Fast and tissue-optimized mapping of magnetic susceptibility and T2* with multi-echo and multi-shot spirals

Abstract

Gradient-echo MRI of resonance-frequency shift and T2* values exhibit unique tissue contrast and offer relevant physiological information. However, acquiring 3D-phase images and T2* maps with the standard spoiled gradient echo (SPGR) sequence is lengthy for routine imaging at high-spatial resolution and whole-brain coverage. In addition, with the standard SPGR sequence, optimal signal-to-noise ratio (SNR) cannot be achieved for every tissue type given their distributed resonance frequency and T2* value. To address these two issues, a SNR optimized multi-echo sequence with a stack-of-spiral acquisition is proposed and implemented for achieving fast and simultaneous acquisition of image phase and T2* maps. The analytical behavior of the phase SNR is derived as a function of resonance frequency, T2* and echo time. This relationship is utilized to achieve tissue optimized SNR by combining phase images with different echo times. Simulations and in vivo experiments were designed to verify the theoretical predictions. Using the multi-echo spiral acquisition, whole-brain coverage with 1 mm isotropic resolution can be achieved within 2.5 min, shortening the scan time by a factor of 8. The resulting multi-echo phase map shows similar SNR to that of the standard SPGR. The acquisition can be further accelerated with non-Cartesian parallel imaging. The technique can be readily extended to other multi-shot readout trajectories besides spiral. It may provide a practical acquisition strategy for high resolution and simultaneous 3D mapping of magnetic susceptibility and T2*.

Introduction

Recent advances in scanner hardware and signal processing techniques have allowed the diagnostic value of the image phase (or resonance frequency shift) to be better exploited. For example, Rauscher et al. demonstrated that image phase reveals contrast in the deep nucleus regions that is not observed in the corresponding magnitude images (Rauscher et al., 2005); Duyn et al. reported that the phase images provide better contrast at the white matter and gray matter interface than the magnitude images (Duyn et al., 2007, Lee et al., 2010). Image phase also allows susceptibility maps to be derived. Compared to image phase, susceptibility is an intrinsic local tissue property that can be measured quantitatively. Quantitative susceptibility measurement potentially allows assessment of iron and myelin tissue contents (Li et al., 2011). Iron content in the deep nucleus is an indicator for various neurodegenerative diseases (Haacke et al., 2005) such as Parkinson's disease (PD), Alzheimer's disease (AD) and Huntington's disease.

Quantitative susceptibility mapping requires 3D volume coverage. Commonly, the image phase information is obtained with a 3D spoiled-gradient echo sequence (SPGR). However, due to the desired long echo time (TE) and high isotropic resolution, whole brain imaging is often a prolonged process. For instance, it takes approximately 20 min to acquire images with 1 mm isotropic resolution (192 × 192 × 120 volume) and a TR of 52 ms. Such long scan time gives rise to many motion related artifacts. This long scan time becomes impractical for multi-orientation based susceptibility mapping (Liu et al., 2009, Wharton and Bowtell, 2010) and for susceptibility tensor imaging (Liu, 2010). Although under-sampling techniques such as parallel imaging (Pruessmann et al., 1999, Wu et al., 2009) and compressed sensing (Lustig et al., 2007, Wu et al., 2011) may be potentially incorporated to reduce scan time, the reconstructed image is always afflicted with artifacts such as noise amplification and loss of image contrast and details. Furthermore, brain tissues typically have a wide range of frequency shifts and T2* values, resulting in different signal behavior for different tissue types. For example, deep nuclei exhibit positive frequency shift and short T2* while white matter shows negative frequency shift and relatively long T2*. Consequently, optimal measurement of frequency shift and magnetic susceptibility cannot be achieved with the standard SPGR method. In summary, faster and tissue-optimized imaging techniques are necessary to resolve these important issues.

Here, we propose a multi-echo multi-shot technique to achieve faster image acquisition and tissue-optimized signal-to-noise ratio (SNR). Although a spiral trajectory is utilized in the current implementation, other multi-shot trajectories, such as multi-shot EPI and read-out segmented EPI, can be readily incorporated. Due to its efficient use of the field gradient system, the spiral readout trajectory allows the k-space to be traversed rapidly and efficiently. In addition to fast imaging benefit, spiral imaging also has other intrinsic properties compared to Cartesian trajectories, such as SNR efficiency and inherent refocusing of motion- and flow-induced phase error (Delattre et al., 2010). Due to these properties, spiral imaging has been widely employed in cardiac imaging (Ryf et al., 2004), diffusion tensor imaging (Liu et al., 2004), coronary artery imaging (Kressler et al., 2007) and functional MRI (Hu and Glover, 2007, Truong and Song, 2008) when high temporal resolution is required and motion is a potential problem. The drawback of spiral sampling compared to Cartesian sampling, however, is the increased susceptibility to off-resonance artifacts. When a short readout time is used, the off-resonance distortion can be limited and image phase information can be captured equally well as in Cartesian sampling. To improve and optimize SNR, we derive an optimal strategy to combine multi-echo data following the signal behavior that is specific to the underlying tissue properties (frequency shift and T2*). Multi-echo acquisition during the otherwise unused time interval does not require extra scan time. Multi-echo fMRI has been shown to increase the sensitivity of BOLD signal detection (Posse et al., 1999) whereas the use of multiple echo data set in the SWI shows improved SNR and contrast-to-noise (CNR) in the vascular regions (Denk and Rauscher, 2010).

The aim of this work is to achieve fast and accurate phase, susceptibility and T2* imaging based on a multi-echo spiral sequence. The temporal characteristics of the phase SNR at different echo times is investigated and used to optimally combine phase images and achieve tissue-optimized SNR. Phase imaging at a very high resolution (0.5 mm isotropic) using the proposed method is demonstrated at 3.0 T.