Qualitative Comparison of 3-T and 1.5-T MRI in the Evaluation of Epilepsy

Pramit M. Phal; Alexander Usmanov; Gary M. Nesbit; James C. Anderson; David Spencer; Paul Wang; Jonathan A. Helwig; Colin Roberts; Bronwyn E. Hamilton

Disclosures

Am J Roentgenol. 2008;191(3):890-895. 

In This Article

Discussion

It is estimated that 60% of epilepsy patients have a focal syndrome and that in 25% of those cases, epilepsy is not controlled with anti convulsive medication.[8] Medically refractory epilepsy can be debil itating and is associated with considerable morbidity.[9] For patients with an epi leptogenic focus identifiable at imaging, surgery offers the potential for long-term cure.[1,2,3] When clinical signs of seizure and electroencephalographic and PET findings are concordant with anatomic localization on MRI, surgery can be safely undertaken in most patients.[9] Studies of dedicated high-resolution 1.5-T MRI in the evaluation of epilepsy show utility in localization and characterization of structural abnormalities.[1]

The results of our study support the clinical supposition that use of 3-T MRI increases the rates of lesion detection and accurate characterization of lesions. MRI at 3 T also yields better contrast resolution of the gray-white matter junction, a finding particularly relevant for detection of subtle focal dysplasia of the cortex. Data from our odds ratio comparison imply that a 3-T MRI examination is 2.57 as likely as a 1.5-T examination to depict a structural abnormality and that correct characterization of the abnormality is 2.66 times as likely on 3-T studies as it is on 1.5-T studies, presumably contributing to a more accurate diagnosis. MRI at 3-T with its intrinsically greater signal-to-noise ratio combined with advances in parallel processing has considerable diagnostic value. The ability to produce high-resolution thin-slice whole-brain images in a practical examination time (< 1 hour) is a strong advantage on 3-T MRI because it facilitates detailed anatomic evaluation with minimal artifacts.

Image acquisition at 3 T combined with parallel processing makes 3D volume acquisition at nearly isovoxel resolution through the entire brain a practical reality. Although technically feasible on our clinical 1.5-T MRI units, this sequence was less desirable from a diagnostic standpoint owing to degradation in image quality due to increased noise and time constraints. High-resolution 3D volume isovoxel acquisitions facilitate multiplanar reformation in any plane, which is important for accurate lesion differentiation from normal gray-white matter structures. Reformatting a gyrus with in-plane orient ation into a perpendicular orthogonal plane can be critical for avoiding the volume-averaging effects that result in overcalling gray-white matter thickening or indistinct ness. These sequences are also highly desirable for neurosurgeons in preoperative planning.

Since the completion of our study, high-resolution 3D T2-weighted and FLAIR techniques have become commercially available for our 3-T MRI units. Although we aim to explore the advantages of such techniques at 3 T, they are not practically feasible at 1.5 T because of time constraints; therefore, the techniques are not directly comparable.

Our results are concordant with findings reported for 3-T MRI with eight-channel phased-array surface coils in the evaluation of focal epilepsy.[10] The major gains in diagnosis in our study not surprisingly were related to detection of cortical malformations, because many of our patients are referred for follow-up MRI because of failures of imaging localization with 1.5-T MRI. These abnormalities are often subtle; thin slices are needed to avoid partial volume effects and to detect subtle areas of gray-white matter blurring and indistinctness.[11,12] The improved gray-white matter contrast at 3 T in our study highlights the importance of this factor. MRI at 3 T was excellent for depiction of the gray-white matter junction (tissue contrast) in our study, showing statistically significant improvement over 1.5-T MRI.

The limitations of high-field-strength imaging are well known, including a propensity to certain imaging artifacts, such as sus ceptibility and a perceived sensitivity to motion. Although usually undesirable, great er susceptibility effects at 3 T can be ad vantageous, as when detection of abnormal vessels or previous hemorrhage is relevant. Detection of two cavernous malformations in our study was improved by susceptibility effects, which were greater at 3 T than at 1.5 T.

Disadvantages of imaging at 3 T include longer T1, increased acoustic noise, greater power deposition, and greater device incompatibility.[13,14,15,16] Although motion artifacts were found to be similar at 1.5 and 3 T, other imaging artifacts were notably fewer at 3 T in our study. We did not anticipate this finding, which might have been related in part to the relatively young age of the patient population, who have minimal or no susceptibility problems related to previous intracranial surgery or hemorrhage.

Our study had several limitations. The retrospective nature of the review, the indications for a second MRI examination at 3 T, and the need to exclude patients without directly comparable sequences may have introduced selection bias. Epilepsy patients undergo 3-T MRI at our institution for various reasons, including normal or equivocal findings on 1.5-T MRI, clarification of lesion characterization, surgical planning, and scheduling constraints. However, because normal or equivocal findings on 1.5-T MRI should theoretically increase the likelihood of normal findings on 3-T MRI (implying a lower pretest probability of disease), repetition of imaging at 3 T should adversely affect rather than improve the likelihood of identification of a structural lesion. By contrast, we found more than twofold improvement in lesion detection on 3-T compared with 1.5-T MRI.

Because we could not control for the timing of the 1.5-T and 3-T examinations, there might have been differences in visualization of certain conditions that change over time, such as tumor growth and conspicuity changes relat ed to interim developmental myelination.[17,18] We also did not blind individual radiologists to viewing both sets of MR images at the same time. Although this factor can introduce bias, it was unavoidable given the nature of our study, which was direct assessment of differences in image quality.

Identical protocol, acquisition times, and coil type were not practically possible in this retrospective study. We do not routinely use dedicated surface coils or multichannel coils for our 1.5-T MRI units in part because of image degradation from inadequate signal-to-noise ratio. Most of the patients in our practice did not have specific localizing information, rendering accurate surface coil placement impossible. Phased-array surface coils in the past have been limited by smaller volumes of coverage and inherently heterogeneous signal intensity over the field of view, precluding visualization of anatomic detail of the brain parenchyma outside the coil isocenter. Surface coils work well in the evaluation of mesial temporal sclerosis, for example, in which technologists can routinely position the coil over the temporal lobes in a reproducible manner. Finally, given the high proportion of pediatric epilepsy in our practice, a large number of patients undergo sedation or anesthesia for MRI, making coil changes during a whole-brain epilepsy examination undesirable.

In the assessment of new imaging technology, an important consideration is whether improvements in image quality have a clinically beneficial effect.[9] Most patients with focal epilepsy in this study had superior lesion localization and diagnosis with 3-T MRI. Most of the patients who underwent surgery had substantial clinical improvement or resolution of seizures.

In conclusion, MRI at 3 T was superior to 1.5-T MRI in the detection and accurate characterization of structural brain abnormalities in a group of patients undergoing whole-brain epilepsy evaluation at our institution. Compared with 1.5-T MR images, whole-brain 3-T MR images are of better quality, do not require surface coils or specific knowledge of lesion location, and are not limited by technical artifacts. Imaging at 3 T should be strongly considered in the evaluation of patients with focal epilepsy and previously equivocal or normal findings on 1.5-T MRI.

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