As a person breathes, the liver glides smoothly up and down. When treating a tumor with radiation, this motion must be accounted for: the beam must hit the target, not healthy tissue. Usually, breathing is "captured" using special computed tomography (CT), taking a series of frames over one breath cycle. But there is another method — free-breathing MRI, which lasts several minutes and is merged into a single averaged image. The doctors in this study asked: can such a single MRI scan tell us where the tumor is at any moment of breathing?
They took data from 54 patients with liver tumors and aligned the free-breathing MRI with each frame of the respiratory CT. The quality of alignment was assessed numerically, like comparing two fingerprints.
Within 30–60% of the cycle, the match remained high, but at the extremes, full inhalation and full exhalation, the images diverged noticeably. Two independent doctors confirmed these results, rating the clinical quality of fusion on a five-point scale: for middle phases, scores never dropped below 4, but for extremes they fell.
The conclusion was clear: free-breathing MRI is like a long-exposure photograph, where only what stayed still for a long time appears sharp. It shows the middle of breathing well, but cannot replace a full "movie" of the entire cycle. Therefore, for precise radiation targeting in liver cancer treatment, doctors should combine such MRI with respiratory CT or other methods that track organ motion at every moment.
The liver is one of the most mobile organs during breathing: its dome can shift by 2–3 centimeters, so precise tumor irradiation without accounting for breathing is like hitting a target that constantly sways.