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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to neurosurgery in general, and in particular to a
method and system for the accurate anatomical localization of functional
activity in the human brain during a neurosurgical procedure. It has been
long known that the sensory, motor, and cognitive functions of the brain
are organized in anatomically distinct locations. However, with respect to
any particular brain function there is considerable variation in
anatomical localization between individuals. During a neurosurgical
procedure it would be of obvious value to be able to reliably determine
whether complete or partial removal of a particular region of the brain
would cause a subsequent functional deficit in the patient. For example,
before surgical resection, or removal, of a region of abnormal tissue in a
patient's temporal lobe causing repeated epileptic seizures, it would be
desirable to localize the language region of the particular patient's
brain in order not to damage or destroy the subsequent ability of the
patient to speak because of inadvertent resection of a functionally active
region of tissue in the temporal lobe. Other types of neurosurgical
procedures, such as tumor resection, would also benefit from a precise
method for functional localization before the neurosurgical procedure is
actually performed.
Current methods for localization of brain function may be divided into two
general types: invasive and noninvasive. While noninvasive methods of
imaging such as positron emission tomography, regional cerebral blood
flow, magnetoencephalography, and brain electrical activity mapping have
been used for general anatomical localization of brain function, each of
these methods has a significant flaw precluding accurate and effective use
for localization in the neurosurgical setting. Positron emission
tomography, which uses radioactive positron-emitting isotopes such as
.sup.18 F-fluorodeoxyglucose to create functional maps of brain metabolism
and blood flow, does not possess the spatial resolution necessary for
precise neurosurgical localization. Likewise, the measurement of regional
cerebral blood flow using .sup.133 xenon is implemented using large
crystal detectors and thus provides very low spatial resolution. The
anatomical relationship of non-invasively generated brain electrical and
magnetic activity maps to the underlying electrophysiological generators
is variable and may be confounded by differences in the spatial
orientation of the underlying generators.
There are, however, two invasive methods which are currently used for
functional localization in the brain during neurosurgery. These are a)
electrocorticographic recording of the electroencephalogram (EEG) and a
sensory evoked potentials, and b) electrical stimulation of the brain.
During electrocorticography the spontaneous ongoing electrical activity of
the brain is monitored using an array of electrodes placed on the surface
of the brain and the extremely low-amplitude analog signals, on the order
of microvolts, are recorded with galvanometric pens on continuous chart
paper. The traces are then analyzed visually and the location of maximum
activity or of particular EEG patterns is estimated. Evoked potentials are
recorded by monitoring the EEG with an array of electrodes, time-locking
digitization of the analog EEG signals to the presentation of a repetitive
sensory stimulus, and averaging multiple stimulus trials to reduce
background noise. Components of the EEG signal which are temporally
synchronized with the stimulus are augmented by the signal averaging
process, while components of the EEG which are not temporally synchronized
are largely eliminated by the averaging process. Multiple channels of
averaged signals are then presented on a video monitor or on chart paper,
the traces are analyzed visually, and the location of maximum electrical
activity evoked by the sensory stimulus is estimated.
The disadvantage of electrocorticography is that while some spontaneous EEG
patterns are well correlated with anatomical landmarks, these patterns may
not be present in all patients. Also, for most brain functions of interest
no specific EEG pattern has been identified. The disadvantage of recording
evoked potentials for neurosurgical localization is that these potentials
are mainly useful in defining the somatosensory region, and are of limited
use in the localization of other higher brain functions. In addition, both
electrocorticography and evoked potential measurements are susceptible to
volume conduction effects, in which activity may be recorded at sites
relatively distant from the actual electrophysiological generators.
