Pars imaging refers to medical imaging techniques that examine the pars interarticularis, a small bone segment in the spine. This area is located between the superior and inferior articular processes of a vertebra, forming a narrow connection within the vertebral arch. This focus allows professionals to identify issues not clearly visible with general spinal imaging.
Reasons for Pars Imaging
Pars imaging is often recommended for individuals, especially adolescents or athletes, with persistent lower back pain. Repetitive lower back overextension, common in sports like gymnastics, football, and weightlifting, stresses the pars interarticularis. This can lead to a weakness or stress fracture, known as spondylolysis.
Spondylolysis can also occur due to genetic factors, where some individuals are born with naturally thinner vertebrae, making them more susceptible to fractures. If a pars fracture weakens the vertebra enough, it can cause the injured vertebra to slip forward over the one below it, a condition called spondylolisthesis. This slippage can compress spinal nerves, leading to symptoms like shooting pain, tingling, or numbness in the legs.
Imaging Methods Used
Several imaging techniques evaluate the pars interarticularis, each offering unique insights. Plain X-rays are often the initial step, providing basic views of the spine, including anteroposterior, lateral, and oblique projections. While X-rays can show established defects, their ability to detect early stress reactions before a complete fracture forms is limited, with only about 30-38% of early defects being visible. Oblique views can sometimes show a “Scottie dog” sign, where a break in the pars appears as a collar around the dog’s neck.
Computed Tomography (CT) scans offer more detailed bony anatomy and are considered a reliable method for visualizing spondylolytic lesions. CT scans can reveal the absence of a complete bony ring at a vertebral level, indicating a pars defect. Magnetic Resonance Imaging (MRI) is valuable for showing soft tissues and can detect stress reactions in the bone marrow before a fracture is visible on X-rays.
Single Photon Emission Computed Tomography (SPECT) scans use a small amount of radioactive material to highlight areas of increased bone activity, which can indicate active bone changes like stress reactions or early fractures. SPECT is more sensitive than X-rays for detecting bone stress and can help localize the injury site. Combining SPECT with CT (SPECT-CT) can provide both functional information about bone activity and detailed anatomical views, offering a comprehensive picture of the pars lesion.
What Pars Imaging Reveals
Pars imaging scans reveal specific indicators of injury or defects in the pars interarticularis. These images help diagnose stress reactions, which show as bone marrow edema on MRI, or actual fractures and defects, known as spondylolysis. Early stages of injury might only show increased bone activity on a SPECT scan or bone marrow edema on MRI, even before a clear fracture line is visible on X-rays or CT.
The imaging also reveals the presence and degree of vertebral slippage, or spondylolisthesis, which occurs when a fractured pars allows a vertebra to shift forward. Slippage extent is typically measured using lateral X-ray views. Identifying the presence and severity of these conditions guides treatment. This can range from rest and physical therapy to bracing or, in some cases, surgical intervention.