22 Questions
What is the primary reason why CT image resolution is often expressed in terms of spatial frequency?
It is easier to understand than object size
What is the minimum number of pixels required to represent one line and its interspace?
Two pixels
What is the relationship between the absolute object size that can be resolved by a CT imaging system and the spatial frequency at the limiting resolution?
The object size is equal to one half the reciprocal of the spatial frequency
What is the purpose of the four test sections of the phantom designed by the Physics Commission of the American College of Radiology (ACR)?
To evaluate a number of CT image quality factors
What is the limitation of any imaging system in terms of spatial resolution?
The size of the pixel
What is the representation of a 1-mm object separated by a 1-mm interspace in terms of line pairs?
2 mm/lp
What is the effect of thinner slice thicknesses on spatial resolution?
It allows better spatial resolution
What is the modulation transfer function (MTF) often used to express?
Spatial Resolution
What happens to anatomy that does not lie totally within a slice thickness?
It is not resolved due to partial volume averaging
How does the design of prepatient and predetector collimation affect spatial resolution?
It affects the level of scatter radiation and influences spatial resolution
What is the unit of measurement often used to express spatial frequency in CT imaging systems?
Line pairs per centimeter (lp/cm)
What is the edge response function (ERF) used to express?
The ability of the CT imaging system to reproduce a high-contrast edge
How is the measured edge response function (ERF) transformed mathematically?
Into a modulation transfer function (MTF)
What happens to the image obtained from a low-frequency bar pattern compared to a high-frequency bar pattern?
It appears more similar to the object
What is the main reason for the loss of faithful reproduction with increasing spatial frequency?
Limitations of the imaging system
What does the Modulation Transfer Function (MTF) measure?
The ratio of image to object at a given spatial frequency
What is the MTF value if the image is a faithful representation of the object?
1
What does an MTF curve that extends farther to the right indicate?
Better spatial resolution
What does an MTF curve that is higher at low spatial frequencies indicate?
Better contrast resolution
What is the significance of an MTF value of 0.1?
Limiting resolution of most CT imaging systems
What is the relationship between spatial frequency and image contrast?
Image contrast decreases with increasing spatial frequency
What can be inferred from an MTF curve?
Both spatial and contrast resolution
Study Notes
CT Imaging System Performance
- Imaging system A has better spatial resolution than B, with a 0.1 MTF at 5.2 lp/cm compared to 3.5 lp/cm for B.
Spatial Resolution
- The absolute object size that can be resolved by a CT imaging system is equal to one half the reciprocal of the spatial frequency at the limiting resolution.
- A system resolution of 1 mm means a 1-mm object separated by a 1-mm interspace.
Evaluating CT Image Quality
- Important measures of imaging system performance that can be evaluated with test objects include artifact generation, contrast resolution, and spatial resolution.
- The Physics Commission of the American College of Radiology (ACR) designed a phantom with four test sections to evaluate CT image quality factors.
Limitations of CT Imaging Systems
- No imaging system can do better than the size of a pixel.
- One line and its interspace require at least two pixels, so the image is blurred due to limitations of the CT imaging system.
- Spatial resolution is a function of pixel size, with smaller pixels resulting in better resolution.
Factors Affecting Spatial Resolution
- Thinner slice thicknesses allow for better spatial resolution.
- Voxel size in CT also affects spatial resolution.
- The design of pre-patient and pre-detector collimation affects the level of scatter radiation and influences spatial resolution.
Modulation Transfer Function (MTF)
- The MTF is a mathematical expression that describes the ability of a CT imaging system to reproduce a high-contrast edge accurately.
- The MTF can be derived from the edge response function (ERF).
- The MTF is often cited to express the spatial resolution of a CT imaging system.
Converting Line Pairs per Centimeter to Line Pairs per Millimeter
- 1 lp/cm = 1 lp/10 mm = 0.1 lp/mm
Image Fidelity and MTF
- Image fidelity is measured by determining the image contrast at various spatial frequencies.
- A higher MTF value indicates better spatial resolution and contrast resolution.
- An MTF curve that extends farther to the right indicates better spatial resolution, while a higher curve at low spatial frequencies indicates better contrast resolution.
Evaluating the performance of CT imaging systems, including spatial resolution and image quality metrics.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
