Caren van Roekel

48 Chapter 2 real-time estimation of damage to tumors or healthy liver, which could, in the future, lead to MR-guided radioembolization with simultaneous quantification for treatment optimization. The latter is useful for research into the dynamics and stability of implanted microspheres. The first human MR quantification was performed as part of the phase 1 HEPAR I trial (15). Of the injected mean total of 523 mg (range, 438-640 mg), MR quantification detected a mean of 431 mg (range, 236-666 mg) of microspheres in the liver, or 89±19%. As surgical clips distorted the quantification, the detection improved to 96±13% when these patients (5/14) were excluded. For this quantification, a map of T 2 * values of the liver was made before and after radioembolization. After subtraction (with inherent registration errors), the amount of microspheres was computed. 7. FOLLOW-UP IMAGING AND RESPONSE IDENTIFICATION 7.1. Imaging with CT and MRI After radioembolization, follow-up imaging is usually performed at three-month intervals (1). It is important to consider the changes in tumor appearance that may be a consequence of radioembolization treatment itself. For example, the tumor may increase shortly after therapy, due to acute necrosis, edema and hemorrhage. Also, inflammation and fibrosis adjacent to the tumor may be falsely interpreted. These changes may lead to a rim enhancement around the tumor, which resembles vital tumor. Furthermore, the presence of tumor necrosis has to be taken into account when measuring tumor size. Because of these typical radioembolization-induced changes, CT and MRI protocols should at least include contrast-enhanced imaging, using an arterial and portal-venous contrast delay. Unenhanced images can be helpful to distinguish between hemorrhage and vital tumor tissue. With MRI, different sequences add to the diagnosis of a lesion. T1 sequences allow for the detection of fat and other substances with a high T1 signal, such as hemorrhage. T2 sequences are used to distinguish between solid and cystic lesions. Diffusion weighted imaging (DWI) evaluates the free motility of water molecules. The mobility is restricted by cell membranes and tissues with different cellularity have differences in water molecule movement. Malignancies are generally highly cellular and water molecule motility is