Diffusion tensor imaging of time-dependent axonal and myelin
degradation after corpus callosotomy in epilepsy patients
Luis Concha,aDonald W. Gross,bB. Matt Wheatley,cand Christian Beaulieua,*
aDepartment of Biomedical Engineering, Faculty of Medicine and Dentistry, 1098 Research Transition Facility,
University of Alberta, Edmonton, AB, Canada T6G 2V2
bDivision of Neurology, Department of Medicine, University of Alberta, Edmonton, AB, Canada T6G 2V2
cDivision of Neurosurgery, Department of Surgery, University of Alberta, Edmonton, AB, Canada T6G 2V2
Received 15 December 2005; revised 11 March 2006; accepted 10 April 2006
Available online 9 June 2006
Axonal degeneration of white matter fibers is a key consequence of
neuronal or axonal injury. It is characterized by a series of time-related
events with initial axonal membrane collapse followed by myelin
degradation being its major hallmarks. Standard imaging cannot
differentiate these phenomena, which would be useful for clinical
investigations of degeneration, regeneration and plasticity. Animal
models suggest that diffusion tensor magnetic resonance imaging (DTI)
is capable of making such distinction. The applicability of this
technique in humans would permit inferences on white matter
microanatomy using a non-invasive technique. The surgical bisection
of the anterior 2/3 of the corpus callosum for the palliative treatment of
certain types of epilepsy serves as a unique opportunity to assess this
method in humans. DTI was performed on three epilepsy patients
before corpus callosotomy and at two time points (1 week and 2–4
months) after surgery. Tractography was used to define voxels of
interest for analysis of mean diffusivity, fractional anisotropy and
eigenvalues. Diffusion anisotropy was reduced in a spatially dependent
manner in the genu and body of the corpus callosum at 1 week and
remained low 2–4 months after the surgery. Decreased anisotropy at 1
week was due to a reduction in parallel diffusivity (consistent with
axonal fragmentation), whereas at 2–4 months, it was due to an
increase in perpendicular diffusivity (consistent with myelin degrada-
tion). DTI is capable of non-invasively detecting, staging and following
the microstructural degradation of white matter following axonal
D 2006 Elsevier Inc. All rights reserved.
Keywords: DTI; Wallerian degeneration; MRI; Tractography; Epilepsy
Wallerian degeneration (WD), described originally in 1850
(Waller, 1850) and extended by Ranvier (Ranvier, 1878) and
Ramo ´n y Cajal (Ramo ´n-y-Cajal, 1928), is characterized by a series
of events caused by neuronal injury that ultimately lead to the
fibrosis and atrophy of the affected neuronal fibers. Such changes
occur both upstream and downstream from the site of the lesion
and therefore produce axonal changes in locations distant to the
primary lesion. In the central nervous system (CNS), the acute
phase of degeneration is composed primarily of fragmentation and
dying-back of the axons (George and Griffin, 1994b; Kerschen-
steiner et al., 2005), whereas the chronic stage is characterized
mainly by the slow and progressive degradation and phagocytosis
of the myelin sheaths (George and Griffin, 1994a).
The demonstration, characterization and staging of the changes
seen in axonal degeneration by means of a non-invasive approach
would be of great importance in the clinical setting. In peripheral
nerve degeneration, clinical magnetic resonance imaging (MRI)
shows T2 signal hyperintensity of the nerve in sites distant from
the precipitating injury as soon as 24 h from onset (Bendszus et al.,
2004). However, due to the slow and progressive nature of axonal
degeneration in the CNS, T2 signal changes distant from the lesion
are not evident during the first 4 weeks (Kuhn et al., 1989;
Khurana et al., 1999). At 4–14 weeks following injury, the white
matter tracts undergoing degeneration become hypo-intense on T2-
weighted images due to loss of myelin proteins (whereas myelin
lipids remain intact), which produces a hydrophobic environment.
As the myelin lipids are digested and gliosis ensues, the tissue
becomes hydrophilic, causing increased signal intensity on T2-
weighted images (Kuhn et al., 1989). Using magnetization transfer
imaging, Lexa et al. (1993) demonstrated abnormalities in feline
white matter within the first 2 weeks of degeneration, prior to the
appearance of T2 changes.
