Clinical Radiology (2001) 56: 302±309doi:10.1053/crad.2000.0651, available online at http://www.idealibrary.com on

Multi-slice Technology in Computed Tomography

PETER DAWSON, WILLIAM R. LEES

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 formany years to come. This article describes this new technology, indicating its provenance and its positionin the evolution of CT. While it does not seek to be a physics and engineering text, enough detail of theseare given to allow an informed discussion of the many advantages and a few potential problems associatedwith the technology. A discussion of a number of applications and a brief consideration of contrastenhancement regimens and the possible need for their modi®cation are presented. Dawson, P. & Lees,W. R. (2001). Clinical Radiology 56, 302±309. # 2001 The Royal College of Radiologists

Key words: helical/spiral, computed tomography, multi-slice.

When ®rst introduced nearly 30 years ago [1], computedtomography (CT) ushered in a new paradigm in X-rayimaging and was a considerable in¯uence in the laterdevelopment of magnetic resonance imaging (MRI). MRI,with its high contrast sensitivity and resolution and lack ofionizing radiation, then looked set soon to supplant CTalmost completely, if not in quite the short time scaleenvisaged by MRI's more enthusiastic proponents.The advent of spiral/helical CT [2,3,4] changed this

outlook somewhat. There had been a logical developmentalprogression from ®rst generation linear and rotary move-ments 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 systemswith no linear but, rather, continuous rotary movement oftube and detector (Fig. 1c), to fourth generation complete3608 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 ofthe tube so that a whole volume, rather than serial discreteslices, could be acquired in one complex movement (Fig. 2).Single-slice acquisition times had decreased in the course ofthis 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 variousarguments were advanced to the e�ect that spiral technol-ogy could be associated with a reduced radiation dose tothe patient, CT in both old and new forms still undoubtedlyrepresented a signi®cant radiation burden [5]. In fact, itrepresented the largest contribution of all diagnosticprocedures and the greatest single contribution to the

non-natural total population burden and geneticallysigni®cant dose. However, it now o�ered such power andversatility by way of acquisition of large anatomicalvolumes in a single breath-hold; examination of smalleranatomical volumes at high spatial resolution, seamlessvolume data sets, and the capacity to perform meaningful`multi-phase' CT studies, that its future, compared to MRIor 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 notmuch less. The `centrifugal' force acting on the X-ray tubeduring a 0.5 s rotation exceeds 10 g. Higher speeds wouldrequire the development of a ®xed anode system, which isimpractical. The other obvious evolutionary change was toincrease the number of detectors by introducing a multiplecontiguous detector arc system and to utilize a beam whichis also `fanned' in the z-direction (patient axis) to a degreedepending on how many detector arcs are used (Fig. 3). Thisimmediately demands another leap in the mathematicaldemands of image reconstruction and introduces a numberof other di�culties discussed below. The ®rst step in thisdirection was taken by Elscint with its `Twin' machine in1993 which had just two contiguous detector arcs. Thisdevelopment led to a halving of any scan time, all otherthings being equal, since it may be seen as either acquiringtwo slices at a time or as covering twice the z-axis distanceper rotation.

This `multi-slice' technique [6±10] has been extended byseveral commercial companies (Table 1) to four simul-taneous slices. Actually, all the commercial systems employmany more than four contiguous detector arcs in theirsystems, ranging from eight to 34, but, for reasons discussedbelow, only a maximum of four contiguous slices can beselected for acquisition in practice. These systems will be

0009-9260/01/040302+08 $35.00/0 # 2001 The Royal College of Radiologists

Author for correspondence and guarantor of study: Prof. PeterDawson, Department of Imaging, The Middlesex Hospital, MortimerStreet, London W1N 8AA, U.K. Fax: 01494 728222; E-mail:phd728222@aol.com

MULTI-SLICE TECHNOLOGY IN COMPUTED TOMOGRAPHY 303

discussed in a little more detail below but just one exampleof what is achievable is described here. In such a `multi-slice' system with four slice acquisitions and with a 0.5-srotation, the scanning speed will be eight times that of acurrent state-of-the-art 1-s rotation single detector ringspiral/helical system. The liver, for example, could beexamined with, say, 5-mm collimation (z-axis resolution) inless than 6 s.

