Clinicians and radiologists interested in MR T1ρ imaging and nasopharyngeal carcinoma.Nasopharyngeal carcinoma (NPC) is one of the most common malignant tumors affecting the population in southern China. The standard treatment for NPC is radiotherapy (RT), which involves irradiation of the inferior part of the temporal lobes. Radiation induced encephalopathy is one of the most serious complications of RT for treatment of NPC. However, most radiotherapy patients undergoing long-term clinical follow-up exhibit little change in their temporal lobes according to conventional MRI examination. T1ρ presents the spin-lattice relaxation time constant in the rotating frame, which has been found to be sensitive to the interaction between the marcomolecules and bulk water. The changes of T1ρ values precede the morphological changes dectected by routine MRI. The purpose of our study was to detect microstructural changes of gray matter (GM) and white matter (WM) between patients with NPC who had and had not received RT by using MR T1ρ imaging.

This prospective study was approved by the local ethics committee, and written informed consent was obtained from each participants.

Subjects and data acquisition: 15 patients (Mean age 51.1± 9.8 yrs; age range 35-67 yrs; 5F/10M) with NPC who had been diagnosed by histopathology and treated with RT and 15 control patients (Mean age 47.1±13.1 yrs; age range 24-67yrs; 5F/10M) with NPC who had been newly diagnosed and previously untreated were recruited. All MR scans were performed on a 3.0T MR scanner (Philips Achieva, the Netherlands) with a 8 channel head coil to receive the signal. T1ρ was performed using 3D Turbo Spin-Echo (TSE) pulse sequence, scanning parameters were as follows: TR/TE= 4800ms/229ms, FOV = 250×250mm2, flip angle = 90°, matrix = 240×240, slice thickness = 1.8mm, number of slices = 100, spin lock frequency = 500 Hz, spin lock time = 0, 20, 40, 60, 80, 100 ms respectively. The entire volume of each subject’s brain was imaged in the sagittal plane using a T1-weighted 3D volumetric pulse sequence with 150 continuous slices.

Data analysis: All the DICOM images were converted to NIFTI format using MRIcron. Non-brain tissues of T1ρ images and 3D T1WI images were removed by FSL software respectively. For each subject, T1-weighted images and T1ρ images were first oriented in the same direction. Then T1-weighted images were co-registered to the T1ρ images. Next, the co-registered T1-weighted images were segmented into GM, WM and cerebrospinal fluid images. Finally, the T1ρ images were normalized by the matrix of standard space (Figure 1). Regions of interest for major white matter tracts were defined by an intersection of the individual spatially normalized WM probability map thresholded at 50% with the JHU ICBM-DTI-81 white matter labels atlas supplied with FSL. Cortical gray matter regions were similarly defined by the intersections of the individual spatially normalized GM and WM probability maps (respectively, each thresholded at 50%) with the Harvard-Oxford cortical atlas. Quantification analyses of WM and GM in the regions of interest were extracted by software based on MATLAB 2013a programs.

Statistical analysis: Independent sample T-test was performed on T1ρ value in GM and WM respectively between NPC patients after RT and before RT by using SPSS software. We used a statistical significance level of p<0.05.

There were no significant differences in T1ρ values on GM between NPC patients before RT and after RT (p>0.05). Compared to NPC patients before RT, NPC patients after RT revealed higher T1ρ values in bilateral superior cerebellar peduncle and right inferior cerebellar peduncle (Table 1, p<0.05). This is the first study, to our knowledge, to investigate microstructural changes of GM and WM in NPC patients after RT by means of MR T1ρ imaging. We found that the NPC patients who had received RT with normal conventional MRI findings exhibited RT-induced brain damages in some regions of WM. However, the present study didn’t show early radiation-induced changes in GM. It might be related to the insufficient number of cases. We plan to recruit adequate number of patients and have further investigation on the pathogenenesis of radiation-induced structural damage in normal-appearing brain tissue in future.Abnormal microstructure changes of white matter had already happened in NPC patients after RT even when the routine MRI findings are negative or nonspecific. MR T1ρ imaging can be used to detect early radiation-induced changes of white matter following RT for NPC patients.