Dual-energy imaging, other techniques hint of clinical developments and yet another revolution
It's probably too soon to proclaim the demise of detector size one-upmanship and CT slice wars. But vendors are starting to go their own way with respect to scanner design, and bigger may not always be better in the future.
After rapid developments in multislice CT, outshining 64-slice scanners is a tall task, yet brand new designs have already emerged. The price tags of some of these newer devices could supersede 64-slice by far, and customers will be demanding value for their money, said Dr. Elliot K. Fishman, director of diagnostic imaging and body CT at Johns Hopkins University in Baltimore.
"Sixteen-slice CT machines are terrific. Sixty-four-slice scanners are incredible. To get beyond that, vendors need to show us what they can do better that is worth all that extra money," he said.
Looking beyond competitive slicing, radiologists are advised take note of upcoming clinical developments that could be made possible with the new CT scanners, such as tissue characterization with dual-energy scanning, whole-organ perfusion with wide area detector CT, and visualization of low-contrast lesions at a low radiation dose with inverse geometry CT.
SKINNING THE CAT
In recent history, vendors built bigger detectors to improve scanning capabilities and expand anatomical coverage.
"Over the last 20 years, spiral CT has been the king. The main difference has been how many slices you can do at any one time. With the new scanners, vendors are going in different directions instead of taking the same path. We now see different flavors of CT," said Dr. Jeffrey Mendel, chair of radiology at Caritas St. Elizabeth's Medical Center in Boston.
Vendors are seeking to solve similar fundamental application challenges, notably tissue characterization and large organ perfusion.
"Everyone is trying to skin the cat, but we are taking different approaches," said Brian Duchinsky, general manager of CT business for GE Healthcare.
The release of the Siemens Somatom Definition scanner with two x-ray sources mounted onto a single gantry at right angles symbolizes the shift in strategy.
"There has never been a [commercial] scanner with two x-ray tubes. Right away, that changes everything. You can argue how good that will be, but clearly you have a totally new design," Fishman said.
The dual-source feature of the system is thought to be particularly useful for cardiac applications, because temporal resolution is improved to a point where organs with motion can be imaged effectively without beta blockers.
Dr. Stephan Achenbach presented experience with 100 patients at the 2006 Stanford University Multidetector-Row CT symposium. Beta blockers were not administered in this patient series, even though heart rates ranged from 53 to 109 bpm, yet images were free of motion artifact, and 98% of vessel segments were visualized, Achenbach said. Visualization included the right coronary artery, which is typically the most severely affected by motion artifacts.
"In the first 100 patients, image quality is extremely impressive," said Achenbach, an associate professor of cardiology at the University of Erlangen in Germany.
The system may also be capable of performing dual-energy scanning, an as yet unproven technique that has captured the imagination of all of the major manufacturers, as well as academic researchers.
Dual-energy scanning, using two x-ray beams created at different energy levels, might be used to isolate bone and blood vessels, as in the case of complex vasculature near the skull or across the knee, or even distinguish between different types of plaques. Much of its potential, however, is speculative.
"Whether a technique like tissue characterization will work is unclear at this point, but it is an interesting idea that will drive research and potential opportunity," Fishman said.
BREAKING NEW GROUND
Tissue characterization, achieved using contrast-enhanced dual-energy CT, could prove groundbreaking in the oncology setting to look at vascularity of lesions.
"So many of the new drugs in oncology revolve around angiogenesis. It has been shown that looking at vascularity with CT can predict aggressiveness of tumors. There is also interest in picking up lesions, such as liver lesions, earlier," Fishman said.
A technique for characterizing tumors or response to therapy would have a positive impact on patient care in private practice and academic settings alike. It would be particularly useful in separating bones and skull from carotid arteries in the circle of Willis automatically, Fishman said.
The concept of dual energy is far from new. In the early 1980s, researchers experimented with performing two separate scans at different energy levels on a single tube system. But technical problems precluded development.
With newer CT technology, however, scan times are shorter, and it is possible to scan multiple locations with one detector, as well as to cover 40 cm in one rotation, according to GE. It is now possible to switch kV energies in two back-to-back rotations very quickly on a single-tube system.
Pending clearance by the FDA, GE is set to introduce a single-tube scanner with dual-energy capabilities at the upcoming RSNA meeting. A prototype system has been tested on 19 patients in the carotid, pelvic, pulmonary, and abdominal regions of the body.
The company will also offer dual energy and other new technical improvements as a hardware and software upgrade at a cost for higher end scanners in its installed base. A software-only upgrade is in development.
