The Role of Imaging in Radiation Therapy Planning: Past, Present, and Future

Table 2

Sources of MR distortion, correction, and method for assessment.

Level of distortion	Distortion source	Distortion correction	Assessment of residual distortion
System	Gradient nonlinearity	Gradient coil design (HW); Model-based corrections (SW)	Geometric fidelity phantoms and assessment tools
	Static field (B0); inhomogeneity	Magnet design and static-field shims (HW)	B0-field map in phantoms
Patient	Static-field (B0); inhomogeneity	Gradient-shim coils (HW); Patient-specific B0 field correction (SW)	B0-field map in the patient
	Transmit-field (B1); inhomogeneity	Multiple RF transmit channels (HW); Patient-specific B1 field correction (SW)	B1-field map in the patient
Sequence	Gradient switching; (eddy currents)	Gradient preemphasis (HW/SW); Protocols with low gradient slew rates (SW)	Visual inspection