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Multi-slice Technology in Computed Tomography
Clinical Radiology (2001) 56: 302±309 doi:10.1053/crad.2000.0651, available online at http://www.idealibrary.com on
Multi-slice Technology in Computed Tomography P E T E R D AW S O N , W I L L I A M R . L E E S Department of Imaging, UCL Hospitals, London, U.K. Received: 30 June 2000
Revised: 28 September 2000 Accepted: 28 September 2000
Multi-slice systems represent a considerable advance in CT and will assure the future of the technique for many years to come. This article describes this new technology, indicating its provenance and its position in the evolution of CT. While it does not seek to be a physics and engineering text, enough detail of these are given to allow an informed discussion of the many advantages and a few potential problems associated with the technology. A discussion of a number of applications and a brief consideration of contrast enhancement regimens and the possible need for their modi®cation are presented. Dawson, P. & Lees, # 2001 The Royal College of Radiologists W. R. (2001). Clinical Radiology 56, 302±309. Key words: helical/spiral, computed tomography, multi-slice.
When ®rst introduced nearly 30 years ago [1], computed tomography (CT) ushered in a new paradigm in X-ray imaging and was a considerable in¯uence in the later development of magnetic resonance imaging (MRI). MRI, with its high contrast sensitivity and resolution and lack of ionizing radiation, then looked set soon to supplant CT almost completely, if not in quite the short time scale envisaged by MRI's more enthusiastic proponents. The advent of spiral/helical CT [2,3,4] changed this outlook somewhat. There had been a logical developmental progression from ®rst generation linear and rotary movements pencil beam systems (Fig. 1a), to second generation ( fewer) linear and rotary movements fan beam systems (Fig. 1b), to third generation wider angle fan beam systems with no linear but, rather, continuous rotary movement of tube and detector (Fig. 1c), to fourth generation complete 3608 detector ring and moving tube-only systems (Fig. 1d). Now, in the spiral/helical systems, the table/patient moved continuously (in the z-direction) during rotation of the tube so that a whole volume, rather than serial discrete slices, could be acquired in one complex movement (Fig. 2). Single-slice acquisition times had decreased in the course of this sequence of developments from 5 min to less than 1 s. Of course, demands on X-ray tubes, and on the mathematician, increased at each step. While various arguments were advanced to the eect that spiral technology could be associated with a reduced radiation dose to the patient, CT in both old and new forms still undoubtedly represented a signi®cant radiation burden [5]. In fact, it represented the largest contribution of all diagnostic procedures and the greatest single contribution to the Author for correspondence and guarantor of study: Prof. Peter Dawson, Department of Imaging, The Middlesex Hospital, Mortimer Street, London W1N 8AA, U.K. Fax: 01494 728222; E-mail: [email protected] 0009-9260/01/040302+08 $35.00/0
non-natural total population burden and genetically signi®cant dose. However, it now oered such power and versatility by way of acquisition of large anatomical volumes in a single breath-hold; examination of smaller anatomical volumes at high spatial resolution, seamless volume data sets, and the capacity to perform meaningful `multi-phase' CT studies, that its future, compared to MRI or any other technique, was assured. What further progress could be made? Rotation times, already under 1 s, could be reduced to perhaps 0.5 s but, for mechanical and electronic reasons, probably to not much less. The `centrifugal' force acting on the X-ray tube during a 0.5 s rotation exceeds 10 g. Higher speeds would require the development of a ®xed anode system, which is impractical. The other obvious evolutionary change was to increase the number of detectors by introducing a multiple contiguous detector arc system and to utilize a beam which is also `fanned' in the z-direction ( patient axis) to a degree depending on how many detector arcs are used (Fig. 3). This immediately demands another leap in the mathematical demands of image reconstruction and introduces a number of other diculties discussed below. The ®rst step in this direction was taken by Elscint with its `Twin' machine in 1993 which had just two contiguous detector arcs. This development led to a halving of any scan time, all other things being equal, since it may be seen as either acquiring two slices at a time or as covering twice the z-axis distance per rotation. This `multi-slice' technique [6±10] has been extended by several commercial companies (Table 1) to four simultaneous slices. Actually, all the commercial systems employ many more than four contiguous detector arcs in their systems, ranging from eight to 34, but, for reasons discussed below, only a maximum of four contiguous slices can be selected for acquisition in practice. These systems will be # 2001 The Royal College of Radiologists
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Fig. 1 ± Four generations of incremental CT machines. (a) First generation linear and rotating pencil beam system; (b) linear and rotating fan beam system; (c) rotation only (tube and detectors) wide angle fan beam system; (d) fourth generation 3608 detector ring-rotating tube system.
Table 1 ± Manufacturers of current multi-slice CT systems Manufacturer
System name
Detector array
General Electric
Lightspeed
Matrix array (20 mm)
Siemens
Somatom Plus 4 Volume Zoom
Adaptive array (20 mm)
Marconi
MX 8000
Adaptive array (20 mm)
Toshiba
Aquilion
Matrix/Adaptive array (32 mm)
discussed in a little more detail below but just one example of what is achievable is described here. In such a `multislice' system with four slice acquisitions and with a 0.5-s rotation, the scanning speed will be eight times that of a current state-of-the-art 1-s rotation single detector ring spiral/helical system. The liver, for example, could be examined with, say, 5-mm collimation (z-axis resolution) in less than 6 s. In fact, for a variety of reasons which will be touched on below, not quite this advance in speed should usually be
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Fig. 2 ± The spiral/helical CT principle. The patient moves continuously in the axial (z) direction during rotation of the gantry. A single detector arc is used and if there is no table movement a single CT slice is obtained during a gantry rotation of thickness determined by the collimation used. Fig. 4 ± Pro®le of the various detector ring geometries used in the commercial systems. (a) Consists of 16 identical detector rings (matrix array); (b) and (c) exploit ring detector widths of variable size [`adaptive' (b) and `matrix/adaptive' (c) arrays] ± see discussion.
