3D hyperpolarized He‐3 MRI of ventilation using a multi‐echo projection acquisition

Abstract

A method is presented for high-resolution 3D imaging of the whole lung using inhaled hyperpolarized (HP) He-3 MR with multiple half-echo radial trajectories that can accelerate imaging through undersampling. A multiple half-echo radial trajectory can be used to reduce the level of artifact for undersampled 3D projection reconstruction (PR) imaging by increasing the amount of data acquired per unit time for HP He-3 lung imaging. The point spread functions (PSFs) for breath-held He-3 MRI using multiple half-echo trajectories were evaluated using simulations to predict the effects of T 2 * and gas diffusion on image quality. Results from PSF simulations were consistent with imaging results in volunteer studies showing improved image quality with increasing number of echoes using up to 8 half-echoes. The 8-half-echo acquisition is shown to accommodate lost breath-holds as short as 6 sec using a retrospective reconstruction at reduced resolution and also to allow reduced breath-hold time compared with an equivalent Cartesian trajectory. Obstructive lung disease is highly heterogeneous (1), requiring 3D coverage at sufficient resolution to detect loci of disease. Hyperpolarized gas (HP) He-3 MRI with 3D isotropic resolution will likely improve depiction and analysis of lung structures and ventilation defects by reducing partial volume effects and providing true cubic voxel size for segmentation and measurement. However, challenges remain, as the polarization is non-recoverable, and sampling time is limited by the breath-hold duration.HP He-3 imaging has been performed using non-Cartesian 2D trajectories such as spiral (2) and undersampled projection acquisition to allow depiction of HP He-3 in the lungs using short breath-holds (3-6). These techniques provide a method of detecting regional ventilation differences without the use of the ionizing radiation associated with CT (7). Breath-hold spin-density imaging of the lungs in 3D has been demonstrated using Fourier techniques (8), a stack of spirals (9), and cyclindrical techniques (10). The use of functional HP He-3 data has shown exciting prospects in the setting of radiation treatment planning for lung cancer, for which 3D isotropic data will aid in registering anatomic with functional images of the lungs (11). A 3D stack of stars acquisition comprised of undersampled 2D projection acquisition in-plane and Fourier encoding in the third dimension has also been demonstrated to provide improved 3D visualization for short breath-holds as well as detection of gas trapping (6). However, each of these methods has tradeoffs with respect to spatial and temporal resolution as well as sensitivity to motion for depiction of dynamic processes, complex lung structure, and accommodating loss of breath-hold during the acquisition.A fully 3D projection acquisition trajectory and projection reconstruction (PR) has advantages for HP He-3 lung imaging as it allows the flexibility not only to perform spin-density imaging at isotropic resolution, but also to acquire functional informatio...

Dose and pulse sequence considerations for hyperpolarized 129Xe ventilation MRI

“…Although 3D radial acquisition, in principle, requires π more views of k-space than Cartesian imaging, the sequence is remarkably robust to undersampling (8,9). Thus, in practice, fully isotropic 3D image data can be undersampled 10× to reduce breath-hold durations or be reconstructed with retrospective accounting for premature exhalation (10). Additionally, such acquisitions are isotropic and have no artificial intensity weighting imposed by the order of slice acquisition.…”

Lung imaging in asthmatic patients: The picture is clearer

“…In recent studies the volume of He/N 2 mixture was adjusted to the subject's TLC to normalize the inflation volume across subjects. 94, 106 To further investigate effects of inflation volume, respiratory maneuvers, such as forced expirations 107 and deep inspirations, 86 can be performed in conjunction with the interventions such as exercise challenge ( Cover Figure ).…”

Slice profile effects in 2D slice-selective MRI of hyperpolarized nuclei

“…A nonconstant divergent signal for SPGR-VFA with hyperpolarized samples is often interpreted as a consequence of imperfect flip angle calibration and B 1 transmit coil inhomogeneity. While this is certainly the case for 3D sequences [2] and 2D projection imaging with a very weak or absent slice selection gradient [11,18], this work demonstrates that the effect is inherent to slice-selective 2D imaging by virtue of flip angle variation across the slice.…”

“…While sequences based on non-Cartesian k-space trajectories [1][2][3] promise faster acquisitions and efficient use of available polarization due to non-uniform k-space sampling, 2D and 3D Cartesian sequences remain popular as they are robust, readily implemented and images are easily reconstructed.…”