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