Week 4 Lecture 2 Flashcards
scatter is absorbed by?
- collimators and/or detectors
scatter detection is affected by?
- placement of detectors
- shape of the detectors
- size of the detector aperture
do spacing bars play a role in distance measurements?
yes. they contribute to distance measurements
*what size of detector aperture absorbs more scatter?
short, wide? long, narrow?
short and wide
↑ aperture size = (↓/↑) spatial resolution
↑ aperture size = ↓ spatial resolution
*due to the decrease in volume averaging of the pixels
what are the four characteristics that determine detector quality?
- quantum detector efficiency (QDE)
- high stability
- fast response time
- wide dynamic range
what determines a detector’s stability?
calibration requirements
QDE
capture, absorption and conversion of radiation energy (into electrical energy)
what describes stability?
consistency in response to radiation
what is afterglow?
it creates lag time which is undesirable for a detector’s response time
wide dynamic range
detector’s ability to measure different signals
what type of detectors are found in conventional scanners?
*ionization (xenon gas) detectors
xenon gas detectors are found in SDCT, as well as MDCT. t/f
false - it is not used in MDCT
what are the 3 advantages to ionization detectors?
- lower cost
- high stability
- response time (no lag/afterglow)
what are the disadvantages to xenon gas detectors?
- requires constant pressurization
- ↓ QDE (~50-87% QDE)
why is QDE only 50-87% in xenon gas detectors?
due to the aluminum casing and spacing between the plates
what type of detectors are used in modern systems?
solid state/scintillation detectors
what is the flow of energy conversion in a scintillation detector?
x-ray energy to light energy to electrical energy
what is used in scintillation detectors to convert the light into electrical energy?
photocathodes or photodiodes
what are the advantages to solid state detectors?
increases absorption efficiencies (94-100% QDE)
what are the disadvantages to the scintillation detectors?
- can exhibit afterglow
- temperature and moisture sensitive
- requires spectral matching
what are the characteristics of a scintillation detector?
- high light output
- high x-ray stopping power
- good spectral match
- fast response time
what is an amplifier needed for in a solid state detector?
to ensure that the x-ray energy captured will be equal to the light energy produced
what is spectral matching?
when energy conversion results in the proper colour of light emitted
detector arrangements play a role in determining how many image slices can be recorded with a single gantry rotation. t/f
true
SDCT = ____ beam
MDCT = _____ beam
SDCT - fan beam
MDCT - cone beam
detectors in a SDCT vs. MDCT.
SDCT: side by side and in a single row
MDCT: an array forming rows and columns
z axis in SDCT vs. MDCT
SDCT: wide z-axis
MDCY: narrow z-axis
what is the largest allowable slice thickness in a SDCT?
LESS THAN the full detector’s width
***MDCT:
collimation + # of detectors correlates to?
slice thickness + # of slices
what are the two main configurations of the MDCT?
- matrix array
- adaptive array
matrix array aka?
fixed or uniform array
adaptive array aka?
non-uniform or hybrid array
matrix array
goal is to improve spatial resolution
matrix array’s voxels?
isotropic resulting in symmetry in slice thickness
adaptive array’s voxels
anisotropic resulting in slice thickness variation
(thinner in the center and wider at the periphery)
***image slice thickness can’t be smaller than the slice thickness selected during acquisition
what are the advantages to MDCT?
- more slices per 360 degree rotation which reduces the scan time, which can help with imaging motion
- can combine detector elements
*retrospective slice thickness
*MDCT obtained slices can have their slice thickness manipulated after acquisition
are thicker or thinner slices better to improve the appearance of 3d reformatted images?
thinner slices
*acquisition slice thickness must be selected before scan acquisition
what are some considerations to selecting slice thickness?
- size of scanned area
- tube heat limits
- reformatting capabilities
- what are you looking for in an image? (spatial resolution)
image slice thickness (=/≠/≥/≤) scan slice thickness
image slice thickness ≥ scan slice thickness
*acquisition slices (can/not) be combined to create larger image slices
they CAN be combined
acq. slices combined to = larger image slices
retrospective slice incrementation
ability to change the starting point of an image slice
retrospective slice incrementation applies to both axial and helical scans. t/f
false.
helical scans only
