Gradient-echo sequence

Figure 2.2.14 shows cross-sections of salmon acquired with a gradient echo sequence (TR = 30 ms, TE = 15 ms, slice thickness = 4 cm, image matrix = 128 x 128, total acquisition time = 3.84 s). For the female salmon, roe were clearly visualized as dark regions because the T2 is shorter than that of the surrounding tissues. For the male salmon, milt was observed instead of roe. However, the image intensity of the milt was similar to that of the surrounding tissues. 

Fig. 10. Sequence for the assessment of magnetization transfer (MT) effects. Images recorded without (a) and with MT prepulses (b) have to be subtracted. In the presented example a standard gradient-echo sequence is used for MT imaging. The MT prepulse is Gaussian-shaped with a flip angle of 600° and 1.5 kHz off-resonance frequency.

Hall and co-workers have imaged overripe melons suffering from internal necrosis with both spin-echo and gradient-echo sequences at 2 T (86 MHz). 

Figure 11 shows the i 2 dependence on the particle size for systems containing a constant amount of magnetized material, without refocusing pulse (like in a gradient echo sequence), and with 7 different echo times used in CPMG sequences. 

The MRA techniques discussed in the following base on gradient echo sequences and can generally be divided into flow-dependent and flow-independent ones. The main flow dependent techniques are time-of-flight MRA (TOF-MRA) and phase contrast MRA. CE-MRA, in contrast, is blood flow-independent, although flow phenomena can still influence... 

Time-of-flight MRA is based on gradient echo sequences with very short repetition times (Laub and Kaiser 1988). The repeated HF excitations cause a relative spin saturation - i. e. signal reduction - in stationary tissue, while inflowing unsaturated blood is depicted with high signal. A saturation... 

For PI, basically all fast and strongly T2 w sequences can be used. In the early days of PI, measurements were performed using fast, T2 w gradient-echo sequences, particularly the fast low angle shot... 

Table 18.2 summarizes the sequences and MR techniques which in our experience can be proposed (as mandatory or optional) in the diagnostic management of veno-occlusive disorders of the brain. Venous MRA can either be performed with the time-of-flight (TOF) or with the phase-contrast (PC) technique. In addition to the tomographic images, a flow sensitive gradient-echo sequence should be obtained if CVST is in question. As a fast screening examination we prefer a TOF 2D FLASH sequence (Table 18.2 Fig. 18.4, see 18.6c), oriented 90 degree to the flow direc-...

The vast majority of experiments now measure the temperature-dependent phase changes in images obtained using gradient echo sequences. In such a sequence, with an echo time TE, and spatially dependent temperature change Ar(x, y, z), the phase change is given by ... 

Figure 3.48. A gradient echo sequence suitable for -axis field gradient shinitning. The pulsed field gradients provide the spatial encoding whilst the delay x encodes the static field inhomogeneity as signal phase.

Detection of Microbleeds. A second disadvantage of using CT rather than MR imaging for stroke assessment is the decreased sensitivity for detection of cerebral microbleeds compared with gradient-echo sequences. Microbleeds detected on T2 -weighted MR imaging, however, have been shown not to be a contraindication to thrombolysis [97]. 

Hermier, M., et al., MRI of acute post-ischemic cerebral hemorrhage in stroke patients diagnosis with T2 -weighted gradient-echo sequences. Neuroradiology, 2001. 43(10) p. 809-15. 

Most centers, including our own, use an EPl gradient-echo sequence for PWl. Our PWl sequence is described in detail in 1, and enables imaging of the majority of the brain every 1.5 s. Our sequence requires 120 s of imaging time, and uses 20 mL of contrast material injected at 5 mL/s. 

Fig. 3.6a-C. MDCT in portal venous phase (a) and MRI with a Tl-w 3D gradient-echo sequence after gadolinium-injection (b) and a T2 -w gradient-echo sequence after injection of the SPIO contrast agent ferucarbotran (c) in a male patient suffering from a colorectal carcinoma in whom atypical resection of a liver metastasis in the right lobe of the liver had been performed previously. Note the postoperative bilioma (marked by the asterisk). In the MDCT examination no further liver lesions can be detected, whereas the gadolinium-enhanced MRI faintly shows a hypervascular lesion arrow). The SPlO-enhanced MRI scan clearly depicts a newly developed metastasis with a high contrast between lesion and adjacent liver parenchyma...

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