Image quality and radiation dose in planar imaging — Image quality figure of merits from the CDRAD phantom

• Purpose: A contrast-detail phantom such as CDRAD is frequently used for quality assurance, optimization of image quality, and several other purposes. However, it is often used without considering the uncertainty of the results. The aim of this study was to assess two figure of merits (FOM) originating from CDRAD regarding the variations of the FOMs by dose utilized to create the x-ray image. The probability of overlapping (assessing an image acquired at a lower dose as better than an image acquired at a higher dose) was determined. Methods: The CDRAD phantom located underneath 12, 20, and 26cm PMMA was imaged 16 times at five dose levels using an x-ray system with a flat-panel detector. All images were analyzed by CDRAD Analyser, version 1.1, which calculated the FOM inverse image quality figure (IQFinv) and gave contrast detail curves for each image. Inherent properties of the CDRAD phantom were used to derive a new FOM h, which describes the size of the hole with the same diameter and depth that is just visible. Data were analyzed using heteroscedastic regression of mean and variance by dose. To ease interpretation, probabilities for overlaps were calculated assuming normal distribution, with associated bootstrap confidence intervals. Results: The proportion of total variability in IQFinv, explained by the dose (R2), was 91%, 85%, and 93% for 12, 20, and 26cm PMMA. Corresponding results for h were 91%, 89%, and 95%. The overlap probability for different mAs levels was 1% for 0.8 vs 1.2mAs, 5% for 1.2 vs 1.6mAs, 10% for 1.6 vs 2.0mAs, and 10% for 2.0mAs vs 2.5mAs for 12cm PMMA. For 20cm PMMA, it was 0.5% for 10 vs 16mAs, 13% for 16 vs 20mAs, 14% for 20 vs 25mAs, and 14% for 25 vs 32mAs. For 26cm PMMA, the probability varied from 0% to 6% for various mAs levels. Even though the estimated probability for overlap was small, the 95% confidence interval (CI) showed relatively large uncertainties. For 12cm PMMA, the associated CI for 0.8 vs 1.2mAs was 0.1–3.2%, and the CI for 1.2 vs 1.6mAs was 2.1–7.8%. Conclusions: Inverse image quality figure and h are about equally related to dose level. The FOM h, which describes the size of a hole that should be seen in the image, may be a more intuitive FOM than IQFinv. However, considering the probabilities for overlap and their confidence intervals, the FOMs deduced from the CDRAD phantom are not sensitive to dose. Hence, CDRAD may not be an optimal phantom to differentiate between images acquired at different dose levels. © 2019 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine.

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Radiologi och bildbehandling (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Radiology, Nuclear Medicine and Medical Imaging (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Medicinteknik -- Medicinsk bildbehandling (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Medical Engineering -- Medical Image Processing (hsv//eng)

Nyckelord

CD-curve

contrast detail phantom

figure of merit

IQF

planar imaging

precision

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