In the last decade, there has been great interest in studying the
microstructural environment of neural tissues by measuring the
anisotropic diffusion of water molecules via MRI (Moseley et al.,
1990). Normally, axonal membranes and myelin pose barriers to
water displacement, such that water preferentially diffuses along
the direction of the axons (Beaulieu, 2002). Given that the
structural integrity of the axons governs the uneven displacement
of water molecules (i.e., anisotropic diffusion), it is feasible to
utilize DTI as a means to obtain information on the axonal state. As
1053-8119/$ - see front matter D 2006 Elsevier Inc. All rights reserved.
* Corresponding author. Fax: +780 492 8259.
E-mail address: email@example.com (C. Beaulieu).
Available online on ScienceDirect (www.sciencedirect.com).
NeuroImage 32 (2006) 1090 – 1099
axons degenerate and break down with subsequent degradation of
myelin, the barriers that normally hinder the diffusion of water
across the axons disappear, allowing a more spatially uniform
profile of water displacement (i.e., isotropic diffusion) (Beaulieu et
Previous diffusion MRI studies have demonstrated axonal
degeneration in animal peripheral (Beaulieu et al., 1996; Stanisz
et al., 2001) and central (Schwartz et al., 2003; Song et al.,
2003; Schwartz et al., 2005) nervous systems. There is
considerable evidence showing that myelin is a barrier to water
diffusion and that its degradation (Beaulieu et al., 1996; Song et
al., 2005) or absence (Gulani et al., 2001; Song et al., 2002)
causes an increase in diffusivity perpendicular to the long axis of
the fibers, a phenomenon that occurs rather late in the
degenerative process. This abnormal diffusion pattern, consistent
with chronic degeneration, has been demonstrated in humans
(Pierpaoli et al., 2001; Glenn et al., 2003) and could be the
underlying reason for the low diffusion anisotropy described in
other series (Wieshmann et al., 1999; Werring et al., 2000;
Thomalla et al., 2005; Thomas et al., 2005). The acute phase of
the degeneration, invisible to conventional MR imaging and
characterized by the fragmentation of the axons, reduces the
diffusivity parallel to the principal axis of the fibers, as
demonstrated using animal models (Ford et al., 1994; Beaulieu
et al., 1996; Song et al., 2003). In a previous human study,
parallel diffusivity was shown to be reduced 9 T 4 days after
stroke in the pyramidal tract, distally from the primary lesion
(Thomalla et al., 2004). However, to our knowledge, a pro-
spective study examining the time course of the full diffusion
tensor after axonal injury has not been performed in humans.
Corpus callosotomy is a palliative surgical procedure performed
in epilepsy patients with disabling seizures that do not respond to
medication. During the surgery, the corpus callosum (typically the
anterior two thirds) is transected in order to prevent the spread of
epileptic activity from one hemisphere to the other and thus limit the
2002). The well-localized nature of the lesion, as well as the
complete transection of the tract by corpus callosotomy, serves as a
unique opportunity to study the evolution of axonal degeneration in
vivo in a single white matter tract of considerable dimensions, as
compared to its pre-surgical state.
course of water diffusion during degeneration of axons in a large
white matter bundle, secondary to a well-localized and complete
injury, namely, corpus callosotomy in epilepsy patients, and (ii) to
relate the diffusion abnormalities with the known underlying stages
of axonal degeneration in the human brain.
Subjects and methods
Approval of the research protocol was obtained from the
University of Alberta Health Research Ethics Board and informed
consent was obtained from all participants.
Three patients with medically intractable epilepsy, with the
predominant seizure pattern being drop attacks, as well as one
healthy individual (27 years old), were included in the study. Drop
attacks are characterized by sudden loss of control over muscle
tone (either tonic or atonic), which causes the individual to
abruptly fall to the ground.
