Adding Line Between Two Lines?

M48
METRIC,LZ
VER,1
FMAT,2
T00

[code]....

I want to add some text beside the text which contain "T" I have the same value below the line DETECT,ON but i just want to add text to the lines between FMAT,2 and DETECT,ON. Can i use indexOf methode to get the lines between FMAT,2 and DETECT,ON

For now i used this code to add text

Dim filename As String = strFileName
Dim tfLines() As String = System.IO.File.ReadAllLines(filename)
If File.Exists("C:kaya.txt") Then

[code]....

I want the text to be added to the T value above the line DETECT,ON. because this code add the text beside T whenever it finds the T value. how add line only between line FMAT,2 and DETECT,ON?