Magnetic resonance microimaging of liquid water distribution in sugar maple wood below fiber saturation point

Abstract

Magnetic resonance (MR) microimaging was used to determine the distribution of liquid water in sugar maple wood (Acer saccharum Marsh.). Two moisture desorption tests were applied using saturated salt solutions at 21°C. Desorptions were accomplished between 58 and 96% RH starting from the full saturation state and from the FSP. Each moisture sorption condition at equilibrium was associated with a MR microimaging scan. Signal intensity (represented by false colors in the MR images) allowed visualization of the concentration of liquid water distributed into the wood structure. In most cases, the presence of liquid water was noticed in samples coming from the full saturation state at moisture contents below FSP. This result shows the coexistence of liquid and bound water even at moisture contents below the FSP. The remaining liquid water in the wood appears to be located principally in the lumina of the least accessible libriform fibers.

Full text

MAGNETIC RESONANCE MICROIMAGING OF LIQUID WATER
DISTRIBUTION IN SUGAR MAPLE WOOD BELOW FIBER
SATURATION POINT
Roger E. Herna
´ndez*{
Professor
Claudia B. Ca
´ceres
MSc Student
Centre de Recherche sur le Bois
De
´partement des Sciences du Bois et de la Fore
ˆt
2425 rue de la Terrasse
Universite
´Laval
Que
´bec, QC, Canada, G1V 0A6
(Received September 2009)
Abstract. Magnetic resonance (MR) microimaging was used to determine the distribution of liquid
water in sugar maple wood (Acer saccharum Marsh.). Two moisture desorption tests were applied using
saturated salt solutions at 21C. Desorptions were accomplished between 58 and 96% RH starting from
the full saturation state and from the FSP. Each moisture sorption condition at equilibrium was associated
with a MR microimaging scan. Signal intensity (represented by false colors in the MR images) allowed
visualization of the concentration of liquid water distributed into the wood structure. In most cases, the
presence of liquid water was noticed in samples coming from the full saturation state at moisture contents
below FSP. This result shows the coexistence of liquid and bound water even at moisture contents below
the FSP. The remaining liquid water in the wood appears to be located principally in the lumina of the
least accessible libriform fibers.
Keywords: Magnetic resonance microimaging, sugar maple, liquid water, fiber saturation point.
INTRODUCTION
The understanding of the hygroscopic behavior
of wood is essential to achieve its optimal utili-
zation. One of the most important features of
wood hygroscopicity is the concept of the FSP
because it governs the changes in different wood
properties (Siau 1995). The FSP was first de-
fined by Tiemann (1906) who established it
as the moisture content at which the cell walls
are saturated with bound water and the cell
cavities have lost all liquid water. Many years
later, studies on wood moisture sorption (Goulet
and Herna
´ndez 1991; Herna
´ndez and Bizon
ˇ
1994; Herna
´ndez and Pontin 2006; Almeida
and Herna
´ndez 2006a, 2006b) have questioned
this concept of FSP. They reported the existence
of a phenomenon called “hysteresis at satura-
tion” that affects the moisture sorption of sugar
maple wood above 63% RH (Goulet 1968;
Herna
´ndez and Pontin 2006). This hysteresis
implies that during desorption, the loss of bound
water begins before all liquid water is removed
from the wood.
Furthermore, Stamm (1964) added that the FSP
is the moisture content at which physical prop-
erties such as swelling, shrinkage, and mechan-
ical properties start to change. However, some
studies that have associated moisture sorption
tests at higher levels of RH with these properties
(Herna
´ndez and Bizon
ˇ1994; Herna
´ndez and
Pontin 2006; Almeida and Herna
´ndez 2006a,
2006b) have shown that they began to change
at equilibrium moisture contents (EMC) well
above the FSP. Therefore, liquid water still
remained in wood when the loss of bound water
began. Almeida and Herna
´ndez (2006b) have
suggested that the remaining liquid water could
* Corresponding author: Roger.Hernandez@sbf.ulaval.ca
{SWST member
Wood and Fiber Science, 42(3), 2010, pp. 259-272
#2010 by the Society of Wood Science and Technology

