TY - JOUR T1 - Quantitative 124I-PET in the presence of high 131I-activities JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 99 LP - 99 VL - 54 IS - supplement 2 AU - Poul-Erik Braad AU - Søren Hansen AU - Poul Hoilund-Carlsen Y1 - 2013/05/01 UR - http://jnm.snmjournals.org/content/54/supplement_2/99.abstract N2 - 99 Objectives Peri-therapeutic 124I-PET/CT is interesting for radionuclide therapy. Unfortunately, image quality is complicated by dead time effects and random coincidence rates from high 131I activities. Before applying 124I-PET in the management of thyroid cancer, we examined the effect of high 131I-activities on 124I-PET measurements. Methods 124I-spatial resolution was measured from line sources in water. Quantitation accuracy and background variability was determined in an 18 cm high, 20 cm diameter cylindrical phantom filled with an aqueous mix of 25 MBq 124I and 1000 MBq 131I. Six spherical inserts with volumes ranging from 0.52 to 26.52 ml were filled with a homogeneous aqueous solution of 124I (0.45 MBq/ml) and 131I (22 MBq/ml) and placed inside a non-radioactive background of the NEMA/IEC phantom. 124I-activity was assessed in reconstructed PET/CT images and the activity recovery of each lesion was determined. Measurements were performed repeatedly during a month on the decaying phantoms using a GE Discovery 690 PET/CT system. Reconstructions were corrected for prompt gamma coincidences and compared to 124I- and 18F-acquisitions. Assessments of 124I- and 131I-activities were compared to gamma ray spectroscopy measurements. Results The spatial resolution was 4.6 mm and unaffected by background 131I. Image uniformity was also unchanged, whereas image noise was slightly increased by 131I. The visual image quality was reduced compared to 18F- but comparable to 124I-alone images. At high count rates, 131I reduced the activity assessments by up to 7% compared to 124I-alone measurements. Activity recovery curves were of similar shape, albeit with an activity recovery reduction at high count rates similar to that observed in the cylindrical phantom. Conclusions The quality of 124I-PET images in the presence of high 131I-activities was excellent and image quantification good, though slightly reduced at high count rates. Thus, quantitative 124I-PET during radioiodine therapy is feasible. ER -