Magnetic Resonance Imaging

There was an initial expectation that magnetic resonance (MR) imaging would provide a more accurate assessment of nodal status than CT. Unfortunately, however, studies have shown that signal characteristics and relaxation times do not reliably distinguish between malignant and benign lymph nodes [1315]. Similarly to CT, MR relies predominantly on size criteria for determining nodal status. Thus, it is not surprising that MR and CT have been shown to have comparable accuracy in the assessment of mediastinal nodal metastases in patients with NSCLC (Fig. 3) [16].

Despite similar accuracy, there are several relative advantages and disadvantages of MR compared to CT [3,11,15]. The multiplanar capability of MR provides superior assessment of the aorticopulmonary window and subcarinal nodal stations, regions that are often difficult to evaluate on axial images (Fig. 4). The ease with which MR differentiates lymph nodes from vascular structures provides another potential advantage of this technique, particularly in the assessment of hilar lymph nodes (Fig. 3). For this reason, MR is more accurate than CT in the assessment of hilar nodes, especially when vascular enhancement is suboptimal or absent on CT scans [17]. Thus, MR may be helpful as a problem-solving tool for inconclusive CT cases.

Two relative disadvantages of MR should be noted [3,11,15]. First, calcification within lymph nodes may be overlooked at MR imaging, thus potentially resulting in the misdiagnosis of a benign, calcified lymph node as concerning for metastatic disease. Second, because of the poorer spatial resolution of MR compared to CT, a group of discrete, adjacent normal-sized lymph nodes may occasionally blur together on MR images and appear as a single large nodal mass, which may be erroneously diagnosed as metastatic disease.

Due to the limited ability of CT and MR to accurately characterize lymph nodes on the basis of anatomic criteria, there has been growing interest in physiologic methods of nodal assessment. The next section of this chapter describes recent advances in physiologic imaging of lymph nodes with MR and FDG PET.

Figure 3 Anatomic imaging of mediastinal nodes with CT and MR. (A) Contrast-enhanced CT image shows an enlarged, 2.5-cm short-axis-diameter, precarinal lymph node (arrow). At biopsy, this lymph node was proven malignant. (B) T1-weighted axial MR image shows enlarged precarinal lymph node (arrow) and enlarged right hilar nodes, with signal intensity similar to skeletal muscle. Note improved visibility of hilar nodes on MR compared to CT. (C) T2-weighted axial MR image shows enlarged precarinal lymph node (arrow) and right hilar nodes, both of which are characterized by bright signal intensity. The MR signal intensity characteristics do not reliably distinguish benign and malignant lymph nodes. (From Ref. 11.)

Figure 3 Anatomic imaging of mediastinal nodes with CT and MR. (A) Contrast-enhanced CT image shows an enlarged, 2.5-cm short-axis-diameter, precarinal lymph node (arrow). At biopsy, this lymph node was proven malignant. (B) T1-weighted axial MR image shows enlarged precarinal lymph node (arrow) and enlarged right hilar nodes, with signal intensity similar to skeletal muscle. Note improved visibility of hilar nodes on MR compared to CT. (C) T2-weighted axial MR image shows enlarged precarinal lymph node (arrow) and right hilar nodes, both of which are characterized by bright signal intensity. The MR signal intensity characteristics do not reliably distinguish benign and malignant lymph nodes. (From Ref. 11.)

Figure 4 Assessment of mediastinal nodes in coronal plane with MR imaging. (A) Posteroanterior (PA) chest radiograph reveals a poorly defined left upper lobe mass and an abnormal convex contour of the aorticopulmonary window (arrow). (B) Coronal Tl-weighted MR image of the thorax demonstrates enlarged lymph nodes in the aorti-copulmonary window (closed arrow) as well as within the subcarinal region (open arrow). (From Ref. 3.)

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