|Fergus V. Coakley, MD|
The development of multislice (multidetector) CT represents a major advance in the ability to rapidly scan large imaging volumes with thin sections. While the temporal resolution of multislice CT is now comparable to MRI, the spatial resolution remains superior to MRI, and can be close to isotropic, and the extent of coverage exceeds MRI. Multislice CT may appear to be simply an incremental improvement in spiral CT technology. This view of multislice CT is incomplete, since the unprecedented capability to rapidly acquire numerous thin sections in multiple phases of enhancement provides both great challenges for image handling and storage, as well as great opportunities for three-dimensional postprocessing and display (Figure 1). It is possible that multislice CT will become the key factor in promoting wide dissemination of PACS technology and three-dimensional soft-copy image interpretation. Multislice CT emerged in the late 1990s, following the introduction of single detector spiral CT in the early 1990s. The technology is spreading rapidly. Already, approximately 40% of the estimated 14,000 CT scanners in the United States are multislice. Of the remainder, 50% are single detector spiral scanners and 10% are conventional scanners. It is expected that more than 85% of new scanners sold in 2002 will be multislice in type (vendor, personal communication). While the clinical ramifications of multislice CT may not become clear for several years, these early indicators of market penetration suggest the impact will be considerable, and therefore a brief overview of the current and potential status of multislice CT is appropriate. This review will discuss the technologic basis of multislice CT in a historical context, current clinical applications of multislice CT, and the possible future impact on clinical practice.
|Benjamin M. Yeh, MD|
CT was invented in 1972 by Godfrey Hounsfield, with initial clinical installations in 1974. The first CT scanner developed by Hounsfield in his laboratory at EMI required several hours to acquire the data for a single slice, and took several days to reconstruct the corresponding image. Data acquisition and image reconstruction became progressively faster during the 1970s and 1980s, although the speed of the scanners remained limited by the need for “stop-start” slice-by-slice acquisition. That is, in conventional CT, an axial slice is generated by rotating an x-ray tube and detector array in a 360° circle around the patient. After a 360° rotation, the rotating gantry reverses direction to prevent disruption of the tethered cables that transfer the data from the detector array to the computer. Such sequential slice acquisition limits the speed of conventional CT, prevents volumetric data acquisition, results in slice misregistration, and limits temporal resolution so that multi-phase volumetric scanning is not possible. The development of spiral (or helical) CT in the late 1980s represented a technologic breakthrough. In spiral CT, data is carried from the rotating gantry to the computer by slip rings, which allow continuous gantry rotation and data transfer. Scanning can be performed while the patient is moved slowly but continuously through the gantry. The ability to continuously scan allows for “non-stop” volumetric data acquisition. Data is gathered on a three-dimensional volume in a spiral fashion. Images are reconstructed from the data volume. Physical and clinical performance studies of spiral CT were first reported at the 1989 Radiological Society of North America meeting. 1 The first commercially available spiral CT scanner was released in 1990. 2
|Figure 1. CT urogram in a patient with right-sided primary ureteropelvic junction obstruction. Contrast was administered in two boluses. An initial bolus was administered in order to opacify the pelvicaliceal systems (asterisks). Asymmetric fullness of the right pelvicaliceal system is evident. Thin-section multislice CT arteriography was performed after administration of a second bolus. The arterial system is well opacified, and an accessory right renal artery (arrow) crossing the ureteropelvic junction is visible.|
Spiral CT was an important technological advance, but many limitations and compromises remained. The rotational speed of the gantry was generally one rotation per second, and clinical results suggested that a pitch (ratio of longitudinal distance moved by the tabletop during one gantry rotation to beam collimation) greater than two was undesirable. As a result, either large volumes could be covered, or thin sections acquired, but not both. Another CT scanner employed a dual array of two side-by-side detector rows, and represented an early version of multislice technology. 3 However, while this scanner was an improvement over single detector spiral CT in terms of coverage, 4 substantial limitations remained. It was not until 1998 that a scanner with four detector rows became commercially available. Modern multislice CT using four or more detector rows essentially abolishes the remaining obstacles to rapid isotropic volumetric imaging by utilizing multiple side-by-side detectors simultaneously. The most recent scanners use up to 16 detector rows. Such multislice CT systems can collect up to four slices of data in about 350 milliseconds and reconstruct a 512 by 512 matrix image from millions of data points in less than a second. An entire body cavity (brain, chest, or abdomen) can be scanned in 5 to 10 seconds using the most advanced multislice CT systems. Faster imaging with modern scanners is due not only to multiple detector rows, but also to increased gantry rotational speed. Many commercial scanners are now capable of a complete gantry rotation in 0.5 seconds or less.
