This study was designed to assess the diagnostic efficiency and image quality of a 20-BH PET acquisition compared with a 20- and 300-FB PET for stage IA pulmonary adenocarcinoma. The 20-BH PET resulted in reduced breathing-induced artifacts on PET images of the lung. The main findings are as follows: (a) 20-BH PET acquisition provided an increased nodule SUVmax and TBR across the cohort, regardless of its morphological characteristic, size and distance from the pleura; (b) 20-BH PET acquisition provided very closely MTV of nodules compared with BHCT; (c) there was an inverse correlation between nodule distances from pleura and %ΔSUVmax; and (d) when 20-BH PET was performed, the diagnostic performance of fused PET/CT in stage IA pulmonary adenocarcinoma was higher than that of BHCT, and 20-BH PET/CT provided a higher accuracy than 20- and 300-FB PET/CT.
The accuracy of lung nodule characterization with PET is affected by several physical factors of the PET scanner, such as system resolution and spatial resolution, particularly when the delineation of the target lesion is affected by the respiratory motion. The total-body PET scanner used in the study implemented the pixilated LYSO crystals, with a size of 2.76 × 2.76 × 18.1 mm3, yielding an excellent spatial resolution of ~ 3mm according to the NEMA NU 2-2018 [12]. Moreover, the AFOV of this total-body PET scanner is up to 194cm which can effectively detected the photons emitted from the patients. The elongated AFOV leads to an approximately 40-fold increase in the effective sensitivity according to the simulations [13]. The signal-to-noise ratio (SNR), a measure representing the PET image quality, is proportional to the square root of the product of the scanner sensitivity, injected activity, and the total acquisition duration. As such, the improved sensitivity can improve SNR, or allow for short acquisition duration while maintaining the image quality. Therefore, a 20-s BH PET acquisition is feasible with the total-body PET/CT scanner while maintain a good image quality.
Due to the limited sensitivity of the PET scanners with a standard AFOV of ~ 30cm, efforts have been made to accurately characterize the lung nodules. First attempt is to increase the acquisition duration. In a previous study on the feasibility of the BH PET acquisition in lung cancer, patients were instructed to hold their breath as long as possible to detect more counts and improve the image quality. Patients whose breath lasted fewer than 29 seconds were excluded and the study demonstrated that more than 81.9% (95/116) of the enrolled patients can successfully provide a breath-hold PET acquisition ranging from 30–143 seconds [19]. If the acquisition duration was further reduced, we believed that the proposed PET breath-hold acquisition can make a high probability in completing the scan. Our data suggest that a single 20-s BH acquisition is feasible for most patients.
Advantage of the proposed BH PET protocol included the short time for the additional BH PET acquisition, a high probability of technical success in completing the study in cooperative patients, simple post-processing and no additional radiation exposure.
In another study, three consecutive BH PET acquisitions were performed instead of a single BH acquisition. Moreover, SUVpeak was used to provide a more accurate characterization of lung nodules than SUVmax to minimize the effect of the statistical noise due to the low counts [10]. The repeated acquisitions will increase the total acquisition duration and require post-processing if applied in the clinical practice. All the above-mentioned limitations can be overcome by the total-body PET/CT scanner, where enough counts can be collected within 20 seconds.
Previous studies have reported an improved accuracy of the lesion uptake and metabolic volume when an external device was utilized for respiratory gating [19–21]. Respiratory motion has been successfully corrected for lung nodules either benign or malignant. PET reconstruction is performed using PET raw data divided into several slot bins between each gate signal. In order to maintain a constant image quality, the acquisition duration should be multiplied by the number of the slot bins. Therefore, the PET acquisition duration has been increased and patient comfort and throughput has been decreased. Moreover, the method requires additional time for patient setup and introduces additional radiation exposure to the operators. Alternatively, a data-driven respiratory gating (DDG) method has been introduced [22, 23]. It represents an efficient breakthrough in terms of respiratory motion correction without additional work for patient setup. DDG method can increase the SUVmax of the lesion and decrease the threshold-defined lesion volume. Moreover, it can even provide performance superior to that of the external device-based method. Likewise, DDG method also requires multiple acquisition duration to detect enough counts to yield a comparable image quality.
Our results showed an increased SUVmax and TBR of lesions of all lung nodules, this finding is consistent with previous studies [10, 19, 24]. Our data also analyzed the effects of the characteristics of nodules on the SUVmax and TBR, and the SUVmax and TBR increased regardless of its morphological characteristic, size and distance from the pleura. However, there was an inverse correlation between distance from the pleura and %ΔSUVmax and %ΔTBR, for subpleural nodules (≤ 10 mm in distance), the %ΔSUVmax and %ΔTBR are higher, and that means 20-BH PET significantly improved SUVmax in subpleural nodules (≤ 10 mm in distance). In addition, the increasing amplitude of SUVmax and TBR increases for nodules ≤ 10 mm is greater than the increase for nodules༞10 mm. This should enable 20-BH PET to be used in the assessment of subpleural and smaller nodules.
Our data showed that metabolic volumes of lung nodules are decreases on average by applying breath-hold technique. Figure 4 shows subpleural lesion was blurred and expanded inaccurately due to the influence of breathing on 300- and 20-FB PET images, but on 20-BH PET images, the quantitative MTV as well as the boundary of the lesion were effectively corrected. Consistent with previous study [23],these differences are likely due to the subpleural nodules, which are close to the pleura and heavily affected by respiration.
The effect of breath-hold technique on nodule evaluation and diagnostic performance was assessed using semi-quantitative and visual criteria. Our results showed that using 20-BH PET/CT achieved a higher diagnostic efficiency than CT alone or 300- and 20-FB PET/CT, and suggested that the 20-BH PET/CT fusion may present advancement in stage IA pulmonary adenocarcinoma applications.
Limitations:
There are several limitations in our study. Firstly, only patients with stage IA pulmonary adenocarcinoma was enrolled, which kept the homology of all cases; in the further study, different pathologic types of lung nodules will be enrolled to evaluate the differential diagnosis value of 20-BH PET/CT on lung nodules. A second limitation is the small sample size, and it is necessary to perform a large scale study to improve reliability and validity of 20-BH PET/CT. Thirdly, holding breath for 20-s is not feasible for all patients especially for patients with pulmonary dysfunction, motion correction technology will be used to reduce motion artifacts in cooperative patients in further studies.