Inhomogeneous Sodium Accumulation in the
Ischemic Core in Rat Focal Cerebral Ischemia by
Victor E. Yushmanov, PhD,1*Alexander Kharlamov, MD, PhD,1Boris Yanovski, MD,1
George LaVerde, MD, PhD,2Fernando E. Boada, PhD,2and Stephen C. Jones, PhD1–3
Purpose: To test the hypotheses that (i) the regional heter-
ogeneity of brain sodium concentration ([Na?]br) provides a
parameter for ischemic progression not available from ap-
parent diffusion coefficient (ADC) data, and (ii) [Na?]brin-
creases more in ischemic cortex than in the caudate puta-
men (CP) with its lesser collateral circulation after middle
cerebral artery occlusion in the rat.
Materials and Methods:23Na twisted projection MRI was
performed at 3 Tesla. [Na?]brwas independently determined
by flame photometry. The ischemic core was localized by
ADC, by microtubule-associated protein-2 immunohisto-
chemistry, and by changes in surface reflectivity.
Results: Within the ischemic core, the ADC ratio relative to
the contralateral tissue was homogeneous (0.63 ? 0.07),
whereas the rate of [Na?]brincrease (slope) was heteroge-
neous (P ? 0.005): 22 ? 4%/h in the sites of maximum
slope versus 14 ? 1%/h elsewhere (here 100% is [Na?]brin
the contralateral brain). Maximum slopes in the cortex were
higher than in CP (P ? 0.05). In the ischemic regions, there
was no slope/ADC correlation between animals and within
the same brain (P ? 0.1). Maximum slope was located at the
periphery of ischemic core in 8/10 animals.
Conclusion: Unlike ADC,
ischemic lesion heterogeneity.
23Na MRI detected within-core
Key Words: rat brain; focal ischemia; permanent MCAO;
ADC;23Na MRI; tissue sodium
J. Magn. Reson. Imaging 2009;30:18–24.
© 2009 Wiley-Liss, Inc.
THE APPARENT DIFFUSION coefficient (ADC) of tissue
water is an established MRI marker for initial ischemic
damage to the brain (1). Because ADC alone is insuffi-
cient to characterize stroke severity, the diffusion/per-
fusion mismatch, that is, the area showing significant
cerebral blood flow (CBF) deficit without corresponding
ADC decrease, is commonly regarded as a candidate for
tissue salvageability (1,2). Lesion evolution studies (3,4)
and quantitative positron emission tomography (5,6)
suggested, however, that the mismatch may not accu-
rately estimate “tissue-at-risk,” and that both the ADC-
defined ischemic core and the mismatch area do not
have unique flow/metabolism counterparts and may
contain brain tissue at various stages of the ischemic
process. Additional complications include potential
permanent or transient (“pseudonormalization”) ADC
reversibility, which may be accompanied by selective
neuronal loss in the lesion core after early reperfusion
(1,3,7). To better characterize the threshold phenomena
resulting in the ADC/CBF mismatch in stroke, the re-
gion-specific ADC responses to cerebral perfusion def-
icit have been described previously in different models
of focal ischemia in rats (8–10). Novel MRI approaches
to monitor the progression of ischemic damage and
refine the detection of the ischemic penumbra include
mapping of cerebral metabolic rate of oxygen utilization
(11), blood-oxygen-level-dependent MRI (12) and pH-
weighted MRI (13).
23Na MRI has been considered as a possible marker
for brain tissue viability after stroke (14–16). Recently,
23Na MRI has been proposed as a means to determine
the stroke onset time for establishing patient eligibility
for thrombolytic therapy (17).23Na MRI timing of stroke
is based upon the linear increase in brain sodium con-
centration ([Na?]br) in affected areas (18,19) in the first
several hours. In an earlier study by Jones et al (17), no
comparison of23Na MRI with ADC was made in a model
of cortical stroke not involving caudate putamen (CP).
