Single photon absorptiometry (SPA) is a quantitative bone densitometry technique that measures the amount of photon attenuation caused by a bone segment, typically the distal radius or calcaneus. The method employs a mono‑energetic photon source—commonly a low‑energy gamma emitter such as ^125I (27 keV) or ^109Cd (22 keV)—and a detector positioned opposite the irradiated bone. By comparing the detected photon count with and without the bone in the beam path, the instrument calculates the bone mineral content (BMC) of the region of interest.
Principle of operation
- Photon source: A sealed radionuclide provides a constant flux of single‑energy photons.
- Collimation: Both source and detector are collimated to define a narrow, well‑controlled beam that passes through a predefined bone area.
- Counting: The detector records the number of photons that traverse the bone; attenuation follows the exponential law $I = I_0 e^{-\mu t}$, where $μ$ is the linear attenuation coefficient of bone mineral and $t$ is the effective thickness.
- Calibration: Measurements are calibrated against phantoms of known mineral density, allowing conversion of photon counts to BMC (typically expressed in g cm⁻²).
Clinical applications
- Assessment of osteoporosis: SPA was among the first methods used to quantify bone loss, especially in peripheral sites where early changes can be detected.
- Monitoring therapy: Repeated SPA measurements can track changes in bone mineral density in response to anti‑resorptive or anabolic treatments.
- Research: Used in epidemiological studies to investigate risk factors for low bone mass.
Historical context
- Introduced in the 1960s and widely adopted throughout the 1970s and 1980s.
- Served as the clinical standard before the development of dual‑energy X‑ray absorptiometry (DXA) in the late 1980s.
Advantages
- Low radiation dose: The photon energies employed result in minimal patient exposure (typically < 0.01 mSv per measurement).
- Portability: Early SPA devices were relatively compact, facilitating use in outpatient or community settings.
- Specificity for cortical bone: The technique primarily assesses cortical bone, which is useful for certain diagnostic contexts.
Limitations and current status
- Limited to peripheral sites: SPA cannot assess central skeletal sites (e.g., lumbar spine, hip) where fracture risk is highest.
- Resolution: The spatial resolution is lower than that of DXA and quantitative computed tomography (QCT).
- Technological obsolescence: With the superior precision, multi‑site capability, and broader regulatory acceptance of DXA, SPA has largely been replaced in routine clinical practice. A few research laboratories may still use SPA for specific methodological investigations.
Safety considerations
- The radionuclide source is sealed and requires regular monitoring for leakage or decay; standard radiation protection protocols are applied.
Summary
Single photon absorptiometry is a legacy bone densitometry method that quantifies bone mineral content by measuring attenuation of mono‑energetic photons through peripheral bones. While historically important for the early detection and monitoring of osteoporosis, its use has largely been superseded by dual‑energy X‑ray absorptiometry and other advanced imaging modalities.