In fact, for a variety of reasons which will be touched onbelow, not quite this advance in speed should usually be

Fig. 1 ± Four generations of incremental CT machines. (a) First generation linear and rotating pencil beam system; (b) linear and rotating fan beamsystem; (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 4Volume Zoom

Adaptive array (20 mm)

Marconi MX 8000 Adaptive array (20 mm)

Toshiba Aquilion Matrix/Adaptive array(32 mm)

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sought in practice but the potential is, nevertheless,remarkable.

DETECTOR GEOMETRY

Three di�erent detector array geometries are used bydi�erent manufacturers (Fig. 4). Those used by Marconiand Siemens are identical, the former having acquiredElscint technology by take-over and the latter via a researchcollaboration. The di�erent designs have an e�ect on theminimum slice thickness available and the number of slicesavailable at this minimum width, the range of choice of slicethickness and the maximum volume/z-axis distance whichmay be scanned in any one system rotation. The slicethickness and number of contiguous slices (up to four) arechosen by beam collimation and by electronic selectionand/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. Thefour central rings may be selected to give a 4 � 1.25 mmsimultaneous slice acquisition, or signals from pairs of

contiguous rings can be summed to allow 4 � 2.5 mmsimultaneous slice acquisitions; summing the signals ofthree and four detector rings yields 4 � 3.75 and 4 � 5 mmsimultaneous 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 eachdetector yielding a 2 � 0.625 mm slice acquisition.

Geometrical considerations dictate that there is adi�culty with detector array designs such as this. Figure 5may be taken to be the 16 � 1.25 mm system con®guration.This shows that only for the innermost detector arcs are theX-rays close to perpendicular to the z-axis. For the outerdetector arcs the rays fall more obliquely and, duringrotation are `smeared' within the patient in a double cone.The so-called cone angle, y, in Fig. 5 is in fact about 1degree and so is greatly exaggerated in this drawing toemphasis the point. As shown in the right half of Fig. 5, ifthe outer detector (here 1.25 mm) bands are selected, thebroadening during rotation results in an e�ective slicethickness of some 3 mm. This may be shown by simplegeometrical considerations and calculations based onFig. 5. This is unfortunate since it means we cannot inpractice perform, say, 16 � 1.25 mm simultaneous slicesusing system 4 (Fig. 4a). However, combining the signals ofgroups of four (4 � 5 mm) results in much reduceddistortion of the selected nominal 5-mm slice thickness asalso shown in the left half of Fig. 5.

These geometrical considerations clearly indicate theconsiderable, though not necessarily insurmountable,di�culties involved in building systems with more detectorarcs covering a greater z-axis distance because of thedistorting e�ects of increasing the cone angle, y, and it isclear why there are limitations on the choice and/or numberof simultaneous contiguous slices which may be selected forsimultaneous acquisition.

With this immutable geometrical framework in mind,two manufacturers, Siemens and Marconi have developedan `adaptive' array detector (AAD; Fig. 4b). Here the

Fig. 2 ± The spiral/helical CT principle. The patient moves continu-ously in the axial (z) direction during rotation of the gantry. A singledetector arc is used and if there is no table movement a single CT sliceis obtained during a gantry rotation of thickness determined by thecollimation used.

Fig. 3 ± The detector geometry of a multi-slice spiral/helical CTsystem. Sixteen detector rings are illustrated. The size of the `coneangle', y, is exaggerated.

Fig. 4 ± Pro®le of the various detector ring geometries used in thecommercial systems. (a) Consists of 16 identical detector rings (matrixarray); (b) and (c) exploit ring detector widths of variable size[`adaptive' (b) and `matrix/adaptive' (c) arrays] ± see discussion.

