Dr. Leighton Mark was invited to comment on this issue.

Drs. Nakada and Jinkins have expressed seemingly differnet interpretations for the imaged structures labels in Figure 1 from Dr. Nakada's paper (1). Dr. Nakada identifies the labeled structure as the indusium griseium (IG), whereas Dr. Jinkins believes these elevations represent the medial longitudinal striae of Lancisii (MLS). These anatomic issues, related to what is essentially a technical discussion, are effectively presented, but the reader may still wonder if the structure is the IG or MLS.

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fig 1

left. Coronal T2-weighted MR image of a specimen showing a very thin, nearly uniformly thick IG (long arrow), with the more lateral component (arrowhead) that extends into the callosal sulcus (short arrow). Note the focally elevated contour of the more medial portion of the IG. [CG= cingulate gyrus] (SE, 1000/35/4 [TR/TE/excitations], FOV=2 cm, 1.0-mm slice thickness, 256 × 256 matrix)fig 1 right. Coronal T2-weighted MR image of a different specimen showing a thickened appearance to the paramidline portion (long arrow) of the IG with a thinner, more lateral section (arrowhead) that can still be seen extending into the callosal sulcus (short arrow). The thickened component of the IG also exhibits a slight, focally convex superior margin, (SE, 1000/35/4 [TR/TE/excitations], FOV=2 cm, 1.0-mm slice thickness, 256 × 256 matrix)

It is diffuclt for me to reach a simple conclusion, but I would instead like to frame the question from the perspective of some of the associated anatomic and technical nuances related to this issue, which I feel are more complicated than simple anatomic identification.

It is generally agreed that the IG represents gray matter and the MLS are white matter fiber tracts. Furthermore, the MLS can cause a small focal elevation on the surface of the IG in a paramidline position. The MLS are frequently considered vesigial, but concern about sparsely myelinated fibers contributing to the gray matter MR appearance may not be an issue, because the MLS are probably fully myelinated with a normal relationship of axonal diameter to myelin sheath thickness (2). The IG covers the entire surface of the body of the corpus callosum from the midline to the callosal sulcus, but it can be nodular and vary in thickness, appearing maximally just under 1.0 mm near the midline with rapid tapering on either side, and can thin out to just several cell layers thickness (Personal communication: Douglas C. Miller, M.D., Ph.D., Director, Neuropathology; Director, Pathology Residency Programs, NYU Medical Center/Bellevue Hospital). There are, however, unresolved but related questions regarding the difference/similarities in the IG appearance between populations that differ in age, sex, race, and geography.

A 3-T MR unit would offer an advantage with increased signal to noise, but depiction of brain anatomy becomes a more complicated issue. The gray/white contrast can decrease at this higher field strength because of the disproportionate T1 signal increase of the gray and white matter. The T2 signal intensity of cerebral tissue is less affected especially when very strongly T2-weighted techniques are used. The fast spin-echo technique, however, offers special challenges, including artifactual changes in k-space at shorter TE and a widened point spread function, affecting the delineation of subtle anatomy. In addition, the use of a 12-cm field of view (FOV) with a 512 × 512 matrix creates maximal resolution of about 0.23 mm, which introduces volume averaging confounds for portions of the IG smaller than 0.23 mm. The slice thickness is also an important component of the issue of volume averaging and is not mentioned in Nakada's article.

My own previous anatomic study of the IG was biased toward the use of a routine spin-echo technique at 1.5-T field strength to avoid some of the mentioned problems. The signal to noise is then significantly augmented by the use of a 2.5-cm single-turn inductively coupled solenoid surface coil. This is used with a 2.0-cm FOV, 256 × 256 matrix, and 1.0-mm slice thickness for a maximal pixel resolution of 0.07 mm. Figures 1 left and right are images obtained with this technique. Figure 1 left shows a fairly uniform, very thin gray matter appearance of the IG layered on the superior surface of the corpus callosum, covering the midline and extending into the callosal sulcus region. The focally elevated contour of the IG near the midline is probably related to the MLS, which is difficult to distinguish from the adjacent white matter of the corpus callosum. Figure 1 right shows a slightly thickened appearance of the IG closer to the midline but a very thick component is still visible near the callosal sulcus. This portion of the IG also reveals a slightly elevated contour.

In summary, I find it difficult to evaluate the very small anatomy in question owing to the article's images obtained with those particular technical parameters, and I am unable to decide confidently if the labeled structure represents only the IG or MLS. It is entirely possible, however, for both seemingly discordant opinions to be correct or at least consistent with my own observations. The focal thickening of the gray matter on the superior surface of the corpus callosum, therefore, may represent the IG (though the rest of the IG is not visible but is probably present, though very thin), and the focally elevated contour of the IG may be related to the underlying MLS, thickened IG, or both, though a focally thickened IG is not necessary to produce the focal convexity.

Dr. Nakada's article and the controversy it generates indicate the need for further refinements in in vivo microantomic MR imaging, and his work should be considered a positive step in that direction.