Method and system for cardiac computed tomography

System and method are set forth enabling reconstruction of images of desired "frozen action" cross-sections of the heart or of other bodily organs or similar objects undergoing cyclic displacements. Utilizing a computed tomography scanning apparatus data is acquired during one or more full rotational cycles and suitably stored. The said data corresponding to various angular projections can then be correlated with the desired portion of the object's cyclical motion by means of a reference signal associated with the motion, such as that derived through an electrocardiogram--where a heart is the object of interest. Data taking can also be limited to only the times when the desired portion of the cyclical motion is occurring. A sequential presentation of a plurality of said frozen action cross-sections provides a motion picture of the moving object.

BACKGROUND OF INVENTION 
This invention relates generally to medical diagnostic apparatus and 
methodology and more specifically relates to X-ray scanning apparatus and 
methodology of the type associated with computed tomography. 
Within recent years, much interest has been evidenced on the part of 
medical diagnosticians in the field now widely known as "computed 
tomography." In a typical procedure, an X-ray source and detector 
apparatus are positioned on opposite sides of the portion of the patient 
which is intended for examination. In early prior art these paired 
elements are made to transit across the body portion to be examined while 
the detectors measure the X-ray absorption at the plurality of 
transmission paths defined during the transit process. Periodically as 
well, the paired source and detector means are rotated to a different 
angular orientation about the body and the transit process repeated. A 
very high number of absorption values may be yielded by procedures of this 
type and the relatively massive amounts of data thus accumulated are 
processed by a digital computer which cross-correlates the absorption 
values to thereby derive absorption values for a very high number of 
points (typically in the thousands) within the section of the body being 
scanned. This point by point data can then be combined to enable 
reconstruction of a matrix (visual or otherwise) which constitutes an 
accurate depiction of the density function of the bodily section examined. 
The skilled diagnostician, by considering one or more of such sections, 
can often diagnose various bodily ailments such as tumors, blood clots, 
etc. 
Later developments in the computed tomography field are demonstrated in the 
copending application of John M. Pavkovich and Craig S. Nunan, Ser. No. 
643,894, filed Dec. 23, 1975 and entitled "Tomographic Apparatus and 
Method for Reconstructing Planar Slices From Non-Absorbed Radiation," and 
in the copending application of John M. Pavkovich entitled "Apparatus and 
Method for Reconstructing Data" filed Dec. 23, 1975 under Ser. No. 
643,896. Both of these applications are assigned to the same assignee as 
is the present application. 
The apparatus disclosed in the last cited applications utilizes a fan beam 
source of radiation coupled with application of a convolution method of 
data reduction with no intervening reordering of fan rays, to thereby 
eliminate the errors and delay in computation time which would otherwise 
be involved in such reordering. The radiation source and detector means 
are positioned on opposed sides of the portion of the patient being 
examined and these elements are made to rotate through a revolution or 
portion thereof while the detectors measure the radiation absorption at 
the plurality of transmission paths defined during the rotational process. 
In a typical apparatus embodiment of the Pavkovich et al type of apparatus, 
an assembly is provided which is rotatable about an axis extending along a 
central opening defined therein, together with means for positioning the 
bodily portion to be examined within a central opening so that the axis of 
assembly rotation is perpendicular to a thin, generally planar section of 
the body portion being scanned. A source of penetrating radiation, i.e., 
of X-rays or gamma rays is mounted on the assembly toward one side thereof 
a provides radiation in the form of a fan beam. Means are provided for 
rotating the assembly so that the fan beam impinges upon the bodily 
portion at a plurality of incident directions. Detection is enabled by 
means positioned on the assembly opposite the source, which thus detects 
non-absorbed radiation proceeding laterally along the section. 
In general, computed tomography apparatus of the foregoing type has found 
its principal application to examination of bodily structures or the like 
which are in a relatively stationary condition. For example, currently 
available computed tomographic apparatus yields tomographic images of the 
beating human heart which suffer from degradation because of cardiac 
motion. 
SUMMARY OF INVENTION 
Now in accordance with the principles of the present invention, a system 
and method is provided which enables highly effective "stop action" or 
"frozen" images to be generated of cross-sections through organs of the 
body or the like undergoing cyclic displacements as, for example, images 
of the human heart during selected phases of the cardiac cycle. 
