United States Patent £191
`Chance
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,972,331
`Nov. 20, 1990
`
`[54] PHASE MODULATED
`SPECTROPHOTOMETRY
`
`[75]
`
`Inventor: Britton Chance, Philadelphia. Pa.
`
`[73} Assignee: Nim, Inc., Philadelphia. Pa.
`[21} Appl. No.: 307,066
`Feb. 6, 1989
`[22} Filed:
`Int. Cl.5 ......................... G06F 15/00; A61B 5/00
`[51]
`[52] U.S. CL .................................... 364/550; 128/633;
`364/413.09
`[58] Field of Search ................... 364/413.09, 497, 554,
`364/575, 525, 550; 356/318, 319, 39, 40, 346,
`333; 250/213 VT, 281; 455/611, 612; 128/633,
`634, 637
`
`[56]
`
`References ated
`U.S. PATENT DOCUMENTS
`
`3,638,640 2/1972 Shaw - - - - - - 128/633
`4,138,727 2/1979 M a n t z - - - - - - 356/346
`4,510,938 4/1985 Jobsis .................................. 128/633
`4,576,173 3/1986 Parker et al ........................ 128/633
`4,800,495
`l/1989 Smith ............................. 364/413.09
`4,800,885 1/1989 Johnson .............................. 128/633
`4,807,630 2/1989 Malinouskas ........................ 128/633
`4,819,646 4/1989 Cheung et al ....................... 128/633
`4,819,752 4/1989 Zelin ................................... 128/633
`4,824,242 4/1989 Frick et al ........................... 128/633
`
`4,827,938 5/1989 Parker _ _ _ _ _ ....... 128/634
`4,832,484 5/1989 Aoyagi et al.
`... 128/633
`
`OTHER PUBLICATIONS
`Chance et al., Time-Resolved Spectroscopy of Hemo(cid:173)
`globin and Myoglobin in Resting and Ischemic Muscle,
`Analytical Biochemistry 174, pp. 698-707, (1988).
`Primary Examiner-Kevin J. Teska
`Attorney, Agent, or Firm-Woodcock Washburn Kurtz
`Mackiewicz & Norris
`ABSTRACT
`[57]
`The present invention to provides methods and appara(cid:173)
`tus for studying photon migration using signal modula(cid:173)
`tion techniques such as time, frequency and phase mod(cid:173)
`ulation. The photon migration data may then be con(cid:173)
`verted, using the principles of time-resolved spectros(cid:173)
`copy, to determine the concentration of an absorptive
`constituent in a scattering medium, such as the concen(cid:173)
`tration of hemoglobin in a brain of other tissue. The
`methods and apparatus of the present invention provide
`as a specific embodiment, a dual wavelength phase
`modulation system which allows the clinical application
`of time resolved spectroscopy in a commerically feasi(cid:173)
`ble embodiment.
`
`16 Claims, 3 Drawing Sheets
`
`Kenwood 321
`MH
`104
`
`102
`
`800nm LaserOiodl
`
`PMT
`Oetector
`
`Petitioner Apple Inc. – Ex. 1052, p. 1
`
`

`

`U.S. Patent Nov. 20, 1990
`
`Sheet 1 of3
`
`4,972,331
`
`R 928 Detector
`
`200.05 MHz
`
`II
`
`Laser Diode
`760nm
`4mW
`
`200MHz
`SSB MOO
`)
`(JF Sh
`
`17
`
`50KHz
`
`FIG. IA
`
`Lock-in
`Amplifier
`SR-510
`
`"'\ 26
`phase
`amplitude)
`
`R928 Detector
`14
`and HV supply
`,-----r,:::;t,..._;~',;;;..6 - - - -
`
`Aeousto-Optical
`Modulator
`
`IO
`
`CW laser
`628-SOOnm
`0.5-IOmW
`lSL-f55)
`
`200.05 MHz
`
`18
`
`_ 22
`
`- - - - - - - - - - - - - - ~·
`
`50KHz
`
`FIG. I
`
`19
`
`phase \26
`mplitudel
`
`l Lock-1n
`0·l {~''nPlifier
`24.,~$R-SIO
`
`Petitioner Apple Inc. – Ex. 1052, p. 2
`
`

`

`U.S. Patent Nov. 20, 1990
`
`Sheet 2 of3
`
`4,972,3,31
`
`810nm
`79421
`
`750nm
`--46--, 79403
`200Hz
`Electronic Switch
`
`Homomatsu
`R41645A
`MCP-PMT
`(Cs-0-~9l
`H.V.
