7T CEST MRI: A potential imaging tool for the assessment of tumor grade and cell proliferation in breast cancer.

Objectives: To investigate the feasibility of chemical exchange saturation transfer (CEST) MRI in patients with breast carcinomas and possible correlations between magnetization transfer asymmetry (MTR _asym ) values and histological features, such as tumor grade and the Ki-67 proliferation index.
Materials and Methods: Nine healthy subjects and 18 female patients were enrolled for this study. The imaging protocol for the patients consisted of diffusion-weighted imaging (DWI), CEST imaging, and T1-weighted, contrast-enhanced (CE)-MRI. CEST was performed using a 3D gradient echo (GRE) sequence, employing eight pre-saturation pulses of a duration of 50 ms and a duty cycle (DC) of 80%, with a mean amplitude of the saturation pulse train of 1 μT. The Z-spectrum was plotted and MTR _asym values calculated for the frequency of the maximum of MTR _asym curve, were correlated with the Ki-67 proliferation index and apparent diffusion coefficient (ADC). Patient data were statistically assessed using the Games-Howell post-hoc and Pearson's correlation test.
Results: Different tumor types had asymmetry peaks at different positions of Z-spectrum. MTR _asym (mean ± SD) (%) calculated for G1 (3.0 ± 0.3; range: 2.70-3.50) was not significantly lower than for G2 (4.50 ± 1.30; range: 3.20-6.50; p = 0.066). In contrast, the increase in MTR _asym between G1 and G3 (6.40 ± 1.70; range: 4.80-9.80) lesions was significant (p = 0.007). No significant difference was observed between G2 and G3 with regard to MTR _asym (p = 0.089). There was a strong positive correlation between the MTR _asym , and Ki-67 proliferation index (r = 0.890; p = 0.001), while there was a moderate negative correlation between MTR _asym and ADC values (r = -0.506; p = 0.027).
Conclusions: Calculated MTR _asym demonstrates a strong positive correlation with tumor proliferation and has the potential to become a valuable biomarker for breast tumor characterization.
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