Magnetic resonance imaging (MRI) has long been the standard method for viewing articular cartilage. Articular cartilage is smooth, white tissue that covers the end of bones where they meet to form joints. This cartilage can be easily damaged or impacted by normal wear and tear. MRIs are extremely valuable for viewing changes to this tissue, specifically changes that occur due to osteoarthritis and trauma to the bone joint.
New technology involving improved radio frequency coils and pulse sequences has helped clinicians achieve higher-quality microscopic images of connective tissue. The impressive high-resolution capabilities of MRI scanners offer doctors the opportunity to observe cartilage degeneration in its early stages and diagnose the change sooner.
Understanding the Imaging Process
The foundation of MRI scanning relies on a strong magnet and radio waves. The magnet creates a magnetic field, which aligns the hydrogen atoms within a patient’s body. When a radio frequency pulse is added, the atoms are disrupted and start to spin. Once the radio wave is removed, the atoms return to their equilibrium state, and emit energy signals. An MRI machine can detect and interpret these signals, using them to create a map of the molecules within the body. In this way, an MRI creates a highly detailed image of the inside of the body.
MRI machines vary in magnetic strength which is measured between 0.5 and 7.0-Tesla (T). Higher Tesla numbers represent higher magnetic strength. MRI machines with a higher signal strength generally provide the clearest, most vivid images.
These high-resolution capabilities are critical when diagnosing conditions involving the minute details of a patient’s joints. Today’s most advanced MR machines with 3.0T strength give physicians the tools they need to observe changes in articular cartilage much earlier than previously possible. This enables doctors to identify joint problems earlier, possibly before they even occur, and potentially change the course of progression
Proton-Density Weighted Imaging
The two basic types of MRI scans are known as T1-weighted and T2-weighted. T1-weighted images highlight fat tissue within the body. T2-weighted images highlight both fat and water within the body.
The modern MRI method of choice for viewing articular cartilage is “fat suppressed, proton-density weighted imaging” (PDWI) using a 2D fast spin echo sequence (FSE). This method of radio frequency pulse sequences falls in the middle of T1 and T2-weighted imagery.
PDWI implements pulse sequences that can image both fat and water, while providing a relatively high contrast between joint fluid and cartilage, and between cartilage and subchondral bone. Joint fluid is imaged as a high-intensity region, and normal cancellous bone is shown as a low-intensity region. This view helps the physician easily detect unwanted joint fluid in injured cartilage, and if fluid is present in damaged bones.
The proton-density weighted method also offers a fat suppression method that displays small lesions clearly even if they are covered by fat. This process uses several imaging planes and 2D acquisition to ensure the correct diagnosis and to avoid missing lesions.
To further clarify the existence of any lesions in the cartilage, the MR machine can take cross-sectional images of the region and reconstruct them for a 3D visual, enabling physicians to evaluate the entire volume of the articular cartilage. This process also utilizes isotropic voxel imaging, which implements 3D gradient echo sequences, ensuring the accurate reconstruction of the cartilage.
Researchers have also found that PDWI technology, when combined with T2 mapping, provides physicians with an even more accurate evaluation of early cartilage degeneration. Many of today’s advanced MRI machines are equipped with T2 mapping functions in case more effective cartilage imaging is needed.
The ability to utilize the PDWI method of MR imaging is significant in the US, as over 30 million adults have developed osteoarthritis, a degenerative joint disease2. Physicians now have the means to accurately observe an injured area, and determine if joint degeneration and articular cartilage damage have occurred. If damage is discovered early enough, physicians can recommend treatment to help reduce the risk of developing osteoarthritis.
The Significance of Early Treatment
Articular cartilage contains an abundance of the molecule proteoglycan (PG), which helps maintain the structure of the cartilage. While PG strengthens the collagen fiber network within articular cartilage, it is also low in blood vessels and cell density, limiting its ability to heal.
Signs of pre-osteoarthritis:
- Degeneration of articular cartilage
- Irregular collagen arrangement
- Increased water content
Osteoarthritis is the most prevalent form of arthritis in the US, costing patients hundreds of dollars in treatments each year. As obesity and the percentage of elderly is on the rise, the cost of treatment is expected to rise as well.
There is no cure for osteoarthritis. Surgery, such as joint replacement, is the only effective treatment. However, early diagnosis allows professionals to detect and treat the condition before structural changes occur to the cartilage. Prevention is the best approach to treating osteoarthritis, and today’s advanced MRI techniques provide the sensitivity and flexibility to make an early diagnosis a reality.
Learn more about advances in MRI for articular cartilage.
MRI INTERPRETATION T1 V T2, Radiology Masterclass, https://www.radiologymasterclass.co.uk/tutorials/mri/t1_and_t2_images, (accessed 25 June 2018)
OSTEOARTHRITIS, 2018, Centers for Disease Control and Prevention, https://www.cdc.gov/arthritis/basics/osteoarthritis.htm, (accessed 25 June 2018)