`Suppl
`
`0
`
`30
`Kenwood 321
`MH
`,...._--.---t--1220.
`34
`ll44 to 440t.tHzl
`Kenwood 21 32
`220 050
`~M~t1~z-+---"---+--i 220.050 MHz
`220.000
`_ _ ......___50K ___ Hz Referen~
`MHz
`36
`Output
`52 ~...__ ....
`50Hz
`SO KHz
`L..--------~ MiHr - - -~ - -~ Lock-in
`Amplifier
`Log Output
`
`__ ......__ _ _,
`
`54
`
`Fl G.2
`
`200Hz
`Reference
`
`E~~k.!1~
`
`Am lif ier
`56
`
`Phase Lock
`
`204
`Kenwood 321
`- - - - - - -~ FM/SSB
`Phase Shifted
`Ree
`Signal
`
`PMT
`,jcp
`Detector
`
`,220
`
`FIG. 4
`
`Difference} 58
`of Logs
`
`206
`
`3KHz
`Carrier
`
`208
`Phase
`Detector
`and Filter
`
`Absorption
`Output
`
`Petitioner Apple Inc. – Ex. 1052, p. 3
`
`

`

`U.S. Patent Nov. 20, 1990
`
`Sheet 3 of3
`
`4,972,3,31
`
`Time
`
`soon m Laser Diode
`
`14
`Kenwood 321
`..------+---' 220.030 MHz
`
`220.030
`MHz
`
`10
`_ _........_I...--.
`PMT
`20.000
`Detector MHz
`
`30KHz
`
`KHz
`
`II
`PhoseOiff ...,a=•--1
`760/800µ.
`0.05-1 Hz
`
`FIG. 3
`
`Petitioner Apple Inc. – Ex. 1052, p. 4
`
`

`

`1
`
`4,972,331
`
`2
`many situations, the capability to quantify hemoglobin
`concentration for both continuous and pulsed light
`techniques greatly extends their applicability to clinical
`studies. See U.S Pat. Application Ser. No. 266,166, filed
`5 Nov. 2, 1988, "Optical Coupling System for Use in
`Monitoring Oxygenation State Within Living Tissue";
`and U.S. Application Ser. No. 287,847, filed Dec. 21,
`1988, "Methods and Apparatus For Determining the
`Concentration of a Tissue Pigment Of Known Absor(cid:173)
`bance, In Vivo, Using the Decay Characteristics of
`Scattered Electromagnetic Radiation", both of which
`are fully referenced above.
`
`PHASE MODULATED SPECTROPHOTOMETRY
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`This application is related to co-pending applications,
`Ser. No. 266,166, filed Nov. 2, 1988, in the name of
`Britton Chance, entitled, "Optical Coupling System for
`Use in Monitoring Oxygenation State Within Living
`Tissue", which is hereby incorporated by reference as if 10
`fully set forth herein; Ser. No. 266,019, filed Nov. 2,
`1988, in the name of Britton Chance, entitled, "A User(cid:173)
`Wearable Hemoglobinometer For Measuring the Meta(cid:173)
`bolic Condition of a Subject", which is hereby incorpo(cid:173)
`rated by reference as if fully set forth herein; and Ser. 15
`No. 287,847, filed Dec. 21, 1988, in the name of Britton
`Chance, entitled, "Methods and Apparatus For Deter(cid:173)
`mining the Concentration of a Tissue Pigment Of
`Known Absorbance, In Vivo, Using the Decay Charac(cid:173)
`teristics of Scattered Electromagnetic Radiation", 20
`which is hereby incorporated by reference as if fully set
`forth herein.
