Continuous Positive Airway Pressure (CPAP) will be administered using FDA-approved equipment.

CPAP requires subjects to wear a plastic pressurized mask/apparatus, which is connected via a filter and hose to a pump capable of supplying variable pressure (typically on the range of 4-20 cm H2O). The investigators plan to use a full-face mask to prevent subjects from reducing pressure by opening their mouths. If this cannot be tolerated or properly fitted, or is not compatible with the CPAP ventilator, subjects would use a nasal mask only and be asked to keep their mouths closed.

Breathing motion at baseline and at initial (higher but tolerable) CPAP settings will be characterized using fast multi-slice, multi-phase 2D gradient-echo or spin-echo MRI sequences in the sagittal or coronal imaging planes over a large field of view covering the thoracic and upper-abdominal region. Specifically "4D-MRI" (or time-resolved MRI) will be generated from retrospective sorting of multi-slice 2D MRI. For these analyses, anatomic reference points (e.g., bright vessels) in multiple organ systems will be identified on the images, contoured or segmented; subsequently 3D motion patterns will be extracted for all anatomical references/features. Numerical simulations will be performed to determine hypothetical radiation target volume changes (cm^3) at these anatomic locations assuming spherical tumors with a given set of tumor diameters (cm).

Time permitting, the CPAP setting will be reduced to approximately half of the nominal/tolerable/high CPAP setting. Using the same MRI sequences and retrospective sorting technique, 4D-MRI will be generated for this lower CPAP setting. Analogous motion characterization analyses (to outcome measure 1) will be carried out for the lower CPAP setting (relative to baseline 4D-MRI).