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(Radiology. 2000;214:217-221.)
© RSNA, 2000

Neuroradiology

Heschl and Superior Temporal Gyri: Low Signal Intensity of the Cortex on T2-weighted MR Images of the Normal Brain1

Takashi Yoshiura, MD, PhD 2, Shuichi Higano, MD, PhD, Ana Rubio, MD, PhD, David A. Shrier, MD, Wingchi E. Kwok, PhD, Soji Iwanaga, MD and Yuji Numaguchi, MD, PhD

1 From the Departments of Radiology (T.Y., S.H., D.A.S., W.E.K., S.I., Y.N.) and Pathology (A.R.), University of Rochester, 601 Elmwood Ave, Box 648, Rochester, NY 14642. Received August 20, 1998; revision requested October 28; revision received April 22, 1999; accepted April 27. Supported in part by the Kodak Visiting Scientist Cooperative Program. Address reprint requests to D.A.S.


   Abstract
TOP
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To study the normal signal intensity pattern in the primary auditory cortex (first Heschl gyrus [HG]) and the surrounding cortices in the superior temporal gyrus (STG) and middle temporal gyrus (MTG) on T2-weighted magnetic resonance (MR) images.

MATERIALS AND METHODS: Coronal T2-weighted fast spin-echo MR images in 30 neurologically normal patients (60 hemispheres) were retrospectively analyzed. Two raters evaluated the cortical signal intensity of the first HG and the neighboring STG and compared them with those of the MTG and the subcortical white matter. The cortical signal intensities between the first HG and the STG were also directly compared. Coronal MR images, which included images of the anterior and posterior halves of the first HG, were evaluated separately.

RESULTS: All first HGs were hypointense to the MTG and were either iso- or hypointense to the STG. Cortical hypointensity was especially prominent in the posterior half; the first HG was isointense to the white matter in 33 (55%) hemispheres. The STG was hypointense to the MTG in 54 (90%) hemispheres and in the anterior halves of 36 (60%) hemispheres.

CONCLUSION: These findings demonstrate lower signal intensity of the cortex on T2-weighted images in the first HG and surrounding STG compared with that of the MTG.

Index terms: Brain, anatomy, 134.91 • Brain, cortex, 134.121411, 134.91 • Brain, iron • Brain, MR, 134.121411, 134.91


   Introduction
TOP
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
The human primary auditory cortex is located in the posterior part of the supratemporal plane (1–6). Anatomically, it largely corresponds to the transverse temporal gyri, or Heschl gyri (HGs) (1–7). The morphology of the auditory cortex has been described as highly variable, and there may be two or more HGs in one hemisphere (8,9). Findings from microanatomic studies have shown that when there are multiple HGs, cytoarchitectonic primary auditory cortex (ie, Brodmann area 41 [10]), is limited to the most anterior gyrus (first HG) (11). Lesions in the HGs occasionally cause auditory symptoms, which are associated with abnormal neuronal activities (12,13).

For studies of patients with such symptoms, it is necessary to identify the HG at preoperative imaging studies. Findings from recent studies have shown that the HG can be identified by its characteristic shape on magnetic resonance (MR) images (8). For further characterization on MR images, we studied the cortical signal intensity of the HG, the surrounding superior temporal gyrus (STG), and the middle temporal gyrus (MTG) in normal brains on T2-weighted images.


   MATERIALS AND METHODS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
We (T.Y. and S.H.) retrospectively reviewed coronal T2-weighted fast spin-echo brain MR images in 30 consecutive patients (15 male patients, 15 female patients; age range, 10–58 years; mean age, 27 years ± 13 [SD]). The criteria for inclusion were normal findings at (a) neurologic examination and (b) brain MR imaging. Indications for the MR study included suspected seizure disorders (n = 24), psychiatric problems (n = 5), and headache (n = 1).

Images were obtained by using a 1.5-T clinical MR unit. The imaging parameters were as follows: 4,000/96–104 (repetition time msec/echo time msec), two or three signals acquired, echo train length of 12, 16–18-cm field of view, 256 x 256 matrix, 3-mm section thickness, 0-mm gap.

