Rounded corners with NSMutableAttributedString

Question

My question, I think, is very simple, but I could not find anything with Google. I use NSMutableAttributedStringto change the styles in the string and, for example, I use this code to change the background color:

    [string addAttribute:NSBackgroundColorAttributeName value:[UIColor colorWithRed:0.4 green:0.8 blue:1 alpha:1] range:NSMakeRange(0,18)];

And my question is: how can I get around the corners of this background?
Thanks in advance.