U.S. patent number 3,576,997 [Application Number 04/762,051] was granted by the patent office on 1971-05-04 for particle accelerator employing a revolving electric field for generating x-rays.
This patent grant is currently assigned to Intelligent Instruments, Inc.. Invention is credited to Peter E. Slavin.
United States Patent |
3,576,997 |
Slavin |
May 4, 1971 |
PARTICLE ACCELERATOR EMPLOYING A REVOLVING ELECTRIC FIELD FOR
GENERATING X-RAYS
Abstract
A stereoscopic image is obtained from an X-ray scanning
instrument in which the patient is moved along the axis of a
rotating X-ray source with an X-ray receiver located oppositely the
source and rotating with it to generate a helical scanning motion
with respect to the patient. The X-ray receiver generates
electrical signals which are recorded on magnetic tape and the tape
is employed to operate a stereo video system. A compact particle
accelerator employing a revolving electrical field is provided for
producing highly directional and monochromatic X-ray beams.
Inventors: |
Slavin; Peter E. (Cambridge,
MA) |
Assignee: |
Intelligent Instruments, Inc.
(Woburn, MA)
|
Family
ID: |
25063973 |
Appl.
No.: |
04/762,051 |
Filed: |
September 24, 1968 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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469505 |
Jul 6, 1965 |
3432657 |
|
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Current U.S.
Class: |
378/137; 378/143;
315/505; 250/493.1; 976/DIG.444 |
Current CPC
Class: |
H01J
35/04 (20130101); H05H 13/00 (20130101); G21K
5/10 (20130101); A61B 6/032 (20130101); A61B
6/4085 (20130101); A61B 6/503 (20130101) |
Current International
Class: |
A61B
6/03 (20060101); H01J 35/04 (20060101); G21K
5/10 (20060101); H01J 35/00 (20060101); H05H
13/00 (20060101); G03b 041/16 () |
Field of
Search: |
;250/49.5 (O)/
;250/97,90,103,93 ;313/55 ;328/233 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Birch; A. L.
Parent Case Text
REFERENCE TO RELATED APPLICATION
This is a division of U.S. Pat. No. 3,432,657 filed July 6, 1965
and entitled "X-Ray Scan."
Claims
I claim:
1. A particle accelerator comprising:
a. at least two pairs of pole pieces arrayed about a closed path
along which particles are accelerated,
b. an evacuated housing enclosing at least said path,
c. a particle source within said housing,
d. a conductive member disposed within said housing, at the axis of
said path and of a polarity repulsive to said particle,
e. means for applying alternating voltages to said pole pieces, the
voltages for one pair being out of phase with the voltages for
another pair to produce a revolving electric vector for
accelerating said particles about said path,
f. means for generating a DC magnetic field perpendicularly through
the plane of said path to maintain said particles in said path,
g. means for generating a variable magnetic field perpendicularly
through the plane of said path to provide stability to said
particles within the orbital plane, and,
h. means for generating a DC electrical field between said member
and the periphery of said path to provide stability of said
particles within the orbital plane and to limit the size of said
path.
2. An accelerator according to claim 1 including a target disposed
across said path, said target being of a material adapted to
generate X-rays upon impingement by said particles.
3. An accelerator according to claim 2 wherein said target
comprises a relatively thick stratum of aluminum formed with an
opening therethrough in register with said path and a relatively
thin stratum of gold mounted to one side of said aluminum and
covering said opening.
4. An accelerator according to claim 3, including a stratum of
silicon mounted to said target for filtering said X-rays.
5. An accelerator according to claim 2 wherein said target
comprises a matrix and a plurality of uranium atoms dispersed in
said matrix.
6. An accelerator according to claim 2 wherein said target
comprises a relatively thick stratum of gold and a relatively thin
stratum of molybdenum.
Description
This invention relates generally to X-ray equipment and to the use
of X-rays to yield information, usually pictorial in content, on
the internal structure of objects such as the absorption structure
of objects within a human body, for example. More particularly this
invention is directed towards improved methods and apparatus for
yielding more precise information to the examiner, presented in a
more graphic manner, more suitable for control by the examiner,
with less of a radioactive dose to the patient, and with a lower
cost of storing this information.
Present X-ray methods and equipment in the medical field, and
usually in the nonmedical field also, "flood" the body to be
examined with X-rays from one direction. The rays then pass through
the body, are partly absorbed, and finally impinge on a film. This
film after `developing` yields an image (the shadow graph) which is
related to the internal structure--viz. the bones, the organs, etc.
In recent years some work has been done with replacement of the
film by a fluorescing screen (image intensifier), which is then
scanned by a television camera to give a television-type image in
place of the film image.
This invention differs from equipment and techniques presently in
use in that it embodies:
1. A transmitter producing a fine beam or ribbon of X-rays which
transmitter is adapted to move in a circular or elliptical path as
the body under examination moves along a straight line along the
axis of the rotating transmitter.
2. A receptor for the X-rays which emerge from the body, this
receptor rotating directly opposite to the transmitter, and
producing an electrical signal (or signals) which is recorded on
magnetic tape.
3. A magnetic tape storage system having a more economical (less
bandwidth) specification than that of the television scheme.
4. A visual presentation which has three major characteristics;
namely, better resolution of fine detail, and particularly that of
the scanning center, ability to select the viewing angle from any
one of eight sectors of one complete circumference, and ability to
give depth (stereoscopic) to the image.
While the transmitter in one embodiment is a radioactive isotope,
the invention also includes an improvement in the form of a new
kind of small (tube) particle accelerator which yields energies
which can be controlled in the 20 kev. to 200 kev. (thousands of
electron volts) range. This tube accelerator embodies a revolving
electric field for acceleration and a second (static) electric
field for horizontal stability, and employs operating principles
different from the large-scale (megavolt) accelerators such as
cyclotrons.
