Advanced Image Acquisition- unit 1- chapter 31 post processing

what are three general approaches to processing a digital image?

spatial operations, intensity operations, and frequency operations

How are pixels sorted in spatial operations

by their location

What are the three types of spatial processing?

point processing, area processing, and global processing

What else is associated with spatial operations?

kernels

How are pixels sorted in intensity operations?

By their pixel values, or gray levels.

What processes are included in intensity operations?

Histogram construction and analysis, gradation processing using Look-Up Tables (LUTs), and windowing.

How are objects or details sorted in frequency operations?

by their size

What are examples of frequency operations?

Edge enhancement and smoothing.

What is an example of point processing?

digital subtraction

What happens during digital subtraction?

A pixel value is subtracted from the value of the same pixel from another image.

What are examples of area processing?

Electronic masking/cropping, zoom, and magnification.

How does area processing create a zoom effect?

The values of pixels in the area of interest are reassigned to a group of pixels, producing a zoom of that area.

What are examples of global processing?

Rotating, flipping, and inverting an image.

What domain does all gradation processing take place in?

the intensity domain

What happens to input pixel values during pre-processing?

They are rescaled or "re-mapped" to consistent output values.

What is the purpose of rescaling pixel values?

To align brightness so it is consistent across images.

How often does automatic rescaling occur in digital x-ray imaging?

It occurs in every digital x-ray image.

Besides brightness, what can rescaling partially correct?

gray scale/contrast

What is gradation?

A gradual passing from one tint or shade to another; it relates to gray scale.

what is gradient?

The rate of change in an ascending or descending scale, or the steepness of a slope.

What is gradient or gradation processing named after?

The gradient curve of an image.

What does the X-axis of a gradient curve represent?

The amount of exposure received at a point on the image receptor.

What does the Y-axis of a gradient curve represent?

The pixel value assigned to the corresponding amount of exposure.

What does the left-to-right position of a gradient curve indicate?

image brightness

What does the steepness or slope of a gradient curve represent?

image contrast

Why do computers use Look-Up Tables (LUTs) instead of gradient graphs?

Computers use numbers rather than graphs, so LUTs are used to process the data.

What is the gray center (GC)?

the average gray level

If two LUTs have the same gray center but different contrast, what has remained unchanged?

window level

If the gray center remains the same but contrast changes, what has changed?

window width

what is window level?

the midpoint of displayed brightness

what is window width?

The number of displayed shades of gray, which controls contrast.

What is a gradation curve graph?

A visual representation of a Look-Up Table.

Why is a gradation curve graph useful?

It makes the Look-Up Table easier for humans to visualize.

What does the curve on a gradation graph mathematically represent?

The mathematical function, or algebraic formula, of the computer algorithm being applied to the input pixel values.

What determines the exact shape of a gradation curve?

The manufacturer's algorithm.

What are examples of intensity formulas that can control window width?

Gamma transformation, log transformation, and image negative.

In the intensity formulas, what does "r" represent?

The input value.

In the intensity formulas, what does "s" represent?

the output value

What does a function curve with a slope steeper than 45° indicate?

A slope greater than 1, representing an increase in contrast.

What type of contrast scale is produced by a slope greater than 1?

Short-scale, high contrast.

What does a function curve with a slope shallower than 45° indicate?

A slope less than 1, representing a decrease in contrast.

What type of contrast scale is produced by a slope less than 1?

Long-scale, low contrast.

What does right-to-left movement of the gradient curve represent?

Changes in image brightness.

What windowing control corresponds with right-to-left movement of the curve?

window level

When is gradient processing used during default processing?

During default pre-processing when anatomical LUTs are applied.

When else is gradient processing used?

Any time the displayed image is windowed by the operator.

In frequency operations, what are the two ways an image can be viewed?

As a collection of pixels located in space and as a collection of waves with different frequencies.

How can the gray levels of a row of pixels be graphed?

Like an exposure trace diagram.

How can a row of pixels be represented when visualized in terms of exposure?

as a wave

What becomes the amplitude, or height, of the waves?

The gray level or brightness of each pixel.

What determines the length of each wave?

The size of the object, or the number of pixels occupied by the object.

What does the length of a wave determine?

its frequency

Why are the corners of an exposure trace rounded instead of perfectly sharp?

because of penumbral blur

How are large objects represented in terms of wavelength and frequency?

