Respiratory-Correlated (RC) vs. Time-Resolved (TR) Four-Dimensional Magnetic Resonance Imaging (4DMRI) for Radiotherapy of Thoracic and Abdominal Cancer

Abstract

Recent technological and clinical advancements of both respiratory-correlated (RC) and time-resolved (TR) four-dimensional magnetic resonance imaging (4DMRI) techniques are reviewed in light of tumor/organ motion simulation, monitoring, and assessment in radiotherapy. For radiotherapy of thoracic and abdominal cancer, respiratory-induced tumor motion, and motion variation due to breathing irregularities are the major uncertainties in treatment. RC-4DMRI is developed to assess tumor motion for treatment planning, whereas TR-4DMRI is developed to assess both motion and motion variation for treatment planning, delivery and assessment. RC-4DMRI is reconstructed to provide one-breathing-cycle motion, similar to 4D computed tomography (4DCT), the current clinical standard, but with higher soft-tissue contrast, no ionizing radiation, and less binning artifacts due to the use of an internal respiratory surrogate. Recent studies have shown that its spatial resolution has reached or exceeded that of 4DCT and scanning time becomes clinically acceptable. TR-4DMRI is recently developed with an adequate spatiotemporal resolution to assess tumor motion and motion variations for treatment simulation, delivery and assessment. The super-resolution approach is most promising since it can image any organ/body motion, whereas RC-4D MRI are limited to resolve only respiration-induced motion and some TR-4DMRI approaches may more or less depend on RC-4DMRI. TR-4DMRI provides multi-breath motion data that are useful not only in MR-guided radiotherapy but also for building a patient-specific motion model to guide radiotherapy treatment using an non-MR-equipped linear accelerator. Based on 4DMRI motion data, motion-corrected dynamic contrast imaging and diffusion-weighted imaging have also been reported, aiming to facilitate tumor delineation for more accurate radiotherapy targeting. Both RC- and TR-4DMRI have been evaluated for potential clinical applications, such as delineation of tumor volumes, where sufficiently high spatial resolution and large field-of-view are required. The 4DMRI techniques are promising to play a role in motion assessment in radiotherapy treatment planning, delivery, assessment, and adaptation.

A modern review of the uncertainties in volumetric imaging of respiratory‐induced target motion in lung radiotherapy

“…More extensive details on the background and development of 4D-MRI acquisition that are beyond the scope of this manuscript can be found in several recently published review articles. 93,[132][133][134]…”

Enhanced super‐resolution reconstruction of T1w time‐resolved 4DMRI in low‐contrast tissue using 2‐step hybrid deformable image registration

“…Four-dimensional magnetic resonance imaging (4DMRI) has been recently studied for MR-based radiotherapy planning and MR-guided treatment delivery. [7][8][9] 4DMRI may appear in four forms: RC-based 4DMRI, dynamic 2D cine, dynamic 3D cine, and time-resolved (TR) 4DMRI. RC-4DMRI, similar to 4DCT, does not provide multi-breath motion simulation, [10][11][12][13] dynamic 2D cine MRI does not provide volumetric motion images, [14][15][16] and dynamic 3D cine cannot provide clinically acceptable spatial resolution.…”

“…Four‐dimensional magnetic resonance imaging (4DMRI) has been recently studied for MR‐based radiotherapy planning and MR‐guided treatment delivery 7–9 . 4DMRI may appear in four forms: RC‐based 4DMRI, dynamic 2D cine, dynamic 3D cine, and time‐resolved (TR) 4DMRI.…”

A super‐resolution framework for the reconstruction of T2‐weighted (T2w) time‐resolved (TR) 4DMRI using T1w TR‐4DMRI as the guidance

“…Li et al reported a 3D-cine-guided, superresolution (SR) method to reconstruct T1w TR-4DMRI using DIR to combine two complementary image sets: adequate temporal resolution free-breathing (FB: 5 9 5 9 5 mm 3 at 2 Hz) 3D cine and high spatial-resolution breath-hold (BH: 2 9 2 9 2 mm 3 ) 3D static images. [17][18][19] Recently, this SR approach has also been implemented using cascade deep learning technique with limited training data for MR-guided adaptive radiotherapy. 20 The SR approach does not assume motion periodicity, and therefore can handle both regular and irregular motions, including breathing irregularities and nonrespiratory motions.…”

“…This novel SR approach combines three sets of MR images, rather than two in previous studies. [17][18][19] First, high-resolution (HR) T2w MRI was acquired using navigator-triggered 3-bin RC-4DMRI scan within the same imaging session of T1w HR BH and low-resolution (LR) FB 3D cine acquisitions. Second, mild binning artifacts in T2w MRI were automatically identified and corrected based on voxel intensity correlation (VIC) among adjacent slices using 2D DIR.…”