Imaging Technique

Lymphoscintigraphy for lymphatic mapping involves dynamic imaging (flow imaging) and static (late) imaging. The dynamic part of the study visualizes the flow of the radioactive tracer through the lymphatic duct to the first lymph node it encounters. Dynamic imaging is essential because of the above-mentioned fact that some of the tracer may pass through to end up in second-tier or even subsequent nodes. Visualization of higher-echelon nodes is useful in some circum-stances—forexample,when thepurposeof lymphoscintigraphy is toidentify all internal mammary nodes for adjuvant radiotherapy in breast cancer patients, but it is troubling in lymphatic mapping. The surgeon does not want to remove all radioactive nodes but only those that receive drainage directly from the primary tumor site. Without the visible lymphatic duct identifying nodes that receive drainage directly from the injection site, the first-tier (sentinel) nodes cannot reliably be distinguished from the secondary (nonsentinel) nodes in which the surgeon is not interested.

A large-field-of-view gamma camera is used with a low-energy, high-resolution, parallel-hole collimator. For most tumor locations, the patient lies supine in a comfortable manner. The gamma camera is positioned in front of the patient so that an anterior view is obtained. The camera is positioned over the most likely drainage basin so that the lymphatic duct coming from the primary lesion site will be depicted. A double-headed gamma camera enables one to obtain a simultaneous lateral or oblique view.

There are several ways to perform the dynamic imaging. The principle is that a number of serial images of short duration are obtained. A satisfactory approach is the following. Dynamic acquisition of 60 frames of 20 s in a matrix of 128 X 128 X 16 is begun immediately after injection of the tracer. In this fashion, one obtains a stack of sequential images over a period of 20 min, three images per minute. Each image contains the information on the lymphatic flow over a time frame of 20 s. The computer can then play back these consecutive images quickly one after the other, like a movie. On the computer display, one can see the tracer flow through the lymphatic duct. Within a few minutes, the sentinel node is visualized. The consecutive images can also be projected on top of each other, combined into one 20-min image. This approach allows a better identification of lymphatic ducts. A duration of 20 min is usually sufficient to collect the required information [29,39,40]. Massaging the skin at the injection site or in-between the injection site and the nodal basin stimulates the flow of the tracer when a lymphatic channel is not immediately apparent. Acquisition may be continued when the sentinel node is not identified within 20 min.

Static imaging is performed after completion of the dynamic imaging and is repeated a few hours later (Fig. 1). Anterior and lateral views are obtained, preferably in a total-body mode. These can be complemented by oblique views if necessary. An acquisition time of 5 min in a 256 X 256 X 16 matrix is satisfactory. The first set of static images usually shows both the lymphatic duct and the sentinel node. The second set of static images is obtained at 2-4 h postinjection when the radioactivity has settled down [23,41,42]. By this time, it has cleared the lymphatic channel. These late images depict radioactivity remaining at the primary lesion site, the radioactivity in the sentinel node, and sometimes radioactivity in second-tier nodes. Images obtained even later rarely show a different pattern.

The tracer is accumulated in the sentinel lymph node(s), but most of it stays behind at the injection site. Very little goes to other tissues. The images show a number of hot spots in a dark background. Therefore, it is difficult to determine exactly where the radioactive nodes are located in the body. The body

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