The x-ray source of the present invention comprises a charged particle beam generator and a vacuum enclosure assembly. The charged particle beam generator includes only a single electrical connection for providing high voltage to the electron gun. The power for the active circuits in the high voltage terminal of the charged particle beam generator is provided by a unique isolation transformer that has minimal losses and generates controlled magnetic flux. The generated charged particle beam is controlled through a series of dynamic and static focus coils and moved across the inner face of the target by a stepping coil assembly comprising x and y deflection coils as well as an x step and preferably a y step coil. Further, to minimize power usage, a control grid pinches off the charged particle beam during the stepping of the beam.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to the field of charged particle beam 
generators and x-ray tubes, and more specifically, scanning beam x-ray 
sources. 
2. Description of Related Art 
Real-time x-ray imaging is increasingly being required by medical 
procedures as therapeutic technologies advance. For example, many 
electro-physiologic cardiac procedures, peripheral vascular procedures, 
PTCA procedures (percutaneous transluminal catheter angioplasty), 
urological procedures, and orthopedic procedures rely on real-time x-ray 
imaging. In addition, modern medical procedures often require the use of 
instruments, such as catheters, that are inserted into the human body. 
These medical procedures often require the ability to discern the exact 
location of instruments that are inserted within the human body, often in 
conjunction with an accurate image of the surrounding body through the use 
of x-ray imaging. 
A number of real-time x-ray imaging systems are known. These include 
fluoroscope-based systems where x-rays are projected into an object to be 
x-rayed and shadows caused by relatively x-ray opaque matter within the 
object are displayed on the fluoroscope located on the opposite side of 
the object from the x-ray source. Scanning x-ray tubes have been known in 
conjunction with the fluoroscopy art since at least the early 1950s. Moon, 
Amplifying and Intensifying the Fluoroscopic Image by Means of a Scanning 
X-ray Tube, Science, Oct. 6, 1950, pp. 389-395. 
Reverse-geometry scanning beam x-ray imaging systems are also known. In 
such systems, an x-ray tube is employed to generate x-ray radiation. 
Within the x-ray tube, an electron beam is generated and focussed upon a 
small spot on the relatively large anode (transmission target) of the 
tube, inducing x-ray radiation emission from that spot. The electron beam 
is deflected (electromagnetically or electrostatically) in a raster scan 
pattern over the anode. A small x-ray detector is placed at a distance 
from the anode of the x-ray tube. The detector typically converts x-rays 
which strike it into an electrical signal in proportion to the detected 
x-ray flux. When an object is placed between the x-ray tube and the 
detector, x-rays are attenuated by the object in proportion to the x-ray 
density of the object. While the x-ray tube is in the scanning mode, the 
signal from the detector is inversely proportional to the x-ray density of 
the object. 
Examples of known reverse-geometry scanning beam x-ray systems include 
those described in U.S. Pat. No. 3,949,229 to Albert; U.S. Pat. No. 
4,032,787 to Albert; U.S. Pat. No. 4,057,745 to Albert; U.S. Pat. No. 
4,144,457 to Albert; U.S. Pat. No. 4,149,076 to Albert; U.S. Pat. No. 
4,196,351 to Albert; U.S. Pat. No. 4,259,582 to Albert; U.S. Pat. No. 
4,259,583 to Albert; U.S. Pat. No. 4,288,697 to Albert; U.S. Pat. No. 
4,321,473 to Albert; U.S. Pat. No. 4,323,779 to Albert; U.S. Pat. No. 
4,465,540 to Albert; U.S. Pat. No. 4,519,092 to Albert; and U.S. Pat. No. 
4,730,350 to Albert. 
In a typical known embodiment of a reverse-geometry scanning beam system, 
an output signal from the detector is applied to the z-axis (luminance) 
input of a video monitor. This signal modulates the brightness of the 
viewing screen. The x and y inputs to the video monitor are typically 
derived from the signal that effects deflection of the electron beam of 
the x-ray tube. Therefore, the luminance of a point on the viewing screen 
is inversely proportional to the absorption of x-rays passing from the 
source, through the object, to the detector. 
Accordingly there is a need for a source of x-rays that is both safe and 
economical as well as capable of being able to be positioned quickly and 
accurately across the face of a target anode. 
SUMMARY OF THE INVENTION 
The x-ray source of the present invention comprises a charged particle beam 
generator and an anode assembly. The preferred charged particle beam 
generator is an electron beam source comprising a single direct electrical 
connection for providing voltage to the electron gun. The power for the 
active circuits in the high voltage terminal of the electron beam source 
is provided by a unique isolation transformer that has minimal losses and 
generates controlled magnetic flux. 
The generated electron beam is controlled through a series of dynamic and 
static focus coils and moved across the face of the target anode by a 
stepping coil assembly comprising x and y deflection coils as well as an x 
step and preferably a y step coil. Further, to minimize power usage, a 
control grid pinches off the electron beam during the stepping of the 
beam. 
The entire x-ray source is packaged in a small form factor with sufficient 
safety features and that will allow for mounting of the source in 
traditional C-arms for use in medical applications without fear of danger 
to the patient or the treating physician. 
These and many other objects and advantages of the present invention will 
become apparent to those of ordinary skill in the art from a consideration 
of the drawings and the description of the invention contained herein. The 
principles of the present invention may be employed in any application, 
medical or industrial.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning to FIG. 1, an embodiment of a presently preferred x-ray source 
employed in a reverse geometry scanning beam x-ray imaging system is 
diagrammed. The x-ray source 10 preferably comprises an x-ray tube and a 
high voltage electron beam source. The high voltage electron beam source 
is preferably connected to an adjustable high-voltage power supply capable 
of generating approximately -70 kV to -120 kV. At this voltage level, 
scanning x-ray source 10 produces a spectrum of x-rays ranging to 120 keV. 
Scanning x-ray source 10 includes deflection coils 20 under the control of 
a scan generator 30. An electron beam 40 generated within high-voltage 
terminal 803 is scanned across a grounded anode target 50 in a 
predetermined pattern. For example, the predetermined pattern may be a 
raster scan pattern, a serpentine (or "S" shaped) pattern, a spiral 
pattern, a random pattern, a gaussian distribution pattern centered on a 
predetermined point of the target anode, or such other pattern as may be 
useful to the task at hand. Presently preferred is the serpentine (or "S" 
shaped) pattern which eliminates the need in a raster scan pattern for 
horizontal "fly back." 
As electron beam 40 strikes anode target 50 at focal spot 60, x-rays 70 are 
emitted in all directions. For simplicity, only a portion of the x-rays 
are shown. The x-rays preferably pass through a collimator toward the 
object 80 to be investigated. To optimize system performance of the 
presently preferred embodiment, a cone of x-ray photons should be 
generated that will diverge in a manner that will just cover the 
multi-detector array 110. This is preferably accomplished by placing a 
collimating element between the anode target 50 of the x-ray source 10 and 
the multi-detector array 110 and more preferably between object 80 and 
x-ray source 10. A more detailed explanation of the system parameters can 
be found in copending U.S. patent application Ser. No. 08/386,861, which 
has been incorporated herein by reference in its entirety. 
The presently preferred configuration for this collimating element is a 
grid of x-ray transmissive cylinders or apertures 140. Collimation grid 90 
is designed to permit passage to only those x-rays whose axes are in a 
path that directly intersects the multi-detector array 110. Collimation 
grid 90 preferably does not move with respect to multi-detector array 110 
while the system is in operation. Thus, as electron beam 40 is scanned 
across anode target 50, at any given moment there is only a single x-ray 
pencil beam 100 which passes through object 80 to multi-detector array 
110. 
The output of multi-detector array 110 is processed and displayed by 
control electronics/monitor 34 as an intensity value on a display monitor 
as described in copending and incorporated U.S. patent application Ser. 
No. 08/836,861, which has been incorporated herein by reference in its 
entirety. 
FIG. 2 is a cross-sectional diagram of the presently preferred scanning 
beam x-ray source 10 which comprises an electron beam source 112 and a 
vacuum envelope assembly 176. 
Electron beam source 112 is comprised of two aluminum flanged cylinders 114 
and 116 bolted to central aluminum cylinder 118. Rear endplate 120, 
fabricated from aluminum with two sealed openings 134 and 136, is bolted 
to the rear of aluminum flanged cylinder 116. Front endplate 138, 
fabricated of aluminum with a sealed central apertured ceramic disc 128, 
is bolted to the front of aluminum flanged cylinders and the central 
cylinder. This method of construction permits electron beam source 112 to 
contain an insulating fluid within its confines, with ceramic disc 128 
forming a seal between electron beam source 112 and vacuum envelope 
assembly 176. It is to be understood that any suitably designed housing is 
within the contemplation of the current inventions. 
