|Kirsten Boedeker, PhD, DABR
Senior Manager, Quantitative Image Quality
Canon Medical Systems Corporation
*1 Adaptive Iterative Dose Reduction, *2 Forward projected model-based Iterative Reconstruction SoluTion
The Contrast-to-Noise Ratio (CNRs) is a measure of image quality that, in one of its simplest forms, is determined by taking the absolute value of the difference in mean CT number of an object from its background and dividing it by the standard deviation of noise (SD) of the background.
This expression for CNR is useful for image quality comparisons when sources of spatial resolution variation, as well as signal and noise texture variation, are held constant. However, when spatial resolution, signal power and/or noise texture vary significantly, this CNR metric is too simple to be an accurate measure of image quality. For example, note that in the two images in Figure 1, both images have similar image quality and low contrast detectability, yet very different CNR values. Both images are reconstructed with the same kernel and AIDR 3D reconstruction settings, but the image on the right is a 1024 × 1024 image generated with the Aquilion Precision’s HR mode and the image on the left is a 512 × 512 image generated in NR mode.
Equation 1 Signal-to-noise ratio formula.
Figure 7 This SHR mode (on the right) lung scan reveals greater anatomical and nodule detail than conventional resolution imaging (on the left).
Disclaimer: Any reference to X-ray exposure is intended as a reference guideline only. The guidelines in this document do not substitute for the judgment of a healthcare provider. Each scan requires medical judgment by the healthcare provider about exposing the patient to ionizing radiation. In clinical practice, the use of the AIDR 3D and FIRST (Forward projected model-based Iterative Reconstruction SoluTion) features may reduce CT patient dose depending on the clinical task, patient size, anatomical location and clinical practice. A consultation with a radiologist and a physicist should be made to determine the appropriate dose to obtain diagnostic image quality for the particular clinical task.
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