³¹P MRS Technique

What must you do differently to perform ³¹P- instead of ¹H-spectroscopy? 
RF Architecture and Coils for ³¹P Spectroscopy
All hardware components in the RF system must be tuned and matched to the ³¹P Larmor frequency (which is only 40% of the ¹H frequency). Accordingly, a complete second radiofrequency (RF) "front end" is required for phosphorus spectroscopy, adding considerable cost and complexity to the system. 
Although "P-only" surface coils are still occasionally used, most centers employ coils sensitive to both ³¹P and ¹H resonances. Such dual-tuned coils come in flat, wrap-around, and birdcage versions. They may be be receive-only or transmit-receive.  Dual-tuned coils are not trivial to construct as inductive interaction between the two components occurs, making it difficult to design a coil that functions optimally for both nuclei. 
Dual-tuned head coil

Dual-tuned ³¹P-¹H transmit-receive birdcage head coil
(from rapidbiomed.com)
Advantages of dual-tuned coils include: 1) ability to perform combined ³¹P and ¹H spectroscopy without moving the patient; and 2) improved shimming, anatomic localization, and voxel placement, all of which must be performed using ¹H (because the ³¹P signal is too weak). ​
³¹P Spectroscopic Imaging Sequences
Compared to the target molecules for ¹H spectroscopy, ³¹P metabolites generate much smaller signals and have relatively shorter T2 relaxation times. Their signals are even further reduced due to J-coupling modulations by nearby ¹H nuclei. Accordingly, ³¹P MRS is optimized by using pulse sequences with very short TE values. PRESS-based sequences, the workhorses of ¹H spectroscopy, are not typically used for ³¹P MRS as their minimum TE's are too long. Several ³¹P methods are in common use:
  • Non-localized FID. Also known as "pulse and acquire", this method uses an RF-pulse with FID recording and surface coil sensitivity profile to determine the volume of interest. Most commonly used for musculoskeletal spectroscopy.
  • Single voxel spectroscopy (SVS). Localization using ISIS (or sometimes STEAM) technique.
  • 2D or 3D Chemical Shift Imaging (CSI). Generally incorporates a non-selective (broadband) RF pulse with phase-encoding localization and recording of FIDs.​
Ancillary Signal Enhancement Techniques 
³¹P spectral peaks are intrinsically small due to low nuclear sensitivity. Their magnitude is even further reduced from line splitting due to J-coupling with nearby ¹H nuclei. Two ancillary methods involving selective RF-irradiation of ¹H nuclei are commonly used in ³¹P spectroscopy to improve signal quality. 
  • Proton Decoupling. Unwanted J-coupling interactions can be disrupted by applying RF-pulses tuned to the ¹H frequency during the period of signal acquisition.
  • Nuclear Overhauser Enhancement (NOE). If selective ¹H RF-excitation is applied in the period before spectroscopic signal acquisition, the NOE effect can boost the signal from certain ³¹P metabolites as much as 50-100%. Both NOE and proton decoupling are described more completely in later Q&A's.

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References
      Andrade CS, Otaduy MCG, Park EJ, Leite CC. Phosphorus-31 MR spectroscopy of the human brain: technical aspects and biomedical applications. Int J Cur Res Rev 2014; 6:41-54.     
     Grist TM, Jesmanowicz A, Kneeland JB, et al. Doubly tuned local coils for MRI and MRS at 1.5T. Magn Reson Med 1988; 6:253-264.
     Luyten PR, Bruntink G, Sloff FM, et al. Broadband proton decoupling in human ³¹P NMR spectroscopy. NMR Biomed 1989; 1:177-183.
Related Questions
     After hydrogen, why is phosphorus the most widely used nucleus for spectroscopy? 
​     How does the ISIS technique work and why is it preferred over PRESS for phosphorus spectroscopy? 
     What is decoupling? Is it required for phosphorus spectroscopy? 
     What is NOE? How and when should it be used for phosphorus MRS?