In Nemeth, the italic type-form indicator (46) affects only the next letter. The type-form "ends" after the letter -- no explicit terminator is needed, not even a space.
In your example, however, the type-form of the imaginary number i should be disregarded. The only time you would retain the italics in the transcription would be if there is another mathematical "i" (in regular type) in the vicinity that means something different from "imaginary number." In that case, the type-form would be retained in order to distinguish the two different i's.
Italics are applied only if the type-form has mathematical significance, according to the provisions of Section 34.b which says the following. "When any material, mathematical or literary, is printed in non-regular type that has no mathematical significance, the variant type form must not be represented in the transcription. Frequently, it is the practice to print the letters of all formulas throughout a book in italicized type. This practice must not be carried over to the transcription unless the author has specifically distinguished between two meanings of the same letter, assigning one meaning to the letter in regular type and another to the letter in italic type. In addition, a variant type form is often used, particularly at the lower grade levels, for the sole purpose of attracting the reader's attention. Such variant type forms must also <u>not</u> be represented in the transcription."