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S. Department of Physics, University of Surrey, Guildford, GU2 5XH, UK. Dr. S. J. Doran. Burst: Applications in Ultra-Rapid Imaging and Quantitative Diffusion Measurement. Simon J Doran Department of Physics, University of Surrey. Acknowledgements. Marc Bourgeois (ICR) - PowerPoint PPT Presentation
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Burst: Applications in Ultra-Rapid Imaging and Quantitative Diffusion Measurement
Simon J Doran
Department of Physics, University of Surrey
S Dr. S. J. Doran Department of Physics,University of Surrey,Guildford, GU2 5XH, UK
Acknowledgements
• Marc Bourgeois (ICR)
• Claudia Domenig (UniS)
• Andy Dzik-Jurasz (ICR)
• Martin Leach (ICR)
• David Collins (ICR)
• Claudia Wheeler-Kingshott (IoN)
• Roger Ordidge (UCL)
Summary
• Introduction to Burst and background Basic concept
Historical survey
• Single-shot Burst imaging Burst variants, SNR comparison and choice made
Problems to overcome
Comparison of techniques in phantoms and in vivo
• Quantitative diffusion imaging extra-cranially Application of Burst for diffusion measurements
Early results and analysis
Comparison of Burst and other techniques at 1.5 T
Introduction: Basic concept
• Burst is a rapid imaging technique, first proposed by Hennig in 1988.
• A series of low angle pulses creates a train of echos, which can be used to form an image.
Burst pulse train(64 low-angle pulses)
180o Train of 64 echos
Gread
Gphase
Advantages of Burst
• Using a slice-selected Burst sequence, all the signals can come from pure spin-echoes.
Little geometric distortion or “signal drop-out” in regions of large susceptibility change
Better off-resonance properties than EPI — no need for fat-sat
• Less rapid gradient switching than EPI dB/dt issues not a problem from a safety point of view
Can be acoustically very quiet
• Lower RF power deposition than HASTE
• Extremely robust no shimming or set-up period required 20
30
40
50
60
70
80
90
100
1 2 3 4 5 6 7 8 9 10
Ambient Noise
EPI
Burst
dB
Disadvantages of Burst
• Low signal-to-noise
Intrinsically low SNR due to low flip angle pulses
(Relatively) high acquisition bandwidth (e.g., 10 s / point), but still much lower than EPI at 4.7 T
• Signal Decay
Diffusion and T2 during both excitation and read periods
This is both a disadvantage (for single-shot imaging) and a valuable feature (for diffusion imaging)
Literature survey: (1) Basic sequence
• This type of pulse sequence has been extensively studied in a non-imaging context.
Kaiser, Bartholdi and Ernst, J. Chem. Phys. 60, 2966 (1974)
Hennig et al. MRM, 3, 823 (1986)
• First images published in 1993 by Hennig and Hodapp (MAGMA, 1, 39-48) and Lowe and Wysong (JMR 101, 106)
• Burst is a variant on the DANTE sequence
spectroscopy: Morris and Freeman,JMR 29, 433 (1978)
cardiac tagging: Mosher and Smith,MRM 15, 334 (1990)
Data: McVeigh and Atalar
Literature survey: (2) Linear approximation
• Easiest theoretical treatment by assuming linear approximation, i,e., each pulse causes one echo. However, this works only for very low pulse angles.
• In practice, the Bloch equations are non-linear and higher order echoes occur.
• Interference between spin and stimulated echoes reduces the echo amplitudes.
Simulations from Zha and Lowe, MRM 33, 377 (1995)
Literature survey: (3) Phase modulation
• We can also look at the problem in the frequency domain. We get a small signal because only a small fraction of the sample is excited.
One pixel
• Zha and Lowe (MRM 33, 377 (1995)) showed that by suitable phase-modulation of the low-angle pulses, one can excite the sample almost completely and obtain the desired echo train.
One pixel
Literature survey: (4) Optimisation
• Several authors have considered optimisation of the Burst excitation pulse train.
Le Roux et al. Chirp pulses Proc. 10th SMRM , 238 (1991)
Zha and Lowe, OUFIS, MRM 33, 377 (1995)
van Gelderen et al. JMR B, 107, 78 (1995)
Heid, MRM 38, 585 (1997)
• The bottom line is that for N excitation pulses, i.e., N echoes, the pulse flip angle should be at most
2 N
as opposed to / 2N for the non-optimised pulse train.
• For 64 pulses, this equates to 11.25° still poor SNR
Literature survey: (5) Burst variants
• Burst can be seen as “simply” a means of generating multiple echoes.
• As such it can be incorporated into many standard sequences.
