U.S. patent number 3,867,634 [Application Number 05/349,242] was granted by the patent office on 1975-02-18 for body portion support for use with penetrating radiation examination apparatus.
This patent grant is currently assigned to EMI Limited. Invention is credited to Godfrey Newbold Hounsfield.
United States Patent |
3,867,634 |
Hounsfield |
February 18, 1975 |
Body portion support for use with penetrating radiation examination
apparatus
Abstract
Apparatus is disclosed for examining a body by means of
radiation such as X or Y radiation. The body to be examined is
surrounded by a liquid such as water and is protected therefrom by
means of a flexible member which surrounds the body. In the event
that the body comprises the skull of a human patient, the flexible
member is conveniently formed as a hat, and means are provided for
holding the patient in a desired disposition with respect to the
apparatus during the examination.
Inventors: |
Hounsfield; Godfrey Newbold
(Newark, EN) |
Assignee: |
EMI Limited (Hayes, Middlesex,
EN)
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Family
ID: |
27259634 |
Appl.
No.: |
05/349,242 |
Filed: |
April 9, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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212778 |
Dec 27, 1971 |
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861538 |
Aug 21, 1969 |
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Foreign Application Priority Data
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Aug 23, 1968 [GB] |
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40317/68 |
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Current U.S.
Class: |
378/18;
378/20 |
Current CPC
Class: |
A61B
6/032 (20130101); A61B 6/501 (20130101) |
Current International
Class: |
A61B
6/03 (20060101); A61B 6/00 (20060101); G06T
11/00 (20060101); G01n 023/00 () |
Field of
Search: |
;250/358,359,360,439,451,456,490,491,503,505,510 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lindquist; William F.
Parent Case Text
This application is a continuation-in-part of my application Ser.
No. 212,778 filed Dec. 27, 1971 which is a continuation of my
application Ser. No. 861,538 filed Aug. 21, 1969 and now abandoned.
Claims
1. Apparatus for examining part of a body by means of penetrating
radiation, such as X- or .gamma.-radiation, comprising a source of
said radiation, support means, transmissive of said radiation,
disposed to support said body part in a predetermined position
relative to said source, detector means disposed to receive
radiation emergent from said body part, and a scanning structure
for causing said source and detector means to execute rotational
scanning motions relative to the body part, wherein said support
means includes a reservoir for a liquid medium, the wall of said
reservoir facing in the direction of the axis of the rotational
scanning motion being formed of pouched, flexible material so that
the body part to be examined, when inserted into the pouched wall,
can be surrounded by the liquid medium in the reservoir, while
exposed to said radiation, and means for varying the amount of said
liquid medium in said reservoir to expel air from the pouch when
the body part is inserted
2. Apparatus according to claim 1 adapted for the examination of
the head of a human patient, and wherein said reservoir wall is
constructed in the form of a hat having a peripheral flange, around
the open end thereof,
3. Apparatus according to claim 2 wherein said support means
further includes a rigid support member, disposed within said
reservoir so as to support said head at a predetermined angle in
relation to said source and
4. Apparatus according to claim 3 wherein said support member
comprises a
6. Apparatus utilizing penetrating radiation such as X- or .gamma.-
radiation to evaluate a variable absorption coefficient with
respect to said radiation in a plane within a body, including a
source of said radiation and detecting means for detecting a beam,
radiated from said source, after passage through said body in said
plane, said detecting means and said source on the one hand and
said body on the other hand being relatively movable so that output
signals can be obtained representing the absorption by said body of
beams at a plurality of angularly and laterally spaced positions,
and means for utilizing said output signals to effect said
evaluation, said apparatus further comprising a scanning frame on
which said source and said detecting means are mounted, facing each
other across an aperture in which said body can be positioned so
that said beams of radiation pass through said body in said plane,
said aperture being formed through a rigid wall member which forms
part of a reservoir for a liquid medium, the absorption of said
radiation exhibited by said liquid medium being similar to the
absorption of said radiation by said body, said aperture being
sealed by a flexible member, transmissive to said radiation and
impervious to said liquid medium, which is disposed to surround
said body in the vicinity of said plane, means being provided for
controlling the pressure exerted on said body by said liquid
medium, acting through said flexible member, by permitting
variation in the amount of said liquid medium in said reservoir,
said source and said detecting means being mounted for orbital
scanning movement about an axis normal to said plane and also for
lateral scanning movement to cause said detecting means to pick up
radiation successively from respective beam paths through the body
which are disposed across the lateral extent of said body in said
plane, and means for producing interrelated orbital and lateral
scanning movements of said source and said detecting means in such
a way that for each of a series of successive increments of said
orbital scanning movement a lateral scanning movement occurs from
which there is derived a set of output signals corresponding to the
absorption suffered by said radiation on traversing said beam
paths, successive sets of such output signals being derived after
said successive increments of orbital movement.
