DOI:10.2214/AJR.07.2800
AJR 2008; 191:W52-W57
© American Roentgen Ray Society
Diffusion Tensor Imaging in Idiopathic Acute Transverse Myelitis
Joon Woo Lee1,
Kyung Seok Park2,
Jae Hyoung Kim1,
Ja-Young Choi3,
Sung Hwan Hong3,
Seong-Ho Park2 and
Heung Sik Kang1
1 Department of Radiology, Seoul National University Bundang Hospital,
Gyeonggi-do, Korea.
2 Department of Neurology, Seoul National University Bundang Hospital, 300
Gumi-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 463-707, Korea.
3 Department of Radiology, Seoul National University College of Medicine, Seoul,
Korea.
Received June 29, 2007;
accepted after revision August 9, 2007.
Supported by grant No. 02-2006-040 from the Seoul National University
Bundang Hospital Research Fund.
Address correspondence to K. S. Park
(pks1126{at}chol.com).
WEB
This is a Web exclusive article.
Abstract
OBJECTIVE. Our study was based on our hypotheses that in idiopathic
acute transverse myelitis (ATM), fractional anisotropy (FA) values would be
abnormal not only in the T2-hyperintense lesion but also in the surrounding
normal-appearing spinal cord and that the abnormal FA values in the spinal
cord could be related to clinical outcome.
SUBJECTS AND METHODS. Sagittal diffusion tensor imaging (DTI) was
performed in 10 patients with idiopathic ATM (four men, six women; mean age,
45 years; age range, 20–66 years) and 10 sex- and age-matched normal
volunteers. FA measurements were made in the spinal cord at three levels:
lesion, proximal normal-appearing spinal cord, and distal normal-appearing
spinal cord. The grade of FA decrease (mild, less than 10% decrease [(FA
normal – FA pt) x 100 / FA normal]; moderate,
10–20%; severe, more than 20%) was related to the clinical outcome,
which was determined by a neurologist using Paine's scale of normal, good,
fair, or poor.
RESULTS. Mean FA values in patients were significantly lower than
those in normal volunteers in lesions (0.5328 vs 0.7125, p = 0.002)
and distal normal-appearing spinal cord (0.6676 vs 0.7720, p =
0.0137). All three patients with a mild FA decrease or increase in distal
normal-appearing spinal cord showed a normal or good outcome, but all three
patients with a severe FA decrease in distal normal-appearing spinal cord
showed a fair outcome, among the eight patients to whom steroid treatment was
given.
CONCLUSION. FA values in lesions and in distal normal-appearing
spinal cord significantly decreased in patients with idiopathic ATM, and FA
decrease in distal normal-appearing spinal cord might be related to clinical
outcome.
Keywords: acute transverse myelitis • diffusion tensor imaging • spine
Introduction
Diffusion tensor imaging (DTI) is an MRI technique that evaluates the
scalar properties of the diffusion of extracellular water molecules within
white matter fibers
[1–3].
Fractional anisotropy (FA), a parameter derived from DTI computations,
reflects the global anisotropy of the analyzed structure. FA values depend on
the water diffusion in the extracellular space along the axon fibers
[1–3].
Parameters such as myelination and axonal membrane thickness and changes in
extra- or intracellular components can affect FA values. The closer the FA
value is to 1, the more anisotropic this structure is
[1–5].
DTI provides unique quantitative information pertaining to structural and
orientational features of the CNS tissue. Although DTI is not in routine
clinical use, it has proven to be an invaluable tool for detecting subtle
damage to white matter that appears normal on conventional T2-weighted MR
images
[1–10].
Application of DTI to the brain has provided characterization of
microstructural changes in multiple sclerosis, schizophrenia, dyslexia,
trauma, amyotrophic lateral sclerosis, stroke, white matter injury of preterm
infants, and aging
[10–20].
Applications of DTI to the spinal cord have been reported in multiple
sclerosis, spinal cord compression, cervical spondylosis, and astrocytoma
[1–4,
6–8,
21–26].
It has been previously reported that DTI is more sensitive than T2-weighted
imaging for detecting white matter involvement
[1–4,
6–8,
10,
23–25,
27].
