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THORACIC AND CARDIOVASCULAR IMAGING Table of Contents   
Year : 2002  |  Volume : 12  |  Issue : 2  |  Page : 207-212
Review article : Imaging of pulmonary thromboembolism


Department of Imaging, Breach Candy Hospital and Research Centre Mumbai, India

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Keywords: Pulmonary Thromboembolism, Ventilation Perfusion Scan, Spiral CT Angiography

How to cite this article:
Kohli A, Rajput D, Gomes M, Desai S. Review article : Imaging of pulmonary thromboembolism. Indian J Radiol Imaging 2002;12:207-12

How to cite this URL:
Kohli A, Rajput D, Gomes M, Desai S. Review article : Imaging of pulmonary thromboembolism. Indian J Radiol Imaging [serial online] 2002 [cited 2019 Dec 9];12:207-12. Available from: http://www.ijri.org/text.asp?2002/12/2/207/28446

   Introduction Top


Acute pulmonary thromboembolism is the third most common cause of death in hospital patients. The three most important risk factors are deep venous thrombosis, immobilization and major surgery. Acute pulmonary thromboembolism is a major diagnostic dilemma for the clinician and is one of the commonest non-or misdiagnosed diseases. An early and accurate diagnosis is extremely important as a wide range of effective therapeutic options are available thereby reducing the morbidity and mortality. The clinical presentations have a wide variation ranging from minor symptoms to massive embolism with dyspnoea, right heart failure and shock. Not only is it important to detect symptomatic pulmonary embolism but also the asymptomatic, as these could be a precursor of massive embolizations with a high mortality rate. Further it is also important not to obtain a false positive diagnosis, as the treatment of pulmonary thrombo-embolism is anticoagulation, which is associated with multiple adverse effects and complications. The gold standard investigation is a pulmonary angiogram. Unfortunately this test is invasive with possible morbidity and mortality. In view of the clinical limitations and gravity of the disease there is need for an accurate, low risk, highly sensitive, highly specific, non-invasive, easily and rapidly performed, cost effective and widely available screening test. It is also important that the test be able to exclude other diseases that may mimic pulmonary thromboembolism.

The modalities available are

  1. Conventional Chest X-Ray
  2. Ventilation Perfusion Scans
  3. Spiral CT Pulmonary Angiography
  4. MR Pulmonary Angiography
  5. Pulmonary Angiography



   Conventional Chest X-Ray Top


The conventional chest x-ray is the commonest investigation performed in a cardiac or respiratory emergency. In acute pulmonary thromboembolism it may be normal or non-specific. Direct visualization of the clot is not possible on a chest x-ray but there are indirect signs, which may help to suggest the diagnosis. There may be a focal area of regional oligaemia (Westermark sign). The central pulmonary arteries may be prominent - (Fleischner sign). Pleural based opacities representing infarcted or atelectic lung (Hampton's humps) may be seen. Other signs are of right heart enlargement, concomitant pleural effusion and elevation of the hemidiaphragm. These findings are extremely non-specific and only rarely can a diagnosis be established on these findings. The main utility of the chest x-ray is as an aid in the interpretation of ventilation-perfusion scans.


   Ventilation Perfusion scans Top


Perfusion scans are based on the demonstration of radioactive particles being blocked in the pulmonary capillaries. Tc-99 macroaggregated albumin particles are used. 6-8 views in the anterior, posterior, right lateral, left lateral, right and left posterior oblique planes are obtained. A normal perfusion scan has a negative predictive value of 98% [1]. Unfortunately this high sensitivity is associated with a very low specificity. All positive scans need to be validated by chest x-rays and complimented with ventilation scans. The ventilation scans essentially are to differentiate primary perfusion defects, (mismatches) from secondary perfusion defects (matches). If all mismatch defects are considered as findings of Pulmonary Embolism then the sensitivity is 98% and specificity 10%. If, on the other hand, only large mismatch defects are considered then the sensitivity decreases to 41% but specificity increases to 97% (1). This has resulted in classifying scintigraphic results into five categories, high probability, intermediate probability, low probability, very low probability and normal.


