by Karolyn Senica, MD

There are multiple orthopedic imaging techniques used to diagnose and evaluate musculoskeletal conditions. These include x-rays, CT scan, and MRI. This article will discuss the various techniques, and briefly, how these images are obtained. It will also describe the advantages and disadvantages of the various modalities and how they are used to evaluate common orthopedic conditions.

X-rays are the most commonly used imaging modality. They were first discovered in 1895, and they were produced with an x-ray machine or tube. Electrons, which are negatively charged subatomic particles, are accelerated and strike a metal target within the tube. This produces an x-ray beam which is projected through the body and onto an image detector. Objects that are more dense, such as bone, absorb more radiation while objects that are less dense, such as soft tissue, absorb less radiation.

Images can be detected with analog (x-ray film) or digital systems directly on the computer. Some advantages of digital radiography include: immediate availability of images, enhancement of images such as magnification, contrast, and taking direct measurements for templating. This system also leads to fewer retakes of x-rays and often times, a decreased radiation dose to the patient. It also offers storage of images on the computer without having to store volumes of x-ray films.

Besides being the most common imaging modality, x-rays are relatively inexpensive. X-rays can be used to diagnose fractures and dislocations, and also to assess fracture healing. Tumors and infections can also be identified on plain x-rays. One of the most common conditions we use x-rays for is to diagnosis arthritis and to assess its severity. They can also be used to assess alignment of total joint arthroplasties and to check for any evidence of loosening. Orthopedic hardware such as plates, screws, and rods can also be visualized on x-ray. We can then determine if there is any malalignment, breakage, or loosening. X-rays can also be used in the spine to evaluate for arthritis, degenerative disc disease, and curvature or scoliosis. Real-time imaging or fluoroscopy can also be used to perform injections and aspirations of joints.

Some disadvantages of x-rays are radiation exposure. X-rays are avoided in pregnancy unless absolutely necessary. X-rays are not helpful for identifying soft tissue injuries to ligaments, tendons, or muscle.

CT scans or computed tomography uses x-ray beams to take multiple slices of an object. These multiple images are reassembled to generate an image and can even be reconstructed on the computer to make a 3-dimensional image. Multiple images do improve resolution and provide soft tissue imaging as well. There is no magnification associated with CT scanning.

Some disadvantages of CT scanning include increased radiation exposure and artifact noted on the images around metal orthopedic hardware or total joint prostheses. CT scans in Orthopedics are used to evaluate complex fractures of the knee, pelvis, wrist, etc., for surgical planning. They can sometimes detect occult fractures which are not visible on plain x-rays or fractures that are sometimes hard to identify on plain film such as those involving the pelvis or sternoclavicular joint. They can be used with contrast dye to identify problems in the shoulder (rotator cuff tear) or knee (meniscal tear) in patients who are unable to have an MRI scan. CT scans can also be used as guidance to perform a bone biopsy.

MRI or magnetic resonance imaging involves using a tube surrounded by a giant circular magnet to obtain images. Our bodies are composed mostly of water, H2O, which contains hydrogen atoms. A strong magnetic field is sent through our bodies and changes the orientation of these hydrogen atoms. When they relax to their baseline state, there is energy released. This is picked up by a scanner and a computer to produce images. MRI scanning provides excellent imaging of soft tissues including tendons, ligaments, menisci, and muscle. It also can be used to evaluate the bone marrow. Since it uses a large magnet for production of images, there is no radiation exposure.

MRI scans can detect changes in the bone marrow that are seen in fractures, bone bruises, infections, or stress reactions. They can be used to identify bone and soft tissue tumors. MRI scans can also be performed with contrast (gadolinium) injected into the shoulder or hip joint to detect labral tears. Before having an MRI scan, patients are screened for any metal implants in their bodies. The strong magnet can cause electrical devices such as pacemakers and metal pumps to malfunction. Metal foreign bodies near the eye or brain could potentially move causing blindness or brain injury. Metallic orthopedic hardware and total joint prostheses attached to the bone are stable and can be imaged. All metal, such as jewelry, earrings, piercings, watches, hearing aids, cell phones, etc. are removed from your body prior to entering the scanning room.

Disadvantages of MRI scanning include poor image quality related to patient motion artifact. Patients need to remain very still during the scan. This is sometimes difficult given the time required for scanning and because of claustrophobia. Patients are often given some Valium an hour prior to the exam to help with relaxation. Also, artifact from metallic hardware and prostheses can reduce image quality. MRI scanning is not approved during pregnancy.