Accordingly this invention then also relates to the production of
X-rays, especially monochromatic (one energy) and directional beams
of such rays.
In the nonmedical field there are X-ray beams which are slowly
moved around a material at the center, to give information on
structure of their crystals. Such examination method is different
from the present invention in that:
a. the interest is not an absorption image, but in those angles
between transmitter and receptor where the crystal structure gives
strong "lines" of X-rays in phase, and
b. the information rate is very slow, and is usually recorded
manually.
Accordingly, it is an object of this invention to provide means to
obtain, store and manipulate the information on a body's X-ray
absorption in such a manner as to be able to select direction of
view, to select a fraction of the body's cross section for special
examination, and to be able to command a radiographic image in
depth.
It is a further object to provide means to store this information
on tape in the most economical manner, and to be able to compare
this electrical picture with a similar electrical picture (i.e.
subtraction), and to be able to produce an accurate and detailed
visual picture for as long a time as required by the examiner.
Yet another object is to provide a small particle accelerator, of
low energy level (100 kev.), low cost, and low power
consumption.
Yet a further object is to provide means for the efficient
conversion of these 100 kev. particle energies into X-rays of a
known energy (frequency) and direction; and with the intensity
liable to control.
And another object is to provide a system for producing the maximum
information on the body examined with a minimum of radioactive
dosage (roentgens) to that body.
In the drawings:
FIG. 1 is a view in perspective of an X-ray scanning apparatus made
according to the invention,
FIG. 2 is a view in perspective, somewhat schematic of the system
shown in FIG. 1,
FIG. 3 is a fragmentary perspective view showing details in the
construction of the transmitter and its support,
FIG. 4 is a sectional side elevation of the isotope transmitter of
FIG. 3,
FIG. 5 is a fragmentary perspective view partly broken away showing
details in the construction of the rotor,
FIG. 6 is a fragmentary sectional side view of the rotor
assembly,
FIG. 7 is a fragmentary perspective view of the sensitive elements
in the receptor,
FIG. 8 is a schematic diagram of a modification of the
invention,
FIG. 9 is a schematic diagram of a particle accelerator as employed
in the FIG. 8 modification,
FIG. 10 is a phase diagram for the coils of FIG. 9,
FIG. 11 is a combination perspective view of the accelerator and a
plot of the electrical potential across the maximum diameter of the
orbit,
FIG. 12 is a sectional side elevation of the accelerator,
FIG. 13 a--c are vector diagrams for particles in the
accelerator,
FIG. 14 is a top plan view somewhat schematic of the
accelerator,
FIG. 15 is a side elevation of the target shown in FIG. 14,
FIGS. 16 and 17 are sectional views of two modified targets for use
in the accelerator,
FIG. 18 is a perspective view of another modified target,
FIG. 19 is a perspective view of the crystal lattice in the FIG. 18
embodiment,
FIG. 20 is a sectional end elevation of still another modified
target,
FIG. 21 is a perspective view of yet another modified target,
FIG. 22 is a perspective view of a modification of the invention
with transmitter and receptor rotating in separate coaxial
orbits,
FIG. 23 is a schematic plan view of the FIG. 22 embodiment,
FIG. 24a and b are waveform diagrams of the deflecting voltages for
the CRT used in the FIG. 22 embodiment,
FIG. 25 is a schematic diagram illustrating how effective
magnification is obtained with the invention of FIG. 1,
FIG. 26a, b and c are views showing the different images produced
from the FIG. 25 arrangement,
FIG. 27 a and b are curves showing the response of filters used in
the system,
FIG. 28 (a) is a schematic diagram of an arrangement for storing
information from the X-ray unit onto magnetic tape and,
FIG. 28 (b) shows a modified tape for use with the FIG. 28 (a)
system,
FIG. 29 is a more or less schematic plan view of a system for
producing a stereo image from the X-ray apparatus.
There follows a detailed explanation of the construction and
operation of this invention:
In FIGS. 1--7 there is given the main elements of this system. An
X-ray transmitter 10 is shown mounted on a rotor 12 to revolve
about a center 14 which is within a body 16 to be examined. An
X-ray receptor 18 is mounted to the rotor 12 diametrically opposite
to the transmitter 10. A beam of X-rays 20, which may also be
parallel beams in a "ribbon," passes from the transmitter 10,
through the body 16 at cross section 22, and what is left after
absorption then goes to the receptor 18. The X-ray energy is
converted by the device 24 into an electrical signal. The device 24
may be a scintillation material (as NaI) with a photomultiplier, or
a direct converter (as a p--n junction), or a mosaic such that the
ribbon of X-rays is resolved into, say, seven electrical signals.
The signal or signals then modulate radio frequency (RF), and are
passed via an antenna 26 to a tape recorder 28. This RF link is
needed as the rotating receptor element should not rely upon a
commutator to pass on its signal (s) to the fixed recording
apparatus.
As the transmitter and receptor revolve, the central body 16 such
as a human patient lying on a dolly 29 slowly moves axially, as
shown by arrow 30 along tracks 31. The dolly is adapted to be
adjusted laterally and vertically to position the area of interest
close to the axis. If we assume a ribbon of seven beams each
0.010-inch diameter, then the axial feed rate is 0.070 inch per
revolution of the transmitter-receptor pair. If this revolution
period is from 20 to 50 msec., then the body movement is about 2
inches per second. The scanning beam (or ribbon) generates a helix
with relation to the body.
The information rate on each beam element then corresponds to the
bandwidth needed for the image intensifier--TV camera system; where
the complete image must be scanned in a fraction of a second. This
means a high bandwidth of about 1 MHz. for equivalent resolution.
Since each system gives thousands of `records` each year tape cost
is a principal economic factor, and bandwidth is the major reason
for high tape cost. The length of tape per record will be closely
equal for these two systems as the helical or circular scan is in
action longer; but the TV camera system has to have the tape run up
to a higher speed (say 120 in./sec.) before recording starts.
