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Advanced Modeled Iterative Reconstruction (ADMIRE) Facilitates Radiation Dose Reduction in Abdominal CT

Posted Sep 30, 2018 Updated Apr 10, 2023
By Stephan Ellmann MD , Ferdin Kammerer MD , Thomas Allmendinger PhD , Matthias Hammon MD , Rolf Janka MD , Michael Lell MD , Michael Uder MD , Manuel Kramer MD
9 min read

Rationale and Objectives

This study aimed to determine the potential degree of radiation dose reduction achievable using Advanced Modeled Iterative Reconstruction (ADMIRE) in abdominal computed tomography (CT) while maintaining image quality. Moreover, this study compared differences in image noise reduction of this iterative algorithm with radiation dose reduction.

Methods

Eleven consecutive patients scheduled for abdominal CT were scanned according to our institute’s standard protocol (100 kV, 289 reference mAs). Using a proprietary reconstruction software, CT images of these patients were reconstructed as either full-dose weighted filtered back projections or with simulated radiation dose reductions down to 10% of the full-dose level and ADMIRE at either strength 3 or strength 5. Images were marked with arrows pointing on anatomic structures of the abdomen, differing in their contrast to the surrounding tissue. Structures were grouped into high-, medium-, and low-contrast subgroups. In addition, the intrinsic noise of these structures was measured. That followed, image pairs were presented to observers, with five readers assessing image quality using two-alternative-forced-choice comparisons. In total, 3000 comparisons were performed that way.

Results

Both ADMIRE 3 and 5 decreased noise of the anatomic structures significantly compared to the filtered back projection, with an additional significant difference between ADMIRE 3 and 5. Radiation dose reduction potential for ADMIRE ranged from 29.0% to 53.5%, with no significant differences between ADMIRE 3 and 5 within the contrast subgroups.The potential levels of radiation dose reduction for ADMIRE 3 differed significantly between high-, medium-, and low-contrast structures, whereas for ADMIRE 5, there was only a significant difference between the high- and the medium-contrast subgroups.

Conclusion

Although ADMIRE 5 permits significantly higher noise reduction potential than ADMIRE 3, it does not facilitate higher levels of radiation dose reduction. ADMIRE nonetheless holds remarkable potential for radiation dose reduction, which features a certain dependency on the contrast of the structure of interest. Applying ADMIRE with a strength of 3 in abdominal CT may permit radiation dose reduction of about 30%.

Background

Computed tomography (CT) of the abdomen is an indispensable tool for clinical diagnosis.Nonetheless, radiation dose exposure raises several widely known concerns. Hence, according to the as-low-as-reasonably-achievable principle, radiation dose should be reduced while preserving diagnostic image quality.

As radiation dose reduction in CT examinations leads to increased image noise (given all other parameters constant) and thus impaired diagnostic image quality, one very promising technique for radiation dose reduction is the implementation of iterative reconstruction (IR) algorithms for noise reduction. In contrast to traditional weighted filtered back projection (FBP), IR algorithms can be considered as feedback mechanisms, with a simulator of the CT physics in the feedback loop. The feed forward loop updates the reconstruction image, based on deviations between the measured and the simulated scans . One of the most recent commercially available IR algorithms is the Advanced Modeled Iterative Reconstruction (ADMIRE; Siemens Healthineers, Forchheim, Germany), which uses IR from the raw data domain in combination with noise reduction in the image space in a hybrid manner. It applies statistical modeling in the raw data domain, followed by back projection, application of statistical model in the image domain (regularization), and forward projection. The resulting pseudo raw data are compared to the measured data and iteratively reinserted into the loop afterward . Compared to its predecessor SAFIRE (Sinogram-affirmed Iterative Reconstruction), the analysis incorporates not only nearest neighbor data but also a larger neighborhood at an anatomically reasonable length scale with respect to clinical applications . The ADMIRE strength setting primarily controls the level of noise reduction the algorithm aims at and therefore an increase in ADMIRE strength should allow higher radiation dose reduction potential.

