Mohammad Dawood

Respiratory gating in positron emission tomography: A quantitative comparison of different gating schemes

Respiratory gating is used for reducing the effects of breathing motion in a wide range of applications from radiotherapy treatment to diagnostical imaging. Different methods are feasible for respiratory gating. In this study seven gating methods were developed and tested on positron emission tomography (PET) listmode data. The results of seven patient studies were compared quantitatively with respect to motion and noise. (1) Equal and (2) variable time-based gating methods use only the time information of the breathing cycle to define respiratory gates. (3) Equal and (4) variable amplitude-based gating approaches utilize the amplitude of the respiratory signal. (5) Cycle-based amplitude gating is a combination of time and amplitude-based techniques. A baseline correction was applied to methods (3) and (4) resulting in two new approaches: Baseline corrected (6) equal and (7) variable amplitude-based gating. Listmode PET data from seven patients were acquired together with a respiratory signal. Images were reconstructed applying the seven gating methods. Two parameters were used to quantify the results: Motion was measured as the displacement of the heart due to respiration and noise was defined as the standard deviation of pixel intensities in a background region. The amplitude-based approaches (3) and (4) were superior to the time-based methods (1) and (2). The improvement in capturing the motion was more than 30% (up to 130%) in all subjects. The variable time (2) and amplitude (4) methods had a more uniform noise distribution among all respiratory gates compared to equal time (1) and amplitude (3) methods. Baseline correction did not improve the results. Out of seven different respiratory gating approaches, the variable amplitude method (4) captures the respiratory motion best while keeping a constant noise level among all respiratory phases.

Lung motion correction on respiratory gated 3-D PET/CT images

Motion is a source of degradation in positron emission tomography (PET)/computed tomography (CT) images. As the PET images represent the sum of information over the whole respiratory cycle, attenuation correction with the help of CT images may lead to false staging or quantification of the radioactive uptake especially in the case of small tumors. We present an approach avoiding these difficulties by respiratory-gating the PET data and correcting it for motion with optical flow algorithms. The resulting dataset contains all the PET information and minimal motion and, thus, allows more accurate attenuation correction and quantification.

List Mode–Driven Cardiac and Respiratory Gating in PET

Gating methods acquiring biosignals (such as electrocardiography [ECG] and respiration) during PET enable one to reduce motion effects that potentially lead to image blurring and artifacts. This study evaluated different cardiac and respiratory gating methods: one based on ECG signals for cardiac gating and video signals for respiratory gating; 2 others based on measured inherent list mode events. Methods: Twenty-nine patients with coronary artery disease underwent a 20-min ECG-gated single-bed list mode PET scan of the heart. Of these, 17 were monitored by a video camera registering a marker on the patients abdomen, thus capturing the respiratory motion for PET gating (video method). Additionally, respiratory and cardiac gating information was deduced without auxiliary measurements by dividing the list mode stream in 50-ms frames and then either determining the number of coincidences (sensitivity method) or computing the axial center of mass and SD of the measured counting rates in the same frames (center-of-mass method). The gated datasets (respiratory and cardiac gating) were reconstructed without attenuation correction. Measured wall thicknesses, maximum displacement of the left ventricular wall, and ejection fraction served as measures of the exactness of gating. Results: All methods successfully captured respiratory motion and significantly decreased motion-induced blurring in the gated images. The center-of-mass method resulted in significantly larger left ventricular wall displacements than did the sensitivity method (P < 0.02); other differences were nonsignificant. List mode–based cardiac gating was found to work well for patients with high 18F-FDG uptake when the center-of-mass method was used, leading to an ejection fraction correlation coefficient of r = 0.95 as compared with ECG-based gating. However, the sensitivity method did not always result in valid cardiac gating information, even in patients with high 18F-FDG uptake. Conclusion: Our study demonstrated that valid gating signals during PET scans cannot be obtained only by tracking the external motion or applying an ECG but also by simply analyzing the PET list mode stream on a frame-by-frame basis.

Respiratory Motion Correction in 3-D PET Data With Advanced Optical Flow Algorithms

The problem of motion is well known in positron emission tomography (PET) studies. The PET images are formed over an elongated period of time. As the patients cannot hold breath during the PET acquisition, spatial blurring and motion artifacts are the natural result. These may lead to wrong quantification of the radioactive uptake. We present a solution to this problem by respiratory-gating the PET data and correcting the PET images for motion with optical flow algorithms. The algorithm is based on the combined local and global optical flow algorithm with modifications to allow for discontinuity preservation across organ boundaries and for application to 3-D volume sets. The superiority of the algorithm over previous work is demonstrated on software phantom and real patient data.

Detection of respiratory tumour motion using intrinsic list mode-driven gating in positron emission tomography

PurposeRespiratory motion of organs during PET scans is known to degrade PET image quality, potentially resulting in blurred images, attenuation artefacts and erroneous tracer quantification. List mode-based gating has been shown to reduce these pitfalls in cardiac PET. This study evaluates these intrinsic gating methods for tumour PET scans.MethodsA total of 34 patients with liver or lung tumours (14 liver tumours and 27 lung tumours in all) underwent a 15-min single-bed list mode PET scan of the tumour region. Of these, 15 patients (8 liver and 11 lung tumours in total) were monitored by a video camera registering a marker on the patient’s abdomen, thus capturing the respiratory motion for PET gating (video method). Further gating information was deduced by dividing the list mode stream into 200-ms frames, determining the number of coincidences (sensitivity method) and computing the axial centre of mass of the measured count rates in the same frames (centre of mass method). Additionally, these list mode-based methods were evaluated using only coincidences originating from the tumour region by segmenting the tumour in sinogram space (segmented sensitivity/centre of mass method). Measured displacement of the tumours between end-expiration and end-inspiration and the increase in apparent uptake in the gated images served as a measure for the exactness of gating. To estimate the accuracy, a thorax phantom study with moved activity sources simulating small tumours was also performed.ResultsAll methods resolved the respiratory motion with varying success. The best results were seen in the segmented centre of mass method, on average leading to larger displacements and uptake values than the other methods. The simple centre of mass method performed worse in terms of displacements due to activities moving into the field of view during the respiratory cycle. Both sensitivity- and video-based methods lead to similar results.ConclusionList mode-driven PET gating, especially the segmented centre of mass method, is feasible and accurate in PET scans of liver and lung tumours.

Optimal number of respiratory gates in positron emission tomography: A cardiac patient study

Respiratory gating is the method of dividing the data from a tomographic scan with respect to the respiratory phase of the patient. It enables more accurate images by reducing the effects of motion blur and attenuation artifacts due to motion. However, it induces image degradation due to higher noise levels as the number of events per gate is reduced. Due to lack of systematic studies in this regard, different numbers of gates are being used in the scientific and clinical practice. The present study aims at examining the relationship between the respiratory signal, the number of gates required for accurate motion detection, and the level of noise with two different methods of gating: (1) Amplitude-based gating and (2) time-based gating. Patient data with a wide range of motion are used for the study. The results show that time-based gating underestimates the real respiratory displacement by up to 50%. The optimal number of gates is 8 for amplitude- and 6 for time-based gatings. The noise properties remain the same with either method but noise increases with increasing number of gates.

Organ Movement Reduction in PET/CT using Dual-Gated Listmode Acquisition

PURPOSE Imaging of moving organs using the PET leads to blurred images due to long acquisition times. Simultaneous cardiac and respiratory gating of list-mode PET/CT is evaluated with the aim to improve image quality and assess the organ movement. METHODS We performed a N-13 ammonia PET/CT scan with a human volunteer, using the Siemens Biograph Sensation 16 scanner with list-mode acquisition. For ECG gating we used the scanners integrated ECG device. Respiratory gating was done with the BioVet pneumatic sensor system. RESULTS The sorting of the list-mode data post acquisition produced the desired matrix of eight cardiac times eight respiratory images. Organ movement could be measured in the series of gated PET images. The quantitation of tracer uptake in the myocardium showed artifacts due to the CT-based attenuation correction. CONCLUSION Double gating is feasible in human PET/CT scans using a list-mode-based scan protocol. The image quality can be enhanced using double gated list-mode acquisition in PET/CT Attenuation correction protocols in PET using a single not gated fast CT introduces artifacts in moving organs.

Evaluation of respiratory and cardiac motion correction schemes in dual gated PET/CT cardiac imaging