High-voltage cable 122, extending from an external high-voltage power 
source (not shown), supplies a potential preferably variable between -70 
kV to -120 kV to generate an electron beam. The energy of this electron 
beam is between 70 kev and 120 keV which corresponds to the potential 
applied through high voltage cable 122. The preferred method of connecting 
high voltage cable 122 to the high voltage terminal assembly 803 is 
through use of molded epoxy cable receptacle 142 which has an integral 
metal mounting flange. Receptacle 142 passes through opening 134 and is 
sealed to end plate 120 with an O-ring seal. High voltage cable 122 is 
fitted into a strain relief sleeve 156 fastened by screws through integral 
flange 158 to the integral metal flange of cable receptacle 142. A rubber 
end piece 162, preferably ethylene propylene rubber, is shaped to conform 
with the conical orifice in cable receptacle 142 and is molded directly to 
the end of high voltage cable 122. For assembly, rubber end piece 162 is 
coated with silicone grease and is tightly compressed into the orifice in 
cable receptacle 142 to minimize electrical breakdown along the interface 
between rubber end piece 162 and cable receptacle 142. High voltage cable 
122 contains electrostatic shielding (not shown) which is connected to 
ground within sleeve 156. Electrical contact is established between high 
voltage cable 122 and contact plate 146 by a conductive rod, thereby 
forming an electrical connection through conducting spring 152 to high 
voltage terminal assembly 803. Conducting spring 152 is preferably 
received in an indentation in the high voltage terminal endplate 154 of 
high voltage terminal assembly 803. 
An insulating medium preferably surrounds high voltage terminal assembly 
803 to allow small distances between the high voltage terminal assembly 
803 and the outer walls of the electron beam source 112. Preferably, the 
insulating medium should be able to allow a high electrical potential of 
at least -120 kV to be impressed across this distance and maintained 
without electrical breakdown. The presently preferred insulating medium is 
sulphur hexafluoride gas (SF.sub.6), which is preferably maintained at a 
pressure of approximately 60 psig and at a temperature less than 
60.degree. C. Other insulating media, such as transformer oil, can also be 
employed in place of SF.sub.6. 
Preferably fitted within aperture 136 of rear endplate 120 is a feedthrough 
assembly 164 through which eight fiber optic cables enter electron beam 
source 112. For purposes of illustration only, the eight fiber optic 
single cables are shown as a single cable 168 in FIG. 2. The fiber optic 
cables 168 are preferably sealed into feedthrough assembly 164 by 
embedding them in epoxy resin in order to prevent leakage of the SF.sub.6 
gas. 
High voltage terminal assembly 803 is preferably insulated to withstand the 
applied high voltage by means of ceramic disc 128 set within the front 
plate 138. High voltage terminal assembly 803 is also preferably 
mechanically supported by means of ceramic disc 128 only, to form a 
cantilever. 
An isolation transformer 744 supplies power to the components within the 
high-voltage terminal assembly 803. The secondary 1271 of the isolation 
transformer 744 is located within the high-voltage terminal 803. The 
primary 1270 is disposed coaxially around the secondary 1271, but is 
physically separated from the secondary 1271 by an insulating gap filled 
with the SF.sub.6 insulating medium. The isolation transformer 744 is more 
fully discussed in the detailed descriptions of FIGS. 6-10. 
Vacuum envelope assembly 176, which is preferably at ground potential, 
generally comprises the entire structure depicted in FIG. 2 to the right 
of the front endplate 138. The interior of the vacuum envelope assembly 
176 forms the pathway for the electron beam 40 from the high voltage 
terminal assembly 803 to the anode target 50. A tapered cylinder ring 262 
extending from the front endplate 138 of the electron beam source 112 is 
welded to disc 264. An accelerating anode 184 with an axial through-hole 
is preferably screwed to the center of disc 264. Hereinafter, anode target 
50 and accelerating anode 184 are referred to as target 50 and anode 184, 
respectively, for simplicity and clarity. 
The interior of the vacuum envelope assembly 176 is maintained at a reduced 
pressure, preferably less than 10.sup.-7 mm Hg. Vacuum envelope assembly 
176 is initially evacuated by means of a negative pressure source mounted 
on a vacuum stand attached to tube and flange assembly 183. During initial 
evacuation, vacuum envelope assembly 176 is preferably baked out at an 
elevated temperature (&gt;200.degree. C.) to outgas all items on the 
interior. During this high temperature bake-out, all components of 
electron beam source 112, except the front end plate 138, are preferably 
removed from x-ray source 10 so that they are not damaged by the high 
temperature. After bake-out, x-ray source 10 is reassembled and 
conditioned, or high voltage processed, by operating the x-ray source at 
greater than normal voltage and current. The vacuum envelope assembly 126 
is sealed off from the vacuum stand by sealing the tube of assembly 183 
using a conventional pinch-off tool. Thereafter the reduced pressure in 
vacuum envelope assembly 176 is preferably maintained through the use of 
getter-ion pump 182. Alternatively, vacuum envelope assembly 176 can be a 
"sealed" tube design which consequently eliminates the need for a 
getter-ion pump. 
Electron gun 198 protrudes from the high-voltage terminal 803 through 
ceramic disc 128 into the vacuum envelope assembly 176. Electrode 126 
preferably extends from the ceramic disc 128, surrounding the emitting end 
of electron gun 198. Electrode 126 and anode 184 are shaped to control the 
electrostatic field configuration in the accelerating space between 
electrode 126 and anode 184, thereby ensuring that electron beam 40 is 
correctly focussed through the axial hole in anode 184. Additional shaping 
of electrode 126 controls the electrostatic field configuration across the 
surface of ceramic disc 128 so that the chance of electrical breakdown 
across the surface of disc 128 is minimized. On reaching anode 184, the 
electron beam 40 has acquired an energy expressed in electron volts 
substantially equal numerically to the voltage applied between electron 
gun 198 and anode 184. In its continuing path to target 50, electron beam 
40 is preferably not subjected to any additional axial forces so upon 
impact at focal spot 60, the energy of electron beam 40 is essentially the 
same as that acquired at anode 184. 
After leaving the axial hole of anode 184, electron beam 40 passes through 
a magnetic focus lens assembly 186 which is preferably a thin lens design 
comprising a cylindrical steel magnetic circuit with a U-shaped section. 
Static focus coil 185 is preferably wound on coil form 272 within this 
magnetic circuit. Dynamic focus coil 187 is preferably located within the 
magnetic circuit air gap and is preferably wound on a bobbin shaped coil 
form 270. Dynamic focus coil 187 is preferably wound with substantially 
fewer turns of wire than static focus coil 184 so that dynamic focus coil 
187 has a low inductance, thus permitting the current flowing in the 
dynamic focus coil 187 to be changed rapidly. Currents flowing in static 
focus coil 185 and dynamic focus coil 187 cause electron beam 40 to be 
brought to a focus at focal spot 60. When used with a collimation grid, 
the size of the focal spot 60 is important. It should be small enough to 
maximize the transmission of x-ray flux through the apertures in 
collimation grid 90 but, if it is too small, the resulting excessively 
high power density concentrated in focal spot 60 could cause local melting 
of the surface of target 50. It has been found that a focal spot size of 
0.3 mm is preferred when x-ray source 10 is used in conjunction with the 
collimation grid disclosed in U.S. patent application Ser. No. 08/386,861, 
which has been incorporated herein by reference in its entirety. 
From the magnetic focus lens assembly, the path of electron beam 40 is 
preferably controlled by a two-part magnetic deflection system comprising 
slow deflection yoke 190 and fast deflection yoke 188 disposed coaxially 
around ceramic cylinder 180. The deflection yokes are described more fully 
in connection with the detailed descriptions of FIGS. 11-15. Ceramic 
cylinder 180 is preferably formed of a ceramic material, as opposed to 
metal, because the rapidly changing magnetic fields produced by the 
deflection yokes, 190 and 188, would induce eddy currents in a metal 
cylinder which would inhibit penetration of the magnetic fields and so 
interfere with the accurate deflection of electron beam 40. Ceramic 
cylinder 180 is preferably formed of alumina, coated on the inside with a 
thin high-resistance coating of a nickel-chromium alloy which serves to 
prevent the build up of an electrostatic charge which will cause 
undesirable deflections of electron beam 40. The resistance of this 
coating is preferably high, and is preferably 1,000 ohms when measured 
between the two ends of ceramic cylinder 180, to minimize induced eddy 
currents. Stainless steel bellows 178 provides strain relieving mechanical 
connecting means to ceramic cylinder 180 to avoid the application of 
stress to the ceramic cylinder caused by, for example, mechanical 
misalignment. 
