Optimizing ventilation-perfusion lung scintigraphy: Parting with planar imaging

V/Q SPECT and SPECT/CT in Pulmonary Embolism

Ventilation and perfusion (V/Q) lung scintigraphy has been used in the assessment of patients with suspected pulmonary embolism for more than 50 y. Advances in imaging technology make SPECT and SPECT/CT feasible. This article will examine the application and technical considerations associated with performing 3-dimensional V/Q SPECT and the contribution of a coacquired CT scan. The literature tends to be mixed and contradictory in terms of appropriate investigation algorithms for pulmonary embolism. V/Q SPECT and SPECT/CT offer significant advantages over planar V/Q, with or without the advantages of Technegas ventilation, and if available should be the preferred option in the evaluation of patients with suspected pulmonary embolism.

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.

A Brief History of Lung Ventilation and Perfusion Imaging Over the 50-Year Tenure of the Editors of Seminars in Nuclear Medicine

The ventilation/perfusion lung scan has been in continuous use for approximately half a century, the same lifetime as Seminars in Nuclear Medicine. Remarkably, the founding Editors-in-Chief have continued to guide the journal over this entire period. In this Feschrift issue celebrating their enormous contribution, we review the history of the lung scan, its highs and lows, the transition from planar to SPECT/CT V/Q scans, and the future that is in store in this age of multimodality functional imaging. We concur with the published view of one of the retiring editors (LMF) that V/Q scintigraphy is indeed alive and well and has a definite future in clinical medicine.

V/P SPECT as a diagnostic tool for pregnant women with suspected pulmonary embolism

Purpose

The purpose of the study was to assess the prevalence of pulmonary embolism (PE) and other lung diseases among pregnant women with suspected PE and to calculate the radiation exposure to patient and fetus in this population. As a secondary aim, we evaluated the negative predictive value of a normal ventilation/perfusion single photon emission computed tomography (V/P SPECT) examination in pregnancy.

Methods

We studied all 127 pregnant women who had suspected PE and had undergone V/P SPECT at our institution in the course of a 5-year period. Radiation exposure to patient and fetus and the negative predictive value of a normal V/P SPECT examination were also measured.

Results

V/P SPECT identified PE in 11 women (9 %). Moreover, in 15 women (12 %) the examination revealed pneumonia (in 2 cases in addition to PE) and in 1 woman signs of airway obstruction were revealed. Among the 116/127 women (91 %) where PE was ruled out by V/P SPECT, none was diagnosed subsequently with PE or deep venous thrombosis (DVT) during the same pregnancy or puerperal period. For P SPECT, the calculated fetal absorbed dose was < 0.6 mGy,and the calculated breast absorbed dose 0.6 mGy. For V SPECT, the calculated fetal absorbed dose was < 0.014 mGy and the breast absorbed dose 0.25 mGy.

Conclusion

The prevalence of PE was low (9 %) among pregnant women with suspected disease. Pneumonia was diagnosed in 12 % of patients. The negative predictive value of V/P SPECT was high, and the radiation exposure from V/P SPECT was low both for fetus and patient.

Contribution of V/Q SPECT to planar scintigraphy in the diagnosis of pulmonary embolism

AIM

To evaluate the feasibility of V/Q SPECT and analyze its contribution to planar V/Q lung scintigraphy in the diagnosis of pulmonary embolism (PE).

MATERIAL AND METHODS

A total of 109 patients with suspected PE showing Wells score>2 and elevated D-dimer were studied. The V/Q could not be completed in 7 patients, so they were excluded. Ventilation and perfusion scans were done using Technegas and (99m)Tc-MAA. Planar study included 8 projections on a 256×256 matrix and 128 projections on a 128×128 matrix were acquired for the SPECT study, applying an iterative method. Planar images were interpreted according to modified PIOPED criteria, and SPECT by the guidelines of the EANMMI. The results with both techniques were compared.

