Ventilation Scintigraphy With Radiolabeled Carbon Nanoparticulate Aerosol (Technegas): State-of-the-Art Review and Diagnostic Applications to Pulmonary Embolism During COVID-19 Pandemic

ABSTRACT

Invented and first approved for clinical use in Australia 36 years ago, Technegas is the technology that enabled ventilation scintigraphy with 99m Tc-labeled carbon nanoparticles ( 99m Tc-CNP). The US Food and Drug Administration (FDA) has considered this technology for more than 30 years but only now is getting close to approving it. Meanwhile, more than 4.4 million patients benefited from this technology in 64 countries worldwide. The primary application of 99m Tc-CNP ventilation imaging is the diagnostic evaluation for suspicion of pulmonary embolism using ventilation-perfusion quotient (V/Q) imaging. Because of 99m Tc-CNP's long pulmonary residence, tomographic imaging emerged as the preferred V/Q methodology. The FDA-approved ventilation imaging agents are primarily suitable for planar imaging, which is less sensitive. After the FDA approval of Technegas, the US practice will likely shift to tomographic V/Q. The 99m Tc-CNP use is of particular interest in the COVID-19 pandemic because it offers an option of a dry radioaerosol that takes approximately only 3 to 5 tidal breaths, allowing the shortest exposure to and contact with possibly infected patients. Indeed, countries where 99m Tc-CNP was approved for clinical use continued using it throughout the COVID-19 pandemic without known negative viral transmission consequences. Conversely, the ventilation imaging was halted in most US facilities from the beginning of the pandemic. This review is intended to familiarize the US clinical nuclear medicine community with the basic science of 99m Tc-CNP ventilation imaging and its clinical applications, including common artifacts and interpretation criteria for tomographic V/Q imaging for pulmonary embolism.

Dual energy window imaging for optimisation of P/V ratios in VP SPECT

Purpose

Ventilation–perfusion single-photon emission computed tomography (VP SPECT) plays an important role in pulmonary embolism diagnosis. Rapid results may be obtained using same-day ventilation followed by perfusion imaging, but generally requires careful attention to achieving an optimal count rate ratio (P/V ratio) of ≥ 3:1. This study investigated whether the ratio of counts simultaneously acquired in adjacent primary and Compton scatter energy windows (E_ratio) on V SPECT was predictive of final normalised perfusion count rate (PCR_norm) on P SPECT using [^99mTc]Tc-macroaggregated albumin (MAA), thus allowing for optimisation of P/V ratios.

Methods

Same-day VP SPECT studies acquired using standard protocols in adult patients during a 2-year period (training dataset) were assessed. Studies were included provided they were acquired with correct imaging parameters, and injection site imaging and laboratory records were available for quality control and normalised count rate corrections. Extraction of DICOM information, and linear regression were performed using custom Python and R scripts. A predictive tool was developed in Microsoft Excel. This tool was then validated using a second (validation) dataset of same-day studies acquired over a subsequent 7-month period. Accuracy of the prediction tool was assessed by calculating the mean absolute percentage error (MAPE).

Results

Of 643 studies performed, the scans of 342 participants (median age 30.4 years, 318 female) were included in the training dataset, the analysis of which yielded a significant regression equation (F(1,340) = 1057.3, p < 0.0001), with an adjusted R² of 0.756 and MSE of 0.001089. A prediction tool designed for routine clinical use was developed for predicting final P/V ratio. Of an additional 285 studies, 198 were included in the second (validation) dataset (median age 29.7 years, 188 female). The Excel-based tool was shown to be 91% accurate (MAPE: 9%) in predicting P/V ratio.

Conclusion

The relationship between the ratio of simultaneously acquired counts in adjacent energy windows on V SPECT and perfusion count rate after administration of a known activity of [^99mTc]Tc-MAA can be linearly approximated. A predictive tool based on this work may assist in optimising the dose and timing of [^99mTc]Tc-MAA administration in same-day studies to the benefit of patients and workflows.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-021-00417-z.

Individual calculation of perfusion activity based on ventilation efficiency for V/P-SPECT

AIM

 Implementation of the individual calculation of perfusion activity to ensure the guideline-compliant ratio of perfusion to ventilation (P/V-ratio) of ≥ 3 in the diagnosis of acute pulmonary embolism (PE) using V/P-SPECT.

