403: Unit 3
the number of millimeters of information reconstructed and displayed in the final image
DFOV
the display device used in CT is the ___ ____ __ or ___ ___
cathode ray tube (CRT) or television monitor
the solutions to the mathematical problems require ___ ___ or ___ ___ in order to reconstruct the image in CT
computer programs; reconstruction algorithms
___ improves the image quality by the removal of blur
convolution
___ is a digital imaging processing technique that modifies images using filter function- a general purpose algorithm
convolution
____ is the process of applying a filter to the data
convolution
_____ removes blur thus improving image quality
convolution
filtered back projection is also referred to as the ____ method
convolution
the process of applying a filter function to an attenuation profile
convolution
in the reconstruction filter, a predesigned ____ ___ (spatial filter) is used to acquire data to modify depiction of detail
function curve
what are some graphic aids
girds, arrows, annotation, histograms
EX: fourier transform, which uses filtering in the frequency domain
global operations
the entire input image is used to compute the value of the pixel in the output image; can provide edge enhancement, image sharpening, image restoration
global operations
___ ___ as part of the software program for image manipulation in CT. example is ROI
graphic aid
the brightness value of each samples pixel is assigned an integer based on the strength of the signal called
gray level
when generating the digital image, we assign a __ __ to each pixel, which allow us to differentiate structures on the image.
gray scale
the ___ ___ ___ can then be displayed on the image console and recorded on film or other media
gray scale image
when referring to the final step in image formation, the digital image is not displayed for viewing, it must be converted to a ____ scale or ____ ____.
gray; brightness image
the stronger the signal, the __ the numerical value of the integer
greater
the CT window should always be displayed on the ___ ___
hard copy
as mAs levels increase, so does the amount of ___ being produced
heat
____ pass filtering emphasizes high spatial frequencies
high (sharp filter)
small filaments cannot withstand what
high mA
two types of filters used in filtered back projection
high pass filter (sharp) low pass filter (smooth)
a graph of the pixels plotted as a function of the gray level. they indicate the overall brightness and contrast on an image
histogram
primary component capable of performing several functions ex: reading and writing data, archiving storage on tape or disk, transmit images
host computer
is reconstructed data the convolved data that has been back-projected into the image matrix to create the CT image you see on a monitor
image data
converting an analog image into numerical data for processing by the computer. what are the 3 steps
image digitization scanning, sampling, quantization
___ ___ is the third and final process of data processing in CT
image display
how or where the image is displayed (computer monitors, tv monitor
image display
the 3rd and final step in the CT process
image display, manipulation, storage, recording, and communication
____ ____ ____ are used by several modalities in the radiology department
image processing techniques
processor responsible for high speed processing of the input digital data
image processor
what is the disadvantage of multiplanar reconstruction
image quality
The mathematical problems in CT are solved by computer programs resulting in ___ ___, thus called reconstruction algorithms
image reconstruction
the ___ ___ process is controlled by algorithms, the fourier transform, convolution and interpolation
image reconstruction
another name for multiplanar reconstruction
image reformatting or image reformation
the digitized image is held in storage/memory for further processing
image storage
a small window ____ image contrast because a specific difference in CT number is represented by a greater difference on the gray scale
increases
as WW decreases, contrast ____
increases
increasing mA ___ the number of electrons that produces the xray photons
increases
prospective reconstruction is the ____ ____ process that results in an image
initial reconstruction
an _____ (CT number) is assigned to each amplified signal (positive or negative whole number)the value of the ____ is determined by the strength of the signal emanating from the patients body (pixels)
integer
____ is used in spiral with filtered back projection
interpolation
a mathematical technique used to determine the value of a function from known values on either side of it.what is the simplest method
interpolation; linear interpolation
____ ____ undergoes pre-processing, it undergoes correction and some reformatting must be completed before the 2nd step of image reconstruction can be done
raw data
convolution kernels can only be applied to ___ ___
raw data
image reconstruction is creating an image from ___ ___
raw data
DFOV determines the size of the image seen on the ___ and reproduced on ___ ___
monitor; hard copy
retrospective use ___ ___ to create additional images after the initial exam is completed
raw data
the result of preprocessed scan data
raw data
in CT the _____ are sampled when x-ray pass through them
voxels
short scan times diminish __ ___ and the effects of ___ ___ in the heart, bowel, etc.
