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ObjectiveThe objective of this study is to visualize the MR imaging characteristics of the natural foramina in the middle cranial fossa and to demonstrate imaging findings of invasion of nasopharyngeal carcinoma through these foraminas.Methods1. Sixty patients with normal appearances of pharynxnasalis and skull base, who received head MR imaging due to other reasons, was enrolled in this retrospective study by reviewing the PACS. The MR imaging was performed at a Siemens Avanto 1.5 T system. The imaging protocols included transverse T1 weighted imaging and T2 weighted imaging, and coronal T2 weighted imaging. The shape and the size of foramen ovale (FO),foramen rotundum (FR),foramen spinosum (FS), pterygoid canal (PC),pterygopalatine fossa (PPF) and foramen lacerum (FL) were described and measured. The difference of their size between both sides and sexes was analyzed using student t test. A p value less than 0.05 was considered to indicate a significant difference. The statistic analysis was performed with SPSS 13.0 for windows.2. One hundred fifty-four consecutive patients with histopathologically proven NPC received MR imaging for local staging before radiotherapy. Local invasion of NPC, especially through the the natural foramina in the middle cranial fossa, was evaluated by two radiologists in consensus.Results1.On transverse MR images, the shapes of FO were variable, mostly as orbicular-ovate shape. The content in FO appeared as iso-intensity on TIWI, slightly hyperintensity on T2WI. The mean maximum diameter and the mean minimum diameter were 6.5±0.8mm and 3.8±0.7mm, respectively. The FR was classified as canal and foramen types on axial images, and presented as round or oval shape on coronal images. The content in FR appeared as iso-intensity on T1WI, slightly hyperintensity on T2WI.The mean length and diameter were 4.9±1.6mm and 2.5±0.5mm, respectively. The shape of FS was small and displayed as round shape on axial images, slightly anfractous canal types on coronal images. The maximum and minimum diameter of FS was 3.2mm and 1.7mm, respectively.50% FS displayed perspicuously,its content appeared as hypointensity on T1WI and T2WI.64% pterygoid canal displayed perspicuously and presented as straight canal on axial images frequently, presented as round shape on coronal images.The content in PC appeared as iso-intensity on T1WI, slightly hyperintensity on T2WI.The maximum and minimum length and diameter of anterior and posterior opening of pterygoid canal was 17.8mm,10.1mm,4.2mm,1.0mm, 2.6mm and 1.0mm, respectively. PPF presented as various shape on different plane, its content appeared as hyperintensity on T1WI and T2WI.The mean posteroanterior diameter was 2.9+1.0mm,2.4±0.5mm,3.8±0.9mm,3.4±1.0mm on pterygoid plates-plane, pterygoid process-plane, pterygoid canal-plane and foramen rotundum-plane, respectively. The FL was triangle -like shape on axial images,and surrounded by the sphenoid bone, clivus basilaris and apex partis petrosae of temporal bone,its content appeared as hypointensity on T1WI、T2WI.The mean length of three side of FL was 8.1±1.7mm,13.8±3.0mm,12.1±2.3mm, respectively. Statistical analysis showed that all these parameters’difference between the gender and lateral were not significant(p>0.05).2. Seventy- five patients with natural foramina involvement was found in 154 NPC patients on MRI examination(48.7%), including foramen ovale (24 cases),foramen rotundum(15 cases),foramen spinosum(2case), pterygoid canal(66 cases),pterygopalatine fossa(55cases) and foramen lacerum(47 cases), respectively. Single natural foramina involvement was 11 cases. Multiple natural foramina involvement was 64 cases. The form of natural foramina invasions in NPC principally presented as destruction of bone, spreading along natural foramina,spreading along nerve,erosion by tumor directly and so on.3. There were 26 cases with dural involvement in NPC, which presented as dural thickening and contrast enhancement,traverse through foramen ovale, foramen lacerum and foramen rotundum 22,16 and 9 cases, respectively.Conclusion1. MRI with the characteristic of multiparameter and multiplane imaging could visualized clearly normal morphous of natural foramina in middle cranial fossa underside skull base, especially for foramen ovale,foramen rotundum,pterygopalatine fossa and foramen lacerum, which is useful for evaluation of skull base disease.2. NPC can local invaded through natural foraminas in the middle cranial fossa with high frequency and the natural foramina provide pathway for NPC’s intracalvarium invasion. The involvement of the natural foramina can be demonstrated by MR imaging. MRI plays an important role in assessment of local staging of NPC and severs as first line imaging modality in diagnosis and staging of NPC.
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