X-ray computed tomography at Abstract X-ray computed tomography (CT), introduced into clinical practice

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    Phys. Med. Biol. 51 (2006) R29R43 doi:10.1088/0031-9155/51/13/R03


    X-ray computed tomography

    Willi A Kalender

    Institute of Medical Physics, University Erlangen-Nuernberg, Henkestr. 91, D-91052 Erlangen,Germany

    E-mail: willi.kalender@imp.uni-erlangen.de

    Received 28 February 2006, in final form 4 April 2006Published 20 June 2006Online at stacks.iop.org/PMB/51/R29

    AbstractX-ray computed tomography (CT), introduced into clinical practice in 1972,was the first of the modern slice-imaging modalities. To reconstruct imagesmathematically from measured data and to display and to archive them indigital form was a novelty then and is commonplace today. CT has shown asteady upward trend with respect to technology, performance and clinical useindependent of predictions and expert assessments which forecast in the 1980sthat it would be completely replaced by magnetic resonance imaging. CT notonly survived but exhibited a true renaissance due to the introduction of spiralscanning which meant the transition from slice-by-slice imaging to true volumeimaging. Complemented by the introduction of array detector technology inthe 1990s, CT today allows imaging of whole organs or the whole body in 5 to20 s with sub-millimetre isotropic resolution. This review of CT will proceedin chronological order focussing on technology, image quality and clinicalapplications. In its final part it will also briefly allude to novel uses of CT suchas dual-source CT, C-arm flat-panel-detector CT and micro-CT. At present CTpossibly exhibits a higher innovation rate than ever before. In consequence thetopical and most recent developments will receive the greatest attention.

    1. Short historical introduction

    Computed tomography became feasible with the development of modern computer technologyin the 1960s, but some of the ideas on which it is based can be traced back to the first half ofthat century. In 1917 the Bohemian mathematician Radon (1917) proved in a research paperof fundamental importance that the distribution of a material or material property in an objectlayer can be calculated if the integral values along any number of lines passing through thesame layer are known. The first applications of this theory were developed for radioastronomyby Bracewell (1956), but they met with little response and were not exploited for medicalpurposes.

    0031-9155/06/130029+15$30.00 2006 IOP Publishing Ltd Printed in the UK R29

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    The first experiments on medical applications of this type of reconstructive tomographywere carried out by the physicist A M Cormack, who worked on improving radiotherapyplanning at Groote Schuur Hospital, Cape Town, South Africa. Between 1957 and 1963,and without knowledge of previous studies, he developed a method of calculating radiationabsorption distributions in the human body based on transmission measurements (Cormack1963). He postulated for radiological applications that it must be possible to display even themost minute absorption differences, i.e. different soft-tissue structures. However, he neverhad the chance of putting his theory into practice and only learned of Radons work muchlater, a fact that he regretted by stating that earlier access to this knowledge would have savedhim a lot of work. While familiarizing himself with Radons work, Cormack discovered thatRadon himself had been unaware of even earlier work on the subject by the Dutch physicistH A Lorentz, who had already proposed a solution of the mathematical problem for thethree-dimensional (3D) case in 1905 (Cormack 1992).

    While Cormacks work later was highly acknowledged as an essential contribution to thedevelopment of CT, there were others who deserve mention just the same. Oldendorf (1961)published his truly pioneering work in CT in 1961. Kuhl and Edwards (1963) introducedthe concepts of emission computed tomography in 1963; although reconstruction efforts werelimited to backprojection only, emission CT thus predated transmission CT. Filtered backprojection was first described by Bracewell and Riddle (1967). Descriptions of algebraicreconstruction techniques were published by Gordon et al (1970) in 1970 and by Hermanet al (1973), descriptions of Fourier reconstruction techniques by Bates and Peters (1971) in1971. The key paper on the filtering functions necessary for CT reconstruction was the workof Shepp and Logan (1974).