be entrapped in cells associated with the small-
est capillaries connecting the wood lumina. This
could correspond to the openings of the simple
pit membranes situated in the radial parenchyma
cells given that these ray elements are consid-
ered as the least permeable flow paths in hard-
woods (Wheeler 1982; Siau 1995).
The distribution and concentration of liquid
water in wood tissues at different equilibrium
sorption conditions are therefore fundamental
points in the understanding of wood–water rela-
tionships. It has been demonstrated in this
respect that
1
H nuclear magnetic resonance
(NMR) is one of the most effective nondestruc-
tive methods for monitoring water movement
(Hsi et al 1977; Riggin et al 1979; Brownstein
1980; Menon et al 1987; Flibotte et al 1990;
Araujo et al 1992; Hartley et al 1994; Labbe
´
et al 2006; Almeida et al 2007; Thygesen and
Elder 2008).
In
1
H NMR, there are two different relaxation
processes: spin–lattice relaxation (longitudinal
relaxation), T
1
, and spin–spin relaxation (trans-
verse relaxation), T
2
. The signal from water pro-
tons in wood cells can relax with different rates
and can be easily divided into three principal
components: solid wood, cell-wall water, and
lumen water (Riggin et al 1979; Araujo et al
1993). Several
1
H NMR studies have shown the
usefulness of spin–spin relaxation (T
2
) for the
characterization of water in wood (Riggin et al
1979; Brownstein 1980; Menon et al 1987,
1989; Flibotte et al 1990; Araujo et al 1992,
1993; Almeida et al 2007; Thygesen and Elder
2008). Moreover, it appears that T
2
relaxation is
also sensitive to cell size, cell wall thickness,
and wood cell proportions (Brownstein and Tarr
1979; Menon et al 1987, 1989; Flibotte et al
1990; Araujo et al 1993).
Most
1
H NMR experiments have been made
using softwood species that have a simpler ana-
tomical structure as compared with hardwoods.
Thus, three different T
2
water populations have
been differentiated: a fast T
2
that represents all
cell-wall water, a medium T
2
that corresponds to
water in earlywood lumina, and a slow T
2
that
represents water in latewood lumina (Menon
et al 1987; Flibotte et al 1990; Araujo et al
1992; Hartley et al 1994; Labbe
´et al 2006;
Thygesen and Elder 2008). For hardwoods,
there are four principal wood elements that can
contain liquid water in their lumina: vessel ele-
ments, fibers, and axial and radial parenchyma.
Nevertheless, their disposition, size distribution,
and proportions are highly variable depending
on species. Almeida et al (2007) carried out
1
H
NMR measurements with two temperate hard-
woods, sugar maple and beech, and one tropical
hardwood, huayruro, at different equilibrium
moisture contents. They observed three T
2
com-
ponents: a slow T
2
that represents liquid water
located in the lumina of the vessel elements, a
medium T
2
that corresponds to liquid water
located in the lumina of fiber and parenchyma
elements, and a fast T
2
that represents bound or
cell-wall water as found in softwoods. Their
results also showed that, even at equilibrated
conditions, liquid water was present at EMC
values lower than the FSP. All liquid water was
lost at 11.5% EMC for sugar maple, 11.4%
EMC for beech, and 18% EMC for huayruro.
The next step is to achieve the visualization of
this liquid water distribution in an image. Cur-
rently, magnetic resonance imaging (MRI) has
become a reliable tool in wood imaging. MRI
has been used to visualize the internal structure
of wood (Hall and Rajanayagam 1986; Hall et al
1986; Cole-Hamilton et al 1995; Merela et al
2005; Oven et al 2008), evaluate water flow
and distribution during drying (Olson et al
1990; MacMillan et al 2002; Meder et al 2003;
Rosenkilde et al 2004), visualize moisture
movement in wood composites (van Houts et al
2004, 2006), differentiate healthy tissue from
decayed wood (Flibotte et al 1990; Kuroda et al
2006), and observe water drainage in wood
(Almeida et al 2008). MRI is defined by differ-
ent parameters that regulate the image quality
permitting proper interpretation of the image
characteristics. The image contrast depends on
the variation of longitudinal relaxation time
(T
1
)
,
transverse relaxation time (T
2
), and proton
density (water protons) in the sample. The effect
260 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)