Localization of brain function using electrical stimulation is implemented
through the direct application of short current pulses of alternating
polarity to different regions of the brain and observing effects of the
stimulation on the patient such as muscle twitching during stimulation of
the motor region, or obliteration of the ability to speak during
stimulation of the language region. Electrical stimulation is useful in
the localization of the somatosensory, language, and other regions of
higher brain function, but is very time-consuming, and the resolving power
of the technique is limited by the size and number of the stimulating
electrodes. In addition, the technique of electrical stimulation has been
criticized as physiologically artificial, in the sense that being
subjected to externally applied electrical current is obviously not a
normal physiological condition of the brain. Also, the possibility of
current leakage to tissue other than that which is being stimulated cannot
be excluded. Finally, the possibility exists that repetitive electrical
stimulation of the brain may induce harmful seizures or cause permanent
damage to brain tissue.
It is thus obvious that the present state of the neurosurgical art does not
offer a dependable, accurate system for localization of brain function
which can be used during a neurosurgical procedure. It follows that a
system and method for functional localization which combined high spatial
resolution, anatomical accuracy, and real time data acquisition and
analysis would satisfy an urgent need in current neurosurgical practice.
It is the object of the present invention to provide such a system and
method.
SUMMARY OF THE INVENTION
The present invention is a system and method for anatomical localization of
functional activity in the human brain during neurosurgery. The invention
is based on the principle that when a region of the brain is functionally
activated by either sensory stimulation of the patient or by the patient's
performance of a cognitive task, regional blood flow, regional blood
concentration, and regional total hemoglobin concentration all locally
increase in the activated area of the brain. (Performance of a cognitive
task is possible because neurosurgical patients are often awakened during
surgery precisely for the purpose of testing sensory or cognitive
responses.) The local increase in blood flow during functional activation
is in fact even greater than the increase in other widely used measures of
localized brain function such as glucose metabolism, and is thus a highly
sensitive indicator of functional activity.
The system and method described here exploits the sensitivity of regional
blood flow and blood concentration in brain tissue to functional
activation in discrete regions of the brain. The system is comprised of an
electronic flash illuminator, optical bandpass filters for sequential
illumination of the exposed brain surface with predetermined wavelengths
of monochromatic light, a video camera for generating images of the
illuminated brain surface, a video processor for conversion of the linear
video signal to a log signal directly representing reflectance of the
brain surface, an image processor for digitization and analysis of the
log-encoded video images and for computation of regional total hemoglobin
concentration, a computer and software for control of the entire
measurement process, a keyboard and monitor for user interaction with the
computer, a high-resolution RGB color video monitor for display of
computed functional activity maps, and a stimulus generator for
stimulating the patient in order to cause changes in regional total
hemoglobin concentration for localizing sensory functional activity of the
brain.
The present system and method localizes functional activity by using
reflectance spectrophotometry to quantitate regional total hemoglobin
(oxyhemoglobin plus deoxyhemoglobin) concentration at a large number of
individual points on the surface of the exposed brain during functional
activation, and comparing those levels with baseline levels of regional
total hemoglobin concentration measured during a preactivation period. A
reflectance measurement of regional total hemoglobin concentration is
implemented by sequentially illuminating the exposed brain surface with
predetermined wavelengths of monochromatic light, acquiring images of the
illuminated brain surface at each wavelength with a high-resolution video
camera, converting the linear video signal from the camera to a
logarithmic signal directly representing reflectance values, digitizing
the log-encoded video images and analyzing them using image processing
algorithms, and converting the resulting data into maps representing
regional total hemoglobin concentration on the brain surface, with each
computed map pixel representing a point on a regular grid on the exposed
brain surface. By subtracting a map generated during a baseline
preactivation period from a map generated during sensory stimulation or
cognitive task performance, a third map is generated which represents
differences in regional total hemoglobin concentration which have occurred
during functional activation. This image is coded, using a standard color-
or gray-scale, to represent levels of change in hemoglobin concentration
over the exposed brain surface, and is displayed on a high-resolution RGB
color video monitor.
The invention thus provides the neurosurgeon with maps of functional
activation, in real time, which delineates the regions of the brain which
are most active in their response to particular sensory or cognitive
stimuli. Regions of the brain which are highly activated manifest large
increases in regional blood concentration, while nonactivated regions of
the brain show little or no change. The neurosurgeon is thus guided in his
decision as to the location and amount of brain tissue he may safely
remove.
