Noise and Artifacts

Prevention of Artifact

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  To reduce patient motion:

  • Prepare the patient before the scan.

  • Eliminate any communication barriers so patient can follow instructions. The services of a translator may be necessary.

    • Prepare the patient to anticipate the warm sensation of contrast dye injection and not be alarmed by it (to attempt to avoid heart rate [HR] increase during the scan).

    • Have the patient practice breath-holding until successful.

    • Instruct the patient not to swallow during scanning.

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  Use β-blockers to avoid HR acceleration during the scan.

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  To avoid excessive R-R variation:

  • Exclude patients with frank arrhythmias.

  • Use β-blockers to suppress HR acceleration.

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  Use optimal kilovolts and milliamps.

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  To optimize sampling during diastasis:

  • Although use of retrospective gating increases radiation exposure, it provides more versatility and robustness in reconstructing images from different phases (e.g., 70%, 75%, and 80% phases of the cardiac cycle) to find one without motion-related artifacts.

Each study should be assessed for noise level and artifacts before image interpretation is initiated because this establishes a baseline sense of image adequacy and may justify the exclusion of ectopic cardiac cycles.

Many studies will have more than one form of artifact.

Noise

With CT scanning, multiple mechanisms can affect image quality. One of these is image noise. Noise can be defined or measured as the standard deviation of Hounsfield unit voxel values within a homogeneous phantom, usually a water phantom. Usually, the lower the noise, the higher the signal-to-noise ratio (SNR), and the better the image quality is. A number of factors affect the amount of noise, including technical parameters, patient factors, and image reconstruction and post-processing. Technical parameters of kilovoltage potential (KvP), milliamps, and exposure time influence the amount of noise within the image. Patient size—for example, morbid obesity—can attenuate the number of x-ray photons reaching the detector and increase the amount of noise, therefore often requiring an increase to the standard parameters of kVp and mA. Noise also can be modulated after image acquisition by altering the reconstruction kernel, with sharper kernels generating noisier images, as well as by increasing the slice thickness, which generally increases the SNR of an image ( Fig. 5-1 ).

Types of Artifacts