J. Biomedical Science and Engineering, 2010, 3, 1099-1107 JBiSE
doi:10.4236/jbise.2010.311143 Published Online November 2010 (http://www.SciRP.org/journal/jbise/).
Published Online November 2010 in SciRes. http://www.scirp.org/journal/jbise
A novel NMR instrument for the in-situ monitoring of the
absolute polarization of laser-polarized 129Xe
Aktham Asfour1,2
1INSERM/Grenoble University, Grenoble Institut des Neurosciences, Grenoble, France;
2Grenoble University, Grenoble Electrical Engineering Lab, Grenoble, France.
Email: Aktham.Asfour@g2elab.grenoble-inp.fr
Received 16 July 2010; revised 25 July 2010; accepted 30 July 2010.
A new fully digital and home-built NMR (Nuclear
Magnetic Resonance) spectrometer working at very-
low magnetic field (4.5 mT) is presented. This spec-
trometer was initially dedicated for the in situ meas-
urement of the absolute polarization of hyperpolar-
ized 129Xe. It allows detection and acquisition of
NMR signals of proton (1H) at 190 kHz and of hyper-
polarized xenon-129 (HP 129Xe) at 50 kHz. In this new
NMR instrument, we replaced as much analog elec-
tronics as possible by digital electronic and software.
Except for the power amplifier and the preamplifier,
the whole system is digital. The transmitter is based
on the use of a Direct Digital Synthesizer (DDS)
board. The receiving board allows direct digitaliza-
tion of the NMR signals thanks to an 8-bits ana-
log-to-digital converter (ADC) clocked at 100 MHz.
Decimation is preformed to dramatically improve the
ADC resolution so as the final achieved effective reso-
lution could be as high as 14-bits at 5 MHz sampling
frequency. NMR signals are then digitally down-
converted (DDC). Low-pass decimation filtering is
applied on the base-band signals (I/Q) to enhance
much more the dynamic range. The system requires
little hardware. The transmitter and the receiver are
controlled using Labview environment. It is a versa-
tile, flexible and easy-to-replicate system. This was
actually one of underlying ideas behind this devel-
opment. Both 1H and hyperpolarized 129Xe NMR
signals were successfully acquired. The system is used
for the measurement of the absolute polarization of
hyperpolarized 129Xe in hyperpolarizing experiments
for the brain perfusion measurements. The high de-
gree of flexibility of this new design allows its use for
a large palette of other potential applications.
Keywords: NMR; Very-Low Field; Digital System; 1H;
Hyperpolarized 129Xe; Absolute Polarization; Brain Per-
fusion Measurements
It’s well known that xenon is an inert gas which is char-
acterized by a high solubility in lipids. It diffuses freely
in biological tissues and especially through the blood-
brain barrier. Spin-exchange with optically pumped ru-
bidium vapors increases the nuclear polarization of xe-
non-129 (129Xe) by several orders of magnitude above
the polarization at thermal equilibrium, resulting in
hyperpolarized 129Xe (HP 129Xe). This HP 12 9 Xe can be
used as magnetic resonance tracer because of its
NMR-enhanced sensitivity. It becomes thus a promising
new exogenous NMR/MRI (magnetic resonance imag-
ing) tracer for cerebral imaging studies in both humans
and small animals [1,2]. Actually, several studies, carried
out in our laboratory and by some groups in the world,
have already demonstrated that the NMR/MRI of in-
jected or inhaled HP 129Xe may allow quantitative
measurement of the cerebral blood flow (CBF) with high
spatial and temporal resolutions [2-5]. These resolutions
could be higher than those currently available with the
“gold standard” of 133Xe radionuclide technique (radio-
active method) [1]. Moreover, a major advantage of MRI
is its noninvasive nature.
The CBF is a physiological parameter that is defined
as the blood supply to the brain in a given time and per
mass unit of brain tissue1. This parameter is tightly regu-
lated to meet the brain’s metabolic demands. The meas-
urement of changes in both global cerebral blood flow
(gCBF)2 and regional cerebral blood flow (rCBF)3 is of
great value for functional brain studies as well as for the
diagnosis of a large number of brain diseases (Alzheimer,
epilepsy, Parkinson, ischemia…).
