Benefits Study of a Respiratory Gating Protocol for 18F-FDG PET: Application on the Liver
18-fluoro-2-deoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT)
became a major imaging modality for management of patients with suspected hepatic cancer
notably because of its ability to detect distant metastases. Hepatic resection is known to
be the only curative treatment in a subset of patients suffering liver neoplasm. Eligibility
for hepatic surgery lays on conventional staging comprising contrast-enhanced CT, magnetic
resonance imaging or ultrasonography of the liver. Recently, 18F-FGD PET/CT has been
introduced as a staging modality in the preoperative work up.
However, many physiological issues could affect PET interpretation. Indeed, some lesions
could be missed by the physician due to histology of the lesion (e.g. in case of moderate or
well-differentiated hepatocellular carcinoma (HCC) or mucinous carcinoma). Unlike, local
inflammations could be wrongly considered as malignant.
Another issue for liver examination with PET imaging is the respiratory motion as it
produces blurring in reconstructed images. Therefore, some lesions may be missed or
underestimated. This motion issue is well known for thoracic imaging and various methods
were proposed to deal with it. We have developed a respiratory motion compensation method
where gated PET events are selected on the base of a breath-hold CT (CT-based).
To our knowledge, no sensitivity study was performed to assess the usefulness of gated
acquisitions in term of lesions detection at abdominal stage. W In this trial, we apply our
gating method on the liver to compare lesion-per-lesion sensitivity of clinical (Ungated)
and CT-based PET images.
All acquisitions (Ungated and CT-based) are performed on a whole-body PET/CT system.
- Whole-body PET/CT (Ungated session) The Ungated acquisition consists in a whole-body,
free-breathing CT (110 kV; 85 mAs; pitch: 1) followed by standard multistep PET (3
minutes per step), as used in routine clinical practice in the department.
- Respiratory-gated PET/CT (CT-based session) The CT-based method consists in an
additional single-step, 10-minute List Mode respiratory gated PET acquisition followed
by an end-expiration breath-hold CT (110 kV; 50 mAs; pitch: 2) added to the end of the
clinical protocol, with continuous respiratory signal recording during these
To summarize respiratory-gated PET processing, the breath-hold CT sequence is visible on the
respiratory signal as a plateau. A selection range is placed around this plateau to select
only the PET events which correspond to the same position as that of tissues at the time of
After compensation for random coincidences, all 3-dimensional (3D) sinograms are
FORE-rebinned into two dimensions and scatter-corrected. The attenuation coefficients at 511
keV are calculated from the CT acquisition, in order to correct for tissue self-attenuation.
Ungated volumes are corrected with the free-breathing whole-body CT scan and CT-based
volumes are corrected with the end-expiration breath-hold CT. All PET volumes are
reconstructed using AWOSEM with the following parameters: 4 iterations, 8 ordered subsets in
168 x 168 x 81 matrices (4.06 mm x 4.06 mm x 2 mm). Finally, a 3D isotropic Gaussian post
filter with a full width at half maximum of 5 mm was applied.
Each PET examination is blindly and independently analysed by experienced nuclear medicine
physicians, i.e. neither the type of image nor the patients' information are known. Each
clinician has to report the number of lesions detected in both Ungated and CT-based PET
datasets. Should it be the case, for each lesion, they are asked to precise its location
according to the Couinaud segmental classification and its maximum standardized uptake value
Surgical procedure and histopathological analysis:
During surgery, intraoperative ultrasound are performed in order to detect and localize all
liver lesions. Surgery is aimed at obtaining disease-free resection margins. The type of
liver resection (hepatectomy or wedge(s)) is at the surgeon discretion as well as the use of
radiofrequency tumour ablation. When available, surgical specimen are analysed in the
department of histopathological analysis of our institution to determine the type of cancer
and the location of the resected lesions.
Endpoint Classification: Efficacy Study, Intervention Model: Single Group Assignment, Masking: Single Blind (Outcomes Assessor), Primary Purpose: Diagnostic
number of detected uptakes on PET images
Observers have to analyse Ungated and/or CT-based PET images. They have to report, for each uptake they see, the corresponding liver segment (according to Couinaud segmental classification).
Marc-Etienne Meyer, Pr
France: Afssaps - Agence française de sécurité sanitaire des produits de santé (Saint-Denis)