Method to Simultaneously Calibrate Flip Angles and Measure T1 of Hyperpolarized Gas during a Single Breath-hold
Jianping Zhong1,2, Weiwei Ruan1, Xianping Sun1, Chaohui Ye1,2, and Xin Zhou1

1State Key Lab Magnet Resonance & Atom & Mol Phys, Wuhan Inst Phys & Math, Chinese Acad Sci, Wuhan, China, People's Republic of, 2School of Physics, Huazhong University of Science and Technology, Wuhan, China, People's Republic of

Synopsis

In hyperpolarized gas MRI, the accurate flip angle calibration and T1 measurements are important. Traditional flip angle calibration methods are time-consuming and suffer from polarization losses during T1 relaxation. In this study, we propose a method to simultaneously calibrate flip angles and measure T1 in vivo during a breath-hold time of less than 4 seconds. The so-called single-breath method is magnitude based and simple to use. We demonstrate the accuracy of this method and contrast it with traditional methods. The results of the calibration verified that it is robust and repeatable.

Purpose

In hyperpolarized gas MRI, the accurate calibration of flip angles is important for maximizing the SNR per unit time[1]. However, the calibration of flip angles is sensitive to relaxation effects. Moreover, the spin-lattice relaxation time (T1) of the gas can reflect the partial oxygen pressure, which is an important parameter related to the efficiency of gas exchange in the lung[2]. The measurement of T1 also requires a pulse flip angle calibration[3]. Most previous methods generally require more than two breaths: one for the flip angle calibration and another one for the T1 measurement. A previous study[4] by Max et al. provided a method for the simultaneous estimation of T1 and the flip angle in a single scan through acquisition at non-regular time intervals. However, the long acquisition time needed for mounting time intervals limits its usefulness during in vivo applications. In this work, we propose a novel method to simultaneously calibrate flip angles and measure T1 of hyperpolarized gas during a short single breath-hold in vivo.

Methods

Our proposed method for calibration in a single-breath is shown in Fig. 1a. First, we set a transmitter gain TG1 to get a small flip angle θ1 in the range 3°-6°. After 8 excitations by θ1, the transmitter gain is changed to a smaller value TG2 to obtain a larger flip angle θ2. Subsequently, we set TG3~TGm to obtain gradually larger flip angles θ3m, 8 excitations for each angle. After acquiring all of the transverse signals by the N excitations, we can fit the unknown flip angles using one of two methods. The first method involves fitting the m angles to $$$S_{k}=S_{1}\cdot\left(\cos\theta\cdot\exp\left(-TR/T_{1}\right)\right)^{k-1}$$$ with k=1-8, by setting an initial T1 value, where TR is repetition time. Once the value θ1 is fitted, the flip angle θ2 can be fitted to $$$\theta_{2}=\arctan\left[\tan\theta_{1}\cdot\exp\left(TR/T_{1}\right)\cdot S_{9}/S_{8}\right]$$$. We denote the flip angle θ2 calibrated from the first method as θ2′, and the one calibrated using the second method as θ2″. If θ2′ is equal to θ2″, this means that the initial T1 value is correct. If θ2′ is not equal to θ2″, this means that the initial T1 value is incorrect, and we should change the T1 to another value. The correct T1 value can be derived from the m angles to minimize the measurement error. Then the correct flip angles can be fitted with the correct T1. All experiments were performed on a 7 T animal MRI scanner with a homebuilt 8-leg rigid transmit-receive birdcage coil. The traditional multi-breath method was used for contrast. In the traditional flip angle measurement experiment, 16 different flip angles are measured with 16 breaths, and 112 excitations for each angle. T1 is measured by varying the delay time between trigger and the 1st RF excitation, with 7 different delay times in 7 breaths. The normal breaths are ventilated with pure oxygen, where different T1 values are acquired by using different xenon gas pre-wash times. Both the flip angle and T1 are measured in a single breath-hold by the method described in this study. The single-breath method was repeated 5 times for testing its robustness. TR was 67 ms for balloon and 33.7 ms for rat. The breath-hold time needed was 4 seconds for rat.

Results

The result of flip angle calibration is shown in Table 1. The parameters were those in $$$TG_{\theta}=TG_{90°}+A\cdot\log_{10}{90/\theta}$$$. Xenon pre-wash time was 0 in all measurements. T1 was set to 14 s in balloon, and 5 s in rat, when calibrated by multi-breath method. The result of T1 measurement is shown in Table 2, where the traditional method was repeated twice for the 0-time pre-wash in balloon, and once for others.

Discussion

The calibration results of both flip angles and T1 by the single-breath method described in this study are accurate. As shown in Table 1, the TG90° in rat lung is smaller than that in balloon, which is because of the RF stacking in rat tissues. The parameter A is close to 20, but not exactly equal to 20, which is an interesting phenomenon. The measured T1 values with 0-time pre-wash are shorter than those with the 1-time pre-wash, which reflects different oxygen pressures. The deviations of 5 measurements are small, thus confirming the robustness of this method.

Conclusion

This study proposed a novel method to simultaneously calibrate flip angles and measure T1 of hyperpolarized gas during a short single breath-hold in vivo.

Acknowledgements

No acknowledgement found.

References

[1] Zhao L et al. JMR Series B 1996;113(2):179-183. [2] Rizi RR et al. MRM 2004;52(1):65-72. [3] Moller HE et al. MRM 2001;45(3):421-430. [4] Puckeridge M et al. JMR 2012;222:68-73.

Figures

Fig. 1. (a) The time sequence of calibration method. (b) The corresponding value of transverse signals S1~SN obtained at every excitation.

Table 1. The result of flip angle calibration

Table 2. The result of T1 measurement in different xenon gas pre-wash time



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3299