Vendors hope large bores will entice CT customers

While MR manufacturers have attempted for more than a decade to improve patient comfort by means of clam-shaped scanners and short-bore magnets, the concept of increased space for patients in CT scanners is a new one.

It began with the arrival in 2003 of GE's LightSpeed RT, a quad-slice CT with an extra-wide bore. Siemens, Philips, and Toshiba introduced wide-bore CTs the following year.

The extra space, which extends 80 cm or more, provides room for oncologists to simulate radiation therapy. As a result, these scanners are being positioned as replacements for the single-slice, large-bore CT simulators now in widespread use around the world. They are not, however, limited to oncology.

Siemens Medical Solutions markets its Sensation Open as a way to image the growing number of obese people in the U.S. The company has developed a special table that supports patients weighing as much as 615 pounds. The applications in bariatrics join others in trauma and intervention, as well as oncology.

"This is a scanner for everything," said Murat Gungor, U.S. head of product management for Siemens CT.

The Sensation Open can complete whole-body scans in 22 seconds. This reduces exam time, a patient comfort issue in diagnostics and radiation therapy planning and a critical consideration for trauma patients. The open bore increases patient access in interventions, supports patient positioning for radiation therapy planning, and affords added room for life-sustaining equipment for trauma patients.

The idea is starting to catch on. Siemens' wide-bore CT is installed at 20 sites, eight of which are in the U.S., and GE has installed almost 100 LightSpeed RTs worldwide. Philips has its Brilliance CT Big Bore Oncology system in place at four beta sites and plans to begin routine shipments in June. Toshiba will install the first of its Aquilion LB (large bore) scanners this summer.

The majority of these systems will be used in oncology, due to the need in that area and the general usage patterns for CT, Gungor said. Cancer diagnosis is a huge part of the workload in most medium to large-sized hospitals.

"If you look into the clinical usage of a typical CT scanner in a typical setup, almost 50% of the examinations have an oncology focus," he said. "A scanner of this type has to focus on the radiation planning side but also provide excellent image quality to support diagnostic oncology."

Software tools turn wide-bore scanners into CT simulators. The Brilliance CT Big Bore Oncology includes localization software for marking isocenters directly on the CT console, said Rafael Vaello, Philips' global field marketing manager for oncology imaging. It also provides virtual fluoroscopy for virtual simulation and respiratory correlated imaging.

A key feature of the LightSpeed RT is its 4D capability.

"We can do respiratory tracking, generating full respiratory cine movies of motion," said Steve Kohlmyer, GE's manager of radiation therapy products. "You can use that data for radiation planning for tumors that move."

Multislice CTs provide fast scan times and wide coverage. Philips' Big Bore Oncology product furnishes a true 60-cm field-of-view and covers 24 mm of body area with each subsecond rotation. This increased precision translates into more accurate targeting of the tumor, according to Vaello.

Other vendors claim even larger FOVs that stretch to 80 cm or more. According to Siemens, the FOV on its Sensation Open extends to the edges of its 82-cm bore. This pushes technology to its mathematical limits, using advanced algorithms and every possible projection, Gungor said.

As wide-bore CTs proliferate, the question arises as to why all CTs are not equipped with a patient-friendly diameter. The reason is the trade-offs that wide-bore designs impose.

GE, Philips, and Toshiba have rearranged the geometry of their gantries, pushing components away from the center of rotation. This makes more room for the patient but can be problematic.

"One of the great challenges in going to a much larger bore is the g- force exerted on components," said Doug Ryan, director of Toshiba's CT business unit.

The CT gantry works like a centrifuge, whirling components such as x-ray tubes and detectors around a circular track. The greater their distance from the center of the circle, the greater the force exerted on them.

Philips designed its Big Bore gantry not only to withstand this increased force but to operate efficiently along a wider diameter, Vaello said. The company redesigned the slip ring around which the x-ray tube and other components rotate, adapted the subsystem for data transfer, bolstered the means for anchoring components, and adjusted the balance of the gantry.

Finding that the increased g force could be a source of vibration, Toshiba eliminated the tilt mechanism built into the gantry, thereby increasing its stability and eliminating the vibrations.

"This gives us a center of rotation closer to the floor, which gives us more stability," Ryan said.

Increased centrifugal force is less of a concern for GE, as its LightSpeed RT rotates once each second, compared with competing systems' subsecond rotation times. Kohlmyer noted, however, that the mechanics of the gantry are designed to withstand faster rotational speeds.

"We have plans to release software that will allow our installed base to go up to a half-second rotational time at some point in the future," he said.

The increased distance between source and detector produces another issue: how to capture enough photons to the detector for a high-quality image.

"When you back the tube out, you reduce the efficiency of the x-ray tube to the detector," Kohlmyer said. "It is an inverse square law: The farther your source is from the detector, the more flux you need, or the more power you need to get the same flux."

One solution is to push more current through the x-ray generator, which can produce slightly more dose than multislice CTs designed to serve radiological applications, Vaello said.

"But it is minimal compared to one monitor unit (the measure of machine output for a linear accelerator) that we give in treatment, so it is not significant," he said.

GE addressed the problem this spring by releasing new electronics on the RT platform to reduce noise in the image.

Siemens sidestepped the problem altogether. It kept components in the Sensation Open gantry in exactly the same places as those in its superpremium model, Sensation 64, said Jan Chudzik, Sensation Open product manager. The x-ray generator was redesigned to be more compact but powerful enough to handle bariatric patients. Most of the space savings result from the removal of a bulky filter that took up space more than it added benefit.

"This filter was for resolving ultrahigh contrast structures, like the inner ear," Chudzik said. "Even without it, we have a resolution under 0.4 mm."

To broaden the applications that the Sensation Open might serve, Siemens included high-end components such as the premium-design Straton x-ray tube and z-Sharp capability, which allows the scanner's 20 detector rows to generate 40 slices per rotation.

The minimal trade-offs in widening the aperture of the Sensation Open have raised the possibility that company designers will do the same for all its high-performance CTs. Gungor said there is no certainty that they will but no reason to believe they won't.

Philips is considering manufacturing wide-bore scanners for applications outside oncology, building on the Big Bore Oncology system platform to create a system specifically for bariatric patients.