First Veterinary Study Links MRI-Visible “High-Intensity Zones” to Cystic and Granulation Lesions in Dogs

High-Intensity Zones in Dogs with Lumbosacral Intervertebral Disc Degeneration: Insights from MRI and Histopathological Findings

S. Amir Kamali, Michelle Teunissen, Dirk Hendrik Nicolaas van den Broek, Elisabeth M. Burgers, Guy C. M. Grinwis, Keita Ito, Marianna A. Tryfonidou, Björn P. Meij

Background:
Degenerative lumbosacral stenosis (DLSS) is a multifactorial cause of back pain in dogs, frequently associated with intervertebral disc (IVD) degeneration. A high-intensity zone (HIZ) is a specific MRI finding in humans indicative of annular fissures and possible discogenic pain. This study aimed to identify and classify HIZs in dogs with lumbosacral IVD degeneration using MRI, and to correlate these findings with histopathological features, thereby evaluating their clinical and diagnostic relevance.

Methods:
A retrospective review of 838 lumbosacral MRI scans from dogs presented between 2013 and 2023 was conducted. Fifty-seven dogs with a visible dorsal annular HIZ at L7-S1 were included. HIZs were classified on T2-weighted MRI using a modified version of a human classification system (Teraguchi et al.), and signal intensity was assessed on T1-weighted sequences. Histological analysis was performed on dorsal annulus fibrosus (AF) tissue from 11 dogs undergoing decompressive surgery, with further immunohistochemistry for IBA1, Factor VIII, and cytokeratin to characterize cellular components.

Results:
HIZs were present in 6.8% of dogs with IVD degeneration and were most commonly round in shape (43%). The majority were isointense on T1W MRI, with only 14% appearing hyperintense. Contrast-enhanced T1W MRI detected HIZ in 3 of 22 cases. Dynamic and follow-up MRIs showed HIZ variability over time—some appeared, persisted, or resolved. Histology revealed two HIZ types: (1) granulation tissue with neovascularization and macrophages (8/11), and (2) cystic lesions with inflammatory infiltration (3/11). The round and rim HIZ morphologies on MRI corresponded with the shape and content of these histological subtypes. Immunohistochemistry confirmed macrophage infiltration and vascular proliferation as prominent features.

Limitations:
This was a retrospective, single-institution study with no correlation between HIZ findings and clinical signs due to inconsistent or unavailable records. Sample size for histology was small, and not all MRI signal types (e.g., T1 hyperintense HIZs) were represented in tissue analysis. Additionally, no control group of clinically normal dogs was included, and concurrent orthopedic conditions could confound interpretations.

Conclusions:
This study provides the first veterinary characterization of HIZs in dogs, identifying distinct morphologies and associated histopathological lesions. These findings suggest HIZs represent annular tears with granulation or cystic changes, potentially contributing to discogenic pain. The MRI classification system applied may enhance diagnostic evaluation of lumbosacral spine disease in dogs, though further studies are needed to determine the clinical significance of these lesions.

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