# Bone SPECT/CT: Added Value and Clinical Applications

## Overview

SPECT/CT combines three-dimensional scintigraphic data (SPECT) with anatomic detail from CT, dramatically improving the specificity of bone scintigraphy compared with planar imaging alone. It resolves equivocal findings on planar bone scan in 30 to 50% of cases and is increasingly considered essential for any equivocal bone scan finding.

## Technical Principles

SPECT acquisition involves a 360-degree rotation of the gamma camera around the patient, collecting 60 to 120 projections. Reconstruction uses iterative methods such as OSEM, with attenuation and scatter correction derived from the CT data. The CT component may be low-dose (non-diagnostic) or diagnostic quality depending on clinical need. Fusion images allow precise anatomic localization of bone scan abnormalities. The acquisition time is approximately 15 to 25 minutes for SPECT plus CT.

## Advantages Over Planar Bone Scintigraphy

### Improved Lesion Localization

Planar imaging suffers from overlapping structures that make precise localization difficult, especially in the spine, pelvis, and skull base. SPECT/CT eliminates this overlap through tomographic data acquisition, and the CT component provides an anatomic reference. This is particularly important in the spine, where SPECT/CT can differentiate vertebral body uptake (which raises concern for metastasis) from facet joint uptake (which is typically degenerative).

### Improved Specificity

The CT morphology of a bone lesion guides the interpretation of metabolic activity. A sclerotic lesion with uptake suggests an osteoblastic metastasis or healing fracture. A lytic lesion with uptake suggests an aggressive process such as metastasis or infection. Degenerative changes visible on CT with corresponding uptake are benign. A fracture line on CT with linear uptake confirms a fracture rather than metastasis.

### Improved Sensitivity

SPECT/CT detects small lesions that are obscured by overlapping structures on planar imaging. It provides better contrast resolution in three dimensions and identifies lesions in anatomically complex areas such as the skull base, sacroiliac joints, and sternum.

<image>Planar bone scan showing equivocal uptake in the lumbar spine (left), resolved by SPECT/CT demonstrating uptake localized to a vertebral body sclerotic lesion consistent with metastasis (right)</image>

## Clinical Applications

| Clinical Application | SPECT/CT Finding | CT Correlation | Diagnosis |
|---|---|---|---|
| Facet arthropathy | Uptake at facet joints | Hypertrophic facets, osteophytes | Degenerative (benign) |
| Vertebral body metastasis | Uptake in body/pedicle | Sclerotic or lytic lesion | Malignant |
| Compression fracture | Endplate uptake | Fracture deformity, height loss | Trauma/osteoporosis |
| Spondylolysis | Pars interarticularis uptake | Cortical defect at pars | Stress fracture |
| Disc space infection | Endplate uptake bilaterally | Erosions, disc narrowing | Osteomyelitis/discitis |
| Stress fracture (foot) | Focal bone uptake | Linear fracture line | Stress fracture |
| Prosthetic loosening | Tip or interface uptake | Periprosthetic lucency | Loosening |

### Spine Pathology

The spine is the most common indication for bone SPECT/CT. It differentiates degenerative facet arthropathy (uptake at facet joints with corresponding CT changes) from vertebral body metastasis (uptake in the body or pedicle with CT showing a lytic or sclerotic lesion). Compression fractures show uptake at the endplate with fracture deformity on CT. Spondylolysis produces uptake at the pars interarticularis with a cortical defect on CT. Disc space infection shows uptake at the endplates with disc space narrowing and erosions.

### Foot and Ankle

The complex anatomy of the foot, with overlapping tarsal bones, makes planar imaging unreliable. SPECT/CT localizes uptake to specific bones and joints. Applications include stress fracture localization in the navicular, calcaneus, and metatarsals; tarsal coalition (calcaneonavicular or talocalcaneal); osteochondral lesions of the talus; and arthritis of specific joints such as the subtalar, talonavicular, and calcaneocuboid joints.

### Knee

SPECT/CT differentiates patellofemoral arthritis from tibiofemoral disease, identifies bone bruise patterns associated with meniscal pathology, localizes stress fractures of the tibial plateau or femoral condyle, and evaluates prosthetic component loosening by localizing uptake to the cement-bone interface.

