IMAGE QUALITY Flashcards
(23 cards)
What are the scanning parameters controlled by the operator?
- mA level
- Scan Time
- Slice Thickness
- FOV
- Reconstruction Algorithm
- kVp
- Pitch
FOV stands for Field of View, which determines the area scanned.
What does mAs stand for and how is it calculated?
mAs = mA x Scan Time
mAs controls the quantity of x-ray photons produced.
What is the effect of a higher mAs on image quality?
- Lower Image Noise
- Higher Patient Dose
- More heat in the x-ray tube
Higher mAs should be optimized to reduce patient dose.
What distinguishes small and large filament sizes in x-ray tubes?
Small Filament: * Concentrated focal spot * Reduces penumbra * Used for lower mA settings
Large Filament: * Tolerates higher mA * Slightly reduced resolution * Used for higher mA settings
Penumbra refers to the geometric unsharpness around the edges of the image.
Which body areas are more prone to motion artifacts?
- Cardiac Imaging
- Abdomen
- Chest
Shorter scan times help reduce motion artifacts.
What is the relationship between mAs settings and heat generation in scanners?
Higher mAs settings generate more heat.
Modern CT systems have improved heat dissipation capabilities.
What is kVp and its impact on contrast in CT imaging?
kVp controls the beam quality and has less direct impact on contrast in CT.
Some CT systems use a fixed kVp, typically 120 kVp.
How does reducing mAs while holding kVp constant affect radiation dose?
Reduces radiation dose.
Reducing kVp while holding mAs constant also reduces dose but may limit penetration.
What is Automatic Tube Current Modulation (ATCM)?
A software-driven approach to adjusting tube current (mAs) based on patient anatomy, optimizing radiation dose while maintaining image quality.
ATCM adjusts mAs in real time during each gantry rotation.
What factors affect mAs selection?
- Body parts
- Scanner capabilities
- Patient cooperativeness
These factors help determine the optimal mAs for imaging.
What are the two critical aspects of image fidelity?
- Detail representation (Spatial Resolution)
- Ability to detect subtle density variations (Contrast Resolution)
Image fidelity refers to how accurately the scanner represents anatomical structures.
What is the purpose of using a line pair phantom in measuring spatial resolution?
Counts the number of visible line pairs per centimeter (lp/cm) to assess spatial resolution.
A higher line pair count indicates better spatial resolution.
What does Modulation Transfer Function (MTF) Analysis measure?
It measures how well a system transfers different spatial frequencies from object to image.
Higher MTF values indicate better resolution.
Define spatial frequency.
Refers to the number of line pairs per unit; large objects have low spatial frequency while small objects have high spatial frequency.
CT imaging has lower spatial resolution compared to conventional radiography.
What are the two types of spatial resolution?
- In-Plane Resolution (XY direction)
- Longitudinal Resolution (Z direction)
In-plane resolution is affected by pixel size and matrix size, while longitudinal resolution is influenced by slice thickness.
How does slice thickness affect spatial resolution?
Thinner slices improve spatial resolution; thicker slices increase volume averaging, which can reduce detail.
Volume averaging occurs when multiple tissues of different densities are included in a single pixel.
What is the impact of motion artifacts on spatial resolution?
Patient movement reduces spatial resolution.
Shortened scan time helps mitigate the effects of involuntary and overt patient motion.
What is contrast resolution?
The ability to differentiate a structure that has a slight density variation from its surroundings.
Also known as low-contrast detectability or low-contrast sensitivity.
How is contrast resolution measured?
Assessed using phantoms with cylindrical objects of varying sizes and densities (4-10 HU difference from background).
Observers detect the smallest distinguishable object.
What role does noise play in contrast resolution?
Noise creates undesirable fluctuations in pixel values, appearing as a grainy effect; higher noise leads to lower contrast resolution.
Signal-to-Noise Ratio (SNR) is used to describe image quality.
What is temporal resolution?
Measures how quickly data can be acquired and reconstructed, reported in milliseconds (ms).
Lower values indicate better temporal resolution.
What factors affect temporal resolution?
- Gantry Rotation Speed
- Detector Channel Configuration
- Data Acquisition and Reconstruction Speed
Faster gantry rotation improves temporal resolution.
Why is optimizing temporal resolution important in cardiac imaging?
Essential for reducing motion artifacts from a beating heart and ensuring clear imaging of coronary arteries.
High temporal resolution is also critical for CT Angiography (CTA) and perfusion studies.
