Mr Imaging Imaging Plane

Although axial imaging is often adequate, additional or alternative imaging planes may be warranted depending on the anatomic region that is being studied. For

Figure 1 Malignant portal adenopathy in a patient with colon cancer. (A) Unenhanced T2-weighted image shows enlarged, hyperintense nodes (arrow) surrounding the portal vein. (B) Postferumoxtran-10 image shows a heterogeneous uptake (arrow) in medial nodes with the areas of darkening representing preserved nodal macrophages; the areas of retained hyper-intensity correspond to tumor infiltration. The lateral node (curved arrow) is completely replaced with tumor. Note darkening of the liver because of contrast medium uptake by the reticuloendothelial system of the liver (Kupffer cells).

Figure 1 Malignant portal adenopathy in a patient with colon cancer. (A) Unenhanced T2-weighted image shows enlarged, hyperintense nodes (arrow) surrounding the portal vein. (B) Postferumoxtran-10 image shows a heterogeneous uptake (arrow) in medial nodes with the areas of darkening representing preserved nodal macrophages; the areas of retained hyper-intensity correspond to tumor infiltration. The lateral node (curved arrow) is completely replaced with tumor. Note darkening of the liver because of contrast medium uptake by the reticuloendothelial system of the liver (Kupffer cells).

example, when evaluating pelvic nodes in patients with prostate cancer, additional imaging in an oblique plane parallel to the psoas muscle allows surgeons to precisely locate the lymph nodes in relation to the obturator nerve—an important surgical landmark (Fig. 3). Furthermore, this imaging plane enables the nodes to be optimally distinguished from vessels. Surgeons usually remove nodes from the so-called "obturator fossa,'' an area anterior and slightly posterior to the obturator nerve. An extended, more aggressive dissection is indicated when there is evidence on the postferumoxtran-10 MRI of nodal disease more than 2 cm posterior to the nerve.

Slice Thickness

Imaging with thin sections (3-4 mm) allows robust nodal detection and anatomical localization. Thin section imaging also improves nodal characterization by minimizing partial volume artifacts and aids in delineating hilar fat, a potential source for interpretation error (Fig. 4). High-spatial resolution imaging is useful for accurate

Figure 2 Benign pelvic lymph node in a patient with bladder cancer. (A) Unenhanced axial T2*-weighted MR image shows a hyperintense right external iliac node (arrow). (B) Axial MR image obtained early (8 hours) after administration of ferumoxtran-10 shows a slight, heterogeneous drop in signal intensity (arrow), which may be misinterpreted as malignant infiltration. (C) Delayed axial MR image obtained at the optimal 24-hours time point shows a homogeneous drop in signal intensity within the node (arrow). This finding indicates benignity, which was confirmed at surgery. Source: From Ref. 12.

characterization and detection of small metastatic foci within the nodes, and an in-plane resolution of at least 0.6 x 0.6 mm is advised.

Choice of Pulse Sequences

Ferumoxtran-10 shortens both T1 and T2* relaxation times. Shortening of T1 increases signal intensity and shortening of T2* decreases signal intensity in images that are appropriately weighted. Therefore, it is important to select a pulse sequence that is sensitive to either the T1 or the T2* effects of ferumoxtran-10. Choosing a pulse sequence sensitive to both can mask the presence of the contrast medium in lymph nodes. As the main emphasis is on lymph node characterization, the main sequence of choice is the T2*-weighted gradient-echo sequence. The sequence parameters for the T2*-gradient-echo sequence (appendix) are selected to enhance T2* sensitivity while concurrently

Figure 3 Oblique imaging for delineating the obturator fossa. (A) Coronal Tl-weighted 3D-GRE MRI. The plane of Figure 3B (parallel to the psoas muscle) is indicated by the line. (B) Tl-weighted SE MR image, obtained 24hours postferumoxtran-10 (which is insensitive to iron oxide particles), shows normal size nodes (arrows and circle) of intermediate signal intensity. (C) T2*-weighted MEDIC MR image (which is sensitive to iron oxide) in the same plane shows low signal intensity in normal nodes (arrows), and high signal intensity in a 6 mm size metastatic node (within circle) in the internal iliac region. (D) The obturator fossa (green) around the obturator nerve (solid green line) is indicated. This is the routine area of node dissection in patient's prostate cancer. As the metastatic node is behind the obturator fossa (in the internal iliac region), the urologist should be informed about this finding preoperatively. (See color insert for Fig. 3B.)

Figure 3 Oblique imaging for delineating the obturator fossa. (A) Coronal Tl-weighted 3D-GRE MRI. The plane of Figure 3B (parallel to the psoas muscle) is indicated by the line. (B) Tl-weighted SE MR image, obtained 24hours postferumoxtran-10 (which is insensitive to iron oxide particles), shows normal size nodes (arrows and circle) of intermediate signal intensity. (C) T2*-weighted MEDIC MR image (which is sensitive to iron oxide) in the same plane shows low signal intensity in normal nodes (arrows), and high signal intensity in a 6 mm size metastatic node (within circle) in the internal iliac region. (D) The obturator fossa (green) around the obturator nerve (solid green line) is indicated. This is the routine area of node dissection in patient's prostate cancer. As the metastatic node is behind the obturator fossa (in the internal iliac region), the urologist should be informed about this finding preoperatively. (See color insert for Fig. 3B.)

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