JAERI-Conf 95-021

PB06
3D Visuali zation of Fast Light Emissio n Phenom ena by Dynam ic CT
Mikiko AIDA,

Takahiro KOZAWA, Toru UEDA, Toshiaki KOBAYASHI,
Mitsuru UESAKA and Kenzo MIYA

Nuclear Engineerin g Research Laboratory , the University of Tokyo,
22-2, Shirakata- shirane, Tokai-mur a, N aka-gun, Ibaraki, JapanX COS 0 +where r

Abstracty sin 0, S =-X sin 0 +y COS 0.

fan beam
rojection

3D visualizati on of Cherenkov radiation in water generated by an electron beam from the 28 MeV linacof University of Tokyo was performed . We introduced the light emission computed tomograph y theory. We adopted the fan beam projection using arotating mirror, a collimator , an optical fiber and aphoto-mul tiplier. Cherenkov radiation passing throughthe collimator enter a photo-mul tiplier via an opticalfiber. The optical fiber should be shielded against X
rays. At the first stage, we succeeded in reconstruc ting the image of distributio n of Cherenkov radiationin water, with the nanosecon d time resolution.

yS

opticalfiber!.Introduc tion
Recently, Computed Tomograp hy( CT) is progressin gremarkabl y in the field of the diagnostic technology
and medical treatment technology. Dynamic lightemission CT is the way of 3D visualizati on of a luminous body using the algorithm of CT. For now, it
had obtained the distributio n of the emission that wasemitted from a fluorescent tube or a discharge tube,
with the millisecond time resolution. The purposeof this research is to enhance the time resolution ofthis CT and to get the dynamic image of distributio nof light emission phenomen a in nano- and picosecond time domains. At the first stage, we measured
Cherenkov radiation in water generated by an electron beam from the linac of University of Tokyo, andreconstruc ted the image of distributio n of the emission by the light emission CT theory.
2.Theory of CT
In order to obtain the CT image of a body, we need toget projection data in all directions. Assuming thatf(x,y) is the distributio n of light emission from thebody, the projection data in the Bdirection, p( r, B), in
the r-s coordinate which rotate e angle from the x-y
coordinate are given as follows (see Fig.l),p(r,B)==

I:parallel beam
projection
Fig.l: Parallel beam projection and Fan beam
projection
This equation is called the Radon transforma tion. Applying this transforma tion to the angle range 0 ::; e <211', the projection data are made. This projection
method is called the parallel beam projection (solidlines), while there is another method, the so-called
fan beam projection (broken lines). To reconstruc t theimage from projection data, we backprojec t the projection data in all directions. Actually we perform
Radon inverse transforma tion discretely. There arethree methods for the backprojec tion such as the 2DFourier transform method, the filtered back projection method and the convolutio n method. Here, we
use the convolutio n method since we can choose anappropriat e filter function to get a clear image. The
projection data are multiplied by filter function h(r)and we carry out the convolutio n to get corrected projection data. We can reconstruc t the image of f(x,y)
by backprojec ting this corrected data (l).

f(x,y)= 171: jrmax p(r,B)h(x cosB+ysi nB-r)drd Bf(r cos e- s sine' r sine+ s cos B)ds

1:1:

0

f(x,y)o(x cosB+ys inB-r)dx dy-263--rmax= L LP(n~r, m~B)h(n~r)
mn

JAERI-Conf 95-021

We use the Shepp and Logan filter function as follows,h(ntlr)
3.Ex:perimental setup
The experimental setup is shown in Fig.2. The energyof an electron beam is 28MeV, macro pulse width is
2.5ns, electric charge is 3.2nC/pulse. The electron
beam passed through a glass tube. The glass tube
is filled with water and the water is shielded by Ti
windows at the both ends. When the electron beam
passes through it, we take Cherenkov radiation in water from the side of the glass tube through a collimator. The light signal passes through the collimator,
an optical fiber, a photo multiplier with multi channel plate(MCP-PM P), and is measured and processed
at a transient digitizer. The diameter and the lengthof the glass tube are ¢50mm and 175mm, respectively.
The inner diameter ofthe collimator is ¢1mm and theouter diameter and the length of the fiber is ¢600J.lm,and 10m respectively. We applied -2kV to the photo
multiplier. The x:yz coordinate is defined as shownin Fig.2. The measurement plane is vertical to thez axis, and we carried out the fan be~m projection
by using the rotating mirror. Then, we move the optical equipment along the z axis, to obtain the datafor 3D reconstruction. The mirror area is a 18mmsquare, and the mirror is fixed and centered at theaxis of the rotation stage to reflect the light into thecollimator. By using this mirror, we could avoid thatthe optical fiber faces directly, the water tube whereX rays are generated, so as to reduce the light noise
generated by X ray irradiation in the optical fiber. Bytaking account of axisymmetry of the water tube, wedidn't scan the collimator in the azimuthal direction
and scanned the mirror only in the range of 24° at
a half side of the fan beam projection. The step angle is 3°, and the measurement plane is changed bylOmm step from the point of z=19mm to z=119mm.
Wemeasured the light noise data by cutting the visible light at the entrance of the collimator by a paper
beforehand and subsrtucted them from all measuredata.

4.Result
Because of axisymmetry of the configuration, we measured only the least necessary data, and assigned themto 31 fan beam projection data in the mirror angle
range of ±45° at 72 azimuthal angles around the water tube. Under this assumption, we calculated thereconstruction image. We show the reconstruction
image of the distribution of Cherenkov radiation inwater as shown in Fig.3. It is the view in the x:-ycross section at z=29mm After we reconstructed in
the x:-y image at each z position, we integrated themwith respect to the x: coordinate and time, the y-z image as shown in Fig.4. For comparison, we show thepicture of Cherenkov radiation taken by a CCD camera as shown in Fig.5. It looks that strongly radiatedarea in the reconstruction is more widely distributed
than in the photograph. It can be attributed to lackof spatial resolution in the x:-y plane due to the large
radius of the the optical fiber.

0

Fig.3:x:-y cross sectional image (z=29mm)
1175mm0
Cherenkov radiation

Triggercollimator

Fig.5:Picture of Cherenkov radiation in water

Fig.2:Ex:periment setup-264-JAERI-Conf 95-021

Next, we show the time variation of Cherenkov radiation in the y~z plane at x=O as shown in Fig.6(a),.....(c).
Fig. 7 shows the time variation of maximum projection
data at z=19mm. It is clear that the electrons lose
their energy near the entrance of the electron beamand the resultant emission area is localized there.

In this experiment, we manually positioned the mirror and other optical equipments and it is thoughtthat there are rather position errors. But now we are
manufacturing a automatic measurement system asshown in Fig.8. The system has a column made of Ph
to cover the body to shield the optical fiber against
X. rays, and the column has a small window for light
extraction into the collimator. It rotates keeping thewindow to face the mirror. By using this system, wecan get fan beam projection data with high spatial
resolution quickly. Now, because we use the photomultiplier as the detector of light, the time resolution is limited to nanosecond, but we will introduce
a streak camera to enhance the time resolution to picosecond soon.
rotative stage motor
X ray shield (Pb)0(a):t=l.Onsmeasured
bod0\·.... ·• ···::::\)?;,

one axis/stage motor

Fig.8:3D Automatic Measurement System(b ):t=2.0ns(peak )5.Conclusion

0(c):t=4.0ns
Fig.6:Time variation of Cherenkov radiation in they-z plane.

(bA 28 MeV electron beam generated by the linac passed
through the glass tube filled with water. Then we
took the Cherenkov radiation in water by the collimator with the rotating mirror, the optical fiber and thephoto multiplier. Using the measured data, we calculated the reconstructed image of each section which
is vertical to the beam line by the light emission CTtheory. Finally, then we succeeded in getting the 3D
image of the radiation distribution with the nanosecond time resolution. We used the convolution method
for numerical reconstruction where the Shepp and Logan filter function is used. Hereafter we attempt toenhance both spatial and time resolutions by introducing the 3D automatic measurement system andthe streak camera.

(a0.0

Reference0.0 2.0 4.0
1.0{l)Tsuneo SAITO : "Image Processing Algorithm",0 hmikagakusha, 1993.

[ns]

Fig.7:Time variation of maximum projection data at
z=19mm-265-