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Siemens Medical Solutions, Computed Tomography, Forchheim, Germany
Correspondence and requests for reprints should be addressed to Gerhard Kohl, M.S., Siemens Medical Solutions, CT, 1 Siemens Street, Forchheim 91301, Germany. E-mail: gerhard.kohl{at}siemens.com
ABSTRACT
Computed tomography has revolutionized diagnostic radiology with the introduction of spiral CT in the early 1990s, allowing for the first time the acquisition of volume data without the danger of misregistration or double registration of anatomical details. The next revolution occurred in 1998 when all major CT manufacturers introduced multislice CT (MSCT) systems, which typically offered simultaneous acquisition of four slices, providing considerable improvement towards the goal of isotropic three-dimensional imaging. The most recent generation of MSCT systems acquire 64 slices per rotation, enabling a whole body CTA with 1,500 mm scan range and an isotropic resolution of down to 0.4 mm in only 22– 25 s. The tube and the detector measurement system belong to the most important system components of a CT system, having a large influence on system performance. For example, new rotating envelope tube design principles allow for faster rotation times and for double z-sampling techniques in order to increase resolution. The applied dose is ultimately the limiting factor for the improvement of image quality and increase in isotropic resolution. In order to make best diagnostic use of the applied dose, sophisticated dynamic dose adaptation techniques to patient size and geometry have been developed.
Key Words: computed tomography design • computed tomography principles • multislice computed tomography • spiral computed tomography
Computed tomography (CT) was introduced in the early 1970s and has revolutionized not only diagnostic radiology but also the whole field of medicine. The introduction of spiral CT in the early 1990s constituted a fundamental evolutionary step in the development and ongoing refinement of CT imaging techniques (1, 2). Until then, the examination volume had to be covered by subsequent axial scans in a "step-and-shoot" mode. Axial scanning required long examination times because of the interscan delays necessary to move the table incrementally from one scan position to the next, and it was prone to misregistration of anatomic details (e.g., pulmonary nodules) because of the potential movement of relevant anatomic structures between two scans (e.g., by patient motion, breathing, or swallowing). With spiral CT, the patient table is continuously translated while scan data are acquired. A prerequisite for spiral scanning was the introduction of slip-ring gantries, which eliminated the need to rewind the gantry after each rotation and enabled continuous data acquisition during multiple rotations. For the first time, volume data could be acquired without the danger of misregistration or double registration of anatomic details. Images could be reconstructed at any position along the patient axis (longitudinal axis), and overlapping image reconstruction could be used to improve longitudinal resolution. Volume data became the very basis for applications, such as CT angiography (CTA) (3), which has revolutionized noninvasive assessment of vascular disease. The ability to acquire volume data also paved the way for the development of three-dimensional image processing techniques, such as multiplanar reformations, maximum intensity projections, surface shaded displays, or volume-rendering techniques, which have become a vital component of medical imaging today.
Ideally, volume data are of high spatial resolution and isotropic in nature (i.e., each image data element [voxel] is of equal dimensions in all three spatial axes), as a basis for image display in arbitrarily oriented imaging planes. For most clinical scenarios, however, single-slice spiral CT with 1 s gantry rotation time is unable to fulfill these prerequisites. To avoid motion artifacts and to use the contrast bolus optimally, spiral CT body examinations need to be completed within a certain time frame, ordinarily one patient breathhold (25–30 s). If a large scan range, such as the entire thorax or abdomen (30 cm), has to be covered within a single breathhold, a thick collimation of 5 to 8 mm must be used. Although the in-plane resolution of a CT image depends on the system geometry and on the reconstruction kernel selected by the user, the longitudinal (z-) resolution is determined by the collimated slice width and the spiral interpolation algorithm. Using a thick collimation of 5 to 8 mm results in a considerable mismatch between the longitudinal resolution and the in-plane resolution (ordinarily 0.5–0.7 mm depending on the reconstruction kernel). With single-slice spiral CT, the ideal of isotropic resolution can only be achieved for very limited scan ranges (4).
