System and method for dynamic background subtraction

An image acquisition system and method for generating a subtracted image motion picture as an imaging device moves relative to a subject includes an imaging device for first generating a sequence of first image frames as the imaging device moves along an imaging path under operator control and later generating a sequence of second image frames as the imaging device automatically moves along the same imaging path under processor control. Motion along the imaging path can be duplicated by sensing and storing information indicative of the position of the imaging device relative to the subject as a function of time and later commanding servo motors connected to the imaging device using that position information. Each frame in the first and second sequence of frames has position information stored therewith identifying the location along the imaging path at which each frame was generated. The first and second sequence of frames may then be aligned using the position information and subtracted on a frame-by-frame basis to obtain a subtraction image motion picture essentially free of background interference to detect change over time. The first and second motion pictures may be made at different points of time under similar circumstances either with or without the use of a contrast medium, and subtracted to obtain a difference picture to highlight changes over time.

FIELD OF THE INVENTION 
The present invention relates to image acquisition systems in the medical 
field and in particular to an image acquistion system for obtaining a 
substrated image motion picture of a subject wherein an imaging device 
which generates the image of the subject moves relative to the subject 
during image generation and storage. 
BACKGROUND OF THE INVENTION 
Image acquisition systems for visualization of various body organs and 
systems such as the cardiovascular system have won wide acceptance. For 
example, coronary cine or video angiograms are generated to discern the 
morphology and function of the coronary arterial vessels. However, other 
structures such as ribs and vertebra (background) diminish the ability to 
view the coronary arterial vessels. Consequently, various methods have 
been devised to decrease the background and enhance the areas of interest. 
For example, photographic or digital subtraction has been used to minimize 
or eliminate background. Background subtraction, as conventionally 
practiced, generally requires that two images be generated and recorded. 
The first image is usually generated without highlighting the vessels of 
interest with a contrast medium. The resultant picture or film is called a 
scout film. The second image is then generated and a second film obtained 
after the contrast medium has been injected into the vascular system so 
that the arterial vessels are highlighted. Common features of the first 
and second films are eliminated by subtracting the first film image from 
the second film image using well-known video (electronic), optical 
(photographic) or digital subtraction techniques. 
In order to effect such subtraction, it is generally necessary to make two 
adjustments. The first adjustment is the registration of the first and 
second films by which corresponding elements of the two films are put into 
congruence. This is most simply done by translation and rotation of one of 
the images relative to the other. If there is any appreciable movement of 
the body between the two images, a more complex intraimage transformation 
may be required. 
The second adjustment is the transformation of image density values into 
units of exposure. This is desirable so that the subtracted image will 
faithfully represent the vessel whether the background is dark or light. 
Of course, the above techniques are useful in any of a number of different 
types of image acquisition systems such as cine, video, cut film, CAT, 
scan or ultrasound image acquisition systems. 
Heretofore video, optical or digital subtraction techniques required that 
the image generator, such as the camera for a cine system or the image 
intensifier for a television or video system, and the subject remain 
stationary relative to each other during the entire time that the first 
and second films or digitized picture were generated. Indeed, one of the 
problems which has existed in prior subtraction techniques has been the 
necessity of assuring that the subject remains perfectly stationary and 
immobile during the entire time that the images are generated and stored. 
Even slight relative movement between the imaging device and the subject 
creates misregistration between the first film and the second film, thus 
resulting in blurring and a consequent lack of definition in the resultant 
subtracted image film. 
This has also prevented extensive use of subtraction techniques in 
angiography because it has been general angiographic practice to pan the 
camera or other imaging device during the injection of the contrast 
medium. By this means, the flow of the contrast medium can be observed 
through the arterial bed without sacrificing details. When it is desired 
to use this panning process substantial amounts of contrast medium must be 
injected into the subject to get the desired degree of definition of the 
arteries without an unacceptacle level of background interference. This 
can be dangerous to the subject. 
A procedure recently adopted to alleviate the danger of high contrast 
medium concentrations has been to take a number of pictures of a subject 
with a decreased amount of contrast medium and thereafter register and 
average the multiple pictures to provide the degree of definition desired. 
Such a technique was the subject of our patent application entitled "Image 
Averaging for Angiography by Registration in Combination with Serial 
Images," U.S. Pat. No. 4,263,916, of which this application is a 
continuation-in-part. This technique is very useful but was particularly 
directed to multiple pictures of the subject at a fixed position relative 
to the imaging device. Such is not the case when the imaging device pans 
the subject. 
Of course, various other techniques are known for combining multiple images 
to form a composite image. However, such techniques have not been 
heretofore used in conjunction with image subtraction where there is 
relative movement between the imaging device and the subject during a 
panning procedure. 
