Respiratory motion impacts over 50% of all PET/CT procedures and can have a drastic effect on image quality that can lead to quantitative inaccuracies when tracking patients over time. Image blurring due to motion degrades image quality, which may decrease small lesion detectability and result in poorly defined gross tumor volume or volume inaccuracies when using PET for radiation therapy (RT) or surgical resection guidance as well as therapy monitoring. Therapeutic monitoring relies heavily on SUV response assessment and current criteria do not factor in the effect of respiratory motion on SUV calculations. Not accounting for respiratory motion could lead to missed lesion detection, incorrect disease staging and a decreased accuracy in radiation therapy target volume delineation, leading to higher dose delivery to normal tissue. There are external gating device solutions whose clinical impact is proven, but, unfortunately in many institutions these devices are not routinely implemented as they are perceived as complex and time-consuming. It is also possible that external devices may not detect short, shallow breaths in the patient’s breathing pattern. Additionally, organ movement may not be accurately represented by the external body movement since external devices focus solely on chest wall motion—motion can occur in the abdomen and pelvis, as well.
A new approach, MotionFree, a data-driven gating technology, uses the raw PET imaging data to seamlessly integrate respiratory motion correction for all patients in every exam, without impacting clinical workflow by requiring the use of external devices.
Challenges when using external devices
“The biggest issue with external respiratory gating devices is that we have to place them on the patient,” says Daniel McGowan, PhD, Principal Clinical Scientist at Oxford University
Hospitals NHS Foundation Trust. “The way that the surface of the body moves may not be representative of what is going on inside the body.”
Dr. McGowan explains that external gating devices capture one dimension of movement in the patient’s abdomen and assumes the lung moves in the same way, which may not always be true.
Patients may also not tolerate external devices and become uncomfortable, leading to additional movement; in larger-sized patients, their body could block the line of sight between the device and the camera at the foot of the scanning bed. This has been the experience at University Hospital Zurich for Martin Huellner, MD, senior physician and PET/CT/MR imaging specialist, and Josephine Trinckauf, BS, senior technician, nuclear medicine. In fact, the hospital stopped using the device nearly eight years ago in the general population and only utilized the device for PET/CT imaging of lung malignancies prior to RT.
“The main challenge in tracking respiratory motion was the external device,” says Dr. Huellner. “It required precise placement on the patient and could be complicated to install.”
In very sick patients, including those who had difficultly lying on their back, the addition of an abdominal device could make the exam even harder to tolerate. In some cases, the patient’s position blocked the line of sight between the device and the camera at the foot of the scanning bed, impacting the acquisition of patient motion data.
Using the external device required more than one step—the placement of the device—according to Trinckauf. She would have to also check the external device computer that all the patient data was entered correctly. Patients often need reassurance when positioning the device as to the purpose of the camera at the foot of the bed. Plus, with the external device she had to verify that the signal was not being lost and that it captured the waveforms. This additional patient set-up time may further expose the technologist to radiation being emitted by the patient after administration of the radiopharmaceutical.
Not only was the device complicated to use, but Trinckauf says it further complicated the imaging results. “It could be difficult to send the data from the device to the workstation.” This could also compromise the radiologist’s reporting workflow.
To address these patient experience and compliance issues and provide a reliable, real-time and true physiologic respiratory analysis, GE Healthcare developed MotionFree. Using PET sinogram data, MotionFree is a principal component analysis-based data-driven respiratory gating technology that takes a signal over time (list mode) and decomposes it into a set of functions by detecting regions with different kinetics that explain the maximum variation in the data over time. The data is prospectively processed leading to fully converged reconstruction, SUVs and SNR, and optimal signal recovery for small lesions or features affected by motion.
PET digital gating
At Oxford, Dr. McGowan and colleagues embarked on a study to evaluate MotionFree and compare it to the existing standard of care, Real-Time Position Management (RPM) (Varian Medical Systems, Palo Alto, CA). With an anthropomorphic phantom and a NEMA IEC Body phantom filled with 18F and placed on a platform that simulates respiratory motion in the axial direction with realistic waveforms, Dr. McGowan scanned the phantoms on a Discovery™ PET/CT 710 system. He captured motion waveforms with the RPM and then retrospectively reconstructed the data using MotionFree to obtain the sinogram waveforms. The two waveforms were analyzed with Pearson’s correlation coefficients, reconstructed using Q.Static (quiescent period gating) and analyzed for measurement of recovery percentage and background variability.
Using phantoms in his study provided a “ground truth,” says Dr. McGowan. “With a phantom, we know exactly the lesion size. In a patient, we can see the lesion becoming easier to see when we account for respiratory motion but we don’t know the ‘real’ truth regarding the exact lesion size.”
Dr. McGowan and co-authors published the results of their phantom respiratory motion study and reported that MotionFree had a similar performance to the RPM with a correlation coefficient greater than 0.97.1 They concluded MotionFree provided a reliable respiratory gating signal that could eliminate the need for external gating equipment and enable routine respiratory gating in clinical practice.
“This technology identifies the area of greatest movement and corrects for it,” Dr. McGowan says. “It looks at how the organ moves and removes the limitation that organ movement may not be accurately represented by external body movements.”