Strategies to achieve more substantial volume coverage with improved longitudinal resolution include the simultaneous acquisition of more than one slice at a time and a reduction of the gantry rotation time. The ability at that time to decrease rotation times substantially was limited by mechanical forces on the rotating part of the gantry and also by the need to increase X-ray flux accordingly. Because most of the flux the X-ray tube is producing was blocked by the collimation of the X-ray tube window, this flux could be used by multiple detector rows with no additional energy cost. Interestingly, the very first medical CT scanners were two-slice systems, such as the EMI head scanner (EMI, London, UK) introduced in 1972 or the Siemens SIRETOM introduced in 1974 (Siemens Medical Solutions, Erlangen, Germany). With the advent of whole-body fan beam CT systems for general radiology, two-slice acquisition was no longer used. Apart from a dedicated two-slice system for cardiac applications, the IMATRON C-100 (General Electric Healthcare, Waukesha, WI), introduced in 1984, the first step toward multislice acquisition in general radiology was a two-slice CT scanner introduced in 1993 (Elscint TWIN; Philips Medical Systems, Best, The Netherlands). In 1998, all major CT manufacturers introduced multislice CT (MSCT) systems, which typically offered simultaneous acquisition of four slices at a rotation time of 0.5 s, providing considerable improvement of scan speed and longitudinal resolution and better use of the available X-ray power (5–7). These developments were quickly recognized as revolutionary improvements that would eventually enable users to do real isotropic three-dimensional imaging. Consequently, all vendors pushed toward more and more slices, effectively rendering the number of slices into the most important performance characteristic of a CT scanner.
Simultaneous acquisition of M slices results in an M-fold increase in speed if all other parameters, such as slice thickness, are unchanged. This increased performance of MSCT compared with single-slice CT allowed for the optimization of a variety of clinical protocols. The examination time for standard protocols could be significantly reduced, which proved to be of immediate clinical benefit for the quick and comprehensive assessment of trauma victims and noncooperative patients, and for breathhold imaging. Alternatively, the scan range that could be covered within a certain scan time (e.g., breathhold time) was extended by a factor of M, which is relevant for oncologic staging or for CT angiography with extended coverage, for example of the lower extremities (8) or the coverage of the complete lung. The most important clinical benefit, however, proved to be the ability to scan a given anatomic volume within a given scan time with substantially reduced slice width, at M times increased longitudinal resolution. This way, for many clinical applications the goal of isotropic resolution was within reach with four-slice CT systems. Examinations of the entire thorax (9) or abdomen could now routinely be performed with 1- or 1.25-mm collimated slice width. MSCT also dramatically expanded into areas previously considered beyond the scope of third-generation CT scanners based on the mechanical rotation of X-ray tube and detector, such as cardiac imaging with the addition of ECG gating capability. With a gantry rotation time of 0.5 s and dedicated image reconstruction approaches, the temporal resolution for the acquisition of an image was improved to 250 ms and less (10, 11), which proved to be sufficient for motion-free imaging of the heart in the mid- to end-diastolic phase at slow to moderate heart rates. With four simultaneously acquired slices, coverage of the entire heart volume with thin slices and ECG-gating within a single breathhold became feasible, enabling noninvasive visualization of the coronary arteries (12–15).
Despite all these promising advances, clinical challenges and limitations remained for four-slice CT systems. True isotropic resolution for routine applications had not yet been achieved, because the longitudinal resolution of about 1 mm does not fully match the in-plane resolution of about 0.5 to 0.7 mm in a routine scan of the chest or abdomen. For large volumes, such as CTA of the lower extremity run-off (8), thicker (e.g., 2.5 mm) collimated slices had to be chosen to complete the scan within a reasonable time frame. Scan times were often too long to allow image acquisition during pure arterial phase. For a CTA of the circle of Willis, for instance, a scan range of about 100 mm must be covered. With four-slice CT, at 1-mm collimated slice width, pitch of 1.5, and 0.5-s gantry rotation time, this volume can be covered in about 9-s scan time, not fast enough to avoid venous overlay assuming a cerebral circulation time of less than 5 s. For ECG-gated coronary CTA, stents or severely calcified arteries constituted a diagnostic dilemma, mainly because of partial volume artifacts as a consequence of insufficient longitudinal resolution (15).
