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Quantitative Image Analysis of Optical Sensor Surfaces for Movement Monitoring

V Sauret PJ Sharrock CJ MooreDepartment of North Western Medical Physics, Christie Hospital,

Manchester, UKEmail: chris moore@physics cr man ac uk

Abstract

The use in clinical situations of a near real-timeprototype opto-electronic dynamic 3 0 sugace sensor h sbeen investigated. Variations within optical sensor systemheight-maps show that noise amounts to1 microns onsnapshots of smooth body surfaces. It is not increasedbyimage processing software removing triangulation spotartefact. Presence of hairs o a body surface degrades

the image qualiry, but still w ithin tolerance 800 microns,typically the size of in-slice C Tpi xel) . The accuracy of thesystem is 150 microns, the repeatability 300 microns;off-surface points should however be ma sked.Feasibiliw of dynamic 3 imaging of a body sugace isdemonstrated. Visualisation and analysis of the entirechest surface movement during breathing is achievedusing the optical sensor system. Such monitoring andquantification of patient movement is directly relevant togated radiotherapy and post-treatment verifications.Analysis of surface movement height variations overtime) in terms of m ean and standard deviation shouldalso be included in patient set-up procedures beforeradiotherapy treatment.

1 Introduction

In radiotherapy the treatment planning of a tumour isusually based on anatomical information from a singleCT study that freezes all body external and internalmovements. The tumour is to receive more than9 5 ofthe prescribed radiation dose while adjacent organs mustbe spared. Such customisation of the radiation dose-volume to the exact shape of the tumour site does meanthat any patient movement becomes of criticalimportance. Therefore two major problems must beresolved within Conformal Radiotherapy: a precise andrepeatable positioning of the patient set-up must beachieved as well as an accurate monitoringof patient

movement during treatment. Current practice is todocument the patient position on the CT bed using bodysurface tattoos, etc... This position must then be repeatedlater in the treatment room before irradiating the patient

target volumes accordingly to the treatment plan. Thelack of accuracy and difficulty ofthe re-positioning of thepatient have been highlighted inthe literature and it hasbeen shown that the useof 3D optical sensing of the bodysurface may well overcome this problem [I]. Anotherapplication of the optical sensor system is in monitoringbody surface movement during treatment. This isparticularly relevant to a) gated radiotherapy, whereirradiation occurs only when the patient is in the best

anatomical orientation during, for example, therespiratory cycle, and b)to verify, post-treatment, that theradiation dose and target sites were reached according tothe treatment plan.

A prototype optical sensor system has been installed atthe Christie Hospital for the applications above. It hasrecently been adjusted and tested. The aimof this paper isto:

Study the variations in the resulting optical sensorheight-map, in particular noise level, depending oncomponent present on the surface imaged, such ashairs, on the size of the field of view and imageprocessing;Show the level of accuracy and details to which bodysurface position and movement canbe tracked in the

treatment room using the optical sensor, once at set-UP.

2 Methods

2 1 Optical sensor system description

The core of the optical sensor system is aninterferometer based on a helium-neon laser that producesa red beam that is split and then polarised before beingchannelled into two adjacent o ptical fibres. The fibres areused to project an interference pattern onto the patientinside a treatment room. One of the two beamcomponents has its optical path length variedelectronically using a piezo-mounted mirror whereby the

interference fringe pattern can be altered under computercontrol. The fringe projection is phase-modulated by thebody surface. A conventional CCD camera with zoomlens and filter views the fringe pattern. The optical sensor

0-7695-1195-3\01 10.00 2001 IEEE162
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system can be set to recorda single frameor a series offrames (sequence) captured at a rate of typically 25frames per second. The whole optical sensor system isradiation resistant.