RESULTS

V/Q planar scintigraphy and SPECT could be performed in 102 patients. V/Q planar scintigraphy was considered "diagnostic" in 39 of the 102 patients, and "non-diagnostic" in 63. Of the 39 "diagnostic" studies, 31 were reported as high probability of PE and 8 as normal. Of the 63 "non-diagnostic", 26 corresponded to intermediate, 29 to low, and 8 to very low probability. The SPECT study was "diagnostic" in 97 and indeterminate in only 5. All patients with a high probability planar scintigraphy had a positive SPECT. In the 8 patients with a normal planar scintigraphy SPECT was negative in 5 and positive in 3. In the 63 patients with a "non-diagnostic" planar scintigraphy SPECT was "diagnostic" in 58 of them, positive in 17 and negative in 41.

CONCLUSION

V/Q SPECT is a feasible technique as it was performed in 102 of the 109 patients who were enrolled in the study (94%). The addition of V/Q SPECT to planar V/Q decreases the number of "non-diagnostic" reports from 62% in planar scintigraphy to 4.9% in SPECT. Therefore, V/Q SPECT should be included in the diagnosis approach of PE due to its high diagnostic yield.

Imaging of acute pulmonary embolism

Acute pulmonary thromboembolism (PE) is a cardiovascular emergency associated with significant morbidity and a 5-35 % mortality for untreated pulmonary embolism. If promptly diagnosed and treated, the mortality rate can be significantly reduced. Diagnosis of acute PE continues to be a clinical challenge, with diagnostic imaging playing an important role. This review discusses the clinical challenges of diagnosing acute PE, presents an evidence-based review of the current tests and ever-evolving imaging technology, and highlights special considerations related to radiation dose, contrast media use, and pregnant patients.

Ventilation/perfusion lung scintigraphy: what is still needed? A review considering technetium-99m-labeled macro-aggregates of albumin

Lung perfusion scintigraphy (LPS) with technetium-99m-labeled macro-aggregates of albumin (Tc-99m-MAA) is well established in the diagnostic of pulmonary embolism (PE). In the last decade, it was shown that single-photon emission computer tomography (SPECT) acquisition of LPS overcame static scintigraphy. Furthermore, there are rare indications for LPS, such as preoperative quantification of regional lung function prior to lung resection or transplantation, optimization of lung cancer radiation therapy, quantification of right-left shunt, planning of intra-arterial chemotherapy, and several rare indications in pediatrics. Moreover, LPS with Tc-99m-MAA is a safe method with low radiation exposure. PE can also be diagnosed by spiral computer tomography (CT), ultrasound, magnetic resonance angiography, or pulmonary angiography (PA, former gold standard). The present review considers all these methods, especially spiral CT, and compares them with LPS with respect to sensitivity and specificity and gives an overview of established and newer publications. It shows that LPS with Tc-99m-MAA represents a diagnostic method of continuing value for PE. In comparison with spiral CT and/or PA, LPS is not to be defeated as mentioned also by the most actual Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) II reports. This applies in particular to chronic or recurring embolisms, whereas currently spiral CT may be of greater value for major or life-threatening embolisms. At present, LPS cannot be replaced by other methods in some applications, such as pediatrics or in the quantification of regional pulmonary function in a preoperative context or prior to radiation therapy. LPS still has a place in the diagnostics of PE and is irreplaceable in several rare indications as described earlier.

Prospective evaluation of the negative predictive value of V/Q SPECT using 99mTc-Technegas

OBJECTIVE

To verify the negative predictive value of pulmonary ventilation/perfusion scintigraphy with single photon emission computed tomography (V/Q SPECT) in ruling out pulmonary thromboembolism.