All evaluations were based on the consensus of two neuroradiologists (T.Y. and S.H.). MR images were obtained with regular clinical window width and level settings and were reviewed by the two readers. The HG was anatomically identified as an {Omega}- or heart-shaped protrusion in the supratemporal plane (8). The raters were aware that there may have been two or more HGs in one hemisphere. Since the primary auditory cortex is located in the first HG (most medial gyrus on the coronal images), only the first HG and the surrounding STG were further evaluated.

For each hemisphere, the number of contiguous section levels in which the first HG was imaged was counted. Evaluations of cortical signal intensities on the coronal MR images were performed separately for the anterior and posterior halves of the first HG. The parts of the temporal lobe imaged in these two groups of coronal images will be hereafter referred to as the anterior and posterior divisions.

The following three comparisons were performed: (a) The cortical signal intensity of the first HG was compared with that of the MTG and the subcortical white matter. This was scored as 0 when the HG was hyperintense to the MTG, 1 when the HG was isointense to the MTG, 2 when it was hypointense to the MTG but hyperintense to the white matter, and 3 when it was isointense to the white matter. (b) The cortical signal intensity of the STG was compared with that of the MTG and the white matter and was scored with the same method. (c) The cortical signal intensity of the first HG was directly compared with that of the STG and was scored as 0, 1, or 2 when the HG was hyper-, iso-, or hypointense, respectively.

Comparisons of the scores between the right and left hemispheres and between the anterior and posterior divisions were performed by using the Mann-Whitney test.

We imaged formalin-fixed brain specimens to compare the findings with those on in vivo images. The brain was taken from a 60-year-old man who died of nonneurologic disease; it was fixed in 10% buffered formalin for approximately 1 week. The posterior two-thirds of the right temporal lobe was harvested. The specimen was imaged with the same 1.5-T MR unit and a 9 x 7-cm rectangular surface coil. A T2-weighted fast spin-echo sequence was used (3,000/96, three signals acquired, echo train length of 12, 8-cm field of view, 256 x 256 matrix, 2-mm section thickness, 0-mm gap).

Imaging was performed in an orientation perpendicular to the long axis of the temporal lobe. In addition, to compare the cortical signal intensities with the minimum partial volume effects, the excised HG, STG, and MTG were imaged. The three gyri were removed from the temporal lobe as approximately 1-cm–thick fragments and were placed together in a 3 x 3-cm area. Special care was taken to determine the imaging planes that were perpendicular to all three gyri.


   RESULTS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
The HG was readily identified on the coronal MR images in each hemisphere. A single HG was identified in 24 (80%) right and 21 (70%) left hemispheres; two HGs were seen in six (20%) right hemispheres and nine (30%) left hemispheres. Two patients had two HGs bilaterally. No hemisphere had three or more HGs. The first HG was seen over four to 10 (mean, 7 ± 2) section levels in the right hemispheres and over three to 11 (mean, 7 ± 2) section levels in the left hemispheres.

The results of the cortical signal intensity characterization of the first HG and the STG are summarized in Tables 1–3. Comparisons of signal intensities in the right and left hemispheres revealed no significant difference in any cortical area. Thus, only combined results from both hemispheres will be described in the rest of this article. Figure 1 shows MR images in a representative patient.


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  		TABLE 1. Cortical Signal Intensity on T2-weighted Images: First HG versus MTG and White Matter
 

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  		TABLE 2. Cortical Signal Intensity on T2-weighted Images: STG versus MTG and White Matter
 

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  		TABLE 3. Cortical Signal Intensity on T2-weighted Images: First HG versus STG
 


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  		Figure 1a. Coronal T2-weighted fast spin-echo images (4,000/104, three signals acquired) obtained in a 43-year-old woman. (a) Posterior parts of the bilateral HGs (H) show lower signal intensity of the cortex compared with that of the STGs (S) and the MTGs (M) and are isointense to the white matter. The STGs are hypointense to the MTGs bilaterally. (b) Anterior portions of HGs (H) are isointense to the STGs (S) and hypointense to the MTGs (M). The left HG is split into two branches anteriorly. Note the cortical signal intensities of the anterior parts of the HGs are higher than those of the posterior parts shown in a.