Description
This invention relates to apparatus for examining a body by means
of radiation such as X or .gamma. radiation.
The apparatus according to the invention can be used to assist in
the production of radiographs in any convenient form, such as a
picture on a cathode ray tube or other image forming device, a
photograph of such a picture, or a map of absorption coefficients
such as may be produced by a digital computer and on which
"contours" may subsequently be drawn.
In the method of, and apparatus for, examining a body described in
my application Ser. No. 212,778, radiation is directed through part
of the body, from an external source, in the form of a set of
pencil beams or rays toward detector means disposed on the opposite
side of the body part to the said source. Each ray is detected
after it has passed through the body and the absorption of
radiation by contents of the body disposed along the path in the
body followed by each ray is determined. The source and detector
means are orbited, relative to the body so that radiation is
directed, in sets of rays, through a plane of the body from a
plurality of different directions. In this way the absorption or
transmission coefficients of the elements in a two-dimensional
matrix of elements notionally disposed in said plane of the body
can be determined, provided sufficient rays are directed through
the body.
It is an object of this invention to provide apparatus for
examining a body by means of radiation, such as X or .gamma.
radiation, in which means are provided for reducing discontinuities
in absorption encountered by the radiation in travelling from a
source through a body to a detector.
It is another object of this invention to provide apparatus for
examining a body by means of radiation, such as X or .gamma.
radiation, including means for excluding air from the region
surrounding said body.
It is a further object of the invention to provide apparatus for
examining a body by means of radiation, such as X or .gamma.
radiation, in which means are provided for surrounding said body
with a liquid medium.
It is a still further object of this invention to provide apparatus
for examining the skull of a human patient by means of radiation,
such as X or .gamma. radiation, and including means for supporting
the patient's head in a flexible bag surrounded by a liquid
medium.
It is yet a further object of the invention to provide apparatus
for examining the skull of a human patient and including a flexible
bag adapted to seal an aperture in a reservoir containing liquid,
and resilient seat means for so holding the patient in relation to
said reservoir that his head is comfortably disposed in and
supported by said bag in a predetermined location with respect to a
source of radiation such as X or .gamma. radiation, and detector
means, said source and detector means being located externally of
said reservoir and on opposite sides of the patient's head.
According to the present invention there is provided apparatus for
examining part of a body by means of penetrating radiation, such as
X- or .gamma.- radiation, comprising a source of said radiation,
support means, transmissive of said radiation, disposed to support
said body part in a predetermined position relative to said source,
detector means disposed to receive radiation emergent from said
body part, and a scanning structure for causing said source and
detector means to execute translational and rotational scanning
motions relative to the body part, wherein said support means
includes a reservoir for a liquid medium, the wall of said
reservoir facing in the direction of the axis of the rotational
scanning motion being formed of pouched, flexible material so that
the body part to be examined, when inserted into the pouched wall,
can be surrounded by the liquid medium in the reservoir, while
exposed to said radiation, and means for varying the amount of said
liquid medium in said reservoir to expel air from the pouch when
the body part is inserted.
In order that the invention may be clearly understood and readily
carried into effect, the same will now be described with reference
to the accompanying drawings in which:
FIG. 1 shows the kind of picture produced by conventional X-ray
apparatus,
FIGS. 2a, 2b, 2c, 2d, 2e and 2f illustrate the principle of the
technique utilised in my aforementioned application Ser. No.
212,778,
FIG. 3 shows one method of scanning used in said technique,
FIG. 4 shows in block form suitable apparatus for carrying out said
technique,
FIG. 5 shows an alternative method of scanning,
FIGS. 6a and 6b illustrate diagrammatically the construction of the
scanning means of apparatus according to two other examples of said
technique, and show, in outline form, an example of the present
invention,
FIG. 7 illustrates a modification of FIG. 6a,
FIG. 8a illustrates, partly in block form, the apparatus embodying
the scanning means illustrated in FIG. 7,
FIGS. 8b, 8c and 8d are diagrams useful in explaining the operation
of parts of the apparatus shown in FIG. 8a,
FIGS. 9a, 9b, 9c and 9d illustrate the application of weighting
factors to elements of the picture,
FIG. 10 shows, in perspective view, part of an apparatus in
accordance with another example of my present invention and,
FIGS. 11 and 12 show cross-sectional views through part of the
apparatus according to said example of my invention.
Referring to FIG. 1, this shows a body 1 containing a bone 2 and a
tumour 3. Also shown are a source of X-rays 4 and an X-ray film 5.
