Magnetic resonance imaging and advanced imaging assessment of the growth plate in the adolescent and young adult

Sammanfattning: The growth plate is a cartilaginous structure located between the metaphysis and epiphysis in long bones. It is the centre for longitudinal growth. Longitudinal growth has a biphasic pattern with a peak during foetal and early postnatal life and another peak during puberty. Growth finally stops after the pubertal growth spurt during late adolescence. Longitudinal bone growth and the skeletal maturation process have traditionally been evaluated with radiographs. MRI has become an alternative since it does not use radiation, visualizes both bone and cartilaginous tissue, and may visualize growth velocity using diffusion tensor imaging (DTI). The overall aim of this thesis is to investigate how best to image the growth plate and which professionals are most qualified to do the assessment. The following studies were conducted in service of this objective. Study I: MRI of the knee was performed in 410 individuals and cartilage-dedicated sequences as well as T1-weighted images were obtained. The images were blindly analyzed by general and pediatric radiologists and their observer agreements were compared. Cartilage-dedicated sequence showed greater agreement than T1 and a higher agreement was seen among pediatric radiologists. Study II: growth plates were imaged at five anatomical sites in 958 individuals using a cartilage dedicated sequence in 1.5 T MRI scanner. The closure of the growth plate was compared to age, sex, pubertal development, BMI and physical activity to see if any of these factors affected the closure of the growth plate. Skeletal maturation occurs in ascending order, from the calcaneus to the distal radius, and correlates with sex, sexual maturation, and BMI but not physical activity. Study IV: the third study of a human population, this study analyzed DTI of the growth plates of the knee in 159 individuals in a 3 T MRI scanner. The DTI metrics and tractography had a relatively linear relationship with chronological age but a different pattern was seen when the same metrics were compared to skeletal maturation. Tractography was seen in the mature growth plate and should therefore be approached with caution in the later stages of skeletal maturation]. Study III: an animal study performed to validate the results from the previous studies. Twelve rabbits were imaged with MRI, micro computed tomography and histology to compare MRI with the other modalities. All modalities performed equally well and validated that MRI can be used to assess the growth plate. In conclusion, the growth plate fuses progressively with age in ascending order and is preferably imaged with a cartilage dedicated sequence and assessed by a pediatric radiologist. The animal model verified that DTI can be used to evaluate the skeletal maturation process and that tractography can be used to assess activity in the growth plate. Tractography seems promising to assess the activity of the open growth plate but must be approached with caution in the later stages of skeletal maturation process of the growth plate.