In/ J Rndmron Ontr,/o~y Bml Phn Vol. Printed I” the USA All rights reserved.

0360-3016/90 $3.M) + .oO Pergamon Press plc

19. pp. 1589-1607

CopvnghtrC1990

0 Special Feature NON-STANDARD

CT SCANNERS: THEIR ROLE IN RADIOTHERAPY

s. WEBB Institute

of Cancer

Research

and Royal Marsden

Hospital,

Downs Rd., Sutton, Surrey SM2 5PT UK

In the past 10 years a number of groups worldwide have investigated the extent to which the imaging requirements for radiotherapy planning may be met by the development of non-standard computed tomography (CT) scanners. Some of these groups have constructed apparatus based on a radiotherapy simulator gantry, others around a linac or a special purpose gantry. The performance of these systems is reviewed and the extent to which they have justified their promise has been assessed. The major use of simulator-based machines has been in planning radiotherapy in the thorax where tissue inhomogeneity corrections are important. Linac-based machines yield images useful for checking the positioning of patients at the time of therapy. Interest at the Royal Marsden Hospital culminated in the construction of a simulator-based CT system which achieved its present form in late 1985. The importance of the subsequent clinical study lies in our conclusive evidence that planning conservative radiotherapy for the breast without multiple-level CT performed in the treatment position can have implications for local control of disease, pulmonary damage, and cosmesis. CT scanners, Imaging in radiotherapy

mograms for assisting with radiotherapy treatment planning. Watson (72) later wrote, “undoubtedly they offer a valuable field of application in the plotting of individual therapy dosage charts.” The pioneering Japanese radiologist Takahashi (66) ( 1957) knew in the 1940’s of the relationship between the sinogram and the cross sectional image. He built a “Rotation Radiograph,” a machine for taking (analogue) sinograms, and then showed many different methods (including hand backprojection and optical backprojection) for “reconstructing from projections.” In Figure 1 one such pencil backprojection is shown with the display technique of the time. Takahashi and Matsuda (67) pointed out how important it was to have cross sectional information with the patient supine, on a flat table, and in the treatment position. They rejected the idea that images obtained with the patient in the vertical position were relevant to supine treatment. (We might recall that vertical axial transverse tomography was popular for imaging the lungs.) The point of this preamble is that although such data might have been crude, the delineation of external contour and gross internal anatomy

INTRODUCTION

History of‘imaging.fiw twatment planning After the first commercial CT scanner was announced in 1972 ( 1, 5 1). it was not long before body scanners were developed (47, 48). Both head and body scanners were used to generate “therapy CT scans,” images of internal tumors and anatomy, useful for radiotherapy treatment planning (14, 52). Unfortunately, it is all too easily assumed that diagnostic quality CT scans are a prerequisite of good treatment planning (64) and it is often forgotten that there were many attempts to generate cross-sectional data and use them for treatment planning long before commercial CT was available. The early connection between the geometries for tomographic imaging and rotation radiotherapy was commented on in historic patents including those by Baese (3) and Portes and Chausse ( 53). As early as the late 1930’s, Watson (73) patented the technique of axial transverse tomography, commenting that the cross-section data thus obtained were preferable to conventional longitudinal to-

Presented at the 17th International Conference of Radiology, Paris, July 1989. Acknowledgemen&-The work at the Institute of Cancer Research and Royal Marsden Hospital on the development of imaging based around a radiotherapy simulator was partly funded by MRC grants G 9781265 and G80/0709/3K. A very large number of people contributed to the developments including M. 0. Leach (joint project leader for G80/0709/3K), R. E. Bentley, F. Duck, R. A. Fox, S. C. Lillicrap. J. Milan, R. D. Speller. H. Steere. K. Maureemootoo. R. Kent, P. Newbery. S. King.

H. Hodt, M. Phillips, P. Collins, D. Hill, J. Phelps. J. R. Yamold, M. A. Toms, J. Gardiner, and D. Parton. The work on Megavoltage imaging at this centre is due to W. Swindell. D. Lewis. P. Evans, and E. Morton. I am grateful to C. J. Kotre. A. T. Redpath, W. Swindell, D. G. Lewis, V. Smith, and S. Grindrod for giving me further details of their imaging systems. I should like to thank P. Evans, J. R. Yarnold. W. Swindell. D. Lewis, and M. 0. Leach for comments on this review. Accepted for publication 21 June 1990.