MULTI-SLICE TECHNOLOGY IN COMPUTED TOMOGRAPHY 305

detectors are of increasing z-axis width the further they liefrom the centre (y � 0) of the array. Another manufacturer,Toshiba, has made a less dramatic step in this direction withits 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 and4 � 1 mm selections are obtained using collimation allow-ing radiation to fall on half of each central detection arrayand on the medial two-thirds of the 1.5 mm detector arc.Four 2.5 mm selections are made by collimation, allowingradiation to fall on the two 2.5 mm rings and on the two 1and 1.5 mm rings; the signals from the latter beingcombined electronically.Quite how tolerable the distortions introduced by the

greater cone angle of this system will be with some sliceselections remains to be seen. This is a highly technical areabut some generalization may be made. The adaptive arraysdo not represent a complete response to the slice widthdistortion problem and, indeed, if detector arrays coveringgreater z-axis distances are yet to be developed, the matrixrather than the adaptive arrays will be the basis.The whole matter of detector array e�ciency of these

various systems remains to be determined, especially as, in

addition to the detector material and its properties, it isclearly dependent on such novel factors as the size ofinsulator gaps between detector arcs (an e�ective `deadspace') as well as on the total number of detectors.

PITCH

The concept of pitch in spiral systems has come to be wellunderstood. 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 bySiemens, GE and Toshiba, but not Marconi, is:

Pitch0 � Table movement per rotation

Detector z ÿ collimation

The existence of two de®nitions holds the potential forconfusion. Some examples may help:

(1) If 4 � 5 mm (� 20 mm) slices are obtained with a tablespeed of 20 mm per rotation, then, on the ®rstde®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 atable speed of 20 mm per rotation, then, on the ®rstde®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-linearmanner. This is a factor which sets a practical limit to someof the exaggerated theoretical claims being made for thespeeds of these systems, as will be discussed further. Thepitch also in¯uences slice pro®le [11,12]. An illustration ofsome di�erent pitches in a four-multi-slice system is shownin Fig. 6.

EXAMINATION SPEEDS

We have seen that for four-slice mode the dataacquisition time may be simply one-quarter times that ofa single-slice spiral CT system set at the same slice

Table 2 ± Slice width combinations possible with the various systems

General Electric Siemens/Marconi Toshiba

2 � 0.6254 � 1.25 4 � 0.54 � 2.5 4 � 1 4 � 14 � 3.75 4 � 2.5 4 � 24 � 5 4 � 5 4 � 32 � 10 2 � 8 4 � 8

2 � 10

Fig. 5 ± If too many simultaneous slices are selected, the outer oneswill be considerably distorted with loss of e�ective spatial (z-axis)resolution, as shown in the right half. Where thicker slices are selectedthe distortion is minimized, as shown in the left half.

306 CLINICAL RADIOLOGY

collimation and with the same rotation speed. If the multi-slice CT system has a 0.5-s rotation speed, but the singleslice CT machine had a rotation speed of 1 s then atheoretical increase of speed of eight times is available.However, as we have also seen, it may be best in theinterests of image quality optimization to use a pitch of,say, three rather than four and this will yield a six timesincrease in speed. Hu et al. [12] have shown in studies onone commercial system that a two to three times increase involume acquisition rate as compared with a single-slicesystem is fully compatible with comparable image quality.Thereafter some loss is entailed in increasing acquisitionspeed.The top speeds available should only be used in specialist

applications (see below). The real `speed' advantage of thesesystems lies in their ability to obtain more modest volumestudies at high resolution during, where appropriate, asingle breath-hold.Reconstruction times range from 0.5 to 2 s per image for

axial slice reconstructions. Such rapid reconstructions arevital given the huge amount of data and number of imageswhich may be generated. For example, suppose a multi-slicescanner has a rotation time of 0.5 s and a reconstructiontime per image of 0.5 s and is used to perform a4 � 2.5 mm scan of a 40 cm z-axis body length. If axialslices are reconstructed at 0.5 mm, then we can say:

Image acquisition time is (400/4 � 2.5) � 0.5 � 20 sNumber of images is 400/(2.5) � 160Reconstruction time is 160 � 0.5 � 80 s.