The interest in comparison of different brain regions
stems from the presence of collateral circulation in cor-
tex, unlike CP where collateral circulation is absent
In this study, we hypothesize that (i) the regional
heterogeneity of [Na?]brincrease provides an additional
“functional” parameter for assessing brain ischemia,
1Department of Anesthesiology, Allegheny-Singer Research Institute,
2MR Research Center, Department of Radiology, University of Pitts-
burgh School of Medicine, Pittsburgh, Pennsylvania.
3Department of Neurology, Allegheny-Singer Research Institute, Pitts-
Contract grant sponsor: National Institutes of Health; Contract grant
Dr. LaVerde’s current address is UPMC Mercy Hospital, Department of
Medicine, Pittsburgh, PA 15219.
*Address reprint requests to: V.E.Y., Department of Anesthesiology,
Allegheny-Singer Research Institute, 320 East North Avenue, Pitts-
burgh, PA 15212-4772. E-mail: firstname.lastname@example.org
Received December 31, 2008; Accepted April 13, 2009.
Published online in Wiley InterScience (www.interscience.wiley.com).
JOURNAL OF MAGNETIC RESONANCE IMAGING 30:18–24 (2009)
© 2009 Wiley-Liss, Inc.
which is not available from ADC data, and (ii) ischemic
cortex is characterized by more intense [Na?]brincrease
than CP in the rat model of focal ischemic stroke.
MATERIALS AND METHODS
Approval for animal use was obtained from the appro-
priate institutional committee and was consistent with
the “Principles of laboratory animal care” (NIH publica-
tion No. 86-23, revised 1985). Ten normally fed male
Sprague-Dawley rats weighing 320 ? 36 g (mean ? SD)
were used. Anesthesia was induced with 3% isoflurane,
and maintained with 1.0% to 2.5% isoflurane, 30%
oxygen, and balance nitrous oxide, administered by
means of endotracheal tube and artificial respiration
(Model 681, Harvard Apparatus, South Natick, MA).
Femoral arterial and venous catheters were inserted.
Inside the magnet, an MR compatible ventilator (MRI-1,
CWE, Ardmore, PA) was used. Arterial blood pressure
was continuously monitored from a femoral artery us-
ing a strain gauge transducer (DT-XX, Viggo Spec-
tramed, Miami, FL) and recorded on a polygraph
(Gould, Cleveland, OH). An appropriate maintenance
level of isoflurane was determined by monitoring the
blood pressure response to tail pinch. Body tempera-
ture was maintained at 37°C by a servocontrolled sys-
tem consisting of a rectal temperature probe and a
heating blanket outside the magnet or a thermostated
water jacket inside the magnet. Immobilization was im-
plemented with 0.4 mg/kg pancuronium bromide in-
jected intramuscularly at 60-min intervals (on the
bench) or continuously infused intravenously at 0.4
mg/kg/h (delivered at 1 mL/h) inside the magnet. To
ensure physiological stability, arterial blood pH and
gases (PaCO2, PaO2) were measured (ABL-3, Radiome-
ter America, Westlake, OH) before surgery, at different
phases of surgery, and at regular intervals during MRI;
in total, typically, at 4–7 time points. With the blood
volume per sample being of ? 65 ?L, the total volume of
withdrawn blood was not hemodynamically significant.
Middle Cerebral Artery Occlusion (MCAO)
In eight animals, permanent focal cerebral ischemia
was produced by insertion of an intraluminal suture.
The 3-0 monofilament poly-L-lysine coated nylon su-
ture was inserted 20–21 mm through the internal ca-
rotid artery and further into the circle of Willis, occlud-
ing the middle cerebral artery (MCA) at its origin (23). In
two other animals, MCA transection and bilateral com-
mon carotid artery occlusion (MCAT) was performed as
described previously (17,18).
inside a 5-cm-diameter, 5-cm-long dual-tuned dual-
quadrature birdcage transmit/receive radiofrequency
(RF) coil (24) in the animal cradle with a recirculating
water bed and fittings for respiratory and anesthesia
gas supply. Images were obtained on a 3 T whole body
scanner (General Electric Medical Systems, Milwaukee,
23Na/1H MRI, the animal’s head was positioned
WI) within a field of view (FOV) of 50 ? 50 ? 50 mm. The
typical experimental timeline was as follows: stroke in-
duction – scout imaging – first ADC map – multiple23Na
MRI (every 5.3 min) – B1mapping – multiple23Na MRI –
second ADC map – multiple23Na MRI.
