532 Optimizing crusher gradients when using multiple presaturation pulses in Arterial Spin Labelling (ASL)

Scientific Poster

532 Optimizing crusher gradients when using multiple presaturationpulses in Arterial Spin Labelling (ASL)

D. A. Holm (Hvidovre/DK)K. Sidaros (Hvidovre/DK) Topic: Perfusion

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Purpose

The importance of the placement and number of presaturation pulses for improving ASLperfusion measurements has previously been established1 . Increasing the numberadversely causes incomplete static tissue subtraction (offset) effects in simulations, aswell as phantom and in vivo experiments. This study shows that static tissue suppressioncan be improved by using multiple presaturation pulses, without the adverse effects, byappropriate selection of crusher gradient area

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Methods and Materials

SimulationsThe Bloch equation was solved by numerical integration using Matlab. Simulations were carried out whilestepping the values of the presaturation crusher before inversion (presat) as well as the size of theinversion crusher. Simulations showed that the size of the crusher placed after the inversion pulse (postsat)did not affect the offset. Based on the results a set of optimal as well as suboptimal crusher values wereselected for the FAIR and PICORE experiments (see table 1 ). Suboptimal crushers were selected from theareas of the simulation that showed the largest offset.

MeasurementsPICORE2 and FAIR3 measurements were carried out on a Siemens Magnetom Trio 3TMR scanner. Measurements were performed on a phantom (T1/T2 1204/90) as well as in vivo (N=4). The crusher areas used can be found in table 1.

Presat crusher

Inversion crusher

Postsat crusher

PICORE optimal

43.14 82.82 41.41

PICORE suboptimal

82.82 131.0 41.41

PICOREsuboptimal 2

20.00 28.00 41.41

FAIR optimal 35.18 99.38 41.41

FAIR suboptimal

21.95 19.88 41.41

TABLE 1Crusher area in mT/m*ms. Presat crusher is the presaturation crusher placed before the 180o tagging pulse, while postsat is the

presaturation crusher placed after the 180o pulse.

The imaging parameters for the phantom experiments were:TE=20ms, TI=1000ms, TR=5000ms, FOV=196mm, 64x64 matrix, five 5mm slices with a spacing of0.5mm, acq 22. The gap between inversion and imaging slices (tag gap) was 5 or 10mm.

The imaging parameters for the in vivo experiments were: TE=20ms, TI=1200ms, TR=2000ms, FOV=196mm, 64x64 matrix, ten 5mm slices with a spacing of 0.5mm,acq 100-120. Tag gap 5 or 10mm. A reference scan, which was assumed to give complete static tissuesubtraction, was acquired with a tag gap of 13mm.

All images were divided by an M0 measurement which was performed as a single acquisition standard EPImeasurement with TE=20 ms.

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RF pulsesThe RF pulses were sinc pulses with a hanning filter except for the inversion pulse which was a hyperbolicsecant pulse. The duration of the presaturation pulses was 12.8 ms and the bandwidth was 1250 Hz. The 90o imaging pulse was 5.12 ms and the bandwidth 2031 Hz. The parameters for the 180o hyperbolicsecant pulse were mu=10, beta=800 and the duration was 15 ms. The inversion slab had a width of 10 cm in thePICORE experiment while it was the area of the slices plus one half of the tag gap on each side for the FAIRexperiments. An example of the presaturation part of a PICORE sequence is shown in figure 1.

Annotation: Example of a PICORE sequence. The saturation pulses before and after the inversion pulse are sinc pulses with ahamming filter while the inversion pulse is a hyperbolic secant pulse

FIGURE 1PICORE sequence example. The saturation pulses before and after the inversion pulse are sinc pulses with a hanning filter whilethe inversion pulse is a hyperbolic secant pulse.

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Results

SimulationsSimulations were performed while stepping the size of the crusher before the taggingpulse as well as the crusher of the tagging pulse.

