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Evaluating 8-independent channel shimming strategies to drive a 16-channel loop-dipole transceiver body imaging array at 7.0 Tesla
M. Arcan Erturk1,2 and Gregory J. Metzger1

1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, 2Restorative Therapies Group, Medtronic, Minneapolis, MN, United States

Synopsis

Majority of the installed 7.0T systems have 8-independent transmit channels, therefore additional hardware changes are necessary to fully utilize higher channel count transceiver arrays (i.e. 16-channel loop-dipole body imaging array, 16LD). Here, we investigated three different 8-independent channel phase shimming strategies to drive 16LD and compared against fully independent 16-channel phase-only shimming. 8-independent channel shimming while transmitting power from all 16 array-elements with pre-determined phase difference between loop and dipole elements on the same block causes only about 10% drop in B1+ efficiency compared to 16-independent channel transmit.

Purpose

High channel count (≥16) transceiver arrays with good geometrical decoupling performance (S21 < -10dB) can be achieved by combining loop and dipole elements for 7.0T body imaging [1,2]. However, a majority of 7.0T MRI systems are equipped with 8-independent transmit channels. The goal of this work is to compare the transmit performance inside the body at 7.0T with phase-only shimming when using 8 versus 16 independent transmit channels when using a 16-channel loop dipole transceiver array [1].

Methods

A state-of-the art 16-channel combined loop-dipole transceiver array (16LD) [1] was modeled using Sim4Life (ZurichMedTech, Zürich, Switzerland) and simulated at three different body imaging locations (pelvis, torso, chest) in three Virtual Population human body models (Duke; Ella; and morphed Fats model with BMI=29, Fats29 [3]), Figure 1. Four imaging region-of-interests (ROIs) per human model are investigated (bilateral hips, kidneys and heart in all models, prostate in male models, uterus in the female model) yielding 12 unique ROIs in total (12 unique simulation configurations). ROIs are grouped into two: Group A: heart, prostate and uterus; Group B: bilateral hips and kidney ROIs. Four phase-only shimming strategies are employed:

  1. Gold-standard: 16-independent phases for each unique configuration; 12x16=192 degrees-of-freedom (DOF).
  2. Option 1: 8-independent phases for each loop-dipole block in each unique configuration, and a constant and identical phase difference between loops and dipoles on the same block; 12x8+1=97 DOF.
  3. Option 2: 8-independent phases for each loop-dipole block in each unique configuration, and 8 independent phase difference values between loops and dipoles on the same block that are kept constant between different configurations; 6x8+8=56 DOF per ROI group. Performed for Group A and B separately.
  4. Dipole-only: 8-dipoles are driven with independent phases, loops are not transmitting; 12x8=96 DOF.

Phase-only shimming in 12 unique configurations are performed using fully independent 16-channel excitation, Option 1, Option 2 and dipole-only 8-channel excitation. Shimming cost function is in the form of: min(λ.CoV-B1_eff), where CoV is the coefficient of variation of B1+ (field non-uniformity term), B1_eff is the average B1+ inside the ROI and λ is a trade-off constant between efficiency and uniformity terms. SAR is not included in the cost function because unlike B1+ it cannot be directly measured using an MRI scanner. For every shim solution, CoV, B1+ efficiency and B1+ SAR efficiency are calculated and normalized against gold-standard shimming.

Results

The phase differences between loops and dipoles on the same block are listed in Table 1. Optimal phase for Option 1 is 90° which is in accordance with Reference [1], where B1+ efficiency of combined loop-dipole excitation is higher when the phase difference between loop-dipole on the same block is around 60-120°. B1+ efficiency and B1+ SAR efficiency for Option 1, 2 and dipole-only shimming strategies normalized against the gold-standard shimming at same CoV (field uniformity) levels are plotted in Figure 2 and 3 for Group A and B, respectively. On average, 10.9% and 5.4% reductions in B1+ efficiency are observed for Option 1 and 2, respectively, compared against the gold-standard shimming. It is worth noting that some of the Option 1 and 2 shim B1+ SAR efficiency values are higher than fully independent 16-channel shim because SAR term was not included in the cost function. Normalized B1+ and SAR efficiency of dipole-only 8-channel transmit is 72.0% and 81.5%, respectively, lowest among investigated shimming strategies. Sample B1+ efficiency maps along axial slices inside the pelvis of the Fats29 model for four different shimming strategies are shown in Figure 4.

