Chapter 11 CT I Flashcards
how is power to the x-ray tube supplied?
slip ring technology
highest tube current
1000 mA
shortest rotation time for 360 degree of x-ray tube
0.3 s
typical is 1 s
focal spot sizes
1.2 mm and 0.6 mm
relative x-ray output
mAs
absolute output depends also on tube design and kV
how is the heel effect minimized?
anode-cathode axis is perpendicular to imaging plane
cost of CT x-ray tube
200 k
max power to CT x-ray tubes
100 kW
-use of 0.6 mm focal spot requires power reduced to 25 kW
heat deposition into anode
100 kJ every s
heat lost from x-ray tube
10 kJ/s, ten times less than rate of heat deposition
anode heat capacities
5 MJ, 10x higher than for normal radiographic and fluoro tubes
what can happen if CT are operated for a long time?
tube overheating
filtration on CT vs radiography and fluoro
heavier
reduces beam hardening effects
bow tie filter
minimize beam hardening differences
transmission through thinner body regions result in higher detector signals- prevent this with bow tie filter
low Z material (Teflon)
reduce scatter
reduce patient dose
antiscatter collimation
thin lamellae
between the detector elements
why do CT detectors have high data output rates (bandwidths)
multi-slice scanners
short gantry rotation times
photon starvation
small number of photons reaches detector
when are CT not quatum mottle limited?
when x-ray signals are low and electronic noise becomes significant
characteristics of CT scintillators
-low after-glow characteristics
-rapid signal decay
-scintillation detectors are coupled to light detectors
-high quantum efficiency (absorb most of the incident x-ray energy)
typical distance from x-ray tube focus to isocenter
60 cm
magnification of objects at patient center
X2
third generation CT system
-both x-ray tube and detector rotate around patient
FOV for fan beam of 50 degrees
50 cm diameter
how many detectors in fan beam?
up to 1000
~ 1.2 m from focal spot
detector offsets
-central rays acquired at 0 and 180 degrees do not overlap, but are offset by half detector width
-offset improves data sampling and spatial resolution
how many axial slices in a typical single rotation?
64
detector width of 0.625 mm
total beam width = 64 * 0.625 = 40 mm
such a scanner would cover a 32 cm long abdomen in 8 rotations
what are CT images?
2D patterns of x-ray attenuation coefficients
what do you call the x-ray transmission that each individual detector measures?
ray
what is a projection?
collection of rays for all detectors in each slice at a given tube angular projection
how many individual data points in a projection?
1000
corresponds to number of detectors (each has a ray)
how many projections are acquired for a single rotation of the x-ray tube?
1,000
what is sinogram
plot of projection vs x-ray tube angle
-each reconstructed slice requires a complete sinogram
axial scanning vs helical scanning
axial: table and patient are stationary while the tube rotates and acquires the necessary projection data
helical: patient moves through the gantry at same time as x-ray tube rotates. Reduces CT scan time
pitch
table increment distance/ (x-ray beam width)
pitch < 1 is oversapled
pitch > 1 is undersampled
interpolation algorithms for helical CT
at any given long patient axis position, helical CT scanning provides one of the 1,000 projections that are required
The 999 missing lines are obtained with interpolation algorithms with projections upsteam and downstram of the location of interest
-after interpolation, a sinogram is generated at eah z-axis location where image reconstruction is to be performed
effective mAs in helical scanning
true mAs/CT pitch
-increasing effective mAs increases patient dose and reduces image mottle