This is an incidental finding in a study requested to evaluate for an intracranial metastatic disease, in the context of known lung cancer.
Here we have multiple MR sequences demonstrating a cystic lesion along the posterior aspect to the right cerebellar hemisphere, extending to the midline.
The right cerebellar hemisphere is displaced anteriorly.
However, there is otherwise no significant associated mass effect.The mass follows CSF signal on all sequences. And there is notably no associated restricted diffusion. The mass is hyper-intense on T2, completely suppresses on FLAIR，and there is no associated enhancement.
These findings are consistent with an arachnoid cyst. Epidermoid cysts are in the differential of cystic intracranial lesions, and will classically demonstrate restricted diffusion and incomplete suppression of signal on FLAIR.
Epidermoid cysts also tend to engulf surrounding structures and have scalloped margins, whereas arachnoid cysts are well circumscribed, and displaced adjacent vessels and nerves. The arachnoid cysts are most commonly found in the middle cranial fossa, but may arise in a variety of locations.
【Arachnoid cyst 蛛网膜囊肿】
这是在评估一位肺癌患者颅内转移情况时偶然发现的病变。
多种MRI序列表明病灶为囊性病变，沿右侧小脑半球后方生长，越过中线。右侧小脑半球向前受压移位。但是，未见明显的占位效应，肿块内信号和脑脊液信号一致，没有弥散受限。肿块在T2高信号，压水相完全抑制，无明显强化。
以上征象均提示蛛网膜囊肿，该病变需要与表皮样囊肿等颅内囊性病变鉴别。脑内表皮样囊肿典型表现是明显的弥散受限和压水相不完全抑制，其常包绕周围结构，边缘呈扇贝样压迹。而蛛网膜囊肿边缘光滑局限，会使周围神经血管移位。蛛网膜囊肿最常好发于中颅窝，但也可位于其他部位。
Notes:
1. intracranial [ˌɪntrə'kreɪnɪrl] adj. 头颅内的，颅骨内的
2. metastatic [ˌmetə'stætɪk] adj. 转移性的; 变态的; 变形的; 由转移所致的
3. cystic ['sɪstɪk] adj. 胞囊的，膀胱的，胆囊的
4. cerebellar [serə'belə] adj. 小脑的
5. CSF=Cerebro-Spinal Fluid 脑脊液
6. arachnoid [ə'ræknɔɪd] adj. 蛛网状的，蛛网膜的
7. epidermoid [ˌepə'dɜ:ˌmɔɪd] adj. 表皮状的，像表皮的
8. engulf [ɛnˈɡʌlf] vt. 吞没; 吞食; 包住; 狼吞虎咽
9. cranial [ˈkreniəl] adj. 头盖的，头盖形的
10. fossa ['fɒsə] n. 窝，凹
11. scalloped ['skɑləpt] 扇形的
来源：每天朗读一段医学影像学英语文章
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