PURPOSE Cardiac imaging suffers from both respiratory and cardiac motion. One of the proposed solutions involves double gated acquisitions. Although such an approach may lead to both respiratory and cardiac motion compensation there are issues associated with (a) the combination of data from cardiac and respiratory motion bins, and (b) poor statistical quality images as a result of using only part of the acquired data. The main objective of this work was to evaluate different schemes of combining binned data in order to identify the best strategy to reconstruct motion free cardiac images from dual gated positron emission tomography (PET) acquisitions. METHODS A digital phantom study as well as seven human studies were used in this evaluation. PET data were acquired in list mode (LM). A real-time position management system and an electrocardiogram device were used to provide the respiratory and cardiac motion triggers registered within the LM file. Acquired data were subsequently binned considering four and six cardiac gates, or the diastole only in combination with eight respiratory amplitude gates. PET images were corrected for attenuation, but no randoms nor scatter corrections were included. Reconstructed images from each of the bins considered above were subsequently used in combination with an affine or an elastic registration algorithm to derive transformation parameters allowing the combination of all acquired data in a particular position in the cardiac and respiratory cycles. Images were assessed in terms of signal-to-noise ratio (SNR), contrast, image profile, coefficient-of-variation (COV), and relative difference of the recovered activity concentration. RESULTS Regardless of the considered motion compensation strategy, the nonrigid motion model performed better than the affine model, leading to higher SNR and contrast combined with a lower COV. Nevertheless, when compensating for respiration only, no statistically significant differences were observed in the performance of the two motion models considered. Superior image SNR and contrast were seen using the affine respiratory motion model in combination with the diastole cardiac bin in comparison to the use of the whole cardiac cycle. In contrast, when simultaneously correcting for cardiac beating and respiration, the elastic respiratory motion model outperformed the affine model. In this context, four cardiac bins associated with eight respiratory amplitude bins seemed to be adequate. CONCLUSIONS Considering the compensation of respiratory motion effects only, both affine and elastic based approaches led to an accurate resizing and positioning of the myocardium. The use of the diastolic phase combined with an affine model based respiratory motion correction may therefore be a simple approach leading to significant quality improvements in cardiac PET imaging. However, the best performance was obtained with the combined correction for both cardiac and respiratory movements considering all the dual-gated bins independently through the use of an elastic model based motion compensation.

Effective Methods to Correct Contrast Agent–Induced Errors in PET Quantification in Cardiac PET/CT

In combined PET/CT studies, x-ray attenuation information from the CT scan is generally used for PET attenuation correction. Iodine-containing contrast agents may induce artifacts in the CT-generated attenuation map and lead to an erroneous radioactivity distribution on the corrected PET images. This study evaluated 2 methods of thresholding the CT data to correct these contrast agent–related artifacts. Methods: PET emission and attenuation data (acquired with and without a contrast agent) were simulated using a cardiac torso software phantom and were obtained from patients. Seven patients with known coronary artery disease underwent 2 electrocardiography-gated CT scans of the heart, the first without a contrast agent and the second with intravenous injection of an iodine-containing contrast agent. A 20-min PET scan (single bed position) covering the same axial range as the CT scans was then obtained 1 h after intravenous injection of 18F-FDG. For both the simulated data and the patient data, the unenhanced and contrast-enhanced attenuation datasets were used for attenuation correction of the PET data. Additionally, 2 threshold methods (one requiring user interaction) aimed at compensating for the effect of the contrast agent were applied to the contrast-enhanced attenuation data before PET attenuation correction. All PET images were compared by quantitative analysis. Results: Regional radioactivity values in the heart were overestimated when the contrast-enhanced data were used for attenuation correction. For patients, the mean decrease in the left ventricular wall was 23%. Use of either of the proposed compensation methods reduced the quantification error to less than 5%. The required time for postprocessing was minimal for the user-independent method. Conclusion: The use of contrast-enhanced CT images for attenuation correction in cardiac PET/CT significantly impairs PET quantification of tracer uptake. The proposed CT correction methods markedly reduced these artifacts; additionally, the user-independent method was time-efficient.

A mass conservation-based optical flow method for cardiac motion correction in 3D-PET

PURPOSE Cardiac positron emission tomography (PET) images usually show two kinds of artifacts: the limited resolution of PET leads to partial volume effects and the motion of the heart induces blurring. These phenomena degrade the PET images and induce errors in the quantification. One method of reducing this problem is to use gated PET data. However, the reduction of information per phase leads to an increase in noise on the reconstructed images. Alternatively, the PET data have to be corrected for motion and partial volume effects. METHODS Optical flow methods have been shown to accurately estimate the motion between PET image frames. These methods assume that the brightness of the objects remains constant between the frames. This condition is not fulfilled in cardiac PET data because the brightness of the cardiac muscle tissue (myocardium) is not accurately resolved due to the partial volume effect. Therefore, the use of a newly developed optical flow method based upon the conservation of mass condition is proposed to correct the cardiac PET data. Mass conservation is applicable to PET images as the total activity in the field of view may be assumed to remain almost constant, if the data are precorrected for radioactive decay. Two variants of the method using the quadratic and the nonquadratic penalization are presented. The methods were evaluated with respect to correlation coefficient, myocardial thickness and the blood pool activity in the left ventricle on phantom data and on 14 patient image volumes. RESULTS The proposed methods showed that the cardiac motion can be efficiently corrected despite partial volume effects. The correlation coefficient between the image volumes increased from 0.87 to 0.98 on average. The change in myocardial thickness was reduced from 28% to 3%. The variation in blood pool activity was reduced from 80% to 8%. The algorithm needed only about 4 s for execution. CONCLUSIONS A mass preserving optical flow method of cardiac motion correction in 3D PET data has been presented and tested on phantom as well as patient data. The results show that the motion was corrected for all datasets effectively.