As electron beam 40 is deflected in the desired scanning pattern across the 
face of target 50, the length of the electron beam path will vary. To 
compensate for this, the strength of the magnetic focus lens assembly 186 
is preferably varied in synchronism with the scan to maintain the optimal 
size of focal spot 60. This is preferably accomplished by operating the 
static focus coil 185 at a fixed current. The small changes in strength of 
the field generated by dynamic focus coil 187 required to maintain the 
optimal size of focal spot 60 are achieved by modulating the current 
flowing in dynamic focus coil 187 in synchronism with the currents flowing 
in deflection yokes 188 and 190. The preferred means to control and drive 
the currents in the dynamic focus coil 187 and static focus coil 185 are 
discussed more fully in copending U.S. patent application Ser. No. 
08/386,861, which has been incorporated herein by reference in its 
entirety. 
X-rays are produced when electron beam 40 strikes target 50, which is 
preferably a circular plate with an active diameter of 25.4 cm (10 in). A 
collimation grid 90 containing an array of x-ray transmissive apertures is 
preferably disposed between target 50 and multi-detector array 110. Target 
50 and collimation grid 90 are discussed more fully in conjunction with 
the detailed description of FIG. 3. 
Infra-red temperature sensor 192 monitors target 50 for excessive 
temperature conditions through viewing window 194 located in a wall of end 
bell assembly 266 opposite target 50. Excessive temperature conditions on 
target 50 may arise, e.g., if a malfunction causes electron beam 40 to 
dwell for too long in one spot on target 50, instead of being scanned 
across its face. Infra-red sensor 192 preferably detects for excessively 
high temperatures by monitoring the amount of, or spectral shifts in, the 
luminosity of the face of target 50. The response time of sensor 192 is 
preferably of the order of one microsecond to avoid target burn-out. 
Cooling jacket 196 and cooling plate 197 are preferably mounted on the 
exterior front wall and exterior perimeter walls of the end bell assembly 
266, to remove heat generated by electrons which are back scattered from 
target 50 during normal operation of scanning beam x-ray source 10. Heat 
is removed from cooling jacket 196 and cooling plate 197 by use of a 
cooling fluid, preferably Fluorinert.TM., available from 3M Corporation, 
which is preferably circulated through an external heat exchanger (not 
shown). 
In the preferred embodiment, end bell assembly 266 is fabricated from 
stainless steel, conical in shape and double walled so that the cooling 
function can be achieved by circulating a cooling fluid in the space 
between the internal and external walls, thus the need for cooling plate 
197 is consequently eliminated. The apex angle of the conical end bell 
assembly 266 preferably conforms with that of the conical volume swept out 
by electron beam 40 while the radial dimensions of the inside wall of the 
cone are such as to provide preferably 1.2 cm spacing to the conical 
volume swept out by electron beam 40. This preferred shape reduces the 
internal surface area and the enclosed volume of end bell assembly 266 and 
the time required to evacuate the vacuum envelope assembly 176 to an 
acceptably low pressure. 
FIG. 3 depicts a magnified diagrammatic view of the preferred target 50 and 
collimation grid 90 assembly. Target 50 preferably comprises a target 
layer 129 supported by beryllium target support 130. A preferred 
construction of target layer 129 is a first layer of niobium 51 
approximately 1 micron thick applied to target support 130 to which is 
then applied a second layer of tantalum 52 approximately 5 microns thick. 
The preferred method of application for niobium 51 and tantalum 52 is by 
sputtering. Alternative methods include chemical vapor deposition, 
evaporation and ion plating. Niobium layer 51 functions as a resilient 
layer which has a coefficient of thermal expansion between those of 
beryllium and tantalum to help prevent the formation of stress cracks in 
the tantalum layer 51, which may be caused by the high instantaneous 
temperature difference between the beryllium and the tantalum at focal 
spot 60 with consequent differential expansion between the tantalum and 
the beryllium substrate which can cause cracking. In an alternative method 
for application of the target layer 129 to the target support 130, the 
coating process can be performed at an elevated temperature so that 
subsequent cooling produces a compressive stress in the target layer 129 
to reduce the operating tensile stress in target layer 129 at focal spot 
60 by an amount approximately equal to the initial compressive stress. 
Another embodiment is a layer of tantalum deposited directly on the target 
support 130. Yet another embodiment is a target layer 129 of an alloy of 
tungsten and rhenium. Still another embodiment is a target layer 139 of 
tungsten. In each of these embodiments an intermediate layer of a 
resilient material such as niobium may be used. Tungsten, tantalum and 
tungsten-rhenium are preferred materials for target layer 129 because they 
have high atomic numbers, making them efficient producers of x-rays, 
coupled with high thermal conductivity, high specific heat and high 
melting point. The thickness of target layer 129 is preferably selected to 
correspond with the distance traveled in the material by electrons of the 
highest operating energy. In an alternative embodiment, a lesser thickness 
is preferably used for target layer 129. In the first described embodiment 
when the x-ray tube is operated at the low end of its operating range, for 
example 70 kV, electrons which strike the target will not fully penetrate 
target layer 129, and the x-rays generated will then be attenuated as they 
pass through the remainder of the target layer 129. For a fixed electron 
beam power the x-ray flux at 70 kV is about 30% of that at 100 kV so it is 
desirable to choose the thickness of target layer 129 based on the range 
of electrons in that material at 70 kV in order to maximize the x-ray flux 
at 70 kV while accepting a slightly lower electron beam power to x-ray 
flux conversion efficiency at 100 kV. The conversion efficiency at 100 kV 
will nevertheless be greater than that at 70 kV. 
Beryllium is presently preferred for target support 130 because it 
possesses relatively high thermal conductivity and it combines a low 
attenuation for x-rays with the high mechanical strength required to 
minimize the mechanical deflection of target support 130 caused by 
atmospheric and coolant pressures. The thickness of target support 130 is 
preferably about 0.5 cm. 
Collimation grid 90 preferably comprises a circular array, 25.4 cm (10 in) 
in diameter of regularly spaced vertical columns and horizontal rows of 
apertures 140 with 166 apertures in both vertical and horizontal 
diameters. The total number of apertures 140 in collimation grid 90 is 
preferably about 21,642. The axis of each aperture 140 points towards the 
center of multi-element detector array 100 (FIG. 1). While x-rays 
generated from focal spot 60 travel in all directions, collimation grid 90 
provides a barrier which attenuates all those not directed towards 
detector array 110. The preferred collimation grid with alternative 
embodiments is described more fully in copending patent application Ser. 
No. 08/386,861, which has been incorporated herein by reference in its 
entirety. 
A cooling chamber 350 is preferably disposed between target 50 and 
collimation grid 90. Cooling chamber 350 is preferably 0.2 cm thick and 
may be adapted to carry water, forced air or other types of cooling fluid. 
The presently preferred coolant is a liquid Fluorinert.TM. which is 
available from 3M Corporation. The coolant flows through cooling chamber 
350 to absorb the heat dissipated by electron beam 40 as it strikes target 
50. The coolant then passes through an external heat exchanger where it is 
cooled before being recirculated to cooling chamber 350. 
FIG. 4 is an exploded view of the components of high voltage terminal 
assembly 803. As described more fully in connection with FIG. 2, an 
approximate -70 kV to -120 kV potential is preferably applied to high 
voltage terminal assembly 803 through spring 152, which is fitted into an 
indentation in the outer side of high voltage terminal endplate 154. 
Circuit board 214 preferably contains the fiber optic communication 
circuits for the components of high voltage terminal assembly 803. Eight 
fiber optic communications cables are preferably connected to circuit 
board 214 through a plug-in feedthrough assembly 166 in high voltage 
terminal endplate 154. The preferred fiber optic communications circuits 
are described more fully in copending patent application Ser. No. 
08/386,861, which has been incorporated herein by reference in its 
entirety. 
Because of the high voltage potential applied to the high voltage terminal 
assembly 803, an isolation transformer 744 is preferably employed to 
supply power to the components within the high voltage terminal assembly 
803. The secondary coil assembly 1271 of isolation transformer 744 is 
affixed to aluminum cylinder ring 226, which is shown bolted to the high 
voltage terminal endplate 154 and circuit board housing 212. A flat washer 
of conducting silicone rubber 288 is preferably compressed between one end 
of secondary coil assembly 1271 and the high voltage terminal endplate 
154, providing electrical conductivity between the two components. 
