Week 9 - Image Processing And Reconstruction Flashcards
How is a CT image reconstructed?
Using a set of xray beam projections at different angles
The data that is sitting in the computer waiting to be made into an image
Raw data
The thousands of bits of data acquired by the system with each scan
Raw data
T/F
The terms raw data and scan data are interchangeable
True
The process of using raw data to create an image
Image reconstruction
What are the two types of image reconstruction?
Prospective
Retrospective
The reconstruction that is automatically produced during scanning
Prospective reconstruction
The process of the same raw data being used later to generate new images
Retrospective reconstruction
-to form an image the computer assigned a HU to each pixel
-the average of all attenuation measurements for that pixel
-the 2D pixel represents the 3D portion of the patients tissue
Image data
-A finite set of unambiguous steps performed in a prescribed sequence to solve a problem
-a set of rules or directions for getting a specific output from a specific input
Algorithms
T/F
In CT, algorithms are used to solve many mathematical equations necessary for conversion of acquired information into visible light
True
T/F
Algorithms are the basis for most computer programming
True
A mathematical method of estimating the value of a unknown value on either side of the function
Interpolation
Assumes that the unknown points fall along a straight line between two known points
Linear interpolation
*simplest type
When is interpolation used in CT?
In image reconstruction process and the determination of slices in spiral/helical CT imaging
A systematic method of collecting data from the patient
Data acquisition
What are the components used for data acquisition?
Xray tube and digital image detectors
What must an analog signal be converted into a digital signal for data acquisition?
Because a digital computer is used in imaging systems
What is digital imaging processsing?
Conversion of an input image to an output image
Performed by a digital computer that takes an input digital image and processes it to produce an output digital image by using the binary number system
Image processing
Input and output images are analog
Analog processing
Discrete input and output image
Digital processing
What is required for the conversion of analog to digital image?
Digitizations system
Continuous images
Analog
Numerical representation or images of objects
Digital image
Performed by a digital computer that takes an input digital image and processes it to produce an output digital image by using binary number system
Image processing
ADC
Analog spiral converter
DAC
Digital analog converter
Sends digital data for digital image processing by a digital computer
-reduces noise in the input image
-enhances the sharpness of the input image
-changes the contrast of the input image
ADC
_______ must be first converted into an analog signal before it can be displayed on a monitor for viewing by the observer
Output digital image
*function of DAC
*this image can be stored and archived
T/F
CT is based on reconstruction processes whereby a digital image is changed to a visible image
True
What domains do images exist in?
-spatial location domain
-frequency domain
What domain are images displayed for viewing in?
Spatial location domain
What domain are xray and CT images displayed in?
Spatial location domain
Domain demonstrated by the number of times a signal changes per unit length
Frequency domain
What domain are MRI and ultrasound images displayed in?
Frequency domain
In a frequency domain, what do small and large structures produce?
Small structures - high frequency (image detail)
Large structures - low frequency (contrast information)
T/F
Images can be converted between domains using the Fourier transform
True
What domain do radiologists and technologist view images in?
Spatial frequency domain
What domain would a physicist likely view an image in?
Frequency domain
What is the major reason for converting images domains?
To facilitate image processing that can enhance or suppress certain features of the image
T/F
An image with many edges contains many high frequency components
True
T/F
Images of more uniform structures (water) contain low frequency components
True
T/F
Frequency components can be selectively changed to enhance an image
True
Increases amplitude of low frequency components (smoothing)
Low frequency
Low pass
Increases amplitude of high frequency components (edge enhancement)
High frequency
High pass
What are considered characteristics of the digital image?
-matrix
-pixels
-voxels
-bit depth
-2D array of numbers
-M x N (columns x rows)
-matrix size = DFOV (in CT)
*larger image requires more processing time and storage
Matrix
-make up the matrix squares
-contains a numerical value representing brightness
-these numbers best represent the tissue imaged
-represent the characteristic atomic number and mass density
Pixels
T/F
Larger matrix size and small pixel (same FOV) allows for better spatial resolution
True
-pixels in a digital image represents the information contained in a volume of tissue in the patient
-volume element
-are assigned brightness levels
Voxel
Y/F
Every pixels in the M x N matrix is represented by k binary digits
True
T/F
CT is done with more bit depth than general xray
True
What does matrix size affect?
Spatial resolution (detail)
*larger matrix = smaller pixel = better detail
What does bit depth affect?
of shades of grey (contrast resolution)
Conversion of analog image into numerical data for processing by the computer
Image digitization
What are the 3 steps of image digitization?
Scanning
Sampling
Quantization
Division of picture into small regions, or scanning (pixels)
Image digitization - scanning
-measures the brightness of each pixels in the image
-light projected onto transparency of pixel and detected by a photomultiplier behind picture
*output of the photomultiplier tube is an electrical signal (analog)
Image digitization - sampling
*electrical signal obtained from sampling is assigned an integer based on the strength of the signal
-brightness of each pixel is assigned an interference (0, +, -) called a grey level
-results in range of numbers or grey levels with a precise location
-greyscale: total number of grey levels
Image digitization - quantization
*results in an array of numbers representing the analog image that was scanned, sampled and quantized
*then sent to the computer for further processing
T/F
Undersampling causes degradation of the image
True
*leads to aliasing (artifact, moire)
-samples the analog signal at various times to measure its strength at different points
-the more points samples, the better representation of the signal
ADC
What are the two characteristics of ADC?
Accuracy - division of the signal into bits also enhances accuracy (bits represent shades of grey on the image)
Speed - time it takes to digitize the analog signal
T/F
In ADC, speed and accuracy are inversely related
True
*it takes more time to digitize an signal with higher accuracy
What is segmentation?
Involves isolating and highlighting objects of interest, followed by categorizing the objects in the image
Creating images from other images or non-image data
*image reconstruction techniques are the basis for CT image production
Ex. 3D imaging
Image synthesis
What are the 3 image processing techniques?
-point operations
-local operations
-global operations
*the image processing algorithms alter the pixel intensity values
-value of input image is mapped on the corresponding output image pixel
-algorithms allow all pixels to be scanned
-grey level mapping : also called windowing (most common point processing technique)
-uses LUT, plots output and input grey levels against eachother
Point operations
*grey level mapping modifies histogram of part or all pixels
-output image pixel determined from a small area of pixels around the corresponding input pixel
-does not scan each pixel
-uses surrounding pixels
-can be called area/group processing
Local operations
-also called edge enhancement
-spatial location is converted to spatial frequency
-using a high pass filter on spatial frequency
-high pass filter suppresses low spatial frequencies, producing a sharper output image
Spatial location filtering: high pass filtering
-smoothing the image
-output image appears blurred
-reduces noise and displayed brightness levels
-image detail is compromised
Spatial location filtering: low pass filtering
-entire input image is used to complete the value of the pixel in the output image
-uses filtering in the frequency domain rather than space domain
-uses Fourier transform
Global operations
What can the Fourier transform provide?
-edge enhancement
-image sharpening
-image restoration
-to modify spatial position or orientation of pixels in the image
-algorithms change position rather than intensity of the pixels
Geometric operations
What reconstruction algorithm does CT use?
Filtered back projection
What is the major problem with the filtered back projection algorithm?
Includes noise and streak artifacts
