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  Thomas G. Flohr, PhDStefan Schaller, PhDKarl Stierstorfer, PhDHerbert Bruder, PhDBernd M. Ohnesorge, PhDU. Joseph Schoepf, MD Published online before print 10.1148/radiol.2353040037 Radiology 2005;  235:756–773 Abbreviations:  AMPR  adaptive MPRECG  electrocardiographyFWHM   full width at half maximumMPR  multiplanar reformationSSP  section-sensitivity profile3D  three-dimensional 1 From Siemens Medical Solutions, CTDivision, Forchheim, Germany (T.G.F.,S.S., K.S., H.B., B.M.O.); Departmentof Diagnostic Radiology, Tu¨bingenUniversity, Germany (T.G.F.); and De-partment of Radiology, Medical Uni-versity of South Carolina, 169 Ashley Ave, Charleston, SC 29425 (U.J.S.).Received January 7, 2004; revision re-quested March 9; revision received April 26; accepted May 24.  Addresscorrespondence to  U.J.S. (e-mail: schoepf@musc.edu  ).U.J.S. is a medical consultant to Sie-mens Medical Solutions, CT Division,Forchheim, Germany. © RSNA, 2005  Multi–Detector Row CTSystems and Image-Reconstruction Techniques 1 The introduction in 1998 of multi–detector row computed tomography (CT) by themajor CT vendors was a milestone with regard to increased scan speed, improvedz-axis spatial resolution, and better utilization of the available x-ray power. In thisreview, the general technical principles of multi–detector row CT are reviewed asthey apply to the established four- and eight-section systems, the most recent16-section scanners, and future generations of multi–detector row CT systems.Clinical examples are used to demonstrate both the potential and the limitations of the different scanner types. When necessary, standard single-section CT is referredto as a common basis and starting point for further developments. Another focus isthe increasingly important topic of patient radiation exposure, successful dosemanagement, and strategies for dose reduction. Finally, the evolutionary steps fromtraditional single-section spiral image-reconstruction algorithms to the most recentapproaches toward multisection spiral reconstruction are traced. © RSNA, 2005 Supplemental material:  radiology.rsnajnls.org/cgi/content/full/2353040037/DC1 Computed tomography (CT) was introduced in the early 1970s and has revolutionized thepractice not only of diagnostic radiology but also of the whole field of medicine. CT wasthe first technology to marry a computer to a medical imaging machine, the first to displayx-ray images as cross sections, and the first modality to herald a new era of digital imaging.A glossary of terms used in this review is available online in Appendix E1 ( radiology .rsnajnls.org/cgi/content/full/2353040037/DC1 ). EVOLUTION OF SPIRAL CT: FROM ONE SECTION TO 16 The introduction of spiral CT in the early 1990s constituted a fundamental evolutionarystep in the development and ongoing refinement of CT imaging techniques (1,2). For thefirst time, volume data could be acquired without misregistration of anatomic detail.Volume data became the basis for applications such as CT angiography (3), which hasrevolutionized the noninvasive assessment of vascular disease. The ability to acquirevolume data also paved the way for the development of three-dimensional (3D) image-processing techniques such as multiplanar reformation (MPR), maximum intensity pro-jection, surface-shaded display, and volume-rendering techniques (4), which have becomea vital component of medical imaging today.Ideally, volume data are of high spatial resolution and are isotropic in nature: Eachimage data element (voxel) is of equal dimensions in all three spatial axes, and this formsthe basis for image display in arbitrarily oriented imaging planes. For most clinicalscenarios, however, single-section spiral CT with a 1-second gantry rotation is unable tofulfill these requirements. To prevent motion artifacts and optimally utilize the contrastagent bolus, body spiral CT examinations need to be completed within a certain timeframe of, ordinarily, one breath hold (25–30 seconds). If a large scan range such as theentire thorax or abdomen (30 cm) has to be covered within a single breath hold, a thickcollimationof5–8mmmustbeused.Whilethein-planeresolutionofaCTimagedependson the system geometry and on the reconstruction kernel selected by the user, the Special