Detection And Assessment Of Glioma Using Magnetic Resonance Amide Proton Transfer Imaging At 3T

Glioma is the most common brain malignant tumor, which has the characteristics of poor prognosis and short survival period. Biomedical imaging plays a key role in tumor identification and therapeutic assessment. With the advantages of good soft tissue contrast, superior spatial resolution, flexible selection of imaging planes and multi-parametric imaging ability, magnetic resonance imaging(MRI) has been regarded as the optimal imaging method in clinical diagnosis and treatment evaluation of glioma. Compared with the conventional morphological MRI methods, MR molecular imaging can capture alterations of biological tissues at cellular level, leading to more sensitive detection of diseases. MR amide proton transfer(APT), one subset of chemical exchange saturation transfer(CEST) imaging, can be used to detect the amide protons of endogenous mobile proteins and peptides that are abundant in tumor, providing a novel noninvasive way for tumor identification.Closely associated with imaging parameters, APT effect is important in noninvasively determination of lesion type and histopathological grades. In this study, technical advancement of APT imaging was performed to improve imaging efficiency by optimization of imaging parameters and data acquisition scheme at 3T MR system. Novel Z-spectrum analysis approach was proposed to remove the influence of magnetic transfer(MT) and nuclear overhauser effect(NOE) for accurate APT effect quantification. The developed imaging and analysis methods were then applied in a glioma rat model. Their feasibilities in glioma detection and assessment were evaluated with comparing to histological results. Computer simulations and phantom studies show that the maximal APT effect could be achieved with the imaging parameters of duty cycle = 0.5, RF duration = 7 ms, flip angle = 180°and averaged B1 = 1.3 ?T. Meanwhile, in vivo experiment studies indicates the optimal imaging parameters of duty cycle = 0.5, RF duration = 15 ms, flip angle = 270°and averaged B1 = 1.0 ?T. The proposed variable-density Z-spectrum sampling scheme was demonstrated to be superior to the conventional sampling method within the same scan time. For example, the mean relative error percentage could be reduced from 12.6% to 8.0% on a simulated phantom with SNR of 50 and the labile proton-to-water ratio of 1/2000. The results imply the ability of the proposed sampling scheme in improvement of imaging efficiency if similar quantification accuracy is desired. Glioma animal studies show that MT and direct water saturation(DWS) effects could be extracted by fitting Z-spectra with two Lorentzian functions. With regarding the residual signals between 3.25~3.75 ppm as contributions from the amide protons, influences of MT and NOE could be efficiently removed and pure APT effect would be attained. Experimental results demonstrates the superior ability of the proposed APT imaging method in tumor detection and therapeutic assessment, such as in distinguishing tumor and radiation necrosis, compared to conventional MR imaging techniques(e.g., T1 and T2 imaging), which helps to provide important information for individualized treatment management and hence is of great clinical importance.