Bipolar gradients

If temis of higher order than linear in t are neglected, the transverse magnetization evolves in the presence of the first bipolar gradient pulse according to (equation Bl.14.2 and equation B 1.14.61 ... 

The zeroth moment, mQ, was mentioned above. It is nothing more than the total area of the gradient [see Figure 1.12(a)]. If the area vanishes, the signal is independent of spin position. This is the case, for instance, for a bipolar gradient with positive and negative lobes of equal area [see Figure 1.12(b)]. 

Fig. 1.19 Spin-echo based pulse sequence to each gradient pulse, A the separation between encode velocity change. The gradients are each pair of bipolar gradient pulses and tm the stepped pair-wise independently (2D VEXSY) mixing time between the bipolar gradient pairs, or simultaneously (1 D VEXSY). For a VEXSY The opposite polarity of the bipolar gradient experiment, 7q to k4 are usually applied along pair is realized by an inversion 180° pulse, the same spatial direction. 8 is the duration of...

In an NMR/MRI flow experiment, we would like to measure parameters such as velocity without regard to the starting position of the particle. Thus, mo is always set to zero. The moments m, are under the control of the experimenter in that they are manipulated by the choice of the time dependence of the gradient G. Thus, it is easy to see that m0 can be set to zero by simply making sure that the time integral of the gradient is zero. The easiest way to accomplish this is to have a bipolar gradient of equal absolute amplitude and duration. 

Fig. 21.1 Pulse sequence to selectively observe solvent-exposed amide protons with TROSY (SEA-TROSY). Narrow and thin bars represent 90 and 180° rf pulses, respectively. Unless specified otherwise, pulse phases are along the x axis. The pulsed field gradients are of 500 ms duration with strengths of gi =20 G cm-1, g2 = 30 G errf1, g3 = 40 G errf1, g"4 = 15 G cm-1, g5 = 55 G errf1. The bipolar gradient gj is 0.5 G errf1 and is used to avoid...

Wu, D. H., Ghen, A. D., and Johnson, C. S., An improved diffusion-ordered spectroscopy experiment incorporating bipolar-gradient pulses, /. Magn. Reson., Ser A, 115, 260,1995. 

Fig. 4. Modified X/Y IMPEACH-MBC pulse sequence used for 19F/15N shift correlation according to Ref. 27. The notation of 90° and 180° pulses is as before. The (d/2 — 180°(Y) — d/2) element represents a variable delay that is incremented concurrently with the decrementation of the accordion delay vd. Pulse phases are x, unless specified x = — x 2 = x, — x 3 = x, x, — x, — x = , — x, — x, x. The bipolar gradients Gs flanking the 180°(Y) pulse can be set to arbitrary power levels, and the relative strengths of the coherence selection gradients G and G2 are determined by G2/G1 =2 Yy/Tx-...

Therefore, the residual phase shift of the MR signal for a magnetic moment undergoing uniform motion with velocity v for a set of bipolar gradients, +g, of duration 6 separated by a time A is - rg(5A (i.e., the measured phase shift is linearly proportional to the velocity). 

An example of a velocity-encoding method is the bipolar-gradient method.35 Here, two identical pulses are applied during the dephasing and rephasing periods of a spin-echo sequence. Since the two pulses are in opposite polarity with respect to the time domain, the phase shift (p(t) in Eq. (28) is dominated by the velocity term, i.e. the first-order term of t in the expansion. 

FIf. 18. Pulse sequence methods for controlling radiation damping. (A) Gradient recalled echoes in the evolution period. The bipolar gradient pulse uses only very weak -gradients ( 0.2 G cm" ). (B) Bipolar gradient pulses between the acquisition of data points in the... 

The final sequence also included an additional longitudinal eddy-current delay and bipolar gradients to compensate for eddy-current effects. Vastly improved results were achieved in terms of the absolute value of D measured and the temperature dependence of D, compared to the standard non-compensated sequence. 

An artifact nniqne to PC MRA is that phase shifts exceeding 180° are interpreted as slow flow in the opposite direction. This aliasing artifact leads to incorrect determination of flow direction. In order to avoid this, a velocity encoding parameter (VENC) is selected which represents the maximum expected flow velocity in the imaging volnme. This value adjusts the strength of the bipolar gradients to prevent phase shifts from exceeding 180°. 

There have been no reported applications of the J, J-HMBC experiment published to date. There has, however, been one report of a COSY-type artefact observed in -J. J-HMBC spectra of a cyclopentafurnanone. The COSY-type responses observed are displaced in Fj as a function of the choice of Jscaie-Removing the bipolar gradients flanking the BIRD pulse in the A2 interval of the STAR operator and superimposing a CYCLOPS phase cycle on the BIRD pulse completely suppresses the COSY-type response artefacts associated with the -J, J-HMBC experiment. ... 

F nre 9.6. Deuterium lock signal behaviour during the STE sequence recorded with (a) monopolar and (b) bipolar gradient pulses. Lock signal refocusing is apparent in (b) leading to a minimal and only momentary perturbation from the two gradient pulse pairs. 

Figure 9.7. H lineshapes observed for the STE sequence recorded with (a) monopolar and (b) bipolar gradient pulses.

Figure 9.24. Solvent signal suppiession by a diffusion filter. The sample is 1 mM lysozyme in 50 50 H20 D20 and suppression in (a) was achieved using a H stimulated-echo-based sequence employing a diffusion delay A of 50 ms, bipolar gradient pairs of 4 ms total duration ( ) and gradient strengths of 0.43 Tm. The standard spectrum is shown in (b).

One example of this approach is the constant-time HSQC-IDOSY [72] experiment (Fig. 9.37) that combines the diffusion and constant-time periods and incorporates the encoding-decoding bipolar gradient pairs within the INEPT transfer steps. Additional gradients are utilised to provide coherence selection and/i quadrature detection according... 

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