Advantages of the present invention include higher spatial and temporal
resolution than other functional imaging techniques, the ability to
analyze the acquired image data and compute maps in real time during the
neurosurgical procedure, the elimination of the need for radioactive
isotopes or external radiation sources such as x-rays to perform the
measurements, the absence of any physical contact between the apparatus
and the patient's brain, and a wide range of image resolution and field of
view.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the entire system.
FIG. 2 is a block diagram of the video processor.
FIG. 3 is a circuit diagram of the variable-slope log amplifier section of
the video processor.
FIG. 4 is a flow chart of the map computation process.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram of the present invention. The system comprises an
electronic flash illuminator 2 for illuminating the exposed surface of the
brain 1, a collimating lens assembly 3 for collimating the light output of
the electronic flash, a filter positioner 4 for positioning optical
filters in the collimated light beam of the electronic flash illuminator,
a filter wheel 5 for holding the optical filters, two optical
narrow-bandpass thin-film interference filters 6 and 7, a high-resolution
CCD video camera 8 for generating images of the illuminated brain surface,
a video processor 9 for conversion of the linear video output signal 17
from the CCD video camera into a logarithmic video signal 18 directly
representing units of reflectance, a video frame digitizer, random-access
memory, and arithmetic processor 10 for capturing, storing, and performing
image processing computations on the logarithmically transformed video
signal from the video processor, a computer and software 11 for operation
of the image processor and control of image acquisition, analysis, and
display, a keyboard and multisync monitor 13 for interaction with the
computer, a high-resolution RGB color video monitor 12 for map display,
and a multi-mode sensory stimulator 14 for stimulation of the patient.
A detailed description of each section of the system is now presented.
I. Electronic Flash
The electronic flash 2 may be a standard photographic electronic flash unit
such as a Vivitar 285HV. Because subsequent filtration of the electronic
flash beam by the narrow-bandpass interference filters 6 and 7
significantly attenuates the flash output illumination level, the flash
unit must be powerful enough to transmit sufficient light through the
filters to allow the use of small lens apertures by the video camera. The
use of small lens apertures insures s adequate depth-of-field in the video
image of the brain, and the high illumination level minimizes the possibly
confounding effect of ambient operating room light on the measurements. A
collimator lens assembly 3 is used to collimate the light beam before
bandpass filtration, since passage of off-axis light rays through the
thin-film interference filters results in a shift of the filter bandpass
center frequency. A TTL-level negative-going digital pulse, used to
trigger the electronic flash, is provided by an output line 16 from the
parallel port of the computer 11. The pulse is applied to the cathode of a
photo-silicon controlled rectifier (SCR) opto-isolator, which may be a
Harris MCS2400GE, and which electrically isolates the computer circuitry
from the high-voltage trigger circuit of the electronic flash. The
photo-SCR comprises a light-emitting-diode (LED) and a photosensitive SCR
packaged in the same integrated circuit but galvanically isolated from
each other. When a digital "0" LOW signal is applied to the cathode of the
LED, current flows through the LED, which emits light. The light turns on
the optically isolated SCR which in turn triggers the electronic flash.
The electronic flash trigger pulse from the computer is synchronized by
the system software to coincide with the first line of a new odd field in
the video frame. The video camera then outputs a complete video frame (two
video fields), which is then processed, digitized and analyzed.
II. Filters and Filter Positioner
Since the method of quantitating hemoglobin in the present invention
depends on accurate reflectance measurements at two wavelengths of
monochromatic light, two ultra-narrow bandpass filters 6 and 7 are used to
filter the electronic flash light illuminating the brain. These filters,
produced with thin-film interference technology, may be Ditric custom
thin-film interference filters with bandpass center frequencies at 569.0
nm (#ENI 1-0) and 586.0 nm (#ENX 1-0) respectively, with 50% transmission
bandwidths of 1.8 nm, and with 10% transmission bandwidths of 2.6 nm. The
filters are placed in the illuminating beam of the electronic flash
illuminator 2 to produce the monochromatic light needed for accurate
spectrophotometric analysis.
