View
234
Download
5
Tags:
Embed Size (px)
Citation preview
Statistical image reconstruction• Intro: SPECT, PET, CT• MLEM• (back) projection model• OSEM• MAP
– uniform resolution– anatomical prior– lesion detection
CTCT
yT II0 e ( )dL
yE (x)L e
( )dx
d
dx
PETPET
yE (x)dxL e
( )dL
jj ijlii eby
j jiji ay
SPECTSPECT
j jiji ay
Sinogram
position
projection angle
sinogram
MLEMmaximumlikelihood
expectationmaximisation
computing p(recon | data) difficult inverse problem
computing p(data | recon) “easy” forward problem
one wishes to find recon that maximizes p(recon | data)
Bayes:
p(recon | data) = p(data | recon) p(recon)
p(data)
datarecon
~
Maximum Likelihood
Maximum Likelihood
p(recon | data) ~
p(data | recon)
projection Poisson
j j
ijiji say
j = 1..Ji = 1..I
i i
yiy!y
ye
ii
i
ii )y|y(p
i
iiii )!yln(yylnyln(p(data | recon)) = L(data | recon) = ~
p(data | recon)recon data
Maximum Likelihood
i
iii yylnyL(data | recon)
i i
iiij
j y
yya
)(Lfind recon:
J..1j,0sa
saya
i k ikik
k ikikiij
Iterative inversion needed
j
ijiji say j
ijiji say
Expectation Maximisation
i
i
iij
i ijj
newj y
ya
a1
• produces non-negative solution
ML-EM algorithm:
ji ij
jj
newj
La
• can be written as additive gradient ascent:
• several useful alternative derivations exist
• only involves projections and backprojections (“easy” forward operations)
Optimisation transferOptimisation transfer
i
iii yylnyL(data | recon)
j
ijiji say
ij jiji af
i j
jijijg
(data | recon)
In every iteration:
with L(data | current) = (data | current)
current
L
likelihoodlikelihood
new
Expectation Maximisation
current
MEASUREMENT
REPROJECTION
COMPAREUPDATE RECON
likelihood
iteration
Iterative Reconstruction
iterationiteration
h00189
FBPFBP
MLEMMLEM
FBP vs MLEM
uniform Poisson
FBP vs MLEM
uniformuniform
FBPFBPFBPFBP MLEMMLEMMLEMMLEM
Poisson
FBPFBPFBPFBP MLEMMLEMMLEMMLEM
Poisson
FBP vs MLEM
8 iter8 iter 100 iter100 iter FBPFBPtrue imagetrue image
sinogramsinogramwithnoise
withnoise
smoothedsmoothed
MLEM: non-uniform convergence
(back) projection model:model for image resolution
resolution model: simulation
projection backprojection
no noiseno noise
Poisson noisePoisson noise
mlemmlem
mlemmlem
resolution model: simulation
no noise Poisson noise
resolution model: simulation
no noise Poisson noise
compute: estimated sinogram – given sinogram = “unexplained part of the data”
compute: estimated sinogram – given sinogram = “unexplained part of the data”
resolution model: simulation
compute sum of squared differences along vertical lines
resolution model: simulation
MLEM withsingle rayprojector
MLEM withsingle rayprojector
MLEM withGaussiandiffusionprojector
MLEM withGaussiandiffusionprojector
(back)projection in SPECT
3D-PET FDG: OSEM, no resolution model
3D-PET FDG: OSEM, with resolution model
(back)projection in PET
after 8 iterations
• assume full convergence 0)|y(L
0)|yy(L
0)|y(L
yy
)|y(L
kj
22
can be used to estimate• impulse response• covariance matrixof ML-solution
first derivatives are zero
likelihood is maximized
yy
)|y(L)|y(L 21
kj
2
small change of the data...
after 8 iterations
Simulation:• SPECT system with blurring (detector and collimator): about 8 mm.• reconstructed with and without resolution modelling• post-filter to have same target resolution• compare CNR in 4 points
— Point 1— Point 2— Point 3— Point 4
target resolution
8 1612
gain in contrast to noise ratio due to better resolution model
gain in contrast to noise ratio due to better resolution model
4
2
accurate modeling of the physics:
• larger fraction of the data becomes consistent better resolution
• larger fraction of the noise becomes inconsistent less noise
we gain twice! but computation time goes up...
(back) projection model
OSEM
ordered subsetsexpectation maximisation
Hudson & Larkin, Sydney
Reference
2 4 8 16 25 50 100 200
Subsets...
Filtered backprojection of the subsets.
