Local validation of the use of Evolution for Bone for bone SPECT imaging.

PURPOSE: In order to locally validate the technique, a retrospective review of a cohort of randomly selected single-photon emission computed tomography (SPECT) bone scans reconstructed with ordered subsets expectation maximization (OSEM) and Evolution for Bone was undertaken. MATERIALS AND METHODS: Thirty consecutive bone SPECT patient data sets (17 spine, nine pelvis, and four spine and pelvis) were chosen. Poisson resampling was used to simulate reduced count data at 50, 75, and 100% of the original number of counts. Evolution for Bone applied resolution recovery to the reduced count images. All images were compared with the original OSEM images, currently used as the standard for clinical use. A qualitative blinded assessment was made by two independent observers, who assessed for noise, contrast, and resolution. RESULTS: Both radiologists saw an improvement in resolution (P = 0.776), noise (P = 0.007), and image quality with all data sets, compared with images processed purely with OSEM and viewed in Volumetrix. However, they completely disagreed on contrast, as the two radiologists scored contrast differently; however, the results are understandable. CONCLUSION: Images with 50, 75, and 100% of the original counts viewed using Evolution for Bone have improved image quality compared with images processed purely with OSEM and viewed in Volumetrix. Evolution for Bone therefore has great potential in departments for reducing either patient doses, waiting lists, or both.

Use of measured scatter data for the attenuation correction of single photon emission tomography without transmission scanning.

PURPOSE: Attenuation correction is essential for reliable interpretation of emission tomography, however, the use of transmission measurements to generate attenuation maps is limited by availability of equipment and potential mismatches between the transmission and emission measurements. The authors present a first step toward a method of estimating an attenuation map from measured scatter data without a transmission scan. METHODS: A scatter model has been developed that accurately predicts the distribution of photons which have been scattered once. The scatter model has been used as the basis of a maximum likelihood gradient ascent method to estimate an attenuation map from measured scatter data. In order to estimate both the attenuation map and activity distribution, iterations of the derived scatter based algorithm have been alternated with the maximum likelihood expectation maximization algorithm in a joint estimation process. For each iteration of the attenuation map estimation, the activity distribution is fixed at the values estimated during the previous activity iteration, and in each iteration of the activity distribution estimation the attenuation map is fixed at the values estimated during the previous attenuation iteration. The use of photopeak data to enhance the estimation of the attenuation map compared to the use of scatter data alone has also been considered. The algorithm derived has been used to reconstruct data simulated for an idealized two-dimensional situation and using a physical phantom. RESULTS: The reconstruction of idealized data demonstrated good reconstruction of both the activity distribution and attenuation map. The inclusion of information recorded in the photopeak energy window in the attenuation map estimation step demonstrated an improvement in the accuracy of the reconstruction, enabling an accurate attenuation map to be recovered. Validation of the results with physical phantom data demonstrated that different regions of attenuation could be distinguished in a real situation and produces results that represent a promising first step toward the use of scatter data to estimate the activity distribution and attenuation map from single photon emission tomography (SPECT) data without a transmission scan. CONCLUSIONS: The technique presented shows promise as a method of attenuation correction for SPECT data without the need for a separate transmission scan. Further work is required to further improve the method and to compensate for the assumptions used in the scatter model.

Investigation of normal ranges for left ventricular ejection fraction in cardiac gated blood pool imaging studies using different processing workstations.

BACKGROUND: An investigation has been undertaken to calculate normal ranges for left ventricular ejection fraction (LVEF) measured by the gated blood pool (GBP) technique. A common set of normal studies was used at 11 hospitals within the south of England to assess the variability of results and normal ranges. METHODS: Normal studies were identified by retrospective review of patients who had undergone a GBP study and echocardiogram at the Royal United Hospital, Bath. Patients who had left ventricular function qualitatively identified as normal on echocardiogram and normal wall motion for the GBP were included. In total, 64 datasets were found to match the criteria. All the studies were made anonymous prior to being distributed to the participating hospitals. The upper and lower limits for normal ejection fraction were defined for each system using the 95% confidence limits around the mean value, before and after normalizing the results to remove systematic differences between the processing systems. RESULTS: The lower cut-off for normal function varied between 40 and 51%. Analysis of the individual operator results gave an inter-operator standard deviation of 4.2 and an intra-operator standard deviation of 2.7. CONCLUSIONS: It is recommended that all studies should be processed at least twice and an average taken to minimise these sources of uncertainty. If there is a significant difference the study should be reprocessed. Due to random differences between results from different systems it is suggested that an equivocal range be used between clearly normal and abnormal function.

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.