Electronically scanned x-ray tomography system

An x-ray source for use in computerized tomographic applications focuses an electron beam on an arcuate anode ring to produce an x-ray beam which passes through a planar slice of the subject under study. Electromagnetic focusing and directing of the electron beam acts to produce the same effects as those produced by a single x-ray source which is mechanically rotated about the patient. Because it is very difficult to focus and to bend an electron beam having a cross section with other than cylindrical symmetry, and because it is highly desirable that the cross section of the electron beam have a rectangular cross section as it impinges upon the anode, an electromagnetic means is disposed adjacent the electron beam path between the anode and the beam bending coils to oscillatorily deflect the beam so that it effectively exhibits a substantially rectangular cross section, the long dimension of the rectangle always pointing toward the system axis. This configuration permits either a more intense x-ray source or, alternately, an x-ray source with a smaller projected spot size.

BACKGROUND OF THE INVENTION 
This invention relates to computerized tomographic x-ray sources and in 
particular to such sources in which the x-ray beam direction is 
electronically controlled. 
In a computerized tomographic imaging system, a beam of x-rays, often 
having a planar fan shape, is directed through the object under study. 
Various portions of the body absorb x-ray energy to a greater or lesser 
degree depending upon a number associated with each point in the body 
called the coefficient of x-ray absorption. Present day tomographic 
scanners typically exhibit a resolution in which such "point" is a square 
approximately 1 mm on a side. To gather sufficient information to 
determine the coefficient of absorption at points within the body, the 
x-ray beam is projected through the body from a plurality of different 
views. These views are typically spaced at regular angular increments in a 
plane. This plane determines the slice through the body for which an image 
is generated. This image is actually a pictorial representation of the 
x-ray coefficient of absorption associated with points in the body. In the 
resulting picture which is typically displayed on a cathode ray tube, the 
differing values of these coefficients are associated with different 
levels on a gray scale and/or with different colors to produce a 
false-color image. 
Early computerized tomography scanners were only used for head scans 
because of their slow speed. Because the cranial organs undergo minimal 
movements, their motions posed no problem for these scanners. However, 
because of the great medical diagnostic advantages offered by computerized 
tomographic x-ray images, scans through other bodily organs are desired, 
and in particular scans through moving bodily organs, such as the heart, 
are highly desirable. For example, such heart scans are useful for 
determining the effectiveness of coronary artery bypass surgery. Because 
of the relatively rapid movement of the organs of the thorax and abdomen, 
it is desirable to collect absorption and data from several hundred views 
in less than 1 second. At present, relatively high speed computerized 
tomographic scanning is accomplished by disposing one or more conventional 
x-ray tubes along a circular rotating gantry which revolves about the 
patient at speeds of less 1 revolution per second. The radius about which 
these tubes revolve is approximately 1 meter and is determined essentially 
by human dimensional constraints. Because of this relatively large radius 
and because of the desire to have a rotation speed of approximately 1 
revolution per second or less, unacceptably large g forces are exerted on 
the rotating x-ray source which itself often contains, for cooling 
purposes, a rotating anode. 
To avoid the difficulties associated with mechanical rotation of the x-ray 
source, certain tomography systems employ an electronically scanned 
electron beam in order to allow much faster movement of the x-ray source 
point. Examples of certain features of such electronically scanned systems 
are found, for example, in U.S. Pat. No. 4,122,346 issued Oct. 24, 1978 to 
H. Enge and in U.S. Pat. No. 4,130,759 issued Dec. 19, 1978 to J. Haimson. 
A common feature of these systems is the relatively long distance between 
the electron gun source and the anode target. Because an electron beam 
comprises, by definition, particles which exhibit the same electrical 
charge, there is a natural tendency for the electron beam to openly 
diverge due to space charge forces. If the electron beam divergence is not 
controlled, insufficient electron beam energy arrives at the anode target. 
Moreover, the electron beam must be passed through bending coils which 
produce further aberrations from a convergent beam. Not only must the beam 
be non-divergent for proper bending and focusing, but the cross section of 
the electron beam should optimally be circular, that is, the beam should 
have cylindrical symmetry. In contrast, however, it is highly desirable 
that the electron beam cross section immediately prior to impingement upon 
the anode, be rectangular with the long dimension of the rectangle 
pointing toward the system axis. The rectangular beam cross section at 
this point is desirable for two reasons. First, because of the typical 
angle of impingement with the anode target, the cross section of the 
resultant x-ray beam source can be made to appear square, as viewed from 
the body or object under study. Second, an electron beam with a 
rectangular cross section distributes its energy more uniformly across the 
face of the anode target. Thus, by using a rectangular focal spot, a 
higher beam wattage is permissible without anode overheating, and the 
effective focal spot size (which causes loss of image spatial resolution 
if too large) is no larger than that of a square focal spot. However, if 
the electron beam, as emitted from an electron gun, were to have such a 
rectangular cross section, conventional focusing and bending coils would 
not properly function to produce the desired rectangular focal spot on the 
anode target. 