In the case of a moving organ such as the heart, the effects of the
old and this new system differ. In the old system the outline would
be a trifle blurred owing to movement over a fraction of a second.
In this new system the outline is "traversed" in one-tenth the
time, so it will be sharp, but have an axial component bent in the
direction of motion.
FIG. 2 shows a signal going to the grid 32 of a cathode ray
oscilloscope--similar to a TV receiver tube. The intensity of an
electron beam gun 34 is modulated by this signal, with horizontal
and vertical deflection signals applied to plates 36 and 38. The
horizontal deflection is in synchronism with the rotation of the
beam, and its length corresponds to a 45.degree. arc of this
circle. The vertical deflection is in synchronism with the axial
movement of the body 16.
In FIG. 3 the transmitter elements, which in this embodiment are a
radioactive isotope 40 and a crystal 42, are shown between spaced
walls 44 and 46. These walls of steel provide a radiation shield in
the horizontal direction, and also support tracks or races 48 on
which the transmitter and receptor run. The gamma radiation 50 from
the isotope 40 is absorbed (in part) by the crystal 42, which then
reradiates a characteristic (Bragg plane) X-ray wavelength in a
certain direction 52 towards the body 16. These X-rays do not form
a fine beam, but by use of collimating walls they are eventually
narrowed to such a beam.
In FIG. 4 there is given a side section through this transmitter,
with the isotope 40 and the crystal 42. The entire small assembly
in a case 54 is moving in the direction indicated by the arrow 56.
Absorbing material 58 collimates the X-rays in the plane of the
beam.
For resolution of the order of 0.01 inch, the beam response will
approach 10 KHz. Then, keeping the statistical (time) variation of
decay below noise level, and assuming a minimum of energy focusing
at the crystal, then the disintegrations per second should be about
10.sup.12. An isotope such as cerium-144 having the correct energy
levels and a convenient half-life (290 days per cerium) will give
this rate for a fraction of a mg. Isotope cost would likely be less
than the cost of power consumed by the usual X-ray transmitter of
today.
In FIG. 5 there is shown the relationship between a fixed outer
shell 60 and a rotatable inner ring 62. This inner ring carries the
transmitter 10 and the receptor 18, and moves on bearings 64
between itself and the shell. These bearings may be ball or roller
type. At high revolution rates the force corresponding to
centrifugal acceleration may be of the order of 20,000 lbs. The
body to be examined lies at the center of the ring 62, so that a
driving motor shaft would involve extending the ring to a drum and
yet preserving the shielding. A better solution is to drive the
ring by using it as part of a linear induction motor, or by the use
of the ring as a turbine. This latter arrangement is shown in FIG.
5, where a nozzle 66 pushes a jet of gas 68 (such as compressed
air) against blades 70. It would take a few seconds to get up to
speed, 2 seconds for the X-ray scan; and a few seconds deceleration
with negative power (braking).
FIG. 6 gives a cross section of the sidewalls 44 and 46, the shell
or end wall 60, and the moving ring assembly 62. The transmitter 10
is directly opposite to the receptor 18; both are a part of the
ring assembly; ideally their masses balance. The propelling nozzle
66 is shown coming through the sidewall 46, with the gas jet 68
pushing the ring. The rotation rate during scan need not be exact,
since at 45.degree. intervals a synchronism pulse is placed with
the electrical signal out. This pulse may be generated by
photoelectric or magnetic proximity of the receptor device 24 to
fixed markers such as 72. The receptor converts the X-ray signal
into modulated radio frequency which is then broadcast to a fixed
ring antenna at 74. This telemetry link is relatively simple, as
the distance of the radiated signal is of the order of 1 inch; and
the radiation shielding prevents any RF interference elsewhere.
Shoulders 76 on the sidewalls give additional horizontal
collimation.
FIG. 7 shows a ribbon of X-rays arriving at the sensitive surface
24. Such a ribbon is basically seven beams (78 to 90) spaced about
0.01 inch apart. The surface 24 is a mosaic, each element of which
converts one of the ribbon beams into an electrical signal. There
are two advantages to a ribbon of seven beams as opposed to one
beam. First, it takes less time to examine a certain axial length.
Secondly, each beam can be assigned one channel of an eight-channel
magnetic tape, resulting in storage economy and ease of
manipulation, (as will be shown).
It should be noted that the mosaic elements can be of high
efficiency--scintillation type as sodium-iodide has a relative
(quanta) efficiency of about 2.0. Its response is reasonably flat
over a wide energy range, its response time is in nanoseconds
(10.sup..sup.-9), and its output is linear. In the case of the
image intensifier the image "dot" has to be stored for some
milliseconds (say, as charge on the image plate of the vidicon
camera) before it is translated into an electrical signal. This
means a loss of sensitivity per X-ray quantum, and a nonlinearity
in the camera chain known as the "gamma."
In FIG. 8 there is shown a means for reduction of the dosage
absorbed by the body 16 still further. This system diagram assumes
a transmitter 92 of small mass which is essentially a particle
accelerator and a target. Such a transmitter may be controlled both
for intensity and energy level (in steps) of the emerging X-ray
beam. While it would reduce the radioactive shielding needed, the
mass of the (moving) transmitter would likely increase. This system
will be of special use for cases where dosage is all-important.