The present invention provides for such subtraction even though there is 
relative movement between the imaging device and the subject by providing 
apparatus whereby the motion between the imaging device and the subject 
can be sensed, stored and later duplicated so that frame-by-frame 
background subtraction can be done. The present invention is applicable in 
the fields of radiography, angiography, digital angiography, ultrasound 
scanning, nuclear imaging, CAT scanning or any other type of imaging. 
More specifically, the present invention comprises apparatus and methods 
for sensing and recording relative position parameters between the imaging 
device and the subject over a period of time to describe completely the 
motion geometry between the imaging device and the subject along a 
movement path selected by the operator. The positional parameters are then 
used to generate commands which drive a servo motor system whereby all or 
part of the imaging device is moved to duplicate the operator's original 
movement path. Reference to the imaging device herein includes the support 
table for the subject, an illumination source and an illumination receiver 
and movement of the imaging device means movement of one or more 
components of the imaging device to effect relative movement between the 
subject and the illumination receiver. 
In accordance with the invention, a first sequence of images is recorded to 
define a first motion picture as movement occurs along the movement path 
under operator control and a second sequence of images is recorded to 
define a second motion picture as subsequent movement occurs along the 
movement path under automatic control. 
The first and second motion pictures may be made immediately after each 
other in which case the first motion picture is made while a contrast 
medium is in the arterial bed of the subject and the second motion picture 
is made after the contrast medium has been substantially diluted or 
otherwise purged from the arterial bed. 
Alternatively, the first and second motion pictures may be made with a 
substantial time interval of even several years therebetween so that the 
subtracted image motion picture will be a difference motion picture 
highlighting the changes which occurred during the time interval between 
the first and second motion pictures. With respect to this latter case, if 
a contrast medium is to be used, as is generally desired in radiographic 
imaging, then both the first and second motion pictures should be 
generated while the contrast medium is in the arterial bed. Otherwise, 
neither the first nor the second motion picture should be made while there 
is contrast medium in the arterial bed. 
The individual frames of both the first and second motion pictures, whether 
or not taken with a contrast medium present are then associated with a 
particular position along the movement path so that every first motion 
picture frame is associated with a corresponding second motion picture 
frame by aligning the frames according to correspondence of the associated 
position information. While there would necessarily be some non-congruence 
due, for example, to subject respiration, movement and cardiac cycle, by 
techniques such as timing or multiple or very slow second motion picture 
runs, it is possible to get or select second motion picture frames which 
are in good registration with the first motion picture frames. 
Translation, rotation and more complex transformations may also be used in 
accordance with the invention to insure satisfactory infraframe 
registration. Similarly, recording or image intensifier information and 
step wedge information enables suitable nonlinear subtraction to be 
carried out. 
SUMMARY OF THE INVENTION 
The present invention comprises an image acquisition system for obtaining a 
subtracted variable density motion picture of selected features of a 
subject where there is relative movement between the subject and a 
receiving means while the motion picture is being taken. The system in 
accordance with the invention includes a source of illuminating energy 
such as an X-ray tube, an ultrasonic source or other suitable means for 
illuminating the selected feature of the subject and a means for receiving 
the illuminating energy and generating a variable density image therefrom. 
A support for the subject is positioned between the illuminating energy 
source and the receiving means. Means are provided for selectively 
changing the relative position between the support and receiving means to 
define a first relative movement path therebetween with position sensing 
means interconnected for sensing the relative position between the support 
and the receiving means as the receiving means and support move relative 
to one another. The position sensing means generates position signals 
representative of the relative position along the first relative movement 
path. Means are provided for thereafter storing the sensed position 
signals and subsequently generating position control signals from these 
stored position signals. 
The position control signals are provided to a means for driving either the 
support or the receiving means to vary the relative position between the 
support and the receiving means in response to the position control 
signals to thereby define a second relative movement path which is 
substantially duplicative of the first relative movement path. 
Means are also included for generating a first motion picture of the 
variable density image of the selected feature of the subject during the 
relative movement along the first relative movement path and generating a 
second motion picture of the variable density image of the selected 
feature of the subject during the relative movement along the 
substantially duplicative second relative movement path. The first motion 
picture is defined by a plurality of first frames where each first frame 
defines a picture taken at a specific geometric position along the first 
relative movement path. Similarly, the second motion picture has a 
plurality of second frames where each second frame defines a picture taken 
at a specific geometric position along the second relative movement path. 