Fig. 3 ± The detector geometry of a multi-slice spiral/helical CT system. Sixteen detector rings are illustrated. The size of the `cone angle', y, is exaggerated.
sought in practice but the potential is, nevertheless, remarkable. DETECTOR GEOMETRY
Three dierent detector array geometries are used by dierent manufacturers (Fig. 4). Those used by Marconi and Siemens are identical, the former having acquired Elscint technology by take-over and the latter via a research collaboration. The dierent designs have an eect on the minimum slice thickness available and the number of slices available at this minimum width, the range of choice of slice thickness and the maximum volume/z-axis distance which may be scanned in any one system rotation. The slice thickness and number of contiguous slices (up to four) are chosen by beam collimation and by electronic selection and/or summation of detector signals. As an example, consider the con®guration in Fig. 4a. Each detector ring is 1.25 mm wide in the z-direction. The four central rings may be selected to give a 4 1.25 mm simultaneous slice acquisition, or signals from pairs of
contiguous rings can be summed to allow 4 2.5 mm simultaneous slice acquisitions; summing the signals of three and four detector rings yields 4 3.75 and 4 5 mm simultaneous slice acquisitions, respectively. By taking the signals of eight together (8 1.25 mm 10 mm), on either side of the mid-line, 2 10 mm simultaneous slices may be obtained. Beam collimation allows the selection of half of each detector yielding a 2 0.625 mm slice acquisition. Geometrical considerations dictate that there is a diculty with detector array designs such as this. Figure 5 may be taken to be the 16 1.25 mm system con®guration. This shows that only for the innermost detector arcs are the X-rays close to perpendicular to the z-axis. For the outer detector arcs the rays fall more obliquely and, during rotation are `smeared' within the patient in a double cone. The so-called cone angle, y, in Fig. 5 is in fact about 1 degree and so is greatly exaggerated in this drawing to emphasis the point. As shown in the right half of Fig. 5, if the outer detector (here 1.25 mm) bands are selected, the broadening during rotation results in an eective slice thickness of some 3 mm. This may be shown by simple geometrical considerations and calculations based on Fig. 5. This is unfortunate since it means we cannot in practice perform, say, 16 1.25 mm simultaneous slices using system 4 (Fig. 4a). However, combining the signals of groups of four (4 5 mm) results in much reduced distortion of the selected nominal 5-mm slice thickness as also shown in the left half of Fig. 5. These geometrical considerations clearly indicate the considerable, though not necessarily insurmountable, diculties involved in building systems with more detector arcs covering a greater z-axis distance because of the distorting eects of increasing the cone angle, y, and it is clear why there are limitations on the choice and/or number of simultaneous contiguous slices which may be selected for simultaneous acquisition. With this immutable geometrical framework in mind, two manufacturers, Siemens and Marconi have developed an `adaptive' array detector (AAD; Fig. 4b). Here the
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Table 2 ± Slice width combinations possible with the various systems General Electric 2 0.625 4 1.25 4 2.5 4 3.75 45 2 10
Siemens/Marconi
Toshiba
4 0.5 41 4 2.5 45 28 2 10
41 42 43 48
addition to the detector material and its properties, it is clearly dependent on such novel factors as the size of insulator gaps between detector arcs (an eective `dead space') as well as on the total number of detectors. PITCH
The concept of pitch in spiral systems has come to be well understood. With single-slice systems it is de®ned as: Pitch
Table movement per rotation Collimation (slice thickness)
An alternative de®nition, adopted for multi-slice systems by Siemens, GE and Toshiba, but not Marconi, is: Pitch0 Fig. 5 ± If too many simultaneous slices are selected, the outer ones will be considerably distorted with loss of eective spatial (z-axis) resolution, as shown in the right half. Where thicker slices are selected the distortion is minimized, as shown in the left half.
detectors are of increasing z-axis width the further they lie from the centre (y 0) of the array. Another manufacturer, Toshiba, has made a less dramatic step in this direction with its array (Matrix/Adaptive) (Fig. 4c) which is also larger, 32 mm as opposed to 20 mm for the other manufacturers. Table 2 shows simultaneous slice thickness acquisitions possible with the various systems. The 2 0.5 mm and 4 1 mm selections are obtained using collimation allowing radiation to fall on half of each central detection array and on the medial two-thirds of the 1.5 mm detector arc. Four 2.5 mm selections are made by collimation, allowing radiation to fall on the two 2.5 mm rings and on the two 1 and 1.5 mm rings; the signals from the latter being combined electronically. Quite how tolerable the distortions introduced by the greater cone angle of this system will be with some slice selections remains to be seen. This is a highly technical area but some generalization may be made. The adaptive arrays do not represent a complete response to the slice width distortion problem and, indeed, if detector arrays covering greater z-axis distances are yet to be developed, the matrix rather than the adaptive arrays will be the basis. The whole matter of detector array eciency of these various systems remains to be determined, especially as, in
Table movement per rotation Detector z ÿ collimation
The existence of two de®nitions holds the potential for confusion. Some examples may help: (1) If 4 5 mm ( 20 mm) slices are obtained with a table speed of 20 mm per rotation, then, on the ®rst de®nition the pitch is 20/20 1. On the second de®nition, it is 20/5 4. (2) If 2 10 mm ( 20 mm) slices are obtained with a table speed of 20 mm per rotation, then, on the ®rst de®nition the pitch is 20/20 1. On the second de®nition, it is 20/10 2. It is the ®rst de®nition which must be used as an indicator of dose, as will be discussed later. As with single slice spiral, the image quality declines as pitch (however de®ned) increases, though in a non-linear manner. This is a factor which sets a practical limit to some of the exaggerated theoretical claims being made for the speeds of these systems, as will be discussed further. The pitch also in¯uences slice pro®le [11,12]. An illustration of some dierent pitches in a four-multi-slice system is shown in Fig. 6. EXAMINATION SPEEDS
We have seen that for four-slice mode the data acquisition time may be simply one-quarter times that of a single-slice spiral CT system set at the same slice
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of image quality), intermediately high-speed CT covering moderately large volumes in short periods (while maintaining image quality) and high z-axis resolution CT (isotropic voxels) of more modest volumes in short times. An outline of some applications under these headings is given below.
Ultra High-speed CT Fig. 6 ± An illustration of some dierent pitches. At higher pitch, overlap is eliminated. At lower pitch it is an important factor ± see discussion.