In this system emerging (unabsorbed) X-rays 94 are held at a
minimum value, just above the noise level in a receptor 96. This
action is as follows: let us suppose that at a certain instant of
the scan a very small fraction of the X-rays coming in at 98 is
absorbed by the body 16. There the X-ray intensity out at 94 will
rise and the electrical signal out of the receptor 96 and amplifier
100 will also increase. This increase is then fed back negatively
to the transmitter via lead 102, causing the transmitter to
decrease its particle and X-ray density. Then, since body 16
absorbs the same fraction of a lesser incident X-fay intensity, the
dosage rate is reduced. At the same time the error signal which
represents this absorption is sent to a recorder 104. In essence
this is a regulator servoloop. Even if the frequency response of
the loop is limited to 100 Hz. there should be a dosage reduction
of an order of magnitude.
In FIG. 9 there is shown a schematic diagram of some of the
essential elements of a very small particle accelerator. The energy
levels desired are low by nuclear standards; 0.1 to 0.3 of mev., or
less. Pole pieces 106 and 108 produce an electrical field between
them in an evacuated envelope owing to the alternating voltage on
winding 110. Another pair of poles 112 and 114 yield a second field
in space quadrature to the first, and driven by winding 116. The
voltages on these windings are in time quadrature, so that the net
result is an electrical field which rotates in accord with Euler's
law: e.sup.i =cos.theta.+jsin.theta.. If such a field be applied to
deflecting a charged particle (in place of accelerating as here)
the resultant path would be a circle, (such as a Lissajous pattern
on a C.R.T.).
In FIG. 9 a charged particle 118 is subject to accelerating forces
120 and 122, which combine to give a resultant force 124. This
force 124 is tangent to a circle on which the particle is
travelling, the circular motion arising from a magnetic field at
right angles to the plane of FIG. 9.
A pin 126 is at the orbit center, and radius 128 of the particle
path is in proportion to the particle's velocity. For ease of
stabilizing the orbit the particles are "launched" from 130 rather
than start at almost zero velocity at the center 126. At any given
instant there will be a large number of particles travelling on
orbits of different diameters, but (since the periods are closely
equal) travelling in phase with the revolving electric field. In
this respect this is the analogue of the revolving magnetic field
and rotor of a motor. At typical conditions the field frequency
will be 3 MHz. and the orbit diameter for a proton at the kev.
level about 2 inches. In FIG. 10 it may be seen that the wave for
the winding 110 lagging 45.degree. on its peak, i.e. at line 132
gives field 122. Sine wave for the winding 116, leading 45.degree.,
gives field 120. The particle may then accelerate or pick up an
energy of 100 volts per orbit, and after 1,000 orbits, be at the
100 kev. energy level. Relativity effects at this energy level are
negligible; and addition of 0.1 percent to the mass of a proton,
for example. The electrical pole faces are curved outward slightly
to take care of this effect, and to increase stability. Such
systems of charged particles moving in circular tracks are not
uncommon. In nuclear physics the diameter of these tracks is at
least 10 times what this invention proposes; and the energies
sought are in mev. (millions of electron volts) rather than in a
fraction of one mev. Further, as will be pointed out, the
accelerating and control fields are different. Any such system has
three main "stability" problems:
1. Phase Stability; the ability to bring a particle a bit behind or
ahead of the rest of the pack into exact synchronism.
2. Vertical Stability; ensures that a particle vertically off the
plane of rotation is subject to a restoring force. It is important
that the vertical limits of the track be kept close together, as
these limits define the gap in a magnetic field.
3. Horizontal Stability; the center of the circular track should
not drift as the particle spirals outward.
In this invention there are (4) fields; they are (with their
function):
F1. the alternating electric field (of FIG. 9) for accelerating the
particles.
F2. the direct DC electric field (of FIG. 11) for horizontal
stability and limiting the track diameter.
F3. a direct magnetic field 134 of FIG. 12 for giving the
deflecting force to the particle to keep it on a circular
track.
F4. a changing magnetic field 136, 138 and 140 of FIG. 12 to give
vertical stability.
While there are small interactions between these fields (for one,
an energy transfer between the alternating electric and magnetic
fields), essentially the effect on the particle can be taken one
field at a time.
First, consider the accelerating field of FIG. 9 in relation to
phase stability. Let the particle shown at 118 be actually at point
142, i.e. be a bit behind. It is then apparent that the
accelerating force vector 124 at this instant, when placed on 142,
will not be a true tangent; but will point inwards by a few
degrees. That is, the slow particle at 142 will be pushed in a bit
to a smaller radius, i.e. to a lower velocity orbit; and as a
result will tend to "catch up." In like manner a leading particle
will be pushed a bit outwards, and this will enter a higher
velocity orbit where it will tend to fall behind until it is in
exact synchronism.
In FIG. 11 there is shown a perspective view of the orbit, and the
electric potential across the maximum diameter of that orbit. Pin
126 is at the orbit center, and it has a large electric field
(potential difference) between itself and a ring 144. The shape of
this field is given by lines 146; where the maximum 148 is reached
close to the ring diameter. The vector 150 is field potential and
128 is radius. This field is in such a direction that the particle
is repulsed from the central pin. It is then evident that if the
orbit (circle) "slips" in a horizontal direction, the central push
outwards is such as to lengthen the shortened radius on one side,
i.e. to restore the orbit center to the true center, the pin 126.
If the ring 144 potential be kept greater than the accelerating
field DC potential (at 112, 196, 114 and 108) the field maxima will
be close to 148. From stability calculations it can be shown that
the ratio of the maximum orbit radius to minimum should not be
excessive, so that (typically) this ratio will be 10 times and the
particles will be "launched" with an energy of a few kev.
In FIG. 12 there is given a side view of the plane of orbit 152,
with the center pin 126, ring 144 and two positions of a particle
(153, 153'). These are shown between the poles of a magnet which
gives the direct field 134 and the changing field 136, 138 and 140.
The direct field is of uniform strength across the orbit diameter,
while the changing field is bent as shown such that its strength
inside the ring 144 is greatly reduced. This is due to the fact
that the ring has induced in it a current which gives an opposing
field within the ring 144, (though of course the direct field of
134 does not induce any such opposing field).