|Figure 2. CT arteriogram in a 26-year-old patient being evaluated as a potential living related right hepatic lobe donor. An aberrant right hepatic artery is seen arising from the superior mesenteric artery. CT arteriography allows such clinically important arterial variants to be identified without the need for invasive catheter angiography.|
Current applications of multislice CT have naturally focused on those areas where rapid near-isotropic volumetric imaging is beneficial, particularly CT angiography (Figure 2). For example, the advantages of multislice CT angiography have been described in the assessment of pancreatic carcinoma, 5 aortoiliac arterial disease,6 and peripheral vascular disease. 7 Nonangiographic applications include the quantification of coronary artery calcium, 8 detection of small hepatic lesions, 9 and virtual colonoscopy. 10 The ability to acquire scans of the abdomen during multiple phases of enhancement is particularly helpful when both arteriographic and parenchymal phases of enhancement are required for complete evaluation. For example, the arteriographic phase is critical for the assessment of arterial encasement in pancreatic cancer, but parenchymal enhancement phases are required for demonstration of the primary tumor and detection of hepatic metastases (Figure 3). Rapid coverage of the chest with thin sections is very valuable during CT pulmonary angiography in patients suspected of pulmonary embolism, because many of these patients have poor breath-holding capacity. Pulmonary angiography has largely replaced ventilation-perfusion scans in many centers that have multislice CT.
|Figure 3A. Multislice CT performed during the arteriographic phase of enhancement in a 67-year-old patient with pancreatic cancer. The superior mesenteric artery (arrow) is well opacified, and is not incased by tumor.|
A potential source of confusion that arises with respect to multislice CT is the definition of pitch. Pitch can be defined in one of two ways: either as the ratio of longitudinal distance moved by the tabletop during one gantry rotation to beam collimation, or the ratio of longitudinal distance moved by the tabletop during one gantry rotation to slice thickness. In single detector spiral CT, these two definitions are the same, since the slice thickness is the beam collimation. In multislice CT, the collimated beam is split into several slices, and therefore the definitions of pitch are different. For example, a four-channel multislice scanner acquiring 2.5-mm-thick slices (beam collimation of 10 mm) at a tabletop speed of 15 mm per 1-second gantry rotation could be defined as having a pitch of 6 (longitudinal distance moved by the table-top during one gantry rotation/slice thickness = 15/2.5 = 6) or 1.5 (longitudinal distance moved by the tabletop during one gantry rotation/beam collimation = 15/10 = 1.5). There is no current consensus on which definition is preferable. While most manufacturers use the ratio of longitudinal distance moved by the table-top during one gantry rotation to slice thickness, there are arguments related to basic science and calculation of radiation dose that suggest the alternative definition is more appropriate. In any event, it is important to clarify which definition of pitch is being utilized when describing CT protocols and parameters for multislice scanners.
