Method for processing X-ray images

In order to reconstruct a composite X-ray image of larger size than the basic images obtained by radiography of the blood circulation system, the invention comprises the steps of picking-up and digitizing a predetermined number of basic images in respect of a number of relative positions between the patient-supporting table and an X-ray unit, in storing these images in a memory and in reconstructing a composite image, for example by transferring selected portions of the basic images into a collecting memory.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for processing X-ray images with the 
main object of reconstructing a composite image, at least one dimension of 
which is appreciably larger than the visualization field of the receiver. 
By way of example, the receiver employed in this instance can be a 
luminance amplifier of a type known per se. The invention finds a 
preferred field of application in angiography, in particular for 
visualizing at least a large proportion of a patient's blood vessels and 
circulatory system. 
2. Description of the Prior Art 
In angiography, it is extremely useful to be able to display on a single 
document a large part of the arterial and/or venous system such as, in 
particular, the entire lower arterial and venous system. In the past, 
examinations of this type have been performed by means of an installation 
comprising an X-ray source placed at a considerable height above the 
patient in order to irradiate the entire region to be visualized as well 
as a film transfer mechanism for taking a series of adjacent photographs 
corresponding to different portions of this region. This system suffers 
from the double disadvantage of high operating costs arising from the 
considerable lengths of photographic film used and also arising from 
extensive irradiation of the patient. Furthermore, it is not always easy 
to read the document by reason of the variations in quality of the 
successive images. More recently, with the appearance of luminance 
amplifiers which tend to replace films, it has been proposed to take a 
number of photographs in respect of different relative positions between 
the source-receiver assembly and the patient-supporting table and to 
reproduce these photo-graphs in adjacent relation on a single photographic 
document of distinctly smaller size than the series of photographs 
described above for an equivalent resolution of the image. Furthermore, 
the X-radiation dose applied to the patient is reduced to an appreciable 
extent. However, this type of presentation calls for an effort of mental 
reconstruction of the image on the part of the physician, which is 
considered as a restrictive condition. 
SUMMARY OF THE INVENTION 
The invention relates to an improvexent in the second type of method for 
preparation of the X-ray document described in the foregoing. 
More specifically, the invention therefore relates to a method for 
processing X-ray images obtained in respect of different relative 
positions between a source-receiver assembly and a support for a body to 
be radiographed along a predetermined path, of the type which comprises 
storing each image in memory in the form of digital data which are 
representative of pixels of said images. The method essentially comprises 
the steps of obtaining overlapping images, in locating and storing said 
corresponding relative positions, in reading parts of the digital data of 
certain images, if necessary after processing, said parts being a function 
of said relative positions, and in converting the digital data of said 
parts to video signals for displaying a composite image formed by the 
joining-together of at least certain predetermined X-ray images aforesaid. 
For the visual display of the reconstructed composite image by means of the 
television monitor, it is only necessary to displace this composite image 
over the television screen at low speed. 
The basic elements of the invention as defined in the foregoing also permit 
the application of a novel method of formation of composite images 
involving the use of conventional techniques of logarithmic subtraction of 
images in which "masks" and images to be processed are produced in 
alternate sequence as the contrast product progresses within the patient's 
body. 
In more precise terms, the invention is therefore concerned in addition 
with a method for processing images of a body within which a contrast 
product flows approximately in a direction parallel to the path aforesaid. 
The distinctive feature of the method in accordance with the foregoing 
definition lies in the fact that it comprises performing a single scan 
between said source-receiver assembly and the support for said body along 
said predetermined path, in picking-up two images and storing them in 
memory in respect of each relative position aforesaid, one image being 
produced in the absence of contrast product within the field of said image 
and the other image being produced in the presence of contrast product in 
part of said field, in carrying out in a manner known per se the 
subtraction of the two images picked-up at each relative position and in 
reading the aforementioned parts of the digital data of the images 
resulting from said subtractions. 
It will be readily apparent that many alternative procedures are open to 
choice for carrying out the method in accordance with the invention. It is 
possible in particular to begin by reconstructing the two composite images 
in which the "masks" or images picked-up without any contrast product are 
grouped together in one composite image whilst the images picked-up with a 
contrast product are grouped together in the other composite image before 
performing the subtraction of the two composite images and displaying the 
result.

