March 23, 2009

Big Picture
By Dan Harvey
Radiology Today
Vol. 10 No. 6 P. 16

CT scanning has evolved to the point where the entire brain can be scanned in one rotation. The swift acquisition of a large volume of data has promising implications for stroke care and other neurologic applications. All that data also presents a challenge to a facility’s PACS infrastructure.

Whole-brain perfusion studies done using high-end CT scanners can provide neuroradiologists with an encompassing study that depicts the entire brain and its blood flow. These whole-brain CT studies foster an effective treatment design for patients presenting with ischemic vascular disease, and clinicians are finding it especially useful in the emergency department setting, where physicians often encounter urgent stroke-related situations.

For acute stroke patients, whole-brain perfusion studies help physicians readily identify the penumbra zone (a major brain-injury zone), as well as the hypoperfused brain tissue that’s particularly at risk for infarction, says Kazuhiro Murayama, MD, a radiologist at Fujita Health University in Japan. “This capability can provide essential information for the planning of reperfusion therapy,” he says.

“The high-end CT scanners advance both the diagnosis and treatment planning by providing the requisite physiologic brain evaluation coupled with anatomic evaluation of the blood vessels,” explains Anthony Mancuso, MD, radiology department chairman at Shands at the University of Florida in Gainesville, which implemented a 320-detector row CT scanner in October 2008.

Taking the issue deeper into inner space, Mancuso points out, “High-end CT brain perfusion enables physicians to look at any part of the organ. You’re not dealing with selected slices as you would with 64-slice scanners, which can only pick out about four slices through the brain.”

256-Detector Study
Brain perfusion applications are complex. Working at Fujita Health University, Murayama has actively engaged in a study designed to evaluate the viability of 256-detector row CT technology as applied to whole-brain perfusion. During that period, he witnessed the technology’s transition from a prototype to a commercially available imaging vehicle.

For their study, Murayama and colleagues sought to evaluate the feasibility and potential diagnostic application of whole-brain perfusion CT performed with a 256-detector row CT system toward the assessment of ischemic cerebrovascular disease. At the study’s outset, the 256-detector system was a Toshiba Medical Systems prototype.

Independent readers retrospectively evaluated perfusion data collected from January to April 2006 and acquired from 11 cases in 10 subjects: six men and four women with an average age of 64.3. All the subjects had occlusive cerebrovascular disease (including eight cases of large-artery atherosclerosis, two cases of cardioembolism, and one case of no infarction). Scanning parameters included 0.5-millimeter section thickness, 912 channels by 256 rows, 128-millimeter scanning range, one-second scanning speed per rotation, and 240-millimeter field of view. The total scanning time per patient ranged from 50 to 60 seconds.

Results, as reported in the January issue of Radiology (“Whole-Brain Perfusion CT Performed With a Prototype 256-Detector Row CT System: Initial Experience”), were significant. Researchers reported that perfusion CT performed with the 256-detector system effectively assesses the entire brain with the administration of one contrast bolus.

“We found that ischemic regions, including remote perfusion abnormalities such as crossed cerebellar diaschisis, could be identified in one examination,” Murayama says.

The multinational study also involved The Johns Hopkins Hospital. “We all performed the same studies at the same time and in the same way, and we pooled our experience,” says Kieran J. Murphy, MD, who, at the time of the study, was an associate professor of radiology and an investigator in the scanner testing at Johns Hopkins. Along with Murayama and Kazuhiro Katada, MD, he worked with Toshiba to test the technology.

The study’s specific take-home points included the following:

• Whole-brain perfusion analysis permits the identification of remote perfusion abnormalities such as crossed cerebellar diaschisis that may have clinical implications.

• Whole-brain perfusion assessment with the higher spatial resolution (compared with techniques such as SPECT) enhances perfusion CT capabilities in evaluating the full extent of perfusion abnormalities, which is necessary for effective treatment planning for patients with steno-occlusive or thromboembolic cerebrovascular disease.