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r

-- --_ ~~--

._s

Fig. 1. (a) Takahashi’s apparatus for reconstructing the outline of an object from a Discontinuous Rotatogram (discontinuous sinogram). A represents the location of the tube focus at radiography, C is a point representing the rotation centre, Rg is the Discontinuous Rotatogram (which is incrementally translated vertically) and P is a sheet of paper (which rotates in synchrony) (From Ref. (66). (b) An example of the cross section built up from Discontinuous Rotatography. (see Fig. la for the method of generating this). L represents lung, P pleura. SC scapula, V spine, C cavity outer wall. and D cavity inner wall, S sternum. Bottom is a particularly pleasing diagrammatic representation (From Ref. (66)).

such as the lungs was of adequate accuracy for treatment planning. Although diagnostic-quality CT data are certainly required for many radiation treatment plans, many plans could be adequately drawn up with coarser data. Of course, in Takahashi’s day, the reconstruction methods were very time-consuming, and with no computer planning possible, using the data in hand planning was also

December

1990, Volume

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6

tedious. The historian of CT can find many prophetic early experiments but there is little evidence that the images obtained were used for treatment planning. Frank ( 18) in Budapest patented optical analogue computed tomography decades before such methods were refined experimentally or the reconstruction techniques could be implemented by digital electronic computers. In Russia, a CT scanner was designed around 19.57 (34. 68, 69). However, until commercial CT in 1973. it was largely with axial transverse tomography that the main hope lay for achieving images for treatment planning. In 1969 Kishi c/ al. (33) produced a machine with the important change ofhaving the patient lying down in the treatment position instead of sitting on a vertical rotating chair. These images were of course no more than blurred tomograms. After 1972 it became quickly fashionable to regard diagnosticquality CT scanners as essential for aiding treatment planning. However, a small body of people developed simpler CT machines to assist with SPECIFIC treatment planning problems, and it is with their work that this review is concerned.

The term NDCT is used here to mean all those computed tomography machines whose aim is to produce CT cross-sections of lower quality-that is, poorer spatial and electron density resolution-than obtained by state-ofthe-art commercial CT scanners. The latter were, of course, developed for diagnostic purposes, their use in treatment planning being inspired by lateral thinking afterwards. An abundance of terms appear in the literature for NDCT machines. including “simple CT scanners,” CT scan“alternative CT scanners, ” “non-commercial ners.” etc. We prefer the term NDCT because it immediately differentiates the purpose of such machines and is a valid generic term. It removes the sense that NDCT scanners are in some way inferior: as we shall see. they can and do provide enough data for the purpose for which they were designed. Most NDCT machines were designed and built in University and Hospital Departments rather than by commercial companies. Hence in reviewing their construction and capabilities, we are faced with a miscellany of technology. No two machines are alike, although some use common detector technology. Faced with the difficulty of comparing machines that are very unalike, we shall review each independently and try to avoid meritocratic comparisons of performance. Indeed, the data to make such comparisons are not complete. Table 1 summarises data of this kind and I am grateful to individual constructors for providing much previously unpublished data. In some cases the figures in this table have been updated from those previously published for each system corresponding to improvements that have been made. The use of the term circa 19xx is to give a rough indication of when the work at a particular center began. Beginnings are often

