Cerebral TOF angiography at 7T: Impact of B1 (+) shimming with a 16-channel transceiver array

Abstract

Purpose: Time-of-flight (TOF) MR imaging is clinically among the most common cerebral noncontrast enhanced MR angiography techniques allowing for high spatial resolution. As shown by several groups TOF contrast significantly improves at ultrahigh field of B0 = 7T, however, spatially varying transmit B1 (B1 (+)) fields at 7T reduce TOF contrast uniformity, typically resulting in suboptimal contrast and reduced vessel conspicuity in the brain periphery.

Methods: Using a 16-channel B1 (+) shimming system, we compare different dynamically applied B1 (+) phase shimming approaches on the radiofrequency excitation to improve contrast homogeneity for a (0.5 mm)(3) resolution multislab TOF acquisition. In addition, B1 (+) shimming applied on the venous saturation pulse was investigated to improve venous suppression, subcutaneous fat signal reduction and enhanced background suppression originating from MT effect.

Results: B1 (+) excitation homogeneity was improved by a factor 2.2-2.6 on average depending on the shimming approach, compared to a standard CP-like phase setting, leading to improved vessel conspicuity particularly in the periphery. Stronger saturation, higher fat suppression and improved background suppression were observed when dynamically applying B1 (+) shimming on the venous saturation pulse.

Conclusion: B1+ shimming can significantly improve high resolution TOF vascular investigations at ultrahigh field, holding strong promise for non contrast-enhanced clinical applications.

Figure 1

Location of the 3 overlapping TOF slabs, indicated in colored boxes, and corresponding positions of the 7 B₁⁺ calibration slices. 3 slices are located within each TOF slab while slices 3 and 5 are shared by two TOF slabs each.

Figure 2

|B₁⁺| maps calculated for subject 3 for the different shim settings (I-V), normalized by the mean |B₁⁺| within the ROI of each shim setting.

Figure 3

Native sagittal TOF images of subject 3 for B₁⁺ shim settings II, IV and V. A high B₁⁺ magnitude in the frontal brain area for the lowest slab (compare Fig. 2) reduces the static tissue signal for shim IV and introduces a significant intensity change between bottom and middle slab. This was recovered using 3 calibration slices for each TOF slab (setting V) as demonstrated in c). In comparison, a constant shim setting used for all 3 TOF slabs (setting II) avoids intensity variations at the junction of 2 adjacent slabs.

Figure 4

a–c) Quantitative summary of mean |B₁⁺|, CV and efficiency for different B₁⁺ shim settings (I–V), averaged over each of the 3 TOF slabs (white+gray bars) and average over all slabs (black bars) for all subjects. Diagram d) displays the squared inverse of the mean |B1+| of all three slabs relative to the CP mode, a quantity that is proportional to the global SAR for a given flip angle.

Figure 5

50 mm thick axial, coronal and sagittal MIP views obtained from receive profile corrected native TOF images in 2 subjects performed with saturation RF pulses applied in CP-like B1+ shim setting and with (a+c): CP like B1+ shim setting I and (c+d) with B1+ shim setting II used for the excitation RF pulse. Stronger contrast in particular for the smaller vessels can be appreciated when using shim setting II (see arrows). The last row in (a–d) shows thinner (30 mm) axial MIP images of the same datasets but without correction of the receive profile. The diagrams in (e+f) show line plots through the MIP images at the location indicated by the white horizontal dashed line.

Figure 6

Estimated B₁⁺ magnitude and corresponding efficiency for the different B₁⁺ shim settings (A–C) applied on the venous saturation RF pulse. All B₁⁺ shim settings were calculated based on ROIs (indicated in black) drawn in the upper 4 slices; the inner ROI indicates a region which was excluded for optimization.

Figure 7

Axial, coronal and sagittal MIP images of TOF acquisitions with CP-like shim used for excitation and a) without venous saturation applied (a), and with saturation applied using B₁⁺ shim setting A (b), setting B (c) and setting C (d). Red arrows signal suppression of subcutaneous fat signal. Yellow arrows indicate area with background signal reduction through MT.

Figure 8

Top row: native TOF images acquired with a CP-like shim setting. a) no saturation was applied, b–c) B₁⁺ shim settings A–C were applied for the saturation pulse. Bottom row: signal intensity ratio images of images b–c) normalized by image a).