collimation and with the same rotation speed. If the multislice CT system has a 0.5-s rotation speed, but the single slice CT machine had a rotation speed of 1 s then a theoretical increase of speed of eight times is available. However, as we have also seen, it may be best in the interests of image quality optimization to use a pitch of, say, three rather than four and this will yield a six times increase in speed. Hu et al. [12] have shown in studies on one commercial system that a two to three times increase in volume acquisition rate as compared with a single-slice system is fully compatible with comparable image quality. Thereafter some loss is entailed in increasing acquisition speed. The top speeds available should only be used in specialist applications (see below). The real `speed' advantage of these systems lies in their ability to obtain more modest volume studies at high resolution during, where appropriate, a single breath-hold. Reconstruction times range from 0.5 to 2 s per image for axial slice reconstructions. Such rapid reconstructions are vital given the huge amount of data and number of images which may be generated. For example, suppose a multi-slice scanner has a rotation time of 0.5 s and a reconstruction time per image of 0.5 s and is used to perform a 4 2.5 mm scan of a 40 cm z-axis body length. If axial slices are reconstructed at 0.5 mm, then we can say: Image acquisition time is (400/4 2.5) 0.5 20 s Number of images is 400/(2.5) 160 Reconstruction time is 160 0.5 80 s. When higher z-axis resolution scans are performed these numbers go up but it is clear that the scanning and reconstruction times (leaving aside any o-line sophisticated work) are short. Within well-funded and well-staed CT units in some countries patient throughput may be signi®cantly increased. In the NHS this advantage will probably not be exploited since most time is spent getting the patients to the CT unit and preparing them. CLINICAL APPLICATIONS
Headline top speeds of multi-slice instruments are impressive and there are some applications where it is useful to run them at their limits. However, in general it is more sensible to view them as oering an unprecedented trade-o between ultra-high speed CT (with some sacri®ce
Though it involves some loss of image quality, this may be useful in such circumstances as multiple trauma cases and unco-operative patients. For example, using 4 5 mm slice selection and a pitch ( prime) of 6 (entailing some image quality loss), and with a 0.5-s rotation time, a 1200-mm long (z-axis) body segment can be scanned in only 20 s. Or, to take another example, the lungs (300 mm) may be scanned at 4 5 mm slice selection with the same pitch of 6 and 0.5 s rotation time can be scanned in only 5 s, obviously in a single breath-hold.
Intermediately High-speed CT Here image quality is not sacri®ced by use of a pitch0 of, say, 3. With 4 5 mm slice selection and 0.5-s rotation time, pelvis, abdomen and chest (say 700 mm) may be covered in only [700/(4 5)] 0.5 4/3 23 s. Such a protocol would provide a remarkably good whole-body survey examination on a single breath-hold. It could, of course, be timed with respect to the infusion of contrast medium so as to cover the liver during the portal venous enhancement phase.
High Z-axis Resolution CT (Isotropic) Any number of examples might be given but consider: A high resolution (isotropic) CT examination of the thorax with 4 1 mm slice selection at 0.5-s rotation time and pitch ( prime) 6 would allow coverage of moderate z-axis distance of 300 mm in 25 s. For the mediastinum, 5-mm slice reconstructions can be made and if high resolution examination of the lungs is required, 1.5±2 mm slice reconstructions could be made by slice summation/averaging. The original isotropic data set can be used for three-dimensional reconstructions including virtual bronchoscopy and endoscopy, or for HRCT of the lungs in any plane if desired. Similarly, an upper abdominal CT examination (150 mm length) could be carried out to include, say, pancreas and kidneys using similar parameter choices in less than 15 s. The isotropic data could be used to generate high-quality CTA, including renal arteriography, if acquired at a suitable time with respect to the administration of contrast medium. The liver alone could be examined at 4 5 mm in less than 5 s and at 4 2.5 mm in less than 10 s. Consequently, a hepatic arterial phase CT study can be achieved which is truly likely to be completed in this phase throughout and this can be followed after a suitable delay by portal venous phase imaging.
MULTI-SLICE TECHNOLOGY IN COMPUTED TOMOGRAPHY
Of course, all this can only be achieved if the timing of data acquisition with respect to the administration of contrast is accurate and in this regard bolus timing software will be invaluable. RECONSTRUCTIONS
These systems are very demanding as regards the requirements on image reconstruction algorithms. The conventional 1808 and 3608 interpolation approaches used in single-slice CT will simply not do when applied to multislice data sets. Many artefacts of interpolation are introduced and the variation in z-axis sensitivity for dierent slices inevitably associated with the ®nite cone angle is problematic. New interpolation algorithms have been developed by the manufacturers. These tend to work best when applied to data obtained using certain pitch selections. Consequently, manufacturers' advice on pitch and algorithm combinations should be taken at least until experience is gained. A steep learning curve in optimization of the use of these CT systems seems in prospect. The speed of implementation of the reconstruction algorithms is of the order of 0.5±2 s per slice. IMAGE QUALITY
Broadly speaking, the image quality of a reconstructed axial slice should be much the same whether from a singleor multiple-slice instrument but some caveats must be entered. The variable z-axis sensitivity for dierent slices and the need, ideally, to match choice of pitch to reconstruction algorithm have been alluded to. The manufacturers of the GE system, for example, oer a choice between two pitch/algorithm combinations: a pitch0 ( pitch 0.75) of 3 for optimum image quality (HQ mode) and a pitch0 ( pitch 1.5) of 6 for speed (HS mode). Generally speaking, a pitch less than 4 ( pitch0 less than 1) is required to obtain single slice spiral image quality equivalence. While multi-slice spiral interpolation artefacts are an important issue, especially in tissues with rapid z-axis direction change and in patient or organ movement, the increased system speed tends to mitigate these eects. One point of considerable importance is that with the higher z-axis spatial resolution acquisitions now possible, e.g. 4 0.5 mm, the z-axis spatial resolution is the equivalent of that in the axial plane; and, since the acquisition is of a volume rather than of separate slices, we have `isotropic' image data sets ideal for threedimensional reconstructions of various kinds and for virtual endoscopy and bronchoscopy. Regarding the more conventional axial slice reconstructions, these can be made at lesser resolution than that set by the original slice thickness choice. Thus, an acquisition could be made using 4 1 mm slice selection and the complete data set used for three-dimensional reconstructions, but axial slices could be reconstructed for display and
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general diagnostic purposes at, say, 5 or 10 mm by combining/averaging contiguous slices. This not only introduces some useful signal averaging with signal-tonoise improvement but also tends to minimize the partial volume eects often seen in thicker slices. It also oers the not unimportant advantage of reducing the number of images for review in hard copy format. It should be noted that the reverse is also true in some cases. For example, with the GE instrument it is possible to perform the examination at 5-mm slice thickness but to reconstruct at 1.25 mm. DOSIMETRY
