Identifying attenuation in SPECT images is a critical component of reading images with the highest clinical confidence possible. Today, some advanced nuclear cameras offer attenuation correction, which removes body artifacts and ultimately provides more uniform images. Without this feature, however, there is another alternative technique commonly used to identify attenuation.

Attenuation shift is a method which attempts to move the questionable attenuation seen in the images by repositioning the patient (i.e. from supine to prone) in order to validate its representation. The camera itself is not correcting the attenuation. It is the movement of the attenuation in the additional images that attempts to confirm its existence.

Challenges with the shift technique

During imaging, the patient is transitioned from a supine to a prone position. For Myocardial Perfusion Imaging, this changes the position of the diaphragm and the heart in relation to the camera’s detectors. Although attenuation shift is an accepted method, there are some notable challenges with this technique.

Once the patient is repositioned, the attenuation pattern may change, which leads to the question of consistency. It can be a complicated process, requiring the patient to be repositioned for additional images, and the result may not be reliable.

Attenuation exists in all images; it’s simply a question of where it is located. If the additional images show the movement of the attenuation to a different area, the reading physician may assume it is attenuation. However, the potential exists that the attenuator was not able to be moved enough to confidently come to that conclusion. By changing the patient’s position, variability is introduced, which is problematic for the process.

If your camera does not provide attenuation correction, the shift technique is an acceptable option. However attenuation correction is ultimately the best method for optimum clinical confidence and is proven to reduce the number of false positives.