Published ahead of print on November 1, 2007
doi: 10.3174/ajnr.A0802
American Journal of Neuroradiology 29:363-365, February 2008
© 2008 American Society of Neuroradiology
Technical Note
BRAIN
Comparison of Optimized and Standard Diffusion-Weighted Imaging at 1.5T for the Detection of Acute Lesions in Patients with Transient Ischemic Attack
A. Bertranda,
C. Oppenheima,
C. Lamyb,
S. Rodrigoa,
O. Naggaraa,
J.L. Masb and
J.F. Medera
a Department of Neuroradiology, Université Paris Descartes, Centre Hospitalier Sainte-Anne, Paris, France
b Department of Neurology, Université Paris Descartes, Centre Hospitalier Sainte-Anne, Paris, France
Please address correspondence to Catherine Oppenheim, Departement of Neuroradiology, Université Paris Descartes, Centre Hospitalier Sainte-Anne, 1 rue Cabanis, 75014 Paris, France; e-mail: c.oppenheim{at}ch-sainte-anne.fr
SUMMARY: The high rate of normal diffusion-weighted imaging
(DWI) in patients with transient ischemic attack (TIA) raises
the question as to its sensitivity for detecting small ischemic
lesions. We compared standard and optimized DWI in 36 consecutive
patients with TIA. Optimized DWI was positive in more patients
than standard DWI (19 versus 16;
P < .001) and showed more
lesions (56 versus 42;
P = .002). At 1.5T, optimizing DWI decreases
the rate of false-negative DWI in patients with TIA.
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Introduction
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Although diffusion-weighted imaging (DWI) has improved the detection
of acute ischemic stroke, false-negatives are not uncommon in
small infarction, brain stem location, or imaging performed
early after onset.
1 Increasing the magnetic field strength allows
the detection of additional small bright dots on DWI in stroke
patients.
2 In these patients, higher sensitivity of DWI for
small lesions can also be obtained at 1.5T.
3 Given that most
patients are still being imaged on 1.5T MR units, our aim was
to determine whether these latter results could be replicated
in patients with transient ischemic attack (TIA).
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Technique
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MR imaging studies were performed on a 1.5T MR unit (GE Healthcare,
Milwaukee, Wis). Standard DWI was acquired using a spin-echo
single-shot echo-planar sequence with the following parameters:
TR/TE, 6400/86.3 ms; FOV, 240
x 240 mm; matrix, 128
x 128 ms;
20
x 24
x 6-mm-thick contiguous sections; NEX = 2;
b = 0–1000
mm
2/s along 3 diffusion directions; and acquisition time = 51
seconds. Optimized DWI consisted of 40 3-mm-thick sections,
acquired using similar parameters except for the following:
NEX = 3; diffusion directions = 9; gap = 0.3 mm; and acquisition
time = 6 minutes.
This was a retrospective study of patients admitted to a stroke unit during a 4-month period. Fifty-three consecutive patients were referred for clinical suspicion of TIA after the exclusion of patients with isolated amaurosis fugax. Five patients were excluded because of MR contraindication and 4 others because the image quality of either standard or optimized DWI was not diagnostic, due to magnetic susceptibility or motion artifacts. Of the 44 remaining patients, all except 8 underwent standard and optimized DWI during the initial MR examination. The final study group consisted of 36 patients.
After correction of distortions, all of the images were randomly reviewed by 2 readers blinded to the clinical data, fluid-attenuated inversion recovery images, and apparent diffusion coefficient (ADC) maps. DWI lesions were defined as hyperintense regions after exclusion of T2 shinethrough effects due to hyperintensities on T2-weighted echo-planar images (b = 0 mm2/s). Cases of discordance between readers were resolved by consensus. For each patient with lesions seen on both DWIs, a region of interest was centered in the largest area of diffusion hyperintensity (region of interest area = 28 ± 18 mm2, mean ± SD) and mirrored on to the contralateral hemisphere to obtain a ratio of signal intensity (rSI) and ADC (rADC). Volume and topography of all of the additional lesions depicted on optimized DWIs were noted.
SPSS 14.0 (SPSS, Chicago, Ill) was used for statistical analysis. Agreements were assessed by calculating the 
coefficient and its 95% confidence interval (CI) for the dichotomous data. After consensus, the number of patients with at least 1 lesion, total number of lesions, rSI, and rADC were compared between standard and optimized DWIs by using Fisher exact test for binary variables, paired Wilcoxon test for the total number of lesions, and Student t test for continuous variables. A value of P 
.05 was considered significant.
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Results
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The 36 patients consisted of 21 men and 15 women (mean age,
66 ± 13 years). The median duration of symptoms was 2
hours and 50 minutes (range, 1 minute to 23 hours). The median
delay from onset to MR imaging was 34 hours (range, 3 hours
to 8 days), with 13 patients imaged within 12 hours. TIA symptoms
were as follows: aphasia (
n = 8), motor weakness (
n = 18), isolated
sensory disturbance (
n = 3), other visual deficits (
n = 3),
brain stem symptoms (
n = 3), and cerebellar symptoms (
n = 1).
