A cyclotron microbeam and its application to biomedical samples

Nuclear Instruments and Methods North-Holland. Amsterdam

A CYCLOTRON Hiroko

in Physics

MICROBEAM

KOYAMA-ITO,

The Institute of Mtdicnl Scirmr.

Eiko

Research

AND WADA

B22 (1067)

ITS and

APPLICATION Akira

205

205-209

TO

BIOMEDICAL

SAMPLES

IT0

The Univrrsity of Tokyo. Minato-ku. Tokyo 108, Japan

The experimental setup to produce a cyclotron microbeam of 27 MeV u particles and 25 MeV protons is described. Using an ordinary doublet of quadrupolc magnets with an aperture diameter of 2.5 in. as a focussing lens, a beam line with demagnification factors D,., = -3.3 and D,,, = - 16 was constructed. A beam spot with a diameter of less than 15 km fwhm and an intensity of 0.06 nA was obtained for an extracted beam current of 300 nA. The experimental result is compared with the result of ray trace calculations. The microbcam of 27 MeV LYparticles is applied to the PIXE positional analysis of biomedical samples and that of 25 MeV protons is used to measure the density distributions in the samples based on the computed tomography (CT) principle. The experimental results of microPIXE and proton CT measurements on the eye from a mouse with a hereditary cataract are presented

1. Introduction The IMS cyclotron (CS-30. TCC Corp., USA) produces beams of 27 MeV cy particles and 25 MeV protons etc. Previously, the focused beam of 27MeV cy particles has been used for PIXE positional analysis with a lateral spatial resolution around 100 pm [l-3]. It was impossible to obtain a smaller beam spot due to the poorly aligned quadrupole magnet then used. So, the beamline setup is rebuilt to produce a 20 km beam spot using a well aligned doublet of quadrupole magnets. The experimental results are compared with the results of ray trace calculations. The microbeam with the improved lateral spatial resolution is now applicable to measure the distributions of minor elements in a mouse ocular lens with a diameter of 1-2 mm. The 25 MeV proton beam, focussed using the same beam line setup, is applicable to proton computed tomography (CT) to measure the density distribution in a biomedical sample with a diameter smaller than the range of the beam. Since the CT measuring system of the first generation type was developed as reported previously [4]. it has been applied to several biomedical samples.

2. Microprobe 2.1.

and its application

Beam line setup

The layout of the beamline setup with the calculated beam path is shown in fig. 1. The upstream collimator defines the object beam size. Both the position and the aperture size can be changed by four microscrews. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

The second collimator limits the beam divergence acceptance. The collimated beam is then focussed on the target by a doublet of quadrupole magnets with an aperture diameter of 2.5 in. (Varians Corp.). The object and image distances are set to 55.5 and %cm, respectively. Computer calculations were performed for this beam optics using a ray trace code [S]. The obtained demagnification factors and aberration coefficients up to the third order are given in table 1. where notations by Grime et al. [6] are adopted. that is, the expanded coefficients of the image beam size (RHS) by the incident beam parameters (LHS) are tabulated. The z axis is taken along the axis of the quadrupole magnets. The parameters of the beam divergence, 0 and 4, are the angles of particles projected on the x-z plane and y-z plane. respectively. The coefficients used in the expansion of the fringing field falloff of a quadrupole magnet were obtained by fitting the measured field values along the optical axis to the formula in the ray trace code. The results for the realistic magnetic fields with fringing fields are compared with the uniform fields with an effective length of 14.68 cm (sharp edge) specified in the manual. Except for the spherical aberration coefficients, the other coefficients are almost unchanged. The effects of these aberrations on the beam size are tabulated in table 2 for an input beam divergence of 1.5 mrad and the assumed beam parameters. Beam line parameters are adjusted by observing the beam spot on a plastic scintillator 50 )*rn in thickness by a x50 microscope from behind. A pair of magnetic coils installed at the exit of the quadrupole doublet are used to scan beams i.500 )I-m both vertically and horizontally. For a wide range scanning in air. a mechanical x-y stage is used. II. BIOLOGICAL/MEDICAL

APPLICATIONS

H. Koyama-lto et al. I Application of a microhram

206

c 221rm

4

to biomedical samples

555c.m

1

6xlmml

Fig. 1. Layout

Table 1 Calculated

aberration

coefficients”’

of the beam

for the IMS microprobe

optics

and the beam

system. Quadrupole

Coefficient

Type

path.