Discussion/Conclusion

In this work, we investigated 8-independent channel phase shimming strategies to utilize a state-of-the art 16-channel loop-dipole array [16LD, 1], because majority of the 7.0T systems utilize 8-channel transmit. Our simulation results show that 8-channel shimming while transmitting power from all 16 array-elements with pre-determined phase difference between loop and dipole elements on the same block underperform 16 fully-independent shim control by about 5% and 10% for Option 2 and 1 shimming strategies, respectively. Previous work [1] demonstrated that 16LD has >20% higher B1+ transmit efficiency compared to 10-channel fractionated dipole antenna [5] and 16-channel microstrip-line [6] arrays; and results presented here reinforces that even when using a limited 8-independent channel shimming approach, 16LD will outperform other body imaging arrays [5-6] in terms of B1+ transmit efficiency. Furthermore, more sophisticated parallel transmission management strategies [7] have the potential to compensate for the 10% penalty due to 8- vs 16-independent channel transmission.

Acknowledgements

Supported by: NCI R01 CA155268, NIBIB P41 EB015894.

References

1. Ertürk MA et al. MRM 2017;77:884-94.

2. Ertürk MA et al. ISMRM 2017;p1222.

3. Gosselin et al . Phys Med Biol 2014;59/18:5287-5303.

4. Metzger, GJ et al. MRM 2008;59(2):396-409.

5. Raaijmakers AJ et al. MRM 2016;75:1366-74.

6. Snyder CJ et al. MRM 2012;67:954-964.

7. Wu, X et al. MRM 2013;70(3):630-638.

Figures

Figure 1. Nine Sim4Life simulation setups are shown (top row: Duke model, middle row: Ella model, bottom row: Fats29 model). Loop-dipole blocks are numbered sequentially with blocks 1-4 placed on the posterior and 5-8 placed on the anterior side. (Left column) Bilateral hip and prostate/uterus ROIs, (middle column) bilateral kidney ROIs and (right column) heart muscle ROIs are investigated yielding 12 unique simulation configurations.

Figure 2. (Top) Relative B1+ transmit efficiency and (bottom) relative B1+ SAR efficiency for Option 1, Option 2 and dipole-only shimming strategies (blue: Option 1, yellow: Option 2, gray: dipole-only) normalized against gold-standard 16-independent are plotted for ROIs in Group A. Simulation configuration (body model and ROI) are listed on the right of the figure. B1+ efficiency is 83.5±3.4% and 91.6±4.7% for Option 1 and 2 shimming strategies, respectively. Normalized B1+ SAR efficiencies are 85.7±8.6% and 90.7±7.2% for Option 1 and 2 shimming strategies, respectively for Group A imaging targets.

Figure 3. (Top) Relative B1+ transmit efficiency and (bottom) relative B1+ SAR efficiency for Option 1, Option 2 and dipole-only shimming strategies (blue: Option 1, yellow: Option 2, gray: dipole-only) normalized against gold-standard 16-independent are plotted for ROIs in Group B. Simulation configuration (body model and ROI) are listed on the right of the figure. Normalized B1+ efficiencies are 94.7±1.6% and 97.6±1.2% for Option 1 and 2 shimming strategies, respectively. Normalized B1+ SAR efficiencies are 106.7±6.7% and 94.4±6.7% for Option 1 and 2 shimming strategies, respectively for Group B imaging targets.

Table 1. Phase difference between loop and dipole elements in the same block for Option 1 and Option 2 shimming strategies are listed. For 16-independent shimming, phase difference between loop and dipole elements are optimized for each simulation configuration, whereas phase difference between loop and dipole elements on the same block are kept constant between 12 simulation configurations and is 90° for Option 1. Phase difference between loop-dipole elements on the same block for Group A and B ROIs for Option 2 shimming are also listed.

Figure 4. B1+ efficiency maps along axial slices inside the pelvis of the Fats29 model for four different phase-only shimming approaches are shown (ROI: prostate). Average B1+ efficiency inside the prostate and peak 10g-averaged SAR are also displayed. Gold-standard (16-independent) shimming provides the highest efficiency followed by Option 2 shimming strategy.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
1509