Review 756      R    a      d      i     o      l     o     g      y    longitudinal (z-axis) resolution along thepatient axis is determined by the col-limated section width and the spiralinterpolation algorithm. Use of a thickcollimation of 5–8 mm results in a con-siderable mismatch between the longitu-dinal resolution and the in-plane resolu-tion,whichis0.5–0.7mm,dependingonthereconstructionkernel.Thus,withsin-gle-section spiral CT, the ideal of isotro-pic resolution can only be achieved forvery limited scan ranges (5).Strategies to achieve more substantialvolume coverage with improved longitu-dinal resolution include the simulta-neous acquisition of more than one sec-tion at a time and a reduction in thegantry rotation time. Interestingly, thefirst medical CT scanners were two-sec-tion systems, such as the EMI (England)head scanner, introduced in 1972, andthe Siemens Siretom (Erlangen, Ger-many), introduced in 1974. With the ad-vent of whole-body fan-beam CT systemsfor general radiology, two-section acqui-sition was no longer used. Apart from adedicated two-section system for cardiacapplications, the Imatron C-100 (Ima-tron, San Francisco, Calif), which was in-troduced in 1984, the first step towardmultisection acquisition in general radi-ology was a two-section CT scanner in-troduced in 1993 (Elscint TWIN; Elscint,Haifa, Israel) (6). In 1998, several CTmanufacturers introduced multi–detec-tor row CT systems, which provided con-siderable improvement in scanningspeed and longitudinal resolution andbetter utilization of the available x-raypower (7–10). These systems typically of-fered simultaneous acquisition of foursections at a gantry rotation time of 0.5second.Simultaneous acquisition of   m  sectionsresultsinan m -foldincreaseinspeedifallother parameters (eg, section thickness)are unchanged. This increased perfor-mance of multi–detector row CT relativeto single-section CT allowed the optimi-zation of a variety of clinical protocols.The examination time for standard pro-tocols could be substantially reduced,which proved to be of immediate clinicalbenefit for the quick and comprehensiveassessment of trauma patients and unco-operative patients (11). Alternatively, thescanrangethatcouldbecoveredwithinacertain time was extended by a factor of  m,  which is relevant for oncologic stag-ing or for CT angiography with extendedcoverage (eg, the lower extremities) (12).The most important clinical benefit,however, proved to be the ability to scana given anatomic volume within a givenscan time with substantially reduced sec-tion width at  m  times increased longitu-dinal resolution. Because of this, the goalof isotropic resolution was within reachfor many clinical applications. Examina-tions of the entire thorax (13) or abdo-men could now be routinely performedwith a 1.0- or 1.25-mm collimated sectionwidth.Despitethesepromisingadvances,clinical challenges and limitations re-mained for four-section CT systems. Trueisotropic resolution for routine applica-tions had not yet been achieved, becausethe longitudinal resolution of about 1mm does not fully match the in-planeresolution of about 0.5–0.7 mm in a rou-tine examination of the chest or abdo-men. For large volumes, such as for CTangiography of lower extremity vessels(12), thicker (eg, 2.5-mm) collimated sec-tions had to be chosen to complete thescan within a reasonable time frame.Scan times were often too long to allowimage acquisition during a purely arterialphase. For CT angiography of the circleof Willis, for instance, a scan range of about 100 mm must be covered (14).With four-section CT at a collimated sec-tion width of 1 mm, pitch of 1.5, andgantry rotation time of 0.5 second, thisvolume can be covered in about 9 sec-onds, not fast enough to avoid venousoverlay, assuming a cerebral circulationtime of less than 5 seconds. (Note: Thedefinition of pitch for multi–detectorrow CT is discussed later in this review.)As a next step, the introduction of aneight–detector row CT system in 2000enabled shorter scan times but did notyet provide improved longitudinal reso-lution (thinnest collimation, eight sec-tions at 1.25 mm). The latter wasachieved with the introduction of 16–detector row CT (15), which made possi-ble the routine acquisition of substantialanatomic volumes with isotropic submil-limeter