A Primer on Medical Imaging Technology
Imaging is at the core of accurate medical diagnosis. Even with an excellent clinical examination, objective evidence to confirm and/or exclude findings is essential before treatment. Imaging includes various modalities, each with unique benefits and risks. What is the difference between various imaging techniques? How exactly do these machines produce an image of one’s interior anatomy? We break them down for you.
X-ray: An x-ray is easily accessible, can visualize a large area, and is an excellent first imaging examination to determine the presence or absence of a bone or joint abnormality, soft tissue mass or calcification. X-rays send electromagnetic waves through the body. As different structures (muscle, bone, fat, tumor) absorb these rays at different levels, the output of the rays on a film provides a landscape of the internal structures. X-rays use ionizing radiation, which can be dangerous in high doses, so limiting the area of exposure and limiting dose are essential.
MRI: Magnetic resonance imaging uses a high-strength magnet rather than ionizing radiation to produce high-resolution images of anatomy. The MR machine captures the energy from water molecules in your body and interprets them into an image. MRI produces superior detail of soft tissues, including the articular cartilage, tendon and ligament pathology, and peripheral nerves, in addition to bones. While MRI is one of the safest diagnostic imaging examinations, as with any magnet, and with one as large and strong as used in MRI, there are safety considerations such as for those with pacemakers, which are a contraindication to MRI. Newer uses of MRI include detecting reactions to indwelling orthopedic implants such as knee or hip replacement (arthroplasty) and noninvasive MR angiography, which allows for detection of abnormalities to regional blood vessels.
CT Scan: Computed tomography uses ionizing radiation, but sends out a series of narrow radiation beams rather than one fan-shaped beam as an x-ray does. By taking numerous scans of thin layers of the body, CT builds an image of internal structures and can display these images in various planes, including 3D. It provides detailed information about bony structures, joints and soft tissue structures. CT can also be used to guide procedures such as bone biopsies, joint injections and aspirations (removal of air, fluid or bone fragments in the body). The same safety considerations regarding radiation apply to a CT scan as to an x-ray.
Ultrasound: Ultrasound uses high-frequency sound waves to produce images and provides information about specific soft tissue structures and blood flow in vessels and within the soft tissues. An ultrasound machine sends out sound waves and receives echoes that outline the structures the sound waves are reaching. These echoes are turned into images based on the time and strength with which the echo returns. Ultrasound can also guide procedures such as cyst aspiration, tendon sheath injections and soft tissue biopsies. Ultrasound is very safe and extremely sensitive to soft tissue pathology.
Discogram: A discogram is the injection of contrast fluid, into the intervertebral disc under fluoro (x-ray) guidance in an attempt to locate disc pathology associated with patient symptoms. This procedure can be very helpful in determining if symptoms are specifically related to a disc abnormality.
Myelogram: A myelogram is the injection of contrast into the spinal canal under imaging guidance. Specialized x-ray views provide detailed information about the spinal cord and nerve roots. It can detect compression or displacement of these structures due to herniated discs, bone spurs, thickening of ligaments, or a shift in the bones or joints of the spine.
If you have any questions about these imaging techniques and their safety, consult with your physician or radiology technician.
Richard Fleury is executive director of Radiology at Hospital for Special Surgery and serves as executive director for the New York Roentgen Society, the New York City chapter of the American College of Radiology.