The impact of respiratory motion on tumor quantification and delineation in static PET/CT imaging

Role of FDG-PET scans in staging, response assessment, and follow-up care for non-small cell lung cancer

The integral role of positron-emission tomography (PET) using the glucose analog tracer fluorine-18 fluorodeoxyglucose (FDG) in the staging of non-small cell lung cancer (NSCLC) is well established. Evidence is emerging for the role of PET in response assessment to neoadjuvant therapy, combined-modality therapy, and early detection of recurrence. Here, we review the current literature on these aspects of PET in the management of NSCLC. FDG-PET, particularly integrated (18)F-FDG-PET/CT, scans have become a standard test in the staging of local tumor extent, mediastinal lymph node involvement, and distant metastatic disease in NSCLC. (18)F-FDG-PET sensitivity is generally superior to computed tomography (CT) scans alone. Local tumor extent and T stage can be more accurately determined with FDG-PET in certain cases, especially in areas of post-obstructive atelectasis or low CT density variation. FDG-PET sensitivity is decreased in tumors <1 cm, at least in part due to respiratory motion. False-negative results can occur in areas of low tumor burden, e.g., small lymph nodes or ground-glass opacities. (18)F-FDG-PET-CT nodal staging is more accurate than CT alone, as hilar and mediastinal involvement is often detected first on (18)F-FDG-PET scan when CT criteria for malignant involvement are not met. (18)F-FDG-PET scans have widely replaced bone scintography for assessing distant metastases, except for the brain, which still warrants dedicated brain imaging. (18)F-FDG uptake has also been shown to vary between histologies, with adenocarcinomas generally being less FDG avid than squamous cell carcinomas. (18)F-FDG-PET scans are useful to detect recurrences, but are currently not recommended for routine follow-up. Typically, patients are followed with chest CT scans every 3-6 months, using (18)F-FDG-PET to evaluate equivocal CT findings. As high (18)F-FDG uptake can occur in infectious, inflammatory, and other non-neoplastic conditions, (18)F-FDG-PET-positive findings require pathological confirmation in most cases. There is increased interest in the prognostic and predictive role of FDG-PET scans. Studies show that absence of metabolic response to neoadjuvant therapy correlates with poor pathologic response, and a favorable (18)F-FDG-PET response appears to be associated with improved survival. Further work is underway to identify subsets of patients that might benefit individualized management based on FDG-PET.

External radioactive markers for PET data-driven respiratory gating in positron emission tomography

PURPOSE

Respiratory gating is an established approach to overcoming respiration-induced image artefacts in PET. Of special interest in this respect are raw PET data-driven gating methods which do not require additional hardware to acquire respiratory signals during the scan. However, these methods rely heavily on the quality of the acquired PET data (statistical properties, data contrast, etc.). We therefore combined external radioactive markers with data-driven respiratory gating in PET/CT. The feasibility and accuracy of this approach was studied for [(18)F]FDG PET/CT imaging in patients with malignant liver and lung lesions.

METHODS

PET data from 30 patients with abdominal or thoracic [(18)F]FDG-positive lesions (primary tumours or metastases) were included in this prospective study. The patients underwent a 10-min list-mode PET scan with a single bed position following a standard clinical whole-body [(18)F]FDG PET/CT scan. During this scan, one to three radioactive point sources (either (22)Na or (18)F, 50-100 kBq) in a dedicated holder were attached the patient's abdomen. The list mode data acquired were retrospectively analysed for respiratory signals using established data-driven gating approaches and additionally by tracking the motion of the point sources in sinogram space. Gated reconstructions were examined qualitatively, in terms of the amount of respiratory displacement and in respect of changes in local image intensity in the gated images.

RESULTS

The presence of the external markers did not affect whole-body PET/CT image quality. Tracking of the markers led to characteristic respiratory curves in all patients. Applying these curves for gated reconstructions resulted in images in which motion was well resolved. Quantitatively, the performance of the external marker-based approach was similar to that of the best intrinsic data-driven methods. Overall, the gain in measured tumour uptake from the nongated to the gated images indicating successful removal of respiratory motion was correlated with the magnitude of the respiratory displacement of the respective tumour lesion, but not with lesion size.