   Spiral CT Angiography Top


The introduction of spiral CT and CT angiography of the pulmonary arteries has finally ended the search for a highly sensitive and accurate test to detect pulmonary arterial thrombi. With CT angiography it is possible to noninvasively depict endoluminal thrombi in central as well as second to fourth division pulmonary arteries. Pulmonary thromboembolia appear as intraluminal filling defects. A Spiral CT Angio of the pulmonary arteries is performed from the apices to the bases of the lung. 50 cc of nonionic Intravenous contrast is injected at a flow rate of 3-4 ml/sec, usually with a scan delay of 20-25 seconds. Scan collimation is 3mm with a pitch of 2. In slower spiral scanners or if the patient is dyspnoeic, it may be difficult to achieve this coverage, then images may be obtained from the level of the aortic arch to the inferior pulmonary veins. Spiral CT Pulmonary Angio directly demonstrates the pulmonary thrombus as a well defined hypodensity in the pulmonary artery. Additionally, ancillary findings may be present such as a pulmonary infarct, which appears as a well defined wedge shaped sub pleural parenchymal density consolidation. The apex is seen to point to the pulmonary artery, with the base along the pleural surface. Occasionally cavitation may be seen in the consolidation. The presence of atelectasis and effusions are non-specific findings

Spiral CT and ventilation -perfusion scans investigate totally different aspects of acute pulmonary embolism. Spiral CT directly visualizes thrombi in the central pulmonary arteries. Scintigraphy demonstrates vascular occlusions in the lung periphery by demonstrating lack of radioactivity distal to the occlusion. Usually central thrombo-emboli coexist with peripheral vascular occlusions. Therefore both tests frequently lead to concordant positive and negative results. Discordant results may occur if there are central nonoccluding pulmonary thromboembolia only. In this case the spiral CT will be positive and the scintigraphic findings negative. This is commonly the cause for difference in sensitivity and specificity between Spiral CT Pulmonary Angiography.

Spiral CT has additional advantages of determining the age of the thrombus as well as demonstrating other pathologies, which may simulate pulmonary thromboembolism clinically such as pneumonia, pneumothorax or aortic dissection. The sensitivity of CT Angio now closely matches the more invasive gold standard of pulmonary angiography. Several recent articles have demonstrated a sensitivity of 90 % for CT angio which compares with the sensitivity of pulmonary angiogram (PIOPED study).[1]


   Limitations of Spiral CT Pulmonary Angio Top


The main limitation of Spiral CT Pulmonary Angio is in the evaluation of arteries below the segmental level. This is due to their small vessel diameter, limited spatial resolution, insufficient enhancement and spatial orientation of vessels and thrombi. Unfortunately Ventilation perfusion scans and conventional pulmonary angiography also share this limitation. What is important to evaluate is the clinical significance of subsegmental pulmonary arterial thrombi. Small sub segmental thrombi are rarely of clinical importance. In one study only 1.6% of subsegmental pulmonary thrombi progressed to symptomatic pulmonary embolism in 6-12 weeks.[2]. In the PIOPED series, emboli in segmental and subsegmental vessels showed no difference in outcome between treated and untreated cases. [3]. Only when the patients cardio-pulmonary reserve is compromised can small emboli be of clinical significance.


   Interpretation pitfalls of Spiral CT Angio Top


Pitfalls in interpretation of Spiral CT Pulmonary Angio do occur frequently, these may be due to technical factors or related to anatomical factors. Technical factors may produce psuedo filling defects. These may be due to inappropriate selection of injection parameters, flow rate, concentration, scan delay or breath hold. Anatomical landmarks and variants especially intersegmental nodes frequently causing filing defects. Care should be taken that filing defects are intra vascular and not intersegmental nodes.


   Spiral CT Pulmonary Angiogram versus Ventilation/ Perfusion scans Top


There are numerous studies which have compared the efficacy, sensitivity, specificity, positive and negative predictive values of Spiral CT versus Ventilation/Perfusion scans and Spiral CT Pulmonary angiogram has been found to be superior than Ventilation-Perfusion scans.


   MR Angiography Top


Over the last few years, like spiral CT, there have been tremendous technical advancements in MRI, as regards to both hardware and software developments. This has resulted in improved demonstration of the pulmonary arteries especially following contrast administration. Similar to spiral CT, thrombi appear as filling defects in the pulmonary arteries on MRI. Thrombi of different ages present with different signal intensities on nonenhanced spin echo imaging. There are significant limitations in the use of MRI. It is expensive, not easily available and critically ill patients are difficult to monitor in a MRI suite. MRI may not be possible in dyspnoeic and claustrophobic individuals. MRI has advantages over spiral CT in that it is able to acquire images in the sagittal and coronal planes and smaller volumes of contrast media need to be used which is also not nephrotoxic. Another advantage of MRI is that MRA of the pulmonary arteries can be combined with MR venography of the pelvic and femoral veins. MRV has a very high accuracy in the detection of DVT. In a one hour study MRI can provide a comprehensive workup of patients with suspected pulmonary embolism and deep venous thrombosis. The main drawback of MRI today is that it is not as sensitive as spiral CT in the detection pulmonary thromboembolia. Scottsman et al evaluated 63 consecutive patients of suspected Pulmonary embolism. Spiral CT had a sensitivity of 75% and specificity of 89%, compared to MRI with a sensitivity of 46% and specificity of 90%.[6] The role of MRI today is as a second line of investigation if spiral CT is contraindicated.