SUMMARY OF THE INVENTION 
In accordance with a preferred embodiment of the present invention, 
electromagnetic deflection means are disposed adjacent to the electron 
beam path between the anode ring and the bending and focusing coils to 
oscillatorily deflect the electron beam so that the beam thereafter 
effectively exhibits a substantially rectangular cross section. While 
magnetic deflection means may be employed, a high frequency electrostatic 
deflector exhibiting cylindrical symmetry about the central system axis is 
preferably employed. The frequency of voltage applied to the electrostatic 
deflector is chosen to be at least as high as the frequency with which 
different views of the subject under study are taken. The peak deflection 
voltage is selected so as to insure that the electron beam impinges upon a 
substantially rectangular portion of the anode target. While a 
sinusoidally varying deflection voltage may be employed, a sawtooth 
waveform is preferred since then the various portions of the rectangular 
anode spot receive the same average power. Additionally, while the 
preferred electrostatic deflection means may be disposed external to the 
housing containing the electron beams, it is preferred that the 
electrostatic deflection means be employed within the housing. The x-ray 
source of the present invention is readily employable in a tomography 
system which operates to produce high speed images of any desired human 
bodily organ.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1 there is shown a computerized tomography imaging system employing 
an electronically scanned x-ray source. In such a system, mechanical 
rotation of the x-ray source is not required and the scanning to achieve 
data collection from differing views is performed electronically, 
primarily through the action of bending coils 13. The x-ray source and the 
apparatus for producing the electron beam are contained within vacuum 
housing 10. Disposed at one end of housing 10 is electron gun 11 which 
operates to accelerate a stream of electrons to an average kinetic energy 
of approximately 70 kev, corresponding to a peak beam voltage of 100 to 
150 Kv. The cross section of the electron beam is circular. Because the 
electron beam comprises a stream of like-charge particles, there is a 
tendency for the beam to diverge as is suggested in FIG. 1. If such 
divergence were permitted to continue, the electron beam would not 
converge onto a focal spot on a targe anode. Accordingly, focus coil 12 is 
provided in accordance with conventional techniques, the coil 12 acts to 
cause a convergence of electron beam onto a relatively small target area 
of the anode. It is to be specifically noted at this point, that the 
electron beam cross section still exhibits cylindrical symmetry, that is, 
it has a circular cross section. After passage through focusing coil 12, 
the electron beam, typically having a current of approximately 500 
milliamperes, passes through magnetic bending coil 13 which bends the beam 
by an angle of approximately 35.degree. away from the central system axis 
L, as shown. The orientation of the bending magnetic field can be rapidly 
shifted; that is, there are two orthogonal sets of bending coils which are 
normally driven by sinusoidally varying currents which are 90.degree. out 
of phase, so as to produce, at a constant bend angle of approximately 
35.degree., a uniform rotation of the focal spot around the system axis L. 
That is to say, the focal spot uniformly rotates in a plane perpendicular 
to the system axis. Still exhibiting cylindrical symmetry, the electron 
beam 14 thereafter impinges upon the anode ring 15. As indicated in the 
figure, the entire traverse of the electron beam is contained within the 
vacuum housing 10, a cross section of which is shown in FIG. 1. The 
housing 10 is generally cylindrical in shape, particularly in that portion 
containing the electron gun. However, for a full appreciation of the shape 
of the vacuum housing 10, it is desirable to view FIG. 1 and FIG. 2 
together. 
The electron beam 14, having been focused and bent by coils 12 and 13 
respectively is caused to impinge upon targets affixed to anode rings 15 
which typically comprise a metallic conductive material, the target 
portion of which is typically tungsten. The anode ring 15 is maintained at 
a high positive voltage with respect to the electron source. The 
particular anode ring illustrated in FIG. 1 has disposed thereon a 
sequence of four target regions, each of which may be employed 
independently. In operation, the electron beam can be made to impinge on 
any one of these four, or more, anode targets so as to produce an x-ray 
fan beam in any one of four parallel, distinct contiguous planes. This 
permits consecutive image slices to be generated, each of said slices 
being separate and perpendicular to central system axis L. FIG. 3, to be 
more particularly described below more completely illustrates the 
interaction between the electron beam and the anode to produce x-rays. 