By filtering the Fourier transform of the fringe imagedata, the phase componmt can be extracted andunwrapped to provide spatial information across the fieldof view. This information emerges in the form of height-map data where all points in the map are calibratedagainst and referenced to the treatment machine isocentre,via a triangulation spot. During this image processing,atriangulation spot artefact appears[2] however softwarehas recently been developed further and allowed removalof such artefact [3]. A narrow field of view allowsimaging of 20cmx20cm surfaces while a large field ofview covers 40cmx40cm. The display windows are either5 12x512 pixels or 440x440 pixels.

Rigid, remote ceiling-mounting of the interferometer/viewing camera and beam divergence ensure opticalstability and a clear wide-field coverage of the patient forall patient and treatment machine orientations. It isinstalled in a radiotherapy tieatment room at the Christie

Hospital; the surface height-maps are displayed on amonitor located outside the treatment room.

2 2 Optical sensor performance tests

The effect on the optical sensor height-map quality ofsurface roughness was invesligated by studying the heightvariation within small surface patches in a series ofmeasurements on the flattest areas of various surfaces.Variations of height over such small patches are thusrepresentative of noise. The objects imaged werea whiteplaster cast of human back, the back of Volunteer I , thechest of Volunteer 1 and the chest of Volunteer 2 .Volunteers 1 and 2 were White Europeans. The chest ofVolunteer 1 was hairy, w hile: that of Volunteer 2 was not.

The plaster cast represented a perfect white surface. Theeffect on the image quality of the size of the field of viewwas also assessed by using both narrow and large fieldsof view. Lastly, the effec : of he ight-m ap ,proce ssingsoftware correction ,w as investigated for the chest ofVolunteer 2.

Static imaging was used, o determine. the -resolutionand reproducibility of the:, optical sensor, T he testconsisted in repeatedly ima ging a truly static clinicallyproportioned test object (R AN DO ) lying on the treatmentmachine bed. Th em ean and standard deviation of eachpoint of the RANDO surface height-map were thencalculated.

In the current project, dynamic3D imaging is used formonitoring the surface movement. Hence the sensor wasused to capture a sequene (AV1 format) of fringedsurface images of RAND O moved manually up and downthe treatment bed, in order to assess the stability andcorrect implementation of iterated frame processing.

Dynamic imaging of the fringed surface images of thefront torso of a test person breathing normally was alsoperformed and the surface movement during the breathingcycle was quantified.

3 Results

Flattest areas on the back surface were seen onboththe cast and Vo lunteer 1 on the right and left sidesof thespine, about 5 cm away from it. On the chest of bothvolunteers, the flattest areas were about2 cm above thesternum. They are shown in Figure 1 The maximumheight variation in a small patch located in the flattestareas and the standard deviation fromthe mean heightwithin the patch are summarised in Table1. 10x10-pixelpatches were used for images with the narrow field ofview; 5x5-pixel patches for images with the large field ofview. Height variations for the cast and the human backare similar. Values for the chest of Volunteer 1 suggeststhat the presence of body hairs increased these variations(Tab. 1). Figure I C also shows that this chest surfaceheight-map contains ripples. Their height is in the order

of a millimetre. The contrast of clarity between FiguresIb , I C and Id suggests that body hairs create these.Measurements on the non-hairy back and chest surfacesare similar (T ab. 1); noise level is in the orde r of a tenthof a millimetre. The field of view used for Volunteer2 islarger than the whole width of the chest (Fig. I d and le).Degradation of the quality of the image at the edge of thechest and large scale off-surface random noise arevisible. The software correction applied to the opticalsensor height-map of the chest of Volunteer2 (Fig. le)has effectively removed the triangulation spot artefactthat is present in Figures 2 to 4, without increasingnoticeably the level of noise (Tab.I . Figure 2 considersboth antero-posterior (AP) and lateral views of RANDO,at the pelvic-abdominal level. These represent the two

extremes of human surface vanation . The top portions ofFigure 2 show the mean of three repeated 440x440-pointheight-map s for AP and lateral views. The lower portionsshow standard deviation of the mean. Standard deviationsfor o n-surfa ce points range from 0.05 to 0.25.The mean standard deviation is approximately0.1 3mmand 0.04 for the AP and lateral surfac es respectively.Again, off-surface random noise is *visible. Similaranalysis of 700 height-maps of RAND O from the frameprocessing of a 20 second -sequence recording showedthat resolution and reproducibility over hundreds offrames equals that for single shot imaging.

