Ouhsc - Ct Test 1

Front 1

What can CT do that x-ray cannot that greatly enhances the diagnostic quality of the image?

Back 1

Combine the capabilities of computers & x-rays to create 3D or multiplanar images

Front 2

What Greek word is 'Tomography' originated from?

Back 2

Tomos (meaning section)

Front 3

When were the first transverse axial tomography cross-section images obtained?

Back 3

1937

Front 4

Definition of Conventional Tomogram

Back 4

An image of a section of the patient that is oriented parallel to the IR created by the tube & IR moving simultaneously to blur out the anatomy over & under the anatomy of interest

Front 5

Limitations of Conventional Tomography

Back 5

- all unwanted areas/planes cannot be removed
- blurring of anatomy may cause a decrease in spatial resolution
- difficulty w/ contrast resolution w/ soft tissues

Front 6

What's the most common use for tomography now?

Back 6

Urography (ex IVP)

Front 7

Digital Tomosynthesis

Back 7

- computer assisted tomographic images
- uses several static exposures taken at different angles
- produces images by reconstruction of image data by post acquisition algorithms

Front 8

Why is digital tomosynthesis better than conventional tomography?

Back 8

It allows for image modification & can create image reconstruction of any plane

Front 9

What are the main advantages of CT over conventional radiography (diagnostic x-ray)?

Back 9

- elimination of superimposed structures
- ability to differentiate small differences in density
- better quality images

Front 10

What does CAT stand for?

Back 10

Computerized Axial Tomography

(only able to scan axial plane)

Front 11

What does each cross-sectional slice on a CT represent?

Back 11

A specific plane in the patient

Front 12

The thickness of the cross-sectional slice is referred to as its __________

Back 12

Z axis

Front 13

Pixels

Back 13

Individual elements of a matrix which is considered as a whole when developing an image

Front 14

How many dimensions does a pixel have?

Back 14

2

x axis (width)
y axis (height)

Front 15

How many dimensions does a voxel have?

Back 15

3

X, Y, & Z Axis

Front 16

Attenuation

Back 16

The degree to which an x-ray beam is reduced by an object

Front 17

Beam absorption increases in _________ (denser/less dense) tissue resulting in _______ (more/fewer) photons hitting the detectors, which produces __________ (higher/ lower) signals

Back 17

Denser; Fewer; Lower

Front 18

X-ray beam quality is determined by...

Back 18

beam's energy (keV)

Front 19

X-ray beam quantity is determined by...

Back 19

mAs (milliampere per second)

Front 20

Which beam energy is ideal, homogenous or heterogenous?

Back 20

Homogenous (actual beam energy is heterogenous)

Front 21

How can filtration help make the beam energy more homogenous?

Back 21

Uses different materials of differing densities that allows transmission of high energy photons but fully absorbs low energy photons (reduces radiation dose)

Front 22

What happens when there's attenuation of a homogenous x-ray beam?

Back 22

- number of x-ray photons decreases in a linear manner
- energy of beam remains constant

Front 23

What happens when there's attenuation of a heterogenous x-ray beam?

Back 23

- low photons absorbed first, results in uneven energy
- overall quality (energy) of the beam changes as a result of the low energy photon attenuation resulting in a higher average energy of the beam
- can detect a beam average (mean) based on all energies involved

Front 24

As tissue thickness increases, the number of photons of the heterogenous beam ___________ (increases/decreases) through attenuation & the mean energy of the x-ray beam _____________ (increases/decreases)

Back 24

Decreases; Increases

Front 25

Photons that pass through objects unimpeded are represented by a _________ (black/white) area on the image

Back 25

Black

Front 26

When photons are completely absorbed by an object, the corresponding area on the image is __________ (black/white)

Back 26

White

Front 27

The number of photons that interact depends on the object's.......

Back 27

Thickness / Density / Atomic Number

Front 28

How does a CT scanner prevent superimposition of the anatomy?

Back 28

Uses quantitative measurements and is able to tell how thick or how dense an individual structure is, as well as detect structures that would superimpose it

Front 29

Air in the lungs is considered what type of attenuation?

