Evaluation of the combined effects of target size, respiratory motion and background activity on 3D and 4D PET/CT images

Abstract

Gated (4D) PET/CT has the potential to greatly improve the accuracy of radiotherapy at treatment sites where internal organ motion is significant. However, the best methodology for applying 4D-PET/CT to target definition is not currently well established. With the goal of better understanding how to best apply 4D information to radiotherapy, initial studies were performed to investigate the effect of target size, respiratory motion and target-to-background activity concentration ratio (TBR) on 3D (ungated) and 4D PET images. Using a PET/CT scanner with 4D or gating capability, a full 3D-PET scan corrected with a 3D attenuation map from 3D-CT scan and a respiratory gated (4D) PET scan corrected with corresponding attenuation maps from 4D-CT were performed by imaging spherical targets (0.5-26.5 mL) filled with (18)F-FDG in a dynamic thorax phantom and NEMA IEC body phantom at different TBRs (infinite, 8 and 4). To simulate respiratory motion, the phantoms were driven sinusoidally in the superior-inferior direction with amplitudes of 0, 1 and 2 cm and a period of 4.5 s. Recovery coefficients were determined on PET images. In addition, gating methods using different numbers of gating bins (1-20 bins) were evaluated with image noise and temporal resolution. For evaluation, volume recovery coefficient, signal-to-noise ratio and contrast-to-noise ratio were calculated as a function of the number of gating bins. Moreover, the optimum thresholds which give accurate moving target volumes were obtained for 3D and 4D images. The partial volume effect and signal loss in the 3D-PET images due to the limited PET resolution and the respiratory motion, respectively were measured. The results show that signal loss depends on both the amplitude and pattern of respiratory motion. However, the 4D-PET successfully recovers most of the loss induced by the respiratory motion. The 5-bin gating method gives the best temporal resolution with acceptable image noise. The results based on the 4D scan protocols can be used to improve the accuracy of determining the gross tumor volume for tumors in the lung and abdomen.

Motion artifacts occurring at the lung/diaphragm interface using 4D CT attenuation correction of 4D PET scans

“…The presence of motion reduces this value. ( 24 ) A 4D imaging technique can be evaluated in terms of its ability to increase NRC towards the levels observed when no motion is present. …”

“…While it has been reported that it is feasible to minimize motion artifacts by using 4D PET corrected by 4D CT in patients in a clinical setting, ( 21 – 23 ) the addition of a 4D CT adds considerable complexity, as well as increased patient dose. As a consequence, clinical centers may avoid these complications by using 4D PET imaging protocols with attenuation correction based on 3D CT. A number of studies have compared different methods for 4D PET attenuation correction in patients ( 21 ) and phantoms, ( 24 – 26 ) as well as simulated data based on 3D PET and 4D CT in patients. ( 13 ) As expected, these studies conclude generally that 4D PET reduces motion artifacts, and that 4D PET corrected by phase matched 4D CT is the most quantitatively accurate approach.…”

Phased Versus Midventilation Attenuation-Corrected Respiration-Correlated PET for Patients with Non-Small Cell Lung Cancer

“…Time-or phase-averaged CT data have been used for AC (18,19) of PET and RCPET. Others have used only particular CT phases for AC of gated and ungated PET (20). The use of midventilation CT has been investigated with respect to its use in radiotherapy treatment planning for lung cancer patients (21,22), but not for AC of RCPET.…”

Assessing and accounting for the impact of respiratory motion on FDG uptake and viable volume for liver lesions in free‐breathing PET using respiration‐suspended PET images as reference

“…Previous approaches to reducing motion artifacts have included four-dimensional PET (4D PET) with 4D CT. Studies using these techniques have reported inaccuracies associated with tumor motion and their impact on SUV, tumor delineation, and localization. [3][4][5] Although 4D PET improves image quality significantly, substantial residual motion artifacts remain in the retrospectively reconstructed 4D PET and 4D CT, primarily due to irregularities in patient breathing. [6][7][8][9][10] There are two main types of 4D PET scan and reconstruction, including respiratory-gated and motion-compensated 3D PET: (1) scanning throughout breathing cycles and sorting raw data into multiple bins of respiratory phases, or scanning only one particular phase with respiratory gating [11][12][13] and (2) scanning breathing cycles and applying a deformable model derived from 4DCT to reconstruct a single phase scan without suffering from low quantum or long acquisition.…”