Further highlighting this possible scenario is another published study by Dr. McGowan and his colleagues that reported MotionFree identified solitary or additional pulmonary lung metastases in a patient who underwent FDG PET for colorectal cancer staging. Using standard reconstruction techniques, Figure 3. In a published phantom respiratory motion study, Dr. McGowan and colleagues concluded that MotionFree provided a reliable respiratory gating signal that could eliminate the need for external gating equipment and enable routine respiratory gating in clinical practice. 4 Clarity magazine • A GE Healthcare publication CLINICAL VALUE MOTIONFREE PET/CT a 6 mm pulmonary nodule identified on CT and located posteriorly in the right lower lobe was indistinct from background activity and assigned a Standard Uptake Value in the volume of interest (SUVmax) of 1.9. However, after reprocessing the PET images with MotionFree, the lesion was reclassified as FDG-avid with a SUVmax of 2.8. Other small pulmonary and hepatic metastatic lesions were also more conspicuous with increased SUVmax.2 “This case demonstrated that by applying MotionFree, we could see things in PET that we couldn’t see in static imaging and that supports the need to correct for motion,” Dr. McGowan adds.
The first clinical results
During the clinical evaluation of MotionFree, University Hospital Zurich used it in tandem with RPM on 15 patients and then used MotionFree exclusively on another 125 patients. When Trinckauf used the RPM, she often had to explain to the patients what it was being used for. With MotionFree, since the patient couldn’t see it, they didn’t worry about it… it was seamless in the workflow. The shortened patient set-up time also reduced Trinckauf’s exposure to radiation.
“I was surprised at how easy MotionFree was to learn and use,” she adds. In most instances, the techs adapted well to the software-based, data-driven gating technique after using it on just a few patients.
More importantly, Trinckauf discovered that if MotionFree was not utilized for a patient, the images could still be retrospectively processed for motion correction after the exam was completed.
While the reduction in set-up time is less than with external gating devices, it is the resulting image quality that most impressed Dr. Huellner.
“I would use MotionFree all the time with any PET study,” he says. “We found the contrast of the organs was better defined in several patients. In particular, the kidneys and the liver were better visualized.”
Poor definition of both organs can cause additional artifacts, Dr. Huellner explains, and leads to uncertainty regarding whether there is homogeneous uptake or reduced uptake on the border of the organ. Also, good definition of the organ on the PET image that appears similar to what is seen on the CT image improves the overall impression for the radiologist.
“I was more confident in my diagnosis, staging and reporting when I reviewed the MotionFree gated images, especially in the upper abdomen, lung and liver,” Dr. Huellner says.
He believes he reported fewer questionable findings in the liver using MotionFree and, as important, could better visualize more tiny, subcentimeter lesions. While it is too early to know if a patient’s treatment plan was altered as a result of visualizing these tiny lesions, he recalls one case of a patient with lung cancer where he found metastases in both lobes of the liver—something he didn’t expect to see.
There was also a noticeable difference in SUVs. In the tiny lesions, and depending upon the type of lesion, there were instances of the SUVmax increasing by 30% or more.
“With MotionFree, we more closely approached reality—the SUV was more reliable and that may influence follow-up exams where clinicians often rely on the SUV,” Dr. Huellner adds. When images are more clear, he can also be more efficient in reading through the studies.
In several cases that Dr. Huellner compared RPM, MotionFree and static (not corrected for motion), he reported similar results between RPM and MotionFree. Both respiratory motion techniques outperformed static imaging in terms of lesion detectability and organ definition.
Plus, the solution works automatically. “The beauty of MotionFree is that it automatically corrects for motion,” explains Dr. McGowan. MotionFree auto detects when respiratory motion is present and corrects for it. With RPM, the staff had to choose which bed positions to apply respiratory motion correction. This led to longer scanning times for those bed positions as it is common to scan for double the standard time when respiratory motion correction is applied.
In fact, Dr. McGowan retrospectively looked at the average number of gated bed positions and discovered that with MotionFree, it may be possible to reduce the number of gated bed positions and effectively shorten scan times.
The average number of gated bed positions was 1.2 with MotionFree, yet Oxford typically gated 2 bed positions with RPM.3 With patients where respiratory motion is not detected by MotionFree, there is no need to extend the scanning time for those bed positions, improving patient throughput on the scanner.
“You don’t know up front if a patient is breathing a lot, nor do you know if there is something in the liver or lung that you don’t already know about,” Dr. McGowan says. “With MotionFree, because it is automatic, there is no reason not to use it for every patient all the time. This capability is a big step forward.”
1. Walker MD, Bradley KM, McGowan DR. Evaluation of principal component analysis-based datadriven respiratory gating for positron emission tomography. Br J Radiol 2018; 91: 20170793. Open access.
2. Morley NCD, McGowan DR, Gleeson FV, Bradley KM. Software Respiratory Gating of Positron Emission Tomography–Computed Tomography Improves Pulmonary Nodule Detection. Am J Respir Crit Care Med, Jan 15, 2017; 195(2):261–262. Open access.
3. Walker MD, Morgan AJ, McGowan DR. EP-1016 Evaluation of Novel Data Driven Respiratory Gating Waveforms. Proceedings of the 2018 European Association of Nuclear Medicine. Eur J Nucl Med Mol Imaging (2018). https://doi.org/10.1007/s00259-018-4148-3