`
`BACKGROUND OF THE INVENTION
`The application of the basic dual wavelength princi- 25
`ple to detect hemoglobin and myoglobin changes in
`tissue began with the work of G.A. Millikan in his stud-
`ies of the cat soleus muscle, and the work of Millikan
`and Pappenheimer who detected hemoglobin deoxy(cid:173)
`genation in the human ear lobe. Multiwavelength in- 30
`struments have been developed; these instruments use
`either a multiwavelength laser diode light source or a
`time shared filter technique, in which high precision is
`sought through various algorithms which deconvolute
`background signals, oxidized and reduced cytochrome 35
`signals, and oxy- and deoxyhemoglobin signals. Such
`instruments are oxy-complex and often have difficulty
`obtaining light sources with wavelengths appropriate to
`the algorithms that have been developed, or they have
`such low light levels that photon counting is necessary. 40
`They are generally in the price range of $80,000 and
`have produced much experimental data in the literature
`on neonates and adults. The basic problem of such
`methods is that the optical pathlength is not known ab
`initio but is calculated by reference to animal models 45
`where the hemoglobin can be removed and cytochrome
`directly studied. Transferability of such data from the
`animal model to the human is one difficulty that had to
`be overcome prior to the invention of time-resolved
`spectroscopy, where the pathlength is measured di- 50
`rectly. See U.S. Pat. Application Ser. No. 266,166, filed
`Nov. 2, 1988, "Optical Coupling System for Use in
`Monitoring Oxygenation State Within Living Tissue,"
`fully referenced above.
`Continuous wave spectroscopy (CWS) of tissue he- 55
`moglobin has the demonstrated advantages of great
`simplicity and sensitivity, as well as affording an "early
`warning" of tissue hypoxia. The application of picose(cid:173)
`cond-pulse time-resolved spectroscopy (TRS) to tissue
`in order to determine optical pathlengths, quantify the 60
`changes in hemoglobin concentration, and determine
`the actual concentration values of hemoglobin and cy(cid:173)
`tochrome has great applicability to clinical studies of
`tissue hypoxia. Moreover, time-resolved spectroscopy
`used in conjunction with continuous light spectropho- 65
`tometry offers a means of calibrating the optical path(cid:173)
`length which photons travel as they migrate through
`tissue. While trend indication can be of great value in
`
`SUMMARY OF THE INVENTION
`It has now been found that the principles of dual
`wavelength spectrophotometry may be applied to time(cid:173)
`resolved spectrophotometry choosing a carrier fre(cid:173)
`quency at a value in which the time characteristic is
`compatible with the time delay of photon migration
`from input to output through a scattering medium.
`The present invention provides methods and appara(cid:173)
`tus whereby a modulated waveform is transmitted to a
`scattering medium and detected after migration there(cid:173)
`through. The detected waveform will have been altered
`and may thus be compared to the initial waveform. For
`example, a waveform is phase shifted by the delay in
`migration through the scattering medium. Thus, in a
`preferred embodiment, the phase of the waveform is
`modulated and the phase shift is detected. The differ(cid:173)
`ence in phase shift between two waveforms of emitted
`electromagnetic radiation having different, known
`wavelengths can then be processed to determine the
`concentration of an absorptive constituent such as he(cid:173)
`moglobin.
`It is an object of the present invention to provide
`methods and apparatus for studying photon migration
`using signal modulation techniques such as time, fre(cid:173)
`quency and phase modulation. It is a specific object of
`the present invention to provide methods and apparatus
`whereby phase modulated spectrophotometry (PMS)
`may be utilized in conjunction with continuous wave
`spectrometry (CWS) to determine the critical value of
`an absorptive pigment such as hemoglobin, at the point
`the PCr/P1ratio begins to decrease. It is another object
`of the present invention to provide as a specific embodi(cid:173)
`ment, a dual wavelength phase modulation system
`which will allow the clinical application of the advan(cid:173)
`tages of time resolved spectroscopy in an economical
`and commercially feasible embodiment.
`
`BRIEF DESCRIPTION OF THE ORA WING
`FIG. 1A illustrates a simplified single wavelength
`phase modulated spectrophotometer made in accor(cid:173)
`dance with the present invention.
`FIG. 1 is a block diagram of another embodiment of
`a single wavelength phase modulated spectrophotome(cid:173)
`ter made in accordance with the present invention.
`FIG. 2 is a block diagram of another embodiment of
`a phase modulated spectrophotometer made in accor(cid:173)
`dance with the present invention.
