AMERICAN RADIOLOGY SERVICES

A New Vision, MRI of the Knee

By: By Carlton C. Sexton, MD and Andy Trovinger, RT-R, MR

Whether used to simultaneously evaluate intra-articular and extra-articular structures in the setting of acute injury, or to diagnose the source of atraumatic swelling, pain, or mass, knee MRI is the single most powerful imaging tool for triaging patients to proper treatment. Scans are faster. Patients are more comfortable. Images are better. Creative ways to disseminate scans and reports to referring physicians and patients are made possible by new computer connectivity and by the Internet.

The new generation of scanners substantially reduces the claustrophobia that troubled previous scanners, while maintaining the image quality provided by a high field system. Our state-of-the-art magnets have a bore diameter 20% larger and a tunnel length of 70 cm, half that of earlier models. While our patients' knees are being imaged, they can now see all around them with their head and chest outside the scanner. A combination of new phased array surface coils, actively shielded high performance gradients, and very high field homogeneity improves scan quality and provides increased signal while finer reconstruction matrices allow us to resolve articular cartilage and layers of the collateral ligament while keeping table time at less than 30 minutes.

Image interpretation is filmless; our practice network allows for rapid image transfer to any of our American Radiology Services work stations for specialty consultation. We are developing routes of rapid dissemination of images to our referring physicians via the Internet and via permanent CD. Finding the right technology solution aids the flow of accurate and timely information.

ACUTE TRAUMA

For the patient with an acutely injured knee, MRI provides a full assessment not only of potential meniscus, cruciate, and collateral ligament tears but also of occult osteochondral fractures; MRI allows for early prognostication and for treatment planning. With accuracy, positive predictive value, and negative predictive value exceeding 90% for meniscus tears, scans allow an appropriate choice between arthroscopic intervention and conservative treatment.

Likewise, the status of the anterior cruciate ligament is predicted with nearly 90% accuracy. Although the diagnosis usually is made confidently by physical exam, MRI detects associated meniscus tears (75% of cases) and osteochondral fractures (20% of cases) that may alter surgical approach.

Knee pain from acute or repetitive subacute trauma may result in a bone bruise or stress fracture requiring rest and rehabilitation rather than arthroscopy. MRI recognizes marrow edema and subtle trabecular fracture lines not visible on x-rays.

ARTHRITIS

Osteoarthritis more commonly affects the knee than any other joint. The pathological changes, including fragmentation and denuding of articular cartilage, bone sclerosis, osteophyte and cyst formation can all be evaluated by MRI. Involvement of each of three compartments can be graded. The medial knee joint is most often affected, followed by the patellofemoral joint (especially the lateral patella) and least commonly the lateral joint. Technological improvements with each successive generation of MRI scanners allow for better visualization of cartilage, using spin echo, gradient echo, and fat suppression techniques. Such visualization allows for the differentiation of destruction of articular cartilage from meniscus tears. The prognosis for arthroscopic meniscectomy when accompanied by significant damage to articular cartilage is more guarded. MRI may provide evidence that guides treatment toward total knee replacement rather than meniscectomy.

In younger patients with osteochondritis dissecans, the challenge for MRI is to identify subtle defects and to locate, size, and predict separation of the osteochondral fragment from underlying epiphysis. When MRI reveals intact overlying cartilage, treatment is usually conservative. When the articular fragment is loose or in marginal continuity with underlying bone, drilling, pinning, or excision may be more appropriate. Contrast resolution with current scanners can take advantage of existing joint fluid around the fragment rather than requiring the need for injection of contrast into the joint in this pediatric patient population.

Synovitis can be mapped around the knee joint including using IV contrast to enhance this hypervascular tissue. Joint fluid can be differentiated from synovium as a source of swelling. The etiology of inflammatory arthritis is usually apparent without the need for MRI, whether from rheumatoid arthritis, crystal or septic arthritis; in the setting of infection, however, MRI has a role in the identification of osteomyelitis or abscess. Pigmented villonodular synovitis, an unusual fibrous proliferation of synovium with histiocytes and hemosiderin laden macrophages has a characteristically low signal on both T1 and T2 weighted sequences because of blood products. A nodular form of the disease can mimic a meniscus tear, causing locking of the joint. The lesion is often extra-articular, at the margin of the infrapatellar bursa, and not easily visualized by arthroscopy.

MASSES

A popliteal cyst, also known as a Baker's cyst or gastrocnemius semimembranosus bursa, is the most common mass about the knee. Ultrasound can confirm the fluid nature of the lesion; since these cysts result from fluid distention of the knee joint, MRI is more effective in the search for a causative injury of internal structures key to treatment and resolution. MRI can answer why as well as what for popliteal cysts. Lesions such as benign lipoma, chronic hematoma, popliteal artery aneurysm, and solid masses can be excluded. Popliteal cysts can rupture, allowing synovial fluid to dissect down the calf between the gastrocnemius and soleus, setting up an inflammatory process that can simulate deep venous thrombosis or even a neoplasm. Present generation MRI scans with improved surface coils can survey a large body part such as the calf without sacrificing signal and can document this irritating spread of joint fluid.

While these common causes of periarticular swelling are usually confidently diagnosed by MRI, other soft tissue and bone masses, benign and malignant, are frequently nonspecific on MRI. Nevertheless, MRI is important in assessing the border of the lesion. MRI can identify invasion of the joint, the deep muscle compartments, and the neurovascular structures of the popliteal fossa.

CONCLUSION

Physicians, from orthopedists to rheumatologists to oncologists, have relied on MRI to diagnose disease of the knee since the first scanners were ramped up in our practice over 15 years ago. Using new high and low field MRI techniques, we now scan more patients more comfortably and arrive at diagnoses with more confidence. Is articular cartilage denuded rather than the meniscus torn? Is the posterior lateral corner disrupted along with an anterior cruciate ligament tear significantly jeopardizing knee stability? Is the pulsatile mass an aneurysm or a normal popliteal artery made more palpable because of a deep mass? MRI has the answer.