To some, the protocol is the most daunting part of performing fetal MRI. Once you understand the needs of the physician and the various pathologies you wish to diagnose, it becomes quite simple to design an imaging protocol. Whether you perform breathholds for any part of the examination is really up to the individual facility. It is often found unnecessary, given today’s fast imaging techniques.
Most of these sequences lend themselves well to T2 contrast, which is of primary concern. Gross pathologies, particularly those of the neurologic system, are typically displayed with such contrast.
If your goal is to achieve T1-weighting, techniques like 3D VIBE, 2D FLASH and TurboFLASH can be employed with moderate breathholds (below 25 seconds). Typically, the focus of T1-weighted imaging is the fetal brain and largely to determine the presence of blood byproduct.
The 2D techniques have the added benefit of physiologic triggering through the use of 2D PACE.
The mainstay of any fetal MRI protocol is TrueFISP (True Free Induction with Steady State Precession) and HASTE (Half Fourier Single Shot Turbo Spin Echo). While TrueFISP contrast is derived from a relationship of T1/T2 due to multiple signal sampling, it can be considered significantly T2-weighted.
HASTE, on the other hand, can be thought of in terms of a more traditional Turbo Spin Echo technique with a very long echo train (roughly half the number of phase encoding steps). The contrast presented by HASTE is more a ‘pruist’ concept of T2-weighting.
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