Contrast, image noise

The detectability of critical defects with CT depends on the final image quality and the skill of the operator, see figure 2. The basic concepts of image quality are resolution, contrast, and noise. Image quality are generally described by the signal-to-noise ratio SNR), the modulation transfer function (MTF) and the noise power spectrum (NFS). SNR is the quotient of a signal and its variance, MTF describes the contrast as a function of spatial frequency and NFS in turn describes the noise power at various spatial frequencies [1, 3]. 

Unlike conventional images, DW images are highly sensitive and specific in the detection of hyperacute and acute infarctions [30, 33-35] (Table 7.3). They are very sensitive to the decreased diffusion of water that occurs early in ischemia, and they have a much higher contrast-to-noise ratio compared with CT and conventional MRI. Reported sensitivities range from 88 to 100%, and reported specificities range from 86 to 100%. A lesion with decreased diffusion highly correlates with infarction. 

Scharfetter, H., Hollaus, K., Rosell-Ferrer, J., Merwa, R., 2006b. Single-step 3-D image reconstruction in magnetic induction tomography theoretical limits of spatial resolution and contrast to noise ratio. Ann. Biomed. Eng. 34, 1786-1798. 

Contrast resolution characterizes a C-arm CT systems capacity to resolve soft tissue differences. For good low-contrast imaging results, it is advantageous to acquire a high number of views, because this reduces streak artifacts. To further reduce noise in appUcations such as head scans, an additional option may be to increase system dose. 

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