Simultaneous B₀‐ and B₁+‐Map acquisition for fast localized shim, frequency, and RF power determination in the heart at 3 T

Abstract

High-field (!3 T) cardiac MRI is challenged by inhomogeneities of both the static magnetic field (B 0 ) and the transmit radiofrequency field (B 1 1). The inhomogeneous B fields not only demand improved shimming methods but also impede the correct determination of the zero-order terms, i.e., the local resonance frequency f 0 and the radiofrequency power to generate the intended local B 1 1 field. In this work, dual echo time B 0 -map and dual flip angle B 1 1-map acquisition methods are combined to acquire multislice B 0 -and B 1 1-maps simultaneously covering the entire heart in a single breath hold of 18 heartbeats. A previously proposed excitation pulse shape dependent slice profile correction is tested and applied to reduce systematic errors of the multislice B 1 1-map. Localized higher-order shim correction values including the zeroorder terms for frequency f 0 and radiofrequency power can be determined based on the acquired B 0 -and B 1 1-maps. This method has been tested in 7 healthy adult human subjects at 3 T and improved the B 0 field homogeneity (standard deviation) from 60 Hz to 35 Hz and the average B 1 1 field from 77% to 100% of the desired B 1 1 field when compared to more commonly used preparation methods. The increased signal-to-noise ratio at the static magnetic field (B 0 ) of 3 T may improve cardiac MRI and MR spectroscopy applications that are constrained by limited signal-to-noise ratio at 1.5 T (1,2). However, cardiac MR at 3 T is challenged by increased B 0 inhomogeneity due to susceptibility. Noeske et al. (3) reported B 0 field variations over the left ventricle of 6 130 Hz. Furthermore, the overall increased B 0 inhomogeneity makes the determination of the on-resonance frequency f 0 more challenging. The combination of increased B 0 inhomogeneity and potential off-resonance in the heart may lead to image artifacts and distortions, especially when applying balanced steady-state free precession sequences (4) or fat suppression.One way to improve B 0 field homogeneity is to perform localized shimming followed by a frequency scout prescan, in which a series of single-shot images with incremental f 0 are acquired (5). The f 0 that yields best image quality is then visually selected by the operator and used for the subsequent high-resolution scan. Another solution is to determine both localized second-order shim corrections and localized on-resonance frequency f 0 at the same time, based on an acquired B 0 -map (6).In addition to B 0 inhomogeneity, both numerical simulations (7) and measurements (8) have shown that the transmit radiofrequency (RF) field B 1 þ in the heart is more inhomogeneous at 3 T as compared to 1.5 T. Conventional methods to measure B 1 þ-maps, such as the dual angle method (9,10) or the actual flip-angle imaging (AFI) method (11), are rather time consuming and may not easily be used for the heart because of blood flow and cardiac/respiratory motion. Recently, the saturated double-angle method (SDAM) to acquire a B 1 þ-map covering the heart within a single breath hold ...

Accelerating Parallel Transmit Array B1 Mapping in High Field MRI With Slice Undersampling and Interpolation by Kriging

“…More generally, the kriging algorithm may help to shorten any MRI sequence, as long as the measured field varies slowly in space. For instance, it may be used for imaging inhomogeneity [37], receive sensitivity [38], or even temperature [39]. The kriging technique can also be used for offline post-processing.…”

Minimum acquisition methods for simultaneously imaging T1, T2, and proton density with B1 correction and no spin-echoes

“…However, this does not hold for FA>90°[29] where signals from the edge of the profile can greatly exceed sin( q .θ) at the slice center (Fig. 6)[30]. For full or slab-selective 3D MRI[31], the middle slice of an excited slab is typically unperturbed by the slice profile, and yields accurate results even with short, truncated slab-selective excitation pulses.…”

“…(10). In addition to a standard ‘sinc’ pulse excitation, the Philips’ scanner can provide ‘spredrex’ excitation pulses (Fig.6a), which have an improved slice profile at high FA across the center of the slice, as compared to a truncated ‘sinc’ pulse (Fig.6d) [30]. A Bloch equation simulation of the effect of slice profile imperfections on T 1 was performed by digitizing ‘spredrex’ pulses into 996 rectangular segments, with the pulse length, amplitude, and gradient strength set according to the scanner’s pulse viewer tool and T 2 =50ms and 100ms.…”

Myocardial T2* Mapping with Ultrahigh Field Magnetic Resonance: Physics and Frontier Applications

“…The frequency shift across the heart obtained at 7.0 T compares well with previous 3.0 T studies which reported a peak-to-peak off-resonance variation of (176 ± 30) Hz over the left ventricle and (121 ± 31) Hz over the right ventricle with the use of localized linear and second-order shimming [86]. The use of an enhanced locally optimized shim algorithm, which is tailored to the geometry of the heart, afforded a reduction of the peak-to-peak frequency variation over the heart from 235 to 86 Hz at 3.0 T [87]. Another study showed a peak-to-peak off-resonance of (71 ± 14) Hz for short axis views acquired at 1.5 T [88] using global shimming.…”