For the distinction between positive and negative DWIs, the 2 readers agreed in all of the cases on optimized DWI, whereas = 0.89 (95% CI, 0.73–1.04) for standard DWI. After consensus, a total of 56 lesions on optimized DWI and 42 lesions on standard DWI was identified, with an average of 1.6 versus 1.2 lesions per patient for optimized and standard DWI, respectively (P = .002). The agreement between standard and optimized DWI was = 0.83 (95% CI, 0.65–1.40) for the distinction between positive and negative DWI and = 0.60 (95% CI, 0.41–0.91) for the number of lesions. None of the lesions seen on standard DWI were missed on optimized DWI. Of the 20 patients with normal standard DWI, 3 had lesions on optimized DWI (Fig 1), 2 of which were imaged within 12 hours from onset. These were frontal or temporal cortical lesions that matched clinical symptoms in 1 patient and displayed a 20% ADC decrease for the 2 others. Overall, optimized DWI improved the detection of at least 1 lesion per patient compared with standard DWI (19 versus 16; P < .001). This remained significant when the analysis was restricted to the patients imaged within 12 hours from onset (P = .02). Of the 16 patients who had recent ischemia identified on standard DWI, 7 had 1–3 additional lesions on optimized DWI, located in the same arterial territory as that seen on standard DWI. Optimized DWI showed 14 additional lesions in 10 patients (Fig 2). These were small (mean ± SD = 45 ± 41 mm3), preferentially located in the cortex (n = 11), and occasionally in brain stem (n = 2) or deep white matter (n = 1). According to the Trial of Org 10172 in Acute Stroke Treatment criteria, a vascular cause was identified in 10 patients, including 7 patients with a positive standard DWI and 1 with ischemia seen only on optimized DWI. The rSI of ischemic lesions was significantly (P = .002) higher on optimized DWI (1.77 ± 0.60) than on standard DWI (1.54 ± 0.45). The rADC was similar on optimized DWI (0.80 ± 0.19) and on standard DWI (0.82 ± 0.15; P value not significant). In patients imaged within 12 hours from onset, the rSI was still higher on optimized DWI but was no longer significantly different from rSI measured on standard DWI (P = .08).
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Fig 1. A 59-year-old man referred for aphasia and sensorimotor weakness of the right face and upper limb that lasted 90 minutes. Optimized DWI showed 2 punctate cortical lesions in the left middle cerebral artery territory, not visible on standard DWI. These lesions were located in the left primary sensory motor cortex, corroborating clinical symptoms.
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Fig 2. A 51-year-old man referred for ataxia and left facial paresthesia that lasted 20 minutes. Standard DWI demonstrated a subtle punctuate hyperintensity in the brain stem. Optimized DWI confirmed a recent ischemic brain stem lesion and revealed additional bright dots in the right cerebral peduncle, matching clinical symptoms.
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Discussion
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DWI is becoming an important technique for optimum management
of patients with TIA.
4 The DWI sequence used in clinical practice
covers the brain in less than a minute, which conforms to the
time constraints of acutely ill patients with stroke. These
constraints, however, are less crucial in patients with TIA,
who are occasionally imaged while the deficit has already partially
resolved and often after complete regression of neurologic symptoms.
Increasing the acquisition time devoted to DWI to improve the
detection of small ischemic lesions is, thus, conceivable. We
implemented such a DWI sequence by reducing the section thickness
by half to minimize the partial volume effect and increasing
the number of diffusion directions and the signal intensity
average to improve the angular resolution and signal intensity-to-noise
ratio. These modifications improved the sensitivity for the
detection of recent ischemic lesions in patients with TIA at
1.5T. The additional lesions identified were, without exception,
small. We speculate that these lesions were not seen on the
corresponding standard DWI because of the lower signal intensity
and contrast-to-noise ratios, which, together with partial volume
averaging of the adjacent normal tissue, precluded the diagnosis
on the standard DWI. In our series of 36 patients with TIA,
optimized DWI yielded approximately 8% higher sensitivity and
allowed 3 patients to receive the diagnosis of acute stroke.
This suggests that slight improvement of DWI parameters can
result in clinically relevant findings in patients with suspected
TIA. Indeed, the presence of a DWI lesion confirms the vascular
origin of the deficit and is associated with a greater risk
for future stroke and functional dependence.
5 We extend to TIAs
the results obtained by others in acute stroke patients, with
a slightly different optimized DWI sequence on a 1.5T unit.
3 Although these authors did not report any stroke patient with
bright dots exclusively visible on optimized DWI and normal
standard DWI, optimized DWI showed twice as many lesions as
standard DWI. In line with our findings, they reported a frequent
cortical location of supplementary lesions. We speculate that
the high proportion of cortical lesions that we observed may
partly be due to the eddy currents distortion correction algorithm
that are more accurate when at least 6 diffusion directions
are acquired. The relative signal intensity ratio of lesions
on optimized DWI was significantly greater than that measured
on standard DWI, whereas ADC decrease (
20%) did not significantly
differ on both sequences. This suggests that improving the angular
resolution by using 9 instead of 3 diffusion directions does
not solve the problems of ADC measurements in very small lesions.
High-field DWI MR improves the detection of small ischemic lesions in subacute stroke. However, image distortions at 3T are substantial and may interfere with diagnosis.2 Until advanced acquisition strategies to reduce geometric distortion are developed and high-field MR imaging systems become widely available, a large proportion of patients with TIA will keep being imaged on 1.5T imaging operating systems and may, thus, benefit from optimized DWI. Optimized DWI may be a useful adjunct when facing a doubtful signal intensity change on standard DWI because of the excellent interobserver agreement that we reported. It may also be the only way to confirm a transient neurologic deficit as being ischemic when standard DWI fails to show acute lesion.
In conclusion, our study suggests that optimization of DWI acquisition parameters at 1.5T increases the sensitivity of DWI for the detection of small ischemic lesions, which are the hallmark of TIA-related lesions.
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References
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- Benameur K, Bykowski JL, Luby M, et al. Higher prevalence of cortical lesions observed in patients with acute stroke using high-resolution diffusion-weighted imaging. AJNR Am J Neuroradiol 2006;27:1987–89[Abstract/Free Full Text]
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Received May 27, 2007;
accepted after revision July 1, 2007.
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