Iterative reconstruction in head CT: image quality of routine and low-dose protocols in comparison with standard filtered back-projection

Abstract

Background and purpose: IR has recently demonstrated its capacity to reduce noise and permit dose reduction in abdominal and thoracic CT applications. The purpose of our study was to assess the potential benefit of IR in head CT by comparing objective and subjective image quality with standard FBP at various dose levels.

Materials and methods: Ninety consecutive patients were randomly assigned to undergo nonenhanced and contrast-enhanced head CT at a standard dose (320 mAs; CTDI, 60.1) or 15% (275 mAs; CTDI, 51.8) and 30% (225 mAs; CTDI, 42.3) dose reduction. All acquisitions were reconstructed with IR in image space, and FBP and images were assessed in terms of quantitative and qualitative IQ.

Results: Compared with FBP, IR resulted in lower image noise (P ≤ .02), higher CNR (P ≤ .03), and improved subjective image quality (P ≤ .002) at all dose levels. While degradation of objective and subjective IQ at 15% dose reduction was fully compensated by IR (CNR, 1.98 ± 0.4 at 320 mAs with FBP versus 2.05 ± 0.4 at 275 mAs with IR; IQ, 1.8 versus 1.7), IQ was considerably poorer at 70% standard dose despite using the iterative approach (CNR, 1.98 ± 0.3 at 320 mAs with FBP versus 1.85 ± 0.4 at 225 mAs with IR, P = .18; IQ, 1.8 versus 2.2, P = .03). Linear regression analysis of CNR against tube current suggests that standard CNR may be obtained until approximately 20.4% dose reduction when IR is used.

Conclusions: Compared with conventional FBP, IR of head CT is associated with significant improvement of objective and subjective IQ and may allow dose reductions in the range of 20% without compromising standard image quality.

Fig 1.

CNR in nonenhanced (A) and contrast-enhanced (B) head CT with the use of various tube currents and reconstruction of data by FBP or IR. In the boxplot diagrams, the line across the middle of the box identifies the median sample value; boxes extend from the 25th to the 75th quartile, and whiskers, down to the lowest and highest values.

Fig 2.

Regression plot of CNR against tube current with IR and FBP. According to linear regression equation, the x-intercept is at a tube current of 255 mAs when y is at a standard 1.98 CNR (320-mAs FBP).

Fig 3.

Subjective grading of WM-GM differentiation in nonenhanced (A) and contrast-enhanced (B) head CT with use of various tube currents and reconstruction of data by FBP or IR. Data are presented as means and ranges.

Fig 4.

Example of image quality at 320 (A and D), 275 (B and E), and 225 (C and F) mAs. At all dose levels, use of IR (D−F) is associated with a considerable reduction of noise and enhancement of image quality. As demonstrated in our study, this is achieved without significant loss of image sharpness. While image quality at 85% dose and IR (E) is similar to the standard of reference (A), a 30% dose reduction results in substantial increase of noise despite using IR (F).