Similarly, another flat washer of conducting silicone rubber 290 is 
preferably compressed between the end of secondary coil assembly 1271 and 
the circuit board housing 212 to provide electrical conductivity between 
these two components. Circuit board housing 212 preferably comprises a 
deep drawn aluminum can with a large diameter axial hole formed in the can 
end plate. Contained within circuit board housing 212 is a circuit board 
stack comprising three circuit boards 206, 208 and 210. Circuit boards 
206, 208 and 210 preferably contain all the electrical components which 
are necessary for operation of electron gun 198. Specifically these 
circuit boards preferably contain a low-voltage heater power supply, a -2 
kV fixed power supply and a 0 to -2 kV variable-voltage power supply. Each 
circuit board is circular in shape and contains a solid "I" shaped 
aluminum heat sink 216 which protrudes from its surface. The circuit 
boards 206, 208 and 210 are bolted together to form a compact stacked 
assembly with electrical connection between each board achieved by means 
of mating connectors mounted on each board. The fiber optic circuitry 
located on circuit board 214 preferably connects to a connector on circuit 
board 210 via a ribbon cable which extends axially through the center of 
secondary coil assembly 1271 and through the hole in the end of circuit 
board housing 212. High voltage terminal support member 202 is fabricated 
from aluminum in a conical shape with a rear flange fitted with two groups 
of three threaded rods 204 diametrically opposed on the rear of the 
flange. The three circuit boards 206, 208 and 210 are shown mounted to 
support member 202 by inserting threaded rods 204 through holes located on 
heat sinks 216. Circuit board housing 212 slides over circuit boards 206, 
208 and 210 so that threaded rods 204 protrude through holes in the end of 
circuit board housing 212. Circuit boards 206, 208 and 210 are thus 
shielded from the effects of the intense electric field which exists on 
the external surface of circuit board housing 212. The assembly comprising 
circuit boards 206, 208, 210 together with circuit board housing 212 are 
preferably held together by nuts applied to the ends of threaded rods 204. 
Referring to FIG. 2, high voltage terminal support member 202 is 
preferably bolted to flange 224 on electron gun 198. 
Referring to FIG. 4, electron gun 198 is preferably mounted within the 
frontal aperture of high voltage terminal support member 202. Referring to 
FIG. 2, a metal sleeve is preferably mounted within the central aperture 
of ceramic disc 128, and welded to this metal sleeve is a flanged vacuum 
tube 221. Flange 224 on electron gun assembly 198 is shown bolted to the 
flanged section of vacuum tube 221 with an intermediate copper gasket 
which provides a seal between the SF.sub.6 contained in electron beam 
source 112 and the high vacuum in vacuum envelope assembly 176. This 
arrangement allows for a simplified procedure for replacement of electron 
gun 198. Feedthrough leads 222 on electron gun assembly 198 pass through 
ceramic insulating disc 218 to make connection to the internal electrodes 
of electron gun 198. 
Electron gun 198 preferably comprises a heater coil embedded in 
electron-emitting cathode 220, with cathode 220 mounted behind control 
grid 200. The entire electron gun structure is preferably supported from 
the feedthrough leads 222 on the vacuum envelope assembly 176 side of 
ceramic disc 218. Feedthrough leads 222 also provide electrical connecting 
means to the electrodes within the structure of electron gun 198. The 
presently preferred cathode 220 is a cylindrical piece of porous tungsten 
impregnated with low work-function materials which readily emit electrons. 
Such cathodes are known as dispenser cathodes and are available from 
Spectromat Inc. Employment of an impregnated tungsten cathode permits the 
use of a small diameter cathode since the electron beam current density 
obtainable from such a cathode is substantially higher than that from a 
pure metal emitter such as a tungsten filament. Because the focal spot 60 
is preferably small, the electron source is also preferably small. The 
embedded heater coil is energized by an electric current generated by a 
low-voltage heater power supply within high voltage terminal assembly 803 
which flows through two of the feedthrough leads 222. The heater coil 
preferably raises the temperature of cathode 220 to approximately 
1100.degree. C., which is the temperature at which the preferred cathode 
220 emits the required electron beam current. These electrons are 
accelerated to an energy between 70 keV and 120 keV in the gap between 
electron gun 198 and anode 184 by the action of the negative high voltage 
applied to electron gun 198. 
Control grid 200 preferably comprises a cylindrical electrode surrounding 
cathode 220 with an apertured end plate positioned slightly in front of 
the surface of cathode 220. The electron beam 40 emitted from cathode 220 
can be varied in intensity by the application of a voltage to control grid 
200, such voltage being of negative polarity with respect to cathode 220. 
In the preferred embodiment, application of -2 kV from a fixed potential 
power supply in high voltage terminal assembly 803 through feedthrough 
leads 222 to control grid 200 completely inhibits the flow of electron 
beam 40. Application of a variable potential in the range 0 to -2 kV to 
control grid 200 from a variable voltage power supply in high voltage 
terminal assembly 803 varies the intensity of electron beam 40 over the 
range of 0 to 60 mA. X-ray source 10 is preferably operated in a pulsed 
mode such that electron beam 40 is pulsed on rapidly for a time period 
relating to the electron beam scanning mode. This is preferably achieved 
by means of two solid state switching circuits contained within the 
circuit boards 206, 208 and 210. Each switching circuit preferably 
comprises a series-connected string of field effect transistors which can 
be turned on and off by means of command signals conveyed through fiber 
optic cables 168. 
Referring to FIG. 5, the components in circuit boards 206, 208 and 210 
which generate heat, such as power transistors and voltage regulator 
components 217, are preferably attached to heat sinks 216. The three 
circuit boards are stacked and heat sinks 216 are clamped together by 
means of threaded rods 204. Heat dissipated in heat sinks 216 by 
components 217 is preferably conducted to high voltage terminal support 
member 202. Most of the heat will then be removed by convection of the 
SF.sub.6 gas and thence to the outer walls of electron beam source 112. 
SF.sub.6 gas under pressure is the preferred heat exchange medium and 
natural convection forces are enhanced by circulation of the gas caused by 
the high electric field. Some of the heat from terminal support member 202 
will also be removed by conduction through ceramic disk 128. 
FIG. 6 is perspective diagram of a preferred isolation transformer 744, 
which supplies power for the components within high voltage terminal 
assembly 803. The secondary coil assembly 1271 of isolation transformer 
744 is preferably located within high voltage terminal assembly 803. 
Because of the high voltage applied to high voltage terminal assembly 803, 
the primary coil assembly 1270 of preferred isolation transformer 744 is 
disposed coaxially around the secondary coil assembly 1271, physically 
separated from the secondary coil assembly 1271 by a distance of 
approximately 4 cm(1.6"). 
Isolation transformer 744 preferably operates at a frequency of 60 kHz 
although other operating frequencies can be employed. The potential at the 
secondary coil is preferably 30V R.M.S. In the preferred embodiment, both 
the primary coil assembly 1270 and the secondary coil assembly 1271 each 
have a ferromagnetic core. The preferred material for the ferromagnetic 
cores is ferrite, chosen for its low loss properties when operating at 60 
kHz, although other low loss materials such as compressed powdered iron 
can also be employed. Isolation transformer 744 preferably operates with 
both primary and secondary coils resonant at the operating frequency by 
means of low loss capacitors connected across the coil connections. This 
improves the coupling between primary and secondary coils and eliminates 
the need for the 60 kHz power source to provide the out-of-phase 
magnetizing current. The resonating capacitors have a capacity of 
approximately 0.1 .mu.F each. 
The ferromagnetic core of secondary coil assembly 1271 is preferably formed 
of a cylinder of ferrite material. In the preferred embodiment, to reduce 
cost, the core is formed from a series of ferrite bars 230, preferably 
numbering twenty, with each bar abutting neighboring bars effectively 
forming a cylinder of ferrite within secondary coil 234. 
As shown in FIG. 7, cylindrical coil form 232 preferably encircles ferrite 
bars 230, which are attached to coil form 232 by means of double sided 
adhesive foam tape 292. Coil form 232 is preferably formed of acrylic 
plastic although other electrically insulating materials with adequate 
thermal properties could be employed. Copper wire, preferably low loss RF 
wire known as Litz wire, is wound around the central part of the outer 
face of coil form 232 to form the secondary coil 234. In the preferred 
embodiment, there are 13 turns of wire in secondary coil 234 although the 
preferred number of turns is not germane to the essence of the invention 
and the actual number of turns depends on the particular usage 
requirements of the transformer. The number of turns depicted in FIG. 7 
for secondary coil 234 is for purposes of illustration only and should not 
be considered the number of turns actually employed in the present 
invention. 
