1
Q
Algorithm
A
- a precise set of steps to be performed in a specific order to solve a problem
- in CT reconstruction algorithms are used by the computer to solve the many mathematical equations necessary for information from the detector array to be converted to information suitable for image display
- “a set of rules or directions for getting a specific output from a specific input”
- an algorithm must always terminate after a finite number of steps
2
Q
Interpolation
A
Estimating the value of an unknown function using the known value on either side of the function
-a mathematical method of creating missing data
3
Q
Hard disk (or hard drive)
A
- essential component of CT systems
- number of images that the hard disk can store varies according to make and model
- saves information (data)
4
Q
What are the principal components in a computer
A
- input device ex keyboard, mouse, touch sensitive plasma screen
- output device ex monitor, laser camera, printer, archiving equipment such as optical disks or magnetic tape
- central processing unit (CPU)
- memory
5
Q
Input and output devices
A
Pieces of computer hardware designed to feed data into the computer or accept processed data from the computer
- input devices ex: keyboard, mouse, touch sensitive plasma screen, and CT detector mechanisms
- output devices ex: monitor, laser camera, printer, and archiving equipment such as optical disks or magnetic tape
6
Q
Central processing unit
A
Component that interprets computer program instructions and sequences tasks
- referred to as the “brain” of the CT system
- gets the data it needs from RAM, processes it, and writes new data back to the RAM in a continuous cycle
7
Q
Scan data and raw data are used interchangeably
A
- all of the thousands of bouts of data acquired by the system with each scan are called raw data
- used interchangeably to refer to the data sitting in the computer waiting to be made into an image
8
Q
The process of using raw data to create an image is called?
A
Image reconstruction
- the same raw data may be used later to generate a new image because it includes all measurements obtained from the detector array, a variety of images can be created from the sam data (this is called retrospective reconstruction)
- raw data storage requires much more computer storage space than that of image data
9
Q
Image data
A
Those which result once the computer has processed the raw data
- one hounsfield unit value is assigned to each pixel (this value, or density number, is the average of all attenuation measurements for that pixel) the proportional amount of x-ray energy that passes through anatomy and strikes the detector
- once the value has been assigned to each pixel an image can be formed
- data manipulation is limited
10
Q
Adaptive statistical iterative reconstruction
A
- there are a large variety of algorithms used, but each starts with an assumed image, computes projections from the image, compares it with the original projection data, and updates the image on the basis of the difference between the calculated and actual projections
- called adaptive statistical interactive reconstruction algorithms
- used to extract additional image clarity and suppress noise
- improve image quality by improving low-contrast detectability
- shown to reduce patient dose by as much as 50% compared to back projection methods
11
Q
Scan field of view (SFOV)
A
- selecting the SFOV determines the area, within the gantry, from which raw data is acquired
- scan data always acquired around the Isocenter, the patient must be positioned in the center of the gantry
- determines the number of detector cells collecting data
- anything outside the SFOV is not imaged because no data are collected beyond this circle
- parts of patient located outside the SFOV may cause inaccuracies in the image, called out-of-field artifacts
- manifest in image as streaking, shading, and incorrect HU numbers
12
Q
Display field of view
A
Determines how much of the collected raw data is used to create an image (the section of data selected for display on the image)
- changing the DFOV will affect image quality by changing the pixel size
- works in a similar way to zoom on a camera and can be used to show the entire area or to display a specific region of interest in greater detail
- the DFOV cannot be larger than the SFOV
13
Q
Choosing the optimal display field improves the detectability of ___
A
Abnormalities
14
Q
Selecting too large a DFOV makes the image appear unnecessarily ____
A
Small
- harder to visualize if its smaller
- more data included in each pixel and special resolution decreases
15
Q
