Spatial Localization of NMR Signals by Narrowband Inversion

Surface coils for exciting and detecting NMR signals are often used in place of the traditional solenoid or Helmholtz coils in situations where only a small region of a large sample is of interest (I). In particular, this is the situation in many in vivo studies (1-5). Typically, it is desirable to observe signals from a single organ without interference from the surrounding tissue. The degree of spatial localization achieved with a surface coil in a single-pulse experiment is often insufficient, however, requiring that the organ be surgically exposed (6-9). A unique characteristic of a surface coil is the fact that it produces a very inhomogeneous radio frequency field within the sample. In general terms, the rf amplitude decreases with distance from the coil (1). This suggests the possibility that a higher degree of spatial localization may be achieved with surface coils if the signals are excited by a pulse sequence that is inherently very sensitive to rf amplitude (10-18). We have recently developed rf pulse sequences that invert nuclear spin populations only over a narrow range of rf amplitudes (18). We call such sequences narrowband inversion sequences. They are similar in form, although opposite in function, to the composite pulses for broadband inversion originally proposed by Levitt and Freeman (19-24). With a surface coil, a narrowband inversion sequence inverts spins in a small region in space. Here we propose an experimental method that exploits narrowband inversion in conjunction with surface coils to produce spatial localization of NMR signals. We call the method NOBLE (Narrowband for Localization of Excitation). Let P be a narrowband inversion sequence and R be a “read” pulse or pulse sequence. As illustrated in Fig. 1, NOBLE consists of the following steps: