Artery selective 3D TOF by Using Asymmetrically RF-shimmed Pre-saturation pulse with 4-channel RF Transmit at 3T
Kosuke Ito1, Atsushi Kuratani1, Yukio Kaneko2, and Masahiro Takizawa1

1Healthcare Company, Hitachi Ltd., Chiba, Japan, 2Research and Development Group, Hitachi Ltd., Tokyo, Japan

Synopsis

A technique of ICA-selective 3D TOF imaging using asymmetrically RF-shimmed Pre-saturation pulse is presented. By using 4-channel RF transmit coil, spatially asymmetric RF transmission is achieved. Then apply RF shimming parameter to Pre-saturation pulse, the saturation effect becomes spatially asymmetric, and achieved selectively visualize blood flow from each ICA. This technique provides high contrast between blood and brain parenchyma because it does not make TR longer.

Purpose

The purpose of this study is imaging blood flow from each internal carotid artery (ICA) by using RF shimming with 4-channel RF transmit coil at 3T.

Introduction

RF shimming1 is routinely used in high field MRI. Recently, to improve B1 homogeneity in specific region or to reduce SAR, spatially asymmetric RF transmission using RF shimming with 4-channel RF transmit coil has been reported2-4. In this study, we applied asymmetrically RF-shimmed pre-saturation (PreSat) pulse in 3D TOF imaging to selectively image blood flow from left or right ICA. ICA-selective 3D TOF imaging with applying 2D excitation to PreSat pulse has already been presented5. The 2D excitation can select blood flow from each ICA, but 2D excitation also causes long duration of PreSat pulse leading to long TR and long scan time. Especially, the long TR sometimes lowers image contrast between blood and brain. The presented asymmetrically RF-shimmed PreSat pulse is expected to visualize blood flow from each ICA without using longer TR.

Materials and Methods

Experiments

Experiments were conducted using 3T whole body MRI system (Hitachi, Ltd.). B1 maps were acquired using 4-channel RF transmit and receive coil6. A 16-channel receive only head coil was used for 3D TOF scan.

Calculation of RF shimming parameters for PreSat pulse

Figure 1 shows the slice of PreSat pulse with blue rectangle. The slice of PreSat pulse is located at just below the FOV. B1 map at the slice was acquired with multi Td method7, 8. RF shimming parameters were calculated as follows. 1. Set two ROIs (ROIT and ROIF) to corresponding to locations of two ICAs. The size of ROIs was 23 mm × 23 mm (Fig. 2). 2. RF shimming parameters were determined to minimize mean of B1 in ROIT, subject to mean of B1 in ROIF is equal to mean of B1 with QD transmission. As B1 in ROIF becomes small, blood flow from ROIF was expected not to be saturated by the PreSat pulse. Thus blood flow from ROIF is expected to be visualized.

3D TOF scan

5 healthy volunteers were evaluated. Scan parameters were as follows; TR 20 ms, TE 3.3 ms, FA 18 deg, and matrix 352 x 320 x 118. Scan time was 4:22. 3D TOF was performed with and without PreSat pulse. The PreSat pulse was applied to left or right ICA.

Evaluation

B1 mean in ROIT and ROIF were evaluated. Signal intensity of two ICAs is also compared to signal intensity of white matter.

Results and Discussions

Figure 2 shows B1 maps at PreSat slice. (a): ROIT is set at left ICA, and (b): ROIT is set at right ICA. In both cases, spatially asymmetric B1 distribution is achieved. Table 1 shows means of B1 in ROIT and ROIF. Mean of B1 in ROIF is smaller than that in ROIT by 87%. Figure 3 shows MIP images of a volunteer, (a): without PreSat pulse, (b): ROIT is set at left ICA, and (c): ROIT is set at right ICA. In the image without PreSat pulse, blood flow from both left and right ICA are imaged, and with PreSat pulse, blood flow from ROIT is saturated, so blood flow from left or right ICA is selectively imaged. In table 2, signal intensity of white matter and left and right ICA are summarized. Signal intensity of white matter and ICA of ROIF was almost same between with and without PreSat pulse. By applying PreSat pulse, the signal intensity of blood flow from ROIT becomes smaller than that of white matter, so no blood signal is seen in the MIP images. These results show that presented asymmetrically RF shimmed PreSat pulse minimizing the B1 in ROIF enables selectively imaging blood flow from only left or right ICA.

Conclusions

ICA selective TOF imaging by using asymmetrically RF-shimmed PreSat pulse was presented. The results demonstrates this imaging technique effectively visualize blood flow from left or right ICA selectively. This technique provides high contrast between blood and brain parenchyma because it does not make TR longer.

Acknowledgements

No acknowledgement found.

References

[1] J. Nistler et al. ISMRM 2007; 15: 1063. [2] K. Ito et al. ISMRM 2014; 22: 942. [3] Y. Kaneko et al. ISMRM 2013; 21: 2756. [4] Y. Kaneko et al. ISMRM 2015; 24: 3132. [5] T. Nishihara et al. ISMRM 2012; 20: 2497. [6] Y. Soutome et al. ISMRM 2013; 21: 2756. [7] K. Ito et al. ISMRM 2013; 21: 2598. [8] K. Ito et al. ISMRM 2013; 21: 2599.

Figures

Fig. 1. Position of PreSat pulse (blue line) (a): Sagittal view, and (b): Coronal view.

Fig. 2. B1 maps at slice of PreSat pulse (a): ROIT is at left ICA, and (b): ROIT is at right ICA. ROIT is shown as blue box and ROIF is shown as white box.

Fig. 3. MIP images of 3D TOF scan (a): without PreSat pulse, (b): ROIT is at left ICA, and (c): ROIT is at right ICA

Table 1. Mean of B1 in ROIT and in ROIF for Left and Right ICAs (mean ± standard deviation of 5 volunteers)

Table 2. Signal intensity in ICA with and without PreSat pulse compared to white matter (mean ± standard deviation of 5 volunteers)



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3177