of these three factors is always present, but it is
possible to emphasize one and minimize others
to obtain a specific image contrast (Kastler
1997). The most important property in wood
tissues for MRI is the water content. As the
moisture content decreases, the signal intensity
will be lower. In fact, as the proton density
decreases, the T
2
values are faster and the signal
is more difficult to obtain. However, the actual
effect on the image intensity will depend on the
imaging method and acquisition parameters
(MacMillan et al 2002). The main objective of
this work was to use MRI to visualize the distri-
bution of liquid water in wood samples that were
equilibrated during desorption at several mois-
ture contents below the FSP.
MATERIALS AND METHODS
Experiments were carried out on sugar maple
(Acer saccharum Marsh.) sapwood that was
equilibrated in a conditioning room at 60% RH
and 20C. The samples were turned to obtain
small 3.6-mm-dia cylinders (transverse to the
grain) 20-mm long (parallel to the grain). The
test material had an average basic wood density
(oven-dry mass/green volume) of 556 kg/m
3
with a coefficient of variation of 2.5%.
Sorption Tests
The experiments consisted of moisture sorption
tests combined with MR microimaging measure-
ments. Fifty-four samples were prepared and
distributed in nine matched groups. Five groups
were destined for desorption experiments from
full saturation. The other four groups were
assigned to desorption experiments from the
FSP. To avoid internal defects caused by fast
adsorption, all samples were saturated using a
mild procedure (Naderi and Herna
´ndez 1997;
Almeida and Herna
´ndez 2007). All specimens
were thus conditioned over a KCl-saturated salt
solution (86% RH) for 10 da. The full saturation
groups were then placed over distilled water for
10 da. The fiber saturation groups were also con-
ditioned over distilled water until they achieved
the equilibrium condition after 16 da. Finally, the
full saturation groups were immersed in distilled
water until their maximum moisture content was
reached by cycles of vacuum and atmospheric
pressure.
All groups were then placed in sorption vats set
at 21C with a temperature control of 0.01C
during extended periods (Herna
´ndez and Bizon
ˇ
1994). This permits a precise control of RH in
the various desiccators serving as small sorption
chambers. For each point of desorption, one des-
iccator containing six samples was used. One
sample was used for the MR microimaging scan
and the other five samples for EMC determina-
tion. All nine desorption conditions were carried
out over saturated salt solutions in a single step
procedure (Table 1). To determine equilibrium,
the specimens were weighed periodically with-
out being removed from the desiccators. Equi-
librium was reached after at least 30 da of
sorption. Once the EMC was reached, the first
sample was placed into a 200-mm long, 5-mm
outside diameter NMR sample tube. For better
RH control during desorption, NMR tubes had
been placed inside the desiccator at the begin-
ning of the test. A Teflon dowel, 175-mm long
and 4-mm dia, was then inserted in the tube to
minimize the air space with which the wood
could interact. A tight cap was used to seal the
tube. Finally, the tube was placed in a 25-mm-
thick Styrofoam box to minimize any change of
hygrothermal conditions during transportation
to the University of Montreal where the MR
Table 1. Characteristics of the moisture sorption condi-
tions used in this experiment.
Sorption
condition
State of
sorption
Saturated salt
solution
Nominal RH
(%)
Equilibration at 21C from full saturation state
9 Desorption K
2
SO
4
96
8 Desorption ZnSO
4
90
7 Desorption KCl 86
6 Desorption NaCl 76
5 Desorption NaBr 58
Equilibration at 21C from FSP
4 Desorption K
2
SO
4
96
3 Desorption ZnSO
4
90
2 Desorption KCl 86
1 Desorption NaCl 76
Herna
´ndez and Ca
´ceres—MAGNETIC RESONANCE MICROIMAGING OF SUGAR MAPLE WOOD 261

