KEY POINTS
- This prospective imaging study included 10 healthy volunteers who underwent two MRI sessions on separate days during free breathing, non-invasive mechanical ventilation at 30 breaths/min, and ventilation at 60 breaths/min with 15 cm H₂O PEEP. Each condition was imaged for 350 seconds using alternating coronal and sagittal cine-MRI at 2.7 Hz.
- Respiratory motion was quantified voxel-by-voxel after deformable registration of the liver, spleen and kidneys. The main metric was median caudal-cranial organ motion after exclusion of the most extreme 5% of respiratory positions.
- Liver motion fell from a group median of 29 mm during free breathing to 16 mm with 30 breaths/min and 9 mm with 60 breaths/min plus PEEP. The reductions with both ventilation strategies versus free breathing were statistically significant (P<.01).
- Similar reductions occurred throughout the upper abdomen: spleen motion decreased from 31 to 20 to 12 mm, right-kidney motion from 22 to 14 to 9 mm, and left-kidney motion from 19 to 14 to 10 mm across free breathing, 30 breaths/min and 60 breaths/min plus PEEP, respectively.
- Increasing ventilation frequency further reduced motion beyond NIMV30 for the liver and spleen (P<.018), while differences between the two ventilation strategies were not significant for both kidneys. Motion measurements were reproducible across the two MRI sessions, with no significant between-day differences.
- All volunteers completed the sessions comfortably and remained hemodynamically stable, with no desaturation, hypocapnia or hypercapnia. The right-diaphragm navigator also correlated significantly with near-maximum liver motion across breathing strategies, supporting internal consistency of the measurement approach.
- Translation to radiotherapy remains unproven. Motion was measured in healthy volunteers using two-dimensional MRI, not during actual treatment, and through-plane motion may have caused underestimation for the spleen and left kidney. Whether reduced motion allows smaller ITV/PTV margins, removes the need for gating, or shortens treatment time must be tested clinically.
CLINICAL TAKEAWAY
Rapid, mechanically imposed breathing could substantially reduce respiratory motion during abdominal radiotherapy, particularly when breath-hold or continuous gating is difficult. The magnitude of reduction is impressive, but this remains a volunteer feasibility study rather than evidence for margin reduction or routine replacement of existing motion-management strategies.