November 17, 2008
Dynamic Volume CT: A Macroevolution?
By Jagat Narula, MD, PhD
Vol. 9 No. 23 P. 10
During the past 15 years, there have been dramatic improvements in CT technology. Recent advances have enabled clinicians to effectively visualize the coronary arteries that were too difficult to image with previous generations of scanners.
It was expected that CT would become an important diagnostic modality in the management of coronary artery and cerebrovascular disease. The advent of helical CT scanning in the early 1990s made it possible to routinely view volumetric images of an entire organ. Helical scanning came from combining the continuous tube and detector rotation allowed by slip-ring technology with constant motion of the patient table through the gantry. To reconstruct an entire anatomic volume, several gantry rotations are pieced together like a mosaic.
But this technique had several limitations. With the patient continuously moving through the gantry, each portion of the acquired dataset was imaged at different moments in time. This was of little concern for the imaging of static organs. However, the lack of temporal uniformity in organs such as the heart and brain, which change over time due to motion or the passage of contrast media, was a critical drawback and resulted in unacceptable motion and registration artifacts in reconstructed images.
A significant innovation in helical CT imaging occurred later in the decade: multidetector row CT (MDCT).1 By dividing the detector into thin, submillimeter sections, multiple neighboring z-locations could be acquired simultaneously. Combined with a helical acquisition, large areas of the anatomy could be rapidly imaged. With the submillimeter detector rows, it became possible to reconstruct these datasets into thin-slice images. With four- and 16-detector row systems, the number of detector rows acquired per rotation was the same as the number of slices that could be reconstructed per rotation. Therefore, the word slice became synonymous with detector row, and the coverage of a system could be deduced from the number of detector rows and the width of each row. Further advancements in technology led to the 64-detector row system, making cardiac and cerebral CT imaging possible.2,3 And coronary atherosclerotic plaques could be imaged in an accurate and reproducible manner.
While it has become possible to image nonstationary organs with MDCT scanners, the need to piece the anatomy together from multiple gantry rotations over time still renders datasets acquired with a spiral acquisition temporally nonuniform. Further, these datasets are typically acquired with a 75% to 80% acquisition overlap, which leads to an increased radiation dose to the patient. Concerns have been raised regarding the long-term consequences of ionizing radiation, which has limited the application of CT coronary angiography to specific populations with coronary artery disease.4,5
Dynamic volume CT is the concept introduced to define data acquired by the latest generation of CT scanners, including 256- and 320-detector row systems. Dynamic volume CT, which is capable of imaging the entire organ with isotropic resolution in a single gantry rotation, is not the same as multislice CT, since it covers the entire organ as a complete volume with no table motion. The data are reconstructed as a true volume rather than individual slices, and these volumes are temporally uniform since the entire dataset is acquired in a single rotation.6,7
Dynamic volume CT offers substantial advantages over multislice CT for both cardiac and cerebrovascular applications. In a patient with a suspected stroke, a conventional 64-slice system involves acquiring a helical, noncontrast, static image of the brain followed by a static helical CT angiography with a total effective dose of about 6 millisieverts. Although the anatomy is effectively demonstrated, the temporal details of cerebral perfusion are somewhat limited. Dynamic volume CT, on the other hand, allows a full volumetric brain acquisition with a total effective dose of about 3 millisieverts, as well as the acquisition of multiple low-dose dynamic volumes over a 60-second time frame during contrast injection to quantify cerebral blood flow. The regional deficits in cerebral blood flow can be identified.
Similarly, to minimize motion artifacts during coronary angiography, the conventional multislice scanners use low-pitch helical acquisitions to image the heart, with each rotation overlapping the previous one to a great extent. As such, not only does the data need to be acquired over multiple heart beats resulting in up to 15 millisieverts of radiation burden but beat-to-beat variation may degrade image quality.
Dynamic volume CT may overcome all these challenges with a 16-centimeter craniocaudal coverage in a solitary rotation lasting less than one heart beat and minimal radiation dose of 2 to 4 millisieverts.8-10 It also eliminates the need of oral and intravenous beta-blockers, used frequently in conventional multislice acquisitions, to achieve a slow, steady heart rate and enables effective imaging of patients in atrial fibrillation. With a serial volumetric imaging of the heart, the assessment of myocardial perfusion has also become feasible.11 Most significantly, it is expected that a substantial reduction in radiation burden would likely and logically broaden the indications of CT angiography for the definitive coronary arterial assessment in asymptomatic but high-risk individuals toward primary prevention of acute coronary events.
Evolution has been defined as a gradual process of change to a more complex and usually better form. Evolution, as a biologic process, has traditionally been described as macroevolution and microevolution driven by natural selection. Microevolution is a variation within a kind, not an upward evolution from simplicity into complexity as supposed by Darwinian evolutionary theory. Macroevolution is the transition from one kind of an organism into another that involves fundamental changes. These changes are unlikely to happen during a single life and require a series of genetic mutations.12
Taking cues from the biologic process of evolution, the advent of dynamic CT technology is a radical improvement over its predecessors and shows greater promise in the assessment of coronary artery and cerebrovascular disease. When appropriately used, it has the potential to replace several tests with one exam and has the capability to lower overall healthcare costs through faster, safer, and accurate diagnoses.
— Jagat Narula, MD, PhD, is a professor of medicine and the chief of cardiology at the University of California, Irvine.
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12. All About Science. What is evolution? Available at: http://www.allaboutscience.org/what-is-evolution-faq.htm