3d Display

JOURNAL OF DISPLAY TECHNOLOGY, VOL. 4, NO. 4, DECEMBER 2008 437

Volumetric 3D Display for RadiationTherapy Planning

Jason Geng, Senior Member, IEEE

(Invited Paper)

Abstract—In current clinical practice, radiation therapy plan-ning (RTP) has often been treated as a two-dimensional (2D)problem, mainly due to the limitations in visualization technologyavailable to date. The slice-by-slice display format makes it dif-ficult to visualize the path of radiation beam not perpendicularto the axis of the CT slices. This discourages consideration oftreatment plans that utilize radiation beam out of the transverseplane. Human body anatomical structures are inherently three-di-mensional (3D) objects, and tumors and tissues/organs involved inthe RTP are all of 3D shapes. A clear understanding of 3D spatialrelationships among these structures, as well as the anatomicimpact of 3D dose distributions, is essential for designing andevaluating radiation therapy plans.

We have recently made an important breakthrough in the high-resolution volumetric 3D display technology and have made an ini-tial attempt to apply it to RTP applications. By “volumetric 3Ddisplay,” we mean that each “voxel” in the displayed 3D imagesis located physically at the ( ) spatial position where it issupposed to be, and emits light from that position to form real3D images in the eyes of viewers. We have demonstrated the fea-sibility of our system design by building full-scale prototypes andachieved a multi-color, large display volume, true volumetric 3Ddisplay system with a high resolution of over 10 million voxels ina portable design. This type of true 3D display system is able topresent a 3D image of a patient’s anatomy with transparent skin,providing both physiological and psychological depth cues to on-cologists in perceiving and manipulating radiation beam configu-ration in true 3D fashion, thus offering a unique visualization toolto ensure the safety, effectiveness, and speed of the RTP process.

The volumetric 3D display technology holds promise to signifi-cantly enhance the accuracy, safety, and speed of RTP procedures.Such an “understanding at a glance” capability is necessary to keepthe clinicians from becoming bogged down in details, as he/shewould be if provided only with conventional 2D display of CT sliceswith overlaid isodose lines.

The main focus of this paper is to provide technical details onthe volumetric 3D display system we developed, and present someinitial results on its capability of displaying true 3D images. Whilethe system design framework of applying such technology into RTPis introduced, its full scale clinical applications to RTP is still anongoing effort and will be reported later in other publications.

Manuscript received January 08, 2007; revised February 21, 2008. First pub-lished June 10, 2008; current version published November 19, 2008. This workwas supported in part by the National Institutes of Health under Grant R44CA80577-02A1, by the Department of Energy DE-FG02-98ER82588, by theU.S. Air Force F08635-97-C-0034, by DARPA DAAH01-97-C-R169, BMDODASC60-98-C-0018, by the National Science Foundation DMI-0124322, andby NASA NAS13-01039. The content of this document does not necessarilyreflect the position or the policy of the sponsors, and no official endorsementshould be inferred.

The author resides in Rockville, MD 20852 USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JDT.2008.922413

Index Terms—Interactive visualization, medical imaging, radia-tion therapy planning, volumetric 3D display.

I. INTRODUCTION

T HIS paper documents our theoretical study and experi-mental demonstration of a revolutionary volumetric three-

dimensional (3D) display technique. We also present a frame-work for applying this true 3D display technology to radiationtherapy planning (RTP). Although we are still in the early stageof development, the ultimate goal of this investigation is to de-velop a clinically viable volumetric 3D display technology formedical image visualization in general.

The proposed volumetric “life-like” 3D image display tech-nique relies upon a display media that is a true 3D volume in-stead of a 2D flat screen. Each volume element (called “voxel,”analogous to a pixel in a 2D image) in the displayed 3D im-ages locates physically at the ( ) spatial position where itis supposed to be and emits light from that position to form real3D images in the eyes of viewers.

The volumetric 3D display we developed is fundamentallydifferent from conventional 3D rendering visualization tech-nique, where the object is displayed on a 2D flat screen with3D rendering for depth perception. It is also different from 3Dstereo video or head-mounted display (HMD), where the 3Dperception is created with a pair of polarized glasses or displayscreens. The volumetric 3D display technology projects 3D im-ages directly into true 3D space that does not require special3D glasses to view it. Viewers can walk around the 3D imageand look at it from all different directions with realistic depthjust as looking at the real physical object. Such 3D display pro-vides both physiological and psychological depth cues to humanviewers for truthfully perceiving objects in 3D space.

Furthermore, with realistic 3D representations of medical im-ages in many imaging modalities (CT, MRI, PET, Ultrasound,etc), viewers can interact with the life-sized volumetric 3D im-ages being displayed, via handheld pointer and/or other user–in-terface devices, as if the true 3D virtual patient were there witha transparent skin and visible internal anatomic structures. Theunique capabilities of walk-around viewing and direct interac-tion with the displayed 3D images could greatly simplify ourunderstanding of the complexity of 3D objects and spatial rela-tionship among them.

We have recently made an important technical breakthroughin implementing the high-resolution volumetric 3D display.Using the spatial light modulator (SLM), high power visiblelasers and precision fabrication of helical screen, we have

1551-319X/$25.00 © 2008 IEEE

438 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 4, NO. 4, DECEMBER 2008

achieved a multi-color, large display volume, true lifelike 3Ddisplay system with a high resolution of over 10 million voxelsin a portable design. We have demonstrated our high-resolutionvolumetric 3D display concept by building a full-scale proto-type that can display complex 3D images.

The volumetric 3D display is a revolutionary concept for RTPand medical image visualization in general. The major innova-tions of our approach are twofold.

1) We have developed a functional hardware platform anda high-speed data interface that enables a high-resolutionvolumetric 3D image with over 10 million voxels to bedisplayed at a rate of 20 frames per second. We have de-signed, fabricated, and tested all hardware components andsoftware package associated with the display prototypesystem. The dynamic volumetric 3D display capability notonly makes the interactive RTP possible, but also opensdoors to many new applications in medical image visual-ization arena.

2) We have developed a novel conceptual framework ofthe “Interactive RTP Environment,” and built a set ofprototype hardware/software. The Interactive RTP Envi-ronment enables direct interactions between the displayedvolumetric 3D image of patient anatomic structure andtumor, and the simulated treatment beam configuration.The intuitive interactions help radiation therapy plannersdetermine suitable beam directions and parameters tomaximize the tumor coverage and minimize the exposureof normal tissues during a planning session.

