Keywords
pleural effusion - pleural effusion volume - contrast-enhanced computed tomography
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.[3]
[4]
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
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.[6]
[7]
[8] 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
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 measured[8] ([Fig. 1]).
Fig. 1 Diagram depicting quantification of PE.
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 (Lus) 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 (Aus) was measured. The pleural effusion volume was calculated by multiplying Lus and Aus
[8] ([Fig. 2] and [Table 1]).[11]
Table 1
Standardized grading method for pleural effusions[11]
|
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.
|
|
Fig. 2 Diagram showing PEV calculation.
Pleural Fluid-Thoracic Drainage
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
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.[18]
[19] 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.[22]
[23]
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.[26]
[27] 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 tissue[29]
[30] 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
Benign Pleural Effusion
On chest X-ray, diffuse pleural thickening appears smooth and continuous pleural density
that extends over at least 25% of the chest wall. The costophrenic angle is usually
blunted, and laterally there is a slight increase in the radiographic density seen
on the chest X-ray.[33] The American Thoracic Society uses the presence of costophrenic angle blunting to
differentiate it from widespread pleural plaque for diagnosis of the diffuse pleural
thickness.[34]
On ultrasound, most pleural collections are hypoechoic or anechoic collections demarcated
by the lung echogenicity and visceral pleura. Pleural septate effusions are exudates,[35] whereas hypoechoic effusions can be exudates or transudates,[35]
[36] and pleural thickening can be seen in the depths.[37]
On CT scan, diffuse visceral pleural thickening is characterized as a continuous sheet
of pleural thickening that is more than 8 cm long, 5 cm wide, and 3 mm thick that
can occur in pleural plaques.[38] In contrast to pleural plaques, the diffuse pleural thickening borders are tapered.[39]
Malignant Pleural Effusion
On chest X-ray, malignant pleural thickening typically appears irregular and nodular
opacities can be seen in the lung perimeter. In 60% of cases, pleural effusions are
seen as unilateral, and in 5% cases, bilateral pleural effusion.[40]
On ultrasound, malignant solid pleural tumors appear as a homogenous well-delineated
sheet-like lesion with breath > 1 cm in diameter.[41] Pleural effusions with pleural nodularity and diaphragmatic thickness > 7 mm and
parietal pleural thickness > 10 mm were found to have a high positive predictive value
and specificity for underlying cancer in one of the studies.[42]
On CT scan, parietal pleural thickening, mediastinal pleural thickening, nodular pleural
thickening (>1 cm), and circumferential pleural thickening can be seen.[43]
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.
