Week 4: Image Quality and Artifacts Flashcards
(23 cards)
Spacial Resolution
The ability to visualize small parts. The ability to see the spaces between small parts. Detail or sharpness. Depends on the quality of the raw data and the reconstruction method.
How do we increase Spacial Resolution
Increase Rotation Time: obtains more data but increases dose
Decrease Slice Thickness: creates less volume averaging, represents the part with higher fidelity.
Decrease Pitch: fewer rotations per table movement create more over-scanning, increasing detail.
Partial Volume Effect
Data form entire section thickness averaged to form image. Small structures lost due to averaging of large amounts of data (more than one type of data contained in a voxel). May obscure any objects smaller than voxel width (section thickness)
To prevent: use thinner sections and overlapping sections.
Pixel and Matrix Size
Large matrix and small pixels increase spacial resolution. Making DFOV smaller “zooms in” and allows the number of pixels to be used on a smaller area, decreasing pixel size.
Low Contrast Resolution
Systems ability to differentiate a structure that varies only slightly in density form its surrounding. CT scanners can detect objects with only a 0.5% contrast variation.
Noise
Degrades contrast resolution, undesirable fluctuations in pixel value in an image of homogenous material. We use the standard deviation within a ROI to determine how much noise there is.
What limits noise?
Increasing technical factors: mA reduces quantum mottle and kV increase part penetration.
Increasing slice thickness: more photons reach the detector increasing the signal to noise ratio.
Decrease patient thickness: we have no control one this, larger people ar harder to image.
Temporal Resolution
The scanners ability to record clear images of objects in motion. Controlled by rotation speed, number of detector channels and pitch.
How to increase contrast resolution during acquisition?
CM is very common in CT it: temporarily increase the atomic number of the part it travels through. Timing is very important for the exam to match the flow of contrast is crucial.
How to increase temporal resolution?
Increase scan speed.
What controls scan speed?
Gantry rotation time (how quickly the gantry can rotation), number of detector channels (how much coverage is possible per rotation) and pitch (what is the ratio o table speed to gantry rotation).
EKG Gated Cardiac Studies
Sometimes called 4D imaging, adds time as a dimension creating a functional study. Takes sequential scans at the same part of the heart cycle as the machine moves down the anatomy. To remove motion while heart beats.
Patient Motion Artifacts
Voluntary and involutary. Shading, ghosting, streaking. Banding artifact (due to heart irregularities). Pulsation artifact (due to vasculature).
Edge Gradient Effect
Due to extreme difference in composition of adjacent structures (barium in colon, metal items). Streaks and shading, light or dark.
To prevent: use thinner slices or use different type of CM (lower HU).
Out of Field Artifacts
Occurs when anatomy extends outside of SFOV or unwanted anatomy in SFOV. Attenuates and hardens the beam confusing image reconstruction process.
To prevent: select SFOV larger than patient, move arms away from imaging area.
Beam Hardening
Occurs when the beam encounters a very dense structure. Broad dark bands or streaks between dense objects.
To prevent: bow tie filters used routinely, increase kV, beam hardening error correction algorithms.
Partial Volume Artifact
Related to partial volume averaging but is seperate. Structure is only scaled during part of the rotation around the patient.
Aliasing
Inadequate data or under sampling. Not enough projections collected for the body part = inaccuracies in sharp edges and small objects.
To prevent: slow gantry rotation, reduce pitch.
Metallic
Due to beam hardening, partial volume artifact, edge gradient and aliasing all combined.
To prevent: avoid metal, increase kV, decrease slice thickness and change window.
Ring Artifact
One or more detectors out of alignment/calibration. Error multiplied around 360 degree to create ring.
To prevent: routine detector calibrations.
Windmill Artifact
As the beam gets wider with more detector rows available, this artifact has become more common. Occurs often with faster pitch and more active detector rows. Most prominent on the periphery of the acquisition row. Algorithms written to remove it.
Stair Step
Occurs when MPRs are created from thick helical sections, reconstruction not smooth, step like edges.
To prevent: use thinner slices.
Segmentation/ROI Error
Structures can be selectively edited out of image but strictures may be removed accidentally.
To prevent: be cautious when reformatting.