II. BRIEF SURVEY OF PRIOR ART ON VOLUMETRIC

3D DISPLAY TECHNIQUES

In this section, we provide a brief survey of a number of 3Dvolumetric display techniques that have been intensively devel-oped in the past.

A. Solid-State Up-Conversion

One of the fundamental requirements for a volumetric 3D dis-play system is to have entire display volume filled with mate-rials that can be selectively excited at any desired locations. Toachieve this goal, one can have two independently controlled ra-diation beams which activate a voxel only when they intersect.While an electron beam cannot be used for such purpose, a laserbeam can, provided that a suitable material of display mediumcan be found. A process known as two-photon up-conversioncan achieve this objective (U.S. Patent 4 041 476 by Swainson,1977, U.S. Patent 5 684 621 by Downing, 1997). Briefly, thisprocess uses the energy of two infrared (IR) photons to pumpa material into an excited level, from which it can make a vis-ible fluoresce transition to a low level. For this process to beuseful as a display medium it must exhibit two-photon absorp-tion from two different wavelengths so that a voxel is turnedon only at the intersection of two independently scanned lasersources. The materials of choice at the present time are the rareearths doped into a glass host known as ZBLAN. ZBLAN isa flurozirconate glass whose chemical name stands for ZrF4-BaF2-LaF3-AlF3-NaF. The two-photon up-conversion conceptfor 3D volumetric display is quite promising, since it requires no

moving parts. Major difficulties to produce a practically useful3D display using this approach are its scale-up capability andability to display multiple colors.

B. Gas Medium Up-Conversion

Another 3D display based on the up-conversion concept em-ploys the intersection of two laser beams in an atomic vapor,and subsequent omnidirectional florescence from the intersec-tion point (U.S. Patent 4 881 068 by Korevaar, 1989). Two lasersare directed via mirrors and - scanners towards an enclosurecontaining an appropriate gaseous species (rubidium vapor, forexample), where they intersect at 90 deg. By itself, either lasercauses no visible fluorescence. However, where both lasers areincident on the same gas atoms, two step excitation results inflorescence at the intersecting point. By scanning the intersec-tion point fast enough, a 3D image can be drawn in the vapor.The eye cannot see changes faster than about 15 Hz, so that ifthe image to be displayed, it is repeatedly drawn faster than thisrate; the image will appear to be steady, even though light maybe originating from any one point in the volume from only asmall fraction of the time.

The advantage of this 3D display concept is its scalability: Itcan be built in almost any desirable size without significantlyincreasing the complexity of the system. The technical difficul-ties in implementing this concept including the requirement ofvacuums chamber, requirement for maintaining certain temper-ature, limitation of number of voxels by the speed of the scan-ners, and eye-safe problem of laser beams.

C. Rotating LEDs Array

One of the earliest volumetric 3D displays was designedby Schipper in 1963 (U.S. Patent 3 097 261). It consists of arotating electroluminenscent panel with embedded high-speedlight emitter array. By controlling the timing of - addressingof the light emitter array and the rotation of the panel, 3Dimages can be formed within the volume swept by the rotatingpanel. In 1979, Berlin developed an innovative approach tosolving the high-bandwidth data transmission problem of thisdesign using optical link and replaced the light emitters withhigh speed light-emitting diode (LED) matrix (U.S. Patent4 160 973 by Berlin, 1979). This system uses LEDs arrays thatare rotated to sweep out a 3D volume. The resolution of thisvolume is a function of number and density of LEDs mountedon the rotating planar array, the speed of rotation and the rate atwhich the LED can be pulsed.

D. Cathode-Ray Sphere (CRS)

The Cathode-Ray Sphere (CRS) concept was originallydeveloped by Ketchpel in 1960s (U.S. Patent 3 140 415 byKetchpel, 1960) and recently implemented by researchersat New Zealand (US Patent 5 703 606 by Blundell, 1997).The voxels are created by addressing a rapidly rotatingphosphor-coated target screen in vacuum by electron beamssynchronized to the screen’s rotation. The view of this rotatingmulti-planar surface depends on the clarity of the glass enclo-sure and the translucency of the rotating screen. Another imagequality issue is the interaction between the phosphor decay rayand the speed of the rotation of the screen.

GENG: VOLUMETRIC 3D DISPLAY FOR RADIATION THERAPY PLANNING 439

Fig. 1. A brief survey of various 3D display technologies.

E. Varifocal Mirror and High Speed Monitor

A very clever method of 3D display employs the strategy offorming optical virtual 3D images in space in front of viewer(U.S. Patent 4 130 832 by Sher, 1978). The varifocal mirrorsystem consists of a vibrating circular mirror along with ahigh-speed monitor. The monitor is connected to a woofersuch that the woofer can be synchronized to the monitor. Aflexible, circular mirror is attached to the front of the woofer,and the monitor is pointed toward the mirror. With the vibra-tions from the woofer, the mirror changes focal length and thedifferent points being displayed on the monitor seem to appearat different physical locations in space, giving the appearanceof different depths to different objects in the scene being dis-played. Variable mirror based 3D display systems are primarilylimited by the size of the mirror and updating rate of images,since this mirror has to vibrate.

F. Laser Scanning Rotating Helix 3D Display

Extensive attempts have been made by researchers at TexasInstruments Incorporated (US Patent 5 042 909, 5 162 787, byGarcia, 1991) to develop a 3D display device based on laserscanning and rotating (helical) surface. Lasers scanning 3D dis-plays operate by deflecting a beam of coherent light generatedby a laser to a rotating helical surface. Timing modulation of thelaser beam controls the height of the light spot that is producedby the laser on the rotating surface. The deflectors include de-vices such as polygonal mirrors, galvanometer, acousto- opticsmodulated deflectors, and micro-deformable mirrors. There areseveral problems with this 3D display mechanism that have pre-vented it from becoming commercially feasible.