motion artifacts; intrinsic motion
kVp defines the ____ or average image of the xray beam
quality
kvp or average energy
quality
measures content of trabecular bone in osteoporosis (bone mineral density) BMD
quantitative CT
most sensitive of all x-ray techniques
quantitative CT
mA defines the ___ of the xray beam and scan time is added to this.
quantity
the analog signal is also _____ and changes into a digital array for input into the computer
quantized
when WL moves towards higher CT numbers (generally ______) more numbers with lower values (generally ____) are displayed
white; black
___ windows (44-2000 HU) should be used to encompass tissues of greatly different attenuation within the image
wide
the ____ is the range of numbers which produce the full shades of gray from black to white
window
___ ___ represents the medium gray scale
window level
is the CT number in the middle of the WW, is ____ referred to as a _____ _____ because it represents water
window level; 0; reference point
___ ___ determines the maximal number of shades of gray that can be displayed on CT monitor
window width
the picture contrast can be changed with 2 control mechanisms
window width and window level
__ __ is the absorption measurement range expressed in _____
window width; HU
a method by which the gray scale can be manipulated using the CT number of the image which can be adjusted by who
windowing; viewer
air is _____ and is a reference point also
-1000
What is the process of events after signals leave detectors:
-CT detectors -preprocessing -reformed raw data -convolution with filter -image reconstruction algorithm -reconstructed images of CT #'s -image storage, display, recording, archiving
There are two steps of the filtered back projection process which are:
-applying a filter to raw data -back projection
the smaller matrices do what
-reduce the memory requirements -allow faster reconstruction -improve spatial resolution
water is the reference material used for assignment of CT numbers. It is assigned the value of ___
0
early CT scanner produced scans made up an __x__ matrix
80x80
what are the basic steps of the reconstruction process:
1. an object is scanned and measurement or scan data is acquired. It must be preprocessed before image reconstruction. It is preprocessed to correct for beam hardening, bad detector readings, errors in measuring the data, or scatter. 2. We then have raw data, which is what we get from the preprocessed scan data. This represents images in the spatial domain. Data from a minimum rotation of 180 degrees is required for adequate reconstruction to occur. Raw data is subject to the image reconstruction algorithms 3. Changing these images from the spatial domain to the frequency domain requires the use of an algorithm. 4. Images in this domain is useless to us and anyone, except physicists, so we need to convert them to useful clinical data which is the CT or MR image 4. The image reconstruction algorithm used by current CT scanners is filter back projection algorithm. It includes both filtering and back projection. 5. Image data or reconstructed date is convoluted date that has been filtered back projected into an image matrix to create CT images that are displayed on a CT monitor. -various digital filters are available to suppress noise and improve image detail
what is the three step process of CT
1. data acquisition 2. image reconstruction 3. image display
what are the steps of QCT:
1. prescan localization image is obtained. This is the scout or pilot scan, or topogram 2. slices are selected from the scout and the midvertebral planes are examined 3. transverse axial images are obtained at the time, a reference phantom is positioned and scanned with the patient. The phantom contains water and bone equivalent parts 4. An automatic contour tracing of cortical and trabecular regions of interest are obtained 5. computer calculates the mean values of the ROI 6. image graphic output is obtained, it shows the BMD values plotted as function of age. The BM content is determined and compared to normal values
steps of filtered back-projection
1.) all projection profiles are obtained2.) logarithmic values for the data are obtained3.) logarithmic values are multiplied by a digital filter or convolution filter to generate a set of filtered profiles4.) these filtered profiles are back-projected5.) filtered projection are added, resulting in the cancellation of positive and negative components, producing a blur free CT image
what are the 4 steps in CT data processing
1.) measurement of scan data that arises from the detectors undergoes preprocessing2.) scan data are converted into digital image characterized by CT numbers3.) reconstruction techniques or algorithms accomplish the task of converting the transmitted data into a CT image4.) image storage is the final step in CT data processing.
