
ARTICLE IN PRESS 362
R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
The spatial position and orientation of the gamma camera are measured by encoders placed inside the five arm couplings. 2.3. Surgical instrument tracking The idea, on which the system of guide of the surgical tool is based, is to obtain a system able to provide real-time information about the position and the orientation of the instrument with respect to the pathology, without modifying in any way the traditional procedure adopted by the medical staff. A contactless guide system based on the use of a couple of stereoscopic cameras [15] has been developed. This
Fig. 1. Arm system.
system provides position and orientation of the surgical instrument, thanks to a set of catadioptric markers. The fundamental request of the system is the necessity to insure that the position of the patient does not change from the beginning of scintigraphy. An important feature of the described solution is that the system is able to provide realtime information about the relative position between the tool tip and pathology: the physician can therefore choose the access trajectory with complete freedom, compatibly with anatomic structures. The system used for the optic localization of the markers is composed of a couple of stereoscopic cameras. These cameras are able to find the three-dimensional coordinates of a tracker present in the shared vision field. The tracker is composed by a light aluminum structure equipped with four spheres with catadioptric effect. Lighting up the scene with a monochromatic radiation (in this case near infrared with wavelength 880 nm), coaxial with cameras, and equipping the cameras with an interferometric filter (centered on the same wavelength), the spheres are much brighter than the background, almost independent of scene lighting conditions. The registration of the stereoscopic cameras with the mechanical arm can be achieved simply by mounting a tracker on the arm’s last link and measuring the tracker three-dimensional coordinates both in the arm and in the stereo camera reference systems. Using the transducers mounted on the cinematic chain, it is possible to obtain the transformation matrix (GAT in Fig. 2), which describes the tracker position and orientation in the arm coordinate system. The coordinates of the same tracker are then measured using the stereo cameras and GST can be found. Once the tracker coordinates are known, both in the arm and in the stereo camera datum systems, the transformation between the two systems (GAS in Fig. 2) can be found.
Fig. 2. (a) Main component of the tracker; (b) stereovision guidance system.
ARTICLE IN PRESS R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365
363
Fig. 3. Armpit laser scanning and reconstruction.
Now scintigraphy can be carried out and the threedimensional coordinates of pathology can be found in arm datum system. Then, thanks to the already performed system registration, the pathology coordinates can be found in stereo camera reference system. At this point measuring the coordinates of the tracker with the stereo cameras it is possible to obtain the position of the surgical tool in the camera datum system. Thus the pathology and the tool tip, both known in the camera datum system, can be shown on a computer monitor, allowing the surgeon to have information about relative position in real time.
2.4. Laser scanner The goal of the laser scanner is to obtain the shape of the surface of the body district involved in the operation. We choose the laser triangulation as, reconstruction technique, because it guarantees in this application a very high reliability, which is the main goal in the medical measures [16–18]. The scanner is conceptually divided in two different parts: the profilometer and the motion system. It would be possible to move the profilometer in different ways: we choose the rotation, because it allows to carry out a more compact device, in fact this feature is very important in this application, because it must be easily used in the operating theatre. The profilometer comprises a camera, a laser source and a support structure. The firewire camera was a grayscale, equipped with a 2/300 CMOS sensor (resolution 640 480 pixels) [19] and a 12 mm focal length optics, it allows to attain the high frame rate of 50 frames per second. In order to acquire a complete profile for every grabbed image, a line generator diode laser module has been used (Fig. 3). For safety reasons we choose 1 mW power laser and also it has been necessary to choose a wavelength comprised in the visible spectrum: it has been opted in particular for l ¼ 670 nm, since in this region the spectral response of the chosen camera is good. In order to integrate the profilometer system its coordinates must be registered with arm coordinates system.
3. Results and discussion Fig. 4 shows a 57Co raw flood field irradiation image of MGC. As visible, all the 10 10 scintillating elements are clearly identified, even the corner ones. The mean width of the central elements is (0.3770.09) mm, for the border zone it reaches (0.6270.22) mm while the cornes ones have a width of (2.270.4) mm. The mean peak to valley ratio has a value of 273737 in the central zone, 104717 in the border elements and 1.770.6 for the corner pixel. To assess the accuracy of the scintigraphic system (MGC and position system), a plexiglass phantom (shown in Fig. 5) was used. A 1 mm diameter 57Co (185 MBq) source was placed at different depth (70, 50, 30, 10 mm) from the external surface. We used triangulation techniques to localize the source. The overall uncertainty of the described system is due to a number of different sources:
the overall accuracy of the system composed of the MGC and the mechanical arm was experimentally estimated to be about 2.5 mm; the stereo cameras have an uncertainty of 1/1000 of the FOV. The FOV is 250 250 250 mm3, so the uncertainty is 0.25 mm; the uncertainty on the surgical tool position and orientation is composed of the uncertainty due to stereo cameras and the uncertainty due to tool–tracker relation. It is important to underline that the tracker has to be mounted on the instrument as near as possible to the tip, in order to minimize the effect of the tracker orientation uncertainty on the tool tip localization uncertainty; the laser scanner has a working volume of 180 180 180 mm3, where the resolution is lower than 0.5 mm; it is characterized by high values of reliability and repeatability. The scanner metrological qualification, carried out on skin-compatible known-geometry objects, has shown that the worst results are obtained with measurands characterized by very high values of curvature. The extended uncertainty (99% confidence level) [20] for a cone reconstruction is less than 1 mm,
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Fig. 4. (a) Scintillating array close up; (b) raw flood field irradiation image of the MGC; (c) a cross section of the image above.
for a cylinder or a plane is less than 0.5 mm. These data show that the uncertainty introduced by the laser scanner is lower than the gamma camera one. In conclusion the overall uncertainty is approximately equal to the MGC uncertainty, estimated to be about 2.5 mm. 4. Conclusions
Fig. 5. Plexiglass phantom used to assess scintigraphic system accuracy (measures in mm).
An auxiliary system prototype, dedicated to surgical navigation, has been proposed. Single technologies, i.e. scintigraphic device, positioning system, tool tracking and laser scanning, have been focused and tested together to achieve development of an integrated navigation system. The overall uncertainty, as discussed in the previous section, is mainly due to the scintigraphic system (slightly greater than the scintillating element size in the usual working condition). However, it is compatible with the foremost surgical application.
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The described system shows some advantages if compared to the classic techniques:
achievement of a functional diagnostics instead of a morphologic one, thanks to the use of images that come from the MGC with high resolution; definition of a semi-automatic procedure for stereotaxis localization of the pathology with many images taken with a known position and orientation of the MGC; introduction of an optic system for the guide of the surgical tool, which follows the localization of the pathology. The proposed system is able to give real-time information to the surgeon about tool tip coordinates with respect to the lesion; surgeon has to learn a simple procedure, which allows him to have real-time visualization of the pathology, the body district shape and surgical instrument positions, without changing the traditional way to operate; great versatility that allows to use the same system and the same procedure in many kinds of tests for cancer detection.
To sum up, the system will guarantee a clear improvement in surgical technique accuracy. In the future we will develop a computer-assisted radioguided surgery system like the one described in this paper, with a large-area gamma camera and a better spatial resolution. This new system will be suitable for scintigraphics techniques on neurosurgery and cardiac surgery. References [1] U. Veronesi, G. Paganelli, V. Galimberti, Lancet 2 (1997) 335. [2] U. Veronesi, G. Paganelli, G. Viale, et al., J. Natl. Cancer Inst. 91 (1999) 368.
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