Forty years after physicist Allan Cormack and electrical engineer Godfrey Hounsfield won a Nobel Prize for inventing computed tomography as we know it, the modality continues to generate new or improved uses and iterations. In fact, the program for RSNA 2019 is dotted with sessions on the latest and greatest advances in this durable imaging technology. RBJ spoke with several researchers who are still uncovering or perfecting applications for CT.
PET/CT Goes Digital
It makes eminent sense when buying a new imaging machine to want the most up-to-date technology, which is why the experts agree digital is the way to go when replacing your aging analog PET/CT scanner. Hybrid PET/CT devices have been around for the past 20 years, and they’ve enjoyed exponential growth, but advances over the past few years like silicon photomultipliers (SiPMs) and longer axial fields of view are now enabling gains in image quality and patient experience that far exceed what imagers could achieve previously.
The replacement of analog photomultiplier tubes with SiPMs has been grabbing a good deal of industry attention. “I can see only advantages to going digital because evidence suggests it will result in much better sensitivity, resolution and potentially timing resolution than using analog techniques,” says Osama Mawlawi, PhD, deputy chair of the department of imaging physics and chief of nuclear medicine physics at MD Anderson Cancer Center in Houston, who will present on the latest PET/CT imaging technologies at RSNA 2019.
SiPMs address the challenges of localizing and uniformly sensing low-light signals generated from the PET detectors. This becomes particularly advantageous with time of flight imaging, in which timing of the photon arrival is critical to localizing the site of radiopharmaceutical uptake in tissue. These capabilities translate into higher image quality for more accurate tumor detection.
“Several vendors are starting to jump on to the digital bandwagon,” observes Mawlawi, “and as that happens prices will become lower and lower, reaching those of analog technology.”
Larger axial field of view is another rapidly growing field in hybrid PET imaging.
Historically, PET scanners had an axial coverage of around 15 to 16 cm, requiring whole-body imaging to be performed in overlapping chunks. Extending the axial field of view to 25 to 30 cm results in increased sensitivity due to the larger patient coverage, leading to improved image quality and physician interpretation. The improvement in sensitivity can in turn be traded off with lower patient radiation dose and higher patient throughput.
“The sensitivity of the scanner increases by approximately the square of the axial extent,” Mawlawi explains. “This implies you will be able to inject less [radiopharmaceutical], scan for shorter periods of time or keep the same scanning time but get far better image quality because of the much higher number of events you can detect in each image.”
Shortening the scan time, he adds, is another important benefit to large axial field of view since it leads to improved patient satisfaction.
Cone-Beam CT for Breast Imaging
Walk into any dentist’s office today and there’s a good chance you’ll see a cone-beam CT (CBCT) scanner. This technology also has made headway in orthopedic imaging (mostly for the knee, unweighted and weighted sequentially) and angiographic suites, as well as for head and neck scanning and radiation therapy. When it comes to breast imaging, however, the FDA has approved only one CBCT breast scanner, and a small number of systems are installed in the U.S. Most hospitals and radiology practices continue to rely on MRI, ultrasound, tomosynthesis and traditional mammography for breast screening.
That’s why the message that John Boone, PhD, professor of radiology and biomedical engineering at UC Davis Health in Sacramento, California, has for the field is disruptive to say the least.
“Cone-beam breast CT is coming to a breast imaging clinic near you soon and will replace mammography five years down the road,” says Boone, who is also chief of medical physics at UC Davis. His lab has spent the past 20 years developing CBCT for breast imaging, and he is founder and shareholder of a company formed to commercialize a breast CT system based on his research. Making clear that commercialization was not his endgame, Boone maintains that “the images are outstanding, the research is compelling, and I firmly believe breast CT imaging should be available to all women today.”
The differences between CBCT and standard CT are vast, and Boone will flesh some of them out at RSNA 2019.
“The benefit of cone-beam CT over traditional mammography and even tomosynthesis is that it is a truly 3D modality that allows you to generate axial, sagittal and coronal images of the breast at high resolution,” he tells RBJ, “and see through the density of the breast by doing thin-section imaging in all planes.”