CONCLUSION

Respiratory information can be assessed from list-mode PET/CT through PET data-derived tracking of external radioactive markers. This information can be successfully applied to respiratory gating to reduce motion-related image blurring. In contrast to other previously described PET data-driven approaches, the external marker approach is independent of tumour uptake and thereby applicable even in patients with poor uptake and small tumours.

Accurate localization of incidental findings on the computed tomography attenuation correction image: the influence of tube current variation

This observer performance study assessed lesion detection in the computed tomography attenuation correction image, as would be produced for myocardial perfusion imaging over a tube current (mA) range. A static anthropomorphic chest phantom containing simulated pulmonary lesions was scanned using the four available mA values (1, 1.5, 2 and 2.5) on a GE Infinia Hawkeye 4. All other computed tomography acquisition parameters remained constant throughout. Twenty-seven cases showing zero to four lesions were produced for a free-response receiver-operating characteristic method. Image observations were completed using our novel web-based ROCView software under controlled conditions. The Jackknife alternative free-response receiver-operating characteristic (JAFROC) figure of merit was used for significance testing, wherein a difference in lesion detection performance was considered significant at P values less than 0.05. Twenty readers with varying computed tomography experience (0-24 years) evaluated 108 images using an ordinal scale to score confidence. The JAFROC analysis showed that there was no statistically significant difference in performance between mA values (P=0.439) for this sample of observers. In conclusion, no significant difference in lesion detection performance was seen between the mA values. This suggests that there is no value in using anything other than the lowest mA value for the investigation of incidental extracardiac findings.

Modeling and Simulation of 4D PET-CT and PET-MR Images

The driving force in the research and development of new hybrid PET-CT/MR imaging scanners is the production of images with optimum quality, accuracy, and resolution. However, the acquisition process is limited by several factors. Key issues are the respiratory and cardiac motion artifacts that occur during an imaging session. In this article the necessary tools for modeling and simulation of realistic high-resolution four-dimensional PET-CT and PET-MR imaging data are described. Beyond the need for four-dimensional simulations, accurate modeling of the acquisition process can be included within the reconstruction algorithms assisting in the improvement of image quality and accuracy of estimation of physiologic parameters from four-dimensional hybrid PET imaging.

Challenges and opportunities in patient-specific, motion-managed and PET/CT-guided radiation therapy of lung cancer: review and perspective

The increasing interest in combined positron emission tomography (PET) and computed tomography (CT) to guide lung cancer radiation therapy planning has been well documented. Motion management strategies during treatment simulation PET/CT imaging and treatment delivery have been proposed to improve the precision and accuracy of radiotherapy. In light of these research advances, why has translation of motion-managed PET/CT to clinical radiotherapy been slow and infrequent? Solutions to this problem are as complex as they are numerous, driven by large inter-patient variability in tumor motion trajectories across a highly heterogeneous population. Such variation dictates a comprehensive and patient-specific incorporation of motion management strategies into PET/CT-guided radiotherapy rather than a one-size-fits-all tactic. This review summarizes challenges and opportunities for clinical translation of advances in PET/CT-guided radiotherapy, as well as in respiratory motion-managed radiotherapy of lung cancer. These two concepts are then integrated into proposed patient-specific workflows that span classification schemes, PET/CT image formation, treatment planning, and adaptive image-guided radiotherapy delivery techniques.

A three dimensional drive system for use with fillable emission phantoms for SPECT and PET imaging

Respiratory motion artefacts pose significant challenges for imaging of the lung and thorax. Dynamic phantoms have previously been applied to the study of respiratory motion, however, most moving platforms have been capable of movement in either one or two dimensions only. We describe a moving platform suitable for SPECT-CT and PET-CT imaging. The platform allows a fillable emission phantom to simulate rigid motion in three dimensions. Elliptical periodical motion of 1.5 cm in all three orthogonal planes was simulated using a series of cams moving a baseplate up and across a slope of 45°. The frequency of movement can be varied manually between 5 and 25 cycles per minute in a known calibrated and reproducible manner (This encompasses the range of physiological respiratory motion). Preliminary studies demonstrated that the phantom can be used to identify motion parameters and for the qualitative assessment of motion blurring in reconstructed images.

