Image Quality (CTBC 8) Flashcards
Spatial Resolution
(Sharpness)
Clarity level of small parts, spaces b/w small parts, edges of parts
Spatial Resolution & Pitch
Decreased pitch requires less interpolation and increases potential spatial resolution in z-axis (reformations)
Spatial Resolution & Matrix Size
Larger matrix = more pixels
More pixels allow for greater detail
Decreased acquisition slice thickness produces _______ spatial resolution in z-axis (reformations)
Higher
Reconstructing with thin slices –> _______ spatial resolution in z-axis (reformations)
highest
Edge-enhancing algorithms (bone/lung) _______ sharpness & spatial resolution
increase
Spatial Resolution & DFOV
Smaller DFOV creates smaller pixel dimensions w/ability to resolve smaller parts
Increasing spatial resolution can have the effect of increasing the appearance of _______ _______
Image Noise
Eventually the appearance of image noise will disguise any increase in _______ ________
Spatial Resolution
Smoothing algorithms are utilized instead of edge-enhancing algorithms when…
Decrease in apparent image noise is needed
Unit for measuring spatial resolution: visualization of more line pairs indicates higher spatial resolution
Line-pairs per mm (lp/mm) [or line-pairs per cm (lp/cm)]
Contrast Resolution (2)
Ability of CT machine to create visible difference in brightness levels (aka “contrast”)
Ability of the scanner to visually discriminate between objects with similar physical density
Sensitivity of the System
Ability to create visible contrast b/w objects with very similar LAC
Increasing kVp causes more uniform penetration of patient and _______ contrast
Decreases
Measurable difference in photon intensity striking detector array
Differential Attenuation
Factors influencing display contrast (3+)
kVp
Filtration (applied?)
Patient Morphology
Etc.
Decreasing kVp results in (3) _______ Scatter, _______ Differential Attenuation, _______ Contrast Resolution
Less Scatter
More Differential Attenuation
Increased Contrast Resolution
Smaller patients result in _______ Scatter, _______ Differential Attenuation, _______ Contrast Resolution
Less Scatter
More Differential Attenuation
Increased Contrast Resolution
Window Width controls the range of displayed tissues thereby changing
Displayed Contrast
Image Noise typically results from
Inadequate exposure to detectors
Higher Standard Deviation (within ROI) indicates _______ variance and, therefore, _______ image noise
Higher Variance–> Higher Image Noise
2 methods for quatifying noise
Contrast Resolution Test
Measure Standard Deviation using a Simple ROI
Decreased Image Noise usually results in _______ patient dose, _______ spatial resolution
Increased Patient Dose
Decreased Spatial Resolution
Increasing kVp increases detector exposure and _______ image noise
Reduces Image Noise
Increasing mA increases detector exposure and _______ image noise
Reduces Image Noise
Increasing Rotation Time _______ detector exposure and _______ image noise
Increases Detector Exposure
Reduces Image Noise
Increasing (acquisition or reconstruction) Slice Thickness _______ image noise
Reduces Image Noise
Smoothing Algorithms (e.g., standard) _______ image noise
Reduce Image Noise
Temporal Resolution
Ability to produce still images of moving parts (similar to shutter speed of film camera: faster avoids blurring)
Decreasing Rotation Time _______ temporal resolution
Increases Temporal Resolution
Increasing Acquisition Thickness _______ total scan time, thus _______ temporal resolution
Decreases total scan time, Increasing temporal resolution
Increasing Pitch _______ total scan time, thus _______ temporal resolution
Decreases total scan time, Increasing temporal resolution
Half-scan algorithms
Use only 180 deg of scan info to reconstruct complete data (thus doubling scan speed), enhancing temporal resolution
Certain algorithms improve _______ _______ by reducing motion artifacts
Temporal Resolution
Gating technology diminishes _______ _______, _______ temporal resolution
cardiac motion
increasing temporal resolution
CT number Accuracy
Assurance that specific materials (and, in turn, specific tissues) generate specific CT numbers
HU Water
0
HU Air
-1000
Air Calibration typically corrects problems with:
HU accuracy
Linearity
Relationship b/w attenuation properties of material and resulting CT number
As attenuation (tissue density) increases, CT numbers increase at a _______ rate
Linear rate
A linearity test phantom includes several cylinders with varying materials of differing _______
Density
For a Linearity Test Phantom, the resulting graph (of LAC for the respective cylinders) should be _______.
Linear
Uniformity
Ability of scanner to yield same CT number for same substance regardless of ROI-position
Factors impacting Uniformity (3)
Beam Hardening
Scatter Radiation
Limited Stability of Scanner
Capping Artifact
Higher CT numbers in center (of uniform phantom)
Cupping Artifact
Lower CT numbers in center (of uniform phantom)
