To evaluate the potential of lower dose CTA for detecting acute PE.
METHOD AND MATERIALS
39 adults (13M/26F) evaluated in the emergency room (ER) with "standard dose” PE CTA (Siemens Sensation 4, kV=120, effective mAs (EffmAs)=180, rot time=0.5s, collim=1.0mm, section tkn=1.25mm, bolus trk with 125cc of 370mg iodine/ml IV contrast) were prospectively enrolled: 19 consecutive positive (for PE) and 20 consecutive negative exams. Simulated ½-dose (90 EffmAs) and ¼-dose (45 EffmAs) studies were reconstructed from standard dose raw CT data by mathematically adding Gaussian noise. The 78 studies (39 at ½-dose and 39 at ¼-dose) were randomized and two readers who were unaware of the study parameters interpreted each exam for presence/absence of PE, overall image quality (adequate v. limited), and subjective causes for degradation in image quality (noise, motion artifact, poor contrast bolus). Test characteristics were computed at the 95% confidence interval (CI) for detection of PE using the standard dose radiology report as the gold standard. The overall image quality and the subjective causes for image degradation were compared (½-dose vs. ¼-dose) using 99% CI on the difference of the proportions.
At ½-dose, reader 1 reproduced the gold-standard; reader 2 (TP=18,TN=19,FP=1,FN=1) had sensitivity (Se)=95% (95% CI[72-100%]), specificity (Sp)=95% (95% CI[73-100%]), Positive Predictive Value (PPV)=95% (95% CI[72-100%]), and Negative Predictive Value (NPV)=95% (95% CI[73-100%]). At ¼-dose, reader 1 (TP=18,TN=19,FP=1,FN=1) had Se=95% (95% CI[72-100%]), Sp=95% (95% CI[73-100%]), PPV=95% (95% CI[72-100%]), NPV=95% (95% CI[73-100%]); reader 2 reproduced the gold-standard. In the two FN and the two FP cases, overall image quality was “limited” and both noise and motion were present. For both readers, there was no statistical difference (99% CI) between the ½-dose and ¼-dose exams with respect to the proportion of studies with adequate image quality, or the proportion of studies degraded by noise, motion artifact, or poor contrast bolus.
Large reductions in patient dose may not significantly compromise the interpretation of ER PE CTA.