Copper rings 236 and 238 are preferably placed around both edges of coil 
form 232. The coil form 232 is preferably longer than ferrite bars 230 by 
a small amount, preferably about 0.2 cm (0.08"). Copper rings 236 and 238 
form two short-circuited single-turn coils which completely encircle the 
upper and lower edges of coil form 232. In the preferred embodiment, 
copper rings 236 and 238 are formed from 0.125" OD copper tubing and have 
a diameter approximately equal to that of coil form 232. Copper rings 236 
and 238 are attached to the upper and lower edges of coil form 232 by 
means of adhesive Kapton.TM. tape, available from Dupont. The entire 
assembly, comprising ferrite bars 230, coil form 232, copper rings 236 and 
238, and the secondary coil 234 is then preferably wrapped toroidally with 
adhesive Kapton.TM. tape 300 to provide an electrically insulating 
protective barrier for secondary coil 234. 
An electrostatic shield 240 is formed of insulated copper wire wound 
closely and toroidally around tape 300. The wire is preferably 24 AWG 
copper magnet wire. The top and bottom surfaces of shield wires 240 are 
preferably treated to remove the insulation and expose the bare copper. A 
bead of solder 294 is preferably applied circumferentially around the bare 
copper wire surfaces to provide an electrical connection between adjacent 
wires at the top and the bottom. A conducting silicone rubber washer 288, 
preferably 0.15 cm (0.060") thick, is preferably placed along the top 
surface of solder bead 294 and another identical washer of conducting 
silicone rubber 290 is preferably placed along the bottom solder bead. 
Aluminum ring 226, containing bolt holes along the upper and lower edges, 
is preferably attached to the inner surface of secondary coil assembly 
1271 by silicone rubber 228. Secondary coil assembly 1271 is attached to 
the rest of high voltage terminal assembly 803 by bolting aluminum ring 
226 to circuit board housing 212 and the high voltage terminal endplate 
154 (FIG. 4). Conducting silicone rubber washers 288 and 290 are 
preferably compressed between the secondary coil assembly 1271 and the 
high-voltage terminal endplate on one end, and the circuit board housing 
212 on the other end, providing electrical conductivity between these 
components. 
Cylindrical coil form 242 is the innermost layer of primary coil assembly 
1270, as shown in FIG. 8. Coil form 242 is preferably formed of acrylic 
plastic although other electrically insulating materials with adequate 
thermal properties could be employed. Copper wire, preferably low loss RF 
wire known as Litz wire, is wound around the central part of the outer 
face of coil form 242 to form the primary coil 246. In the preferred 
embodiment there are 11 turns of wire in primary coil 246 although the 
number of turns is not germane to the essence of the invention and the 
actual number of turns depends on the particular usage requirements of the 
transformer. The number of turns depicted in FIG. 8 for primary coil 246 
is for purposes of illustration only and should not be considered the 
number of turns actually employed in the present invention. 
Double sided adhesive foam tapes 296 and 298 are attached circumferentially 
to the upper and lower extremities of the outer face of coil form 242. 
Preferably attached to the outside of double sided adhesive foam tapes 296 
and 298 is the primary coil ferromagnetic core material. In the preferred 
embodiment, the ferromagnetic core comprises a series of ferrite bars 252, 
preferably numbering 37, with each bar abutting neighboring bars 
effectively forming a cylinder of ferrite outside primary coil 246. 
Copper rings 248 and 250 are preferably placed around both edges of coil 
form 242. Coil form 242 is preferably longer than ferrite bars 252 by a 
small amount, preferably about 0.2 cm (0.08"). Copper rings 248 and 250 
form two short-circuited single turn coils which completely encircle the 
upper and lower edges of coil form 242. In the preferred embodiment, 
copper rings 248 and 250 are formed from 0.125" OD copper tubing and have 
a diameter approximately equal to that of coil form 242. Copper rings 248 
and 250 are attached to the upper and lower edges of coil form 242 by 
means of adhesive Kapton.TM. tape. The entire assembly, consisting of 
ferrite bars 252, coil form 242, copper rings 248 and 250, and the primary 
coil 246 is then preferably wrapped toroidally with adhesive Kapton.TM. 
tape 302 to provide an electrically insulating barrier for primary coil 
246. 
An electrostatic shield 304 is preferably formed of insulated copper wire 
wound closely and toroidally around tape 302. The wire is preferably 24 
AWG copper magnet wire. The top and bottom surfaces of shield wires 304 
are treated to remove the insulation and expose the bare copper. Beads of 
solder 306 and 308 are then applied circumferentially around the bare 
copper wire surfaces to provide an electrical connection between adjacent 
wires at the tope and the bottom. A conducting silicone rubber washer 310, 
preferably 0.15 cm (0.060") thick, is placed along the top surface of 
solder bead 306 and a similar washer of conducting silicone rubber 312 is 
placed along the bottom solder bead 308. 
The outer face of primary coil assembly 1270 is preferably affixed to 
central aluminum cylinder 118 with silicone rubber 254. Aluminum cylinder 
118 contains bolt holes along its upper and lower edges and bolts to 
flanged aluminum cylinders 114 and 116 to form the outer wall of electron 
beam source 112. Conducting silicone rubber washers 310 and 312 are 
preferably compressed between the flanged portions of aluminum flanged 
cylinders 114 and 116 along both the top and bottom edges of primary coil 
assembly 1270. Conducting silicone rubber washers 310 and 312 thereby 
provide an electrically conductive path between flanged aluminum cylinders 
114 and 116 and electrostatic shield 304. 
Referring to FIG. 9, an alternative embodiment of primary coil assembly 
1270 is shown. Like the preferred embodiment of FIG. 8, a cylindrical 
acrylic plastic coil form 348 preferably forms the inner structure of 
primary coil assembly 1270. Primary coil 350 is preferably wound around 
the outer face of coil form 348, preferably forming 11 turns. Two bands of 
adhesive double backed tape 352 and 354 affix a series of ferrite bars 356 
to the upper and lower outer circumference of coil form 348. The outer 
face of primary coil assembly 1270 is preferably affixed to central 
aluminum cylinder 118 using silicone rubber 254. The number of turns 
depicted in FIG. 9 is for purposes of illustration only. 
The embodiment depicted in FIG. 9 employs an alternative electrostatic 
shield arrangement from that depicted in FIG. 8. This alternative 
electrostatic shield arrangement employs a resistive electrically 
conductive paint such as a colloidal graphite paint available from Acheson 
Colloids Inc., over the interior circumference 244 of coil form 348. As in 
the previous preferred embodiment, electrical connection to the graphite 
paint is made by conducting silicone rubber washers compressed against 
flanged aluminum cylinders 114 and 116. 
FIG. 10 illustrates the typical path of magnetic field lines which couple 
primary coil assembly 1270 with secondary coil assembly 1271 in the 
preferred isolation transformer 744. As depicted, currents induced in the 
copper eddy current shield rings confine the magnetic field and minimizes 
coupling into the various support structures, thereby minimizing excessive 
power losses in those support structures. Unlike conventional power 
transformers, magnetic field containment is achieved without a 
ferromagnetic return circuit. 
As shown in FIG. 10 the preferred isolation transformer 744 substantially 
confines the magnetic flux .PHI. within the area defined by ferrite bars 
230 and 252 and the copper rings 236, 238, 248 and 250. Ferrite bars 230 
and 246, by virtue of their high permeability, provide a low reluctance 
path for the magnetic field so that the field travels preferentially in 
the ferrite bars rather than in the material on either side of the ferrite 
bars. 
Eddy current shield rings 236, 238, 248 and 250 function as magnetic field 
clamps, substantially confining the magnetic flux .PHI. within the 
boundaries shown in FIG. 10. This field clamping occurs because the 
magnetic flux .PHI. induces eddy currents in the copper rings which in 
turn generate opposing magnetic fields which effectively "push in" 
magnetic flux field lines to within the above stated boundaries. Thus the 
typical path followed by magnetic flux .PHI. will extend from ferrite bar 
252 towards ferrite bar 230 and will be curved with a bulge midway between 
copper rings 250 and 236 then returning from ferrite bar 230 towards 
ferrite bar 252 in a similar shaped path between copper rings 238 and 248. 
The ferrite bars together with the eddy current shield essentially 
function as a magnetic return circuit while maintaining physical 
separation between the primary coil assembly 1270 and the secondary coil 
assembly 1271. 
