Chapter 13 Ct Bushberg

Front 1

What do 2D picture elements correspond to?

Back 1

3D voxel elements

Front 2

What does each pixel in a CT image display?

Back 2

Average xray attenutation properties of the tissue in the corresponding voxel

Front 3

What is a ray?

Back 3

Single transmission measurement through the patient made by a single detector at a given moment in time

Front 4

What is a projection or view?

Back 4

Series of rays that pass through the patient in the same orientation

Front 5

What type of geometry to most CT scanners use?

Back 5

fan beam geometry for the acquisition and reconstruction process

Front 6

What is the purpose of a CT scanner?

Back 6

To aquire a large number of transmission measurements through the patient at different positions

Front 7

What is the formula for the xray intesity of the atteunated xray beam?

Back 7

It=Ioe^(-ut)
ln (Io/It)=ut
With a preprocessing step, t cancels out and this reduces the dependency of CT image on machine dep parameters (Io and It)

Front 8

What is used to produce CT images?

Back 8

CT reconstuction algorithm; filtered backprojection is most common

Front 9

How does backprojection work?

Back 9

the u value for each ray is smeared along the same path in the image of the patient-->areas of high/low attenuation reinforce each other-->builds up image in the computer

Front 10

Describe data aquistion and process of computing a CT image:

Back 10

Make transmission measurements through the object at multiple angles-->reverse aqusition geometry and backproject on a digital matrix

Front 11

Describe the first generation of CT scanners:

Back 11

Rotate/translate pencil beam system (only 2 detectors measure xrays through 2 different slices): Xray tube and single detector (per slice) translates across the FOV (160 rays across 24 cm)-->produces a series of PARALLEL rays-->system rotates slightly-->tranlates back across at a slightly different angle-->repeat at 1 degree intervels across 180 degrees

Front 12

Advantages and disadvantages of pencil beam technology of first generation CT scanners

Back 12

Advantages: efficient scatter reduction, b/c scattered beam deflected away from the pencil beam is not measured by a detector

Front 13

Describe second generation CT scanners:

Back 13

-Fan beam technology with an array of 30 detectors
-increases scatter compared to pencil beam technology
-Narrow fan angle of 10 degrees
-In theory, 30x reduction in scan time expected-->not realized b/c ↑ data point acquistion-->↑ image quality

Front 14

What kind of geometry does projection radiography use?

Back 14

Open beam geometry

Front 15

Describe 3rd generation CT scanners:

Back 15

Rotate/Rotate system:
-↑ to >800 detectors
-↑ angle of fan beam arc to interrogate entire span the patient-->eliminates translational motion
-mech. joined xray tube and detector array rotate together around patient w/o translation
-↓ scan times

Front 16

Compare scan times:
1. 1st gen
2. 2nd gen
3. 3rd gen
4. 4th gen

Back 16

1. 270 sec
2. 18 sec
3. 5 sec
4. 1/2 sec

Front 17

Disadvantge of third generation CT scanners:

Back 17

Ring artifacts

Front 18

what causes ring artifacts of 3rd generation scanners?

Back 18

-In rotate/rotate geometry-->each detector corresponds to a "ring" on an image
-central detectors provide info about small diameter rings
-peripheral detectors about large diameter rings
-vertex of "fan" is xray focal spot
-if detector is miscalibrated (electronic drift), tainted data leads to ring artifact b/c there are differences in gain b/w detectors so (g1 should = g2 but if miscalibration then g1≠g2
-even slight imabalances affect the ut that is backprojected-->ring artifact

Front 19

How do 4th generation CT scanners overcome problems with ring artifact of 3rd generation scanners?

Back 19

Rotate/stationary system:
Detectors are in a stationary 360 ring around the patient-->requires more detectors

Front 20

Describe the difference b/w source fan of 3rd generation CT scan and detector fan of 4th generation CT scan:

Back 20

Source fan-xray tube is the apex of the fan (source fan)
Detector fan-normalize the data so the apex of a fan is at the individual detector

Front 21

Describe the difference in data acquistion b/w 3rd and 4th generation CT scanners:

Back 21

3rd generation: fan data acquired by detector array simultaneously
4th generation: fan beam data acquired over time needed for xray tube to rotate through the arc angle of the fan each detector acts as its own reference detector

Front 22

In 4th generation CT scanners what is the significance of each dtector acting as its own refence detector?

Back 22

Each detector has its own gain; g cancels out in the equation and eliminates ring artifact

Front 23

Describe the 5th generation CT scanner:

Back 23

Stationary/Stationay:
-used in cardiac cine
-large tungsten arc encircles the patient-->lies opp. the detector ring
-high energy e- ring strikes tungsten-->produces xrays
-50msec scan times

Front 24

In the 1990's what did the design of the 3rd and 4th generation scanners incorporate?

Back 24

slip ring technology so gantry can rotate continously w/o tethering from wires

Front 25

Describe 6th generation CT scanners:

Back 25

-helical CT
-gantry does not have to be stopped after each slice is acquired b/c table moves continously
-sometimes entire scan can be done in one breath hold

Front 26

Describe 7th generation CT scans:

Back 26

-Multiple detector array
-WIDER collimator spacing
-more xrays produced by xray tube are used to produce an image

Front 27

What does the use of multiple detector array overcome due to opening up the collimator

Back 27

-In conventional, single detector array ↑ collimator, ↑ slice thickness-->↑ utilization of xray beam but ↓ spatial resolution
-with multiple detector array-->slice thickness determined by detector size and not by collimator

Front 28

Describe xenon detectors:

Back 28

High pressure xenon gas in long thin cells b/w two metal plates

Front 29

How do you overcome detector -inefficiency of a gas detector compared to a solid one?

Back 29

-Make it very thick (>6 cm) to compensate for low density
-Make metallic septa very thin to reduce dead space--> ↑ geometric efficiency

Front 30

What is geometric efficiency?

Back 30

fraction of primary xrays exiting a patient that strike an active detector

Front 31

Why must xenon detectors be positioned in a fixed orientation wrt xray source?

Back 31

Long, thin ionization plates of xenon detecor are highly directional

Front 32

Why can xenon detectors not be used in fourth generation CT scanners?

Back 32

xenon detectors must be positioned in a fixed orientation wrt to x-ray source; so only used in third generation systems

Front 33

How do xenon detectors work?

Back 33

voltage is applied across electrodes; xrays interact with the xenon atoms and cause ionization-->ions move to the electrodes-->electronic signal is amplified and digitized-->numerical value is directly proportional to x-ray intensity striking the detector

Front 34

Describe the physical makeup of a solid state detector

Back 34

scintillator coupled tightly to a photodetector (photodiode)

Front 35

How does a solid state detector work?

Back 35

scintillator emits visible light when stuck by xrays-->light reaches the photodiode

Front 36

What type of scintillators are used in solid state CT detectors?

Back 36

CdWO4, yttrium, gadolinium

Front 37

Why do solid state CT detectors have better xray absorption than xenon detectors?

Back 37

Increased atomic number

Front 38

What somewhat reduces geometric efficiency of detector elements?

Back 38

Small gaps b/w detector elements put there to reduce "crosstalk" b/w detector elements

Front 39

What makes solid state CT detectors capable of xray detection over a wide range og angles?

Back 39

Top surface of solid-state CT detectors are essentially flat (unlike xenon detectors)

Front 40

What type of scanners are solid state detectors used in?

Back 40

4th and higher generation 3rd

Front 41

What is a multiple detector array?

Back 41

Assembly of multiple solid-state detector array modules

Front 42

How do you practically adjust the slice width with a multidetector array system?

Back 42

Grouping one or more detector units together; to combine the signal-->detectors are wired together

Front 43

Why do multiple detector array scanners make use of third generation geometry?

Back 43

Fan angle in 3rd gen CT scanners is 60 degrees and in 4th gen. is 360 degrees so would need a lot more (60x) as many detectors as in a third generation system

Front 44

What determines slice thickens in a single detector array CT system?

Back 44

Physical collimation of xray beam with two lead jaws; as gap b/w lead jaws widens, slice thickness increases

Front 45

What places an UPPER LIMIT on slice thickness in a single detector array system?

Back 45

Width of detectors in the single detector array

Front 46

What are the advantages of increasing slice thickness?

Back 46

1. # of detected photons increases linearly with increased slice thickness
2. Increases SNR (contrast resolution)

Front 47

What are the disadvantages of increasing slice thickness?

Back 47

Decreases spatial resolution; thin slices improve spatial resolution and decrease partial volume averaging

Front 48

What will partially compensate for loss of xray photons due to thin slices?

Back 48

Increasing the mA

Front 49

What is the slice sensitivity profile?

Back 49

Contrast decreases as object moves to the edge of a a slice;

Front 50

In single detector array, what is the shape of slice sensitivity profile due to?

Back 50

-finite width of the xray focal spot
-penumbra of the collimator
-projection angle imaging

Front 51

Why does helical scanning have a slightly broader slice sensitivity profile?

Back 51

due to translation of the patient during the scan

Front 52

In a multidetector array scheme, what is slice thickness determine by?

Back 52

Width of detectors in the slice thickness dimension

Front 53

How do you change the width of detectors in multidetector array scheme?

Back 53

binning different numbers of detector elements together (ie summing electronic signals of adjacent detector elements)

Front 54

In conventional 4 slice axial scanning (ie not helical) describe what happens to 2 the slices at the edge of the scan?

Back 54

inner edge of each slice is determined by the inner edge of the detector; outer edge determined by the collimator penumbra or the outer edge of the detector depending on the adjustment of the collimator

Front 55

In helical imaging, what is te difference in detector array contribution?

Back 55

-Each detector array contributes to the image reconstruction
-slice sensitivity profile for each detector array needs to be similar to prevent artifact
-do this by adjusting collimation so that focal spot-collimator blade penumbra falls outside the edge of the detectors-->leads to higher radiation with but reduces artifact

Front 56

When is pitch used?

Back 56

helical scan protocols

Front 57

What is the formula for pitch for a single detector array?

Back 57

Collimator pitch=table movement (mm) per 360 degree rotation of the gantry/collimator width (mm) at the isocenterW

Front 58

What does pitch influence?

Back 58

-radiation dose to the patient
-image quality
-scan time

Front 59

What does a pitch of 1 mean?

Back 59

number of CT views acquired when averaged over the long axis of the patient is comparable to the number acquired with continguous axial CT

Front 60

What does a pitch of <1 mean?

Back 60

slight improvmenet in image qulaity and higher radiation dose to the patient;

Front 61

What are common pitches used?

Back 61

pitches up to 1.5 commonly used-->manufacturers spend time trying to develop pitch >1

Front 62

What is the significance of a pitch >1?

Back 62

-Some degree of partial scanning of the patient
-faster scan time, less motion, less radiation
-sometimes less radiation

Front 63

What is the minimum requirement to produce an adequate CT image?

Back 63

180 degrees + fan angle

Front 64

What is the formula for detector pitch and collimator pitch for multiple detector array?

Back 64

Detector pitch=table movement (mm) per 360 degree rotation of the gantry/detector wideth (mm)

Collimatory pitch=Detector pitch/N where N is number of detector arrays

Front 65

What is a sinogram?

Back 65

way data acquired for one CT slice can be displayed before reconstruction
Y axis: views (each projection angle), X axis: rays (different rays in each projection)

Front 66

What would a bad detector in a third-generation scaneer show up as on a sinogram?

Back 66

Vertical line

Front 67

How do you calculate the number of data point?

Back 67

Rays/view x view

Front 68

What does each affect?
a. number of rays used to reconstruct a CT image
b. number of views

Back 68

a. radial component of spatial resolution (goverened mostly by spacing and wideth of ray data)
b. circumferential component of spatial resolution (number of views used to reconstruct the image)

Front 69

What happens to images if you reduce the ray sampling?

Back 69

low resolution, blurred images

Front 70

How do you compensate for decreased number of rays?

Back 70

Increase detector aperture size-->blurs images

Front 71

Artifact created by too few veiws?

Back 71

View aliasing-->exacerbated by objects with sharp edges

Front 72

What happens to data scquired by a CT scanner before reconstruction?

Back 72

preprocessing:
1. calibration data from air scans
2. Geometric efficiencies correction

Front 73

What dies the reconstructed value in each pixel correspond to?

Back 73

liner attenutation coefficient for the corresponding voxel

Front 74

What does helical imaging allow compared to conventional axial scanning?

Back 74

-CT images can be reconstructed at any position along the length of the scan to w/i 1/2 pitch (slice thickness of each edge of the scanned volume)
-production of additional overlapping images with no additional dose to the patient (interleaved images)

Front 75

What is the significance of interleaved images?