The most serious problem is the limitation on the maximumnumber of voxels that can be displayed. Due to the nature ofsequential (non-parallel) laser scanning, only one spot of lightcan be displayed at any given moment. All the activated imagevoxels have to be addressed, one by one, by the scanning ofsingle laser beam in time-multiplex fashion. The time neededfor steering the laser beam and to stay on the voxel position toproduce sufficient brightness poses an upper limit to how manyvoxels it can display. To increase the number of voxels, mul-tiple channel lasers and scanners could be used. However, many

attempts to increase the spatial resolution have hampered withhigh cost and bulky hardware design.

Fig. 1 summarizes various research and development effortson 3D display. Recently, there is a surge of research activitieson volumetric 3D display that promise to bring high resolution( 100 million voxels) display into reality [1]–[11].

III. VOLUMEVIEWER 3D DISPLAY CONCEPTP

In this section, we provide detailed technical discussions onthe VolumeViewer 3D display design and its implementation.

A. Principle of the “Multi-Planar” Volumetric 3D Display

Fig. 2 illustrates the principle of the “Multi-planar” volu-metric 3D image display using a high-speed 2D image projectorand a moving screen. Suppose that a sweeping screen can becontrolled to move back and forth along the direction at afrequency higher than 20 Hz. Within the time period of eachsweeping motion, frames of 2D image patterns are projectedby the high-speed 2D image projector. The moving screen inter-cepts 2D image projections at different positions along axis,forming a stack of spatial image layers in true 3D space. If thecycling speed of the moving screen is sufficiently high, and the2D image projector can produce sufficient number of 2D imagesections during each pass, human observers are able to perceivea true volumetric 3D image floating in the 3D space withoutflicker, due to the residual effect of human eyes.

The “multi-planar” volumetric 3D display principle is by nomeans a complex concept. However, implementation has beendifficult due to lack of suitable high-speed image projector,clever mechanism to produce sweeping screen motion, andhigh brightness light sources. There has been a number ofattempts been made to build such cumbersome system withoutsuccess. A physically flat screen sweeping at 20 Hz createsserious problems of mechanical design, balance, vibration, andnoise. Conventional liquid crystal projector can only achievea switching rate of few hundred hertz, leading to a very lowspatial resolution. High power light source has been veryexpensive and cumbersome. All these factors contribute toa slow progress of volumetric 3D display techniques usingmulti-planar principle.


Fig. 2. Principle of the “multi-planar” volumetric 3D display using fast 2D projection and a moving screen.

Fig. 3. Volumetric 3D display concept using a fast SLM and a rotating helix screen.

B. Concept of the “VolumeViewer” 3D Display

We propose a new generation of the multi-planar volumetric3D display system, taking advantages of rapid advances in mate-rial, laser, and semiconductor fabrication technologies. With thenewly developed ferrorelectric liquid crystal spatial light modu-lator (SLM)technology, theswitchingspeedofSLMreachesover3000 fps for a SLM of 256 by 256 pixels or higher resolutions.Such a fast SLM can be used as a high-speed image pattern gen-erator to produce volumetric 3D pictures, providing a powerfultool to revolutionize thestate-of-the-artof3Ddisplay.Fig.3 illus-trates a design concept of our SLM/helix volumetric 3D display.

In Fig. 3, light rays produced by a light source projector ,passing through filter and collimating lenses , impinge on apolarizing beam splitter cube . Due to the polarization char-acteristics of the beam splitter, the polarized light rays are re-flected by the beam splitter and projected onto a SLM . Theimage data shown on the SLM is generated by a host computer

. The SLM is able to alternate image patterns at high framerate (i.e., over 3000 fps). When a pixel on SLM is turned ON,the light will be reflected back to the beamsplitter cube, whilewhen the pixel is turned OFF, the projected light on this pixelwill be absorbed by the SLM and will not be reflected. The pat-terns on the SLM are therefore able to control the patterns of the


reflected light rays. The reflected light rays with encoded SLMimage patterns transmit through the beam splitter cube.

An optical projection lens system is employed to projectthe image patterns towards a spinning helix screen, marked as

. The light spots projected on the helix screen intersect thehelix surface at different heights depending on different rotatingangles of the helix, thus form 3D voxels in 3D space (the displayvolume ). Each section of the helix surface is described by thefollowing mathematical equations:

If rotation is synchronized via a motor driving the helixscreen with the switching timing of the SLM , such that 3Dimage patterns are shown in the 3D space with a high refreshrate (e.g., 20 Hz), naked eyes perceive it as a 3D volumetricimage. No special eyewear is required to view such 3D imagefloating in true 3D space, just as a real object is placed there.

C. Advantages of the Proposed SLM/Helix 3D Display System

• Inherent Parallel Architecture for Voxel-Addressing: In-stead of using single laser beam to address all the voxels(such as the NRaD scanning laser system) the SLM/Helixsystem use 256 by 256 (or more) light rays to address si-multaneously voxels, thus overcomes the bottleneck in pro-ducing high resolution 3D images encountered by otherapproaches.

• High Spatial Resolution: The maximum number of voxelsthat can be generated by the SLM/Helix display dependsupon the spatial resolution of SLM and the spinning speedof helix. With the currently available SLM technology, aSLM with 1024 by 1024 pixel and 300 000 frames persecond switching speed is available. The resolution of pro-posed 3D display can take advantage of the rapid advancesof SLM technology.

• Simple Structure and Easy to Build: Other than the rotatinghelix, there is no other scanning or moving part. The op-tical design and alignment are not difficult. The system canbe built using commercial off-the-shelf (COTS) products,which leads to shorter development period and low cost.

• No Special Viewing Glasses or Helmet are Needed byViewers: The volumetric images are displayed in true3D space with almost 360 degree viewing angle, whichpreserve all physiological and psychological depth cues ofhuman visual system. Viewers can walk freely around themonitor to see the 3D images, just as if the real 3D objectwere sitting there.

• Implementation of Full Color Display is Straightforward:Just use three SLMs for Red, Green, and Blue respectively,and the color of voxels can be automatically controlled.Another way to implement color display is even simpler:use Red, Green, and Blue light projector, and synchronizethe timing of three projectors with a high speed SLM.

D. VolumeViewer 3D Display Prototype

Fig. 4 presents an overall system design configuration ofthe newly designed and prototyped volumetric 3D display. We

Fig. 4. Prototype of the VolumeViewer™ 3D display.