Means for aligning the first frames in the first motion picture with the 
second frames in the second motion picture are provided whereby each 
aligned first frame and second frame represent variable density images at 
substantially corresponding geometric positions relative to the subject 
along the respective first and second relative movement paths. Finally, 
means are provided for subtracting each second frame taken along the 
second path from each corresponding aligned first frame taken along the 
first path for defining a plurality of subtracted variable density frames 
to define the subtracted variable density motion picture.

DETAILED DESCRIPTION 
The present invention encompasses an image acquisition system and method 
for generating a first motion picture as relative motion occurs between an 
imaging device, which includes an illumination source, a support table for 
the subject and an illumination receiver, and a subject and automatically 
controlling either or both of the illumination receiver and support table 
to duplicate the movement path at a later time while the imaging device is 
generating a second motion picture. The frames of the first and second 
motion pictures are then aligned and registered and then subtracted to 
obtain a subtracted image motion picture where only the differences 
between the first motion picture and the second motion picture are 
visible. 
The resultant subtracted image motion picture can be used in a number of 
ways. For example, the apparatus and method is useful in angiography where 
a contrast medium is injected into a subject and the first motion picture 
made while the contrast medium is in the arterial bed of the patient or 
subject. The first motion picture can therefore be made as, for example, 
the imaging means pans a stationary subject wherein only dilute solutions 
of contrast medium are used. The second motion picture is then generated 
along the same movement path as soon as the contrast medium has become 
sufficiently diluted or has otherwise been purged from the arterial bed of 
the subject. Generally the second motion picture can be made within 
several minutes after the first motion picture has been made. The 
resultant subtracted image motion picture is then a motion picture of the 
arterial bed highlighted by the contrast medium flowing therethrough. The 
subtraction process substantially eliminates the density variations due to 
background images of the bone and tissue. 
The present invention can also be used to generate a subtracted image 
motion picture which highlights the differences in the subject's 
physiology which have occurred over time such as the degeneration or 
healing of particular organs of the subject. Such a subtracted image 
motion picture showing differences can be made by first positioning a 
subject on a support table and thereafter generating the first motion 
picture as the operator moves the illumination receiver, support table or 
both along an operator selected first movement path. The first motion 
picture may be generated with or without a contrast medium present in the 
arterial bed of the subject. 
At a later time, hours, weeks, or even years later, the subject is again 
positioned on the support table at a position as nearly identical as is 
possible to that which existed when the first motion picture was taken. 
The original operator's selected movement path is then automatically 
duplicated under computer control by commanding movement of the 
illumination receiver, support table or both to duplicate the movement 
previously selected by the operator. If the first motion picture was 
generated with a contrast medium in the arterial bed, then the second 
motion picture should preferably also contain the contrast medium in the 
arterial bed of the subject. In general a contrast medium will be utilized 
when the imaging acquisition system is radiographic. However, contrast 
medium is not used when the imaging device is an ultrasonic system. Of 
course, it will be appreciated that even with radiographic image 
acquisition systems a contrast medium is not absolutely necessary to 
generate a subtracted motion picture indicative of the differences in 
certain organs of a subject which have occurred over an interval of time. 
Specific apparatus for effecting relative movement between the imaging 
device and the subject are well known. For example, the bilateral 
suspension system produced by General Electric Medical Systems Department 
provides an apparatus whereby a fluorocon (illumination receiver) can be 
positioned along three orthogonal translational axes. Once the fluorocon 
is so positioned, the support table moves with the fluorocon remaining 
stationary. 
Other systems permit the illumination receiver to move along up to three 
orthogonal translational axes or up to three orthogonal rotational axes. 
In such systems, the support table remains stationary while the 
illumination receiver moves relative to the support table and subject. It 
is therefore possible, utilizing such systems, for the operator to move 
the illumination receiver in one or more of up to six different degrees of 
freedom. 
In addition to moving the support table, the illumination receiver or both 
to define the movement path, it is also possible to move the source of 
illuminating energy. Generally, the source of illuminating energy is 
positioned on one side of the subject and the illumination receiver is 
positioned on the other side of the subject. It may be necessary to move 
the illuminating energy source as the illumination receiver, for example, 
moves so that the illumination receiver will not move out of the field of 
energy emanating from the illuminating energy source. 
Although any one of a number of different mechanisms can be incorporated to 
provide for translational or rotational motion of either the illumination 
receiver or the support table and suitable position changes of the 
illuminating energy source as required, the system to be described 
hereafter and illustrated in FIG. 1 provides for a stationary support 
table and a movable illumination receiver where movement is permitted 
about one or more of three orthogonal translational axes and three 
orthogonal rotational axes. It will be appreciated, however, that in 
accordance with the invention any other movement mechanism is possible and 
the system illustrated in FIG. 1 is to be taken as being only illustrative 
of the invention. 
