title: Attenuation of acute and chronic pulmonary emboli.
abstract: OBJECTIVE
  To compare retrospectively the attenuation coefficients of acute and chronic pulmonary embolism (PE).
  
  
  METHODS
  Institutional review board approval was obtained, and informed consent was waived. The study was compliant with requirements of the Health Insurance Portability and Accountability Act. All patients with chronic PE, from July 2001 to January 2004, were identified via a radiology report search system; of the 39 identified, 25 were excluded because the thrombi were too small to measure or were obscured by streak artifacts or because there was no corroborative evidence of chronic PE. Of 27 consecutive patients with acute PE who were also identified, two were excluded because of streak artifacts. The final study group included six women and eight men with chronic PE (mean age, 50 years; range, 26-76 years) and 11 women and 14 men with acute PE (mean age, 61 years; range, 33-83 years) (P = .01 for age). Images were acquired with a four-detector row computed tomographic scanner and 1.25-mm collimation. Two readers made independent attenuation measurements of the largest thrombus in each patient at a workstation. Statistical analysis included calculation of means and standard deviations, the t test, and the Bland-Altman test.
  
  
  RESULTS
  Reader 1 found mean attenuation of 90 HU +/- 30 (range, 54-155 HU) for chronic PE and 33 HU +/- 15 (range, 6-63 HU) for acute PE (P < .001). Reader 2 found mean attenuation measurements of 83 HU +/- 32 (range, 32-135 HU) for chronic PE and 33 HU +/- 14 (range, 13-65 HU) for acute PE (P < .001). The mean attenuation for both readers was 33 HU for acute PE (95% confidence interval: 26, 41 HU) and 87 HU for chronic PE (95% confidence interval: 66, 107 HU). The Bland-Altman test demonstrated agreement between readers.
  
  
  CONCLUSIONS
  The mean attenuation measurement in chronic PE is significantly higher than in acute PE.
authors.affiliation: Department of Thoracic Radiology, Massachusetts General Hospital and Harvard Medical School, Founders 202, 55 Fruit St, Boston, MA 02115, USA. cwittram@partners.org
authors.forename: Conrad
authors.initials: C
authors.lastname: Wittram
authors.forename: Michael M
authors.initials: MM
authors.lastname: Maher
authors.forename: Elkan F
authors.initials: EF
authors.lastname: Halpern
authors.forename: Jo-Anne O
authors.initials: JA
authors.lastname: Shepard
mesh_terms: D000208:Acute Disease; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000792:Angiography / Q000379:methods*; D002908:Chronic Disease; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011655:Pulmonary Embolism / Q000000981:diagnostic imaging*; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed