Functional nuclear medicine imaging with single-photon emission CT (SPECT) in combination with anatomical CT has been commercially available since the beginning of this century. The combination of the two modalities has improved both the sensitivity and specificity of many clinical applications and CT in conjunction with SPECT that allows for spatial overlay of the SPECT data on good anatomy images. Introduction of diagnostic CT units as part of the SPECT/CT system has also potentially allowed for a more cost-efficient use of the equipment. Most of the SPECT systems available are based on the well-known Anger camera principle with NaI(Tl) as a scintillation material, parallel-hole collimators and multiple photomultiplier tubes, which, from the centroid of the scintillation light, determine the position of an event. Recently, solid-state detectors using cadmium-zinc-telluride became available and clinical SPECT cameras employing multiple pinhole collimators have been developed and introduced in the market. However, even if new systems become available with better hardware, the SPECT reconstruction will still be affected by photon attenuation and scatter and collimator response. Compensation for these effects is needed even for qualitative studies to avoid artefacts leading to false positives. This review highlights the recent progress for both new SPECT cameras systems as well as for various data-processing and compensation methods.

PURPOSE: The energy spectrum for a cadmium zinc telluride (CZT) detector has a low energy tail due to incomplete charge collection and intercrystal scattering. Due to these solid-state detector effects, scatter would be overestimated if the conventional triple-energy window (TEW) method is used for scatter and crosstalk corrections in CZT-based imaging systems. The objective of this work is to develop a scatter and crosstalk correction method for (99m)Tc/(123)I dual-radionuclide imaging for a CZT-based dedicated cardiac SPECT system with pinhole collimators (GE Discovery NM 530c/570c). METHODS: A tailing model was developed to account for the low energy tail effects of the CZT detector. The parameters of the model were obtained using (99m)Tc and (123)I point source measurements. A scatter model was defined to characterize the relationship between down-scatter and self-scatter projections. The parameters for this model were obtained from Monte Carlo simulation using SIMIND. The tailing and scatter models were further incorporated into a projection count model, and the primary and self-scatter projections of each radionuclide were determined with a maximum likelihood expectation maximization (MLEM) iterative estimation approach. The extracted scatter and crosstalk projections were then incorporated into MLEM image reconstruction as an additive term in forward projection to obtain scatter- and crosstalk-corrected images. The proposed method was validated using Monte Carlo simulation, line source experiment, anthropomorphic torso phantom studies, and patient studies. The performance of the proposed method was also compared to that obtained with the conventional TEW method. RESULTS: Monte Carlo simulations and line source experiment demonstrated that the TEW method overestimated scatter while their proposed method provided more accurate scatter estimation by considering the low energy tail effect. In the phantom study, improved defect contrasts were observed with both correction methods compared to no correction, especially for the images of (99m)Tc in dual-radionuclide imaging where there is heavy contamination from (123)I. In this case, the nontransmural defect contrast was improved from 0.39 to 0.47 with the TEW method and to 0.51 with their proposed method and the transmural defect contrast was improved from 0.62 to 0.74 with the TEW method and to 0.73 with their proposed method. In the patient study, the proposed method provided higher myocardium-to-blood pool contrast than that of the TEW method. Similar to the phantom experiment, the improvement was the most substantial for the images of (99m)Tc in dual-radionuclide imaging. In this case, the myocardium-to-blood pool ratio was improved from 7.0 to 38.3 with the TEW method and to 63.6 with their proposed method. Compared to the TEW method, the proposed method also provided higher count levels in the reconstructed images in both phantom and patient studies, indicating reduced overestimation of scatter. Using the proposed method, consistent reconstruction results were obtained for both single-radionuclide data with scatter correction and dual-radionuclide data with scatter and crosstalk corrections, in both phantom and human studies. CONCLUSIONS: The authors demonstrate that the TEW method leads to overestimation in scatter and crosstalk for the CZT-based imaging system while the proposed scatter and crosstalk correction method can provide more accurate self-scatter and down-scatter estimations for quantitative single-radionuclide and dual-radionuclide imaging.

A revised geometric representative model of the lower part of the colon, including the rectum, the urinary bladder and prostate, is proposed for use in the calculation of absorbed dose from injected radiopharmaceuticals. The lower segment of the sigmoid colon as described in the 1987 Oak Ridge National Laboratory mathematical phantoms does not accurately represent the combined urinary bladder/rectal/prostate geometry. In the revised model in this study, the lower part of the abdomen includes an explicitly defined rectum. The shape of sigmoid colon is more anatomically structured, and the diameters of the descending colon are modified to better approximate their true anatomic dimensions. To avoid organ overlap and for more accurate representation of the urinary bladder and the prostate gland (in the male), these organs are shifted from their originally defined positions. The insertion of the rectum and the shifting of the urinary bladder will not overlap with or displace the female phantom's ovaries or the uterus. In the adult male phantom, the prostatic urethra and seminal duct are also included explicitly in the model. The relevant structures are defined for the newborn and 1-, 5-, 10- and 15-y-old (adult female) and adult male phantoms. METHODS: Values of the specific absorbed fractions and radionuclide S values were calculated with the SIMDOS dosimetry package. Results for 99mTc and other radionuclides are compared with previously reported values. RESULTS: The new model was used to calculate S values that may be crucial to calculations of the effective dose equivalent. For 131I, the S (prostate<--urinary bladder contents) and S (lower large intestine [LLI] wall<--urinary bladder contents) are 6.7 x 10(-6) and 3.41 x 10(-6) mGy/MBq x s, respectively. Corresponding values given by the MIRDOSE3 computer program are 6.23 x 10(-6) and 1.53 x 10(-6) mGy/MBq x s, respectively. The value of S (rectum wall<--urinary bladder contents) is 4.84 x 10(-5) mGy/MBq x s. For 99mTc, we report S (testes<--prostate) and S (LLI wall<--prostate) of 9.41 x 10(-7) and 1.53 x 10(-7) mGy/MBq x s versus 1.33 x 10(-6) and 7.57 x 10(-6) mGy/MBq x s given by MIRDOSE3, respectively. The value of S (rectum wall<--prostate) for 99mTc is given as 4.05 x 10(-6) mGy/MBq x s in the present model. CONCLUSION: The new revised rectal model describes an anatomically realistic lower abdomen region, thus giving improved estimates of absorbed dose. Due to shifting the prostate gland, a 30%-45% reduction in the testes dose and the insertion of the rectum leads to 48%-55% increase in the LLI wall dose when the prostate is the source organ.