title: The relevance of MRI for patient modeling in head and neck hyperthermia treatment planning: a comparison of CT and CT-MRI based tissue segmentation on simulated temperature.
abstract: OBJECTIVE
  In current clinical practice, head and neck (H&N) hyperthermia treatment planning (HTP) is solely based on computed tomography (CT) images. Magnetic resonance imaging (MRI) provides superior soft-tissue contrast over CT. The purpose of the authors' study is to investigate the relevance of using MRI in addition to CT for patient modeling in H&N HTP.
  
  
  METHODS
  CT and MRI scans were acquired for 11 patients in an immobilization mask. Three observers manually segmented on CT, MRI T1 weighted (MRI-T1w), and MRI T2 weighted (MRI-T2w) images the following thermo-sensitive tissues: cerebrum, cerebellum, brainstem, myelum, sclera, lens, vitreous humor, and the optical nerve. For these tissues that are used for patient modeling in H&N HTP, the interobserver variation of manual tissue segmentation in CT and MRI was quantified with the mean surface distance (MSD). Next, the authors compared the impact of CT and CT and MRI based patient models on the predicted temperatures. For each tissue, the modality was selected that led to the lowest observer variation and inserted this in the combined CT and MRI based patient model (CT and MRI), after a deformable image registration. In addition, a patient model with a detailed segmentation of brain tissues (including white matter, gray matter, and cerebrospinal fluid) was created (CT and MRIdb). To quantify the relevance of MRI based segmentation for H&N HTP, the authors compared the predicted maximum temperatures in the segmented tissues (Tmax) and the corresponding specific absorption rate (SAR) of the patient models based on (1) CT, (2) CT and MRI, and (3) CT and MRIdb.
  
  
  RESULTS
  In MRI, a similar or reduced interobserver variation was found compared to CT (maximum of median MSD in CT: 0.93 mm, MRI-T1w: 0.72 mm, MRI-T2w: 0.66 mm). Only for the optical nerve the interobserver variation is significantly lower in CT compared to MRI (median MSD in CT: 0.58 mm, MRI-T1w: 1.27 mm, MRI-T2w: 1.40 mm). Patient models based on CT (Tmax: 38.0 °C) and CT and MRI (Tmax: 38.1 °C) result in similar simulated temperatures, while CT and MRIdb (Tmax: 38.5 °C) resulted in significantly higher temperatures. The SAR corresponding to these temperatures did not differ significantly.
  
  
  CONCLUSIONS
  Although MR imaging reduces the interobserver variation in most tissues, it does not affect simulated local tissue temperatures. However, the improved soft-tissue contrast provided by MRI allows generating a detailed brain segmentation, which has a strong impact on the predicted local temperatures and hence may improve simulation guided hyperthermia.
authors.affiliation: Hyperthermia Unit, Department of Radiation Oncology, Erasmus MC - Cancer Institute, Groene Hilledijk 301, Rotterdam 3008 AE, The Netherlands.
authors.forename: René F
authors.initials: RF
authors.lastname: Verhaart
authors.affiliation: Biomedical Imaging Group of Rotterdam, Department of Medical Informatics and Radiology, Erasmus MC, Dr. Molewaterplein 50/60, Rotterdam 3015 GE, The Netherlands.
authors.forename: Valerio
authors.initials: V
authors.lastname: Fortunati
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Groene Hilledijk 301, Rotterdam 3008 AE, The Netherlands.
authors.forename: Gerda M
authors.initials: GM
authors.lastname: Verduijn
authors.affiliation: Department of Radiology, Erasmus MC, Dr. Molewaterplein 50/60, Rotterdam 3015 GE, The Netherlands.
authors.forename: Aad
authors.initials: A
authors.lastname: van der Lugt
authors.affiliation: Biomedical Imaging Group of Rotterdam, Department of Medical Informatics and Radiology, Erasmus MC, Dr. Molewaterplein 50/60, Rotterdam 3015 GE, The Netherlands.
authors.forename: Theo
authors.initials: T
authors.lastname: van Walsum
authors.affiliation: Biomedical Imaging Group of Rotterdam, Department of Medical Informatics and Radiology, Erasmus MC, Dr. Molewaterplein 50/60, Rotterdam 3015 GE, The Netherlands.
authors.forename: Jifke F
authors.initials: JF
authors.lastname: Veenland
authors.affiliation: Hyperthermia Unit, Department of Radiation Oncology, Erasmus MC - Cancer Institute, Groene Hilledijk 301, Rotterdam 3008 AE, The Netherlands.
authors.forename: Margarethus M
authors.initials: MM
authors.lastname: Paulides
mesh_terms: D055592:Biophysical Phenomena; D003198:Computer Simulation; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000628:therapy*; D006801:Humans; D006979:Hyperthermia, Induced / Q000379:methods* / Q000706:statistics & numerical data; D008279:Magnetic Resonance Imaging; D013696:Temperature; D013813:Therapy, Computer-Assisted / Q000379:methods / Q000706:statistics & numerical data; D014057:Tomography, X-Ray Computed