Back 75

-allows placement of additional images along the patient so that the exam is almost uniformly sensitive to subtle abnormalities.
-but increases reconstruction time

Front 76

What should you not confuse with the ability to reconstruct CT images at short intervels (with helical CT) compared to axial resolution

Back 76

Just b/c 5 mm images can be reconstructed every 1mm, does not mean 1 mm spatial resolution is achieved

Front 77

What is simple backprojection

Back 77

Mathematical process based on trig designed to emulated the acquisiton process in reverse; each ray in each view represents an individual measurement of u

Front 78

Describe the process of simple backprojection:

Back 78

Empty image matrix (all pixels set to zero)-->u value from each ray in ALL views smeared or backprojected onto image matrix-->so u is added to each pixel in a line through the image corresponding to the ray's paths

Front 79

What is a limitation of simple backprojection

Back 79

Characteristic 1/r blurring as a by product

Front 80

How do you solve the characteristic blurring of simple backprojection?

Back 80

filtering step called filtered backprojection

Front 81

What is filtered backprojection?

Back 81

mathematically reverses image blurring and restores image to an accurate representation of the object that was scanned

Front 82

What does the mathematical filtered step involve?

Back 82

Convolving the projection data with a convolution kernal

Front 83

What is a convolution kernal?

Back 83

shape of the filter function in the spatial domain (where the filtered step occurs); different kernal used in soft tissue and bone imaging etc

Front 84

What do you use to convert a function expressed in the spatial domain (mm) into the spatial frequency domain (cycles/mm)

Back 84

fourier transformation

Front 85

What type of filter compensates for the 1/r blurring function?

Back 85

Lak filter or ranmp filter b/c 1/r blurring function in spatial domain becomes 1/f blurring function in the freq. domain and L(f)=f therefore 1/f x f=1

Front 86

When does the Lak (ramp) filter work the best?

Back 86

When there is no noise in the data but there is always xray quantum noise so if you use the Lak filter image will be very noisy

Front 87

What filter is used to reduce high requency noise in a final CT image?

Back 87

Shepp-Logan filter-->incorporates some roll-off at higher frequencies and decreases high frequency noise

Front 88

What has even better high frequency noise suppression from the Shepp-logan filger?

Back 88

Hamming filter has even more pronounced high-frequency roll-off

Front 89

If you reduce noise (greater roll-off at high frequencies) what do you sacrifice?

Back 89

decreased spatial resolution, decreased noise-->used to image soft tissues (liver)

Front 90

What does kind of kernal bone imaging use?

Back 90

Less high frequency roll off-->increased noise, increased spatial solution

Front 91

what is the formula to convert each number in a CT pixel CT (x,y)?

Back 91

CT (x,y)=[1000 * u (x,y)]-uwater/uwater

Front 92

What does each correspond to:
1. u (x,y)
2. uwater
3. CT (x,y)

Back 92

1. floating point number of the (x,y) pixel before conversion
2. attenutation coefficient of water
3. Hounsfield unit

Front 93

What is the value of uwater for xray energies usually used in CT?

Back 93

0.195 (almost o)

Front 94

What do CT numbers and CT images derive contrast form?

Back 94

-physical properites of tissues that influence Compton scatter
-Ex Ct images made with highly filtered, high KV energy beam 75keV: for muscle 91%, fat 94%, bone 74% interactions are compton scattering

Front 95

What physical properties does compton scatter depend on?

Back 95

-Density (linearly related to u)-->plays the dominant role
-electron density (depends on Z/A) and hydrogenous tissues are visualized better

Front 96

What are some ways to postprocess CT images?

Back 96

1. widow/level
2. Multiplaner reconstruction (ex sagittal and coronal reformats)

Front 97

Why do you have to window and level?

Back 97

CT images possess 12 bits of gray scale (4096 shades of gray) and laser imagers used for filming CT studies has 8 bits--> window/level b/c 12 bit images must be reduced to 8 bits

Front 98

What does the window width mean?

Back 98

Shades of gray

Determines the contrast-->narrower window=greater contrast

Front 99

What is the level?

Back 99

Center of the shades of gray

Front 100

How do you make sagittal and coronal multiplaner reformatted images?

Back 100

combine the x and y axis data along the z axis-->mismatch in spatial resolution occurs-->compensates for this by interpolation-->MPR images maitain proper aspect ratio-->but cannot improve image resolution in the z direction-->leads to blur on this axis

Front 101

How can you improve blur in the z direction of MPR reformatted images?

Back 101

thinner slice thickness down to 1 mm

Front 102

What are the 2 types of 3D reconstructions?

Back 102

-Volume rendering
-Reprojection

Front 103

What does volume rendering require?

