B1 Power Optimization for Chemical Exchange Saturation Transfer Imaging: A Phantom Study Using Egg White for Amide Proton Transfer Imaging Applications in the Human Brain

Abstract

The chemical exchange saturation transfer (CEST) effect on an egg white (EW) suspension was investigated for optimization of magnetization transfer (MT) power (B_1,rms) and pH dependency with the addition of lactic acid. Applying a higher MT pulse, B_1,rms, Z-spectrum shows higher asymmetry and the magnetisation transfer ratio (MTR)_asym signal increases to around 1-3.5 ppm, indicating a higher CEST effect. Amide proton transfer (APT) at 3.5 ppm shows a signal elevation in MTR_asym with the application of higher B_1,rms power and high pH. In addition, the hydroxyl proton signal in MTR_asym increases as pH is reduced by lactic acid. In Z-spectrum of B_1,rms at 1.0 μT and 2.0 μT, the dependence on CEST effect of amide proton and hydroxyl proton could be observed by using an EW suspension phantom. The CEST MT power was optimized on the EW suspension phantom with pH dependency and further confirmed on volunteers. In addition, APT imaging at 3.5 ppm using B_1,rms at 1.0 μT performed on two human brains with different pathophysiological conditions indicated appropriate ATP effect.

Keywords: B0 correction; B1 power; amide proton transfer; amide proton transfer effect on human brain; chemical exchange saturation transfer; egg white; pH dependency.

Fig. 1.

Chemical exchange saturation transfer (CEST) images of egg white (EW) before and after B₀ correction around the central frequency of water (from 1.0 to −1.0 ppm) at each saturation pulse power. Left images show before B₀ correction, and right images show after B₀ correction. The upper row represents 0.4 μT, the middle row represents 1.0 μT, and the bottom row represents 2.0 μT. Gray scale bar on the right indicates signal intensity in arbitrary units.

Fig. 2.

(a) Z-spectra of control egg white (EW) at each saturation pulse power. Z-spectrum; X axis shows frequency offset, Y axis shows the ratio of the water signals with and without saturation (I/I₀), where I_sat and I₀ are the imaging signal intensities measured with and without magnetization transfer (MT) pulse saturation. (b) asymmetric magnetization transfer ratio (MTR_asym) curves of the control EW phantom at each saturation power. MTR_asym curve; X axis shows frequency offset, Y axis MTR_asym value. These MT saturation powers in the plots are shown as: open circle, 0.4 μT saturation power; open square, 1.0 μT saturation power; open triangle, 2.0 μT saturation power.

Fig. 3.

Z-spectrum and asymmetric magnetization transfer ratio (MTR_asym) curve of two different pH egg white (EW)_lactic acid (Lac) samples and EW sample at three saturation powers: (a, b) at 0.4 μT; (c, d) at 1.0 μT; (e, f) at 2.0 μT; and (g) an enlarged view of (f) near 3.5 ppm. Green squares indicate pH 7.0 EW_Lac sample, blue triangles indicate pH 6.2 EW_Lac sample, and open dots indicate pH 8.8 EW sample Z-spectrum: X axis shows frequency offset, Y axis shows measured value of the ratio of the water signal with saturation and without (I/I₀). MTR_asym curve: X axis shows frequency offset, Y axis shows MTR_asym value.

Fig. 4.

Asymmetric magnetization transfer ratio (MTR_asym) images at 1.0 ppm and 3.5 ppm at each saturation pulse power. The pH 6.2 egg white (EW)_lactic acid (Lac) sample shows the highest value, at 1.0 ppm. The EW sample shows a higher value at 3.5 ppm. The low-intensity artifact is visible in all samples of 0.4 μT at 1.0 ppm (white arrow). The high-intensity artifact is visible in some samples of 1.0 and 2.0 μT at 1.0 ppm (black arrow). No artifacts are apparent in any samples at 3.5 ppm. The upper row represents 0.4 μT, the middle row, 1.0 μT, and the bottom row, 2.0 μT.

Fig. 5.

Brain amide proton transfer (APT) effect at B₁ at 0.4 μT (a) and 1.0 μT (b) on a 28-year-old male healthy volunteer. The source image (c) and an asymmetric magnetization transfer ratio (MTR_asym) of region at red circle (d).

Fig. 6.

The source image and amide proton transfer (APT) image around 3.5 ppm obtained from a 30-year-old male patient with grade 2 oligoastrocytoma. The tumor is identified in both source and APT images. The APT image indicates about 2% elevated signal as compared to the normal area.

Fig. 7.

The source image and amide proton transfer (APT) image around 3.5 ppm obtained on a 71-year-old patient with malignant lymphoma. The malignant lymphoma lesion is shown in both source and APT images. Note that an elevated APT value of about 5% is observed in the tumor area as compared to the normal area, indicating greater malignancy of amide proton.