TABLE IPERFORMANCE OF THE VOLUMETRIC 3D DISPLAY PROTOTYPE

dubbed this prototype system the VolumeViewer™. Inside thetransparent hemispherical dome is a rotating helix forming a3D image display volume of 7” height and 20” in diameter.

There are nine pieces of reflective mirrors with large di-mensions of optic surfaces. If fabricated using conventionalthick glasses, these mirrors would be heavy-weighted, fragile,and difficult to assembly with acceptable accuracy of opticalalignment. We adopted a state-of-the-art mirror fabricationtechnology that forms large piece of flat mirror using framedthin films with high reflectivity. These thin-film mirrors haveonly 10% of weights as their glass mirror counterparts, and canbe built to fit various difficult geometric dimensions. Due totheir light weight and flexible dimensions, we can easily mountthem into optically aligned modules, thus saves us tremendouseffort in the final stage of the optical alignment in the systemintegration. Table I lists major performance specification of theVolumeViewer Prototype system.


Fig. 5. SLM structure.

IV. DESIGN AND FABRICATION OF THE PCI INTERFACE BOARD

ALLOWING FOR DYNAMIC 3D IMAGE DISPLAY

A. Primary Objectives of the PCI Interface Board Design

Although our initial success in developing the original proto-type system represented the state-of-the-art true volumetric 3Ddisplay technology then in terms of achieving high spatial res-olution, the updating rate of 3D images in original system wasstill slow. To update a displayed 3D image into a new frame of3D image, the host PC has to upload the data set of the new 3Dimage to the SLM driver via a parallel port. This data transmis-sion of a single frame of 3D image usually takes about 20 s, dueto the size of 3D dataset and the slow speed of the PC parallelport. Such a low updating rate certainly prevents our current 3Ddisplay system design from being used in many dynamic inter-active 3D display applications, such as radiation therapy plan-ning sessions.

Therefore, one of the main efforts of this investigation is todesign and fabricate a PCI interface board to eliminate the bot-tleneck of 3D image transmission between host PC and SLMchip. Primary goals of this PCI interface board are:

1) to achieve 3D image updating at a rate up to 20 images persecond from host PC to SLM chip;

2) to increase the frame rate of 2D image displayed on theSLM;

3) to allow for multiple color 3D display.

B. Spatial Light Modulator (SLM)

SLMs, devices that alter the temporal and spatial character ofa light beam, can be either optically or electrically addressed.Optically addressed SLMs often require bulky support equip-ment and additional light sources. To obtain high frame rate ofimage projection for volumetric 3D display, we propose to usethe electronically controllable, fast ferroelectric liquid crystal(FLC) reflective spatial light modulator. The device is built atopa planarized 0.6 m CMOS SRAM backplane with 15 m pixelpitch and 87% fill factor. A thin layer of FLC material is sand-wiched between a metal conductor and a glass window coatedwith a transparent conductive layer such as indium–tin–oxide(ITO). When a voltage is applied across the FLC layer the fastaxis of the bi-refringent FLC material is forced into one of twopossible states: ON or OFF (the image on the SLM is binary).

The structure of a SLM is depicted in Fig. 5. It is a speciallydesigned integrated circuit housed in a 49-pin ceramic PGApackage. The effective area of the SLM consists of 65 536 FLC

cells arranged as a square of 256 by 256 array with total di-mension of 5 5 mm approximately. The device achieves betterthan 25% optical throughput when used with collimated laserlight and better than 100:1 contrast ratio when oriented for am-plitude modulation. A better than 100:1 contrast ratio of SLMprovides a fairly good image quality. The device can be operatedas fast as 5 kHz with complete switching of the liquid crystal.

C. Design the PCI Interface Board to Control the SLM Chip

Controlling the SLM’s operation is very similar to addressinga Static Random Access Memory (SRAM) chip. The interfaceboard contains an on-board microprocessor, memory for up to512 frame 2D images, circuitry for controlling the SLM, andcircuitry for communicating with host PC computer. We useC++ and VXD (a low-level assembly) software to manipulatethe image data and to transfer them into the image buffer on thecontroller, which in turn sends the image sequence to the SLMin a predetermined high frame rate. Fig. 6 illustrates the blockdiagram of our Interface Board design.

1) Microprocessor: The size of 3D data sets is inherentlyhuge. In order to transfer huge amount of 3D data in higherspeed among the host computer, on-board image memory, andthe SLM chip, the microprocessor must have the high-speeddata transferring unit, such as DMA, Interrupt Unit etc. The mi-croprocessor must also have a PCI interface and other controlunits for connecting with PC and communicating with otherstandard facilities. By careful design comparison, we selectedthe Intel 80960RP as the CPU of the board. The 80960RP is aPCI IO processor with 352 BGA pins. It has many units for datacommunication. Its DMA Controller, Address Translation Unit(ATU), Message Unit (MU), Memory Controller and other con-trol units are suitable for the design, and it has an available andcompleted embedded software system, so it fulfills the designfeatures.

2) Memory: We need a higher speed RAM on the board as abuffer to store 3D images. In the design two memory groups areused to display dynamic 3D images and make the other displayfunctions. By considering the speed, volume and stability, wechose eight SRAM, MCM6246 chips, as the image memory onthe board and divided them into two groups, each has 2 Mbytes.This image memory can implement all features described above.

3) FPGA (Field Programmable Gate Array): Due to com-plex operation of the PCI interface board, thousands of gatesand flip-flops are needed to fulfill the desired functions. Dozensof buses with 32 bits have to switch each other, which is impos-sible to be laid on an area-limited printed circuit board directly.


Fig. 6. Block diagram of the PCI interface board.

Fig. 7. Block diagram of FPGA1.

Fig. 8. Block diagram of FPGA2.

We employed advanced FPGA technology which allows for asoftware programmable functionality on hardware chips. TwoXilinx XCS40 (each with 40,000 gates) are used and their func-tions are illustrated in Figs. 7 and 8. Due to space limit, a totalof 24 complex circuit diagrams implemented by FPGAs cannotbe included. Fig. 9 gives an example of programmable functionsimplemented by the FPGA1. Powerful FPGAs make it possiblefor us to design a compact PCI board with the desired features.

