Echo times

Human skin is the largest organ in the human body. It is fundamentally important to health as the semi-permeable barrier - the first line of defence - between the body and the external world. However, it remains relatively inaccessible to conventional magnetic resonance imaging, firstly because it is thin and therefore requires high spatial resolution, and secondly because it is characterized by relatively short T2 relaxation times, particularly in the outermost stratum comeum. Conventional studies have not usually achieved a resolution better than 70-150 pm, with an echo time of the order of a millisecond or so. As a planar sample, skin has proved amenable to GARField study where it has been possible to use both a shorter echo time and achieve a better spatial resolution, albeit in one direction only. Such studies have attracted the interest of the pharmaceutical and cosmetic industries that are interested in skin hydration and the transport of creams and lotions across the skin. 

Fig. 2.4.8 Skin profiles measured in the palm region as a function of the echo time. The profile amplitude is the value of the weighting function obtained for different echo times tE = 35 ( ), 70 (O), 100 (A), 140 (V), 180 and 220 ps (I>). The indices /, if, j and jf were recalculated for each subsequent experiment keeping tE/f = 2.1 ms, tEj = 2.1 ms, and...

Fig. 2.9.2 Radiofrequency, field gradient and current distributions requires a three-dimen-ionic current pulse sequences for two-dimen- sional imaging sequence [see Figure 2.9.1(a)] sional current density mapping. TE is the Hahn and multiple experiments with the orientation spin-echo time, Tc is the total application time of the sample relative to the magnetic field of ionic currents through the sample. The 180°- incremented until a full 360°-revolution is pulse combined with the z gradient is slice reached. The polarity of the current pulses...

For the experiments in type C catalysts, the pellets were overfilled with cyclohexane and initially cooled to 230 K. They were then reheated in steps of 1 K and allowed to equilibrate for 10 min before each measurement. The signal was determined from 32 accumulations with an echo sequence of 20 ms echo time to ensure that the signal from the plastically crystalline phase of cyclohexane had decayed fully. The typical heating curves of cyclohexane in the fresh and coked catalyst are displayed in Figure 3.3.3(a) As the temperature is increased, larger and... 

Fig. 3.4.12 Three-dimensional rendered spin-echo image of water filled cracks in a cement paste specimen [13]. Three cracks are visible in the image a large triangular crack in the forefront, a smaller crack in the bottom left corner and a sheet-like structure at the top of the image. Water droplets can also be observed condensing on the cement paste surfaces. The measurement parameters were FOV 20 x 20 x 20 mm, acquisition points 128 x 128 x 64, nominal resolution 156 x 156 x 312 pm, echo time 2.7 ms, repetition time 500 ms and acquisition time 270 min.

We can perform spatially resolved Carr-Purcell-Meiboom-Gill (CPMG) experiments, and then, for each voxel, use magnetization intensities at the echo times to estimate the corresponding number density function, P(t), which represents the amount of fluid associated with the characteristic relaxation time t. The corresponding intrinsic magnetization for the voxel, M0, is calculated by... 

Figure 4.1.1 CPMG pulse sequence designed for three-dimensional imaging. TE is echo time, and Gi, G2 and G3 represent the gradient magnetic fields along the directions of zlt z2 and z3, respectively.

Pfeuffer, J., Tkac, I., Provencher, S. W. etal. Toward an in vivo neurochemical profile quantification of 18 metabolites in short-echo-time H NMR spectra of the rat brain. /. Magn. Reson. 141 104-120,1999. 

Simister, R. J., Woermann, F. G., McLean, M. A. et al. A short-echo-time proton magnetic resonance spectroscopic imaging study of temporal lobe epilepsy. Epilepsia 43 1021-1031, 2002. 

Most clinical examinations apply robust spin-echo or fast spin-echo sequences. These types of sequences provide tissue contrast changes by variation of the chosen repetition time TR (time interval between succeeding RF excitations) and echo time TE (time delay between RF excitation and signal acquisition). 

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