Constrained modeling for spectroscopic measurement of bi-exponential spin-lattice relaxation of water in vivo

The ¹H NMR water signal from spectroscopic voxels localized in gray matter contains contributions from tissue and cerebral spinal fluid (CSF). A typically weak CSF signal at short echo times makes separating the tissue and CSF spin-lattice relaxation times (T₁) difficult, often yielding poor precision in a bi-exponential relaxation model. Simulations show that reducing the variables in the T, model by using known signal intensity values significantly improves the precision of the T₁ measurement. The method was validated on studies on eight healthy subjects (four males and four females, mean age 21 ± 2 years) through a total of twenty-four spectroscopic relaxation studies. Each study included both T₁ and spin-spin relaxation (T₂) experiments. All volumes were localized along the Sylvian fissure using a stimulated echo localization technique with a mixing time of 10 ms. The T₂ experiment consisted of 16 stimulated echo acquisitions ranging from a minimum echo time (TE) of 20 ms to a maximum of 1000 ms, with a repetition time of 12 s. All T₁ experiments consisted of 16 stimulated echo acquisition, using a homospoil saturation recovery technique with a minimum recovery time of 50 ms and a maximum 12 s. The results of the T₂ measurements provided the signal intensity values used in the bi-exponential T₁ model. The mean T₁ values when the signal intensities were constrained by the T₂ results were 1055.4 ms ± 7.4% for tissue and 5393.5 ms ± 59% for CSF. When the signal intensities remained free variables in the model, the mean T₁ values were 1085 ms ± 19.4% and 5038.8 ms ± 113.0% for tissue and CSF, respectively. The resulting improvement in precision allows the water tissue T₁ value to be included in the spectroscopic characterization of brain tissue.