Critical review of SPECT imaging in pulmonary embolism

Modern imaging of acute pulmonary embolism

The first-choice imaging test for visualization of thromboemboli in the pulmonary vasculature in patients with suspected acute pulmonary embolism (PE) is multidetector computed tomography pulmonary angiography (CTPA) - a readily available and widely used imaging technique. Through technological advancements over the past years, alternative imaging techniques for the diagnosis of PE have become available, whilst others are still under investigation. In particular, the evolution of artificial intelligence (AI) is expected to enable further innovation in diagnostic management of PE. In this narrative review, current CTPA techniques and the emerging technology photon-counting CT (PCCT), as well as other modern imaging techniques of acute PE are discussed, including CTPA with iodine maps based on subtraction or dual-energy acquisition, single-photon emission CT (SPECT), magnetic resonance angiography (MRA), and magnetic resonance direct thrombus imaging (MRDTI). Furthermore, potential applications of AI are discussed.

Ventilation/perfusion SPECT for the diagnosis of pulmonary embolism: A systematic review

BACKGROUND

Ventilation/perfusion (V/Q) single-photon emission computed tomography (SPECT) has largely replaced conventional planar V/Q scan in nuclear medicine departments for pulmonary embolism (PE) diagnosis. However, the diagnostic performance of the test and its role in the diagnostic management of acute PE are still a matter of debate.

OBJECTIVE

The primary aim was to establish the diagnostic accuracy (sensitivity, specificity) of V/Q SPECT for PE diagnosis. The secondary aim was to review the clinical outcomes of patients investigated for PE suspicion with a standardized algorithm based on V/Q SPECT.

METHODS

We conducted a systematic review of diagnostic accuracy and management outcome studies involving patients evaluated with V/Q SPECT for suspected acute PE. We searched from inception to June 23, 2020, MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials for diagnostic accuracy studies, randomized controlled trials, and observational cohort studies. The methodological quality and risk of bias of eligible studies were assessed using the Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2) and the Risk of Bias in Nonrandomized Studies of Interventions tools.

RESULTS

We identified 13 accuracy studies and one prospective outcome study. Eleven diagnostic accuracy studies were deemed at high risk of bias in at least two of the four domains of QUADAS-2 evaluation and a further two studies raised concerns regarding the applicability of results, precluding the meta-analysis for accuracy indices. The only prospective cohort study demonstrated critical risk of bias.

CONCLUSIONS

Although V/Q SPECT has been widely implemented in daily clinical practice, the exact diagnostic performance of V/Q SPECT for PE is still unknown. This systematic review clearly identifies knowledge gaps and sets the agenda for future research.

New developments and future challenges of nuclear medicine and molecular imaging for pulmonary embolism

Although widely validated, current tests for pulmonary embolism (PE) diagnosis, i.e. computed tomography pulmonary angiography (CTPA) and V/Q planar scintigraphy, have some limitations. Drawbacks of CTPA include the radiation dose, some contra indications and a rising concern about a possible overdiagnosis/overtreatment of PE. On the other hand, V/Q planar scintigraphy has a high rate of non-diagnostic tests responsible for complex diagnostic algorithms. Since the PIOPED study, imaging equipment and radiopharmaceuticals have greatly evolved allowing the introduction of techniques that improve imaging of lung ventilation and perfusion. Single photon emission computed tomography (SPECT) and SPECT/CT techniques are already largely used in daily practice and have been described to have greater diagnostic performance and much fewer non-diagnostic tests as compared with planar scintigraphy. However, they have not yet been firmly validated in large scale prospective outcome studies. More recently, it has also been proposed to image pulmonary perfusion and ventilation using positron emission tomography (PET), which has an inherent technical superiority as compared to conventional scintigraphy and may provide new insight for pulmonary embolism. Regardless of modality, these new thoracic imaging modalities have to be integrated into diagnostic strategies. The other major challenge for venous thromboembolism diagnosis may be the potential additional value of molecular imaging allowing specific targeting of thrombi in order, for example, to differentiate venous thromboembolism from tumor or septic thrombus, or acute from residual disease. In this article, the new imaging procedures of lung ventilation perfusion imaging with SPECT, SPECT/CT and PET/CT are discussed. We also review the current status and future challenge of molecular imaging for the in vivo characterization of venous thromboembolism.

Pulmonary Scintigraphy for the Diagnosis of Acute Pulmonary Embolism: A Survey of Current Practices in Australia, Canada, and France

There are currently no data published regarding the proportion of nuclear medicine centers using SPECT or SPECT/CT rather than planar ventilation/perfusion (V/Q) imaging in patients with suspected acute pulmonary embolism (PE). Furthermore, the reporting criteria used for interpretation of both planar and SPECT V/Q scans are variable and data are lacking regarding which criteria are commonly used in various centers. The aim of this study was to assess current practices regarding the performance and interpretation of lung scintigraphy across 3 different countries.

METHODS

A short online survey composed of simple multiple-choice questions was distributed to nuclear medicine departments in Australia, Canada, and France during the period April to December 2014. The survey covered image acquisition, interpretation criteria for SPECT and planar images, and use of pseudoplanar images and radiopharmaceuticals. Information was initially solicited by 2 sets of e-mails, which pointed to the survey internet link. Departments were subsequently contacted by telephone. A single response per department was consolidated.

RESULTS

Three hundred thirty-one responses were collected (Australia, 74; Canada, 48; and France, 209). Twenty-eight percent of centers indicated use of V/Q planar imaging alone whereas 72% of centers included some form of SPECT in their acquisition protocol for evaluation of PE, specifically V/Q SPECT in 36%, V/Q SPECT/CT in 29%, Q SPECT/CT in 2%, and both V/Q planar and SPECT in 5%, with a strong variability among countries. The most commonly used criteria for SPECT interpretation were the those of the European Association of Nuclear Medicine (60%). Criteria used for planar interpretation were heterogeneous (European Association of Nuclear Medicine criteria, 35%; Prospective Investigation of Pulmonary Embolism Diagnosis study, 29%; no standardized criteria, 21%). Sixty-three percent of departments used pseudoplanar images in addition to SPECT images.

CONCLUSION

In the 3 countries surveyed, SPECT has largely replaced planar imaging for evaluation of PE, with almost half of the SPECT studies incorporating a CT acquisition. Criteria used for interpretation are inconsistent, especially for planar imaging.