Feasibility study of a computer-assisted radioguided surgery system

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Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365

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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,�

aInstitute of Biomedical Engineering, CNR, Via Salaria Km 29.300, C.P. 10 00016 Monterotondo, Rome, ItalybLi-tech srl, Lauzacco Pavia di Udine (UD), Italy

cPolitecnico di Milano, IV Facolta di Ingegneria, Dipartimento di Meccanica, Milan, ItalydDepartment 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 surgeryconsists of acquiring images with large Field Of View(FOV) gamma cameras and then searching the radio-activity peak with a radiation counter during operation.

Lack of images in the operating theatre may causemisinterpretation and/or be time consuming also for well-trained equips. For example, during Sentinel Lymph-NodeDissection (SLND) [1,2] the standard method that uses

e front matter r 2007 Elsevier B.V. All rights reserved.

ma.2007.09.029

ing author. Institute of Biomedical Engineering, CNR, Via

300, C.P. 10 00016 Monterotondo, Roma, Italy.

72923; fax: +39 6 90672692.

ess: [email protected] (A. Soluri).

drawings on skin and probes with acoustic signaling, cansometimes miss Sentinel Lymph-Node (SLN), or detectioncan sometimes be long and cumbersome [3]. Similarprocedures are applied in other surgical treatments (para-thyroid, osteomielitis, etc.). Thus, groups engaged in thedevelopment of new detectors studied alternative ways oflesions detection with portable, small-size imaging devices.These devices can be useful in different fields of radio-guided surgery [4–6] as Radioguided Occult LesionLocalization (ROLL) [7] technique in which radiolabeledalbumin is used to localize nonpalpable breast lesions.Since 1998 some of the authors have studied an alter-

native method based on portable, high-resolution, newconcept gamma cameras [8–13], named Imaging Probe (IP).Successive improvements of IP since 1998 and long-time

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experience in using these cameras during operation havebrought us to suggest a particular surgical procedure, withthe aim of making more precise the SLND and significantlyshorter the withdrawal time of SLN. By themselves, nuclearimages do not show anatomic reference points: this issometimes a problem for surgeons, because their location isneither immediate nor instinctive, also when radioisotopeand Rx images had previously been fused, because surgicalcutting changes the reference points. The aim of this workhas been the development of a system prototype used as anauxiliary tool in radioguided surgery. To this purpose, eachelement of the system cooperates as follows: the MiniGamma Camera (MGC) allows to determine in an accurateway the 3D positioning of the lesion; the passive dedicatedpositioning system and the contactless scalpel trackingsystem allow to determine real-time the relative positionbetween the lesion and the scalpel in a well-known referencesystem. The use of the laser scanner provides 3D reconstruc-tion of the anatomy in the same reference system. In thisway the surgeon has complete knowledge about thepathology and the operator tools, which allows him todetermine the operation strategy and to optimize thesurgical work flow.

In this article, medical aspects will be neglected andattention will be focused on the engineering ones.

2. Equipment and method

The proposed system mainly comprises an MGC, a custommechanical positioning system, a laser scanner and a devicededicated to the guide of the surgical tool. During themeasurement the gamma camera must be very close to theskin: this device was put on a robotic mechanical arm providedwith encoders in order to guarantee its accurate positioningand to express the measures in a well-known reference system.

The 3D body district reconstruction, expressed in a well-known reference system, performed by using the scanner,allows to know the relative position between the lesion andthe 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 withthe surgeon.

At this point the system is ready for the computer-assisted radioguided surgery. The steps above can berepeated 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

which every crystal is inserted in a square hole of aparallel collimator module creating an integrated colli-mator–scintillator structure [13].Additional collimation modules can be added depending

on the desired performances of the final detector. Resultsreported in this study have been obtained using a detectorwith a 24mm collimator added to the 6mm collimator–scintillator module.Collimator modules are made of pure Tungsten and have

200-mm-thick septa with square holes of the same size ofcrystals. The scintillator structure consists of 10� 10 CsI(Tl)crystals 2.05� 2.05� 5.0mm3 (Spectra Physics–Hilger Crys-tals, UK) coated with a 100mm white reflective epoxy layeron their five blind surfaces. The FOV of such a structure is24.5� 24.5mm2.The collimator–scintillator assembly is matched to an

