U.S. patent application number 10/801961 was filed with the patent office on 2004-09-09 for method and apparatus for cardiac imaging with minimized cardiac motion artifact.
Invention is credited to Budoff, Matthew J., Mao, Songshou.
Application Number | 20040176681 10/801961 |
Document ID | / |
Family ID | 31947141 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040176681 |
Kind Code |
A1 |
Mao, Songshou ; et
al. |
September 9, 2004 |
Method and apparatus for cardiac imaging with minimized cardiac
motion artifact
Abstract
The present invention provides for a method and apparatus for
acquiring cardiac images having minimized motion artifact by
triggering an image-acquisition scan at a point during the
quiescent segment of each cardiac cycle. The method of the present
invention comprises: measuring the length of the R-R interval of a
cardiac cycle; calculating the R-T segment length based on gender
and R-R interval length; identifying an optimal scan starting point
of the cardiac cycle based on R-R interval length, R-T segment
length and scan speed; and triggering the image-acquisition scan at
this starting segment. The method is implemented by an apparatus,
namely a cardiac imaging device that has image-acquisition speeds
of about 15-300 ms. The apparatus comprises a transmitter that
generates the image-acquisition signal, an input console, and an
ECG gating device that synchronizes the trigger of
image-acquisition scans with the starting points determined by the
above method.
Inventors: |
Mao, Songshou; (Torrance,
CA) ; Budoff, Matthew J.; (Redondo Beach,
CA) |
Correspondence
Address: |
ORRICK, HERRINGTON & SUTCLIFFE, LLP
4 PARK PLAZA
SUITE 1600
IRVINE
CA
92614-2558
US
|
Family ID: |
31947141 |
Appl. No.: |
10/801961 |
Filed: |
March 15, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10801961 |
Mar 15, 2004 |
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09833461 |
Apr 11, 2001 |
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6708052 |
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Current U.S.
Class: |
600/413 ;
600/428 |
Current CPC
Class: |
A61B 6/503 20130101;
A61B 6/541 20130101; A61B 5/7285 20130101; A61B 6/027 20130101 |
Class at
Publication: |
600/413 ;
600/428 |
International
Class: |
A61B 005/05 |
Claims
What we claim is:
1. A method for acquiring images of the heart during cardiac cycles
comprising: measuring the length of at least one of said cardiac
cycles prior to triggering at least one image-acquisition scan; and
triggering said image-acquisition scan at a point of said cardiac
cycle having minimized motion, whereby said point is based on said
cardiac cycle length.
2. A method for acquiring an image of the heart of a patient by
triggering an image-acquisition scan at a point of a cardiac cycle
having minimized motion, comprising: measuring the length of the
R-R interval of said cardiac cycle; calculating the length of the
R-T segment of said cardiac cycle to determine the quiescent
segment of said cardiac cycle; identifying an optimal scan starting
point of said cardiac cycle; and triggering said image-acquisition
scan at said optimal scan starting point.
3. The method of claim 2 wherein the step of calculating said R-T
segment length includes determining the gender of said patient and
said R-R interval length.
4. The method of claim 3 wherein the step of calculating said R-T
segment length comprises applying the algorithm
0.143.times.RR+224.2, where the gender of said patient is male.
5. The method of claim 3 wherein the step of calculating said R-T
segment length comprises applying the algorithm
0.157.times.RR+221.2, where the gender of said patient is
female.
6. The method of claim 2 wherein said optimal scan starting point
is identified in part by said R-R interval length, said R-T segment
length, and the speed of said image-acquisition scan.
7. The method of claim 6 wherein the step of identifying said
optimal scan starting point comprises applying the algorithm
RT.+-.X, where said X value depends on said R-R interval length and
the speed of said image-acquisition scan.
8. The method of claim 2 wherein the speed of said
image-acquisition scan is within the range of about 15 ms to about
75 ms.
9. The method of claim 2 wherein the speed of said
image-acquisition scan is within the range of about 76 ms to about
150 ms.
10. The method of claim 2 wherein the speed of said
image-acquisition scan is within the range of about 151 ms to about
225 ms.
11. The method of claim 2 wherein the speed of said
image-acquisition scan is within the range of about 226 ms to about
300 ms.
12. The method of claim 2 whereby said optimal scan starting point
may dynamically vary with each cardiac cycle.
13. The method of claim 12 wherein acquiring said image of the
heart is independent of the consistent heart rate of said
patient.
14. A cardiac imaging apparatus that acquires an image of the heart
of a patient by triggering an image-acquisition scan at a point of
the cardiac cycle having minimized motion comprising: a transmitter
that generates said image-acquisition scan; an input console,
whereby said input console is adapted to receive information
regarding an optimal scan starting point of said cardiac cycle, and
said optimal scan starting point is based in part on the length of
the R-R interval of said cardiac cycle, the length of the R-T
segment of said cardiac cycle, and the speed of said
image-acquisition scan; an ECG gating device that is connected to
and adapted to communicate with said transmitter and said input
console, whereby said gating device triggers said image-acquisition
scan at said optimal scan starting point.
15. The apparatus of claim 14 wherein said ECG gating device
includes software adapted to measure said R-R interval length,
calculate said R-T segment length, and identify said optimal scan
starting point.
16. The apparatus of claim 14 comprising a magnetic resonance
imaging device.
17. The apparatus of claim 14 comprising a spiral computer
tomography scanner.
18. The apparatus of claim 14 comprising an electron beam
tomography scanner.
19. The apparatus of claim 15 wherein said optimal scan starting
point may dynamically vary with each cardiac cycle.
20. The apparatus of claim 14 wherein said ECG gating devices
receives and stores the gender of said patient.
21. The apparatus of claim 14 wherein said ECG gating device
receives and stores the speed of said image-acquisition scan.
22. The apparatus of claim 14 wherein said R-T segment length is
based on said R-R interval length and the gender of said
patient.
23. The apparatus of claim 22 wherein said R-T segment length is
calculated by the algorithm 0.143.times.RR+224.2 where the patient
is male.
24. The apparatus of claim 22 wherein said R-T segment length is
calculated by the algorithm 0.157.times.RR+221.2 where the patient
is female.
25. The apparatus of claim 14 wherein said optimal scan starting
point is identified by the algorithm RT.+-.X, where said X value
depends on said R-R interval length and said image-acquisition scan
speed.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to cardiac imaging
and, in particular, acquiring cardiac images having minimized
motion artifact with high-speed imaging devices. The invention
relates to a method and apparatus for acquiring these cardiac
images by prospective gating based in part on the length of a
cardiac cycle, gender of the subject being imaged and the imaging
speed.
BACKGROUND OF THE INVENTION
[0002] Acquiring clear images of the heart is typically impeded by
cardiac motion and coronary artery motion caused by the rhythmic
beating of the heart. The resulting loss of resolution causes
blurring or streaking, called motion artifact, which diminishes the
diagnostic value of these images.
[0003] Efforts have been made to minimize cardiac motion artifact.
Scanning protocols may be adapted to attempt to limit the motion
reflected in the cardiac image. Alternatively, patients may simply
be instructed to lie still and hold their breath during the scan to
reduce respiratory motion, or patient restraints and supports may
be used to limit general body motion. In addition, some attempts
have been made to shorten image acquisition time by actively
increasing the subject's heart rate and/or increasing the speed of
the image-acquisition scan generated by the cardiac imaging device.
As an alternative, ECG gating techniques have developed which
involve equipping the cardiac imaging apparatus with an ECG gating
device that synchronizes the image-acquisition scans with specific
phases of the cardiac cycle.
[0004] ECG gating relies on the electrocardiographic signals that
represent the rhythmic contraction of the heart's atria and
ventricles. These signals originate from electrical pulses of the
sinoatrial (SA) node, which spread over the atria and ventricles
and cause them to contract, resulting in a complete cycle of the
heart's contractions. Thus, a recorded ECG waveform represents the
cardiac cycle, and is comprised of a set of discrete
electrocardiographic signals corresponding to the muscular
contraction and relaxation of the atria and ventricles.
Specifically, the R-R interval measures the period of the heart
beat, the P-R segment corresponds to the time from the onset of
atrial contraction to the onset of ventricular contraction; the R-T
segment approximately measures ventricular contraction or systole;
and the T-R interval measures ventricular relaxation, or
diastole.
[0005] Many known ECG gating techniques involve "retrospective
triggers" that coordinate the scanning of images with different
electrocardiographic signals of the cardiac cycle to obtain a full
set of scans over a number of cardiac cycles. Then, the scans are
"sorted" by computerized means according to the phase of the cycle
during which they were taken to construct separate images of each
phase of the cardiac cycle. Thereafter, the technician or physician
selects the clearest image from this series with the least motion
artifact for diagnostic purposes.
[0006] ECG gating techniques also involve efforts to "prospectively
trigger" an image-acquisition scan starting with a specific phase
of the cardiac cycle, typically at 40-50% of the R-R interval and
at 70-80% of the R-R interval. These percentages allegedly
correspond to quiescent points of the cardiac cycle where cardiac
motion is at a minimum.
[0007] There are several problems associated with known ECG gating
techniques. First, the traditional techniques generate scan
triggers at pre-determined, fixed percentages of the cardiac cycle
regardless of the heart rate of the subject being imaged during the
scanning procedure. Quiescent points, however, vary with heart
rate, so that using a fixed percentage for all subjects regardless
of heart rate is ineffective.
[0008] Moreover, with many known techniques, the clarity of the
resulting images depends on the type of imaging device being used
and the speed of the image-acquisition scan it generates. For
instance, one ECG gating technique involves using 40-50% of the R-R
interval to trigger image-acquisition scan for coronary artery
screening or coronary angiography with electron beam tomography
(EBT), which has an ultrashort image acquisition time (50-100 ms).
This technique may not be as effective with scanning devices having
longer acquisition times, such as MRI and spiral CT scanners
(100-500 ms).
[0009] Finally, because the traditional techniques generate
triggers at predetermined percentages of the R-R interval, the
length of each cardiac cycle must be the same during the scanning
procedure so that the image-acquisition scan is triggered at
precisely the right time for each heart beat. Thus, these
techniques produce images with minimized motion artifact only when
the heart being imaged has a consistent heart rate, usually
measured in beats per minute. ECG gating is not effective for those
subjects that have irregular heart rates, or whose heart rates
increase or decrease during the imaging procedure, either because
of a physical condition or disease or because of stress resulting
from the imaging procedure.
[0010] Thus, it is an object of the present invention to provide a
method and apparatus for acquiring diagnostically valuable cardiac
images of the heart having minimized motion artifact via
prospective gating by triggering an image-acquisition scan starting
at a point of a cardiac cycle, where this point is calculated, in
part, by the length of the cardiac cycle. It is yet another object
of the present invention to acquire cardiac images having minimized
motion artifact with imaging devices having a wide range of scan
speeds. Finally, it is an object of this invention to acquire these
diagnostically valuable cardiac images in a manner wherein the
quality of the resulting images is not dependent on consistent
heart rate and, in fact, may dynamically vary as necessary with
each heart beat.
SUMMARY OF THE INVENTION
[0011] The present invention relates to a method and apparatus for
acquiring an image of the heart by triggering an image-acquisition
scan starting at a point of the cardiac cycle having minimized
motion. The optimal point for triggering an image-acquisition scan
is determined by measuring the lengths of the cardiac cycles of the
subject being imaged. The optimal trigger point of each cardiac
cycle depends on the length of that cycle; thus, this point varies
as the length of the cardiac cycle changes. Thus, preferably, the
present method and apparatus measures each cardiac cycle during the
scanning procedure and dynamically determines and adjusts the
optimal trigger point from one heart beat to the next as necessary.
Thus, in the preferred embodiment, the diagnostic value of the
images obtained pursuant to this invention does not rely on the
subject having a constant heart rate throughout the duration of the
scanning procedure.
[0012] The method disclosed by the present invention acquires a
cardiac image with minimized motion artifact by measuring the
length of the R-R interval of a particular heart beat, calculating
the length of the R-T segment to determine the quiescent segment of
the cardiac cycle, identifying an optimal scan starting point of
the cardiac cycle, and triggering an image-acquisition scan at this
point.
[0013] The calculation of the R-T segment length is based in part
on the gender of the subject and the length of the R-R interval.
For men, the R-T length is calculated by the algorithm
0.143.times.RR+224.2; for women, the R-T length is calculated by
the algorithm 0.157.times.RR+221.2. In both cases, the R-R
interval=1000 ms.times.60/heart rate (ms). The quiescent segment,
which corresponds to the period of minimized cardiac motion
velocity, is late systole to early diastole, and approximates the
end of the R-T segment.
[0014] The optimal scan starting point, which is within the
quiescent segment, is based on the image-acquisition scan speed and
the subject's heart rate. The present method contemplates the use
of scan speeds that fall within the range of about 25 ms to about
250 ms. As an example, the optimal scan starting point is at 25-50
ms before the end of the R-T interval where the speed of the scan
protocol is 25-100 ms for any heart rate. However, this scan
starting point is suboptimal where the scan protocol speed is over
150 ms and the subject's heart rate is less than 61 beats per
minute. In this case, mid-diastole provides for better scan points
because the longer quiescent segment is at 60-80% of the R-R
interval. Thus, the optimal scan starting point will vary with each
subject's heart rate and the imaging device used. The optimal scan
starting point in the R-R interval is identified by the algorithm
RS=RT.+-.X, where RS refers to the length of time from the peak of
the R wave to the scan starting point, the length of the RT segment
is determined by the formulas set forth above, and the "X" value
depends on the scan speed, as listed in table 1 herein.
[0015] The present method is implemented by a cardiac imaging
apparatus that is capable of generating image-acquisition scans
within the range of about 15 ms to about 300 ms, namely MRI
devices, spiral CT scanners and EBT scanners. The apparatus
comprises a transmitter that generates the image-acquisition scan,
an input console that receives parameters used in implementing the
above method, and an ECG gating device that is connected to and
adapted to communicate with the transmitter and the input console.
This gating device synchronizes the triggering of image-acquisition
scans with specific points of the cardiac cycle. The ECG gating
device includes hardware that receives electrical signals
representing the cardiac cycle and triggers image-acquisition
scans. The ECG gating device also preferably includes software that
operates the gating hardware and is adapted to implement many of
the steps described above, i.e., to measure the length of the R-R
interval, calculate the length of the R-T segment and to identify
the optimal scan starting point. Alternatively, these steps towards
identification of the optimal scan starting point may be
implemented manually by the physician or technician overseeing the
scanning procedure who can relay the calculated optimal scan
starting point to the ECG gating device via the input console.
Regardless of how this optimal scan starting point is relayed to
the gating hardware, it electronically triggers the transmitter to
release an image-acquisition scan at that point.
[0016] This and further objects and advantages of will be apparent
to those skilled in the art in connection with the drawings and the
detailed description set forth below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a flowchart of the method of the present
invention.
[0018] FIG. 2 is a block diagram of the apparatus of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] The present invention discloses a method for acquiring an
image of the heart by triggering an image-acquisition scan at a
point in the cardiac cycle having minimized motion artifact,
whereby the trigger point is based on the length of the cardiac
cycle. As seen in FIG. 1, the method is implemented by a cardiac
imaging apparatus 10 that receives the subject's
electrocardiographic signals, receives the optimal scan starting
point of each heart beat, and triggers the image-acquisition scan
at that point.
[0020] This invention contemplates the use of a cardiac imaging
apparatus capable of generating high-speed image-acquisition scans,
i.e., magnetic resonance imaging devices (MRI's) provided by
companies such as GE and Siemens, electron beam tomography (EBT)
scanners provided by companies such as Imatron, and spiral computer
tomography (spiral CT) scanners provided by GE, Siemens, Toshiba,
Picker, Phillips, Marconi and other manufacturers. These devices
vary in the components involved in acquiring images.
[0021] These devices also share several general features in common.
Cardiac imaging apparatus 10 contemplated by the present invention,
whether it comprises a MRI device, a spiral CT scanner or EBT
scanner, includes a transmitter 12 that generates an
image-acquisition electrical scan 14. In an MRI, transmitter 12
comprises an RF pulse transmitter; in spiral CT and EBT scanners,
transmitter 12 comprises a x-ray tube and x-ray gun, respectively.
