Multi-Wavelength, Depth Resolved, Scattering and
Pathlength Corrected in-vivo Near-Infrared Spectroscopy of
Brain Tissue.
Ilias Tachtsidis1, Terence S. Leung1, Arnab Ghosh2, Martin Smith1, 2, Chris E. Cooper3, Clare E. Elwell1
1
Department of Medical Physics & Bioengineering, Malet Place Eng. Bldg., University College London, Gower St., London WC1E 6BT, UK
Email: iliastac@medphys.ucl.ac.uk
2
Neurocritical Care Unit, National Hospital for Neurology and Neurosurgery, University College London Hospitals, Queen Square, London, UK
3
Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester Essex, CO4 3SQ, UK
Abstract: For resolving concentrations of tissue chromophores in the human adult brain with
near-infrared spectroscopy it is advantageous to calculate the light scattering and absorption, at
multiple wavelengths with some depth resolution. We report a novel methodology that combines
multi-distance frequency and broadband spectrometers and we show preliminary results in a
healthy young adult during hypo- and hypercapnia.
2010 Optical Society of America
OCIS codes: (170.3890) Medical optics instrumentation; (170.6510) Spectroscopy, tissue diagnostics
1. Introduction
In non-invasive in-vivo near-infrared spectroscopy (NIRS) of brain tissue, the quantification of haemoglobin, water
and cytochrome-c-oxidase concentrations may be improved with high depth sensitivity reflectance measurements,
multi-spectral data and the ability to separate the effects of absorption from those of scattering.
We have previously described a Hybrid Optical Spectrometer (or pHOS) and methodology [1], which (i) uses a
multi-distance (3.0, 3.5cm) frequency domain (MDFD) system to measure the absorption coefficient (µa) and the
reduced scattering coefficient (µs’) of tissue at a selection of discrete wavelengths (690, 750, 790, 850nm); (ii)
assumes a power law wavelength dependence of scattering to interpolate and extrapolate µs’ over all wavelengths in
the spectral window of interest; (iii) uses a multi-distance broadband spectrometer (MDBBS) to measure the
attenuation slope over a wide spectral range (504-1068nm) and at a number of source detector spacings (2.0, 2.5,
3.0, 3.5cm); (iv) corrects the attenuation slope using the previously calculated µs’ and the spatially resolved
spectroscopy (SRS) algorithm and hence determines a depth resolved absorption (µaSRS); (v) scales the calculated
µaSRS with the MDFD measured µa to obtain an absolute multi-wavelength measurement of absorption (µaHybrid).
In recent years particular effort has been devoted to the development of instruments that provide multiwavelength µa and µs’ for measuring the optical properties of breast tissue either using single distance combined
frequency and broadband domain spectrometers [2], or time resolved systems utilising a supercontinuum laser
source [3]. In this paper we report on the use of our novel multi-distance frequency and broadband domain system to
measure brain oxygenation, haemodynamics and metabolism during changes in cerebral oxygenation secondary to
carbon-dioxide induced changes in cerebral blood flow (CBF) in a healthy adult.
2. Experimental Methods
The pHOS comprises the MDFD spectrometer, which is a modification of the commercially available OxiplexTSTM
from ISS Inc (Champaign, IL, USA); and the MDBBS, which is based on a lenses spectrograph and a front
illuminated CCD camera (PIXIS:512f, Princeton Instruments). Figure 1(a) shows a picture of the experimental setup along with the pHOS with the optodes placed securely over the left frontal hemisphere.
Synchronously with the optical measurements, non-invasive, multimodal physiological data was collected
continuously. In particular left middle cerebral artery blood flow velocity (Vmca) was measured using transcranial
Doppler ultrasonography (ScanMed) and the changes in end tidal CO2 (EtCO2) were monitored from the expired
gases (COSMO, Novametrix). Here we present results from one healthy young adult (male, 22 years old) who after
5min of baseline breathing hyperventilated to reduce EtCO2 for 5min (Hypocapnia). Following a second 5-minute
period of baseline breathing, CO2 was added to the inspired gases to increase EtCO2 (Hypercapnia). Figure 1(b)
shows the EtCO2 signal and the Vmca response for the entire protocol.
