What does a Hounsfield unit (HU) value of 76 indicate?

Hounsfield Unit 76: Interpretation and Clinical Significance

A Hounsfield unit (HU) value of 76 falls within the range of soft tissue density and is most consistent with acute blood, muscle, or soft tissue, but is significantly below the threshold for bone density assessment and indicates severely compromised bone quality if measured in vertebral trabecular bone. 1

Context-Dependent Interpretation

The clinical significance of HU 76 depends entirely on the tissue being measured:

In Bone Density Assessment

• HU 76 indicates severe osteoporosis and high fracture risk when measured in vertebral trabecular bone 2
• This value falls well below established osteoporosis thresholds: HU values <110 are significantly correlated with osteoporosis in vertebral bone 2
• For lumbar vertebrae specifically, HU thresholds for osteoporosis are: L1 ≤110, L2 ≤100, L3 ≤85, and L4 ≤80 3
• HU 76 would indicate osteoporosis at any lumbar level and represents severely compromised bone quality 3
• In cervical spine, HU values <326.5 indicate osteoporosis with 88.9% specificity and 63.2% sensitivity 4
• HU values >160 demonstrate significant reduction in osteoporosis risk, while HU <110 correlates with osteoporosis 2

In Soft Tissue Evaluation

• Acute blood typically measures 40-70 HU, so 76 HU could represent acute hemorrhage with higher hematocrit or early clot formation 1
• Muscle and other soft tissues fall within this range on unenhanced CT 2

In Lung Parenchyma Assessment

• For lung density evaluation, emphysema is characterized by abnormally low attenuation areas when window levels of -600 to -800 HU are used 2
• HU 76 would be far too high for lung parenchyma and would indicate consolidation or mass rather than normal aerated lung 2

Critical Clinical Implications for Bone Quality

If HU 76 is measured in vertebral bone, this represents a surgical risk factor requiring intervention:

• Pedicle screw loosening risk is significantly elevated with low HU values in the screw trajectory 5
• For female patients, HU <153.5 in screw trajectories predicts pedicle screw loosening with good diagnostic performance (AUC 0.880) 5
• Additional augmentation techniques (such as cement augmentation) are recommended to prevent hardware failure when HU values are this low 5

Important Caveats

Never interpret HU values on contrast-enhanced scans for tissue characterization that requires unenhanced values 1

• Contrast enhancement artificially elevates HU measurements and invalidates bone density assessment 1
• In patients with severe lumbar arthritis, standard measurements may be falsely elevated due to osteophytes and facet joint sclerosis, making HU 76 even more concerning as it suggests true severe osteoporosis despite degenerative changes 6, 3
• Approximately 40% of women aged 55 and 85% of those >75 years have spine osteoarthritis that can spuriously increase BMD values, so low HU despite this suggests profound bone loss 6

Measurement Technique Considerations

Proper HU measurement requires systematic technique to avoid bias:

• Measurements should be obtained from the middle axial CT image of the vertebral body in trabecular bone 3
• Avoid single-slice measurements; CT slice thickness may introduce bias due to overprojection of dense components 1
• Multiple measurements across vertebral levels improve accuracy 3, 4