Large objects → long waves → low frequency.

How are small details represented in terms of wavelength and frequency?

Small details → short waves → high frequency.

What happens when small wavelengths are added to large wavelengths?

A complex waveform is created.

What happens when the peaks of different waves coincide?

An amplified spike is created in the larger wave.

What can a complex waveform represent in an image?

Different-sized objects occurring together, such as two small objects with one overlapping a large object.

What is Fourier analysis used for?

To break a complex waveform into its component wavelengths.

What does breaking a complex waveform into component wavelengths allow us to do?

Effectively separate large structures from small structures.

What does frequency processing allow us to do with different image layers?

Perform different operations on individual frequency layers.

How can frequency processing enhance structures of a particular size?

By choosing the frequency corresponding to that structure size and boosting it.

What can be done to a selected frequency layer to enhance it?

The amplitude of all signals in that layer can be boosted before the layer is placed back into the image stack.

What happens when the amplitude of a selected frequency layer is boosted?

Details of that particular size become more visible relative to the other layers.

What is multiscale processing?

Processing that decomposes an image into 8 or more separate frequency bands.

What can happen to each frequency band during multiscale processing?

Each can undergo different filtering methods and parameters before all the bands are added back together to create the final image.

How many frequency bands can multiscale processing divide an image into?

Eight or more separate frequency bands.

Who develops the algorithms used for multiscale processing?

Equipment manufacturers.

What is Philips' multiscale processing system called?

UNIQUE

What does Philips UNIQUE stand for?

Unified Image Quality Enhancement.

What is Agfa's multiscale processing system called?

MUSICA

What does Agfa MUSICA stand for?

Multi-Scale Image Contrast Amplification.

What is Fuji's multiscale processing system called?

MFP

What does Fuji MFP stand for?

Multi-Objective Frequency Processing.

What is band-pass filtering?

Filtering in which any selected frequency layer can be left out when the image is reconstructed.

What does high-pass filtering filter out?

Large structures, which are the lower-frequency layers.

What is high-pass filtering often called at the console?

edge enhancement

What does high-pass filtering or edge enhancement bring out?

smaller details

How does high-pass filtering make smaller details more visible?

By suppressing larger background areas.

What is the trade-off of high-pass filtering/edge enhancement?

The image becomes slightly noisier.

What does low-pass filtering filter out?

Small structures, which are the higher-frequency layers.

What is low-pass filtering often called at the console?

smoothing

What is the benefit of low-pass filtering or smoothing?

it cleans up noise

What is the trade-off of low-pass filtering/smoothing?

The image becomes slightly blurrier and fine edges may be lost.

What four things can be done to an individual layer during multiscale processing?

It can be boosted, suppressed, removed, or subjected to treatments such as gradient processing or noise reduction.

Can gradient processing be applied to an individual frequency layer?

Yes

Can noise reduction be applied to an individual frequency layer?

yes

Why is frequency processing a major advantage of digital imaging?

Individual image layers can be manipulated in ways that were not possible with analog/film imaging.

How does digital frequency processing compare with analog/film imaging?

Frequency processing is a major advantage of digital imaging over analog/film imaging because individual frequency layers can be selectively processed.

What is an example of a special post-processing feature?

Dual Energy Subtraction.

What is Dual Energy Subtraction capable of doing?

Separating a digital image into a tissue-only image and a bone-only image.

How can Dual Energy Subtraction aid diagnosis?

It de-superimposes obstructing anatomy.

What two initial images are needed to create separate soft-tissue and bone images?

A high-energy image and a low-energy image.

How do the high-energy and low-energy images differ?

They have very different contrast characteristics.

What are the two methods for obtaining the high- and low-energy images used in Dual Energy Subtraction?

A double exposure using two different kVp settings or a single exposure using a special receptor system with a filter between imaging plates.

How is the double-exposure method of Dual Energy Subtraction performed?

Two exposures are made using two different kVp settings.

Approximately how much time is required between exposures when using the double-exposure method?

About 200 milliseconds, or 1/5 of a second.

Why is approximately 200 milliseconds needed between the two exposures?

The system needs time to switch the kVp.

What disadvantage can occur because of the delay between exposures?

Patient motion may occur.

How does the single-exposure method of Dual Energy Subtraction work?

A single exposure is made using a special receptor system containing a filter placed between two or more imaging plates.