PET/CT imaging has been the standard for lung cancer staging, but a new pilot study reveals that PET/MR imaging could provide comparable diagnostic image quality while cutting radiation dose by 75 percent compared with diagnostic contrast-enhanced PET/CT.
“Our preliminary data indicate that simultaneous PET/MR offers an alternative modality in thoracic imaging and reduces radiation dose by 75 percent, from about 28 mSv with standard-dose contrast-enhanced whole-body PET/CT—including an additional CT scan of the lung in inspiration—to about 7 mSv with whole-body PET/MR,” according to Nina F. Schwenzer, M.D., an assistant professor, Department of Radiology, Eberhard-Karls University, Tuebingen, Germany, and lead author of the study published in the August 2012 issue of Radiology.
In the study, 10 patients who had or were suspected of having lung cancer underwent standard clinically indicated fluorine 18 fluorodeoxyglucose (FDG) PET/CT with a whole-body scan from the skull base to mid-thigh level and underwent whole-body PET/MR imaging immediately afterward.
Results showed that local tumor staging was feasible with simultaneous PET/MR imaging. “In seven out of 10 patients, a similar tumor stage was found at PET/CT and PET/MR imaging,” according to researchers. “In all patients, higher tumor-to-liver ratios were found in PET at the later time point, probably because of increasing FDG uptake in the pulmonary masses.”
For a number of years, MR imaging of the lung was not considered feasible due to the low proton density of lung tissue and limited spatial resolution. But new technological advances now allow fast T1-weighted gradient-echo sequences to depict pulmonary nodules in the range of 3-5 mm, according to researchers. Nevertheless, detection of small pulmonary nodules remains a challenge for MR imaging.
Whole-body PET/MR imaging systems were introduced into the clinical arena in 2009, said Christina Schraml, M.D., a study co-author and a radiologist in the Department of Radiology at Eberhard-Karls University. “Especially where superior soft tissue contrast is needed, innovative sequences with parallel imaging offer MR imaging an increasing role in the clinical practice of tumor staging.
“Our results show that work-up of pulmonary masses by simultaneous PET/MR imaging is feasible with diagnostic imaging quality in all patients,” Dr. Schraml added. “Also in most patients, PET/MR imaging provided equivalent diagnostic assessment of pulmonary lesions concerning staging compared with PET/CT findings.”
While results are promising, researchers point out that the study findings should be considered preliminary due to the small sample size. Researchers also named economic and data analysis barriers to using the hybrid modality PET/MR in clinical practice.
“Due to economic considerations in the health system, an adequate throughput of patients is mandatory for adopting PET/MR,” Dr. Schwenzer said. “However, high-resolution MR and functional imaging is time-consuming, limiting the number of patients per day.
“Moreover, MR images are challenging to read because radiologists and nuclear medicine physicians are confronted with a number of image series’ for each body region instead of the well-known, whole-body scans in PET and CT,” Dr. Schwenzer continued. “Therefore, data handling and visualization strategies need to be developed to optimize the reading workflow of this new hybrid modality.”
The attenuation correction of the PET datasets poses another problem, according to Dr. Schwenzer. Because all commonly used MR-based methods for attenuation correction ignore bone, an additional inaccuracy—the underestimation of standardized uptake value—is introduced in PET quantification, she said. “Anatomically more exact attenuation maps need to be developed to allow accurate lesion quantification in all body regions,” she said.
Researchers also stress that the hybrid modality is not recommended for all patients, including those with metallic implants such as cardiac pacemakers or those who are obese or claustrophobic. In addition, patient selection for PET/MR needs to be undertaken with care to ensure benefit from the new modality. For example, patients with tumors of the pulmonary apex would benefit from the high soft tissue contrast of the MR component with comparable information from PET compared with PET/CT, according to researchers.
In terms of further studies, “one goal will be the response evaluation of new targeted therapies,” Dr. Schraml said. “Here, PET/MR could be the ideal diagnostic tool for personalized molecular medicine. PET/MR might be especially suitable for patients with prostate cancer, brain tumors and neurodegenerative disorders, as well as pediatric patients.”
The new hybrid modality also offers a new chance to bring nuclear medicine and radiology closer together, according to researchers. “We hope that radiologists and nuclear medicine physicians, individually in daily practice and collectively in medical societies, will take advantage of the PET/MR hybrid modality to advance the emerging field of multimodal imaging.”
To access the study, “Pulmonary Lesion Assessment: Comparison of Whole-Body Hybrid MR/PET and PET/CT Imaging—Pilot Study,” in the August 2012 issue of Radiology, go to radiology.rsna.org/content/264/2/551.full.
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