  Purpose: We aim to develop a robust 4D MRI method for large FOVs enabling the
extraction of irregular respiratory motion that is readily usable with all MRI
machines and thus applicable to support a wide range of interventional
settings.
  Method: We propose a 4D MRI reconstruction method to capture an arbitrary
number of breathing states. It uses template updates in navigator slices and
search regions for fast and robust vessel cross-section tracking. It captures
FOVs of 255 mm x 320 mm x 228 mm at a spatial resolution of 1.82 mm x 1.82 mm x
4mm and temporal resolution of 200ms. A total of 37 4D MRIs of 13 healthy
subjects were reconstructed to validate the method. A quantitative evaluation
of the reconstruction rate and speed of both the new and baseline method was
performed. Additionally, a study with ten radiologists was conducted to assess
the subjective reconstruction quality of both methods.
  Results: Our results indicate improved mean reconstruction rates compared to
the baseline method (79.4\% vs. 45.5\%) and improved mean reconstruction times
(24s vs. 73s) per subject. Interventional radiologists perceive the
reconstruction quality of our method as higher compared to the baseline (262.5
points vs. 217.5 points, p=0.02).
  Conclusions: Template updates are an effective and efficient way to increase
4D MRI reconstruction rates and to achieve better reconstruction quality.
Search regions reduce reconstruction time. These improvements increase the
applicability of 4D MRI as a base for seamless support of interventional image
guidance in percutaneous interventions.