METHODS

V/Q SPECT using 99mTc-Technegas was performed on 584 patients to rule out pulmonary thromboembolism between October 2004 and July 2005. Pulmonary thromboembolism was defined as any clear-cut vascular mismatch, regardless of size. Indeterminate scans were defined as cases having matching vascular type defects with a corresponding X-ray abnormality, or cases with equivocal mismatches. Other patterns were considered negative for pulmonary thromboembolism. Outcome data was gathered >3 months after the scan. Absence of pulmonary thromboembolism was defined as any patient still alive at least 3 months after the scan, with no anticoagulation treatment and no proof of pulmonary thromboembolism by other techniques, either at the time of the scan or during follow-up, or death by other causes.

RESULTS

One hundred and eight patients (19%) had a positive pulmonary thromboembolism reading, 18 (3%) an indeterminate study, and 458 (78%) patients had a negative reading for pulmonary thromboembolism. There were 189 patients with an abnormal chest X-ray. The mean follow-up time was 165 days. Of the 458 patients classified as negative for pulmonary thromboembolism, patients receiving chronic anticoagulation for other causes were excluded from follow-up (n=53), which left 405 patients for final analysis. There were no pulmonary thromboembolism-related deaths in the negative group. Six patients were identified as false negatives. The negative predictive value is estimated at 98.5%.

CONCLUSION

SPECT pulmonary scintigraphy using 99mTc-Technegas demonstrates a high negative predictive value and a low indeterminate rate.

Multislice computed tomography perfusion imaging for visualization of acute pulmonary embolism: animal experience

The purpose of our animal study was to evaluate a new computed tomography (CT) subtraction technique for visualization of perfusion defects within the lung parenchyma in subsegmental pulmonary embolism (PE). Seven healthy pigs were entered into a prospective trial. Acute PE was artificially induced by fresh clot material prior to the CT scans. Within a single breath-hold, whole thorax CT scans were performed with a 16-slice multidetector-row CT scanner (SOMATOM Sensation 16; Siemens, Forchheim, Germany) before and after intravenous application of 80 ml of contrast medium with a flow rate of 4 ml/s, followed by a saline chaser. The scan parameters were 120 kV and 100 mAs(eff), using a thin collimation of 16x0.75 mm and a table speed/rotation of 15-18 mm (pitch, 1.25-1.5; rotation time, 0.5 s). Axial source images were reconstructed with an effective slice thickness of 1 mm (overlap, 30%). A new automatic subtraction technique was used. After 3D segmentation of the lungs in the plain and contrast-enhanced series, threshold-based extraction of major airways and vascular structures in the contrast images was performed. This segmentation was repeated in the plain CT images segmenting the same number of vessels and airways as in the contrast images. Both scans were registered onto each other using nonrigid registration. After registration both image sets were filtered in a nonlinear fashion excluding segmented airways and vessels. After subtracting the plain CT data from the contrast data the resulting enhancement images were color-encoded and overlaid onto the contrast-enhanced CT angiography (CTA) images. This color-encoded combined display of parenchymal enhancement of the lungs was evaluated interactively on a workstation (Leonardo, Siemens) in axial, coronal and sagittal plane orientations. Axial contrast-enhanced CTA images were rated first, followed by an analysis of the combination images. Finally, CTA images were reread focusing on areas with perfusion deficits indicating PE on the color-coded enhancement display. Subtraction was feasible for all seven studies. In one animal, opacification of the pulmonary arteries was suboptimal owing to heart insufficiency. In the remaining six pigs, a total of 37 perfusion defects were clearly assessable downstream of occluded subsegmental arteries, showing lower or missing enhancement compared with normally perfused lung parenchyma. Indeterminate findings from CTA showed typical PE perfusion defects in four out of six cases on CT subtraction. Additionally, 22 peripheral triangular-shaped enhancement defects were delineated. Nine of these findings were reclassified as definitely being caused by PE on second reading of the CTA data sets. Our initial results have shown that this new subtraction technique for perfusion imaging of PE is feasible, using routine contrast delivery. Dedicated examination protocols are mandatory for adequate opacification of the pulmonary arteries and for optimization of data sets for subsequent subtraction. Perfusion imaging allows a comprehensive assessment of morphology and function, providing more accurate information on acute PE.