(a)
(b)
EtCO2 (KPa)
12
Hypercapnia
Hypocapnia
10
8
6
4
2
Vmca (cm/sec)
0
0
200
80
400
600
800
1000
1200
1400
Hypercapnia
Hypocapnia
70
60
50
40
30
20
10
0
0
200
400
600
800
1000
1200
1400
Time (seconds)
Fig. 1. (a) Experimental set up showing the pHOS. (b) Physiological data showing the CO2 changes and the Vmca.
The MDFD system measures µa and µs’ for four wavelengths, from these measurements we calculate the
differential pathlength factor (DPF) using Eq. 1 [4]. The DPF allow us to calculate the pathlength at 3.5cm optode
distance; the 790nm pathlength is then used in the Modified Beer-Lambert (MBL) law to calculate the changes in
concentrations of oxy-, deoxy- haemoglobin (∆[HbO2], ∆[HHb]) and oxidised minus reduce cytochrome-c-oxidase
(∆[oxCCO]) using the changes in the light attenuation from the furthest detector (3.5cm) of the MDBBS system.
The MDBBS used SRS methodology to measure the slope of light attenuation versus distance (dA/dρ) across the
spectral range 690-860nm (Fig. 2(b). Eq. 2, derived from the diffusion equation, describes the relationship between
dA/dρ, µa and µs’ upon which SRS methodology is based [1]. The MDFD provided µs’ values for the four
wavelengths, and assuming a power law relationship for µs’ (µs’=bλ-a, where a and b are free parameters in the fit)
µs’ was derived for the full spectral range (Fig. 2(a)). µaSRS was then calculated for all wavelengths by substituting
µs’ in Eq. 3 (Fig. 2(c)). In addition the absolute µaHybrid was obtained by using the MDFD derived µa for the four
wavelengths to scale the calculated MDBBS µaSRS by minimising the mean squared difference (Fig. 2(d)).
2 µa
(2)
∂A
1 
2
=
⋅  3 ⋅ µ a ⋅ µ s , + 
∂ρ ln 10 
ρ
µa =
(3)
(a)
11.0
0.93
10.5
0.92
ρ (OD/cm)
∂A/ ∂ρ
DPF =
3µs'
µ s’ (cm-1)
(1)
10.0
9.5
9.0
8.5
8.0
7.5
(b)
0.91
0.90
0.89
0.88
0.87
0.86
0.85
0.84
0.83
680
7.0
680
700
720
740
760
780
800
820
840
860
700
720
Wavelengths (nm)
740
760
780
800
820
840
860
Wavelengths (nm)
(c)
0.080
(d)
0.13
0.075
0.12
µ aHybrid (cm-1)
µ aSRS (cm-1)

∂ A( λ ) 2 
⋅  ln 10 ⋅
− 
, 
∂ρ
ρ
3⋅ µs 
1
0.070
0.065
0.060
0.055
0.050
680
700
720
740
760
780
800
Wavelengths (nm)
820
840
860
0.11
0.10
0.09
0.08
0.07
680
700
720
740
760
780
800
820
840
860
Wavelengths (nm)
Fig. 2. Measurements on the head during baseline, hypocapnia and hypercapnia; (a) power law fit of the MDFD µs’; (b) the attenuation slope
as measured by MDBBS; (c) the calculate MDBBS µaSRS using the derived µs’ from MDFD; (d) the absolute µaHybrid after scaling with MDFD µa.
3. Results
From the changes in light attenuation (MDBBS) and using the continuous measured pathlength for 790nm
(MDFD) the changes in concentrations of [HbO2], [HHb] and [oxCCO] were calculated by fitting from 740 to
900nm and correcting for the wavelength dependency of DPF (see Fig. 3). The absolute µaHybrid (fitting from 700 to
2
860nm) was used to calculate the brain tissue oxygenation (StO2=[HbO2]/ [HbO2]+ [HHb]) assuming a constant
water concentration and changes in the concentration of [HbO2], [HHb] and [HbT]=[HbO2]+ [HHb] (Fig. 4).