A study has shown the application of DTI in idiopathic acute transverse
myelitis (ATM) [27]. It was
the study about DTI in inflammatory diseases of the spinal cord that included
multiple sclerosis, sarcoidosis, transverse myelitis, and
polyradiculoneuritis, but there were only two cases of transverse myelitis
[27]. Except for that study,
there have been no reports of the application of DTI in idiopathic ATM. To our
knowledge, there has also been no study relating FA values in the spinal cord
with the clinical outcome of the patient after treatment.
Our hypotheses were that in idiopathic ATM, FA values would be abnormal,
not only in the T2-hyperintense lesion but also in the surrounding
normal-appearing spinal cord because DTI has been known to be more sensitive
for detecting subtle white matter change than T2-weighted imaging and that the
abnormal FA values in the spinal cord could be one of the severity indicators
related to clinical outcome because DTI is known to reflect microstructural
white matter changes.
Subjects and Methods
Subjects
This study was approved by the institutional review board, and informed
consent was obtained. Patients with idiopathic ATM with a lesion in the
cervical spinal cord who visited the department of neurology in our hospital
during a 1-year period from January 2006 to December 2006 were prospectively
enrolled in this study. Exclusion criteria were uncertain diagnosis for
idiopathic ATM, a lesion in the thoracic spinal cord, and refusal to undergo
the DTI scan. Clinical assessment of patients was performed by one
neurologist. The diagnosis was based on the criteria of the Transverse
Myelitis Consortium Working Group
[28]. According to the
criteria, nine patients had definite idiopathic ATM and one had possible
idiopathic ATM. There were four male and six female patients, whose age ranged
from 20 to 66 years (mean age, 45 years). The interval between symptom onset
and MRI ranged from 0.25 to 11 months (mean, 3.8 months). All but two patients
were treated with IV methylprednisolone (1 g/d for 5 days). MRI was performed
before methylprednisolone treatment in eight patients. Clinical outcome after
IV methylprednisolone treatment was evaluated by one neurologist using Paine's
scale: 1, normal, full recovery; 2, good, gait essentially normal but mild
urinary symptoms or minimal sensory and upper motor neuron sign; 3, fair, mild
spasticity but independent ambulation, urgency of micturition, or constipation
with some sensory signs; 4, poor, unable to walk or severe gait disturbances,
absence of sphincter control, and sensory deficit
[29]. Sex- and age-matched
normal volunteers with no history of neurologic disorders and with a normal
neurologic examination underwent the same scanning procedure as the patients.
Clinical features of the patients are outlined in
Table 1.
DTI Technique
A 1.5-T MR scanner (Gyroscan Intera, Philips Healthcare) was used. Head and
neck coils were applied to all subjects. The sensitivity encoding (SENSE)
single-shot echo-planar imaging (EPI) with pulse sequence and SENSE factor 2
was used for the sagittal DTI in the cervical spinal cord with b value, 900
s/mm2; number of diffusion gradient directions, 15; number of
excitations, 5; and slice thickness, 4 mm
[30]. The diffusion gradient
strength was 30 mT/m, the foldover direction was anteroposterior, and the fat
shift direction was posterior. The TR/TE was 7,000/100; matrix, 112 x
128; and field of view, 224 x 224 mm. The slice thickness was 4 mm.
Measurements
After sending all source images of the DTI to a PC, one radiologist who was
blinded to clinical outcomes and history measured FA in the cervical spinal
cord by PRIDE software (Philips Healthcare). In patients, FA measurements were
made in the spinal cord at three different levels (lesion, proximal
normal-appearing spinal cord, and distal normal-appearing spinal cord) by
using regions of interest (ROIs) and the most accurate B0 axial image. Lesion
level was determined as the center of a T2-hyperintense lesion in the spinal
cord. Proximal normal-appearing spinal cord level was determined as a level
with the T2-isointense spinal cord and one spine segment proximal to the
cranial end of the T2-hyperintense spinal cord lesion. Distal normal-appearing
spinal cord level was determined as a level with the T2-isointense spinal cord
and one spine segment distal to the caudal end of the T2-hyperintense spinal
cord lesion (Figs. 1A,
1B,
1C, and
1D). Special attention was paid
in ROI selection to avoid partial volume effect, magnetic susceptibility
effects, and motion artifacts. To maintain the same ROI for the same spinal
cord, we carefully placed a circular ROI in side the spinal cord as large as
possible and without containing CSF outside the spinal cord. After FA
measurements, the three levels of measurements were recorded in each patient,
and FA measurements were also made at the same levels on the matched normal
volunteers. The ROIs in patients and normal volunteers were matched at each
level. Levels and FA values are summarized in
Table 2.
