Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Computerized tomographic X-ray or gamma ray scanners
(CT scanners) reconstruct an image representing a single tomo- '~
gram of the radiation absorptivity of tissues from data
collected from numerous coplanar scan lines. The widest appli-
cation of CT scanners thus far has been for brain studies.
Being stationary when supported in the CT scan circle, all
parts of the brain generally remain in the same location during
; each of the numerous scans required for constructing~a single
tomographic image. However, involuntary muscular activity
makes accurate image reconstruction of other parts of the body
difficult. This problem is presented with both basic types of
CT scanners, namely, traverse-and-rotate type CT scanners and
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purely-rotational-type CT scanners.
Ileart structures, for example, are in constant
motion. While the heart period is on the order of one second,
distinct physiological phases of the cardiac cycle, for
example, the periods referred to as end systole (ES) and end
diastole (ED) last on the order of 1/20 and 1/5 of a second,
respectively. That is, if all of the scan lines needed to
reconstruct an image of the heart could be produced in less ~ -
than 1/20 of a second, the motion of the heart would be
effectively frozen during either of these periods. This speed,
however, is difficult for conventional CT scanners which
normally require from about 5 seconds to several minutes to
Collect the scan data for a single image.
The objectives of cardiac imaging in general are
visualizing the sizes of the cardiac chambers, estimating
contractilities of the chambers, comparing chamber wall
motions, locating aneurysms and areas of myocardial infraction
and detecting mitral stenosis. Most of these objectives are,
of course, difficult to attain using conventional exposed film
X-ray techniques because the differences in absorption or
density of heart tissues and blood is not sufficient to
confidently distinguish these features at safe radiation
dosages and because a tomogram or cross-sectional slice image
is not generated.
An electrocardiogram (ECG) is produced by recording
the amplitude of electrical activity associated with the heart
muscle versus time. In ultrasound imaging, the ECG signal has
been used before as a synchronizing device for producing a
stop-action image of the heart. See, for example, U.S. Patent
No. 3,954,098, issued May 4, 1976 to D. E. Dick et al. Some
ultrasound imaging systems have used
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computerized sorting and assembling multiple images per nominal
heart cycle with recorded data from several heart cycles.
Ultrasound imaging differs fundamentally from X-ray imaging.
X-rays are not normally reflected detectably by tissue; that portion
which is not absorbed is merely transmitted. All conventional X-ray
imaging machines operate in the transmission mode. While ultrascund
imaging can be carried out in the transmission mode in some
instances, conventional ultrasound cardiac imaging, particularly
ECG-gated imaging, is only done in the reflecting or echo mode.
Ultrasound imaging involves pinpointing each partially reflecting
surface for a given pulse of sound energy by measuring the round
trip transit time for reception of the echoes, just as in sonar. A
Single pulse of radiation, however, in the X-ray transmission moce
results in a single datum describing the total absorption
encountered over the entire path of the X-ray beam; that is, the
location of structures is not identifiable from one pulse. ~
Summary of the Invention ,
The present invention relates to a method and radiographic
apparatus for imaging a planar slice of a patient's heart. The
apparatus comprises a CT scanner which has a source of radiation,
scan means for scanning the beam of radiation from the source
relative to the heart, data generating means for generating image
data from radiation that has traversed the planar slice of the
heart, processing means for processing the image data to reconstruct
a tomographic image of the planar slice. The apparatus further
comprises cardiac cycle monitoring means for producing a repeatirg
pulse signal indicative of the same functional point in each
successive cardiac cycle and control means responsive to the pulse
signal for causing image data generated during the same selectable
phase in successive cardiac cycles to be reconstructed. The control
means includes a selectable delay means for delaying the pulse
signal for a selectable duration thereby selecting the selectable
phone in the cardiac cycle.
