Should I capitalize 'X' in 'X-ray?'

I write an upper case 'X' whenever I write 'X-ray.'

However, the other days, my boss told me something like "when the 'x-ray' isn't placed in the top of the sentence, the lower case 'x' is better, isn't it?"

Then I searched for various pages, and found that both cases are used.

As a general tendency, the upper case 'X' seems to be used (especially when the 'X-ray' is used as a noun or an adjective).

Also, as a result that I examined 200 theses that include the word 'x-ray' on Google Scholar, it's found that the number of 'X-ray' is about three times as many as 'x-ray.'

Therefore, I will continue to use a capitalized 'X,' like "An X-ray CT image is..."

I would appreciate it if you could share any thoughts on that with me.

X-ray の X は大文字にすべきか否か

私は「X-ray」を書くとき、「X」を大文字で書いています。

しかし先日、「文中なら x-ray のように小文字で書くべきではないだろうか」と指摘を受けました。

そこで色々なサイトを探してみたのですが、どうやらどちらも使われるということがわかりました。

しかし傾向としては、大文字の「X」が使われることが多いようです（特に名詞形と形容詞形において）。

また、Google Scholar で「X-ray」を含む論文を200本調べたところ、おおよそ 3対1 の割合で大文字の「X」が多かったです。

そのため、今後も私は「An X-ray CT imaging is...」のように大文字の「X」を使おうと考えています。

もし何かアドバイスがありましたら、是非ご教授願いたい思いです。

According to this document (Capitalization and Acronyms in Physics, page 16) http://www.ifsc.usp.br/comissaopq/Caps &Acronyms.pdf
in US you can write it either way, "X-ray" or "x-ray". But, in UK they tend to write X in capital letter.

• Then I searched for various pages, and found that both cases are used.
• Then I searched for various pages (or: Then I did a little research), and found that both cases are used.

To be honest, to a native english speaker, there is not much difference.
I feel like especially in America, we are getting more and more lazy about English grammar and capitalization. This is something I have never thought about before with 'X-ray'!