The impact of noisy attenuation maps and patient motion on human-observer performance at Ga-67 lesion detection in SPECT

The authors have demonstrated an improvement due to attenuation correction (AC) at the task of lesion detection in thoracic SPECT images. However, increased noise in the transmission data due to aging sources or very large patients, and misregistration of the emission and transmission maps, can reduce the benefits of AC and may result in a loss of lesion detectability. The authors investigated the impact of noise in and misregistration of transmission data, on the detection of Ga-67 thoracic lesions. Human-observer LROC methodology was used to assess performance. Both emission and transmission data were simulated using the MCAT computer phantom. Images were reconstructed using OSEM incorporating AC and detector resolution compensation. Clinical count levels were used in the emission data. The transmission-data noise levels ranged from zero (noise-free) to 32 times measured clinical levels. Transaxial misregistrations of 0.32, 0.63, and 1.27 cm between emission and transmission data were also examined. Results indicate that a 20-fold increase in the noise was required to eliminate the benefit afforded by AC but that smaller increases in noise could be detrimental, especially for low-contrast lesions. Misregistration errors are also a concern as even small errors here greatly reduce the performance gains of AC.