Unit 5 Exam - Postprocessing & Technical Factors

Postprocessing and Digital Technique Summary (exam 03/04)

Postprocessing

Improves image appearance further based upon need/interest  

What are the three domains postprocessing is performed in?

Spatial, intensity, and frequency

Spatial Domain

Manipulates pixel value based on their location within image matrix; includes changes in orientation, magnification, and digit subtraction

Kernels only work in what domain?

Spatial

Global Processing

Orienting/mirroring (affects every pixel)  

Area (local) Processing

Zoom/magnification (affects only pixel values within a particular subsection)

Point Processing

Subtraction (changes one pixel value at a time based on its location)

Detail Processing

Kernels correct for del drop out and noise (pass vertically/horizontally over vales in the matrix and average based on location)

Intensity Domain

Manipulates pixel values based upon their shade of gray; includes all gradient processing

Gradation (gradient) Processing

Adjusts brightness and contrast gradually

Contrast changes what on the gradient curve?

Slope (body)

Brightness changes what on the gradient curve?

Shoulder and toe

Data Clipping

Bit depth limits the amount of gradation

What happens if you manipulate images before sending images to PACS?

Loss of information from original image and reduces amount of manipulation a radiologist can make

Equalization (dynamic range compression)  

Removal of “extreme shades” from the images gray scale to better see gray shades  

Advantages of equalization

Saves repeats and reduces file size

Frequency Domain

Manipulates pixel values based on how small/large of a structure they represent; sorts by size of an object or how many pixels the object takes up  

Large objects

Lower frequency, longer wavelength

Smaller objects

Higher frequency, shorter wavelength

Frequency Filtering

Suppressing pixel values based upon their frequency

High pass filtering

Suppresses low frequencies (pixels appearing the most/larger areas); we can see smaller objects better (pixel values that occur in fewest amounts)

Low pass filtering  

Suppresses high frequencies (pixels not appearing often/smaller areas); we can see larger objects better (pixel values that appear the most)

When is low pass frequency beneficial?

With noise

Another name for high pass filtering

Edge enhancement

Another name for low pass filtering

Smoothing

What does high pass filtering do to contrast?

Increase

What does low pass filtering do to contrast?

Decrease

Black Surround Mask

Applies solid black frame or border around every image to eliminate veil glare for the radiologist

What are other names for black surround mask?

Shuttering or cropping

Orientation/Mirroring

Flipping image side-to-side or up-and-down 

Annotations

Electronic information added after image acquisition

Magnification/Zoom

Used for visualization only, not more manipulating image quality

Stitching

Ability to seamlessly join multiple images (ex. scoliosis, scanogram leg length)

Reversal/Black Bone Filter

Image reversal; all pixel values are reversed

Virtual Grids

Filters out scatter signal; eliminates appearance of scatter by increasing contrast

What does virtual grids allow you to do with technique?

Lower mAs (lower patient dose)

What is EI derived from?

Median pixel value in the VOI

EI

A numerical value for the amount of receptor exposure; quantity of photons striking the IR

AAPM

American Association of Physicists in Medicine

American Association of Physicists in Medicine

Created to identify a standard method of calculating and displaying adequacy of exposure reaching a detector; created in 2009 AAPM task group 116

K_IND – indicated equivalent air kerma

The radiation dose measurement for the incoming photon beam at the detector surface 

K_TGT – target equivalent air kerma

The radiation dose measurement for a proper exposure to the detector

Deviation Index (DI)   

Showing how much over or under exposure occurred; the difference between K_IND and K_TGT 

What is a perfect DI?

0

What DI can be an ALARA issue?

+3

Technologist Goal

Optimal signal with lowest patient dose

Two most common factors that affect digital image quality  

Positioning errors and image noise

Dose Creep

Increased exposure technique to decrease chance of image noise and repeats

Photon Starvation

Inadequate exposure to the DELs, results in noise

Data Drop

Data elements in detector are overwhelmed with photon energy and become incapable of recognizing high-energy values. Data drop detracts from optimal image quality

How much overexposure can digital imaging systems accomodate for?

400-500% (4-5x)

How much underexposure can digital imaging systems accomodate for?

25%

What does doubling mAs do to absorbed dose?

2x

What does 15% increase in kVp do to ESD?

33%

How do we measure patient dose?

Air kerma

What is air kerma measured in?

Gy or mGy/min

DAP Calculators

Equipment mounted to the tube (between collimation and patient) that measures the dose to the patient and amount of exposed anatomy

How do you find DAP?

Air Kerma x field size

What is DAP measured in?

mGy- cm²

Optimal technique for patient

High kVp and low mAs

For a manual technique (on a variable kVp system), adjust the kVp by ______ for every ______ cm difference in patient measurement

2, 1

What do you do to density with a low EI on AEC?

Increase

What do you do to density with a high EI on AEC?

Decrease

Doing what to mAs will halve or double the exposure strength to the IR?

Halving or doubling

Doing what to kVp is the same as halving or doubling the exposure strength to the IR?

Increase or decrease by 15%

When is a grid used?

Part > 10 cm and kVp > 60

Exposure matienence formula

mAs₂ = mAs₁ x SID₂²/SID₁²

Grid conversion formula

mAs₂ = mAs₁ x GCF₂/GCF₁