T o obtain information o n surface movement, suchasmean and standard deviations, requires extraction ofallthe frames contained in the AV1 sequence. Such processtakes 15 minutes for a one-minute long sequence. Thisisusing a current computer power of 90GB RAID-0 diskarray, P3-1GHz processor, 1GBRAM. Recording of thepatients body surface during a pelvic or lung

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Figure 1.440~440 ixels surface height-map ofa) plaster cast ofa human back, b) the back of Volunteer1, c) the chest of Volunteer1 d) the chest of Volunteer2, e) the chest of Volunteer 2 after software correction. The black lines contour the flattest areas; the arrows

point at the triangulation spot artefact. The range of the ordinate is 250mm.

ObjectM a x height variation(mm)SD from the mean height (mm)Remarks

Fig. la Fig. lb Fig. I Fig. Id Fig. leCast back Back 1 Chest 1 Chest 2 Chest 20.46M.21 0.34M.11 1.01M.41 0.3M.12 0 . 3 M . 1

0.09M.03.1 1M.05 0.08M.02 0.26M.1 1 0 08M 04No hair Hairs N o hair Software correction

Figure 2 Static tests onRANDO pper: 440x440-pixel surface height-map.a) AP imaging, ordinate range:1 mm b) lateralimaging, ordinate range: 120 mm. Lower: Standard deviation.c AP imaging, ordinate range: 25mm. d) lateral imaging, ordinate

range: 12 mm. Note the high noise comes from off-surface points.

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radiotherapy treatment produces a sequence, about15-minute long, which occupies about30 GB of disk space.Visualisa tion of vertical disiplacements ofRAND0 as aseries of height-map surfaces (MPEG format) showedthat the iterated frame processing is stable and error-free.TKe narrow field of view used for the sequence showingthe in vivo movement of the front torso surface whenbreathing covered the lower part of the chest. Analysis ofthe frames show s that, as the chest surface height movesin the breathing cycle, the inhalation m aximum varies by2-2.5cm whilst the exhalation minimum is consistentlyreproduced.

In all figures, the verylow angle of illumination usedfor surface rendering may exiiggerate the surface marksby shadowing.

4. Discussion

The optical sensor system investigated in this paper isthe first of its kind installed in a hospital radiotherapytreatment room. Its use is intended for patient movementmonitoring in situations where modest motion occurs,such as during breathing, and for patient set-up. It wastherefore important to quantify the errors on the resultingheight-maps and assess its limitations, when imagingbody surfaces.

Noise contained in body surface height-maps amountsa tenth of a millimetre Ion a single sensor image(snapshot) of smooth surfaces. The optical signal receivedand transformed into height information by the opticalsensor system is impaired when hairs are present on thebody surface because they do not reflect structured lightas well as skin. It is therelbre difficult to remove theripples that they create on the surface height-maps.Imaging of a hairy chest appears to contain near threetimes the level of noise of non-hairy surfaces. This ishowever still a small error: under 0.5 mm; a technical

solution is to shave the patient, which is an actionregularly done in medical practice. Imaging with a fieldof view wider than the bo dy :surface creates large errors atthe edge of the surface arid off-surface points. Thephenomenon also occurs when imaging with a narrowerfield of view but covering only edges of the body surface.This is due to the lack of fringe information. Thislimitation should be overcome using adequate filteringand masking. The software correction developed in[3]removed successfully triangiilation spot artefact withoutincreasing noise level. The standard deviation obtained inthe static tests agree well with the measurements madeindependently in the optics laboratory at Liverpool JohnMoore University, where the optical sensor was firstdeveloped 121. Largest devialions are, as expected, linkedto high surface gradients. Working system accuracy isestimated to 0.15 mm and repeatability to 0.30 mm, inboth the laboratory and the treatment room. The opticalsensor performances on image quality are thus generally

better than what can be obtained using CT or MRimaging systems to quantify the height variations overlarge areas of body surface. To give an idea of the scaleof the sensor height-map deviations, it is worthremembering that a single CT-image usedfor treatmentplanning will have a single pixel that scalesapproximately0.8 mm.

