A coil assembly (11) comprising two coil sections (13a and 13b). Each of the sections (13) consists of a combination of a spiral coil (15) having a variable coil pitch arranged in a saddle shape configuration. The saddle shape has a central portion (17) and a pair of outer coextending portions (19). The coil sections (13) are located opposite each other with the central portions (17) of the respective sections disposed in parallel spaced apart relationship and the corresponding coextending portions (19) of the respective sections disposed in coplanar alignment with each other so that the sections combine to circumscribe a scanned volume within which the target substance is to be disposed.