Back 29

Low attenuation (gives off high signal)

Front 30

Hounsfield Units (CT Numbers)

Back 30

- quantify the degree of beam attenuation
- important for describing & assigning linear attenuation coefficients for individual tissues (translated into CT numbers)

Front 31

Order of HU Tissues from +1000 to -1000

Back 31

Bone (+1000) / Blood / Brain Matter / Water / Fat / Air (-1000)

Front 32

What structure would produce a +2000 HU?

Back 32

Metallic pacemakers

Front 33

Volume Averaging

Back 33

Process in CT by which different tissue attenuation values are averaged to produce 1 less accurate pixel reading

Affected by slice thickness; great impact on how voxel is displayed

Front 34

CT Numbers & subsequently shades of gray for individual pixels are determined by...

Back 34

The average of attenuation for that pixel or voxel area

Front 35

Are pixels always square?

Back 35

No; matrices aren't always squares

Front 36

Matrix Size

Back 36

Number of pixels in matrix

Common Sizes: 256x256, 512x512, 1024x1024 pixels

Front 37

Larger matrices containing more pixels will have ______ (high/low) resolution

Back 37

High

Front 38

Raw Data

Back 38

All the data collected (scan data); measurements taken from the detectors

Requires more memory for storage than Image Data

Front 39

Image Data

Back 39

Once the raw data have been segmented & averaged so that each pixel has an associated HU

Requires 1/5 of computer storage space as does raw data

Front 40

Step-and-Shoot Scan Mode ("Axial" or "Slice by Slice")

Back 40

- individual, distinct acquisition sequence
- involves a "start", tube goes around & makes measurements, tube stops & table moves
- still used today for some head CT scans

Front 41

Helical Scan Mode ("Spiral" or "Volumetric")

Back 41

- most commonly used today
- constant tube rotation & activation accompanied by constant table movement
- think of a Slinky shape

Front 42

Imaging plane that divides body into right & left sections is called....

Back 42

Sagittal

When R & L sections are of equal size, referred to as mid-sagittal or median plane

Front 43

The process of CT image creation can be broken down into 3 general segments:

Back 43

1) Data Acquisition
2) Image Reconstruction
3) Image Display

Front 44

Basic Scheme of Data Acquisition

Back 44

Production of x-ray beam, beam emitted from tube, beam geometry, passes through patient's body, attenuated, remaining photons strike detector

Front 45

Beam Geometry

Back 45

Path the x-ray beam takes after it exits the x-ray tube

Front 46

2 Major Components of Data Acquisition

Back 46

Gantry & Patient Table

Front 47

Gantry

Back 47

- ring-shaped part of scanner
- contains most functional parts of scanner
- have exterior controls to move patient table & control gantry tilt

Front 48

Slip Rings

Back 48

- permits gantry frame to rotate continuously (makes helical scans possible)
- 2 designs: cylinder & disk
- has higher resolution & accuracy
- good for biopsies

Front 49

Generator

Back 49

- produce high voltage for the creation of x-ray photons
- CT machines use high frequency generators
- kVp is responsible for determining energy & speed that electrons move

Front 50

CT Cooling Systems

Back 50

- 99% of x-ray energy is converted to heat
- newer CT machines have HU capabilities in the millions, have efficient cooling systems
- older machines had longer exam times, ran out of HU 'space' (sometimes exams had to be stopped to cool machine off)

Front 51

Heat Units (HU)

Back 51

- calculated as: mAs x kVp x 1.45 = HU
- 1.45 used for high frequency generators

Front 52

How are HU calculated for helical scans?

Back 52

Use mA & the total scan time to determine the mAs, then use normal equation

Tube is always on during entire scan, doesn't have chance to cool between scans

Front 53

What's the most common type of filter used in CT machine?

Back 53

Bow tie filter

Front 54

Which axes do the collimators in the CT machine line up with?

Back 54

X & Z axes

Front 55

Where is the first set of collimators located on the CT tube?

Back 55

Inferior to the x-ray tube; shapes the beam to cover only the thickness of the slice which the technologist has set

Reduces penumbra; pre-patient collimation

Front 56

Where is the second set of collimators located on the CT tube?

