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CROSS REFERENCE TO RELATED APPLICATIONS
This invention is related to co-pending application Ser. No. 852,419 filed
Nov. 17, 1977, entitled Unidimensional Nuclear Medical Imager and assigned
to the assignee of the present invention which is hereby incorporated by
reference. The invention also relates generally to the study of periodic
organ motion and more particularly to the study of periodic cardiovascular
system operation and most specifically to the study of myocardial wall
motion.
Because the cardiac function is one of periodic motion between contraction
(called systole) and relaxation (called diastole) numerous methods of
study have been provided by which the quality of cardiac function is
studied through observation of its periodic motion. However, the
criticality of cardiac function gives rise to a number of difficulties in
observing the function. For example, in many instances the heart being
evaluated is one which is already subject to a number of difficulties and
may, indeed, be functioning in a very weak or abnormal condition. Under
such circumstances, an evaluation technique which imposes additional
stresses upon the cardiac function subjects the patient to substantial
risk of further damage or even mortality. Simply stated, the heart which
is most in need of evaluation may also, due to the patient's condition, be
the heart which is most susceptible to damage by many methods of cardiac
study.
In addition, even the most "healthy" cardiac system may be unexpectedly
damaged by subjection to severe stress. Also, many forms of cardiac
evaluation may involve the administration of substantial doses of high
level energy such as X-ray or ultrasound. Quite apart from their effect
upon the cardiovascular system, these high energy doses may also produce
damage of surrounding or interposed tissue. Effective cardiac study is
often rendered even more difficult by additional problems associated with
the position of the heart within the subject's anatomy. For example, the
human heart is positioned within the protective cavity of the "rib cage"
and is partially obscured as a result. In addition, from some viewing
angles, the interposed organs, such as the lungs and diaphragm, make
evaluation with some techniques from these angles difficult.
The great need for effective cardiac study has led practitioners in the art
to develop a number of systems or methods for cardiac evaluation, each of
which attempts to surmount one or more of these problems. While each
method is effective to some degree, each also represents a compromise
between the quality of data achieved and the degree of stress or potential
for injury imposed on the subject by the testing procedure.
Perhaps the most commonly used method of cardiac function observation is
that generally referred to as contrast angiography or cardiac
catheterization in which a catheter is used to inject an X-ray opaque
material into the cardiac blood pool or the coronary artery system.
Conventional X-ray observation techniques are then employed to produce a
photographic image of the blood pool in the former case or coronary artery
system in the latter. This method is generally used because of the quality
of image produced, however, it does have an associated morbidity and to a
small degree mortality. As can be imagined, the injection of the X-ray
opaque material from the catheter under pressure imposes a great stress
upon the cardiac function. This stress limits the availability of this
procedure to patients having existing cardiac problems. In addition to the
hazardous aspects, the catheterization procedure is uncomfortable and the
equipment utilized is not portable necessitating moving the patient to the
equipment. Also, the X-ray dosage used is high further limiting the
frequency of safe patient examination. Finally, it has been determined
that many of the X-ray opaque materials used in contrast angiography
produce subsequent changes in the patient's physiology. This, of course,
is extremely undesirable. The dangers and difficulties associated with
cardiac catheterization combine to limit it as a general cardiac
diagnostic tool.
Another "X-ray" system of cardiovascular motion study is that known as
radarkymography in which a fluoroscope is used in conjunction with video
tracking. Again, however, problems of high radiation dosage, lack of
machine portability and the limited number of views obtainable with this
method make its eventual use as a basic heart evaluation tool unlikely.
Nuclear angiography, which involves the study of heart motion by injection
into the blood stream of a radioisotope which is traced by its emission of
gamma radiation, may also be used to study cardiac function. Different
radioisotopes may be selected some of which remain within the cardiac
blood pool while others are assimilated by the myocardium (i.e. the
cardiac muscle structure). In either case, a scintillation camera is
positioned adjacent the heart at the desired angle and periodically
"exposed" to a selected portion of the cardiac cycle. Most commonly, a
gated-image corresponding to the end of the diastole interval together
with one for the end of the systole interval are sought. Cardiac motion is
then determined to some extent by a qualitative comparison of the gated
end-diastole and end-systole images. Successive exposures over many
cardiac cycles are required because of the low level radiation typical of
the safely administered radioisotopes. While the gated image technique of
nuclear angiography avoids many of the difficulties associated with
cardiac catheterization or radarkymography, the images achieved are often
difficult to interpret meaningfully on a regional basis. Further, the
interpretation is generally limited to qualitative results and the
relative durations of systole and diastole cannot be observed.