Therefore, referring to FIG. 1, one possible six degree of freedom system 
10 which may be used in accordance with the invention incorporates a pair 
of elevated spaced-apart stationary rails 12 and 14 with a beam 16 movably 
mounted on the rails 12 and 14 to provide translational movement along an 
axis 18 parallel to the rails 12 and 14. A telescopic column 20, having an 
end 24 remote from the beam 16, is movably attached to the beam 16 by any 
suitable means to provide translational movement along an axis 22 parallel 
to the beam 16 and orthogonal to the axis 18. The telescopic column 20 has 
a remote end 24 to which the illumination receiver 26 is rotatably 
attached. The telescopic column moves vertically up and down along an axis 
28 which is orthogonal to both axes 18 and 22. 
The illumination receiver 26 is mounted to the end 24 of the telescopic 
column 20 to be rotatable about one or more of up to three orthogonal 
axes. For example, the illumination receiver 26 may be attached to the 
telescopic column 20 to be rotated about the vertical axis 28; to be 
tilted about a tilt axis 34 which is perpendicular to the vertical 
rotation axis 28; and pivoted about a pivot axis 36 which is perpendicular 
to both the vertical rotation axis 28 and the tilt axis 34. 
The particular interconnection and support mechanisms by which the 
illumination receiver can be rotatably mounted to rotate, tilt and pivot 
about orthogonal rotation axes are well known and will not be specifically 
described herein. Of course, it will be appreciated that many other 
possible mechanical linkages are possible within the scope of the present 
invention to provide for translation motion along one or more orthogonal 
axes or rotational motion about one or more rotational axes to effect 
relative movement between the illumination receiver and the subject 31 on 
the table 30. 
While mechanical apparatus are available which enable an operator to 
selectively move either the support table 30 or the illumination receiver 
26 in any of up to six degrees of freedom, it has not been heretofore 
known to generate a first motion picture as the position of the 
illumination receiver 26 is moved along a first path relative to the 
subject 31 and thereafter to retrace that movement path to generate a 
second motion picture which can be aligned and registered and thereafter 
subtracted from the first motion picture on a frame-by-frame basis to 
achieve a substracted image motion picture of a subject while there is 
relative motion between the subject 31 and the illumination receiver 26. 
Therefore, in accordance with the invention, apparatus illustrated in FIG. 
1 includes a plurality of position sensing devices whereby the 
translational position of the illumination receiver along each 
translational axis 18, 22 and 28 and the angular position about each of 
the rotational axes 28, 34 and 36 during the movement of the illiumination 
receiver by an operator is sensed. In one embodiment, the sensors may be 
suitable servo motorswhich are well known in the field of servo systems. 
It is well known that servo motors can serve as either a signal generator 
or a motor depending on whether the servo is caused to rotate in response 
to an external mechanical force or in response to an electrical signal 
respectively. Thus, by suitably connecting servo motors to detect or cause 
translational and rotational movement along or about each of the 
translational and rotational movement axes, it is possible to generate a 
position signal for each which varies with time and is indicative of the 
movement path along each axis. By digitizing and storing each of these 
signals and then later using the stored signals to command the servos, it 
is possible to duplicate substantially the original movement path selected 
by the operator. 
Thus, referring again to FIG. 1, a suitable rail servo motor 50 may be 
fixed to the movable beam 16 and positioned to engage the stationary rail 
12 so that as the movable beam 16 moves along the stationary rail, the 
rail servo 50 will rotate thereby generating a signal indicative of the 
position of the movable beam 16 relative to the stationary rail 12. Such a 
mechanical interrelationship may be provided by suitable linear gear teeth 
13 along the rail 12 or any other suitable means. 
Alternatively, a position signal may be generated indirectly by a suitable 
sensor. One such indirect sensor may be provided by a resistive strip 
positioned along the entire length of one of the stationary rails where 
the resistive strip has a resistance gradient along its length. A pressure 
applying flange could then be fixed to the movable beam and positioned to 
press against the resistive strip and complete a circuit. As the beam 16 
is moved, the position at which pressure is applied would change and the 
resistance would also change. The variation in voltage across the resistor 
strip would thus be indicative of the position of the movable beam 16 
relative to the stationary rails 12 and 14. Of course, any other suitable 
mechanism could be provided whereby a signal was generated either directly 
or indirectly by analog or digital means to indicate the position of the 
movable beam 16 along the stationary rails 12 and 14. 
In a similar manner, a beam servo 52 fixed to the telescopic column 20 may 
be provided to engage a gear 17 or other mechanism along the movable beam 
16 to generate an electronic signal indicative of the position of the 
telescopic column 20 along the movable beam axis 22. 