microimaging scans were made. The tube was
weighed before transportation and at the begin-
ning of the MR microimaging scan to detect any
changes in moisture content.
Magnetic Resonance Microimaging Tests
1
H MRI experiments were performed at 600 MHz
on a 14.1-T Bruker Avance 600WB spec-
trometer equipped with a microimaging probe.
The system was also equipped with three orthog-
onal field gradient coils permitting a maximum
gradient of 30 G/mm along the z axis (ie parallel
to the main magnetic field) and 20 G/mm in the
x-y plane. A standard slice-selective spin-echo
imaging sequence was used to acquire images of
liquid water inside wood samples. Preliminary
tests were made to select the best parameter con-
figuration that permitted us to obtain a suitable
quality image. A 1-mm-thick slice was selected
by the use of sync-shaped selective pulses.
Images were acquired using an accumulation of
1024 scans to obtain 100 100 pixel images
with a field of view of 4 mm, leading to a nom-
inal in-plane resolution of 40 mm. An echo time
(TE) of 2.3 ms and a repetition time (TR) of
200 ms were used, leading to an experimental
time of about 340 min for each image. All exper-
iments were performed at 21C.
Transverse and tangential images were obtained
for each sorption condition. For the transverse
image, the slice was selected at the middle of the
longitudinal axis of each specimen. From this
image, a perpendicular slice was taken at the
middle to provide an image of the tangential–
longitudinal plane of wood. The MR microimag-
ing data acquisition, reconstruction, analysis, and
visualization were made using ParaVision 4.0,
Bruker’s digital image processing software com-
puter package. The intensity of the images was
adjusted using the sample having the higher
moisture content. This permitted us to visualize
and compare the water concentration differences
among the MR microimages. A color scale was
chosen to obtain a clearer differentiation of the
changes in signal intensity.
Environmental Scanning Electron
Microscopy and Scanning Electron
Microscopy Tests
The same specimens used for MR microimaging
scans were cross-cut with a sharp circular saw at
the same position where these scans were made
(at the middle height). This freshly cut end-grain
was then carefully cleaned with a razor blade
mounted onto a microtome. This was done to
match the MR microimages with the electron
micrographs for comparison purposes. Environ-
mental scanning electron microscopic (ESEM)
pictures of end-grain surfaces were taken for
each sample coming from the full saturation
desorption test with a JEOL JSM6360LV micro-
scope. Afterward, the same specimens were
oven-dried for 2 h, mounted on standard alumi-
num stubs with silver paint, and coated with
gold/palladium in a sputter-coater to obtain
scanning electron microscopic (SEM) images
using a JEOL 840-A microscope.
RESULTS AND DISCUSSION
Wood Hygroscopicity
The desorption curves of sugar maple wood
obtained at 21C are given in Fig 1. The upper
curve at high RH corresponds to the boundary
desorption (from full saturation) and the lower
corresponds to desorption from FSP. In all
cases, the standard errors of the EMC data do
not exceed the symbol size shown. The differ-
ence between both curves at high RH is the
result of the hysteresis at saturation phenome-
non. Goulet and Herna
´ndez (1991) attributed
such difference to the presence of liquid water
during desorption. Several authors have con-
firmed this statement and relate it to the entrap-
ment of liquid water in the most impermeable
cells of wood, particularly radial parenchyma
cells (Herna
´ndez and Bizon
ˇ1994; Herna
´ndez
and Pontin 2006; Almeida and Herna
´ndez
2006a, 2006b). These elements are considered
as the least permeable flow paths in hardwoods
(Wheeler 1982; Siau 1984). A paired Student’s
t-test was made to compare the EMC means
262 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)

between the two desorption tests for each RH
(P¼0.05). The results indicate that differences
between EMC means were statistically signifi-
cant for 96, 90, and 86% RH, although they were
similar at 76% RH (Table 2). This could indicate
that the entire loss of liquid water was already
accomplished at 76% RH, which corresponds to
approximately 17.2% EMC. This assumption
will be confirmed later after examination of
MR microimages of the samples.
The EMCs obtained in desorption starting from
the full saturation state were also compared with
those of previous studies performed on the same
wood (Fig 2). Goulet (1968) and Herna
´ndez and
Bizon
ˇ(1994) used larger samples of 15 (T) 
15 (L) 45 (R) mm and 20 (R) 20 (L) 
60 (T) mm, respectively. Their EMC values at
58% RH are similar with that obtained in the
present work. At higher RH, the EMC values
became increasingly different as RH increased.
These differences would be almost entirely attrib-
utable to the amount of liquid water remaining
in wood. Larger volumes of liquid water would
remain entrapped in larger samples (Goulet 1968;
Herna
´ndez and Bizon
ˇ1994) than in smaller ones
(Almeida et al 2007; present work). On the other
hand, there were smaller differences in EMC
between the results reported by Almeida et al
(2007) and those of the present work, although
both studies used a similar sample size but with
different orientations. A radially oriented 4-mm-
dia cylinder (L T) and 20-mm long (R) was
used by Almeida et al (2007). EMCs in these two
studies were even different at 58% RH in which
liquid water is virtually absent. Such differences
Figure 1. Equilibrium moisture content (EMC) as a function of RH for sugar maple wood at 21C (standard errors did not
exceed the symbol size).
Table 2. Equilibrium moisture content as a function of RH at 21C for different sample orientations of sugar maple wood.
RH (%)
Longitudinal
a
Radial
a
Tangential
a
Desorption from Desorption from Desorption from
Full saturation (107.5%) FSP (42.4%) Full saturation (114.2%) FSP (34.1%) Full saturation (109.3%) FSP (33.1%)
96 31.1 Aa
b
28.9 Ba 31.6 Aa 28.2 Bb 31.7 Aa 27.5 Bc
90 25.9 Aa 23.6 Ba 25.9 Aa 23.8 Ba 25.6 Aa 23.3 Ba
86 21.8 Aa 20.7 Ba 22.4 Aab 21.5 Bb 22.7 Ab 21.5 Bb
76 17.1 Aa 17.4 Aa 17.5 Aa 17.5 Aa 17.6 Aa 17.0 Ba
58 12.7 a 13.1 a 12.8 a
a
Means of five replicates.
b
Means within a row followed by the same letter are not significantly different at the P¼0.05 level. Uppercase letters are for desorption type comparison for
each orientation separately. Lowercase letters are for orientation type comparison for each desorption type separately.
Herna
´ndez and Ca
´ceres—MAGNETIC RESONANCE MICROIMAGING OF SUGAR MAPLE WOOD 263