   Pulmonary Angiography Top


Pulmonary angiography is considered to be the gold standard in the evaluation of pulmonary thromboembolism. This is an invasive procedure with an incidence of 1.5% serious complications [7]. Selective catheterization of the right and left pulmonary arteries is done. Images are obtained in two views usually on a DSA machine. Acute pulmonary emboli are demonstrated as intraluminal filling defects, peripheral occlusion of pulmonary vessels and/or wedge shaped perfusion defects. To improve the detection of small pulmonary emboli dedicated techniques are now available, such as cine angiography, balloon occlusion angiography and superselective angiography.

Many studies using spiral CT angiography have demonstrated a sensitivity and specificity for spiral CT angiogram to match that of pulmonary angiogram. The limitations of both spiral CT angiography and pulmonary angiography are also comparable. It is reported that 10% of spiral CT examinations will be inconclusive compared to 12% for pulmonary angiograms. 3% of spiral CT angiograms will be technically inadequate compared to 4% for pulmonary angiograms. [1]

In the past as well as currently in many centres, the work up for suspected pulmonary thromboembolism was a clinical examination, chest radiograph and ventilation perfusion scans. If these were inconclusive venous doppler of the lower limbs was performed. If still inconclusive then pulmonary angiography was performed. The superiority of CT over ventilation-perfusion scans is well documented. For these reasons spiral CT angio is now considered as a first line investigation. Ventilation - perfusion scans are recommended as a first line investigation only if spiral CT is not available. MR angiograms are recommended as a first line investigation only if spiral CT is contraindicated. Only in individuals with cardio-pulmonary compromise where there is a high index of suspicion, of sub segmental pulmonary embolism and the spiral CT Pulmonary angiogram is negative, a Ventilation-Perfusion scan and or pulmonary angiography is required.

 
   References Top

1.PIOPED investigations (1990) value of ventilation/perfusion scan in acute pulmonary embolism: Results of prospective investigations of pulmonary embolism diagnosis (PIOPED) J Am med ASSOC 263: 2753 - 2759.  Back to cited text no. 1    
2.Henry JW, Relyea B, Stein PD (1995) Continuing risk of Thromboemboli among patients with normal pulmonary angiograms. Chest 107:1375-1378  Back to cited text no. 2    
3.Stein PD, Henry JW, Relyea B (1995) untreated patients with pulmonary embolism. Outcome, clinical and laboratory assessment. Chest 107:931-935  Back to cited text no. 3    
4.Mayo JR, Remy-Jardin M, Muller NL, etal. (1997) Pulmonary Embolism: Prospective comparison of spiral CT with ventilation - perfusion scintigraphy. Radiology 205:447-452  Back to cited text no. 4    
5.Van Rossum AB, Pattynama PM, Mallens WM, Hermans J, Heijermann HG, (1998) Can helical CT replace scintigraphy in the diagnostic process in suspected pulmonary embolism? A retroelective -prolective cohort study focusing on total diagnostic yield. Eur Radiol8:90-96  Back to cited text no. 5    
6.Socstman HD, Layish DT, Tapson VF, Spritzer CE et al (1996) Prospective comparison of helical CT and MR imaging in clinically suspected acute pulmonary embolism. Lournal of magnetic Resonance imaging 6: 276-281  Back to cited text no. 6    
7.Stein P, Athanasoulis C, Alavi A, Greenopan R, Halest. Complications and validity of pulmonary angiography in acute pulmonary embolism. Circulation 85: 462 - 468.  Back to cited text no. 7    

Top
Correspondence Address:
A Kohli
Department of Imaging, Breach Candy Hospital and Research Centre Mumbai
India
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Source of Support: None, Conflict of Interest: None


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    Figures

[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8]

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[Pubmed]



 

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