Purpose: To quantify B 1 transmission-field inhomogeneity in breast imaging of normal volunteers at 3T using 3D T 1 -weighted spoiled gradient echo and to assess the resulting errors in enhancement ratio (ER) measured in dynamic contrast-enhanced MRI (DCE-MRI) studies of the breast.
Materials and Methods:A total of 25 volunteers underwent breast imaging at 3T and the B 1 transmissionfields were mapped. Gel phantoms that simulate preand postcontrast breast tissue T 1 were developed. The effects of B 1 -field inhomogeneity on ER, as measured using a 3D spoiled gradient echo sequence, were investigated by computer simulation and experiments on gel phantoms.Results: It was observed that by using the patient orientation and MR scanner employed in this study, the B 1 transmission-field field is always reduced toward the volunteer's right side. The median B 1 -field in the right breast is reduced around 40% of the expected B 1 -field. For some volunteers the amplitude was reduced by more than 50%. Computer simulation and experiment showed that a reduction in B 1 -field decreases ER. This reduction increases with both B 1 -field error and contrast agent uptake.Conclusion: B 1 transmission-field inhomogeneity is a critical issue in breast imaging at 3T and causes errors in quantifying ER. These errors would be sufficient to reduce the conspicuity of a malignant lesion and could result in reduced sensitivity for cancer detection.
The purpose of this work is to quantify the accuracy of pharmacokinetic parameter measurement in DCE-MRI of breast cancer at 3 T in relation to three sources of error. Individually, T1 measurement error, temporal resolution and transmitted RF field inhomogeneity are considered. Dynamic contrast enhancement curves were simulated using standard acquisition parameters of a DCE-MRI protocol. Errors on pre-contrast T1 due to incorrect RF spoiling were considered. Flip angle errors were measured and introduced into the fitting routine, and temporal resolution was also varied. The error in fitted pharmacokinetic parameters, K(trans) and v(e), was calculated. Flip angles were found to be reduced by up to 55% of the expected value. The resultant errors in our range of K(trans) and v(e) were found to be up to 66% and 74%, respectively. Incorrect T1 estimation results in K(trans) and v(e) errors up to 531% and 233%, respectively. When the temporal resolution is reduced from 10 to 70 s K(trans) drops by up to 48%, while v(e) shows negligible variation. In combination, uncertainties in tissue T1 map and applied flip angle were shown to contribute to errors of up to 88% in K(trans) and 73% in v(e). These results demonstrate the importance of high temporal resolution, accurate T1 measurement and good B1 homogeneity.
The variation of the native T 1 (T 10 ) of different tissues and B 1 transmission-field inhomogeneity at 3 T are major contributors of errors in the quantification of breast dynamic contrastenhanced MRI. To address these issues, we have introduced new enhancement indices derived from saturation-recovery snapshot-FLASH (SRSF) images. The stability of the new indices, i.e., the SRSF enhancement factor (EF SRSF ) and its simplified version (EF 0 SRSF ) with respect to differences in T 10 and B 1 inhomogeneity was compared against a typical index used in breast dynamic contrast-enhanced MRI, i.e., the enhancement ratio (ER), by using computer simulations. Imaging experiments with Gd-DTPA-doped gel phantoms and a female volunteer were also performed. A lower error was observed in the new indices compared to enhancement ratio in the presence of typical T 10 variation and B 1 inhomogeneity. At changes of relaxation rate (DR 1 ) of 8 s 21 , the differences between a T 10 of 1266 and 566 ms are <1, 12, and 58%, respectively, for EF SRSF , EF 0 SRSF , and ER, whereas differences of 20, 8, and 51%, respectively, result from a 50% B 1 field reduction at the same DR 1 . These quantification techniques may be a solution to minimize the effect of T 10 variation and B 1 inhomogeneity on dynamic contrast-enhanced MRI of the breast at 3 T. Magn Reson Med 67:531-540,
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