Quantification in positron emission tomography (PET) and single photon emission tomographic (SPET) relies on attenuation correction which is generally obtained with an additional transmission measurement. Therefore, the evaluation of the radiation doses received by patients needs to include the contribution of transmission procedures in SPET (SPET-TM) and PET (PET-TM). In this work we have measured these doses for both PET-TM and SPET-TM. PET-TM was performed on an ECAT EXACT HR+ (CTI/Siemens) equipped with three rod sources of germanium-68 (380 MBq total) and extended septa. SPET-TM was performed on a DST (SMV) equipped with two collimated line sources of gadolinium-153 (4 GBq total). Two anthropomorphic phantoms representing a human head and a human torso, were used to estimate the doses absorbed in typical cardiac and brain transmission studies. Measurements were made with thermoluminescent dosimeters (TLDs, consisting of lithium fluoride) having characteristics suitable for dosimetry investigations in nuclear medicine. Sets of TLDs were placed inside small plastic bags and then attached to different organs of the phantoms (at least two TLDs were assigned to a given organ). Before and after irradiation the TLDs were placed in a 2.5-cm-thick lead container to prevent exposure from occasional sources. Ambient radiation was monitored and taken into account in calculations. Transmission scans were performed for more than 12 h in each case to decrease statistical noise fluctuations. The doses absorbed by each organ were calculated by averaging the values obtained for each corresponding TLD. These values were used to evaluate the effective dose (ED) following guidelines described in ICRP report number 60. The estimated ED values for cardiac acquisitions were 7.7 x 10(-4) +/- 0.4 x 10(-4) mSv/MBq.h and 1.9 x 10(-6) +/- 0.4 x 10(-6) mSv/MBq.h for PET-TM and SPET-TM, respectively. For brain scans, the values of ED were calculated as 2.7 x 10(-4) +/- 0.2 x 10(-4) mSv/MBq.h for PET-TM and 5.2 x 10(-7) +/- 2.3 x 10(-7) mSv/MBq.h for SPET-TM. In our institution, PET-TM is usually performed for 15 min prior to emission. SPET-TM is performed simultaneously with emission and usually lasts 30 and 15 min for brain and cardiac acquisitions respectively. Under these conditions ED values, estimated for typical source activities at delivery time (22,000 MBq in SPET and 555 MBq for PET), were 1.1 x 10(-1) +/- 0.1 x 10(-1) mSv and 1.1 x 10(-2) +/- 0.2 x 10(-2) mSv for cardiac PET-TM and SPET-TM respectively. For brain acquisitions, the ED values obtained under the same conditions were 3.7 x 10(-2) +/- 0.3 x 10(-2) mSv and 5.8 x 10(-3) +/- 2.6 x 10(-3) mSv for PET-TM and SPET-TM respectively. These measurements show that the dose received by a patient during a transmission scan adds little to the typical dose received in a routine nuclear medicine procedure. Radiation dose, therefore, does not represent a limit to the generalised use of transmission measurements in clinical SPET or PET.

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Standardized uptake values (SUV) are commonly used in FDG PET to characterize suspicious high uptakes. To better understand the reliability and the limits of SUV, we studied the accuracy of uptake estimates as a function of a number of parameters using phantom data acquired on the CPET. Methods: Using the Data Spectrum thorax phantom in which spheres were inserted, we studied the effect of the sphere sizes, of out-of-the-field-of-view activity (OFOVA), of the emission scan duration and of attenuation and partial volume effect (PVE) corrections upon biases in tumor-to-normal tissue uptake ratio (TNR) estimates. Considering a specific acquisition and processing protocol as a reference, we determined the changes in TNR when modifying this protocol. Results: For a true TNR of 8, estimated TNR were strongly dependent on lesion size, but also on the acquisition and processing protocol. Depending on the method used to derive the attenuation map, estimated TNR could change by more than 50% for lung spheres less than 2 cm in diameter. Using PVE correction, TNR increased by a factor greater than 2 for spheres less than 2 cm in diameter. The very method used to estimate sphere activity from the reconstructed images could also change the TNR by more than 50%. Even with CT-based attenuation correction and PVE correction, TNR estimates remained underestimated by more than 20% in lesions less than 2 cm in diameter. Conclusion: Biases in uptake estimates strongly depend on the acquisition and processing protocols. This suggests that comparing SUV between studies make sense only if the scanner and the acquisition and processing protocols are strictly identical.