GE Healthcare eXplore Locus SP, Руководство по обслуживанию

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Chapter 3
Troubleshooting
Section 3.1
Overview
3.1.1 Avoiding Common Errors
Wrong filter : When a low density filter is used in combination with a high density specimen, beam
hardening can occur. Beam hardening occurs because the beam is not filtered before it goes
through a material with high attenuation.
When a high density filter is used in combination with soft tissue, the signal-to-noise ratio can be
such that resolution is reduced considerably. When a dense filter is used in combination with soft
tissue (tissue with low variation in the CT values of the different structures), the combination of the
energetic, filtered beam with the low density, low variation tissue results in an image in which the
structures are very hard to distinguish from each other.
Current too high: When the current is too high, the resulting projection appears similar to a film
camera image that has been overexposed.
Current too low: When the current is too low, the resulting projection appears similar to a film
camera image that was taken in low light.The resolution is very low.
Voltage too high: When the voltage is too high, the resolution can be low, and high dynamic range
errors can occur. A high dynamic range error appears as a dark spot in the midst of a bright area
which is not accounted for by the structure that was scanned.
Voltage too low: When the voltage is too low, a great deal of the beam is absorbed instead of
penetrating the material. The resulting projection shows low resolution, high noise, and sometimes
beam hardening.
Exposure time too long: When the exposure time is too long, too many photons appear on the
same exposure. The result is similar to the current being too high. If you need to use a long
exposure time, compensate by decreasing the current and voltage.
Exposure time too short: When the exposure time is too low, the signal-to-noise ratio is low. This
in turn reduces resolution. If you need to use a short exposure time, compensate by using frame
averaging to increase the number of frames used in each final projection file.
Bubbles in suspension medium: Bubbles in the suspension medium cause several problems.
They can be a distracting visual element in the final images. Also, because they unevenly change
the density of the suspension medium, they can compromise the usability of the final data.