Comparison between 2D and 3D gradient‐echo sequences for MRI of human lung ventilation with hyperpolarized ³He

Abstract

Images of hyperpolarized 3 He were acquired during breath-hold in four healthy volunteers with the use of an optimized 3D gradient-echo sequence. The images were compared with existing 2D gradient-echo methods. The average SNR from a 13-mm-thick slice in the peripheral lung was 1.4 times greater with 3D. In the airways the average SNR was 1.7 times greater with 3D. The higher SNR of 3D was particularly evident when regions of unimpeded gas diffusion, such as the major airways, were imaged with thin slices. This is because diffusion dephasing due to the slice-encoding gradient is minimized with a 3D sequence. The in vivo experimental findings were substantiated with experiments on phantoms of free gas, which showed more than four times the SNR with 3D compared to 2D. Most clinical studies of hyperpolarized (HP) gas ventilation have imaged the lungs using 2D, multislice, low-flipangle, gradient-echo sequences (1,2). A recent abstract (3) demonstrated impressive images of ventilation that were acquired with a 3D cylindrical k-space trajectory. The only other studies of 3D imaging with HP gas were conducted in animal models (4 -7). In this work, the factors that affect the SNR and spatial resolution of 3D gradient-echo sequences with rectilinear phase encoding were investigated. Experiments were performed on gas phantoms and healthy human subjects at breath-hold. The observed SNR and spatial resolution in the images were quantitatively compared with theory and the results from 2D experiments. THEORYDuring breath-hold, the transverse magnetization, M t (n), after the nth RF pulse is given by[1]Equation [1] accounts for polarization loss due to RF depletion by a pulse of flip angle , and also includes a longitudinal relaxation term exp(-nTR/T 1 ). The dominant contribution to the 3 He T 1 in vivo is from the dipolar interaction with paramagnetic oxygen (8). Spoiling of any residual transverse coherence is also assumed. The weighting of the SNR in the images (the k y ϭ 0 point) due to H(k y ) is thus given by sin( ). If this is considered with the diffusion attenuation effect of the slice encoding gradient, exp(-b s D) (9), and the inherent SNR weighting of 3D vs. 2D (10), then the theoretical SNR of 3D vs. 2D for HP gas is given byEquation [2] assumes an identical bandwidth, TR, and gas polarization for both 2D and 3D. Furthermore, a thin slice is assumed, and interslice polarization mixing from gas diffusion in 2D experiments is not included. The diffusion of gas out of the slice has previously been studied in phantom experiments (16,17). Assuming an infinite medium with no boundaries, the spatial mixing of gas between a single slice (the profile of which is denoted by f(z,t)) and its surroundings can be solved as a function of time from the 1D diffusion equation ‫ץ‬f/‫ץ‬t ϭ D͑‫ץ‬ 2 f/‫ץ‬z 2 ͒. The analytical solution is the convolution integral of the initial slice profile from the first view, f(z,0), with a Gaussian of variance given by 2 ϭ 2Dt. MATERIALS AND METHODS SimulationsComputer simulations were performed f...

3He MRI in mouse models of asthma

“…The diffusion problem has begun to receive attention in clinical 3 He imaging, where its careful consideration can yield measurable improvements in signal-to-noise ratio (SNR) (13)(14)(15). Here we show that for small animal imaging, the problem can be largely eliminated by starting the image acquisition at the center of Fourier space.…”

Functional lung imaging using hyperpolarized gas MRI

“…Recently, 3D acquisitions have been developed for ventilation imaging (63), but for most imaging applications, a 2D multislice, SPGR sequence (TR/TE ϭ 5 msec/1.2 msec) is used to acquire images in the coronal or axial plane. Low flip angle RF pulses (7-10°) are used with RF spoiling to minimize signal saturation and steady-state artifacts (64).…”

“…At present, dynamic techniques are 2D and are performed with a second inhalation of HP gas in which the first inhalation is used for static ventilation imaging as described above. Efforts to combine dynamic and static ventilation imaging in a single scan and to extend these dynamic techniques to 3D are areas of future development (63).…”

An open-access, very-low-field MRI system for posture-dependent 3He human lung imaging

“…In future work, we plan to implement slice selection, which will improve the visual quality of 2D anatomical images. We note, however, that slice selection must be employed carefully in quantitative functional studies such as pulmonary oxygen partial pressure and Apparent Diffusion Coefficient (ADC) mapping, due to the rapid diffusion of polarized 3 He spins into the slice of interest [53,54].…”