Multiple regression method for pulmonary apparent diffusion coefficient measurement by hyperpolarized ³He MRI

Abstract

Purpose:To develop and validate a new multiple regression technique for the separation of flip angle effect in pulmonary apparent diffusion coefficient (ADC) measurement. Materials and Methods:Hyperpolarized 3 He MRI (HP 3 He MRI) ADC measurements were performed on phantom, pig, and human models. The diffusion-sensitization sequence is modified from a standard gradient echo (GRE) sequence with a nonlinear progression in the bipolar gradient amplitude with each image. In the self-diffusion phantom experiment, four images were acquired with base gradient factor b 0 ϭ 0.15 second/cm 2 ; in the pig and human experiment, six images were acquired with base gradient factor b 0 ϭ 1.4 second/cm 2 . Results:The self-diffusion coefficient measured in the phantom experiment was 1.98 Ϯ 0.16 cm 2 /second. The measured uncertainty curve was consistent with the theoretically predicted curve. The measured in vivo ADC values (three coronal slices in the supine direction) were 0.20/ 0.16/0.13 cm 2 /second and 0.20/0.18/0.16 cm 2 /second for pig and human experiments, respectively. Conclusion:With the introduction of a nonlinear progression in the diffusion-sensitization gradients, the multiple regression technique is capable of separating the flip angle effect in ADC measurement. In addition, this technique can perform a rigorous measurement uncertainty analysis and provide the optimal scan parameters that yield best noise performance.Key Words: apparent diffusion coefficient; multiple regression method; hyperpolarized HYPERPOLARIZED HELIUM-3 MRI (HP 3 He MRI) shows promise as an ideal imaging tool for quantitative assessments of pulmonary parameters; this technique is sensitive, regional, and noninvasive. More importantly, it is capable of providing both structural and functional lung information. Current applications include static imaging of the airways (1-6), diffusion measurements (7-13), regional ventilation studies (14,15), and regional oxygen measurements (16 -20).In its gaseous form, an individual 3 He atom traverses a sample using Brownian random motion. The average projected distance along an individual axis, l, that an individual atom moves during the time interval is l ϭ ͙ 2D. The diffusion coefficient D is specific to the gas mixture. At atmospheric pressure (1 bar) and room temperature (ϳ25°C), when 3 He atoms diffuse freely in a pure helium environment, D is around 1.8 cm 2 /second (21). The diffusivity of helium is significantly lower in air, averaging around 0.75 cm 2 /second (22), due to the larger collision cross-section of nitrogen and oxygen molecules. In other words, the mean distance traveled by a helium atom in air before colliding with another atom is shorter, compared to a pure helium environment, when the larger nitrogen and oxygen atoms are present. We note that even during a short MRI acquisition time, of say 1 msec, a helium atom diffusing through air in free space will travel an average of 0.4 mm from its starting position. In the lungs the average alveolar radius is of the order of 135-250 m so inh...

Biomedical imaging with hyperpolarized noble gases

“…where T bipolar is the time from the beginning of the first to the end of the second gradient of the pair (the other gradient timing parameters β, δ and are shown in figure 4), G is the gradient amplitude, and γ is the gyromagnetic ratio (Yu et al 2007).…”

A Robust Method for Estimating Regional Pulmonary Parameters in Presence of Noise

“…Second, 3He depolarizes in the presence of oxygen due to dipolar interactions between the two molecules. The depolarization time constant for this process is given by (17):…”

Simultaneous measurement of pulmonary partial pressure of oxygen and apparent diffusion coefficient by hyperpolarized 3He MRI

“…Structural HP 3 He MRI has relied heavily on a measurement called the apparent diffusion coefficient (ADC), which is a function of alveolar size and geometry, to characterize the lung's microstructure at the alveolar level (9–13). This technique utilizes the fact that the atoms of 3 He gas have a restricted diffusion capability in the lung because the alveolar size (∼200 μm) is smaller than the free diffusion mean distance during a time period encoded into the imaging pulse sequence.…”