Radial imaging: Jakob et al., 36, 557 (1996)
SSFP: Heid, Proc. 8th ISMRM, 1499 (2000)
STEAM: Cremillieux et al. MRM 38, 645 (1997) (6 64 64 images in 210 ms)
Burst pulse train(16 low-angle pulses)
180o Train of 16 echos
Gread
Gphase
4
HASTE (BASE): van Gelderen et al. MRM 33, 439 (1995); Zha et al. Proc 5th ISMRM, 1820 (1997)
Burst pulse train(9 low-angle pulses) Multiple trains of 9 echos
Gread
RF
N / 18
EPI (URGE-EVI): Heid, Proc. 3rd ISMRM, 98 (1995)
Burst pulse train(9 low-angle pulses) Train of 9 echos
Gread
RF
N / 9
FLASH (URGE): Heid et al., MRM 33, 143 (1995)
Multi-refocusing Burst: (1) Excitation
• The original design of Burst sequence has two major problems with its excitation scheme:
The entire sample is excited by the train of hard pulses, so multi-slice acquisitions are not possible.
Image profile
Theoretical profile from -pulse frequ-ency response
Pixel NumberSig
na
l in
ten
sit
y /
arb
. u
nit
s
Although overall RF energy deposition is relatively small, the peak power required is excessive, because it needs to be applied as a short pulse.
Multi-refocusing Burst: (2) Selective excitation
• A solution to both problems is found by using selective excitation (van Gelderen et al. MRM 33, 439 (1995))
• However, this removes several of the key advantages of Burst. Now the sequence becomes noisy, is highly demanding on the gradients and we get some artifacts.
Gread
Gslice
N
RF
Gread
Gslice
N / 2
RF
Unipolar scheme Bipolar scheme
RF frequency offset inverted
Multi-refocusing Burst: (3) Excitation artifacts
• In the presence of B0-inhomogeneities, the bipolar scheme gives rise to slice-definition inconsistencies. Not noticeably a problem.
• However, we do see significant differences in echo phase.
Echo number
Ech
o ph
ase
/ rad
Echo number
Ech
o ph
ase
/ rad
Slice offset = 0
Slice offset = 60 mm
Raw Corrected
Multi-refocusing Burst: (4) Echo phase
• The most significant problem in developing the multi-refocusing Burst sequence at 1.5 T on the Siemens Vision is the unwanted variation in echo phase.
• A standard FT reconstruction algorithm assumes that, in the absence of the phase-encoding, all echoes have the same phase.
• In fact, we observe the phase to change in the following ways:
continuously during an echo train
discontinuously between echo trains
alternating when we use the bipolar slice selection
with an amplitude of variation that depends on the slice offset from isocentre
• The cause of these phase variations is still uncertain, but may be an eddy current effect.
Multi-refocusing Burst: (6) Echo phase examples
Continuous phase change during a single readout of 64 echoes
Alternating phase during a single readout of 64 echoes (bipolar slice gradient), small slice offset
Alternating phase during a single readout of 64 echoes (bipolar slice gradient), large slice offset
(NB Phase needs unwrapping!)
Discontinuous phase change during a multi-refocusing readout of 12 6 echoes
Multi-refocusing Burst: (6) Echo phase artifact
• Uncorrected, the echo phase problem gives rise to a serious artifact.
• With suitable correction, using a non-phase encoded echo train, the artifact can be mostly removed, but the remaining artifacts still degrade the performance of the sequence.
• The major unsolved problem is to achieve the correction in regions of the body that move between the non-phase encoded scan and the “image” scan.
Early results at 1.5 T
• Comparison of original OUFIS with “off-the-shelf” EPI on Siemens Vision.
• Slice deliberately chosen to highlight problems with EPI.
• Poor resolution and SNR, but excellent geometric fidelity, particularly around air spaces
• Note the difference in contrast.
64-pulse OUFIS
128 128 EPI
Very (!) early results at 4.7 T
• Results after approximately two days on 4.7 T system in factory environment
• Single-shot 642 image (partial Fourier, reconstructed to 64 112) acquired at 4.7 T
• Poor resolution and SNR, but excellent geometric fidelity, particularly around air spaces
• Note: no need to shimCompare the EPI acquired at the same time (shimmed to get best results on top slice).
Recent comparison at 1.5 T
• “Original” Burst (OUFIS)100 ms, 3.63.6 mm2, TEeff~10 ms, SNRN=2.3, SL=7mm
• Refocussed Burst238 ms, 1.8 1.8 mm2, TEeff~25 ms, SNRN=8.3, SL=7 mm
• EPI248 ms, 1.8 1.8 mm2, TEeff~90 ms, SNRN=27, SL=7 mm
• HASTE344 ms, 1.25 1.25 mm2, TEeff=?, SNRN=48, SL=7 mm
Refocused Burst“Original” Burst
EPI HASTE
SNRN = SNR / ( (acq. time)1/2. (pixel area) )
Extra-cranial imaging at 1.5 T: pelvis
HASTE
EPI + “fat sat”
“New” Burst
• Pelvic imaging is important for diagnosing rectal and prostate cancers.
• HASTE is currently the method of choice for single-shot imaging, but RF power deposition is a potential problem.
• EPI is not widely used because of the presence of fat.
• Burst works “quite well”.
SNR: Comparison with EPI
• Ignoring artifacts, the key relationship is SNR sin / BW 1/2.
