Estimating Relative Levels of Linolenic Acid with Magnetic Resonance Spectroscopy of Fat Allylic Protons at 3 T
Abstract
Purpose
To exploit differences in J-coupling evolution of the ≈2 ppm fat allylic protons (neighbor the two ends of the double bond chain) to estimate relative linolenic acid content at 3 T. Oleic (mono-unsaturated), linoleic (di-unsaturated) and linolenic acid (tri-unsaturated) are the three primary unsaturated components of edible oils.
Methods
A 3 T Philips MRI scanner was used with a Point RESolved Spectroscopy (PRESS) sequence (first echo time, TE1, 17 ms) to measure the response of fat allylic protons from oleic, linoleic and linolenic acid. TE values ranged between 40 ms and 300 ms in steps of 10 ms. A TE that resulted in relatively low yield for oleic and linoleic acid but a significantly higher output for linolenic acid was identified. Measurements were obtained with the identified TE and with a TE of 70 ms (resolves allylic resonance from that of the ≈2.2 ppm resonance) from oils of varying linolenic acid content (linseed, walnut, canola, soybean and peanut oil).
Results
A TE of 220 ms was found to be effective for identifying the presence of linolenic acid. J-coupling evolution resulted in inverted allylic signal for all three constituents. The allylic resonance areas obtained with a TE of 220 ms normalized to that acquired with a TE of 70 ms were -0.07, -0.22 and -2.8 for oleic, linoleic and linolenic acid, respectively. The higher signal yield for linolenic acid is likely a result of more J-coupling evolution rewinding due to weaker coupling. A correlation was found between linolenic acid contribution to total unsaturation and allylic signal area obtained with a TE of 220 ms normalized to that from a TE of 70 ms (R2=0.99).
Conclusion
Allylic fat proton signal obtained with PRESS with a TE of 220ms can provide an indirect means of estimating relative linolenic levels at 3 T.