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Methods

Patient Examination

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Image Reconstruction

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Statistical Analysis

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Results

Scan Parameters

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Inter- and Intraobserver Agreement

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Classification of the Anatomic Structures

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Figure 1, (a) Boxplot displaying differences between the attenuation of the analyzed anatomic structures and their surrounding tissue as a measure of contrast: adrenal glands (AGs), common hepatic artery (CHA), common hepatic duct (CHD), falciform ligament (FL), common bile duct (CBD), and duodenal wall (DW). (b) Boxplot of the structures grouped into high contrast (consisting of AG and CHA), medium contrast (consisting of CHD and LF), and low contrast (consisting of CBD and DW). Whiskers indicate the 1.5 interquartile range. Significant Hounsfield unit (HU) differences are marked with asterisks, whereas nonsignificant differences are marked with n.s.

TABLE 1

HU Comparison of Different Anatomic Structures—Statistical Analysis

P Values AG CHA CHD LF CBD DW AG 0.2329 <.0001 <.0001 <.0001 <.0001 CHA <.0001 <.0001 <.0001 <.0001 CHD >.9999 .0002 <.0001 LF .0004 <.0001 CBD .3301

AG, adrenal gland; CBD, common bile duct; CHA, common hepatic artery; CHD, common hepatic duct; DW, duodenal wall; HU, Hounsfield unit; LF, falciform ligament.

P values derived from a Tukey’s multiple comparison test in conjunction with an analysis of variance for the contrast of different anatomic structures. Also see Figure 1a . An additional comparison between grouped high-, medium-, and low-contrast structures revealed only significant differences (all P values < .0001; see Fig 1b ).

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Noise Analysis

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Figure 2, ADMIRE 5 reduced noise significantly more than ADMIRE 3 in all three contrast subgroups.

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Calculation and Comparison of the Indecision Points

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Figure 3, Sigmoid regression curves for ADMIRE 3 and ADMIRE 5 reconstructions of high-, medium-, and low-contrast structures with the radiation dose on the axis of abscissae and the preference ratio (PR) on the axis of ordinates. Dotted lines indicate 95% confidence intervals. The radiation dose level corresponding to a PR of 0.5 is the indecision point (IP), the radiation dose reduction potential is 100% − IP. The regression curve equations and R 2 values are given in addition.

TABLE 2

Indecision Points of ADMIRE 3 and 5 for High-, Medium- and Low-contrast Structures

ADMIRE 3 ADMIRE 5 IP 95% CI_R_ 2 IP 95% CI_R_ 2 High contrast 47.5 44.6–50.3 0.987 46.5 44.2–48.9 0.992 Medium contrast 61.8 56.1–67.6 0.954 60.9 54.7–67.0 0.950 Low contrast 71.0 64.9–77.2 0.961 66.1 54.6–77.7 0.885

CI, confidence interval.

A numerical presentation of Figure 4 , depicting indecision points (IPs) of ADMIRE 3 and ADMIRE 5 reconstructions for high-, medium-, and low-contrast structures along with their 95% confidence intervals. R 2 values are given in addition to goodness-of-fit quantification.

Figure 4, Stacked column chart displaying indecision points (IPs) of ADMIRE 3 (gray columns) and ADMIRE 5 (white columns) reconstructions for high-, medium-, and low-contrast structures. Dotted columns display dose reduction potential (100% − IP). Error bars indicate 95% confidence intervals. Brackets and P values are given to illustrate significant and nonsignificant differences. Significant differences are marked with asterisks, whereas nonsignificant differences are marked with n.s. Also compare to Table 2 .

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Figure 5, Exemplary panel showing high-, medium-, and low-contrast structures (adrenal gland, common hepatic duct, and common bile duct, respectively, all marked by an arrow ) in a full-dose weighted filtered back projection, a full-dose ADMIRE 3 and ADMIRE 5 reconstruction, at their ADMIRE 3 indecision points (IP, 48% for high-, 62% for medium-, and 71% for low-contrast structures), and at a radiation dose level of 10%, reconstructed with ADMIRE 3 and a weighted filtered back projection. In addition, the left adrenal gland (high-contrast structure) is also visible in the medium-contrast slices (marked by an arrowhead ).

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Discussion

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Conclusion

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Academic Radiology, Volume 25, Issue 10
Journals General Radiology
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