Pathlength (cm)
Hypocapnia
Hypercapnia
32
30
28
26
24
690nm
790nm
750nm
850nm
22
∆ [Concentrations] (µ
µ M)
(a)
34
(b)
1.5
Hypocapnia
Hypercapnia
1.0
0.5
0.0
-0.5
-1.0
[HbO2]
[oxCCO]
[HHb]
-1.5
0
200
400
600
800
1000
1200
1400
0
200
400
Time (seconds)
600
800
1000
1200
1400
Time (seconds)
Fig. 3. (a) Pathlength measurements using the MDFD µa and µs’. (b) Changes in brain tissue concentrations of [HbO2], [HHb] and [oxCCO].
(a)
Hypocapnia
[HbO2]
8
[HHb]
(b)
65
Hypercapnia
Hypocapnia
Hypercapnia
63
[HbT]
61
6
StO2 (%
%)
∆ [Concentrations] (µ
µ M)
10
4
2
59
57
55
53
51
0
49
-2
47
StO2
45
-4
0
200
400
600
800
1000
Time (seconds)
1200
1400
0
200
400
600
800
1000
1200
1400
Time (seconds)
Hybrid
Fig. 4. (a) Changes in concentrations and (b) absolute brain tissue oxygenation calculated using the µa
.
4. Conclusion
The results in Fig. 3(b) and 4(a) demonstrate the use of our pHOS system to investigate oxygenation, haemodynamic
and metabolic signals in a healthy adult head. The combination of multi-distance frequency domain and broadband
spectrometers allows one to measure pathlength continuously and hence correctly scale the changes in
concentrations (Fig. 3) and to obtain multi-wavelength, depth resolved measures of absorption (Fig. 4). This
approach is especially important for NIRS measurement of the adult brain where broad wavelength coverage and
increased sensitivity to deeper layers are required. These preliminary results show that during hypercapnia the
changes in haemoglobin concentrations calculated with the conventional MBL method (Fig. 3(b)) are smaller than
those calculated using the hybrid SRS method (Fig. 4(a)), possibly indicating different contributions from the intra
and extra cerebral layers. This technology is currently being used for studies in human adult volunteers and in
critically ill brain-injured patients. Further work on algorithm development to fully incorporate all of the measured
signals is also underway.
5. References
[1] I. Tachtsidis, M. Kohl-Bareis, T.S. Leung, M. Gramer, B. Tahir, C.E. Cooper and C.E. Elwell, "A hybrid multi-distance phase and broadband
spatially resolved algorithm for resolving absolute concentrations of chromophores in the near-infrared light spectrum: application on to dynamic
phantoms," Biomedical Topical Meetings (The Optical Society of America), BSuE76, Florida, USA (2008).
[2] F. Bevilacqua, A.J. Berger, A.E. Cerussi, D. Jakubowski, and B.J. Tromberg, "Broadband absorption spectroscopy in turbid media by
combined frequency-domain and steady-state methods," Applied Optics 39, 6498-6507 (2000).
[3] A. Bassi, A. Farina, C. D’Andrea, A. Pifferi, G. Valentini, and R. Cubeddu, "Portable, large-bandwidth time-resolved system for diffuse
optical spectroscopy,” Optics Express 15(22) (2007).
[4] S. Fantini , D. Hueber, M.A. Franceschini, E. Gratton, W. Rosenfeld, P.G. Stubblefield, D. Maulik and M.R Stankovic, "Non-invasive optical
monitoring of the newborn piglet brain using continuous-wave and frequency-domain spectroscopy, " Phys. Med. Biol. 44,1543-1563 (1999).
Acknowledgments The authors would like to thank the EPSRC (EP/D060982/1) for the financial support of this work. This work was
undertaken at University College London Hospitals and partially funded by the Department of Health's National Institute for Health Research.