Using a single tube for dual-energy scanning is more challenging from a technical perspective, but it could save acquisition and maintenance costs associated with the two-tube model.
Philips is also committed to developing means of scanning tissues at different energies. The company has developed a prototype system with a "multi-energy detector" that scans tissues at different energies simultaneously. Unlike the other models, the Philips design does not require scanning of the same area twice and therefore has the potential to limit radiation dose, said James Fulton, vice president of global marketing for CT.
Researchers at Hadassah Medical Organization in Jerusalem have been working with a prototype and, pending acceptance, will present research with 300 patients at the RSNA meeting.
WIDE AREA DETECTOR CT
It's possible that a new 256-slice wide area detector CT system from Toshiba will ultimately incorporate dual or multisource scanning, according to Dr. Kazuhiro Katada, the company's lead physician consultant in CT.
Katada, who is also a professor and chair of radiology at Fujita Health University School of Medicine in Japan, claimed the introduction of 256-slice CT has nothing to do with traditional slice wars. The minimum number of rows to cover a whole organ in a single rotation is 256, and beyond 64 slices, there would be little value in going to 128 rows, he said.
"One of the biggest misconceptions is that increasing the number of rows is done to further improve helical scanning. That is not the case. We are trying to get away from helical," Katada said. "We are in the early stages of a paradigm shift."
Researchers presented the first clinical data with the prototype system, including a whole-heart scan produced in 1.5 seconds, at the 2006 American College of Cardiology meeting.
The system allows full volumetric imaging of the heart in one 0.5-second rotation. However, researchers in this particular study looked at perfusion of the heart over three rotations of 0.5 second each.
The 256-slice system helps avoid misregistration or misalignment artifacts, because data may be acquired in a single scan, Katada said. As of the middle of this year, Katada's institution had collected heart and brain data for 49 patients, acquiring 20 scans for each patient, and the results look promising.
"Whole-brain CT perfusion is very exciting. CT angiography and CT perfusion studies come from exactly the same data. There is a perfect match of morphology and functional imaging," Katada said.
True volume acquisition will be the next major breakthrough in CT, according to Mendel.
"Scanners with 256 or more rows will allow acquisition of an entire organ with a single nonhelical rotation. This opens up many new vistas including true volume-acquired 3D CT vascular imaging," Mendel said.
For example, in the brain, this will allow visualization of the entire circulation, including the capillary and venous phases, with a single injection as well as complete separation of the arterial and venous anatomy, he said.
"Bone subtraction is likely to become routine," Mendel said. "Similar vascular detail should be available in other organs."
There are significant advantages for wide area detector CT in the management of acute cerebral stroke, Katada said.
Only 3% to 4% of the 700,000 stroke patients in the U.S. each year make it to intravenous thrombolysis, the only FDA-approved therapy, within three hours of stroke onset, said Dr. Michael Lev, director of emergency neuroradiology at Massachusetts General Hospital in Boston.
"Imaging can measure how much of the brain is already dead versus how much of the brain may die in the future. This is going to be a very powerful imaging tool, with the potential to treat many hundreds of thousands more patients up to eight or nine hours after stroke onset with the same IV thromobolytic therapy.
I can't underscore enough how important imaging is going to be in this application," said Lev, speaking at the Stanford Multidetector-Row CT meeting in June.
Katada agrees with this assessment.
"The role of imaging is very important for stroke patients, and the modality of choice should be 256-row wide area detector CT," he said.
Researchers also say that 256-slice wide area detector CT is potentially very valuable in whole-organ liver perfusion studies. The need for such an application is likely to grow in importance as the number of patients diagnosed with hepatitis C continues to increase worldwide.
INVERSE GEOMETRY MODEL
Meanwhile, novel ways to achieve scanning in a single rotation are emerging. Researchers at Stanford University have been testing a prototype inverse geometry CT (IGCT) system.
With conventional CT, a point source of x-rays and a large detector array collect projection data along a large number of ray paths, with lines connecting the point source with each detector element.
In contrast, the system at Stanford has many separate sources with a much smaller detector array. X-rays are generated from hundreds of source locations, and information is acquired separately from each source with the array of detectors. Initial results were published this year (Med Phys 2006;33(6):1867-1878).
"This is a different approach for CT acquisitions from the ones we use now. Preliminary results with IGCT are positive, with no conebeam artifacts, very high isotropic spatial resolution, and excellent dose efficiency," said Norbert Pelc, ScD, a professor of radiology at Stanford.