Image contrast is the different between the values in different parts of the image that enables you to distinguish different tissue types for instance.įor instance an example of a low contrast task is distinguishing a soft tissue lesion in a background of other soft tissue in the abdomen. What is image contrast?Ĭontrast in medical images such as x-ray or CT images is given by different tissue properties that are measured as different values in the medical images such as different CT numbers (Hounsfield numbers). These measures help us understand how well the diagnostic task can be performed, even when noise is present in the images. In this section we will discuss the concepts of Signal to Noise Ratio (SNR) and Contrast to Noise Ratio (CNR). In this case we call the tumor the image signal of interest and the task is differentiating it from the background tissue.
Therefore, the optimal images will always have a reasonable amount of noise (otherwise the scan is acquired at a radiation dose that is too high or in the case of MR the scan time is too long).Ī very common clinical task in x-ray or CT is to determine if there is something undesirable in the image such as a tumor or another type of lesion. In x-ray and CT imaging the trade-off is the radiation dose the patient receives, while in MRI it is scan time. In all diagnostic imaging there is a trade-off between noise and image acquisition parameters. Signal to Noise Ratio (SNR) Definition and Contrast to Noise Ratio (CNR) Definition
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