Characterizing Lesions — Dual-Energy CT Provides a New View in Oncology Imaging
By Dan Harvey
Vol. 13 No. 10 P. 18
It’s been said that two heads are better than one. In the world of CT, two energies may prove better than one for many applications.
“Clinical benefits have been proven,” says Jakub Mochon, a CT product manager with Siemens, which developed and markets the SOMATOM Definition dual-source CT system. Dual-energy CT has become part of the daily clinical imaging routine in some facilities, proving useful for conditions such as kidney stones, gout, musculoskeletal injuries, atherosclerosis, stroke, pulmonary embolism, brain hemorrhage, and cardiovascular disease. It doesn’t take a great leap of the imagination to realize how the capabilities could transfer to characterizing lesions in oncology imaging.
Dual-energy CT can be achieved with two X-ray sources and detector systems operating at different energy levels or by using one source and detector and rapidly switching between two energy levels to produces two images of the same anatomy.
“With conventional CT, we acquire one image at a time in a single energy setting on a scanner. But dual-energy CT makes it possible to acquire two images simultaneously at two different energies,” says Sunit Sebastian, MD, director of the body imaging division and an assistant professor of radiology at the University of Mississippi Medical Center. “That’s the overarching principle. Even though different vendors might accomplish this in different ways, they get to the same place. What excited everyone was that we could see more, and thus learn, more than with conventional CT. It has stretched the capabilities of conventional CT.”
Old Idea Made Feasible
The concept of dual-energy CT is not new, according to Mochon, who says, “It goes back more than 30 years, but recent developments have made it possible to implement it in the clinical setting.” What initially held it back were certain technical deficiencies, which included motion-related misregistration and image noise related to older, deficient tube technology, according to Mochon. Older systems also had reduced spatial resolution compared to new systems.
“It’s a case of where the idea was in place, but the available technology wasn’t,” Sebastian says. “But in the past four decades, technology has advanced, and dual-energy CT is a by-product.”
Siemens’ dual-energy scanner uses two X-ray tubes and two detectors opposed at a 90-degree angle. Other manufacturers, such as GE Healthcare, Philips, and Toshiba, utilize fast kilovolt-switching approaches to collect images at two energy levels with one tube and detector.
The technology, combined with digital subtraction techniques, can help vascular surgeons remove bone from 3D images to improve their view of a patient’s vasculature. Similarly, iodinated contrast can be removed from images, allowing users to take a patient’s contrast-enhanced exam and process it into a noncontrast image without exposing the patient to a second round of radiation.
“Dual-energy CT is an important emergency department tool that’s used every day to image patients with suspected kidney stones,” Mochon says. “They complain about abdominal pain, and a CT scan can reveal a kidney stone.”
Dual energy could take CT to the next level by being able to characterize a cancerous lesion. “Sure CT will reveal the stone, but dual energy reveals the composition. Is it a uric acid kidney stone? Identification leads to the aggression level of treatment. So it goes deeper than problem identification; it goes right to the cause or the composition of the problem and the subsequent treatment,” Mochon says. “We’re talking about an additional dimension added to analysis.”
In oncology imaging, doctors are using dual energy in an attempt to improve lesion detection and characterization. What has been done with kidney stones strongly hints at what can be done with cancerous lesions.
“It’s an indication about how dual-energy CT can be widely applied and not just to noncancer patients but to those suffering from cancer,” Sebastian adds. “More information about the lesion can impact treatment. The physician asks, ‘Do you treat with medicine or treat with surgery?’ The path is made much clearer. A very common conundrum can indicate treatment options.”
“Currently, some of the applications are oncologic but indirectly,” says Rajan T. Gupta, MD, an assistant professor of radiology at Duke University School of Medicine in Durham, North Carolina. “With dual-energy CT, one of the biggest considerations involves utilizing the properties of tissues at two different energy levels to see how soft tissue behaves differently from calcium and from when iodine is used and to see if hypervascular lesions are more conspicuous on lower energy levels than on higher.”
With the liver, for example, research has revealed that some hypervascular lesions are better seen on 80-kVp vs. 140-kVp images, which has obvious implications for cancer imaging, he points out.
“Additionally, when we look at the pancreas, sometimes it can be difficult to delineate a subtle adenocarcinoma against the background of the organ. Again, due to the different tissue properties to say that these lesions are more conspicuous at lower energy levels. So this data can be extracted from the scan as already performed. For the kidney, one of the biggest things you do to determine if a lesion is suspicious—and one of the things we’ve been able to do—is by theoretically subtracting the iodine content, you can determine if there is indeed an enhancing lesion such as a renal cell carcinoma or if it is just a hyperdense cyst.”
Two Studies in One
This potential for performing the equivalent of two exams in one scan could prove to be a major benefit. Often, if the initial imaging is indeterminate, a second study is needed, with or without contrast, to gather more information.
“If you did the same kind of study on the dual-energy scanner, you could just theoretically subtract out the iodine and figure out what the lesion looked like before iodine administration,” he says. “This has implications about reduced radiation dose and minimizing follow-up because sometimes you can get all of the information off of a single scan as opposed to bringing the patient back.”
Gupta also points to the adrenal gland. “That’s another possibility, as the glands commonly have benign or malignant lesions. We could use dual energy to separate the benign from the malignant by determining if the lesion has fat inside. Such a differentiation can have significant implications for cancer care.”
Siemens has incorporated some of these ideas that may prove valuable in oncology imaging into its Optimum Contrast software. “We can enhance tissue differentiation. In the abdominal organs—such as the pancreas, which presents subtle differentiation—a tumor is more easily identified, whereas on conventional CT, you wouldn’t be able to see it. But with greater tissue enhancement, you get better differentiation from the tumor and surrounding normal tissue,” Mochon says.
Thus, a tumor that might have escaped notice is apparent. “Users can even see the smallest lesions surrounding an organ, which provides the clinician a much better grip on understanding the disease extent,” he says. “As much as we’ve improved cancer treatment and survival rates, we can always do something better.”
“With dual energy, if you try to do a contrast-enhanced study of the liver, for instance, and if you are trying to assess if there are tumors, you may want to do a multiphase acquisition,” Mochon adds. “That involves a contrast study and a noncontrast study in two phases. Virtualized enhancement enables generation of a noncontrast study out of the contrast study. This eliminates a step: You don’t have to do the additional noncontrast study which, from the concerns about the X-ray dose, is definitely beneficial for the patient.”
Before moving to the University of Mississippi, Sebastian engaged in research that involved applying dual-energy CT to colon cancer screening at Emory University School of Medicine in Atlanta. The research involved a model construction of the human colon in its natural state. Researchers hoped this model would help them better evaluate any advantages of dual-energy screening, most importantly differentiating polyps from the fecal matter in the colon, which can play havoc with imaging results in colon screening.
The colon model included 17 lesions that simulated polyps and six that simulated fecal matter. Abdominal radiologists found that they could make the necessary differentiation using the dual-energy scans. The ability to perform CT colonography without the unpleasant bowel preparation needed for single-source CT colon exams and for optical colonoscopy could relieve patient concerns and possibly increase colon cancer screening rates, Sebastian notes.
“A noninvasive diagnostic tool just became better,” he says about colon scanning attached to dual energy, “and our research moves in that direction.”
Sebastian admits that this is an area—and a disease—that he is particularly passionate about. “Colon cancer is a largely preventable disease. It’s quite common, and screening is important. Caught early, it’s easily treatable. New techniques such as dual energy will lead to enhanced uptake, as you’re talking about a procedure that is non-invasive and involves almost no discomforting preparation. And therein lays the secret about dual energy CT: It will decrease the incidence of a most common cancer.”
Dual-energy technology is expanding; there are more than 12 FDA-cleared applications and systems, and industry observers expect this number will only increase.
“I don’t possess a crystal ball,” Sebastian says, “but if you want me to predict, I’d say that in the next five years most CT imaging will be dual energy. The technique will entrench itself in oncology because there is great potential. The aforementioned challenges in bringing it to market already have been met, as evidenced by current applications. So now we need to look at the further promise. Technology improves. Applications increase. In the imaging area, that’s a given. So I anticipate greater utilization.”
Gupta believes dual-energy imaging will grow as oncology imaging expands to the imaging of biomarkers. “We could be looking at a next frontier in oncologic imaging involving the imaging of biomarkers,” he explains. “Knowing that different materials behave differently at different energy levels. By imaging the biomarkers, we might have a better idea as to how cancers actually behave, or react to treatment.That may be the next step for dual-energy CT for oncologic imaging.”
— Dan Harvey is a freelance writer based in Wilmington, Delaware.