Thirty-six out of the 57 (63.2%) patients were males and 21 (36.8%) were females. The encountered pleural diseases were subdivided according to lesions types based on their imaging findings; 40 patients showed pleural effusion and 41 showed pleural thickening and neoplastic lesions.
From the 40 patients with pleural effusion, 4 (10%) had bilateral effusion and 36 (90%) patients had unilateral effusion; 21 (52.5%) on the right side and 15 (37%.5) on the left side.
According to the cytology results, 14 (40%) patients were diagnosed with malignant pleural effusion, while the other 26 (60%) patients had benign pleural effusion.
Morphological assessment of pleural effusion was classified according to cross-sectional imaging into either free or encysted, where 20 (50%) patients showed free effusion and 20 (50%) patients showed encysted effusion.
On correlating the morphological assessment with the cytology results, 10 (50%) of the 20 patients showing encysted effusion were found to have malignant effusion and 10 (50%) had benign effusion. On the other hand, from the 20 patient with free effusion, 14 (70%) were benign and 6 (30%) were malignant (Fig. 1).
According to signal intensity analysis, in 39 (97.5%) patients, the fluid displayed low T1, bright T2, and STIR signal intensities, while in the remaining one (2.5%) patient which had hemothorax, the fluid displayed high T1 and low T2 and STIR signals (Fig. 2).
Qualitative assessment of the DWI and ADC maps revealed that 39 (97.5%) patients with pleural effusion showed no evidence of diffusion restriction, whereas the one (2.5%) patient diagnosed with empyema showed diffusion restriction being bright on DWI with corresponding dark area on the ADC map (Fig. 3).
Pleural effusion ADC mean values ranged from 0.82 × 10−3 mm2/s to 4.83 × 10−3 mm2/s with average 3.35 ± 0.78× 10−3 mm2/s.
Malignant effusion ADC mean values ranged from 2.09 × 10−3 mm2/s to 3.96 × 10−3 mm2/s with average 3.08 ± 0.55 × 10−3 mm2/s, while, benign effusion ADC mean values were between 0.82 × 10−3 mm2/s and 4.83 × 10−3 mm2/s with average 3.5 ± 0.82 × 10−3 mm2/s.
Hence, the ADC mean values of malignant pleural effusion were not significantly different from benign pleural effusion (P = 0.09) making MRI unable to differentiate between them.
From the 41 patients showing pleural thickening and neoplastic lesions, 40 (97.6%) had unilateral lesions; 21 (51.2%) on the right side and 19 (46.3%) on the left side.
Patients were grouped according to histopathological diagnosis into benign pleural lesions and malignant pleural lesions.
Thirteen (31.7%) out of the 41 patients had benign pleural lesions; 1 (7.7%) patient had pleural lipoma, 2 (15.4%) had calcified pleural plaques, and 10 (76.9) had benign pleural thickening, while 28 (68.3%) patients had malignant pleural lesions; 7 (25%) patients were diagnosed with malignant pleural mesothelioma (Fig. 4) and 21 (75%) with pleural metastasis (Fig. 5).
According to imaging findings, lesions were characterized by morphology and signal intensity in conventional sequences in addition to quantitative and qualitative assessment in DWI and ADC maps.
Morphological assessment of the pleural lesions was based on their thickness (> 1 cm or < 1 cm) and contour (smooth or nodular).
Regarding pleural thickness, 28 (68.3%) lesions were > 1 cm in maximum thickness and 13 (31.7%) lesions were < 1 cm.
Nodular contour was also depicted in 28 (68.3%) lesions, while 13 (31.7%) lesions showed smooth contour.
Correlation between histopathological diagnosis and imaging findings revealed that 10 (76.9%) lesions out of the 13 lesions with thickness < 1 cm were found to be benign and 3 (23.1%) lesions were malignant. On the other hand, from the 28 lesions with thickness > 1 cm, 25 (89.3%) were malignant and 3 (10.7%) were benign.
Similarly, 10 lesions out of the 13 (76.9%) lesions with smooth countour were found to be benign in nature and 3 (23.1%) lesions were malignant, while from the 28 lesions with nodular contour, 25 (89.3%) were malignant and 3 (10.7%) were benign.
Comparing the imaging morphology of the pleural lesions with their histopathological results, MRI was found to be able to discriminate benign from malignant lesions by using morphological features (contour and thickness) with sensitivity 89.29%, specificity 76%, positive predictive value 89%, negative predictive value 76.92%, and accuracy 85.37%.
According to MRI signal intensity, 40 (97.6%) lesions displayed low T1, while the remaining fat containing lesion (2.4%) showed high T1 signal.
In the T2 sequence, 29 (70.7%) lesions elicited high signal and the other 12 (29.3%) lesions showed low signal, while in the STIR sequence, 28 (68.3%) lesions displayed high signal and 13 (31.7%) showed low signal.
Qualitative assessment of the DWI and ADC maps identified 28 (68.3%) lesions showing diffusion restriction being bright on DWI with corresponding dark area on the ADC map, while 13 (31.7%) lesions did not show restricted signal in DWI.
All 28 lesions with diffusion restriction were found to be malignant after reviewing their histopathology results, while the other 13 lesions which showed non-restricted signal in DWI were benign.
Pleural lesions ADC mean values ranged from 0.45 × 10-3 mm2/s to 3.18 × 10−3 mm2/s with average 1.45 ± 0.71× 10−3 mm2/s.
The malignant pleural lesions showed ADC mean values ranging from 0.45 × 10−3 mm2/s to 2.84 × 10−3 mm2/s with average 1.10 ± 0.53× 10−3 mm2/s, while benign lesions ADC mean values were between 1.80 × 10−3 mm2/s to 3.18 × 10−3 mm2/s with average 2.19 ± 0.42× 10−3 mm2/s.
The ADC mean values in patients diagnosed as malignant pleural mesothelioma ranged from 0.45 × 10−3 mm2/s to 1.17 × 10−3 mm2/s with average 0.84 ± 0.22 × 10−3 mm2/s.
In patients diagnosed with pleural metastasis, the ADC mean values ranged from 0.50 × 10−3 mm2/s to 2.84 × 10−3 mm2/s with average 1.19 ± 0.58 × 10−3 mm2/s.
Therefore, this study found that the ADC values of malignant pleural diseases were significantly lower than that of benign lesions (P < 0.001). However, the ADC mean values of malignant pleural mesothelioma were not significantly different from that of pleural metastasis (P = 0.090).
ROC curve analysis revealed that ADC mean value of 1.68 × 10−3 mm2/s as a cutoff value can differentiate malignant from benign pleural diseases with sensitivity 89.3%, specificity 100%, positive predictive value 100%, negative predictive value 81.2%, and accuracy 92.68% with P value < 0.001 indicating high statistical significance.
The results of morphological and functional assessment to differentiate benign from malignant pleural lesions were compared and illustrated in details in Fig. 6.
Evaluating the associated lymph nodes in patients with malignant pleural mesothelioma showed that 5 patients had intra-thoracic lymph nodes; in 2 (40%) of them, the lymph nodes were > 1 cm, while in the other 3 (60%), the lymph nodes were ≤ 1 cm and were considered as reactive lymph nodes and not metastatic.
However, all lymph nodes ≤ 1 cm and > 1 cm showed restricted signal in DWI.
From the 7 patients diagnosed with malignant pleural mesothelioma, associated osseous erosions were depicted in 2 patients, chest wall invasion in 3 patients and vascular invasion in 1 patient.