1 in2 R8900-00-C12 (Hamamatsu, Japan) [14] PositionSensitive Photomultiplier Tube (PSPMT), which hasexternal dimensions of 26.2� 26.2� 27.2mm3, active areaof 23.7� 23.7mm2 and a 0.8-mm-thick glass window; thephotocathode is bialkali and 10-stage metal channeldynodes provide a charge amplification of about 106, at�800V.The PSPMT charge readout is provided by a resis-

tive chain that reduces output signals to 4. Signals aresampled with a proprietary data acquisition board pluggedvia USB bus to a PC. The acquisition system provides 4input up to 20Msamples/s per channel with a 12-bitresolution.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 positioningsystem has been developed (Fig. 1) equipped with a set ofhigh-accuracy transducers, capable of measuring theposition of the arm. The arm can reach any positionwithin a sphere of radius of about 60 cm, with theexception of the volume taken by the arm support. Theworking volume is wide enough to allow the use of thesystem in all diagnostic procedures listed above, with apositioning uncertainty lower than 1mm in the worstworking condition.The arm has been designed in order to allow medical

staff to obtain the gamma camera positioning in thesimplest way. This means that the mechanical arm ischaracterized by a low inertia and a counter-weight systemable to null the gamma camera weight effect on thebalance. Once the gamma camera has been positioned, it ispossible to lock the arm in a very simple way. Encum-brance has been minimized by mounting gamma camera atthe end of the mechanical arm; in this way, all encum-brances are far from the operative area. The system hasbeen balanced so that the displacement, in the workingvolume, occurs without working versus gravity force,but simply by overcoming inertia of the involved masses.

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The spatial position and orientation of the gamma cameraare measured by encoders placed inside the five armcouplings.

2.3. Surgical instrument tracking

The idea, on which the system of guide of the surgicaltool is based, is to obtain a system able to provide real-timeinformation about the position and the orientation of theinstrument with respect to the pathology, without modify-ing in any way the traditional procedure adopted by themedical staff.

A contactless guide system based on the use of a coupleof stereoscopic cameras [15] has been developed. This

Fig. 1. Arm system.

Fig. 2. (a) Main component of the track

system provides position and orientation of the surgicalinstrument, 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 fromthe beginning of scintigraphy. An important feature of thedescribed solution is that the system is able to provide real-time information about the relative position between thetool tip and pathology: the physician can therefore choosethe access trajectory with complete freedom, compatiblywith anatomic structures.The system used for the optic localization of the markers

is composed of a couple of stereoscopic cameras. Thesecameras are able to find the three-dimensional coordinatesof a tracker present in the shared vision field. The tracker iscomposed by a light aluminum structure equipped withfour spheres with catadioptric effect. Lighting up the scenewith a monochromatic radiation (in this case near infraredwith wavelength 880 nm), coaxial with cameras, andequipping the cameras with an interferometric filter(centered on the same wavelength), the spheres are muchbrighter than the background, almost independent of scenelighting conditions.The registration of the stereoscopic cameras with the

mechanical arm can be achieved simply by mounting atracker on the arm’s last link and measuring the trackerthree-dimensional coordinates both in the arm and in thestereo camera reference systems. Using the transducersmounted on the cinematic chain, it is possible to obtain thetransformation matrix (GAT in Fig. 2), which describes thetracker position and orientation in the arm coordinatesystem. The coordinates of the same tracker are thenmeasured 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 transforma-tion between the two systems (GAS in Fig. 2) can be found.

er; (b) stereovision guidance system.

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Fig. 3. Armpit laser scanning and reconstruction.

R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365 363

Now scintigraphy can be carried out and the three-dimensional coordinates of pathology can be found in armdatum system. Then, thanks to the already performedsystem registration, the pathology coordinates can befound in stereo camera reference system.

At this point measuring the coordinates of the trackerwith the stereo cameras it is possible to obtain the positionof the surgical tool in the camera datum system. Thus thepathology and the tool tip, both known in the cameradatum system, can be shown on a computer monitor,allowing the surgeon to have information about relativeposition in real time.

2.4. Laser scanner

The goal of the laser scanner is to obtain the shapeof the surface of the body district involved in theoperation.