Nuclear medicine imaging, including PET and SPECT, has traditionally been used in clinical practice, Murayama points out. Also, MRI that combines diffusion and perfusion analysis of the brain parenchyma proved useful for assessing the infarct core and ischemic penumbra in the entire brain. “However, MRI and SPECT, when used to evaluate patients with acute stroke, demonstrate some substantial limitations. The main one is poor accessibility in medical facilities. CT has higher accessibility, as well as shorter scanning time, which often makes it the modality of choice in evaluating acute stroke patients,” he says.

But the relatively narrow range of brain tissue covered is the main limitation in the evaluation of cerebral perfusion with older CT scanners. The newer systems with more detectors and the ability to scan a larger volume address that concern. Murphy, now chief of medical imaging at the University of Toronto, says high-end CT scanners, as they exist today (and as applied to whole-brain perfusion), can serve emergency departments around the world and demonstrate their value in several important ways.

“Not only are the systems effective in identifying acute stroke, as well as cardiac or aortic abnormalities, they are also low maintenance and much more readily available than MRI technology,” he says. “They’re ideal for the emergency environment, as they’re cheaper to install and don’t entail complex service requirements.”

Also, the technology comes without patient compatibility issues, such as when a patient with a pacemaker can’t undergo an MRI. “The systems are low baggage,” says Murphy, “and they’re high yield and high signal with a price tag about a third of the cost of an MRI unit.”

Murphy adds that high-end CT scanners provide two more advantages over previous CT models: The postprocessing of images is accomplished more quickly and easily, and the procedure involves reduced contrast volume, a factor particularly important for elderly patients and people with renal insufficiency.

Radiation Dose
In the study conducted by Murayama and colleagues, the researchers reported that the maximum effective radiation dose was 4.6 millisieverts for 25 scans performed with 80 kilovolts, 80 milliamperes, and 1 second per rotations. The minimum effective dose was 3.5 millisieverts for 19 scans performed with the same parameters. Conversely, in head examinations performed with 64-detector row CT at Fujita, the radiation dose was 60 milligrays for routine nonenhanced CT, 214 milligrays for perfusion CT, and 53.2 milligrays for 3D CT angiography (CTA).

Putting this into plain terms, Murphy says the total dose for dynamic head scans at 4.6 millisieverts is lower than conventional alternatives. Dardinger adds that the 320-detector row system provides more information faster and with less radiation than any other CT available on the market, including the 64, 128, 256, or dual-source systems. “That’s one of the main reasons we chose the AquilionONE system,” he says.

Specifically, the whole-brain CT perfusion with the 320-detector row system includes CTA in the same acquisition; other systems require separate acquisitions to get the same information. “The AquilionONE uses a very low-dose protocol, actually a lower dose than 64-detector scanners, even though it acquires five times the brain tissue volume,” says Dardinger. “At our hospital, the total dose for our CT brain and perfusion and CTA with the AquilionONE averages to 4.25 to 4.5 millisieverts.”

Most other CT brain and perfusion packages without CTA expose the patient to more than 6 millisieverts of radiation, he adds.

 “I want to evaluate the clinical usefulness of whole-brain perfusion CT using a 320-row area detector CT scanner, developed by Toshiba, in treatment planning for patients with hyperacute cerebral infarction,” says Murayama.

320-Detector Implementations
The new 320-detector row CT scanners offer clear information quickly, which is crucial for helping stroke patients.

“It provides us with split-second imaging capability, which enables us to determine if a patient is experiencing a stroke, clot, or hemorrhage,” says Jeff Dardinger, MD, director of imaging at St. Elizabeth Medical Center’s Vascular Institute in Edgewood, Ky., where the technology was implemented in August 2008. “Since the acquisition, we’ve been performing brain perfusion studies in our emergency room. It has changed our stroke imaging.”

The medical center implemented Toshiba Medical System’s 320-detector row CT system, the AquilionONE, which is described as the first-ever dynamic volume scanner. The system deploys 320 ultrahigh resolution detectors (0.5 millimeters in width) and can image the entire brain in a single, 0.35-second gantry rotation. Speed is crucial in the emergency room setting, particularly for stroke patients and their “time is brain” urgency.