Back 56

Above the detectors; reduces amount of scatter radiation

Pre-Detector/Post Patient Collimation; controls slice thickness of attenuated beam

Front 57

2 Types of CT Detectors

Back 57

Xenon Gas Ionization Chamber
Solid-State Crystal Variety (scintillation detectors)

Front 58

Newer CT Detector Technology

Back 58

Capable of determining the number of x-ray photons that hit it & the energy of the photons (quality)

Front 59

Ideal CT Detector Characteristics

Back 59

Efficiency at capturing, absorbing, & converting x-ray photons to usable signal / stability / rapid response time / large dynamic range / small size / durable

Front 60

Xenon Gas Ionization Chamber

Back 60

- gas ionization detectors use small compartments filled w/ xenon
- directly produces an electric signal current (no intermediate process); ionization produces electric signal
- introduced w/ 3rd Generation Scanners
- detector chamber contains charged metal plates (+ and - opposing each other) that collect the ejected electron

Front 61

How does the x-ray photon ionize xenon?

Back 61

Removes an electron from an orbital shell

Front 62

Advantages of Xenon Gas Ionization Chamber

Back 62

- no afterglow
- very stable
- most sensitive to x-rays along long axis of chamber, which creates its own post-patient collimation
- cheaper

Front 63

Afterglow

Back 63

Continuation of light emission after radiation has stopped

Front 64

Disadvantages of Xenon Gas Ionization Chamber

Back 64

- wears out quicker
- not as efficient
- requires higher amounts of radiation to produce signal

Front 65

Scintillation Detectors

Back 65

- ceramic crystal coupled to a photomultiplier tube or photodiode
- x-ray photon strikes crystal, emits a flash of light (scintillation); a response to ionization
- tube or diode increases intensity of light & converts it into an electric analog signal, which becomes digitized

Front 66

What type of crystals did early scintillation detectors use?

Back 66

Sodium Iodide (NaI); left a slight afterglow

Front 67

What type of crystals are used in today's detectors?

Back 67

Calcium Tungstate; leaves little afterglow

Front 68

Scintillation Detector Characteristics

Back 68

- photodiode operates 0.5-250 nanoseconds
- 99% efficiency at converting captured x-rays into signal
- sensitive to temp & moisture
- highly efficient
- Cons: expensive

Front 69

Plug-In Detector Modules

Back 69

- detectors arranged into clusters or modules to improve replacement of detectors (i.e. 40 solid state detectors in 1 module)
- 4th gen scanners can have as many as 4800 detectors (cost a lot)

Front 70

Data Acquisition System (DAS)

Back 70

- measures photons that pass through the patient & strike the detectors
- converts analog to digital signal
- located inside gantry near detectors

Front 71

Scanner Generation

Back 71

- configuration of x-ray tube to detector
- 3rd generation is most widely used today

Front 72

1st & 2nd Generation CT Machines

Back 72

- Translate/Rotate Movement
- very small x-ray beam used to gather info in 1st, fan shaped beam in 2nd

Front 73

3rd Generation CT Machine

Back 73

- rotate/rotate movement; tube & detectors move in unison around patient
- fan beam w/ detector array, eliminates translate portion of x-ray beam acquisition
- uses slip ring technology, able to do helical & volumetric scans
- multislice detectors
- high heat capacity anode

Front 74

4th Generation CT Machine

Back 74

- rotate only; tube rotates within detector ring
- fan beam, detectors surround perimeter of gantry opening & remain stationary; slip ring technology used
- used for first hybrid PET/CT scanners

Front 75

4th Generation - Nutating Ring

Back 75

- x-ray tube located outside ring of detectors
- x-ray tube angled inward to aim beam at detectors, located farther away from patient, reducing radiation dose
- moves ring out of the way of tube so x-ray beam doesn't pass through ring on its way to patient; appears to 'wobble'

Front 76

5th Generation CT Machine

Back 76

- stationary only; electron beam scanning
- electron gun produces electrons which are focused & deflected to strike stationary tungsten targets
- scan time 0.05 seconds; may be ECG triggered

Front 77

6th Generation CT Machine

Back 77

- dual source CT; uses 2 tube/detector array arrangements offset at 90 degrees from each other
- 0.30 sec/slice acquisition time
- cardiac scanners have up to 320 slices

Front 78

Incrementation/Feed/Step/Index

Back 78

Process of moving the table within the gantry by a specified measure

Front 79

Scannable Range

Back 79

Degree to which a table can move horizontally

Front 80

What is referred to with the phrase "referencing the table"?

