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Medical Radiation Sciences 2 > Computed Tomography > Flashcards

Computed Tomography Flashcards

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1
Q

Can we take tomograms in general radiography?

A

Yes

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2
Q

What is the relationship between the angle used in conventional linear tomography, and the slice thickness?

A

Conventional linear tomography is inversely proportional to slice thickness

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3
Q

To see another area of the body in conventional tomography, what do we have to do?

A

Change the falcrum (either the height of the tube or the patient)

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4
Q

With a smaller angle used, will that produce a thinner, or a thicker slice?

A

Thicker slice

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5
Q

With a larger angle, would you have a thinner or thicker slice?

A

A thinner slice

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6
Q

Where is the pivot point/fulcrum in conventional tomography?

A

It is a point on the body

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7
Q

What is the focal/object plane?

A

The area that we will see in best resolution

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8
Q

What is the exposure angle?

A

The angles when the tube actually starts and stops exposing

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9
Q

Which is larger, the tomographic angle or the exposure angle?

A

The tomographic angle

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10
Q

What does axial tomography refer to?

A

Conventional tomography

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11
Q

Which way is the tube positioned in conventional tomorgraphy?

A

Parallel to the long axis of the body

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12
Q

What type of images can you aquire in conventional tomography?

A

Saggital or coronal imags

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13
Q

In what position would the patient have to be placed ot acquire sagittal images in conventional tomography?

A

In a lateral position

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14
Q

Which way is the tube directed in computed tomography?

A

Perpendicular to the long axis of the body

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15
Q

What images can computed tomography acquire?

A

Trans axial or transverse images

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16
Q

Who developed Computed tomography?

A

Developed by Gondfrey Housfield

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17
Q

In which way does first generation CT scanners acquire images?

A

Images aquired in axial slices

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18
Q

What type of beam is used in first gen CT scanner?

A

Narrow beam

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19
Q

What is the slice thickness determined by in first generation CT scanners?

A

Determined by beam width

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20
Q

Describe the process of how an image is obtained in first generation scanners:

A
  1. The tube at detector scan across the subject to create a slice (Translation)
  2. The x ray tube and detector rotate 1 degree and scan again in the opposite direction (Rotation)
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21
Q

Where are the transmission measurements collected in first generation CT scanners? What is this set of measurements called?

A

-Transmission measurements are collected at the detector at many locations as is more across the patient
-This set of measurements is called a view

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22
Q

How many rays per view is each translation measured?

A

160 rays per view

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23
Q

How many views are collected and over how many degrees?

A

180 views aer collected over 180 degrees

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24
Q

What creates translate-rotate motion?

A

Linear translation+incremental rotation

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25
Q

How long does it take to create a single image in first generation CT scanners?

A

5-6 minutes

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26
Q

What element makes up the detector in first generation CT scanners?

A

NaI (sodium iodide)

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27
Q

What is one modification that was made to first generation scanners to reduce time?

A

-Two adjacent detectors and a 26 mm wide x ray beam (in the slice thickness directon)
-This allowed for simultaneously collected data for 2 slices

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28
Q

What type of beam is used in 2nd generation scanners?

A

Fan beam

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29
Q

T/F

The 2nd generation scanners have a detector array of 5-30 detector cells

A

True

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30
Q

T/F

The second generation scanners use a translate and rotate method at 50-100 degree increments

A

The second generation scanners use a translate and rotate method at 5-10 degree increments

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31
Q

What are the pros of 2nd generation scanners?

A

-Increase in speed and imaging time
-Are able to image the rest of the body as opposed to first gen
-Scans could be performed within a breath hold for most patients (reduced motion artifacts)

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32
Q

What are the cons of 2nd generation CT scanners?

A
  1. Heavy x ray tube and associated electronics needed to be moved without causing significant vibration
  2. Scan times of 20 seconds or longer
  3. Expensive
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33
Q

What type of beam is used in 3rd generation scanners?

A

Fan beam covering entire patient width

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34
Q

What type of acquesition is used in 3rd generation scanners?

A

Rotate and rotate acquesition

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35
Q

What type of detector array is used in 3rd generation scanners?

A

Curvilinear detector array

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36
Q

What are the major disadvantages of 3rd generation CT scanners?

