Dual-energy CT (DECT) and MR are destined to play a much larger role in pulmonary imaging in the not-so-distant future, according to researchers who are investigating new applications for the technologies.
“We are clearly modifying our way of scanning patients,” said Martine Rémy-Jardin, M.D., Ph.D., head of the Department of Thoracic Imaging, the University Hospital, Lille, France, who presented new findings on emerging thoracic applications at RSNA 2012. “There is no longer just a single energy source and one way to administer contrast material. There are new ways to use CT technology.”
Six years of clinical investigation using DECT led Dr. Rémy-Jardin to a number of new insights about the technology. She said DECT offers major advantages in material decomposition and the elimination of artifacts; for example, DECT can be used to suppress image artifacts around the superior vena cava due to iodine-containing contrast agents during chest CT.
“High-energy CT will suppress these artifacts, improving our ability to analyze the lymph nodes when staging cancer,” Dr. Rémy-Jardin said. “Alternatively, we can use low-energy CT to increase the level of attenuation in the vessels. This allows us to use less iodine, which is especially useful for patients with renal impairment.”
DECT also improves upon routine CT scanning by providing more than anatomical information. “DECT scanning can provide both morphology and functional information based on the same data set,” Dr. Rémy-Jardin said.
DECT effectively generates perfusion and ventilation images similar to those produced by scintigraphy, which can be helpful in detecting acute and chronic pulmonary embolisms (PEs) and especially distal lesions, she added.
“DECT is especially helpful when looking for small clots that are very difficult to see,” she said. “Instead of looking for the clot, we look for the consequence of the occlusion by observing perfusion.”
Applied to thoracic imaging, DECT can improve the diagnosis of PE, lung malignancies and parenchymal diseases.
While chest MR has played a very small role in routine clinical care up to now, it is “potentially a very powerful technique for chest imaging,” according to Jens Bremerich, M.D., a professor of radiology and head of cardiothoracic imaging at the University of Basel Hospital, Switzerland, who offered an update and new developments on thoracic MR imaging at RSNA 2012.
Technical limitations—including longer exam times and lower spatial resolution of MR compared to that of CT and the low-proton density of the lung—have historically limited the use of MR for pulmonary applications. However, the latest generation of MR scanners is capable of overcoming a major limitation of MR: magnetic field inhomogeneities at air-tissue interfaces in the lung, Dr. Bremerich said.
“Major MR manufacturers are all producing new scanners with strong, powerful gradients that enable single images to be acquired much faster and with very short echo times,” he said.
As a result, MR now surpasses CT for tissue characterization in the chest. “Using the variety of MR sequence options, from T1 and T2 to diffusion-weighted and fat-saturation, it’s possible to characterize a pulmonary mass and nail down a differential diagnosis,” he said. “That’s not possible with CT. With CT, you’re simply able to detect a mass and ascribe it a benign or malignant appearance.”
Even current MR scanners using diffusion-weighted sequences are capable of characterizing masses in the chest, making pulmonary MR a viable alternative to nuclear medicine studies, Dr. Bremerich said. He cited a January 2012 study in European Radiology demonstrating that diffusion-weighted MR imaging performed as well as PET/CT in pre-operative staging of non-small-cell lung cancer.
“PET/CT is the non-invasive gold standard for detecting malignancies of the chest,” said Dr. Bremerich. “However, this study showed that diffusion-weighted MR performed equally well.”
MR also offers possibilities for evaluating lung function, including:
“Pulmonary MR offers the potential to cover several relevant aspects of the pulmonary evaluation in one examination,” Dr. Bremerich said. Pulmonary MR is unlike cardiac MR procedures, he added, which are technically difficult to administer. MR offers other advantages over PET and PET/CT, including absence of radiation and greater availability. “PET/CT isn’t universally available but many institutions have an MR scanner,” he said.
Unfortunately, radiologists’ perception of pulmonary MR is lagging behind both new technology and research, Dr. Bremerich concluded. “Many radiologists still think pulmonary MR isn’t useful,” Dr. Bremerich said. “That has to change.”
To access an abstract of the study, “Preoperative Staging of Non-small-cell Lung Cancer: Comparison of Whole-Body Diffusion-weighted Magnetic Resonance Imaging and 18F-Fluorodeoxyglucose-Positron Emission Tomography/Computed Tomography,” in European Radiology, go to www.ncbi.nlm.nih.gov/pubmed/22772365.
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