CT Flashcards
CT mA, kVp and focal spot vs Xray
Main diff
HIGHER mA in CT
CT kVp 80-120 Xray 50-120
focal spot CT 0.6-1.2 Xray 1.0-1.2
CT filters
material
copper or aluminum
Bow tie filters
purpose? material?
compensate for uneven attenuation of beam by patient (head)
low Z materials
heel effect in CT
minimized - anode cathode axis positioned perpendicular to imaging plane
CT scatted reduction
collimator?
septa?
collimator at both the xray tube and at detector
“defines section thickness on a single slice”
anti-scatter septa (like grid)
iterative reconstruction
effect on dose
allows more noise
(less dose same quality)
Modern vs dino CT detectorys
scintillator vs Gas-filled
what determines number of slices obtained simultaneously?
number of detectors in Z direction
“isotropic resolution”
MDCT can do non-axial recons without stretching pixels
minimum slice thickness determined by?
detector element aperture width
kVp and mA adjustments in modern CT
using scout before acquisition
on the fly adjustment with continuous modulation
“ray”
total xray attenuation along a particular line from focal point to single detector
“projection”
all rays at a given tube position/angle of xray tube
‘kind’ of xrays used in CT?
highly filtered, high kV
Matrix size in CT?
bits?
shades of gray?
512 x 512
12 bits
2 ^12 = 4096 shades of gray
pixel size = ?
better res = ?
FOV/MATRIX
better res = larger matrix
Pitch equation
Table movement / beam width
relationship bt HU and Xray attenuation
increase in 10HU = 1% increase in x-ray attenuation
effect of keV on HU
low keV –> higher HU
more attenuation thanks to k edge
contrast HU at 140 kV = 100HU
at 80 kV = 400HU
filtration and HU’s
increases average energy (beam hardened)
higher energy beam –> lower HU’s
phenomenon of ‘cupping’ with lower HU in center as beam is filtered by body
Window ‘level’ =
‘width’
level = middle of grayscale, want it at attenuation of thing youre lookin at
width = width… want it narrow if things look same (gray and white matter)
what HU is black if level set at 100 and window 300
less than -50
Typical presets (W,L)
Brain
W 80, L+40
Typical presets (W,L)
Lung
W 1500, L-400
Typical presets (W,L)
Abdomen
W 400, L +50
Typical presets (W,L)
Bone W 1600, L +500
Process of making an image in CT
starts with detector…
Raw data
I
processing (filter back or iterative)
I
Mathematic filter ‘Kernel’ applied
I
Ready to read/store
Acquisition
Axial
Stationary table, tube on, picture, tube off
table moves, tube on, another picture
Better Z resolution with no partial volume effect along long axis
Acquisition
helical
table moves, tube on
way faster
effects of increased beam width
faster
less motion
INCREASES PARTIAL VOLUME
DOESNT affect dose (faster but larger area)
more kVp –> ?
Increased dose (always in CT)
Decreased noise
Kernel
trade off decision between spatial resolution and noise
sharp kernel = better res, more noise (bone)
soft kernel = low noise, reduced res (brain)
Cardiac
Prospective pro/con
step and shoot - R-R interval
Less radiation
Can’t do functional imaging
ALWAYS axial
Cardiac
Retrospective
CAN do functional
scanning whole time
higher radiation
Dual energy kVp’s?
80 and 140 kVp
SNR and xray flux (mA)
Signal is directly related twice the xrays, twice the signal
Noise (also concerned with scatter, not just mottle), changes by square root of change. Twice the xrays square root 2 x the noise
EXAMPLE quadruple the mA, what is change in SNR
4 times the signal, square root of 4 times the noise
4/2 = SNR doubles
to improve SNR
mA?
rotation time?
kVp?
slice thickness?
pixel size?
pitch?
Higher SNR with
higher mA
Longer rotation time
higher kVp
thicker slices
bigger pixels
LOWER pitch
CT vs xray
contrast and spatial resolution
CT has waaay better contrast res
CT spatial res not so much
Why is contrast res so good in CT?
limiting factor for contrast res
minimal scatter hitting detector
tight collimation (twice)
windowing
NOISE limits contrast res
ENEMY OF CONTRAST RES =
NOISE
Spatial res
in CT = ?
mammo = ?