1In healthy brain of adult humans, the CBF is regulated to an average
of 50 milliliters of blood per 100 grams of brain tissue per minute.
2Value of the CBF measured over all the brain.
3Value of the CBF measured within a specific brain region.
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
Nevertheless, for such measurements, especially those
of the rCBF, to become quantitatively reliable by the
MRI of HP 129Xe, one must first overcome several ex-
perimental and instrumental challenges. One of these
instrumental challenges concerns the development of
specific radiofrequency (RF) probes for these MRI ex-
periments. In a recent work, we have actually addressed
a solution through a new design of an RF double-tuned
(100 MHz-27 MHz) RF volume coil actively-decoupled
from a single-tuned (27 MHz) receive-only RF surface
coil [6].
In this paper, we deal with another challenge that con-
cerns the experimental set-up for polarization build-up
of the hyperpolarized 129Xe. This set up is relatively
complicated and it requires some know-how.
To help the reader in better understanding the MRI of
hyperpolarized xenon and to illustrate the scope of this
paper, one may need to review some NMR basic princi-
Consider a population of N NMR-sensitive (non-zero
spin) nuclei (1H, 3He, 129Xe...) subjected to a uniform
static magnetic field 0
. In the microscopic scale, this
population will be divided into two categories according
to some energy levels considerations. We denoted these
categories N (low energy level which corresponds to
spins having the same orientation as 0
) and N
(high energy level which corresponds to spins having
orientation opposite to 0
), with NN N
We define the absolute polarization,
, of this popu-
lation by the Eq . 1:
 
Assuming that N and N follow Boltzmann sta-
tistics, one can demonstrate, under usual values of mag-
netic field B0 and absolute temperature T, that the abso-
lute polarization can be expressed by the Eq.2:
where h the Planck constant,
k the Boltzmann con-
stant and
the gyro-magnetic ratio which character-
izes the nucleus.
Under these conditions, we say that the population of
nuclei is under thermal (or Boltzmann) equilibrium.
Therefore, the obtained polarization is sometimes called
thermal polarization.
The value of the polarization is generally small. For
example, at B0 = 1 T and at room temperature, the proton
(1H) polarization is only about 10-5 for the usual in vivo
proton concentrations.
In a basic NMR experiment, the population of nuclei
will be subjected to short pulse (called excitation pulse)
of a second sinusoidal magnetic field B1, applied at the
characteristic Larmor frequency 00
to the
sample. At the end of the excitation pulse, a resonance
signal (or the NMR signal) at the same frequency is re-
ceived. This signal (S) is directly proportional to the po-
larization P according to the Eq.3:
The NMR signal is processed to be used for obtaining
a “fingerprint” of the environment of the nucleus being
studied. It is clear that a high signal level is necessary to
achieve the required sensitivity in most MRI applica-
For the in vivo proton MRI, the large proton concen-
trations (N) and the use of high magnetic fields (0
B) to
increase the polarization allow for an exploitable signal
level and acceptable sensitivity.
Unlike the case of proton NMR/MRI, the in vivo
NMR signal of injected or inhaled 129Xe is not exploit-
able. Actually, at thermal equilibrium, and for a same
magnetic field B0, the NMR signal of a 129Xe population
is about 10000 times lower than the one that could be
obtained from the same volume of protons. This is be-
cause of the intrinsic lower gyro-magnetic ratio (4 times
lower), the intrinsic lower thermal polarization and
lower density of the xenon (see Eqs.2 & 3).
To compensate for these limitations, hyperpolarizing
techniques have been successfully used to dramatically
increase the polarization level of xenon before using it
for the NMR or MRI in-vivo experiments. The hyperpo-
larization is actually a physical process that increases the
polarization level without the need to increase the mag-
netic field B0. Typically, the polarization, and conse-
quently the NMR signal, is enhanced by a factor of 105.
The injected of inhaled HP 129Xe xenon becomes then a
very interesting NMR tracer for brain perfusion meas-
In our laboratory, the 129Xe is hyperpolarized by spin-
exchange with Rubidium (Rb) optically pumped by laser
at 795 nm [7]. The simplified set-up is illustrated by
Figure 1. About 0.1 g of Rb is introduced in a 100-ml
Pyrex cell, which has subsequently filled with a mixture
of He, N2, and natural Xe (26% 129Xe) at 5 bars at room
temperature. The cell is heated to about 120°C, set in a
4.5 mT magnetic field produced by Helmholtz coils, and
exposed to a circularly polarized laser of 795 nm wave-
length. This laser will polarize the Rb electronic spins.