As can be seen, images of the bone and tumour are produced on the
film, but partly superimposed. The tone of any point on the film is
dependent on the product of the coefficients of transmission of all
the elements lying between that point and the X-ray source. Thus if
the bone 2 has the lowest coefficient of transmission, the tumour 3
the second lowest and the surrounding material the highest, the
X-ray image comprises a dark patch where the bone and tumour are
superimposed, a lighter patch due to the bone not superimposed on
the tumour and a still lighter patch due to the tumour not
superimposed on the bone. These are surrounded by a light area
where neither bone nor tumour is present. Also as the differences
between the co-efficients of transmission of tumour and normal
tissue are small, the differences in tone between the different
parts of the X-ray picture are slight and difficult to detect using
such a method.
Referring now to FIG. 2, the body, bone and tumour are denoted by
the same references as in FIG. 1. The X-ray source is replaced by a
source 6 which may alternatively be of gamma rays but is preferably
of X-rays. It differs from source 4 in that it produces a beam of
small cross section area or a ray as it might also be called, for
example 3 m.m square or diameter, and preferably includes a
collimator to reduce scatter of the rays. The X-ray film 5 has been
replaced by a detector 7, which may be a scintillator and a
scintillation counter and which preferably also includes a
collimator. The body 1 is scanned by the beam in one plane only,
the plane being 3 m.m thick in this example, in a direction not
only linearly across the plane, but at a plurality of angles round
the plane, the detector 7 being so mounted that it is always
pointing towards the source 6. FIG. 3 illustrates the scanning in
more detail. If only a single scan across the plane were performed,
the result would merely be equivalent to a conventional X-ray
picture of that plane, all the objects on a line between source 6
and detector 7 being superimposed. However by performing a large
number of scans, sufficient information can be derived to enable
the coefficient of absorption of the material in each 3 m.m cube of
material in the plane to be calculated and the co-ordinates of its
position in the plane determined. Although only three scans are
shown in FIG. 3 it will be appreciated that many more would be
required in practice.
In each position of the beam the detector 7 determines the
transmission of the X radiation by a path of relatively small
cross-sectional area through the body. The plane under examination
is regarded as a two dimensional matrix of elements and the
directions and numbers of the paths is such that each element of
the matrix is intersected by a group of paths, which paths
intersect different groups of elements.
From the transmissions by all the paths, a series of simultaneous
equations is built up represented by the discrete output signals,
derived from the radiation traversing all the respective paths and
by means of a digital computer provides the absorption coefficient
of each element of the matrix. The outputs of the computer may be
used to produce a picture or other representation of the section in
any convenient form. Successive parallel planes may be examined in
this way, and a picture of each planar slice produced to build up a
picture of the entire body or a larger section of it. The slices
may be examined in sequence or simultaneously by using a number of
X-ray sources and detectors in parallel. FIGS. 2b to 2f show the
pictures resulting from examination of planar slices 5b to 5f of
body 1.
FIG. 4 shows a block diagram of the apparatus for producing
pictures from the outputs from detector 7. The output from detector
7 is applied to an amplifier and counter 8 which produces a digital
output representing the number of counts in each reading. The
output from 8 is converted to logarithmic form in a logarithmic
converter 9 whose output is stored in a punched-tape or magnetic
tape recorder 10 before being transmitted to a digital computer 11
for processing. The computer 11 produces for each 3 m.m cube of a
planar slice of body 1 a digital number representing the absorption
coefficient of the material within that cube. These digital numbers
may be converted to analogue form in digital-to-analogue converter
12 and applied to a tone printer 13 to produce a picture.
Alternatively, the computer outputs may be retained in digital form
for comparison by pattern recognition techniques, with other
digitised pictures.
To achieve the required result, the absorption along each path is
deduced from the transmission by each path and a knowledge of the
initial intensity of the beam or ray entering each path. The
logarithmic converter 9 is used to provide a linear output so that
the total absorption along a path is equal to the sum of the
absorption in each small element along the path. Let 100 parallel
paths be used for each of 400 directions spaced equally over
180.degree.. The computer 11 has then 40,000 figures to process,
each representing the total absorption along a given path. Consider
the section divided into 100 .times. 100 similar meshes as on
Cartesian graph paper. Each mesh represents an element of the body,
but the term mesh will be used in the following mathematical
consideration for convenience. The computer 11 is then programmed
to give the absorption for each of the 10,000 meshes.
Consider a ray which passes through a set of n = 100 meshes through
none of which a ray has previously passed. Let the total absorption
be Z dB. The computer then allocates a provisional value of Z/100
to each of the meshes. Now suppose that, at a later stage, a ray
passes through another set of 100 meshes the absorption in some or
all of these meshes having already been allocated. Let the sum of
the figures already allocated be Z.sub.1 whereas the new
measurement gives a total absorption Z.sub.2. It will be
appreciated that Z.sub.1 constitutes a reconstruction of the output
signal Z.sub.2 derived from the last approximations to the
absorptions of the respective meshes. Then a correction (Z.sub.2 -
Z.sub.1)/100 is added to the figures already appearing in each of
the meshes. This process is then continued for all the 40,000 rays.