Patient 1, a 40-year-old woman, suffered tonic drop attacks and
underwent investigation and subsequent left frontal lobe resection
in 1995, which provided little clinical improvement. Clinical MRI
demonstrated the left frontal cavity secondary to her prior surgery
without any other obvious abnormalities. EEG video telemetry
failed to demonstrate localized or lateralized ictal or interictal
epileptic abnormalities. She was studied with our imaging protocol
1 week before surgery, and 9, 47 and 120 days following the
Patient 2 (33-year-old male) suffered atonic drop attack
seizures. Clinical MRI, ictal SPECT and EEG video telemetry
failed to lateralize or localize the patient’s epileptic focus. He was
imaged for the present study at days ?7, 6 and 95 from surgery.
Patient 3 (54-year-old male) presented with atonic drop attacks.
His clinical MRI showed a large regional malformation of cortical
Fig. 1. Diffusion tensor tractography of the corpus callosum prior to corpus callosotomy. (A) The full extent of the corpus callosum of Patient 2 was depicted
using tractography on the pre-operative DTI data set and is shown overlaid on a midsagittal, high-resolution, T1-weighted image. The genu, body and splenium
of the corpus callosum were virtually dissected and further analyzed (highlighted in orange). (B) The genu and body of the corpus callosum are shown overlaid
on a coronal slice in a three-dimensional view similar to the one used in Figs. 2–5.
L. Concha et al. / NeuroImage 32 (2006) 1090–1099
development (polymicrogyria) of the left central region that was
not considered amenable to surgical resection. He was imaged at
days ?7, 8 and 60 relative to callosotomy.
The three patients underwent surgery in which the anterior two
thirds of the corpus callosum were bisected using a parasagittal
approach. The first post-operative DTI data sets were obtained as
soon as surgical staples were removed (6–9 days). At 1 year
clinical follow-up, all patients had experienced a dramatic
improvement in seizure control. Our healthy control was also
imaged three times at 2-month intervals to ensure reproducibility of
the diffusion measurements.
DTI was performed on a 1.5-T Siemens Sonata using a single-
shot EPI-based sequence (63 slices, 2 mm thickness with no inter-
slice gap; TR = 10 s, TE = 88 ms; 6 diffusion directions, b = 1000 s/
mm2; 8 averages; 128 ? 128 matrix, phase partial Fourier = 6/8,
zero-filled to 256 ? 256; FOV = 256 ? 256 mm, acquired voxel
size: 2 ? 2 ? 2 mm3, interpolated to 1 ? 1 ? 2 mm3; scan time =
9:30 min). We also acquired standard and cerebral–spinal fluid
(CSF)-suppressed (FLAIR) T2-weighted fast spin echo images
(voxel size = 0.7 ? 1 ? 5 mm3; TR/TE/TI = 5850/99/0 ms and
7450/94/2400 ms, respectively).
Image processing and data analysis
Diagonalization of the diffusion tensor yielded three
eigenvalues (k1–3) and eigenvectors that provided the three-
dimensional information about the diffusivity of water mole-
cules per voxel (Basser et al., 1994). The largest eigenvalue
(i.e., k1) is equivalent to the diffusivity parallel to the principal
axis of the fibers (k||), whereas perpendicular diffusivity is
expressed as k– = (k2 + k3)/2. Two important diffusion
parameters are derived from the eigenvalues, namely, the mean
apparent diffusion coefficient (ADC), which is the average of
the three eigenvalues and represents the bulk diffusivity of
water molecules, and fractional anisotropy (FA, ranging from 0
to 1), a normalized ratio of diffusion directionality. All the
quantitative diffusion maps were generated in DTIstudio (Johns
Eachpatient’snon-diffusion-weightedimages(i.e., b =0s/mm2)
acquired prior to surgery served as a template to which their
corresponding post-operative images were linearly co-registered
and re-sliced using SPM2 (Ashburner and Friston, 2003). The
image transformations were extended to the quantitative diffusion
measurement maps (i.e., FA, ADC, k||and k–).
