Reinventing ventilation/perfusion lung scanning with SPECT

A Technical Overview of Technegas as a Lung Ventilation Agent

Technegas is a carbon-based nanoparticle developed in Australia in 1984 and has been in widespread clinical use, including SPECT imaging, since 1986. Although ^81mKr offers the ideal ventilation properties of a true gas, Technegas is considered preferred in more than 60 countries for ventilation imaging yet has limited adoption in the United States. In March 2020, a new U.S. Food and Drug Administration application was lodged for Technegas, and the impending approval warrants a detailed discussion of the technical aspects of the technology for those for whom it is new. Technegas is a simple yet versatile system for producing high-quality ^99mTc-based ventilation studies. The design affords safety to patients and staff, including consideration of radiation and biologic risks. Technegas is the gold standard for the ventilation portion of SPECT-based ventilation-perfusion studies in pulmonary embolism and several respiratory pathologies. When approved by the U.S. Food and Drug Administration, Technegas will extend advantages to workflow, safety, and study quality for departments that adopt the technology.

V/Q scanning using SPECT and SPECT/CT

Planar ventilation-perfusion (V/Q) scanning is often used to investigate pulmonary embolism; however, it has well-recognized limitations. SPECT overcomes many of these through its ability to generate 3-dimensional imaging data. V/Q SPECT has higher sensitivity, specificity, and accuracy than planar imaging and a lower indeterminate rate. SPECT allows for new ways to display and analyze data, such as parametric V/Q ratio images. Compared with CT pulmonary angiography, SPECT has higher sensitivity, a lower radiation dose, fewer technically suboptimal studies, and no contrast-related complications. Any nuclear medicine department equipped with a modern hybrid scanner can now perform combined V/Q SPECT with CT (using low-dose protocols) to further enhance diagnostic accuracy. V/Q SPECT (with or without CT) has application in other pulmonary conditions and in research.

Planar images reprojected from SPECT V/Q data perform similarly to traditional planar V/Q scans in the diagnosis of pulmonary embolism

OBJECTIVE

To compare the diagnostic interpretation of traditional ventilation/perfusion (V/Q) planar images with that of planar-like images reprojected from single-photon emission computed tomography (SPECT) data sets.

METHODS

Retrospective data from patients who had undergone both planar and SPECT imaging were used to generate anonymized reprojected planar images, which were compared with traditional planar V/Q images. Two consultants interpreted both sets of images for 81 patients following a proforma. We assessed the agreement in the final diagnosis between the two imaging methods and between the two clinicians. We also compared the number, nature, and localization of defects, as well as image quality. Finally, we compared the diagnosis made using planar methods with the original diagnosis made using SPECT.

RESULTS

There was excellent agreement in diagnosis both between the two planar methods (κ=0.93) and between the two consultants (κ=0.91). Similar numbers of defects were detected, with fewer matched defects being reported in the reprojected group by one of the clinicians. Localization of defects and image quality were similar for the two imaging methods. Six additional pulmonary embolisms were diagnosed using SPECT data.

CONCLUSION

We have shown that the performance of reprojected planars from SPECT V/Q was similar to that of traditional planars. These results have potential important implications for patient workflow in busy nuclear medicine departments, as well as for patient comfort.

Filtering in SPECT Image Reconstruction

Single photon emission computed tomography (SPECT) imaging is widely implemented in nuclear medicine as its clinical role in the diagnosis and management of several diseases is, many times, very helpful (e.g., myocardium perfusion imaging). The quality of SPECT images are degraded by several factors such as noise because of the limited number of counts, attenuation, or scatter of photons. Image filtering is necessary to compensate these effects and, therefore, to improve image quality. The goal of filtering in tomographic images is to suppress statistical noise and simultaneously to preserve spatial resolution and contrast. The aim of this work is to describe the most widely used filters in SPECT applications and how these affect the image quality. The choice of the filter type, the cut-off frequency and the order is a major problem in clinical routine. In many clinical cases, information for specific parameters is not provided, and findings cannot be extrapolated to other similar SPECT imaging applications. A literature review for the determination of the mostly used filters in cardiac, brain, bone, liver, kidneys, and thyroid applications is also presented. As resulting from the overview, no filter is perfect, and the selection of the proper filters, most of the times, is done empirically. The standardization of image-processing results may limit the filter types for each SPECT examination to certain few filters and some of their parameters. Standardization, also, helps in reducing image processing time, as the filters and their parameters must be standardised before being put to clinical use. Commercial reconstruction software selections lead to comparable results interdepartmentally. The manufacturers normally supply default filters/parameters, but these may not be relevant in various clinical situations. After proper standardisation, it is possible to use many suitable filters or one optimal filter.

Pulmonary enhancement imaging with dual energy CT for the detection of pulmonary embolism in a rabbit model: comparison to perfusion planar scintigraphy, SPECT and SPECT-CT modalities

RATIONALE AND OBJECTIVES

Pulmonary enhancement imaging (PEI) derived from dual-energy computed tomographic (CT) imaging has been used to detect perfusion defects from pulmonary embolism (PE). The purpose of this study was to compare the ability of PEI, planar, single photon-emission CT (SPECT) perfusion scintigraphy, and SPECT-CT fusion images to detect perfusion defect in a PE rabbit model.

MATERIALS AND METHODS

A PE model was made by injecting Gelfoam into the femoral veins of rabbits (n = 16). After 2 hours, 16 experimental rabbits and three control rabbits underwent contrast-enhanced dual-energy CT scans, from which PEI and CT pulmonary angiography were created, and planar, SPECT, and SPECT-CT fusion images were then obtained and evaluated. Pathologic determination of locations and numbers of lung lobes with PE were recorded. The sensitivity and specificity of the above-mentioned modalities were calculated using the histopathologic results as reference standards.

RESULTS

Emboli were present in 31 pulmonary lobes and absent in 64 lung lobes in histopathologic analysis. With the histopathologic findings as the gold standard, sensitivities and specificities of PEI, planar, SPECT, and SPECT-CT fusion images to detect PE were 100% and 96.9%, 71.0% and 84.4%, 77.4% and 90.6%, and 74.2% and 93.8%, respectively. McNemar's tests showed that PEI had higher diagnostic accuracy for the detection of PE than three scintigraphic images (all P values < .05), while three scintigraphic images had similar diagnostic accuracy (all P values = NS).

CONCLUSIONS

This study demonstrates that PEI from dual-energy CT imaging can provide higher diagnostic accuracy for detecting PE than planar, SPECT, and SPECT-CT fusion images in a rabbit model.