This process gives a rough approximation, but to obtain better
accuracy, the computer must repeat it a number of times, say
five.
Consider a single section of the body in the xy-plane in which the
absorption per unit distance in dB at the point x, y is z.
Let
z = f(x,y)
Now consider a single set of rays all parallel to the y-axis and
spaced equally by intervals .DELTA.x. The rays are arranged to have
a width rather greater than .DELTA.x so that some overlapping
occurs. The optimum beam width is determined empirically. For
methematical purposes it is assumed that the change of absorption
through any interval .DELTA.x is negligible. We now suppose that
the section of the body to be examined is bounded on two sides by
the x- and y-axes and is square in shape so that it can be divided
into M elementary squares with edges parallel to the axes.
The complete total of rays can be divided into sets each of which
consists of parallel rays or effectively parallel rays at a given
angle or mean angle. The sets of discrete output signals derived
from the rays in each set are treated in the computer in sequence.
However, since there are only about 100 .times. 100 meshes and
about 400 angles are employed within 180.degree., rays at
neighbouring angles must include some of the same squares and their
absorption will not, therefore, be independent. If the sets were
therefore taken in angular succession the lack of independence
would clearly lead to a slower convergence than if they were
independent.
The computer is therefore arranged, by programming, to take the
different angular subsets in a pseudo random order with large
angular gaps, of say 40.degree., between successive sets of rays.
The sequence is intended to ensure that every angle is included,
but not repeated, within the 400 directions. Rays close together in
angle then appear far part in the computer scanning sequence.
The accuracy of detection by detectors such as 7 is limited so that
the raw data contain errors and therefore, as the complete cycle of
100 .times. 400 measurements is analysed four or five times by the
computer, the resultant figures for the meshes tend to oscillate.
It has been found that this can be avoided by multiplying the later
corrections by a factor which is less than unity and falls steadily
for successive cycles.
The procedure may be represented mathematically as follows. The
true continuous distribution function is given by equation
z = f(x,y) (1)
Let the distribution function reached at some stage of the work
be
z' = g(x,y) (2)
which is a discontinuous function since z' must have the same value
over each mesh.
Now consider an arbitrary ray passing through n meshes. If z.sub.r
is the mean value of z through the r th mesh and Z is the total
absorption (or attenuation) of the ray in dB as measured
##SPC1##
The value of z' for each mesh will also be known from the previous
work. If no ray has passed through a given mesh z' is put equal to
zero.
The mean square error for all the meshes along the path of the ray
will be written E where ##SPC2##
and it is required to choose new values z.sub. r " to replace
z.sub.r ' in order to minimise E.
There is no reason to favour one mesh rather than another, and
therefore a constant C is added to z.sub.r ' where C is independent
of r and must be obtained from the additional information provided
by Z. Hence
z.sub. r " = z.sub. r ' + C (5)
hence the new value of n E will be ##SPC3##
The minimum value of E is obtained where C is equal to the mean
value of z.sub.r - z.sub.r ' or ##SPC4##
In other words the correction applied is equal to the mean value of
the error. If none of the n meshes has previously appeared all the
z.sub. r ' are put equal to zero so that
z.sub. r " = Z/n (9)
In other words the attenuation is, at first, uniformly distributed
among the meshes.
For the s th ray equation (8) becomes ##SPC5##
If there is a total of S rays there will be a total of S equations
for a complete cycle. If m is the number of rays in a set of
parallel rays and N is the number of angles
S = m N (11)
if q is the number of complete cycles used by the computer, the
total number of ray operations is q S.
Since the number of rays S per cycle is several times the number of
meshes M, the number S of equations will be several times the
number M of independent equations.
Difficulties arise in finding a system which traces through the
picture matrix an equivalent beam or ray as it has been called
heretofore which has effectively constant width, and which also
includes the correct number of picture elements along its length.
Both of these requirements are essential for the accurate computer
calculations which are to follow.
The two worst cases are shown in FIG. 9a, where in one case a beam
centre line CL1 passes through the squares of the matrix
perpendicularly and the centre line of the beam passes through the
centre of the squares, in the other case the beam centre line CL2
passes between the squares. The latter case would add up to twice
as many squares as the former, when the squares along the length of
the beam are added up, and would clearly give an error of 2:1.