The genu, body and splenium of the corpus callosum were
depicted (subdivisions 2, 4 and 7, respectively, according to
Fig. 2. Diffusion measurements obtained at three time points in non-affected tracts. The genu, body and splenium of the corpus callosum were depicted on a
healthy volunteer (A–C) on three occasions, at 2-month intervals (splenium not shown). Notice that diffusion measurements show very little variation
throughout time, as expected in a healthy individual. The splenium of the corpus callosum in Patient 3 (D–F), which was not transected during surgery, was
depicted using tractography on the pre-surgery data set and overlaid on the registered quantitative diffusion maps obtained post-operatively at 8 and 60 days. As
expected, there is little variation in FA measurements throughout time, as compared to the pre-operative values.
L. Concha et al. / NeuroImage 32 (2006) 1090–1099
Witelson (1989) using tractography, a novel computational tech-
nique in which fiber bundles are reconstructed three-dimensionally
according to DTI data (Fig. 1). Tractography was performed on the
tracking (FACT) algorithm (Mori et al., 1999). The fiber-tracking
propagated according to the principal eigenvector until a voxel with
an FAvalue <0.25 was reached or if the tract deviated by more than
70- between adjacent voxels. In order to perform the ‘‘virtual
dissection’’ of the callosal portions, the computed tracts had to
intersect two large, user-defined regions drawn manually on the 2D
images. The first region was located on the midsagittal slice for the
three portions of the corpus callosum studied, whereas the second
was located on the frontal pole on a coronal slice, the white matter
roughly underlying the motor cortex in an axial slice, and the
occipital pole in a coronal slice, for the genu, body and splenium,
respectively. Tractography of the genu in Patient 1 was performed
slightly more posterior than in the other cases in order to avoid the
pre-existing surgical resection of the left frontal lobe.
The tracts obtained from the pre-operative DTI data sets were
used to extract quantitative parameters from the pre-operative and
the registered post-operative diffusion maps. In order to avoid
measuring areas of very low diffusion anisotropy post-operatively,
such as the CSF taking the space the corpus callosum occupied prior
to the surgery, voxels that had an FAvalue below 0.25 in any of the
images were not considered for analysis in any of the data sets from
1. The diffusion measurements of the portions of the tracts within
T20 mm from the midline were averaged to obtain summary
data (voxels with FA <0.25 at any time point were not analyzed
in any of the data sets). This region was most affected following
surgery, with diffusion abnormalities becoming less evident
further away from the lesion. As can be seen in Figs. 2 and 3 and
Table 1, registration and re-slicing of the diffusion maps
introduced little variation in repeated measurements of healthy
tracts. In our control subject, the three portions of the corpus
callosum, measured at three time points, showed an average
coefficient of variability in FA, ADC and k||of 1% whereas that
of k–was 4%. Overall, the body of the corpus callosum showed
the smallest variability in our control subject (FAvaries by 1%),
whereas the genu and splenium showed variations in FA of 2%.
2. In order to look at the spatial distribution of diffusion parameters
parameters from the diffusion maps and non-diffusion-weighted
theypropagatedaway fromthe midline.
3. Thesignalintensityonthenon-diffusion-weightedimages(b =0s/
mm2), being heavily T2 weighted, was used to assess T2 changes
over time. In order to account for scanner variability in different
imaging sessions, we normalized the tissue T2 signal intensity of
each voxel to the CSF signal. CSF signal was defined as the mean
Fig. 3. Mean diffusivity of the corpus callosum in a healthy individual. The genu and body of the corpus callosum are color-coded according to the mean
apparent diffusion coefficient (ADC) as measured at 2-month intervals. There is very little variation in diffusivity over time. The areas of high diffusivity in the
midline, clearly evident in the genu, likely correspond to cerebral spinal fluid pulsation artifacts, as cardiac gating was not employed in this study. The inferior
portion of the body of the corpus callosum also showed the same artifact but is not evident from this viewing angle. These artifacts do not affect our findings, as
these areas are at the transection site and thus were not analyzed.