A
  1. Lenth of time needed to acquire image data
  2. Ring artifact
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37
Q

T/F

2nd generation scanners have pre-patient collimators

A

False; 3rd generation scanners have prepatient collimators

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37
Q

What are the types of rings that can appear in 3rd generation CT images?

A
  1. Complete
  2. Partial
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38
Q

What does the ring artifact appear as in an image?

A

Appears as bright or dark circular bands

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39
Q

What is the ring artifact a result of?

A

A faulty detector

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40
Q

Describe ring artifact:

A

If one detector is not working, as it goes around the patient, it creates a ring of either dark or bright lines due to a lack of signal in that area

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41
Q

What is used to reduce scatter incident on the detector array?

A

Pre-detector grids reduce scatter

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42
Q

Does CT imaging use sheilding?

A

No

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43
Q

What is the downside of computed axial tomography?

A

It is not great for imaging the chest

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44
Q

T/F

In CAT scans, the images are obtained in uniform slices.

A

True

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45
Q

What is the type of acquesition does CAT use to acquire images?

A

Step and shoot method

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46
Q

Describe the process of how an image is obtained in CAT scan imaging:

A

-The CT table/couch moves and remains stationary while the x ray tube rotates within the gantry
-Obtains image through rows of detectors

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47
Q

T/F

With 5th generation CT scanners, there is interscan delay as the table moves to the next location.

A

False; With CAT scans this occurs

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48
Q

Why do 3rd generation MDCT scanners have improved temporal resolution?

A

Since the multi slice/row detector systems increase volume coverage

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49
Q

How are scanners refered to by?

A

By the number of detector rows

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50
Q

Why is slip ring technology needed?

A

-Since after each 360 degree rotation, cables connecting rotating components required that rotation stop, and reverse the direction (respolling)
-During this time, there is no x ray exposure

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51
Q

What is the result of scanners with no slip ring technology?

A

-Interscan delays
-Long proceedure times
-Increased patient motion
-Poor temporal resolution

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52
Q

What are slip rings?

A

They are a set of parallel, stationary ciruclar, electrically conductive rings in contact with multiple electrically conductive brushes or blocks

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53
Q

How do rotating components of CT scanners recieve electricity?

A

The generator supplies voltage to the ring, which transfers it to rotating components via brushes that glide in the contract grooves of the ring

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54
Q

What are the functions of slip ring technology?

A
  1. Provide the electrical power to operate tube and detector
  2. Provide scanning instructions to the gantry components
  3. Transfer detector signal into image reconstruction computer
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55
Q

What are the advantages of slip ring technology?

A
  1. Facilitates continuous rotation of the x-ray tube
  2. No interscan delay
  3. Less motion artifacts, quicker scans and improved temporal resolution
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56
Q

Describe how helical scanning works in computed tomography:

A
  1. As the tube rotates continuously, the patient is translated continuously throught the gantry opening
  2. This results in CT scanning in a spiral/helical geometry (slices at a slight tilt)
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57
Q

What axis are the beginning and end slices located at?

A

The same z axis at different points

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58
Q

In helical scanning, how do we get rid of the slant and blur in the image?

A

The computer uses interpolation to take out slant and blur

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59
Q

T/F

Third gen CT scanners use filtration of the x ray beam

A

True

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60
Q

What is the function of the filter in 3rd generation CT scanners?

A
  1. To make the image roughly uniform by compensating for the attenuation of the lateral edges of the beam
  2. To remove long wavelenth x rays as they only contribute to dose
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61
Q

What would occur if filtration was not present in 3rd generation CT scanners?

A

-Increase in dose
-The computer would give incorrect information about attenuation

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62
Q

T/F

Fourth generation scanners are routinely used clinically

A

False; they are not

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63
Q

How are images obtained in 4rth generation scanners?

A

By using a rotating x-ray tube placed within a stationary circular detector array

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64
Q

What are the 3 major steps of CT image formation?

A
  1. Data acquesition
  2. Image reconstruction
  3. Image display
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65
Q

Define data acquisition:

A

The collection of x ray transmission measurements from the patient

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66
Q

Define data aquesition system (DAS) with respect to CT scanners:

A

This refers to the detectors electronics positioned between the detector array and the computer

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67
Q

Once the image is obtained, how is the image reconstructed?