most systems have __ filaments
2
changes in mAs of less than ____ may not make a visible difference on the image
20%
the range is +1000 to - 1000 for a total of what
2000 numbers
the human eye can differentiate __-__ shades of gray
25-30
small detectors= medium detectors= large detectors= x-large detectors=
25cm; 35cm; 48cm; 50cm
small filament for less than ____ mA
350
with MDCT, scan time is the time it takes for the xray tube to make a ___ degree rotation (0.5-2 seconds)
360
special software gathers information from transaxial scan data to display 3D on a 2D tv screen
3D rendering
the standard matrix sizes are either ___x___ or ____x____
512x512 or 1024x1024
kvp ranges
80-140
major components of basic image processing system
ADC; image storage; image display; image processor; host computer; DAC
structures are not superimposed, can discriminate subtle tissue differences, data can be manipulated (reformatted), low contrast resolution is possible, all CT slices have a defined thickness of 0.5-10mm, and the high sensitivity to very small xray attenuation differences, includes all information about structures within a given slice, conventional tomos, blurs surrounding structures, displays very well slight attenuation differences, and reduces scatter radiation which reduces scatter (darkening) on the image are all advantages of what
CT
algorithms, which are reconstruction techniques accomplish the task of converting the transmitted data into ___ ___
CT images
the raw data is reconstructed by the computer to create an image in which each pixel is assigned a numberical value known as
CT number
a ___ ___ is assigned to represent attenuation properties of the tissues in each pixel. These numbers are related to the linear attenuation coefficients of the tissue that comprise the slcie
CT numbers
the ___ ____ is an image contrast control
CT window
fourier transform is also known as ____ ____
fourier transformation
pixel size can be computed from the ___ and the ___ ___
FOV; matrix size
CT numbers are expressed in ___ ___
Hounsfield Units
___ ___ is a practical alogirthm used in some scanners an example is analogy for hearing
fourier transform
scan time= the time it takes for the x-ray tube to make a full 360 degree rotation even though many slices can be produced.usually takes how long?
MDCT; 0.5-2 seconds
what are the 3 caterogies of 3D rendering
MIP, SSD, VR
when referring to the most common algorithms, the fourier transform is used in ____
MRI
___ algorithm has edge enhancement, provides greater detail, has more noise, lower mAs, and a short scale of contrast
bone
SCAN TIME= the time the x-ray beam is on for the collection of data for each slice. usually 360 degreescan be overscan or partial
SDCT
scan field of view is important for minimizing ___ and it is the reconstruction circle, which is a circular region from where the transmission measurements are recorded during scanning
SFOV
What are the two types of reconstruction filters:
Sharp and smooth
when referring to back projection, since we only have a few projections, the image will not be an exact replica of the original image with things such as:
blurry images and grainy appearance
retrospective is not the same as reconstruction because those images are created from originally _____ ____
acquired data
____ is an operating system for the CT computer
algorithm
when reconstructing an image, reconstruction techniques or ____ accomplish the task of converting the transmitted data into CT images, which is held in the ___ ____
algorithms; K space
minimum number required to create image free of the sampling artifact
aliasing
what is the advantages of multiplanar reconstruction
allows visualization of specific structures such as optic nerve or lesions in relation to surrounding structures and helps localize lesions, foreign bodies, intraarticular bone fragments. to determine true extent of fractures or lesions
in interpolation, numerical representation is converted to ____ to get the image
analog
analog signal is converted into a digital form by the digitizer
analog to digital converter
a specific shade of gray is assigned to each pixel and is based on the ___ profiles of the xray beam from tissue located at each point in the slice
attenuation