We choose the laser triangulation as, reconstructiontechnique, because it guarantees in this application a veryhigh reliability, which is the main goal in the medicalmeasures [16–18]. The scanner is conceptually divided intwo different parts: the profilometer and the motionsystem. It would be possible to move the profilometer indifferent ways: we choose the rotation, because it allows tocarry out a more compact device, in fact this feature is veryimportant in this application, because it must be easily usedin the operating theatre.

The profilometer comprises a camera, a laser source anda support structure. The firewire camera was a grayscale,equipped with a 2/300 CMOS sensor (resolution 640� 480pixels) [19] and a 12mm focal length optics, it allows toattain the high frame rate of 50 frames per second.

In order to acquire a complete profile for every grabbedimage, a line generator diode laser module has been used(Fig. 3). For safety reasons we choose 1mW power laserand also it has been necessary to choose a wavelengthcomprised in the visible spectrum: it has been opted inparticular for l ¼ 670 nm, since in this region the spectralresponse of the chosen camera is good. In order tointegrate the profilometer system its coordinates must beregistered with arm coordinates system.

3. Results and discussion

Fig. 4 shows a 57Co raw flood field irradiation image ofMGC. As visible, all the 10� 10 scintillating elements areclearly identified, even the corner ones. The mean width ofthe central elements is (0.3770.09) mm, for the borderzone it reaches (0.6270.22)mm while the cornes ones havea 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 and1.770.6 for the corner pixel.To assess the accuracy of the scintigraphic system (MGC

and position system), a plexiglass phantom (shown inFig. 5) was used. A 1mm diameter 57Co (185MBq) sourcewas placed at different depth (70, 50, 30, 10mm) from theexternal surface. We used triangulation techniques tolocalize 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 theMGC and the mechanical arm was experimentallyestimated to be about 2.5mm; � the stereo cameras have an uncertainty of 1/1000 of

the FOV. The FOV is 250� 250� 250mm3, so theuncertainty is 0.25mm;

� the uncertainty on the surgical tool position and

orientation is composed of the uncertainty due to stereocameras and the uncertainty due to tool–trackerrelation. It is important to underline that the trackerhas to be mounted on the instrument as near as possibleto the tip, in order to minimize the effect of the trackerorientation uncertainty on the tool tip localizationuncertainty;

� the laser scanner has a working volume of

180� 180� 180mm3, where the resolution is lower than0.5mm; it is characterized by high values of reliabilityand repeatability. The scanner metrological qualifica-tion, carried out on skin-compatible known-geometryobjects, has shown that the worst results are obtainedwith measurands characterized by very high values ofcurvature. The extended uncertainty (99% confidencelevel) [20] for a cone reconstruction is less than 1mm,

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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.

Fig. 5. Plexiglass phantom used to assess scintigraphic system accuracy

(measures in mm).

R. Massari et al. / Nuclear Instruments and Methods in Physics Research A 583 (2007) 360–365364

for a cylinder or a plane is less than 0.5mm. These datashow that the uncertainty introduced by the laserscanner is lower than the gamma camera one.

In conclusion the overall uncertainty is approximatelyequal to the MGC uncertainty, estimated to be about2.5mm.

4. Conclusions

An auxiliary system prototype, dedicated to surgicalnavigation, has been proposed. Single technologies,i.e. scintigraphic device, positioning system, tool trackingand laser scanning, have been focused and tested togetherto achieve development of an integrated navigation system.The overall uncertainty, as discussed in the previoussection, is mainly due to the scintigraphic system (slightlygreater than the scintillating element size in the usualworking condition). However, it is compatible with theforemost surgical application.

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The described system shows some advantages if com-pared to the classic techniques:

�

achievement of a functional diagnostics instead of amorphologic one, thanks to the use of images that comefrom the MGC with high resolution; � definition of a semi-automatic procedure for stereotaxis

localization of the pathology with many images takenwith 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 thepathology. The proposed system is able to give real-timeinformation to the surgeon about tool tip coordinateswith respect to the lesion;

� surgeon has to learn a simple procedure, which allows

him to have real-time visualization of the pathology, thebody 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 cancerdetection.

To sum up, the system will guarantee a clear improve-ment in surgical technique accuracy.

In the future we will develop a computer-assistedradioguided surgery system like the one described in thispaper, with a large-area gamma camera and a better spatialresolution. This new system will be suitable for scinti-graphics techniques on neurosurgery and cardiac surgery.

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