However, in the x-ray source portion of the system in FIG. 1, the electron 
beam arrives at the anode having a small circular cross section. Because 
of the nature of this cross section and the angle of the anode target, the 
x-ray beam appears to be emanating from a source which has a squashed, 
elliptical shape. The minor axis of the apparent x-ray source ellipse is 
generally directed at a slight angle, toward the patient. The major axis 
of this ellipse is generally oriented in a direction parallel to a tangent 
to the anode ring structure. Because of the necessity for x-ray source 
collimators 17, a portion of the resulting x-ray beam signal is lost. 
For purposes of illustration, the electron beam in FIG. 1 is shown as it 
impinges upon a lowermost target of the anode ring 15. However, anode ring 
15 is actually an arcuate structure whose shape is more readily perceived 
in FIG. 2. In general, the electron beam focal spot rotates in response to 
signals applied to the bending coils 13 as to always impinge upon the 
targets affixed to the arcuate anode ring structure. These structures are 
angled with respect to the incident electron beam so as to produce a peak 
flux of x-ray photons directed up through source collimator 17 through the 
patient 20 and to detector ring 16 where the x-ray intensity level, having 
been modulated by absorption in the body 20, is converted to electrical 
signals for ultimate analysis by computer means which operate to generate 
the images as described above. 
It is also to be noted that the shielding typically comprising a heavy 
metal conductive sheet is disposed between the subject 20 and the bending 
coils 13 for additional patient safety. 
FIG. 2 illustrates the computerized tomographic imaging system of FIG. 1 as 
seen from the feet of the subject 20. Anode ring 15 and the vacuum housing 
10 are clearly shown as arcuate structures from this perspective. In a 
typical tomographic imaging system for human patients, the anode ring 15 
has a diameter of approximately 200 cm and extends for a circular angle of 
approximately 210.degree. in a plane normal to the system axis L. This 
angular extension is sufficient to provide a full 180.degree. scan with 
some overlap. The anode ring comprises a conductive metal and at least a 
portion of the anode ring is fashioned into angled target surfaces, 
preferably comprising a heavy metal such as tungsten. The x-rays that are 
produced by impact of the electrons on the anode target surface produce 
x-ray fan beams 30 having an approximate fan angle of 30.degree. as shown. 
Such a fan beam is typically configured to subtend a field of view 25 
having a diameter of approximately 50 cm. Within this field of view, 
typical patient cross section 21 is shown. The beam of x-ray photons 
having been selectively modulated by body portions of varying density 
thereafter impinges upon detector 16. The detector functions to convert 
the intensity levels of the impinging x-ray fan beam into directly related 
electrical signal levels. Such a detector may typically comprise crystals 
of bismuth germinate disposed between tungsten collimator plates and 
optically coupled to photomultipliers which produce the desired electrical 
signals. These electrical signals are then converted to digital form and 
are processed by digital computer means to produce numbers indicative of 
the coefficient of x-ray absorption exhibited by a small area of the 
subject under study. The size of this area determines the resolution of 
the imaging system which is typically approximately 1 mm.times.1 mm. 
FIG. 3 illustrates the desired relationship between an impinging electron 
beam 14 upon anode 15 giving rise to x-rays 30. The cross section of the 
electron beam in the figure is rectangular as shown by rectangular section 
19. FIG. 3 is representative of the geometries employed in conventional 
x-ray tubes to help insure that the anode is not overheated. The length of 
the rectangular cross section is typically five to ten times its width. 
The x-rays are utilized at an angle .alpha., of between 6.degree. and 
13.degree. as shown in FIG. 3, so that when viewed from the x-ray detector 
16 the projected x-ray focal spot appears square. For example, for an 
electron beam cross section fives times its width, the projected focal 
spot appears perfectly square when .alpha.=11.5.degree.. This method of 
using an electron beam with a rectangular cross section allows a suitable 
increase in x-ray output for a given degree of anode heating. For example, 
when the electron beam cross section is a rectangle with a length-to-width 
ratio of approximately 5, the allowable x-ray output increases by 
approximately five times. This is particularly true for brief, intensely 
loaded situations where the instantaneous heating of a local tungsten 
target surface is a limiting factor. In conventional x-ray tubes the 
rectangular cross section of the electron beam is readily assured by 
suitable collimation structures. However, in an electronically scanned 
tomography system as shown in FIG. 1, it is necessary that the electron 
beam possess a circular cross section for passage through the focusing and 
bending coils. More importantly, use of the bending coils produces 
undesirable effects if the beam has a rectangular cross section. A 
rectangular beam could be readily bent by the 35.degree. angle, but the 
long axis of the rectangular focal spot would not in general always point 
at the system axis. Instead, as the focal spot is deflected along the 
210.degree. angle (measured in the plane perpendicular to the system 
axis), the long direction of the focal spot points in directions far 
removed from the system axis. Additionally, even if the cross section of 
the electron beam entering the bending magnetic field were uniformly 
circular, asymmetries caused by the bending method magnetic structures 
produce a focal spot which is essentially a 2.times.3 mm ellipse. Thus, 
even though a rectangular focal spot is highly desirable, this result is 
not achievable in the system described in FIGS. 1 and 2 alone. 
