Rationale and Objectives
During aging, there is evidence of microstructural changes in certain cortical and subcortical brain regions. Diffusion tensor imaging (DTI) is used to study age related microstructural changes in the acoustic pathway.
Materials and Methods
Twenty healthy volunteers (mean age 28.5 years) and 15 healthy volunteers (mean age 61.3 years) were examined using a 1.5-T MR system with a high-resolution T1-weighted sequence and an integrated parallel imaging technique DTI Echo-planar-imaging (EPI) sequence. For reliability, 10 subjects underwent a second examination 2 days later. The fractional anisotropy (FA) and the apparent diffusion coefficient (ADC) were measured in six brain regions of the auditory pathway.
Results
We found no left/right asymmetry in the selected brain structures. There were no significant differences ( P < .05) in the ADC and FA in the lateral lemniscus and medial geniculate body of young and elderly subjects. However, FA was significantly increased ( P < .05) in the inferior colliculus and decreased in the auditory radiation, the superficial temporal gyrus, and the transverse temporal gyrus in the elder subjects than in the younger ones. There were no significant differences in anisotropy in subsequent examinations in the younger individuals.
Conclusions
These findings suggest evidence of age-related changes in the acoustic pathway. These changes are associated with a decrease in anisotropy mainly in the cortical grey and white matter rather than in the subcortical regions. Our DTI measurements were reproducible.
The aging brain shows a variety of micro- and macroscopic changes, which can result in some degree of functional decline. During aging, the auditory system undergoes numerous changes in its structure, function, and neurochemistry ( ). Age-related hearing changes have usually been attributed to changes in the cochlea, inducing a loss of sensory cells, atrophy of stria vascularis, and a loss of spiral ganglion cells ( ). However, there is strong evidence that changes in the ability of hearing is also due to changes in the central auditory system ( ). The anatomic structure of the acoustic pathway is organized as follows: The anatomic organization of the auditory system consists of a large ensemble of subcortical nuclei, connected with each other and with the auditory cortex through ascending and descending white matter fiber pathways, processing both serial and parallel information within the auditory system. The cochlear nerve connects the cochlear with the anterior and posterior cochlear nucleus in the brainstem. Then, the majority of fibres cross to the contralateral side (90%) and approximately 10% run ipsilaterally to connect the auditory nuclei with the superior olivary complex. The fibers of the pathway reach the medial geniculate body, which is the thalamic auditory relay station and are there transmitted bilateral toward the primary auditory cortices. The gyri directly involved in the auditory perception are the transverse temporal gyri (of Heschl) and the superior temporal gyrus. Diffusion tensor imaging (DTI), a recently emerged, exciting variation of magnetic resonance imaging (MRI), provides a unique opportunity to visualize the integrity of tissue microstructure and quantify the diffusion of water in various tissues noninvasively ( ). Tissues, which have a random microstructure, such as cerebrospinal fluid, show isotropic diffusion properties ( ). This means diffusion is equal in all directions. In contrast, in tissues that have a regularly ordered microstructure, such as brain white matter, water molecules behave in a sorted fashion, with a predominant motion direction and a given orientation, indicating a marked anisotropy in the diffusion properties ( ). The microstructural tissue changes can be expressed as FA (fractional anisotropy), which has no dimension and as mean diffusitivity (apparent diffusion coefficient, ADC) with ×10 −3 mm 2 /s ( ). These indices can be seen as complementary for the evaluation of brain tissue. Whereas ADC is a measure of the directionally averaged magnitude of diffusional motion of water molecules (= related to integrity of membranes), FA describes the degree of anisotropy the process of molecular diffusion (= degree of structural alignment) ( ). Previous DTI studies have shown white matter declines in normal aging and various neuropsychiatric disorders ( ). Two earlier studies investigating the acoustic pathway by the means of DTI were focused on patients with sensineuronal hearing loss ( ) and the FA value of Heschl’s gyrus in a normal population ( ). However, to the best of our knowledge, there is no previous study, investigating the aging of the acoustic pathway.
The major goal of this study was to evaluate the aging of structures of the acoustic pathway by means of measuring the mean diffusivity and anisotropy values in young healthy adults in comparison to elderly healthy adults. Therefore, we designed a DTI acquisition protocol, using integrated parallel imaging technique (iPAT) to investigate the anisotropy of the acoustic pathway. To analyze the reliability of anisotropy measurements, we investigated the reproducibility of our DTI measurements, using a second follow-up examination of our younger study group in the same experimental settings.
Material and methods
Subjects
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Image Acquisition
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Image Analysis
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Region of Interest
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Statistical Analysis
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Results
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Table 1
Comparison of the ADC and FA Values for the Different Brain Regions of the Left and Right Hemisphere in the Younger Study Group
Comparison of Right and Left Hemisphere: Younger Subjects Region ADC (× 10 −3 mm 2 /s) FA Right Left_P_ Value Right Left_P_ Value Lateral lemniscus 0.805 (± 0.01) 0.801 (± 0.01) .198 0.32 (± 0.05) 0.32 (± 0.04) .979 Inferior colliculus 0.537 (± 0.02) 0.534 (± 0.03) .272 0.17 (± 0.03) 0.16 (± 0.02) .084 Medial geniculate body 0.644 (± 0.02) 0.653 (± 0.06) .758 0.21 (± 0.02) 0.21 (± 0.02) .758 Acoustic radiation 0.708 (± 0.07) 0.700 (± 0.05) .317 0.71 (± 0.03) 0.72 (± 0.02) .149 Heschl’s gyrus 0.779 (± 0.01) 0.755 (± 0.04) .262 0.37 (± 0.03) 0.37 (± 0.03) .463 Superior temp gyrus 0.762 (± 0.05) 0.743 (± 0.05) .174 0.37 (± 0.03) 0.37 (± 0.02) 0.943
ADC: apparent diffusion coefficient; FA: fractional anisotropy.
