Giulia Corsini, Barbara Jardim Gomes, Josep Brocal, Rodrigo Gutierrez-Quintana, Claudia Mallol
Background
Bone marrow is the primary hematopoietic organ, transitioning from red (hematopoietically active) to yellow (fatty and inactive) marrow during development. While this conversion has been studied in canine long bones and pelvis, the skull has remained uncharacterized. Understanding normal skull bone marrow maturation is vital for distinguishing normal physiological changes from pathology on MRI. This study aimed to define the MRI features and timeline of normal skull bone marrow development in dogs and propose a model for red-to-yellow marrow conversion.
Methods
A multicenter retrospective study included 40 dogs that underwent 1.5-Tesla head MRI for idiopathic or unknown-origin epilepsy between 2015 and 2023. Dogs were divided into four age groups: <6 months (“puppy1”), 6–11 months (“puppy2”), 12–18 months (“young adult”), and 18 months–6 years (“adult”). Two board-certified radiologists, blinded to clinical data, evaluated signal intensity, homogeneity (graded 1–3), and contrast enhancement across four skull regions: calvarium, skull base, temporomandibular joint (TMJ), and maxillofacial bones. Linear mixed models and permutation tests assessed effects of age, bone type, and gender on marrow characteristics.
Results
MRI intensity and homogeneity increased with age, reflecting progressive marrow fat deposition. The calvarium and skull base converted earliest, showing predominantly fatty (grade 3) marrow by 6 months, while TMJ and maxillofacial bones converted later, reaching homogeneously fatty marrow by 12 months. A rostro-caudal conversion gradient was observed within these regions. All dogs <6 months exhibited strong contrast enhancement of red marrow, which declined with age and was absent in adults. Periosteal enhancement followed a similar pattern. Gender showed a mild influence, with males more likely to exhibit higher-intensity (fatty) marrow. Interobserver agreement was substantial to almost perfect across evaluations.
Limitations
The small, diverse sample size limited breed- or size-specific analyses. The retrospective design excluded advanced sequences (e.g., fat suppression), which might have improved red–yellow marrow differentiation. Brachycephalic dogs presented technical challenges due to thicker cortices and smaller marrow spaces. Phenobarbital use in some epileptic dogs could not be entirely ruled out as an influence, although no hematologic abnormalities were present.
Conclusions
This study provides the first MRI-based model of skull bone marrow maturation in dogs. Conversion from red to yellow marrow occurs early, beginning in the calvarium and skull base by 6 months and completing in most skull bones by 12–18 months. Contrast enhancement and periosteal vascularity decrease with age, mirroring marrow conversion. These normative data establish key age-related imaging benchmarks that will help veterinarians distinguish normal developmental variations from bone marrow pathology in clinical practice.
MRI of a 10-week-old crossbreed dog from the “puppy1” group. (A) Sagittal T2W image. (B) transverse T2W image. (C) Transverse T1W image precontrast. (D) Transverse T1W image postcontrast at the level of the TMJ. The calvarium (arrows), skull base (triangle), and TMJ (star) were classified as intermediate on T2W images (A, B). On T1W images (C), the calvarium (arrows) was classified as low, and the skull base (triangle) and TMJ (star) as intermediate. The calvarium and TMJ were graded as 1 (homogeneous), and the skull base as 2. All showing strong contrast enhancement (D) (arrows, triangle, and stars).
How did we do?
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