If the DFOV is too small it may exclude necessary patient ____
A
Anatomy
16
Q
_____ refers to a selected circle in the center of the gantry. Raw data are acquired and calibrated for any object that lies within this circle. The entire scan circle or any portion of the circle may be selected to display on the monitor. The size of the circle that is displayed is called the ____
A
SFOV, DFOV
17
Q
Display monitors
A
- output device allows the information stored in computer memory to be displayed
- displayed in back and white or color
- display device is usually either a cathode ray tube (CRT) or some form of flat panel such as a TFT LCD (thin-film transistor, liquid crystal display)
- CRT is basically a standard television set with some modifications that improve image resolution (hotter and less durable than LCD)
- LCD produce higher luminance and higher spatial resolutions
- DAC change the digital signal from the computer memory back to an analog format so that the image can be displayed on the monitor
18
Q
Cameras
A
- output device that transfers the image from the monitor to the film (multi format camera)
- usually a laser camera (bypass the image on the display monitor and transfer the data directly from the computer bypassing the video system entirely, thereby significantly improving image quality)
- film used in CT consists of a single emulsion that is sensitive to either the light-emission spectrum of the video screen phosphor (for the multi format camera) or to the laser light beam
19
Q
Gray scale
A
- a display processor assigns a certain number of HU to each level of gray
- the number of HU assigned to each level of gray is determined by the window width
- the gray scale is used to display CT images. This system assigns a certain number of HU to each shade of grey
- assigns higher HU values lighter shades of grey
- lower values are represented by darker shades
20
Q
Window width
A
- determines the number (QUANTITY) of HU represented on a specific image
- the software assigns shades of gray to CT numbers that fall within the range selected
- all values higher than the selected range appear white, and any value lower than the range appears black
- “by widening the width” (increasing the WW) more numbers are assigned to each shade of gray
21
Q
_____ assigns the quantity of pixel values to the gray scale. ____ determines the center pixel value in the gray scale
A
WW, WL
22
Q
Window level
A
- selects the center CT value of the WW
- window level and window center mean the same thing
- selects which hounsfield numbers/ values are displayed as shades of gray on the image
23
Q
True or false: the window level should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest
A
True!
24
Q
Wide window widths
A
- best for imaging tissue types that vary greatly, when the goal is to see all the various tissues on one image
- encompass greater anatomic diversity, but subtle density discrimination is lost
- decrease image contrast, they suppress display of noise on an image
- common practice to widen window width when patients are obese or when there are metallic artifacts
25
True or false: Tissue types with similar densities should be displayed in a lower, or narrow window width
- provide greater density discrimination and contrast
| - great for displaying brain
26
Single detector row systems
- opening or closing the collimator controls the slice thickness by controlling the portion of the detectors width that is exposed to incoming x-rays
- opening the collimation beyond the upper limit would do nothing but increase dose to the patient and scatter radiation
- radiation emitted from the collimated x-ray source is referred to as a fan beam
- each gantry rotation produces data for a single slice
- collimators slightly closed results in a thinner slice
27
Multi-detector row systems
- a single rotation can produce multiple slices
- provides longer and faster z axis coverage per gantry rotation
- faster
- slice thickness is determined by a combination of the x-ray beam width (controlled by the collimators) and the detector configuration (can combine different numbers of individual detector elements together ex if one detector row will provide slices 1.25 mm thick, grouping two detector rows together will provide slices 2.5mm thick)
- radiation emitted from the collimated x-ray source in these systems is referred to as a cone beam
- can be used for either axial or helical data acquisitions
- in some systems the width of the detector rows are uniform
- in others, the width of the detector row is variable, with the rows thinner in the center and wider at the periphery
28
When are axial scans typically used?