MATERIAL AND METHODS

 50 consecutive V/P-SPECT examinations, in which a standard activity of 160 MBq was applied for perfusion imaging, are evaluated retrospectively. Based on this patient group an activity factor is determined, which provides a correlation between the applied perfusion activity and the expected perfusion counts of the gamma camera. Using the mean activity factor, the perfusion activity required for a P/V-ratio of four is calculated using the previously acquired ventilation count rate. This is applied prospectively to the 100 subsequent examinations.

RESULTS

 The mean perfusion activity factor is (54.56 ± 10.13) cps/MBq. The individually calculated perfusion activities range from 80 MBq to 200 MBq with an average value of (146.9 ± 35.3) MBq and a median of 140 MBq. The individual activity calculation thus reduced the mean perfusion activity by 8.2 % and the median by 12.5 %. In addition, the individual calculation reduced the proportion of P/V ratios < 3 from 14 % to 0 % and the proportion of P/V ratios > 5 from 24 % to 19 %.

CONCLUSION

 The presented method for the individual calculation of perfusion activity offers a simple way to ensure a guideline-compliant P/V-ratio. Furthermore, unnecessarily high perfusion activity as a result of inadequate ventilation can be avoided.

Semi-automated Analysis of Ventilation-Perfusion Single-Photon Emission Tomography in the Diagnosis of Pulmonary Embolism – Does it add extra value?

Aim Diagnosis of pulmonary embolism using V/P-SPECT may include the application of advanced image-processing techniques to identify V/P-mismatches. Aim of this study was to evaluate the benefit in clinical decision making in the diagnosis of pulmonary embolism.by whether adding to conventional reading a software that automatically calculates and visualizes the ventilation/perfusion-quotient pixel by pixel.

Methods 63 consecutive patients with a clinical suspicion of PE who underwent V/P-SPECT were included in this retrospective study. Images were randomly ordered both for standard as well as for software-assisted reading using V/P-quotients. Studies were read independently by 2 experienced and 2 inexperienced raters. Diagnostic performance and observer agreement of all readers and both reading methods were determined.

Results Expert observers consistently achieved a high diagnostic accuracy both in conventional as well as in software-assisted reporting (sensitivity: 0.94 vs. 0.94, specificity: 0.96 vs. 0.97, LR+: 17.32 vs. 28.86, LR– stayed constant at 0.06). For inexperienced readers, diagnostic performance improved: sensitivity raised from 0.74 to 0.85 and specificity from 0.86 to 0.95, LR+ raised from 5.20 to 15.69, LR– decreased from 0.31 to 0.16. Inter-rater reliability (Fleiss’ κ) improved from 0.63 to 0.86 by using V/P quotient.

Conclusion Benefit from a software-tool that calculates V/P-ratio automatically is only small when used by experienced physicians If inexperienced readers use the software, the diagnostic accuracy increases. Images generated by automated calculation of V/P-mismatches are easy to read and their use might help to standardize and objectify interpretation of V/P-SPECT in the diagnosis of PE.

Research on the method of controlling the 99mTc-Technegas inhaled during 1-day pulmonary ventilation/perfusion imaging

OBJECTIVE

Our purpose was to establish a simple and feasible method for monitoring and controlling the Tc-Technegas inhaled to improve the success ratio of imaging and ensure the imaging quality.

MATERIALS AND METHODS

The relationship between the success ratio and the pulmonary ventilation counting rate (VCR) of 113 cases, the activity of perfusion imaging agents injected and the pulmonary perfusion counting rate (PCR) of another 114 cases were analysed retrospectively. And combined with the relationship between the surface radioactivity monitoring value and the SPECT probe counting rate of a pulmonary model, the effective range of the VCR and the surface radioactivity monitoring value were determined. Two hundred fifty cases with Tc-Technegas inhaled monitored and controlled were used to verify the reliability and practicability of this method.

RESULTS

The VCR of the ventilation/perfusion imaging with deep venous thrombosis imaging and the ventilation/perfusion imaging without deep venous thrombosis imaging was in 1.0-3.0 kct/s and 1.0-2.0 kct/s when the monitoring values of handheld radiation monitor was within the range of 60-170 µSv/h and 60-110 µSv/h, respectively. The success ratio of the V/Q-Only increased from 48.9% (43/88) of the control group to 80.8% (122/151) of the experimental group. The VCR in the two groups was examined by the non-parametric Mann-Whitney U test (P < 0.001), which indicated that there was a significant difference between the experimental group and the control group.

CONCLUSION

The external monitoring method established in this study was of great significance in improving the success ratio of 1-day pulmonary ventilation/perfusion imaging and ensuring the image quality.