filtered back projection is used from the recon of images when the ___ ___ is acquired through multiple projections
attenuation information
levels should be centered near the ____ ____ of the tissues of interest
average attenuation
with ___ projection, the attenuation profiles are added together
back
___ ___ is not the same as filtered back projection
back projection
a __ __ is a technique that adds together the attenuation information that is collected from all projections
back projection
the process of converting the raw data from the attenuation profile to a matrix. a technique that adds together all attenuation information from all projections
back projection summation method
____ ____ is the process of converting the raw data, from the attenuation profile, to a matrix. Its a technique that adds together the attenuation information from all projections. And the calculation is used for ___ or the final CT images from raw data
back projection; recon
analytic reconstruction algorithms are developed to overcome limitations of ___-___ and ____ ____
back-projection; iterative algorithms
correct errors in measurement data from things such as what
beam hardening, bad detector readings, scatter radiation
image areas having CT numbers less than the lower window settings are displayed as ____ and those with values greater are displayed as _____
black; white
when referring to convolved data, first, raw data must be filtered using a mathematical filter or kernel. this process is known as the ___ ___
convolution technique
when referring to the most common algorithms, the filtered-back projection is ____ and is most common in _____
convolution; conventional
image reconstruction algorithm is filtered back projection, which includes both filtering and back projection
convolved data
as WW increases, contrast ___
decreases
decreasing kvp, holding mas constant
decreases dose
decreasing mas, holding kvp constant
decreases dose
___ is slice thickness
depth
___ algorithm has slightly less edge enhancement than bone algorithm, has greater detail, and less noise than bone algorithm
detail
sampling is done by ____
detectors
in image reconstruction, scan data is converted into __ __ which is characterized by CT numbers relative to the ___ ____ ___
digital form; linear attenuation coefficient
when reconstructing an image, first the raw data is processed and the scan is converted to ___ ___ ___ ___
digital image CT numbers
digital image held in memory can be displayed on a TV monitor which work with analog signals
digital to analog converter
the ___ ____ is converted to a gray scale image for interpretation by the radiologist and for viewing
digital value
global operations can provide what
edge enhancement, image sharpening, and image restoration
if selecting both a scanned field of view and a displayed field of view, the displayed field of view must be ___ __ or ___ ___ than the scan field of view, never ___
equal to or less than; larger
examples of hard copy
film-laser printer, optical disk, CD
the purpose of the ___ is the enhance the important parts of the image before back projection is performed
filter
applying a ___/___ that enhances certain characteristics in the raw data, before back projection, helps make a more perfect image
filter/kernel
When referring to the basic steps or the reconstruction process, the raw date must be filtered with a ___ or ____. this process is known as the ____ technique
filter; kernel; convolution
___ ___ projection can depict the original object in the final CT image, if a sufficient number of projections are present and the proper recon filter is applied
filtered back
Most CT systems today use a reconstruction method called ____ ____ ____ for reconstructing a CT image from the raw data
filtered back projection
when referring to reconstruction algorithms, analytic methods include what
filtered back projection or Fourier reconstruction
____ is accomplished by convolution which removes the blurring which normally arises from back projection convolution
filtering
in order to produce a clincally useful image, the numerical representation of the image must be converted back into analog form by a DAC to facilitate the ___ ____ in the ___ ___
final steps in the CT process
if the CT number is not within the CT window setting what happens?