`FIG. 3 is a block diagram of the preferred embodi(cid:173)
`ment of the spectrophotometer of the present invention.
`FIG. 4 is a block diagram of an alternate embodiment
`of the spectrophotometer of the present invention.
`
`Petitioner Apple Inc. – Ex. 1052, p. 5
`
`

`

`4,972,331
`
`3
`DETAILED DESCRIPTION
`The time, frequency, or phase of a signal may be
`modulated. Phase modulation appears to be a conve(cid:173)
`nient implementation of the time-released spectroscopy 5
`(TRS) technique discussed above. In FIG. lA, a single
`wavelength spectrophotometer using the principle of
`phase modulation is shown. In this embodiment, a fre(cid:173)
`quency generator 17, operating at 200 MHz, excites a
`4mW laser diode 11, which emits light at a wavelength 10
`of 760 nm. The light is conducted to the subject 20 via
`optic fiber 15. After the light has migrated through the
`tissue, it is detected. Preferably this detector is com(cid:173)
`prised of a photomultiplier tube and its associated volt(cid:173)
`age supply 16; one such device is the Hamamatsu R928. 15
`The frequency generator 17 also receives an input
`from a 50 kHz oscillator 19, transmitting a 200.05 MHz
`reference waveform, which is input into the detector
`16. Accordingly, the output waveform 22 from the
`detector 16 is at a carrier frequency equal to the differ- 20
`ence, i.e., 50 kHz. The waveform 22 from the detector
`16 and a reference waveform from the oscillator 19 are
`fed into a phase and amplitude detector 24. In this em(cid:173)
`bodiment, the phase and amplitude detector 24 is a
`lock-in amplifier. The output of the lock in amplifier are 25
`signals representative of the phase shift and amplitude
`of the detected signal. These signals are then processed
`and related to the relative concentration of an absorbing
`constituent, such as hemoglobin.
`In the embodiment of FIG. 1, a helium-neon laser 30
`light source 10 is connected to a wide band acousto(cid:173)
`optical modulator 12 operating at 200 MHz. The acous(cid:173)
`to-optical modulator 12 frequency modulates the light
`emitted by the laser 10. The light is conducted via a
`fiber optic light guide 14 to the forehead of the subject 35
`20 as shown, or other region to be studied. Signals about
`3-6 cm from the location of the input waveform are
`received by a detector 16, for example a Hamamatsu
`R928. The dynodes are modulated by a 200.050 MHz
`signal 18 so that a 50 kHz hetrodyne signal 22 will be 40
`obtained and can be fed into a lock-in amplifier 24, such
`as a PAR SR510. As above the reference frequency for
`the lock-in amplifier is obtained from the 50 Hz differ(cid:173)
`ence between the two frequencies. The phase shift be(cid:173)
`tween the transmitted and detected waveforms is mea- 45
`sured with high precision and the output waveforms,
`shown at 26, are plotted as an analog signal on a strip
`chart recorder to allow the user to follow the variations
`in the propagation of light through the brain or other
`tissue. A logarithmic conversion of the signal is then 50
`obtained. The result is linearly related to the change in
`concentration of an absorptive pigment, such as hemo(cid:173)
`globin.
`Referring to FIG. 2, there is shown a block diagram
`of a simplified embodiment of a dual wavelength phase 55
`modulation spectrophotometer made in accordance
`with the present invention. Unlike the single wave(cid:173)
`length system of FIG. lA, this embodiment allows the
`determination of the concentration of an absorptive
`constituent on an absolute basis. The embodiment of 60
`FIG. 2 is similar to that depicted in FIG. lA, except
`that light is transmitted to the subject at two discrete
`wavelengths.
`FIG. 2 illustrates a dual wavelength embodiment of
`the apparatus of the present invention. In this embodi- 65
`ment, the laser diode light is amplitude modulated and
`the phase shift caused by photon migration is measured
`by an optical detector, a mixer, and a phase detector.