OSEM
01
2
3
410
40
1 iteration of 40 subsets(2 proj per subset)
1 iteration of 40 subsets(2 proj per subset)
OSEM
MLEM-iterations
1 OSEM iteration with 40 subsets
0 1 2 3 4 10 40
Reference
0 1 2 3 4 10 40
OSEM
s1s2
s3s4
no noise (and subset balance)
with noise
Convergence to limit cycleConvergence to limit cycle
Solutions:
• apply converging block-iterative algorithm: sacrifize some speed for guaranteed convergence
• gradually decrease the number of subsets
• ignore the problem (you may not want convergence anyway)
OSEM
ML
initialimage
64x164x1 1x641x64truetrue differencedifference
OSEM
MAPmaximum a posteriori
• short intro• MAP
• uniform resolution• anatomical priors• lesion detection
MAP
computing p(recon | data) difficult inverse problem
computing p(data | recon) “easy” forward problem
one wishes to find recon that maximizes p(recon | data)
Bayes:
p(recon | data) = p(data | recon) p(recon)
p(data)
datarecon
~
MAP
Bayes: p(recon | data) ~ p(data | recon) p(recon)
ln p(recon | data) ~ ln p(data | recon) + ln p(recon)
posteriorposterior likelihoodlikelihood priorprior
- penalty- penalty
local prior or Markov prior:
Gibbs distribution:
p(reconj | recon) = )N(EexpZ1
jjj
p(reconj | recon) = p(reconj | reconk, k is neighbor of j)
ln p(reconj | recon) = j Ej(Nj) + constant
j
k
MAP
ln p(reconj | recon) = j Ej(Nj)
jNk
kj )(E
j – kj – k
E(j – k)E(j – k) quadratic
Huber
Geman
MAP vs smoothed ML
MLEMMLEM smoothedMLEM
smoothedMLEM
MAP withquadratic prior
MAP withquadratic prior
When postsmoothed-MLEM and MAPhave same resolutionsame resolution, they have same covariancesame covariance!When postsmoothed-MLEM and MAPhave same resolutionsame resolution, they have same covariancesame covariance!
Use non-uniform “prior” to smooth• more where likelihood is “strong”• less where likelihood is “weak”
Use non-uniform “prior” to smooth• more where likelihood is “strong”• less where likelihood is “weak”
Likelihood provides non-uniform information:• some information is destroyed by
• attenuation• Poisson noise• finite detector sensitivity and resolution• ...
Likelihood provides non-uniform information:• some information is destroyed by
• attenuation• Poisson noise• finite detector sensitivity and resolution• ...
MAP with uniform resolution
MAP with uniform resolution
• equivalent to post-smoothed MLEM
• prior improves condition number:– MAP converges faster than MLEM:
• fewer iterations required!
• but more work per iteration
T1T1 GreyGrey WhiteWhite CSFCSF
prior knowledge,valid for severaltracer(FDG, ECD, ...)
prior knowledge,valid for severaltracer(FDG, ECD, ...)
• CSF: no tracer uptake• white: uniform, low tracer uptake• grey: higher tracer uptake,
possibly lesions
• CSF: no tracer uptake• white: uniform, low tracer uptake• grey: higher tracer uptake,
possibly lesions
MAP with anatomical prior
• smoothing prior in gray matter (relative difference)• Intensity prior in white (with estimated mean)• Intensity prior in CSF (mean = 0)
MLEMMLEM MRIMRI MAPMAP
MAP with anatomical prior
MAP with anatomical prior
Theoretical analysis indicates that
PV-correction with MAP-reconstruction is superiorsuperior to
PV-correction with post-processed MLEM
phantomphantom
ml with resolutionmodeling
ml with resolutionmodeling
make anatomicalregions uniform
make anatomicalregions uniform ml-pml-p
map withanatomical priorand resolutionmodeling
map withanatomical priorand resolutionmodeling
mapmapsinogramsinogram
projection with finiteresolution(2 pixels FWHM)
MAP with anatomical prior
ml-pml-p
mapmap
MAP with anatomical prior
MAP with anatomical prior
MAP yields better noise characteristics
than post-processed MLEM
MAP and lesion detection
human observer studyhuman observer study
MAP and lesion detection
MLEM MAP
moresmoothing
moresmoothing
higher higher
observerscore
observerscore
moresmoothing
moresmoothing
higher higher
observerresponsetime
observerresponsetime
MLEM MAP
MAP and lesion detection
(non-uniform quadratic) MAPseems better for lesion detection
thanks