Caren van Roekel

47 Radioembolization time constraint on the imaging procedure. Conversely, just after administration of 166 Ho, the gamma photons are so abundant, that they invoke detector dead- time; the recorded photon produces a pulse of a certain duration during which no second pulse can be detected. Detector dead-time is seen when the activity in the scanner is greater than 1420 MBq (14). In practice, patients undergo a SPECT when the activity of their 166 Ho microspheres is between 250-500 MBq, between 3 and 6 days after treatment, depending on the infused activity. 6.2.2. 166 Ho MRI Quantification of 166 Ho using MR techniques is possible because of the local field inhomogeneities it induces. It allows for imaging of higher resolution, is independent of radioactivity, and can be combined with anatomical scans of different kinds to show tumor boundaries and deposition of microspheres, but its application is limited to tissue without air (i.e., not in lungs and gastrointestinal organs) and metal (staples from prior hepatectomies). In MR imaging, the nuclear magnetic moment of the nuclei of hydrogen atoms is aligned along a constantly applied magnetic field, after which a radiofrequency pulse disturbs its alignment. The resulting realignment of the nuclei after this pulse induces an electrical current in the receiver coil, which can be reconstructed to form an image. The realignment can be separated in a longitudinal (expressed in T 1 ) and a transversal vector (T 2 ). The T 2 vector from tissue decays quickly (several milliseconds) which is accelerated by the presence of Ho, while its effect on the T 1 is little to none. The decrease in T 2 * invokes a lower signal on gradient echo MR images; the microspheres show up as dark areas on these images. A linear relation has been shown between the disturbance in the T 2 signal decay and the concentration of Ho present (16). This relationship, called the relaxivity (R 2 *), depends on the strength of the main magnetic field of the scanner and the Ho content. A calibration phantom with known Ho microspheres concentrations enables the translation of the differences in R 2 * into units of microsphere concentrations, which convert into local activity using the activity per sphere. As MR imaging of Ho is independent of radioactivity, it is possible during or immediately after infusion, but also after several months. The first enables 2