A system of cardiac evaluation using ultrasound energy, also called "M"
mode, has also been developed in which high frequency sound energy is
directed into the patient's anatomy and returing "echoes" are evaluated to
produce an image. The ultrasound system is perhaps the most desirable
approach in the sense that it is the least invasive of all the known
systems. However, a number of problems arise which severely restrict the
views available using ultrasound techniques. Most importantly, the
position of the heart adjacent the lungs, rib cage and diaphragm restrict
ultrasound evaluation to a narrow "window" which excludes many critical
areas such as the cardiac apex.
Accordingly, it is a general object of the present invention to provide an
improved method of cardiac motion study. It is a more particular object of
the present invention to provide a less invasive method of cardiac motion
study which facilitates the quantitative real-time evaluation of regional
myocardial motion.
BREIF DESCRIPTION OF THE FIGURES
The features of the present invention which are believed to be novel are
set forth with particularity in the appended claims. The invention,
together with further objects and advantages thereof, may best be
understood by reference to the following description taken in connection
with the accompanying drawings, in the several figures of which like
reference numerals identify like elements, and in which:
FIG. 1 is a block diagram representation of a system for nuclear kymography
constructed in accordance with the present invention.
FIG. 2 is a more detailed depiction of gamma camera 12.
FIG. 3 shows a single axis collimator for use in the present invention
system of nuclear kymography.
FIG. 4 shows a single axis scintillation camera in accordance with the
present invention.
FIG. 5 is a pictorial layout of a cardiac ventricle and overlying imaging
area along the minor ventricular axis.
FIGS. 6A thru 6C depicts nuclear kymogram and ECG signals obtained in
accordance with the present invention.
FIG. 7A is a pictorial layout of a cardiac ventricle and overlying imaging
area along the major ventricular axis.
FIGS. 7B thru 7E depict ECG signals and nuclear kymograms of cardiac
contraction for systems having differing ejection fractions.
SUMMARY OF THE INVENTION
A method of producing a quantitatively evaluatable visual representation of
periodic regional organ motion, as for example, a cardiovascular system,
comprises the steps of causing energy to radiate from at least a portion
of the organ; receiving the radiated energy as positional events along a
predetermined imaging axis as a function of time; accumulating a plurality
of successive positional events in synchronism with the periodic organ
motion; and displaying the accumulated plurality of successive
recordations to form a continuous two-dimensional image, one of the
dimensions depicting time and the other depicting regional organ motion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a block diagram representation of the present invention system
of nuclear kymography. An organ of interest 10, in this case a human
heart, within a patient anatomy 9 is positioned beneath a gamma camera 12.
A radiation means 21 (shown diagrammatically coupled to heart 10) depicts
the well-known process in which a radioisotope is injected into the blood
stream (not shown) of patient anatomy 9 to cause accumulation of the
radioactive substance within either the region, or interior, of heart 10.
Radiation means 21 may include, for example, the injection of a
radioisotope which collects within the cardiac blood pool as in the case
of blood pool kymography, or alternatively a radioisotope which
assimilates within the myocardial walls in the case of myocardial
kymography.
In either case, however, radiant energy 11 in the form of gamma radiation
propagates outwardly from heart 10 toward camera 12. The radiated gamma
energy is received by gamma camera 12 and therein converted by means
described below in more detail to electrical signals which are coupled to
an image axis selector 13. A signal matrix (not shown) within selector 13
processes the output signal of camera 12 producing an electrical signal
which includes a recordation of positional events occurring within a
restricted field of examination 12a. The second dimensional components
within the restricted field are "compressed" in the second dimension
direction such that only one dimensional events along the imaging axis are
present in the output signal. This one dimensional positional event output
signal is applied to a vertical axis deflection system 15 of a CRT 18
within a display system 14. In addition, image axis selector 13 also
produces an unblanking signal which is applied to a Z axis intensity
modulation system 16 of CRT 18.
Electrocardiogram means (ECG) 20 are responsive to heart 10 and produce a
conventional ECG signal which is applied to the synchronization input of a
horizontal-axis sweep system 17 of CRT 18 and to an ECG signal display 23.
