Gradient Echo Imaging

Typical flip angles are in the order of 15° for the slice-selective excitation pulses. The intensity of the FID after such a pulse corresponds to 25% (sin 15°) of the intensity after a 90° pulse. However, more than 96% (cos 15°) of the longitudinal magnetization are preserved, enabling fast repetition rates. The method can be employed in combination with the backprojection imaging scheme and with spin-warp imaging by phase encoding of spatial information. 

After about 10-20 cycles of the experiment, the magnetization reaches a steady state, where it is in equilibrium with relaxation and selective excitation pulses. Conditions 

10 [Haal] Timing diagram of the 2D FLASH sequence for fast imaging. The method employs slice-selective excitation pulses of the order of 15°. 

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The spin-echo images showed no obvious differences between ripe and overripe samples, but the gradient-echo images of the over-ripe melons showed decreased intensity. It was surmised that the increased air spaces in the overripe tissue intensified the internal magnetic susceptibility gradients, whose dephasing effects can be refocused by a spin-echo but not by a gradient-echo. 

Figure 7.5 [15] Susceptibility contrast in EPDM samples at 363 K (a) 2D Hahn-echo image acquired with the sequence of Figure 7.4a (b) 2D gradient-echo image acquired...

Lin DD, Filippi CG, Steever AB, Zimmerman RD (2001) Detection of intracranial hemorrhage comparison between gradient-echo images and b(0) images obtained from diffusion-weighted echo-planar sequences. AJNR Am J Neuroradiol 22 1275-1281... 

Fig. 13.5. T2 -weighted gradient echo images in a patient with biopsy proven cerebral amyloid angiopathy (CAA). Extensive low signal abnormality is noted on the brain surfaces and in the parenchyma indicating the previous hemorrhages. Periventricular high intensity lesions, a common finding in CAA is also noted...

Fig. 13.6. FLAIR (top row) and T2 -weighted gradient echo images (bottom row) in a patient with hypertension showing a typical lesion pattern. Note subcortical lesions including lacunar lesions with severe tissue destruction and CSF like, low signal intensity (e.g. lateral to the anterior horn of the left lateral ventricle) on FLAIR images. On T2 -weighted images residuals from hemorrhagic lesions and small bleeds are appreciated mainly in the basal ganglia...

Hti. 7.2.10 Susceptibility contrast in commercial EPDM samples, (a) 2D projection acquireil by spin-warp imaging, (b) 2D projection acquired by gradient-echo imaging. The images were measured at a temperature of 363 K. Adapted from BIUR with permission from Hiithig Gmbh. 

Because H linewidths in many elastomers are of the order of 3 kHz or less, spin-echo and gradient-echo imaging techniques can be applied. Contrast is introduced by suitable filters like Ti and double-quantum filters or by use of the spectroscopic dimension. Parameter images of T2, the double-quantum signal intensity, or the quadrupolar coupling strength are evaluated and rescaled according to theory or experimental calibration data (cf. Section 7.1.6). 

Glover, G. H., 1993, Gradient Echo Imaging, in the American Association of Physicists in Medicine (AAPM) Monograph No. 21 The Physics qfMRI, P. Sprawls and M. Bronskill, eds., American Institute of Physics, New York, pp. 188-205. 

Figure 14 (A) Planning images for magnetic resonance spectroscopy (MRS) of the brain top and middle spin-echo image TE = 96 ms, TR = 3 s. Bottom gradient echo image TE = 1.9 ms, TR = 116 ms. (B) Chemical shift metabolic image (CSI) of the brain overlaid on the anatomic image. Spectra are displayed for the selected regions. Chemical shift range is from 4.3 to 0.5 ppm. (Spectra were obtained by Dr JA Hopkins, GE Medical Systems and reproduced by permission of GE Medical Systems, Milwaukee, Wl.)...

With conventional spin echo techniques, those features that will alter image contrast are the pulse repetition time (TR) and the echo delay time (TE). With inversion recovery sequences, it is the inversion time (TI) which has the greatest effect while with gradient echo imaging, it is the flip angle that is most important (Barnewolt and Chung 1998 Bradley 1999 Anupindi and Jaramillo 2002 Bitar et al. 2006). 

Metallic objects will produce their own local magnetic field and this will cause marked distortion of the MR image. This is more pronounced on gradient echo imaging. 

Fig. 5.11. Oblique spoiled gradient echo image of the distal humerus showing a fracture extending across the physeal growth plate and through the unossified cartilage...

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