D. Fabrication and Test the PCI Interface Board

Using the newly designed SLM device interface board, wehave achieved a maximum transmission rate of about 2200 fpsand the resolution of each frame of 2D images is 256 by256 pixels. At the target 3D image refreshing rate of 7 imagesper second, we are able to produce 157 frames of 2D imagesfor each 3D image cube. This enhanced speed of image transfereffectively increases the spatial resolution of our volumetric3D display to by by million voxels.In comparison to the use of original SLM drive unit, themaximum spatial resolution is about by by

million voxels.Furthermore, the 3D image-refreshing rate was about 20 sec-

onds per image versus the 20 images per second of currentsystem equipped with the new PCI interface board. The suc-cess of the PCI interface board allows us to perform dynamic3D image display, and makes the application of our 3D displaytechnology to RTP practically possible.

E. Software Drive Development Using VXD Techniques

The software for the board is divided into host computer pro-grams and the 80960RP microprocessor programs. The hostcomputer programs include a Windows based work studio and aVxD (Virtual Device Driver) program, as shown in Fig. 10. The80960-based program includes the embedded programs storedin PROM or the executable code downloaded from the host com-puter to the 80960RP program RAM. In fact the board can becontrolled from host computer and 80960RP.

When the host computer is turn on, BIOS of the computerfinds the PCI board, so that the Windows 95/98 can get the infor-mation from the Intel 80960RP. Then the model of (OperationSystem) OS loads the GTI3DD.VxD into the computer memory.During loading GTI3DD.VxD, OS communicates with the VxDto decide the resources allocation. After all VxDs are loaded, OSbuilds up a table to save the results.


Fig. 9. An example of detailed design diagram inside the FPGAs.

Fig. 10. Operation of the VxD for the PCI interface board.

V. SWITCHABLE DISPLAY VOLUME—A NEW OPTICAL DESIGN

ENABLING BOTH FULL AND HALF HELIX DISPLAY VOLUMES

In our previous system design and experiments, we projectedthe 3D images onto a small portion (less than one half of a helix,see the left drawing in Fig. 11) of the sweeping volume producedby the helix. A large portion of the useful volume produced bythe sweeping helix was wasted. In our latest effort, we havesignificantly improved the system optical design to project the3D images into the entire helix volume (see the right drawingin Fig. 11), thus increasing the size of the display volume toentire sweeping volume of the rotating helix – 20” (508 mm) indiameter and 7” (178 mm) in height.

Fig. 11. Comparison of previous and current 3D image projection schemes.

A. Overall Optical Configuration Design and Tradeoff: FrontProjection Versus Rear Projection

There are two possible overall optic configurations for theSLM/Helix system design: Front projection configuration(FPC) versus rear projection configuration (RPC). Previous3D display systems, such as NRaD 3D display, use the frontprojection configuration. This means that laser beams areprojected onto the rotating helix surface in the same side asviewers view the 3D images [Fig. 12(a)]. The front projectionconfiguration makes it easier to implement a driving system


(a) (b)

Fig. 12. Comparison of overall optical configuration for the SLM/helix 3D display. (a) Front Projection Configuration (FPC). (b) Rear Projection Configuration(RPC).

for the rotating helix. It also provides convenience in adjustingoptical system setups and modifying other components orsubsystems. However, the disadvantages of the front projectionconfiguration include the following.

(1) It leads naturally to an overhead projection configura-tion so a compact system design (as a portable desktopdisplay, for instance) is difficult to achieve.

(2) Viewers may possibly block the projection of laserbeams.

(3) Since system components are spread out, maintaining anaccurate optical alignment is difficult.

We have developed the “rear projection system (RPS)” designfor our SLM/Helix 3D display. In a RPS configuration, laserbeams are projected onto the rotating helix screen from below,while viewers look 3D images from above [see Fig. 12(b)]. Thehelix surface is made of semi-transparent material so it transmits50% light and reflects 50% light. This rear-projection approacheliminates the unwieldy overhead mirrors from the NRaD’s de-sign and allows the lasers, scanners, optics, and the helix tobe packaged together as a single compact mobile 3D displayunit. Since the helix surface transmits as well as reflects light inomni-direction, the viewing angle of the voxels in a 3D imageis very large (almost true walk-around viewing angle and groupviewing capability).

A major advantage of using the RPS configuration is that itis possible to achieve a compact system design. All the compo-nents of the SLM/Helix system can be packaged into a cabinetwith 3D display volume on the top. From the viewpoint of finalcommercial product design, RPS is a much better system designconfiguration for a volumetric 3D display device. Compact andstylish desktop 3D display unit can be built.

B. Optical Layout of the SLM/Helix System With a Full Helix3D Display Volume

Due to structural constraints of the Rear Projection Config-uration, entire displayable volume of the helix cannot be fully

Fig. 13. Optical layout of switchable 3D display volume: When the electron-ically controlled swing mirror is “On”, the 3D image occupies the entire helixvolume, while when the swing mirror is “Off”, the 3D image occupies one halfof the helical display volume.

illuminated by an image projector via single light path. The mo-tion control components (motors, encoders, etc.) would blockportion of images located close to the rotating axis of the helix.

To solve this problem, we invented a new optical layout thatemploys split light paths. As shown in the Fig. 13(a), the imageprojection coming out from the SLM projector is first reflectedby the electronically controllable “swing mirror” (labeled as M),to a 45 mirror “A” towards upward. The image is then split inhalf by a pair of mirrors B and B’. The light path on the leftsubsequently goes through mirrors C, D and E to illuminate theleft half of the helix volume. In a similar fashion, a symmetriclight path on the right goes through mirrors C’, D’, and E’ toilluminate the right half of the helix volume. Fig. 13(b) presentsa 3D view of this dual light path arrangement. The dual lightpath optical layout bypasses the motion control unit (motor andencoder) and is able to deliver the image projection that coversentire displayable volume on the helix (except for the centralaxis).

When the swing mirror is on the “off” position, the light pro-jection coming out from the SLM projector is reflected by themirror “ ”, and the entire image ray will pass only the path


Fig. 14. (a) 3D image data consisting of a voxel cube. (b) 3D image data issliced into� helical slices conformal to the shape of helix screen at� differentlocations.

of A, B, C, D, E, towards to the one half of the helix volume.This display mode is often needed to offer viewers the flexi-bility of seeing 3D image in a higher voxel density and higherimage brightness. With the same projected light energy, smallerthe display volume, brighter the image.