The optical sensor system provides both dynamicvisualisation and quantitative information on the positionof thousands of points of the surface over time. Suchsurface movement description is needed in radiotherapy.In the work presented above, dynamic 3D imaging wasprocessed and analysed off-line. Improvement on thespeed of analysis for continuous movemen t monitoring isfunction of the computer power. It is clear that reasonablesampling of the images of the sequence would acceleratethe analysis presented above, e.g. extraction of only oneimage every 5 frames (i.e. 5 frames per second). Nearreal-time analysis would benefit gated radiotherapy.Analysis of the body surface movement over a minute interms of mean and SD surfaces is suggested for patientset-up. Re-positioningof the patient in the treatmentroom

as for during CT examination should be done on a surfacesnapshot representative of todays patient movement. T hemean and SD are likely to vary daily with the level ofstress and wellbeing of the patient which affect breathingpatterns and muscular tensions. This creates time-dependant variations of the patients body surface. Thelimits of acceptable movement (mean n SD) will be theobject of an extensive investigation undertaken togetherwith experienced radiographers.A major shortcoming ofthe previously proposed set-up procedures using theoptical sensor system as an alternative to set-up based onas little as three co-planar surface tattoos or severalphysical markers attached across the body surface, is thatthey did not include the above statistical study[ I ] Theamount of data storage can be a potential limitation to

movement recording of all patients but could besignificantly reduced by using 128x128 frames, that stillcontain several thousands points. These data areimportant for post-treatment verifications and introduceknowledge on potential patient movement patterns intotreatment planning. Display of the height variations ofsurfaces has applications in particular in chest topologyand breathing control studies. These are potentiallyrelevant to radiotherapy treatments of breast and lungtumour which movement may be linked to the bodysurface position. Display of both real-time movingsurfaces (via the optical sensor system) and the static CTimages used for treatment planning is interesting forunderstanding the reality of the radiotherapy process. Thebody surface movements could in particular be controlledbased upon deviations from a desired irradiationgeometry andor dose plan. In turn, related biologicaleffects should be studied. The measurements presentedabove of body surface height variations duringin vivo

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breathing dem onstrate that breathing ca n be comp ensatedfor dynamically in radiotherapy by monitoring the entirepatient surface, with substantial applications for thedevelopment of respiratory gated radiotherapy.

5 Conclusion

Static surface height-maps derived from the opticalsensor system investigated in this paperare more accuratethan surface information derived from CT imaging.Dynamic imaging of body surface also providesquantitative monitoringof surface movement. This shouldbe exploited for patient surface monitoring in breathingstudies and gated radiotherapy, and for patient set-up.

[ l ] C.J. Moore and P.A. Graham, 3D dynamic body surfacesensing and CT-body matching: a tool for patient set-up andmonitoring in radiotherapy, Computer Aided Surgery 5 , pp

[2] F. Lilley, M.J. Lalor and D.R. Burton, Robust fringeanalysis system for human body shape measurement,OpticalEngineering 39(1), pp 187-195,2000[3] C.J. Moore et al. Dynamic background correctionin 3Dtbody surface sensing visualisation, submitted toIEEEIV2000 MedVis conference, London, July 2001.

234-245,2000

6 Acknowledgements

W e thank Francis Lilley for allowing us to use hisdynamic sequence ofRANDO.

6 References

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