Quantitative diffusion imaging of adipose tissue in the human lower leg at 1.5 T

Diffusion-weighted spin-echo echo-planar imaging was developed and applied for assessment of diffusion coefficients of adipose tissue in human lower leg on a 1.5-T whole-body MR scanner. Because of the higher molecular weight of triglycerides, apparent diffusion coefficients (ADCs) of adipose tissue are approximately two orders of magnitude smaller compared with water, leading to the necessity of using high b-values up to 50,000 sec/mm(2) and an echo time of 240 msec for sufficient diffusion-related signal attenuation. ADC maps of adipose tissue in the human lower leg were derived for diffusion encoding along orthogonal spatial directions in six healthy volunteers. Mean diffusion coefficients in the tibial bone marrow amounted to (1.81 ± 0.10) × 10(-5) mm(2) /sec (left-right), (1.96 ± 0.10) × 10(-5) mm(2) /sec (anterior-posterior), and (1.96 ± 0.20) × 10(-5) mm(2) /sec (head-feet), respectively. Pixel-wise calculated ADC values of subcutaneous adipose tissue showed a distinctly higher variation with the smallest ADC values similar to those measured for tibial bone marrow. Some subcutaneous adipose tissue regions showed increased signal attenuation at higher b-values resulting in ADC coefficients up to 4.2 × 10(-5) mm(2) /sec. It must be noted that diffusion measurements with extremely high b-values in vivo are extremely sensitive to incoherent motion effects in tissue. Nonetheless, it could be shown that in vivo diffusion imaging of adipose tissue in human lower leg is possible at 1.5 T in acceptable measurement time of a few minutes. Potential future applications of fat diffusion imaging are seen in temperature measurements in adipose tissue, detection of free fatty acids in white or brown adipose tissue in case of high lipolysis, differentiation of macro- and microvesicular steatosis, or assessment of the mobility of intramyocellular lipids.