in desorption behavior could in part be explained
by an eventual effect of the orientation of speci-
mens on EMC. To test this hypothesis, an addi-
tional sorption test was carried out using samples
1) with their longer axis oriented following the
rays; and 2) following the growth rings. Table 2
shows the EMC means and the means compari-
son test (unpaired Student’s t test) obtained.
Some statistically significant differences in EMC
among orientations are shown. As discussed sub-
sequently, water condensation occurred during
the adsorption step over distilled water (during
moisturizing wood up to FSP). Judging by the
different nominal FSPs reached, this condensa-
tion was different for the three types of samples
(Table 2). As a result, variable volumes of liquid
water could even remain in the three types of
samples at 96% RH. On the other hand, slight
differences in EMC at 86% RH occurred between
the longitudinal samples and the others. In con-
trast to the radial and tangential samples, longitu-
dinal samples were taken from different pieces of
wood, which can explain the differences in EMC.
However, for most of the cases, the EMC values
obtained for the three orientations were statisti-
cally similar. Therefore, the orientation of the
sample does not appear to affect EMC. Differ-
ences in EMC between Almeida et al (2007) and
the present work must therefore be explained by
other sources of variation.
Magnetic Resonance Microimaging Analysis
Previous
1
H NMR studies in wood have con-
firmed that analysis of the transverse relaxation
times (T
2
) permits the separation of water into
three populations: bound water or cell-wall
water with faster T
2
; liquid water with medium
T
2
; and liquid water with slower T
2
(Menon et al
1987, 1989; Flibotte et al 1990; Araujo et al
1992, 1993; Almeida et al 2007). T
2
can also
characterize water in different internal compart-
ments because it is related to the size and pro-
portion of woody tissues (Brownstein and Tarr
1979; Menon et al 1987; Araujo et al 1992;
Almeida et al 2007). Thus, such results give a
theoretical basis for the study of the distribu-
tion of water in wood by MRI at EMCs below
the FSP.
Nevertheless, there are some limitations that
should be considered when performing MRI
analysis. First, the image intensity is dependent
on the relationship among radiofrequency pul-
ses, relaxation times (T
1
and T
2
), and proton
density (r) of water in the material of study
Figure 2. Comparison of equilibrium moisture content values of different studies obtained in desorption from full
saturation state for sugar maple wood.
264 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)

