Whole-brain B1+ mapping for bias correction in quantitative imaging

At a field strength of 3T, the accuracy of quantitative maps of MR parameters (Magnetization Transfer, longitudinal relaxation time T1, Proton Density) is severally affected by inhomogeneities of the transmit field B1+ (see figure 1) (1). A rapid and robust MR pulse sequence was developed to accurately map the B1+ field over the entire brain (2, 3). This sequence is run as part of the multi-parameter scanning protocol (1, 4) and is used in order to achieve highly accurate MR quantitative maps (http://www.fil.ion.ucl.ac.uk/Research/physics_info/QuantMRI_VBM.html)

 

Fig 1

Figure 1. a) R1 (=1/T1) map uncorrected for spatial variations of the B1+ transmit field (b). c) R1 map corrected for the B1+ inhomogeneities visible in b).

 

Challenges of B1+ mapping - optimization of the method at 7T

The rapid acquisition of highly accurate whole-brain B1+ maps is particularly challenging at high field strengths (≥3T) due the large range of B1+ and B0 inhomogeneities present over the brain. The B1+ mapping method that we developed is based on the acquisition of 3D EPI spin-echo and stimulated echo images (2, 3, 5). Extensive development work was done at 7T in order to: 1. Enhance the dynamic range of the method to improve its robustness against large B1+ deviations 2. Reduce the sensitivity of the method to B0 inhomogeneities (3). The main effects of these improvements can be seen in figure 2: 1. The accuracy of the method is improved around the temporal lobes and cerebellum where the B1+ field is particularly low. 2. The accuracy of the method is improved in regions such as the Orbito Frontal Cortex where strong B0 inhomogeneities are present. As a result of these improvements and extensive use of parallel imaging, accurate whole-brain B1+ maps could be acquired at 7T within a short acquisition time (4min34s).

 

Fig 2

Figure 2: Reductions in B1+ inaccuracy of the method due to its enhanced dynamic range (a) & its improved robustness against off-resonance effects (b).

 

Implementation at 3T

The optimization of the 3D EPI B1+ mapping method at 7T was adapted for 3T applications. At 3T, 11 nominal values of the spin-echo RF pulse are taken between 230° and 130° to allow for sufficient dynamic range of the method (total acquisition time: 3min) and RF pulses with maximal amplitude and bandwidth are used to minimize off-resonance precession effects. Figure 3 shows an application of the R1 maps extracted from the multi-parameter protocol and corrected using highly accurate B1+ maps (6). The aim of this study was the identification of functional brain regions using in-vivo myeloarchitecture. A number of functional regions could be clearly identified, exhibiting higher R1 values than their surrounding due to their higher myelination. While variations in R1 values between highly myelinated regions and their surrounding could be detected robustly across all subjects, the amplitude of these variations is particularly small (∼1%), emphasizing the importance of accurate B1+ mapping for in-vivo myeloarchitecture studies.

 

Fig 3

Figure 3. Inflated R1 maps sampled at 50% through the cortical layer. Variations in R1 values reflect changes in myelination and enable the identification of functional regions using in-vivo histology. The small –but robustly detected- variations in R1 values between highly myelinated regions and their surrounding illustrate the high level of accuracy of the B1+ mapping method.

 

Primary contact

Nikolaus Weiskopf (n.weiskopf «at» ucl.ac.uk)

 

References

[1]        Helms G, Dathe H, Dechent P. Quantitative FLASH MRI at 3T using a rational approximation of the Ernst equation. Magn Reson Med 2008;59(3):667-672.

[2]        Lutti A, Hutton C, Finsterbusch J, Helms G, Weiskopf N. Optimization and validation of methods for mapping of the radiofrequency transmit field at 3T. Magn Reson Med 2010;64(1):229-238.

[3]       Lutti A, Stadler J, Josephs O, Windischberger C, Speck O, Bernarding J, Hutton C, Weiskopf N. Robust and Fast Whole Brain Mapping of the RF Transmit Field B1 at 7T. PLoS One 2012;7(3).

[4]       Helms G, Dathe H, Kallenberg K, Dechent P. High-Resolution Maps of Magnetization Transfer with Inherent Correction for RF Inhomogeneity and T-1 Relaxation Obtained from 3D FLASH MRI. Magnetic Resonance in Medicine 2008;60(6):1396-1407.

[5]       Akoka S, Franconi F, Seguin F, Lepape A. Radiofrequency Map of An NMR Coil by Imaging. Magnetic Resonance Imaging 1993;11(3):437-441.

[6]       Sereno MS, Lutti A, Weiskopf N, Dick F. In vivo parcellation of the human cortical surface using quantitative T1 mapping and retinotopy. Submitted 2012.

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