- when acquisition speed is not a major concern
- when optimal resolution is required
- used for studies in which slices are gapped, or when the exposure will be interrupted
29
Pitch
- a parameter commonly used to describe the CT table movement throughout a helical scan acquisition
- most commonly defined as the travel distance of the CT scan table per 360 rotation of the x-ray tube, divided by the x-ray beam collimation width
30
Pitch in SDCT
- table speed and slice thickness are directly related in a helical scan process
- when the table moves a distance that is equal to the slice thickness during each gantry rotation, the pitch is described as 1
- describes the relationship of the table speed to the slice thickness
- as the pitch increases, so does the slice angle
- as pitch increases, the effects of interpolation become more pronounced ex image unsharpness and effective slice thickness blooming
- information is collected for each table position regardless of pitch
- as pitch increases fewer data are acquired for each table position
31
Increasing the pitch in a SDCT will
- result in a scan covering more anatomy lengthwise for a given total acquisition time
- reduce radiation dose to the patient (if other scan parameters are held constant)
32
A decrease in pitch will
- slow down the table speed
- a pitch of less than 1 will result in overlapping slices
- decreasing the pitch will decrease the amount of anatomy covered per unit time and increase the radiation dose to the patient
- a pitch of less than 1 is not commonly used and on some SDCT systems it is not an available option
33
when to extend the pitch beyond 1 in SDCT
- (recused image time and patient dose with longer pitch times)
- lose image sharpness but not very significantly
- these disadvantages are minimal unless exceeding 1.5
- pitch is increased slightly to allow an entire area to be covered in a single breath hold
- when the tube heat limits the length of a helical scan acquisition
- more than 1.5 used in CT angiography (where scan speed is paramount) when the few seconds that the increased pitch saves will make a major impact on the outcome
34
Pitch in MDCT systems
- simultaneous data acquisition from parallel rows of detectors requires rapid table advancement during scanning
- pitch still defined as the relationship between slice thickness and table travel per rotation
- beam pitch can be defined as table movement per rotation divided by beam width
35
In general, in both SDCT and MCDT systems a pitch between ____ is most common
1-1.5
36
Increasing the pitch in MDCT systems
- affected the same way as SDCT
- shorten the total acquisition time for a given distance covered, although there will be some penalty in image noise related to fewer data being acquired (ex under-sampling)
- usually small effect at levels less than 1.5
- decrease radiation dose to the patient provided the technique remains constant
- extended pitch values may result in an increase in streak artifact
37
Changing slice incrementation retrospectively
- helical data allow the slice incrementation to be changed, retrospectively
- this allows the creation of overlapping slices, without increasing the radiation dose
- in some situations, changing the slice incrementation can reduce the partial volume effect resulting in a more accurate image
- because the data is acquired in a helical fashion, we can retrospectively change the portion of data used, which produces a new image
38
MDCT: reconstructed slice thickness
- offer opportunities for retrospectively changing slice thickness that are not available on SDCT systems
- slice choices available for reconstructed slice thickness are not unlimited
- the thinnest images that can be reconstructed for a data set are predetermined by the slice thickness used for data acquisition
- the image thickness (how the data are reconstructed) may be greater than the slice thickness (how the data were acquired), but the image thickness should not be less than the slice thickness
39
Dual source CT scanners
- the x-ray tubes and detectors need only rotate 90 degrees to get a full data set instead of 180 degrees in older systems and 360 degrees in current systems to generate more data
- these scanners have two X-ray tubes (sources) and two detector arrays within the gantry, spaced at a 90 degree angle
40
Fourier transform
-used to reconstruct images of a patients anatomy in CT and MRI
41
Convolution
-digital image-processing technique used to modify images through a filter function
42
Iterative algorithms
- "an interactive reconstruction starts with an assumption (ex that all points in the matrix have the same value) 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"
- advantages: reduce image noise, and minimize the higher radiation dose inherent in the filtered back-projection algorithm
43
Analytic reconstruction algorithms
- developed to overcome the limitations of back-projection and iterative algorithms
- used in modern CT scanners
- two analytic reconstruction algorithms are the fourier reconstruction algorithm and filtered back-projection
44
Filtered back projection
- aka convolution method
- projection profile is filtered or convolved to remove the typical star like blurring that is characteristic of the simple back-projection technique
45
Fourier reconstruction
- used in MRI but not in modern CT because it requires more complicated math than the filtered back-projection algorithm
- a radiograph can be considered an image in the spatial domain (shades of grey representing various parts of anatomy) with the fourier transform, this spatial domain image the radiograph represented by the function f(x,y) can be transformed into a frequency domain image represented by the function F(u,v)
- in addition this image can be retransformed into a spatial domain image with the inverse fourier transform
- advantages: image frequency domain can be manipulated (ex edge enhancement or smoothing) by changing amplitude of frequency components, computer can perform those manipulations (digital image processing), frequency information can be used to measure image quality
46
What are the 4 data types?