As a next step, the introduction of an eight-slice CT system in 2000 enabled shorter scan times, but did not yet provide improved longitudinal resolution (thinnest collimation 8 x 1.25 mm). The latter was achieved with the introduction of 16-slice CT (16, 17), which made it possible routinely to acquire substantial anatomic volumes with isotropic submillimeter spatial resolution. Improved longitudinal resolution goes hand in hand with considerably reduced scan times that enable high-quality examinations in severely debilitated and severely dyspneic patients (Figure 1). For patients with suspicion of ischemic stroke, both the status of the vessels supplying the brain and the location of the intracranial occlusion can be assessed in the same examination (18). Additional brain perfusion CT permits differentiation of irreversibly damaged brain tissue from reversibly impaired tissue at risk (19). Examining the entire thorax (350 mm) with submillimeter collimation requires a scan time of approximately 11 s. Because of the short breathhold time, central and peripheral pulmonary embolism can be reliably and accurately diagnosed (20, 21). A 16-slice CT enables whole-body angiographic studies with submillimeter resolution in a single breathhold. Compared with invasive angiography, the same morphologic information is revealed (22, 23). ECG-gated cardiac scanning with 16-slice CT systems benefits from both improved temporal resolution achieved by gantry rotation times down to 0.375 s and improved spatial resolution.
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With the most recent generation of CT systems, CT angiographic examinations with submillimeter resolution in the pure arterial phase will become feasible even for extended anatomic ranges. A CTA of the carotid arteries and the circle of Willis with 64 x 0.6 mm collimation, 0.375-s rotation time, and pitch 1.5 requires only 5 s for a scan range of 350 mm (see the clinical example in Figure 2 [p. 499]). A whole-body submillimeter CTA with 1,500-mm scan range, 64 x 0.6 mm collimation, 0.375-s rotation time, and pitch 1.2 to 1.4 is completed in only 22 to 25 s.
Very useful up-to-date information regarding MSCT is also readily available on the Internet—for example, at the U.K. Medical Devices Agency (MDA) CT website, www.medical-devices.gov.uk or at www.ctisus.org.
PRINCIPLES OF A STATE-OF-THE-ART MSCT SYSTEM DESIGN
The fundamental demands on a modern volume scanner can be summed up in the following two requirements: continuous data acquisition (the possibility to reconstruct images at any z-position); and ability to scan a long distance in short time without compromising longitudinal (z-) resolution. The first requirement calls for a spiral acquisition. The breakthrough of spiral MSCT since 1998 is caused by the fact that it is able to fulfill the second requirement: the maximum z-resolution is defined by the detector size alone rather than by a combination of spiral pitch and detector collimation as in single-slice spiral. Moreover, multislice spiral scanners provide the new feature of allowing specification of the z-resolution in the reconstruction phase, after the scan has been done.The technical challenges of MSCT are manifold. A detector capable of measuring several thousand channels at a time has to be built; the data have to be transferred to the image reconstruction system; and a suitable reconstruction algorithm has to be devised.
The basic system components of a modern third-generation CT system are shown in (Figure 3). Third-generation CT scanners use the so-called rotate–rotate geometry, in which both the X-ray tube and the detector rotate about the patient (Figure 4). In an MSCT system, the detector comprises several rows of 700 and more detector elements that cover a scan field of view of usually 50 cm. The X-ray attenuation of the object is measured by the individual detector elements. All measurement values acquired at the same angular position of the measurement system are called a projection of view. Typically, 1,000 projections are measured during each 360° rotation.
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State-of-the-art X-ray tube and generator combinations provide a peak power of 60 to 90 kW, usually at various, user-selectable voltages (e.g., 80, 100, 120, and 140 kV). Different clinical applications require different X-ray spectra and hence different kilovolt settings for optimum image quality or best possible signal-to-noise ratio at lowest dose. As an example, CT angiographic examinations generally benefit from lower tube voltage, resulting in a higher contrast resolution for CTA using iodine-based contrast material.
