Rationale and Objectives
Application of a previously developed model-based algorithm on hyperpolarized (HP) 3 He magnetic resonance (MR) dynamic projection images of phantoms was extended to investigate the utility of HP 3 He MR imaging (MRI) in quantifying airway caliber changes associated with asthma.
Materials and Methods
Airways of seven volunteers were imaged and measured using HP 3 He MRI and multidetector-row computed tomography (MDCT) before and after a methacholine (MCh) challenge. MDCT data were obtained at functional residual capacity and 1 L above functional residual capacity.
Results
Comparison of the resultant data showed that HP 3 He MRI did not match MDCT in measuring the ratios of airway calibers before and after the MCh challenge in 37% to 43% of the airways from the first six generations at the two lung volumes tested. However, MDCT did yield the observation that 49% to 69% of these airways displayed bronchodilation following MCh challenge.
Conclusion
The current implementation of HP 3 He MRI did not match the MCh-induced postchallenge-to-prechallenge airway caliber ratios as measured with MDCT. Elevated parenchymal tethering due to bronchoconstriction-induced hyperinflation was proposed as a possible explanation for this airway dilation.
High-resolution computed tomography (HRCT) has been the imaging modality of choice for assessing airways in asthma studies ( ). Numerous algorithms have been developed to process the acquired computed tomographic data ( ). Sophisticated software has been successfully developed for the visualization and three-dimensional (3D) rendering of the airway tree ( ). However, the use of ionizing radiation makes computed tomography (CT) undesirable in the event that repeated imaging is necessary to follow the progress of disease.
Hyperpolarized (HP) 3 He magnetic resonance imaging (MRI) has been used to visualize ventilation distribution in both animals ( ) and humans ( ) during breathhold of HP 3 He. HP 3 He MRI has also been used to visualize the airways by acquiring the MR images as the gas travels down the respiratory airway tree of both animals ( ) and humans ( ). HP 3 He MRI was able to observe correlations between ventilation dropouts and airways with low MR signal in broncho-challenged human subjects. Whether HP 3 He MRI could be an effective tool in evaluating airway constriction/dilation in asthmatics has become a subject of interest ( ).
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Methods
Subject Enrollment
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MRI Hardware and Imaging
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Computed Tomography Hardware and Imaging
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Protocol
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Table 1
Maximum Methacholine Dose Administered to Each Subject During Imaging Experiments
Subject Methacholine dose (mg/ml) Hyperpolarized 3 He Magnetic Resonance Imaging Multidetector-row Computed Tomography HEAL3 25.0 25.0 HEAL4 25.0 25.0 ASTH1 10.0 5.0 ASTH 8 0.3125 0.156 ASTH 13 0.3125 5.0 ASTH 16 5.0 25.0 ASTH 21 25.0 25.0
HEAL, healthy; ASTH, asthmatic.
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Image Postprocessing
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Results
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Table 2
Signal-to-Noise Ratio in the Trachea and Main Bronchi from Selected Hyperpolarized 3 He Magnetic Resonance Image Before and After Bronchial Challenge
Subject Tracheal SNR Main Bronchi SNR ⁎ Pre-MCh Post-MCh Pre-MCh Post-MCh HEAL3 67.7 89.7 70.9 71.9 HEAL4 120.8 108.1 75.1 62.4 ASTH1 133.4 97.1 79.7 38.5 ASTH 8 114.3 113.7 72.3 39.7 ASTH 13 98.4 92.2 75.4 67.8 ASTH 16 86.9 59.5 51.5 36.9 ASTH 21 107.8 94.7 90.7 63.8
MCh, methacholine; SNR, signal-to-noise ratio; HEAL, healthy; ASTH, asthmatic.
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Table 3
Number of Airways Visible and Their Response as Observed Through Multidetector-Row Computed Tomography
Airway Generation Airways Visible with Both Imaging Modalities ⁎ CT Measurements Available Both at Pre-MCh and Post-MCh † CT Measurements Indicate Airway Dilation Due to Bronchial Challenge † FRC FRC+1L FRC FRC+1L Trachea 7 7 7 6 6 First 14 14 14 13 14 Second 21 21 21 16 19 Third 43 34 42 17 27 Fourth 26 16 20 7 13 Fifth 16 5 14 3 9 Sum 127 97 118 62 88
CT, computed tomography; FRC, functional residual capacity; 1L, 1 liter; MCh, methacholine; MDCT, multidetector-row computed tomography.
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Discussion
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Measurement Accuracy
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Potential Experimental Sources of Difference Between MRI and MDCT Measurements
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Pathophysiological Implications
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Conclusion
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