C. Real-Time Dynamic 3D Image Data Generation

Although we have developed the dynamic 3D displaycapability for our volumetric 3D display, fast 3D image datapreparation for real-time applications remains a challengingtask. The 3D image preparation for our display can be illus-trated in Fig. 14. A set of 3D data is represented as a 3D datacube, as shown in Fig. 14(a). The preparation tasks includescaling and orientating the image, and “slicing” the 3D datacube using helical surfaces, as shown in Fig. 14(b). The sliceddata set (a stack of 2D images) can then be sent directly toSLM chip for projection. In general, the size of 3D image isinherently huge (20 Mbytes each, for example) and most 3Dimage processing tasks takes a long time to complete usingoff-the-shelf PC computer.

In this project, we have discovered a new approach to greatlyincrease the speed of the 3D data processing for displaying thedose distribution. In RTP application, the locations of tumor andcritical organs are known via preprocessing. In the real-time dis-play operation, we only need to adjust the color of voxels onthese objects to reflect the dose values on these voxels resultingfrom real-time dose calculation. We call these voxels the “activevoxels”. Usually, the number of the active voxels is only a smallpercentage (i.e., 5%) of all voxels. All the data corresponding toother voxels remain unchanged. By processing only the “activevoxels”, significant amount of time can be saved, thus real-timedynamic display of the changing 3D image is feasible.

We have preliminarily implemented this “active voxels”approach on a prostate tumor visualization experiment. Thetumor can be tuned “On” and “Off” by viewer using a mouse ata response time of about 0.1 s.

D. Example of Volumetric 3D Display Images

Fig. 15 shows an example of true 3D image displayed on theVolumeViewer prototype. Note that due to the nature of true3D image, it is very difficult to present the true 3D nature ofthe display on flat media such as on a flat paper. However, theobservers who have had opportunity to see the true 3D displayall appreciate the unprecedented capability of providing bothphysiological and psychological depth cues to human viewersto truthfully perceive 3D objects in volumetric images.

Fig. 15. Pictures of a volumetric 3D image (human head) displayed on ourVolumeViewer prototype system.

Fig. 16. Radiation therapy planning: irradiate a tumor using multiple radiationbeams while sparing neighboring tissues from radiation damage.

VI. TRUE 3D DISPLAY FOR RADIOTHERAPY PLANNING

A. Basic Concept of Radiation Therapy Planning

The primary goal of a radiation therapy treatment is to deliver ahigh and uniform dose to the tumor while keeping the dose to theneighboring healthy tissues and radiation-sensitive organs as lowas possible. Fig. 16 schematically illustrates the basic conceptand constraints in the radiation therapy planning. A cross sectionof a body anatomy with a circular tumor is shown. If the tumor isirradiated from only one direction with a cylinder beam (labeledas “beam1”), all the healthy tissue along the beam path are ex-posed to approximately the same dose as the tumor. If, instead,we use multiple beams (the beam1 and beam2, for example),the dose deposited on the tumor would be approximately sev-eral times of the dose exposed to the healthy tissue. Using morebeams in different directions can lead to further improvementsof the dose distribution, and a very sharp dropoff of the dose inthe tissue surrounding the tumor region can be achieved.

The planner(s) of a radiation therapy procedure should care-fully select the beam configuration in order to achieve the besttreatment result. By “beam configuration” we mean a set of pa-rameters including the number of beams, spatial orientation ofeach beam, beam angels, intensity, beam weights and cross-sec-tion shape of each beam, etc. The “best treatment result” isjudged by the maximum dose distribution on the tumor and theminimum dose distribution on surrounding healthy tissues. Theradiation therapy planning is an interactive process where theplanner has to produce, evaluate, modify, and compare severalalternative plans based on available information regarding thepatient’s anatomy, tumor characteristics and planner’s knowl-edge and clinical experience [12]–[21].

B. Why Use True Volumetric 3D Display inRadiation Therapy Planning?

1) Human Anatomy is Inherently 3D: The true volumetric3D display technology offers unambiguous spatial relation-


ship among the 3D structures allowing viewers to perceive3D anatomical structure correctly and quickly. In the radia-tion therapy applications, the ability to visualize 3D internalstructures, as if the patient had transparent skin, allows theoncologist to select beam angles, weights, and field shapes thatwill minimize inclusion of radiosensitive organ/tissues with thebeam. More importantly, the volumetric 3D display capabilityreveals the complex spatial relationship among these body partsin a true 3D physical space, providing the planner a much moreeffective way to comprehend the complex spatial relationshipsbetween tumor and surrounding healthy organs, as well as thedose distribution in 3D space.

Displaying dose coverage as color objects also has significantadvantages over existing technology. In existing practice, dosedistribution can either be viewed slice-by-slice on sectional im-ages or as a 3D rendered color object displayed on a computerscreen. Therefore, the operators either view all slices to get thedose coverage or perform 3D rendering at many viewing an-gles to get the complete picture of dose coverage. Either way,it would take a long time or the picture has to be completed inthe operator’s mind since neither method can show the completepicture at once. In contrary, when displayed with an interactivelifelike 3D display monitor, the complete picture of dose cov-erage could be presented for the planner.

2) Limitation of Conventional Display Techniques: In con-ventional RTP practice, planning has often been treated as atwo-dimensional (2D) problem, mainly due to the limitationsin imaging/display technology and resources. Conventional“slice-by-slice” display of CT or MRI data while providing de-tailed anatomic information imposes serious limitations on theradiation treatment planning process. First, the slice-by-slicedisplay format makes it difficult to visualize the path of anyradiation beam not perpendicular to the axis of the CT slices.This discourages the consideration of all treatment plans thatutilize radiation beam out of the transverse plane. Second, bydisplaying the radiation iso-doses on each CT slice, the meritsof multiple competing treatment plans can be compared onlyin a piecewise fashion. Experience has shown that under theseconditions, it is not always easy either to recognize the besttreatment plan or to suggest useful modifications. Finally, forbrachy therapy treatment, the conventional CT format mayoffer ambiguous information as to the location of the implant.It may be impossible to determine whether a radioactive seedseen on one CT slice is the same as that seen on an adjacentslice.