Pursuant to the present invention, and utilizing computed tomography 
apparatus of the aforementioned type, raw projection data is collected in 
consequence of one or more continuous cycles of rotation of the scanner 
portion of the said apparatus, the said data being stored for use in the 
reconstruction process. By suitable correlation of the stored data with 
that portion or phase of the cyclic movement for the body organ which is 
to be examined, the desired cross-sectional image may be reconstructed. In 
addition, animated presentation is obtained by rapidly projecting 
cross-sections of sequential positions of the object throughout its cycle 
of movement. 
In order to enable this correlation, means are provided for positively 
identifying the gathered data with portions of the organ cycle. In the 
case of the heart, for example, the ECG signal may be appropriately 
identified with the scanner projection angle, as for example, by means 
directly marking the ECG record with indicia representing the associated 
projection angle; or the ECG reference points may be included with the 
stored data or separately stored so that same may be thereafter identified 
with the projection angles corresponding to the portion of the cycle 
sought to be investigated. 
In application of the invention to analysis of a cardiac cycle, the R--R 
interval for each cardiac cycle is typically subdivided into seven equal 
segments. Within any cycle the segments are all of equal length. The CT 
angle for projections corresponding to the same segment from each of the 
cardiac cycles are then identified, i.e., by the techniques 
aforementioned. In reconstructing the final seven images, however, the 
angular projections from two successive segments in each of the recorded 
beats are preferably utilized, as such procedure yields a considerable 
increase in number of available projections, thereby substantially 
improving generation of image density data--with resultant improvement in 
image "quality" without any severe degradation resulting from blurring. 
The number of angular projections available pursuant to the invention, may 
similarly be increased by increasing the total number of scan rotations 
utilized, e.g., to 3 or 4 or more. 
In another aspect the data, instead of being taken throughout the 
rotational scanning, can be taken only at the times when the desired 
portion or portions of the cylical motion is occurring. This is 
particularly useful when only a single or relatively few cylical positions 
are known in advance to be the only positions of interest. Exposure to 
X-rays is minimized by this approach.

DESCRIPTION OF PREFERRED EMBODIMENT 
In FIG. 1 herein an external perspective view appears, the view being 
somewhat simplified in nature, and setting forth scanning apparatus 10 of 
a type suitable for use with the invention. This view may be considered 
simultaneously with FIG. 2. In general, these Figures disclose only prior 
art details of the devices illustrated therein, and will serve primarily 
to establish an environment for illustrating the present invention. 
Apparatus 10 comprises generally an external casing 12, within which a 
frame (not seen) supports a rotatable assembly 16, better seen in FIG. 2. 
Scanning apparatus 10 forms part of a computed tomography system, the 
remaining elements of which principally include control, image 
reconstruction elements, and image display elements, most of which are 
contained at a control and image reconstruction station. Apparatus 10 is 
in communication with the said station by various control lines, as 
schematically indicated at link 18 in FIG. 1, which is to say that digital 
information developed in consequence of the scanning operations effected 
by apparatus 10 are furnished to such station; and the latter in turn 
provides both control information for actuating apparatus 10, as well as 
the various power and excitation potentials, e.g., for the radiation 
source, the motor, and for other elements which are present in said 
apparatus. 
Rotatable assembly 16 includes an outer cylinder 22 of stainless steel or 
other metal, and is adapted to be rotated in direction 24 about its 
central axis 26 by means of a motor 28, the drive wheel of which bears 
against a drive collar 32 secured about cylinder 22. Wheel 30 may thus 
include a rubber surface 34 or the like which by virtue of its high 
coefficient of friction is effective in causing non-slip rotation of 
cylinder 22. Other drive mechanisms may be similarly utilized. For 
example, a timing belt driven by motor 28 may engage with a suitable track 
about cylinder 22 to effect the desired rotation. 
The central opening 36 of assembly 16 serves to receive a patient 54 who is 
to be examined within apparatus 10. A sleeve 38 of plastic or the like, is 
secured to casing 12 and provides a stationary reference frame--which has 
certain advantages, especially psychologically, for the patient who is 
positioned within opening 36. 
