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Sample radial spokes from -kmax to kmax Trajectory design considerations (resolution, #shots)

Reconstruction, PSF and streak-like aliasing

SNR considerations

Undersampling

3D Projection

359

kx

ky

Projection-Reconstruction (PR) Sequence

RF

Gz

Gy

Gx

Signal

TE

360

sin()

cos()

Shortest TE (~0) of any sequence

Low first-moments

Fastest way to reach high-spatial frequencies

Impact of delays

Can do odd/even sampling

Impact of ramp sampling

361

kx

ky

RF

Gz

Gy

Gx

Signal362

kx

ky

sin()

cos()

Often Replace w/ Non-Selective

Excitation

ky

kx

Projection (PR): Design Considerations

Same as Cartesian

Same as Cartesian

Number of angles?

May undersample

363

Radial/Projection: Recon and SNR Usually use gridding

Density (D = kmax/kr)

compensate by multiplying by 1/D =kr

How does this affect SNR?

More samples required to cover a given area

Noise variance is altered by gridding reconstruction

Noise is colored (Speckle or salt and pepper)

Efficiency:

364

~ 0.87 for Uniform-density projections

=AqR

A DRA 1/D

See example later...

Projection-Reconstruction PSF / Undersampling PSF has a ring of aliasing (less coherent)

Intuitition: No preferred direction for coherent peak

Undersampling tends to result in streak artifacts

365

Fully-sampled PSF Undersampled PSF

From Scheffler & Hennig, MRM 1998

Undersampled PR: Streak Artifacts

With reasonable undersampling the artifact is often benign

366

J.Liu, A.Lu, A.Alexander, J.Pipe, E.Brodsky, D.Seeber, T.Grist, W.Block, Univ Wisc.

Radial Out - Odd vs Even #Spokes Odd N is a half-

Fourier trajectory

Difficult to do homodyne, but quadrature aliasing

367

ky

kx

Even, Magnitude Odd, Magnitude

Even, Real Part Odd, Real Part

Global Delay: Shift center

Inter-axis: May miss center

Global Delay: Shifts along traj.

Inter-axis: Warping of trajectory

368

Gy

Gx

kykx

ky

kx

Off-Resonance Effects

369

ky

kx

ky

kx

Full Projection (- to variation)

ReferencePSF

ReferencePSF

3D Projection-Reconstruction Encode in kx, ky, kz (, end-point parameterization)

Density (1/kr)2, compensate by multiplying by kr2

SNR efficiency now 0.75 (See example later)

Can undersample more though!

370

J.Liu, A.Lu, A.Alexander, J.Pipe, E.Brodsky, D.Seeber, T.Grist, W.Block, Univ Wisc.

Temporal Radial Patterns Reduction R gives a fully-sampled

image with R lower resolution

Vary spatio-temporal resolution

KWIC: k-space weighted image contrast

Field map generation by delaying odd vs even lines

Golden-angle increment:

Arbitrary Ns

Non-Cartesian, requires gridding reconstruction

Incoherent undersampling artifact (similar to CS)

Short TE (and UTE) imaging

2D and 3D options

No phase-encoding ~ can be efficient

Off-resonance causes blurring

SNR efficiency loss due to high-density near center, but resampling the center can be advantageous

372

Spiral

Low first-moments

Archimedean, TWIRL, WHIRL

Variable-density

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Archimedean Spiral Radius proportional to angle

Provides uniform density with N interleaves

Extreme case: single-shot with N=1

kx

kyk(t) = N

2 FOVei

Challenge is to design (t) to meet constraints

374

max

=2

Nkmax

FOV

Stopping point:

Archimeden Spiral Design

Begin with spiral equation:

Differentiate to obtain dk/dt and d2k/dt2

Amplitude limit: dk/dt < /2 Gmax

Slew limit: d2k/dt2 < /2 Smax

375

k(t) = N2 FOV

ei

Solution Options

1. Approximations for (t) (Glover 1999) Consider slew-limited and amplitude-limited regions

2. Solve numerically at each point Find all limits, use active limit

Can include circuit model easily

(Meyer, King methods ~ 1995)

Both methods allow variable density design (Kim 2003)

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Example Spiral Waveforms

377

Slew-Limit at First

Archimedean|k| linearly increasing

Moments are small at low |k|

Variable-Density Spiral

Undersample outer k-space

Vary spacing (1/FOV) with |k/kmax| or

Increase spacing along trajectory

378

k(t) = N2FOV()ei

Spiral Point-Spread Functions Swirl artifacts from

undersampling

Again, odd/even selection applies

Variable Density: Less coherent

379

Even, Magnitude Odd, Magnitude

Odd, Real PartEven, Real PartVariable Density (Brightened)

Spiral Imaging Sequence

RF

Gz

Gy

Gx

Signal380

TE

Off-Resonance Sensitivity Uniform-density - ~ A|k|2

Off-resonance correction in recon

381

Minimal # Interleaves, Scan Time

High SpatialResolution

1. Choose tolerable readout duration2. Trade scan time (#interleaves) for spatial resolution

0.5 3.0Spatial Resolution (mm)

0 s

0.6 s

Tem

pora

l Res

olut

ion

382

TWIRL / WHIRL Faster start, with radial segment

TWIRL: Jackson 1990

WHIRL: Pipe 1999

Non-archimedean spiral

constrained by trajectory spacing

Faster spiral, particularly for many interleaves

whirl.m on website

383

1/FOV

Archimedean:kr direction

1/FOV

WHIRL: perpendicular to trajectory

WHIRL vs Archimedean Spiral

384

10% Shorter

WHIRL

Archimedean Spiral

25 interleaves, 24cm FOV, 1mm resolution

Spiral in - Spiral out

Useful for delayed TE

Perhaps also for spin echo

time

Glover 2000

385

3D Methods: Spiral Stack, TPI, ConesStack of Spirals

kz

kxky

kx,y

kz(Irarrazabal, 1995)

386

Many variations (spherical stack of spirals) Density-compensated cones, TPI 3D design algorithms get very complicated

Spiral rewinders

Phase-encode/rewind

spoilers

Consider 3D rotation again

Arbitrary path

Speed always helps efficiency

387

Approaches to Rewinder Design

Goal: Bring G and k to zero quickly

Just use trapezoids Problem:

How much power to use on each axis

Finite segment method solutions (Meyer 2001)

Convex Optimization

k-space

388

Delays can have a few effects:

Mis-mapping of center of k-space, causes a low-frequency cloud

In outer k-space, delays cause rotation of the image

Other effects can be similar to radial

Often measure actual gradient waveforms (later I hope)

Study in homework!

389

Spiral Summary

Center-out: TE~0, Low first-moments

Archimedean, TWIRL, WHIRL, variable-density

PSF with circular aliasing, swirl-artifact outside

Off-resonance sensitivity, correct in reconstruction

Variations: Spiral in/out, 3D TPI, 3D Cones

Rewinder design

390