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The method comprises sensing the same functional point in each
su~essive cardiac cycle and producing a pulse signal, delaying the
pulse signal for a selectable duration, generating and scanning at
least one beam of radiation relative to the heart, generating image
data indicative of the attenuation of radiation crossing through the
planar slice, processing the image data to construct the tomographic
image, and controlling the processing of image data with the delayed
signal. In this manner the image data that is generated during the
same selected plane in successive cardiac cycles is processed to
construct the tomographic image of the planar slice of the heart in
the selected phase. `~
Brief Description of the Drawings
Fig. 1 is an ECG waveform;
Fig. 2 is a block diagram illustrating an ECG-controlled CT
scanner system according to the invention;
Fig. 3, appearing on the first sheet of drawings, is a schematic
diagram indicating the CT scanner beam's traverse in relation to the
patient's heart;
Fig. 4 is a schematic diagram of a traverse-and-rotate-type
fan-beam CT scanner illustrating the beam pattern intersecting the
patient's heart;
Fig. 5 is a schematic representation of the beam pattern of a
purely-rotational-type CT scanner; and ~;
Figs. 6, 7 and 8 are detail block diagrams illustrating
different systems of ECG-gating for a purely-rotational-type CT
scanner.
Description of the Preferred Embodiments
The ECG waveform shown in Fig. 1 presents features designated by
the letters P, Q, R, S and T. The group of features Q, R and S is
referred to as the QRS complex, in which the R-feature or R-wave is
the most prominent, highest amplitude feature of the entire ECG.
Moreover, the narrow pulse width of the QRS complex and in
particular the R-wave, provides a digital clock pulse for timing the
cardiac cycle.
The cardiac cycle is usually defined as beginning with
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the R-wave and continuing until the occurrence of the next
R-wave. Heart functions are characterized by two distinct
periods called systole and diastole. In systole, the heart
- muscle is contracting the volume of the left ventricle to pump
the contents out through the aortic valve. During diastole,
the left ventricle is filling through the mitral valve. At
the end of systole (ES), the left ventricle has its smallest
volume since it has contrac~ed to pump blood out. The end of
' diastole (ED) is the point at which the left ventricle has
its largest volume since it is filled with blood ready to
be pumped out. Thes~ two extremes of heart function, end
of systole and end of diastole, are of interest, for example,
in determining fractional ejection, i.e., the ratio of
minimum-to-maximum ventricular volume. Each of these fea-
tures, end of systole and end of diastole, lasts for an inter-
val on the order of 1/10 second and occurs once every cardiac
cycle.
Fig. 2 shows a conventional traverse-and-rotate-
type CT scanner having a scan circle 12 defining a scan plane
in which the patient 14 is positioned such that the scan plane
preferably intersects the left ventricle, left atrium or aortic
root of the heart. The mechanical operation of the traverse-
and-rotate mechanism and the beam shutter is controlled by a
scanner controller 16. An external pacemaker 18 may be employed
to stabilize the cardiac period of a patient with irregular heart
rate. The ECG signal from the patient is applied by an isolation
amplifier 20 to a QRS complex detector 22 whose output is a
digital timing pulse corresponding to each R-wave of the
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patient's live ECG signal. The items 20 and 22 are commercially
available units, for example, Hewlett Packard Corporation,
Model Nos. 7807C and 7330A. The output of the QRS detector
22 is fed to a delay timing circuit 24 which provides a trigger
pulse to the CT scanner controller 16.
If the objective is to acquire an image of the heart
at the end of diastole with the left ventricle fully expanded,
the trigger pulse is timed to be applied to the scanner con-
' troller 16 sufficiently in advance of the end of diastole so
; 10 that the scanner controller 16 can begin the traverse of ~he
radiation beam, as shown in Fig. 3, from point xO so that by
the time the beam travels distance Dl to position xl where it
first begins to intersect the heart, the heart will be in the
;~ end of diastole stage. Of course, the beam must be traversing
at a rate sufficient to traverse the width of the heart in thispresentation in approximately less than 1/10 of a second. Thus,
the distance D2 from point xl to the point x2 would determine
the minimally acceptable speed of traverse. The consequence
, of traveling too slowly through the distance D2 in Fig. 3
would be a blurring of some of the moving heart structures.