DETAILED DESCRIPTION OF THE INVENTION 
The installation of FIG. 1 comprises a patient-supporting table 11 and an 
X-ray unit 12 comprising an X-ray source 13 placed opposite to a luminance 
amplifier 14 which constitutes the X-ray image receiver mentioned earlier. 
The X-ray unit 12 and/or the table 11 are displaceable in translational 
motion along a predetermined rectilinear path in a direction parallel to 
the table. The luminance amplifier 14 is read by a television camera 15, 
the video output of which is connected to a television monitor 16 (direct 
connection 31) and to an analog-to-digital converter 17, the digital 
output of which is coupled with an image memory M.sub.O. Measuring means 
18 for measuring the relative displacement between the table 11 and the 
X-ray unit 12 are connected to a computing unit 20 for controlling an 
address generator 21 which initiates reading of selected portions of the 
memory M.sub.O. The address generator 21 is also coupled with a selecting 
keyboard 23 used by the operator for selecting a predetermined number of 
images prerecorded in the memory M.sub.O and for reading said images line 
by line and pixel by pixel, in a predetermined order. The memory M.sub.O 
is coupled with a collecting memory M.sub.1 in which certain images or 
parts of images may be transferred under the control of the address 
generator 21 and of the keyboard 23. The output of the memory M.sub.1 is 
connected to a digital-to-analog converter 25, the output of which 
controls the monitor 16 and a reprographic means 26 which preferably 
consists in this case of a device of a type known per se for reproduction 
of video images by laser printing. By way of example, a device which is 
well suited for this application is proposed by the Soro Company. 
All the means which have just been described are individually known except 
for the computing unit 20 which will hereinafter be described in greater 
detail. This unit receives via the measuring means 18 data representing 
the relative positioning between the table 11 and the unit 12 at the 
moments of picking-up of images. These data can be stored in a memory 
storage unit 20.sub.a and the computing unit 20 is arranged and programmed 
for controlling the address generator 21 in such a manner as to carry out 
selective readout of the memory M.sub.O and thus to read only the data 
which are representative of the pixels located in specific regions of the 
different images as a function of the data stored in the memory 20.sub.a. 
FIG. 2 illustrates this selective readout. In accordance with conventional 
practice, the luminance amplifier delivers a circular image which is 
characterized by the position of its center Yi and by its constant 
diameter. In the case of all images read and stored, the positions of 
these centers Yi with respect to a common origin are stored in memory 
(memory 20.sub.a). Since the diameter of each image is known and 
preprogrammed, the overlap values between adjacent images can readily be 
determined by means of these data alone in order to decide on which pixels 
are to be read from each portion of the memory M.sub.O in which the data 
relating to a given image are written with a view to transmitting them to 
the memory M.sub.1. For example, if the images are read line by line 
(which corresponds to the conventional mode of display produced by 
scanning of the television monitor 16) and if the lines of two adjacent 
images which pass through the centers of these images correspond 
respectively to the positions Yi and Yi+1 along the path mentioned 
earlier, the line A-A' which joins the two points of intersection of the 
limits of the images along the same path will correspond to the position 
(Yi+Yi+1)/2 and will be the boundary line to be adopted between the two 
images. It will therefore only be necessary to collect the digital data 
from the lines of the video image located on one side of said line A-A' in 
an image (FIG. 2) and to collect the digital data from the lines located 
on the other side of the saxe line A-A' in the other adjacent image and to 
transfer these selected data into the memory M.sub.1 in order to 
reconstruct the desired composite image from a plurality of adjacent 
elementary images. 