FIG. 13a applies to a particle on the orbit plane 152 at 153 and
FIG. 13b is for a particle a bit above this plane. In (a) it is
seen that the particle motion 124 (out of the page) and the field
vector 154 are in such relative directions that the (motor) force
vector 156 is towards the center. At (b) there is given the same
particle motion, with a magnetic field component now in direction
158. It is then seen, by rotating the vectors 156 and 154 of (a)
through 90.degree., that a force vector 156 is now towards the
orbit plane, i.e. downwards. This is then in a direction to restore
the particle to the correct plane. For a position below this plane
the correcting force will be upwards since the changing field
component runs the opposite way.
In (c) the particle is at 153', i.e. moving "into the paper." Then
a field vector 158 and motion 124' evidently give the usual force
159 towards center. The FIG. 13 (d) gives motion 124', field vector
160 and correcting force vector 162; derived from the vector set of
(c) just as (b) was from (a). It will be noted that this changing
field is only a fraction of the strength of the initial direct
field 134. If this direct field 134 has one polarity and the
particles one charge polarity, then this changing field must change
in one direction only (that of FIG. No. 12, for example) for the
length of the "scan."
This small particle accelerator is, of course, nothing like the
megavolt units which weigh many tons such as the cyclotron,
betatron, syncrotron, etc. The difference lies not only in size but
in principle of operation, in particular:
1. The central pin giving a repulsing electric field.
2. The ring for shaping the peak electric field and the alternating
magnetic field.
3. The use of a revolving electric vector for acceleration.
For example, a cyclotron has a deflecting magnetic field, but it
does not have the three features above--it makes use of a pulsed
"kik" between two plates to boost the electron velocity.
A betatron uses a varying magnetic (cross) field for acceleration;
it has poor horizontal and vertical focusing ability, and the field
has to build up to high peak strengths over an interval of very
many particle orbits.
A syncrotron, despite "banking" of the field, has mediocre
positional stability, and it has poor phase stability--the
accelerating frequency has to be "synced" (increasing RF) very
exactly. Alternating gradients give good focus but introduce
synchronizing problems.
Before going on to discuss the manner in which these rotating
particles are turned into X-rays, let us review the normal method
as used today. In this method the electrons are linearly
accelerated and caused to strike a thick target, often of tungsten
material. These electrons arrive at the target with a small
distribution of energy (due to variation at the start of
acceleration) and direction (due to focus method). The target is of
a high atomic number so as to have a large cross section; it is
thick so that none of the electron energy will be lost by passing
through the target. The efficiency of such a conversion is rather
low, say 3 percent in going from electrical energy to X-ray quanta.
This involves high loss and high heating in the target material, so
that this material must have a high thermal conductivity and an
external coolant (as water) supplied. For example, 100 kv. energy
in the linear beam and 100 ma. means 10 kw. of power dissipation
for a fraction of a second.
The ray energies (wavelengths) and the direction of these rays
depend primarily upon the change of momentum vectors of the
electrons as they are slowed down or stopped by the target atoms.
This stopping process is mostly due to these atom's electrons. If
the beam electron comes close enough to a target electron and if
the beam electron has sufficient energy (velocity) so that its
coupling period is less than that for the target wave function,
then such electron is decelerated, i.e. a vector. This then appears
as electromagnetic radiation, i.e. X-ray. Part of this radiation
will be associated with an energy "jump," such as a shift of the
electrons on the k shell. There are thus peaks in the radiation
spectrum. Another part of this radiation will be absorbed in
surrounding atoms of this thick target material, and reemitted
after some loss.
The target material itself has no particular alignment of its atoms
nor of its interatom exchange forces. In brief, then, the path of
any beam electron in the target will be a zigzag one involving even
(a few) reversal of momentum vector. Thus, the radiation will be of
a very broad spectrum, with no sharp peaks, and having a strength
(intensity in quanta) in any direction only weakly related to the
sine of the angle of incidence. The strength of a particular
frequency in a particular direction may be increased by the use of
a crystal (as calcite) which gives phase reinforcing in that
direction; but of course this is not done in present medical X-ray
practice, where a "flood" in a very broad direction (cone) is
used.
As an introduction to several features of the present X-ray
transmitter a very basic sketch is given in FIG. 14. The electron
118 is travelling on a critical orbit 164. This is the orbit at
which the ring 144 and field 138 of FIG. 12 act such that the
radius 128 no longer increases, or increases very slowly. Now the
energy and velocity vector on this orbit are very well defined,
principally by the equation: P =erB. (P is momentum, e is charge, r
is radius, and B is perpendicular magnetic field.) A thin peak 166
is the target supporting structure, and 168 (FIG. 15) is the
target. Several interesting comments may be made in connection with
the target.
1. The target 168 may be extremely thin. The base material 166 will
support it--and may act as a heat sink.
2. Most electrons will go through this target, but there will be no
loss of efficiency as such electrons will simply come around again
in a very short time. For sake of illustration let us assume 10 ma.
of electron current travelling in a "bunch" once around the orbit
each 0.1 microsecond. Perhaps only one-thousandth of this current
i.e. 10 na. will be "captured" in the target at any one orbit; but
this will be more than enough to give a dot (pulse) of X-ray if it
is highly directional (i.e. towards the receiver.)
3. The directivity can be made very high because of the lack of
scatter of the bombarding electrons inside the target, and much
less double emission. Further directivity can be had by an
orientation of the target material--either by introduction of
strain and/or magnetic axes.
4. The X-ray receiver (for example, a mosaic of NaI) is more
sensitive than film, or film preceded by an image intensifier. Its
decay time may be matched to the orbit period, and there is much
less problem of cross leakage than in a scintillation screen type
of intensifier.