Given the prevailing anxieties about the contribution of CT to the radiation burden of the population, this is an important issue [5]. Basically, the considerations are the same for a multi-slice as for a single-slice scanner but with a handful of complications. When pitch is less than 1 ( pitch0 5 4) there is overlap of slice irradiation during rotation to some extent. This will tend to increase dose but, since all information is used in image reconstruction, an image with equal signal to noise characteristics can be obtained in these circumstances by reducing mAs per rotation. As pitch increases, overlap is eliminated (Fig. 6). Comparisons of dosimetry from dierent manufacturers' machines should only be made on the basis of equivalent de®nitions of pitch as well as equivalence of other imaging parameters. Broadly speaking, the dosimetry of single- and multi-slice machines in studying the same body volume with identical collimation is the same. However, one can easily see how the speed of these machines, even when sensibly limited in practice in the interests of image quality optimization, may lead to the performance of multiple acquisitions in dierent phases which could not have been contemplated before. While some of this might be justi®able in clinical management terms, much may not and the associated increased radiation burden must be borne in mind. These systems oer such speed of image acquisition that close to real-time CT ¯uoroscopy in anatomical slabs up to 20 mm thick can be achieved, though there are questions about the usefulness of this particular technique and about the high radiation burden associated with it [13]. DEMANDS ON X-RAY TUBES
Single-slice spiral CT systems made greater demands on X-ray tubes than earlier incremental machines. The radiologist might sensibly fear that these `powerful' multislice CT systems will make correspondingly powerful demands on tubes. The issues are complex and the demands on the tube are a function of several parameter choices for the CT examination. However, simply speaking, the tube total output required for a given body volume acquisition is the same whether it is acquired more slowly (single-slice spiral) or more quickly (multi-slice spiral). With a choice of
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pitch less than 1 to optimize image quality, there is, as indicated earlier, overlap of data with consequent repeat exposures. This means that mAs per rotation may be reduced without loss of image quality but with reduced tube demand and, as discussed, radiation dose. However, yet again, the temptation to do more and to perform multiple examinations of the same block of tissue in dierent phases of contrast enhancement, rather than a single coverage will have its own eects on tube lifetime per case as it will have on dose per examination. IMAGE STORAGE AND REVIEW
One issue generated by this new technology is that of the storage of the huge numbers of images which it can generate. To take just one example, consider the thorax study discussed above: 4 1 mm at pitch 6 and 0.5-s rotation time with a scan length of 300 mm. This will generate 300 images. In multiphase abdominal±pelvic scans the number can easily be 500. Whether storage is on disc, in hard copy form or by transfer to a PACS system, there are clearly storage-space and storage-cost implications on a greater scale than so far encountered with CT. This raises a number of issues. How much data should be downloaded to hardcopy, how much reporting should be `soft-copy' and how much data should be digitally stored long-term, in whatever form? Each department will have to make its own decisions but in doing so must take account of some basis facts. The cost of laser copier ®lm is £3 per sheet. For our hypothetical example above of 300 images, and assuming 20 images per sheet, the cost would be (300/ 20) £3 £45 the cost of hard-copying any manipulated images (e.g. three-dimensional reconstructions). If, say, 3-mm axial slice reconstructions were made, and if only these were hard-copied, the cost would be reduced to £15. The cost of digital storage can be estimated as follows. One optical disk will store 5000 images and costs £17. Such a disk will store some 17 or so of the 300-image studies cited above. Our own experience is that many or most studies generate more than 300 images so it is clear that most units will use at least one disk per working day. This is a modest cost but does not represent the whole story as far as costs are concerned. Disks must be stored and data archiving and retrieval demands signi®cant operator input and disrupts work ¯ow. Comparison with old archived data is extremely time-consuming. Of course, if the original data is all stored long-term and hard copy is also made the costs are additive. Our own approach is to perform both higher and lower resolution axial slice reconstructions from the primary data set, to carry out `soft-copy' reporting using the larger number of the former and to generate hard copy of the smaller number of the latter. Every CT Unit will evolve its own policies which will be adapted with experience. It should be noted that multiple workstations will be needed, at not inconsiderable cost, to allow soft-copy reporting and clinico-radiological conferencing. Some will argue that PACS where available will provide the answer but such a claim may be simplistic. PACS
provides better work ¯ow management with pre-fetching and automated retrieval but currently has diculty handling large data sets. The entry costs of even limited PACS systems are very high. CONTRAST AGENT ENHANCEMENT REGIMENS
There is no doubt that the advent of faster third and fourth generation CT systems have made it necessary to examine contrast enhancement regimes and to consider how to tailor them to optimize enhancement of the examination. The subsequent introduction on a wide scale of (singleslice) spiral systems caused yet more confusion. It seems sensible to ask at this early stage, before these multi-slice systems are in widespread use, whether there is a need for the further modi®cation of enhancement regimens. It is important to realize immediately that some things are immutable, such as the fact that the cortical nephrogram will appear early and that the portal venous phase will be delayed some 50±60 s after the start of any contrast medium infusion. Such considerations answer the questions about timing but leave unresolved the issues of injection volumes and concentrations (total dose) and injection rate. Some simple immediate thoughts are possible. We already know that slower infusion rates than those commonly used, e.g. 3, 4 or 5 ml/s, are inadequate. Could faster injections of, say, higher concentrations of agent with earlier CT data acquisition be appropriate in some cases? Could smaller total volumes of contrast agent delivered faster with earlier data acquisition be an option in some applications of these new faster systems? Some careful thought and considerable experience will be needed before these issues can be resolved. CLINICAL EFFECTIVENESS
No studies of outcomes or of clinical eectiveness have yet been performed. What can already be said is that the apparent advantages of multi-slice technology of greater speed, versatility and isotropic spatial resolution oer considerable appeal to radiologists and clinicians and would appear to broaden the repertoire of CT. The seemingly inexorable onward march of MRI will not be halted but CT may have been given a new lease of life. REFERENCES 1 Houns®eld GN. Computerised transverse axial scanning (tomography). I Description of system. Brit J Radiology 1973; 46:1016±1022. 2 Kalender WA, Seissler W, Klotz E, Vock P. Spiral volumetric CT with single breath-hold technique, continuous transport and continuous scanner rotation. Radiology 1990; 176:181±183. 3 Kalender WA, Polacin A. Physical performance characteristics of spiral CT scanning. Med Phys 1991; 18:910±915. 4 Brink JA. Technical aspects of helical (spiral) CT. Radiol Clin North Am 1995; 33:825±841. 5 Jessen K, Shrimpton PC, Geleijus J. Dosimetry for optimisation of patient protection in computed tomography. Appl Rad Isotopes 1999; 50:165±172.
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6 Fuchs T, Krause JU, Wolf H, Kalender WA. Experimental row and cone-beam CT: theoretical considerations, simulations, evaluation of image quality of four-slice spiral CT. Radiology phantom measurements and patient studies. Radiology 1999; 1999; 213(P): 317. 213:283±289. 7 Flohr T, Schaller S, Ohnesorg BM, Klingenbeck-Regn K, Kopp AF. 11 Hu H, Fox SH. The eect of helical pitch and beam collimation on AF. Evaluation of image artifacts in multi-slice CT. Radiology the lesion contrast and slice pro®le in helical CT imaging. Med Phys 1999; 213(P): 317. 1996; 23:1943±1954. 8 Hu H. Multi-slice helical CT: scan and reconstruction. Med Phys 12 Hu H, He HD, Foley WD, Fox SH. Four multi-detector-row 1999; 26:5±18. helical CT: image quality and volume coverage speed. Radiology 9 McCullough CH, Zink FE. Performance evaluation of a multi-slice 2000; 215:55±62. CT system. Med Phys 1999; 26:2223±2230. 13 Silverman SG, Tuncali K, Adams DF et al. CT ¯uoroscopy-guided 10 Kachelrie ûM, Kalender WA. Dose reduction by generalized abdominal interventions: techniques, results and radiation 3D adaptive ®ltering for conventional and spiral single-, multiexposure. Radiology 1999; 212:673±681.
Multi-slice Technology in Computed Tomography P E T E R D AW S O N , W I L L I A M R . L E E S Department of Imaging, UCL Hospitals, London, U.K. Received: 30 June 2000
Revised: 28 September 2000 Accepted: 28 September 2000
Multi-slice systems represent a considerable advance in CT and will assure the future of the technique for many years to come. This article describes this new technology, indicating its provenance and its position in the evolution of CT. While it does not seek to be a physics and engineering text, enough detail of these are given to allow an informed discussion of the many advantages and a few potential problems associated with the technology. A discussion of a number of applications and a brief consideration of contrast enhancement regimens and the possible need for their modi®cation are presented. Dawson, P. & Lees, # 2001 The Royal College of Radiologists W. R. (2001). Clinical Radiology 56, 302±309. Key words: helical/spiral, computed tomography, multi-slice.
When ®rst introduced nearly 30 years ago [1], computed tomography (CT) ushered in a new paradigm in X-ray imaging and was a considerable in¯uence in the later development of magnetic resonance imaging (MRI). MRI, with its high contrast sensitivity and resolution and lack of ionizing radiation, then looked set soon to supplant CT almost completely, if not in quite the short time scale envisaged by MRI's more enthusiastic proponents. The advent of spiral/helical CT [2,3,4] changed this outlook somewhat. There had been a logical developmental progression from ®rst generation linear and rotary movements pencil beam systems (Fig. 1a), to second generation ( fewer) linear and rotary movements fan beam systems (Fig. 1b), to third generation wider angle fan beam systems with no linear but, rather, continuous rotary movement of tube and detector (Fig. 1c), to fourth generation complete 3608 detector ring and moving tube-only systems (Fig. 1d). Now, in the spiral/helical systems, the table/patient moved continuously (in the z-direction) during rotation of the tube so that a whole volume, rather than serial discrete slices, could be acquired in one complex movement (Fig. 2). Single-slice acquisition times had decreased in the course of this sequence of developments from 5 min to less than 1 s. Of course, demands on X-ray tubes, and on the mathematician, increased at each step. While various arguments were advanced to the eect that spiral technology could be associated with a reduced radiation dose to the patient, CT in both old and new forms still undoubtedly represented a signi®cant radiation burden [5]. In fact, it represented the largest contribution of all diagnostic procedures and the greatest single contribution to the Author for correspondence and guarantor of study: Prof. Peter Dawson, Department of Imaging, The Middlesex Hospital, Mortimer Street, London W1N 8AA, U.K. Fax: 01494 728222; E-mail: [email protected] 0009-9260/01/040302+08 $35.00/0
non-natural total population burden and genetically signi®cant dose. However, it now oered such power and versatility by way of acquisition of large anatomical volumes in a single breath-hold; examination of smaller anatomical volumes at high spatial resolution, seamless volume data sets, and the capacity to perform meaningful `multi-phase' CT studies, that its future, compared to MRI or any other technique, was assured. What further progress could be made? Rotation times, already under 1 s, could be reduced to perhaps 0.5 s but, for mechanical and electronic reasons, probably to not much less. The `centrifugal' force acting on the X-ray tube during a 0.5 s rotation exceeds 10 g. Higher speeds would require the development of a ®xed anode system, which is impractical. The other obvious evolutionary change was to increase the number of detectors by introducing a multiple contiguous detector arc system and to utilize a beam which is also `fanned' in the z-direction ( patient axis) to a degree depending on how many detector arcs are used (Fig. 3). This immediately demands another leap in the mathematical demands of image reconstruction and introduces a number of other diculties discussed below. The ®rst step in this direction was taken by Elscint with its `Twin' machine in 1993 which had just two contiguous detector arcs. This development led to a halving of any scan time, all other things being equal, since it may be seen as either acquiring two slices at a time or as covering twice the z-axis distance per rotation. This `multi-slice' technique [6±10] has been extended by several commercial companies (Table 1) to four simultaneous slices. Actually, all the commercial systems employ many more than four contiguous detector arcs in their systems, ranging from eight to 34, but, for reasons discussed below, only a maximum of four contiguous slices can be selected for acquisition in practice. These systems will be # 2001 The Royal College of Radiologists
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Fig. 1 ± Four generations of incremental CT machines. (a) First generation linear and rotating pencil beam system; (b) linear and rotating fan beam system; (c) rotation only (tube and detectors) wide angle fan beam system; (d) fourth generation 3608 detector ring-rotating tube system.
Table 1 ± Manufacturers of current multi-slice CT systems Manufacturer
System name
Detector array
General Electric
Lightspeed
Matrix array (20 mm)
Siemens
Somatom Plus 4 Volume Zoom
Adaptive array (20 mm)
Marconi
MX 8000
Adaptive array (20 mm)
Toshiba
Aquilion
Matrix/Adaptive array (32 mm)
discussed in a little more detail below but just one example of what is achievable is described here. In such a `multislice' system with four slice acquisitions and with a 0.5-s rotation, the scanning speed will be eight times that of a current state-of-the-art 1-s rotation single detector ring spiral/helical system. The liver, for example, could be examined with, say, 5-mm collimation (z-axis resolution) in less than 6 s. In fact, for a variety of reasons which will be touched on below, not quite this advance in speed should usually be
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Fig. 2 ± The spiral/helical CT principle. The patient moves continuously in the axial (z) direction during rotation of the gantry. A single detector arc is used and if there is no table movement a single CT slice is obtained during a gantry rotation of thickness determined by the collimation used. Fig. 4 ± Pro®le of the various detector ring geometries used in the commercial systems. (a) Consists of 16 identical detector rings (matrix array); (b) and (c) exploit ring detector widths of variable size [`adaptive' (b) and `matrix/adaptive' (c) arrays] ± see discussion.
