1. CT-based surrogates of pulmonary ventilation in lung cancer:
A voxel-level comparison with HP gas MRI
Tahir BA1,2, Hughes PJ2, Hart KA1, Marshall H2, Swinscoe JA1, Wild JM2, Ireland RH1,2 and Hatton MQ1,
1Academic Unit of Clinical Oncology and 2Academic Radiology, University of Sheffield, UK
Image Acquisition
Introduction
Deformation vector fields for shallow and large respiratory differences for 4D-CT reference dataset
CT and MRI immobilisation and patient positioning CT
MRI
Although dose escalation can be employed to improve local control, it is limited by the incidence of radiation induced pneumonitis. Potentially localised measures of lung function can assist in preferential sparing of functional lung during the treatment planning process. This ‘functional lung avoidance planning’ concept has been applied using several pulmonary functional imaging modalities including single photon emission computed tomography [1] and helium-3 magnetic resonance imaging (3He-MRI) [2].
As an alternative, a novel computed tomography (CT) method, (‘ventilation CT’), has been proposed which could provide lung ventilation information directly from CT treatment planning scans, acquired at two different respiratory states, solely by image processing [3].
Before clinical use, the physiological accuracy of ventilation CT must be validated against established modalities such as hyperpolarised 3He-MRI.
Previously, Mathew et al. attempted to validate ventilation derived from 4D-CT against 3He-MRI [4]. Their study was limited by significant differences in acquisition settings between CT and MRI including breathing manoeuvres (tidal breathing vs breath-hold), patient couch positioning and time interval between scans (1.5 weeks).
6 NSCLC patients due to have radiotherapy underwent expiration and inspiration breath-hold CT.
On the same day, 3He-MRI and 1H-MRI were acquired in the same breath and at the same inflation state as the inspiratory CT, acquired using a 1L bag of room air [5].
Both CT and MRI were acquired supine, arms raised.
CT and MRI acquisition
Shallow difference
Large difference
Inspiration CT
Expiration CT
3He MRI
1H MRI
Ventilation CT
Registration
CT vs 3He MRI
comparison
Expiration CT
Warped expiration CT
Inspiration CT
Same-breath
1H & 3He MRI
Warped 3He MRI
Ventilation CT computation
(Apply transform of 1H MRI to CT to 3He MRI)
Workflow of comparison method of ventilation CT and 3He MRI
Objectives
To present an imaging protocol for acquiring pulmonary CT that can be used to calculate ventilation surrogates for direct comparison with hyperpolarised gas 3He-MRI.
To compare the spatial correlation of ventilation CT & 3He MRI in a cohort of lung cancer patients.
Image Registration and Processing
Corresponding anatomical landmark identified on inspiration and expiration CT
Methods
Theory – Computation of CT-based surrogates of ventilation
Inspiration
Expiration
Results
Expiration CT was registered to inspiration CT via an initial pre-alignment affine stage followed by a diffeomorphic transform using Advanced Normalization Tools (ANTs) registration software [6].
To reduce computational time and improve the spatial alignment of the lungs by avoiding high contrast structures such as the ribs, the registration was limited to voxels within the lungs.
The lungs were contoured using the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA, USA).
Registration accuracy was validated using a reference 4D-CT data set consisting of end-inhale and end-exhale phases for 6 patients with 100 expert anatomical landmarks defined on both images [7].
Ventilation CT images were calculated from voxel-wise intensity differences in Hounsfield unit (HU) values [3].
1. Acquire inspiration & expiration CT
Can be from 4DCT or breath-hold
2. Perform deformable
image registration
3. Calculate a ventilation metric
(Other methods are available!)
4. Apply post-processing
Visual examination indicated accurate registration of inspiratory and expiratory breath-hold CT.
Quantitative results demonstrated registration error of 1.1±0.2mm (mean±SD) for the reference dataset.
Successful registration enabled computation of ventilation CT images at the inspiratory state and direct comparison with 3He-MRI.
The median (range) Spearman’s coefficient was 0.65 ( ).
CT vs 3He
0.650.15 
p < 0.05 CT
3He MRI
Conclusions
This work demonstrates the feasibility of acquiring CT and 1H / 3He-MRI in similar breath-holds and posture such that registered data can be used to compare CT ventilation and 3He-MRI ventilation maps.
Initial results show moderate correlation between ventilation CT & 3He MRI, suggesting that ventilation CT may also be influenced by non-ventilatory factors.
To ensure that 3He-MRI was in the same spatial domain and inflation state as the CT ventilation surrogate, the former was registered to the inspiratory CT data via same-breath anatomical 1H-MRI, also using ANTs [8].
The transformation from 1H-MRI to CT was applied directly to 3He-MRI allowing direct comparison of 3He-MRI and CT ventilation in corresponding regions of interest located within the lungs as defined by a 1H MRI lung mask.
Spearman coefficients were used to assess voxel-level correlation.
Acknowledgements:
References:
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[2] Ireland RH, et al. Int J Radiat Oncol Biol Phys 2007;68:273-81
[3] Guerrero TM, et al. Int J Radiat Oncol Biol Phys 2005;62:630-34
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[7] Vandemeulebroucke J, et al. Med Phys 2011;38:166-78
[8] Tahir BA, et al. Phys Med Biol 2014;59: