Lymph nodes are small, bean-shaped glands that are part of the body’s immune system. They filter lymph fluid and house immune cells that fight infections and diseases. Magnetic Resonance Imaging (MRI) is often used to examine lymph nodes, providing detailed images of their structure and surrounding tissues to help diagnose various conditions. MRI offers high contrast resolution, making it valuable for assessing these structures.
Normal Lymph Node Appearance on MRI
Normal lymph nodes appear small on MRI. While size varies by location, they are under 10 mm in their short-axis diameter. For instance, retroperitoneal nodes measure up to 10 mm, while retrocrural nodes are smaller, around 6 mm.
Normal nodes exhibit an oval or kidney-bean (reniform) shape. A distinguishing feature is a fatty hilum, an indentation where blood vessels enter and exit. This fatty hilum appears bright on T1-weighted MRI sequences.
Normal lymph nodes show specific signal intensities on different MRI sequences. They are intermediate in signal on T1-weighted images, appearing hypointense to surrounding fat and isointense to muscle. On T2-weighted images, they are bright or moderately hyperintense. After contrast administration, normal lymph nodes show uniform enhancement.
Identifying Abnormal Lymph Nodes on MRI
Abnormal lymph nodes show changes in size, shape, and internal structure on MRI. While enlargement can indicate abnormality, size alone is not always a definitive sign of malignancy, as normal-sized nodes can still contain small metastases, and inflammatory nodes can also be enlarged. For instance, a short-axis diameter of 10 mm or more is considered suspicious for malignancy, though this can vary by nodal site and patient age.
A change in shape is another indicator of abnormality, with malignant lymph nodes lose their normal oval or reniform shape and becoming more rounded. This is referred to as spherical transformation. Conversely, reactively enlarged lymph nodes retain their elongated, oval shape.
The loss or effacement of the normal fatty hilum is a sign of internal structural change. This absence can suggest infiltration within the node. Internal heterogeneity, such as uneven internal signal, necrosis (cell death), or cystic changes, indicates abnormality. Necrosis, appearing as very high signal intensity on T2-weighted images, is an indicator of malignancy, particularly in larger metastatic nodes.
Changes in signal intensity on T1- and T2-weighted images, or altered enhancement patterns after contrast administration, suggest abnormality. Malignant nodes may show irregular, peripheral, or highly avid enhancement, with a longer time to peak enhancement and slower washout compared to normal nodes. Clustering or matting, where multiple nodes fuse together, is another feature suggestive of malignancy, especially in patients with a known primary tumor. These features help radiologists differentiate between benign inflammatory or reactive nodes, which retain their hilum and exhibit uniform enhancement, and malignant nodes, which are rounded with a loss of hilum and heterogeneous internal characteristics.
Interpreting Lymph Node Findings on MRI
Radiologists interpret lymph node findings on MRI by considering factors beyond size. The patient’s clinical history, symptoms, and other imaging results are integrated into the assessment. This comprehensive approach allows for a more accurate classification of lymph nodes as benign-appearing, indeterminate, or suspicious for malignancy.
Radiologists use specific MRI characteristics, such as the node’s size, shape, internal architecture, and signal intensity, to form their conclusions. For example, a rounded shape, loss of the fatty hilum, or heterogeneous internal signal are indicators that raise suspicion. The presence of central necrosis within a node, which appears as a very high signal intensity on T2-weighted images, is a key indicator of malignancy.
After an MRI reveals abnormal lymph node findings, several steps may follow. These can include follow-up imaging to monitor changes in the nodes over time, especially for indeterminate findings. A biopsy, often guided by ultrasound, may be performed to obtain a tissue sample for definitive diagnosis when malignancy is suspected.
The MRI findings are also correlated with other diagnostic tests, such as blood tests and physical examination findings, to provide a complete picture of the patient’s condition. While MRI is an effective tool for lymph node evaluation due to its excellent soft-tissue contrast, it does have limitations. It may not always be definitive, and some findings might require further investigation to reach a precise diagnosis.