Sprites

Other products of pharmacological interest in this connection are dealt with by Hunt and Renshaw, by Lee, van Arendonk and Chen, by Bender, Sprites and Sprinson,i by Morrison, and by Work . It should be noted that Amanita spp. also contain toxins, on which a considerable amount of work has been done, especially with A. phalloides. ... 

SPRITE is termed a pure phase encode technique because spatial encoding occurs through the application of variable amplitude magnetic field gradients (which yield spatially varying frequencies) applied for fixed periods of time. Variable frequency with a fixed evolution or encoding time yields a variable signal phase. 

The SPRITE [16] technique was developed to reduce the dangerous mechanical vibrations and noise produced during an SPI experiment. In addition to the reduced noise and mechanical vibration, the experiment is much faster than SPI and the image intensity is still easily understood. However, one experimental shortcoming is a magnetic field gradient duty cycle which is even more demanding than an SPI experiment. 

The SPRITE signal equation is identical to Eq. (3.4.1) and it is possible to acquire density weighted images in the same manner as an SPI experiment. The TR in a SPRITE experiment is only a few milliseconds, which is similar to the Tj of a concrete specimen. Therefore to remove this influence from the image, a can be... 

Fig. 3.4.2 Schematic description of the three-dimensional SPRITE imaging technique. Gz, Gx and Gy are the phase encode magnetic field gradients and are amplitude cycled. A single data point is acquired at a fixed encoding time tp after the rf excitation pulse from the free induction decay (FID). TR is the time between rf pulses. Notice that Gx is ramped (+GZ max to -GXt max) and one /c-space point is acquired for each value of the magnetic field gradient. Gy and Gz are on during the Gx magnetic field gradient ramp and turned off at the end.

Fig. 3.4.3 (a) Two-dimensional k-space acquisition using SPI or SPRITE. The k-space data acquisition is indicated numerically. High magnetic field gradient amplitudes are applied at the extremities of k-space. (b) A generic two-dimensional centric scan SPRITE method. The... 

Fig. 3.4.4 Schematic description of the one-dimensional double half k (DHK) SPRITE technique. The phase encode magnetic field gradient, Gz, ramped through half of /(-space beginning at the center and a single data point is acquired at a fixed time (tp) after the rf excitation pulse. The second half of /(-space is acquired after a 5T time delay. The time between rf pulses is TR.

The SPI and the SPRITE techniques have been applied to numerous concrete material problems. Some of these applications are presented in the following discussion. It is very important, however, to point out that these measurement techniques are not limited to cement-based material problems, but are fairly general in application. They can be, and have been, applied to many different material science problems. 

Cano Barrita [27] cast concrete specimens with w/c of 0.6, dried the specimens at 38 °C and 20% relative humidity, then measured the penetration of water in a capillary uptake type of experiment. A 3D centric scan SPRITE measurement was selected, as an image could be acquired in 150 s and the image would therefore be weighted only by the T2 decay. 3D images were acquired at various exposure times and the central 2D image slice was extracted from the data to measure the penetration depth with time. 

Fig. 3.4.9 Two-dimensional slice images taken from a three-dimensional Spiral-SPRITE water uptake experiment [27]. (a) Initially dry,...

It is well known that the melting point of water confined to small pores is depressed [30, 31]. Therefore in concrete as the temperature decreases, the amount of frozen water will increase. Under normal temperature variation not all water in the pore structure will be frozen. The change from water in the liquid form to solid ice drastically reduces the Tfk of hydrogen (T2 ice <9 ps [32]). Ice will not be observed in an image, even with the SPRITE techniques, and our experimental images will be maps of unfrozen water distribution. 

The SPRITE class of measurements is required for MRI measurements of the low sensitivity, low concentration and short signal lifetimes of these nuclei. The absolute sensitivity of 35C1 is only 0.4% and 23Na is 9% compared with 3H [36]. 

It is important to note that sodium and chlorine are both quadrupolar nuclei and Eq. (3.4.1) is therefore not true for SPI/SPRITE measurement of these species however, Eq. (3.4.2) is correct when centric scan SPRITE methods are employed. 

This provides one more illustration of the value of the centric scan SPRITE methods. 

Fig. 3.4.14 One-dimensional SPRITE image of the sodium distribution in mortar specimens...

The centric scan, one-dimensional, DHK SPRITE measurement was used to study the ingress of lithium. This measurement technique was selected due to the low absolute sensitivity of 7Li (27% of [36]), the small amounts that are present and the short signal lifetimes (bulk Tx of 10 ms and T2 of 120 ps). In addition to the robust, quantitative nature of this technique, lithium is a quadrupolar nucleus and interpretation of the image intensity is more complex than spin % nuclei. Once again Eq. (3.4.2) is quantitatively correct for even quadrupolar nuclei due to the fact the longitudinal steady state does not influence the image intensity. 

Figure 3.4.16 shows one-dimensional DHK-SPRITE profiles of lithium penetrating into dry mortar. In addition to the determination of the penetration depth of lithium shown at various times, there is an overall increase in signal intensity at a given point, which is attributed to the filling of the various pore sizes in time. The... 

The SPI and SPRITE class of measurements for imaging short MR signal lifetimes are quantitative and have signal equations that are readily understood. The centric SPRITE methods are much faster and feature signal equations that are easier to interpret. This feature makes these measurements more readily density weighted and better suited to imaging quadrupolar nuclei. 

Fig. 3.7.6 DDIF spectra and SPRITE MRI images of Berea obtained in different saturation states. (A) The DDIF spectra during cocurrent imbibition at different water saturation (Sw) levels. Note the similar shape of DDIF spectra at different Sw. (B) The DDIF spectra during counter-current imbibition acquired at different water saturation levels. Note the change in the DDIF spectral shape for the different saturation levels. (C, D) A pair of images show 2D longitudinal slices from 3D...

Conical-SPRITE MRI data sets obtained during co-current imbibition. The time interval between the two images was 10.5 min. The images show a piston-like water penetration. (E, F) 2D slices from a 3D Conical-SPRITE MRI data set obtained during counter-current imbibition. The overall water saturation was 26.3%. The penetrating waterfronts have not reached the sample center. Figure from Ref. [65] with permission. 

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