Quadrature image

Quadrature images Any imbalances between the two channels of a quadrature detection system cause ghost peaks, which appear as symmetrically located artifact peaks on opposite sides of the spectrometer frequency. They can be eliminated by an appropriate phase-cycling procedure, e.g., CYCLOPS. 

Quadrature images in the Fi dimension can be suppressed by expanding the 8-step phase cycle to 32 steps or 16 steps, respectively, using CYCLOPS [20] or 2-step CYCLOPS [21]. In the CYCLOPS scheme, the phases of all pulses are simultaneously incremented by 90°, 180° and 270°. In the 2-step CYCLOPS scheme, the incrementation of the pulse phases is limited to the 90° step. 

Fig. 4a,b spectra of natural abundance L-alanine obtained with the DUMBO decoupling sequence in a a 2D experiment and b a ID experiment with observation windows inserted between the DUMBO cycles. In a and b cjr/27r was 9 kHz and 12 kHz, respectively, o i/27r = 100 kHz and the external field corresponded to a H Larmor frequency of 500 MHz. The observation window in b was 5 ps. The stars in b are quadrature images (Reproduced with permission from Ref. [82])... 

Figure 3.25. Quadrature images are unwanted mirror-image artefacts that arise from spectrometer imperfections. Here an image of CHCI3 can be seen at about one-half the height of the carbon satellites in a single scan spectrum.

Quadrature receive coils receive correlated signals but uncorrelated noise from the two receive channels. The result is a >/2 improvement in ignal-to-noise ratio over linear coils. In addition, quadrature imaging reduces diagonal shading in images. 

The dotted line indicates a third possible coherence pathway. If the initial 90° pulse is imperfect, there will be some residual z magnetization which will be raised to coherence level -l-l by the second 90° pulse. Suppression of this pathway requires two extra steps, yielding a four-step phase cycle. Finally, if one also wishes to incorporate a CYCLOPS cycle for 2 quadrature image suppression, the total phase cycle is 4 x 4 = 16 steps. The number of scans in a 2D experiment should be some whole number multiple of the number of steps in the phase cycle. However, with COSY, the sensitivity is high enough that 16 scans are usually more than necessary to acquire good spectra and thus the phase cycle determines the minimum time for the experiment. Fortunately, pulsed field gradient sequences have overcome this problem. 

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