In vivo measurement of spin-spin relaxation time for localized PRESS magnetic resonance spectroscopy (MRS) of lipids

Accumulation of excess abdominal visceral fat has been shown to play a crucial role in the development of cardiovascular disease and obesity-related disorders like diabetes mellitus, hyperlipidemia, and hypertension.
In the last years, rodent models have gained an important role in the investigation of adipose tissue (AT) biology and disorder. In this line, non-invasive differentiation and characterization of various AT deposits in the living animal is a current challenge. MRS at high magnetic field strength is a potential tool to learn more about the pathophysiological role of lipids in vivo.

The aim of the present study was to derive the spin-spin relaxation times T2 of 9 different proton resonances occurring in lipid spectra in vivo. With these values the correction factors M₀/M_TE (with the magnetization M_TE = M₀ x exp(-TE/T2), TE = echo time) for the peak integrals of the 9 different peaks can be calculated for specific TE. These correction factors can then be used to correct in vivo PRESS spectra to derive the true values for saturation and unsaturation of the triacylglycerols in lipids.

Localized MR spectra were generated in male Wistar rats with a 7 Tesla small animal scanner with a 30 cm bore (BioSpec 70/30, Bruker BioSpin MRI GmbH, Ettlingen, Germany) equipped with a mini imaging gradient coil system (gradient strength 400 mT/m) and a ¹H transmit-receive quatrature coil with 72 mm inner diameter.

We used a localized PRESS sequence with a repetition time of 1.8 s and a series of different TE (TE = 12 – 50 ms) to calculate T2. We determined T2 for 9 different peaks (in the range from 0.9 – 5.35 ppm) by fitting the monoexponential model function M_TE to the measured signal intensities at the different TE.

As results we got values for the spin-spin relaxation times T2 in the range from 24.7 ms to 57.4 ms. These values result in correction factors M₀/M_TE in the range from 2.25 to 1.42 for the 9 different peak integrals arising in lipid spectra derived with localized PRESS spectroscopy with TE = 20 ms.

We have measured 9 different T2-relaxation times of the protons of lipids in rats in vivo. These values have been used to correct localized PRESS spectra measured in vivo to obtain the true values for saturation and unsaturation of the fatty acids of AT deposits in experimental animal models. Prospectively, this method is also applicable in human studies using clinical MRS.