Non-standard

CT scanners

fuzzy dates; these are largely deduced from submission dates for first papers or written communications. Shorter reviews are to be found in Webb (75) and Dobbs and Webb (14). The work of Kak et al., BailJ? et al. (circa 197.5). A number of workers built NDCT systems based around the use of an image intensifier (II) as X ray detector. The idea appears to have been first suggested in a German patent by Dummling (16). Since an image intensifier is generally available on a hospital simulator, the intention was (to quote Kak et al., (29)) “a hospital could itself put together a computerised tomographic system.” Kak cl ul. were among the first to investigate this possibility. Kak coupled an image intensifier to a vidicon TV tube and recorded real-time fan-beam x-ray projections. Since the fluoroscopic data could be digitized to only 16 grey levels, spatial and temporal averaging was required to increase the signal to noise ratio of the projections. A video disk and scan converter were used to slow down the data rate before digitisation by an analogue to digital converter. The resulting projections (60- 180 in x; 80 elements long) were represented by a 7 bit deep word (i.e., quantisation to 128 levels). Kak’s method took account of the response of the image intensifier detector. The data were obtained in fan beam geometry by collimating the x-ray source to a narrow fan spanning just 8 TV lines. At the time of this work (circa 1975). reconstruction from fan-beam projections had received little attention and Kak et al. decided to treat the beams as if they were parallel (not a bad approximation with a IO” fan angle). One of the intents of

0 S. WEBB

1591

Kak ~1 al. was to point out the advantage of fan-beam data collection over the more commonly found rotatetranslate data capture geometry. He addressed the reconstruction problem with numerous computer simulations. Kak et al. generated experimental tomograms. first of phantoms (rotated on an axis relative to a stationary tube and detector), and later of a dead rat (with the rat stationary and the source and detector rotating). The latter result is shown in Figure 2. The intended application was not discussed in detail, but the work is a cornerstone of later similar developments (see text) and although spatial and density resolutions are not quoted. the images show clearly the internal lung contours. bones. and external outline (7). Other papers from this group showed that performance close to that of a DCT scanner could be obtained from a fluoroscopic system (0.5 cm spatial resolution, 0.5RI density resolution. slice thickness of 1 mm. and skin exposure of 2 mGy). Indeed, Baily (5, 6) reports that the fluoroscopic system outperformed an EMI unit in delineating objects that had minimal differential attenuation from their surroundings. An almost perfect correlation was found between the EMI CT number and the corresponding value from the fluoroscopic DCT system. One should remember that at this time. commercial CT scanners were rather slow and some of Baily’s interest (4) was in taking advantage of the rapidity of fan-beam scanning as well as the potential for obtaining slices at multiple levels in the body. Using a phantom with rods of differing contrast (and of various sizes) Baily ~‘t ul. (7) showed that all rods

Fig. 2. Experimental data from the scanner constructed by Kak et al. (a) approximate location of the thoracic section for which the tomogram was made. (b) tomogram with no correction for the air transmission value. (c) tomogram with the air transmission value corrected-using the null attenuated x-ray path outside the body. (d) identification of features in (c). (From Ref. (29)) 0 1977 IEEE

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Biology 0 Physics

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Table 1. Physical aspects of NDCT Prinicpal author of system

Kak

Hanis0n

Redpath

Kijewski

Amot

Smith

Half-fan

Fan

IlfTV

II +

Cadmium tungstate + photodiodes

NA

NA

(2nd

machine)

(an = analogue system) (dig = digital system)

Scanner

geometry

Fan-beam

Fan-beam

Half-fan

Half-fan

(an)

Full fan (dig) Type

of detector

Fhmroscopic

FIuoroscopic

screen (II/

screen (II/

Fluoroscopic

lsocon)

(CSI

screen

TV

II + Vidicon)

vidicon) Number

of detectors

Number

NA

of projections

NA

60-180

in 7r

NA

81 in 180or60in240

360 (f)

in 360”

Equivalent

143 in 360’ (dig)

180 full in

240 hemi

in 360”

1500 in 360” (an)

360-720

in 360”

220-440

in 220’

360” Source to axis distance

100

IO0

80

16’

25”

42’

30

44

(cm) Useful

x-ray

fan-angle

impinging Reconstruction diameter

100

on detector circle

20 (dig)

(cm)

40

54.4

40 (an)

Patient aperture diameter (cm) Spatial resolution (mm)

78

7s

3.1

3.1 (an):

1.1-3.1

2.3

(dig) Density

resolution

(%)

IO

3.5 (an);

15.4-3.2

3-5

0.5

(dig)

I .8

(dig for 1500

projections) Scan time

Real time

52 s

Im

hours (dig)

I5 s (360”)

30 s

9 s (220”)