Annotation: Simulated offset in percent of M0 vs. crusher area for the PICORE experiment. The left figure shows the offset for a taggap of 5 mm while the right shows the offset for 10 mm tag gap.

FIGURE 2Simulated offset in percent of M0 vs. crusher area for the PICORE experiment. The left figure shows the offset for a tag gap of 5mm while the right shows the offset for 10 mm tag gap.

As figure 2 shows the offset is comparable to the size of the perfusion signal (about 1%of M0) for small presat crusher areas as well as if the area of the presat crusher matchesthe area of the crusher for the tagging pulse. The offset is smaller at a tag gap of 10 mmthan 5 mm which is expected as the effect of imperfect inversion slice profiles is reducedas the tag gap is increased.

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Annotation: Simulated offset in percent of M0 vs. crusher area for the FAIR experiment. The left figure shows a tag gap of 5 mmwhile the right shows 10 mm tag gap.

FIGURE 3Simulated offset in percent of M0 vs. crusher area for the FAIR experiment. The left figure shows a tag gap of 5 mm while theright shows 10 mm tag gap.

For the FAIR experiment a large offset is again seen for small presat crusher areas andequal crusher areas (figure 3 ). The offset increases with tag gap for the FAIR experimentwhich might be explained by the inversion and presaturation profiles being less idealwhen their width increases.

Phantom experimentsThe simulation results were tested using phantom measurements. Only one of thecrusher areas were stepped in these experiments as it is not realistic to acquire imagesat all the crusher areas tested in the simulations.An example is shown in figure 4.

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Annotation: Offset in percent of M0 vs. presat crusher area. The red line is the results from the simulation while the blue line is theresult from the phantom measurement. The bars are SD.

FIGURE 4Offset in percent of M0 vs. presat crusher area. The red line is the results from the simulation while the blue line is the result fromthe phantom measurement. The bars are SD.

As the figure shows there is some qualitative agreement between the simulation andphantom results. Discreptanties exist and are most likely caused by B1inhomogenies. Similar agreement was found for most experiments.

In vivo experimentsThe images divided by M0 for a single subject is shown in FIGURE 5+6.

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Annotation: Perfusion weighted images from the PICORE experiment for a tag gap of 5/10 mm. The images are shown as imagesminus reference image (tag gap 13 mm) divided by M0.

Figure 5Perfusion weighted images from the PICORE experiment for a tag gap of 5/10 mm. The images are shown as images minusreference image (tag gap 13 mm) divided by M0.

Figure 5 shows the difference in perfusion estimates between the experiments and thereference scan for optimal as well as suboptimal crusher values. The offset from thereference image is quite small and is seen to decrease when the tag gap is increasedwhich is consistent with the simulation results. There is little difference between themeasurements using optimal and suboptimal crusher areas.

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Annotation: Perfusion weighted images from the FAIR experiment for a tag gap of 5-10 mm. The images are shown as imagesminus reference image (tag gap 13 mm) divided by M0.

Figure 6Perfusion weighted images from the FAIR experiment for a tag gap of 5-10 mm. The images are shown as images minusreference image (tag gap 13 mm) divided by M0.

In the FAIR experiment the perfusion images are very different for the optimal andsuboptimal crusher areas. The large offset seen in the suboptimal images is most likelycaused by unwanted refocused coherence pathways. The offset for the optimal crusherareas is seen to be larger at a tag gap of 10 mm than at 5 mm which is also consistentwith the simulations results.

In vivo results - multiple subjectsTo further investigate the offset seen in the initial experiments more subjects wereincluded and statistics were performed on four different ROIs. White matter anterior,white matter posterior, gray matter anterior and gray matter posterior. As the WM/GMcontrast in the PWI is very small the ROIs were drawn on the M0 images. Minormovement was seen between the different experiments and therefore the ROIs weredrawn so they were homogenous within all images. Partial volume effects are expectedas the inplane resolution is 3x3 mm.