It follows from the foregoing that the design concept of the computing 
circuit 20 is within the capacity of any one versed in the art since it is 
limited to the use of simple computing means for receiving the stored 
values in the memory 20.sub.a and means for controlling the address 
generator 21. From a structural standpoint, the memory 20.sub.a can form 
part of the memory M.sub.O whilst the computing circuit 20 can be wired or 
formed by a subprogram. The memory M.sub.1 is read cyclically while being 
controlled by an address generator 28 (refreshable memory) for permitting 
continuous display on the television monitor 16 after conversion of the 
digital data (digital-to-analog converter 25) to a video signal. Since the 
capacity of the memory M.sub.1 is that of a number of images of the same 
size as the television monitor, the address generator 28 is programmed so 
as to produce a relative displacement of one or a number of lines at each 
read operation, thereby ensuring that the entire composite image passes 
slowly across the screen of the television monitor 16. The memory M.sub.1 
can also be dispensed with if the address generator 21 is programmed so 
that the useful portions of the image displayed at each instant may be 
read successively and directly from the memory M.sub.O. It is also wholly 
possible without thereby departing from the scope of the invention to 
choose different geometrical limits between the overlapping images. For 
example, it is possible to record all the pixels of the first image and 
then the pixels of the second image which are located outside the 
perimeter of the first image, and so forth. The computing circuit is then 
modified so as to determine and convert to digital data the equation of 
the circular arc which marks the boundary between the two images and so as 
to select the pixels by means of a classification of the addresses of 
these latter with respect to said boundary curve. As will be readily 
apparent, the method can readily be transposed for the reconstruction of 
composite images from square or rectangular images. 
FIG. 3 illustrates an advantageous application of the method in accordance 
with the invention for the use of a number of sequences of image pickup 
and storage corresponding to successive scans between the table 11 and the 
X-ray unit 12. It is assumed for example that there have thus been formed 
four groups A, B, C, D each comprising four overlapping images. Thus each 
group reconstructs the entire lower portion of a patient's body for a 
complete examination of the circulatory system. These different images are 
picked-up approximately in the same positions (these positions being four 
in number in the example illustrated) along the path of displacement and 
are stored in the memory M.sub.O. 
The operator then has the possibility of displaying said images one by one 
by means of a direct connection 30 which is switchable between the output 
of the memory M.sub.O and the digital-to-analog converter 25. Thus the 
operator can select from each group those images which can most readily be 
utilized and can transfer the corresponding information into the memory 
M.sub.1 while at the same time forming the "cutouts" which result from the 
method defined in the foregoing. Thus in the example shown in the 
drawings, the operator has successively selected the images B.sub.1, 
A.sub.2, A.sub.3 and C.sub.4. It will of course be understood that, if 
provision is not made for a memory M.sub.1, the selected digital data can 
be read cyclically and directly from the memory M.sub.O as mentioned 
earlier. Independently of the visual display of the composite image which 
is contained in the memory M.sub.O and which passes slowly across the 
screen of the television monitor 16, the reprographic means 26 can be 
utilized at any moment for instantaneously delivering a printout of the 
same image. 
The invention also covers an additional application which makes use of 
well-known techniques for processing X-ray images by subtraction of two 
images, one of the images or "mask" being picked-up without any contrast 
product (that is to say prior to arrival of said contrast product in the 
field of the receiver 14) and the other image being picked-up after 
arrival of the contrast product in said field. The means for processing by 
subtraction can be integrated with the installation of FIG. 1 without any 
difficulty. The initial step consists in injecting a dose of contrast 
product into the patient. A single scan is then performed between the 
X-ray unit 12 and the table 11 while picking-up two images for each 
position chosen. Prior to recording of any image, the field of view 
corresponding to this image is continuously observed by radioscopy (direct 
connection 31 between the output of the camera 15 and the television 
monitor 16). After injection of the product, the first image is picked-up 
and stored in memory, preferably a short time before said contrast product 
penetrates into the field of the image under observation. A second image 
is then picked-up and stored in the same position when the contrast 
product has spread over a certain part of the field of the image. A 
subtraction of these two images picked-up in the same position makes it 
possible to improve the visual display of the circulatory system in the 
portion corresponding to the presence of the contrast product. This 
portion is stored in the memory M.sub.1 . A new relative position is then 
sought between the X-ray unit and the table with a view to ensuring that 
the image displayed in radioscopy is substantially free of contrast 
product. The same operations are then repeated for the purpose of 
transferring another portion of the composite image processed in the 
memory M.sub.1, and so on in sequence. It will readily be apparent that 
certain operations may be reversed. For example, it is wholly feasible to 
reconstruct a complete composite "mask" image (from all the images 
picked-up without any contrast product and stored in memory throughout the 
sequence) and a complete composite image with contrast product before 
subtracting one composite image from the other.