Fig. 3 ± The detector geometry of a multi-slice spiral/helical CT system. Sixteen detector rings are illustrated. The size of the `cone angle', y, is exaggerated.
sought in practice but the potential is, nevertheless, remarkable. DETECTOR GEOMETRY
Three dierent detector array geometries are used by dierent manufacturers (Fig. 4). Those used by Marconi and Siemens are identical, the former having acquired Elscint technology by take-over and the latter via a research collaboration. The dierent designs have an eect on the minimum slice thickness available and the number of slices available at this minimum width, the range of choice of slice thickness and the maximum volume/z-axis distance which may be scanned in any one system rotation. The slice thickness and number of contiguous slices (up to four) are chosen by beam collimation and by electronic selection and/or summation of detector signals. As an example, consider the con®guration in Fig. 4a. Each detector ring is 1.25 mm wide in the z-direction. The four central rings may be selected to give a 4 1.25 mm simultaneous slice acquisition, or signals from pairs of
contiguous rings can be summed to allow 4 2.5 mm simultaneous slice acquisitions; summing the signals of three and four detector rings yields 4 3.75 and 4 5 mm simultaneous slice acquisitions, respectively. By taking the signals of eight together (8 1.25 mm 10 mm), on either side of the mid-line, 2 10 mm simultaneous slices may be obtained. Beam collimation allows the selection of half of each detector yielding a 2 0.625 mm slice acquisition. Geometrical considerations dictate that there is a diculty with detector array designs such as this. Figure 5 may be taken to be the 16 1.25 mm system con®guration. This shows that only for the innermost detector arcs are the X-rays close to perpendicular to the z-axis. For the outer detector arcs the rays fall more obliquely and, during rotation are `smeared' within the patient in a double cone. The so-called cone angle, y, in Fig. 5 is in fact about 1 degree and so is greatly exaggerated in this drawing to emphasis the point. As shown in the right half of Fig. 5, if the outer detector (here 1.25 mm) bands are selected, the broadening during rotation results in an eective slice thickness of some 3 mm. This may be shown by simple geometrical considerations and calculations based on Fig. 5. This is unfortunate since it means we cannot in practice perform, say, 16 1.25 mm simultaneous slices using system 4 (Fig. 4a). However, combining the signals of groups of four (4 5 mm) results in much reduced distortion of the selected nominal 5-mm slice thickness as also shown in the left half of Fig. 5. These geometrical considerations clearly indicate the considerable, though not necessarily insurmountable, diculties involved in building systems with more detector arcs covering a greater z-axis distance because of the distorting eects of increasing the cone angle, y, and it is clear why there are limitations on the choice and/or number of simultaneous contiguous slices which may be selected for simultaneous acquisition. With this immutable geometrical framework in mind, two manufacturers, Siemens and Marconi have developed an `adaptive' array detector (AAD; Fig. 4b). Here the
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Table 2 ± Slice width combinations possible with the various systems General Electric 2 0.625 4 1.25 4 2.5 4 3.75 45 2 10
Siemens/Marconi
Toshiba
4 0.5 41 4 2.5 45 28 2 10
41 42 43 48
addition to the detector material and its properties, it is clearly dependent on such novel factors as the size of insulator gaps between detector arcs (an eective `dead space') as well as on the total number of detectors. PITCH
The concept of pitch in spiral systems has come to be well understood. With single-slice systems it is de®ned as: Pitch
Table movement per rotation Collimation (slice thickness)
An alternative de®nition, adopted for multi-slice systems by Siemens, GE and Toshiba, but not Marconi, is: Pitch0 Fig. 5 ± If too many simultaneous slices are selected, the outer ones will be considerably distorted with loss of eective spatial (z-axis) resolution, as shown in the right half. Where thicker slices are selected the distortion is minimized, as shown in the left half.
detectors are of increasing z-axis width the further they lie from the centre (y 0) of the array. Another manufacturer, Toshiba, has made a less dramatic step in this direction with its array (Matrix/Adaptive) (Fig. 4c) which is also larger, 32 mm as opposed to 20 mm for the other manufacturers. Table 2 shows simultaneous slice thickness acquisitions possible with the various systems. The 2 0.5 mm and 4 1 mm selections are obtained using collimation allowing radiation to fall on half of each central detection array and on the medial two-thirds of the 1.5 mm detector arc. Four 2.5 mm selections are made by collimation, allowing radiation to fall on the two 2.5 mm rings and on the two 1 and 1.5 mm rings; the signals from the latter being combined electronically. Quite how tolerable the distortions introduced by the greater cone angle of this system will be with some slice selections remains to be seen. This is a highly technical area but some generalization may be made. The adaptive arrays do not represent a complete response to the slice width distortion problem and, indeed, if detector arrays covering greater z-axis distances are yet to be developed, the matrix rather than the adaptive arrays will be the basis. The whole matter of detector array eciency of these various systems remains to be determined, especially as, in
Table movement per rotation Detector z ÿ collimation
The existence of two de®nitions holds the potential for confusion. Some examples may help: (1) If 4 5 mm ( 20 mm) slices are obtained with a table speed of 20 mm per rotation, then, on the ®rst de®nition the pitch is 20/20 1. On the second de®nition, it is 20/5 4. (2) If 2 10 mm ( 20 mm) slices are obtained with a table speed of 20 mm per rotation, then, on the ®rst de®nition the pitch is 20/20 1. On the second de®nition, it is 20/10 2. It is the ®rst de®nition which must be used as an indicator of dose, as will be discussed later. As with single slice spiral, the image quality declines as pitch (however de®ned) increases, though in a non-linear manner. This is a factor which sets a practical limit to some of the exaggerated theoretical claims being made for the speeds of these systems, as will be discussed further. The pitch also in¯uences slice pro®le [11,12]. An illustration of some dierent pitches in a four-multi-slice system is shown in Fig. 6. EXAMINATION SPEEDS
We have seen that for four-slice mode the data acquisition time may be simply one-quarter times that of a single-slice spiral CT system set at the same slice
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of image quality), intermediately high-speed CT covering moderately large volumes in short periods (while maintaining image quality) and high z-axis resolution CT (isotropic voxels) of more modest volumes in short times. An outline of some applications under these headings is given below.