`
`4
`The dual frequency time sharing system is comprised of
`stable oscillators 30,32, such as Kenwood Model #321
`for 220 Hz; the oscillator system preferably used can
`generate waveforms from 144 to 440 MHz (Kenwood
`TM721A). Continuous variation of the frequency is
`possible, although, as will be understood by one of
`ordinary skill, the three frequencies mentioned, 144, 220
`and 440 MHz, are adequate for the purposes of initial
`studies and other applications. The oscillators 30,32 are
`set 50 Hz apart and the difference frequency is detected
`by a mixer 34 to obtain a reference phase signal 36, as
`shown. A 200 Hz electronic switch 38 alternately ex(cid:173)
`cites laser diodes 40,42, nominally operating at between
`about 750-60 nm and 800-10 nm, to emit 220 MHz
`modulated light which is conducted by fiber optic
`guides 44,46, preferably about 3 mm in diameter, to the
`surface of the head of the subject 20, or other region to
`be examined.
`In order to achieve satisfactory operation at 220
`MHz, it has been found that the most cost effective
`detector 48 for this purpose is Hamamatsu R928. A
`more advantageous device, however, is the Hamamatsu
`R1645u, which is a microchannel plate tube having 120
`picosecond transit time spread, and a high gain; that is,
`a two-stage microchannel plate photomultiplier 48. This
`tube, which is capable of current amplification of
`5X 105 (57 dB) is similar to those used for pulsed time
`measurements in time resolved spectroscopy (TRS)
`studies, and is considered to be ideal for these purposes.
`See, Chance et al., ''Time-Resolved Spectroscopy of
`Hemoglobin and Myoglobin in Resting and Ischemic
`Muscle," Analytical Biochemistry 174, 698-707 (1988).
`tube 48 is connected to a high voltage supply 50 which
`has an output of about 3400 volts, in order to ensure
`high gain. The photomultiplier tube 48 can be con(cid:173)
`nected to the brain or other tissue area by the fiber optic
`guides 44,46 or may be directly connected and placed in
`a housing isolated from ground potential, as illustrated.
`As above, the detector 48 is attached to the subject 20
`and is connected to a mixer 52, which down converts
`the 220 MHz output of the detector 48 to a 50 kHz
`signal by mixing with a 220.050 MHz signal from the
`oscillator 32. A lock-in amplifier 54 determines the
`phase of the exiting waveform. The lock-in amplifier 54
`also obtains the logarithm of the signal. This signal is
`then fed to a second phase detector/lock-in amplifier 56
`which determines the difference between the signals at
`each of the two wavelengths, this signal 58 is directly
`proportional to the concentration of an absorptive pig(cid:173)
`ment, such as hemoglobin. This embodiment may be
`used on neonate, as well as adult brains.
`A preferred embodiment of a time-shared, dual wave(cid:173)
`length laser diode phase modulation spectrophotometer
`is illustrated in FIG. 3. In this embodiment, a pair of
`laser diodes 100,102 are excited in parallel by a stable
`frequency generator 104 (Kenwood 321) at 220 MHz.
`Each of the diodes 102,104 generates electromagnetic
`radiation of a different wavelength, preferably 760 nm
`and 800 nm. The electromagnetic radiation is time
`shared by a vibrating mirror 105, which illuminates a
`single fiber optics probe at a modulating frequency,
`preferably about 60 Hz. The synchronization of the
`motion of the mirror 105 and the 60 Hz phase detector
`120 (explained below) is accomplished using an electri(cid:173)
`cal coupling of the reference voltage in the 60 Hz lock(cid:173)
`in amplifier 120. Thus, electromagnetic radiation at
`each wavelength is synchronized between emission and
`detection.
`
`Petitioner Apple Inc. – Ex. 1052, p. 6
`
`

`

`4,972,331
`
`5
`One of ordinary skill will note that the spectropho(cid:173)
`tometer of FIG. 3 differs from the embodiment depicted
`in FIG. 2 in that the latter embodiment uses a carrier
`modulation system to code the excitation power of one
`laser from another, while the embodiment of FIG. 3 5
`continuously switches between the output light from
`two lasers excited at the same frequency.
`The time shared 760/800 nm light is applied to the
`subject 20 via an optic fiber 106. Several centimeters
`away, an output probe 108, preferably comprising a 10
`second fiber of relatively large area will pick up the
`light which has migrated through the subject and illu(cid:173)
`minates a photo detector 110, which is a suitable photo(cid:173)
`multiplier tube (Hamamatsu 928) or a microchannel
`plate detector (Hamamatsu R1645u). The light col- 15
`lected is phase shifted from input oscillations by the
`time delay in photon migration between input and out(cid:173)
`put.
`A second oscillator 114 which generates a 220.030
`MHz waveform is connected to a mixer 112. The 20
`220.000 MHz output of the detector 110 is also con(cid:173)
`nected to the mixer. As a result, the phase modulation
`frequency is downshifted to 30 kHz, which is a conve(cid:173)
`nient frequency for lock-in detection. This signal is
`input to a phase detector 116, which is preferably a 25
`lock-in amplifier. A second input to the phase detector
`116 is obtained by connecting an output from the
`220.000 MHz oscillator 104 and the 220.030 MHz oscil(cid:173)
`lator 114 to a mixer 118 to obtain an unshifted 30 kHz
`signal which is used as a phase reference. Thus, the 30
`lock-in amplifier 116 operates with a reference phase
`obtained directly from
`the frequency generators
`104,114 and a phase modulated input obtained by pho(cid:173)
`ton migration through the subject 20.
`The phase of the signal output will vary between the 35
`phase due to light propagation at 800 nm and the phase
`due to light propagation at 760 nm. The output of the
`lock-in amplifier 116 is thus a 60 Hz waveform, the
`amplitude of which bears the phase information at the
`two wavelengths. The output of the phase difference 40
`detector 116 then is connected to the same waveform as
`that which drives the 60 Hz vibrating mirror 105. The
`output of the phase detector may be obtained by using
`switch contacts on the vibrating reed modulation which
`alternatively connects opposite phases of the 60 Hz 45
`waveform to the integrating network, each one at the
`peak of the waveform of the output phase detector. The
`output is put into a differential amplifier to record the
`difference of the amplitude of the two parts of the 60 Hz
`waveform, corresponding to the 760 nm and 800 nm 50
`phase shift. This phase difference output is suitably
`filtered from 0.05-1 Hz and provides a running time
`record of the changes in hemoglobin concentration by
`dual wavelength time-resolved spectroscopy.
`The advantage of the system illustrated by FIG. 3 is 55
`that it affords a single light guide input to the subject
`operated from two laser diodes which are continuously
`operated at the same oscillator frequency. Thus, spuri(cid:173)
`ous phase differences in frequencies associated with
`excitation are minimized. That is, no differential phase 60
`shift is expected between the 760 nm and 800 nm signals.
`Thus, the 30 kHz difference signal would represent the
`true phase delay between these two wavelengths.
`Moreover, phase noise in this region would be mini(cid:173)
`mized by the differential detector 116. The photomulti- 65
`plier tube detector 110 can be of any adequately fast
`type, since the mixing function is separated from the
`detector. The lock-in amplifier technique obtained to
`
`6
`derive the difference of the phase and amplitude of the
`two signals has the highest signal to noise ratio possible
`for this type of equipment.
`The principles of time-shared dual wavelength spec(cid:173)
`trophotometry, together with lock-in technology, fol(cid:173)
`lows the principles employed in dual wavelength spec(cid:173)
`trophotometry generally. However, the present inven(cid:173)
`tion provides a vastly improved device, since the carrier
`frequency of 220.000 MHz is sufficiently fast to measure
`photon migration times between input and output with
`a characteristic time of about 5 nanoseconds to be ob(cid:173)
`served. Therefore, the sensitivity of the system dis(cid:173)
`closed is high, approximately 70° per nanosecond or 3°
`per centimeter change of pathlength, as observed in
`experimental models.
`The application of the principles of dual wavelength
`spectrophotometry to time-resolved spectrophotome(cid:173)
`try involves the choice of a carrier frequency at a value
`in which the time characteristic is compatible with the
`time delay of photon migration from input to output.
`The device disclosed achieves the result of precisely
`measuring the absorbance changes in photon migration,
`over a specified distance, e.g., over approximately one
`meter, as contrasted, to the continuous light method in
`which photon migration is measured over all possible
`path lengths. A path length of approximately one meter
`is preferably selected in order to ensure exploration of
`all parts of the brain for brain bleeding studies. Obvi(cid:173)
`ously, higher frequencies would select smaller portions
`of the brain which are more localized to the input-out(cid:173)
`put configuration.