In a conventional tube design, an anode plate of typically 160- to 220-mm diameter rotates in a vacuum housing (Figure 5, top). The heat storage capacity of anode plate and tube housing (measured in megaheat units) determines the performance level: the bigger the anode plate, the larger the heat storage capacity, and the more scan seconds can be delivered until the anode plate reaches its temperature limit. Typically, a conventional state-of-the-art X-ray tube has a heat storage capacity of 5 to 9 megaheat units, realized by thick graphite layers attached to the backside of the anode plate. The heat produced in the anode during X-ray emission is mainly dissipated by thermal radiation, and only a small percentage by thermal conduction (see Figure 5, top [p. 499]). Constructive efforts aim at increasing both heat storage capacity and heat dissipation rate (e.g., by increasing the anode diameter, by using circular grooves in the anode support to increase the contact area for improved cooling, or by using special liquid-metal vacuum bearings that allow for a faster anode rotation). An alternative design is the so-called rotating envelope tube (Straton; Siemens, Forchheim, Germany; see Figure 5, bottom, for a schematic drawing, and Figure 6 [p. 500] for a picture). The anode plate constitutes an outer wall of the rotating tube housing; it is in direct contact with the cooling oil and can be effectively cooled by thermal conduction. This way, a very high heat dissipation rate of 5 megaheat units/min is achieved, eliminating the need for heat storage in the anode, which consequently has a heat storage capacity close to zero. Because of the fast anode cooling, rotating envelope tubes can perform high-power scans in rapid succession. With the Straton tube, five full-power 10-s spirals are possible within 100 s. Ultimately, the performance of both conventional and rotating envelope tubes is limited by the maximum heat dissipation of the CT system itself. Because there are no moving parts and no bearings in the vacuum, the tube design can be small and compact (anode diameter, 12 cm) and has the potential better to withstand the high gravitational forces associated with gantry rotation times of less than 0.4 s. Because of the central rotating cathode, permanent electromagnetic deflection of the electron beam is needed to position and shape the focal spot on the anode. The versatile electromagnetic deflection is a prerequisite for the double z-sampling technology used in a recently introduced 64-slice CT system.
With increasing number of detector rows and decreasing gantry rotation times, the data transmission systems of MSCT scanners must be capable of handling significant data rates: a four-slice CT system with 0.5-s rotation time roughly generates 1,000 x 700 x 4 x 2 bytes = 5.6 megabytes (MB) of data per rotation, corresponding to 11.2 MB/s; a 16-slice CT scanner with the same rotation time generates 45 MB/s; and a 64-slice CT system can produce up to 180 to 200 MB/s. The acquired signal of every detector element is during this process digitized by analog-digital (A/D) converters in the submillisecond range with typically 16-bit resolution.
This stream of data is a challenge for data transmission off the gantry and for real-time data processing in the subsequent image reconstruction systems. In modern CT systems, contactless transmission technology is generally used for data transfer, which is either laser transmission or electromagnetic transmission with a coupling between a rotating transmission ring antenna and a stationary receiving antenna. In the image reconstruction computer images are reconstructed at a rate of up to 40 images/s using special array processors.
Modern CT systems generally use solid-state detectors. Each detector element consists of a radiation-sensitive solid-state material (e.g., cadmium tungstate or rare earth-based material such as gadolinium-oxide, gadolinium oxi-sulfide, or yttrium-gadolinium- oxide with suitable dopings), which converts the absorbed X-rays into visible light. The light is then detected by a Si-photodiode. The resulting electrical current is amplified and converted into a digital signal. Key requirements for a suitable detector material are good detection efficiency and very short afterglow time to enable the fast gantry rotation speeds that are essential for ECG-gated cardiac imaging. Gas detectors, such as the Xe-detectors used in previous generations of single-slice CT systems, have become obsolete because of their inherent limitations. To achieve acceptable detection efficiency, the Xe-chambers had to be designed deep enough to absorb and convert most of the X-ray quanta. This resulted in great challenges to manufacture such detectors, especially in combination with a multirow design.
