Optimization of scan parameters in pulmonary partial pressure oxygen measurement by hyperpolarized ³He MRI

Abstract

The dependence of hyperpolarized (HP) 3 He T 1 on local oxygen concentration provides the basis for measuring the partial pressure of oxygen (pO 2 ) and oxygen depletion rate (R) in the lungs. Precise measurements of this type are difficult because the oxygen effect manifests itself through a decay of signal, leading to noisy images at the end of the series. The depolarization caused by RF excitation pulses further complicates the problem. It is therefore important to optimize scan parameters, such as measurement timing and flip angle, to obtain accurate and reproducible measurements. This work presents a new singleacquisition technique in conjunction with the multiple regression fitting method for data evaluation. Analytical expressions for the measurement uncertainties are derived. A total of four types of single-acquisition timing schemes are investigated; simulation shows a large uncertainty variation between these schemes (pO 2 : 7.5-30.2%; R: 47.4 -173.7%). A basic procedure for optimizing scan parameters is then described. A phantom experiment was conducted to verify the simulation results. Repeated in vivo measurements with the optimal scheme in a rabbit experiment showed that average variation of global mean is 6.2% for pO 2 and 12.0% for R, and that the average variation of percentiles (10th, 25th, 50th, 75th, and 90th) is 8. In recent years, hyperpolarized helium-3 magnetic resonance imaging (HP 3 He MRI) has been researched extensively as a promising tool for quantitatively assessing pulmonary parameters (1-6). This technique is sensitive, regional, and noninvasive. It is also capable of providing both structural and functional information on lung tissue. Structural HP 3 He MRI has used the apparent diffusion coefficient (ADC), which is a function of alveolar size and geometry, to characterize the lung's morphology at very small spatial scales (7-17). The pulmonary oxygen measurement, which is an example of functional imaging, yields regional alveolar partial pressure of oxygen (pO 2 ) and oxygen depletion rate (R). These values provide a marker that is sensitive to both ventilation abnormality and impaired oxygen exchange efficiency between the lung and the bloodstream (18 -25).The HP 3 He MRI pulmonary oxygen measurement has not been studied as intensively as the ADC measurement. To the best of our knowledge, only one reproducibility study has been reported, performed only at a few manually-selected regions of interest (23). There are two likely reasons for this relative lack of activity. First, while ADC is a structural parameter that is relatively stable over various pulmonary physiological conditions (e.g., breathing patterns, inhaled gas composition, and ventilation-perfusion ratios), pO2 and R are functional parameters strongly dependent on the state of these conditions. We can therefore expect simpler and more repeatable ADC measurements because the necessity to monitor or control physiological state is not as strong as that in oxygen measurement. Second, the ADC measurement is typically base...

A multislice single breath‐hold scheme for imaging alveolar oxygen tension in humans

“…Assuming a slowly varying p A O 2 , and that 3 He depolarization in the lung is dominated (20) by: (i) the dipoledipole interaction of 3 He nuclei with the magnetic moments of O 2 molecules (Eq. 2), and (ii) tipping by RFpulses used in imaging (with flip-angle a), 3 He magnetization available to the nth acquired image in the series can be expressed as (16):…”

“…This double-acquisition approach was later replaced with an improved single acquisition scheme (Fischer et al (10)), albeit on a single-slice basis. Yu et al extended Fischer's scheme to multiple slices and optimized the RFpulse flip-angle to minimize p A O 2 uncertainty using a nonlinear timing pattern (16). However, only three slices (dorsal, middle and ventral) were imaged due to breathhold time constraints and the chosen timing scheme.…”

Rapid 3-D mapping of hyperpolarized 3He spin-lattice relaxation times using variable flip angle gradient echo imaging with application to alveolar oxygen partial pressure measurement in rat lungs

“…Indeed, this mechanism can be used to advantage by permitting estimation of the regional alveolar oxygen partial pressure (p A O 2 ) [3]. p A O 2 maps have been obtained in animals [4][5][6][7][8][9] and humans [10,11] using a variety of hyperpolarized 3 He imaging approaches and breathing schemes [12][13][14]. In diseases, where ventilation and/or perfusion is strongly impaired, regional variations in p A O 2 are expected and may be used to compute ventilation/perfusion mismatch [7].…”

Regional Alveolar Partial Pressure of Oxygen Measurement with Parallel Accelerated Hyperpolarized Gas MRI

“…This separation can be accomplished by acquiring identical image sets, in which only the RF power is varied [7], although more efficient use of available gas and superior immunity to subject motion can be achieved by using a variable inter-scan acquisition delay scheme during a single breath-hold [8]. Optimization of the timing scheme has been undertaken, and the effect on breath-hold duration and extracted P A O 2 uncertainty has been well studied [8,10,11]. …”

“…The purpose of this work is to further optimize the hybrid single acquisition P A O 2 imaging timing scheme [9,10] to include parallel accelerated acquisition. P A O 2 imaging utilizes signal intensity differences between successive images, and is therefore highly sensitive to noise and to RF-induced depolarization; thus, it is a particularly attractive application for parallel imaging.…”