R2*-ρ Imaging on Rat Allograft Cardiac Transplantation with Acute Rejection: A Preliminary Study

Introduction: Cellular imaging of the immune response by MRI has been used to reveal the existence of particular cells in the tissue, which has several important applications in biomedical research. One important application is the detection of the rejection severity in cardiac allograft transplantation. Non-invasive methods using ultra-small superparamagnetic iron oxide (USPIO) and micrometer-sized superparamagnetic iron oxide (MPIO) particles on T2*-weighted images have been shown to be sensitive to acute and chronic rejection [1][2][3]. Using this approach of labeling macrophages by iron oxide particle, the rejection tissue can be visible in areas with decreased signal intensity on T2*-weighted images. However, the exact measurement of the rejection level requires accurate quantification of the iron particles, and the signal decreases on T2*-weighted image is not linearly related to the iron concentration, making it hard to perform quantification and to specify the severity of the rejection. To improve the quantification, R2* value has been shown to linearly correlated with the iron concentration, but it is well known that R2* is vulnerable to susceptibility artifact, and thus it only offers a non-specific quantification of the iron particles. In this study, we propose a new imaging modality called R2*-ρ, which is obtained by multiplying the R2* value by the spin density (ρ) to reduce the susceptibility artifact. To accurately estimate the R2* value and the spin density, a more accurate R2* estimation model was used to carry out the parameter estimation [4]. The proposed method is applied to acute cardiac rejection on rat allograft model, and both pre-contrast and post-USPIO images were acquired to perform a comparison and to evaluate the performance of the proposed method.