简述牙及牙周组织正常X线影像。
参考答案：
【X线表现】
釉质：机体中钙化最高和最坚硬的组织，X线片显示的影像密度最高，似帽状覆盖于牙冠部牙本质的表面。
牙本质和牙骨质：牙本质X线影像密度较釉质稍低。牙骨质被覆于牙根表面，是一层很薄的组织，在X线片上所显示的密度与牙本质不易区别。
牙髓腔：为密度低的影像，髓腔可分为冠部的髓室及根部的根管。随着年龄的增长牙本质沉积，髓腔逐渐狭窄。因此，老年人的根管和髓室较年轻人的根管和髓室小而细。
牙槽骨：在X线片上显示的密度较牙低。上牙槽骨的密质骨薄，松质骨多，骨小梁数目多，因而在X线片呈颗粒状影像；下牙槽骨的密质骨厚而松质骨少，骨小梁数目少，故在X线片骨小梁结构呈网状。
牙周膜和骨硬板：牙周膜显示为包绕牙根的、连续不断的低密度线条状影像，其宽度均匀一致；骨硬板在X线片上显示为包绕牙根的、连续的、致密的线条状影像。
【X线片示例】
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| They crowded inside the storm door-two children in torn coats. "Any old 1 , lady?" I was busy. I wanted to say 2 - until I looked down at their feet. Thin little shoes, wet with snow. " 3 and I'll make you a cup of hot coffee." There was no conversation. Their wet 4 left marks upon the floor of the fire place. I served them coffee and toast with jam to keep the 5 out. Then I went back to the kitchen and started 6 on my household budget. The silence in the front room 7 me. I looked in. The girl held the empty cup in her hands, 8 it. The boy asked in a flat voice, "Lady... are you rich?" "Am I rich? Mercy, no!" I looked at my 9 slipcovers (沙发套). The girl put her cup back in its saucers (茶托) - 10 . "Your cups match your saucers." Her voice was old, with a 11 that was not of the stomach. They left then, holding their bags of papers 12 the wind. They hadn't said thank you. They didn't 13 to. They had done 14 that. Plain blue pottery cups and saucers. But they 15 . I tested the potatoes and tasted the gravy. Potatoes and brown gravy, a roof over our heads, my 16 with a good steady job-these things matched, too. I moved the chairs back from the fire and 17 the living room. The dirty prints of small shoes were 18 wet upon my floor. I let them be. I want them there 19 I ever forget again how very 20 I am. | ||||
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