Ultra-low dose CT attenuation correction for PET/CT

A challenge for positron emission tomography/computed tomography (PET/CT) quantitation is patient respiratory motion, which can cause an underestimation of lesion activity uptake and an overestimation of lesion volume. Several respiratory motion correction methods benefit from longer duration CT scans that are phase matched with PET scans. However, even with the currently available, lowest dose CT techniques, extended duration cine CT scans impart a substantially high radiation dose. This study evaluates methods designed to reduce CT radiation dose in PET/CT scanning. We investigated selected combinations of dose reduced acquisition and noise suppression methods that take advantage of the reduced requirement of CT for PET attenuation correction (AC). These include reducing CT tube current, optimizing CT tube voltage, adding filtration, CT sinogram smoothing and clipping. We explored the impact of these methods on PET quantitation via simulations on different digital phantoms. CT tube current can be reduced much lower for AC than that in low dose CT protocols. Spectra that are higher energy and narrower are generally more dose efficient with respect to PET image quality. Sinogram smoothing could be used to compensate for the increased noise and artifacts at radiation dose reduced CT images, which allows for a further reduction of CT dose with no penalty for PET image quantitation. When CT is not used for diagnostic and anatomical localization purposes, we showed that ultra-low dose CT for PET/CT is feasible. The significant dose reduction strategies proposed here could enable respiratory motion compensation methods that require extended duration CT scans and reduce radiation exposure in general for all PET/CT imaging.

The clinical significance and management of lesion motion due to respiration during PET/CT scanning

Lesion movement during positron emission tomography (PET) scan acquisition due to normal respiration is a common source of artefact. A PET scan is acquired in multiple couch positions of between 2 and 5 min duration with the patient breathing freely. A PET-avid lesion will become blurred if affected by respiratory motion, an effect similar to that created when a person moves in a photograph. This motion also frequently causes misregistration between the PET and computed tomography (CT) scan acquired for attenuation correction and anatomical correlation on hybrid scanners. The compounding effects of blurring and misregistration in whole-body PET/CT imaging make accurate characterization of PET-avid disease in areas of high respiratory motion challenging. There is also increasing interest in using PET quantitatively to assess disease response in both clinical reporting and trials. However, at this stage, no response criteria take the effect of respiratory motion into account when calculating the standardized uptake value on a PET scan. A number of different approaches have been described in the literature to address the issue of respiratory motion in PET/CT scanning. This review details the clinical significance of lesion movement due to respiration and discusses various imaging techniques that have been investigated to manage the effects of respiratory motion in PET/CT scanning.

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

For PET/CT, fast CT acquisition time can lead to errors in attenuation correction, particularly at the lung/diaphragm interface. Gated 4D PET can reduce motion artifacts, though residual artifacts may persist depending on the CT dataset used for attenuation correction. We performed phantom studies to evaluate 4D PET images of targets near a density interface using three different methods for attenuation correction: a single 3D CT (3D CTAC), an averaged 4D CT (CINE CTAC), and a fully phase matched 4D CT (4D CTAC). A phantom was designed with two density regions corresponding to diaphragm and lung. An 8 mL sphere phantom loaded with 18F-FDG was used to represent a lung tumor and background FDG included at an 8:1 ratio. Motion patterns of sin(x) and sin4(x) were used for dynamic studies. Image data was acquired using a GE Discovery DVCT-PET/CT scanner. Attenuation correction methods were compared based on normalized recovery coefficient (NRC), as well as a novel quantity "fixed activity volume" (FAV) introduced in our report. Image metrics were compared to those determined from a 3D PET scan with no motion present (3D STATIC). Values of FAV and NRC showed significant variation over the motion cycle when corrected by 3D CTAC images. 4D CTAC- and CINE CTAC-corrected PET images reduced these motion artifacts. The amount of artifact reduction is greater when the target is surrounded by lower density material and when motion was based on sin4(x). 4D CTAC reduced artifacts more than CINE CTAC for most scenarios. For a target surrounded by water equivalent material, there was no advantage to 4D CTAC over CINE CTAC when using the sin(x) motion pattern. Attenuation correction using both 4D CTAC or CINE CTAC can reduce motion artifacts in regions that include a tissue interface such as the lung/diaphragm border. 4D CTAC is more effective than CINE CTAC at reducing artifacts in some, but not all, scenarios.