1 lp/mm
11lp/mm
Effect on spatial res
focal spot
Smaller better
Effect on spatial res
Mag
mag blurs, worsens spatial res
Effect on spatial res
Detector aperture size
Larger aperture improves Z axis spatial res
NO EFFECT ON X, Y
Effect on spatial res
Pixel size, DFOV, matrix size
pixel size = DFOV/matrix
bigger matrix, smaller DFOV = smaller pixel size
smaller pixel size = BETTER SPATIAL RES
CT X-Y spatial res
Z spatial res
X-Y determined by focal spot
Z determined by detector size
pixels, Matrix, FOV and contrast vs spatial res
constant matrix, decreased FOV will make pixels smaller
better spatial res
WORSE CONTRAST RES (fewer photons per pixel)
BIGGER FOV, bigger pixels, worse spatial res better contrast res (more photons per box)
CT dose distribution
body vs head
Body
middle gets 50% of skin
Head
All the same
CTDI
“weighted”
“volume”
radiation dose, normalized to beam width
“weighted” = 1/3 central CTDI + 2/3 Peripheral CTDI
“volume” = Weighted CTDI / PITCH
CTDI vol
Pitch and dose
proportional
2.0 halves the dose
DLP = ?
CTDI Vol x length of scan
CT effective dose
unit?
k x DLP
k = body part constant
effective dose unit = Sv
Body phantom size
what if patient bigger or smaller
32 cm
size 32
bigger patient = OVER estimated dose
smalle patient = UNDER estimated dose
‘average’ dose and effective dose (CTDI) for
CT head
CT abd
Peds belly
CT head = 58 mGy (1-2 mSv)
CT abd = 18 mGy (8-11mSv)
Peds belly = 15 mGy
ACR Established reference values (75th percentile)
Head
adult belly
peds belly
Head = 75 mGy
Adult belly = 25 mGy
Peds belly = 20 mGy
above these doses, “should be investigated and reduced if possible”
Risk of radiation induced cancer per dose?
adult per Sv
child per Sv
adult = 5% per Sv
Child = 15 % per Sv
0.5% per Sv over 50 yo
2 view chests per CT radiation wise
100
embryo dose in CT abd/pel ?
30mGy
CT artifacts
Cupping
Dark center of image 2/2 beam hardening
(center sees higher average energy)
CT artifacts
beam hardening dark bands/streaks
in the setting of two dense objects
objects that pass through one are less attenuated than those that pass through both
results in dark streaks between them
CT artifacts
How to fix beam hardening
Filtration - pre-hardening to remove lower energy components or bow tie filter
Calibration correction- using a phantom to allow the detector to compensate
iterative software
Partial volume
2 patterns
how to fix?
dense object protrudes partially into the widthof an xray beam. Beam diverges, this looks like shading adjacent to the object
Dense thing and low attenuation thing each occupying half of 3d voxel. machine averages together and it look intermediate
ex = skull base averaging with csf or brain to look like blood
Fix with thinner slices
Photon starvation
typically shoulders or other high attenuating areas result in streaking when beam travels horizontally through shoulders
Fix = automatic modulation, more mA there
Under sampling
Insufficient number of projections
leads to mis-registration
View aliasing - under sampling between projections, see fine stripes radiating from the edge of a dense object
fix = slow rotation speed
Ray aliasing - undersampling within a projection. See strips appearing close to structre
Metal artifact
fixes
Beam hardening, partial volume, aliasing
worse with higher Z (fe, platinum)
FIX = increase kVp, THINNER SLICES
Incomplete projection
body parts or IV contrast outside field but still attenuating xrays
Ring artifact
calibration error of defective detector
recalibrate or replace
helical artifact in axial plane
top of skull, anatomy changing rapidly in Z direction
worse with higher pitch
lower pitch, use thinner sections