The patient 54 during use of apparatus 10 is positioned upon the top 
surface 42 of a positioning bench 40, the surface 42 being movable along 
axis 26 so as to enable movement of the patient into such apparatus. Bench 
40 may include actuating means which enable incremental advance of same so 
as to facilitate successive transverse scan sections through the body of 
patient 54. The advance of the patient can in some instances also be 
continuous. Such means can also enable movement of the bench in other 
directions to facilitate the patient positioning. 
The forward end of assembly 16 carries a plate 48 at the periphery of which 
is mounted a radiation source 50, preferably comprising an X-ray source 
capable of projecting an X-ray pattern in the form of fan beam 52. Fan 
beam 52 may be yielded by collimator 51 which is positioned in front of 
the X-ray emission source, as is known in the art. Fan beam 52 is 
preferably (though not necessarily) at least as wide as the object to be 
examined which in the present instance constitutes patient 54. 
A collimator/detector assembly generally indicated at 56 and consisting of 
a detector means 58 and a collimator means 60, is mounted directly 
opposite source 50, i.e., toward the opposite edge of plate 48. Although 
other types of detectors suitable for use with X-rays and similar 
electromagnetic radiation may be utilized such as crystal scintillators 
coupled with photomultipliers or photodiodes and so forth, detector means 
58 preferably comprises an array of ionization chambers, effectively 
located in side-by-side relationship and aligned with respective 
passageways formed by the collimator means 60, which chambers may be of 
the xenon or the xenon-krypton type. Detector means 58 is in very close 
physical proximity to a signal processing and conditioning means 64. This 
close proximity has important advantages in minimizing the possibility of 
introducing spurious signals into the various detector channels which can 
arise from the high potentials associated with the X-ray source or the 
like. 
In the case of X-ray diagnosis, the thickness of fan beam 52 as defined by 
the collimators is typically between about 1 mm and 15 mm at the middle of 
the object. It will be understood that as the source-detector array 
undergoes relative rotation with respect to the patient, (continuously 
where exact reconstruction is desired) over a time of approximately 1 to 
15 seconds, readings of absorbed radiation are measured by detector means 
58. Typically, for example, on the order of 301 individual detector cells 
may constitute the detector array 58, which cells are effectively in 
side-by-side relationship. Typically, therefore, a set of measurements may 
be taken at each successive one degree increment of rotation (preferably 
by pulsing the source on at each said one degree position), so that 
360.times. 301 values of measured (transmitted) radiation are obtained 
during each 360 degree cycle or rotation. Pursuant to the present 
invention, the data acquisition may be completed during one relative 
rotation, i.e., 360 degrees (of the system); however, more commonly 
pursuant to the invention, several successive and continuous rotations 
(i.e. each rotation is continuous and there is not stoppage between 
successive rotations) will be involved in the data acquisition process. 
In the normal course of operating systems of the type thus far discussed, 
data from the collimator/detector assembly 56, after suitable processing 
and conditioning, is provided to the control and image reconstruction 
station, and if a cross-sectional image is to be directly obtained the 
said data is convolved and appropriately stored and later back projected 
with other data to provide an output picture which is a replica of the 
thin cross-section of patient 54 which has been examined. It will, of 
course, be understood that the data need not necessarily be converted into 
a visually discernable picture, but can be expressed in other analytical 
forms, i.e., numerically or so forth. 
In FIG. 3 herein a schematic block diagram is set forth illustrating a 
system 100 suitable for cardiac computed tomography pursuant to the 
principles of the present invention. System 100 is shown associated with a 
CT scanning apparatus 10 of the type heretofore discussed. In order to 
simplify and better illustrate operation of the invention, only the 
rotating plate 48 carrying the essential scanner elements appears in 
schematic fashion in the present Figure. These rotating elements, i.e., 
plate 48 and the components carried thereon (e.g., source 50 and the 
detector/scanner assembly 56) will for convenience be henceforth 
collectively identified as scanner 101. 
Pursuant to one aspect of the present invention, center signal generating 
means are associated with scanner 101, as indicated schematically at 102. 