1H diffusion-weighted multislice spin-echo images
(TR/TE of 2000/140 ms, in-plane resolution of 0.2 mm,
eight 3.2-mm-thick slices, diffusion weighting b-factor
values of 0, 93, 372, and 837 s/mm2, scan time per
b-factor was 4.7 min), with the diffusion-sensitizing
gradient applied along each of the Cartesian axes, were
used for reconstruction of ADC trace maps. To mini-
mize the contribution of capillary microcirculation to
ADC, all b-factor values were in the range governed by
molecular diffusion (25).23Na MRI was performed using
a three-dimensional (3D) twisted projection imaging
(TPI) scheme (26) with a voxel size of 0.48 mm3, imaging
time of 5.3 min (eight transients for each of 398 projec-
tions), and the inhomogeneity correction of the B1field
by RF mapping (27,28). An ultra-short TE of 0.4 ms and
a long TR of 100 ms were used to eliminate a quantita-
tion bias resulting from possible changes in relaxation
times in ischemic brain. Cylindrical tubes containing
NaCl solutions at different concentrations (0, 77, 116,
and 154 mM) were placed next to animal’s head and
served as external position and concentration refer-
ences after correction for partial23Na signal saturation
in the solution due to its longer T1(T1? 60 ms, correc-
tion factor 1 ? exp(? TR/T1)). Tissue23Na with T1of
10–30 ms is fully relaxed in these conditions. The23Na
MRI series typically spanned 2 to 4 h within a 1.1- to
7.3-h window after ischemia.
After the end of MRI scanning (typically, 4.5 to 7.3 h
after MCAO), rats were decapitated, and their heads
were immediately frozen in dry ice and stored at ?80°C
to preserve the spatial characteristics of the brain for
further superposition and comparison with MR images.
The brain was chipped out of the skull in a ?20°C cold
box and mounted into the cryostat (?8°C). Twelve to 18
samples of approximately 0.5 mg wet weight were
punched from the ipsilateral and contralateral brain
(19,28) at two or three coronal levels, typically between
?1 and ?4 mm from bregma. The micro-puncher inner
diameter was 0.53 mm, the sampling depth was deter-
mined by examining coronal brain cuts taken every 40
?m, and the samples were precision-weighed using a
Cahn model C-44 microbalance (ATI Orion, Boston,
MA). The sampling location was guided by ADC and
23Na maps of the brain (Fig. 1a,b) and by the change in
surface reflectivity of ischemic tissue (29). Cut-face
photographs of the brain were taken at several levels,
including punched surfaces before and after sampling
(Fig. 1c,d). The 40-?m-thick coronal sections of the
brain at different levels from bregma were mounted on
glass slides and digitized. The infarct size and location
were verified by reflective changes and by immunohis-
(MAP2) antibody in slide-mounted brain sections (29),
as shown in Figure 1e,f. Brain sodium content was
determined by emission flame photometry of punched
Inhomogeneous Na Accumulation in Stroke 19
samples at 589 nm using an IL943 flame photometer
(Instrumentation Laboratory, Lexington, MA).
