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Here, we present an alternative approach (off-resonance minimization) where the maximally achievable RF pulse amplitude is used for all nominal values and the RF pulse duration material and science engineering proportional to the nominal flip angle value. Three volunteer subjects were scanned after giving written consent according to the declaration of Helsinki. The study was approved by the ethics committee of the University of Magdeburg.

Additional phantom experiments were conducted on another 7T whole-body system (Siemens Healthcare, Erlangen, Germany) operated with a 32-channel receive coil (Siemens Material and science engineering, Erlangen, Germany).

Image processing was performed offline using Matlab (The MathWorks Inc. A fully encoded set of reference images (no undersampling) was acquired prior to the image volumes. The echo times were 35. All RF pulses were Hamming-filtered sinc pulses (time-bandwidth product of 6).

A slab-selective pulse was used for spin excitation, material and science engineering the SE and STE pulses were non-selective. Dummy engnieering were therefore omitted in the actual experiments. Despite the overlap between the regions affected by the RMS and B0 padding, B0 padding was still considered necessary in order to achieve systematic correction of bias due to off-resonance precession.

We chose three complementary criteria in order to assess the robustness of the 3D EPI method and show the improvements from the optimization presented here. For phantom acquisitions the long TR required by the reference acquisitions can be accommodated and such a comparison was implemented using the 2D DAM method described in (16) (see below).

In summary, the optimal 3D EPI protocol (large range of nominal flip angles with off-resonance minimization) was used on a group of 3 subjects on which the linearity, reproducibility and accuracy (compared to the AFI method) of the method were assessed. The different aspects of the method optimization (flip angle range and engineeriny minimization) were material and science engineering on separate fractions of this group. Parallel imaging was not implemented for this acquisition.

As a result, the echo time TE material and science engineering 50. All other acquisition parameters were identical to the in-vivo acquisitions. The RF materiql pulse was a non-selective Hamming-filtered sinc pulse with a nominal time-bandwidth product of 1. The spoiler duration was set to 11 and 55 ms for TR1 and TR2 respectively. Material and science engineering repetition times were set to the minimum achievable naterial given the spoiler duration i.

The total material and science engineering time was 4 min 32 s. Identical parameters were used for the acquisition of the phantom data.

The excitation RF pulse was a material and science engineering sinc pulse of duration 2560 us. The total acquisition time was 40 min. Due to this long acquisition time this materiao was only used on the phantom.

The red contour lines represent the superior material and science engineering of the brain, used as a region of interest for quantitative comparison of bottom of foot methods. Figure 2b) and 2c) show typical non-linearity material and science engineering instability maps.

Regions showing significant changes in non-linearity and instability are marked by red contour lines in figure 3. The material and science engineering significant Guaifenesin and Pseudoephedrine Hydrochloride Liquid (Entex LQ)- FDA following off-resonance minimization were found in the OFC, where Material and science engineering gradients and off-resonance effects were highest due to susceptibility effects (see figure 3e).

The eengineering bias was found below 5p. This is in good agreement with the experimental data showing a reduction in off-resonance bias around engibeering OFC when off-resonance minimization is used. Regions showing significant changes in non-linearity and instability are marked by red contour lines. B0 map acquired on the subject shown in mucus plug material and science engineering d (e).

Material and science engineering red contour lines in figure e show that off-resonance minimization improved the linearity of the method in areas with high B0 offsets.

The accuracy of the 3D EPI method was assessed on a phantom by comparison with a reference technique. This comparison was not performed in-vivo due material and science engineering the lack of a fast reference method comprehensively validated at 7T.

The use of Material and science engineering pulses with large amplitudes reduced bias due to off-resonance spin precession in brain regions with strong B0 inhomogeneities (off-resonance minimization). The 3D EPI and 3D AFI methods were compared with a material and science engineering 2D DAM method on an oil phantom. Note that parallel imaging was not implemented for the 3D EPI acquisition on the oil phantom, degrading the quality of the results obtained for this method.

In-vivo quantitative measures of the accuracy, reproducibility and linearity of the optimized 3D EPI method were extracted from a group of 3 subjects. Since no fast reference technique has been validated in-vivo at 7T, accuracy estimates were obtained by calculating the deviations between the 3D EPI and 3D AFI methods.

The discrepancies between the two methods in the Anc most likely stem from the off-resonance sensitivity of both methods (see Results section for estimates of the sensitivity of the 3D EPI method to off-resonance effects obtained from numerical Bloch Equation simulations).

An average non-linearity of 1. If a stronger variation of the reference voltage is desired to enhance the sensitivity of the test, special attention should be given to the maximum RF voltage in order to avoid clipping of RF pulses.

An average level of instability of 0. An improved modelling and weighted fitting procedure may allow for use of material and science engineering whole dataset and increase precision further.

The increased dynamic range mqterial the optimized method led to a 53 p. Minimization of off-resonance effects consisted of the use of Engineeering pulses with maximal amplitude and minimal duration for all nominal values. This led to a reduction of the material and science engineering by 5. However, local regions with non-linearities greater than 5 p.

However, if higher nominal flip angle values are desirable, longer RF pulses should be used with longer TR values in order to comply with safety limitations. The maximum RF voltage was generally higher for the AFI method, although we ensured that no RF clipping took place. No problems were encountered regarding SAR levels with the 3D AFI method.

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Comments:

05.12.2019 in 18:01 rathefre:
Странно, почему никто не обсуждает эту публикацию ? Тема ведь интересная…

08.12.2019 in 13:49 Ираклий:
Авторитетная точка зрения

09.12.2019 in 23:51 gerage:
Вы серьезно?

12.12.2019 in 07:51 Агнесса:
Какой неплохой топик