Image Quality

What factors can the operator control?

Milliamperage (mA)

Scan time

Kilovoltage peak (kVp)

Slice thickness

Field of view

Reconstruction algorithm

Pitch (helical scanning)

What factors affect the total beam exposure in CT

mA setting

Scan time

kVp

For the factors that affect the total beam exposure in CT, which factor is held constant

kVp’s are generally held constant

For the factors that affect the total beam exposure in CT, which factor is varied

mAs is varied to control patient dose and image quality

What is mA

The current of electrons flowing from the cathode to the anode

How does mA work?

Electrons are boiled off when the filament is heated

X-ray photons are produced when electrons strike the anode

Increasing mA will increase the number of photons

What is scan time and how does it work

The time the x-ray beam is on for the collection of data for each slice

Most often, it is the time required for the gantry to make a 360-degree rotation

Shorter scans help to reduce motion on the image

What is the relationship between scan time and mAs

mAs = mA x scan time

Using a higher mA setting allows for a shorter scan time while maintaining the same mAs

Shorter scan time helps reduce patient motion

mAs is limited by heat

Recommended mAs settings are programmed into each exam protocol

What happens to SNR when mAs is doubled

SNR increases by approximately 40%

How do 5 mm and 2.5 mm slices compare

A 5 mm slice contains twice as many photons as a 2.5 mm slice, improving contrast resolution

What is the formula for pixel size

Pixel size = DFOV / matrix size

What is kVp and how does it work

AKA tube voltage

Controls the quality and energy of the beam

What standard kVp values are used in CT

Adults: 120-140 kVp

Pediatrics: Usually 80 kVp

What is the uncoupling effect and how does it affect dose

When using digital technology, the image quality is not directly linked to the dose, so even when mA or kVp setting that is too high is used, a good image still results

This effect can make it difficult to identify when a dose that is higher than necessary is used

Dose Creep: Gradual increase in techniques over time because high dose images still look acceptable

What is Automatic Tube Current Modulation (ATCM) and how does it work

An equipment (software) option that will make changes in tube current (mA) based on the estimated attenuation of the patient at a specific location

These estimations are derived from scout views done in both the AP and lateral projections or from the previous slices. From these views, the mA will be programmed to vary by location along the length of the patient

The exact details of the option vary by manufacturer

What is slice thickness and what is its impact on image quality?

The slice thickness is how the data was acquired

Slice thickness has a significant impact on image quality

What is the difference between the slice thickness and image thickness

Slice thickness is how the data was acquired

Image thickness is how the data is reconstructed (processed)

Looks at raw data versus image data

What is Scan Field of View

Determines the area, within the gantry for which all raw data are acquired

Acquired around the gantrys isocenter

What is the Display Field of View

Determines how much and what section, of the collected raw data are used to create an image

What are Reconstruction Algorithms

Depending on the manufacturer, is also called algorithm, convolution filter, FC filter or filter

Applied only to raw data

Changing the reconstruction algorithm will change the way the raw data are manipulated to reconstruct the image

Windowing does not change the algorithm; it will only change the way the image is viewed

What is pitch

The relation of table speed to slice thickness

The travel distance of the CT scan table per 360 rotation of the x-ray tube, divided by the x-ray beam collimation width

Know about tube arc and what is the most common tube arc

360 degrees is the most common tube arc

This is data collected from 1 360-degree rotation of the x-ray tube (2 samples 180 degrees apart)

400 degrees is used for overscanning

This was used in 4th generation scanners to reduce motion artifact by collecting overlapping data

Define Image Quality

A comparison of the image to the actual object

In CT, image quality is directly related to its usefulness in providing an accurate diagnosis

What 2 features are used to assess how well the image represents the real anatomy

Spatial Resolution (detail) - the ability to resolve (as separate objects) small, high-contrast objects

Contrast Resolution - the ability to differentiate between objects with very similar densities as their background (low contrast detectability)

What is spatial resolution?

The ability to resolve (as separate objects) small, high-contrast objects

Can also be called detail

What is contrast resolution

The ability to differentiate between objects with very similar densities as their background (low contrast detectability)

What are the 2 ways to measure spatial resolution

Direct:

• by using a line pairs phantom

• Counts the number of strips

• Spatial Frequency- the number of line pairs visible per unit length

• Small objects=high spatial frequency

• Large objects=low spatial frequency

Modulation Transfer Function:

• The ratio of the accuracy of the image compared with the actual object being scanned

• Most commonly used method in CT and DR

• Indicated image fidelity

• Scale from 0-1

  • 0= blank image, no info

  • 1= object reproduced exactly

• As the object size increases, MTF increases

What factors affect spatial resolution

Pixel size (matrix size, DFOV)

Slice thickness

Reconstruction algorithm

Focal spot size

Pitch (because of slice thickness)