Sharp

field

edge

Fringing

Demagnification

-3.3 - 15.7

-3.2 -15.9

Chromatic

68 65 -8.4 8.4 -41 41 -184 49 47 ~ 177 -0.5 -1.0 -0.2 -1.4

6X 65 -8.5 8.5 -42 42 -187 51 48 -178 -1.5 -17 -3.8 -7.7

Parasitic,

rotation

Parasitic,

excitation

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0, + in mrad; 6=percentage change in particle change in excitation of quadrupole n

energy<

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of quadrupole

n about

optical

field

axis.

H. Koyama-lto et al. I Application of N microbeurn to biomedical samples In the analysis in air. the beam is extracted into atmosphere through a 7.5 km thick kapton film. The beam spot observed by the microscope is not so much blurred as long as the distance between a focus point and the exit film is kept less than 5 mm. Fig. 2 shows the beam profile under well adjusted conditions, which was obtained by scanning the cy beam across a cross of 13 Pm o.d. wolfram wire in vacuum. The counts of the X-rays emitted at 135” with respect to the beam arc plotted as a function of the beam position. From the figure the diameter of the beam is estimated to be smaller than 14 km fwhm and 42 km fwtm, which is almost consistent with the calculations in table 2. The typical beam intensity was 60 pA for the extracted LYbeam of 300 nA, which is sufficient when the distributions of minor elements in a thick sample are measured. As the cyclotron itself is capable to produce an extracted LYbeam of up to 30 PA. it is feasible to increase the intensity of the microbeam if necessary. 2.2. Application cataract

to the mouse

eye of a hereditary

The external microprobe of an (Y beam is used to measure the distributions of minor elements in mouse eyes. The excised eye, which is mounted on a sample holder with the aid of an embedding compound, is frozen in isopentan cooled by liquid nitrogen. Then, each sample is sectioned by a cryomicrotome until the sagittal plane of the eye, which includes the axis of lens, is exposed. At the time of beam analysis, the sample holder which was stored in liquid nitrogen is transferred to a frozen stage. The stage is cooled by circulating liquid nitrogen and is encapsuled in a box

100

Object

207

Table 2 Effect of aberrations on beam size”‘. Type

x I&m)

Chromatic Parasitic, rotation

(xl&?) (xl&,) (xl&?) Parasitic, excitation (XI&,) (X[HF?) Spherical”’ (x10’} (x10$‘)

Y (ml 20.3 -1.3 I.3 -8.3 2.2 -5.1 -57

(YIN)

(Yl@,) (YIQJr) (Yl@,) I’i@:l,’ (j:i8’6)

‘I)For H=+=l.S mrad. Assumed parameters: 0.2%~; ~,=~~=0.03%; ~,=p~=i).l mrad. ‘) With fringing field value.

19.5

-6.1 6.1 2.1 1;; -I3 #=AE/E=

with a window of 1.5 km thick mylar film. The box is fixed to a mechanical x-y stage. The sample thus prepared and kept at low temperature is considered to maintain the intact distributions of elements. The surface of the sample is set perpendicular to the beam. and the X-rays are detected in the 135” direction to the beam. The diameter of a mouse eye is around 2mm, so the sample is thick enough for the PIXE analysis of the minor elements in the lens, such as sulfur, chlorine, potassium and calcium. Since the cy beam is energetic, the longitudinal spatial resolution is determined by the penetrability of the emitted X-rays. Fig. 3 shows the minor elements in the saggital plane of the eye from a mouse of a hereditary cataract (BALBcica-caica) as compared with a normal one (BALc). The age is four months and the lens in vivo was opaque. In the opaque lens, calcium is found on the surface around the equator. Its maximum concentration is estimated to be more than 1 wt.% as esti-

:

FWTM=44pm

-40

-20

0

20

40

-40

Horizontal Position (WI)

-20 Vertical

0 Position

20

40

(pm)

Fig. 2. Measured beam profiles. II. BIOLOGICAL/MEDICAL

APPLICATIONS

H. Koyama-lto

208

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able limit. The distribution of sulfur is considered to b) reflect the distribution of the protein in the lens, which ,,5 ~~~i~~~~ijlr:,:l.;.~:,;~~ is to be compared with the result of the density