In accordance with a preferred embodiment of the present invention, the 
benefits of a rectangular focal spot are achieved by adding a high 
frequency electrostatic reflector 40 as shown in FIG. 4. This deflector is 
cylindrically symmetrical about the system axis L, so that it, in effect, 
spreads out the power of the electron beam into a rectangular focal spot, 
the long dimension of which always points at the system axis L. The 
optimum voltage waveform for use with the electrostatic reflector 40 is a 
sawtooth wave with a fast return, as shown in FIG. 5. A sinusoidally 
varying voltage wave may also be employed but at a lesser anode power than 
with the sawtooth wave, because the anode locally overheats when the 
sinusodially varying deflection momentarily stops at the maxima and minima 
of the sine wave. 
Although the preferred embodiment of the present invention employs an 
electrostatic structure to produce the deflection desired in the electron 
beam, it is also possible, although less desirable, to employ a time 
varying magnetic field to achieve similar results. 
It is also desirable in the system illustrated in FIG. 4, that the 
deflection frequency be rapid enough so that the focal spot motion, due to 
the deflection, does not degrade the spatial resolution of the tomographic 
imaging system. In this respect, it is to be noted that such systems 
typically possess a sample period of approximately 40 microseconds. This 
sample period is the time between successive views, that is between 
successive locations of the x-ray focal spot along the 210.degree. 
circular extension of the anode ring. If the deflection frequency is not 
synchronized with this sample period, then the deflection waveform period 
must be made small in comparison with the system sample period. Thus, for 
example, if the period of the deflection sawtooth is equal to 1 
microsecond, the electron spot runs back and forth along the rectangular 
focal spot approximately forty times per view. Such a 1 MHz sawtooth 
waveform is readily achieveable with conventional electronics. However, if 
the deflection sawtooth waveform is synchronized with the clock 
controlling the sample period, then a slower deflection frequency may be 
used. That is, in such a situation, there is no problem with system 
resolution as long as a sample period is equal to an integral number of 
deflection waveform periods. However, care must be taken that the velocity 
of the electron spot does not slow down sufficiently so that the maximum 
instantaneous temperature of the tungsten anode rises above temperatures 
present at higher deflection frequencies. 
By way of example, and not limitation, an electron focal spot having a 2 mm 
diameter may be spread out into a focal spot having a length of 10 mm and 
a width of 2 mm. For example, if the electron beam voltage is 120 kv and 
the electrostatic reflector is approximately 1 m from the tungsten anode 
and if the deflector has a length of 20 cm along the beam and a gap of 2 
cm between the two halves of the deflector, the required peak-to-peak 
deflection voltage is approximately 230 volts which is readily achieved. 
Although this example does not consider relativistic effects, the required 
correction is not large and the resultant peak voltage is still readily 
achieveable by conventional electronics. 
While it is possible to dispose the high frequency electromagnetic 
deflection means outside the vacuum housing, it is not necessary to do so 
and it is preferred that the means be disposed within the housing so as to 
provide greater control over the electron beam movement. 
From the above, it may be appreciated that the present invention provides a 
tomographic imaging system in which there are no moving parts associated 
with the x-ray source and which permits either a more intense x-ray source 
or alternatively, an x-ray source with a smaller projected spot size, for 
greater resolution. Moreover, the x-ray source of the present system does 
not require the electronically scanned electron beam to have azimuthal 
velocities with respect to the arcuate anode ring, as apparently taught in 
the above-mentioned Haimson patent. 
While this invention has been described with reference to particular 
embodiments and examples, other modifications and variations will occur to 
those skilled in the art in view of the above teachings. Accordingly, it 
should be understood that within the scope of the appended claims, the 
invention may be practiced otherwise than is specifically described.