The electronic polarization is transferred from Rb elec-
trons to 129Xe nuclei. For more details about this process,
the interesting reader may refer to [7].
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
Figure 1. A simplified scheme of the hyperpolarizing set-up.
This optical pumping process is achieved outside the
MRI system. In few minutes, about 20 ml of hyperpo-
larized xenon-129 (HP 129Xe) can be obtained. This HP
129Xe could then be inhaled or injected for the in vivo
MRI experiments of brain perfusion measurement.
The achieved polarization is five orders of magnitude
larger than its equilibrium (thermal) value. It is therefore
called hyperpolarization. It is important to notice that
this polarization is not under equilibrium conditions and
its value could not be calculated by the Eq.2. Moreover,
a determination of the obtained polarization using the
theoretical physics of the hyperpolarizing process is not
a trivial task since a lot of experimental parameters and
conditions could influence the obtained value. This value
would be far from the theoretical expected one.
Nevertheless, monitoring the available absolute po-
larization of the gas during the optical pumping and at
the end this process is critical. In fact, one must be able
to quantify the effects of changing temperature, pressure,
gas mixture, laser power or the position and elements of
the laser optical train. The goal is to guarantee a maxi-
mum polarization level or to diagnose eventual problems.
An in- si tu measurement of the available polarization,
during and at the end of the pumping process is then
This measurement could a be performed by NMR
technique since the gas being polarized (inside the cell)
is subjected to the 4.5 mT static magnetic field produced
by the Helmholtz coils. This very-low field is initially
needed to maintain the polarization during the optical
pumping process. The Larmor frequency f0 at this field is
about 52 kHz for xenon and 190 kHz for proton.
However, the NMR signal of the HP 129Xe is only pro-
portional to the polarization P. It does not allow knowing
directly its value. Therefore, in order to a measure this
polarization, one needs to compare the NMR signal (SXe)
of the HP 129Xe with and another NMR reference signal
(Sref) This reference signal could be the NMR signal of
thermal 129Xe (not HP 29Xe) or the NMR signal of a
sample (reference sample) containing another nucleus
such as 1H. Actually, according to the Eq.3, the ratio
between these two signals is given by the Eq .4 :
XeXe Xe
ref refref ref
where Xe
(respectively ref
) and Xe
N (respectively
N) are the gyro-magnetic ratios and the number of
nuclei of the HP 129Xe (respectively of the reference
sample). ref
P is the thermal polarization of the refer-
ence sample. The hyperpolarization, Xe
P, can then be
calculated from the Eq.5:
Xerefref ref
ref XeXe
This equation shows that the polarization of the HP
129Xe is obtained if we measure the NMR signals of both
HP 129Xe and reference sample, since Xe
N andref
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
could be determined from the experimental conditions,
and the thermal polarization ref
P can be calculated
from Eq.2.
Measuring theses NMR signals requires use of an
NMR spectrometer working at 4.5 mT. Such spectrome-
ters for such low working frequency (52 kHz for 129Xe
and 190 kHz for 1H) are not commercially available, and
the development of dedicated low and very-low fre-
quencies (up to several 100 kHz or several 1 MHz)
NMR systems remains far from the experience of most
NMR groups.
In fact, to meet the needs of expanding research pro-
jects and applications, powerful and expensive spec-
trometers or imagers are commercially available. Al-
though, these high field NMR systems have many ad-
vantages, such as high signal-to-noise ratio (SNR) and
high image quality, their use in some specific applica-
tions could be prohibitively expensive. Actually, in many
cases, for a particular purpose, one may need only an
NMR spectrometer having a subset of features of a
standard commercial one. Recently, a number of groups
have worked to develop dedicated low-cost MRI/NMR
systems for a variety of applications, by using compact
low-field (so low-cost) MR magnets. For example, we
proposed in a previous work a complete home-built
digital MRI system working at 0.1 Tesla (which corre-
sponds to a resonance frequency of 4.25 MHz) [8]. This
system was based on the use of high performance DSP
SHARC4 processor, a direct digital synthesizer (DDS)
and a digital receiver typically employed in communica-
tion applications (mobile phone base-stations). Based on
this work, Shen et al. [9] proposed another system
working at 0.3 T (about 13 MHz) and allowing larger
imaging sizes than in [8]. Another work carried out by C.
Michal et al. was focused on the realization of a wide-
band high performance receiver for a home-built NMR
spectrometer working at 55.84 MHz (high field) [10]. In
any case, the development of home-built and dedicated
low and very-low field NMR systems remains the work
of only few research groups arround the world.
In the context of measuring the polarization in hyper-
polarized gases experiments, some research groups have
NMR apparatus. These NMR systems were developed
by modifying high frequency and high cost commercial
spectrometers. One research group has however devel-
oped its own complete NMR system for measuring the
polarization of hyperpolarized helium-3 (3He) [11]. It
was a fully analog system (for both transmit and receive
channels) where authors performed a phase-sensitive
detection of the NMR signal. They used then an oscillo-
scope for signal visualization
Despite the great merit of the original and elegant
electronic solutions developed in [11], the detection of
hyperpolarized 3He signals was relatively not a hard task
since their levels were quite high (at least 10 mV). Actu-
ally, the spectrometer described by Saam et al. did not
allow sufficient dynamic range to detect thermal 3He
(not hyperpolarized 3He) signal or even 1H signal (de-
spite the high density of water) at such very-low field.
Indeed, the detection of these signals is far from trivial at
very-low field since their levels are, at least, four to five
orders of magnitude lower than the signal of hyperpo-
larized 3He. It was however necessary, for Saam et al. to
detect thermal 3He or 1H (water) reference signals to
measure the absolute polarization of hyperpolarized 3He.
In fact, as we have mentioned, the measurement method
consists in comparing the levels of hyperpolarized 3He
signals to another reference signal such as thermal 3He
or 1H signals. Sam et al. have done their comparison at
high field (B0 = 2 T). Nevertheless, in this comparison
(not in situ measurements which is not convenient),
losses in the hyperpolarized 3He polarization during
transport from the hyper-polarizing set-up to the high
field NMR spectrometer may not guarantee accurate
measurements of the initial obtained polarization.
In the present paper, we propose an NMR system that
allows detection of both HP 129Xe and 1H signals at 4.5
mT. The system allows then the in-situ measurement of
the absolute polarization. In fact, the dynamic range of-
fered by our system allows detection of water equilib-
rium NMR signal at very-low field (4.5 mT) so as the
calibration of the spectrometer can be done without
transporting the gas to the high field spectrometer and
consequently without loss in polarization.
It is a fully digital system. One of the underlying ideas
of this work is to make the apparatus versatile and easy
to replicate so as to help the interesting groups to simply
develop such NMR spectrometer. In addition of the
measurement of the absolute polarization, some others
potential applications of this developed spectrometer
will be addressed.
Unlike our first work at 4.25 MHz [8], where we used
very advanced digital electronics and signal processing
techniques which were typically employed for mobile
phone applications, we use here data acquisition boards
(DAQ) since they were adequate at such low frequencies.
Moreover, these DAQ have increased in performance
and the related software made their use quite straight-
The general architecture of our home-built NMR sys-
tem is illustrated by Figure 2. The static field B0 of
4AD2106x from Analog Devices
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
Figure 2. The general architecture of the NMR system.
about 4.5 mT is produced by the Helmholtz pair in
which the Pyrex cell is placed and the optical pumping is
The excitation pulse (at 52 kHz for HP 129Xe and 190
kHz for 1H) is generated by the transmitter (signal gen-
erator board NI 5411 form National Instruments). This
pulse is amplified by a power amplifier and sent, via the
duplexer, to the well-tuned coil (at 190 kHz or 52 kHz)
which generates the excitation field B1.
At the end of the excitation pulse, the same tuned coil
(transmit-receive coil) detect the weak NMR signal. This
signal is transmitted to a low noise preamplifier via the
duplexer. The amplified signal is then received by the
receiving board (NI 5911 from National Instruments) for
digitalization and quadratic demodulation.
