Chapter 6: Image Quality Flashcards
(42 cards)
- parameters that are controlled by the
operator
Scanning Parameter
: controls the number of x-ray photons produced.
mA
: Duration of the x-ray beam is active for
acquisition
Scan time
- a software driven approach to adjusting tube current(mAs) based on patient anatomy.
- helps optimize radiation dose while
maintaining image quality - adjusts mAs in real time during each gantry rotation
- dose reduction 15 to 40 %
Automatic Tube Current Modulation (ATCM)
– affects spatial resolution and noise
Slice Thickness
– determines the scan area
within the gantry
Field of View (FOV)
– affect image clarity and detail
Reconstruction Algorithms
– relationship between the slice thickness and table travel during helical scanning.
Pitch
- refer to the movement and positioning of the x-ray tube and detectors in relation to the patient during a scan. Impacts the quality and accuracy of the images produced
SCAN GEOMETRY
- standard method
- data collected from a full rotation oof the
tube. - two matching sample taken 180 deg apart
improve image accuracy
360 deg
- referred to as half scans
- only half of the available data is used for
image reconstruction - suitable for studies requiring short scan
times
Partial Scan (180 deg + Fan Angle Arc)
- includes an extra 40 deg of rotation beyond the standard 360 deg
- reduces motion artifacts by overlapping
initial and final scan positions - more commonly used in fourth gen CT-scans
400 deg Overscan
- the ability to differentiate between small
objects.
Spatial Resolution (High Contrast Resolution)
- the ability to differentiate objects with
similar densities
Contrast Resolution (Low Contrast Resolution)
- refers to how accurately the scanner
represents anatomical structures.
Image Fidelity
- a phantom with metal strips embedded in
acrylic is scanned. - the number of visible line pairs per
centimeter (lp/cm) is counted. - a line pair consists of metal strip and the
space between the strips. - higher line pair count indicated better spatial resolution
Direct Measurement Using a Line Pair Phantom
- measures how well a system transfers
different spatial frequencies from object to
image. - highest MTF values indicates better
resolution - used to compare system performance over
time or against other systems.
Modulation Transfer Function (MTF) Analysis
- refers to the number of line pairs per unit
- large objects have low spatial frequency,
while small objects have high spatial
frequency.
Spatial Frequency
- resolution within the CT-scan
- affected by pixel size and matrix size
In-Plane Resolution (XY direction)
- resolution along the patient’s body axis
- influenced by slice thickness
Longitudinal Resolution (Z direction)
is an undesirable fluctuation in pixel values.
Appears as a “grainy” or “salt-and-pepper” effect.
Noise
is a major contributor type of noise, caused by insufficient detected photons.
Quantum noise (mottle)
is used to describe image quality.
Signal-to-Noise Ratio (SNR)
- Related to the inherent properties of
scanned tissues.
Subject Contrast
