Effective doses to patients from CT acquisitions on the GE Infinia Hawkeye: a comparison of calculation methods.

BACKGROUND: Nuclear medicine scans may be accompanied by CT acquisitions to provide localization of radioisotope uptake through image fusion and for use in attenuation correction. The effective doses to patients resulting from radioisotope administrations and from diagnostic CT scans are well documented. However, the development of gamma cameras with low dose CT attachments introduces the requirement for calculation of effective doses arising from non-standard CT acquisitions. In this study, the CT function of the GE Infinia Hawkeye was investigated and effective doses were calculated using various methods in order to assess the suitability of standard CT dose calculation methods. METHODS: Dose measurements were performed using Perspex head and body phantoms and the results were used in three calculation methods: (1) the ImPACT CT dosimetry calculator used Monte Carlo dose data to calculate effective doses; (2) organ fractions exposed by each scan were estimated and applied to dose measurements and ICRP tissue weighting factors; (3) standard conversion factors were used with measured and displayed dose indices to provide the simplest method of calculation. RESULTS: The maximum variation in effective dose using each calculation method was within 10% of the mean. Average effective doses from CT scans acquired using the Hawkeye were 0.9 mSv for a chest scan, 1.5 mSv for an abdomen-pelvis scan, and 0.1 mSv for a head scan, all significantly lower than doses resulting from diagnostic CT scans. CONCLUSION: These doses may be used for justification of radiation exposures in accordance with IR(ME)R 2000, in association with the accompanying radioisotope dose.

Measurement of Bremsstrahlung as a practical alternative to use of a liquid scintillation counter for the assessment of low activities of 90Y.

BACKGROUND: The use of 90Y for nuclear medicine therapies has steadily increased over the last 10 years. High administered activities are measured in a calibrated re-entrant ionization chamber, while the most sensitive method of assessment of low activities uses a liquid scintillation counter. This method requires the samples to have an acceptably low quench, and therefore heavily coloured samples must undergo chemical processing before assessment. An alternative method has been investigated to measure low activities of 90Y in a sodium iodide well counter by detection of associated Bremsstrahlung. METHODS: Test samples of 90Y with activities 0.5-730 Bq were measured in a well counter and a liquid scintillation counter, with counting times of 4 h per sample. Both counters were investigated for a relationship between count rate and activity. RESULTS: The efficiency of the well counter was found to be 0.08 count x s(-1) x Bq(-1), for a specified vial and sample volume. This is poor compared with the liquid scintillation counter efficiency of 1.0 count x s (-1) x Bq(-1). The uncertainty in measurement of a sample with unknown activity was calculated for a 4 h count time: +/-8.0% at 730 Bq and +/-45% at 6 Bq for the well counter; and +/-8.0% and +/-8.1%, respectively, for the liquid scintillation counter. These errors are dominated by the initial measurement of activity to determine counting efficiency, using a calibrator with an accuracy of +/-8%. If long counting times of both samples and background radiation are practicable, it has been found that a well counter can successfully be used to assess low activities of 90Y.

Estimated radiation dose to breast feeding infant following maternal administration of 57Co labelled to vitamin B12.

Administration of a radiopharmaceutical may result in a radiation dose to an infant due to ingestion of the radiopharmaceutical secreted in the breast milk. Following a maternal administration of Co labelled to vitamin B12 (cyanocobalamin) as part of a Schilling test an estimate of the absorbed dose to a breast feeding infant was calculated. Milk samples were collected from every feed in the first 24 h, and at approximately 48 and 72 h post-administration. The absorbed dose to the infant's liver (the organ receiving the highest dose) was calculated to be 0.23 mGy. The effective dose to the infant was calculated to be 0.025 mSv, which is considerably lower than the current regulatory limit of 1 mSv. The Administration of Radioactive Substances Advisory Committee advise that the first feed, at approximately 4 h after administration, be discarded. The data show that this was unwarranted, and that the peak concentration of Co in the breast milk occurred at around 24 h.