 


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  		Figure 1b. Coronal T2-weighted fast spin-echo images (4,000/104, three signals acquired) obtained in a 43-year-old woman. (a) Posterior parts of the bilateral HGs (H) show lower signal intensity of the cortex compared with that of the STGs (S) and the MTGs (M) and are isointense to the white matter. The STGs are hypointense to the MTGs bilaterally. (b) Anterior portions of HGs (H) are isointense to the STGs (S) and hypointense to the MTGs (M). The left HG is split into two branches anteriorly. Note the cortical signal intensities of the anterior parts of the HGs are higher than those of the posterior parts shown in a.

 
All first HGs were hypointense to the MTGs in both the anterior and the posterior divisions. The first HG was isointense to the white matter in 33 (55%) hemispheres in the posterior division and in five (8%) hemispheres in the anterior division. (P < .001).

The STG was never hyperintense to the MTG or isointense to the white matter. The STG showed hypointensity to the MTG in 54 (90%) hemispheres in the posterior division and in 36 (60%) hemispheres in the anterior division. (P < .03).

Findings from a direct comparison of cortical signal intensities in the first HG and STG revealed that all first HGs were either hypointense or isointense to the STGs. The first HG was hypointense to the STG in 55 (92%) hemispheres in the posterior division and in 33 (55%) hemispheres in the anterior division.

The cadaveric image of the temporal lobe showed lower cortical signal intensity in the HG compared with that of the STG and MTG (Fig 2a). The image of the excised gyri showed that the HG had the lowest cortical signal intensity, followed by the STG and MTG (Fig 2b), which was consistent with the results of in vivo patient imaging.



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  		Figure 2a. In vitro MR images (3,000/96; three signals acquired) of the right temporal cortices obtained from a 60-year-old man who died of nonneurologic disease. (a) Image shows the HG (H) has significantly lower signal intensity of the cortex compared with that of the STG (S) and the MTG (M). The hypointensity spreads into the adjacent STG, especially in its superior aspect. There is an artifactual crack (arrow) in the STG. (b) Image of the excised HG (H), STG (S), and MTG (M) shows the lowest signal intensity in the HG, followed by the STG and MTG.

 


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  		Figure 2b. In vitro MR images (3,000/96; three signals acquired) of the right temporal cortices obtained from a 60-year-old man who died of nonneurologic disease. (a) Image shows the HG (H) has significantly lower signal intensity of the cortex compared with that of the STG (S) and the MTG (M). The hypointensity spreads into the adjacent STG, especially in its superior aspect. There is an artifactual crack (arrow) in the STG. (b) Image of the excised HG (H), STG (S), and MTG (M) shows the lowest signal intensity in the HG, followed by the STG and MTG.

 

   DISCUSSION
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
Findings from numerous anatomic and electrophysiologic studies (1–6) have shown that the primary auditory cortex is located in the HG. It is known that there are considerable variations in the anatomic configuration of the HG (8,14). The simplest variation is a single HG without a notch or bifurcation. However, the HG is occasionally cleaved by the sulcus intermedius (15). This sulcus is often poorly developed at the medial aspect, which results in a laterally bifurcated HG. When the sulcus is deep and cleaves the HG along its entire length, two HGs are considered to be present, and the sulcus should be called the Heschl sulcus (15).

Bilateral lesions in the HG are known to result in cortical deafness (12,16). In contrast, because of the bilateral (and contralateral dominant) projections from the ears, it is believed that unilateral lesions have no effect on hearing (12); although, findings from some animal studies (17) showed minor and transient hearing loss in the contralateral ear that resulted from such lesions. Thus, the localization of the primary auditory cortex before brain surgery may not be as critical as the localization of the motor and language cortices. However, because lesions in the HG may cause symptoms similar to those caused by lesions in other parts of the auditory pathway (eg, auditory hallucinations) (12,13), radiologists may need to localize the lesions in relation to the HG.