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and its application

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The system for proton CT has a spatial resolution of less than 0.1 mm and an integral density resolution . . ,,.. ,. ..‘.... +:~>~>:‘.‘.:.,’ : better than 0.1% as reported earlier 141. The excised >... . ,..‘y:‘.:: : .‘: ‘.’ -..r-.- ........‘. .:” ,. >:, :_>’ :;: ..,.~.,._ ;: ,. . . . . : .: .,.. ..,. eye is put into a cylindrical sample holder filled with .::.:.:j: :~~.‘..,‘.. .*....,., .’ :. . .,.., ,,,: ;+;*; ‘.. .‘.‘,:‘.:-:....‘., >, ..; :; : ‘. .I%>...: ,, ,: silicon oil for the measurement. The holder is made of ;;y polystyrene, and has an inner diameter of 4 mm and a .. . .. . .. .. . .:>.,: . . . Ij; : : ....... . > .,;: .:.,.:..;: ... .....~‘...~.‘..,.. ‘...’ ... . . :. wall thickness of 0.25 mm. In fig. 4, proton CT images ..:riiiiiliil:ii:ir~~~~~~~~~~~~~’ ::, ‘y;;;;,;,:i.‘,::.,,:::;1.~~~;~~~~~~~~~:::~~~~~~ 1 1: of mouse eyes which show the density distributions in :: ,..: ,: ; .: ..‘.’ .., ,? :.::2:,.*:::.: :_ ,:.~,:::~, ,;, ; :. .. ... .,,. _.:. :: .::‘..: .., ., .: the central slices are shown. A normal mouse is compared with a mouse of a hereditary cataract. The outer circle in the image derives from the wall of the holder. The linear scan of the density along the optical axis of the eye, indicated by a straight line, is overlapped on the image. The central high density circle is the image of the lens. The mouse is of the same strain with the same age as the one used in the previous example in fig. 3. The measured density distributions are consistent with the distributions of sulfur. It is to be noted that in the opaque lens, the density de1 2mm creased in the periphery compared with normal one. The present system of the first generation type is Fig. 3. Distributions of sulfur, potassium and calcium in the now being revised to the third generation type in order saggital plane of the frozen eye, (a) from a normal mouse, (b) from a mouse of a hereditary cataract. The dark area indito shorten the measuring time by two orders of magcates higher concentrations of each element. nitude. In the new system, the linear scans are eliminated by utilizing fan spread beams and a position sensitive detector system which consists of a pair of mated from relative normalization by the continuum microstrip detectors for positional analysis and a thick part of the spectrum between 4.5 and 6.0 keV. In the SSD for energy analysis. normal lens, the level of calcium is below the detect/

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and (b) is from

a mouse

of a hereditary

cataract

H. Koyama-lto et al. I Application of a microbeam

to biomedical samples

209

4. Summary

Acknowledgement

A 20 km beamspot of a rather high energy beam from a cyclotron was obtained using an ordinary doublet of quadrupole magnets. The microbeam is now utilized to microPIXE and proton CT analysis. The above examples demonstrate that microPIXE with a special resolution of a few tens of km is an appropriate tool for measuring the distributions of elements in mm range. Proton CT is a powerful technique of density analysis since it is nondestructive and threedimensional. The changes in the elemental distributions and in the density distribution are both specific phenomena which occur and are associated with the appearance of opacity in an ocular lens. Therefore, the application of the both techniques to the lens is considered to be important in the study of cataract. An attempt to improve the spatial resolution in microPIXE, and so to shorten the measuring time in proton CT is the next to be done.

The authors acknowledge Mr. Y. Kashiwa and Mr. H. Takasaki for their help in the whole course of the experiments.

References H. Koyama-Ito, A. Jahnke,

E. Wada, T. Tsumita and T. Yamazaki, Nucl. Instr. and Meth. 166 (1979) 595. A. Jahnke, T. Shinmmen, H. Koyama-Ito and T. Yamazaki, Chemosphere 10 (1981) 303. H. Koyama-Ito. E. Wada, T. Tsumita, M. Horiuchi and S. Iwata. Nucl. Instr. and Meth. B3 (1984) 625. A. Ito and H. Koyama-Ito, Nucl. Instr. and Meth. B3 (1984) 584. H.A. Enge and S.B. Kowalski, 3rd Int. Conf. on Magnetic Technology (1970) and private communications. G.W. Grime, F. Watt, G.D. Blower J. Takacs, Nucl. Instr. and Meth. 197 (1982) 97.

11. BIOLOGICAL/MEDICAL

APPLICATIONS