Two deflection yokes, fast yoke 188 and slow yoke 190 are preferentially 
employed to move electron beam 40 in the required scan pattern over the 
surface of target 50. Slow deflection yoke 190 preferably comprises saddle 
type X and Y deflection coils wound within the internal slots of a ferrite 
cylinder. Such a construction technique has been used for the deflection 
yokes used with television picture tubes. FIG. 11 shows a diagrammatic 
representation of a preferred fast deflection yoke 188. In FIG. 11, the x 
axis is defined as horizontal and the y axis is defined as vertical when 
FIG. 11 is viewed in its correct orientation. Y-step deflection coils 265 
and 266 and X-step deflection coils 268 and 270 are toroidally wound with 
copper magnet wire in internal slots formed on the inside diameter of 
ferrite ring 286. The coils of fast deflection yoke 188 are preferably 
wound with fewer turns than the coils of slow deflection yoke 190 thus 
ensuring that the coils of fast deflection yoke 188 have substantially 
lower self inductances in comparison with those on slow deflection yoke 
190. This lower self inductance of the coils on fast deflection yoke 188 
makes it possible to effect small fast step changes in the amplitudes of 
the currents flowing in coils 265, 266, 268 and 270 with resultant rapid 
step changes in the position of electron beam 40 on target 50. The 
preferred circuitry to control and drive the current in the coils of the 
fast and slow deflection yokes are discussed more fully in the more 
detailed explanation of FIGS. 16-25, which are set forth below. The number 
of turns depicted in FIG. 11 is for purposes of illustration only and 
should not be considered the preferred number of turns. 
The deflection of electron beam 40 by deflection yokes 188 and 190 results 
in aberrations from ideal performance which increase in effect as the 
deflection angle of electron beam 40 increases. These aberrations cause 
focal spot 60 to depart from circularity as its distance from the center 
of target 50 increases. 45.degree. stigmator coil 784 and 0.degree. 
stigmator coil 786 are preferably employed to correct these aberrations. 
Currents supplied from an external source pass through stigmator coils 784 
and 786 to modify the deflecting magnetic field configuration. The 
amplitudes and directions of these currents is programmed to maintain a 
circular shape for focal spot 60 as it scans over the face of target 50. 
45.degree. stigmator coil 784 and 0.degree. stigmator coil 786 are 
preferably wound toroidally around ferrite ring 286 at the 0.degree. and 
45.degree. positions. The preferred circuits employed to control and drive 
the current in 45.degree. stigmator coil 784 and 0.degree. stigmator coil 
786 are discussed more fully in the more detailed explanation of FIGS. 
16-25, which are set forth below. 
Referring to FIGS. 12 and 13, means are provided to rotationally adjust the 
axes of the deflection yokes such that they are properly aligned in 
relation to the apertures of the collimation grid 90. Endplate 314, which 
is rigidly attached to the end bell assembly 266, contains two rotational 
support members 316 and 317 along its outer face, one on either side of 
the slow yoke 190. Rotational support member 316 contains a C-shaped 
section with an adjustment screw 318 inserted through the upper portion 
and adjustment screw 320 inserted through the lower portion of the 
C-shaped section. 
Slow yoke 190 is clamped between two identical alignment-clamps 326 and 
328. Alignment-clamp 326 contains a flat rectangular tongue which extends 
outward between the upper and lower C-shaped portions of the rotational 
support member 316. Locking screw 322 extends through a groove 324 in the 
alignment-clamp tongue into a mating hole in the rotational support member 
316. The adjustment screws 318 and 320 tighten to form contact with the 
upper and lower surfaces of the alignment-clamp tongue. A similar assembly 
exists on the other side of slow yoke 190 with respect to the other 
alignment-clamp 328. To effect the rotational adjustment of the slow yoke 
190, locking screw 322 on alignment-clamp 326 and a similar locking screw 
on alignment-clamp 328 are loosened to allow free rotational movement of 
the Alignment-clamps 326 and 328. Adjustment screws 318 and 320, along 
with similar adjustment screws for alignment-clamp 328, are then adjusted 
to rotationally position the alignment-clamps 326 and 328, thereby 
effecting a corresponding rotational adjustment for the slow yoke 190 
around the central ceramic cylinder 180. 
A rotational support member 330 containing two rectangular protrusions 
extends and attaches through upper and lower rectangular grooves in 
alignment-clamps 326. Rotational support member 330 contains a C-shaped 
section with an adjustment screw 332 inserted through the upper portion 
and adjustment screw 334 inserted through the lower portion of the 
C-shaped section. A similar rotational support member 331 and locking 
screws 333 and 335 extend and attach to the other alignment-clamp 328. 
Cylinder ring 338, which has the fast yoke 188 mounted along its interior 
surface, is formed with two rectangular adjustment plates 340 and 342 
along its exterior surface. Rectangular adjustment plate 340 extends 
outward between the upper and lower C-shaped portions of the rotational 
support member 330. Locking screw 336 extends through a groove in the 
adjustment plate 340 into a mating hole in the rotational support member 
330. The adjustment screws 332 and 334 tighten to form contact with the 
upper and lower surfaces of the adjustment plate 340. Adjustment plate 342 
is similarly positioned between the upper and lower C-shaped portions of 
rotational support member 331. To effect the rotational adjustment of the 
fast yoke 190, locking screw 336 on adjustment plate 340 and a similar 
locking screw on the other adjustment plate 342 are loosened to allow free 
rotational movement of the cylinder ring 338. Adjustment screws 332 and 
334, along with similar adjustment screws 333 and 335 for adjustment plate 
342 are then adjusted to rotationally position the cylinder ring 338, 
thereby effecting a corresponding rotational adjustment of the attached 
fast yoke 188 around the central ceramic cylinder 180. 
The magnetic focus lens assembly 186 can be positioned axially along the 
length of the vacuum envelope assembly 176 to regulate the minimum 
electron beam spot size on the target 50. Such positioning can prevent 
damage to the target 50 from minimum electron beam spot sizes which are 
overly concentrated, which may burn the target 50. Positioning rod 274 
extends from front endplate 138 to an endplate 314, which is rigidly 
attached to the end bell assembly 266. Five such positioning rods are 
preferably disposed equidistantly along the outside perimeter of the 
endplates 314 and 138. The magnetic focus lens assembly 186 is mounted 
between a front support plate 346 and a rear support plate 344. Preferably 
attached to the front support plate 346 are five rectangular clamps 276, 
each of which encircles a corresponding positioning rod 274. To position 
the focus coil structure 186, locking screws 278 on the clamps 276 are 
released allowing the focus coil structure to slide along the positioning 
rods 274. Once an optimal position is established, the locking screws 278 
are tightened into a locking position. 
Magnetic focus lens assembly 186 can be moved radially to align the central 
magnetic axis of focus lens assembly 186 with the central axis of electron 
beam 40 when electron beam 40 is not deflected by yokes 188 and 190. 
Alignment of focus lens assembly 186 is effected by means of 4 set screws 
(not shown), which protrude radially from threaded holes in plate 346. The 
inner ends of these set screws push against the outer diameter of the 
U-shaped magnetic circuit member. Turning these screws causes the magnetic 
circuit member to move in any radial direction with respect to plate 346. 
For purposes of illustration only, magnetic focus lens assembly 186 is 
shown as a solid in FIG. 12. 
As discussed previously, the electron beam 40 is moved across the face of 
the target 50 in a predetermined scan pattern. Because of the collimation 
grid 90 employed in the preferred scanning beam x-ray imaging system, the 
electron beam 40 is preferably scanned in a "step" pattern. This step 
pattern is used to direct electron beam 40 to a spot on the target 50 that 
is on the axis of a specified collimator grid aperture 140 for a 
designated period of time, and then to rapidly move the electron beam 40 
to another spot on the target 50 directly on the axis of the next 
specified collimator grid aperture 140. Electron beam 40 rapidly moves to 
the next target location to maximize the useful x-ray flux emitted through 
the collimator aperture. 
The electron beam 40 is directed in this step pattern by the fast 
deflection yoke 188 working in combination with the slow deflection yoke 
190. Within the slow deflection yoke 190, the X and Y deflection coils 
function in a conventional manner to apply a varying magnetic field such 
that the electron beam 40 is scanned in a sweep pattern across the target 
50. The width and height of the sweep pattern is regulated by the current 
pattern applied to the X and Y deflection coils. 
