11/20/200611ENGG 167 - F06MEDICAL IMAGINGFriday, Nov. 3Fast Imaging, MR Angiography, and Advanced TechniquesReferences: The Essential Physics of Medical Imaging, Bushberg et al, 2nded., Chapters 14 & 15Principles of Magnetic Resonance Imaging: A Signal Processing Perspective, Liang and Lauterbur, IEEE Press 2000MRI the Basics, Hashemi and Bradley, Williams and Wilkins publ., 1997http://www.cis.rit.edu/htbooks/mri/2Review of Concepts• MRI is sensitive to nuclei with odd mass number or odd charge number•1H is used (high |µ|, high in vivo concentration in H2O)• B0establishes Mz0, individual spins precess at ω = γB0(42.6 MHz / T)• B1(ωrf = γB0) rotates Mz0into transverse plane →Mxy(FID or echo measured)• This excitation is followed by relaxation• Transverse, spin-spin relaxation• T2*, dephasing from intrinsicspin interactions, phase diffusion• T2*, dephasing from stationaryextrinsic B inhomogeneities• Longitudinal, spin-lattice relaxation,return of M to Mz0• T1, transfer of energy to theenvironment (lattice)11/20/200623Excitation, Relaxation and DetectionExcitationRelaxationRotatingFrameLaboratoryFrameDetection4MRI Contrast Summary• T1depends on the efficiencies of tissues to accept energy ∆E=γB0.• T2depends on spin-spin interactions and averaging of the local B fields.• Differences in T1, T2, or PD can be measured using a SE sequence.• TE and TR (and TI) are used to control the weighting.• Gradient echoes can be used for faster imaging experiments.• Forced echoes, can be used for very fast imaging• No T2* refocusing, so data may be T2* weighted.• The flip angle can be used to adjust the contrast.• These concepts form the basis of tissue contrast in MRI.()()()1201Induced ,,,,,,VTT PDBBTE TR TIxyMM t Sk IxSφµ→→ → →→11/20/200635Review: T1 and T2 DependenciesRef: Bushberg6Review: SE Pulse SequencesRef: Bushberg11/20/200647Review: CSE 2DFT Pulse SequenceRef: Bushberg,2xxfekGtγπ=,2yypepekGTγπ=0()RF rBGrωγ=+8Review: Spatial Encoding GradientNarrow-band RF and SE gradient isolate a plane in the object.Combining FE (applied during read-out) and PE (applied prior to read-out) gradients yields,Defining,this can be rewritten() ( )()()pe pe feiG rT iG rtobjectSt re e drγγρ−−=∫ii2fefekGtγπ=2pe pe pekGTγπ=fepekk k=+()()2ikrobjectSk re drπρ−=∫iRef: Liang and Lauterbur()Skis the Fourier Transform of()rρ11/20/200659Spatial Encoding Summary• Spatial encoding is achieved using ∆Bz(x), ∆Bz(y), ∆Bz(z) [∆Bz(r)].• 3 techniques for spatial encoding• Slice encoding (SEG) – narrow-band ωrfand ∆Bzisolate spins where ωLarmor= ωrf .• Frequency encoding (FEG) – A gradient applied during readout encodes spatial information into the frequency components of S(t).• Phase encoding (PEG) – A gradient applied prior to readout encodes spatial information into the phases of the components of S(t).• k-space interpretation• The measured data gives the FT of the spatial object.• k-space can be sampled by applying the FEG with sequential PEG with varying amplitudes.• Inverse FT of k-space gives the image.10The need for speed…http://info.med.yale.edu/intmed/cardio/imaging/techniques/em_spectrum/index.htmlPulse Sequence Acronyms vs. Manufacturer11/20/2006611Gradient Recalled Echo AcquisitionRef: Bushberg12Dual Spin Echo ImagingRef: Bushberg11/20/2006713Multi-slice Spin EchoRef: Bushberg14Fast Spin Echo AcquisitionRef: Bushberg11/20/2006815Echo Planar Image AcquisitionRef: Bushberg16Spiral K-space acquisitionRef: BushbergRef: Liang and Lauterbur11/20/2006917½NEXRef: BushbergRef: Hashemi and Bradley183DFT Acquisition,,acq pe z pe y avgtTRN N N=×××Ref: BushbergRef: Liang and Lauterbur11/20/20061019MRA with time of flight (TOF) taggingRef: BushbergFlow ArtifactSE TOF TaggingReference : The Basics of MRI http://www.cis.rit.edu/htbooks/mri/20Angiography with time of flight taggingRef: Bushberg11/20/20061121Phase dispersionRef: Bushberg22Phase contrast imagingRef: BushbergAxial Brain MRARef: Hornak11/20/20061223Parallel Imaging Techniques()() ()2ikrobjectSk Cr re drπρ−=∫i SMASH – SiMultaneous Acquisition of Spatial HarmonicsSENSEH– SENSitivity EncodingRef: Sodickson, “SpatialEncoding using Multiple RFCoils” NMR in Biomedicine() ( )()()pe pe feiG rT iG rtobjectSt re e drγγρ−−=∫ii24Parallel Imaging TechniquesRef: Sodickson, “SpatialEncoding using Multiple RFCoils” NMR in Biomedicine11/20/20061325Parallel Imaging TechniquesRef: Paschal and Morris, “K-Space in the Clinic”,JMRI 19:145-159 (2004)26fMRI and the BOLD EffectRef: Dougas Noll fMRI primerwww.eecs.umich.edu/~dnoll/primer2.pdf •BOLD-Blood Oxygen Level Dependant•Deoxyhemoglobin is paramagnetic and surrounding tissue is diamagnetic.•This susceptibility difference leads to decreased T2* near deoxyhemoglobin and a corresponding reduction in signal intensity.11/20/20061427fMRI and BOLDRef: Dougas Noll fMRI primerwww.eecs.umich.edu/~dnoll/primer2.pdf • Typically many repetitions of the task and control are performed and the results are averaged to increase SNR.• The need for sensitivity encourages the use of high B0fields.28MR Spectroscopic ImagingPE MRSI Pulse sequence* No gradient applied during readout11/20/20061529MR Spectroscopic ImagingRef: Gordon, “Magnets,molecules,, and medicine”,Phys. Med. Biol 30, 741-770(1985))30Local MR SpectroscopyRef: Gordon, “Magnets,molecules,, and medicine”,Phys. Med. Biol 30, 741-770(1985))11/20/20061631Artifacts – “barrel distortion”Ref: BushbergRef: Hashemi and Bradley32Artifacts – K-space errorsRef: Bushberg11/20/20061733Artifacts – motionRef: http://www1.stpaulshosp.bc.ca/Ref: Hashemi and Bradley“Ghosting” from periodic motionPatient exiting magnet...34Artifacts – motionRef: Bushberg11/20/20061835Artifacts – chemical shiftRef: Hashemi and BradleyRef: BushbergChemical shift in an axial slice through the kidney.Ref: www.mritutor.org36Artifacts – Phase wrap aroundRef: BushbergRef: http://www1.stpaulshosp.bc.ca11/20/20061937Artifacts – Phase wrap aroundRef: Hashemi and Bradley38Artifacts – SusceptibilityRef: http://www1.stpaulshosp.bc.caDental BracesStudded BeltMascaraRef: http://www.mritutor.org11/20/20062039Summary• Spin and gradient echo pulse sequences are combined with gradient