Week 4 Lecture 2

Question 1

scatter is absorbed by?

Answer 1

• collimators and/or detectors

Question 2

scatter detection is affected by?

Answer 2

• placement of detectors
• shape of the detectors
• size of the detector aperture

Question 3

do spacing bars play a role in distance measurements?

Answer 3

yes. they contribute to distance measurements

Question 4

*what size of detector aperture absorbs more scatter?
short, wide? long, narrow?

Answer 4

short and wide

Question 5

↑ aperture size = (↓/↑) spatial resolution

Answer 5

↑ aperture size = ↓ spatial resolution
*due to the decrease in volume averaging of the pixels

Question 6

what are the four characteristics that determine detector quality?

Answer 6

1. quantum detector efficiency (QDE)
2. high stability
3. fast response time
4. wide dynamic range

Question 7

what determines a detector’s stability?

Answer 7

calibration requirements

Question 8

QDE

Answer 8

capture, absorption and conversion of radiation energy (into electrical energy)

Question 9

what describes stability?

Answer 9

consistency in response to radiation

Question 10

what is afterglow?

Answer 10

it creates lag time which is undesirable for a detector’s response time

Question 11

wide dynamic range

Answer 11

detector’s ability to measure different signals

Question 12

what type of detectors are found in conventional scanners?

Answer 12

*ionization (xenon gas) detectors

Question 13

xenon gas detectors are found in SDCT, as well as MDCT. t/f

Answer 13

false - it is not used in MDCT

Question 14

what are the 3 advantages to ionization detectors?

Answer 14

1. lower cost
2. high stability
3. response time (no lag/afterglow)

Question 15

what are the disadvantages to xenon gas detectors?

Answer 15

1. requires constant pressurization
2. ↓ QDE (~50-87% QDE)

Question 16

why is QDE only 50-87% in xenon gas detectors?

Answer 16

due to the aluminum casing and spacing between the plates

Question 17

what type of detectors are used in modern systems?

Answer 17

solid state/scintillation detectors

Question 18

what is the flow of energy conversion in a scintillation detector?

Answer 18

x-ray energy to light energy to electrical energy

Question 19

what is used in scintillation detectors to convert the light into electrical energy?

Answer 19

photocathodes or photodiodes

Question 20

what are the advantages to solid state detectors?

Answer 20

increases absorption efficiencies (94-100% QDE)

Question 21

what are the disadvantages to the scintillation detectors?

Answer 21

• can exhibit afterglow
• temperature and moisture sensitive
• requires spectral matching

Question 22

what are the characteristics of a scintillation detector?

Answer 22

1. high light output
2. high x-ray stopping power
3. good spectral match
4. fast response time

Question 23

what is an amplifier needed for in a solid state detector?

Answer 23

to ensure that the x-ray energy captured will be equal to the light energy produced

Question 24

what is spectral matching?

Answer 24

when energy conversion results in the proper colour of light emitted

Question 25

detector arrangements play a role in determining how many image slices can be recorded with a single gantry rotation. t/f

Answer 25

true

Question 26

SDCT = ____ beam MDCT = _____ beam

Answer 26

SDCT - fan beam MDCT - cone beam

Question 27

detectors in a SDCT vs. MDCT.

Answer 27

SDCT: side by side and in a single row MDCT: an array forming rows and columns

Question 28

z axis in SDCT vs. MDCT

Answer 28

SDCT: wide z-axis MDCY: narrow z-axis

Question 29

what is the largest allowable slice thickness in a SDCT?

Answer 29

LESS THAN the full detector's width

Question 30

***MDCT: collimation + # of detectors correlates to?

Answer 30

slice thickness + # of slices

Question 31

what are the two main configurations of the MDCT?

Answer 31

1. matrix array 2. adaptive array

Question 32

matrix array aka?

Answer 32

fixed or uniform array

Question 33

adaptive array aka?

Answer 33

non-uniform or hybrid array

Question 34

matrix array

Answer 34

goal is to improve spatial resolution

Question 35

matrix array's voxels?

Answer 35

isotropic resulting in symmetry in slice thickness

Question 36

adaptive array's voxels

Answer 36

anisotropic resulting in slice thickness variation (thinner in the center and wider at the periphery)

Question 37

***image slice thickness can't be smaller than the slice thickness selected during acquisition

Question 38

what are the advantages to MDCT?

Answer 38

- more slices per 360 degree rotation which reduces the scan time, which can help with imaging motion - can combine detector elements

Question 39

*retrospective slice thickness

Answer 39

*MDCT obtained slices can have their slice thickness manipulated after acquisition

Question 40

are thicker or thinner slices better to improve the appearance of 3d reformatted images?

Answer 40

thinner slices

Question 41

*acquisition slice thickness must be selected before scan acquisition

Question 42

what are some considerations to selecting slice thickness?

Answer 42

- size of scanned area - tube heat limits - reformatting capabilities - what are you looking for in an image? (spatial resolution)

Question 43

image slice thickness (=/≠/≥/≤) scan slice thickness

Answer 43

image slice thickness ≥ scan slice thickness

Question 44

*acquisition slices (can/not) be combined to create larger image slices

Answer 44

they CAN be combined acq. slices combined to = larger image slices

Question 45

retrospective slice incrementation

Answer 45

ability to change the starting point of an image slice

Question 46

retrospective slice incrementation applies to both axial and helical scans. t/f

Answer 46

false. helical scans only