Caren van Roekel

46 Chapter 2 6.2. 166 Ho imaging Because 166 Ho emits gamma photons, is paramagnetic and has a high attenuation coefficient, it can be detected during and after administration. While radioactive, the gamma photons emitted by 166 Ho can be measured using single-photon emission computed tomography (SPECT) (Figure 8a-c). Independent of radioactivity, holmium is a lanthanide and with its paramagnetic properties allows visualization and quantification using MR imaging, while the high atom weight attenuates X-rays (15, 16). FIGURE 8A-C. Figure a-c show extrahepatic deposition in the falciform ligament (arrows) in a 53-year old patient with hepatic metastases of a neuro-endocrine neoplasm who was treated with 166 Ho-radioembolization. Figures 8a-c show the 166 Ho SPECT/CT, the low dose CT and a planar image of the 166 Ho SPECT/CT. 6.2.1. 166 Ho SPECT/CT Upon decay, 166 Ho emits several gamma photons, most of which are 81 keV (abundance 6.7%), 1379 keV (0.9%) or 1581 keV (0.2%). To quantify 166 Ho accurately, the SPECT detector has to be set to register photons with an energy window of 7.5% around the photon peak of 81 keV (14). However, the bremsstrahlung photons and other photons with high energies scatter and become part of the photo peak window, limiting accurate quantification. Also, the higher keV gamma photons from 166 Ho decay interact with the patient and the detector. Both these effects are partially corrected for by a downscatter window of 118±6% keV, as well as the use of a medium energy collimator. Furthermore, attenuation of the patient’s body causes a decrease in the photons. A Monte Carlo simulation compensates for attenuation, scatter and collimator-detector response, and has been developed to improve quantification (86). Acquisition of a 166 Ho SPECT/CT takes about 30 minutes. When all 166 Ho has decayed to erbium, there are no more gamma photons to image, so there is a