Phase Encoding Steps (res = 4) Clinical MRI Education Lecture 4 will start @ 7am Tue. /4 Thanks for arriving early. Please wait Lecture 4 k-space Sampling Techniques and Applications Chen Lin, PhD Indiana University School of Medicine & Clarian Health Partners The Anatomy of Basic MR Pulse Sequences Filling of k-space Magnetization Preparation Section Chemical Shift Selective Saturation/ Excitation Spatial Selective Saturation Magnetization Transfer (MT), CHESS water suppression Inversion Recovery (IR) k z Data Acquisition Section Slice/Slab Selective Excitation Phase Encoding(s) Echo Generation Spin Echo (SE), Fast/Turbo SE (TSE), Single-shot FSE (HASTE) Gradient Recalled Echo (GRE), Fast GRE, Single-shot GRE (EPI) Diffusion Weighting (DWI/DTI) and Gradient Moment Nulling (GMN) Frequency Encoding Filling of K-space Increment Phase Encoding Magnetization Recovery Section Spoiling Driven Equilibrium G z Frequency Encoded Points (res = 8) k-space and I-space (Image) K-space Data Point and Encoding Magnitude K-space Image space Phase K :
Min K -i,-j = K * i,j K-space Properties Echo/Line/View K i,j K G() t dt Max Symmetry (Hermitian conjugate for real object) -> Allows partial k-space acq. Inner k-space (Low spatial freq.) -> Intensity/Contrast (High SNR) Outer k-space -> High spatial freq. -> Edges/Details (Low SNR) K-space resolution (DK) -> I- space FOV K-space FOV (K max & K min ) -> I-space resolution Full k-space Inner (Central) K-space Outer (Peripheral) K-space K-space Over-sampling Phase / Slice Over-sampling Multiple Averages or Number of Ecitations (NE). Short-term (k-space) average Long-term (I-space) average Phase over-sampling. Overlapping segments/trajectories (Propeller/BLADE). Variable density sampling (Radial/Spiral). max ~ /RES Phase D ~ /FOV Phase
Partial k-space Acquisitions Asymmetric (Fractional) Echo Asymmetric Echo Phase Partial Fourier / Slice Partial Fourier Zero-padding and Interpolation Elliptical Scan Parallel Imaging (SENSE and GRAPPA) Echo Phase / Slice Partial Fourier Question Can one combine asymmetric echo AND partial phase Fourier at the same time? Why? Zero Padding (ZIP / Interpolation) Elliptical Scan / k-space Filters Elliptical Filter Gaussian Filter Hamming Filter
Gibbs Ringing Artifact K-space Truncation Ringing around high contrast objects Abrupt transition in k-space data such as truncation (at the edge of k-space) or acquisition error. Expand k-space coverage Increase the number of encoding steps Increase k-space step size, i.e. reduce FOV Smooth the transition with k- space filter. Artifacts Caused K-space Error Parallel Imaging with Phased Array Ky Truncation -> Ringing White pixel (Spike) -> Wave Odd/Even Modulation -> N/ Ghost (in EPI sequence) Transition/Decay Modulation -> Blurring (in Magnetization Prepared, Steady State or FSE acquisition) Reference Lines Kx View Ordering for D View Ordering for D Sequential 4 5 Centric 7 5 Shorter Eff. TE for FAST or TURBO acquisition k z Linear (Sequential) Centric Square Spiral (Recessed) Elliptical Centric 6 7 8 4 6 8 Reduce the time span for acquiring central k-space data. 4
Contrast Concentration /4/009 Elliptical Centric View Ordering CE-MRA Acquisition Timing k z Ordered according to k-space distance. Optimized for dynamic contrast. Disperse artifacts as pseudo noise. Facilitates skipping of k-space corners and reduce scan time by %. Bolus Injection Patient Specific Transit to K-space Center Artery Vein Fluoro Triggering (RT D @ ~ fps) D with Recessed Elliptical Centric View Order Non-Cartesian K-space Sampling Radial Spiral BLADE/PROPELLOR Re-gridding Cartesian K-space Radial K-space Sampling Radial K-space Higher sampling density for central k-space. Higher spatial and/or temporal resolution. Isotropic in-plane resolution. No phase encoding and no phase wrap at smaller FOV Less sensitive to motion Radial streaks, especially near FOV edge. No Phase Wrap with Small FOV D Radial 7.5 s s 4.5 s Cartesian Radial VIPR Vastly undersampled Isotropic PRojection 8 s.5 s 5 s 8.5 s s 5.5 s 5
Spiral Data Matrix and Spatial Resolution Matrix size: res x res x Zres D interleaved spiral D stack-of-spirals Matrix of 56 x 8 x means: res = 56 Frequency-encoded points. res = 8 Phase-encoded steps. Zres = Phase-encoded steps (in nd direction). [ang et al, MRM 996] K-space Sampling in Dynamic Imaging Segmented acquisition View Sharing and Keyhole technique BRISK (Block Regional Interpolation Scheme for K- space) TRICKS (-Resolved Imaging of Contrast Kinetics) TWIST (-resolved anigography With Stochastic Trajectories) K-T Blast ECG Echo View Cardiac Phases Segmented Acquisition Window TR 4 5 6 7 8 9 0...??? 4 5 4 5... 4 5 n View sharing Radial View Sharing ECG Echo View Cardiac Phases Acquisition Window 4 5 6 7 8 9 0...??? 4 5 4 5... 4 5 TR 4 n Phase Phase Phase. 4 5 = Measured = Shared 6
Higher Resolution Keyhole Imaging C B A C B A B A C A B A C A B A C 4 5 4 6 4 C B A C A B C A B 5 4 Cartesian Real- Cine Echo-sharing 50 lines 55 ms frame rate 00mm x 00mm FOV.mm x 6.0mm resolution Radial Real- Cine Interleaved Echo-sharing 50 lines 55 ms frame rate 00mm x 00mm FOV.mm x.mm resolution B A C B A C 4 B A C 4 6 4 4 5 6 EC-TRICKS TWIST Ky -resolved MRA Contrast Size (%) Density (%) A D C B Kz A B A B A B Kx Dynamic Series MIP ABCD AB AC AD AB AC AD AB AC AD AB Courtesy of. Zhou, PhD PROPELLER or BLADE A low resolution image can be reconstructed from each segment, i.e. PROPELLOR or BLADE. Overlapping at central k-space: Oversampling -> longer scan time and higher SNR Allow co-registration and motion correction between segments Square FOV Streak artifact Thanou! ou may visit www.indiana.edu/~mri for more info about the lectures 7