Horizontal-axis sweep 17 includes well-known CRT display circuitry for the
generation of a time-varying sweep signal which when applied to the
horizontal-deflection axis means of CRT 18 cause the vertical-deflection
axis inputs to sweep in time on viewing screen 19. In addition,
horizontal-axis sweep 17 includes peak detection circuitry which
identifies the "R" wave component of the applied ECG signal and causes the
generated sweep signal to be synchronized to the ECG signal. The use of
the ECG signal to synchronize horizontal sweep 17 is of particular
importance to the present invention because the image thus formed on
viewing screen 19 of CRT 18 represents successive cardiac cycles. As
successive cardiac cycles and horizontal axis sweeps of viewing screen 19
occur, the vertical-axis deflection and intensity modulation caused by the
output signal at image selector 13 produce an image on viewing screen 19
which depicts the spatial distribution of gamma events along the imaging
axis (i.e. restricted field of examination 12a) as a function of time. The
output of ECG display 23 is coupled to a second vertical-axis deflection
24 and to actuate second vertical deflection 24 and provide simultaneous
display of the ECG signal and nuclear kymogram for visual reference.
As in the procedure of nuclear angiography, the level of energy radiation
from heart 10 is maintained at a low level to avoid high patient dosage.
As a result, the data from a substantial number of cardiac cycles must be
accumulated to produce a high quality image. This may be simply
accomplished by using a CRT which is of the well-known persistent or
storage scope variety. However, a more preferable image is produced if a
photo camera 22 having film responsive to the light output of viewing
screen 19 is oriented in a proper relationship to viewing screen 19 for
exposure of the film to successive CRT images. Further, in situations
wherein a digital memory system is available, an even more improved image
is provided if data is stored in the digital memory and conventional image
processing techniques are applied. In accordance with boundary
determination techniques used in other forms of nuclear medicine the
one-dimensional output signal of image axis selector 13 may alternatively
be processed by numeric techniques to present the accumulated plurality of
successive imaging axis events in a numeric format. The use of this
alternative facilitates the generation of quantitative data of cardiac
function such as total movement, maximum segmental velocity, and average
segmental velocity. It will be apparent to those skilled in the art that
nuclear kymographic information may be "imaged" in either format that is
visual display or numeric "read-out". It will be equally apparent to those
skilled in the art that both formats may be concurrently utilized.
Also, at this point, it will be readily apparent to those skilled in the
art that the physical orientation between camera 12 and the to-be-studied
organ (heart 10) may be varied to select any of a large number of angular
projections and bring different aspects of the organ into "view". It will
be similarly apparent that the position of restricted examination field
12a of camera 12 with respect to heart 10 may be varied to view selected
portions of the organ. In each case the resulting display called a nuclear
kymogram provides an image which depicts the cyclical changes of
positional events along the selected imaging dimension on a continuous
real-time basis. In the case of cardiac evaluation, the resultant image
produced clearly shows cyclical regional myocardial wall movement in a
quantitatively assessable format.
Before proceeding to more detailed discussions of the present invention
nuclear kymography, a brief description of an Anger-type scintillation
camera constructed in accordance with the teachings of U.S. Pat. No.
3,011,057 is believed helpful. FIG. 2 shows a simplified pictorial layout
of the major components of a scintillation camera when utilized in
accordance with the present invention. The to-be-evaluated organ (heart
10) is shown pictorially radiating gamma energy 11 in the form of a
plurality of energy rays 31 propagating in a generally radial pattern from
the nuclei of their respective radioisotopes. Because this gamma energy is
radiating in a generally incoherent manner, it is in a form virtually
useless as an imaging source. The gamma energy is not focusible by
conventional optic techniques, therefor a collimator 35 is interposed
between the energy emitting organ and the sensing portions of the camera.
The fabrication and utilization of such collimators is the subject of
considerable variety and refinement within the art; however, all
structures utilized may be generally described as being of a gamma
absorptive material such as lead in which a plurality of axially directed
collimating apertures 36 are defined.
In accordance with basic principles of collimation, the randomly radiating
energy 31 is collimated, or converted, to axially directed energy rays
which pass through the apertures 36 of collimator 35 to impinge a
scintillation crystal 40. Again, as is true of collimators, scintillation
crystals are well-known in the art being set forth, for example, in the
above-described Anger patent. Scintillation crystals are also subject to
some refinement and variation within the art. However, all may be said to
perform the basic function of converting the impinging gamma radiation
energy to optically perceivable light energy. The principle of
scintillation is somewhat complex. However, suffice it to say here that
each time a "bundle" of gamma energy strikes scintillation crystal 40 the
molecules within the crystal scintillate and a "photo event" occurs which
positionally corresponds to the relationship between the original gamma
source (i.e. the radioactive nucleus), one of the collimating apertures,
and the region of the scintillation crystal stimulated.
