MRI sequence optimisation methods to identify cranial nerve course for radiotherapy planning

Abstract

Introduction: Magnetic resonance imaging (MRI) is being increasingly used to improve radiation therapy planning by allowing visualisation of organs at risk that cannot be well-defined on computed tomography (CT). Diagnostic sequences are increasingly being adapted for radiation therapy planning, such as the use of heavily T2-weighted 3D SPACE (Sampling Perfection with Application optimised Contrasts using different flip angle Evolution) sequence for cranial nerve identification in head and neck tumour treatment planning.

Methods: A 3D isotropic T2 SPACE sequence used for cranial nerve identification was adapted for radiation therapy purposes. Distortion was minimised using a spin-echo-based sequence, 3D distortion correction, isocentre scanning and an increased readout bandwidth. Radiation therapy positioning was accounted for by utilising two small flex, 4-channel coils. The protocol was validated for cranial nerve identification in clinical applications and distortion minimisation using an MRI QA phantom.

Results: Normal anatomy of the cranial nerves CI-CIX, were presented, along with a selection of clinical applications and abnormal anatomy. The usefulness of cranial nerve identification is discussed for several case studies, particularly in proximity to tumours extending into the base of skull region. In-house testing validated that higher bandwidths of 600 Hz resulted in minimal displacement well below 1 mm.

Conclusion: The use of MRI for radiation therapy planning allows for greater individualisation and prediction of patient outcomes. Dose reduction to cranial nerves can decrease late side effects such as cranial neuropathy. In addition to current applications, future directions include further applications of this technology for radiation therapy treatments.

Keywords: Magnetic resonance imaging; radiotherapy; radiotherapy planning, computer-assisted; radiotherapy, image-guided.

Figure 1

Coil adaptation for radiation therapy s‐frame mask setup.

Figure 2

Normal cranial nerve anatomy for cranial nerves II–X, highlighted on a T2 SPACE sequence adapted for radiation therapy.

Figure 3

A comparison of an isotropic T2 SPACE (first column) and a typical T2 sequence acquired axially (second column) for radiation therapy planning purposes. The sequences are presented in the axial, coronal and sagittal plane, focusing on CNVII &VIII visibility (red arrows).

Figure 4

A clinical case of a meningioma (outlined in yellow), and the clinically significant nerves identified on a T2 SPACE. Bottom row displays dose distribution, highlighting high‐dose sparing of the identified cranial nerves.

Figure 5

A clinical case of a pituitary adenoma (outlined in yellow), and the clinically significant nerves identified on a T2 SPACE.

Figure 6

A clinical case of a skull base meningioma (outlined in yellow), and the clinically significant nerves identified on a T2 SPACE. Bottom row displays dose distribution, highlighting high‐dose sparing of the identified cranial nerves.