[Metabolically active volumes automatic delineation methodologies in PET imaging: review and perspectives]

PET imaging is now considered a gold standard tool in clinical oncology, especially for diagnosis purposes. More recent applications such as therapy follow-up or tumor targeting in radiotherapy require a fast, accurate and robust metabolically active tumor volumes delineation on emission images, which cannot be obtained through manual contouring. This clinical need has sprung a large number of methodological developments regarding automatic methods to define tumor volumes on PET images. This paper reviews most of the methodologies that have been recently proposed and discusses their framework and methodological and/or clinical validation. Perspectives regarding the future work to be done are also suggested.

Respiratory motion correction for quantitative PET/CT using all detected events with internal-external motion correlation

PURPOSE

We present a method to correct respiratory motion blurring in PET/CT imaging using internal-external (INTEX) motion correlation. The internal motion of a known tumor is derived from respiratory-gated PET images; this internal motion is then correlated with external respiratory signals to determine the complete information of tumor motion during the scan.

METHODS

For each PET/CT data, PET listmode data were phase-gated into five bins and reconstructed. The centroid of a targeted tumor in each bin was determined and correlated with the corresponding mean displacement of externally monitored respiratory motion signal. Based on this correlation, the external motion signal was converted into internal tumor motion information in the superior-inferior direction. Then, the PET listmode data were binned sequentially to multiple 1-s sinograms. According to the converted internal tumor motion signal, each 1-s sinogram was registered to a reference frame, which best matched the helical CT attenuation map based on consistency conditions. The registered sinograms were summed and reconstructed to form an image, corrected for the motion of the specific tumor. In this study, the proposed INTEX method was evaluated with phantom and patient studies in terms of tracer concentration and volume.

RESULTS

The INTEX method effectively recovered the tracer concentration to the level of the stationary scan data in the phantom experiment. In the patient study, the INTEX method yielded a (17 +/- 22)% tumor volume decrease and a (10 +/- 10)% tumor SUVmax increase compared to non-gated images.

CONCLUSIONS

The proposed INTEX method reduces respiratory motion degradation of PET tumor quantification and delineation in an effective manner. This can be used to improve the assessment of response to therapy for a known tumor by minimizing residual motion and matching the attenuation correction, without increasing image noise.

Extension of a data-driven gating technique to 3D, whole body PET studies

Respiratory gating can be used to separate a PET acquisition into a series of near motion-free bins. This is typically done using additional gating hardware; however, software-based methods can derive the respiratory signal from the acquired data itself. The aim of this work was to extend a data-driven respiratory gating method to acquire gated, 3D, whole body PET images of clinical patients. The existing method, previously demonstrated with 2D, single bed-position data, uses a spectral analysis to find regions in raw PET data which are subject to respiratory motion. The change in counts over time within these regions is then used to estimate the respiratory signal of the patient. In this work, the gating method was adapted to only accept lines of response from a reduced set of axial angles, and the respiratory frequency derived from the lung bed position was used to help identify the respiratory frequency in all other bed positions. As the respiratory signal does not identify the direction of motion, a registration-based technique was developed to align the direction for all bed positions. Data from 11 clinical FDG PET patients were acquired, and an optical respiratory monitor was used to provide a hardware-based signal for comparison. All data were gated using both the data-driven and hardware methods, and reconstructed. The centre of mass of manually defined regions on gated images was calculated, and the overall displacement was defined as the change in the centre of mass between the first and last gates. The mean displacement was 10.3 mm for the data-driven gated images and 9.1 mm for the hardware gated images. No significant difference was found between the two gating methods when comparing the displacement values. The adapted data-driven gating method was demonstrated to successfully produce respiratory gated, 3D, whole body, clinical PET acquisitions.