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Annotation: WM = white matter, GM = gray matter, ant = anterior, post = posterior. Numbers 1-4 refer to 4 differentsubjects. The figure shows the median perfusion in the ROI divided by the median perfusion in the same ROI in thereference scan. Bars are SEM of the ROI divided by the same ROI in the reference scan.

FIGURE 6 - PICOREWM = white matter, GM = gray matter, ant = anterior, post = posterior. Numbers 1-4 refer to 4 different subjects.The figure shows the median perfusion in the ROI divided by the median perfusion in the same ROI in the reference scan. Barsare SEM of the ROI divided by the same ROI in the reference scan.

Figure 6 shows the perfusion in the experiment divided by the perfusion in the referencescan. This means that a value of one indicates identical perfusion in the two scans, whilea value of two means that the perfusion signal is twice as high in the experimentcompared to the reference scan. It is expected that the perfusion estimate should behigher in the experiments as the tag gap is only 5 mm compared to 13 mm in thereference scan, which means that the transit time will be lower resulting in more taggedblood reaching the imaging region within the inversion time. Similar to the results seen on the perfusion images little difference is seen between theoptimal and suboptimal crusher values within each subject which could suggest thatPICORE is less sensitive to changes in crusher area.

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Annotation: WM = white matter, GM = gray matter, ant = anterior, post = posterior. Numbers 1-4 refer to 4 differentsubjects. The figure shows the median perfusion in the ROI divided by the median perfusion in the same ROI in thereference scan for slice 1. Bars are SEM in the ROI divided the same ROI in the reference scan. Subject 3 SEM in WM antwas 23.5, cropped for better visualisation.

FIGURE 7 - FAIR - Slice 1 WM = white matter, GM = gray matter, ant = anterior, post = posterior. Numbers 1-4 refer to 4 different subjects.The figure shows the median perfusion in the ROI divided by the median perfusion in the same ROI in the reference scan. Barsare SEM of the ROI divided by the same ROI in the reference scan. Subject 3 SEM in WM ant was 23.5, cropped for better visualisation.

In the FAIR experiment (figure 7 ) the perfusion ratio is seen to be fairly constant between subjects for theoptimal crusher values, while it is very large and varies between subjects for the suboptimal crusher values.This means that suboptimal crushers overestimate perfusion. The effect is present in most ROIs and in allslices. Furthermore the SEM is often higher for the suboptimal crushers which might suggest that the resultoptained using suboptimal crushers are less stable. Consequently it is necessary to optimize the crusherarea if using FAIR with both presat and postsat.

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Conclusion

The choice of crusher values in FAIR can severely affect perfusion measurements. Ifusing multiple presaturation pulses, the area of the different crushers should not match,as this leads to a very large offset. This offset is caused by the tag image, where a spinecho is followed by a stimulated echo.

In the case of PICORE, the effects of imperfect crusher areas are small at a tag gap of 10mm, but can not be ignored for smaller tag gaps.

This study has shown that optimization of crushers can be essential for perfusion measurements if usingpresaturations before and after the inversion pulse. Other experiments including multiple crushers beforeand after the inversion pulse have shown results similar to the ones in the PICORE experiment. In someexperiments the optimization of crusher areas could even lead to reduction of the needed tag gap resultingin an increased SNR.

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References

1Sidaros,K., et al. [2003] Proceedings Eleventh ISMRM, p.2214

2 Wong,E.C., Buxton,R.B., & Frank,L.R. Implemantation of quantitative perfusion imaging techniques for functional brain mappingusing pulsed arterial spin labeling. [1997] NMR Biomed. 10, 237-249.

3 Kim,S.G. Quantification of relative cerebral blood flow change by flow-sensitive alternatinginversion recovery (FAIR) technique: application to functional mapping. [1995] Magn Reson. Med. 34, 293-301

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The Authors

David A Holm, MSc (Hvidovre) (davidh@drcmr.dk)Karam Sidaros, PhD (Hvidovre)

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