5. The cooling problem no longer exists owing to the high
directional efficiency, high conversion efficiency (to useful
spectrum), and the sensitivity of the receiver. It will be noted
that the target dissipates energy only for a few degrees of each
orbit. The heat energy will be only a few watts by a few
seconds--typically less than 10 joules.
6. The electrons which have their velocity vector "bent" by the
target, but go through, will in general fall into an orbit of less
diameter and resume accelerating out to the critical radius once
again. The spiral current of the beam is only about 10 na.,
although they gather in a 10 ma. "bunch" at the critical radius.
The size of the target "spot" bombarded depends upon the coulomb
repulsion between the orbiting electrons, and upon cooling
characteristics. Typically the spot is 0.030 inch in diameter with
the notch 168 about twice this diameter. (the notch being in the
base material only.)
In FIG. 16 there is given a section of one possible target; a
plating 170 of a gold alloy on top of a thin aluminum heat sink
166. Gold is desirable as a target in that it has high thermal
conductivity and a "K" characteristic radiation close to 80.7 kv.
It has a specific gravity of 19.3 (equal to tungsten) and atomic
number 79, so it has good capture cross section. Its absorption
peaks at 8.8 (rel.) at 0.146 (1A. is 10.sup..sup.-10 m.) The
plating thickness cannot approach the bombarding electron
equivalent wavelengths (a fraction of 1 Angstrom), but a few
microinches (say 1,000A.) will reduce scatter to the point where
the main radiation lobes are well defined. The plating is not a
true alloy, but the use of a nickel plating electrode as well as
gold will strengthen the plated layer, and give it some small
degree of orientation.
The stopping cross section of gold at 200 kev. is about
40.times.10.sup..sup.-15 ev. cm..sup.2. A 100 A. thickness will
have about 10.sup.12 to 10.sup.13 atoms and give a cross section of
a least 10.sup.-3 cm. at 100 kev. Thus a particular electron may
orbit 800 times in process of accelerating, and 500 times through
the target before capture.
Gold does have a low melting point (1,600.degree. C.), and so a
minute amount will melt at each contact with the orbiting electron
bunch. This will simply anneal after the bunch has gone through,
although the notch plating may have to be renewed at weekly
intervals. This notch is formed by plating first and then etching
away the base material (aluminum).
There is another possible target based upon the radiation from
gold. This is shown in FIG. 17 where there is a notched structure
similar to FIGS. 15 and 16 and fabricated with an aluminum support
172 for a stratum 174 of gold. A layer 176 either a silicon or
calcite crystal completes the target. It will be understood that
the aluminum provides a support for the gold and acts as a heat
sink for the target while the crystal 176 provides a filtering
medium for the generated X-rays.
There is another sort of target which yields a broad spectrum of
energy combined with very high directivity. That is the use of a
material with oriented grain structure, such as certain 50 percent
nickel 50 percent iron alloys rolled down to strip as low as 0.000l
in. thick. This thickness 180 is shown for magnetic strip 182 in
FIG. 18. Such strips have a low coercive force (typical 0.1
oersteds) and a remanent flux which can be 99 percent of the
saturation flux. There may be several "easy" directions of
magnetization, but the dominant one will lie along an edge 184 of a
cube 186 in FIG. 19. The strip may be etched to give a notch less
than 0.0001 in., and it may be supported on a thin layer of
material having a low (electron) capture cross section.
The essential notion here is that the electron momentum (energy)
change does not entirely depend upon electron initial velocity, but
also how this energy couples in to the spin axes of the target
electrons. These axes are best defined in the material above, owing
to the lineup of the magnetic exchange forces. At the same time the
material must be kept below its curie temperature (about
500.degree. C) or the orientation effects will be lost. Note that
this orientation does not rely upon the electron position, which of
course, cannot be defined to better than the usual Planck number
(.sup.h /mv.sub.o). It is thus possible to modulate the direction
of energy in a particle direction by the use of magnetic alignment
of the crystal structure. Polarizing effects are also possible as
the magnetizing field is reversed.
In FIG. 20 there is shown another sandwich type of target. In this
embodiment a relatively thick stratum 177 of gold which is notched
at 179 serves as a support for a relatively thin stratum 181 of
molybdenum. The notch may be eliminated for single-pass capture
filtering effect due to the difference of absorption edges of
molybdenum and gold.
Finally in FIG. 21 there is shown another type of target in which
uranium atoms 190 are dispersed through a suitable matrix 188 such
as glass or a crystal. The use of a crystal matrix makes possible
the selection of the proper beam direction by rotation of the
crystal. Different energy levels may be selected according to the
selected angle. A glass matrix on the other hand provides no such
crystalline filtering effects insofar as the glass is a series of
long polymer chains and cross chains.
Another variation in the target construction is a gold stratum on a
polyurethane support which would have low absorption and stopping
power. Also atoms of gold may be suspended in a glass matrix in a
manner similar to FIG. 21.
To review the target characteristics: the gold notch and aluminum
heat sink of FIG. 16 provides good directivity, narrow spectrum,
and good thermal characteristics although there is a possibility of
gradual gold loss through sputtering. The addition of a silicon
crystal in FIG. 17 further narrows the spectrum by filtering the
X-rays. The gold and molybdenum sandwich of FIG. 20 provides
excellent thermal characteristics but displays relatively low
efficiency at the medical X-ray spectrum. Otherwise its
characteristics are similar to the target of FIG. 16. The oriented
nickel-iron strip of FIG. 18 provides excellent directivity, but
the spectrum is in the 1.5--1.7 mev. range and as such its primary
applications would be in physics. Finally the use of uranium in a
matrix provides a good spectrum and good directivity although the
directional overall efficiency is low.
It will be seen that FIG. 18 offers applications in the use of a
magnetic force to modulate the direction and/or the polarity of the
X-ray beam. An accelerator at these energy levels must be too
massive to move rapidly on a circular path, but within a narrow
limit the scan beam can be magnetically deflected. For widespread
use this technique will depend upon improvement of design in mosaic
receivers as the image intensifier screens.