Also included in apparatus 10 is input console 16 that allows the
technician or physician to enter parameters used in practicing the
invention and to receive data from the apparatus. Finally, imaging
apparatus 10 also contains ECG gating equipment 18 that is
connected to and adapted to communicate with other components of
apparatus 10, including transmitter 12 and input console 16, to
synchronize image acquisition with the cardiac cycle. This ECG
gating equipment includes electrode leads attached to the subject
and an ECG telemetric system that acquire the electrocardiogram
signals 17. Received signals 17 are then routed to ECG gating
hardware 20 that contains digital and analog connectors and related
circuitry to trigger image-acquisition scan 14.
[0022] EGG gating equipment 18 also preferably includes a software
program 22 that operates hardware 20, and that receives and
processes parameters used in practicing the invention, as described
in more detail below. Program 22 applies these parameters according
to the steps set forth below to calculate the point of each heart
beat at which to trigger image-acquisition scan 14, and instructs
gating hardware 20 of this trigger point. In this way, the
invention may achieve one of the preferred objectives, i.e., to
determine the length of each cardiac cycle during the scanning
procedure and to dynamically vary the optimal scan starting point
as necessary with each heart beat.
[0023] While the description set forth herein refers to this
preferred embodiment, wherein steps 26, 28 and 30 of the claimed
method are implemented automatically for each heart beat by program
22, this invention also contemplates implementing steps 26, 28 and
30 manually, by the physician or technician overseeing the scanning
procedure. The term "manually" includes, by way of example and not
limitation, via manual calculation, calculator or computer-assisted
means. That is, the physician/technician may take the heart rate of
the subject being imaged immediately prior to the scanning
procedure and, from this, calculate the average R-R length using,
for example, the R-R=100 ms.times.60/heart rate (ms) formula
described above. The physician/technician may then calculate the
R-T segment length and the R-S segment length by the steps
described in more detail below.
[0024] Referring to FIG. 2, the method 24 of the present invention
begins with preparing the subject for ECG measurements of the heart
by attaching standard electrode leads to the subject's chest and
electrically connecting them to ECG gating equipment 18. The first
step, step 26, is to measure the length of the R-R interval of a
particular heart beat of the subject. This step is preferably
implemented by software 22 that is programmed to automatically
measure the time period in ms between two R electrocardiogram
waveforms. A normal heart beats 60-80 times/minute, which
translates into about 1000 ms/beat to about 750 ms/beat. However,
there is wide variation in possible R-R waveform lengths.
[0025] Subjects with bradycardyia can have heart rates as slow as
40 (or under) beats/minute, or about 1500 ms/beat, while subjects
with tachycardia can have heart rates as fast as 110 beats/minute,
or 545 ms/beat. The R-R waveform length is stored by ECG gating
equipment 18.
[0026] Next, in step 28, the length of the R-T segment of the heart
beat is calculated. The length of this segment is important
because, as described above, it reflects the period of systole or
ventricular contraction. More importantly, the R-T segment reflects
late systole, and the end of the T wave corresponds to the
quiescent segment of the cardiac cycle. Typically, the value of R-T
length ranges from 24% to 54% of the cardiac cycles in subjects
with heart rates from 35-115 beats per minute.
[0027] In calculating the R-T segment, gender is relevant, i.e.,
women have been found to have longer R-T intervals than men. In
addition, significant correlation was found between heart rate and
the length of the R-T segment. As heart rate increased from about
40 beats/min to about 110 beats/min and the R-R interval length
decreased, the R-T segment--i.e., the period of systole--also
became shorter. Thus, based on these findings, the R-T segment of
the cardiac cycle of a subject is the function of the subject's
gender and R-R interval length. For a male subject, the R-T length
may be calculated by: RT=0.143.times.RR+224.2. For a female
subject, the R-T length calculation is: RT=0.157.times.RR+221.2.
The RT values fall typically between 350-365 ms for subjects with
normal heart rate (60-80 beats per minutes).
[0028] Calculation of the length of the R-T segment may be
implemented by input console 16 and software program 22 where a
technician or physician enters the subject's gender into console
16. Then, program 22 selects the appropriate algorithm based on the
subject's gender and inputs the R-R value measured in step 26. The
R-T segment length is stored by ECG gating equipment 18.