(Callaghan 1991). This relationship is described
by the signal intensity equation:
S¼rð1eðTR=T1ÞÞðeðTE=T2ÞÞ
Olson et al (1990) have demonstrated that the
image intensity is proportional to the quantity
of liquid water present in the wood sample.
Therefore, it is more difficult to obtain a clear
image of wood at lower water concentrations.
Another barrier is the size of the cells, because
diameters smaller than 10 mm are difficult to
observe with MRI (Ko
¨ckenberger 2001). There
are also some restrictions because of the MRI
equipment and parameters used that will directly
influence the quality of the image (MacMillan
et al 2002; Bucur 2003; van Houts et al 2004).
As mentioned previously, the image intensity
was set equally for each type of image to make
them comparable. A color scale was also chosen
to have a better contrast within the image.
Colors varied from red, which shows the highest
intensity (100%), to black that shows nil inten-
sity (0%). The transposition of this to our results
is as follows: from red to yellow indicates liquid
water and from green to black indicates absence
of liquid water. Thus, the limit intensity value
for detecting liquid water (yellow) was about
65% for transverse microimages and 58% for
tangential microimages.
The MRI conditions used permitted us to obtain
signal intensity that was mainly dependent on
the liquid water concentration and the T
2
values.
As discussed subsequently, images revealed pri-
marily the variation in water proton density of
the medium T
2
values. The signal of faster T
2
values was too low to be imaged.
Transverse and tangential MR microimages for
the two desorption tests performed with the longi-
tudinal oriented samples at two RH (96 and 86%)
are shown in Figs 3-6. The EMC at 96% RH was
31.1% for desorption from full saturation (Fig 3a)
and 28.9% for desorption conducted from FSP
(Fig 3b). The difference between the two desorp-
tion experiments is 2.2% MC. The EMC value
obtained from FSP should correspond exclusively
to bound water. However, the adsorption step
over distilled water gave a value of 42.4% MC
(Table 2). According to previous research, the
FSP of sugar maple should correspond to 31 or
30% (Herna
´ndez and Bizon
ˇ1994; Herna
´ndez
2007). Condensation of water vapor apparently
occurred, which implies that small volumes of
Figure 3. Magnetic resonance transverse microimages of the samples equilibrated at 96% RH. (a) From full saturation
with an equilibrium moisture content (EMC) of 31.1%. (b) From FSP with an EMC of 28.9%.
Herna
´ndez and Ca
´ceres—MAGNETIC RESONANCE MICROIMAGING OF SUGAR MAPLE WOOD 265

liquid water may be present in the sample coming
from FSP desorption (Herna
´ndez 2007). This is
confirmed by the fact that Figs 3b and 4b display
some yellow–orange spots (red circles) revealing
the presence of liquid water in the sample. Never-
theless, we can affirm that the difference in color
between the two samples corresponds to the liquid
water present in greater proportion in the spec-
imens coming from full saturation (Figs 3a and 4a).
Figure 3a also illustrates (black circles) that all
liquid water has been removed from the lumina
of vessel elements at 96% RH. This early drain-
age of the pores in sugar maple has been des-
cribed previously (Herna
´ndez and Bizon
ˇ1994;
Herna
´ndez and Pontin 2006; Almeida et al
2007). On other hand, liquid water appears to be
located in the wood tissue surrounding the pores,
because the colors red, orange, and yellow form
Figure 4. Magnetic resonance tangential microimages of the samples equilibrated at 96% RH. (a) From full saturation
with an equilibrium moisture content (EMC) of 31.1%. (b) From FSP with an EMC of 28.9%.
Figure 5. Magnetic resonance transverse microimages of the samples equilibrated at 86% RH. (a) From full saturation
with an equilibrium moisture content (EMC) of 21.8%. (b) From FSP with an EMC of 20.7%.
266 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)

irregular lines parallel to the rays. This exposes
the fiber cavities as being the most likely location
for liquid water. Moreover, the middle area of the
sample seems to be devoid of liquid water
because it shows mostly green and blue colors.
This area corresponds to the limit of a growth
ring, revealing it as an important flow path to
drain liquid water in sugar maple.
As we could also see in Fig 3, Fig 4 shows more
clearly that the rays are completely emptied
of liquid water at 96% RH for both desorption
tests (black circles). Considering the longitudinal
and tangential-oriented samples, the maximum
length of the rays was 4 mm, therefore liquid
water needs to travel a maximum of 2 mm to be
removed. For the radially oriented samples,
the length of rays was 20 mm, and liquid water
will need to travel a maximum of 10 mm to
be removed. The distance for liquid water to
leave the sample will depend on the orientation.
However, Fig 4 shows the rays as one of the
first elements to be drained of liquid water. Sugar
maple has homocellular uniseriate and multi-
seriate rays (Panshin and de Zeeuw 1980). Carl-
quist (2007) found the existence of conspicuous
bordered pits in the tangential walls of procum-
bent ray cells and fewer in their horizontal walls.
This ray walls pitting would facilitate the flow of
liquid water through ray cells. Moreover, ray-
vessel pitting is similar to intervessel pitting
in Acer species (Panshin and de Zeeuw 1980).
The distance (size of the sample) used in
these experiments should not be a barrier for the
drainage of liquid water in ray elements. This
observation contradicts previous hypotheses that
established the lumina of radial parenchyma cells
as the most probable location for the entrapment
of liquid water below the FSP (Goulet and
Herna
´ndez 1991; Herna
´ndez and Bizon
ˇ1994).
Figure 4a, however, shows that liquid water is
located in the lumina of the fibers.
Figures 5a and 6a still show some liquid water
(red circles) present at 86% RH that corresponds
to 21.8% EMC. Figure 7a, however, does not
depict liquid water at 76% RH, showing only
blue and black colors. Thus, all remaining liquid
water was removed at some point between these
conditions. Almeida et al (2007) reported values
of medium T
2
(liquid water) for sugar maple
wood even at 76% RH (16.4% EMC). It is
possible that the remnant of liquid water still
present in wood at this level of RH might not
Figure 6. Magnetic resonance tangential microimages of the samples equilibrated at 86% RH. (a) From full saturation
with an equilibrium moisture content (EMC) of 21.8%. (b) From FSP with an EMC of 20.7%.
Herna
´ndez and Ca
´ceres—MAGNETIC RESONANCE MICROIMAGING OF SUGAR MAPLE WOOD 267

