[MR Imaging Features of Pediatric Bone Marrow]

Abstract

MRI plays a crucial role in bone marrow (BM) assessment, and has very high sensitivity in diagnosing marrow disorders. However, for radiologists who may not frequently encounter pediatric imaging, distinguishing pathologic BM lesion from normal BM can be challenging. Conditions involving the BM in pediatric patients, such as leukemia and metastatic neuroblastoma, often manifest with diverse musculoskeletal symptoms and may be diagnosed using musculoskeletal MRI examinations. Accurate interpretation of pediatric MRI requires not only an understanding of the normal composition of BM but also an awareness of agerelated developmental changes in the marrow and familiarity with conditions that commonly involve pediatric BM. We aim to describe the composition of normal BM and outline the normal and abnormal MRI findings in pediatric BM. Additionally, we aim to present clinical cases of malignant BM disorders including leukemia, neuroblastoma metastasis, and other malignant BM disorders.

Fig. 1. Age-related marrow conversion in long bones.

During the perinatal period, a secondary ossification center appears in the epiphysis/apophysis, where the red marrow is rapidly converted to yellow marrow within 6 months. Secondary ossification centers (arrowheads) with yellow marrow signals are prominently noted at either end of the long bones throughout childhood. During adolescence, marrow conversion begins at the central portion of the diaphysis (*) and spreads bidirectionally towards either end of the metaphysis. Conversion occurs until the late 3rd decade of life when most of the marrow is yellow marrow, with a small amount of red marrow (arrow) remaining in the proximal portion of the metaphysis.

Fig. 2. T1-weighted images of normal marrow conversion of the femur according to age.

A. At 3 months, the marrow is composed of red marrow with an intermediate T1 signal. B. At 6 months, the epiphysis (arrows) shows yellow marrow conversion with a T1 high signal. C. During childhood, marrow conversion starts at the center of the diaphysis (arrows), which spreads bidirectionally to either end of the long bone. D. During adolescence, a heterogeneous T1 signal is noted at the proximal metaphysis (arrows) because of the remaining red marrow.

Fig. 3. T1-weighted images of normal spinal marrow maturation.

A-D. Sagittal T1-weighted images of the vertebrae at (A) 3 days, (B) 7 months, (C) 3 years, and (D) 11 years of age. (A) Oval-shaped vertebral body ossification centers (*) are primarily composed of red marrow, and are hypointense compared to the cartilaginous endplates (arrowheads) and intervertebral discs (arrows). (B) Vertebral body ossification centers (*) appear to be isointense compared to intervertebral discs (arrows). (C) Vertebral bodies (*) appear slightly hyperintense compared to the intervertebral disc, especially around the basivertebral vein (curved arrow). (D) The signal intensity of the vertebral body (*) becomes higher than that of the intervertebral disc.

Fig. 4. Normal versus abnormal marrow of two 2-year-old children on T1-weighted image of hip MRI.

A. Coronal T1-weighted image of the hip in a patient with right hip synovitis shows normal signal intensity of yellow and red marrow in the femur, pelvic bones, and vertebrae. B. Coronal T1-weighted image of the hip in a patient with metastatic neuroblastoma shows diffusely isointense marrow signals at the femurs, pelvic bones, and vertebrae compared to the muscles, which is abnormal considering the extent of marrow conversion seen in a normal 2-year-old.

Fig. 5. Normal versus abnormal marrow of two 2-year-old children on fat saturated T2 weighted image of hip MRI.

A. Coronal fat saturated T2-weighted image of the hip in a patient with right hip synovitis shows intermediate signal intensity of normal marrow, slightly higher than that of the skeletal muscles. B. Coronal fat saturated T2-weighted image of the hip in a patient with metastatic neuroblastoma shows diffusely abnormal high signal intensity of the marrow due to marrow metastasis.

Fig. 6. Spine MRI in a 17-year-old leukemia patient who presented with back pain.

A. T1-weighted image shows a slightly lower marrow signal of the vertebrae compared to that of the intervertebral disc. B. On T2-weighted image, the marrow shows a diffusely higher signal intensity than that of normal marrow. C. Postcontrast fat-saturated T1-weighted image shows homogeneous enhancement of the vertebrae.

Fig. 7. Thigh MRI of a 10-year-old patient who presented with right thigh swelling and pain.

A. Axial T2 short tau inversion recovery image shows intramuscular abscess cavity in right rectus femoris along with myositis and cellulitis, and an abnormally high marrow signal of the femur (arrow). B. Diffusely low marrow signal on T1-weighted image is noted in the femurs and pelvic bones, indicating diffuse infiltrative marrow disease. Following MRI examination, the patient underwent incision and drainage of the thigh with antibiotics treatment, and the abscess was resolved. However, detection of abnormal marrow signal was missed during initial MRI interpretation, and after prolonged cytopenia, the patient was diagnosed with leukemia with bone marrow biopsy 2 weeks after MRI.

Fig. 8. A 5-year-old boy with recurrent fever and left shoulder pain, who was diagnosed with acute lymphoblastic leukemia.