[0029] Next, in step 30, the optimal scan starting point of the
cardiac cycle is identified. From this point, an image may be
acquired with minimized motion artifact. While the end of the R-T
segment has been found to correspond to low cardiac motion, as
described above, step 30 is also based on cardiac cycle length and,
thereby, may factor in potential variation in the length of each
heart beat. Thus, the optimal scan starting point at which to
trigger the image-acquisition scan may preferably change from one
beat to the next. Step 30 also factors in the speed of
image-acquisition scan 14, which depends on the imaging device
being used, and recognizes that this scan speed may effect the
optimal scan starting point for image acquisition. In particular,
step 30 effectively determines this optimal scan starting point
where the length of the image-acquisition scan is between about 15
ms and about 300 ms, including MRI devices provided by companies
such as GE and Siemens, EBT scanners, such as the Imatron EBT
machine, and spiral CT scanners provided by companies such as such
as GE and Siemens. While the fastest scan speeds generated by these
devices to date approximate 50 ms, some have shown the potential of
reaching even faster speeds, such as 25 ms.
[0030] Thus, step 30 calculates the time period from the R wave to
the optimal scan starting point at which to begin the
image-acquisition (referred herein as the R-S interval) by the
algorithm R-S=RT.+-.X. The optimal scan starting point typically
falls before the end of the T wave for most scan protocols. Where
the subject being imaged has an extremely slow heart rate and the
imaging device being used has a very slow scan speed, the optimal
scan starting point has been found to fall beyond the end of the T
wave, as exemplified by Table 1. The value of X is a function of
the length of the particular heart beat and the speed of the scan
being used, and can be determined from the following
distribution:
1TABLE 1 Scan Time Scan Time Scan Time Scan Time RR 50 ms 100 ms
200 ms 250 ms HR Interval (15-75 ms) (76-150 ms) (151-225 ms)
(226-300 ms) beats/minute ms RS = RT .+-. X RS = RT .+-. X RS = RT
.+-. X RS = RT .+-. X <40 1714 RT-25 RT-50 RT + 654 RT + 620
(Mean 35) 41-50 1333 RT-25 RT-50 RT + 458 RT + 432 (Mean 45) 51-60
1090 RT-25 RT-50 RT + 300 RT + 267 (Mean 55) 61-70- 923 RT-25 RT-50
RT-113 RT-122 (Mean 65) 71-80 800 RT-25 RT-50 RT-102 RT-126 (Mean
75) 81-90 706 RT-25 RT-50 RT-88 RT-116 (Mean 85) 91-100 632 RT-25
RT-50 RT-80 RT-106 (Mean 95) 101-110 571 RT-25 RT-50 RT-68 RT-91
(Mean 105) >110 522 RT-25 RT-50 RT-68 RT-89 (Mean 115)
[0031] Step 30 would also preferably be implemented by input
console 16 and software program 22. That is, program 22 would be
adapted to read the speed of the imaging scan or, alternatively,
the technician or physician would input the speed of the
image-acquisition scan into console 16, and program 22 would apply
this scan speed and the value of the R-R interval measured in step
26 to determine the value of X. Then, program 22 would plug in the
length of the R-T segment calculated in step 28 into algorithm
R-S=RT.+-.X to identify the optimal scan starting point.
[0032] The next step, step 32, comprises triggering the
image-acquisition scan at the optimal scan starting point
identified in the above step. As previously described, the
triggering of this scan depends on cardiac cycle length and,
preferably, may vary dynamically from one heart beat to the next.
Step 32 is implemented by ECG gating hardware 20. As soon as
program 22 completes steps 26, 28 and 30, thereby identifying the
optimal scan starting point, it instructs hardware 20 to
electronically trigger transmitter 12 to generate image-acquisition
scan 14 at that point. Alternatively, where steps 26, 28 and 30 are
implemented without program 22, the physician or technician may
relay the optimal scan starting point to hardware 20 via input
console 16, which then triggers image-acquisition scan 14.
[0033] While preferred method and apparatus embodiments have been
shown and described, it will be apparent to one of ordinary skill
in the art that numerous alterations may be made without departing
from the spirit or scope of the invention. The invention is not to
be limited except in accordance with the following claims and their
legal equivalents.
* * * * *