The CBCT machine—essentially a flat-panel x-ray detector that uses a cone-beam radiating from an x-ray source in the shape of a rectangular cone—allows full-field, 360-degree views of the breast with one rotation of the CBCT hardware around the breast. The acquired image data are reconstructed using algorithms to produce high-resolution 3D pictures for the detection of masses and calcifications by the radiologist. By comparison, a conventional CT device, which produces a narrower swath of images per scan (around 40 mm), would require multiple rotations to cover the necessary field of view for breast imaging. Importantly, conventional transaxial CT also would unnecessarily irradiate the lungs and heart.
As Boone has found, the bottom line could well be that CBCT affords better detection and diagnosis of breast cancer, particularly early-stage. “The three-dimensional attribute of CBCT is very powerful,” he says, allowing that, while MRI also offers that capability, it’s considerably costlier than a breast CT. Make that by a factor of around five if you weigh the purchase price of the two imaging machines, he elaborates. Operational costs of MRI also are much higher and scan times longer, he points out. Claustrophobia and metal implants are additional concerns with breast MRI.
Another advantage of breast CT is the fact it’s simply more comfortable for the patient. The woman lies on the scan table and no physical compression of the breast is required. This benefit was highlighted by Boone’s team back in 2008 and underscored by a study recently published in the European Journal of Radiology showing breast CT to be more tolerable than mammography. In the study, “Comparison of Comfort Between Cone Beam Breast Computed Tomography and Digital Mammography” (Zhaoxiang Ye et al., EJR, Nov. 2019), researchers found CBCT of the breast was favored by women with dense breasts, breast tissue density being a primary predictor of discomfort during mammography.
CTA vs. MRA in the Ring—Again
At an expert panel on CT angiography vs. MR angiography at RSNA 2019, let there be no doubt heading in which side radiologist Geoffrey Rubin, MD, MBA, of Duke University will take. Rubin performed one of the first computed tomography angiograms in the world in 1991, and he and his former lab at Stanford University went on to pioneer the development of spiral and multidetector-row CT for imaging the cardiovascular system. In his 22 years at Stanford, he also led the movement from invasive to non-invasive CT-based diagnosis and treatment planning for many vascular disorders.
“CTA really is superior to MRA for the vast majority of clinical indications, mainly because of its expediency and quality consistency,” he says. “It’s a modality that technologists with a broader range of training, competency and expertise can use to great effect when compared to MRA, which requires a bit more tweaking, customization and attention to all kinds of details.”
Pitting CTA against MRA is hardly new terrain. But the contours of the debate have changed to a certain extent, thanks to advances in the technologies underlying each, information from updated studies and the weight of economics on how healthcare is delivered today. Hence the great debate.
“It’s fair to say that both the expense of delivering the CT scan service, as well as the expense to the patient and the payer on the receiving end, are substantially less than those same metrics for MRA,” Rubin says.
Rubin also downplays the incontrovertible fact CTA exposes the patient to ionizing radiation while MRA doesn’t. At a recent conference of the International Society for Computed Tomography (ISCT), the subject of imaging cardiovascular disease in infants came up during a pediatric session.
“You’d think radiation sensitivity would be a major impediment to CT in these patients,” he recalls, “but in fact it was reported at the meeting that CT is the preferred tool because it’s so quick and avoids the need for general anesthesia, which brings greater risk to the babies than radiation exposure.” In the case of patients older than infants who require repeat imaging, he allows that MRA might be a wiser choice to avoid cumulative radiation, but even that argument fades in patients who are older than 70, he contends.
Tough to Beat
Rubin is well aware of the strengths and weaknesses of both modalities. For example, he notes, CTA allows the imager to see smaller branches of blood vessels and to better understand the relationships between those branches and the organ, while MRA offers unique sensitivity for detecting inflammation and edema in vessel walls. In a disease like vasculitis, CTA permits views of thickened walls but doesn’t confirm if they indicate active inflammation or chronic thickening with dense fibrosis. With its ability to show how much water is in the wall and how swollen it is as a consequence, MRA gives the clinician a better handle on how active or advanced the vasculitis is. MRA also is able to characterize dynamic blood flow patterns.