The positioner may be a filter wheel 5 with 2 filter apertures. The
thin-film interference filters are held in place in the filter wheel
apertures by retaining rings which are threaded on their outer
circumference and which mate with threads on the inside circumference of
the filter wheel apertures. The position of the filter wheel is controlled
by a filter positioner 4 which contains a stepping motor driven by a
microcontroller board specifically programmed for the task. The stepping
motor may be an Airpax L82401 and the microcontroller board may be a
Motorola 68HC11 evaluation board. The microcontroller determines the
initial position of the filter wheel by monitoring the status of a
photosensor which is illuminated by an adjacent lightemitting diode only
when a reflective spot on the wheel apparatus is aligned at a unique
rotational angle. The computer specifies the position of the filter wheel
by writing a digital signal 15 to bit 3 of the parallel port most
significant byte, with a "0" selecting the 586 nm filter and a "1"
selecting the 569 nm filter. The microcontroller monitors the position bit
15 and positions the filter wheel accordingly.
III. Video Camera
A high-resolution charge-coupled device (CCD) interline-transfer video
camera 8, which may be a Sony XC-77, is used to photograph the surface of
the brain. The CCD used in this camera is organized as a 768.times.493
pixel device. The horizontal resolution of the CCD is 380 TV lines, and
the overall signal-to-noise ratio of the camera is above 50 dB. Although
the CCD used in this camera and in similar industrial grade video cameras
are not highly linear in their photometric response, the technique used
for image processing in the present system is inherently ratiometric, and
photometric nonlinearities in CCD pixel response are thus almost
completely removed by the measurement technique.
The camera lens 22, which may be a Nikon 55 mm F/2.8 Micro-Nikkor macro
lens, is attached to the video camera with a F-to-C mount adapter. The use
of a macro lens originally designed for use with a 35 mm camera insures
high resolution, excellent sharpness at both the center and corners of the
video frame, and the ability to focus on the brain from short distances
for high image magnification when needed, while the telephoto perspective
of the lens (when it is used with a video camera) allows routine imaging
from a convenient distance in the operating room.
IV. Video Processor
FIG. 2 is a block diagram of the video processor 9. The main function of
the video processor is to transform the linear output signal 17 from the
video camera into a logarithmic signal. There are 3 advantages to the use
of logarithmic rather than linear video signals in the present system: 1)
Optical absorbance and reflectance are intrinsically ratiometric
measurements and are normally expressed as logarithmic units; 2) Log
arithmetic replaces multiplication and division operators used in linear
frame video arithmetic with addition and subtraction operators
respectively, significantly enhancing the speed of video frame arithmetic
operations; 3) Using log signals, an equals number of quantization steps
(bits) is available at any point on the reflectance vs. voltage scale,
while in the digitization of linear reflectance signals most of the
quantization steps are clustered at the high end of the scale.
The video processor is comprised of an input video-speed amplifier 25
configured as a differential amplifier and which may be an Analog Devices
AD844; a sample-and-hold amplifier 26 which is used as a DC level
restoration circuit and which may be a National Semiconductor LF398; a
video-speed op amp 29 configured as an inverting amplifier with unity gain
and which may be an Analog Devices AD844; a video-speed
linear-to-logarithmic amplifier 27 which converts the linear video signal
to a logarithmic current output and which may be a Analog Devices AD640; a
video-speed op amp 55 (shown in FIG. 3) configured as a current-to-voltage
transresistance amplifier and which may be an Analog Devices AD844; a
wideband linear multiplier 30 used as a variable-gain amplifier, which is
connected to the output of amplifier 55, and which may be an Analog
Devices AD539; a video-speed op amp 56 (shown in FIG. 3) configured as a
current-to-voltage amplifier which is connected to the output of the
wideband multiplier and which may be an Analog Devices AD844; a
digital-to-analog-converter 31 whose DC output voltage controls the gain
of the variable-gain amplifier and which may be an Analog Devices AD558;
an analog multiplexer 33 which both restores horizontal and vertical sync
pulses to the log video signal and clamps the log video signal to a preset
maximum voltage and which may be an Analog Devices AD9300; a video-speed
comparator 32 which sets the clamp voltage for the log video signal and
which may be one half of a National Semiconductor LM319; a second
video-speed comparator 34 which selects the horizontal and vertical sync
pulse inputs to the multiplexer at the appropriate time period in order to
restore standard RS-170 sync pulses to the log video waveform and which
may be one half of a National Semiconductor LM319; a sync detector 28
which detects vertical and horizontal sync pulses in the linear video
signal and which may be a National Semiconductor LM1881; and an output
amplifier 35 configured as an inverting amplifier with a gain of -2.00 and
an output impedance of 75 ohms and which may be an Analog Devices AD844.
