Evaluation of the number of SPECT projections in the ordered subsets-expectation maximization image reconstruction method

Simulated Fewer-Angle SPECT/CT Imaging Protocol for Parathyroid Adenoma

A new SPECT/CT protocol for parathyroid imaging detailing fewer image-angle acquisitions (fewer-angle SPECT/CT [FASpecT/CT]) was evaluated for identification of parathyroid adenoma. The motivation for validating this protocol was to be able to use it in the future to decrease patient imaging time in our clinic. Methods: This was a retrospective review of existing data performed as a simulated case control study evaluating 50 parathyroid SPECT/CT scans acquired using the standard 60-stop protocol and the tested 15-stop FASpecT protocol acquired using angular sampling software. Agreement on the final interpretations between imaging methods was evaluated using the McNemar test and the Cohen κ. Interrater reliability among the 3 readers was described for each method using the Fleiss κ interpreted as in the strength-of-agreement guidelines by Landis and Koch. Results: Of the 50 evaluated images, 45 (90%) had concordant final image interpretations between imaging methods. The sensitivity of FASpecT/CT relative to SPECT/CT was 17 of 19 (89.5%; 95% CI, 66.9%-98.7%), and the specificity was 28 of 31 (90.3%; 95% CI, 74.2%-98.0%). Additionally, there was statistically significant substantial agreement between protocols and among readers for each protocol. Conclusion: Adequate diagnostic-quality SPECT/CT images can be acquired using significantly fewer imaging stops given advances in camera quality and processing algorithms such as iterative reconstruction.

Improvement of Quantitative Single-Photon Emission Computed Tomography Image Quality by the New Step-and-Shoot Scan Mode

The step-and-shoot (SS) mode and continuous mode are currently used for single-photon emission computed tomography (SPECT) scan mode, and a new scan mode that combines both modes, step-and-shoot plus continuous (SSC) mode, was developed. It is expected to allow a shorter scan time and lower injected dose because the SSC mode is more sensitive than the SS mode. We confirmed the image quality of this scan mode, including various quantitative correction methods for scatter (SC), attenuation (AC), and resolution recovery (RR) in a phantom study and clinical case study. Image quality was evaluated by the count, contrast-to-noise ratio (CNR), and percent of the coefficient of variation (%CV). Independent of the correction methods, the count, CNR, and %CV of the SSC mode were superior to those of the SS mode. The ACSCRR was the best method, with a maximum increased rate of 66.4% in counts and 57.8% in CNR for the 13-mm sphere and 19.6% in CNR for other sphere sizes. The %CV for the SSC mode was the best for AC and ACRR, which was at 15.1%. With regards to attaining short bone SPECT scan time, the combination of the SSC mode and ACRR or ACSCRR demonstrated the best physical performance.

Fewer-Angle SPECT/CT Blood Pool Imaging for Infection and Inflammation

A new protocol for rapid SPECT/CT blood pool imaging consisting of fewer image-angle acquisitions (fewer-angle SPECT/CT, or FASpecT/CT) was evaluated for localization of focal sites of soft-tissue inflammation, infection, and osteomyelitis. Methods: Immediately after dynamic flow and standard planar blood pool imaging with ^99mTc-methylene diphosphonate, FASpecT/CT was performed with a dual-head γ-camera consisting of 6 steps over 360°, 12 total images with 30° of separation between angles, and 30 s per image, requiring a total imaging time of approximately 3 min. Images were reconstructed using iterative ordered-subset expectation maximization. Before use in a patient-care setting, various FASpecT/CT acquisition protocols were modeled using a phantom to determine the minimum number of stops and the stop duration required to produce a reliable image. Results: FASpecT/CT images provided excellent 3-dimensional localization of spine osteomyelitis, soft-tissue infection of the foot, and tendonitis of the hand and foot using a 3-min image acquisition time. The FASpecT/CT acquisition protocol required 1.3-3.5 min, including camera movement time. This was a reduction of 72%-90% from the time required for the standard 60-angle, 20-s SPECT/CT acquisition. Conclusion: The ability of FASpecT/CT blood pool images to help localize focal sites of hyperemia and inflammation can increase exam sensitivity and specificity. Additionally, using a FASpecT/CT protocol decreases imaging time by up to 90%.

FASpecT/CT, A New SPECT/CT Acquisition With Higher Sensitivity and Efficiency in Radioiodine Thyroid Cancer Imaging

PURPOSE

This article demonstrates the use of a new SPECT/CT acquisition protocol in patients with differentiated thyroid cancer (DTC).

METHODS

SPECT/CT scans (FASpecT/CT) with fewer angle acquisitions were retrospectively reviewed in 30 DTC patients treated with radioiodine at University Hospital, San Antonio, Tex, from July 2017 to March 2019. This FASpecT/CT of 12 versus 60 to 64 sampled views for convention SPECT was made possible by iterative reconstruction.

RESULTS

The FASpecT/CT protocol was judged to increase lesion detection in patients with low count rates. Furthermore, in patients with higher count rates, this technique reduced the acquisition time. FASpecT/CT patient images are shown as case examples in 4 of the 30 patients reviewed.

CONCLUSIONS

This FASpecT/CT acquisition in radioiodine-treated DTC offers the potential of higher sensitivity for metastatic lymph node detection in low count rates and a significant decrease in imaging time in high count rates. These advantages make SPECT/CT imaging more acceptable for patients who have difficulty with longer imaging times, to include the pediatric population.

