4D-MRI methods for radiotherapy

4D-MRI aims to temporally resolve motion in three spatial dimensions. The term was introduced in 2005 by von Siebenthal et al, who developed a method for proton therapy planning. Following the nomenclature of Stemkens et al’s 4D-MRI review, one can distinguish between respiratory-correlated 4D-MRI, which consists of volumetric images depicting different phases of the respiratory […]4D-MRI aims to temporally resolve motion in three spatial dimensions. The term was introduced in 2005 by von Siebenthal et al, who developed a method for proton therapy planning. Following the nomenclature of Stemkens et al’s 4D-MRI review, one can distinguish between respiratory-correlated 4D-MRI, which consists of volumetric images depicting different phases of the respiratory cycle (similar to 4D-CT), and time-resolved 4D-MRI, where the series of image volumes corresponds to different timepoints. Respiratory-correlated 4D-MRI uses a respiratory surrogate signal to assign MRI data to different respiratory phases and needs to be distinguished from 4D flow MRI, which is used in cardiovascular imaging and employs three- dimensional velocity encoding to analyse complex blood flow patterns. Time-resolved 4D-MRI differs from two- dimensional cine MRI, which repeatedly acquires an imaging slice to characterise motion in the imaging plane with high temporal resolution. Three-dimensional cine MRI is sometimes used as a synonym for 4D-MRI and can refer to time-resolved 4D-MRI with low temporal resolution to characterise organ drifts and peristaltic motion in pelvic MR-guided radiotherapy on MR-linacs.

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