2.1. The Pulse Transmitter and the Power
For generating pulse, we used the Direct Digital Synthe-
sis (DDS) technique available with the PC board NI
5411. This technique allows deriving, under digital con-
trol, an analog frequency source from a reference clock
frequency. Compared to analog synthesis techniques,
The DDS produces high frequency accuracy and resolu-
tion, temperature stability, rapid and phase-continuous
frequency switching, and it offers low phase noise.
The transmitting board uses a 32-bit, high-speed ac-
cumulator with a lockup memory and a 12-bit DAC for
DDS-based pulse generation.
This generated pulse is amplified by an op-amp-based
power amplifier stage. The op-amp (AD711KN) is cho-
sen for its high output slew rate. The stage allows more
than 20 V peak-to-peak output measured on a high im-
pedance oscilloscope.
Since the transmitter is loaded by transmit-receive coil,
the design of the op-amp-based power amplifier has to
take care of instabilities that usually appear when the
amplifier is loaded by the capacitive and inductive coil.
2.2. The Transmit/Receive Well-Tuned Coil and
the Duplexer
The well-tuned coil is one of the key elements for a suc-
cessful detection of the weak NMR signals (especially
those of protons). Calculations showed that a quality
factor Q of at least 200 (at 190 kHz) is necessary. We
realized a surface coil of about 400 turns of a Litz wire
with an average diameter of 2 cm and a height of 0.5 cm.
The developed inductance is about 1.3 mH measured at
190 kHz. Even at relatively low frequency such as 190
kHz, the use of a Litz wire was important to achieve
high quality factor Q. The measured quality factor of the
coil was 220 at 190 kHz and 130 at 52 kHz, about two
times and a half that achieved with solid wire of the
same gauge and geometry.
The coil was tuned using fixed capacitors and variable
ones. Notice that the same coil is easily used for the both
frequencies. The only modifications are the tuning ca-
pacitors. To facilitate these modifications, tuning com-
ponents were plug-ins on pin DIP component carriers.
The tuned coil is connected to the duplexer by ordinary
coaxial cable; there are no tuning elements near the coil.
Actually, the tuning elements are placed on the duplexer
printed board.
This duplexer “blinks” the preamplifier during the ex-
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
citation pulse transmission and insulates the transmitter
from the receiver during the receiving period. This du-
plexer, based of the use of a FET transistor switch, is
controlled by a TTL compatible signal generated by the
transmitter board (Figure 2).
2.3. The Low-Noise Preamplifier
The low noise preamplifier is the second key element for
NMR signal receiving at very-low field. The one that we
developed was based on the use of the OP37 low noise
op-amp. It was high gain and low noise. The design of
high gain and low noise preamplifiers is relatively com-
plicated and requires some know-how. Our preamplifier
is gain-programmable between 2 and 2000 (6 dB to 66
dB). We used typically a gain of 60 dB for the detection
of 1H signals and 6 dB for the detection of 129Xe signals.
Optional analog filters could be used or not in adequate
locations between the different stages of the preamplifier
to optimize the SNR and/or the dynamic range.
The output of the preamplifier is directly connected to
input of the receiving board
2.4. The Digital Signal Receiving
The receiving board (NI 5911 from National Instruments)
is the last receiving key element. The input signal is
scaled to match the full input range of an 8-bit ana-
log-to-digital converter (ADC) clocked at 100 MHz. For
our application, a resolution of 8 bits is not sufficient.
The down-sampling allowed by the receiving board can
dramatically enhance the ADC effective resolution. We
used typically a down-sampling factor of 20 so as the
final ADC effective resolution was of 14 bits at 5 MHz
sampling frequency.
After down-sampling, digital down-conversion (DDC)
or digital demodulation was performed by software as
illustrated in Figure 3.
The digital signal is multiplied by digital sine and co-
sine waveforms. The quadratic base-band signals (I/Q)
were obtained after decimation and low-pass filtering.
Actually, since the useful bandwidth of these base-band
signals is no more than 1000 Hz, these low-pass decima-
tion filters, with programmable decimation factors, were
used to improve the final dynamic range and the sig-
nal-to-noise ratio (SNR).
