Diagnostic Images For Musculoskeletal Conditions
March 10, 2016 | Posted by: Sam Menteith
Diagnostic images enable a doctor to look inside and receive information about a patient’s body, without performing surgery.
This information is used together with information provided by a patient’s history, physical examination and other tests and investigations, to help doctors:
- Make a diagnosis – injury and/or disease – that is the cause of a patient’s presenting symptoms;
- Monitor how a condition is responding to treatment;
- Screen for disease before symptoms appear.
The medical speciality that performs diagnostic imaging is called radiology. Each radiological test uses a different physical energy to look inside the body, and has its own benefits, risks and limitations. The following are tests commonly used for musculoskeletal (bone, joint, muscle) conditions. Please note that these tests are used for other medical conditions not mentioned in this article.
Plain x-rays use x-ray radiation (electromagnetic energy). They usually involve the patient standing, sitting or lying, with the x-ray machine on one side and an x-ray film on the other side of the body.
When x-rays pass through the body, they produce images on the film of structures with varying density. High-density structures like bones absorb a high percentage of x-rays, which creates a light grey colour, whereas soft tissues absorb a lower percentage and appear dark grey.
Benefits: X-rays are a fast and easy way to provide amongst the clearest, most detailed views of bones and joint disease such as arthritis. They usually have no immediate side effects.
Risks/Limitations: Provide little information about soft tissues such as muscles and tendons, and joint injuries. There’s also a slightly increased risk of harm due to excessive exposure to radiation.
A CT (Computed Tomography) scan involves using x-rays to create images that are cross-sectional slices through the body. The images may be thought of like fine slices (1 mm thickness) of a loaf of bread. On a computer the slices can be restructured into different planes (front-back, top-bottom of the body etc.). They may also be put back together to create 3-D images.
CT scans involve lying on a scanner and moving head or feet first into a circular opening of a large machine.
Benefits: CT scans are fast and simple. They produce detailed images of bones, particularly for subtle fractures that plain x-rays can miss, joint dislocations and soft tissue damage. They usually have no immediate side effects.
Risks/Limitations: Don’t produce images of muscles, ligaments and joint cartilage as well as MRI. There’s also a slightly increased risk of harm due to excessive exposure to radiation.
MRI (Magnetic Resonance Imaging) uses magnets to create a strong magnetic field and radiofrequency pulses to generate signals from the body. These signals are produced by the protons in water molecules inside the body. All tissues and organs, including bones, contain various amounts of water (and hence protons). The varying signals are detected by an antenna and processed by a computer to produce images. It involves lying on a table inside a large cylinder.
Benefits: MRI creates detailed images of many tissues including muscles, tendons, ligaments, bones, joints and structures of the spine such as discs, spinal cord and nerve roots. They can be particularly helpful for detecting harm due to injuries such as bone bruises, and ligament and meniscus tears. No harmful effects are associated with MRI magnetic fields and radiofrequency pulses. It doesn’t use radiation.
Risks/Limitations: MRI requires patients to lie absolutely still to be effective. Lying inside the cylinder can be claustrophobic and some people require sedation during testing. Also, metal implants can make it difficult to obtain clear images.
Ultrasound uses high frequency soundwave energy that can’t be heard by the human ear. These soundwaves penetrate inside the body. Internal structures reflect the sound, which bounces back out like an echo. Different tissues cause specific echoes that are converted into electrical impulses used to produce images on a computer screen. Ultrasound involves sitting or lying down, and a handheld device is placed and moved around on the skin.
Benefits: Ultrasound produces good images of tendons, muscles, ligaments, nerve entrapments, inflammation and foreign bodies, such as splinters or glass. It’s easy to use and there are no known risks. It doesn’t use radiation.
Risks/Limitations: Difficulty penetrating bone, deep into joints and also deep inside the body of large people. MRI may be preferred in these cases.
Dual energy CT scan
A dual energy CT scan uses both a normal x-ray and also a second, less powerful x-ray to produce images. It’s performed in the identical way as standard CT scans.
Benefits: The advantage over standard CT scans is better quality images of blood vessels and also metal, which may have been used to help repair bones or joints. It may also require a smaller number of images to obtain the same information.
Risks/Limitations: Don’t produce images of muscles, ligaments and joint cartilage as well as MRI. There’s also a slightly increased risk of harm due to excessive exposure to radiation6,8.
A DXA (Dual-energy x-ray absorptiometry)scan is used to measure the density of bone. Like dual energy CT scans, it uses x-rays at two different energy levels to produce images. However, it’s usually performed with the patient lying flat on a table.
Benefits: DXA scans are the best test for measuring bone density, usually for osteoporosis (a disease of thinning of the bone) in the spine, hip and wrist, to help determine bone fracture risk. It uses significantly smaller doses of radiation compared to plain x-rays.
Risks/Limitations: Cannot predict a future fracture, only the risk of a fracture. Images also may be less clear if osteoarthritis (joint disease) is present, in which case CT scan is more accurate. It also involves radiation exposure.
Fluoroscopy produces moving images in real time like a video camera by combining x-rays and an image intensifier machine. It’s used in musculoskeletal condition diagnosis, for example, to guide a needle to inject anaesthesia into a spinal facet joint, to determine if that joint is the cause of pain. It usually involves lying on a table.
Benefit: Moving images in real-time can provide more information compared to static photos produced by plain x-rays and CT scans8.
Risks/Limitations: Doesn’t show cross-section slices. It also involves radiation exposure.
A myelogram is used in diagnosis of the spine. It involves: 1) injecting a dye into spinal canal fluid (that surrounds the spinal cord and nerve roots); 2) x-ray radiation with fluoroscopy or CT scan to produce images. The dye contains a contrasting medium that allows the x-ray radiation to produce the images. It’s usually performed when a patient can’t have MRI.
Benefits: Can help to diagnose spinal problems.
Risks/Limitations: May be no better than MRI images of the spinal cord and nerve root. It also involves radiation exposure. Other small risks include: headache due to needle puncture into the spinal canal fluid and damage such as nerve injury, fluid leakage, Arachnoiditis, infections and allergic reactions to the dye.
A discogram is similar to a myelogram, except the contrast medium is injected into the centre of one or more spinal discs (shock absorber pads between each pair of vertebrae of the spine). This procedure can be painful.
An arthrogram is similar to a myelogram, except the dye is injected into a joint, such as the shoulder, and in some cases MRI is used. It’s particularly effective for assessing integrity of structures within joints such as ligaments and tendons.
Nuclear Medicine Bone Scan
A nuclear medicine bone scan involves:
- Injecting a radioactive material into a vein, which attaches to bones over a period of several hours;
- A special camera (gamma camera) is then used to detect the radioactive material to create images.
Benefits: Nuclear medicine bone scan images show the structure and active cell growth of bones. The scans are often used for follow up after other images have been performed, to help assess inflammatory conditions and progress.
Risks/Limitations: Involves similar radiation dose as a CT scan and fluoroscopy. Rarely, there are allergic reactions.