- measurement data
- raw data
- filtered raw data or convolved data
- image data or reconstructed data
47
Measurement data or scan data
- arise from the detectors
- subject to preprocessing to correct the measurement data before the image reconstruction algorithm is applied
- such corrections are necessary because of errors in measurement data from beam hardening, adjustments for bad detector readings, or scattered radiation
48
Raw data
- result of preprocessed scan data and are subjected to the image reconstruction algorithm used by the scanner
- data can be stored and retrieved as needed
49
Convolved data
- raw data must first be filtered with a mathematical filter, or kernel (process also referred to as the convolution technique)
- improves image quality through the removal of blur
- convolution kernels can only be applied to the raw data
50
Image data
- image data or reconstructed data, are convolved data that have been back-projected into the image matrix to create CT images displayed on a monitor
- various digital filters are available to suppress noise and improve detail
51
Standard algorithm
-usually used before the previous algorithms, especially when a balance between image noise and image detail is mandatory
52
Smoothing algorithms
- reduce image noise and show good soft tissue anatomy
| - used in examinations where soft tissue discrimination is important to visualize very low contrast structures
53
Edge enhancement algorithms
- emphasize the edges of structures and improve detail but create image noise
- used where the fine detail is important ex inner ear, thin slice
54
Cone beam geometry
- for a four-detector row MSCT scanner, the beam divergence from the x-ray tube to the outer edges of the detectors increases
- such a beam is called a cone beam
- within the cone beam, the rays that will be measured by the detectors are tilted at an angle relative to the central plane (plane perpendicular to the long axis of the patient, the z-axis)
- this is called the cone angle
55
As the number of detector rows increase the cone angle becomes ____
Larger!
56
Cone-beam algorithms
- fan beam approximation algorithms require that the data be consistent, that is, the x-ray beam from the tube to the detector and the section being imaged be in the same plane
- this is no longer the case for large cone angles characteristic of MSCT systems with larger than 4 detector rows
- fan beam not very accurate with new MSCT scanners so cone-beam algorithms are used instead
- developed to eliminate the cone beam artifacts
- essentially, several cone-beam algorithms have become available for use with the new generation MSCT scanners, and they fall into two classes: exact cone-beam algorithms and approximate cone beam algorithms
57
Image post processing
- refers to the use of various techniques (image processing software or algorithms) that modify the reconstructed images displayed for viewing and interpretation
- used to enhance the visualization of structures in the image
58
____ is the most common image-processing technique in CT
Windowing
59
The range of CT numbers in the image is referred to as _____
Window width
60
What determines the maximum number of shades of gray that can be displayed on the CT monitor
Window width
61
The ____ is defined as the center or midpoint of the range of CT numbers
Window level
62
As the WW increases, the contrast ____
Decreases
63
As the WW decreases, the contrast becomes ____
Greater
| -the image appears totally black and white
64
True or false: As the WL moves toward the higher CT numbers (generally white), more CT numbers with lower values (generally black) are displayed
True
| -image looks more black
65
When the WL moves toward the lower CT numbers, more CT numbers with higher values are displayed, and the image looks ____
White
66
```
Which of the following changes a time domain signal into a frequency domain signal?
A) fourier transform
B) interpolation
C)convolution
D)algorithms
```
A) fourier transform
67
```
A measure of the total absorption of x-rays along a straight line is referred to as:
A) projection data
B) a projection profile
C) a ray sum
D) a view
```
C) a ray sum
68
The back-projection technique results in:
A) the classical star pattern in the image
B) image blur
C)poor image quality, and therefore a diagnosis cant be made
D) all are correct
D) all are correct
69
```
CT scanners now use the:
A) back-projection algorithm
B) FT reconstruction
C) algebraic reconstruction method
D)filtered back-projection algorithm
```
D) filtered back-projection algorithm
70
```
Image data are obtained:
A) before back projection
B) after back projection
C) after preprocessing
D) after convolution
```
B) after back projection
71
```
The first operation to which raw data are subjected is referred to as
A) convolution
B) back-projection
C) after preprocessing
D) after convolution
```
B) back projection
72
```
Which of the following belongs to the class of analytic algorithms for CT?
A) back-projection
B) filtered back-projection
C) algebraic reconstruction technique
D) all are correct
```
B) filtered back-projection
73
```
Which of the following uses a digital filter to modify an image matrix?
A) algorithm
B) interpolation
C) FT
D) convolution
```
D) convolution
74
HU value for water
0
75
HU value for bone
+1000
76
HU value for air
-1000
77
HU value for metal
+2000 or higher
78
⬆️ atomic number = ______ attenuation
⬆️ attenuation
79
⬆️ energy photons = ____ attenuation
⬇️ attenuation
CT Theory
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