Parallel imaging has become a mainstream component of pediatric body MRI exams and has been covered extensively in other articles[10
]. Further, recent efforts have been devoted to combining parallel imaging with other undersampling strategies for even higher accelerations. These strategies include radial undersampling and model-based iterative image reconstruction, such as compressed sensing [11
]. Here, we will focus on three applications that have the strongest potential to reduce anesthesia need, depth, and duration.
Contrast-enhanced dynamic imaging
Cardiovascular, abdominal, and pelvic imaging protocols routinely incorporate dynamic contrast enhancement sequences. These sequences must contend with fast circulatory dynamics in children and limited breathholding capability; thus imaging speed is vital. [10
] Parallel imaging can largely address the encoding limitation of MRI, i.e. adequate sampling of k-space in a short time. However, parallel imaging does not address the compromised SNR of a short scan; this is best addressed by combining it with higher field strength and high-density receive coils. For the unsedated child, a volumetric acquisition using this approach can be obtained at high resolution in under 10 seconds.
Volumetric (3D) T2-weighted imaging
Conventional T2-weighted imaging employs fast spin echo techniques and requires a lengthy repetition interval for T1 relaxation, so a 3D T2-weighted acquisition is impractically long. However, a very long echo train can be obtained by slowly increasing the refocusing flip angle to counters the effects of T2 relaxation. [14
] At the same time, the scan time of such a technique, despite the longer echo train, is still prohibitive. With parallel imaging however, scan times for the abdomen and pelvis can be under 5 minutes. Additionally, images can be obtained with near isotropic voxel size, permitting reformatting of image data set in arbitrary planes and 3D reconstructions. This approach works particularly well for the sedated child and may reduce the number of sequences in a protocol.
Single shot imaging
The applicability of single shot imaging with T2-weighting (SSFSE, HASTE) to various abdominal and pelvic applications has been debated. For the unsedated child, the speed of these sequences and their robustness to motion are appealing. On the side of clear value is cholangiography and bowel imaging. The more debatable applications include oncologic and gynecologic. The questionable wisdom of substituting a single shot approach to T2 weighted imaging for conventional FSE sequences stems from several factors: (1) reduced SNR, (2) blurring due to T2 decay over the long echo train, and (3) image contrast that may be different from conventional T2. For cholangiography and bowel imaging, the long T2 of bile/bowel lumen mitigates these issues.
For evaluation of solid organs in the abdomen and pelvis, all of these issues may be addressed by the combination of 3T, a good coil, and parallel imaging. In my personal experience, adequate SNR can certainly be obtained at 3T with a 32 channel coil. Further, these coils typically have two plates, anterior and posterior, and each plate consists of roughly a 4x4 array. Thus, for coronal imaging, four-fold acceleration can be obtained and the resulting echo train length is comparable to conventional FSE T2 imaging.