Evaluation of Simultaneous Dual-radioisotope SPECT Imaging Using (18)F-fluorodeoxyglucose and (99m)Tc-tetrofosmin

OBJECTIVES

Use of a positron emission tomography (PET)/single-photon emission computed tomography (SPECT) system facilitates the simultaneous acquisition of images with fluorine-18 fluorodeoxyglucose ((18)F-FDG) and technetium ((99m)Tc)-tetrofosmin. However, (18)F has a short half-life, and 511 keV Compton-scattered photons are detected in the (99m)Tc energy window. Therefore, in this study, we aimed to investigate the consequences of these facts.

METHODS

The crosstalk correction for images in the (99m)Tc energy window involved the dual energy window (DEW) subtraction method. In phantom studies, changes in the count of uniform parts in a phantom (due to attenuation from decay), signal detectability in the hot-rod part of the phantom, and the defect contrast ratio in a cardiac phantom were examined.

RESULTS

For (18)F-FDG in the step-and-shoot mode, nearly a 9% difference was observed in the count of projection data between the start and end positions of acquisition in the uniform part of the phantom. Based on the findings, the detectability of 12 mm hot rods was relatively poor. In the continuous acquisition mode, the count difference was corrected, and detectability of the hot rods was improved. The crosstalk from (18)F to the (99m)Tc energy window was approximately 13%. In the cardiac phantom, the defect contrast in (99m)Tc images from simultaneous dual-radionuclide acquisition was improved by approximately 9% after DEW correction; the contrast after correction was similar to acquisition with (99m)Tc alone.

CONCLUSION

Based on the findings, the continuous mode is useful for (18)F-FDG acquisition, and DEW crosstalk correction is necessary for (99m)Tc-tetrofosmin imaging.

[Comparison of FBP and ML-EM reconstruction used by simulation data; effect of the projection number for the SPECT image]

PURPOSE

The influence of the numbers of projection of SPECT exerting on a re-constructed image cannot be strictly evaluated by phantom studies. Therefore, we compared re-constructed images of the FBP method and the ML-EM method by using simulation data.

METHODS

Simulation data was entered in the image processing software, and the projection data that changed the numbers of projection was made. Afterwards, reconstructed images of the FBP and the ML-EM methods were compared with respect to contrast, %COV, and the NMSE value.

RESULT

When the numbers of projection of the FBP and the ML-EM method were decreased, all of the contrast, %COV, and the NMSE value were more deteriorated than that of the ideal image. Therefore, the image quality of SPECT improves with both FBP and ML-EM methods when there are many numbers of projection. Moreover, the FBP method was excellent in a cold contrast, and the ML-EM method was uniformly excellent. Therefore, an understanding of features and their inspection are effective for the selection of each image reconstruction method.

[Evaluation of optimized injection dose and acquisition time using body mass index for three-dimensional whole-body FDG-PET]

OBJECTIVE

The standardized uptake value (SUV) is a relative measure of tracer uptake in tissue used in (18)F-FDG PET. However, the quality of ordered subset expectation maximization (OS-EM) images is sensitive to the number of iterations, because a large number of iterations leads to images with checkerboard noise. The main advantage of data acquisition in the three-dimensional (3D) mode is the high sensitivity to better exploit the intrinsic spatial resolution and the lower injection dose given to patients. In the 3D mode, the scatter fraction is higher, and, for a given administered dose, the random fraction is higher than that in the two-dimensional mode, which implies that correction methods need to be more accurate. Moreover, in clinical oncology (18)F-FDG PET studies, patients have a wide variety of body shapes and sizes, which may impact image statistics. Consequently, it is necessary to make constant the acquisition (true) counts. The purpose of this study was to optimize injection dose and acquisition time in consideration of body mass index (BMI) for 3D whole-body (18)F-FDG PET.

METHODS

A dedicated PET scanner, SIEMENS ECAT EXACT HR(+), was used to scan images of clinical data. The injection dose for BMI of <14-19, 19-22, 22-25, and 25< (kg/m(2)) were, 92.5 MBq, 111.0 MBq, 129.5 MBq, and 148.0 MBq, respectively. The emission scan time per bed position for BMI of <14-19, 19-22, 22-25, and >25 (kg/m(2)) were, 120, 120, 180, and 240 sec, respectively. A total of 20 patient subjects were evaluated as to true counts per bin (T/bin) of sinogram data and measured activity concentrations for the region of interest in the liver section.

RESULTS

T/bin was stable using an optimized protocol that took into consideration the BMI for any type of body morphology. The overall coefficient of variation was 7.27% for radioactivity concentration. Additionally, Gaussian filtering (8 mm FWHM) after reconstruction by the OS-EM method provided stable SUV values even when the iteration number was increased 30 times over.

CONCLUSION

Optimization of injection dose and acquisition time indicated that BMI was a clinically useful acquisition protocol for 3D whole-body (18)F-FDG PET.

Image distortion and correction in single photon emission CT

The task of single photon emission CT (SPECT) is to visualize the physiological function of various organs with the help of radiopharmaceuticals. But the projection data used for image reconstruction are distorted by several factors, making the reconstruction of a quantitative SPECT image very difficult in most cases. These factors include the attenuation and scattering of gamma rays, collimator aperture, data acquisition method, movement of organs, and washout of radiopharmaceuticals. This review article classifies the causes of the distortion in SPECT images and describes correction methods.