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.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 reported^2-4. 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.

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 method^{7, 8}. RF shimming parameters were calculated as follows. 1. Set two ROIs (ROI_T and ROI_F) 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 ROI_T, subject to mean of B₁ in ROI_F is equal to mean of B₁ with QD transmission. As B₁ in ROI_F becomes small, blood flow from ROI_F was expected not to be saturated by the PreSat pulse. Thus blood flow from ROI_F 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 ROI_T and ROI_F were evaluated. Signal intensity of two ICAs is also compared to signal intensity of white matter.

Figure 2 shows B₁ maps at PreSat slice. (a): ROI_T is set at left ICA, and (b): ROI_T is set at right ICA. In both cases, spatially asymmetric B₁ distribution is achieved. Table 1 shows means of B₁ in ROI_T and ROI_F. Mean of B₁ in ROI_F is smaller than that in ROI_T by 87%. Figure 3 shows MIP images of a volunteer, (a): without PreSat pulse, (b): ROI_T is set at left ICA, and (c): ROI_T 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 ROI_T 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 ROI_F was almost same between with and without PreSat pulse. By applying PreSat pulse, the signal intensity of blood flow from ROI_T 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 ROI_F enables selectively imaging blood flow from only left or right ICA.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.