In order to overcome the above problem the values in each square
are multiplied by a weighting factor which is a function of the
distance from the centre of the square to the centre line of the
beam, i.e., the squares of beam 2 in FIG. 9a would have a weighting
factor of 0.5, the resulting sum of the numbers in the two beams
then being equal.
FIG. 9b shows an intermediate position of the beam in which the
distances from the centre line CLB of the beam to the centres of
the two affected squares in the beam are a and b respectively. The
corresponding weighting factors A and B can be read off the graph,
and when these are added together they must for reasons indicated
above add up to unity. Therefore it follows that the parts of the
curves labelled x must be drawn the inverse of the parts labelled
y, if the beam and hence the weighting curve is to be considered
symmetrical about its centre line.
It can be shown that one requirement for accurate summation of
values of the matrix squares is idealised in FIG. 9c and its
practical equivalent is shown in FIG. 9d using a matrix with a beam
at the same angle.
In FIG. 9c the area abcd is obviously constant at any position of
the intersection of the beams and is a function of the angle of
intersection of the two beams A and B. In FIG. 9d the two
equivalent beams vary in width from one to two squares and a
constant area at intersection would be impossible without the use
of weighting factors. It can be shown that for a given X-ray beam
width there is one weighting curve which fulfils all the
requirements. For example, if the squares contained at the
intersection of the beams in FIG. 9d are multiplied by their
respective weighting factors taken from this curve, they will
produce a sum which is proportional to the area abcd in FIG. 9c.
Any angle of intersection may be chosen and the beam in FIG. 9d may
be intersected anywhere along its length for this condition to
remain true.
The weighting factor curve can be split up into a table of
approximately 20 values to which the computer can refer during
calculations without substantially impairing the accuracy of the
system.
In the example illustrated in FIG. 3 only a single detector 7 is
indicated. If however a fan-shaped or strip beam of radiation is
used, with a group of detectors each for receiving radiation
transmitted by one of the paths of small cross sectional area, some
correction may have to be made in solving the equations for the
effect of Compton scatter but in many cases this can be avoided by
adequate spacing of the detectors.
As was previously mentioned, the differences in absorption between
different materials is very small. However, in accordance with the
invention the contrast of the picture produced can be so arranged
that the full black to white range represents only the small range
of absorption values which is of interest.
It is essential in all X-ray apparatus to ensure that the patient
does not receive an overdose of radiation. In this respect the use
of a scintillator and a scintillation counter is advantageous as
its efficiency and accuracy in detecting X-rays are several orders
better than those of photographic film. The maximum detail
obtainable in a picture is a function of the number of counts per
reading received by the scintillation counter around the edge of
the body. In view of the limitation on the permissible number of
counts per reading, it would not be feasible to produce a picture
having the same order of definition as a television picture when
examining a living body, although a high definition picture of an
inanimate object could be produced. Moreover, in examining living
bodies, it is not normally necessary to have a high definition
picture of the whole body. Apparatus according to the invention can
be used to produce a picture which is of high definition in the
area of immediate interest and of low definition in surrounding
areas. For example, as shown in FIG. 5 the radiation source 6 and
detector 7 may be arranged to perform a circular scan indicated by
the arrow 15 round the edges of the body, which is so positioned
that the area of interest is near the centre of the scan. By
averaging the number of counts over a small angle of rotation, mean
values of absorption for areas enclosed by the angle such as the
area shown shaded, may be calculated. It is clear from FIG. 5 that
near the edges of the body only a relatively small number of large
area elements are being examined, whereas at the centre a large
number of small area elements is examined. Consequently the
resulting picture will have a high definition near the centre and a
low definition towards the edges. In producing the picture, the
points may conveniently be plotted in polar co-ordinates. As in the
example of FIG. 3, a large number of scans is required to produce
sufficient information. In the embodiment of FIG. 5, the additional
scans may be produced by superimposing a slower rotary motion which
shifts the axis of the main rotation so that the centre of the
circle of the main scanning motion traces a circle of small
diameter. This additional scanning motion produces the intersecting
paths for each element of the matrix according to which the body is
examined. The superimposed motion need not be circular and need not
be confined to the centre circle. For example it could be a spiral
starting at the edge of the outer circle progressing rapidly
towards the centre then performing a slow spiral in the region of
the centre. It may be more complicated provided that it achieves
the object of even coverage at the centre.
