B₁ inhomogeneity correction of RARE MRI with transceive surface radiofrequency probes

Abstract

Purpose The use of surface radiofrequency (RF) coils is common practice to boost sensitivity in (pre)clinical MRI. The number of transceive surface RF coils is rapidly growing due to the surge in cryogenically cooled RF technology and ultrahigh‐field MRI. Consequently, there is an increasing need for effective correction of the excitation field ( B1+) inhomogeneity inherent in these coils. Retrospective B1 correction permits quantitative MRI, but this usually requires a pulse sequence‐specific analytical signal intensity (SI) equation. Such an equation is not available for fast spin‐echo (Rapid Acquisition with Relaxation Enhancement, RARE) MRI. Here we present, test, and validate retrospective B1 correction methods for RARE. Methods We implemented the commonly used sensitivity correction and developed an empirical model‐based method and a hybrid combination of both. Tests and validations were performed with a cryogenically cooled RF probe and a single‐loop RF coil. Accuracy of SI quantification and T1 contrast were evaluated after correction. Results The three described correction methods achieved dramatic improvements in B1 homogeneity and significantly improved SI quantification and T1 contrast, with mean SI errors reduced from >40% to >10% following correction in all cases. Upon correction, images of phantoms and mouse heads demonstrated homogeneity comparable to that of images acquired with a volume resonator. This was quantified by SI profile, SI ratio (error < 10%), and percentage of integral uniformity (PIU > 80% in vivo and ex vivo compared to PIU > 87% with the reference RF coil). Conclusion This work demonstrates the efficacy of three B1 correction methods tailored for transceive surface RF probes and RARE MRI. The corrected images are suitable for quantification and show comparable results between the three methods, opening the way for T1 measurements and X‐nuclei quantification using surface transceiver RF coils. This approach is applicable to other MR techniques for which no analytical SI exists.

Empirical Transmit Field Bias Correction of T1w/T2w Myelin Maps

“…A candidate alternative emerged in the course of developing an improved receive field bias correction approach for fMRI data termed the “SEBASED” correction (Glasser et al, 2016a), which enables biological interpretation of fMRI parameter maps (e.g., so that estimated task fMRI betas or variances have a consistent scale across space). In particular, differential transmit field effects were also present in the gradient echo EPI (echo-planar imaging) fMRI images and the spin echo EPI field map images [for the same reason that they are present in the T1w and T2w images as described above, i.e., the signal intensity depends on sin(FA*TF) for gradient echo images and sin 3 (FA*TF) for spin echo images, FA=Flip angle; TF=Transmit Field; Bonny et al, 1998; Collewet et al, 2002; Wang et al, 2004; Wang et al, 2005; Weiskopf et al 2011; Delgado et al, 2020]. For the purposes of fMRI receive bias correction, such transmit effects were nuisances that needed to be appropriately modeled (together with T2* induced susceptibility dropouts) to generate an accurate receive field estimate.…”

“…As such, the signal intensity of the T2w SPACE image is more affected by transmit field inhomogeneities than is the signal intensity of the T1w scan, which has the added benefit of using an adiabatic inversion pulse for the initial 180-degree preparation pulse that is designed to be resilient to transmit field inhomogeneities (Garwood and Ugurbil 1992). Notably, the effect of tbT1w / tbT2w on T1w/T2w varies spatially according to the transmit field and is multiplicative (Bonny et al, 1998;Collewet et al, 2002;Wang et al, 2004;Wang et al, 2005;Weiskopf et al 2011;Delgado et al, 2020) but does not substantially affect image contrast over the range of flip angles at 3T (Mugler 2014). Importantly, this effect is expected to vary across participants based on interindividual differences such as weight, Body Mass Index (BMI), or head size due to differential transmit coil loading and dielectric effects.…”

B1inhomogeneity correction of RARE MRI at low SNR: Quantitative in vivo19F MRI of mouse neuroinflammation with a cryogenically‐cooled transceive surface radiofrequency probe

“…The B 1 fields of the 19 F-CRP were characterized. 23 Separate sets of maps were determined using the low-T 1 uniform phantom and the high- 19 F concentration reference cap as follows:…”

“…21,22 We previously implemented three B 1 correction methods (model-based, hybrid, and sensitivity) for RARE 1 H-MRI and transceive surface RF probes, considerably increasing image homogeneity and significantly reducing errors in signal quantification and T 1 contrast. 23 The low SNR, signal sparsity, and lack of a priori location of the 19 F signal constrain the reliability of signal quantification, even after B 1 correction. A procedure that evaluates the quality of the SI correction and quantification per image voxel is thus crucial.…”

A multimodal analysis of Magnetic Resonance Imaging for the study of brain abnormalities in migraine: gray matter morphometry, white matter integrity and structural connectivity

“…The B 1 inhomogeneities can produce geometric distortions (S. M. . Non-uniform B 1 fields cause a reduction of the effective flip angle (Ramos Delgado et al, 2020). The variation of the effective flip angle is related to a change of the effective b-value.…”