Experimental results on time-resolved reflectance diffuse optical
tomography with fast-gated SPADs
Agathe Puszka*a, Laura Di Sienob, Alberto Dalla Morab, Antonio Pifferib, Davide Continib, Gianluca
Bosoc, Alberto Tosic, Anne Planat-Chrétiena, Lionel Hervéa, Anne Koeniga, Jean-Marc Dintena
a
CEA-LETI, Minatec Campus, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France; bPolitecnico di
Milano, Dipartimento di Fisica, Piazza Leonardo da Vinci 32, Milano I-20133, Italy; cPolitecnico di
Milano, Dipartimento di Elettronica, Informazione e Bioingegneria, Piazza Leonardo da Vinci 32 –
I-20133 Milano, Italy.
ABSTRACT
We present experimental results of time-resolved reflectance diffuse optical tomography performed with fast-gated
single-photon avalanche diodes (SPADs) and show an increased imaged depth range for a given acquisition time
compared to the non gated mode.
Keywords: Time-resolved imaging; Tomography; Image reconstruction techniques; Turbid media; Photon counting;
Photodiodes.
1. INTRODUCTION
In the context of analyzing biological tissue with near-infrared light, the possibility to realize optical probes with small
interfiber distances to study deep layers with reflectance measurements is currently investigated. This specific geometry
is relevant to probe organs whose access is anatomically restricted like the prostate1 for which a single fiber probe is
useful2. But for organs like the breast3, the brain or the muscle4 it can also be preferred to other optodes configurations
for other reasons: small probes are practical to handle for the medical practitioner and easy to position on the patient
compared to rings of fibers for which a wrong placement causes errors on the obtained results.
In the case of reflectance measurements at small interfiber distances, time-resolved techniques enable to reach high
sensitivity to deep layers of the probed tissue by selecting late photons5. We have proposed an image reconstruction
method for Diffuse Optical Tomography (DOT) based on the Mellin-Laplace transform (MLT) of Time-Point Spread
Functions (TPSFs) which allows including the information of late photons in the reconstruction process6. We have
shown that this method can be adapted to the dynamic range of the TPSF, and enables to detect and localize deeper
absorbing inclusions in a diffusing medium for a higher dynamic range7.
Recent developments with fast-gated single-photon avalanche diodes (SPADs) associated to a time-correlated singlephoton counting (TCSPC) setup have shown that it is possible to acquire TPSFs with a high dynamic range in limited
acquisition time compared to the classical non gated mode8, in principle without any limit in the achievable dynamic
range, in practice with the limitation due to the memory effect of the gated detector9. This technique has already been
applied to turbid media for improving the contrast to deep absorbing inclusions in reflectance 10,11. We show here that it
can be used to reconstruct images of the absorption coefficient in turbid media with DOT algorithms. In particular we
demonstrate that the reconstructed depth range and depth localization accuracy of a single absorbing inclusion in a turbid
medium are improved by using the fast-gated mode, for an interfiber distance of 15 mm.
* agathe.puszka@cea.fr, Phone: +33 (0)4 38 78 14 46, Fax: + 33 (0)4 38 78 59 30.
2. EXPERIMENTS
2.1 Setup
The setup is depicted in Figure 1. The source is a 4-wave mixing laser (Fianium, UK) providing picosecond pulses at
820 nm at 40 MHz. The laser beam is attenuated by a self-made variable optical attenuator (VOA) before being injected
in the excitation fiber.
Retro-diffused light is collected by an optical fiber coupled to a 100 µm diameter SPAD. The used SPAD detector and
corresponding electronics were developed at the Dipartimento di Elettronica, Informazione e Bioingegneria of
Politecnico di Milano12. The SPAD module is connected to a TCSPC board (SPC-130, Becker & Hickl GmbH).
The tested probe features two couples of source and detector with the same interfiber distance of 15 mm (couples S1D2
and S2D1) aligned along the x axis (Figure 1). The experiment is performed in a liquid phantom with a movable solid
diffusing and absorbing inclusion. The background is composed of Intralipid and black ink (µ a = 0.1 cm-1 and µs' = 10
cm-1 at 820 nm). The optical properties of the inclusion (cylinder of 8 mm diameter and 6 cm long made of resin, TiO 2
particles and black ink) are µ a = 0.6 cm-1 and µs' = 10 cm-1. This geometry consisting of a one-dimension probe and a
cylindrical inclusion was chosen to mimic a two-dimension configuration, as the employed reconstruction algorithm is
currently developed in 2D. The inclusion is placed in the liquid, perpendicular to the (x, z) plane and at the center of the
2 couples of source and detectors. During the experiment, it is only moved along the z axis at different depths values.
The acquisition is successively done for each couple of source and detector.
TCSPC
SPAD
sync.
Delayer
Laser
λ = 820 nm
40 MHz
Optical
fiber
delay
Optical
fiber
S1 D1 D2 S2
y x
z
depth
VOA
15 mm
15 mm
Figure 1. Experimental setup and schematic view of the probe and phantom (2D cross section in plane (x, z)).
