To better understand how this works in practice, the diagram below shows how a 2-cycle ISIS sequence in one dimension could be used to localize a single planar slab of data.
The major limitations of ISIS result from subtraction errors. These occur primarily due to signal contamination when various portions of the imaged tissue block respond slightly differently to the RF-pulses. Motion artifacts during the 8-cycle acquisition also contribute.
Advanced Discussion (show/hide)»
To improve uniformity of B1 excitation, the RF-pulses used in transmit/receive surface coil implementations of ISIS are typically adiabatic — hyperbolic secant full (180°) or half passage (90°) are good choices. Adiabatic pulses utilize simultaneous amplitude and frequency modulation. They have excellent slice profiles with flat response and no side lobes leading to better edge definition of the voxel.
In addition to 31P spectroscopy, ISIS is also widely used in 15N spectroscopy, whose metabolites, like those of phosphorus, have very short T2 values.
Like PRESS and STEAM, ISIS can also be used in a multi-voxel mode, being combined with 2D or 3D chemical shift imaging techniques with phase-encoding gradients.
Ordidge RJ, Connelly A, Lohman JAB. Image selected in vivo spectroscopy (ISIS). A new technique for spatially selective NMR spectroscopy. J Magn Reson 1986; 66:283-294.
If frequency-encoding cannot be used to determine spatial position, how do you localize an MRS signal?