LESIONS WITH ACCELERATED PARTICLES 329 



the brain (Tilsjar-Lentulis and Tobias, 1959) are only examples of studies 

 currently active. 



One of the major tools of the neurological research worker is electrical 

 stimulation. We have shown during the last few months that radiation, too, 

 can be used for stimulation of nerve activity. For some years it has been 

 clear that specialized structures, such as the retina, can be stimulated with 

 penetrating radiations. Hug (1960) only a year ago demonstrated that ten- 

 tacles of the snail can "feel" the presence of x-rays. Conard (1956) has 

 studied the effect of radiations on contractions of parts of intestine in vitro. 

 In our laboratoiy (Tobias et al., 1961) pulsed beams of alpha particles were 

 used on the cornea to elicit the so-called "corneal blinking reflex." It is clear 

 that deep stimulation of structures of the central nervous system by heavy 

 ion beams is a distinct possibility. Since the diameter of such beams can be 

 made very small, and pulsing of the beams can be accomplished in various 

 ways, a distinct possibility exists that in the future one might be able to 

 scan limited regions of the brain surface with a deflected beam of penetrat- 

 ing particles, delivering a "message" to certain cells in the brain by a pre- 

 arranged spatial and temporal code, without the trauma of surgical 

 intervention. 



Physical Techniques of Producing Lesions 



The physical characteristics of accelerated nucleons have been described 

 in detail (Tobias ct al., 1952; Larsson et al., 1956; Birge et al., 1956; 

 Brustad, 1961). Here we wish to give only an outline of those properties of 

 radiobiologic importance. Usually an attempt is made to produce an almost 

 parallel beam of particles in an accelerator. This task cannot be accomplished 

 perfectly because in the course of acceleration in a cyclotron, radial and axial 

 oscillations occur; in a linear accelerator there are also "defocusing" effects. 

 In a linear accelerator it is possible, with the aid of electromagnetic focusing, 

 to have almost all particles emerge from the machine in a single spot of 

 perhaps 1 mm in diameter and within 0.1% monoenergetic. In the cyclotron, 

 the particles emerge from a nonhomogenous magnetic field, and they are 

 not parallel. In this instance one may focus them to obtain a slightly con- 

 vergent or divergent beam, and one usually applies a slit system to select 

 particles traveling nearly parallel. This way beams of about 0.1 degree 

 angular divergence can be obtained, but a great part of the beam which does 

 not satisfy the criteria of parallelity is not actually used. 



The monoenergetic particles penetrate nearly the same distance in tissue. 

 Figure 1 gives the range energy relationship in water for protons, alpha 

 particles, and for heavier ions as a function of the kinetic energy per 

 nucleon. Also, on this figure are indicated the actual energy and range values 