A
  1. the computer performs the image prospective reconstruction process using raw data and converts into numerical form
  2. Numerical data is converted into electrical signals for the technologist to view on a monitor
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68
Q

Where is the digitized raw data stored?

A

Stored in the RAM (random acess memory) of the computer attached to the CT sytstem

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69
Q

T/F

The storage capcity witin the CT computers is unlimited.

A

False; The storage capcity witin the CT computers is limited

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70
Q

Define efficiency in relation to CT detectors:

A

The ability to capture, absorb and convert x ray photons to electrical signals

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71
Q

What is capture efficiency in relation to CT detectors?

A

The ability to capture photons transmitted from the patient

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72
Q

What is absorption efficiency with respect to CT detectors?

A

The number of photons absorbed by the detector

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73
Q

What is conversion efficiency with respect to CT detectors?

A

How well the detector converts the incoming x ray signal to a digital signal

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74
Q

What are the determinants of capture efficiency in CT detectors?

A
  1. The size of the detector area facing the beam
  2. The distance between two detectors
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75
Q

What does absorption efficiency of CT detectors depend on?

A
  1. Atomic number
  2. Physical density
  3. Size
  4. Thickness of detector face
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76
Q

How does better AE affect SNR and patient dose?

A
  1. Increases SNR
  2. We can use less of a technique
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77
Q

What is response time with respect to CT detectors?

A

The spee with which the detector can detect an x ray event and recover to detect another event

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78
Q

Why is it important that we have a low response time in CT detectors?

A

-To avoid afterglow and detector pile-up

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79
Q

How long should the afterglow typically be in CT detectors?

A

0.01%-100ms after the radiation has been terminated

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80
Q

What is the dynamic range for msost CT scanners?

A

1 million to 1

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81
Q

What is total detector efficiency?

A

The product of capture efficiency, absorption efficiency and conversion efficiency

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82
Q

What is the scintilator crystal in detectors bonded to in CT scanners?

A

They are bonded to photodiodes

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83
Q

What 2 materials can photodiodes be made of in CT scanners?

A
  1. Cadminium tungstate and ceramic material made of high purity rare earth oxides
    OR
  2. Gadolinium oxysulfide with ultrafast ceramic
84
Q

What does the photodiode convert light to in the PMT?

A

It converts light into electrons

85
Q

T/F

CT scanners use a PMT

A

True

86
Q

What are the 2 catagories of multirow CT detectros?

A
  1. Matrix array detectors
  2. Adaptive array detectors
87
Q

What determines the number of slices and the thickness of each slice in CT scanners?

A

Determined by detector configuration

88
Q

Which type of detector has better SR; matrix array detectors or adaptive array detectors? Why?

A

Adaptive array detectors because each slice is smaller

89
Q

What type of multi-row detector has better signal; matrix array detectors or adaptive array detectors? Why?

A

Matrix array detector; because the detector elements are bigger

90
Q

What is the design of adaptive array detectors?

A

They are anisotropic (the cells are not equal, they have different sizes)

91
Q

What is the design of matrix array detectors?

A

They are isotropic (All cells are equal in all dimensions)

92
Q

T/F

CT scanners that are used currently use a detector, and an x-ray source that rotates around the patient.

A

True

93
Q

Where is the rotating detector and x ray tube located within current CT scanners?

A

They are located within the gantry

94
Q

What is the average size of the gantry?

A

70-90 cm

95
Q

What type of force has the frame of the gantry beeen designed to withstand?

A

The frame has been designed to withstand centrifugal forces

96
Q

What is the maximum tilt that a CT gantry can be angled?

A

Gantry angulation tilt may be up to +/- 30 degrees

97
Q

Where are the operator controls for the gantry located in a CT room?

A

1.On the gantry itself
2. In the control room

98
Q

How does the cooling system work with CT scanners?

A
  1. Draws in air from the room through filters to cool electrical components
  2. Warm air is expelled through filters at the top gantry
99
Q

What movements does the CT table/couch allow?

A
  1. Vertical movement
  2. Horizontal movement
  3. Indexing after every x-ray exposure or continiously at a steady pre-set rate
100
Q

What is the weight limit of the CT table/couch

A

450 lbs

101
Q

T/F

CT tables/couches must be made of a high atomic number material

A

False; must be made of a low atomic number material

102
Q

What is the orgin in CT scanners? What is another name for the orgin?