Multimodality molecular imaging of the lung

The continued progression of chronic lung disease despite current treatment options has led to the increasing evaluation of molecular imaging tools for diagnosis, treatment planning, drug discovery, and therapy monitoring. Concurrently the development of multimodality positron emission tomography (PET) / computed tomography (CT), single-photon emission computed tomography (SPECT)/CT, and magnetic resonance imaging (MRI)/PET scanners has opened the potential for more sophisticated imaging biomarker probes. Here we review the potential uses of multimodality imaging tools, the established uses of molecular imaging in nononcologic lung pathophysiology and drug discovery, and some of the technical challenges in multimodality molecular imaging of the lung.

Evaluation of the SUV values calculation and 4D PET integration in the radiotherapy treatment planning system

BACKGROUND AND PURPOSE

To evaluate the SUV calculation and integration of the gated (4D) PET in the iPlan 4.0 treatment planning software (BrainLAB).

MATERIALS AND METHODS

Phantom and patient data for different tracers were used. Two comparisons were performed for each patient: for the delineated VOI, the maximum value of SUV in iPlan was compared with the results from TrueD software. For 10 patients lesion volumes were defined in both systems for a given SUV threshold and differences were calculated. For four patients examined with respiratory gated PET, SUV(max) and volume analysis was performed in each phase of the breathing cycle in the gated and the ungated PET.

RESULTS

Maximum differences of 6% and 10% were found for phantom and patient measurements of SUV(max). For patient data, maximal differences in delineated volume of 10% for ungated and up to 27% for gated PET were found in both systems.

CONCLUSION

This study suggests that for the safe implementation of PET data and delineation algorithms in the radiotherapy planning system, one has to be aware of the differences in SUVs and volumes found in the two systems.

Quiescent period respiratory gating for PET/CT

PURPOSE

To minimize respiratory motion artifacts, this work proposes quiescent period gating (QPG) methods that extract PET data from the end-expiration quiescent period and form a single PET frame with reduced motion and improved signal-to-noise properties.

METHODS

Two QPG methods are proposed andevaluated. Histogram-based quiescent period gating (H-QPG) extracts a fraction of PET data determined by a window of the respiratory displacement signal histogram. Cycle-based quiescent period gating (C-QPG) extracts data with a respiratory displacement signal below a specified threshold of the maximum amplitude of each individual respiratory cycle. Performances of both QPG methods were compared to ungated and five-bin phase-gated images across 21 FDG-PET/CT patient data sets containing 31 thorax and abdomen lesions as well as with computer simulations driven by 1295 different patient respiratory traces. Image quality was evaluated in terms of the lesion SUV(max) and the fraction of counts included in each gate as a surrogate for image noise.

RESULTS

For all the gating methods, image noise artifactually increases SUV(max) when the fraction of counts included in each gate is less than 50%. While simulation data show that H-QPG is superior to C-QPG, the H-QPG and C-QPG methods lead to similar quantification-noise tradeoffs in patient data. Compared to ungated images, both QPG methods yield significantly higher lesion SUV(max). Compared to five-bin phase gating, the QPG methods yield significantly larger fraction of counts with similar SUV(max) improvement. Both QPG methods result in increased lesion SUV(max) for patients whose lesions have longer quiescent periods.

CONCLUSIONS

Compared to ungated and phase-gated images, the QPG methods lead to images with less motion blurring and an improved compromise between SUV(max) and fraction of counts. The QPG methods for respiratory motion compensation could effectively improve tumor quantification with minimal noise increase.