Mean values with standard deviation in parentheses.
P values < .05 indicate significant difference.
Table 2
Comparison of the ADC and FA Values for the Different Brain Regions of the Left and Right Hemisphere in the Elderly Study Group
Comparison of Right and Left Hemisphere: Elder Subjects Region ADC (× 10 −3 mm 2 /s) FA Right Left_P_ Value Right Left_P_ Value Lateral lemniscus 0.802 (± 0.02) 0.810 (± 0.02) .087 0.30 (± 0.01) 0.30 (± 0.02) .569 Inferior colliculus 0.552 (± 0.05) 0.543 (± 0.03) .258 0.18 (± 0.08) 0.18 (± 0.09) .277 Medial geniculate body 0.644 (± 0.02) 0.643 (± 0.02) .563 0.22 (± 0.02) 0.22 (± 0.02) .918 Acoustic radiation 0.737 (± 0.05) 0.725 (± 0.06) .463 0.64 (± 0.03) 0.65 (± 0.04) .277 Heschl’s gyrus 0.755 (± 0.04) 0.751 (± 0.06) .681 0.30 (± 0.03) 0.31 (± 0.03) .196 Superior temp gyrus 0.775 (± 0.04) 0.759 (± 0.06) .059 0.35 (± 0.01) 0.35 (± 0.01) .717
ADC: apparent diffusion coefficient; FA: fractional anisotropy.
Mean values with standard deviation in parentheses.
P values < .05 indicate significant difference.
Table 3
Average ADC and FA Values for the Different Brain Regions of the Younger and the Elder Subjects
Comparison of Younger vs Elder Subjects Region ADC (× 10 −3 mm 2 /s) FA Younger Older_P_ Value Younger Older_P_ Value Lateral lemniscus 0.801 (± 0.02) 0.805 (± 0.03) .658 0.31 (± 0.04) 0.31 (± 0.01) .691 Inferior colliculus 0.540 (± 0.04) 0.547 (± 0.02) .112 0.17 (± 0.02) 0.18 (± 0.06) .0006 Medial geniculate body 0.650 (± 0.05) 0.645 (± 0.02) .323 0.21 (± 0.02) 0,21 (± 0.01) .686 Acoustic radiation 0.700 (± 0.04) 0.732 (± 0.02) .067 0.71 (± 0.05) 0.64 (± 0.02) <.0001 Heschl’s gyrus 0.821 (± 0.01) 0.830 (± 0.03) .441 0.36 (± 0.04) 0.31 (± 0.05) <.0001 Superior temp gyrus 0.764 (± 0.01) 0.810 (± 0.06) .356 0.37 (± 0.02) 0.35 (± 0.03) <.0001
ADC: apparent diffusion coefficient; FA: fractional anisotropy.
Mean values with standard deviation in parentheses.
P values < .05 indicate significant difference.
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Table 4
The P Values of the Follow-Up Examination of the Younger Study Group, Indicating Excellent Reproducibility ( P > .05) for the FA value (a) and the ADC (b)
a. FA P Values ( P < .05) Reproducibility: Younger Subjects Region Right Left 1. Scan 2. Scan_P_ Value 1. Scan 2. Scan_P_ Value Lateral lemniscus 0.32 (± 0.05) 0.28 (± 0.02) .176 0.32 (± 0.04) 0.30 (± 0.01) .240 Inferior colliculus 0.17 (± 0.03) 0.18 (± 0.04) .310 0.16 (± 0.02) 0.19 (± 0.03) .058 Medial geniculate body 0.21 (± 0.02) 0.19 (± 0.03) .091 0.21 (± 0.02) 0.19 (± 0.03) 0.128 Acoustic radiation 0.71 (± 0.03) 0.70 (± 0.01) .147 0.72 (± 0.02) 0.70 (± 0.02) .063 Heschl’s gyrus 0.37 (± 0.03) 0.35 (± 0.02) .35 0.37 (± 0.03) 0.35 (± 0.01) .553 Superior temp gyrus 0.37 (± 0.03) 0.38 (± 0.06) .466 0.37 (± 0.02) 0.37 (± 0.02) .695 b. ADC (× 10 –3 mm 2 /s) P Values ( P < .05) Reproducibility: Younger Subjects Region Right Left 1. Scan 2. Scan_P_ Value 1. Scan 2. Scan_P_ Value Lateral lemniscus 0.805 (± 0.01) 0.811 (± 0.06) .352 0.801 (± 0.01) 0.820 (± 0.06) .080 Inferior colliculus 0.537 (± 0.02) 0.540 (± 0.02) .753 0.534 (± 0.03) 0.552 (± 0.04) .053 Medial geniculate body 0.644 (± 0.02) 0.671 (± 0.04) .061 0.653 (± 0.06) 0.667 (± 0.04) .500 Acoustic radiation 0.708 (± 0.07) 0.691 (± 0.02) .485 0.700 (± 0.05) 0.718 (± 0.05) .159 Heschl’s gyrus 0.779 (± 0.01) 0.775 (± 0.05) .977 0.755 (± 0.04) 0.768 (± 0.01) .798 Superior temp gyrus 0.762 (± 0.05) 0.739 (± 0.03) .078 0.743 (± 0.05) 0.753 (± 0.02) .187
ADC: apparent diffusion coefficient (ADC); FA: fractional anisotropy (FA).
Mean values with standard deviation in parentheses.
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Discussion
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