A CT detector must provide different slice widths to adjust the optimum scan speed, longitudinal resolution, and image noise for each application. With a single-slice CT detector, different collimated slice widths are obtained by prepatient collimation of the X-ray beam (Figure 7).
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For the four-slice CT systems introduced in 1998, two detector types have been commonly used. The fixed-array detector consists of detector elements with equal sizes in the longitudinal direction. A representative example for this scanner type, the GE Lightspeed scanner (General Electric Healthcare), has 16 detector rows, each of them defining 1.25-mm collimated slice width in the center of rotation (6). The total coverage in the longitudinal direction is 20 mm at isocenter; because of geometric magnification the actual detector is about twice as wide. By prepatient collimation and combination of the signals of the individual detector rows, the following slice widths (measured at isocenter) are realized: 4 x 1.25 mm, 4 x 2.5 mm, 4 x 3.75 mm, 4 x 5 mm (Figure 8, top left). The same detector design is used for the eight-slice version of this system, providing 8 x 1.25– and 8 x 2.5–mm collimated slice width.
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The selection of the collimated slice width determines the intrinsic longitudinal resolution of a scan. In a step-and-shoot axial mode, any multiple of the collimated width of one detector slice can be obtained by adding the detector signals during image reconstruction. In a spiral mode, the effective slice width, which is usually defined as the full width at half maximum of the spiral slice sensitivity profile, is adjusted independently in the spiral interpolation process during image reconstruction. Hence, from the same dataset, both narrow slices for high-resolution detail or for three-dimensional postprocessing and wide slices for better contrast resolution or quick review and filming may be derived.
The established 16-slice CT systems have adaptive array detectors in general. A representative example for this scanner type, the Siemens SOMATOM Sensation 16 scanner, uses 24 detector rows (16, 17). The 16 central rows define 0.75-mm collimated slice width at isocenter, the four outer rows on both sides define 1.5-mm collimated slice width (Figure 8, bottom left). The total coverage in the longitudinal direction is 24 mm at the isocenter. By appropriate combination of the signals of the individual detector rows, either 12 or 16 slices with 0.75- or 1.5-mm collimated slice width can be acquired simultaneously. The GE Lightspeed 16 scanner (General Electric Healthcare) uses a similar design, which provides 16 slices with either 0.625- or 1.25-mm collimated slice width. The total coverage in the longitudinal direction is 20 mm at the isocenter. Yet another design, which is implemented in the Toshiba Aquilion scanner (Toshiba Medical Systems Corp., Tokyo, Japan), allows the use of 16 slices with 0.5-, 1-, or 2-mm collimated slice width, with a total coverage of 32 mm at isocenter.
In 2004, the latest generation of MSCT systems providing more than 16 slices was introduced. The Siemens SOMATOM Sensation 64 scanner has an adaptive array detector with 40 detector rows. The 32 central rows define 0.6-mm collimated slice width at the isocenter, the four outer rows on both sides define 1-mm collimated slice width (Figure 8, top right). The total coverage in the longitudinal direction is 28.8 mm. Using a periodic motion of the focal spot in the z-direction (z-flying focal spot), two subsequent 32-slice readings with 0.6-mm collimated slice width are slightly shifted in the z-direction and combined to one 64-slice projection with a sampling distance of 0.3 mm at the isocenter. With this technique, 64 overlapping 0.6-mm slices/rotation are acquired. Alternatively, 24 slices with 1.2-mm slice width can be obtained. The Philips Brilliance 40 scanner (Philips Medical Systems) provides 40 slices with 0.625-mm collimation or 32 slices with 1.25-mm collimation, with coverage of 40 mm at the isocenter (Figure 8, center right). Both Toshiba and GE use fixed-array detectors for their systems. The Toshiba Aquilion scanner has 64 detector rows with a collimated slice width of 0.5 mm. By appropriate combination of the signals of the individual detectors, 32 slices with either 0.5- or 1-mm collimated slice width can be acquired simultaneously. The total z-coverage at the isocenter is 32 mm. The GE VCT scanner has 64 detector rows with a collimated slice width of 0.625 mm, enabling the simultaneous read-out of 64 slices (Figure 8, bottom right), with a total coverage of 40 mm in the longitudinal direction. As a representative example, Figure 9 (p. 500) shows a picture of a detector module of the SOMATOM Sensation 64. Each module consists of 40 x 16 detector pixels and the corresponding electronics. The antiscatter collimators are diagonally cut to open the view on the detector ceramics.