Patient motion

How does a smaller pixel size affect spatial resolution

Increased spatial resolution

How does a bigger pixel affect spatial resolution

Decreased Spatial Resolution

How does using a thinner slice affect spatial resolution

Increased Spatial Resolution

How does using a wider slice thickness affect spatial resolution

Decreased Spatial Resolution

How does using the appropriate detail enhancing algorithm affect spatial resolution

Increases Spatial Resolution

How does the use of a larger focal spot affect spatial resolution

Decreases Spatial Resolution

How does the use of a smaller focal spot affect spatial resolution

Increases Spatial Resolution

How does increased pitch affect spatial resolution

Decreases Spatial Resolution

How does decreased pitch affect spatial resolution

Increases Spatial Resolution

How does patient motion affect spatial resolution

Decreases Spatial Resolution

What is in-plane resolution

Resolution in the x and y direction

What is longitudinal resolution

Resolution in the Z direction

What is an isotropic voxel and how does it affect information when data are reformatted in a different plane

A cube with equal dimensions in the X, Y, and Z directions

When the imaging voxel is equal in size in all dimensions, there is no loss of information when data are reformatted in a different plane

What is the Nyquist Sampling Theorem and how is it related to pixel dimension

Theorem that states because an object may not lie entirely within a pixel, the pixel dimension should be half the size of the object to increase the likelihood of that object being resolved

What is Iterative Reconstruction

A different way to apply an algorithm

Begins with an assumption and then improve the image by continually analyzing scan data and making adjustments until the assumption and real time data are in agreement

Creates less noise and better spatial resolution then filtered back projection

What are deep learning reconstruction methods

Deep learning reconstruction produces significantly lower noise while preserving low-contrast detectability and spatial resolution

Know about low-contrast detectability

The ability to distinguish an object that is nearly the same density as its background

High contrast differences are easier to see

Low contrast differences are more difficult to see

What can contrast resolution also be called

Sensitivity of the system

What is % linear attenuation coefficient and the % difference in CT related to contrast change

The contrast difference between objects

1% contrast difference = 10 HU

CT can differentiate an object with a 0.5% contrast variation

CT has better contrast resolution than conventional x-ray, but MRI is considered superior for contrast resolution

Know about the phantom that is used to measure small differences in densities

Cylindrical phantom

Contains objects with small differences in density

Used to evaluate the system’s ability to distinguish low-contrast objects

What is noise in CT

The undesirable fluctuation of pixel values in an image of a homogeneous material

Appears grainy or like salt and pepper

What is the most common cause of noise

Quantum Mottle (insufficient number of photons)

Fewer photons=more noise

What is SNR

Signal to noise ratio

Higher SNR is better (5:1 is better than 4:1)

What does standard deviation indicate

The amount of variance among pixel values in a designated ROI

The SD of a phantom indicates the degree of noise

Lower SD=less noise and better contrast resolution

What 5 factors affect contrast resolution

mAs/Dose

Pixel Size

Slice Thickness

Reconstruction Algorithm

Patient Size

How does increasing mAs affect contrast resolution

Increases SNR and improves contrast resolution, but increases patient dose

How does pixel size affect contrast resolution

Smaller pixels= more noise and worse contrast resolution

Larger pixels= less noise and better contrast resolution

How does slice thickness affect contrast resolution

Thicker slices = more photons and better contrast resolution, but worse Z-axis spatial resolution

How do reconstruction algorithms affect contrast resolution

Bone (Edge, high pass) = better spatial resolution, worse contrast

Soft tissue (Smooth, low pass) = better contrast and worse spatial resolution

How does patient size affect contrast resolution

Larger patient = lower SNR, more noise, and worse contrast resolution (same technique)

What exam can use a low dose despite reduced contrast resolution

Low-dose CT lung cancer screening because nodules and masses can still be seen

What is subject contrast

The inherent properties of the object scanned (Tech cannot control this)

How does windowing help contrast resolution

For low contrast objects to be visible, the window level must be set near the average density of the object, and the window width must be narrow enough so that only small groups of HU are represented in each shade of gray displayed

What is temporal resolution?

How rapidly data are acquired

What 3 factors control temporal resolution

Gantry rotation speed

The number of detector channels in the system

The speed with which the system can record changing signals

What is temporal resolution measured in

Milliseconds (ms)

When is temporal resolution important

Heart

Angiography

Perfusion studies (contrast studies)

How do manufacturers report spatial resolution

MTF

How do manufacturers report contrast resolution

% linear attenuation coefficient

Sensitivity of the system

SNR (signal to noise ratio)

Standard deviation

What is the main difference between spatial and contrast resolution

Spatial: Detail (small, high contrast objects)

Contrast: Density differences (similar-density objects)