Before going on to examine the nature of the shadowgraph
information, and how it is stored, manipulated and displayed, let
us consider the characteristics of magnetic tapes. The trend
towards image intensifier--orthicon (TV) camera--(TV) tape storage
has been noted. Hospitals find storage of film records in lead
screens a nuisance.
Now the flood of X-ray approach means a certain amount of
distortion, and high cost in the recording chain. That is, the
brief intensifier image is stored as a charge on the image plate of
the camera (orthicon) tube. A scanning beam in this tube changes
this pattern into a time-variant voltage.
First, the cost of magnetic tape and its write-read equipment is
directly related to bandwidth. Thus, for example, a FRI300 with
loop and 300-kc. band costs $10,000 while a 4-megacycle band can go
to $90,000. Then the "flood" method involves at least video
bandwith--where the lowest price is about $15,000. More important,
the video tape costs about $90 for a reel, as opposed to $20 per
reel for 300-kc. tape.
Second, there is more distortion in the receiving channel due not
only to bandwidth but to the extra stage (the camera tube) needed
before aiming at voltage signals for tape recording. Thus the net
nonlinearity of the camera response (due to charge leakage, etc.)
is termed "gamma," and adds to distortion.
Finally, the flood method does not admit of the magnification and
stereoscopic effects which can be introduced using a circular beam
scan. Conventional X-ray units involve bombarding a high density
material such as molybdenum which will cause a broad cone or flood
of X-rays having a variety of energy levels.
The concept of scanning an electron beam across a target material
to produce X-ray is not practical for reasons of efficiency; a
reasonable beam could not be formed. Then given a transmitter of
small mass which moves with the origin of the beam, the first
requisite is that the origin move with a constant velocity so that
the tape can record continuously with no distortion, and so that
high scan rates can be achieved with small motivating power. Hence
a circulator scan.
In FIG. 22 there is a view of such a scan showing a transmitter 194
in a horizontal orbit 196, and a receiver 198 in a parallel orbit
200 below a body 202. A plan view (from the top) is given in FIG.
23. It can be seen from FIG. 23 that on each circle of the
transmitter the vertical beam cuts the body in an arc; and thus,
with horizontal body movement, a "raster" or picture is developed.
This picture when taken from the tape to a cathode ray tube will be
identical to the usual "flat" picture which the flood method
yields.
There are two points of interest here. First, the cathode ray tube
deflecting voltages will not be of the usual "sawtooth"
(triangular) shape; but will be parts of sine waves, as shown in
FIG. 24. Here part 204 of horizontal deflection 206 is used, and
part 208 of vertical deflection 210. This will give the necessary
arc to the tracing beam. The drive mechanism in this case FIG. 23
may seem to be simpler than that where the body lies within the
scanning plane, FIG. 1. However, the receiver 198 has to be kept in
line with the beam, and thus either two separate synchronous motors
for 196 and 200, with servo to keep synchronism and long strip
receivers to take care of angular error; or else a mechanical
(outside) linkage between the transmitter 194 and the receiver
198.
Secondly, the rate of longitudinal body movement depends upon the
rev./sec. of the transmitter and the number of beams in the ribbon.
A typical low rate corresponds to 5 r.p.s. and 10 beams (1 per
track on a 1 -inch tape) of 0.01-inch diameter. This gives a rate
of 0.5 in./sec., in the upper limit. During these exposure periods,
if the body be alive, the organs will move a bit. In a "flood"
technique these movements (say, over 0.5 second) show as a blur of
the outline. In the beam scan system each point on the outline is
scanned in a very brief time; there is thus no blurring, but rather
an outline which is curved to show minimum and maximum positions.
For example, a heart muscle, when not at rest (diastasis), may be
moving at 2.0 cm. per second; and the minimum and maximum volume
will be shown by a scan picture.
The roentgen dose to the body (patient) is much reduced over the
flood method. First, the X-rays, due to the electron orbit, emerge
as pulses lasting a fraction of a microsecond with a period between
pulses 20 to 30 times the pulse duration. This will still give the
maximum resolution which the eye can resolve or the cathode ray
tube reproduce (say, 300 to 500 lines at 70 percent modulation).
Second, the crystal detectors in the receiver are more sensitive
than the screen of an image intensifier. They have no storage or
decay time problem and in theory can detect each quanta of X-ray.
Third, a "flood" of X-ray, even when grid collimated, gives
incident rays to the body at an angle to those desired.
In FIG. 25 there is shown the effective magnification due to a scan
encircling a body 16; this effect has to be tied in with the
description of the stereoscopic image given in FIG. 28.
A body sector 212 is to be examined. As the transmitter passes from
10 to 10', the scanning beam goes from 20 to 20'. A detail 214 of a
certain width has been placed at a large radius from the scan
center 14, and a detail of the same width 216 at a small radius.
Then FIG. 26 (a ) shows the received signal due to these details,
with dark shading representing absorption. It is evident that the
detail 216 close to center gives a signal of long duration 216 (e),
and the detail 214 further away from center gives a short period
signal, 214 (L).
In FIG. 27 (a) there is given the response of an electronic filter,
where 218 represents signal strength and 220 frequency measured on
the X axis. The slow cutoff of 222 gives the signal (image) of FIG.
26 (b), where it is seen that the large radius detail 214 is now
less pronounced 214 (m). The filter characteristic of FIG. 27 (b)
is one of equal response 224 until cutoff frequency of 214 signal
is higher than cutoff; and so 214 (h) disappears. The net effect is
thus one of giving the examiner the option of eliminating
distracting detail whenever he wishes to concentrate on a volume
near the scanning center. The filters are located in the system
either following the receiver or are introduced at a later stage on
the tape readout amplifier. It is then obvious that the higher rate
of rotation (i.e. a larger radii) giving higher frequencies will be
removed and the low frequency information at the focal point will
be emphasized.