When higher z-axis resolution scans are performed thesenumbers go up but it is clear that the scanning andreconstruction times (leaving aside any o�-line sophisti-cated work) are short. Within well-funded and well-sta�edCT units in some countries patient throughput may besigni®cantly increased. In the NHS this advantage willprobably not be exploited since most time is spent gettingthe patients to the CT unit and preparing them.

CLINICAL APPLICATIONS

Headline top speeds of multi-slice instruments areimpressive and there are some applications where it isuseful to run them at their limits. However, in general it ismore sensible to view them as o�ering an unprecedentedtrade-o� between ultra-high speed CT (with some sacri®ce

of image quality), intermediately high-speed CT coveringmoderately large volumes in short periods (while maintain-ing image quality) and high z-axis resolution CT (isotropicvoxels) of more modest volumes in short times. An outlineof some applications under these headings is given below.

Ultra High-speed CT

Though it involves some loss of image quality, this maybe useful in such circumstances as multiple trauma casesand unco-operative patients.

For example, using 4 � 5 mm slice selection and a pitch(prime) of 6 (entailing some image quality loss), and with a0.5-s rotation time, a 1200-mm long (z-axis) body segmentcan be scanned in only �20 s.

Or, to take another example, the lungs (�300 mm) maybe scanned at 4 � 5 mm slice selection with the same pitchof 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 rotationtime, pelvis, abdomen and chest (say 700 mm) may becovered in only [700/(4 � 5)] � 0.5 � 4/3 � 23 s.

Such a protocol would provide a remarkably goodwhole-body survey examination on a single breath-hold. Itcould, of course, be timed with respect to the infusion ofcontrast medium so as to cover the liver during the portalvenous enhancement phase.

High Z-axis Resolution CT (Isotropic)

Any number of examples might be given but consider: Ahigh resolution (isotropic) CT examination of the thoraxwith 4 � 1 mm slice selection at 0.5-s rotation time andpitch (prime) 6 would allow coverage of moderate z-axisdistance of 300 mm in 25 s.

For the mediastinum, 5-mm slice reconstructions can bemade and if high resolution examination of the lungs isrequired, 1.5±2 mm slice reconstructions could be made byslice summation/averaging. The original isotropic data setcan be used for three-dimensional reconstructions includingvirtual bronchoscopy and endoscopy, or for HRCT of thelungs in any plane if desired.

Similarly, an upper abdominal CT examination (150 mmlength) could be carried out to include, say, pancreas andkidneys using similar parameter choices in less than 15 s.The isotropic data could be used to generate high-qualityCTA, including renal arteriography, if acquired at a suitabletime with respect to the administration of contrast medium.The liver alone could be examined at 4 � 5 mm in less than5 s and at 4 � 2.5 mm in less than 10 s. Consequently, ahepatic arterial phase CT study can be achieved which istruly likely to be completed in this phase throughout andthis can be followed after a suitable delay by portal venousphase imaging.

Fig. 6 ± An illustration of some di�erent pitches. At higher pitch,overlap is eliminated. At lower pitch it is an important factor ± seediscussion.

MULTI-SLICE TECHNOLOGY IN COMPUTED TOMOGRAPHY 307

Of course, all this can only be achieved if the timing ofdata acquisition with respect to the administration ofcontrast is accurate and in this regard bolus timing softwarewill be invaluable.

