Universe to Human Brain Keiji Tanaka Et Al. L~ of RIKEN Frontier Research System Have Studied Visual Feature Recognition In

No. 1] Proc. Japan Acad., 71, Ser. B (1995) 1

From Universe to Human Brain

By Minoru ODA, M. J. A.

(Communicated Jan. 12, 1995)

Abstract : A possible non-invasive method of microscopic imaging of X-ray sources included in the brain is described. The principle is basically similar to the Fourier-Transform-Telescope developed for imaging celestial X-ray sources.

Key words : Brain; microscopic imaging; X-ray sources.

Keiji Tanaka et al. l~of RIKEN Frontier Research discovery of its optical counterpart. System have studied visual feature recognition in the The device usually consists of two grids, sepa- monkey's brain with an embedded probe in the cortex. rated by a certain length. An X-ray detector placed It was concluded that the minimum spatial dimension behind the two-layered grids measures the integrated of the brain activity, the column, is approximately 0.1 X-ray flux which represents a spatial Fourier compo- mm. nent of the source structure transformed with the What about for human brain? Of course we can not spatial wave-length determined by the pitch of the embed a probe. But with some non-invasive method it grid. was learnt that with the activity of the brain the blood In the late 1970s the modulation collimators were flow and/or de-oxidization of hemoglobin concentrate flown aboard the balloon at high attitudes and were at certain regions. As for the non-invasive probing into used to produce the X-ray image of the Crab Nebula the human brain, techniques of multiple scattering of from spatial Fourier components at various position light, PET, MRI and the measurement of very weak angles of the Nebula. Also, rotating modulation magnetic field with SQUID have been developed, and collimators had been utilized aboard X-ray astronomy the concentration of the blood flow has been identified satellites, "Hakucho" (launched on 1979) and "Tenma" to a scale of several mm.2~ (launched on 1983), and Solar X-ray astronomy satel- If the spatial resolution of the non-invasive prob- lite "Hinotori" (launched on 1980). (Fig. 3.) The ing is improved, a breakthrough in bridging between rotating modulation collimator produces Fourier com- high level activity of the brain or, say, the mind and ponents for a number of spatial wave numbers and the physical observation may be achieved. In the position angles by which an X-ray image of the sources following, a proposed instrument, which may be called may be reconstructed. "Fourier -Transform-Microscope" (FTM) is described . The Fourier-Transform-Telescope (FTT) was It is a complex structure of multiple modulation realized as a Hard X-ray Telescope (HXT) on board collimators to construct an image of the X-ray source the Solar X-ray satellite "Yohkoh" (launched on 1991), produced in the brain. total sensitive area of the X-ray detector being One often sees the Moire's fringe pattern through approximately 70 square cm. The HXT consists of 64 overlapped lace curtains, grid plates, mesh screens elements of the modulation collimator of different etc. The modulation collimator3~ was conceived as a spatial wave numbers and position angles which device to determine the location, the size and the produce 64 Fourier components on the Fourier u-v shape of celestial X-ray sources from how the sources plane. (Fig. 4.) With the HXT, being aided by the are seen alternatively through the Moire's fringe Maximum Entropy Method (MEM), the X-ray images projected on the sky. (Fig. 1, Fig. 2.) In early period of of typical Solar flares have been produced with 0.5 sec X-ray astronomy, with this device the X-ray source of time resolution and approximately 5 aresecond of was found to be as localized as a "star" and the precise spatial resolution for four energy bands of X-rays. location of bright sources, e.g. SCO X-1 led to the (Fig. 5.) 2 M. ODA [Vol. 71(B),

Fig. 1. Principle of the modulation collimator. Layers of the grid produce the Moire fringe pattern. Transmission with respect to the angle is illustrated.

Fig. 3. The illustration indicates the principle of rotating modulation collimator.

The concept of the FTT may be transferred to the Fourier-Transform-Microscope (FTM) to produce the microscopic image of X-ray or soft gamma ray sources. Modification of the structure of the modulation collimator may be necessary. (Fig. 6.) Unlike the case of FTT producing 2-dimensional X-ray images of astronomical objects at essentially infinite distances, for FTM the sources are of 3-dimensional at near distances. The difference may cause some difficulties. Prof. Kazuo Makishima of the University of Tokyo and his colleagues are undertaking laboratory simulations utilizing an optical systems to investigate the situa- tion. In order to achieve the spatial resolution higher than 100 µm or 10 µm, the advanced manufacturing techniques of the fine grids, like lithography, has to be accommodated and also the techniques of the align- ment of the grids has to be devised. A variety of possible methods of producing the X-ray or soft gamma-ray sources in the brain is represented in Table I and Fig. 7. First, as is utilized for the diagnosis of the heart, radioisotopes like 2o1T1, 123J , 133Xemay be introduced into the blood vein. They Fig. 2. Qualitative illustration of the principle of the radiate X-rays of energy of several tens to over 100 Fourier-Transform-Telescope (FTT) with multi-pitch 2- KeV which may be observed from outside with layer modulation collimator. essentially negligible magnitude of scattering through No. 1] From Universe to Human Brain 3

Fig. 4a (left). A design of multi-pitch and multi-position-angle grids. Fig. 4b (right). Fourier components obtained with the multiple modulation collimators (Fig. 4a) on the u-v plane. the body. oxygen atoms. The muonic atoms then generate hard Secondly a muon beam facility which is under KX-rays. The muonic oxygen atoms generate charac- construction at Rutherford-Appleton-Laboratory in teristic 133 Kev KX-rays. England under the collaboration between the Labora- The third possibility is to utilize X-rays to produce tory and RIKEN being led by Prof. K. Nagamine may fine shadows of blood veins, which contain Iodine, at be utilized. The region in the brain to be observed is just above and below the KX-ray-edge of Iodine: The irradiated with low energy minus-muons which replace difference of the shadows enhance the fine image of the the atomic electons of oxygen atoms producing muonic vein. As the X-ray emitting source, "SPring-8" i.e. a

Fig. 5. An example of the X-ray imago of solars flares obtained with FTT (lower right for hard X-ray together with contour image for visible light) and with an X-ray telescope (upper left for soft X-ray) on board Solar X-ray Astronomy Satellite YOHKOH. 4 M. ODa [Vol. 71(B),

Fig. 7. Illustration of FTM for human brain as stated in the text.

synchrotron radiation facility with 8 Gev electron ring located in Hyogo-prefecture, Japan, may be used. Fig. 6. Design of a unit modulation collimator for FTM. Question of dosage, when radioisotopes are introduced into blood and a part of the brain is irradiated by muon beams or X-ray beams, has to be carefully Table I investigated in relation to the clarity of the image and the time of exposure i. e. temporal resolution. I wish to thank Prof. Masao Itoh and Keiji Tanaka of RIKEN who inspired my interest in neuro- physiology. I also express my gratitude to Prof. Claude Canizares of Center for Space Research and other friends at MIT who offered generous hospitality and encouragement during my short stay at MIT in May, 1994 where I developed the concept described in this note.

References

1) I. Fujita, K. Tanaka, M. Ito, and K. Cheng (1992) Nature vol. 360, no. 6402. 2) e.g. M. Raichle, Scientific American, April 1994. 3) M. Oda (1965) Applied Optics 4, 143. 4) M. Oda (1968) Space Science Reviews 8, 507.