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Two-point modified Dixon (mDixon) turbo spin-echo (TSE) sequence provides an efficient, robust method of fat suppression. In one mDixon acquisition, four image types can be generated: water-only, fat-only, in-phase and opposed-phase images. To determine whether PD mDixon TSE water-only and, by proxy, PD in-phase images generated by one acquisition can replace two conventional PD TSE sequences with and without fat suppression in routine clinical MR examination of the knee. This is a retrospective study of 50 consecutive pediatric knee MR examinations. PD mDixon TSE water-only and PD fat-saturated TSE sequences (acquired in the sagittal plane with identical spatial resolution) were reviewed independently by two pediatric radiologists for homogeneity of fat suppression and detection of intra-articular pathology. Thirteen of the 50 patients underwent arthroscopy, and we used the arthroscopic results as a reference standard for the proton-density fat-saturated and proton-density mDixon results. We used the Kruskal-Wallis rank test to assess difference in fat suppression between the proton-density mDixon and proton-density fat-saturated techniques. We used kappa statistics to compare the agreement of detection of intra-articular pathology between readers and techniques. We also calculated sensitivity, specificity and accuracy between arthroscopy and MR interpretations. Proton-density mDixon water-only imaging showed significant improvement with the fat suppression compared with proton-density fat-saturated sequence (P=0.02). Each observer demonstrated near-perfect agreement between both techniques for detecting meniscal and ligamentous pathology and fair to substantial agreement for bone contusions, and chondral and osteochondral lesions. Two-point mDixon water-only imaging can replace conventional proton-density fat-saturated sequence. When same-plane proton-density fat-saturated and non-fat-saturated sequences are required, proton-density water-only and proton-density in-phase image types acquired in the same acquisition shorten the overall examination time while maintaining excellent intra-articular lesion conspicuity.

MR physics can be a challenging subject for practicing pediatric radiologists. Although many excellent texts provide very comprehensive reviews of the field of MR physics at various levels of understanding, the authors of this paper explain several key concepts in MR physics that are germane to clinical practice in a non-rigorous but practical fashion. With the basic understanding of these key concepts, practicing pediatric radiologists can build on their knowledge of current clinical MR techniques and future advances in MR applications. Given the challenges of both the increased need for rapid imaging in non-sedated children and the rapid physiological cardiovascular and respiratory motion in pediatric patients, many advances in complex MR techniques are being applied to imaging these children. The key concepts are as follows: (1) structure of a pulse sequence, (2) k-space, (3) "trade-off triangle" and (4) fat suppression. This review is the first of five manuscripts in a minisymposium on pediatric MR. The authors' goal for this review is to aid in understanding the MR techniques described in the subsequent manuscripts on brain imaging and body imaging in this minisymposium.

Pediatric body MRI exams often cover multiple body parts, making the development of broadly applicable protocols and obtaining uniform fat suppression a challenge. Volumetric T2 imaging with Dixon-type fat-water separation might address this challenge, but it is a lengthy process. We develop and evaluate a faster two-echo approach to volumetric T2 imaging with fat-water separation. A volumetric spin-echo sequence was modified to include a second shifted echo so two image sets are acquired. A region-growing reconstruction approach was developed to decompose separate water and fat images. Twenty-six children were recruited with IRB approval and informed consent. Fat-suppression quality was graded by two pediatric radiologists and compared against conventional fat-suppressed fast spin-echo T2-W images. Additionally, the value of in- and opposed-phase images was evaluated. Fat suppression on volumetric images had high quality in 96% of cases (95% confidence interval of 80–100%) and were preferred over or considered equivalent to conventional two-dimensional fat-suppressed FSE T2 imaging in 96% of cases (95% confidence interval of 78–100%). In- and opposed-phase images had definite value in 12% of cases. Volumetric fat-water separated T2-weighted MRI is feasible and is likely to yield improved fat suppression over conventional fat-suppressed T2-weighted imaging.

The purpose of this general review is to suggest practical strategies to optimize musculoskeletal MR imaging in children. The changes related to ossification and marrow transformation affect the MRI appearance during development. This review summarizes the normal appearance of the growing skeleton on various pulse sequences, as well as ways to optimize the imaging parameters. Appropriate patient positioning, choice of field of view and imaging coils are essential. There are various tools including intravenous contrast agent administration, fat suppression and parallel imaging that can enhance the depiction of abnormalities, increase speed of imaging, and improve overall quality of the study. Finally, special considerations for imaging at 3 T are also reviewed.

Background. Effective fat suppression is desirable in clinical magnetic resonance imaging. Conventional frequency selective fat suppression is dependent on accurate prescan shimming and is subject to artifacts due to magnetic field inhomogeneity. Quadrature three-point water-fat imaging with direct phase encoding is an alternative technique for fat suppression that has been previously described in adult volunteers and patients. Objective. To evaluate the use of three-point water-fat imaging with direct phase encoding for fat-suppressed MR scans in children. Materials and methods. Sixty-two three-point water-fat imaging studies were performed in 55 children 2 months to 18 years old. T 1-weighted fat-suppressed (water) images from this sequence were compared with frequency selective fat-suppressed images obtained in 15 patients. The reliability and subjective quality of the sequence were assessed in the remaining 47 cases. Results. High-quality fat suppression was achieved in all anatomic sites studied, even where frequency selective fat-suppression failed due to magnetic susceptibility artifact. The three-point water-fat sequence was visually preferred to the frequency selective fat saturation technique in 15/15 cases. Conclusion. Three-point water-fat imaging has replaced the conventional frequency selective technique for fat suppression on T 1-weighted MR imaging at our institution.

An 11-year-old girl with chronic left wrist deformity and pain underwent MRI evaluation. Coronal intermediateweighted sequence with fat suppression showed a triangular deformity with medial tilt of the distal radius and proximal migration of the triangular-shaped lunate (Fig. 1). The triangular fibrocartilage complex (TFCC) demonstrated diffuse thickening and oblique orientation (Fig. 1, arrow). These findings are suggestive of Madelung deformity. Sagittal intermediate-weighted sequence with fat suppression showed diffuse thickening of the radiolunate ligament, suggesting Vickers ligament (Fig. 2).