The delay time pro~ided by the circuit 24 in Fig. 2
is determined ~y three parameters: (1) the speed of the traverse
of the beam; (2) the position of the heart in the scan circle
12; and (3) a predication of when the particular phase of
interest, for example end of diastole, will occur in the average
or nominal cardiac cycle of the patient. The speed of traverse
of the beam is normally a known constant value. However, the
speed can be monitored during the scan to compensate in
successive scans for any variations in the average scan speed.
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Th ~osition of tl~e heart can be determined in two ways: the
heart's position can be considered by adjusting the patient's
positi~n in the scan circle or the patient can be prescanned and the
location of the heart determined by the scanner operator from the
reconstructed image. Prediction of the time that the heart will be
in a particular phase, such as end diastole, requires a knowledge of
the heart rate, as measured by the interval between R-waves, and the
-average time elapsed from an R-wave up to the phase of interest~
This information is derived from the patient's electrocardiogram.
It could also be derived from a phonocardiogram or pressure
measurements. The ultimate objective is to synchronize the CT
cardiac scanning with cardiac contractility. Use of the ECG signal
is a means for inferring the phases of the cardiac contractility
curve~ Contractility measurement devices may be employed to
determine the contractile state of the heart directly and more
accurately.
These parameters are taken into consideration in setting the
delay implemented by the timing circuit 24. After the R-wave signal
from the QRS detector 22, the delay timing circuit 4 pauses before
issuing a trigger pulse for an interval of time which can be
represented as follows:
Time Interval = TED - Dl/Ravg
where TED is the predicted time from a given R-wave to the
beginning of the end of the diastole phase; Dl is the
position of the heart in terms of the distance the beam covers
from the starting point xO until reaching the center wall of
the heart (or some other point of interest); RaVg is the
average speed of the traversing beam; and DlRaVg is the
predicted elapsed time from the beginning of the traverse to
the point where the center beam intersects the center of the
heart.
For any patient the period of the cardiac cycle, as
shown in Fig. 1, from one R-wave to the next R-wave always
varies to some degree. Theitiming of the triggering of a
traverse is based solely on the occurrence of the last R-wave
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and the predicted time for beginning of the end of diastole
or end of systole whichever phase is being imaged. It is
entirely possible that the prediction may not be borne out.
- If the cardiac cycle, which the scanner is preparing to sample,
is one of sufficiently increased or reduced period, the end of
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diastole or end of systole will occur at a significantly
different time. Thus, it is advisable to place tolerance
limits on the cardiac period in order to distinguish acceptable
and unacceptable scan data.
The system of Fig. 2 merely illustrates one form of
digital circuitry foL performing heart period dissemination.
In practice, it may be preferable to implement these functions
with software using the CT scanner computer associated with
image processing the machine control. The output of the QRS
detector 22 is passed to the reset input of a digital counter
26 clocked, for example, at one hundred or one thousand Hertz
by a stable frequency oscillator 28. The parallel binary out-
put of the counter 26 is passed via a latch circuit 30 to a
subtractor circuit 32. The latch 30 operates as a digital
sample-and-hold circuit which holds the count attained by the
counter 26 immediately before being reset by the next R-wave.
The number contained in latch 30 is compared by the subtractor
32 to the number held in storage 34 representing the nominal
period of the pàtient's cardiac cycle. The difference between
the counts for the actual and nominal periods is passed to a
comparator 36. A reference number indicating a tolerance
limit on the difference between the actual and nominal periods
is provided by the limit circuit 38. If the difference exceeds
the limit provided by the circuit 38, the binary comparator
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output alerts the image processing unit 40 associated with the
CT scanner 10 to discard the scan data corresponding to the
irregular period.