Parametric1H ADC maps were generated pixel-wise by
exponential fitting of the diffusion-weighted image in-
tensity versus the b value (30) in MatLab (MathWorks,
Natick, MA).23Na MR images were reconstructed, cor-
rected for inhomogeneity of the B1 field and for the
nonzero noise baseline in the magnitude mode recon-
struction (27), stacked in four dimensions (including
the time dimension), and parametric images of the rate
of23Na signal increase (“slope”) were generated from the
regression of image intensity versus time after stroke
onset) using C and C?? scripts in the UNIX environ-
ment. Selected coronal brain slices (taken every 400
?m) and histological MAP2 stained sections (taken ev-
ery 800 ?m) were digitized, stacked and registered us-
ing ImageJ (31) (available from: Rasband WS, ImageJ,
U. S. National Institutes of Health, Bethesda, Maryland,
http://rsb.info.nih.gov/ij/, 1997-2008) to render volu-
metric reconstructions of the brain. MR images were
23Na images (by performing a pixel-wise linear
aligned with histological 3D images and cut-face pho-
tographs and analyzed using AMIDE software (32). The
[Na?]br values were obtained after MRI calibration
against flame photometry as described elsewhere (28)
by placing cylindrical ROIs in the23Na images at the
positions of punch voids on histological and cut-face
images, as shown in Figure 1. To characterize the rate
of [Na?]braccumulation and ADC deficit in ischemic
brain, [Na?]br and ADC data in ischemic ROIs were
referenced to the corresponding time-averaged control
values of homotopic ROIs.
Parametric and nonparametric one- or two-tailed sta-
tistical tests were applied for significance of correla-
tions and differences, independent or paired as appro-
priate, with a post hoc Bonferroni correction when
multiple comparisons were made, using SPSS for Win-
dows, version 14.0 (SPSS, Chicago, IL). P ? 0.05 was
taken as indicating significance. The errors are pre-
sented as SD or SEM, as indicated. The number of rats
reported in some of the comparisons was less than the
total (ten), because not all parts of the protocol were
successfully completed in all animals.
Physiological variables at different phases of the exper-
imental protocol were in the normal range for all ani-
mals, as summarized in Table 1. Minor fluctuations in
physiological variables were not accompanied by devi-
ations in the23Na time courses. Between the animals,
the23Na slope values did not correlate with mean arte-
rial blood pressure (P ? 0.1).
[Na?] Increase in Ischemic Brain
The ischemic lesion (as defined by the ADC deficit,
changes of surface reflectivity, and MAP2 staining) in-
volved parts of the cortex and CP, as is typical for the
suture MCAO model. Therefore, the changes in [Na?]br
after MCAO were analyzed in the ipsilateral and homo-
topic contralateral frontal cortex, parietal cortex and CP
(Fig. 1).23Na MRI intensity showed a linear increase in
ischemic brain and no statistically significant changes
in contralateral ROIs over time (Fig. 2). Within the
boundaries of the infarct region, sites with an elevated
rate of23Na increase (slope) were observed in all ani-
Physiological Variables at Three Phases of the Protocol*
Start of23Na MRI
End of experiment
*Mean ? SD, n ? 10.
MABP ? mean arterial blood pressure, pH ? arterial blood pH,
paCO2 and paO2, partial CO2 and O2 pressure in arterial blood,
109 ? 10
101 ? 9
90 ? 11
7.38 ? 0.07
7.25 ? 0.09
7.27 ? 0.05
30 ? 10
30 ? 13
40 ? 11
90 ? 20
120 ? 64
100 ? 77
Figure 1. Region-of-interest (ROI) analysis of Na?accumula-
tion and ADC deficit in a rat brain after MCAO. Coronal images
of the brain (at approximately bregma ?0.4 mm) of the rat #6
are shown. a: ADC map where ADC ? 500 ?m2/s in the
ischemic area (left-hand side of the image). b: Pseudocolor-
coded parametric image of the rate of
(“slope”) superimposed over the grayscale
image as an anatomic reference. c,d: Cut-face photograph of
the brain in the cryostat before sampling (c) and after sampling
(d) showing punch holes. A millimeter scale is shown at the
top. e: Cross-section of the 3D reconstruction of the brain from
the 40-?m-thick slices cut at 4.4 h after MCAO. The change in
surface reflectivity of ischemic tissue shows the infarct loca-
tion (outlined by a red dotted line). Cylindrical ROIs (yellow
circles) were placed over the punch holes. f: The absence of
staining in the MAP2-stained slice indicates the ischemic le-
sion. Reference tubes with NaCl solutions were external to the
rat head in the magnet and are not shown in MR images. ROIs
defined in (e) by their correspondence to the punch positions
are shown in images (a–c) as white or colored circles. The
images were aligned and analyzed using AMIDE software.
