Table 1:

Image modality considerations for online and offline ART implementations. Grading system: (least to most advantageous, 1 * to 5 *) based on consensus grading by authors.

ART Imaging Modality Considerations	Computed Tomography (on rails or simulation)	Cone beam Computed Tomography	Megavoltage Computed Tomography	Magnetic Resonance Imaging	Positron Emission Tomography
Contrast - Soft tissue differentiation	Diagnostic quality Same as planning CT scan	Good contrast for large density differences such as bone/ tissue/air. Scatter significantly decreases contrast.	Good contrast for large density differences such as bone/ tissue/air. Scatter significantly decreases contrast.	Excellent soft tissue contrast	No soft tissue differentiation but provides quantitative functional information
	****	***	**	*****	***
Spatial resolution	Same as planning CT scan – sub millimeter, can be limited longitudinally	Same as planning CT scan – sub millimeter, longitudinally typically 1 mm	Same as planning CT scan – sub millimeter, longitudinally typically 2 mm	Similar to planning CT scan – sub millimeter, longitudinally typically 1 mm	Typically few mm in each direction PET, depends on body site
	*****	****	****	****	***
Motion artifacts	Fast scan but motion must be managed to avoid artifacts	Long scan times prone to motion artifacts	Long scan times prone to motion artifacts	Scan times can be long or short - prone to motion artifacts	Very long scan times, prone to blurring from motion
Clinical motion management solutions	Yes	Yes	No	Yes	No
	****	**	**	***	*
Reconstruction artifacts	Prone to hardening artifacts from high Z materials or elongated body shape, motion	Same artifacts as CT, as well as scatter, ring, aliasing, and misalignment artifacts	Same artifacts as CT, zipper artifacts	Susceptibility, motion, distortion	Same artifacts as CT, attenuation correction, motion, CT reconstruction as well as partial volume artifacts
	***	***	***	**	**
Geometry
Anatomy	Anatomy changes from CT, organ localization	Anatomy changes, organ localization	Anatomy changes, organ localization	Anatomy changes, organ localization	Metabolic uptake changes
	****	***	***	*****	****
FOV limitations	60 cm FOV	Up to 50 cm FOV, large FOV results in poor image quality	Up to 50 cm FOV, large FOV results in poor image quality	Up to 50 cm FOV	Up to 70cm FOV
	****	***	***	****	****
Patient position issues	Same as planning CT scan	Can affect image quality depending on position on treatment couch	Can affect image quality depending on position on treatment couch	Bore size may limit patient position, coil placement may limit use of accessories	May not be same as simulation setup, PET scan bore size may limit patient position
	****	***	***	***	****
Truncated structures	Same as planning CT scan	FOV limitations may truncate structures	FOV limitations may truncate structures	FOV limitations may truncate structures and external contour	Same as planning CT scan
Tracking organ motion	Not available during treatment	Not available during treatment	Not available during treatment	Available	Not available during treatment
	*	*	*	****	*
Density
HU table management	Same as planning CT scan	Can build custom HU table 1–2% accuracy in dose calculation	MVCT number, similar to HU table, must be monitored at high frequency	Not available, surrogate needed	Same as planning CT scan if PET-CT
	*****	*****	***	**	*****
Online/Offline ART
Modality for Online	CT on rails	CBCT	Tomotherapy able to sum plans and “dose of the day”	MRI-Cobalt, MRI-Linac	Under development
	***	***	***	****	*
Additional dose to patient	Up to 3 cGy per scan	Up to 10 cGy per scan	Up to 5 cGy per scan	Not applicable	Up to 3 cGy whole body plus CT dose
	***	**	**	*****	**