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Appendix E: Fast MR Imaging Techniques

Patients are positioned supine or lateral decubitus. Patients are typically most comfortable entering the magnet feet first to minimize claustrophobia. If patients are claustrophobic, sedation can be given with sublingual benzodiezepines. Half Fourier single shot RARE (Rapid Acquisition with Relaxation Enhancement) technique is used to obtain T2 weighted images in axial, coronal, and sagittal planes of the fetus. Because fetal motion generally occurs throughout the examination, each acquisition serves as the scout for the subsequent acquisition. A typical sequence uses an echo spacing of 4.2 msec, a TE of 60 msec, an echo train length of 72, 1 acquisition, 4 mm section thickness, 24 x 24 cm field of view, and 192 x 256 acquisition matrix. A 130-degree refocusing pulse is used to minimize the amount of radiofrequency power deposition. The acquisition time per image is only 430 msec per slice. The T2-weighted images are excellent for depiction of fetal anatomy.

      As the RARE sequence is a single slice acquisition technique, it limits artifacts related to maternal and fetal motion, since only the slice in which the motion occurred will be affected. Since motion can occur at any time during imaging, if the fetus moves during the sequence acquisition, this will generally lead to nonvisualization of a portion of the fetal anatomy, but may lead to repeated visualization of a fetal part. For example, if the hand moves in plane with the sequence, it may be visualized more than once during the scan.

      T1-weighted imaging of the fetus is achieved using a fast low angle shot (FLASH) technique. A typical sequence is performed during a maternal breath-hold using a TR/TE 126/4; an 80 degree flip angle; a 24 x 32 cm field of view; a 96 x 256 matrix; 5 mm slice thickness; and one signal acquisition for an acquisition time of 12 seconds. While the images are degraded by motion that occurs during the breath-hold, T1-weighted images are useful for evaluating the liver position in cases of congenital diaphragmatic hernia and for evaluating hemorrhage or fat in a lesion.

      In this web-based atlas, the majority of images are obtained with half Fourier single shot RARE technique. Only those images performed with other techniques are listed in the figure captions.