Chapter 6: Image Quality Flashcards
- 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
describes the relationship between object size and visibility
Contrast-detail curve
Measures how quickly data can be acquired
and reconstructed.
Temporal Resolution
- Essential for reducing motion artifacts from a beating heart.
Cardiac Imaging
- Critical for capturing dynamic contrast flow in vessels.
CT Angiography (CTA)
- Used in stroke assessment to track blood
flow in the brain. - Requires rapid data acquisition to
differentiate perfusion levels
Perfusion Studies
Factors that determine X-ray Beam Exposure
- mA Setting
- Scan Time
- kVp Setting
Effects of a higher mAs:
- Lower Image Noise (better quality)
- Higher Patient Dose (should be optimized)
- More heat in the x-ray tube
- provides a concentrated focal spot
- reduces penumbra (geometric unsharpness)
- used for lower mA setting (<350mA)
Small Filament
- can tolerate higher Ma
- slightly reduced resolution
- used for higher mA setting (>350 mA)
Large Filament
Shorter scan time help ___ motion
artifacts.
reduce
higher mAs settings generate ___ heat
more
kVp has ___ direct impact on contrast in CT
less
increased pitch ____ spatial resolution
reduces
- refers to the ability to differentiate a
structure that has a slight density variation
from its surroundings.
Contrast Resolution
Increasing mAs improves contrast resolution by increasing the number of detected photons. True or False
True
Smaller pixels capture fewer photons,
reducing noise. True or False
False
Faster gantry rotation improves temporal
resolution. True or False
True
Multi-slice CT scanners improve temporal
resolution by collecting more data per
rotation. True or False
True