`For a multiple-scattering medium such as human
`tissue, the only known method for determining the path
`length of transmitted photons is the measurement of the
`time of flight and of the refractive index, from which
`the distance travelled may be calculated. Since this path
`length in the brain is on the order of centimeters, the
`transit time is on the order of nanoseconds or less. A
`direct measurement of such periods in this time domain
`has several fundamental drawbacks. As the required
`time resolution becomes finer, the detection bandwidth
`must increase; signal power at best remains constant,
`while noise power increases proportionally with the
`increasing bandwidth. For sources such as laser diodes,
`where average output power for both pulsed and con(cid:173)
`tinuous operation are nearly the same, signal power
`typically declines when the pulse width is reduced.
`Since the time between probe pulses must be long
`enough for the returning light to decay to approxi(cid:173)
`mately zero, the duty cycle of the pulse train is typically
`low; this implies low average signal power or the use of
`high peak power, which may endanger the skin cover(cid:173)
`ing the tissue being studied. Finally, both the expense
`and difficulty of constructing suitable electronic circuits
`is considerably greater for pulsed than for continuous(cid:173)
`wave systems. As an alternative to time-domain mea(cid:173)
`surement, a CW system may be employed with phase
`measurement taking the place of time intensity, a simple
`calculation based on measurement of the phase shift
`between probe and return light at a single frequency
`yields the characteristic decay time. Such a system has
`the advantages of narrowband modulation and detec(cid:173)
`tion and high average power in the probe signal, yield(cid:173)
`ing a considerable advantage in signal-to-noise ratio and
`therefore in data acquisition time. There is a consider(cid:173)
`able body of literature on this technique of time mea(cid:173)
`surement, particularly as applied to radar, time stan(cid:173)
`dards, and spectroscopy. Perhaps the most relevant to
`
`Petitioner Apple Inc. – Ex. 1052, p. 7
`
`

`

`4,972,331
`
`7
`this application is the literature on the phase-resolved
`measurement of fluorescent decay kinetics.
`Another alternate embodiment of the apparatus of the
`present invention is depicted in FIG. 4. This system
`relies more upon communications technology rather 5
`than NMR technology, and is essentially a single side(cid:173)
`band system where the sidebands are displaced in pro(cid:173)
`portion to the modulation frequency shift required. This
`design places more reliance upon the existing radio
`frequency transmitter/receivers which, at prices of 10
`about $300 per frequency for transmit/receive, is a
`significant advantage.
`A block diagram of a system as described directly
`above is shown in FIG. 4. In this embodiment, a first
`standard communications transmitter-receiver (trans- 15
`ceiver) 200, operating at 220 MHz, is used in the trans(cid:173)
`mit mode to generate a waveform which excites a laser
`diode 202. The transceiver 200 is used in the single side
`band (SSB) mode to provide SSB modulation at 3 kHz.
`This carrier signal is fed back to the transceiver 200 and 20
`into a phase detector/filter 208, which also receives an
`input from a second transceiver 204. As in the previous
`embodiments, the laser diode 202 emits light which is
`conducted to the subject 20 via optic fibers 216.
`The SSB modulated signal is phase shifted by the 25
`delay in migration through the brain. The light is scat(cid:173)
`tered and absorbed as it migrates through the subject 20
`and is received by an optical coupler/fiber assembly
`218. The received light is then transmitted to a detector
`220, either of the photomultiplier tube or the micro- 30
`channel plate type, both of which are discussed above in
`reference to other embodiments.
`The output of the detector 220 is coupled to the RF
`input to the second transceiver 204, i.e., the transceiver
`is used in the receive SSB mode and a phase shifted 3 35
`kHz tone is obtained and connected to the phase detec(cid:173)
`tor filter 208. The output is a 3 kHz phase shifted signal
`which, is input to the second SSB transceiver 204. In
`order to ensure phase coherence, the first transceiver
`200 and the second transceiver 204 form a phase locked 40
`loop. The 3 kHz carrier waveform is also locked to 220
`MHz by frequency dividers 206, thereby locking the
`220 MHz and the 3 kHz phases and allowing the phase
`shift to be determined with high precision. As seen in
`FIG. 4, an output of the transmitter oscillator 200 is 45
`frequency divided by about 7 X 105, to yield a 3 kHz
`signal. The output of the phase detector/filter 208 is
`thus related to the phase shift and, accordingly, is repre(cid:173)
`sentative of the absorption within the subject.
