ISISHow does the ISIS technique work and why is it preferred over PRESS for phosphorus spectroscopy?
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Image-Selected In vivo Spectroscopy (ISIS) was developed in the 1980s and probably remains the best single voxel spectroscopy (SVS) for ³¹P when using transmit/receive surface coils. The recorded signal is a free induction decay (FID). Similar to PRESS and STEAM, ISIS uses a slice intersection strategy for voxel localization. However, the required spectrum is not obtained from a single acquisition, but must be calculated using data from 8 separate RF-pulse cycles. These FIDs are added and subtracted in a particular order to define the volume of interest. A simplified version of the ISIS sequence is shown below:
Simplified pulse-timing diagram of the ISIS sequence
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The sequence includes three slice-selective 180°-inversion pulses and gradients in three orthogonal directions. The 8 separate acquisitions needed to calculate the final ISIS spectrum result from the 2³ = 8 permutations of inversion pulses (off/off/off, off/off/ON, off/ON/ON, etc.). The FID in each cycle is obtained using a nonselective ("hard") 90°-pulse that excites the entire volume under the coil.
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.
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 metabolites of interest in ³¹P spectroscopy have relatively short T2 values, and hence ISIS allows capturing more of this signal than either PRESS or STEAM. During the ISIS localization process, the magnetization remains along the longitudinal axis (± z) except during the short readout period of the FID.
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.
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.
References
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.
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.
Related Questions
If frequency-encoding cannot be used to determine spatial position, how do you localize an MRS signal?
If frequency-encoding cannot be used to determine spatial position, how do you localize an MRS signal?