Dosimetry of transmission measurements in nuclear medicine: a study using anthropomorphic phantoms and thermoluminescent dosimeters

Abstract

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.

18F-FDG PET in Pregnancy and Fetal Radiation Dose Estimates

“…This radiation dose to the fetus represents activity from the administered radiopharmaceutical alone. Because all our studies were performed as PET-only studies with 68 Ge rod source transmission scanning for attenuation correction, the additional radiation exposure from the transmission scanning was negligible (12,13) and was not included in the dose estimates. The observed uptake and time-activity integrals from this study are compared in Table 4 with those from 2 other studies, 1 by Stabin (14), assigned on the basis of observed activity measured in primates, and 1 by Zanotti-Fregonara et al (15), measured in 1 human patient at about 10 wk of gestation.…”

“…for attenuation correction in lieu of CT. This ability facilitates further reduction in the fetal radiation exposure from 18 F-FDG PET in pregnant patients because the radiation dose from a 68 Ge rod source is negligible (12,13). Although the exact values of fetal radiation dose from transmission scanning with a 68 Ge rod source have not been calculated, for comparative purposes the effective dose from this method of scanning for cardiac and brain transmission studies has been estimated to be approximately 0.00077 and 0.00027 mSv/ MBqÁh, respectively.…”

Radiation Dose to Patients From Cardiac Diagnostic Imaging

“…Performed in a minority of nuclear cardiology laboratories, attenuation correction scans performed with either radioisotope sources or low-dose CT have Es that are small compared with those of radionuclide studies. 19,20 Table 3 demonstrates the effect of the tissue weighting factors on effective dose, using the 3 ICRP w T schema, and compares E 1 determined with the use of dose coefficients from ICRP tables with those from manufacturers' PIs. Organ doses for selected protocols are summarized in Table IV in the onlineonly Data Supplement, which lists the organs receiving the highest equivalent doses for each protocol.…”

Minimizing the radiation dose of CT attenuation correction while improving image quality: The case for innovation

“…One limitation of this approach was that the transmission data can be of poor quality; however, iterative reconstruction of the line source data significantly improves the quality of the reconstructed transmissions maps. 22,23 This approach, when sufficient counts are present, provides a very low radiation dose 24,25 with very highquality attenuation maps for SPECT and for PET using a relatively short transmission acquisition (see Figure 1). …”