title: What is the minimal radiation dose that can be used for detecting pleural effusion?
abstract: OBJECTIVE
  The objective of our study was to assess the effect of radiation dose reduction on the detection of pleural effusions, thickening, and calcifications.
  
  
  METHODS
  Forty-five human cadavers (mean age at death, 60 ± 17 [SD] years; male-female ratio, 29:16; mean body mass index, 29 ± 5.7 [SD] kg/m(2)) were scanned at seven different dose levels (CT Dose Index volume [CTDIvol] = 20, 12, 10, 6, 4, 2, and 0.8 mGy) on a 128-MDCT unit (Definition FLASH). Images were reconstructed at a 3-mm slice thickness and 2-mm increment with filtered back projection (FBP) technique. Two chest radiologists independently reviewed all image series for the detection of pleural effusion, pleural calcification, and adjacent parenchymal opacification from atelectasis or consolidation. Objective image noise was measured at each dose level on the pleural effusion using ImageJ software. Data analysis was performed with the Student t test and kappa test.
  
  
  RESULTS
  Pleural effusions were seen in 39 of 45 cadavers on image series acquired at 2-20 mGy. Only 14 of 39 pleural effusions were identified at 0.8 mGy. Pleural effusions were not detected in 25 of 39 cadavers at 0.8 mGy because of photon starvation and increased image noise. Patient size was significantly larger in subjects with undetected pleural effusion than in those with detectable pleural effusion at 0.8 mGy (p < 0.01). Pleural calcifications and thickening (seen at 2-10 mGy images in three of three cadavers) were not identified on 0.8-mGy FBP images. On the other hand, adjacent parenchymal opacification could be assessed at all dose levels. The mean CT numbers of the pleural effusion were significantly lower on 0.8-mGy images than on images obtained at all other dose levels (-21 ± 55 [SD] vs 17.6 ± 19 HU, respectively) (p < 0.001).
  
  
  CONCLUSIONS
  Pleural effusions, thickening, and calcifications can be seen on FBP images reconstructed at a CTDIvol as low as 2 mGy (32-cm body phantom). CT at 0.8 mGy may provide suboptimal information on very small pleural effusions, pleural thickening, and calcifications.
authors.affiliation: 1 All authors: Department of Radiology, Division of Thoracic Imaging, Massachusetts General Hospital, Harvard Medical School, 55 Fruit St, Founders Rm 202, Boston, MA 02114.
authors.forename: Sarvenaz
authors.initials: S
authors.lastname: Pourjabbar
authors.forename: Sarabjeet
authors.initials: S
authors.lastname: Singh
authors.forename: Amita
authors.initials: A
authors.lastname: Sharma
authors.forename: Atul
authors.initials: A
authors.lastname: Padole
authors.forename: Ranish Deedar Ali
authors.initials: RD
authors.lastname: Khawaja
authors.forename: Diego
authors.initials: D
authors.lastname: Lira
authors.forename: Mannudeep K
authors.initials: MK
authors.lastname: Kalra
mesh_terms: D002102:Cadaver; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010996:Pleural Effusion / Q000000981:diagnostic imaging*; D011829:Radiation Dosage; D011857:Radiographic Image Interpretation, Computer-Assisted; D013902:Radiography, Thoracic / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*
keywords: chest CT; pleural effusion detection; radiation dose reduction