Such means may typically comprise a commercially available optical 
coupling. For example, a light source and detector may be positioned such 
that an optical barrier fastened in the rotating scanner 101 will 
periodically interrupt the light path between source and detector 
providing an electrical pulse in line 108 indicating "TOP DEAD CENTER" 
positioning of the scanner 101. As the scanner rotates in the direction 
24, the TOP DEAD CENTER signal is thus enabled in line 108 each time the 
scanner reaches its vertical position. The signal, in turn, proceeds via 
line 108 and is provided to the computer and control logic means 110, 
which may form part of the control and image reconstruction station 
previously referred to. 
In order to concretely illustrate operation of the present invention, it 
may be assumed that the object sought to be examined is the beating heart 
112 of the patient 54 positioned within apparatus 10. Both patient and 
heart are, of course, shown in highly schematic fashion, the heart being 
very much enlarged in order to render the object clear. It may be noted 
here that in the "normal" operation of CT devices as previously discussed 
herein, cross-sectional views are taken by rotating scanner 101 through 
one or more 360 degree rotation and collecting data from detector array 58 
for each of a plurality of angles about such cycle of rotation. For 
example, in a typical procedure as above discussed, such projections may 
be gathered at each successive one degree rotation, to thereby provide 360 
sets of projection data for each full rotation. Since in a typical 
embodiment of apparatus of the type considered herein the detector array 
may include 301 detector cells, it will be evident that throughout the 
course of one rotation a total of 301.times. 360 data values are 
gathered-- and these in turn are provided via the line 114 to the computer 
and control logic 110 which effects the convolution and back projection 
processing of such data, and eventually provides a visual or other 
representation, for example at a display means 116. The data interfeed 
from the rotating scanner is enabled through a slip ring connection 113 in 
line 114. 
If the conventional techniques above set forth are utilized, it is found in 
practice that the heart or other pulsating or cyclically moving organs or 
the like appear as an essentially blurred object, and in a form in which a 
diagnostician cannot recognize structural features or matters of interest. 
Ideally, the diagnostician, to the contrary, would desire to obtain "stop 
action" or static views of the heart in a cross-sectional mode of 
presentation--and especially would the diagnostician desire such stop 
action views to correspond to recognized phases of the cardiac cycle. 
Pursuant to the approach of the present invention, such result is 
completely enabled. 
In accordance with one technique of the present invention, a "normal" scan 
is effected at apparatus 10, but the data proceeding via line 114 to 
computer and control logic 110 is not immediately processed to yield the 
desired images or sections through the heart. Rather such data is 
furnished via the line 118 to a scan data storage means 120, e.g., a 
magnetic disc or other known memory device. Specifically, such data may be 
stored for one or a plurality of 360 degree cyclic rotations of scanner 
101. It is important in this connection to note that the pertinent data is 
thus acquired all at one time with the patient in place, after which the 
patient may be removed from the apparatus and discharged from the 
laboratory or so forth, with the knowledge that all data required for full 
analysis of any particular portion or phase of the cardiac cycle has been 
acquired. In other words, there is no requirement for maintaining the 
patient in place at the apparatus for further examination and analysis. 
The analysis, rather, can be made from the stored data, which pursuant to 
the invention is reconstructed in leisurely fashion to produce the desired 
views. It may also be noted in this connection that the present apparatus 
10 is especially well adapted to this mode of operation, in that the said 
data can be acquired during successive continuing rotations, i.e., the 
apparatus 10 as is disclosed, e.g., in the patent application of Kendall 
Dinwiddie et al. Ser. No. 677,958, filed 4/19/76 and entitled "Tomographic 
Scanning Apparatus," renders clear that such a device can undergo 
continuous successive rotations while continuing to acquire data, this 
being a particular convenience insofar as the present type of operation is 
concerned. 
The basic problem involved in reconstructing slices through the heart 
corresponding to various selected phases of the cardiac cycle, now 
involves a requirement for correlating the slice of interest, i.e., the 
slice associated with a particular point in time in the cardiac cycle, 
with the projections pertinent thereto. 
In order to enable the said correlation, an ECG means 122 is coupled to the 
patient, as schematically indicated at 124, to provide a record of cardiac 
cycle against a time base. At the same time, the correlation with 
projection is enabled by means of the centering signal previously referred 
to as provided in line 108. In the simplest mode of operation of system 
100, an ECG record is schematically shown at 126 advancing from the ECG 
means 122. The centering signal provided in the first instance from line 
108 may be in turn provided from computer and control logic 110 via line 
128 to the ECG means 122, so that a mark 130 is placed on the ECG record 
in correspondence to the scanner achieving its top dead center position. 
