The use of spectral CT imaging in characterization of pleural fluid: a new method to differentiate transudates from exudates

Objective To assess the feasibility of characterizing pleural fluid on the basis of spectral imaging features utilizing spectral CT imaging. Methods Gemstone spectral imaging(GSI) was used to examine 20 pleural fluids filled tubes (11 exudates and 9 transudates ) following diagnostic thoracentesis. Effusions were classified as transudates or exudates using laboratory markers based on Light criteria. CT values on 140 kVp QC image were compared between two groups. Using GSI viewer, various CT spectral imaging parameters (CT values on different energy level, effective-Z, iodine-water concentration,calcium-water concentration and calcium-fat concentration ) were calculated and compared between two groups. The difference of these spectral characteristic parameters was evaluated statistically by independent-samples t test. Results According to Light criteria, the mean CT value on QC image of exudates [ ( 19. 56 ±4. 10) HU ] was higher than that of transudates [ ( 13.44 ±3.46) HU] (t =3.002,P =0. 010).Difference of CT value was found more obvious in the lower keV. On 40 keV images, the difference of CT value of two groups was the largest, the mean value of exudates [ (47.49 ± 14. 60) HU ] was significantly higher than that of transudates[ ( 19. 76 ± 6. 85) HU ] ( t = 5.520, P = 0. 000). While On 140 keV, the mean CT value were (9.76 ±4. 16)and (6.22 ±3. 17) HU and the difference of the two group has no statistically significant difference (t =2. 107,P =0. 050). The mean slope rates of exudates (0.51 ± 0.23)was significantly larger than that of transudates (0. 18 ± 0. 08 ) ( t= 4. 287, P = 0. 001 ). The effective-Z (7. 89 ± 0. 16), iodine-water concentration [ (5. 74 ± 1.28 ) g/L], calcium-water concentration[ (7. 89 ±1.78) g/L] and calcium-fat concentration [ (25.95 ± 1.74) g/L] of exudates were significantly higher than those of transudates [ 7.67 ± 0. 07, ( 1.70 ± 0. 95 ) g/L, (2. 53 ± 1.37 ) g/L, ( 20. 82 ± 1.40 ) g/L ] ( t = 4. 080,6. 998,6. 546,6. 301 ,P ＜ 0. 05 ). Conclusions The spectral curve and spectral imaging parameters of exudates is found to be different from transudates. The low energy spectral imaging plays an important role in the characterization of pleural fluid. Gemstone spectral CT imaging provides a new multiparameter method to differentiate transudates and exudates. Key words: Pleural effusion;  Exudates and transudates;  Tomography, X-ray computed