Adjusting acquisition parameters to account for attenuation in patients with excess adipose tissue

Objectives: There are several acquisition parameters to consider with gamma camera imaging (collimator, positioning, matrix, duration, etc.) that contribute to obtaining the best image possible with a high target to background ratio, the best spatial resolution, and the least scatter possible. Since nuclear medicine is a largely diagnostic modality, a high quality image is essential for a proper diagnosis. The goal of the acquisition is to get as many counts as possible for the best image. In patients with excess adipose tissue, counts are more likely to be attenuated therefore decreasing the quality of the image. If a technologist were to increase acquisition time in this case, would the number of counts be adequate for obtaining a high quality image? The main objective, as a student, is to understand the impact of adjusting certain parameters in imaging and how we as technologists can manipulate the quality of an image without having to give the patient a greater exposure to radiation with a higher dose. This simulation is helpful to see the effect of increasing imaging time.

Methods: A 10 mCi dose of Tc-99m Pertechnetate was placed inside an opaque thorax phantom. An initial static was done for 1 minute to visualize normal uptake. A 3 inch layer of beef tallow was then added to surround the phantom, and 2 statics were taken at 1 minute and 2 minutes. Then another 3 inches of beef tallow (6 inches total) was added to do 2 more statics at 1 minute and 3 minutes. A final layer of 3 inches of tallow was added (9 inches total) along with 2 final statics at 1 minute and 4 minutes.

Results: The image quality remained visually consistent with all 3 increased duration images even though the number of counts was reduced by each layer of beef tallow. Each longer scan demonstrated a significantly better image quality compared to its 1 minute counterpart.

Conclusions: As fat was added to attenuate the Tc-99m Pertechnetate dose, the image quality stayed adequate because the duration of the count acquisition increased to take the attenuation into account. Patients with excess adipose tissue should be imaged for a longer duration of time to get the maximum amount of counts possible and therefore keep the image quality at a good diagnostic quality. Simulating the significance of increasing acquisition time for this population of patients by using an imitation adipose tissue in an educational setting could help students understand how we achieve maximum counts and why they are important. The value of simulations such as this could help students make more efficient decisions, and therefore get better images, when manipulating acquisition parameters in future clinical settings.