30.10.2018 1 Advanced Imaging Techniques Perfusion Imaging Prof. Dr. Frank G. Zöllner Computer Assisted Clinical Medicine Medical Faculty Mannheim Heidelberg University Theodor-Kutzer-Ufer 1-3 D-68167 Mannheim, Germany [email protected]www.ma.uni-heidelberg.de/inst/cbtm/ckm Prof. Dr. Zöllner I Slide 2I 10/30/2018 Learning Goals introduction to perfusion imaging basic MRI principles -> Physics of Imaging Techniques Goals: 1. How does the technique work ? 2. What kind of images do we receive? 3. Where is this applied to ? Slides of the lectures at https://www.umm.uni-heidelberg.de/inst/cbtm/ckm/lehre/index.html
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30.10.2018
1
Advanced Imaging TechniquesPerfusion Imaging
Prof. Dr. Frank G. ZöllnerComputer Assisted Clinical MedicineMedical Faculty Mannheim Heidelberg University
� basic MRI principles -> Physics of Imaging Techniques
� Goals:
1. How does the technique work ?
2. What kind of images do we receive?
3. Where is this applied to ?
� Slides of the lectures at https://www.umm.uni-heidelberg.de/inst/cbtm/ckm/lehre/index.html
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Prof. Dr. Zöllner I Slide 3I 10/30/2018
Reading� Alsop, D. et al. (2015). Recommended implementation of arterial spin-
labeled Perfusion mri for clinical applications: A consensus of theISMRM Perfusion Study group and the European consortium for ASL in dementia. Magnetic Resonance in Medicine, 73(1), 102–116.
� Idea: change the contrast between flowing blood and stationary tissue
Prof. Dr. Zöllner I Slide 8I 10/30/2018
Arterial Spin Labeling (ASL)
� arterial blood used as endogenous, freely diffusible tracer� needs to be labeled (tagged) to be identified
� use of radio frequency inversion pulse
� basic scheme:read out volumelabeling slicelongitudinal magnetization
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Prof. Dr. Zöllner I Slide 9I 10/30/2018
Properties of ASL
� signal change due to the delivered, labeled magnetization is typically about 0.5% - 1.5% of the total signal
� need to average several pairs of tag/control to increase SNR
� ∆M is a perfusion-weighted (PW) image that directly reflects localperfusion
� however, perfusion signal does not only depend on vascular flow but
� Longitudinal relaxation times of blood and tissue,
� proton density,
� the transit time from labeling to imaging region
� the inflow time TI
� only quantification of PWI takes this into account
Prof. Dr. Zöllner I Slide 10I 10/30/2018
ASL - Imaging
� building blocks of ASl sequence
� inversion pulse
� readout
� bolus arrival time
� background suppression
t
Inversion ss/ns readoutQ2TIPSbackgroundinversion
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Prof. Dr. Zöllner I Slide 11I 10/30/2018
ASL – Imaging, Inversion scheme
� Pulsed ASL (PASL):
� use 1 RF pulse (180°) to invert spins� wait t=TI ms
� readout signal
� repeat without labeling slab
t
Inversion ss/ns readoutInversion time TI
Prof. Dr. Zöllner I Slide 12I 10/30/2018
ASL – Imaging, Inversion scheme
� (pseudo) continuous ASL ((P)CASL):
� continuously use RF pulse toinvert spins in thin slabfor labeling time t
� wait post label delay (PLD ~ TI)
� readout signal
� pseudo cont. → 1000 or more shapedRF pulses at a rate of ~ 1/ms
t
(P)CASL labeling readoutpost inversion delay(PLD)
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Prof. Dr. Zöllner I Slide 13I 10/30/2018
ASL – Labeling Schemes
� CASL
Alsaedi et al. Contrast Media Mol Imaging. 2018
Prof. Dr. Zöllner I Slide 14I 10/30/2018
ASL- Labeling Scheme
� EPISTAR (PASL)
Alsaedi et al. Contrast Media Mol Imaging. 2018
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Prof. Dr. Zöllner I Slide 15I 10/30/2018
ASL –Labeling Schemes
� FAIR (PASL)
Alsaedi et al. Contrast Media Mol Imaging. 2018
Prof. Dr. Zöllner I Slide 16I 10/30/2018
ASL - Readout
� readout block Independent of labeling scheme
� EPI, bFFSP, TSE, 3D – GRASE
� selection of readout scheme: SNR, CNR, time constraints
t
readout
courtesey of M.Günther
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Prof. Dr. Zöllner I Slide 17I 10/30/2018
ASL - Readout
� readout block Independent of labeling scheme
� EPI, SSFP, TSE, 3D - GRASE
t
readout
courtesey of M.Günther
Prof. Dr. Zöllner I Slide 18I 10/30/2018
ASL - Readout
� readout block Independent of labeling scheme
� EPI, SSFP, TSE, 3D - GRASE
t
readout
courtesey of F. Zimmer
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Prof. Dr. Zöllner I Slide 19I 10/30/2018
ASL - Readout
� readout block Independent of labeling scheme
� EPI, SSFP, TSE, 3D - GRASE
t
readout
courtesey of M.Günther
Prof. Dr. Zöllner I Slide 20I 10/30/2018
ASL - Readout
Alsop et al. , Magn Reson Med 2015
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Prof. Dr. Zöllner I Slide 21I 10/30/2018
ASL - Readout
� comparison of readout schemes
Nery et al. Diagnostics, 2018
Prof. Dr. Zöllner I Slide 22I 10/30/2018
ASL – Q2TIPS
� perfusion signal depends on inflow time TI
� but also: transit delay /bolus arrival time (BAT)
� time of the labeled blood to arrive at the tissue of interest
� use saturation pulses to shape the bolus, using e.g. „quantitative imaging of perfusion using a single subtraction with thin-slice TI1 periodic saturation” (Q2TIPS)
t
Q2TIPS
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Prof. Dr. Zöllner I Slide 23I 10/30/2018
ASL – Q2TIPS
� TI1 to TI1S consist of a train of 90° excitation pulses for saturation� TI2: inflow time
Luh et al, Magn Reson Med 1999
Prof. Dr. Zöllner I Slide 24I 10/30/2018
ASL – Background Suppression
� when calculating PWI by subtracting control from tag image → signal of static tissue is to be eliminated
� low SNR of the perfusion signal →small fluctuations in the tissue signal can corrupt the ASL signal
� background suppression to avoid signal from stationary tissue
� insert inversion pulses matching T1 of static tissues to besuppressed
t
Inversion ss/ns readoutQ2TIPSbackgroundinversion
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Prof. Dr. Zöllner I Slide 25I 10/30/2018
ASL – Background Suppression
two tissue species with a T1 of 1.5 s (blue) and 1.2 s (green)