Within the fast deflection yoke 188, the X-step and Y-step deflection coils 
264, 266, 268, 270 apply a rapidly moving magnetic field to modify the 
magnetic field generated by the slow deflection yoke 190. The combination 
of the magnetic fields generated by the fast and slow deflection yokes are 
such that the electron beam 40 is deflected in a step pattern across the 
target 50. Fast deflection yokes 188 are preferably employed because 
conventional slow deflection yokes designed to sweep the electron beam 
typically require a large voltage in order to change its current fast 
enough to generate the necessary step pattern, particularly in the 
preferred embodiment of the present invention where the electron beam is 
preferably stepped behind an 166 by 166 array of apertures with a scanning 
frame rate of 30 Hz. The coils in the preferred fast deflection yokes 188 
are wound with shorter lengths and fewer turns than the slow deflection 
yokes 190, allowing fast current changes. 
In a preferred embodiment, the electron beam 40 is deflected in a stepped 
raster scan pattern across the face of the target 50, as depicted in FIG. 
14. The preferred method to deflect the electron beam 40 in a raster scan 
pattern is diagrammed in FIGS. 14A-F. FIGS. 14A depicts a sample linear 
pattern applied to the X-deflection coils 280 and 282, producing a 
conventional X sweep of the target 50 by the electron beam 40. FIG. 14C 
depicts the sawtooth pattern applied to the X-step deflection coils 264 
and 266, which produces the resultant step pattern as shown in FIG. 14E 
when magnetically combined with the X deflection pattern of FIG. 14A. 
FIG. 14B depicts the pattern applied to the Y-deflection coils 276 and 278, 
to produce a conventional Y-sweep of the target 50 by the electron beam 
40. As indicated in FIG. 14D, current is not applied to the Y-step 
deflection coils when scanning in the horizontal flyback mode since the 
period of time required for the electron beam 40 to "flyback" from the end 
of one horizontal row to the beginning of the next horizontal row gives 
the Y deflection coil sufficient reaction time to modify the current in 
its coil such that the electron beam is correctly deflected to the proper 
Y position. 
In an alternate embodiment, the electron beam 40 is deflected in a stepped 
serpentine pattern across the target 50, as depicted in FIG. 15. The 
preferred method to deflect the electron beam 40 in a stepped serpentine 
pattern is diagrammed in FIGS. 15A-F. FIGS. 15A diagrams a sample pattern 
applied to the X-deflection coils 280 and 282, producing an X sweep of the 
target 50 by the electron beam 40. FIG. 15C depict the sawtooht pattern 
applied to the X-step deflection coils, with a mirrored sawtooth pattern 
applied when the electron beam 40 begins scanning the next horizontal row, 
producing the resultant step pattern as shown in FIG. 15E when 
magnetically combined with the X deflection pattern of FIG. 15A. An 
alternate x-step pattern could comprise the use of a negative sawtooth 
pattern during the return horizontal step period, as shown in FIG. 15G. 
FIG. 15B depicts a sample current pattern applied to the Y-deflection coils 
276 and 278, to produce a Y-sweep of the target 50 by the electron beam 
40. The sawtooth Y-step pattern in FIG. 14D is applied when the scanning 
electron beam 40 reaches the end of a horizontal row, producing the 
resultant Y pattern shown in FIG. 14F when magnetically combined with the 
Y deflection coil pattern. 
In another alternative embodiment, electron beam 40 is scanned in a stepped 
serpentine pattern as described in the previous embodiment but the Y-step 
coils are not used when the electron beam reaches the end of a horizontal 
row. The required y direction deflection of electron beam 40 is caused 
using the slow y coils in slow yoke 190. The greater time taken to achieve 
the step from row to row will typically result in a small reduction in 
efficiency of x-ray production. 
The size and shape of the current patterns depicted in FIGS. 14A-F and 
15A-F are shown for illustrative purposes only. The actual current 
patterns applied to the X and Y deflection coils and the X-step and Y-step 
deflection coils are dependant upon many factors, which may include the 
rate of movement of the electron beam, the amount of deflection already 
applied, the number of collimator apertures, the dwell time for each 
collimator aperture location, the number of turns for each coil, and the 
exact placement of the deflection coils. 
FIG. 16 is a partial block diagram of a preferred scanning beam x-ray 
imaging system, showing a preferred beam controller 796 and a portion of a 
C-arm cart. FIG. 16 depicts only a portion of the overall scanning beam 
x-ray imaging system; the other components of this imaging system, and the 
exact placement of FIG. 16 in relation to these other components, are 
discussed more fully in copending appl. Ser. No. 08/386,861, which was 
incorporated herein by reference in its entirety. Accordingly, only select 
aspects of the preferred imaging system from the copending application are 
discussed in the following paragraphs; thus, reference is made to the 
previously mentioned copending application for greater details concerning 
the other components of the preferred scanning beam x-ray imaging system. 
Beam controller 796 preferably controls the focus coils through two 
separate drivers, a static focus driver 774 and a dynamic focus driver 
776. Static focus driver 774 is preferably set only once for a given 
operating voltage of the high voltage power supply. The dynamic focus 
driver 776 adjusts the precise focussing of the electron beam 1240 as it 
scans across a target. 
Beam controller 796 preferably controls the deflection coils through five 
separate drivers: x-deflection driver 778, x-step driver 780, y-deflection 
driver 782, 45.degree. stigmator driver 784, and 0.degree. stigmator 
driver 786. 
The x-deflection driver 778 communicates a conventional linear input 
pattern to the deflection coils via wires 1046 to drive the electron beam 
horizontally across the target whereas the x-step driver 780 communicates 
a novel sawtooth input signal to the deflection coils via wires 1048. The 
net effect is a stepped movement of the electron beam across the target. 
The y-deflection driver 782 communicates a conventional y-deflection 
pattern to the deflection coils via wires 1050 to drive the electron beam 
1240 vertically across the face of the anode. The 45.degree. stigmator 
driver 784 and the 0.degree. stigmator driver 786 and their respective 
coils correct for aberrations in the electron beam spot to maintain a 
circular spot on the target. 
Beam controller interface information, including grid voltage, static focus 
current, current sense select, current sense sample select information and 
current sense sample information, is transmitted to beam controller 
interface 794 from an I/O controller 762 via cable 1080. Current sense 
monitor 788 is preferably used to monitor the output of the beam 
controller drivers to verify their correct operation as well as to measure 
the electron beam current as previously discussed. The preferred I/O 
controller and C-arm cart 811 are disclosed in more detail in copending 
appl. Ser. No. 08/386,861. 
A failure in the deflection system could result in the electron beam not 
scanning across the target in the x direction or the y direction. This 
could result in thermal damage to the target. Deflection fault sensor 770 
preferably receives x-scan and y-scan monitoring information from 
x-deflection driver 778 and y-deflection driver 782. Deflection fault 
sensor 770 preferably transmits a fault status signal to a fail-safe 
controller via fiber-optic cable 1072. If a deflection fault condition 
occurs, fail-safe controller will shutdown the x-ray source. Fail-safe 
controller preferably receives and monitors status information from 
various components of the system and is designed to disable the system 
upon detection of a potential safety problem. If the fail-safe controller 
detects such a potential problem, it will preferably: (1) signal the grid 
controller 738 to disable (turn off) the electron beam; (2) shut down the 
high-voltage power supply 790; and (3) shut down the static focus driver 
774 to defocus the electron beam. 
A tube controller generates scan control data which directs the operation 
of the beam controller 796, thereby controlling the scanning pattern of 
the x-ray source. Tube controller functionally comprises a beam deflection 
lookup table which stores beam deflection data for each point on the 
target anode, programmable scan controller 920, beam transmitter 916, I/O 
transceiver 964, and I/O latch 958. Data from a beam deflection lookup 
table 918 is preferably sent to beam controller interface 794 via a beam 
transmitter 916 and high-speed fiber-optic link 1000. This data includes: 
(1) current sense sample signals; (2) dynamic focus; (3) x-step; (4) 
x-deflection; (5) y-deflection; (6) 45.degree. stigmator; (7) 0.degree. 
stigmator; and (8) "beam on request" signals. Preferably, approximately 
every 1.28 microseconds, a new set of data is sent from the beam 
deflection lookup table 918 to the beam controller interface 794. The 
preferred tube controller and beam deflection lookup table is disclosed in 
more detail in copending appl. Ser. No. 08/386,861. 
FIGS. 17A-B, 18, 19A-C, 20A-C, and 21A-C diagram the control logic within 
the beam controller interface 794, which processes and distributes analog 
coil current control signals to the various coil drivers. The digital scan 
control data generated by the tube controller 807 is optically coupled to 
the beam controller input circuit 1408, which preferably includes the 
optical communications circuit described more fully in copending appl. 
Ser. No. 08/386,861. Beam controller input circuit 1408 outputs eight 
parallel bits of digital scan control data to an eight-bit data bus D 0 . 