23Na signal increase
1H spin-echo MR
20Yushmanov et al.
mals (Fig. 1b), either in the cortex (n ? 7) or in CP (n ?
2), as shown in Table 2. The rat #9 accidentally died
during scanning, and the observation time was too
short (approximately 1 h) for the linear regression to
reach statistical significance. The slope values at the
sites of maximum slope were significantly higher than
other slopes within ischemic cortex or CP in the same
brain (P ? 0.005 by paired t-test). The maximum slope
values averaged over all animals were 22 ? 4%/h
(mean ? SEM), as compared with 14 ? 1%/h in other
ischemic ROIs. The mean value of maximum slope in
CP was 15 ? 1%/h (rats #6 and 7), and in the cortex,
24 ? 5%/h (in other 6 rats). A chi-square test showed
that the cortical location of the sites of maximum slope
was associated with higher slope values at these sites
(P ? 0.04).
Comparative Mapping of ADC and23Na Slope
[Na?]braccumulation in different cortical and subcorti-
cal areas quantitated by23Na MRI was juxtaposed with
the ADC deficit calculated as a ratio of ipsilateral to
homotopic contralateral ROIs, ADCi/ADCc. In contrast
to the observation of “hot spots” of [Na?]brincrease by
23Na MRI, the ADC deficit in the ischemic area was
mostly homogeneous. ADCi/ADCc was 0.63 ? 0.07
(mean ? SD) without statistically significant variations
(P ? 0.7) between different ischemic regions (cortex, CP,
sites of maximum slope) and stable over time (except for
some parts of ischemic CP in rat #6, where ADCi/ADCc
decreased approximately from 0.8 to 0.5 between the
first and second ADC mapping). Figure 1a demon-
strates mostly homogeneous ADC deficit in the isch-
emic area. Figure 3 shows that the values of23Na slope
and ADCi/ADCcmeasured within the same ischemic
ROIs did not correlate either in individual brain or be-
tween the sites of maximum slope in different animals.
Table 3 illustrates this point for all individual brains. In
addition, the [Na?]brdynamics in the individual brains
was approached independently by direct measurement
of [Na?]brat the end of experiment using flame photom-
etry. In this case, the correlation between ADCi/ADCc
and [Na?]brin the same ROIs (also shown in Table 3)
was absent in all rats but one. The only statistically
significant correlation (rat #6) was positive (R ? 0.93),
which suggested the possibility that more Na?accumu-
lated in the regions with moderate ADC deficit than
with the more severe ADC deficit. Thus, in the ROIs
characterized as ischemic by the ADC criterion, neither
the fastest [Na?]brincrease nor the highest [Na?]brat
the end of experiment were accompanied by the stron-
gest ADC depression.
Maximum Na?Imbalance at the Stroke Periphery
After the borders of ischemic core were defined by ADC,
MAP2 staining, and by the reflectivity changes, the site
with a maximum rate of [Na?]br increase within the
ischemic core was identified by23Na MRI as a 3D iso-
contour ROI at the 90% level of the maximum slope. A
Figure 2. Na?accumulation in a typical ischemic brain mon-
itored by23Na MRI. [Na?]brand corresponding linear regres-
sions are shown for the ROIs in ischemic cortex ([Na?]i, cir-
cles), homotopic normal cortex ([Na?]c, squares), and the site
of the maximum slope ([Na?]m, diamonds) in the rat #5. Ta,
time after MCAO.
Maximum23Na Slopes Observed After MCAO
23 ? 6
10 ? 1
46 ? 2
20 ? 1
16 ? 1
14 ? 2
22 ? 1
23 ? 3
aTd, duration of the experiment after MCAO.
bCorrelation coefficient (R2) and statistical significance (P, for the slope difference from zero) of the linear fit.
cThe23Na MRI slope data were excluded because of the movement in the animal holder assembly.
dFit parameters are not shown as lacking statistical significance.
