TO THE EDITOR:

It was with great interest that we read the article about outlining methods for lymphoscintigraphy, and we applaud the additional described method that uses a ¹⁵³Gd-line source (1). It provides an option for those centers that have this capability installed on their SPECT cameras. However, we disagree with the statements in the article concerning the inability of the ⁵⁷Co-flood source method to obtain lateral images. This is simply not so. In fact, we have been using the ⁵⁷Co-flood source method to obtain excellent outlined lateral images for several years (2,3). Simply placing the sheet source on a chair with the patient between the flood source and camera head clearly provides the necessary geometry to produce the desired results.

Although it is conceivable that certain camera configurations (multihead and ring gantry based) could create geometry issues for the sheet source method, geometry issues have not prevented lateral views in our own triple- and dual-head camera systems, which are similar in physical configuration to other systems available. To outline the body of the patient in the lateral view, the sheet can be placed on any suitable surface (adjustable chair or stool below the opposing head[s]) or even hung from suitable attachment points (gantry, swing arm, etc.). One system we use has long hook-and-loop strips to hang the sheet source from the opposing camera head or arm. Outlined images can also be obtained with oblique (45°, etc.) camera angles.

In addition, the lateral images obtained with the ⁵⁷Co-flood source method are superior to the anterior images obtained with this same method. This superiority is due to the greater effective patient cross-sectional distance/mass involved and the resultant increase in attenuation compared with the effective cross-sectional distance with anterior views. With increased attenuation, the patient outline is better defined and the sentinel node is easier to detect against a background of activity that is less than that obtained with anterior views.

Given the lower-energy 122-keV photons of ⁵⁷Co, separate energy windows in the 122-keV range, in addition to conventional 140-keV windows (or upwardly offset windows to reduce scatter for better sentinel node delineation), could be used and printed separately and/or combined to optimize outlining when the ⁵⁷Co-flood source is weak, and to minimize exposure if deliberately using a weak ⁵⁷Co-flood source.

Finally, although the image quality obtained with a ¹⁵³Gd-line source is of very high quality, this method is possible only with select new camera systems and is not as readily available as is a ⁵⁷Co-flood source, which is already present in most nuclear medicine departments.

In discussing outlining methods, the authors did not mention 2 other methods that can be used to outline the patient.

The first is a simple “activity background painting” method in which activity in a 10-mL syringe is waved behind the patient in a pattern that sweeps across a plane parallel to the entire collimator surface, in effect simulating a ⁵⁷Co-flood source. This method uses the same principles of differential attenuation as do other methods. The method can be used when no ⁵⁷Co-flood source is available or when the source has decayed. The method is inexpensive, readily available, and quick. Issues of exposure can be controlled by limiting the activity in the syringe. The main drawback of the method is in the need for a certain level of skill to evenly sweep the activity behind the patient to achieve a uniform outline. Nevertheless, minor irregularities are not a significant distraction, and this method is superior to the method of manually moving a point source around the patient edge while acquiring an image.

The second method uses internal patient scatter from the injection site to generate an outlined image. An additional very-low-scatter energy window is programmed into the camera system (approximately 85–115 keV) to create a body-contour outline using scatter from the injection site (2,4). This image can be combined with the standard ^99mTc energy window images to obtain a composite image delineating the position of the sentinel node in relation to the patient’s body. Drawbacks to this method include poor image quality in many patients, additional complex image-processing steps, and the need to often normalize the image set to obtain usable results (2).

No matter which method is ultimately used for outlining, we strongly believe that the surgeons should be presented with 4 images at a minimum. A set of 2 anterior images should be obtained—an image obtained with no outlining method, and a subsequent same-view image obtained immediately afterward with outlining. A second set of images with and without outlining should be obtained for the lateral views. This allows for a better delineation of the sentinel nodes and will alert the surgeon to any rare artifacts that might be generated by the outlining process and occasional decreases in contrast from activity that shines through from the outlining sources themselves that could hide faint nodes.