Referring to FIG. 6a of the drawing there is represented therein an
X-ray tube 20 from which the rays, when the tube is operating, pass
through two collimators 21 and 22. The collimator 21 is aligned
with a further collimator 23 and the collimator 22 is aligned with
a further collimator 24. Between collimator 22 and 24 is located a
dummy attenuator 25. There is a gap between the collimators 21 and
23 for the location of the object to be X-rayed and in the example
illustrated this gap is occupied by a plastics block 26 having a
central aperture 27 for the body to be X-rayed. Two scintillators
28 and 29 are located at the ends of the collimators 23 and 24
respectively and these communicate via a light pipe 30 with a
photomultiplier 31. A chopper 33 rotatable by an electric motor 34
is arranged to allow beams to pass through the collimators 21 and
22 only alternately to produce scintillations in the scintillators
28 and 29 for detection by the photo-multiplier 31. When the
apparatus is in use, the collimators 21 to 24, the attenuator 25,
the scintillators 28 and 29, the light pipe 30 the photo-multiplier
31, the chopper 33 and the motor 34 are oscillated through the
angle subtended by the block 26. The X-ray source 20 does not take
part in this oscillation because it produces a beam wide enough to
span the block 26. However the whole equipment is arranged to
rotate slowly about the body to be examined by X radiation. Such a
body is represented by the outline 32.
The use of the scintillator 29 and the attenuator 25 provides a
reference for the photo-multiplier 31. The material of the
attenuator 25 is selected to have similar absorption properties to
the body 32 to be examined so that accurate transmission readings
may be obtained from the X radiations which pass through this body
substantially independent of the X-ray source intensity. The
material in the dummy attenuator 25 compensates, to some extent,
for the X-ray tube spectrum drift. The space 27 between the body
and block 26 is filled with a bag containing water so that the beam
intensity received by the scintillator 28 is kept as constant as
possible as it traverses the body 32, thus reducing the range of
the readings which the photo-multiplier 31 has to handle. The
apparatus may be calibrated initially by inserting a round
homogeneous body in the aperture of the block 26. FIG. 6b is a
similar system but the chopper is discarded and two separate
detectors are used for measuring the sources and readings through
the body.
The modification of FIG. 6a which is illustrated in FIG. 7 is
intended to reduce the time required to complete an examination.
According to FIG. 7 a series of photo-multipliers 31.sub.1,
31.sub.2 are used instead of the single photo-multiplier 31 of FIG.
6. The photo-multipliers have a common reference scintillator 29
and light pipe 30. Each photo-multiplier has individual collimators
between it and the source of X-rays 20, the collimators being
denoted by the references 21.sub.1 and 23.sub.1 in the case of the
photo-multiplier 31.sub.1. With this form of the invention the
oscillation of the photo-multipliers and the associated collimating
systems need be only a fraction of that of the apparatus shown in
FIG. 6a. The photo-multipliers could also be arranged slightly
displaced downwards so that six pictures can be taken at one time.
As indicated, in FIG. 8a the outputs of the photo-multipliers
31.sub.1, 31.sub.2 are applied to a series of amplifiers 41.sub.1,
41.sub.2 . . . and thence to a serialiser 42 which feeds the
plurality outputs of the amplifiers in series to an
analogue-to-digital converter 43. The digital output of the
converter 43 is fed to a magnetic tape recorder 44 and thence to a
digital computer 45 which is programmed to compute the absorption
coefficients of the elements of a matrix notionally superimposed on
the body 32 under examination. The co-efficients computed by the
computer 45 are recorded by a further magnetic tape recorder 46
from which they are applied to a picture selector control device
47. The tape produced by the computer 45 may be replaced on the
tape recorder 44, recorder 46 then being unnecessary. The output of
device 47 is applied to a digital-to-analogue converter 48 and
thence to a control circuit 49 which has a manual knob 50 for
controlling the position of the contrast window and another manual
knob 51 for controlling the width of the window. The output of the
control circuit 49 is fed to a display unit 52 which includes a
cathode ray tube having a screen 53. The display unit 52 is
arranged to respond to the output signals of the digital computer
to build up a visual representation of the section of the object
under examination. The term "window" denotes the range of signal
amplitudes which is applied to the unit 52 to form the display, and
the unit 52 is thus such that different absorption coefficients can
be displayed on a scale from black to white. The contrast window
width control knob 51 enables the full scale black to white to be
occupied by a small or large critical range of absorption
coefficients, and the observer may vary the position of the window
by manipulation of the control knob 50. FIGS. 8b, 8c and 8d
illustrate the effect of varying the width and position of the
window. The values of the absorption coefficients are indicated on
the vertical scale in these Figures. FIG. 8b illustrates the case
in which a wide window is used, that is to say in which the
black/white range covers a wide range of values of absorption
coefficients. If signals exceeding peak white are removed, for
example by limiting, only tissue and tumour will show on the
picture. However, as the absorption coefficient of tumour is only
10 percent greater than that of tissue both will appear as grey and
it will be difficult to distinguish between them. FIG. 8c shows the
effect of using a narrow window. In this case it is not possible to
distinguish between bone and tumour but it is easy to distinguish
tissue from both bone and tumour. If signals exceeding peak white
are removed, only tissue will shown up in the picture.