. . 7! and four parallel bits of control data CD to a control 1410, 
which distributes and/or reformats the digital scan control data within 
the beam controller interface 794. The preferred software modules for 
control 1410 are included as APPENDIX A(1). 
Referring to FIGS. 18 and 19A-C, control 1410 preferably outputs 
control signals, via leads 1411 (LD1 and LD2), to instruct the 
x-deflection 1412 to sequentially load parallel bits of digital 
x-deflection coil control data DXDEF from the eight-bit data bus D 0 . . 
. 7!. The x-deflection 1412 essentially manipulates the digital 
x-deflection coil control data DXDEF to generate a smoothly ramping 
triangular waveform at the x-deflection driver 778. Approximately every 
1.28 usec, the x-deflection 1412 preferably converts the parallel bits 
of digital x-deflection coil control data DXDEF to serial bits of digital 
x-deflection coil control data SDX. The serial x-deflection coil control 
data SDX is coupled, via output line 1413, to a twenty-bit serial DAC 1414 
which converts the information to an analog signal that is preferably 
applied to an intermediate x-deflection amplifier 1416. The preferred 
software modules for x-deflection 1412 are included as APPENDIX A(2). 
Approximately every 80 nsec, the x-deflection 1412 mathematically 
manipulates the sequentially acquired items of digital x-deflection coil 
control data DXDEF to calculate an eight-bit x-slope value, which is 
referred to as the x-slope control data XSD. The x-slope control data XSD 
is transmitted to DAC 1418 for conversion to an analog signal, and its 
analog output signal is preferably coupled to a series of intermediate 
x-slope amplifiers 1420. The amplified analog x-slope control signals XSD 
is preferably summed with the amplified analog x-deflection coil control 
data SDX to generate a smoothly ramping output waveform, which is 
amplified by intermediate amplifier 1417 to produce the x-deflection coil 
control signal XDEFL. The x-deflection coil control signal XDEFL is 
preferably output, via output line 1418, to a preferred x-deflection 
driver 778, which is described more fully in connection with the detailed 
description of FIGS. 22A-B and 23A-C. Alternatively, the x-deflection coil 
control signal XDEFL can be coupled, through an amplifier 1419 and a BNC 
connector 1444, to a commercially available amplifier, for example a 
Centronics amplifier, which then drives the current in the x-deflection 
coil. 
Analog y-deflection coil control signals are generated in the same fashion 
and output to a y-deflection driver 782. However, if a raster scan pattern 
is employed, then the serial y-deflection coil control data SDY is 
directly generated by the control 1410, therefore a y-deflection , 
y-slope control data YSD, and related circuitry are not required. 
Control 1410 also outputs control signals, via leads 1421 (LD6, LD7, 
LD8, LD9, and LD10), to instruct the small DAC control 1422 to 
sequentially load x-step control data (XCD), dynamic focus coil control 
data (DFCD), and stigmator control data (SCD) from the data bus D 0 . . . 
7!. Small DAC control 1422 redistributes the XCD and DFCD control 
signals to multi-channel DAC 1426 and redistributes SCD control signals to 
multi-channel DAC 1424. DAC 1424 preferably outputs analog 0.degree. 
stigmator coil control signals to the 0.degree. stigmator driver 786 
through an intermediate 0.degree. amplifier 1428. Analog 45.degree. 
stigmator coil control signals are similarly output to the 45.degree. 
stigmator driver through an intermediate 0.degree. amplifier 1430. DAC 
1426 preferably outputs analog x-step slope control signals XSTEPSLP to 
the x-step driver 780 via output line 1432. Similarly, analog x-step 
amplitude control signals XSTEPAMP are preferably output to the x-step 
driver 780 via output line 1434 and analog dynamic focus coil control 
signals DFOCUS are preferably output to the dynamic focus driver 776 via 
output line 1436. The preferred software modules for small DAC control 
1422 are included as APPENDIX A (3). 
Serial data 1438 preferably receives static focus coil control data 
SDIN from the I/O controller 762. Serial data 1438 couples control 
data SDIN to a DAC 1440, which converts this information to analog static 
focus coil control signals which are sent to the static focus driver 774 
through intermediate focus amplifiers 1442. The preferred software modules 
for serial data 1438 are included as APPENDIX A(4). 
The analog coil control signals from the beam controller interface 794 are 
preferably transmitted to suitable power amplifier circuits within the 
coil drivers to drive the current patterns in their corresponding focus or 
deflection coils. For example, the analog x-deflection coil control 
signals XDEFL from the beam controller interface 794 are preferably 
coupled, via input line 1418, to a preferred x-deflection driver 778 
(FIGS. 22A-B and 23A-C). The XDEFL control signals are applied to a 
control amplifier 1454, which regulates the activity of power amplifiers 
1446 and 1448. The x-deflection driver 778 is preferably a circle bridge 
circuit in which power amplifiers 1446 and 1448 differentially drive both 
ends of the x-deflection coil. The output voltages of the power amplifiers 
1446 and 1448 are coupled, through current sense resistors 1450 and 
current sensor 1447, to the x-deflection coil via output lines 1458 and 
1460. Resistors 1450 sense the current in the x-deflection coil and 
preferably feeds the current information back to regulate the control 
amplifier 1454. The current in the x-deflection coil is also monitored by 
a current sensor 1447, which transmits the measured current, via output 
line 1449, to the current sense monitor 788. Temperature sensor 1445, 
which measures the temperature at the x-deflection driver 778, employs a 
temperature switch 1462 to disable the x-deflection driver 778 if a 
temperature fault condition occurs. The y-deflection driver 782 preferably 
includes a similar circuit to drive the current in the y-deflection coil. 
X-step driver 780, which preferably comprises x-step ramp control switch 
1462, x-step voltage control circuit 1464, and decay control circuit 1468 
(FIGS. 24 and 25A-E), is preferably employed to generate a sawtooth 
current wave form in the x-step coil. The x-step driver 780 is connected 
across the x-step coil via output leads 1472 and 1474. Referring to FIG. 
24, x-step amplitude control signals XSTEPAMP from the beam controller 
interface 794 are preferably applied to x-step voltage control circuit 
1464 to control the voltage level of a VICOR multi-output switching power 
supply (not shown), which supplies an input voltage to the x-step driver 
780 via input line 1470. 
Ramp switch control signals XSTEP.backslash. are preferably applied from 
the control 1410, via input line 1471, to control the operation of the 
x-step ramp control switch 1462. When the x-step ramp control switch 1462 
is switched on, voltage from the VICOR multi-output power supply is 
applied to the x-step coil, allowing the current in the x-step coil to 
ramp up for a specified time period, preferably 1 to 200 nsec. The 
amplitude of the current pattern is determined by the voltage level of the 
VICOR multi-output power supply, which is preferably set by the x-step 
voltage control circuit 1464. 
When the x-step ramp control switch 1462 is switched off, decay control 
circuit 1468 applies a voltage to the x-step coil to control and shape the 
slope of the current decay in the x-step coil. X-step slope control 
signals XSTEPSLP are preferably applied to the decay control circuit 1468 
via input line 1432. An isolation amplifier 1474 is preferably employed to 
optically couple the x-step slope control signals XSTEPSLP to the decay 
control circuit 1468, to avoid potential problems relating to high 
voltages applied to the circuit by the VICOR power supply. The output of 
the isolation amplifier 1474 is preferably coupled to an intermediate 
x-step amplifier 1478. Intermediate x-step amplifier 1478 preferably 
converts the differential output from isolation amplifier 1474 into a 
single ended signal, which is coupled to the inverting input of a control 
amplifier 1476. Control amplifier 1476 manages the voltage across 
transistor 1472, which functions as a variable load, such that the voltage 
applied to the x-step coil during the current decay period produces an 
optimal current decay rate in the x-step coil. If a particular x-ray 
imaging application requires the use of a y-step coil, then a y-step 
driver similar to the x-step driver of FIGS. 24 and 25A-E is preferably 
employed. 
While embodiments, applications and advantages of the invention have been 
shown and described with sufficient clarity to enable one skilled in the 
art to make and use the invention, it would he equally apparent to those 
skilled in the art that many more embodiments, applications and advantages 
are possible without deviating from the inventive concepts disclosed and 
described herein. The invention therefore should only he restricted in 
accordance with the spirit of the claims appended hereto and is not to be 
restricted by the preferred embodiments, specification or drawings. 
APPENDIX A 
This document is an appendix to the U.S. patent application entitled "X-Ray 
Source." This appendix contains program listings for the preferred 
software modules for the programmable logic devices employed in the 
above-identified invention. These software modules are written in ABEL V. 
5.1, from DATAIO Corp., for x86 based IBM PC-compatible computers. 
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