Diffusion parameters in a healthy control
1st scan +2 months +4 months1st scan +2 months+4 months 1st scan+2 months +4 months
Average diffusion measurements of fractional anisotropy (FA), mean diffusivity (ADC), parallel diffusion (k||) and perpendicular diffusion (k–) along the tracts
within T20 mm from midline. Diffusion parameters for the control subject show minimal variability with repeated measurements.
L. Concha et al. / NeuroImage 32 (2006) 1090–1099
As with diffusion parameters, the portions of the tracts that were
replaced with CSF after the transection were not analyzed.
Because the corpus callosum does not function as a structural
component holding thetwo sides of thebrain together, displacement
of the remnant fibers is not expected, making linear registration an
adequate tool that aids the analysis of serial observations performed
on the same subject. Thus, this method yields reproducible results
(Table 1; Figs. 2 and 3). It is also clear from Figs. 2A–C that
diffusion parameters vary along the length of the healthy corpus
callosum at different distances from the midline due to the gradual
spreading of fibers, as well as the encounter with different fiber
populations with varying orientations (fiber crossing).
Qualitative visualization of diffusion changes
As expected, visual inspection of the color-coded tracts showed
minimal changes of diffusion parameters of the splenium of the
corpus callosum (not transected during surgery),as compared totheir
thereliability ofthe coregistration process.In contrast, weobserved a
surgery in the body and genu of the corpus callosum in all patients
(Figs. 4A–C). Furthermore, FA measurements remained low or
continued to decrease at 2–4 months in the genu and body of the
corpus callosum. The mean diffusivity was unchanged or slightly
a pseudo-normalization or increase at 2–4 months (Figs. 4D–F). k||
2–4 months in all the transected tracts (Figs. 5A–C). On the other
hand, k–was slightly increased at 1 week, but it showed a dramatic
increase at 2–4 months in all the affected tracts (Figs. 5D–F). It is
important to note that the most dramatic changes in diffusion
measurements were seen in the areas closest to the resection,
suggesting a centrifugal spread of the abnormalities. The data
obtained from Patient 1 at 47 days following surgery (data not
shown) showed similar findings to those at 4 months.
Quantitative assessment of diffusion parameters
Analysis of the average diffusion measurements from the
sections of the tracts within T20 mm from the midline showed a
Fig. 4. Diffusion anisotropy and mean bulk diffusivity changes due to axonal degeneration following corpus callosotomy. The genu and body of the corpus
callosum of Patient 3 were depicted using tractography on the pre-surgical DTI data sets (Pre) and overlaid on their registered quantitative diffusion maps
obtained at 1 week and 2 months following corpus callosotomy. In both white matter structures, FA values have decreased considerably at 1 week and remain
low at 2 months (A–C). The mean apparent diffusion coefficient (D–F) shows a slight decrease at 1 week, followed by an increase at 2 months. Recall that any
segment of the tracts with an FA <0.25 at any time point was excluded for analysis in all the time points in order to avoid measuring cerebrospinal fluid filling
the space previously occupied by the tracts at the bisection site. For this reason, the post-operative tracts appear truncated near the midline. For clarity purposes,
these segments are shown in the pre-operative tracts (A and D). The same pattern of changes was seen for the other two patients.
L. Concha et al. / NeuroImage 32 (2006) 1090–1099
similar pattern as that seen qualitatively (Fig. 6). As anticipated, the
splenium of the corpus callosum (not transected) showed variability
similar to that seen in our control subject, with coefficients of
variation of FA equal to 3%, 4% and 2% for Patients 1–3,
respectively. Conversely, in all patients the genu and body of the
corpus callosum showed a considerable decrease in diffusion
anisotropy 1 week after surgeryrelative to their pre-operative values
(reduced by 33 T 6%, range: 27–41%). These two portions showed
range: 34–53%). The affected portions (i.e., genu and body) also
4 monthsshowed atrend towards pseudo-normalization in5/6 tracts
(the callosal body of Patient 1 showed an increased k||relative to its
pre-operative value). Perpendicular diffusivity (i.e., k–) showed a
progressive increase in the affected tracts of all patients, with the
FA reductions were seen as soon as 1 week after the surgery and
remained lowat thesubsequenttime points, themechanisms driving
such decreases were different; namely, a decrease in k||is largely
responsible at 1 week, whereas an increased k–and a near-normal
k||are accountable for the low FA at 2–4 months.