• On the Siemens Vision at 1.5 T, we have shown that the SNR of EPI is approximately a factor of 3 higher than our best Burst.
• At higher field, a spin-echo based Burst sequence could be read out at the same BW, whereas the EPI sequence would be likely to require a much higher bandwidth.
Image contrast (1)• What sort of information can we get out of Burst images?
• Contrast properties of Burst images very little studied so far.
2/
0
TTEDb
jjj eeAA
• Sequence is inherently T2 and D weighted. For low flip angles
• By adjusting TE and the read gradient, we can emphasise either T2 or diffusion decay.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35 40
Echo Number
A /
A0
Data for CuSO4
T2 and D double fit
Typical Spectroscopic Data Typical Image Data
Can we use the decay to get D and T2?
ADC and T2 Burst sequence
• A data array of n echoes is acquired for each PE step.
• Each echo, j, corresponds to the same k-space line of the same slice, but with a different ADC and T2 weighting.
• Corresponding echoes in successive arrays are used to reconstruct a given image.
In one scan we collect n images weighted by ADC and T2.
Ph
ase
enco
de
Readout
D, T 2
j = 0
j = n-1
Burst images SE images
ME images
First in vivo images (8 T)
Quality of double-exponential data fit
Echo number Echo number
Echo number Echo number
Aj / A
0A
j / A
0
Aj / A
0A
j / A
0
ROI1
ROI2
ROI3
Typical single pixel fit
First human study (1.5 T)
• Multi-functional rectal carcinoma study
• Images had very poor SNR, so analysis performed on ROIs in tumour.
• Remarkable correlation between tumour ADC and treatment success.
But are we really measuring diffusion?
• The data fit moderately well to a bi-exponential function.
• There are several possible explanations:
genuine IVIM perfusion effect
incorrect T2 correction
motion
many different ADC values in the ROI
Burst single-shot imaging: Conclusions
• Burst has been around for a “long” time (10 years), but has never really caught on.
• It has a number of attractive features, most notably that it can be made almost impervious to susceptibility, giving undistorted images.
• The SNR has been improved by a factor of approximately 30 since the original introduction of the sequence, but is still quite low.
• The contrast of the sequence needs investigating further.
• Our 3-year EPSRC project came to the conclusion that Burst is “almost competitive”, but not quite on the hardware we used.
• Application at higher fields remains an attractive possibility.
Measurement of diffusion with Burst: Conclusions
• Burst gives us a potentially exceedingly time-efficient way of obtaining many b-values in the same measurement.
• The SNR in the original measurements was low, but we have researched a number of ways of improving this.
• There are still a number of technical difficulties with the approach, the most serious of which is motion.
• This makes it as yet unclear whether the values we are getting from Burst are correct or not.
• We have performed extensive phantom and initial in vivo comparisons with three other diffusion imaging sequences: split-echo HASTE, PSIF and segmented EPI.
Choice of Burst sequence to develop
• Aim: medium resolution, single-shot, multi-slice dataset
• Choice made on basis of expected SNR.
Chosensequence
“Original” Burst
Multi-refocusing Burst: (4) Unwanted echoes
• The need for crusher gradients around the 180° pulse can be understood by the use of an extended phase graph.
180 180 180 180
...
• Everywhere that magnetisation crosses the central axis, an echo is formed. (Not all paths from the original - pulses are shown.)
• Higher order echoes are superimposed on the desired spin echoes.
Multi-refocusing Burst: (5) Unwanted echoes
• We can separate out the unwanted echoes by changing the gap between the last - pulse and the first 180°.
180 180 180 180
...
• The unwanted echoes are small, but can be significant.
• A very sensitive test of the efficiency of spoiling is to acquire a non-phase-encoded dataset and FT all the echoes.
• Depending on where the unwanted echoes occur, the effect on the image may be slight or extremely serious.
Multi-refocusing Burst: (7) Segment offset artifacts
• The current implementation of the sequence uses mosaic tiling of the k-space segments.
• Eddy currents and poor performance of the Vision gradient system lead to offsets in the phase-encoding blip gradient of small fractions of kphase.
• These again lead to complicated multiple ghosting artifacts in the phase-encoding direction. The process can be simulated and, in principle, corrected.
Segment 1
Segment 2
Segment 4
Segment 3
kx
ky
Segment 1
Segment 2
Segment 4
Segment 3
kx
ky
Phantom comparison at 1.5 T
SNR=19, SL=10mm SNR=72, SL=10mm SNR=75, SL=5mm
“Original” Burst “New” Burst “Best” EPI
Image contrast (2)
• Do we need T2 contrast to see the activations?
E.g., Hutchinson et al. JMRI 7(2), 361-364 (1997)
• Potential method for getting T2 contrast …
Burst pulse train(64 low-angle pulses)
180o Train of 64 echos
Gread
Gphase
T2 delay
• In practice, this works with the basic Burst sequence, but we have not had any success in achieving T2* weighting with multiply refocused Burst.
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