DMI65: CT Chapter 3 Image Reconstruction

What is a con of using a Smooth Filter to "smooth" the data?

"Smoothing" the data reduces the difference between adjacent pixels (CONTRAST RESOLUTION); this reduces artifacts, however, also REDUCES SPATIAL RESOLUTION minimizes grainy appearance, no edge enhancement (soft tissue contrast), CONTRAST INCREASES/NOISE DECREASES

Describe the image reconstruction process

-DAS reads each ray and measures how much of beam is attenuated (ray sum) -Ray Sums make up a view (views are needed to create an image) -An attenuation profile is created for EACH VIEW (raw data is mapped into the attenuation profile) -Info from all attenuation profiles is projected onto a matrix (back projection)

What are some components of Computer Hardware?

-Hard Disk -Input Devices -Output Devices -CPU

What are the 3 principal types of memory?

1) Read-only memory (ROM)--only macroprocessor of CPU has access 2) Random-access memory (RAM)--read & write, easily modified, TEMPORARY storage, once you turn off, you will lose info 3) Write-Once read many times memory (WORM)--like CD-R's

What is a con to using too large of a DFOV?

A DFOV that is too large makes anatomic structures unnecessarily small

What is a con to using too small of a DFOV?

A DFOV that is too small may exclude important patient anatomy

What are the other terms used for filter functions?

Algorithms Convolution Filters Kernels

What are algorithms?

Algorithms are a finite set of steps performed in a prescribed sequence to solve a problem (software)

What is Back Projection?

Back Projection is the process of converting data from the attenuation profile to a matrix

Which computer hardware contains the microprocessor, the control unit, and the primary memory?

Central Processing Unit

What is the difference between Reconstruction Algorithm and Window Setting?

Changing the algorithm changes the way the RAW DATA are manipulated to RECONSTRUCT the image Window Setting merely changes the way the image is viewed

What are the 2 major components of Image Reconstruction?

Computer Hardware and Software

What is the term used to describe the application of a filter function to an attenuation profile?

Convolution

What is DFOV?

Display Field of view DFOV selection determines how much of the collected raw data are used to create an image Changing the DFOV affects image quality by changing the pixel size (quality decreases as image increases) DFOV works in a manner similar to the zoom on a camera

What is a con of using a Sharp Filter to enhance contrast?

Enhancing contrast by accentuating the difference between neighboring pixels IMPROVES SPATIAL RESOLUTION but at a cost of LOW-CONTRAST RESOLUTION for edge enhancement (skeletal detail), DETAIL INCREASES/NOISE INCREASES

True or False: Filter Functions can be applied to both raw data and image data

False! Filter functions can only be applied to raw data

True or False and Why: DFOV can be larger than the SFOV

False, Because the data selected for the DFOV are a subset of all the scan data available, the DFOV cannot be larger than the SFOV

True or False: Image Data storage requires much more space than Raw Data

False: Raw data storage requires more space than Image data

How is filtering done?

Filtering is done by a series of complicated mathematic steps, often referred to as a reconstruction algorithm

What is the purpose of the Filter in CT?

Filtering is used to minimize streaks on the image that result from back projection

What is the name of the process which uses raw data to create an image?

Image Reconstruction, there are 2 types: Prospective and Retrospective Reconstruction

What is Image Data?

Image data is data that the computer has processed into pixels; if only image data are available, data manipulation is very limited

About how much computer space does Image data require compared to raw data?

Image data requires about 1/5 of computer space as raw data does

What is Software?

Instructions that tell the computer what to do and when to do it

What is Interpolation?

Interpolation is a mathematical method of estimating the value of an unknown function using the known value on either side of the function (used for creating missing data)

What are some pros of using adaptive statistical iterative recon?

It can reduce image noise and is shown to reduce patient radiation dose by as much as 50%

What is "Adaptive Statistical Iterative Reconstruction"?

It is a new method of image recon that computes projections from the image, compares it with the original projection data, and updates the image based on the difference

Relationship of ray sum, view, and image?

It takes many ray sums to make a view and takes many views to make an image in CT

What is an example of an input device?

Keyboard

What is a "View"?

Many Ray Sums make up a View

What is needed to create an image?

Many views are needed to create an image

What is an example of an output device?

Monitor

What are RAS Coordinates?

RAS Coordinates are image coordinates that allow the operator to specify the image within the SFOV that will be displayed on the image; Some manufacturers precede the number by R (right)/L (left), A (anterior)/P (posterior), and S (superior)/ I (inferior)

What is Hardware?

Parts of the computer that can be physically touched

True or False: Parts of the patient located outside of the SFOV may cause out-of-field artifacts on the image

True

What is Prospective Reconstruction?

Prospective Reconstruction is an image that is automatically produced during scanning

What is "Ray Sum"?

Ray Sum is when the DAS "reads" each arriving ray and measures how much of the beam is attenuated

What is Retrospective Reconstruction?

Retrospective Reconstruction refers to using raw data later to create a new image (ie: thinner slices, bone window)

What is SFOV?

Scan Field of View: Selecting SFOV determines the area within the gantry from which the raw data are acquired SFOV is the isocenter of the gantry Selecting SFOV also determines the # of detector cells collecting data

What is the Fourier Transform?

The Fourier transform is an important image-processing tool that decomposes an image into components (Takes analog signal and breaks it down into a digital signal; separates a function into its frequency components)

True or False: Anything outside of the SFOV is not imaged

True

True or False: If storage space in the computer system reaches maximum capacity, the tube will not scan

True

True or False: The grid on the gantry is the same as display on computer monitor

True, the grid on the gantry (512 x 512) is the same as display on computer monitor

What are other names used for DFOV?

Zoom or Target