2.5. The Graphical User Interface
The GUI and control program was developed using
LABVIW environment. The architecture of the program
is open, which lets users build their own modules if
wanted. The program allows the user to choose the exci-
tation frequency (consequently the nucleus: 1H, 129Xe…),
amplitude, and duration of the excitation pulse as well as
the repetition time of a one-pulse NMR sequence. Other
parameters that concern some hardware configurations
Figure 3. Down-sampling and Digital Down-Conversion (DDC) of the NMR signals.
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
of the NI 5411 are not available in the GUI but they
could be modified if required in the LABVIEW Diagram
of the program. The signal acquisition is done according
to the parameters defined by the user in the GUI. These
parameters are the scale of the input, number of points to
be acquired and the down-sampling factor of the ADC.
The effective ADC resolution could be displayed for
The acquired digital data are read from the buffer of
the DAQ board. Quadratic digital demodulation is per-
formed. User can choose the demodulation parameters
(frequency, phase and amplitude) as well as the decima-
tion factor of the final low pass digital filters. The GUI
allows displaying of base-band signals (I/Q) and the
NMR signal spectrum (Fast Fourier Transform of the
NMR signal). Users could add more modules in the pro-
gram to perform others kinds of measurements on the
acquired signal or on its spectrum.
Notice finally that the program contains two modules
which are dedicated to the calibration of the flip-angle of
the excitation pulse and for the transmit/receive coil
tuning and matching.
3.1. Acquired NMR Signals
Figure 4 shows, respectively, the quadratic base-band
HP 129Xe NMR signals and the signal spectrum acquired
at 4.5 mT with the developed spectrometer.
The excitation pulse duration was of the 800 µs with a
repetition time of 2 seconds. The gain of the low-noise
preamplifier was set to 6 dB. The sampling frequency of
the received signal before the DDC was of 5 MHz so as
the effective ADC resolution was of 14 bits. The final
sampling frequency in the base-band was of 10 kHz.
As it was mentioned in the introduction, the meas-
urement of absolute polarization requires comparing the
signal of hyperpolarized gas to a reference signal at
thermal equilibrium at 4.5 mT. However, since equilib-
rium polarization and gas density are very low, these two
signals are far apart in magnitude. An alternative solu-
tion is to compare the hyperpolarized 129Xe signal with
the equilibrium water signal (higher signal level than
129Xe signal thanks to the higher density of water).
The dynamic range offered by our system allowed
detection of such water equilibrium NMR signal at very-
low field (4.5 mT) so as the calibration of the spec-
trometer can be done without transporting the gas to the
high field spectrometer and consequently without loss in
polarization. We used so the developed spectrometer to
acquire 1H signal at 4.5 mT (about 190 kHz of Larmor
The sample used for 1H signal acquisition is a pyrex
Figure 4. The I and Q parts of the 129Xe NMR signal (upper
figure) and its spectrum (lower figure) obtained with the de-
veloped spectrometer at 4.5 mT. No signal averaging was used.
reference cell filled with pure water. This reference cell
has the same shape and the same volume that the one
which usually contains the HP 129Xe.
An example of 1H signals and their spectrum is given
in Figure 5. These signals were collected by the same
coil that was used for collecting 129Xe signals and which
was retuned to 190 kHz by simply modifying the tuning
capacitors. The gain of the preamplifier was set to 60 dB
and 10 signal averages were used. All the other sequence
and acquisition parameters were identical to those used
for 129Xe signals acquisition.
This calculation of the absolute polarization of the HP
129Xe is straightforward using these acquired signals and
the Eq.5.
To illustrate an example, we studied the influence of
the temperature of the cell on the available polarization.
Figure 6 shows the results. In this figure, the value of
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
Figure 5. The I and Q parts of the 1H NMR signal (upper fig-
ure) and its spectrum (lower figure) obtained with the devel-
oped spectrometer at 4.5 mT. Only 10 signal averages were
Figure 6. Influence of the temperature of the cell on the avail-
able absolute polarization. For each temperature, the polariza-
tion was measured after about 2 minutes of optical pumping.
the hyperpolarization is given in % (it represents actually
the quantity 100
This figure suggests that the optimum working tem-
perature for our system is about 90°C. At this tempera-
ture, the available polarization was about 5.8 %. This is
very useful information since our working temperature
was thought to be about 110°C before developing this
control NMR system. Actually, calibration of the opti-
mum temperature was performed at high field (2.35 T)
some years ago and was considered to be unchanged. We
think that changing the laser source and the cell by new
ones (this has been done after high field calibration) has
led to a different optimum temperature. The developed
system allows a convenient tool to monitor very easily
and at each moment the amount of the polarization and
to define the optimum working parameters. We are now
studying the influence of the pressure, the laser power
and the cell in order to obtain the highest polarization
When compared with the common method, which is
based on the use of high field spectrometers, this new
measurement method presents important advantages.