When judged on the broad basis of practicality, however, CTA is the hands-down winner, in Rubin’s estimation. Among the criteria he employs are machine availability, efficiency, ease of use, consistency of quality and patient friendliness.
“The truth is that CTA is easier for the patient to experience since they’re into and out of the scanner much faster, and it’s easier, quicker and less expensive for the radiology department to use,” he says. “I think of MRA as more of a niche application, whereas CTA is the workhorse for imaging so many different parts of the body.”
Consensus Coming on Whole-Body CT?
Over the past 20 years, advanced trauma care has improved dramatically in the U.S. as severe injuries from automobile accidents, falls and other calamities became the leading cause of death in people below the age of 45 and the source of 41 million emergency department visits each year. Multidisciplinary teams of trauma specialists, standardized protocols and technological advances in imaging, particularly CT, have been the linchpins of that improvement. And for the most complex cases of polytrauma, whole-body CT imaging (WBCT) has proven to be a vital tool in the trauma workup armamentarium, offering fast and accurate diagnoses to guide appropriate care management.
Oddly, though, no consensus has emerged on when to use WBCT versus selective CT for scanning specific parts of the body.
“There are guidelines for imaging the cervical spine and for imaging the head, but we’ve never quite defined the boundaries of when it becomes appropriate to do whole-body CT or selective imaging,” says Jeffrey Shyu, MD, MPH, a staff radiologist in the emergency division at Brigham and Women’s Hospital in Boston and lead author of “R-SCAN: Whole-Body Blunt Trauma CT Imaging” (JACR, April 2017). “It’s challenging because there are so many variables that determine if a patient should get complete body imaging or imaging of only a few body parts.”
Underscoring that uncertainly, he notes, is the fact several recent studies have found that WBCT improves outcomes in the major trauma setting, including survival, while other studies found no benefit.
Shyu defines polytrauma as a pattern of injuries involving at least two body parts, including the head, neck, chest, abdomen, pelvis and one or more extremities, with any of the injuries posing a threat to the life of the patient. WBCT typically includes imaging of the head and cervical spine without intravenous contrast, as well as imaging of the chest, abdomen and pelvis after contrast—all acquired during a single acquisition setting. The chest CT is usually obtained in the arterial phase to maximize detection of aortic/arterial injury, while an abdomen and pelvis CT is usually in the portal venous phase for pinpointing visceral organ and vascular injuries.
Even in patients with multiple injuries, the decision to proceed with whole-body imaging must be carefully weighed. Radiation exposure from WBCT is not insignificant, and the use of contrast can present its own set of risks and concerns for some patients. Then there’s the issue of cost: CT imaging is expensive, and whole-body imaging often results in incidental findings, such as lung nodules and renal masses, which can necessitate additional workups and healthcare costs.
“If a patient was in a high-speed automobile collision or fell from a height of 15 feet or more, then the decision to do a whole-body CT scan is not really controversial,” says Shyu, who is also a radiology instructor at Harvard Medical School. “The area where it becomes more of an open question is in the less severe mechanisms of injury.”
Clarity may be on the way. Shyu has been working with other radiologists and surgeons in the trauma field to develop ACR appropriateness criteria guidelines for major blunt trauma. He expects a document to be published by early next year, setting the stage for a medical consensus around when and when not to perform WBCT.
In the meantime, he points clinicians to the resources available through the ACR’s RScan.org website to help make the appropriate decisions around using whole-body CT for polytrauma.
“If a patient with significant multiple injuries is hemodynamically stable, I wouldn’t hesitate to use whole-body CT imaging,” Shyu says. “It becomes more of a judgment call when you’re dealing with minor trauma, and that’s where we expect the new ACR guidelines to provide valuable assistance to radiologists and trauma physicians.”