Ultra High-speed CT Fig. 6 ± An illustration of some dierent pitches. At higher pitch, overlap is eliminated. At lower pitch it is an important factor ± see discussion.
collimation and with the same rotation speed. If the multislice CT system has a 0.5-s rotation speed, but the single slice CT machine had a rotation speed of 1 s then a theoretical increase of speed of eight times is available. However, as we have also seen, it may be best in the interests of image quality optimization to use a pitch of, say, three rather than four and this will yield a six times increase in speed. Hu et al. [12] have shown in studies on one commercial system that a two to three times increase in volume acquisition rate as compared with a single-slice system is fully compatible with comparable image quality. Thereafter some loss is entailed in increasing acquisition speed. The top speeds available should only be used in specialist applications (see below). The real `speed' advantage of these systems lies in their ability to obtain more modest volume studies at high resolution during, where appropriate, a single breath-hold. Reconstruction times range from 0.5 to 2 s per image for axial slice reconstructions. Such rapid reconstructions are vital given the huge amount of data and number of images which may be generated. For example, suppose a multi-slice scanner has a rotation time of 0.5 s and a reconstruction time per image of 0.5 s and is used to perform a 4 2.5 mm scan of a 40 cm z-axis body length. If axial slices are reconstructed at 0.5 mm, then we can say: Image acquisition time is (400/4 2.5) 0.5 20 s Number of images is 400/(2.5) 160 Reconstruction time is 160 0.5 80 s. When higher z-axis resolution scans are performed these numbers go up but it is clear that the scanning and reconstruction times (leaving aside any o-line sophisticated work) are short. Within well-funded and well-staed CT units in some countries patient throughput may be signi®cantly increased. In the NHS this advantage will probably not be exploited since most time is spent getting the patients to the CT unit and preparing them. CLINICAL APPLICATIONS
Headline top speeds of multi-slice instruments are impressive and there are some applications where it is useful to run them at their limits. However, in general it is more sensible to view them as oering an unprecedented trade-o between ultra-high speed CT (with some sacri®ce
Though it involves some loss of image quality, this may be useful in such circumstances as multiple trauma cases and unco-operative patients. For example, using 4 5 mm slice selection and a pitch ( prime) of 6 (entailing some image quality loss), and with a 0.5-s rotation time, a 1200-mm long (z-axis) body segment can be scanned in only 20 s. Or, to take another example, the lungs (300 mm) may be scanned at 4 5 mm slice selection with the same pitch of 6 and 0.5 s rotation time can be scanned in only 5 s, obviously in a single breath-hold.
Intermediately High-speed CT Here image quality is not sacri®ced by use of a pitch0 of, say, 3. With 4 5 mm slice selection and 0.5-s rotation time, pelvis, abdomen and chest (say 700 mm) may be covered in only [700/(4 5)] 0.5 4/3 23 s. Such a protocol would provide a remarkably good whole-body survey examination on a single breath-hold. It could, of course, be timed with respect to the infusion of contrast medium so as to cover the liver during the portal venous enhancement phase.
High Z-axis Resolution CT (Isotropic) Any number of examples might be given but consider: A high resolution (isotropic) CT examination of the thorax with 4 1 mm slice selection at 0.5-s rotation time and pitch ( prime) 6 would allow coverage of moderate z-axis distance of 300 mm in 25 s. For the mediastinum, 5-mm slice reconstructions can be made and if high resolution examination of the lungs is required, 1.5±2 mm slice reconstructions could be made by slice summation/averaging. The original isotropic data set can be used for three-dimensional reconstructions including virtual bronchoscopy and endoscopy, or for HRCT of the lungs in any plane if desired. Similarly, an upper abdominal CT examination (150 mm length) could be carried out to include, say, pancreas and kidneys using similar parameter choices in less than 15 s. The isotropic data could be used to generate high-quality CTA, including renal arteriography, if acquired at a suitable time with respect to the administration of contrast medium. The liver alone could be examined at 4 5 mm in less than 5 s and at 4 2.5 mm in less than 10 s. Consequently, a hepatic arterial phase CT study can be achieved which is truly likely to be completed in this phase throughout and this can be followed after a suitable delay by portal venous phase imaging.
MULTI-SLICE TECHNOLOGY IN COMPUTED TOMOGRAPHY
Of course, all this can only be achieved if the timing of data acquisition with respect to the administration of contrast is accurate and in this regard bolus timing software will be invaluable. RECONSTRUCTIONS
These systems are very demanding as regards the requirements on image reconstruction algorithms. The conventional 1808 and 3608 interpolation approaches used in single-slice CT will simply not do when applied to multislice data sets. Many artefacts of interpolation are introduced and the variation in z-axis sensitivity for dierent slices inevitably associated with the ®nite cone angle is problematic. New interpolation algorithms have been developed by the manufacturers. These tend to work best when applied to data obtained using certain pitch selections. Consequently, manufacturers' advice on pitch and algorithm combinations should be taken at least until experience is gained. A steep learning curve in optimization of the use of these CT systems seems in prospect. The speed of implementation of the reconstruction algorithms is of the order of 0.5±2 s per slice. IMAGE QUALITY
Broadly speaking, the image quality of a reconstructed axial slice should be much the same whether from a singleor multiple-slice instrument but some caveats must be entered. The variable z-axis sensitivity for dierent slices and the need, ideally, to match choice of pitch to reconstruction algorithm have been alluded to. The manufacturers of the GE system, for example, oer a choice between two pitch/algorithm combinations: a pitch0 ( pitch 0.75) of 3 for optimum image quality (HQ mode) and a pitch0 ( pitch 1.5) of 6 for speed (HS mode). Generally speaking, a pitch less than 4 ( pitch0 less than 1) is required to obtain single slice spiral image quality equivalence. While multi-slice spiral interpolation artefacts are an important issue, especially in tissues with rapid z-axis direction change and in patient or organ movement, the increased system speed tends to mitigate these eects. One point of considerable importance is that with the higher z-axis spatial resolution acquisitions now possible, e.g. 4 0.5 mm, the z-axis spatial resolution is the equivalent of that in the axial plane; and, since the acquisition is of a volume rather than of separate slices, we have `isotropic' image data sets ideal for threedimensional reconstructions of various kinds and for virtual endoscopy and bronchoscopy. Regarding the more conventional axial slice reconstructions, these can be made at lesser resolution than that set by the original slice thickness choice. Thus, an acquisition could be made using 4 1 mm slice selection and the complete data set used for three-dimensional reconstructions, but axial slices could be reconstructed for display and
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general diagnostic purposes at, say, 5 or 10 mm by combining/averaging contiguous slices. This not only introduces some useful signal averaging with signal-tonoise improvement but also tends to minimize the partial volume eects often seen in thicker slices. It also oers the not unimportant advantage of reducing the number of images for review in hard copy format. It should be noted that the reverse is also true in some cases. For example, with the GE instrument it is possible to perform the examination at 5-mm slice thickness but to reconstruct at 1.25 mm. DOSIMETRY