It will be understood that since the top dead center position of source 50 
is known and the speed of rotation of the scanner 101 is known, the 
angular location of the source is known at any time. 
Thus it will be clear that by this sort of procedure the ECG record 126, 
itself carries a direct indication or correlation between the phases of 
the cardiac cycle and the angular position of the scanner plate 48 which 
thus enables one to ascertain directly from the ECG record 126 those 
projection angles at which data was acquired bearing upon the cardiac 
cycle of interest. It may incidentally be noted in connection with the 
foregoing discussion, that conventional ECG apparatus may carry means for 
manually marking the record 126 (in the manner of the indicia provided at 
130) which conventional means are commonly used by the physician for 
manually marking the points on the ECG record which he later wishes to 
investigate. In practice, therefore, the commercially available ECG 
devices may be readily modified to accommodate the mode of operation just 
discussed, in that the signal in line 128 (or in line 108) may simply be 
utilized to enable the same marking means as is already present in the 
commercially available ECG unit. 
In addition to the technique just described, more sophisticated data 
identification methods may be utilized. Thus, instead of or in addition to 
placing the correlating information directly on the ECG record, the data 
indicative of reference points in the cardiac cycle may be furnished via 
line 132 from ECG means 122 to the computer and control logic means 110. 
Thus the said means 110 is actually provided with all three of the 
essential sources of information, i.e., the raw detector data at 114, the 
centering signal at 108 and the cardiac phasing signal at 132 which enable 
storage at means 120 of the entire scan data for the one or more cycles of 
rotation of apparatus 10, and storage as well of data indicating the 
correlation of assembly rotational position with the cardiac phases of 
interest. In other words, appropriate data is now stored at means 120 as 
will enable an operator subsequent to patient examination, to insert a 
request at cardiac phase selection means 134, which proceeding via line 
136 and computer and control logic means 110, enables read-out from the 
scan data storage means 120 via line 138, of the projections which will be 
processed to enable the reconstructed image at display means 116 
corresponding to the selected cardiac phase. 
The manner in which the projection data is selected for reassemblage 
pursuant to the present invention, is best understood by reference to FIG. 
4 herein, schematically illustrating the collection of suitable projection 
data by reference to the ECG record 126 in FIG. 3. 
In FIG. 4, the cardiac trace shown in simplified schematic fashion at 140, 
depicts a series of successive "beats," with successive R-waves carrying 
indicia such as "1," "2," etc. Immediately beneath the cardiac record 144, 
are seen the indicia marks 130, indicating as already discussed that the 
scanner 48 has reached its "top dead center" position as previously 
discussed. In a representative operation pursuant to the invention, the 
scanner assembly may undergo a single rotation in 6 seconds or two full 
360 degree rotations in approximately 12 seconds. With a typical patient, 
this period of two rotations will correspond to approximately 14 full 
cardiac cycles. To a rough approximation, this would indicate the 
availability of only 100 projections (i.e., assuming a projection at each 
1 degree of assembly rotation) for association with each of seven slices 
or phases of the cardiac movement. Thus, if one divides the cardiac cycle 
into approximately seven phases (which is a useful representative 
division) then only about 100 projections are actually available for 
reconstructing an image corresponding to each said phase. 
It has been found in general that 100 projections is however, relatively 
inadequate for producing an acceptable reconstructed image of the heart or 
other organ. Here it should be recognized that two countervailing factors 
are involved. One is the interest in limiting the number of projections to 
those associated with as small a time duration as possible, i.e., in order 
to enable maximum "freezing" of the heart action. Weighed against this is 
the countervailing interest of obtaining sufficient numbers of projections 
to enable good development of the image, by which is meant the development 
of good quality data reflecting the density function in the ultimate 
image. To put this matter directly: One is effectively trading off image 
quality (in the sense of resolution and density) against blurring of the 
said image. Pursuant to the present invention, a highly effective 
technique for such trading off is enabled. 
