Researchers discussed the effect that breathing irregularities have on the accuracy of 4D PET scans and outlined a PET imaging method that reduces motion artifacts or image blurring arising from respiratory motion, according to a study presented this week at SNM's 58th Annual Meeting in San Antonio, Texas. The researchers said that non-gated PET imaging with 4D CT may be useful for imaging patients who do not benefit from the use of respiratory gating, most notably patients with erratic breathing.
4D PET/CT can be used to suppress motion artifacts by correlating the PET raw data with the breathing phase; however, the accuracy of each PET phase is dependent on the reproducibility and consistency of the breathing cycle during acquisition.
"Breathing irregularities can lead to significantly underestimated lesion activity in respiratory-gated PET imaging," said Boon-Keng Teo, PhD, assistant professor of radiation oncology at the University of Pennsylvania in Philadelphia. "Non-gated PET imaging corrected with 4D CT may be more effective for imaging patients with irregular breathing. This could potentially lead to a more robust and quantitatively accurate reading of active tumors."
Therefore, the researchers sought to evaluate the impact of breathing amplitude and phase irregularities on the quantitative accuracy of 4D PET. They demonstrated a simple technique using 4D CT to derive quantitatively accurate standard uptake values (SUVs) from a non-gated PET.
The investigators conducted phantom studies to compare respiratory-gated PET imaging with non-gated PET imaging corrected with 4D CT using simulated lesions ranging in diameter from 9 mm to 47 mm. They performed phantom studies with inanimate objects and specialized motion devices that move in a controlled manner to simulate tumors in respiratory motion. The 4D PET and CT studies were conducted in succession with a hybrid PET/CT system. Various degrees of motion irregularities were simulated to study their impact on the accuracy of 4D PET for suppressing motion artifacts.
The researchers compared the accuracy of SUVs derived from gated PET (four bins) for breathing patterns ranging from ideal to highly irregular. The combined effects of motion and partial volume errors were also simulated in a digital phantom to derive a set of correction factors that can be applied to non-gated PET based on the tumor size and motion amplitude.
The study results showed that non-gated PET with 4D CT imaging can be an alternative to respiratory-gated PET imaging for determining tumor activity in patients with highly irregular breathing. For an ideal breather, gated PET produced SUVs that were within 3.4 percent of the true value. As breathing pattern became more irregular, the quantitative accuracy of gated PET decreased. Gated PET underestimated SUVs by 7.3 percent for an average breather and 13.6 percent for a highly irregular breather. Using the simulated correction factors on a non-gated PET, corrected SUVs were within 0.8 percent of the true value.
Teo and colleagues concluded that the breathing irregularities can significantly impact the quantitative accuracy of gated PET/CT. In practice, gated PET still underestimates the true lesion activity due to motion. Applying correction factors to a non-gated PET is potentially more robust against breathing irregularities and can correct for the combined effects of both motion and partial volume errors.
These findings, according to the study authors, could change imaging protocols for patients with uneven breathing and potentially improve overall accuracy of tumor detection, thereby informing clinicians about appropriate treatments and perhaps even surgical planning for better cancer management.