Image Quality in CT (part 2) Flashcards by Josie Curley

What are the four components of CT image quality?

Noise
(Low) contrast resolution
High contrast (spatial) resolution
Temporal resolution

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What is a water phantom?

An Acrylic cylinder with Uniform density that has an Attenuation equivalent to that of water

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What is the HU of water?

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T/F

If an image is created of an object that is known to be of uniform density (water phantom), then all measured points within that image should in theory be the same.

True

(however in reality it isn’t)

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What two measurements appear when a Region of Interest (ROI) tool or cursor is placed over the image?

A mean HU measurement and standard deviation (SD) measurement is obtained.

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T/F

The larger the SD, the higher the image noise.

True

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What does the SD (standard deviation) in an image indicate?

The SD indicates the magnitude of random fluctuations in the CT number

These random fluctuations in the CT number of otherwise uniform materials appear as graininess on CT images.

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Even if we image a perfectly uniform object (e.g., a water filled object) there is still a variation in the Hounsfield units about a mean. Why is this?

This is due to noise

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T/F

Noise degrades the image by degrading high contrast resolution

False; Noise degrades the image by degrading low contrast resolution

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What are the 3 sources of noise in CT?

Quantum noise
Electronic noise
Noise introduced by the reconstruction process (e.g., filtered back projection)

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What is the biggest enemy of low contrast resolution?

Noise

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The number of photons detected will vary randomly about a mean value. What is the variation called?

Noise

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Photon registration by the detectors is what type of process?

Photon registration by the detectors is a stochastic process.

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What 5 technical factors (scan parameters) affects the number of photons detected?

mA
Scan (rotation time)
Slice thickness
Peak kVp
Reconstruction algorithm

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Describe how changes in mA can affect the noise:

Changing the mA value changes the beam intensity and thus, the number of x-rays — proportionally.

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T/F

Scan (rotation) time affects the number of detected x rays proportionally

True

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T/F

An increase in mA decreases the noise

True

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T/F

A decreased scan time increases the signal to noise ratio (decreasing the noise)

True

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T/F

As slice thickness increases, so does the noise.

False; as slice thickness increases, the noise decreases

For example, compared with a slice thickness of 5 mm, a thickness of 10 mm approximately doubles the number of x-rays entering each detector.

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T/F

kVp is directly proportional to noise

False

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T/F

Increasing the kilovoltage reduces image noise

True

(but reduces subject contrast)

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T/F

The Reconstruction Algorithm direclty affects the number of x-rays hitting the dector, thus reducing noise.

False; Reconstruction Algorithm does not affect the numbers of detected x-rays. A reconstruction filter profoundly affects the appearance of noise in the image.

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What is the purpose of the smoothing filter?

To soften noise

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What is the purpose of the sharpening filter?

To increase noise

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What is preffered for images of soft tissue, a smoothing or sharpening filter?

Smooth filter

What is preffered for images of structures with edges and small details, such as bone; a smooth or a sharp filter?

Sharp filter

# T/F As noise increases Low Contrast Detectability decreases

True

What is low contrast resoltion?

How well low-contrast test objects are seen in the presence of typical noise levels

What 8 factors affect low contrast resolutioin?

1. mAs 2. kVp 3. Slice thickness 4. Reconstruction algorithm 5. Pixel size 6. Patient size 7. Inherent tissue properties 8. Use of contrast media

# T/F Dose does not increase linearly with mAs

False; Dose increases linearly with mAs

As mAs increases, what happens to low contrast resolution?

It increases

As kV increases, what happens to low contrast resolution?

It decreases ## Footnote Although the use of a higher tube voltage (kV) results in improved x-ray photon statistics (↑SNR), the quality of the x-ray beam is somewhat compromised because the visibility of low-contrast objects depends on the presence of low-energy photons, which are disproportionally less for the higher tube voltage

What is the downside of increasig Slice Thickness?

It decreases spatial resolution in the z axis

# T/F Slice thickness has a linear effect on the number of x-ray photons available to produce the image

True ## Footnote 5mm slice will have twice the number of photons as a 2.5mm slice.

Although slice thickness can increase SNR, what factor can get in the way and decrease the visiblity of smaller objects?

Volume averaging can reduce the visibility of smaller objects.

What produces better low contrast resolution; smoothening filters or sharpening filters?

Smoothening filters improve low contrast resolution | (at the cost of SR)

# T/F As noise increases, low contrast resolution decreases

True

As Pixel Size decreases, what happens to low contrast resolution? Why?

-Contrast resolution decreases -As pixel size decreases, the number of detected photons per pixel will decrease.Fewer photons per pixel results in a subsequent decrease in contrast resolution.

As patient size increases, what happens to low contrast resolution?

It decreases ## Footnote -For the same technique, larger patients attenuate more x-ray photons, leaving fewer to reach the detectors. -This reduces SNR, increases noise, and results in lower contrast resolution.

How do inherent tissue properties affect low contrast resolution?

The difference in the linear attenuation coefficient of adjacent imaged objects will determine the contrast between those objects

# What is this describing? The scanner’s ability to resolve closely placed objects that are significantly different from their background.

Spatial resolution of a CT scanner

In what two dimensions can the spatial resolution of a CT image be described?

1. in-plane (transaxial) resolution. 2. cross-plane (longitudinal) resolution or z axis sensitivity

What is cross-plane (longitudinal) resolution or z axis sensitivity?

