22- Ct Systems

Front 1

What is a 3rd generation type geometry for modern CT scanners?

Back 1

Tube and opposing detector array are fixed with relative to the detector array and both components rotate around patient.

Front 2

What allows the gantry to rotate continuously with disrupting the electronics?

Back 2

Slip rings allow continuous transmission of electrical energy for power or data

Front 3

what defines a ray?

Back 3

each detector the with focal spot of the xray tube

Front 4

What is the main issue with CT xray tubes that require high output and extended operation?

Back 4

Heat dissipation with water or oil
2 types of cooling designs:
1) Rotating anode, stationary tube envelope
2) Rotating envelope tubes

Front 5

What tube voltages are CT's designed to operate at?

Back 5

80-140 kV
20-100 kW
Focal spot of 0.6-1.2 mm

Front 6

Describe how bow-tie filters work

Back 6

more beam attenuation near edges than at its center. The edge of patients do not require as much radiation as the center to form the image. this reduces scatter and makes the radiation dose more uniform throughout the patient. Limits range of intensities seen by the detector and makes the noise more uniform.

Front 7

What are the 2 types of CT detectors?

Back 7

1) Scintillator
2) Gas detector

Front 8

how does a scintillator detector work?

Back 8

incident photons emit light which can be amplified by a photomultiplier tube or solid state device (photodiode)

Front 9

How does a gas detector work?

Back 9

Photons interact with Xenon in a gas chamber accross which a voltage is applied. Each interaction results in ionization which can be quantified

Front 10

Multislice vs. Single slice

Back 10

Front 11

What are the benefits of a dual source CT design?

Back 11

1) Speed
2) Can obtain attenuation maps using different energies which allow for material specific reconstructions and post processing (ie. contrast can be removed thereby generating pre- and post contrast equivalents on a single scan.... lower patient dose.

Front 12

When referring to CT, why do we use effective mAs instead of conventional definition of radiographic mAs

Back 12

because a given point in the patient is only in the beam for a portion of the time that the tube is turned on.

Front 13

How do kVp and mAs affect the number of photons and their energy distributions?

Back 13

mAs= number of photons
kVp = energy

Front 14

How does mAs affect the image quality?

Back 14

higher mAs = more photons to work with,= less noise = lower standard deviations

Front 15

Define Pitch (aka beam pitch)

Back 15

Table movement per tube revolution/beam width = d/W

Front 16

Example: if the table moves 12 mm in one revolution and the beam width is 8mm, what is the pitch

Back 16

table movement per tube revolution/beam width
12/8 = 1.5

Front 17

What is slice pitch?

Back 17

table movement per tube revoltion/ slice thickness = d/S

Front 18

how do you convert slice width to beam width?

Back 18

slice width x number of detectors

Front 19

What is the effect of pitch on patient dose?

Back 19

the smaller the pitch the more overlapping radiation = larger patient dose

Front 20

What is the effect of slice thickness on image noise and spatial resolution

Back 20

large slices = decreased spatial resolution (in z axis) and decreased noise

Front 21

what is the effect of slice thickness on patient dose.

Back 21

thin slices= larger patient dose.

Front 22

what are the three factors upon which attenuation coefficient depends?

Back 22

type of material, physical density and photon energy

Front 23

What is the beer-Lambert law?

Back 23

the number of photons reaching the detector is related to the sum of the linear attenuation coefficients of each voxel in the ray. Therefor referred to as raysum

Front 24

The set of all CT projections at one slice positions placed side-by side is called a _____________

Back 24

sinogram

Front 25

What are the 2 methods of image reconstruction?

Back 25

1) Analytical- Filtered backprojection (fast and noisy)
2) Iterative (slow and clean)

Front 26

How does iterative reconstruction work?

Back 26

see chart

Front 27

What is the convolution kernal?

Back 27

Filter that is applied to the scan information before it is backprojected to form the 3D matrix

Front 28

When we look at an axial slice what are we really looking at?

Back 28

axial reconstruction of a 3D volume

Front 29

Quesion: if the FOV is 40 cm x 40cm and the image matrix is 1024 x 1024, what is the size of a pixel?

Back 29

p= FOV/m

p= 40 cm (400 mm)/1024

p=0.4 mm

Front 30

what are the x,y and z dimensions of a voxel?

Back 30

x and y = pixel size (determined by FOV and matrix size)

z= slice thickness.

Front 31

True or false: A kernel will change the actual CT number

Windowing and leveling will change the actual CT number

Back 31

True

False

Front 32

Since the CT scan is aquired helically, only a single projection is taken of an axial section. However, multiple views are needed to generate the axial section. How is this overcome?

Back 32

For a 360 information, linear interpolation assumes that the change between 2 points is linear and fills in the gap.

Front 33

How does 180 degree linear interpolation work?

Back 33

raysums measured from any angular position are approximately the same as those measured from a position exactly 180 degrees away. However, in a 180 degree interpolation, the slices used for interpolation are closer together resulting in better spatial resolution.

Front 34

Which is better 180 degree or 360 degree linear interpolation?

Back 34

180 degree

Front 35

Which scan mode has better z resolution? (axial or helical)

Back 35

axial
helical us prone to volume averaging due to linear interpolation.

Front 36

what is the benefit of helical acquisition?

Back 36

speed-especially for time sensitive imaging (ie. PE studies) and when imaging moving anatomy (cardiac work)

Front 37

how do variable kv and mAs systems work

Back 37

acquire frontal and lateral scouts from which attenuation tables are generated which drive the tube current and mas during scan.

new systems can do this on the fly based on information obtained from the leading detector row

Front 38

In CT fluorsocopy what moves and what stays still?

Back 38

the table stays still, the tube moves continuously