Sentinel Node Imaging

Sentinel Node imaging was introduced it served as a means to reduce the number of lymph nodes excised during surgery, thereby reducing the morbidity rate in women undergoing the procedure. The concept is to establish the path that cancerous cells may take along the lymphatic channels, allowing for metastic cells to travel to other organs.

Debate has often been raised as to whether there is a need for imaging or not. Surgeons have requested patients receive the injection and go right to surgery...

The proceeding example demonstrates a reason to image the patient, which is first and foremost to ensure the injection is good and the materials is traveling through the lymphatic channels. But secondary to determine where the radiopharmaceutical is traveling.

This patient presents with right breast cancer, pre-lumpectomy and lymph node dissection. The patient was positioned in a right oblique position and injected with 500 uCi Tc99m sulfa colloid with Lidocaine by subcutaneous injection to the right of the nipple. When the patient was placed under the camera it was noted that there were lymphatic tracks above and below the injection site.

On the next image a lead blocker was placed over the injection site and imaging was continued at 2 minute intervals.

Although the physician was confident in the location of the sentinel node in the right axilla, he wanted confirmation that the lower track was not going to a lymph node but as altered due to the patient's previous surgery in the lower portion of the right breast. The patient was placed in the right lateral position and the right axillary node is easily identified and the lower track appears to remain in the breast tissue. No lead placed over injection site.

To further define the activity in the right breast, the patient was placed in an oblique position with a wedge under the left shoulder and imaged from the right posterior oblique position. This view allows the breast to move away from the patient's torso to further show the activity below the injection site (blue dot at site of lead blocker) is confirmed to be in the breast tissue.

As the radiologist put it, this became an exercise in obliques to best find the view that would confirm the activity was indeed in the breast and not traveling to a lymph node.

Whole Body Bone SPECT

Recently we starting looking at whole body bone spect imaging for metastatic evaluation. Metastatic disease in the vertebral bodies can be difficult to detect on planar whole body imaging, and an abnormal finding does not always mean metastatic disease. It is also difficult to detect small abnormalities in the thoracic vertebrae.1 

In order to acquire a whole body bone spect in an acceptable time frame, the time per bed has to be reduced. Studies were performed and determined that a bone spect can be acquired in as little as 5 minutes and still be acceptable as compared to planar imaging.2

Filtered back projection - the most commonly used method of image reconstruction. the back projection assigns values in the projection to all points along the line of acquisition through the image plane from which it is acquired. This can create an star artifact  consisting of radial lines at the edge of the object, to which a filter is then applied to reduce this artifact.3

Iterative reconstruction - is a more computer intensive program, which uses the original projections and models of the acquisition to predict areconstruction. The predicted projections are used to modify the recontstruction. In the OSEM method the projection data are ordered into subsets. The advantage of this is an increase in computer processing speed.3

Iterative reconstruction has proven to be superior to count poor data, which would create excessive noise from the radial lines when filtered back projection is applied. Our data was acquired at 6 secs per view, 64 views for a 12 minute acquisition time, compared to our standard SPECT acquisition time of 40 minutes.

When the initial study was reviewed it was noted that some problems existed. The first bed (patient head) looked good but the rest of the body offered excessive background noise, especially around the bladder. It was discovered that the iterative reconstruction was applied to the first bed, while filtered back projection had been applied to the remainder of the study.

Whole body coronal slices processed with filtered back projection

After re-processing the data with iterative reconstruction applied to all of the beds we noted another problem. The right half of the skeleton has a brighter, crisper look. It was then discovered that the data selector for processing was for 180 degrees. 

Reprocessed coronal slices with iterative reconstruction, but 180 degree processing

We adjusted the processing protocol to used the 360 degree method and re-processed the data again. This time the study came out with excellent results. (I never knew the system could process half a study?!)

Processing page, applying 5 separate acquisitions into one study

References:

1 - Do short-time SPECT images of bone scintigraphy improve the diagnostic value in the evaluation of solitary lesions in the thoracic spine in patients with extra-skeletal malignancies? 

Kobayashi, et al. Annals of Nuclear Medcine vol 19 No 7, p557-566, 2005.

2 - What is the optimal minimum whole body bone SPECT scan duration that can replace planar scintigraphy? 

Mawlawi, et al, The Journal of Nuclear Medicine vol 48 supplement 2, p120, 2007.

3 - Nuclear Medicine and PET/CT, technology and Techniques.

Christian and Watersum-Rich, Mosby Elevier