There have been only a few radiologic descriptions of the HG, and all of them described the anatomic configurations of this cortex (8,14). Most recently, Yousry et al (8) studied MR images in 50 brains (100 hemispheres) and concluded that the HG can be readily identified on MR images because of its characteristic shape. On sagittal or coronal images, the HG appeared as a protrusion on the supratemporal plane, with variable shapes that include a single {Omega}, a heart, or two separate {Omega}s, depending on the number of HGs and on the absence or presence, depth, and length of the sulcus intermedius (8). On transverse MR images, the HG appeared to be a gyrus (or gyri) that extended anterolaterally from a point posterior to the insula to the convexity (8). They found one HG in 66 (66%) hemispheres, two HGs in 33 (33%) hemispheres, and three HGs in one (1%) right hemisphere. We confirmed that the HG can be easily identified on coronal MR images, and our result regarding the number of HGs was largely consistent with their results.

To our knowledge, there have been no studies on the MR cortical signal intensity of the HG. Our results revealed that on T2-weighted images cortical signal intensity of the first HG was always lower than that of the MTG and was often lower than that of the STG. Low signal intensity of the cortex of the first HG is prominent in its posterior half; it was isointense to the white matter in 50%–60% of the hemispheres. Although the HG can usually be localized on the basis of its shape, this conspicuous low signal intensity on T2-weighted images can be used as an additional landmark for identification.

The origin of the low signal intensity of the cortex of the first HG is unknown. Low signal intensities on T2-weighted images have been reported in other normal cortices, including the motor, sensory (18), and visual (19) cortices. In these reports, low signal intensities were increasingly seen in older patients. Authors of these reports attributed the low signal intensity of the cortex to the age-related physiologic deposition of paramagnetic iron because the negative signal intensity change correlated well with increasing iron content of the cortex (18–20). Findings from a quantitative biochemical study (21) showed that the iron content of the auditory cortex is comparable with that of the motor and visual cortices (Fig 3). Thus, the high iron content seems to be a reasonable explanation for the low signal intensity of the cortex in the first HG on T2-weighted images. However, no studies, including ours, have been conducted to rigorously compare the iron content in the HG, the STG, and the MTG. Further quantitative studies are necessary to validate the contribution of iron to the differences in signal intensity in these cortices on T2-weighted images.



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  		Figure 3. Photomicrograph shows cortical iron deposition in the HG. Multifocal positive staining for iron (blue stains) can be seen in the cytoplasm of oligodendrocytes. The specimen was obtained from a 59-year-old woman who died of a nonneurologic disease. (Pearl iron stain; original magnification, x400.)

 
As an alternative explanation, different cortical architectures may account for the gradation in signal intensity of the cortex over the first HG, the STG, and the MTG on T2-weighted images. von Economo and Koskinas (1,7) described five fundamental types of cortex primarily on the basis of the relative development of granular and pyramidal cells, as follows: agranular-type, frontal-type, parietal-type, granular-type (koniocortex), and polar-type.

As with the primary sensory and visual cortices, the primary auditory cortex (HG) is classified as the granular-type cortex, which is characterized by densely packed small neurons (granular cells), poorly developed laminae III and IV, and a high degree of myelination (1,7) (Fig 4a). Most parts of the STG consist of the parietal-type cortex, which has a granular layer that is less dense than that of the granular-type cortices (1,7) (Fig 4b). The MTG is composed of the frontal-type cortex, in which granular layers are narrow and loose; in layers III and V, pyramidal cells, which are larger than the granular cells, are large and well developed (1,7) (Fig 4c). Thus, the geometric distribution of the signal intensity on T2-weighted images follows the cytoarchitectonic topography in these regions.



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  		Figure 4a. Photomicrographs show the cortical microarchitecture of the HG, the STG, and the MTG in specimens obtained from the same patient as in Figure 3. (Luxol fast blue-cresyl violet stain; original magnification, x40.) (a) In the HG, vertically arranged granular neurons are prominent. Note the higher cell density in this cortex compared with that of the STG and MTG shown in b and c, respectively. Myelin tracts vertically radiate from the deeper cortex. (b) In the STG, granular neurons are less prominent than in the HG shown in a. (c) The cortex in the MTG is characterized by predominant pyramidal neurons. Cell density is the least prominent among the three gyri.

