Tissue relaxation time: in vivo field dependence.

Area of Science:

• Biomedical Engineering
• Magnetic Resonance Imaging
• Physiology

Background:

• Understanding tissue relaxation properties is crucial for Magnetic Resonance Imaging (MRI) applications.
• In vivo measurements provide the most relevant data for biological systems.
• Field strength significantly influences MRI signal characteristics, necessitating studies across a range of values.

Purpose of the Study:

• To measure in vivo spin-lattice (T1) and spin-spin (T2) relaxation times in canine tissues.
• To investigate the dependence of T1 and T2 relaxation times on magnetic field strength (0.3-1.5 Tesla).
• To characterize the relaxation behavior of different tissue types, including muscle, kidney, spleen, and adipose tissue.

Main Methods:

• In vivo measurements were performed on mongrel dogs.
• T1 relaxation times were acquired using a variable inversion time sequence (10-1,280 ms).
• T2 relaxation times were determined using a four-point multiple spin-echo sequence across multiple field strengths (0.3, 0.5, 1.0, 1.35, 1.5 T).

Main Results:

• T2 relaxation times were found to be independent of magnetic field strength across the tested range.
• T1 relaxation times exhibited a field dependence, with linear fits yielding slopes of 400-500 msec/T for water-dominant tissues.
• Adipose tissue showed a lower T1 slope (~150 msec/T), attributed to the distinct spin-lattice relaxation mechanism of -CH2 protons.

Conclusions:

• T1 relaxation times are significantly influenced by magnetic field strength in vivo.
• T2 relaxation times remain relatively constant across different field strengths for the studied tissues.
• Differential T1 relaxation behavior in adipose tissue highlights the importance of proton source and relaxation mechanisms in MRI.