Analysis of diffusion measurements in the genu and body of the
corpus callosum along the tracts as they propagated away from the
pattern of diffusion anisotropy similar to the one reported in a
healthy subject by Mori et al. (2002). Following transection, the
tracts showed a centrifugal spread of diffusion changes, with larger
differences to the pre-operative values in the areas closest to the
lesion (Fig. 7). Indeed, the portions of the tracts within the surgical
lesion itself showed post-operative FAvalues below 0.25, indicating
fluid filling the resection area (these portions were not analyzed).
We did not observe any hypo-intensities on the FLAIR T2-
weighted images either at 1 week or 2–4 months following surgery
in the white matter containing the surgically transected callosal
fibers. On the contrary, increased T2 signal was evident immedi-
ately adjacent to the transection at 1 week, but this hyper-intensity
was not apparent at 2–4 months. However, the white matter distal
to the transection did not show T2 signal abnormalities (neither
reduced nor increased) at any time point following the lesion. In
Patient 1, in which a pre-existing lesion existed (i.e., left frontal
lobectomy performed 10 years prior to our study), the remaining
frontal white matter, belonging partly to the genu, as well as the
contralateral white matter, showed areas of obvious signal hyper-
intensity at all time points.
We qualitatively assessed CSF-normalized T2 signal changes
over time using the non-diffusion-weighted (b = 0 s/mm2) EPI
images (Fig. 8). Similarly to what we observed on the FLAIR T2-
weighted images, a slight increase in T2 signal at 1 week is evident
Fig. 5. Parallel and perpendicular diffusivity before and after corpus callosotomy. The diffusivity of water molecules parallel to the direction of the tracts (i.e.,
k||, A-C) in Patient 3 shows a considerable reduction at 1 week following surgery (as compared to its pre-surgical state) and nearly returns to baseline at 2
months. The sharp decrease in k||seen at 1 week is consistent with the axonal fragmentation occurring at this stage of axonal degeneration. The diffusivity of
water perpendicular to the principal direction of the tracts (k–, D–F), on the other hand, appears only slightly increased at 1 week, but shows a dramatic
increase 2 months following the transection of the axons. Such an increase is consistent with the degradation of myelin, which occurs late in the evolution of
Wallerian degeneration in the central nervous system. Similar findings were seen for the other two patients.
L. Concha et al. / NeuroImage 32 (2006) 1090–1099
in the portions of the transected tracts that are immediately adjacent
to the lesion. However, the extent of the T2 signal changes is more
restricted to the midline, as compared to the changes seen in the
diffusion parameters. At 2–4 months, much of the hyper-intense
T2 signal within the tracts appears to have resolved. The residual
hyper-intensities are not as widespread as the dramatic changes in
the diffusion parameters (Figs. 4 and 5).
Axonal degeneration is characterized by a series of simulta-
neous events. Approximately 30 min from the time of the lesion,
the axons undergo centrifugal disintegration of the cytoskeleton,
which produces sudden and rapid fragmentation of the axons
(George and Griffin, 1994b; Kerschensteiner et al., 2005). This
stage is short-lived, lasting from several hours to days, depending
on the species, length and diameter of the axons, the temperature of
the tissue and the location of the fibers (Lubinska, 1977).
Simultaneous to the cytoskeletal degradation, the myelin sheaths
that surround the axons become less tightly wrapped and
eventually break apart and form ovoids (George and Griffin,
1994a). A couple of weeks following the precipitating injury, the
microglia are activated and digest the axonal and myelin debris.
Myelin phagocytosis in the CNS can last for months and even
years (George and Griffin, 1994a), whereas formation of new
myelin is almost non-existent due to the apoptosis of oligoden-
drocytes in the first few weeks following injury (Crowe et al.,
1997). In the peripheral nerve, in contrast, digestion of the debris is
performed by circulating macrophages, whereas Schwann cells re-
myelinate regenerating axons and the fragmented axons reconnect
(Barron, 2004; Bendszus et al., 2004).