Resolution in the z direction

What is in-plane (transaxial) resolution?

Resolution in the x-y direction

What are the 7 factors that affect the In-Plane Resolution?

1. Focal Spot Size 2. Detector Size 3. Scanner Geometry 4. Field of View 5. Sampling Theorem 6. Reconstruction Algorithm 7. Patient Motion

# T/F Smaller focal spots give higher resolution

True

What are the sizes of the focal spots in CT?

Fine = 0.7 mm Broad = 1.2 mm

What is the relationship between Detector Size (Width) and detail?

Detector Size (Width) Inversely related to detail ## Footnote Increased detail requires decreased size or space between detector elements.

# T/F Detector size or spacing is dependent on the model of CT scanner

True

Increased detail requires what; an increased tube arc or a decreased tube arc?

increased tube arc

# T/F By taking two matching (mirror) samples taken 180° apart, the image is ussually improved.

True

# T/F Partial 180° scans are inferior to standard 360° scans. Why or why not?

True; Only half of the otherwise available data are available to reconstruct the image with partial scans

What is one benefit of partial 180 degree scans?

Decreased dose

What is an overscan?

400° tube arc scan

What makes up the 400° scan?

360° (full scan) + 40° (overscan) = 400° scan.

# T/F Changing the DFOV will NOT alter the size of the image on the screen

False; Changing the DFOV will alter the size of the image on the screen

How does the image matrix divide data with DFOV?

The matrix divides data into squares with an x and y dimension.

By increasing by increasing the DFOV, what happens to the size of each pixel in the image?

Increasing the DFOV, increases the size of each pixel in the image.

# T/F The larger pixel will include more patient data

True

# T/F If an object is larger than a pixel, its density will be averaged with the density of other tissues contained in the pixel, creating a more accurate representation of the object.

If an object is smaller than a pixel, its density will be averaged with the density of other tissues contained in the pixel, creating a less accurate representation of the object.

# T/F Having smaller pixels decreases the volume averaging, thus increasing spatial resolution

True

# T/F The Technologist directly controls DFOV but cannot change pixel size

True

What does the Nyquist Sampling Theorem state?

The pixel dimension should be half the size of the object to increase the likelihood of that object being resolved

# T/F Reconstruction algorithm is indirectly related to spatial resolution.

False; Reconstruction algorithm is directly related to spatial resolution.

# T/F As the spatial frequency of a filter increases the detail increases.

True

What does Z-sensitivity refer to?

Z-sensitivity refers to the effective imaged slice width.

# T/F The wider (in the z-axis) the detector row, the lower the resolution.

True

# T/F The greater the z axis, the more flattening (volume averaging) is necessary.

True

What is the relationship between slice thickness and spatial resolution?

Inversely related to spatial resolution.

# T/F The thicker the slice, the less volume averaging needed

False; The thicker the slice, the more volume averaging

What reduces cross-plane resolution?

Slice Thickness

What is the smallest slice thickness that scanners allow today?

0.5mm

# T/F Two 0.5mm slices added together to display a single 1.0mm slice will retain the spatial resolution of the original 0.5mm slices

True

What is an Isotropic voxel?

A voxel where the x, y & z axis dimensions are equal.

What is the benefit of Isotropic voxels?

There is no loss of information when data are reformatted in a different plane (i.e. coronal or sagittal images).

What anatomy benefits the most from isotropic voxels?

small vascular tortuous structures

What is the disadvantage of thinner slices?

Increased noise

What is Slice Sensitivity Profile (SSP)?

SSP describes the effective slice thickness of an image and to what extent anatomy within that slice contributes to the image.

What pitch results in a broader SSP?

Pitch 2 results in a broader SSP

# T/F Z axis spatial resolution is traditionally described by the slice sensitivity profile (SSP).

True

# T/F Increasing pitch increases the effective slice thickness.

True

# T/F ↓ volume averaging = ↓ spatial resolution

False; ↑ volume averaging = ↓ spatial resolution ## Footnote Increasing the pitch uses data farther from the actual slice position, thus degrading the slice sensitivity profile

# T/F ↓ Pitch = ↑ spatial resolution

True

As pitch decreases, what happens to noise?

Noise decreases

If all other parameters are constant, what is the realtionship between dose and pitch?

Dose is inversely proportional to the pitch (if all other parameters are constant).

As pitch increases, what happens to noise?

Noise increases

What is the scanners goal when imaging a patient?

Scanner’s goal is to maintain image quality

When you increase the pitch, the mA is increased. Why is this?

To keep the effective mAs constant ## Footnote Thus, the dose and image noise remain constant.

# T/F **Increasing pitch increases dose**

False; Increasing pitch does NOT increase dose | DEPENDS IF ITS THE ONLY FACTOR CHANGED

# T/F Dose is affected by the AEC system selection

True

What is temporal resolution?

The temporal resolution of a system refers to how rapidly data is accquired and the ability to freeze motion of the scanned object

**What is the best way to elimate motion impact?**

Increase the scan speed

What is the fastest speed that third generation MDCT scanners are capable of rotating?

Less than 0.3 seconds per gantry rotation

What is the half scan algorithm?

The projection dataset in the view range of 180 degrees plus fan angle (total 220 degrees) are used

How is cardiac motion reduced in a CT scan?

ECG gated acquisition; synchronizing the data acquisition and reconstruction with an ECG signal