RECONSTRUCTIONS

These systems are very demanding as regards therequirements on image reconstruction algorithms. Theconventional 1808 and 3608 interpolation approaches usedin single-slice CT will simply not do when applied to multi-slice data sets. Many artefacts of interpolation areintroduced and the variation in z-axis sensitivity fordi�erent slices inevitably associated with the ®nite coneangle is problematic.New interpolation algorithms have been developed by

the manufacturers. These tend to work best when applied todata obtained using certain pitch selections. Consequently,manufacturers' advice on pitch and algorithm combi-nations should be taken at least until experience is gained.A steep learning curve in optimization of the use of theseCT 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 reconstructedaxial slice should be much the same whether from a single-or multiple-slice instrument but some caveats must beentered. The variable z-axis sensitivity for di�erent slicesand the need, ideally, to match choice of pitch toreconstruction algorithm have been alluded to. Themanufacturers of the GE system, for example, o�er achoice 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) isrequired to obtain single slice spiral image qualityequivalence.While multi-slice spiral interpolation artefacts are an

important issue, especially in tissues with rapid z-axisdirection change and in patient or organ movement, theincreased system speed tends to mitigate these e�ects.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 theequivalent of that in the axial plane; and, since theacquisition is of a volume rather than of separate slices,we have `isotropic' image data sets ideal for three-dimensional reconstructions of various kinds and forvirtual endoscopy and bronchoscopy.Regarding the more conventional axial slice reconstruc-

tions, these can be made at lesser resolution than that set bythe original slice thickness choice. Thus, an acquisitioncould be made using 4 � 1 mm slice selection and thecomplete data set used for three-dimensional reconstruc-tions, but axial slices could be reconstructed for display and

general diagnostic purposes at, say, 5 or 10 mm bycombining/averaging contiguous slices. This not onlyintroduces some useful signal averaging with signal-to-noise improvement but also tends to minimize the partialvolume e�ects often seen in thicker slices. It also o�ers thenot unimportant advantage of reducing the number ofimages for review in hard copy format.

It should be noted that the reverse is also true in somecases. For example, with the GE instrument it is possible toperform the examination at 5-mm slice thickness but toreconstruct at 1.25 mm.

DOSIMETRY

Given the prevailing anxieties about the contribution ofCT to the radiation burden of the population, this is animportant issue [5]. Basically, the considerations are thesame for a multi-slice as for a single-slice scanner but with ahandful of complications.

When pitch is less than 1 (pitch0 5 4) there is overlap ofslice irradiation during rotation to some extent. This willtend to increase dose but, since all information is used inimage reconstruction, an image with equal signal to noisecharacteristics can be obtained in these circumstances byreducing mAs per rotation. As pitch increases, overlap iseliminated (Fig. 6).

Comparisons of dosimetry from di�erent manufacturers'machines should only be made on the basis of equivalentde®nitions of pitch as well as equivalence of other imagingparameters. Broadly speaking, the dosimetry of single- andmulti-slice machines in studying the same body volume withidentical collimation is the same.

However, one can easily see how the speed of thesemachines, even when sensibly limited in practice in theinterests of image quality optimization, may lead to theperformance of multiple acquisitions in di�erent phaseswhich could not have been contemplated before. Whilesome of this might be justi®able in clinical managementterms, much may not and the associated increased radiationburden must be borne in mind.

These systems o�er such speed of image acquisition thatclose to real-time CT ¯uoroscopy in anatomical slabs up to20 mm thick can be achieved, though there are questionsabout the usefulness of this particular technique and aboutthe high radiation burden associated with it [13].