A. T1-weighted sagittal image shows abnormal low signal intensity of the marrow in the humerus. B, C. Fat saturated coronal T2-weighted image (B) shows heterogenously increased marrow signals of the scapula, clavicle and humerus, with (C) heterogeneous enhancement in fat-saturated postcontrast T1-weighted image. Peripheral blood was unremarkable without blasts, and juvenile idiopathic arthritis was suspected clinically. However, due to prolonged fever for one month, the patient underwent bone marrow biopsy and was diagnosed with acute lymphoblastic leukemia.

Fig. 9. A 3-year-old girl presented with right thigh pain, who was diagnosed with metastatic neuroblastoma.

A, B. T1-weighted (A) and fat-saturated T2-weighted (B) images of both thighs show diffusely abnormal marrow signals of the femurs and pelvic bones, representing diffuse infiltrative marrow disease. C. In the postcontrast fat-saturated T1-weighted image, the right femoral shaft shows a lesion (arrowhead) with peripheral rim enhancement, which is considered to be marrow necrosis. D. Abdominal CT reveals a small left adrenal tumor (arrow), which was surgically confirmed as neuroblastoma.

Fig. 10. Pathologic fracture triggered by jumping rope in a 15-year-old girl who was diagnosed with B-lymphoblastic lymphoma/leukemia.

A, B. Sagittal T1- (A) and fat-saturated T2-weighted (B) images show cortical discontinuity along with marrow signal change (arrows) at the medial cuneiform. Note another nodular marrow lesion (arrowheads) with low T1 and high T2 signals. Conservative treatment was done, and further evaluation was not done at that time. One and a half month later, the patient presented with azotemia. C. Renal ultrasonography shows marked globular enlargement with multiple scattered hypoechoic lesions at the cortex. Kidney biopsy confirmed B-lymphoblastic lymphoma/leukemia. D. Note multifocal FDG uptake of lower leg bones/marrow in ¹⁸F-FDG PET. FDG = fluorodeoxyglucose

Fig. 11. A 2-year-old boy with fever and limping, who was diagnosed with metastatic neuroblastoma.

A. On plain radiograph, permeative osteolysis is noted, with cortical destruction in distal tibial shaft. B, C. Coronal T1-weighted image (B) shows abnormal low signal of the marrow of the tibia, with (C) increased signal in sagittal fat-saturated T2-weighted image. D. After gadolinium injection, heterogeneous enhancement of the marrow is noted with bone/bone marrow infarction in the distal tibia (arrowhead). E. Abdominal CT taken 2 months later reveals left adrenal neuroblastoma (arrow) with multiple bone metastasis (not shown here).

Fig. 12. A 5-year-old boy with back pain, who was diagnosed with metastatic neuroblastoma.

A, B. Sagittal T1-weighted (A) and fat-saturated postcontrast T1-weighted (B) images show a diffusely abnormal marrow signal (low T1 signal with increased enhancement) of the spine as well as an epidural mass (arrows) with a non-enhancing bone lesion at L1 (arrowhead). C, D. In the axial view, the primary tumor is conspicuous in the left paravertebral area (arrows) on both T2-weighted (C) and fat-saturated postcontrast T1-weighted (D) images, which was confirmed as neuroblastoma. The peripheral enhancing marrow/bone lesion in L1 (arrowheads) represents bone/marrow infarction in the underlying diffuse marrow metastasis.

Fig. 13. A 14-year-old boy with lymphoma, who presented with lower extremity weakness.

A-C. Sagittal T1-weighted (A), T2-weighted (B), and fat-saturated postcontrast T1-weighted (C) images show diffuse abnormal marrow infiltration of the vertebrae with marrow necrosis, most prominent in the T2 vertebral body (black arrowhead). Note the large enhancing cervical mass (arrows) extending to the prevertebral space and multiple epidural enhancing lesions (white arrowheads). Biopsy of the neck mass revealed lymphoma, which was accompanied by epidural and marrow metastases.

Fig. 14. A 14-year-old girl diagnosed with rhabdomyosarcoma with diffuse bone marrow metastasis.

A-C. Coronal T1-weighted image (A), STIR (B), and fat suppressed postcontrast T1-weighted (C) images show an irregularly enhancing mass from the left perineum to the left pelvic sidewall (white arrowheads) and another mass in the left lower retroperitoneal space (black arrowheads). Biopsy of the inguinal lymph nodes (not shown here) revealed alveolar rhabdomyosarcoma. On T1-weighted image (A), the marrow signals of the humeri, vertebrae, and femurs are diffusely low. On STIR (B), the bone marrow signal in the entire skeleton is heterogeneously increased, (C) with heterogeneous enhancement, suggesting extensive marrow metastasis. STIR = short tau inversion recovery

Fig. 15. Focal periphyseal edema in a 14-year-old girl who presented with knee pain.

A. Sagittal fat suppressed T2-weighted image shows flame-shaped edema-like high signal intensity lesion (arrows) in the metaphysis and epiphysis of the distal femur, centered at the physis. B. On coronal T1-weighted image, the flame shaped lesion (arrows) shows an intermediate signal.

Fig. 16. Normal hematopoietic foci in a child’s foot.

A, B. T1-weighted (A) and fat-saturated T2-weighted (B) images show multiple ill-defined nodular marrow lesions with intermediate signals in the talus and calcaneus, representing normal hematopoietic foci. A small amount of effusion is observed in the talofibular joints.