Parallel Acquisition as a Key for Rapid High Resolution 3He-ADC Imaging

Introduction A large variety of lung diseases, such as emphysema, result in severe destruction of the lung’s alveoli walls. Determining the Apparent Diffusion Coefficient (ADC) of hyperpolarized He-gas in lungs is a proven method of non-invasively probing the integrity of the lung’s microstructure [Chen, Morb1]. The essential problem of the efficiency of 3He ADC-imaging of lungs as the diagnostic tool in comparison with HRCT is the poor localisation of integrity defects caused by typically low spatial resolution of ADC-maps. The low resolution originates from (i) breath hold time restriction limiting the amount of phase encodings and (ii) limited SNR of diffusion encoded image. The solution of both problems can be achieved by using multi-channel phased arrays in combination with parallel imaging acquisition. This increase the SNR of acquired images due to high sensitivity of array elements and, simultaneously, acceleration of image acquisition by reducing the number of phase encoding steps (typically by factor 2-8). The principal advantage of using phased arrays for hyperpolarized He is the possibility to compensate for the unavoidable SNR loss because of parallel acquisition, by increasing the flip angle. The purpose of the current study is to demonstrate the possibilities, which provides the phased array parallel acquisition for improving the efficiency of ADC-measurements with hyperpolarized 3He as well as to compare different methods of image reconstruction (e.g. mSENSE and GRAPPA) in terms of optimal ADC image quality Materials and method The 3He ADC measurements were performed on Siemens Avanto Tim MR-scanner (Erlangen, Germany), and an in-house built 32 channel phased array. Pilot measurements were performed using an ex-vivo pig lung phantom with ca 1.5 full lung capacity. 100ml of hyperpolarized 3He (polarisation P=6570%) mixed with 700 ml N2 has been used. The phantom lungs were preliminary washed out with N2. In-vivo measurements were done on healthy male volunteer with approval of local ethics committee. 250/650 He/N2 mixture was applied using Tedlar bag. Bipolar diffusion encoding SGRE-sequence in 2D and 3D variant was used to acquire images. The imaging parameters are shown in figures caption. Acceleration factors 2 and 4 as well as two methods of reconstruction (mSENSE and GRAPPA) were used.