NA ? not available.
Inhomogeneous Na Accumulation in Stroke21
position of the center of mass of this isocontour ROI was
determined using a corresponding tool in AMIDE. In all
8 animals amenable to analysis (out of 10), the site of
the maximum slope was located near the ischemic core
periphery, i.e., within 30% of the overall lesion exten-
sion. Its positions relative to the lesion were dorsolat-
eral (n ? 3), ventral (n ? 3) or caudal (n ? 2). In the two
other rats, the data were excluded because of animal
movement (rat #2) and the limited extent of the isch-
emic area (rat #4). The average distance between the
centers of mass (as determined in AMIDE) of the lesion
and of the ROI of the maximum slope was 4.1 ? 0.9 mm
(mean ? SEM). Figure 4 presents two examples of the
maximum slope location in ischemic core periphery.
The present study demonstrated that23Na MRI is more
sensitive than ADC for assessing heterogeneity within
the ischemic core in an animal model of permanent
focal stroke. This result is based on the systematic
comparison of dynamics of Na?imbalances with ADC
values within the ischemic core in the absence of reper-
fusion. The heterogeneity of these imbalances suggests
the role of collateral circulation in the physiology and
pathology of ischemic stroke, because the increase in
[Na?]bris due to a delicate interaction of continuing Na?
delivery through trickle flow and impaired egress from
deficit (ADC ratio of ipsilateral to homotopic contralateral
ROIs, ADCi/ADCc) in the same brain ROIs in ischemia. a: In
the typical brain (rat #6), ADC deficit shows no correlation with
slope in ROIs characterized as ischemic by ADC (i.e., ADCi/
ADCc? 0.8). b: ADC deficit in the ROIs of maximum slope of
different rats shows no correlation with the maximum slope
values. Each data point corresponds to an individual animal
for which both parameters were available (n ? 7).
23Na slope (the rate of [Na?]brincrease) and ADC
ADC Deficit (Defined as ADCi/ADCc) and Na?Accumulation in
0.20 ? 0.15
?0.80 ? 0.04
0.60 ? 0.13
?0.05 ? 0.14
?0.20 ? 0.13
0.55 ? 0.09
0.09 ? 0.16
0.54 ? 0.09
0.30 ? 0.06
?0.70 ? 0.11
0.93 ? 0.04
0.58 ? 0.09
0.40 ? 0.10
?0.05 ? 0.18
*Na?accumulation was characterized either by the [Na?]brincrease
rate measured by MRI (Slope, left columns) or, independently, by
[Na?]brat the end of experiment as measured directly by emission
flame photometry ([Na?]EFPright columns). The data were obtained
from ROIs characterized as ischemic by ADC (i.e., ADCi/ADCc? 0.8).
aFor the rat no. 3, ADC data were unavailable.
bSlope values are not statistically significant.
R ? correlation coefficient ? SD, P ? statistical significance of the
correlation, NA ? not available.
Figure 4. Two examples of Na?accumulation at the periphery
of the focal stroke in the rat. ROIs of maximum slope were
maximum slope and are shown as hatched areas. a: Rat #7:
coronal ADC image with the ischemic region (black mask)
defined as ADC ? 500 ?m2/s in the ipsilateral (left) hemi-
sphere. b: Rat #6: coronal section of the 3D reconstruction
from thin slice brain images, which was aligned with brain
MRI. The ischemic region (outlined) was defined by the surface
reflectivity changes. ROIs of maximum slope correspond to the
slope ranges of 13.5 to 15.0%/h (a) and 14.4 to 16.0%/h (b).
23Na MRI at an isocontour level of 90% of the
22Yushmanov et al.
edema because of swelling. Although energy depletion
is a primary cause of cytotoxic brain edema, the result-
ing disturbances of water and ion homeostasis are me-
diated by residual or collateral circulation, which act, in
particular, as a source of Na?(17,18,33).