A

-The orgin is the point at which all 3 axes intersect
-Also called the isocenter

103
Q

What makes up the y-axis?

A

The patient’s body anterior to posterior

104
Q

What makes up the x-axis?

A

The width of the patient

105
Q

What makes up the Z axis?

A

The patients thickness from head to toe

106
Q

What determines the image reconstruction time in CT scanners?

A
  1. Microprocessors
  2. Primary memory
107
Q

What part of the computer is dedicated to pre-processing and image reconstruction?

A

The RAM (random acess memory)

108
Q

What are the cartesian coordinates for 1 and 2? What type of slice is this?

A
  1. Y
  2. Z
    -This is a sagittal slice
109
Q

What are the cartesian coordinates for 3 and 4? What type of slice is this?

A
  1. X
  2. Z
    -This is a coronal slice
110
Q

What is slice thickness?

A

-Z axis
-It is the x ray beam thickness in SDCT

111
Q

What does SDCT stand for?

A

Spectral detector computed tomography

112
Q

Where should our isocenter be when positioning a patient that is either supine or prone?

A

Halfway at the midsaggital/midcoronal plane

113
Q

How many pixels make up the matrix of a CT image?

A

512 x 512 pixels

114
Q

What is a voxel?

A

It is the volume element of a matrix that results in a cube

115
Q

What does each cube of a voxel represent?

A

Each cube represents a different slice of the image

116
Q

What is the SI unit for voxel size?

A

mm^3

117
Q

What determines the pixel size?

A

The X and Y coordinates

118
Q

What happens if FOV is increased for a fixed matrix size?

A

Decreased SR

119
Q

What is another name for a CT scout?

A

-Scantogram/topogram-scout

120
Q

T/F

Scouts have a higher dose than normal x-ray proceedures.

A

True

121
Q

How many images are taken in a CT scout?

A

2 images; AP and PA

122
Q

When is a scout taken and what is the purpose of acquiring these images?

A

-These images are taken prior to the CT acquisition
-These images allows for the operator to plan the extend of the scan and determine the FOV

123
Q

T/F

During a scout, the detector and the x-ray tube are moving.

A

False; they are stationary

124
Q

What determines the attenuation of the x ray beam along a given ray path? What is this known as?

A

-The sum of the linear attenuation coefficient of tissues
-This is called a line integral

125
Q

What determines how much attenuation occurs in each voxel?

A

Image reconstruction

126
Q

What was the mathematical method that Hounsfield developed for the first CT scanner called?

A

Algebraic reconstruction technique

127
Q

What algorithm does the algebraic reconstruction technique belong to?

A

It belongs to a class of algorithms referred to as iterative reconstruction algorithms

128
Q

What algorithm is used now days for image reconstruction?

A

Analytical ersconstruction algorithms
1. Filtered back projection algorithms
2. Interpolation algorithms

129
Q

What 4 angles are voxels collected at?

A
  1. 0 degrees
  2. 90 degrees
  3. 45 degres
  4. 135 degrees
130
Q

What 2 factors does iterative reconstruction compare?

A
  1. Assumed vales
  2. Measured values
131
Q

What method is ART an example of? Why?

A

ART is an example of iterative reconstruction method because it involves the repitition of a mathematical or comutational process

132
Q

What are the disadvantages of the interative reconstruction method?

A
  1. Takes longer to calculate
  2. Need to have a lot of computer power
133
Q

What are the advantages of the interative reconstruction method?

A
  1. Reduced image nose
  2. Allowance to use lower techniques (lower patient dose)
134
Q

What is the standard method of reconstructing CT slices?

A

Filtered-Back projection

135
Q

TF

The smearing back projection takes place across the image at the angle that it was acquired.

A

True

136
Q

How is the blurring inherent in the simple back projection supressed?

A

Through using filtering techniques (i.e. sharpening filter)

137
Q

What is another way to refer to a convolution filter?

A

-Kernel

138
Q

T/F

FBP is also reffered to as a convolution method.

A

True

139
Q

What does a smoothining filter do to SR and CR?

A

SR decreases, CR increases

140
Q

What does a sharpening filter do to SR and CR?

A

SR increases, CR decreases

141
Q

What is being shown here?

A

Ring artifact

142
Q

What does the graph represent?