The whole-brain coverage, Dardinger says, eliminates errors. “Previous higher end scanners only covered a portion of the brain, so if we didn’t position the slab or the angle correctly, we were stuck and had to reperform the study. With whole-brain imaging, you can’t do anything wrong,” he says.

In addition, scanning the whole brain in one image goes beyond producing 3D reconstructions and enables better 4D videos that reveal organ structure, movement, and blood flow. With its October 2008 installation, Shands at the University of Florida became the first facility in the Southeast to equip itself with a 320-detector row CT scanner. “The detector system is so large and wide and rotates so fast, it enables us to acquire data in a targeted area as quickly as a heartbeat,” says Mancuso. “The 16-centimeter detector is large enough to encompass the entire head, and the system is wide enough that you don’t have to move a patient through the beam, as you would with a 64-slice detector.”

Mancuso notes that the 320-detector row CT is unique in that it enables users to keep a patient static and the tube continuously spinning around the same anatomy, which generates vascular maps combined with perfusion maps. The scanner enables rapid stroke diagnosis and, in turn, swift determination of the most appropriate treatment strategy.

Versatile Protocol
One other important finding that came out of the Murayama study is that CTA and venography can be performed with the administration of one contrast medium bolus during a single acquisition with the 256-detector row CT system. Indeed, a considerable advantage 320-detector row technology provides is that CTA is included in the protocol. “You get a regular angiogram, whereas iterations less than 320 provide only an arterial map,” says Mancuso. “But when you’re able to spin the detector around the anatomy without having to move the patient, you get a standard angiogram. It might not have quite the resolution that an angio suite affords, but you get a very good arterial, capillary, and venous phase angiogram with excellent anatomic detail. With that, physicians can easily match the anatomy to the perfusion abnormalities imaged and come up with the best treatment plan according to disease evolution.”

With other CT scanners, if a physician wanted to do a CTA of intracranial circulation, it would be necessary to do another study with a separate contrast injection, adds Dardinger. “But with the AquilionONE, it’s all part of the study. Essentially, users get the CTA component for free. So, we get the brain perfusion and the CTA and then we go down with another 50 [cubic centimeters] of contrast and do the carotids in a dynamic fashion. We can actually see the contrast flow to the carotids.”

With stroke imaging, physicians not only want to see the stroke in the brain but also where it came from. “Being able to scan the brain and the arteries accurately, with 100 [cubic centimeters] of contrast is the perfect stroke protocol,” says Dardinger.

Remaining Challenges
Murayama says the main challenges that still exist with the 256- and 320-detector row scanners involve the large amount of data generated. Mancuso adds that this entails image display and distribution. Because the studies are so complex, one challenge is how to display the images. But the bigger challenge, he says, is how to place the images into an easy-to-read display for transmission over PACS and image distribution systems, so they can be viewed virtually anywhere by radiologists, neurologists, neurosurgeons, and other decision makers.

“That is a very large issue, and facilities looking to implement the high-end CT technology need to make sure that their information system technology is up to the task,” Mancuso says.

With the 320-detector row system in particular, users are faced with enormous amounts of perfusion data that need to be stored and processed separately. “You have to be able to select exactly how you want the images to be displayed, and that has to be an automated process and easily transmittable to PACS,” says Mancuso. “Decision makers no longer run down to the radiology department. That’s not the way medicine works anymore.”

Potential users need to know that these elements aren’t easy to deal with, and that these challenges need to be surmounted for successful implementation and optimal patient care. “Robust IT capabilities are crucial to effective and efficient use of the technology,” says Mancuso. “When you’re dealing with patients suffering acute stroke, you need to get the data out as quickly as possible. Efficiently displayed data needs to be available and potentially available for transport in less than five minutes so that treatment decisions can flow rapidly thereafter.”

— Dan Harvey is a freelance writer based in Wilmington, Del., and a frequent contributor to Radiology Today.