Respiratory-gated PET-CT improves liver tumour imaging

A prospective clinical study conducted by the Hong Kong Baptist Hospital in collaboration with the Hong Kong Polytechnic University has demonstrated the feasibility of implementing respiratory-gated PET-CT for liver malignancies with improved image quality compared with ungated scans.

¹⁸F-FDG PET-CT is an irreplaceable tool for providing medically relevant anatomical and functional radiological information. The acquisition of liver PET-CT images consists of two parts. First, CT data are acquired by scanning the entire body in a few seconds. Secondly, PET data are acquired by using ring detectors, which can last up to 30 minutes. As the subject continues to breathe throughout the procedure, resulting liver motion can lead to inaccurate tumour vs normal tissue localization, dosimetric uncertainty based on a static CT image plan, and increased planning target volume margins, meaning potential overexposure of surrounding normal tissue and a limited maximum allowable dose for the target tumour. [Int J Radiat Oncol Biol Phys 2011;80:938-946; Int J Radiat Oncol Biol Phys 1998;41:939-943; Semin Radiat Oncol 2004;14:19-26]

“Despite recognized respiration-induced issues in thoracic imaging, studies investigating the impact of patients’ breathing motion in PET-CT scan for liver malignancies, especially in a prospective clinical design, are severely scarce,” wrote the researchers. “In this prospective study of 16 patients [male, 9; average age, 65 years], we carried out comparative analyses of gated and ungated PET images of a total of 89 liver lesions in terms of volume measurements, accuracy of maximum and mean standardized uptake values [SUV_max and SUV_mean], and accuracy of total lesion glycoses [TLG].” [Front Oncol 2022;doi:10.3389/fonc.2022.789506]

The PET-CT scanner was equipped with a respiratory gating system that enabled image data sorting. The acquired CT and PET data were equally separated into different respiratory phases, after which respiratory cycle–specific data were used for image reconstruction. [Med Phys 2002;29:366-371] After whole-body ungated CT scan was obtained, an additional respiratory-gated liver CT scan was acquired. The respiratory-gated CT imaging protocol was comparable to those previously reported. [Med Phys 2005;32:874-889; Med Phys 2004;31:333-340]

Implementation of additional gated PET-CT liver scan in routine PET-CT scan was successfully achieved for 92 percent of studied lesions. In the remaining cases, implementation was affected by CT image motion artifact and failure in cine CT image reconstruction. In the gated PET-CT liver study, all scans were acquired in only one bed position and the mean gated PET-CT acquisition time was approximately 23 minutes. All patients were satisfied and duty staff provided positive feedback on the workflow. “Although additional time for patient set-up and scanning was required, respiratory-gated PET-CT can be implemented feasibly and efficiently in clinical routines,” commented the researchers.

The gated PET-CT technique demonstrated a reduction of image blurring on FDG uptake lesions. Gated PET images were sharper, with a more well-defined tumour border. The contour volume was significantly decreased by using the respiratory gating technique (from 4.22 mL to 3.32 mL; p<0.001). Similarly, SUV_max and SUV_mean were significantly improved with the respiratory-gated technique by 19.81 percent and 25.53 percent, respectively, compared with the respiratory-ungated technique. There was no significant difference between gated and ungated images in terms of TLG values.

“The mitigation of image blurring artifact and improvement of lesion volume accuracy with respiratory-gated PET-CT, as well as enhanced classification and identification of liver tumours, should help liver radiotherapy treatment planning in the future,” concluded the researchers.