Positron emission tomography (PET) uses radiotracers to image functional changes for detecting diseases. It is usually more sensitive and can detect disease foci earlier than structural imaging [1]. PET is critical for the staging and assessment of treatment response in many cancers. The standardised uptake value (SUV) is usually used in PET [2, 3]. Accuracy of uptake measurement is vital.
Respiratory motion during scanning can blur images, affect the SUVs of images, and cause bias in its measurement [4]. Many methods for correcting respiratory motion in PET imaging have been reported [5–7]. The key thinking has been to isolate the quiescent period based on the patient’s measured respiratory waveform (amplitude or phase). For example, end-expiration respiratory gating is based on the observation that the patients usually have a quiescent period after end-expiration during the breathing cycle. Only the counts acquired during the quiescent period were considered valid and used for reconstruction [8, 9]. However, these respiratory motion correction methods are seldom used in routine clinical scanning. For PET/CT, whole body scanning usually requires only 2 minutes per bed position, and the speed of scanning has been increasing with the improvement of PET detectors [10], which has limited the effects of respiratory motion.
PET/MRI has been in clinical use for over 10 years [11]. PET/MRI has many advantages over PET/CT, especially improved soft tissue contrast and the elimination of ionizing radiation from CT. The combination of PET and multi-sequence MRI, that is, T1-weighted (T1WI), T2-weighted (T2WI), and diffusion-weighted imaging (DWI), provides more information for diagnosis. However, the characters of multi-sequences scanning for PET/MRI decided the scanning time much longer than that for PET/CT. Even for single-site scanning, the time often approximate 10 minutes. With this increased scanning time, the effects of respiratory motion on PET images will increase, making respiratory motion correction necessary.
The most commonly utilized respiratory motion correction technique is end-expiration respiratory gating. The system in our PET/MRI (Q.Static, GE Healthcare) uses this quiescent part of the respiratory cycle. End-expiratory respiratory gating has two key parameters, offset and width [8], and it relies on the frequency and amplitude of the respiratory cycle (Fig. 1). The offset determines the initial point for the quiescent phase and the width represents the proportion of the quiescent time in the respiratory cycle. Grootjans et al. used a pressure sensor integrated in an elastic belt placed around the patient’s thorax and reconstructed the images with 50%, 35%, and 20% of acquired PET data (respiratory cycle) for respiratory motion correction. Their optimal respiratory-gated images were reconstructed with a duty cycle of 35% [12]. Hope et al. evaluated the effects of end-expiratory respiratory gating for liver PET images using six patients’ 68Ga-DOTATATE PET/MRI scans. Their results suggest that SUVmax improved significantly compared with ungated data, which used the 50% of the respiratory period from accepted breath-holds for the final reconstruction [13]. Another respiratory motion correction technique is multi-bin respiratory gating (Fig. 1), which is used to replay gated images to eliminate respiratory motion by subdividing the data into bins (phases) to better visualize the respiratory cycle. The method does not depend on the amplitude of the respiratory cycle.
The SUVmax calculations are sensitive to noise and values improved with less scan times as image noise increases, which has been demonstrated in previous works [14]. For the PET reconstruction with respiratory motion correction, there are two reasons effecting the SUVs. On the one hand, the reducing of respiratory blurring will increase measured SUVs [15]. On the other hand, the respiratory motion correction isolated the PET counts with consistent respiratory amplitude or phase and dropped others, which means the less PET counts and increased the image noise. However, the comparisons of the two reason on effecting the SUVs were rarely reported.
Here, we compared the two respiratory motion correction methods, end-expiratory respiratory gating and multi-bin respiratory gating in abdominal PET/MRI, which was evaluated with measured SUV both in lesions and the background. The aim of this study was to evaluate two respiratory motion correction methods and further explore how to balance respiratory motion and the lost counts due to respiratory motion correction in abdominal PET/MRI.