have been sufficient to produce a measurable
signal using the parameters applied.
Figures 5b and 6b also illustrate that desorption
from FSP at 86% RH (20.7% EMC) leaves no
more liquid water, because their colors are only
blue and black. Thus, all liquid water that had
been present by water condensation had already
been removed before attaining equilibrium at
86% RH. At 76% RH (Fig 7b) and 58% RH
(not shown), both desorption tests were also
devoid of liquid water.
Comparison of Magnetic Resonance
Microimages and Scanning Electron
Microscope Images
The comparison between MR microimages of
the samples and the corresponding SEM images
of the same sections showed very good agree-
ment. SEM images provide higher resolution
images of anatomical structure displayed in MR
microimages. A typical comparison is presented
in Fig 8 corresponding to the sample equili-
brated at 96% RH from full saturation.
The higher concentration of liquid water (red to
yellow spots) is found in the tissues outlying the
pores distributed all around the sample (Fig 8a).
Hardwoods have two different types of fibers:
libriform fibers that are elongated, commonly
thick-walled cells with simple pits, and fiber
tracheids that are commonly thick-walled cells
with a small lumen, pointed ends and bordered
pit pairs (IAWA 1964). The usual way to differ-
entiate them is by their pitting. These two types
of fibers represent approximately 61% of the
total volume of sugar maple wood and their
average length is about 0.92 mm (Panshin and
de Zeeuw 1980). As mentioned previously, each
pixel in the MR images represents 1-mm depth.
Thus, Fig 8a could illustrate the behavior of one
complete fiber with one tip of another fiber or
two portions of two different fibers.
Carlquist (2001) established that in the evolu-
tion from tracheids to libriform fibers through
the fiber tracheids, the pit membrane diameter
and the borders of pits decrease and there are
sequentially fewer pits. Thus, only fiber tra-
cheids could take part in water transport,
whereas libriform fibers will provide mechan-
ical support. Cirelli et al (2008) established that
fiber tracheids are part of the water-transport
vessel system in sugar maple. Therefore, our
hypothesis is that the liquid water entrapped in
Figure 7. Magnetic resonance transverse microimages of the samples equilibrated at 76% RH. (a) From full saturation
with an equilibrium moisture content (EMC) of 17.1%. (b) From FSP with an EMC of 17.4%.
268 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)