Back 80

When the table readout is set to zero; when the anatomical landmark on the patient is recorded

Front 81

How are computer systems utilized in radiology?

Back 81

- used for patient registration & info
- imaging processes
- reduction in film storage costs
- quick retrieval & shared info

Front 82

Essential Elements of Computer System

Back 82

Hardware, software, people (user)

Front 83

Supercomputers

Back 83

- can do large numbers of calculations quickly
- operated by large corps or governments

Front 84

Mainframe Computers

Back 84

- large networks, high level of input & output
- used by universities or corporations

Front 85

Minicomputers

Back 85

- primarily what's needed to make a CT machine run
- used to perform multiple complex computations w/ high level of input & output

Front 86

Microcomputers

Back 86

- personal computers
- ex. laptops, desktops

Front 87

Computer Hardware

Back 87

- hard disk
- input devices (keyboard)
- output devices (monitor)
- CPU

Front 88

Computer Memory - ROM (Read-Only Memory)

Back 88

- data that needs to be used for running the computer but shouldn't be changed at the risk of having the computer malfunction
- should only be read, not rewritten

Front 89

Computer Memory - RAM (Random-Access Memory)

Back 89

- temporary data storage used for data manipulation by the computer software
- when power is lost or computer is restarted, RAM is erased

Front 90

Computer Memory - WORM (Write-Once Read Many Times)

Back 90

- data written to devices like CDs or DVDs that may not be rewritten
- used to distribute or share image data externally

Front 91

Image Reconstruction

Back 91

Process of using raw data to create an image

Front 92

Prospective Reconstruction

Back 92

Automatically produced during scanning

Front 93

Retrospective Reconstruction

Back 93

Using same raw data later to create a new image

Front 94

After saving the image data, can you revert back to raw data at a later date?

Back 94

No; only saved image data can be used for further reconstruction

Front 95

Algorithms

Back 95

Finite set of unambiguous steps performed in a prescribed sequence to solve a problem

There can be no vagueness in the rules / Rules must be simple & well defined / Must have specific beginning & end

Front 96

Fast Fourier Transform (FFT)

Back 96

Method of analyzing large volumes of data by separating it into several parts, manipulating it, then putting it back together for analysis

Front 97

Interpolation

Back 97

Mathematical method used during reconstruction as a "best guess" for areas where info may be missing, incomplete, or unclear

Front 98

Ray Sum

Back 98

Measurement of the amount of beam attenuation that the DAS "reads" for each arriving ray

Front 99

Attenuation Profile

Back 99

- created for each view; represents the attenuation of beam that occurs as it passes through the anatomy from that perspective
- each profile from each perspective is converted to a back projection by algorithms that manipulate data & apply to matrix

Front 100

What does a back projection alone produce?

Back 100

- streaks or artifacts
- give a round structure, a star-burst appearance
- edges aren't accurately represented

Front 101

Filter Functions - Image Reconstruction

Back 101

- minimizes streaks from back projection
- done by complicated mathematical steps (reconstruction algorithm)
- may use FFT as a way to speed up processing of large amounts of data

Front 102

Convolution

Back 102

Applying a filter to an attenuation profile

Front 103

What type of data can filter functions be applied to?

Back 103

Only Raw data

Front 104

Reconstruction Algorithm vs. Window Setting

Back 104

- changing algorithm changes way raw data is manipulated to reconstruct image
- contrasts w/ changing window setting, which just changes the way the image is viewed

Front 105

Does changing the window setting represent an algorithm change?

