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Second generation iterative model reconstruction provides diagnostic quality images with substantial reduction in radiation exposure

Abstract
Philips CT Clinical Science Philips Healthcare • USA

Halpern EJ, Gingold EL, White H, Read KM.

Purpose
Iterative Model Reconstruction (IMR), as implemented in the second generation Philips software for multislice CT, is a knowledge-based reconstruction with marked reduction in image noise. The purpose of this study was to evaluate the application of IMR to reduce radiation dose in coronary CT angiography (cCTA).

Method and Materials
We evaluated 25 consecutive cCTA studies acquired on a 256-slice iCT scanner with tube current modulation (TCM). TCM lowers systolic phase tube current to 20% of that in diastole, resulting in photon-limited systolic images. Systolic phase images from each study were reconstructed at 40% of the R-R interval with filtered back projection (FBP), first generation iterative reconstruction (iDose⁴) and second generation IMR (Philips Medical Systems; Cleveland, OH). Each case was evaluated by two independent reviewers. The mean and standard deviation (sd) of pixel values were computed in a standardized region of interest in the left ventricle and left main coronary artery. Subjective rating scores were obtained from each reviewer (1-5 scale for poor-excellent) for definition of:
  1. Contours of small coronary arteries (<3 mm)
  2. Small coronary calcifications
  3. Irregularity/contour of non-calcified plaque
  4. Overall diagnostic confidence for presence/absence of stenosis. The three reconstruction techniques were compared with ANOVA, followed by individual paired comparisons. 

Results 
There was no significant difference in mean pixel intensity among FBP, iDose⁴ and IMR (p>0.8). However, image noise within a contrast-enhanced region of interest in the left ventricle was reduced by a factor of 2.4 from FBP to iDose⁴ (sd=348 vs 144,p<0.001) and by a factor of 3.4 from iDose⁴ to IMR (sd=144 vs 42,p<0.001). Image quality scores were significantly better with IMR as compared with iDose⁴ and FBP (scores for IMR: iDose⁴: FBP in each category: arterial contour: 4.5 > 2.7 > 1.6 (p<0.001); coronary calcification: 4.6 > 2.8 > 1.6 (p<0.001); non-calcified plaque contour: 4.0 > 2.6 > 1.6 (p<0.001); overall diagnostic confidence: 4.5 > 3.0 > 1.7 (p<0.001).

Conclusion
Second generation IMR reduces intravascular noise on cCTA by 88% compared with FBP, providing significantly improved image quality at radiation exposure levels 80% lower than those currently used with FBP and iDose⁴.

Clinical Relevance/Application
Second generation IMR reconstruction may allow diagnostic quality cCTA imaging with a substantial reduction in patient radiation exposure.


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Abstract
iCT, IntelliSpace Portal
calcification, Cardiac, Contrast Enhanced, coronary angiography, coronary arteries, dose, FBP reconstruction, iDose4, image quality, IMR, iterative model reconstruction, iterative reconstruction, left ventricle, plaque, stenosis, Vascular
 

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