title: Indeterminate CT pulmonary angiogram: Why and does it matter?
abstract: BACKGROUND
  This study assessed the rate of indeterminate CT pulmonary angiogram (CTPA) in a general hospital and evaluated potential contributing factors to an indeterminate CTPA and patients' eventual clinical outcome.
  
  
  METHODS
  Four hundred and three consecutive CTPA and their finalised report from 01/08/2012 to 05/12/2012 at Waitemata DHB were reviewed retrospectively. Patient demographics, scan parameters and average Hounsfield unit (HU) of the main pulmonary trunk (PT) and likely cause of indeterminate study were documented. Studies were categorised into diagnostic, suboptimal or non-diagnostic studies. Univariate and multiple regression analyses were employed.
  
  
  RESULTS
  Six per cent (n = 24) of the studies were deemed indeterminate which included suboptimal and non-diagnostic studies. Seven pregnant patients were scanned during the study period and four of the seven studies were deemed indeterminate. In univariate analysis, predictors of indeterminate studies included 'weight' (P = 0.022), 'average HU of PT' (P < 0.0001) and 'effective dose' (P = 0.0003). In the multivariate logistic regression model, only 'average HU of PT' was associated with indeterminate studies (OR = 0.99, 95%CI: 0.99, 1.00). Twelve of 24 indeterminate studies had suboptimal pulmonary enhancement with causes including suboptimal timing of contrast bolus, hyperdynamic state due to pregnancy and transient interruption of contrast. Three patients had excessive noise due to body habitus. Eight studies had motion artefacts and one patient had left lower lobe pneumonia which decreased the accuracy. A third of the indeterminate studies were clinically considered as negative by referring clinicians.
  
  
  CONCLUSIONS
  Pulmonary trunk average Hounsfield unit is a predictor of indeterminate CT pulmonary angiogram.
authors.affiliation: Department of Radiology, North Shore Hospital, Waitemata DHB, Auckland, New Zealand.
authors.forename: Ju Hee
authors.initials: JH
authors.lastname: Yeo
authors.affiliation: Planning and Funding, Waitemata DHB, Auckland, New Zealand.
authors.forename: Lifeng
authors.initials: L
authors.lastname: Zhou
authors.affiliation: Department of Radiology, North Shore Hospital, Waitemata DHB, Auckland, New Zealand.
authors.forename: Remy
authors.initials: R
authors.lastname: Lim
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000792:Angiography / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011651:Pulmonary Artery / Q000000981:diagnostic imaging*; D011655:Pulmonary Embolism / Q000000981:diagnostic imaging*; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed; D055815:Young Adult
keywords: 
  CTPA
  ; contrast; indeterminate; pulmonary embolism; suboptimal