Assessment of Pleural Effusion by Means of Imaging Modalities

Introduction

Pleural effusion is the fluid collection in the pleural cavity, which is situated between the parietal and visceral pleura. It is caused on its own or due to parenchymal diseases such as infection, inflammatory disorders, or cancer. Pleural effusion is the leading cause of respiratory mortality and morbidity.¹ A small amount of pleural fluid lubricates the membrane and allows regular breathing in all healthy individuals. The lymphatic drainage, oncotic pressure, and hydrostatic pressures control this delicate fluid balance, and disturbances in either of these systems may lead to the build-up of pleural fluid.² Based on modified Light's criteria, pleural fluid is divided into transudate or exudate. If any one of the conditions is satisfied then the fluid is considered as exudative effusion: the ratio of protein pleural fluid to serum protein is more than 0.5, the ratio of lactate dehydrogenase (LDH)/serum LDH greater than 0.6, and LDH pleural fluid exceeds two-thirds of the upper limit of serum LDH.³⁴

This review summarizes the various evidence-based applications of all imaging modalities in the assessment of pleural illnesses, as well as how their application improves patient safety and accuracy. The review also gives an overview of how chest radiographs, ultrasounds, and CT scans can play an important role in pleural illness.

Characteristics of Pleural Effusion

The normal amount of pleural fluid in the right and left pleural space is 8.4 ± 4.3mL, and in non-smoker individuals, the total pleural fluid per kilogram of body mass is 0.26 ± 0.1 mL/kg.⁵ Formulas have been proposed for the quantification of pleural fluid volume under ultrasound guidance.⁶⁷⁸ Pleural fluid is identified by dynamic signs such as a change in echo-free space during the respiratory cycle,⁹ compressed lungs, or atelectatic and swirling motion in the echo-free space.¹⁰

Quantify Pleural Effusion

The left side image shows that the ultrasound probe is placed at the level of mid-thorax for measuring the cross-sectional area of PE, and the right-side image shows right lung two-dimensional computed tomography (2D CT) reconstruction in one of the patients who had PE. The thin arrows(blue) indicate the PE length (L), and the dashed line (red) indicates the intercoastal gap where the cross-sectional area (A) of PE was measured. Then the length of each paravertebral intercostal gap was measured⁸ (Fig. 1).

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Fig. 1 Diagram depicting quantification of PE.

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Calculation of Pleural Effusion Volume

The patient was positioned in supine. The pleural cavity was visualized in the axial plane by placing the probe in each paravertebral intercostal space. The probe was slided posteriorly to visualize the pleural cavity. The length of PE (L_us) was determined by measuring the distance between upper and lower paravertebral intercoastal spaces where pleural effusion was noted. The frame where the pleural effusion was noted was frozen and the area of PE (A_us) was measured. The pleural effusion volume was calculated by multiplying L_us and A_us⁸ (Fig. 2 and Table 1).¹¹

Table 1 Standardized grading method for pleural effusions¹¹

Grade	Illustration	Landmarks	Intercostal spaces
Grade 1: minimum	Limited to costophrenic sinus	Partially evident diaphragmatic dome	Limited to costophrenic sinus
Grade 2: small	Partially involved lower lung lobe	Completely evident diaphragmatic dome	Intercoastal space 1
Grade 3: Small to medium	Partially collapsed lower lung lobe	Lower lung lobe partially atelectasis, pulmonary hilum not seen	Intercoastal space 2–3
Grade 4: medium	Completely collapsed lower lung lobe	Lower lung lobe atelectasis, pulmonary hilum is seen	Intercoastal space 3–4
Grade 5: large	Partially involved upper lung lobe	Lower lung lobe atelectasis, partially atelectasis of upper lung lobe	Intercoastal space four and more
Grade 6: massive	Fully collapsed lung	Hilum is wholly seen, Whole lung atelectasis.

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Fig. 2 Diagram showing PEV calculation.

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Pleural Fluid-Thoracic Drainage

The patient's position during the entire procedure was up to the sonographer's choice but was generally supine to permit scanning of the lateral hemithorax. The possibility of thoracentesis was identified by a sonographic window where the fluid remained throughout the respiratory cycle. After identifying the suitable sonographic window, the angle of the sonoprobe was noticed, and the skin was marked where the needed depth of penetration was measured from the sonographic image. Under ultrasound guidance, site preparation, and thoracentesis were performed while maintaining the patient's position. During the process, the placement of needle was along a safe penetration axis according to the sonographic image.¹²

Diagnosis of Pleural Effusion

The initial step in diagnosing a pleural effusion is to identify whether it is a transudate or an exudate. When PE is an exudative effusion, additional diagnostic tests such as cytopathology, pleural biopsy, and sometimes thoracotomy can be performed to know a precise diagnosis and proper therapy for the pleural disease. On the other side, if the fluid is transudate, then therapeutic maneuvers performed for pleura are unnecessary and underlying diseases such as nephrosis, congestive heart failure, cirrhosis, or hypoproteinemia must be treated.¹³

Pleural effusion can be identified by chest radiography. Pleural effusion can cause mediastinal shift away from the diseased side seen on chest X-ray.¹⁴ Mediastinal shift can be upper or lower shift. Tracheal shift indicates upper mediastinal shift, whereas a shift in the heart position shows lower mediastinal shift. The trachea and heart shift to the opposite side in conditions such as tuberculous PE and effusion caused by other infective diseases. In case of malignant PE, the tracheal and heart are seen to be centrally or shifted to the same side as the fluid.¹⁵ In moderate pleural effusions, there might be silhouette loss between the heart borders and hemidiaphragms on chest X-ray.¹⁴ On the chest PA radiograph, the presence of pleural fluid around 200 mL is abnormal. On a lateral chest radiograph, only 50 mL of pleural fluid that leads to costophrenic angle blunting is visible.¹⁶ Lateral films can help distinguish between pleural thickening and free fluid because free fluid gravitates to the most dependent area of the chest wall.¹⁷

Ultrasound is more accurate in assessing pleural effusion volume and helps with thoracentesis than chest radiography.¹⁸¹⁹ The pleural effusion was divided as echogenic or non-echogenic during the pre-procedure ultrasound. Echogenic pleural effusions were characterized as PE having debris (white specks) or echogenic cellular material within the effusion. Non-echogenic were characterized as those that had no echogenic material (echo-free, black).²⁰ Ultrasound-guided pleural aspiration should be performed if the effusion is modest or loculated as it provides a safe and accurate means of collecting fluid. Also, ultrasound allows better visualization of fibrous septations than CT scan,²¹ and an additional benefit of being portable allows bedside imaging.²²²³

A contrast-enhanced computed tomography (CECT) thorax scan must be performed before complete drainage of pleural effusion as it allows better visualization of pleural abnormalities.²⁴ CT scan has been found superior to chest radiography in the differentiation of pleural and parenchymal illness.²⁵

MRI has a limited role in diagnosing pleural disease because of poor spatial resolution and motion artifacts.²⁶²⁷ MRI sequences used to image chest are T1-weighted and T2-weighted spin-echo, proton density, or short tau inversion recovery (STIR), and fast spin-echo with fat saturation.²⁸ T1-weighted images are helpful as they show a clear distinction between excess pleural fat and anomalies in the pleural space. T2-weighted images enhance the pleural fluid and provide good contrast between muscle and tumor.²⁸

Pleural Biopsy

Pleural biopsy is the most common method for obtaining diagnostic tissue²⁹³⁰ that has been an accepted choice by patients and physicians due to its ease of use, particularly as an alternative to thoracoscopy in countries with limited healthcare resources.³¹ US-guided biopsy for thoracic lesions near the chest wall may be viable approach than CT-guided biopsy in terms of efficacy and safety.³²

Radiological Appearance

Conclusion

Pleural effusion can be diagnosed using imaging modalities such as chest radiograph, ultrasound, CT scan, and MRI. MRI has a limited role in diagnosing pleural effusion because of poor spatial resolution and motion artifacts. Ultrasound is a feasible imaging modality for diagnosing pleural effusion because there is no radiation involved. Also, it is the safest modality for pleural fluid drainage and biopsy as it provides real-time imaging with better efficacy and more safety to patients.