Likewise, a column servo 54 could be fixed to the telescopic column 20 for 
generating an electronic signal representative of the vertical position of 
the telescopic column 20 along the vertical axis 28 at each point in time 
thereby indicating the vertical height of the illumination receiver. 
In a similar fashion, a rotation servo motor 56, a tilt servo motor 58 and 
a pivot servo motor 60 may be suitably and conventionally mounted to 
generate signals indicative of the angular position of the illumination 
receiver 26 about the rotation axis 28, the tilt axis 34 and the pivot 
axis 36, respectively. 
The resultant time varying analog position signals from the servo motors 
50, 52, 54, 56, 58 and 60 are interconnected to suitable analog-to-digital 
converters 48 which digitize each of the analog position signals. Each 
digitized position signal comprises a series of numbers representative of 
the position of the illumination receiver along a particular movement axis 
at a particular instant of time. 
In operation, an operator simply grasps and manually moves the illumination 
receiver, 26 along a desired movement path relative to the subject 31. As 
the illumination receiver 26 moves, each of the above-described servo 
motors generate analog or digital signals indicative of the position of 
the illumination receiver along each of the three orthogonal translational 
axes and about each of the three orthogonal rotational axes. In accordance 
with the invention, the position signals, whether analog or digital, are 
then stored in a suitable memory such as a computer memory. 
While the operator is moving the illumination receiver relative to the 
subject 31 thereby generating the position signals, the illumination 
receiver is generating and storing a sequence of image frames to define a 
first motion picture. Each frame of the first motion picture is stored 
utilizing a suitable memory such as a computer core, disc, tape cassette 
memory or a photographic memory medium such as photographic film. As 
previously described, the second motion picture is then generated by the 
illumination receiver as the system retraces and substantially duplicates 
the first movement path. 
It is, of course, desired that the subject be in substantially the same 
position on the support table when the first and second motion picture are 
generated. Such would generally be the case where the first motion picture 
and the second motion picture are made within several minutes according to 
the method previously described where the first motion picture is taken 
with a constant medium in the arterial bed of the subject and the second 
motion picture is taken after the contrast medium has either become 
sufficiently dilute or has been purged from the arterial bed of the 
subject. 
On the other hand, in the second situation previously described where the 
time interval between the generation of the first motion picture and the 
generation of the second motion picture is several hours, weeks or even 
years so that the differences by way of either healing or deterioration of 
body organs is shown, it is necessary to position the subject in 
substantially the same position during the generation of the second motion 
picture as existed when the first motion picture was generated. However, 
in accordance with one aspect of the invention, the subject need not be 
placed in exactly the same position on the table so long as the 
approximate position is substantially the same. Specifically, in 
accordance with a registration method which may be used in conjunction 
with the present invention, it is possible to shift each frame up and down 
and right and left and to, in effect, twist each frame a small amount to 
achieve registration between the two corresponding but slightly 
misregistered motion picture frames of the first and second motion 
pictures respectively. Consequently, it is not essential, as in prior 
subtraction systems, to position the subject in exaclty the same location 
on the support table to achieve registration of the images. The specific 
means for achieving such frame-to-frame registration between the first and 
second motion pictures will be described in greater detail hereafter. 
Referring again to FIG. 1, the previously stored position data is outputted 
from the computer memory and is converted back to a time varying analog 
signal in the digital-to analog converter 48 wherein the analog position 
signals are interconnected to drive the rail servo motor 50, the beam 
servo motor 52, the column servo motor 54, the rotation servo 56, the tilt 
servo 58, and the pivot servo 60 respectively, to cause the illumination 
receiver to duplicate the movement path originally followed by the 
operator. The second motion picture is then generated and stored as the 
illumination receiver retaces the original movement path selected by the 
operator in the same manner as when the first motion picture was 
generated. 
Referring now to FIG. 2, a radiographic system in accordance with the 
invention may include, for example, an illumination source such as an 
X-ray tube 70 for providing a source of X-rays, a support table 72 for 
supporting a subject 74 in a beam of X-ray 76, and an illumination 
receiver 78 such as a fluorocon positioned above the subject 74 to receive 
the X-rays 76 passing through the subject 74. The illumination source, 
support table and illumination receiver together comprise the imaging 
device. In such an arrangement, the X-rays passing through the subject are 
variably attenuated depending on the local density of the subject to 
produce a variable density image of the subject which is received by the 
illumination receiver 78. A suitable recording device such as a cine 
camera 80 or a video digitizer 109 and memory 110 generates the first 
motion picture as the illumination receiver and/or support table are moved 
by the operator along the movement path. 