FIG. 8d shows the effect of altering the position of the narrow
window used in FIG. 8c. The tumour now appears as grey while tissue
exceeds peak black and bone exceeds peak white. Consequently if
signals exceeding peak white and peak black are removed, only the
tumour will show up in the picture. It can therefore be seen that
by manipulation of the width control knob 51 and position control
knob 50 the operator can eliminate from the final picture
everything except the material which he wishes to examine. The
display unit may also include means for displaying up to four
representations of different sections at one time and provision may
be made to enable the observer to dwell on one representation. A
long after glow tube may be used the picture being replenished by a
continuous backwards and forwards pass of the tape deck. The
digital computer 45 may be an on line computer and may be remote
from the magnetic tape recorders 44 and 46 being connected thereto
by suitable lines or the like. Alternatively the magnetic tape
recorders may be arranged to store information for computation and
display at desired times.
In some cases it may be more convenient to have a direct display.
This could employ a cathode ray tube store for storing the data in
analogue form. Preferably, the tube should have large values of
screen capacity so that the stored information may be interrogated
without causing any significant change in its value. Such tubes are
commonly used to provide "bright" radar displays. The summation and
computing of values received from the cathode ray tube may be
carried out by a simple accumulator and comparator operating a
serial mode, and the output fed back to the cathode ray tube to
give the necessary small additions to the charge built up over the
screen. A digital computer would therefore be unnecessary.
In the examples which have been described, the detecting means
detect the transmission of radiation through a plurality of paths
which are parallel to the slice being examined. In some cases
however some at least of the paths may be oblique to the slice and
such oblique paths used to determine the transmission or absorption
coefficients of the elements of a three dimensional matrix.
Referring now to FIG. 10, apparatus for examining a body by means
of radiation such as X or .gamma. radiation is shown generally at
54. An apertured outer casing 55 is adapted to rotate in steps of
about 1.degree. around a fixed, apertured inner casing 56. The
outer casing 55 contains a linear channel, enclosed within a
housing 57, in which a source of X or .gamma. rays is located, and
means (see FIG. 11) for causing the source to move to and fro
linearly along the channel. A second linear channel is enclosed
within a housing 58, which is integral with the outer casing 55,
and a detector such as a scintillator crystal is disposed within
the housing 58 and adapted to move to and fro linearly along the
second channel in synchronism with the motion of said source along
the first mentioned channel. The detector is optically coupled to a
photo multiplier.
Referring now to FIG. 11, which represents a diagrammatic
elevational view taken from the rear of the apparatus, a source 76
of a collimated beam of X or .gamma. rays is fixedly mounted upon a
rectangular yoke 77. A detector 78 is mounted on a limb of the said
yoke so as to be located opposite the source 76 and the yoke 77 is
linearly moveable to and fro on runners such as 79 and 80. The
source 76 is clamped to a toothed belt 81 which is driven parallel
to the runners 79 and 80 by means of an electrical motor 82.
The casing 55 carries a toothed gear wheel 83 which is driven, via
a co-operating gear wheel 84, by a second electrical motor 85. Thus
it will be evident that motor 82 causes the source and detector to
traverse linearly across the region of interest (represented by the
dotted circle 86) and that motor 85 causes the casing 55 and the
components supported thereby to orbit about the centre of the
circle 86.
The two motions are synchronised electrically by means of
photo-electric cells as follows. Yoke 77 carries a transparent band
87 which is provided with a series of opaque strips thereon. The
band 87 is interposed between a light source (not shown) and a
photo-electric cell 88, the source and the cell being mounted on
the casing 55 but being stationary relative to the yoke 77. When
the yoke 77 is driven linearly (say from right to left in FIg. 11)
the band 87 moves relative to the light source and the cell 88,
thus causing interruptions of the light energy incident on cell 88.
The cell 88 thus produces a pulsed output signal and this is fed to
a motor control circuit 89 wherein counter means is provided for
counting the pulses. When the counter means, (which may comprise an
integrating circuit adapted to provide an output signal when the
integrated signal exceeds a threshold level) detects that the yoke
is adjacent the leftwards extremity of its travel, it produces a
signal to stop the motor 82 and start the motor 85. The shaft of
motor 85 bears a shutter member 90 which has alternate equiangular
sectors of differing radial extent and which is interposed between
a light source (not shown) and a photo-electric cell 91. When the
cell 91 provides an output signal pulse indicative of a change in
illuminaton thereon (either from dark to light or vice-versa) the
pulse is fed to the motor control circuit 89 via a rectifying
circuit 92. The pulse causes the control circuit to stop motor 85
and start motor 82; the latter being caused to run in the opposite
direction to that in which it ran previously so as to cause the
yoke 77 to traverse from left to right as viewed in FIG. 11. When
the pulses from cell 88 indicate that the yoke is adjacent the
rightwards extremity of its travel, the control circuit 89 stops
motor 82 and starts motor 85 which runs until the photocell 91
provides a pulse indicative of a change of illumination. The motor
85 is then stopped and the motor 82 re-started in reverse to cause
the yoke 77 to traverse from right to left. This procedure
continues until the casing 55 has rotated through at least
180.degree. about the centre of the circle 86.