`The carrier frequency is initially chosen to be 220 50
`MHz; this is sufficiently high to give a detectable phase
`shift for decay times of a few nanoseconds, but low
`enough to be within the bandwidth of a number of
`commercially-available active mixers. Although diode(cid:173)
`ring mixers are readily available up to 36 GHz, they 55
`have significantly less dynamic range than active (tran(cid:173)
`sistor bridge or linear multiplier) designs; a large dy(cid:173)
`namic range is crucial for this type of spectrophotome(cid:173)
`ter system. A heterodyne system is chosen to allow
`multiple optical wavelengths to be transmitted and de- 60
`tected in parallel on individual subcarrier frequencies,
`and to allow phase detection to be carried out within
`the frequency range of commercial phase sensitive de(cid:173)
`tectors, i.e., "lock-in amplifiers". These devices have a
`superb noise figure, linearity, dynamic range, and phase 65
`and amplitude accuracy; their performance is very
`much superior to any phase detector operating directly
`at the RF carrier frequency. Generation of the refer-
`
`8
`ence signals for the lock-in amplifiers by frequency
`division from the master RF oscillator provides ade(cid:173)
`quate phase coherence of all subcarriers and all demod(cid:173)
`ulated signals with respect to the carrier; no phase cali(cid:173)
`bration between wavelengths is required. Frequency
`generation by division also provides minimum possible
`phase noise for a given master oscillator. Should addi(cid:173)
`tional carrier frequencies be required, such as for the
`measurement of multi-exponential decays, the only
`changes required in this design would be the addition of
`a one-by-N RF switch and additional RF oscillators.
`Laser diodes are chosen over thermal sources, for
`their much higher radiance, ease of coupling to optical
`fiber, narrow output spectrum and wavelength stability,
`long life, and ease of modulation at RF frequencies. In
`order to maximize the signal-to-noise ratio of the sys(cid:173)
`tem, and to avoid problems of intermodulation distor(cid:173)
`tion due to laser nonlinearities, single-sideband sup(cid:173)
`pressed-carrier modulation is used. The intermediate
`frequencies are chosen within the range of 10 to 100
`KHz; they must be high enough to allow a realizable Q
`of the single side band filters, but low enough to be
`within the range oflow-cost commercial lock-in ampli(cid:173)
`fiers.
`The heat sinks of the lasers are preferably tempera(cid:173)
`ture-controlled using Peltier coolers and feedback con(cid:173)
`trol. Temperature control is necessary in order to stabi(cid:173)
`lize the wavelength of the lasers, and to allow sufficient
`tuning of the output wavelength to cover the tolerance
`(approx. ±10 nm) of commercial diodes. However, it
`should be noted that post-demodulation detection of
`phase shift substantially eliminates this consideration,
`since neither constant wavelength nor amplitude are
`required.
`The optical system consists of one optical isolator per
`laser, lens assemblies for coupling the laser light into the
`optical fibers, a fiber bundle(s) for transmitting the light
`to and from the subject, a fiber-subject coupler on the
`distal end of the bundle(s), and a light detector assem(cid:173)
`bly.
`The isolators are necessary in order to prevent optical
`feedback into the laser cavity due to reflections from
`the optics or subject; such feedback, even at levels as
`low as - 60 dB, are well known to cause both amplitude
`and phase noise in laser sources.
`The fibers chosen for fabrication of the bundles must
`have sufficiently small dispersion that the phase uncer(cid:173)
`tainty introduced at the nominal 100 MHz modulation
`frequency is much smaller than the phase shifts of inter(cid:173)
`est. At the same time, the maximum possible core diam(cid:173)
`eter and numerical aperture are desired. This simplifies
`and makes more robust the laser-fiber coupling, and
`greatly increases the return light signal col