DOSE AND DOSE REDUCTION
With MSCT, a certain dose increase compared with single-slice CT is unavoidable because of the underlying physical principles. The collimated dose profile is a trapezoid in the transverse direction. This is a consequence of the finite length of the focal spot and the prepatient collimation. In the plateau region of the trapezoid, X-rays emitted from the entire area of the focal spot illuminate the detector. In the penumbra regions, only a part of the focal spot illuminates the detector, whereas other parts are blocked off by the prepatient collimator. With single-slice CT, the entire trapezoidal dose profile can contribute to the detector signal and the collimated slice width is determined as the full width at half maximum of this trapezoid. With MSCT, only the plateau region of the dose profile may be used to ensure equal signal level for all detector slices. The penumbra region has to be discarded, either by a postpatient collimator or by the intrinsic self-collimation of the multislice detector, and represents wasted dose. The relative contribution of the penumbra region increases with decreasing slice width, and it decreases with increasing number of simultaneously acquired slices. This is demonstrated by Figure 10, which shows the minimum width dose profiles for a four-slice CT system and a corresponding 16-slice CT system with equal collimated width of one detector slice. Correspondingly, the relative dose utilization of a representative four-slice CT scanner (SOMATOM Sensation 4) is 70% for 4 x 1–mm collimation and 85% for 4 x 2.5–mm collimation. A comparable 16-slice CT system (SOMATOM Sensation 16) has an improved dose use of 76%, respectively, 82% for 16 x 0.75–mm collimation, and 85%, respectively, 89% for 16 x 1.5–mm collimation, depending on the size of the focal spot (large or small).
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FUTURE PERSPECTIVE OF MSCT
Because of its ease of use and its common availability, general-purpose CT will evolve into the most widely used diagnostic modality for routine examinations, especially in emergency situations or for oncologic staging. CT gives morphologic information only; in combination with other modalities, however, functional and metabolic information can be additionally obtained. This is an important step on the way toward a "one-stop shop" examination; combined systems will gain increasing importance in the near future. The combination of state-of-the art MSCT with positron emission tomography scanners, for instance, opens a wide spectrum of applications, ranging from oncologic staging to comprehensive cardiac examinations.
For general-purpose CT, there will be a moderate increase of the number of simultaneously acquired slices in the near future; the resulting clinical benefits, however, may not be substantial and have to be carefully considered in the light of the necessary technical efforts. Potential further improvement of the spatial resolution will have to be reserved to special applications because of the inevitable increase of dose that has to be applied for adequate signal-to-noise ratio. It will have to go along with the development of more powerful X-ray tubes and generators. Instead of a mere quantitative enhancement of scan parameters with doubtful clinical relevance, the introduction of area detectors large enough to cover entire organs, such as the heart, the kidneys, the brain, or a substantial part of a lung, in one axial scan (
120-mm scan range) could bring a new quality to medical CT. With these systems, dynamic volume scanning would become possible, and a whole spectrum of new applications, such as functional or volume perfusion studies, could arise. Area detector technology is currently under development, yet no commercially available solution so far fulfills the high requirements of medical CT concerning dynamic range of the acquisition system and fast data read-out. The combination of area detectors with sufficient quality with fast gantry rotation speeds is a promising technical concept for medical CT systems.
FOOTNOTES
The color figures for this article are on pp. 499–500.
Conflict of Interest Statement: G.K. is an employee of Siemens Medical Solutions. He personally is not in a position within the company to have a conflicting, direct financial interest.
(Received in original form August 5, 2005; accepted in final form September 21, 2005)
REFERENCES
Related articles in Proceedings of the American Thoracic Society:
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