In FIG. 28 (a) there is given one possible pattern of information
on the magnetic tape. The salient points of storage and display
are:
1. The X-ray image (shadowgraph) is written once only upon the
tape. This image has to be then continuously "read" onto the
cathode ray tube viewing screen.
2. The image is not a changing one, so that the raster (image) rate
at the screen may be lower than the television case. That is, a
long-persistence phosphor.
3. The dark-adapted eye, where the rods are dominant, has a
threshold of about one quanta times two rods times 20 msec. Acuity
is low at this level, for example only at three times threshold
does the hand resolve itself into fingers.
4. While the eye material (rhodopson) has a decay time in
milliseconds the retina loop response may run to 1/4of a
second.
5. In a recent thesis Massa of the Massachusetts Institute of
Technology measured the information rate capacity of the eye. For
example, with 10 elements he obtained about 16 "bits" (information)
per 125 msec.
6. The visual loop is able to amplify contrast in an image, but the
initial image is limited to three orders of density. (exposure to
X-ray of film, etc.) The visual response is to the logarithm of
opacity. The recording response of photofluorescent systems is
distorted by the successive "gammas" (nonlinear sensitivity) in the
chain; especially at the "toe," (lower end).
Let the scan rate be 50 rev./sec., that is a "sector" rate of 400,
and a maximum bit rate of 100 kc. A seven-beam ribbon then
completes a shadowgraph in about 1 second. If we allow 4 seconds
(lower r.p.s.) then the bit rate goes to 25 kc., (Kilocycles).
Considering factors (2) to (5) above we wish to "write" the cathode
ray tube image at about 10 times per second. In order to write over
and over (factor (1) and to increase the original information rate
the tape image may be "written" onto a storage tube, as the C73959
or CK7702. It can then be read out at a higher multiple of the
scanning rate, to give the final visual image. Alternatively the
tape information (image) may be speeded up and repeated by placing
this information on a tape loop which is running at a multiple of
the original inches per second.
The feature of note here is the relatively low info-rate
(bandwidth) on the first tape, making for low storage cost (as
discussed) and ease of manipulation. For example, the resolution of
a storage tube varies inversely with the writing speed; hence lower
info-rates fed in give a better picture. Also, (reference factor
(6) at lower frequencies it is easier to electrically amplify a
narrow range within the full response of "tones" (opacity).
Finally, an examination for small "differences" is facilitated, as
signal subtraction from another signal is less of a problem at low
information rates. For example, signal one (shadowgraph) may be of
a patient's kidney region today, and signal two the same kidney two
months ago.
Returning to FIG. 28 the sector of interest 212 is traversed by a
seven-beam "ribbon" which is sensed by the receiver 18. A tape 228
has eight channels 230 thru 244 and is moving in direction 246.
Channel ("track") 230 is reserved for pulse markers from 248,
occurring once per sector, and with a double mark each revolution.
These pulse markers, which are written on the tape, serve to
trigger each television horizontal line (or group of eight lines).
The sweep rate corresponds to the inches per second setting on the
tape readout. Thus during "record" (a scan period of 1 or 2
seconds) a head 250 is writing seven channels at the same time.
Following "record," the channels may be read out one at a time as
shown by the lower portion of FIG. 28. Here channel 232 is read by
head 250 during one sector time; following which head 252 reads
channel 234 for another sector time. Since only one or two selected
sectors are viewed at any examination, all of the original seven
parallel (sector) information blocks 232 thru 244 are read and
there is one spare sector time before the next seven blocks are
ready. If the tape 228 information be transferred to a loop tape as
shown at 254 in FIG. 28 (b) the read image will repeat itself, as
required by the cathode ray tube. The price paid here is the need
for the eight heads (each of eight tracks) in place of one.
While the circular scan in a plane above the body (FIG. 22)
obviously gives a shadowgraph identical to that of older methods,
it may be objected that if the scan encircle the body (as in FIG.
1), then the shadowgraph may contain more information but is no
longer familiar.
Then FIG. 28 gives the way in which the familiar image is
presented; with the added features of the "bringing out" out
(magnification) of an inner volume, and an apparent (stereoscopic)
depth to the image.
Radiologists today attempt to "bring out" out internal planes by
simultaneous movement of the "flood" transmitter and receiving
plane such that the internal plane is the only one to remain in
focus -- but this is not an ideal solution, no magnification for
one thing. For another, no impression of depth.
In FIG. 29 there is a lighttight box 256, such that a pair of eyes
258, 266 are dark adapted. Now, it is well known that each eye
receives a separate image, and, dependent on focus, these images
are combined by the visual response to obtain a depth impression.
Detail outside of focus region is largely discarded by this
response (by the well-known trick of focusing on a pencil at the
tip of the nose--and then elsewhere). In FIG. 29 the X-ray scan
does just this, projects a separate image to each eye, image 262 to
eye 258, and, on the right, image 264 to eye 260. A screen 266
blocks any image or sight crossover. Beams 268 and 270 act just as
light rope in their focus effect, 270 signal gives 262, and 268
gives 264. And it is evident that 270 beam is nothing but 268 beam
displaced by a small angle (say 45.degree., i.e. displaced in time
on the tape record. For the best stereo effect the circular scan is
made elliptical, such that an axis 272 is about one-quarter more
than an axis 274 in length. Separate images 262 appear in separate
television tubes with each of two images corresponding to a
different angle of viewing as the human eye does. These images
appear on the two video tubes and are generated by the magnetic
tape. The different angle of viewing is due to a small time
displacement as the scanning beam rotates. Horizontal synchronizing
signals are written as pulse markers on the tape.
* * * * *