As movement occurs along the first movement path, servos or suitable 
indirect position sensors 82 generate a plurality of position signals 
indicative of the geometric position of the illumination receiver 78 
relative to the support table 72 and hence the subject 74. Each of the 
position signals is then digitized in a digital-to-analog converter 84 to 
define digitized position signals which are stored in a position parameter 
memory 86. Subsequently, the digitized position information stored in the 
position parameter memory 86 is sequentially outputted in the same order 
that it was stored and is converted to a plurality of analog signals in 
the digital-to-analog converter 88. The resultant analog signals are each 
directed to one of the servo motors 90 to move the illumination receiver 
78 and/or support table 72 along a path substantially the same as the 
first relative movement path. 
As the illumination receiver 78 and/or support table 72 move in response to 
the servos 90 to duplicate the first relative movement path, the images 
are again recorded as before on the recording device to generate the 
second motion picture. 
In the embodiment where cinegraphic motion pictures are generated, the 
first cinegraphic motion picture and the second cinegraphic motion picture 
may be optically aligned on a frame-by-frame basis. A photographic 
substraction process may then be performed on a frame-by-frame basis to 
generate a subtracted cinegraphic motion picutre. The particular theory 
and method of photographic subtraction is fully described in the article 
"Photographic Subtraction, I. Theory of Subtraction Image" by Hardstedt 
and Welander, Acta Raciologica Diagnosis Vol. 16 (1975); and "Photographic 
Subtraction, II. Technical Aspects and Method" by Hardstedt, Rundelius and 
Welander, Acta Radiologica Diagnosis 17 (1976) Fasc. 1 January, both of 
which articles are herein incorporated by reference. 
Alternatively, the first cinegraphic motion picture and the second 
cinegraphic motion picture may be digitized on a frame-by-frame basis in 
cinegraphic digitizer 92 wherein each cinegraphic film is divided into an 
array of picture elements (pixels) where each pixel has a unique x,y 
coordinate address within the frame and each pixel has a numerical value 
representative of the illuminating energy density at that pixel coordinate 
address. Each digitized cinegraphic frame may then be stored in a memory 
94 so that each frame of the first cinegraphic motion picture and each 
digitized frame of the second motion picture are stored in the memory 94. 
The cinegraphic digitizer 94 may, for example, be simply a television video 
camera which is used to transfer the motion picture from a film medium to 
an electronic or video tube medium as the motion picture is being run. The 
video information can then be easily digitized in a conventional manner as 
described in "Computerized Fluoroscopy: Digital Subtraction for 
Intravenous Angiocardiography and Arteriography" by Crummy et al., 
December 1980, AJR:135, pp. 1131-1140, which article is herein 
incorporated by reference. 
The digitized cinegraphic motion picture stored in memory 94 and the second 
cinegraphic motion picture stored in the memory 94 may then be aligned as 
described in our copending application Ser. No. 890,103 filed Mar. 3, 1978 
which is herein incorporated by reference, so that each frame of the 
digitized first motion picture is aligned with the frame of the digitized 
second motion picture which was generated when the relative position 
between the imaging device 78 and the subject 74 along the movement path 
was the same as that at which the aligned first motion picture frame was 
generated. Such alignment may be done manually (visually) prior to the 
cinegraphic digitation so that the first frame of the first cinegraphic 
motion picture will correspond to the first frame of the digitized second 
cinegraphic motion picture. Each subsequent frame of the first cinegraphic 
motion picture and the second cinegraphic motion picture will then be 
likewise aligned provided the first and second motion picture were taken 
at the same film speed. 
Although such alignment may be reasonably good, it is possible and indeed 
likely that the subject 74 will be in slightly different positions in the 
first and second motion pictures. Furthermore, it is likely that the 
individual aligned frames of the first and second motion pictures will 
have been generated at very slightly different geometric locations along 
the first relative movement path. Consequently, there is a need, once the 
above-described frame-to-frame alignment has been made, to provide a means 
of registering the images in each pair of aligned motion picture frames. 
Such registration of each second motion picture frame with each aligned 
first motion picture frame may be achieved using processor 96 by a 
suitable computer algorithum whereby subgroups of pixels in each of the 
second motion picture frames are sequentially shifted up or down until, 
for example, the sum of the difference between corresponding pixel density 
values of the subgroups is at a minimum. This subgroup registration is 
performed on a computer using the Fortran program entitled LANDMARK, a 
listing of which is appended hereto in Appendix A. 