Referring again to FIG. 10, behind the inner casing 56 there is
provided a reservoir (see FIGS. 12 and 13) which is adapted to
contain water and (see FIG. 12) a reversible pump 93 which
communicates via a pipe 94 with a reservoir (not shown) is provided
for pumping water into and out of said reservoir. The aperture in
said inner casing 56 is closed by a resilient, waterproof member in
the form of a bag or "hat" 59 which is re-entrant into the
reservoir and forms part of the front wall of said reservoir. The
head of a patient is inserted into the member 59 as shown in FIG.
10 and is supported by means of a support member (see FIG. 12).
In order to enable the patient's head to be inserted into the
member 9, some of the water in the reservoir is pumped out, thus
causing the diameter of the member 59 to expand. The patient's head
is then inserted and the member 59 is pulled into position over the
patient's head. For this purpose the member 59 may be formed with
peripheral flanges or the like. The position of the patient is
adjusted until the aforementioned plane over which the radiation is
scanned coincides with the plane in the patient's head which it is
desired to examine. This having been done, water is pumped into the
reservoir which causes the member 59 to collapse, thus making
intimate contact with the patients' head, at least in the region
thereof which is to be irradiated, in order to reduce as far as
possible the entrapment of air in said region between the member 59
and the patient's head. This expedient reduces the discontinuities
in absorption encountered by the radiation in the region of the
patient's skull, it having been found undesirable for the radiation
to pass directly through air into the patient's skull.
The patient is supported supine on a couch 60 which is supported,
by means such as legs 61 and 62, from the fixed inner casing 56. In
order that the patient may be firmly positioned throughout the
period of irradiation, a force is applied which tends to urge the
patient's head into the aperture in casing 56 to counteract the
outward force exterted by the pressure of the water on top of the
patient's head. This is achieved in the present apparatus by means
of a sling 63 which passes under the patient's seat. The sling is
secured by flexible members such as 64 (one on either side of the
couch) to support prongs such as 65 which are slidably mounted by
means of a mechanism 66 to respective sides of the couch. The
position of the prongs can be adjusted by rotating a rod 67 by
means of a knurled knob 68, the rod 67 being screw threaded and
adapted to rotate in screw-threaded casing 69 which is fixed to the
couch by means of a bracket 70. Means (not shown) can also be
provided for tilting the couch by raising or lowering the foot
thereof.
FIGS. 12 and 13 represent cross-sectional views through the
aforementioned water reservoir, the head support and the member 59;
the head of a patient being shown in each case to illustrate how
the head can be supported at different angles to enable different
planes therein to be investigated.
In FIG. 12, the reservoir is indicated by the reference numeral 71.
As has been mentioned previously, the reservoir 71 is suitably
constructed of the material known by the Registered Trade Mark
"Perspex". The reservoir 71 is supported from the rear of the inner
casing 56 and, as previously described, the aperture in the inner
casing 56 is closed by the flexible member 59 which is suitably
formed of rubber or the like. Within the reservoir 71 and
surrounding the member 59 is provided a head supporting member 72
which is of tubular form and which is constructed, for example, of
"perspex". The supporting member 72 comprises a conic section and
is open at its right-hand end, as shown in FIG. 12, to permit water
to surround the flexible member 59. To this end, the member 72 may
be formed with holes passing therethrough as indicated, for
example, at 73.
By means of the supporting member 72 being formed of the generally
conical shape shown in FIG. 12, the head can be supported at such
an angle as to permit the investigation of the interior thereof
along planes substantially parallel to the orbito-meatal line as
shown at 74.
In the arrangement shown in FIG. 12, a cylindrical supporting
member 75 is used instead of the member 72, and this permits the
head to be tilted with respect to the apparatus so as to permit the
investigation of the interior of the head to be carried out along
planes skewed with respect to the orbito-meatal line. By this
means, sections low on the back of the head can be
investigated.
It will be appreciated from the foregoing that the aforementioned
matrix of elements is disposed so as to be wholly within the water
i.e., the matrix embraces only the body part and some of the water
surrounding it. By this means, greater accuracy can be achieved in
the derivation of the absorption (or transmission) coefficients of
the elements of the matrix, as compared with an arrangement in
which the body part is surrounded by air.
* * * * *