Given the prevailing anxieties about the contribution of CT to the radiation burden of the population, this is an important issue [5]. Basically, the considerations are the same for a multi-slice as for a single-slice scanner but with a handful of complications. When pitch is less than 1 ( pitch0 5 4) there is overlap of slice irradiation during rotation to some extent. This will tend to increase dose but, since all information is used in image reconstruction, an image with equal signal to noise characteristics can be obtained in these circumstances by reducing mAs per rotation. As pitch increases, overlap is eliminated (Fig. 6). Comparisons of dosimetry from dierent manufacturers' machines should only be made on the basis of equivalent de®nitions of pitch as well as equivalence of other imaging parameters. Broadly speaking, the dosimetry of single- and multi-slice machines in studying the same body volume with identical collimation is the same. However, one can easily see how the speed of these machines, even when sensibly limited in practice in the interests of image quality optimization, may lead to the performance of multiple acquisitions in dierent phases which could not have been contemplated before. While some of this might be justi®able in clinical management terms, much may not and the associated increased radiation burden must be borne in mind. These systems oer such speed of image acquisition that close to real-time CT ¯uoroscopy in anatomical slabs up to 20 mm thick can be achieved, though there are questions about the usefulness of this particular technique and about the high radiation burden associated with it [13]. DEMANDS ON X-RAY TUBES
Single-slice spiral CT systems made greater demands on X-ray tubes than earlier incremental machines. The radiologist might sensibly fear that these `powerful' multislice CT systems will make correspondingly powerful demands on tubes. The issues are complex and the demands on the tube are a function of several parameter choices for the CT examination. However, simply speaking, the tube total output required for a given body volume acquisition is the same whether it is acquired more slowly (single-slice spiral) or more quickly (multi-slice spiral). With a choice of
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pitch less than 1 to optimize image quality, there is, as indicated earlier, overlap of data with consequent repeat exposures. This means that mAs per rotation may be reduced without loss of image quality but with reduced tube demand and, as discussed, radiation dose. However, yet again, the temptation to do more and to perform multiple examinations of the same block of tissue in dierent phases of contrast enhancement, rather than a single coverage will have its own eects on tube lifetime per case as it will have on dose per examination. IMAGE STORAGE AND REVIEW
One issue generated by this new technology is that of the storage of the huge numbers of images which it can generate. To take just one example, consider the thorax study discussed above: 4 1 mm at pitch 6 and 0.5-s rotation time with a scan length of 300 mm. This will generate 300 images. In multiphase abdominal±pelvic scans the number can easily be 500. Whether storage is on disc, in hard copy form or by transfer to a PACS system, there are clearly storage-space and storage-cost implications on a greater scale than so far encountered with CT. This raises a number of issues. How much data should be downloaded to hardcopy, how much reporting should be `soft-copy' and how much data should be digitally stored long-term, in whatever form? Each department will have to make its own decisions but in doing so must take account of some basis facts. The cost of laser copier ®lm is £3 per sheet. For our hypothetical example above of 300 images, and assuming 20 images per sheet, the cost would be (300/ 20) £3 £45 the cost of hard-copying any manipulated images (e.g. three-dimensional reconstructions). If, say, 3-mm axial slice reconstructions were made, and if only these were hard-copied, the cost would be reduced to £15. The cost of digital storage can be estimated as follows. One optical disk will store 5000 images and costs £17. Such a disk will store some 17 or so of the 300-image studies cited above. Our own experience is that many or most studies generate more than 300 images so it is clear that most units will use at least one disk per working day. This is a modest cost but does not represent the whole story as far as costs are concerned. Disks must be stored and data archiving and retrieval demands signi®cant operator input and disrupts work ¯ow. Comparison with old archived data is extremely time-consuming. Of course, if the original data is all stored long-term and hard copy is also made the costs are additive. Our own approach is to perform both higher and lower resolution axial slice reconstructions from the primary data set, to carry out `soft-copy' reporting using the larger number of the former and to generate hard copy of the smaller number of the latter. Every CT Unit will evolve its own policies which will be adapted with experience. It should be noted that multiple workstations will be needed, at not inconsiderable cost, to allow soft-copy reporting and clinico-radiological conferencing. Some will argue that PACS where available will provide the answer but such a claim may be simplistic. PACS
provides better work ¯ow management with pre-fetching and automated retrieval but currently has diculty handling large data sets. The entry costs of even limited PACS systems are very high. CONTRAST AGENT ENHANCEMENT REGIMENS
There is no doubt that the advent of faster third and fourth generation CT systems have made it necessary to examine contrast enhancement regimes and to consider how to tailor them to optimize enhancement of the examination. The subsequent introduction on a wide scale of (singleslice) spiral systems caused yet more confusion. It seems sensible to ask at this early stage, before these multi-slice systems are in widespread use, whether there is a need for the further modi®cation of enhancement regimens. It is important to realize immediately that some things are immutable, such as the fact that the cortical nephrogram will appear early and that the portal venous phase will be delayed some 50±60 s after the start of any contrast medium infusion. Such considerations answer the questions about timing but leave unresolved the issues of injection volumes and concentrations (total dose) and injection rate. Some simple immediate thoughts are possible. We already know that slower infusion rates than those commonly used, e.g. 3, 4 or 5 ml/s, are inadequate. Could faster injections of, say, higher concentrations of agent with earlier CT data acquisition be appropriate in some cases? Could smaller total volumes of contrast agent delivered faster with earlier data acquisition be an option in some applications of these new faster systems? Some careful thought and considerable experience will be needed before these issues can be resolved. CLINICAL EFFECTIVENESS
No studies of outcomes or of clinical eectiveness have yet been performed. What can already be said is that the apparent advantages of multi-slice technology of greater speed, versatility and isotropic spatial resolution oer considerable appeal to radiologists and clinicians and would appear to broaden the repertoire of CT. The seemingly inexorable onward march of MRI will not be halted but CT may have been given a new lease of life. REFERENCES 1 Houns®eld GN. Computerised transverse axial scanning (tomography). I Description of system. Brit J Radiology 1973; 46:1016±1022. 2 Kalender WA, Seissler W, Klotz E, Vock P. Spiral volumetric CT with single breath-hold technique, continuous transport and continuous scanner rotation. Radiology 1990; 176:181±183. 3 Kalender WA, Polacin A. Physical performance characteristics of spiral CT scanning. Med Phys 1991; 18:910±915. 4 Brink JA. Technical aspects of helical (spiral) CT. Radiol Clin North Am 1995; 33:825±841. 5 Jessen K, Shrimpton PC, Geleijus J. Dosimetry for optimisation of patient protection in computed tomography. Appl Rad Isotopes 1999; 50:165±172.
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