Chemical exchange saturation transfer MR imaging of articular cartilage glycosaminoglycans at 3 T: Accuracy of B_0 Field Inhomogeneity corrections with gradient echo method

摘要

Glycosaminoglycan Chemical Exchange Saturation Transfer (gagCEST) is an important molecular MRI methodology developed to assess changes in cartilage GAG concentrations. The correction for B_0 field inhomogeneity is technically crucial in gagCEST imaging. This study evaluates the accuracy of the B_0 estimation determined by the dual gradient echo method and the effect on gagCEST measurements. The results were compared with those from the commonly used z-spectrum method. Eleven knee patients and three healthy volunteers were scanned. Dual gradient echo B_0 maps with different ΔTE values (1, 2, 4, 8, and 10 ms) were acquired. The asymmetry of the magnetization transfer ratio at 1 ppm offset referred to the bulk water frequency, MTRasym(1 ppm), was used to quantify cartilage GAG levels. The B0 shifts for all knee patients using the z-spectrum and dual gradient echo methods are strongly correlated for all ΔTE values used (r = 0.997 to 0.786, corresponding to ΔTE = 10 to 1 ms). The corrected MTRasym(1 ppm) values using the z-spectrum method (1.34% ± 0.74%) highly agree only with those using the dual gradient echo methods with ΔTE = 10 ms (1.72% ± 0.80%; r = 0.924) and 8 ms (1.50% ± 0.82%; r = 0.712). The dual gradient echo method with longer ΔTE values (more than 8 ms) has an excellent correlation with the z-spectrum method for gagCEST imaging at 3 T.