 


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  		Figure 4b. Photomicrographs show the cortical microarchitecture of the HG, the STG, and the MTG in specimens obtained from the same patient as in Figure 3. (Luxol fast blue-cresyl violet stain; original magnification, x40.) (a) In the HG, vertically arranged granular neurons are prominent. Note the higher cell density in this cortex compared with that of the STG and MTG shown in b and c, respectively. Myelin tracts vertically radiate from the deeper cortex. (b) In the STG, granular neurons are less prominent than in the HG shown in a. (c) The cortex in the MTG is characterized by predominant pyramidal neurons. Cell density is the least prominent among the three gyri.

 


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  		Figure 4c. Photomicrographs show the cortical microarchitecture of the HG, the STG, and the MTG in specimens obtained from the same patient as in Figure 3. (Luxol fast blue-cresyl violet stain; original magnification, x40.) (a) In the HG, vertically arranged granular neurons are prominent. Note the higher cell density in this cortex compared with that of the STG and MTG shown in b and c, respectively. Myelin tracts vertically radiate from the deeper cortex. (b) In the STG, granular neurons are less prominent than in the HG shown in a. (c) The cortex in the MTG is characterized by predominant pyramidal neurons. Cell density is the least prominent among the three gyri.

 
The aforementioned differences in the histologic composition of the HG, the STG, and the MTG can alter their T2 signal intensities because of changes in water content and molecular structure, with resultant changes in T2 relaxivity and/or magnetization transfer. In our MR results, the posterior half of the first HG showed significantly lower signal intensity compared with that of the anterior half. Braak (22) extensively studied the cortical cytoarchitecture in the supratemporal plane with a pigment-staining method and found that the core of koniocortex lay roughly in the posterior half of the first HG and was surrounded by a belt of transitional areas with less dense granular cells (ie, the prokoniocortex, which is located anterior to the koniocortex and which spreads toward the primitively organized insular cortex and the parakoniocortex, which covers an area of the STG mainly lateral to the koniocortex). These findings were confirmed by Galaburda and Sanides (23) by means of Nissl staining. These cytoarchitectonic findings seem to match well with our signal intensity findings.

Our study has limitations. First, partial volume effects may not be negligible in our in vivo evaluations. The HG extends mediolaterally and is somewhat oblique with respect to the coronal imaging plane, whereas the STG and MTG are almost perpendicular to the imaging plane. However, our results at in vivo imaging were supported by the results at in vitro imaging, which was performed with minimum partial volume effects. The cortical hypointensity of the STG relative to the MTG cannot be explained by the partial volume effect because of highly similar geometric relationships. Second, since we retrospectively analyzed the patients' MR images, quantitative measurements of T1 and T2 relaxivities were not performed. Further studies with such measurements may reveal a more detailed relationship between the MR imaging signal intensities and the histologic composition in the cortex.

In conclusion, our study findings reveal lower signal intensity of the cortex in the first HG and in the surrounding STG, relative to the MTG on T2-weighted MR images. This finding can help to localize the primary auditory cortex at clinical MR studies. In addition, our study findings suggest that the signal intensity on T2-weighted images reflects the cortical organization in the normal brain.


   Footnotes
 
2 Current address: Department of Radiology, Massachusetts General Hospital, Charlestown, Mass. Back

Abbreviations: HG = Heschl gyrus MTG = middle temporal gyrus STG = superior temporal gyrus

Author contributions: Guarantor of integrity of entire study, T.Y.; study concepts, T.Y., D.A.S., S.H.; study design, T.Y., S.H.; definition of intellectual content, T.Y., D.A.S., Y.N., S.H.; literature research, T.Y.; experimental studies, T.Y., W.E.K.; data acquisition, T.Y., S.I., A.R., W.E.K.; data analysis, T.Y., S.H.; statistical analysis, T.Y.; manuscript preparation, T.Y., D.A.S.; manuscript editing, T.Y.; manuscript review, T.Y., D.A.S., Y.N.


   References
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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