spatial resolution and scan timesof less than 10 seconds for 300 mm of coverage (Fig 1). While in-plane spatialresolution is not substantially improved,the two major advantages of fast multi–detector row CT are a true isotropicthrough-plane resolution and a short ac-quisition time that enable high-qualityexaminations in severely debilitated andseverely dyspneic patients (Fig 1).Traditional CT applications have beenenhanced and strengthened by the remark-able, although incremental, improvementin scanner performance by the additionof more detector rows. Multi–detector rowCT also dramatically expanded into areaspreviously considered beyond the scopeof third-generation CT scanners thatwere based on the mechanical rotation of an x-ray tube and detectors, such as car-diac imaging with the addition of elec-trocardiographic (ECG)-gating capabil-ity. With a gantry rotation time of 0.5second and dedicated image-reconstruc-tion approaches, the temporal resolutionfor acquisition of an image was improvedto 250 msec and less (16,17), whichproved to be sufficient for motion-freeimaging of the heart in the mid- to end-diastolic phase when the patient had aslow to moderate heart rate (ie, up to 65beats per minute [18]). With four simul-taneously acquired sections, coverage of the entire heart volume with thin sec-tions (ie, four sections at 1.0- or 1.25-mmcollimation) within a single breath hold ESSENTIALS  ●  Multi–detector row CT allows substan-tial reduction in examination time for standard protocols, coverage of ex-tended anatomic volumes, and, most important, substantially increased lon-gitudinal resolution by means of re-duced section width. ●  Near-isotropic spatial resolution in rou-tine examinations, which has beenachieved with 16-section CT systems,enables 3D renderings of diagnostic quality and oblique MPRs and maxi-mum intensity projections with resolu-tion similar to that of the transverse images. ●  Scanning at narrow collimation does not markedly increase the radiationdose to the patient, as long as the ef-fective milliampere-seconds level is kept constant. ●  A key challenge for image reconstruc-tion with multi–detector row CT is the cone angle of the measurement rays;this requires novel reconstruction tech-niques such as 3D back projection,AMPR, or weighted hyperplane recon-struction. ●  Z filtering makes it possible to recon-struct images retrospectively with dif-ferent section widths from the same raw CT data set, trading off, in this way, z-axis resolution and image noise. Volume 235    Number 3 Multi–Detector Row CT and Image Reconstruction    757      R    a      d      i     o      l     o     g      y    became feasible. This 1.0–1.25-mm lon-gitudinal resolution combined with theimproved contrast resolution of modernCT systems enabled noninvasive depic-tion of the coronary arteries (19–22). Ini-tial clinical studies demonstrated the po-tential of multi–detector row CT to notonly demonstrate but to some degreealso characterize noncalcified and calci-fied plaques in the coronary arteries onthe basis of plaque CT attenuation(22,23).The limitations of four– and eight–de-tector row CT systems, however, have sofar prevented the successful integrationof CT coronary angiography into routineclinical algorithms: Stents or severely cal-cified arteries constitute a diagnostic di-lemma, mainly because of partial volumeartifacts as a consequence of insufficientlongitudinal resolution (22). For patientswith a higher heart rate, careful selectionof separate reconstruction intervals fordifferent coronary arteries has been man-datory (25). It is almost impossible forpatients with manifest heart disease tocomply with the breath-hold time of about 40 seconds required to cover theentire heart volume (approximately 12cm) with four-section CT. The ongoingtechnical refinement of multi–detectorrow CT, however, holds the promise of gradually overcoming some of these lim-itations. The most important steps to-ward this goal are gantry rotation timesfaster than 0.5 second (26,27) for im-proved temporal resolution and robust-ness of use, 16-section submillimeter ac-quisition for increased longitudinalresolution and shorter breath-hold times,and novel sophisticated approaches forimage acquisition and reconstruction.In this review, ECG-synchronized ex-aminations of