Back 103

segmentation-->ID of specific 2D structrues before 3D reconstruction occurs

Front 104

When is segmentation for 3D reformating easiest to perform?

Back 104

When there is a large difference in CT number between the target structures and adjacent anatomy.

Front 105

Describe the process of volume rendering:

Back 105

Segmentation of CT converts image to one-bit representation-->a 0 if target structure is not in the pixel and a 1 if target is in the pixel-0->once target structures is appropriately segmented-->software calculates a number of surfaces from this data (surface rendering)-->surfaces closest to the viewer obscure surfaces at greater depth

Front 106

What limits volume rendering in real life?

Back 106

Imperfect SNR of CT images

Front 107

How does reprojection work? and why is it used over volume rendering?

Back 107

-uses ray-tracing software through the volume data set from a specified viewing angle-->software displays the noramlized sum of CT numbers from all voxels through which the ray has passed
-saves time compared to volume rendering

Front 108

Compare radiation dose in CT with projection radiography:

Back 108

1. single Ct image acquired in a highly collimated way-->volume of tissue irradiated by a primary beam is less
2. Volume of tissue irrdiated exposed to raditation from all angles-->more evenly distributes the radiation (in chest radiograph for ex, tissue irridated by the entrance beam has > dose than tissue near exit surface)
3. Need a high SNR, so radiation does to the slice volume is higher (use higher kV and mAs)

Front 109

What is the principle interaction in CT and what does this mean in terms of dose?

Back 109

Radiation dose attributable to scattered radiation is significant-->can be higher than the dose from the primary beam-->also get scattered dose from adjacent tissue slices

Front 110

What is the standard for determining radiation in CT?

Back 110

MSAD (multiple scan average dose)-->includes dose attributable to scattered radiation emanating from adjacent slices

Front 111

What is a good approximation for MSAD?

Back 111

CT dose index-radiation dose to any point in a patient+scattered radiation from 7 adjacent slices in both directions (14 slices)

Front 112

What are 2 ways to measure CT dose index?

Back 112

1. TLDs (small thermoluminescent dosimeters) placed in holes along 14 slice thickness increments
2. Pencil ionization chamber-long enough to span the width of 14 7mm contingous slices measures the CTDI of a single slice at the center of the inonization pencil chamber and calculates it by CTDI=fXT*L (measures air kerma and not dose directly)

Front 113

How does CTDI compare to MSAD?

Back 113

underestimates MSAD for smaller slice thicknesses (2mm) b/c a significant amount of rdiation is scatterend beyond the 7 slice thicknesses

Front 114

What is the relationship b/w dose and mAs in CT?

Back 114

Proportional-->doubling mAs, doubles dose

Front 115

How do you calculate dose in helical CT?

Back 115

Use collimator pitch verses detector pitch (in axial scanning)

Dose (helical)=Dose (axial) x 1/collimator pitch

If pitch <1, increases dose; if pitch>1, decreases dose

Front 116

What are the kV and mA typical CT scan?

Back 116

120 kV and 200 mA verses PA chest radiograph is 120 and 5

Front 117

How do you reduce patient dose in CT fluroscopy?

Back 117

Decrease the mA per slice

Front 118

What is the advantage of modulating mA in CT?

Back 118

fewer xray photons needed to penetrate thinner tissues and more to penetrate thicker tissues-->with constant mA the inc. noise in thicker tissues cancels out the inc. SNR in thinner tissues-->so to reduce patient dose, reduce mA

Front 119

For a given scenario, give the change in contrast resolution and SNR:
1. Contrast resolution is linked to what parameter?
2. Doubling the mA
3. Dose
4. Pixel size (FOV)

Back 119

1. SNR
2. Inc. SNR by sq. root of 2-->contrast increases
3. dose inc. linearly with the mA-->inc. SNR and contrast res.
4

Front 120

How does beam hardening artifact occur?

Back 120

Lower energy beam xray ray photons attenutated to a greater degree than higher energy xray photons-->"harder" or higher energy beam as it exists the patient

Front 121

How do you correct for beam hardening?

Back 121

beam hardening correction algorithms

Front 122

What is partial volume averging?

Back 122

-some voxels contain densities of different tissues in one voxel
-usually most pronounced for softly rounded structures that are almost parallel to the CT slice
-Ex near top of head where cranium shares voxels with brain tissue-->details of brain parenchyma are lost b/w large u of bone dominates q

Front 123

How can you compensate for partial volume averaging?

Back 123

Thinner slices