Because adjacent subgroups of pixels may overlap or become spaced apart 
after the above shifting process of individual pixel subgroups, a second 
computer program may be used to further shift individual pixel subgroups 
up or down or sideways to eliminate such gaps and overlaps. This procedure 
may likewise be accomplished on a computer using a Fortran program 
entitled TRANSFOR and a program entitled P1R, a listing of each of which 
is appended hereto in Appendix B. Utilizing such an approach, each frame 
in the second motion picture can be adjusted up and down, sideways, and 
can in effect be rotated or twisted to achieve optimal registration with 
the corresponding frame in the first motion picture. 
Once registration of the two frames has been achieved through the 
above-identified pixel position adjustment procedure, the illumination 
energy density value of each pixel of each second motion picture frame is 
subtracted (either linearly or nonlinearly) from the illumination energy 
density value of the registered pixel of the corresponding first motion 
picture frame to generate a plurality of digitally subtracted frames which 
together define a digitally subtracted motion picture. The digital 
subtraction may likewise be done on a computer using a fortran subtraction 
program entitled SUBTRACX, a listing of which is appended wherein hereto 
as Appendix C. The digitally subtracted motion picture can be displayed on 
a television monitor or other suitable device in a conventional manner. 
Of course, it will be appreciated that digital subtraction of still 
radiographic images is known and that such subtraction may be performed 
linearly or nonlinearly. More specifically, it has been known that images 
may be subtracted to produce a resultant image which presents the features 
of interest separated from features which are the same in each of two 
images. This may be done, for example, in angiography where a scout film 
of the background is first made followed by the making of another film 
after injection of a contrast medium in an artery. By subtracting the 
scout film from the contrast medium film, the background is removed 
leaving only the image of the artery with the contrast medium therein. 
Such subtraction may be done by analog techniques as with video systems or 
by digital techniques if the image is so represented. 
If the densities of the images are well approximated by a linear function 
of the irradiation or other illuminating energy, then simple linear 
subtraction is adequate. This happens, for example, when the range of 
density is small. In general, however, the range of densities is large and 
nonlinear so that a more suitable technique must be employed to obtain a 
more satisfactory subtraction image. 
More specifically, it is known that a chord length through a vessel is a 
function of the incident irradiation which is attenuated when the 
biological vessel is filled with contrast medium. Referring to FIG. 3, 
when there is no contrast medium present in the vessel and the background 
is uniform tissue, the incident radiation is attenuated to I.sub.ST, where 
the ST signifies a scout film with the irradiation passing only through 
tissue. For the same cord through the same vessel under identical 
conditions, with the vessel filled with a uniform mix of contrast medium 
and blood, the attenuated irradiation is I.sub.CT where CT signifies a 
contrast medium through only tissue. If the contrast medium concentration 
is not too great, then the chord length is well approximated by the linear 
function I.sub.ST -I.sub.CT .ident.A.sub.T. 
If the background is somewhat different, i.e., if it includes bone in the 
same ray that includes the vessel cord, then the chord length would again 
be approximated by the differential attenuation I.sub.SB -I.sub.CB 
.ident.A.sub.B, where the B means that there is bone as part of the 
background for both the scout and contrast medium films. 
It is well known that film densities are a nonlinear function of the 
attenuated irradiation. As a result, the chord length which produced an 
attenuation differential of A.sub.T in tissue would produce a film density 
differential of D.sub.ST -D.sub.CT .ident.F.sub.T. An equivalent chord 
length in a background which includes bone would produce a film density 
differential of D.sub.SB -D.sub.CB .ident.F.sub.B. In general, F.sub.T 
does not equal F.sub.B, as illustrated in FIG. 3. 
Given the relation of density and irradiation it is possible to obtain the 
irradiation value I corresponding to any density value D, i.e., I=f(D), so 
that the differential attenuation between the scout and contrast medium 
films results in an unbiased estimate of relative chord length. This 
relation can be empirically derived by use of a suitably constructed wedge 
X-rayed under the same conditions as at least one of the contrast medium 
or scout frames as illustrated by the wedge 85 in FIG. 3. 
In order to obtain an irradiation attenuation A from a differential film 
density F, it is merely necessary to divide the film density differential 
F by the local derivative value of I=f(D). As a result, a value is 
obtained which is proportionate to the chord length whatever the 
background. 
For example, referring to FIG. 3, if the density D of the image is a 
nonlinear function of the irradiation I from the irradiating source, all 
density values across the image can be normalized so that, for example, a 
difference between the density between the one portion of the scout frame 
and a registered portion of the contrast medium frame where the background 
is tissue will be the same as the difference between a portion of the 
scout frame and a registered portion of the contrast medium frame where 
the background is bone. Such normalization can be achieved utilizing the 
curve shown in FIG. 3 by simply dividing F.sub.T which is the difference 
D.sub.ST -D.sub.CT by the slope of the curve I=f(D) at a point along the 
curve between D.sub.ST and D.sub.CT. In general, the slope along the curve 
I=f(D) between the point D.sub.ST and D.sub.CT will be approximately 
constant. 