Back 105

No, it represents the gray scale used for image display

Front 106

Adaptive Statistical Iterative Reconstruction

Back 106

- newer method of image reconstruction
- computes projections from image, compares it w/ original projection data, updates image based on difference
- can reduce image noise
- shown to reduce radiation dose by 50%

Front 107

Scan Field of View (SFOV)

Back 107

- determines area within gantry from which raw data are acquired
- in the isocenter of the gantry
- determines number of detector cells collecting data
- too large SFOV takes up lots of space & slows down image reconstruction time

Front 108

What should the FOV include?

Back 108

All of the pertinent anatomy

Front 109

Display Field of View (DFOV)

Back 109

- determines how much of collected raw data are used to create image
- also called zoom or target (similar to zoom on camera)
- changes affects image quality by changing pixel size
- cannot be larger than SFOV
- correct selection helps improve vision of abnormalities

Front 110

Zoom

Back 110

- improves image size w/o degrading spatial resolution
- uses raw data to reset the scan FOV & apply the matrix to only that area, resulting in small pixel size

Front 111

Magnification

Back 111

- increases image size but degrades spatial resolution & distorts anatomy
- uses image data
- will increase pixel size (decreases resolution)

Front 112

Image Center

Back 112

- image coordinates allow operator to specify area within SFOV that will be displayed on image

Front 113

2 Types of Display Devices

Back 113

Cathode-Ray Tube (CRT) [not many remain]
- heavier, bulkier, hotter, less durable
Some form of flat panel (ex. TFT, LCD)

Front 114

What type of camera do most modern CT systems use?

Back 114

Laser cameras (bypass video system entirely); most require no external processing ('dry' type)

Front 115

Windowing in CT - Definition

Back 115

- involves used Hounsfield scale as a base around which to define contrast & density parameters to be used for image display

Front 116

Window Width

Back 116

- determines NUMBER OF SHADES OF GRAY
- over 4000 shades of gray available on most modern CT scanners

Front 117

Window Level

Back 117

- middle shade of gray within window width
- like to target around main tissue of interest

Front 118

Narrow Window Width = ________ Contrast

Back 118

High (good for brain, bad for abdomen)

Front 119

Why can't images be displayed with a different shade of gray for all 2000 HU?

Back 119

Most monitors only display 256 shades of gray

Human eye can only differentiate a fraction of those shades (40 shades)

Front 120

Gray Scale

Back 120

- used to display CT images; higher density tissue appears lighter & lower density tissue appears darker
- assigns a certain number of HU to each shade of gray (higher HU = lighter shades of gray)

Front 121

Window Width & HU

Back 121

Window width determines the number of HUs represented on a specific image (higher values = white, lower values = black)

Front 122

Increasing the window width assigns _________ (more/less) HU to each shade of gray

Back 122

More

Front 123

Window width selects the ___________ of HU
Window level selects the _________ of HU

Back 123

Quantity
Range

Front 124

Window Level & HU

Back 124

Window level determines which HUs are displayed as shades of gray

Also called window center

Front 125

The window level should be set at a point that is roughly the same value as....

Back 125

The average attenuation number of the tissue of interest

Front 126

When are wide window widths (500-2000) used?

Back 126

Imaging tissue type that varies greatly (abdomen)

Decreases image contrast; suppress display of noise on image

Front 127

When should narrow window widths (50-350) be used?

Back 127

When the area of interest has small attenuation differences between tissues (brain, liver)

White Matter: 20-35 HU
Gray Matter: 35-45 HU

Front 128

Preset Windows

Back 128

- set up during initial setup of CT scanner or after major upgrade
- used as guideline only (think Pirates of the Caribbean...)
- most current machines will automatically set window parameters based on radiologist's preference

Front 129

Region of Interest (ROI)

Back 129

- most often circular, may be elliptic, square or rectangular
- helps measure density of specific areas on an image
- more accurate for measuring tissue density in an arc rather than a single point/pixel

Front 130

HU Measurement & Standard Deviation

Back 130

- SD helps us determine degree of variation in the pixels within ROI
- high SD = large degree of variation in tissue density within ROI
- HU measurement may be affected by volume averaging or image noise