One of the unfortunate consequences of the increased ability to obtain multiphase imaging has been a corresponding increase in confusion with respect to the proper terminology for these studies. 11 It has reasonably been suggested that the term “phase” should not be applied to noncontrast images. 12 It also seems reasonable to use the term “arteriographic phase” for early arterial images in which contrast is primarily within the arteries, and the term “parenchymal arterial phase” for the subsequent period of early tissue enhancement when hypervascular masses are most prominent. Later periods of image acquisition can be referred to as “portal venous” or “delayed,” as appropriate. Until a universal terminology is established, it is probably best to avoid unclear terms such as “dual” or “triple” phase studies, and instead describe the specific phases acquired
|Figure 3B (left). Image obtained in a slightly later phase of enhancement, with a greater degree of parenchymal enhancement. The tumor (asterisk) is predominately hypovascular, without hypervascular arterial enhancement that might suggest an atypical tumor such as neuroendocrine neoplasm. Figure 3C (right). Image obtained in the portal venous stage of enhancement shows a small hypodense lesion (arrow) in the liver, which may be a metastasis. The ability to obtain multiple sequential images in different phases of enhancement is a major advantage of multislice CT, as different parameters are optimally evaluated in different phases of enhancement.|
Multislice CT provides us with enormous opportunities for improved diagnostic imaging, and potentially improved patient care. However, this opportunity does not come without a cost, and an associated challenge is dealing with the huge number of images that these scanners can acquire. This has been aptly described as a “data explosion.” 13 Meeting this challenge will require new and innovative ways of data storage and handling. Printed film is no longer a viable option, and will likely soon be associated with the same quaint technologic nostalgia with which we currently regard typewriters. The fact that overlapping images improve disease detection implies that reconstruction or review of fewer images is not a solution. 14 Similarly, the use of thick sections is not an option to reduce the total number of images, because thinner sections are clinically beneficial. The use of 2.5-mm collimation during CT of the liver has been shown to increase the detection rate of small (1 cm or less in diameter) liver lesions by 46% when compared to 10-mm-thick sections. 9
Not only must technology advance to handle and store these very large image files, but the technology must provide physicians with efficient yet accurate methods to analyze these data sets. Concurrent technologic improvements in digital image storage, display, and volumetric reconstruction will likely lead to novel and more efficient methods of image analysis. Three-dimensional postprocessing shows promise, but the current systems require improvements in several areas. In particular, a greater ease of use is required, so that the average radiologist can rapidly perform three-dimensional postprocessing and review. This will succeed only when the current user interfaces become friendlier and more intuitive. Another fundamental requirement is integration of these new methods of image analysis with PACS, rather than the current inefficient situation where axial images are stored or displayed on PACS, while three-dimensional postprocessing takes place separately on a specialized workstation. Full utilization of both technologies requires that these processes be seamlessly fused, and commercial success appears to await the first vendor to provide such a combined system.
In addition to improved three-dimensional postprocessing, computer aided diagnosis (CAD) is likely to be crucial as a means to handle the flood of data created by multislice CT. While the initial clinical application of CAD will likely be in the field of digital mammography, it seems inevitable that this technology will advance into CT, particularly if lung cancer screening and virtual colonoscopy become more widely performed. Novel methods of image postprocessing and analysis may be developed, as illustrated by the use of sliding maximum intensity projection images in the detection of pulmonary nodules. 15
|Figure 4A (left). CT coronary arteriogram obtained using a four-channel multislice CT scanner with a collimation of 1 mm and a tabletop speed of 6 mm per second. Diffused disease is evident within the left coronary arterial system in one particularly severe stenosis (arrow). Figure 4B (right). Corresponding conventional coronary arteriogram confirms the findings, including the severe stenosis (arrow).|
Noninvasive coronary arteriography can be regarded as the holy grail of diagnostic imaging. Over the last decade, substantial advances in the temporal and spatial resolution of MR angiography have suggested that MRI would be the first modality to allow routine noninvasive coronary arteriography. However, the advances offered by multislice technology are such that CT may yet be the first modality to reach this goal (Figure 4).
Multislice CT represents a major advance in CT technology, and the clinical implications are currently emerging. The daunting number of images produced by modern CT scanners represents a major challenge, although the convergence between the development of multislice CT and the technologic advances in digital storage and manipulation of three-dimensional image sets represent a fortuitous technologic confluence that may permanently change the primacy of axial CT imaging. If this viewpoint is correct, the impact of multislice CT on clinical practice may one day be regarded as revolutionary, rather than evolutionary.
Fergus V. Coakley, MD, is associate professor of radiology and chief, Abdominal Imaging, Department of Radiology, University of California San Francisco.
Benjamin M. Yeh, MD, is clinical fellow, Abdominal Imaging, Department of Radiology, University of California San Francisco.
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