The actual tolerance limits on cardiac period depend
on the phase being imaged. For example, the tolerance for
imaging end of diastole will be smaller than the tolerance for
end of systole since the interval to end of diastole is
generally regarded as proportional to the cardiac period and ;
comes at the very end of the cardiac cycle. Thus, in addition
to setting the delay timing in accordance with the phase of
interest, the tolerance limits for an acceptable cardiac period
should also be adjusted accordingly.
Instead of employing a single beam as shown
schematically in Fig. 3, several CT scanning devices currently
on the market, such as the DELTA-SCAN~ model scanner marketed
by Ohio-Nuclear, Inc., traverse with a fan-shaped pattern of
beams as shown in Fig. 4. The fan-beam pattern covers a width
Wl where it intersects the heart. The consequence of the
width of the beam pattern is that it takes longer for the
entire plurality of beams to traverse the heart from point x
to point x2. As is the case in Fig. 4, if the width of the
beam is approximately equal to D2, i.e. the width of the
heart in the plane in the scan direction, the time relative to
a narrow beam scan will be doubled for a full heart traverse of
all of the beams in the fan pattern. To alleviate this
problem, the effective beam width can be narrowed, for example
to W2, by ignoring or "throwing out" data from several of the
peripheral detectors. For example, the data from the two
outermost detectors on either side can be ignored.
Alternatively, a shutter can be employed as shown in Fig. q to
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block certain peripheral rays thus narrowing the actual
pattern. The effect of either of these remedies is to narrow
the fan-beam width at the heart so that the distance D2 can
be traversed more quickly. The faster the interval D2 is
traversed, the less motion will be present to cause blurring in
the image. Thus, the precision of the stop-action effect can
be increased by omitting data from peripheral beams to achieve
a shorter effective time window. Blocking the beams instead of
ignoring the data from the detectors has the advantage of
eliminating unnecessary X-ray dosage. However, removing
several of the beams will cause a slight increase in the
overall scan time for completing the image.
Cardiac imaging can also be accomplished with a purely
rotatiOnal CT scanner as shown in Fig. 5 wherein ~ is the
rotational axis. In this case, however, since rotation of the
course and detectors is continuous, instead of triggering the
mechanical traverse at the right point so that data is acquired
in the phase of interest, scan data is generated continuously.
The patient's ECG signal can be used as in Fig. 6 to gate the
storage of data by the image processing unit 40. The delay
timing circuit is thus used to open a time window during the
appropriate phase in each cardiac cycle in which data is
collected. Several phases can be gated with the ECG signal or
even a full set of images covering every distinct physiological
point in the cardiac cycle can be generated. These images can
be sequenced then to produce a movie or cine presentation of
the subject's heart. This alternate also includes the
possibility of a semi-circle of stationary detectors and a
rotating source or stationary source(s) and rotating
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detectors.
In order to minimize X-ray dosage to the patient,
the X-ray tube could be gated on and off as indicated by the
gating of X-ray power supply 42 in Fig. 7, or dynamically
shuttered as indicated in Fig. 8 by the gating of beam shutter
control 44, which would open the shutter blocking the beams
only during the physiological phase of interest during each
cardiac cycle.
In addition to cardiac gating, chest motion from
`breathing can be removed from the image by using pulmonary
gating. Scan data would only be stored at times when the
J phase of the cardiac cycle under investigation coincided with
a particular phase of the pulmonary cycle.
The above-described embodiments are intended to be
illustrative, not restrictive. For example, the use of the
interval end of diastole or end of systole is intended to
be illustrative of the use of any interval of interest. More-
over, the same techniques disclosed herein may be applicable
to other physiological functions besides the heart. The
invention is applicable, of course, to any type of beam trans-
mission sub;ect to differential tissue absorption such as
X-rays, ~ rays, etc. The scope of the invention, however,
is defined by the appended claims and all variations which
fall within the range of equivalents thereto are intended to
be embraced therein.