A

The integrated attenuation profile

143
Q

T/F

Sharpening filters increase noise.

A

True

144
Q

For what type of anatomy would we use a sharpening filter?

A

When imaging lungs or bones and looking for small, discrete nodules or fractures

145
Q

For what type of anatomy would we use a smoothening filter?

A

For soft tissues such as the brain or abdomen to look for larger features with mild differences in attenuation

146
Q

Which kernel would you use where the inherent contrast is very high?

A

A sharpening/noisier filter

147
Q

Which kernel would you use where the inherent contrast is very low?

A

Softer kernels

148
Q

What is the defenition of interpolation?

A

A mathematical method of estimating the value of an unknown function using the known values on either side of that function

149
Q

How is interpolation performed?

A

Through a computer program called an interpolation algorithm that takes 2 known values, creates a line aand then chooses a value somwhere inbetween

150
Q

What controls interpolation?

A

Pitch

151
Q

What is pitch?

A

The ratio of the patient table movement (during one revolution) to the width of the x-ray beam

152
Q

If the distance translated by the table equals the slice thickness, what is the pitch ratio?

A

1:1

(same as saying that the pitch is 1)

153
Q

What does a pitch of 2:1 indicated?

A

Thtat the table will move twice the distanceof the slice thickness for each rotation of the gantry

154
Q

What does SDCT stand for?

A

Single detector CT scans

155
Q

In SDCT, what determines the beam width/slice thickness?

A

The width of the collimator

156
Q

In SDCT, what is pitch?

A

The relationship of the table speed to the slice thickness

157
Q

If the pitch is 2, and slice thickness is 5mm, what distance does the CT table travel during one gantry roation?

A

The table travels 10 mm every 5mm

158
Q

For a 4 slice MDCT at a 1.25mm slice thickness and table feed of 6mm per rotation, what is the pitch?

A

1.2

159
Q

What does a pitch of greater than 1 indicate when it comes to patient dose and imaging?

A

Results in extended imaging (missing anatomy) and reduced patient dose

160
Q

What does a pitch of less than 1 indicate when it comes to patient dose and imaging?

A

Results in overlatpping images (less missing anatomy) and a hgih patient radiation dose

161
Q

What is the amount of anatomy imaged in a helical scan with a 20 second acquisition time, a 1 second rotation time, and 2.5 mm slice thickness, 4 slices per rotation, with a pitch of 1.2? What if the rotation is .5 seconds?

A
  1. VI=240
  2. VI=480
162
Q

What is the x ray attenuation coeffficient of the volume of the tissue (voxel) displayed as?

A

Displayed as a level of brightness in its pixel based off of the calculated u

163
Q

What is the CT number of water?

A

0

164
Q

What is the CT number of fat?

A

-30HU

165
Q

What is the CT number of air?

A

-1000

166
Q

What is the CT number of soft tissue?

A

+50

167
Q

What is the CT number of bone/metal?

A

+1000

168
Q

T/F

The CT number of any given pixel is related to the u of all the tissues contained in that voxel

A

True

169
Q

If a tissue attenuates less than water, will it have a negative or a positive CT number?

A

Negative CT number

170
Q

T/F

If a slice is thick, there is less tissue within the voxel.

A

False; If a slice is thin, there is less tissue within the voxel.

171
Q

What is beam hardening due to?

A

The polychromatic beam

172
Q

What is beam hardening?

A

The gradual increase in effective energy of the polychromatic beam as they penetaate deeper into attenuating tissue

173
Q

What is cupping due to?

A

It is due to lower u values being reconstructed for deeper voxels, where the material appears darker in the center of the image

174
Q

What is being shown here?

A

Cupping artifact

175
Q

What is streaking due to?

A

High atomic number artifacts within the image

176
Q

What is CT simulation?

A

The use fo the CT scanner in radiation treatment planning

177
Q

What are the 3 main steps in CT simulation?

A
  1. Scan the patient in the CT scanner
  2. Virtual simulation
  3. Treatment setup
178
Q

What occurs during the step 1 of CT simulation?

A

-Patient positioned and scanned in the same position that would be on the treatment machine
-Image dataset computes dose distributions

179
Q

What occurs in stage 2 of CT simulation?

