Bipolar pulse pairs

Figure 9.4. The (a) BPP-STE and (b) BPP-LED diffusion sequences. The encoding gradients of the stimulated-echo are apphed as symmetrical bipolar pulse pairs of total duration 8 and the LED sequence is extended with an eddy current delay period T. Optional purging pulses Gpi and Gp2 may also been employed.

Figure 9.5. (a) Monopolar gradient pulse and (b) the equivalent bipolar pulse pair (BPP). Dashed vertical lines indicate time points and the period Tg is the gradient recovery delay. 

The bipolar pulsed pair stimulated echo (BPPSTE) pulse sequence was used to measure the water self-diffusion coefficients these were used as indicator of the Ostwald ripening extent in precursor emulsions leading to poly(divinyl-benzene) emulsion-derived solid foams based on a three component amphiphile system. 

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...

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 9.8. The one-shot diffusion sequence. The bipolar pulses Gi are applied as unbalanced pairs in a ratio 1 -l-ail—Q with additional balancing pulses of relative intensity la. An intense purge gradient Gp is applied during the diffusion period D and is balanced by the gradient pulse —Gp during the relaxation delay.

BPP Bipolar pair gradient pulse pair of opposite sign 9.2.3... 

In order to encode displacement as opposed to average position, the gradient is applied in such a maimer as to ensure that ). Generally, this means applying gradient pulses in bipolar pairs or applying uni-... 

Let us now consider the action of the two equal and opposite pulsed gradients (referred to as a bipolar pair) of amplitude +g and length (5, separated by time A, as shown in Fig. 7, and in the absence of relaxation. When there is no motion the first pulse, +g, will cause a phase shift which is proportional to the zeroth moment (Eq (14)) ... 

To allow for gradient pulse recovery delays Tg in the bipolar pairs, Eq. (9.8) for defining the signal amplitude of the stimulated-echo is modified slightly ... 

Figure 9.37. The constant-time-HSQC-IDOSY sequence. The delays T are set to 1/2Jhx as required for the INEPT transfer and the constanttime period 2T remains fixed, its duration being dictated by the desired diffusion time A. The effective ti evolution time is varied by moving the two 180° refocusing pulses within the constant time period (pulses shown with arrows over) and coherence selection is made with the echo/antiecho (E/A) scheme. The diffusion encoding/decoding gradients are applied as bipolar pairs during the INEPT and reverse-INEPT transfer steps.

The afferent pain nerves have a higher threshold and rheobase than sensory and motor nerves. Thus it is possible to stimulate sensory and motor nerves without ehciting pain. Very short pulses of 10—400 ps duration are used, with constant amplitude current up to about 50 mA and treatment duration of 15 min or more. The skin electrodes may be bipolar or monopolar. The position is in the pain region an electrode pair may, for example, be positioned on the skin on the back of the patient, or implanted with thin leads out through the skin. The electrode pair may also be positioned outside the pain area (e.g., at regions of high afferent nerve fiber densities in the hand). 

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