The output 18 of the inverting amplifier is connected to the input of the
MVP-AT image processor 10 as described below.
A detailed description of the video processor is now presented. The
AC-coupled output 17 of the video camera which is in standard RS-170
format is input through a BNC connector and terminated in a 75 ohm
resistor 38. The signal is simultaneously input to the inverting input of
video-speed amplifier 25 configured in differential amplifier mode with a
gain of +1.00 at the noninverting input and -1.00 at the inverting input,
and also to the signal input of a sample-and-hold amplifier 26. The
sample-and-hold amplifier 26 is switched to SAMPLE mode during the "back
porch" period of the video waveform. The output 39 of the sample-and-hold
amplifier is connected to the noninverting input of the video-speed
amplifier 25, effectively setting the output of the amplifier 25 to a
fixed, stable DC voltage level which is independent of the DC component of
the input video waveform. A resistor and capacitor (not shown) are
configured as a single-pole lowpass filter with a -3 dB point of 235 Hz
and connected to the noninverting input of amplifier 25 to smooth
switching transients generated by the sample-and-hold amplifier.
FIG. 3 is a circuit diagram of the variable-slope log amplifier section of
the video processor. The output of amplifier 25, a signal 42 which is the
sum of the inverted input waveform and a fixed DC component, is applied to
the attenuator input of the log amplifier 27. The output of the attenuator
67, reduced in amplitude by 20 dB, is applied to the main signal
processing section of the log amplifier. Both input common and output
common pins 66 of log amplifier 27 are connected to ground. The log common
pin 52 is connected to the grounded positive input pin of video-speed
amplifier 55. The negative signal input 65 of the log amplifier is
connected to the low side of the attenuator differential input. The log
amplifier converts the linear signal into a current output 53 proportional
to the log.sub.10 of the absolute value of the input waveform. The log
amplifier current output 53 is filtered by a resistor 57 and capacitor 58
which form a single-pole lowpass filter with a -3 dB point of 15 MHz. The
filtered current is applied to the inverting input 54 of video-speed
amplifier 55 configured in current-to-voltage transresistance mode, with a
precision 1000 ohm resistor 59 setting the voltage gain of the amplifier.
Thus, the output of op amp 55 is a voltage proportional to the log of the
absolute value of the output signal 42 of amplifier 25. The zero-intercept
point of the log response slope is set by a variable resistor (not shown),
which adds a fixed DC offset to the output signal of amplifier 25.
The log-transformed video signal is next applied to the variable-gain
amplifier section of the circuit. In this circuit application the two
channels of the wideband linear multiplier 30 are operated in parallel so
that both inputs V.sub.Y1 and V.sub.Y2 43 are connected together and both
outputs OUT.sub.1 and OUT.sub.2 63 are connected together. The log voltage
signal output 43 of op amp 55 is applied to voltage inputs V.sub.Y1 and
V.sub.Y2 43 of the multiplier 30. Likewise, both outputs of the multiplier
63 are connected to the inverting input of video-speed op amp 56, which
converts the current output of the multiplier to a voltage. The output 46
of op amp 56 is connected to the feedback inputs of the multiplier
W.sub.1, W.sub.2, Z.sub.1, and Z.sub.2. The input and output common pins
61 are connected to ground. The base common pin 64 is connected to the
grounded positive input of video-speed op amp 56. A 0.1 .mu.F bypass
capacitor 60 is used to improve the high-frequency response of the
multiplier. The output of this circuit configuration at 46 is -V.sub.X
V.sub.Y /2V and is controlled by a positive DC voltage 44 present at the
V.sub.X input of the multiplier within the range of 0 to +3.160 VDC. This
DC voltage is generated by a digital-to-analog converter 31 whose digital
data inputs 19 are connected to outputs D0-D7 of the computer 11 parallel
port lower byte. The output 46 of the wideband multiplier is thus the
instantaneous product of the log voltage output 43 of log amplifier 27 and
the digital value on line 19 present at the inputs to the
digital-to-analog converter 31. The multiplier thus directly controls the
log response slope of the processing circuit. A high control voltage
results in a high-contrast steep log response slope with variations in
brain tissue reflectance generating relatively large amplitude differences
in the video signal, while a low control voltage results in a low-contrast
shallow low response slope, with tissue reflectance variations generating
relatively small amplitude differences in the video signal.
It should be noted that at this point in the circuit, the processed signal
is inverted with respect to the input signal, and must again be inverted
before being connected to the MVP-AT image processor input. In addition,
since the output of the processing circuit must conform to RS-170
standards, and since the negative-going sync pulses of the video signal 17
from the camera are highly distorted by the absolute value logarithmic
response of the log amplifier 27, sync pulses of correct amplitude and
duration must be added to the processed log signal before it is output to
the image processor. The addition of sync pulses of correct amplitude and
duration to the log-transformed signal is performed by the output
multiplexer 33 in conjunction with dual comparators 32 and 34. The output
46 of the multiplier 30 is applied to analog input 2 of multiplexer 33.
One of the two multiplexer digital address bits is generated by each of
the dual comparators 32 and 34. One comparator 34 generates the lower A0
bit 49 of the multiplexer address input. The negative input of comparator
34 is grounded, and the positive input is connected to the output 45 of
video-speed op amp 29 configured as an inverting amplifier with a gain of
-1.00. Amplifier 29 reinverts the previously inverted signal output of the
input amplifier 25, thus generating a correct-polarity, DC-clamped, linear
video signal 45 of the same phase and peak-to-peak amplitude as the input
waveform 17. Thus, when the input waveform 45 to the comparator 34 swings
below ground during the horizontal and vertical sync pulse periods of the
video waveform, the output 49 of the comparator goes low, connecting the
output of video amplifier 25, the inverted, DC-restored input waveform 42,
to the output 51 of the multiplexer. When the waveform swings above
ground, the output of the comparator goes high, connecting the output 46
of the multiplier circuit 30 to the output 51 of the multiplexer. Thus,
the output 51 of the multiplexer is the multiplier output combined with
composite sync signals of correct amplitude and duration. (However, the
polarities of both the signal and sync pulses are inverted). The positive
input of the comparator 32 is connected to a voltage divider whose node 47
is held at -1.00 VDC. The output 50 of this comparator controls the high
address bit A1 of the multiplexer. Thus, when the multiplier output swings
below -1.00 volts, the comparator output goes high, connecting either the
composite portion of the input waveform (when A0 is low) or the -1.00 VDC
divider voltage (when A0 is high) to the output of the multiplexer. The
multiplexer output is thus always clamped to a maximum negative voltage of
-1.00 VDC.
The analog output 51 of the multiplexer 33 is applied to the negative input
of video-speed op amp 35 configured as an inverting amplifier with a gain
of -2.00. Thus, the output 18 of op amp 35 is the log-transformed input
video signal 43 multiplied by the slope control voltage 44. The waveform
at this point is of proper polarity and has vertical and horizontal sync
pulses of s correct amplitude and duration superimposed on it. The signal
18 is connected to the output BNC connector of the video processor through
a 75 ohm terminating resistor and a 1000 .mu.F AC-coupling capacitor. The
signal is finally connected to analog input channel 0 of the MVP-AT image
processor. Since the input resistance of the MVP-AT image processor is 75
ohms, the actual gain of the output amplifier stage is restored to -1.00.