The main advantage is that the new method allows in
situ measurement of the polarization during the optical
pumping process. This is fundamental since it is neces-
sary to quantify the influence of the different parameters
of the process and to adjust them in order to guarantee
the maximum polarization. A second advantage is that
the new method allows measurement without losses in
the polarization. Actually, in our set-up, we observed a
loss of about 2% in polarization during the transport of
the HP 129Xe to the high field spectrometer. Other ad-
vantages are related to the low cost and the simplicity of
use of the developed system which can be used continu-
ously without occupying the high field systems…
The only limitation of the new method could be the
detection of a 1H NMR signal magnetic fields lower than
4.5 mT are used in the polarization set-up (Helmholtz
coil). Actually, we did not try to detect 1H NMR signals
at lower fields. This problem is out of the scope of this
In addition to the measurement of the polarization in
hyperpolarization experiments, there is a large palette of
potential applications of this developed spectrometer.
Theses applications may especially concern NMR non-
invasive measurements. A first potential application
could be the measurement of the quantity of water con-
tained in a given sample. Actually, the NMR signal is
proportional to the density of water within the sample.
One could then quantify the degree of humidity of the
sample. A second application could concern the meas-
urement of both transversal T2 and longitudinal T1 re-
laxation times of the sample. This could find application
A. Asfour / J. Biomedical Science and Engineering 3 (2010) 1099-1107
Copyright © 2010 SciRes. JBiSE
for temperature measurement of biological samples. Ac-
tually, NMR is a very important technique for measuring
temperature of a sample in millikelvin and below [12]
through the temperature dependence of the spin-lattice
relaxation time T1 or through the measurement of mag-
netic susceptibility (measurement of the effective trans-
versal relaxation time *
T). These measurements should
be performed at frequencies below 1 MHz to minimize
power dissipation in the sample. Our developed spec-
trometer is adequate for these applications. Specific
pulse sequences, which are necessary for such measure-
ments could be easily implemented on our system with-
out hardware modifications. This is actually allowed by
flexibility of the design and to the open structure of the
Another potential application of this non exhaustive
palette of applications may concern educational purposes
in the field of electronics, signal processing and bio-
medical engineering. This could be allowed, once again,
thanks to the flexible and open structure of both hard-
ware and software of the spectrometer. In fact, we are
currently building another version of this spectrometer in
the Physics Department of Measurement (Technological
Institute of the University Joseph Fourier at Grenoble).
Practical courses and projects in electronics, signal pro-
cessing, instrumentation and measurements, NMR phys-
ics could be take place during and after the development
of the spectrometer. Students could simulate and build
by themselves the source of the magnetic field (Helm-
holtz coils), the power amplifier, the duplexer, the coil
and the preamplifier. They could also develop their own
applications for the control of the NMR sequence and
the data acquisition and processing under LABVVIEW
Notice finally that this low frequency NMR spec-
trometer could also be dedicated for others applications
with working frequencies as high as 20 MHz, without
any change on the hardware (except for the tuned coil).
The spectrometer is low cost and easily transportable
(for in situ experiments for example). It is also an easy-
to-replicate system. Full developed circuits, part refer-
ences, PC Board, controlling programs and any other
detail about the system can be obtained by simply writ-
ing to author.
We presented a fully home-built NMR spectrometer for
the measurement of the absolute polarization of laser-
polarized gases. The system described here has been in
use in our lab for more than one year and gives a reliable
measurement of the polarization. The flexibility of the
system allows its use for other NMR applications with-
out (or with minor) hardware and software modification.
The authors would like to thank Christoph SEGEBARTH, Director of
the Team 5 of The Grenoble Institute of Neurosciences (GIN) for sup-
porting this work.
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