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Fig. 1A —29-year-old woman with idiopathic acute transverse myelitis
(patient no. 3). T2-weighted sagittal image shows hyperintense lesion in
spinal cord from C2 to C6 level. Lesion level was determined to be center of
T2-hyperintense lesion in spinal cord. Proximal normal-appearing spinal cord
level (PNLSC) was determined as level with T2-isointense spinal cord and one
spine segment proximal to cranial end of T2-hyperintense spinal cord lesion.
Distal normal-appearing spinal cord level (DNLSC) was determined as level with
T2-isointense spinal cord and one spine segment distal to caudal end of
T2-hyperintense spinal cord lesion.
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Fig. 1B —29-year-old woman with idiopathic acute transverse myelitis
(patient no. 3). Diffusion tensor image of fractional anisotropy (FA) map
(B) and color tensor map image (C) show decreased FA in lesion.
FA measurements were made in spinal cord at three different levels (lesion,
proximal normal-appearing spinal cord, and distal normal-appearing spinal
cord) in most accurate B0 axial image (D).
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Fig. 1C —29-year-old woman with idiopathic acute transverse myelitis
(patient no. 3). Diffusion tensor image of fractional anisotropy (FA) map
(B) and color tensor map image (C) show decreased FA in lesion.
FA measurements were made in spinal cord at three different levels (lesion,
proximal normal-appearing spinal cord, and distal normal-appearing spinal
cord) in most accurate B0 axial image (D).
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Fig. 1D —29-year-old woman with idiopathic acute transverse myelitis
(patient no. 3). Diffusion tensor image of fractional anisotropy (FA) map
(B) and color tensor map image (C) show decreased FA in lesion.
FA measurements were made in spinal cord at three different levels (lesion,
proximal normal-appearing spinal cord, and distal normal-appearing spinal
cord) in most accurate B0 axial image (D).
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TABLE 2: Fractional Anisotropy (FA) Measurement in 10 Patients with Idiopathic
Acute Transverse Myelitis (ATM) and 10 Age- and Sex-Matched Normal
Volunteers
|
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With FA values on matched levels of patients (FA pt) and normal
volunteers (FA normal), the percentage of decrease of FA values was
calculated as follows:
On the basis of the percentage of decrease, the FA decrease was graded as mild
(less than 10% by percentage of decrease), moderate (10–20% by
percentage of decrease), severe (more than 20% by percentage of decrease). One
neurologist and one radiologist compared the grade of FA decrease with the
clinical outcome.
Statistical Analysis
Wilcoxon's matched-pairs signed rank test was used for evaluating
differences of FA values in the spinal cord at each matched level (lesion,
proximal normal-appearing spinal cord, and distal normal-appearing spinal
cord) between patients and normal volunteers. Graphpad Instat software
(Graphpad Software) was used for statistical analysis.
Results
Levels and FA values in 10 patients with idiopathic ATM are summarized in
Table 2. FA values in the
spinal cord at three matched levels (lesion, proximal normal-appearing spinal
cord, and distal normal-appearing spinal cord) in 10 patients with idiopathic
ATM and in 10 age- and sex-matched normal volunteers are summarized in
Table 3. Mean FA values for the
lesion level in patients were significantly lower than those in normal
volunteers (0.5328 vs 0.7125, p = 0.002). Mean FA values for distal
normal-appearing spinal cord in patients were also significantly lower than
those in normal volunteers (0.6676 vs 0.7720, p = 0.0137). However,
mean FA values of spinal cord in proximal normal-appearing spinal cord were
not significantly different between patients and normal volunteers (0.6443 vs
0.6307, p = 0.693). FA values in the spinal cord tended to be lower
in the upper cervical level in the normal volunteers. At the C1 level, FA
values in the spinal cord were lower than 0.6 in all normal volunteers and
patients.