Referring back to FIG. 4 there appears at 142, directly beneath the 
representation of the ECG trace, a series of indicia which divide the 
cardiac cycle into successive slices-- which are seven in number. Here it 
should be emphasized that selection of grouping into seven is 
representative of the present approach. The number of such "slices" can be 
increased or decreased within the physician's range of interest. At the 
bottom of FIG. 4, the scanner 101 position during scanning is further 
indicated in angular terms at 144, i.e., for a complete scanner cycle 
running from 0 to 360 degrees. 
Pursuant now to the approach utilized in the present invention, the 
projection data utilized to reconstruct, e.g., slice 1 of the cardiac 
phase, is garnered by collecting in each of the cardiac cycles, the 
projections corresponding to not only slice 1 as shown at 142, but as well 
those projections corresponding to slice 2. This is schematically 
suggested, e.g., at 146, at 148, at 150, etc. Further, it will be noted 
that in reconstructing the image corresponding to slice 2, i.e., slice 2 
as defined at 152 in the Figure, the projection data actually utilized, 
that is the projection data collected, will include not only the 
projections from slice 2, but also those from the adjacent slice 3--this 
point being illustrated in the Figure by the schematic showing at 154, 
156, 158, etc. 
Particularly to be thus noted is that the slices defined at 142 wherein 
association is made of projections, are processed pursuant to the 
invention so that in reconstruction of slice 1, the projections from 1 and 
2 are used; in reconstruction of slice 2, the projections from slices 2 
and 3 are used; etc. In each instance the reconstructed slices are thus 
utilizing data from an adjacent slice--but further the scheme is such that 
half of the projections in each reconstructed slices are common with the 
preceding reconstructed slice, and the other half of such data is common 
with the next reconstructed slice. The scheme is therefore such that for a 
given mode of operation, i.e., rate of rotation and gathering of data, 
etc., and assuming division of the cardiac cycle into a selected number of 
slices, the number of available projections is effectively doubled. The 
net effect of this arrangement is therefore one of providing fully 
acceptable images in terms of blurring, while at the same time providing 
very acceptable image qualities in terms of density and resolution. 
It will of course be understood that the method described in connection 
with FIG. 4 is precisely similar where the automated techniques described 
in connection with FIG. 3 are utilized, i.e., in the technique when 
automated one reads out from the scanner data store 120, those projections 
corresponding to adjacent slices as just described. Such data is then 
provided to the computer and control logic means 110, where it is 
processed in accordance with the convolution and back projection 
techniques which are fully disclosed in the Pavkovich et al applications 
previously referenced herein, to ultimately provide an image or display 
116 which can be on a CRT or is otherwise available for differing 
representations, e.g., by photographs or so forth. 
When the data is taken throughout the full cycle of an object's cyclic 
movement the data can be reconstructed into one or more "still" 
cross-sectional pictures depicting conditions at one or more portions or 
phases of the cycle. In addition the invention also comprises rapidly 
presenting cross-sections of sequential positions of the object throughout 
its cycle of movement, for example, sequential presentation of the 7 
phases of the cardiac cycle shown in FIG. 4. The resulting animated 
presentation of the cyclic movement permits the viewer to detect 
information which may not be discernable from observation of "still" 
cross-sectional views. The animated presentation can be directly on the 
display means 116, or copies of the cross-sections at the various phases 
can be made and presented with a cine projector. 
A somewhat modified aspect of the invention comprises taking data during 
the occurrence of just one phase or a relatively small number of phases of 
the cardiac cycle. For example, if one knows in advance that the only 
phase of interest is phase 2, the computer and control logic 110 is 
connected to source 50 via line 160 to provide selective control for 
pulsing of the source. Thus instead of pulsing the source on at each one 
degree increment for collection of data throughout the cardiac cycle, the 
computer and control logic 110 is programmed to pulse the source on only 
during the times the cardiac cycle is in phase 2. If more data is desired 
for improved image quality the selected pulsing can occur during phases 2 
and 3. Also it should be pointed out that the patient is exposed to much 
reduced radiation during, for example, two revolutions of selective 
pulsing compared to two revolutions of continuous pulsing. 
While the present invention has been particularly set forth in terms of 
specific embodiments thereof, it will be understood in view of the present 
teaching, that numerous variations upon the invention are now enabled to 
those skilled in the art, which variations yet reside within the scope of 
the present invention. Accordingly, the invention is to be broadly 
construed and limited only by the scope and spirit of the claims now 
appended hereto.