A plurality of photomultiplier tubes 41 are positioned in a generally
planar array substantially parallel to scintillation crystal 40. Each time
gamma energy scintillates a portion of crystal 40 causing a photo event in
which light energy radiates from the scintillation crystal area, one or
more of the proximately located photomultiplier tubes are energized. The
light energy produced is of a distributed nature in which a maximum of
light energy is received by the most directly aligned photomultipliers
while a minimum is received by those most remotely aligned. The
photomultiplier array is connected via a first plurality of connections 43
to a Y axis matrix 46 and via a second plurality of connectors 44 to an X
axis matrix 47. In accordance with well-known scintillation camera
principles, the positional coordinates of each photo event along the X and
Y axes are derived by matrices 46 and 47. As a result, the output of
matrices 46 and 47 forms a real-time positional signal of each respective
coordinate in the form of electrical impulse signals the amplitudes of
which depict the positional relationship along the respective coordinate.
The outputs of matrices 46 and 47 are applied to an image axis selector 50
which may, for example, comprise the well-known "region of interest"
feature. Selector 50 provides a restricted area of camera evaluation by
selection of X and Y axis information solely within preset limits. In
performance of the present invention nuclear kymography, events in the
region of interest are converted to events along the imaging by
"collapsing" the information to the imaging axis and ignoring the
orthogonal component. As a result of this process, the output signal of
image axis selector 50 coupled to vertical axis deflection system 15 of
CRT 18 provides a succession of electrical pulse signals the polarity and
amplitudes of which depict the positional or spatial coordinates of each
event along the imaging axis horizontally displayed on a real-time basis.
Not all scintillation cameras are equipped with region of interest imaging,
therefore an alternative method for restriction of camera imaging to a
selected image axis is provided by a specially designed collimator. FIG. 3
shows a special single-axis collimator 55 in which a plurality of
collimating apertures 56 are arranged along a selected imaging axis 57.
Collimator 55 is designed to be otherwise interchangeable with collimator
35 and rotatable within camera 12 and when so interchanged restricts
scintillation of crystal 40 to gamma events occurring along axis 57. In
preferred form, axis 57 is selected to coincide with either of the camera
electronic axes (X or Y). In such case the stimualtion of the remaining
photomultipliers is minimized which simplifies image axis selection and
permits more direct coupling of the selected axis matrix to vertical
deflection system 15 of CRT 18. However, often the desired imaging axis
will not be a major (X or Y) axis. In such case the full image axis
circuitry set forth above is required to perform axis selection data
restriction and "collapsing" of the positional component orthogonal to the
imaging axis.
As mentioned, special collimator 55 is of particular importance in adapting
existing scintillation cameras not having a region of interest capability
to performance within the present invention nuclear kymography. However,
the need for detection of events occurring along a single imaging axis
also makes the use of a single axis scintillation camera, as described in
the above-noted co-pending reference application, of particular advantage.
FIG. 4 shows pictorially the basic structure of a single-axis scintillation
camera in which an elongated collimator 60 defines a plurality of
collimating apertures 61 arranged along a single axis. A similarly
elongated scintillation crystal 62 accommodates a corresponding linear
array of photomultiplier tubes 63 each of which is connected to display
system 14 via a plurality of electrical conductors 65. Because the
photomultipliers are arranged in a linear single-axis array, the complex
system of matrices used in a two-dimensional scintillation camera is no
longer needed. Instead the output signals of photomultiplier tubes 63 are
combined in a position matrix the output of which forms a real-time
recordation of the positional events along the imaging axis.
Turning now to the nature of the present invention nuclear kymogram itself,
FIG. 5 shows a pictorial representation of the left ventricle of a human
heart which defines a long axis 77 terminating in the "apex" 79 and a
short axis 78. Outlines 10a and 10b depict the locations of the left
ventricle myocardial wall structure during the intervals of end-diastole
and end-systole respectively. An imaging dimension, or area, 70 is shown
positioned overlying a portion of heart 10 along short axis 78. The area
bounded by imaging area 70 approximates the region from which photo events
are received and utilized in the present invention nuclear kymography. As
mentioned above, nuclear kymography may be pursued using either of two
methods. The first, blood pool kymography, results when a radioactive
isotope is selected which assimulates within the cardiac blood pool. In
this case the change in radiation density at the boundary between the
cardiac blood pool and the inner surface of the myocardium are observed.