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TABLE 3: Fractional Anisotropy (FA) Values in Spinal Cord at Matched Three Levels
(Lesion, Proximal Normal-Appearing Spinal Cord, and Distal Normal-Appearing
Spinal Cord) in 10 Patients with Idiopathic Acute Transverse Myelitis (ATM)
and 10 Age- and Sex-Matched Normal Volunteers
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Comparison of FA decrease in the spinal cord of patients and outcome
results are summarized in Table
4. Among eight patients to whom steroid treatment was given, there
were three patients with normal, one with good, and four with fair results.
Among four patients with a fair clinical outcome, severe FA decrease was shown
at the lesion level in all (100%) and in distal normal-appearing spinal cord
in three (75%). Among four patients with normal or good clinical outcome,
severe FA decrease was shown in two (50%) at the lesion level and none (0%) in
distal normal-appearing spinal cord. In terms of FA decrease at the lesion
level, FA decrease was mild in one, moderate in one, and severe in eight. Two
cases with mild or moderate FA decrease at lesion level had a normal clinical
outcome. Among eight cases with severe FA decrease at the lesion level, only
two cases showed normal (patient no. 8) or good outcome (patient no. 3, Figs.
1A,
1B,
1C, and
1D), which showed moderately
decreased FA or increased FA in distal normal-appearing spinal cord. In terms
of FA decrease in distal normal-appearing spinal cord and the clinical outcome
of eight patients to whom steroid treatment was given, all three patients with
mild FA decrease or FA increase in distal normal-appearing spinal cord showed
a normal or good clinical outcome, two patients with moderate FA decrease in
distal normal-appearing spinal cord showed a normal or fair outcome, but all
three patients with severe FA decrease in distal normal-appearing spinal cord
showed a fair clinical outcome. There was no relation of FA decrease in
proximal normal-appearing spinal cord to clinical outcome.
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TABLE 4: Comparison of Fractional Anisotropy (FA) Decrease in the Spinal Cord of
10 Patients with Idiopathic Acute Transverse Myelitis (ATM) and Clinical
Outcome
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Discussion
Acute noncompressive myelopathies were first recognized in the 19th
century, and early pathologic studies identified both inflammatory and
vascular causes
[31–33].
ATM is a syndrome characterized by anterior and posterior (hence transverse)
spinal cord impairment resulting in weakness, a lowered sensory level, and
autonomic dysfunction [28,
34–36].
Possible causes include vascular, infectious, neoplastic, collagen vascular,
iatrogenic, and autoimmune abnormalities. In addition, the syndrome can occur
as part of a CNS demyelinating disease such as neuromyelitis optica or,
uncommonly, typical multiple sclerosis. After such recognizable causes are
excluded, idiopathic ATM can be diagnosed
[28,
29,
34–36].
On the basis of expert opinion, the Transverse Myelitis Consortium Working
Group proposed diagnostic criteria for idiopathic ATM
[28]. The clinical
requirements include bilateral sensory, motor, or autonomic dysfunction
referable to the spinal cord, with a clearly defined sensory level that
progresses to the nadir over 4–21 days from onset. Neuroimaging,
preferably MRI, is used to eliminate structural causes. Evidence to support an
inflammatory cause is also required; this may be shown through MRI evidence of
gadolinium enhancement within the cord or by CSF findings of pleocytosis or
IgG index elevation. Patients meeting all diagnostic criteria are considered
to have definite idiopathic ATM, whereas those who do not meet the MRI or CSF
criteria for inflammation have possible idiopathic ATM. The intent of these
criteria is to identify a relatively homogeneous patient cohort for research.
In this study, we used these criteria to identify cases with idiopathic ATM
[28].