There have been rapid advances recently in 3D visualizationtechniques (both software and hardware) to produce 3D effecton 2D display screens. However, CT/MRI data is inherently of3D nature, yet all conventional displays use flat 2D screens orfilms (e.g., CRTs, LCDs, and slices) that lack important depthcues. This fundamental restriction greatly limits the capabilityof oncologist to perceive the complexity of the anatomy andradiation beam configuration, therefore affects the safety, speedand accuracy of the radiation treatment planning process.

3) “Understanding at a Glance”: We believe that the truevolumetric 3D display technique holds the potential to revolu-tionize current clinical practice of 3D treatment planning, and isa logical evolutionary step to the fifth generation technology in

Fig. 17. Hardware setup of the interactive RTP environment.

the history of radiation therapy treatment planning. The inherentcapability of displaying 3D data with true 3D cues allows clini-cians to understand the spatial radiation dose distribution muchmore quickly and easily. Such an “understanding at a glance”is necessary to keep the clinician from becoming bogged downin endless details, as he would be if provided only with conven-tional 2D display of CT slices with overlaid iso-dose lines.

The 3D RTP techniques have received broad clinical accep-tance and has shown in improve clinical outcomes. It is evidentthat improved visualization tools in RTP can make significantimprovements in patient care. The lifelike 3D display providessignificant advancement over the existing 3D rendering tech-nique in that the inherent capability of displaying 3D data withmost true 3D cues allows clinicians to understand the spatial ra-diation dose distribution much more quickly and easily. Such an“understanding at a glance” is necessary to keep the clinicianfrom becoming bogged down in endless details, as he wouldbe if provided only with conventional 2D display of CT sliceswith overlaid iso-dose lines. We believe that the true volumetric3D display technique holds the potential to revolutionize currentclinical practice of 3D treatment planning, and is a logical evo-lutionary step to the next generation visualization technology inthe history of radiation therapy treatment planning.

VII. FRAMEWORK OF THE INTERACTIVE RADIATION THERAPY

PLANNING ENVIRONMENT

We propose an interactive RTP environment framework thattakes full advantage of the true volumetric 3D display capa-bility, as shown in Fig. 17, which illustrates a prototype of theInteractive RTP Environment with gantry, beam simulator, andthe dynamic volumetric 3D display. Note that the gantry canrotate around the hemisphere display volume, and the beamsimulator can adjust its angular position, thus realizing a twodegree-of-freedom positioning capability, which is able to sim-ulate typical beam positions used in RTP.

Fig. 18 provides a flowchart of the interactive RTP processusing our volumetric 3D display technology. Images of patient’sanatomy and cancer/organs are acquired and processed to pro-vide 3D digital models of anatomic structures and cancer or-gans. These data are sent to the volumetric 3D display for visu-alization. An oncologist/planner starts his/her planning process


Fig. 18. Interactive radiotherapy planning environment using volumetric 3D display.

by visualizing directly the true 3D images displayed on the vol-umetric 3D display monitor, just like he can view the patientwith transparent skins. The oncologist can specify the beamconfiguration by define beam parameters or by using the sim-ulated beam simulator hardware that shines a simulated radia-tion beam directly on the anatomic structure and tumor loca-tion. The spatial position and orientation of the simulated beamcan be totally controlled by the oncologist/planner so he/she hasentire 3D freedom to place and adjust the beam configuration.Beam Eyes View (BEV) and Room View can be provided forthe visualization.

Once the planner selects the beam configuration, dose dis-tribution corresponding to this set of beam configuration willbe calculated and the results will be sent to the volumetric 3Ddisplay monitor for visualization. Should the planner decide tomodify the beam configuration based on the visualization re-sults, he can go back to the beam configuration planning stageand define the modified beams.

After the dose distribution of a plan meets the requirement,the system automatically performs the collision avoidance ver-ification, based on the kinematics relationship among the treat-ment machine, couch, and patient’s body shape. The collisionavoidance verification process can be animated and displayedon the volumetric 3D display so the oncologist can visually con-firm the collision-free treatment plan.

Finally, the system will formulate a final radiation therapyplan and compute various quantitative figure of merits (FOM),such as dose-volume histogram, dose statistics, normal tissuecomplication probability (NTCP), and tumor control probability

(TCP), etc. These data can be displayed on the 2D/3D monitorssimultaneously.

We have performed experiments to demonstrate the feasi-bility of this novel “Interactive RTP Environment” concept.Components of the Interactive RTP Environment are describedin the following paragraphs. Results of our initial experimentsare promising, as judged by a number of radiation physicistswho observed the experimental demonstrations in our prototypesystem.

The 3D volumetric images of patient’s anatomic structure andtumor site are displayed on the volumetric 3D display monitor,and a simulated radiation beam mounted on a gantry structureand controlled by the oncologist can illuminate directly the dis-played tumor to observe the radiation effect. Such a beam sim-ulator is able to duplicate the motion similar to that achieves bythe treatment machine and it has position tracking sensors thatrecord the motion of the beam head.

On the other hand, the configuration (orientation and posi-tion) of the displayed patient anatomic image can be controlledby the planner to simulate the realistic patient setup configura-tion. The combination of the displayed volumetric 3D image andthe simulated radiation beam mechanism allows the oncologistto adjust and select interactively the configuration and parame-ters of a beam (divergence, orientation, intensity, and shape) aswell as the patient’s setup position to achieve the best figure ofmerits and to avoid the beam paths that could cause potentialdamages of neighboring healthy tissues.

After beams are selected, computer will generate a treatmentplan, and the 3D dose distribution will be calculated. The 3D


display monitor then superimposes the dose distribution mapswith anatomical structure, allowing the radiation oncologists tofurther review, modify, and approve the radiation therapy plan.The entire planning process is highly intuitive and interactivethus is very easy to learn and master, takes much less time fromoncologists to the RTP, and can achieve better quality of theresulting plan.

We now describe individual components of the proposedframework for the interactive RTP environment.

A. Volumetric 3D Display Monitor

Using a volumetric 3D display monitor in the proposed Inter-active RTP Environment has unique advantages.

• The 3D images of anatomy and tumor organ are floating inthe true 3D space, with the correct 3D spatial relationshipas true objects. The images can be viewed from all direc-tions without needing any special eyeglasses, and indepen-dence from observers’ capability of stereo-vision. Thesefeatures offer the planner high degree of intuition and eas-iness to comprehend patient’s specific anatomic situation.