The second, myocardial kymography, results when a radioactive isotope is
utilized which is assimulated primarily within the myocardial structure
itself. As will be seen, this gives additional information as to muscle
"thickening".
FIG. 6A shows a sample nuclear kymogram resulting from blood pool
kymography in which the distribution of gamma events along imaging
dimension 70 are depicted along the vertical axis while the horizontal
axis depicts time. Each "dot" records a gamma event and the high-density
region of recorded events (shown in FIG. 6 as a "more shaded" area)
defines the outline of the cardiac blood pool. As mentioned, the
configuration of the cardiac blood pool within the myocardial structure
provides an easily interpreted "view" of the inner boundary of the
confining myocardium configuration. The "width" of the blood pool along
imaging dimension 70 (i.e. the vertical axis) shows a maximum character
during the diastole period 74 indicating that the myocardium has assumed
its maximum volume bounded by outline 10a in FIG. 5. Conversely, during
the systole period, the blood pool is at a minimum volume bounded by
myocardial outline 10b giving the kymogram dimension 75 along imaging axis
70.
FIG. 6B shows a typical ECG wave form which, as mentioned, is used to
synchronize the successive cathode ray tube sweeps to the periodic cardiac
cycle. Of particular importance to the present invention system of nuclear
kymography are the high amplitude "peak" portions 71 of the ECG signal
which are typically referred to as "R waves". These "peak" portions are
used to produce a trigger pulse which in turn initiates successive sweeps
of CRT 18. In addition, the kymogram obtained by triggering in response to
ECG "R" signals is easy to interpret since the generally accepted format
of cardiac cycle rests upon common assumption that each cycle is initiated
by the ECG R-wave.
FIG. 6C shows a second nuclear kymogram taken along imaging axis 70 in FIG.
5 in which myocardial rather than blood pool kymography has been pursued.
While the general outline of high density shaded area is similar to that
shown in FIG. 6A for blood pool kymography, a second gradation 73 may be
observed generally along each outer edge of the kymogram high density
area. The second gradation defines the myocardium and therefore the
dimension of the blood pool derived in FIG. 6A corresponds to the "inner"
side of this second gradation. Because this second gradation corresponds
to the myocardium itself, the use of myocardial kymography provides
additional information not always available in blood pool kymography by
recording the changes of myocardium along the imaging axis during the
cardiac cycle. Examination of FIG. 6C, for example, shows the
characteristic myocardium "thickening" during the systole portion of the
cardiac cycle. It should also be noted that the portions of myocardium
within imaging axis 70 are each shown (i.e. the top and bottom gradation
of FIG. 6C) permitting individual evaluation.
Some indication of the effectiveness of the present invention system of
nuclear kymography as a diagnostic tool may be derived by examination of
FIGS. 7A through 7E. FIG. 7A again shows a pictorial representation of a
human heart left ventricle similar to that of FIG. 5. However, in this
instance imaging area 70 now overlies the long axis 77 of heart 10. As in
FIG. 5 outline 10a depicts the myocardial boundary location during
diastole while outline 10b depicts the myocardial boundary during systole.
Also, FIG. 7B shows the ECG wave form similar to that set forth in FIG. 6B
defining high intensity R wave peaks 71 and a mid-cycle "T" wave 72
corresponding to ventricular systole. FIGS. 7c, 7d and 7e show long-axis
blood pool nuclear kymograms of hearts having ejection fractions of 75%,
50% and 25% respectively. An ejection fraction of 75% is generally
considered normal while a 50% ejection fraction is below normal and a 25%
ejection fraction characterizes a critical cardiac malfunction. It should
be noted at this point that, with appropriate equipment calibration, the
nuclear kymograms of FIGS. 7c through 7e may be quantitatively evaluated.
For example, comparison of the respective maximum dimensions 80, 82 and 84
in FIGS. 7c through 7e shows a similar boundary condition or myocardial
"breadth" during the diastole period of cardiac function and also each
side movement individually. However, a similar comparison of the
respective minimum boundary dimensions 81, 83 and 85 shows great
variations of the systole performance. Each dimension and the differences
between may be measured along the vertical axis facilitating quantitative
evaluation. In addition to the quantitative information the kymograms also
show a continuous time function display which yields information as to
both the relative durations of both systole and diastole periods and the
rate of myocardial contraction. The latter is especially useful since it
facilitates determination of myocardial compliance. | | |