The first stages of axonal degeneration in the CNS are virtually
invisible using conventional MRI. Previous reports have docu-
mented T2 signal hypo-intensity 4 weeks after the precipitating
injury (Kuhn et al., 1989; Khurana et al., 1999). In our present
study, however, the only surgery-related T2 signal changes
observed 1 week following the surgery were hyper-intensities
immediately adjacent to the lesion (Fig. 8). We believe these signal
changes were not directly related to axonal degeneration, but to
tissue edema caused by surgical manipulation as these T2 hyper-
intensities mostly resolved by 2–4 months. Using DTI measure-
ments as surrogate markers of axonal state, the early stages of
degeneration are clearly visible both qualitatively and quantita-
tively in regions distant to the precipitating injury, where neither
edema nor inflammation is expected to occur.
Our study after complete surgical axonal transection confirms
previous reports in patients with stroke that reduction in diffusion
anisotropy following axonal injury precedes T2 signal changes
(Thomalla et al., 2004). We observed reduction of diffusion
Fig. 6. Diffusion parameters in the transected genu and body of the corpus callosum. Average diffusion measurements along the tracts within T20 mm from the
midline. Fractional anisotropy (FA) shows a marked decrease at 1 week following surgery, accompanied by nearly normal bulk diffusivity (ADC). Reduction in
FA at this time point is due to a reduction in parallel diffusivity (k||), with only a slight increase in perpendicular diffusivity (k–), consistent with axonal
degradation. At 2–4 months post-surgery, ADC is elevated and FA shows a further decrease; however, the FA decrease is now due to a marked increase in k–,
consistent with myelin degradation, and a pseudo-normalization of k||.
L. Concha et al. / NeuroImage 32 (2006) 1090–1099
anisotropy as soon as 1 week following surgery, accompanied by
nearly pseudo-normal bulk diffusivity (i.e., ADC). Whether or not
such diffusion changes could have been detected earlier is unknown
because the patients included in this study were not imaged before 1
week due to the presence of surgical staples. At subsequent time
points, diffusion anisotropy showed a further reduction whereas an
increase in ADC was evident.
As demonstrated using an in vitro model of Wallerian
degeneration in frog sciatic nerve, axonal and myelin degeneration
causes a decrease in diffusion anisotropy due to reduced k||and
increased k–(Beaulieu et al., 1996). Myelin has been shown to
modulate perpendicular diffusivity (Ono et al., 1995; Gulani et al.,
2001; Song et al., 2005), although it is not the only factor involved
(Beaulieu and Allen, 1994). In a mouse model of retinal ischemia,
Song et al. (2003) performed serial diffusion measurements of the
optic nerve and showed that k||and k–can differentiate axonal
from myelin damage during the course of degeneration. According
to this animal model, k||shows a significant decrease in the first
days of degeneration, which corresponds to the disintegration of
the axonal microstructure, whereas myelin remains intact. Five
days after the initial injury k–increased, which corresponds to the
degradation of myelin sheaths. Likewise, using an ex vivo animal
model of spinal cord injury, Schwartz et al. (2003) demonstrated
significantly increased k–14 weeks after the injury, accompanied
by reduced k||; as in our study, these diffusion abnormalities were
more severe closer to the injury.
Increases of k–have been shown in chronically degenerated
white matter bundles in humans (Pierpaoli et al., 2001; Glenn et al.,
2003). It is very likely that such an increase in k–drives the
reduction in diffusion anisotropy demonstrated in other human
studies of axonal degeneration in the chronic stage (Wieshmann et
al., 1999; Werring et al., 2000; Thomalla et al., 2005; Thomas et
al., 2005). Thomalla et al. (2004) reported significant reductions in
k||in the pyramidal tract within the first 16 days after onset of
stroke, corresponding to the acute phase of axonal degeneration,
namely, the fragmentation and dying-back of axons. In the former
study, there is also a slight increase in k–, suggesting a transition
between the acute and chronic phases described above.
Fig. 7. Diffusion measurements along the genu of the corpus callosum before and after corpus callosotomy. The genu of the corpus callosum depicted in the
pre-operative data set from Patient 3, served as a path through which the pre- and post-operative diffusion measurements were sampled at 1-mm intervals as
they propagated away from the lesion. The reason for the discontinuity of the plots near the midline is that voxels with an FA value <0.25 at any of the time
points were presumed to be fluid (i.e., complete degradation) and were not included for analysis. Whereas FA shows a decrease as soon as 1 week after surgery
and it is further decreased by 2 months (A), the mechanisms underlying the FA reductions are different for each time point. At 1 week after surgery, k||shows a
marked decrease (C) with a nearly normal k–(D), whereas at 2 months after surgery, k||appears nearly normal (C) but k–shows a considerable increase (D).
The pseudo-normalization of k||, along with an increase of k–, cause an overall increase in the mean bulk diffusivity (i.e., ADC) at 2 months (B). The most
marked changes of diffusion parameters occur in the regions closest to the surgical lesion (particularly within the first T20 mm), suggesting a centrifugal spread
of diffusion abnormalities (and presumably axonal degeneration).
L. Concha et al. / NeuroImage 32 (2006) 1090–1099
Examination of the eigenvalues yields interesting and clinically
relevant information on the underlying causes of reduced anisotropy.
In our study, a reduction in k||was observed 1 week following corpus
callosotomy. At this stage of degeneration, the axons break up into
a model of living transgenic mice (Kerschensteiner et al., 2005). The
dimensions of these fragments in human brain are not known. The
fragmentation of axons creates barriers to the longitudinal displace-
the obstacles to radial diffusivity (namely, the axonal membrane and
the small increase in k–at 1 week. As the myelin sheaths fall apart
and axonal membranes become further degraded, water molecules
become more mobile perpendicular to the axons, resulting in an
increase of k–. Consistent with this phenomenon, we observed a
marked increase of k–in the genu and body of the corpus callosum
the first week following surgery). In addition to the myelin
degradation and subsequent increase in k–, the axonal fragments
are cleared, allowing the water molecules to once again diffuse in the
longitudinal direction, normalizing or even increasing k||.
Our serial observations in patients undergoing a highly selective
surgical lesion within a major white matter tract (the corpus
callosum) demonstrate an excellent temporal relationship between
DTI changes and the expected underlying histological processes
(i.e., decreased k||at 1 week when axonal degradation is expected
and elevated k– at 2 months when myelin degradation is
expected). These findings suggest that analysis of the full diffusion
tensor can provide a measure not only of the structural integrity but
also an indication of the timing of injury and the underlying
histological processes following injury to central nervous system
white matter tracts. Diffusion-weighted images, ADC and FA maps
individually are not sufficient to differentiate axonal versus myelin
degeneration (Song et al., 2003; Tyszka et al., 2006).
Although the sample size in our present study is small, the
changes in diffusion parameters due to axonal degeneration were
marked, whereas they were practically non-existent in the repeated
measurements of non-transected tracts. Corpus callosotomy is a
procedure performed rather infrequently due to its palliative nature
and restricted indications. However, the precise bisection and the
long callosal remnants identified with tractography provide an
opportune human situation for the serial characterization of axonal
and myelin degeneration. The present study demonstrates that
analysis of the full diffusion tensor provides reliable and useful
information on the stages of axonal degeneration in a non-invasive
manner using widely available methodology. Accurate information
about the state of white matter bundles affected by disease or trauma
is likely to prove valuable for prognostic and therapeutic purposes.
Operating and salary support provided by the Alberta Heritage
Foundation for Medical Research and Canadian Institutes of Health
MRI infrastructure from the Canada Foundation for Innovation,
Alberta Science and Research Authority, Alberta Heritage Founda-
tion for Medical Research and the University of Alberta Hospital
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Fig. 8. T2 signal intensity changes after corpus callosotomy. The genu and body of the corpus callosum are color-coded according to the T2 signal intensity of
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