Gradient & spin echo sequence

‘Gradient’ and ‘spin echo’ are common terms in an MRI context. Importantly, this is a technique that can be used on a T1, T2 and PD sequence. The gradient & spin echo technique may be regarded as two large families in which multiple variations are possible.

In summary: the gradient technique has a shorter scan time than the spin echo technique and is used among other things for angiography, brain, heart, abdomen and functional MRI. A significant drawback of the gradient technique is susceptibility to artifacts (hemoglobin in blood and prosthesis/osteosynthesis material).

Spin echo technique is an alternative option. The traditional spin echo was commonly used because of its many applications. Nowadays the spin echo has been developed into a faster sequence; the fast spin echo (FSE) and the single shot fast spin echo (SSFSE). Scan time has now been reduced to a few minutes, resulting in fewer movement artifacts. Despite the fact that the spin echo is not as fast (as the gradient), the technique is used frequently because of its image quality. The fast spin echo sequences are used frequently to image the abdomen (e.g. MRCP), pelvis (urogenital) and musculoskeletal system (especially in prosthesis material!).

Practical tips:

• Gradient is a good choice to detect blood products.
• Spin echo technique has fewer unwanted artifacts in prosthesis and osteosynthesis material.

Susceptibility artifact
Magnetic susceptibility means that protons with their own internal magnetization interact with the external magnetic field. In other words: it is the degree to which tissue becomes magnetic as a result of exposure to a magnetic field. When two tissues with different magnetic susceptibilities are close together, local field inhomogeneities may develop. This disruption accelerates dephasing and will eventually lead to loss of signal or distorted images. These susceptibility artifacts develop with metals (depending on the metal type) and natural transitions such as air-tissue (sinuses-brain parenchym) and tissues surrounding bone. Also the blood product hemoglobin may cause susceptibility artifacts on a gradient echo sequence. Despite this undesired phenomenon, it can also be used to characterize lesions. A frequently used gradient echo is the SWI sequence (Susceptibility Weighted Imaging) (fig. 18).