According to our study, FA values in the lesion level and distal
normal-appearing spinal cord significantly decreased in the patients compared
with normal volunteers. Patients with mild FA decrease or FA increase in
distal normal-appearing spinal cord showed a better clinical outcome than
patients with severe FA decrease in the distal level. Patients with mild or
moderate FA decrease at the lesion level also showed a better clinical
outcome. The mechanism of FA decrease in the normal-appearing spinal cord
distal to the lesion in idiopathic ATM is unknown. According to the study of
Renoux et al. study [27], FA
value was different in the surrounding normal-appearing spinal cord in
inflammatory disease of the spinal cord. They suggested that in the spinal
cord, decreased FA values may be related to an increase of the extracellular
space (dysmyelination, axonal loss, unpacking of white matter fibers, and so
forth) as well as a decrease of the intracellular space (edema). In a study of
DTI in multiple sclerosis, Hesseltine et al.
[6] suggested that the
mechanism by which multiple sclerosis affects the normal-appearing spinal cord
is unknown and may relate to wallerian degeneration, a primary ischemic or
vasculitic process, or early local demyelination. According to our study,
severe decrease of FA in distal normal-appearing spinal cord would suggest
severe damage of myelinated fiber in the lesion, causing more severe wallerian
degeneration at the distal level, and might be used as one of the outcome
predictors after treatment.
Wallerian degeneration refers to antegrade degeneration of axons and their
accompanying myelin sheaths resulting from injury to the proximal portion of
the axon or its cell body
[37]. Wallerian degeneration
begins with disintegration of the axon and its myelin sheath after axonal
connection with the neuronal cell body has been interrupted. It is progressive
over a period of weeks to months, followed by an extended period of removal of
the breakdown products of the myelinated axons. In the spinal cord, one would
expect to see wallerian degeneration in the dorsal columns above the lesion
and in the corticospinal tracts below the lesion
[37]. Why the FA change was
more severe in the distal slice than the proximal slice in our study is
uncertain. We found that FA values in the spinal cord of the proximal level in
the normal volunteers were already lower than the distal level, so the
difference of FA values between normal volunteers and patients was minimal.
The proximal cervical spinal cord, like the C1 level, has more
multidirectional fibers in which the FA value can be decreased. A study for
evaluating the normal range of FA values of the spinal cord according to each
spinal level in each age group is also needed to adapt DTI to clinical
practice.
According to our study, a normal-appearing spinal cord showed increased FA
value in some cases. A similar result was also reported in the Renoux et al.
study [27]. According to that
study, FA value could be increased in surrounding normal-appearing spinal cord
in one third of patients with myelitis. They suggested that increased FA could
be caused either by intracellular edema with inflow of the extracellular water
in an axon or by Schwann's cells or decreased extracellular space due to
cellular infiltration by inflammatory cells.
We could not know the normal cervical spinal cord FA value for a given
patient because there could be abnormal pathology in the cervical spinal cord
that was normal-appearing on T2-weighted imaging. Hence, we could not use an
internal reference, for example, comparing the FA value at the lesion with a
normal part of the spinal cord in the same patient. We think that normal FA
values measured from large populations of normal volunteers of different age
groups and sex will be necessary for future clinical application.
Our study has some limitations. First, the number of patients was
relatively small because idiopathic ATM is not common. Further study with a
large number of patients will be needed. Second, visual analysis of an FA map
image was not performed. Currently it is difficult to assess regional
differences of subtle change of FA values in the normal-appearing spinal cord
by an FA map image only. If axial resolution of DTI in the spinal cord
improves in the future, it will be possible to evaluate changes of FA values
in a normal-appearing spinal cord by an FA map image. This study is a
preliminary study suggesting the possible application of DTI in spinal cord
pathology. Third, ROI measurement has potential bias because ROI measurement
was performed by manual placement. To overcome this limitation and maintain
the same ROI for the same spinal cord, we carefully placed a circular ROI
inside the spinal cord as large as possible and without containing CSF outside
the spinal cord.
In conclusion, FA values in lesion and in distal normal-appearing spinal
cord significantly decreased in patients with idiopathic ATM, and FA decrease
in distal normal-appearing spinal cord might be related to the clinical
outcome.
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Abstract
Introduction