the heart and of the car-diothoracic anatomy will be very suc-cinctly discussed, since this topic hasbeen extensively reviewed elsewhere(28). Similarly, advanced 3D postprocess-ing techniques are omitted. In this arti-cle, we will review the general technicalprinciples of multi–detector row CT asthey apply to the established four– andeight–detector row systems, the more re-cent 16–detector row scanners, and gen-erations of CT systems yet to come. Onthe basis of the technologic descriptionof different scanner types and image-re-construction approaches, we providepractical “take-home points” to enablebetter translation into daily clinical prac-tice of the technology and science re-viewed here. Useful up-to-date informa-tion regarding multi–detector row CT isalso readily available on the Internet at,for example, the UK Medicines andHealthcare products Regulatory AgencyCT Web site  (www.medical-devices.gov.uk) or the Advanced Medical Imaging Labo-ratory site  (www.ctisus.org). CURRENT TECHNIQUES System Design  Detector design. —For clinical purposes,different section widths must be avail-able to adjust the optimum scan speed,longitudinal resolution, and image noisefor each application. With a single–de-tector row CT scanner, different colli-mated section widths are obtained bymeans of prepatient collimation of thex-ray beam (Fig 2). For a very elementarymodel of a two-section CT scanner ( m  2, or two detector rows), Figure 2 demon-strates how different section widths canbe obtained by means of prepatient col-limation if the detector is separated mid-way along the z-axis extent of the x-raybeam. For  m  2, this simple design prin-ciple must be replaced by more flexibleconcepts requiring more than  m  detectorrows to simultaneously acquire  m  sec-tions.Different manufacturers of multi–de-tector row CT scanners have introduceddifferent detector designs. In order to beable to select different section widths, allscanners combine several detector rowselectronically to a smaller number of sec- Figure 1.  Transverse sections (top) and coronal MPRs (bottom) from a thoracic examinationillustrateclinicalperformanceofCT.Left:single-section8-mm-thickimages.Middle:four-section1.25-mm-thick images. Right: 16–detector row 0.75-mm-thick images. Differences in diagnosticimage quality are most obvious in the MPRs. With 16–detector row images, the goal of isotropicresolution in routine examinations has been reached. Single- and four-section images weresynthesized from the 16-section CT data. Figure 2.  Illustration shows prepatient collimation of the x-raybeam to obtain different collimated section widths with a single–detector row CT detector.  FOV   field of view. 758    Radiology    June 2005 Flohr et al      R    a      d      i     o      l     o     g      y    tions according to the selected beam col-limation and the desired section width.For established four-section CT sys-tems, two detector types are commonlyused. The fixed-array detector consists of detector elements with equal sizes in thelongitudinal direction. A representativeexample of this scanner type, the Light-speed scanner (GE Medical Systems, Mil-waukee, Wis), has 16 detector rows, eachof them defining a 1.25-mm collimatedsection width in the center of rotation(8,10,29). The total coverage in the lon-gitudinal direction is 20 mm at the iso-center; owing to geometric magnifica-tion, the actual detector is about twice aswide. By means of prepatient collimationand combination of the signals of theindividual detector rows, the followingsection widths (measured at the iso-center) can be realized: four sections at1.25 mm, 2.5 mm, 3.75 mm, and 5.0 mm(Fig 3a). The same detector design is usedfor the eight-section version of this sys-tem and provides eight sections at 1.25-and 2.5-mm collimated section widths.A different approach uses an adaptive-array detector design, which comprisesdetector rows with different sizes in thelongitudinal direction. Scanners of thistype, the Mx8000 four-section scanner(Philips Medical Systems, Best, the Neth-erlands) and the Somatom Sensation 4scanner (Siemens), have eight detectorrows (7,9). Their widths in the longitudi-nal direction range from 1 to 5 mm at theisocenter and allow the following colli-mated section widths: two sections at 0.5mm, four at 1.0 mm, four at 2.5 