Synopsis

Keywords: Image acquisition: Sequences

This presentation will first review the basic functional elements of a pulse sequence, and then discuss pulse-sequence types commonly used for clinical MRI, including gradient-echo and spin-echo based techniques, as well as echo-train implementations of both methods – echo planar imaging and fast/turbo spin echo. Magnetization preparation approaches will also be described, such as used for inversion recovery contrast (e.g., fat-signal suppression (“STIR”) or fluid-signal suppression (“FLAIR”)), fat suppression, or spatial presaturation.

MRI pulse sequences employ a time series of radio-frequency (RF) pulses, magnetic-field gradient pulses and data-sampling periods to generate the data required for the desired images. While there are a great variety of pulse-sequence types, every pulse sequence accomplishes two basic tasks. First, transverse magnetization is created, often in a spatially-localized manner (e.g., for a “slice” or series of slices), using one or more RF pulses, and this magnetization is encoded using gradient pulses so that the spatial positions of the tissues can be determined to form the image. Second, the desired contrast among the tissues is created based on the time of application, duration and other properties of the various RF and gradient pulses.

Following a brief review of the “anatomy” of a pulse sequence, this presentation will discuss the pulse-sequence types commonly used for clinical MRI. These pulse sequences are often divided into two general families – so called “gradient echo” and “spin echo” pulse sequences. The term “echo” refers to the situation wherein transverse magnetization, which has been “dephased” due to magnetic-field gradient pulses or other magnetic-field inhomogeneities, is brought back into alignment to yield a peak in the corresponding signal – the echo. As the name implies, a gradient-echo pulse sequence uses a reversal of applied magnetic-field gradients, which reverses the associated dephasing to create an echo. In contrast, a spin-echo pulse sequence uses a “refocusing” RF pulse to reverse dephasing. A key difference between gradient-echo and spin-echo pulse sequences is that the dephasing effects of only the applied magnetic-field gradients are reversed for gradient-echo imaging, whereas the dephasing effects of both applied magnetic-field gradients and other static-field inhomogeneities (e.g., due to magnetic susceptibility interfaces) are reversed for spin-echo imaging. Depending on the situation, this difference may favor either a gradient-echo or spin-echo pulse sequence. For example, spin-echo imaging may be preferred for imaging near the air-tissue interfaces of the sinuses to avoid regions of signal loss or for long-echo-time (T2-weighted) imaging, whereas gradient-echo imaging may be preferred to improve detection of hemorrhagic blood products or the signals changes associated with deoxyhemoglobin in functional brain imaging studies.

There are multiple “flavors” of gradient-echo pulse sequences, arising from the details of how the RF-pulse phases and encoding gradients are implemented. The three most common variants will be discussed: RF-spoiled, rephased and balanced pulse sequences. Both gradient-echo and spin-echo pulse sequences have “echo-train” implementations, wherein a series of echoes is generated following the excitation RF pulse. The gradient-echo form is called “echo planar imaging” (EPI), wherein rapid, repeated gradient reversals generate a train of gradient echoes. The spin-echo form is commonly called “fast spin echo” (FSE) or “turbo spin echo” (TSE), wherein a series of refocusing RF pulses is used to generate a train of spin echoes.

Finally, magnetization preparation will be discussed. Magnetization preparation refers to the application of RF pulses, and often also gradient pulses, preceding the excitation RF pulse of a gradient-echo or spin-echo pulse sequence. Magnetization preparation provides additional control over the image contrast, often in ways that are challenging (if not impossible) to achieve using the basic pulse sequence alone. Some of the commonly used types of magnetization preparation are inversion recovery (e.g., for fat-signal suppression (“STIR”) or fluid-signal suppression (“FLAIR”)), fat suppression, spatial presaturation, and magnetization transfer.

Acknowledgements

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References

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