In agreement with earlier studies (14,16–19), a linear
increase in [Na?]br was observed during evolution of
cerebral ischemia between 1.1 and 7.3 h after MCAO
(Fig. 2). Effects of focal ischemia may vary with the
brain region (22). The data showed that ischemic cortex
was a more favorable location than CP for Na?accumu-
lation: (i) the sites of maximum slope were found in the
cortex more often (n ? 7) than in CP (n ? 2), and (ii) the
sites of maximum slope located in the cortex had typi-
cally higher slope values than the sites of maximum
slope in CP. These differences may be attributable to
the peculiarities in collateral circulation in the two
brain regions. Even after total occlusion of main nour-
ishing artery, the cortical pial vascular network is able
to provide a collateral supply of blood flow at the border
of that vascular territory (21). On the other hand, the
blood supply to the subcortical CP is of the collateral-
lacking “end artery” type (20). This difference might be
of relevance for [Na?]brincreases because the higher
trickle flow through the collateral network in ischemic
cortex (and less trickle flow in CP) may explain the
higher slope in the cortex.
The possibility of relation of the [Na?]brincrease to the
ADC decrease was examined. No correlation was ob-
served, however, between the slope values or [Na?]br
and ADCi/ADCcin the same ROIs, both between ani-
mals (Fig. 3) and within the same brain (Table 3). The
data in Table 3 show that within the same brain, the
regions of the strongest Na?accumulation did not co-
incide with the most ADC-depressed regions. Moreover,
the variations in ADCi/ADCcwithin the ischemic area of
the brain, although present, were not statistically sig-
nificant. Several previous reports on the fall in ADC
correlated with severity of brain perfusion deficit
(8,10,34) owed, probably, to a better MR sensitivity: (i)
at 4.7 T compared with 3 T in the present study (8,10),
or (ii) in a human brain versus small rat brain (34).
Thus,23Na slope mapping in the ischemic brain pro-
vided better description of the heterogeneity in the isch-
emic core compared with the ADC mapping in particu-
lar experimental setting of our study.
Figure 4 further demonstrates that the site of maxi-
mum slope tends to be located at the periphery of the
ischemic core. Previously, in the MCAT model yielding a
purely cortical stroke (17), the maximum Na?increase
also appeared at the edge of the infarct region.23Na MRI
in nonhuman primate focal cerebral ischemia revealed
the same trend (35). This supports the concept that the
increase in [Na?]broccurs at a maximum rate at the site
of maximum Na?delivery via trickle flow or, alterna-
tively, at the site of maximum swelling and most limited
Na?egress. Supposedly edema is more severe at the
edge of the ischemic region, driven by the more avail-
able collateral flow delivering more Na?(33). Ion imbal-
ances at the edges of the ischemic region manifested
themselves in a more severe decrease of [K?]brin gerbils
(33) and in the MCAT model in rats (36). It should be
underscored that the trickle flow supplying Na?to the
developing “edge edema” is still below the ischemic
threshold, so this edge region belongs to the ischemic
core and must not be confused with ischemic penum-
bra. The presence of slightly more trickle blood supply
may modulate the severity of ischemic damage.
Thus, the rate of [Na?]br increase as measured by
23Na MRI may serve as a complementary tool in assess-
ing ischemic damage and treatment planning. A reliable
regional quantitation of [Na?]br increase in ischemia
was demonstrated using23Na MRI with a small voxel
size in a small animal model. It is reasonable to expect
even better accuracy in the clinical setting where the
requirements to the signal-to-noise ratio imposed by
the voxel size are less demanding.
In conclusion,23Na MRI revealed heterogeneity in the
rate of [Na?]brincrease, or slope, within the ischemic
core in the rat brain. The maximum rate of [Na?]br
accumulation was at the periphery of the ischemic core.
The fastest [Na?]brincrease occurred preferentially in
the cortex rather than CP. The accumulation of Na?by
23Na MRI provides an enhanced physiological charac-
terization of ischemic lesion not available by ADC.
The authors thank Jayashree Kanchana for early ef-
forts in data processing, and Jayjayantee Dasgupta for
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