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Updated: Jun 14, 2026
Diffusion Imaging in the Rat Cervical Spinal Cord
Published on: April 7, 2015
Multiexponential T2, magnetization transfer, and quantitative histology in white matter tracts of rat spinal cord
Adrienne N Dula1, Daniel F Gochberg, Holly L Valentine
1Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA.
Abstract:
Quantitative MRI measures of multiexponential T(2) relaxation and magnetization transfer were acquired from six samples of excised and fixed rat spinal cord and compared with quantitative histology. MRI and histology data were analyzed from six white matter tracts, each of which possessed unique microanatomic characteristics (axon diameter and myelin thickness, in particular) but a relatively constant volume fraction of myelin. The results indicated that multiexponential T(2) relaxation characteristics varied substantially with variation of microanatomy, while the magnetization transfer characteristics remained close to constant. The most-often-cited multiexponential T(2) relaxation metric, myelin water fraction, varied by almost a factor of 2 between two regions with myelin volume fractions that differed by only approximately 12%. Based on the quantitative histology, the proposed explanation for this variation was intercompartmental water exchange, which caused the underestimation of myelin water fraction and T(2) values and is, presumably, a greater factor in white matter regions where axons are small and myelin is thin. In contrast to the multiexponential T(2) relaxation observations, magnetization transfer metrics were relatively constant across white matter tracts and concluded to be relatively insensitive to intercompartmental water exchange.
Insights
Quantitative magnetic resonance imaging (MRI) reveals that myelin water fraction, a key metric, is sensitive to microanatomy variations. Magnetization transfer, however, remains stable, unaffected by water exchange in spinal cord white matter.
Area of Science:
- Neuroimaging
- Biophysics
- Histology
Background:
- Quantitative magnetic resonance imaging (qMRI) is crucial for characterizing white matter.
- Multiexponential T(2) relaxation and magnetization transfer (MT) are common qMRI techniques.
- Understanding the influence of microanatomy on these MRI metrics is essential.
Purpose of the Study:
- To compare quantitative MRI measures (T(2) relaxation, MT) with quantitative histology in rat spinal cord white matter.
- To investigate how variations in microanatomy (axon diameter, myelin thickness) affect these MRI parameters.
- To determine the sensitivity of T(2) relaxation and MT to intercompartmental water exchange.
Main Methods:
- Acquisition of multiexponential T(2) relaxation and MT data from fixed rat spinal cord samples.
- Quantitative histological analysis of six distinct white matter tracts.
- Correlation of MRI-derived metrics with histological data on microanatomy and myelin volume fraction.
Main Results:
- Multiexponential T(2) relaxation characteristics, including myelin water fraction, varied significantly with microanatomical changes.
- Myelin water fraction showed a near twofold change despite only a 12% difference in myelin volume fraction.
- Magnetization transfer metrics remained relatively constant across different white matter tracts.
- Intercompartmental water exchange was proposed to explain T(2) variations, particularly in regions with small axons and thin myelin.
Conclusions:
- Multiexponential T(2) relaxation is sensitive to microstructural variations and intercompartmental water exchange in white matter.
- Magnetization transfer appears more robust and less affected by water exchange, offering potential advantages for stable quantification.
- These findings highlight the differential sensitivity of qMRI techniques to white matter tissue properties.