DEMANDS ON X-RAY TUBES

Single-slice spiral CT systems made greater demands onX-ray tubes than earlier incremental machines. Theradiologist might sensibly fear that these `powerful' multi-slice CT systems will make correspondingly powerfuldemands on tubes. The issues are complex and the demandson the tube are a function of several parameter choices forthe CT examination. However, simply speaking, the tubetotal output required for a given body volume acquisition isthe same whether it is acquired more slowly (single-slicespiral) or more quickly (multi-slice spiral). With a choice of

308 CLINICAL RADIOLOGY

pitch less than 1 to optimize image quality, there is, asindicated earlier, overlap of data with consequent repeatexposures. This means that mAs per rotation may bereduced without loss of image quality but with reduced tubedemand and, as discussed, radiation dose.However, yet again, the temptation to do more and to

perform multiple examinations of the same block of tissuein di�erent phases of contrast enhancement, rather than asingle coverage will have its own e�ects on tube lifetime percase as it will have on dose per examination.

IMAGE STORAGE AND REVIEW

One issue generated by this new technology is that of thestorage of the huge numbers of images which it cangenerate. To take just one example, consider the thoraxstudy discussed above: 4 � 1 mm at pitch 6 and 0.5-srotation time with a scan length of 300 mm. This willgenerate 300 images. In multiphase abdominal±pelvic scansthe number can easily be 500. Whether storage is on disc, inhard copy form or by transfer to a PACS system, there areclearly storage-space and storage-cost implications on agreater 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 storedlong-term, in whatever form? Each department will have tomake its own decisions but in doing so must take account ofsome basis facts. The cost of laser copier ®lm is �£3 persheet. 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 manipu-lated images (e.g. three-dimensional reconstructions). If,say, 3-mm axial slice reconstructions were made, and if onlythese 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-imagestudies cited above. Our own experience is that many ormost studies generate more than 300 images so it is clearthat most units will use at least one disk per working day.This is a modest cost but does not represent the whole storyas far as costs are concerned. Disks must be stored and dataarchiving and retrieval demands signi®cant operator inputand disrupts work ¯ow. Comparison with old archived datais extremely time-consuming. Of course, if the original datais all stored long-term and hard copy is also made the costsare additive.Our own approach is to perform both higher and lower

resolution axial slice reconstructions from the primary dataset, to carry out `soft-copy' reporting using the largernumber of the former and to generate hard copy of thesmaller number of the latter. Every CT Unit will evolve itsown policies which will be adapted with experience. Itshould be noted that multiple workstations will be needed,at not inconsiderable cost, to allow soft-copy reporting andclinico-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-fetchingand automated retrieval but currently has di�cultyhandling large data sets. The entry costs of even limitedPACS systems are very high.

CONTRAST AGENT ENHANCEMENT REGIMENS

There is no doubt that the advent of faster third andfourth generation CT systems have made it necessary toexamine contrast enhancement regimes and to consider howto tailor them to optimize enhancement of the examination.The subsequent introduction on a wide scale of (single-slice) spiral systems caused yet more confusion. It seemssensible to ask at this early stage, before these multi-slicesystems are in widespread use, whether there is a need forthe further modi®cation of enhancement regimens.

It is important to realize immediately that some thingsare immutable, such as the fact that the cortical nephro-gram will appear early and that the portal venous phase willbe delayed some 50±60 s after the start of any contrastmedium infusion. Such considerations answer the questionsabout timing but leave unresolved the issues of injectionvolumes and concentrations (total dose) and injection rate.Some simple immediate thoughts are possible. We alreadyknow that slower infusion rates than those commonly used,e.g. 3, 4 or 5 ml/s, are inadequate. Could faster injectionsof, say, higher concentrations of agent with earlier CT dataacquisition be appropriate in some cases? Could smallertotal volumes of contrast agent delivered faster with earlierdata acquisition be an option in some applications of thesenew faster systems?

Some careful thought and considerable experience will beneeded before these issues can be resolved.

CLINICAL EFFECTIVENESS

No studies of outcomes or of clinical e�ectiveness haveyet been performed. What can already be said is that theapparent advantages of multi-slice technology of greaterspeed, versatility and isotropic spatial resolution o�erconsiderable appeal to radiologists and clinicians andwould appear to broaden the repertoire of CT. Theseemingly inexorable onward march of MRI will not behalted but CT may have been given a new lease of life.

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