it will not be displayed the image
___ reconstruction is a resurfaced concept from the early development of CT
iterative
is an effort to overcome image noise and streak artifact of the filtered back projection algorithm creates simulated projections that are true representation of the scanned object starts with an assumption and compares this assumption with measured values, makes corrections to bring the two into agreement, and then repeats this process over and over until the assumed and measured values are the same or within acceptable limits
iterative algorithms
___ is the intensity or force which water flows through a hose
kVp
the convolution filter is called a
kernel
the filter used is a convolution filter which is a digital processing technique general purpose algorithm called a ___
kernel
____ or _____ matrices refer to the number of pixels within
larger or smaller
DFOV is important in optimizing the detection of ___
lesions
no depths due to superimposition, cannot manipulate data so there is no quantitative analysis, cannot appreciate density differences between subtle tissue densities so there is not really any ability to discriminate contrast refers to what
limitations of radiogrpahy
EX: spatial frequency filtering performed during convolution with a convolution kernel
local operations
the output image pixel value is determined from a small area of pixels around the corresponding input pixel; area processes or group processes
local operations
___ pass filtering emphasizes low spatial frequencies
low (smooth filter)
the current of electrons that flow from the filament to the anode is measured in ___
mA
the signal to noise ratio may be improved by increasing what
mA, kv or scan time
In ct the total x-ray beam exposure is determined by a combination of what? we tend to vary what more than what?
mA, scan time, and KVp vary mA more than KVP
the product of mA and scan time
mAs (tube current)
CT ___ is a complex
math
when generating the digital image, we divide the slice we select into a grid composed of rows and columns called a ____. Each tiny region (box) on the grid is known as a ___. And the volume of the tissue is the ___
matrix; pixel; voxel
scan data that arises from the detector data is subject to preprocessing to correct the measurement data before the image can recon. algorithm is applied
measurement data
____ or ___ ___ that arises from the detectors undergoes pre-processing
measurement or scan data
contrast is optimized with _____ ____ settings
medium WW
algorithms are used in ___ CT scanners
modern
a computer that creates coronal, sagittal, and paraxial images from a stack of contigouous transaxial scansirregular,oblique and other views can be generated
multiplanar reconstruction
when generating the digital image, the total number of pixels in the matrix=
multiply the # of pixels in a row by the # of pixels in a column
___ windows (50-350) should be used to display soft tissues within the structures that contain different tissues of similar densities
narrow
materials with a density less than that of water are assigned ___ values
negative
when referring to image data, various filters are available to suppress ___ and improve ___ such as smoothing algorithms and edge enhancement algorithms (sharp)
noise; detail
thin slice images are sometimes ___ to compensate increase tube output (mA and scan time) moderately
noisy
windowing is a ____ technique that can be modified by different algorithms
nonlinear
algorithm is an ___ __ for the CT computer
operating system
examples of storage/archive
optical disk, CD, pacs
when referring to back projection, at the location where the attenuation profiles add up, the image there is the ____ image
original
reconstruction is different because it comes from ___ ___
original data
retrospective uses a series of __ __ usually from another plane
original slices
if too small- body tissue will be located outside and the accuracy of the attenuation numbers will be diminished, resulting in shading artifacts along the periphery may be produced called what
out-of-field artifacts
examples of electronic transmission
pacs, electronic networking, dicom standard
the ____ plane cuts through the coronal and sagittal planes in a longitudinal direction
paraxial
using a smaller filament size concentrates the focal spot size which will reduce what
penumbra
in filtered-back projection, an array processor calculates values for every ___
pixel
__ __= display filled of view/matrix size
pixel size
In spiral scanning, a calculation must be done to divide the information into ___ ___
planar samples
Every system works on a ____ or ___ ___
platform or operating system
windowing or gray level mapping is done by what operations
point
The least complicated and most frequently used image processing technique is the:value of the input image pixel is mapped to corresponding output image pixels
point operations
image processing techniques 3 types
point operations; local operations; global operations
materials with a density more than that of water are assigned ___ values
positive
In CT, both conventional and spiral, we measure attenuation information by multiple ___, which becomes the ___ ___
projections; raw data
___ ___ is done when an area of interest is known at the beginning of the exam and occurs automatically during the exam
prospective reconstruction
the values of x-ray detector response from all views and rays within a scanthese data are then convolved with the convolution filter function and back-projection to produce a CT image
raw data
A filter is applied to raw data during image reconstruction and it is called ___, ___, or ___. This purpose is to enhance the important characteristics of the image before the back projection is performed
recon; filter; kernel
We ____ every image, but we do not always ____ the image
reconstruct; reformat
another named for DFOV
reconstructed field of view
Raw data must undergo a ___ process to form the final images
reconstruction
We use ____ algorithms in CT
reconstruction
back projection is a ____ algorithm
reconstruction
___ ___ cannot be changed by us, they are part of your system. You can apply different windows
reconstruction algorithms
mathematical filter wherein the process of multiplication of overlapping portion of the filter function are and the detector response curve are used selectively produce a third function used for image reconstruction
reconstruction filter
_____ changes the orientation of the image such as coronal or sagittal
reformatting
the process of recreating images in various body planes
retrospective/prospective reconstruction
the slice of the patient is divided into voxels because the dimension of depth is added to the pixel this is accomplished by what?