2.2 Protocol
During gated acquisitions, the VOA is adapted for each temporal position of the gate in order to reach the allowed
maximum count rate of 106 photons per second and therefore optimize the signal to noise ratio. We choose the temporal
position of the different gates in order to obtain a smooth reconstitution of the full TPSF. 9 gates are used to acquire each
TPSF (Figure 2) with a maximum power increase of around 2 decades between the first and the last gate. 1 second is
acquired for each gate so the acquisition is 9 seconds in total for each couple of source and detector. For comparison
purpose, we reproduce the same protocol with the non gated mode: the gate is positioned in order to acquire the full
TPSF at once and we use an acquisition of 9 seconds per TPSF in order to compare the two modes for the same "photon
counting time".
3. DATA ANALYSIS AND RECONSTRUCTION METHOD
Each TPSF is built by pre-processing the 9 gated acquisitions (Figure 2). These TPSFs are then used as input for an
iterative algorithm reconstructing µa nonlinearly in the 2D plane (x, z) orthogonal to the surface of the medium and
including the line of excitation and detection points (2D see cross section shown in Figure 1). This algorithm is based on
the Mellin-Laplace transform (MLT) of the TPSFs6,7.
4
4
10104
101022
4
10
104
10104
2
102
10
2
102
0
100
0
10
100
10
10
-2
0
-2
10
10-2
0
10100
gate 5
gate 6
gate 7
gate 8
gate 9
gate 1
gate 2
gate 3
gate 4
keep in
analysis
10
10-2
16.276ns 00
16.3
16.3
16.276ns 00
DCR correction
and rescaling
16.3
16.276ns
Building the full
diffusion curve
Figure 2. Data pre-processing steps to build each TPSF from fast-gated acquisitions (couple S1D2).
4. RESULTS
For a total acquisition time of 9 s per couple of source and detector, using the gated mode has enabled to recover the
TPSFs with a dynamic range of more than 5 decades (Figure 3) compared to 3.5 decades with the non gated mode (not
shown here). Figure 3 shows that the information on late photons is crucial to obtain contrast on the deepest inclusions.
102
Photon counts
104
reference
13 mm
19 mm
25 mm
101
102
b)
1.8
2.5 ns
3.1
3.6 ns
2.100
100
100
10-2
a)
0
2
Time
4
2.10-1
6 ns
c)
Figure 3. (a) Acquired signals after pre-processing for the reference and 3 depths of the inclusion (for S1D2), (b) zoom between
1.8 and 2.5 ns: the signal measured for the depth of 25 mm is overlapping the reference, (c) zoom between 3.1 and 3.6 ns: the
signal measured for the depth of 25 mm is clearly below the reference signal in this time window.
The reconstruction algorithm is based on the MLT defined as: f ( p , n )  M ( p , n )  f (t )  
pn
n!


f (t )t n exp( pt )dt where
0
f(t) is the TPSF, p (in ns-1) a real positive and n a positive or null integer. A contrast is calculated as follows
MLTwithout _ inclusion  MLTwith _ inclusion
Contrast 
100 for each MLT order from the TPSFs with and without the inclusion in
MLTwithout _ inclusion
the medium. It provides information on the possibility to detect robustly an absorbing inclusion7.
On Figure 4, we can see that the gated mode enables to reach higher values of contrast on orders of MLT > 10
(containing information on late photons) than with the non gated mode. For the depth of 2.5 cm in particular, the contrast
with the gated mode reaches more than 15 % for some MLT orders whereas it is very noisy and inferior to 10% with the
non gated acquisition. This analysis is consistent with the obtained reconstruction results shown in Figure 5. Whereas the
obtained images for the most superficial inclusion (13 mm) are similar for gated and non-gated modes, the deepest
inclusion at 25 mm is robustly detected and well localized with the gated mode, which is not the case for the non gated
mode.
1.3 cm
Contrast (%)
50
2.5 cm
50
40
40
40
30
30
30
20
20
20
10
10
10
0
0
0
Gated
S1D2
S2D1
Non gated
S1D2
S2D1
-10
5
10
15
20
25
30
-10
5
10
15
Figure 4. Contrast on MLT orders (for p = 3
and S2D1.
ns-1)
Non gated
Gated
0.16
0
0.1
0
0
-20
0
20
30
-10
5
10
10
20
20
30
30
40
40
-20
0
20
Non gated
10
15
20
25
30
MLT order
13 mm
0
µa (cm-1)
0
25
for different depths of the inclusion for gated and non gated modes for S1D2
0
50
m
20
MLT order
MLT order
0
0
1.9 cm
50
50
Gated
0.16
0.1
-20
25 mm
0
0
10
10
20
20
30
30
40
40
0
Non gated
µa (cm-1)
Gated
0
10
10
20
20
30
30
40
40
50
20
-20
0
µa (cm-1)
0.115
50
-20
0
20
50
0.1
-20
0
25 mm
0
20
Figure 5. Reconstruction results in plane (x, z) for the depths of 13 and 25 mm, with gated and non gated modes (red circle: real
inclusion).
5. CONCLUSIONS
We have shown that it is possible to exploit fast-gated measurements to build TPSFs that can be used by DOT
algorithms. For a given acquisition time, using detectors in the fast-gated mode rather than in the classical non-gated
mode enables to detect deeper absorbing inclusions in a turbid medium from reflectance measurements at the interfiber
distance of 15 mm. This fast-gated technique opens the possibility to image an increased depth range in biological
tissues with reflectance DOT in a limited acquisition time, compatible with a clinical application.
20
50
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