The inverted linear video waveform output of input amp 25 is also applied
to an inverting video-speed op amp 29 and the linear reinverted output 45
of amplifier 29 is applied to the input of a video sync detector circuit
28. The sync detector circuit generates separate logic level signals
during both composite sync and back porch time periods. The back porch
digital signal 41 is applied to the SAMPLE input of sample-and-hold
circuit 26, which samples the video waveform during the back porch period
and implements DC restoration of the signal at a fixed DC voltage level
39.
V. Image Processor
The analog log-transformed output signal 18 of the video processor 9, in
standard RS-170 format, is applied to the video input of the image
processor 10, which may be a Matrox MVP-AT with 1 megabyte of memory and
which consists of a video-speed analog-to-digital converter, 32 input
lookup tables, a random-access memory capable of storing 4 video frames
with each video frame consisting of a 512.times.512.times.8-bit pixel
array, an arithmetic processor capable of performing mathematical
operations on a video frame, generally in 1 frame period (30
milliseconds), 32 output lookup tables, 3 digital-to-analog converters at
the image processor output 23 which directly drive a high-resolution RGB
color video monitor 12, and a high-speed microcomputer embedded in the
image processor, which controls imaging functions and processing
algorithms. The Matrox MVP-AT frame memory is supplemented by a Ramagette
memory board with 4 additional megabytes of random-access memory, allowing
the storage of an additional 16 video frames. The image processor and
supplementary memory are designed for use on an industry-standard IBM AT
(or compatible) computer platform, and in the MS-DOS operating system
environment.
Image computation in the image processor is performed using two's
complement arithmetic. The input signal from the video processor is
therefore converted from the offset binary encoding format of the image
processor analog-to-digital converter to two's complement format. This
conversion is implemented through a two's complement lookup table stored
in one of the input lookup table memories. While the digitized data is
initially encoded with 8 bit precision, all subsequent image arithmetic
operations are carried out with 16-bit precision in order to allow for
two's complement overflow at any step in the computation process. Frame
buffers which are normally configured in 8-bit format are therefore
reconfigured to 16-bit format during frame arithmetic computations.
The image processor is configured to operate in a conventional manner, the
details of which should be readily apparent to persons having ordinary
skill in the field of the invention.
VI. Video Data Acquisition and Analysis
The method used for localization of functional brain activity in the
present invention is based on the principle that when an area of brain is
functionally activated, both regional blood concentration and regional
total hemoglobin concentration increase in that area. By creating a
baseline map of regional total hemoglobin concentration over the exposed
surface of the brain before sensory stimulation of, or cognitive task
performance by the patient, and by comparing the baseline map to maps
generated at regular intervals during stimulation or task performance, the
regions which manifest the greatest change in total hemoglobin
concentration can readily be identified.
In the present invention, regional total hemoglobin concentration is
quantitated using a spectrophotometric method which is based on the
difference in reflectance of brain tissue at two isosbestic wavelengths of
hemoglobin. An isosbestic wavelength is one at which the extinction
coefficient of two molecules, in this case oxyhemoglobin and
deoxyhemoglobin, are identical. Two of the isosbestic wavelengths of
hemoglobin are 569 nm and 586 nm. The reflectance of a region of brain
tissue illuminated by monochromatic light at a wavelength of 586 nm minus
the reflectance of the same region illuminated by monochromatic light at a
wavelength of 569 nm is linearly proportional to the total hemoglobin
concentration in the region of tissue being measured. This relationship
may be expressed as:
Hb.sub.n =R586.sub.n -R569.sub.n
where Hb is regional total hemoglobin concentration expressed in arbitrary
units, n is the elapsed time in seconds from acquisition of the baseline
preactivation map, and R is reflectance at the specified isosbestic
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