### Hip

In the hip, SPECT/CT can identify avascular necrosis, which shows a photopenic center with surrounding increased uptake (the donut sign). Femoroacetabular impingement produces uptake at the anterosuperior acetabular rim. Stress fractures of the femoral neck and prosthetic hip evaluation for loosening versus infection are additional applications.

### Temporomandibular Joint (TMJ)

SPECT/CT evaluates condylar hyperplasia by detecting asymmetric uptake. It guides surgical decision-making, as active condylar growth demonstrated by increased uptake may indicate the need for condylectomy. Quantitative comparison of right versus left condylar uptake can be performed.

<image>Bone SPECT/CT of the foot showing focal uptake in the navicular bone on SPECT, with the CT component demonstrating a linear fracture line consistent with a navicular stress fracture</image>

## Solitary Bone Lesion Evaluation

SPECT/CT significantly improves the characterization of solitary lesions detected on planar scans. The combination of CT morphology and metabolic activity allows a more confident diagnosis. This reduces the need for additional imaging, such as MRI or biopsy, in many cases. Studies show that SPECT/CT changes management in 25 to 40% of patients with equivocal planar findings.

## Pre-Surgical Planning

SPECT/CT identifies the precise source of pain in patients with multiple potential pathologies. It can guide targeted injection therapy, including facet blocks and sacroiliac joint injections. It confirms active osteoblastic activity at a site of suspected pathology before surgery. In spondylolysis, it identifies the active (symptomatic) lesion when bilateral defects are present.

## Prosthetic Joint Evaluation

SPECT/CT provides improved localization of periprosthetic uptake, differentiating tip uptake (suggesting loosening) from interface uptake (which may represent loosening or infection). The CT component demonstrates hardware position, periprosthetic lucency, and fractures. Combined with clinical data such as ESR, CRP, and aspiration results, SPECT/CT contributes to a comprehensive assessment.

## Infection

SPECT/CT improves specificity for osteomyelitis by correlating uptake with CT bone changes. It distinguishes soft tissue infection (cellulitis) from bone involvement. In the diabetic foot, it helps separate Charcot changes from infection by CT morphology. In vertebral osteomyelitis, endplate erosions and disc space changes on CT confirm infection.

<image>Bone SPECT/CT of the spine demonstrating focal uptake in the L4 vertebral body with a corresponding sclerotic lesion on CT consistent with osteoblastic metastasis, clearly distinct from adjacent facet joint degenerative uptake</image>

## Quantitative SPECT/CT

Quantitative SPECT/CT is an emerging technique for measuring bone turnover using standardized uptake values for bone agents. It has potential for monitoring treatment response in metastatic bone disease. Standardization of acquisition and reconstruction parameters is ongoing. The approach may eventually provide objective metrics comparable to SUV measurements in PET imaging.

## Clinical Pearls

SPECT/CT should be performed for any equivocal finding on planar bone scan. It is no longer optional.

Facet joint uptake on SPECT/CT is the most common benign cause of spine uptake and is easily identified.

In oncology patients, SPECT/CT reduces unnecessary biopsies by confidently characterizing benign lesions.

For foot and ankle pain evaluation, SPECT/CT is often superior to MRI for identifying the source of bone-related pain.

Low-dose CT is usually sufficient for anatomic correlation. Diagnostic CT adds radiation but may be preferred if no prior cross-sectional imaging exists.

Pedicle uptake on SPECT/CT has high positive predictive value for metastatic disease (the "hot pedicle" sign).

SPECT/CT has been shown to change the final diagnosis in up to 50% of equivocal planar bone scan findings.

## References

- Defined Role of SPECT/CT in Bone Scintigraphy. *Seminars in Nuclear Medicine*, 2015.
- Defined Added Value of SPECT/CT Over Planar Bone Scan. *European Journal of Nuclear Medicine*, 2014.
- Defined Role of SPECT/CT in Spine Imaging. *Journal of Nuclear Medicine*, 2017.
- SNMMI/EANM Practice Guideline for SPECT/CT Bone Imaging.