• The volumetric 3D display is the only information displaymedia that allows for the true 3D interaction between thedisplayed anatomy and tumor organ images and the simu-lated radiation beams. RTP Planner can interactively con-figure the patient position and beams configurations. Theinteractive nature of the volumetric 3D display allows theplanner to modify geometric parameters while viewing di-rectly at the 3D images of anatomic organs, as if the trueobject is there. 3D images are not really useful unless theviewer is able to interact with display in a convenient way.

• The 3D images displayed on our volumetric 3D mon-itor possess the “see-through” feature. This means thatthe foreground images of organs would not occlude thebackground images. This “transparency” feature allowsviewers to see both the tumor and surrounding healthyorgan as well as the treatment beams simultaneously, thusgreatly increase the understanding of 3D spatial relation-ship among these elements.

B. Gantry Motion Fixture

In a “single iso-center” radiation treatment plan, all beamsintersect at the accelerator’s iso-center. To meet this require-ment in our Interactive RTP Environment, we have designedthe system similar to that of gantry on the treatment machine,with one degree-of-freedom motion fixture along the gantry (asshown in Fig. 17) that hosts the simulated beam head. The beamhead can be moved freely around the patient by the plannerduring the interactive planning session. The 3D image of the pa-tient’s anatomy can be manipulated by the planner to simulatethe realistic setup position of the patient in the radiation therapyplanning session.

A position tracking sensor is installed on the gantry to trackthe location of the beam head on the gantry. The sensor outputwill be send to the central computer to calculate the beam con-figuration.

Fig. 19 shows a set of interactive visualization of anatom-ical structure of a prostate and treatment beam configuration.Note that the simulated beam controlled by a planner is able

Fig. 19. Interactive visualization of anatomic structure/prostate andbeam configuration.

to directly interact with the life-size 3D anatomic structure ofa patient, and an optimal beam configuration can be selectedintuitively via interactions. The unique “direct interaction” ca-pability offered by the volumetric 3D display makes it an idealtool for radiation therapy planning.

VIII. CONCLUSION

In this paper, we presented a novel design of true volumetricthree-dimensional display systems that is able to show true vol-umetric 3D images with high volumetric spatial resolution. Wedocumented some of our effort in designing, prototyping andtesting the volumetric 3D display systems, and our initial at-tempt to apply this unique 3D display technology as an aug-mented visualization tool to helping oncologists in selecting thebest radiation treatment plan. Although exciting progresses havebeen made in terms of developing the volumetric 3D displaytechnology, we are still far away from achieving our ultimategoal, which is to develop a clinically viable hardware and soft-ware that will provide unique capability of volumetric 3D visu-alization to aid oncologists in radiation therapy planning withhigher accuracy, effectiveness, convenience, and speed.

Good radiation treatment planning requires that the targetvolume be treated with a high and uniform dose of radiationwhile irradiating normal tissue as little as possible. Judging themerits of a given treatment plan from the conventional 2D dis-play screen can be difficult for radiation oncologists to selectthe best of several alternative treatment plans. The problem be-comes even more difficult if the entire spatial distribution of


the radiation dosage is to be considered, because of the enor-mous amount of 3D data that must be evaluated. We believethat lack of suitable method to simultaneously display 3D dosedistribution superimposed on the relevant anatomy has greatlycontributed to the slow incorporation of 3D considerations intoroutine radiation treatment planning.

The drawbacks of conventional CT or MRI displays can belargely overcome by employing the true volumetric 3D displaytechnology. Such true 3D display system is able to provide bothphysiological and psychological depth cues to oncologists inperceiving and manipulating radiation beam configuration in atrue 3D fashion, thus providing unique visualization tool to en-sure the safety, effectiveness, and speed of radiation treatmentplanning process.

The main focus of this paper is to provide technical detailson the volumetric 3D display system we developed, and presentsome initial results on its capability of displaying true 3D im-ages. While the system design framework of applying such tech-nology into RTP is introduced, its full scale clinical applicationsto RTP is still an ongoing effort and will be reported later in otherpublications.

The field of true 3D display technology is still quite young,comparing to its 2D counterpart that has developed over severaldecades with multi-billion dollar investments. It is our hopethat our preliminary work could provide some stimulations andattractions to more talented researchers from both technicaland clinical background to this fascinating field of research anddevelopment.

ACKNOWLEDGMENT

The authors would like to thank many collaboratorsand supporters who contributed in part to the success ofthis study, among them Dr. J. Rogers, Dr. M. Freedman,Dr. T. DeWeese, Dr. M. Vannier, Dr. S. Li, Dr. D. Frassica,Dr. J. Wong, J. Russell, M. Deis, Dr. P. Zhunag, Dr. Y. Feng,Dr. H. Li, Dr. J. Qiao, Dr. G. Ying, Dr. S. Nerlove, Dr. J. Hen-nessey, Dr. R. Coryells, Dr. K. Narayanan, Dr. P. Srivastava,Dr. L. Quatrano, Dr. H. Baker, and Dr. B. Donoff.

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Jason Geng (SM’89) has over two decades ofexperience in leading the research, development andcommercialization of advanced imaging technolo-gies. He has over 80 technical papers and one bookpublished in the related fields. In 1995, he solelyfounded and served as CEO of Genex TechnologiesInc, a Maryland-based U.S. company specializedin advanced 3D/360-degree imaging and displaytechnologies and products. He has served on reviewpanels for National Science Foundation, NationalInstitutes of Health, and US Army Medical Research

Commands. He taught as adjunct professor in George Washington University,Washington, DC, and New Jersey Institute of Technology, Newark. He isinventor of 20 issued patents and over 20 patent applications.

Dr. Geng received the “Rising Star” award and ranked #291 by Deloitte &Touché on the lists of Fast 500 Growing companies in US and Canada. He alsoreceived prestigious national honors, including the Tibbetts Award from U.S.Government and was ranked #257 as INC magazine’s INC 500 company in2002. He was honored by DARPA as one of the 200 top scientists in USA asthe “Scientist helping America”. He currently serves as the Vice President forIEEE Intelligent Transportation Systems Society (ITSS) and is the chairman ofITSS standard committee and ITSS publications committee.

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