The HPM‑100-50
Hybrid Detector: Increased Dynamic Range for DOT
The bh HPM‑100-50
module combines a Hamamatsu R10467-50 GaAs hybrid PMT tube with the
preamplifier and the generators for the PMT operating voltages in one compact
housing. The principle of the hybrid PMT in combination with the GaAs cathode
of the R10467-50 yields excellent NIR sensitivity, a clean TCSPC instrument
response function, and virtually non-existent afterpulsing. The absence of
afterpulsing results in a substantially increased dynamic range of TCSPC
experiments. We demonstrate the performance of the new detector for the
recording of time-of-flight distributions in experiments of diffuse optical
tomography.
The bh HPM‑100 modules combine a
Hamamatsu R10467 hybrid detector tube [5] with the generator of the -8000 V
acceleration voltage, the generator of the avalanche diode reverse voltage, and
the preamplifier an a compact housing. We have shown before that the principle
of the hybrid detector yields excellent timing resolution, a clean TCSPC
instrument response function, high detection quantum efficiency, and large
detection area [1, 2]. The most intriguing feature of the hybrid detectors is,
however, that they are free of afterpulsing. Afterpulsing has been present in
all photon counting detectors so far. In TCSPC experiments it results in a
signal-dependent background that can be orders of magnitude higher than the
thermal background [1]. The HPM detectors are free of this background and thus
reach an unprecedented dynamic range for any kind of TCSPC measurements.
The hybrid detectors come in two cathode versions.
The HPM‑100-40 has a GaAsP cathode with a detection wavelength range of
300 to 700 nm and about 40% detection efficiency [2]. It is an excellent
detector for fluorescence decay, FLIM, and FCS measurements. For applications
in microscopy please see [3, 4]. The HPM‑100-50 has a GaAs cathode and can
be used from 400 to 900nm [5]. It has a detection efficiency of about 15%
between 500 and 820 nm. Its typical application is in diffuse optical
tomography and fluorescence decay measurement of NIR dyes.
To demonstrate the performance of the
HPM-100-50 the test setup shown in Fig. 1 was used. A scattering phantom was
placed directly between a bh BHL-600 (785 nm) laser and the detector (Fig.
1, left). Filters were inserted to adjust the light intensity and reduce the
sensitivity to daylight. For IRF recording the phantom was replaced with barrel
that contained an additional ND filter and a thin scattering foil (Fig. 1,
right). Both for the phantom measurement and the IRF recording the same
distance between laser and detector was maintained. The photon pulses of the
HPM-100-50 were recorded by a bh SPC-150 TCSPC module [1].
Fig. 1: Test setup. Left: Measurement of time-of-fight distribution in a
phantom. Right: Measurement of instrument response function.
A recording obtained with the HPM‑100‑50
is shown in Fig. 2. For comparison, Fig. 3 shows reference data recorded with a
conventional NIR PMT. A PMC-100-20 cooled PMT module was used for this
measurement. The PMC-100-20 contains an R7400-20 TO-9 PMT within an H573-20
photosensor module [1]. To obtain comparable results the count rates for both
detectors were adjusted to 1 MHz. The dark count rate was 2700 s-1
for the hybrid detector and 900 s-1 for the cooled PMT.
Fig. 2: DTOF
(red) measured at the 30 mm phantom and IRF (blue). HPM‑100-50
hybrid detector
Fig. 3: DTOF
(red) measured at the 30 mm phantom and IRF (blue). PMC‑100-20 NIR
PMT module
At first glance, it can be seen hat the
dynamic range obtained with the HPM-100-50 detector is much higher. Despite of
the higher dark count rate the background in the HPM recording is by about a
factor of 20 lower than for the PMT. A comparison of the two recordings also
shows that the HPM delivers a much cleaner IRF. Photons arriving only 1 ns
after the peak of the DTOF are virtually free of IRF influence.
Conclusions
Compared to conventional NIR-sensitive PMTs
the HPM‑100-50 hybrid detectors deliver a dramatically increased dynamic
range of TCSPC measurements. This is an obvious advantage for data analysis. The
increased dynamic range is important especially for the detection of late
photons. Clean detection of late photons is essential to DOT measurements
because these are more likely to have travelled through deep layers of the
tissue.
Moreover, the HPM‑100-50 have an IRF
free of tails or bumps. IRF measurement in DOT setups is still hampered by a
number of pitfalls. A smaller influence of the IRF is therefore a clear
advantage, no matter whether a diffusion model, or time-gates or moments of
times-of-flight are used to analyse the data.
Considering the fact that the reverse
problem of DOT sets extreme requirements to the raw data hybrid detectors are
likely to improve the reliability at which tissue parameters can be derived.
References
1. W. Becker, The bh TCSPC handbook. 3rd edition, Becker
& Hickl GmbH (2008), available on www.becker-hickl.com
2.
The HPM‑100-40 hybrid detector.
Application note, available on www.becker-hickl.com
3.
Becker & Hickl GmbH, DCS-120 Confocal
Scanning FLIM Systems, user handbook. www.becker-hickl.com
4.
Becker & Hickl GmbH, FLIM systems for Zeiss
LSM 510 and LSM 710 laser scanning microscopes. www.becker-hickl.com
5. Hamamatsu Photonics, R10467 hybrid PMTs,
data sheet.
