Z‐Spectral MRI Quantifies the Mass and Metabolic Activity of Adipose Tissues With Fat‐Water‐Fraction and Amide‐Proton‐Transfer Contrasts

Background Brown adipose tissue (BAT) is metabolically activatable and plays an important role in obesity and metabolic diseases. With reduced fat‐water‐fraction (FWF) compared with white adipose tissue (WAT), BAT mass and its functional activation may be quantified with Z‐spectra MRI, with built‐in FWF and the metabolic amide proton transfer (APT) contrasts. Purpose To investigate if Z‐spectral MRI can quantify the mass and metabolic activity of adipose tissues. Study Type Prospective. Subjects Seven groups of 8‐week‐old male rats, including two groups (n = 7 per group) for in vivo MRI study and five groups (n = 5 per group) for ex vivo validation; 12 young and healthy volunteers with 6 male and 6 female. Field Strength/Sequence The 7 T small animal and 3 T clinical systems, T 2 ‐weighted imaging, Rapid Acquisition with Relaxation Enhancement (RARE) readout based chemical exchange saturation transfer (CEST) Z‐spectral MRI sequence. Assessment Quantified FWF and APT from Z‐spectra in rats before and after norepinephrine (NE) stimulation and in healthy human subjects; ex vivo measurements of total proteins in BAT from rats. Statistical Tests Two‐tailed unpaired Student's t ‐tests and repeated measures ANOVA. P ‐value <0.05 was considered significant. Results Decreased FWF (from 39.6% ± 7.2% before NE injection to 16.4% ± 7.2% 120 minutes after NE injection, P < 0.0001) and elevated APT (from 1.1% ± 0.5% before NE injection to 2.9% ± 0.5% 120 minutes after NE injection, P < 0.0001) signals in BAT were observed with in vivo Z‐spectral MRI in rats injected with NE at 7 T MRI. At clinical 3 T, Z‐spectral MRI was used to quantify the FWF (58.5% ± 7.2% in BAT and 73.7% ± 6.5% in WAT with P < 0.0001) and APT (2.6% ± 0.8% in BAT and 0.9% ± 0.3% in WAT with P < 0.0001) signals in healthy volunteers. APT signals of BAT were negatively correlated with the BMI in humans ( r = 0.71). Data Conclusion Endogenous Z‐spectral MRI was demonstrated to simultaneously quantify BAT mass and function based on its FWF and APT contrasts. Level of Evidence 2. Technical Efficacy Stage 1.