wood at moisture contents below FSP is located
in the libriform fibers, principally in those
located far from the radial parenchyma and ves-
sel elements.
According to Vazquez-Cooz and Meyer (2006),
most libriform fibers in sugar maple have simple
pits with elliptical shapes that are grouped
mainly near the center of the fiber. Thus, their
extremities do not have communication between
cells. In contrast, pits in the fiber tracheids are
rather evenly distributed along the fiber length.
Furthermore, Magendans and van Veenendaal
(1999) stated that complementary pits in neigh-
boring walls of adjacent libriform fibers are not in
perfect alignment as are the complementary pits
of fiber tracheid pit pairs. Cirelli et al (2008)
found no pits or only blind pits connecting
libriform fibers to vessels and no connections
between libriform fibers and fiber tracheids.
Fiber-to-fiber simple pit pairs were scarce in
sugar maple. Therefore, libriform fibers appear
to be more isolated from the other cell groups.
Libriform fibers have also larger lumina than
fiber tracheids and have intercellular spaces that
occur in various patterns, ranging from large
groups to wavy bands (Vazquez-Cooz and Meyer
2006). More recently, Vazquez-Cooz and Meyer
(2008) have discarded the use of the terms libri-
form fibers and fiber tracheids for fiber type
1 and 2, respectively. The IAWA Committee
(1989) considers these terms as corresponding to
different subtypes of libriform fibers.
Figure 9 shows two sections of wood sample
presented in Fig 8, which show fibers with differ-
ent lumen sizes. Good agreement exists between
the distribution of the larger lumen fibers in
Figs 9a and 9b and the distribution of the liquid
water in Fig 8a. Therefore, it is possible to estab-
lish that liquid water is entrapped principally in
the libriform fibers, probably at their lumina
extremities. Furthermore, liquid water could also
be present to some extent in the intercellular
spaces, which are commonly associated with libri-
form fibers (Vazquez-Cooz and Meyer 2006).
The connections between intercellular spaces and
surrounding cells have not yet been studied.
This analysis could help in the interpretation of
the EMC differences between previous studies
Figure 8. (a) Magnetic resonance transverse microimage of one sample equilibrated at 96% RH from full saturation
(31.1% equilibrium moisture content). (b) Scanning electron microscopy image of the same section. The white lines
indicate the position of Figs 9a and 9b. The red rectangle indicates the position of Fig 10.
Herna
´ndez and Ca
´ceres—MAGNETIC RESONANCE MICROIMAGING OF SUGAR MAPLE WOOD 269

(Goulet 1968; Herna
´ndez and Bizon
ˇ1994;
Almeida et al 2007). As mentioned previously,
such differences in EMC could be entirely
because of the quantity of liquid water in the
sample such that larger samples could entrap
more liquid water than smaller ones. Thus, the
longer the longitudinal axis in the sample, the
higher the possibility of capturing liquid water,
which will also increase if the sample has a
larger section (R T). Goulet (1968) and
Herna
´ndez and Bizon
ˇ(1994) used larger sam-
ples than those used in the present work. Larger
samples give them a greater probability to lose
the connectivity among wood cells and conse-
quently entrap more liquid water. However, dif-
ferences can also be attributed to the great
variability in the proportion and distribution of
fiber tracheids and libriform fibers. Panshin and
de Zeeuw (1980) have stated that they vary
between species, among individuals of the same
species, and within a tree.
Figure 8b clearly shows the limit of one growth
ring that represents a region without liquid water
in Fig 8a. That area appears to be formed by
fiber tracheids because they have smaller lumina
(Fig 10). Sparse marginal parenchyma could
also be present as occasional cells (Panshin and
de Zeeuw 1980). Cirelli et al (2008) have found
a good association between fiber tracheids and
vessels connected through bordered pit pairs.
A large pore distribution is also present along
the limit of the growth ring that could indicate
an easy passage for liquid water between the
fiber tracheids and vessels (Fig 8b).
SUMMARY AND CONCLUSIONS
The MR microimaging technique was used to
obtain images of liquid water distribution in
sugar maple wood at equilibrium moisture con-
tents below the FSP. Desorption experiments
from FSP and from full saturation desorption
were carried out at 21C to distinguish the
location of liquid water in the wood structure.
Figure 9. Environmental scanning electron microscopy
transverse images of two different sections of the sample
(a and b) equilibrated at 96% RH (31.1% equilibrium mois-
ture content) from full saturation.
Figure 10. Scanning electron microscopy transverse
image of the limit of growth ring of the sample equilibrated
at 96% RH (31.1% equilibrium moisture content) from full
saturation.
270 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)

The principal conclusions are listed subse-
quently:
1. Liquid water was found below FSP even at
equilibrated conditions. Thus, there is a loss
of bound water even when liquid water is
entrapped in wood. The concept of FSP
should be re-evaluated.
2. MR microimaging is a technique that lends
itself as a powerful tool to visualize liquid
water distribution over a wide range of mois-
ture contents.
3. MR microimages show good evidence that
vessel and ray elements are drained of liq-
uid water before achieving equilibration at
96% RH.
4. MR microimages, and ESEM and SEM
images, suggest that the remaining liquid
water below FSP could be principally en-
trapped toward the ends of the libriform
fibers given that pits in these elements are
predominantly concentrated toward their
centers. Remnants of liquid water could also
be located in the intercellular spaces present
in this wood.
ACKNOWLEDGMENTS
We thank Cedric Malveau for his valuable
help during MRI tests at Montreal University.
This research was supported by the Natural
Sciences and Engineering Research Council of
Canada.
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