rotational scanning of the patient
x is what plane Y is what plane Z is what plane
sagittal; coronal; axial
measures the brightness of each pixel
sampling
when referring to raw data, it is the preprocessed ___ ___, which is subjected to image recon ___ and can be ___ and ____ when needed
scan data; algorithms; stored; retrieved
__ __ is the time it actually takes to image a slice <1-10 secs
scan time
the size of the field within the gantry opening, the number of millimeters of anatomy over which the projection data is collected, selected by the technologist, image data for reconstruction is fathered here, and the detectors are told which data to use and which data to ignore
scanned field of view
preset windows are available on _____
scanners
first step in digitizing an image is the division of the image into small regions this is called
scanning
filtered-back projection is not used for ___ or ____ images since its a single projection
scout or localizer
restrospective is known as what
secondary recon
SFOC, DFOV, window settings, matrix size, slice thickness, scan time and rational arc, radiographic tube output, focal spot and tube geometry are all examples of ____ ___ ___
selectable scan factors
each integer corresponds to a ___ __ ___ on the image
shade of gray
when reducing scatter, it is accomplished by filtering of the transmitted beam at the detector assembly by:
shape, nature, and spaces between each detector
____ filters are used for high contrast areas like the musculosketal. It improves definition of edges of structures, does not minimize grainy looks in the image
sharp
higher mA settings allow for __ scan times, which reduces patient ___
shorter; motion
___ ___ affects perceived detail because of the effect of volume averaging for structures with complex shapes
slice thickness
the ___ the pixel, the greater the detail, but the perception of detail may be reduced
smaller
___ algorithm has little to no edge enhancement, maximum smoothing, low noise level, higher mAs required, and longer scale of contrast
smooth
____ filtering distinguishes contrast between tissues better. It minimizes the grainy appearance of noise in the image. Low contrast regions. Does not improve upon edge definition
smooth
____ algorithm has less edge enhancement than detail algorithm, longer scale of contrast (smoothing), and less noise than none, detail, or standard algorithms
soft
it is required to reconstruct a ___ CT image
spiral
____ algorithm has medium edge enhancement, scale of contrast, and noise level
standard
convolution method requires that the projection profile (data) be filtered or convolved using mathematical filters to remove the typical ___ ___ blurring which occurred when the simple back projection was in use
star like
when referring to back projection, a typical ____ pattern artifact is seen on the image as streaks. This is minimized by a process of ___, which is called ____ or ___
star; filtering; convolution or filtering
DFOV plus the matrix size determines what
the limitations of perceived data
the ___ and ___ the part the, the more mAs is required
thicker and denser
___ ____ (not spiral) deliver more radiation per area than thicker ones; should be used for their value in areas with complex anatomy and high subject contrast, the larynx, parathyroid glands, biliary system
thin slices
reducing scatter is accomplished by the xray being collimated ___:
twice; near the source and beyond the patient
what is interpolation used in?
used in image reconstruction and used in spiral/helical scanning to determine slices
quantitative measurements
volume, distance, diameter
the specific value of each number is determined by the physical density of the tissue in the corresponding
voxel