A

-Virtual simulation software takes the patient’s CT dataset and provides a virtual representation of that dataset in the radiation treatment machine

180
Q

This software is responsible for the contouring of the target and normal structures, placement of the treatment isocenter and the beams. What software is this?

A

Virtual simulation software

181
Q

T/F

Virtual simulation takes place within the time that the patient is present in the CT room.

A

False; takes place after the patient has already been scanned and left the room

182
Q

What occurs during the third stage of CT simulation?

A

The CT simulation software results are used to set up the patient in the treatment machine.

183
Q

What is quantitative CT (QCT)?

A

The most sensitive of all x-ray techniques for the measurement of the mineral content of trabecular bone in osteoperosis.

184
Q

Pic for understanding

A
185
Q

What are the pros of temporally sequential scans? (through entire volume)

A

Can be done on any CT scanner

186
Q

What are the cons of temporally sequential scans (through entire volume)?

A

Risk of patient motion

187
Q

What are the pros of temporally sequential scans (through each tube rotation)?

A

-Reduction in interscan delay between the low and high energy aqueisitions
-Can be done on any scanner

188
Q

What are the cons of temporally sequential scans (through each tube rotation)?

A

-Suseptibility to motion misregistration between the low and high energy scans may limit usefulness for imaging vascular anatomy

189
Q

What reconstruction method has the best temporal resolution?

A

Partial scan reconstructions

190
Q

What is temporally sequential scans through each tube rotation?

A

One tube rotation is performed at each tube potential prior to the table incrementation to minimize the time delay between scans

191
Q

What is temporally sequential scans through the entire volume?

A

Two temporally sequential scans performed to aquire data at 2 different tube potentials

192
Q

T/F

Materials having different elemental compoistions can be represented by the same CT numbers

A

True

(this makes classification of diff tissues difficult)

193
Q

What are the different methods for acquiring dual energy data?:

A
  1. Sequential acquisition of 2 different scans
  2. Ra[id tube potential sqitching
  3. Multilayer detectors
  4. Dual x-ray sources
194
Q

What are the pros of rapid kVp switching?

A

Near simultaneous data acquistion of low and high energy data sets

195
Q

What are the pros of multilayer detectors?

A

Low and high energy data sets are acquired simultaneously

196
Q

What are the cons to multilayer detectors?

A

To achieve noise in the low and high energy images, differnet detector thicknesses are used to keep the signal, and SR decreases

197
Q

What are the pros to dual energy sources?

A

-Tube current (and noise) can be optimized for each tube potential independently
-Filtration can be independently optimized for each tube-detector pair

198
Q

What are the cons to dual x-ray sources?

A

-90 degree phase shift between low and high energy data
-Scatter resulting from one tube may be detected by the detector of the other tube because both tubes are simultaneously energized

199
Q

What transition time is requrired for rapid kVp switching?

A

Less than a millisecond

200
Q

What are 3 cons to rapid kVp switching?

A
  1. Requires rapidly modulating tube current (mA)
  2. Lack of rapidly modulating mA would result in increased noise in the low KV data and excessive radiation dose levels from the high energy projections
  3. Difficult to optimize filtration

(read the rest of the cons)

201
Q

How do multilayer detectors work?

A

-One single high tube potential is used
-Scintillation detectors are layered, which causes the low E data to obe collected by the front/innermost layer
-High E data is collected by the back/outermost layer

202
Q

How do dual x-ray sources work?

A

-2 x ray sources and 2 detectors are mounted on the same gantry positioned 90 degrees from one another

203
Q

T/F

For dual x-ray sources, each x ray source is equipped with its own high V generator. Why or why not?

A

True; so that you can have independant control of kV, mA and be able to adujst the noise

204
Q

What is another way of saying that two objects are placed apart at 90 degrees?

A

Orthogonally positioned

205
Q

What are the 2 types of images that can be acquired from DECT?

A
  1. Virtual monoenergetic images
  2. Material decomposition images
206
Q

What are material decomposition images?

A

Mapping or removing substances of known attenuation characteristics, such as iodine, calcium & uric acid

207
Q

What is being shown here?

A

A portable CT machine

208
Q

Virtual non-contrast images (iodine removed, iodine concentration (iodine maps) calcium suppression (calcium removed) and uric acid suppression are all part of what type of CT imaging?

A

DECT

Medical Radiation Sciences 2 (13 decks)

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