    The first successful practical implementation of the theory was achieved in 1972 bythe English engineer G N Hounsfield, who is now generally recognized as the inventor ofcomputed tomography (Hounsfield 1973). Like his predecessors, Hounsfield worked withoutknowledge of the earlier efforts. His success took the entire medical world by surprise. Heachieved his remarkable breakthrough neither at a renowned university nor with a leadingmanufacturer of radiological equipment, but with the British firm EMI Ltd. His inventiongave EMI, which had until then manufactured only records and electronic components, amonopoly in the CT market that lasted for two years, and the terms EMI Scanner and CTScanner became almost synonymous (figure 1). In 1974 Siemens became the first traditionalmanufacturer of radiological equipment to market a head CT scanner, after which many othercompanies quickly followed suit. A boom followed, reaching its peak in the late seventieswith 18 companies offering CT equipment. Most of these, including EMI, have withdrawnfrom the market by now.

    The first clinical CT images were produced at the Atkinson Morley Hospital in London in1972. The very first patient examination performed with CT offered convincing proof of theeffectiveness of the method by detecting a cystic frontal lobe tumour. CT was immediatelyand enthusiastically welcomed by the medical community and has often been referred toas the most important invention in diagnostic radiology since the discovery of x-rays; itslater development only confirmed these early expectations. Computed tomography hasbecome a very important factor in radiological diagnosis. While only 60 EMI scannershad been installed by 1974, there were more than 10 000 devices in use in 1980, includinga high number of head scanners. In 1979 Hounsfield and Cormack, an engineer and aphysicist, were awarded the Nobel Prize for Medicine in recognition of their outstandingachievements.

    At this point, the development seemed to have reached its peak, and the 1980s saw littletechnological progress. The introduction of spiral CT in 1989 (Kalender et al 1990b) and the

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    Figure 1. Godfrey N Hounsfield (a), an English engineer, developed the first CT scanner and,together with the physicist A M Cormack, received the Nobel Prize for Medicine and Physiologyin 1979. Transverse slice imaging of the brain at low resolution with 80 80 image matricesbecame the standard CT application in the first half of the 1970s (b).

    following developments in x-ray, detector and scanner technology have led to a renewal ofinterest in CT and to a true renaissance of CT, as will be pointed out in detail below. For theyear 2006, the number of clinical installations in operation is estimated to be about 45 000,almost exclusively whole-body spiral scanners. The upward trend is unbroken for the timebeing, and the position of CT in clinical radiology appears consolidated to a higher degreethan ever before.

    2. Developments in CT technology, performance characteristics and applications

    The first clinical CT scanner allowed for the acquisition of single images in 300 s; todaysscanners all feature rotation times below 1 s with the fastest ones offering 330 ms and effectiveimage acquisition times down to the order of 100 ms for partial scans with dual-source systems.Among many other performance features which have improved steadily over the years,it appears that increasing scan speed always received highest priority and was the drivingforce behind CT developments. The basis of modern CT and its success is the fact thatthe increase in speed does apply not only to the acquisition of single images, but also tothe acquisition of image data for complete volumes. Image quality, in particular 3D spatialresolution, the spectrum of clinical applications and many other factors have shown a steadyimprovement over the years just the same (table 1). The underlying technical developmentscan be assigned to a good approximation, although not strictly, to the single decades.

    2.1. The 1970s from head to whole-body imaging

    The development of CT scanners began with Hounsfields experimental set-up, which largelycorresponded to the sketch in figure 2(a). This set-up was termed the first generation ofCT. The first commercial scanners, the so-called second generation, differed only little fromHounsfields scanning system. To speed up scanning and to utilize the available x-ray powermore efficiently detectors were added which entailed going from a pencil beam to a smallfan beam. Both types of scanners functioned according to the translationrotation principlein which the radiation source and the detector scan the object in a linear translatory motionand repeat this procedure successively after a small rotational increment (figures 2(a) and (b)).180 projections were sampled in 1 steps with 160 data points each, i.e. a total of 28 800 data

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    (c) (d)


    Figure 2. Four scanner generations were promoted in the 1970s. Head scanners, which scannedthe patient by translation and rotation of the measurement system with a pencil beam (a) or a smallfan beam (b), and fan beam systems, in which all body sections can be scanned with a continuous360 rotati