Purpose <p>Isthmic spondylolisthesis (IS) poses challenges in bone mineral density (BMD) assessment due to altered biomechanics and localized changes in bone quality. Traditional dual-energy X-ray absorptiometry (DEXA) methods are limited in these patients, prompting exploration of alternative techniques like MRI-based vertebral bone quality (VBQ) scores and CT Hounsfield Unit (HU) values. This study evaluates the diagnostic performance of VBQ and CT HU methods in IS patients and examines regional variations in bone quality.</p> Methods <p>This retrospective study included 109 IS patients and 199 controls. VBQ scores were calculated using signal intensity from T1-weighted MRI at L1‒L4 and S1, while CT HU values were measured from reconstructed CT images at corresponding levels. Diagnostic accuracy for osteopenia and osteoporosis was assessed using receiver operating characteristic (ROC) analysis. Subgroup analysis was performed for L4 and L5 IS patients to explore differences in diagnostic performance across vertebral levels.</p> Results <p>VBQ and CT HU values differed significantly between normal and osteopenia/osteoporosis groups. CT HU at L1‒L4 demonstrated superior diagnostic accuracy (AUC &gt; 0.85) compared to VBQ in identifying osteopenia and osteoporosis. Subgroup analysis revealed reduced diagnostic efficacy of VBQ at L4 IS, likely due to altered biomechanical stress, while CT HU maintained robust performance across all subgroups. Regional variability in bone quality was observed, with distinct adaptations at S1, highlighting its potential as a diagnostic site in advanced IS cases.</p> Conclusion <p>CT HU outperforms VBQ in BMD assessment for IS patients, particularly in L4 IS cases where VBQ shows diminished efficacy. The selection of appropriate opportunistic bone mineral assessment methods can provide clinicians with supplementary information for bone quality evaluation in patients with IS.</p>