In the similar manner, the density value of the portion of the image 
through bone D.sub.CB is subtracted from the density of the registered 
portion of the scout film D.sub.SB yielding a difference F.sub.B which, 
when divided by the average slope or the slope at some point of the curve 
I=f(D) between the points D.sub.SB and D.sub.CB, will yield the same 
normalized density value. Such is the desired result, since the only 
difference in both cases is the fact that the irradiation passes through a 
different background. 
Referring again to FIG. 2, the first and second motion pictures may be 
generated directly from the video signals generated by the illumination 
receiver 78 as when the illumination receiver 78 is a fluorocon. In such 
an embodiment the video signals generated by the illumination receiver 78 
are digitized in an analog-to-digital converter 109 and then stored in a 
video frame and position memory 10. In order to be able to align each 
digitized first motion picture video frame with a corresponding digitized 
second motion picture frame, it is necessary to be above to identify the 
point along the relative movement path at which each frame was generated. 
One method of storing the geometric position data for each video frame is 
to generate a sample enable signal after each frame has been digitized and 
stored in the video and frame position memory 110. The sample enable 
signal then closes a switch 112 which causes the position data to be 
sampled and stored along with the video information in the video and frame 
position of memory 110. Thus, each frame of video information stored in 
the memory 110 has associated with it position information defining the 
position along the movement path at which the video frame was generated. 
Subsequently, when the movement path is duplicated, video information is 
again generated and stored in a similar manner in the video and frame 
position memory 110. Position data is again sampled and stored at the end 
of the generation of each frame of the second motion picture. 
The position data sample for the second motion picture frames may be taken 
by either sampling the digitized command information transferred from the 
position parameter memory 86 to the digital-to-analog converter 88 or 
alternatively, may be the digitized position data from the 
analog-to-digital converter 84 which is indicative of the actual position 
of the illumination receiver. 
Once the sequence of video frames and the associated position of each frame 
has been generated along the first movement path for the first motion 
picture and the second motion picture, the frames of the first motion 
picture and the second motion picture may be processed in a processor 120 
in a manner similar to that previously described. 
More specifically, the various frames of the first motion picture can be 
aligned with the corresponding frames of the second motion picture by 
comparing the position data stored with each set of frame pixel data and 
aligning those frames having substantially the same stored position data, 
thus indicating that the frames were generated at the same relative 
position along the relative movement path. 
It will, of course, be appreciated that the position data of the 
corresponding frames of the first motion picture and the second motion 
picture may not be precisely identical simply becuase it is unlikely that 
the sampling of the position data would have occurred at precisely the 
same points along the movement path for both the first and second motion 
pictures. Thus, the processor 120 may be utilized to round off or truncate 
the position data or to otherwise allow some variance in comparing the 
specific position data of the first and second motion picture frames to 
generate a positive comparison of position data so that corresponding 
frames of the first motion picture and the second motion picture can be 
aligned. 
Once the individual frames of the first motion picture and the second 
motion picture have been aligned, the processor 120 can perform a suitable 
registration adjustment within one or both of the plurality of pairs of 
aligned frames as previously described and thereafter perform either 
linear or nonlinear subtraction as previously described. 
It will also be appreciated that while previous reference has been made to 
registration between aligned frames of the first motion picture and the 
second motion picture, a composite first motion picture may be generated 
and used by first registering and then averaging several adjacent frames 
of motion picture information utilizing the registration routine attached 
hereto. Similarly, a composite second motion picture comprised of a 
plurality of sequential composite frames may be generated and used where 
each composite frame is generated by registering several adjacent frames 
of the first motion picture and then averaging those registered frames in 
accordance with the techniques previously described in conjunction with 
our Application Ser. No. 890,103 filed Mar. 27, 1978. 
It will be appreciated of course that various other embodiments and 
variations may be made in the above-described invention without departing 
from the spirit of the invention wherein apparatus and methods have been 
provided for generating a first motion picture as an illumination receiver 
or support table or both move along a relative movement path, and 
thereafter generating a second motion picture by automatically causing the 
illumination receiver or support table or both to follow the me relative 
movement path as that selected originally by the operator. The first 
motion picture and second motion picture are thereafter subtracted on a 
frame-by-frame basis after suitable alignment and registration to obtain a 
subtracted motion picture of a subject whereby background information 
common to both is eliminated and only the differences appear. 
##SPC1##