mm, fourat 5.0 mm, two at 8.0 mm, and two at10.0 mm (Fig 3b).The selection of the collimated sectionwidth determines the intrinsic longitudi-nal resolution of a scan. In a “step-and-shoot” sequential mode, any multiple of the collimated width of one detector sec-tion can be obtained by adding the de-tector signals during image reconstruc-tion. In a spiral mode, the effectivesection width, which is usually defined asthe full width at half maximum (FWHM)of the spiral section-sensitivity profile(SSP), is adjusted independently in thespiral interpolation process during imagereconstruction. Hence, from the samedata set, both narrow sections for high-spatial-resolution detail or for 3D post-processing and wide sections for bettercontrast resolution or quick review andfilming may be derived.Sixteen-section CT systems usuallyhave adaptive-array detectors. A repre-sentative example for this scanner type,the Somatom Sensation 16 scanner (Sie-mens), uses 24 detector rows (15). The 16central rows define 0.75-mm collimatedsection widths at the isocenter, and thefour outer rows on both sides define1.5-mm collimated section widths (Fig3c). The total coverage in the longitudi-nal direction is 24 mm at the isocenter.By means of appropriate combination of the signals of the individual detectorrows, either 12 or 16 sections with 0.75-or 1.5-mm collimated section width canbe acquired simultaneously. The Light-speed 16 scanner (GE Medical Systems)uses a similar design: It provides 16 sec-tions with either 0.625- or 1.25-mm col-limated section width. The total coveragein the longitudinal direction is 20 mm atthe isocenter. Yet another design, whichis implemented in the Aquilion scanner(Toshiba, Tokyo, Japan), can provide 16sections with either 0.5-, 1.0-, or 2.0-mmcollimated section width, with a totalcoverage of 32 mm at the isocenter. Radiation Dose  Radiation dose and dose efficiency. —Ra-diation exposure to the patient at CT andthe resulting potential radiation hazardhave recently gained considerable atten-tion in both the public and the scientificliterature (30,31). Typical values for theeffective patient dose for selected CT pro-tocols are 1–2 mSv for a head CT, 5–7mSv for a chest CT, and 8–11 mSv forabdominal and pelvic CT (32,33). Thisradiation exposure must be appreciatedin the context of the average annualbackground radiation, which is 2–5 mSv(3.6 mSv in the United States). Despitethe undisputed clinical benefits, multi-section CT scanning is often consideredto require increased patient dose com-pared with the dose from single-sectionCT. Indeed, a certain increase in radia-tion dose is unavoidable owing to theunderlying physical principles.In the x-ray tube of a CT scanner, asmall area on the anode plate, the focal-spot, emits x-rays that penetrate the pa-tient and are registered by the detector. Acollimator between the x-ray tube andthe patient, the prepatient collimator, isused to shape the beam and to establishthe dose profile. In general, the colli-mated dose profile is a trapezoid in thelongitudinal direction. In the umbral re-gion (ie, plateau region of the trapezoid),x-rays emitted from the entire area of thefocal spot illuminate the detector. In thepenumbral regions, only a part of thefocal spot illuminates the detector, whilethe prepatient collimator blocks off otherparts.With single-section CT, the entire trap-ezoidal dose profile can contribute to thedetector signal, and the collimated sec-tionwidthisdeterminedastheFWHMof this trapezoid. The relative dose utiliza-tion of a single-section CT system cantherefore be close to 100%. In most caseswith multi–detector row CT, only theplateau region of the dose profile is usedto ensure an equal signal level for all de-tector elements. The penumbral region isthen discarded, either by a postpatientcollimator or by the intrinsic self-colli-mation of the multisection detector, andrepresents “wasted” dose. The relativecontribution of the penumbral region in- Figure 3.  Illustrations show examples of  (a)  fixed-array and  (b, c)  adaptive-array detec-tors used in commercially available four- and16-section CT systems. Volume 235    Number 3 Multi–Detector Row CT and Image Reconstruction    759      R    a      d      i     o      l     o     g      y  
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