50 s (an) Reconstruction

time

3.5 m

2.5 m

zero (an)

Real time

45 m (PDPI

l/45)

30s + Array

Processor

(3 m/slice

on VAX

780 and FPS 5205 AP) Dose to

sternum (mGy)

Slice width

(mm)

8

5% above the modal dose! When the internal and external outlines are fully used and the plans remade with optimal repositioning of beams and reselection of weights and wedges, a better situation can be restored (C). By selecting 135% as the modal dose, 73% of the area is restored to within +5% of this and there is no significant under- or overdosage. Forty-five percent of the region is in the +5% part and 27% in the -5% part. In Figure 15 the same data are shown on the more familiar “cumulative volume dose distribution-CVDD”. Although this is a single example, this kind of pattern was seen many times and convinced us of the importance of creating cross-sectional data WITH THE PATIENT IN THE TREATMENT POSITION (in our hospital an armsup, elbows out position) and using it in the treatment planning process. We have not yet been able to establish the importance of this from clinical sequelae, but we expect the consequences of ignoring the “tailored cross-section” approach to be substantial. The work sf Green et al. (circa 1987). Green et al. (21)

mner

sclntlllotor

CT system with (b) schematic.

have taken the logical step of constructing a multielement linear detector to be attached to a simulator for CT work. The detector comprises 256 silicon photodiodes whose output is linear with x-ray intensity. The elements are 2 mm in size and the geometry will be such as to give a good field of view (FOV). The reconstruction is performed by the modified fan-beam CBT technique of Webb (74). No clinical data have yet been reported. The work c$SwindeN c’t al. (circa 1978). In Tucson, Arizona and later at the Royal Marsden Hospital, Sutton, Swindell and team have developed megavoltage computed tomography. Strictly speaking, this does not fit into the scope of a review of the development of NDCT scanners based on a radiotherapy simulator, but it is included here because the work is complementary. Its purpose was once again not diagnosis but to provide CT images which assist with radiotherapy. Several aims of the work were identified. First, it was noted that x-ray linear attenuation coefficients derived from a machine operating at Megavoltage energy would be immediately applicable to making tissue inhomogeneity corrections in planning (whereas for a diagnostic machine a conversion is needed). Second, images taken on a linac would automatically ensure that the patient was in the treatment position; the aperture is large and the bed is flat. Also, the emphasis shifts to the delivery itself. the aim being to take images just prior to treatment

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December 1990. Volume 19. Number 6

Fig. 15. CVDD’s for the plans whose dose-area shown in Figure 14.

Fig. 13. CT images ofthe breast (with the patient in the treatment position) taken with the RMH Simulator-based CT system (a) mid-field (b) upper border (c) lower border.

to verify the positioning of the patient by comparing back with CT images at treatment simulation. This provides the connection since these latter images might well be

Fig. 14. Dose-area histograms for the plans using the data shown in Figure 13. See text for description of the assumptions used. MD = modal dose.

histograms

are

obtained with a simulator NDCT scanner. Periodic checks of the patient anatomy become feasible during the course of fractionated radiotherapy. Hendee (26) prophetically wrote, “With improved dose perturbation correction techniques, will not the accuracy of dose distributions be determined primarily by how well the patient geometry used to collect CT data simulates the patient geometry employed for radiation therapy?” Swindell’s work has progressed in several phases (48, 58. 65). These workers used the well-known relationship between image spatial and density resolutions, slice width, beam energy, delivered dose, incident x-ray energy, detective quantum efficiency, and object size (9) to calculate that by relaxing the spatial resolution requirement to some 2 mm, images with a signal to noise ratio greater than 200 could be calculated with a dose of about 8 cGy. When the signal to noise ratio was relaxed to 100, the dose could be as low as 2 cGy. The first system constructed had a multielement detector made of a Pilot-B plastic scintillator coupled to photodiodes. The individual elements were arranged on the arc of a circle of radius 140 cm centered on the x-ray source. One hundred ten fan-beam projections ( I2 bits deep) were collected in a 220” arc and images were reconstructed by a CBP method with a Shepp-Logan filter to 128” matrices using a PDP 1 l/34 computer. Simpson ef al. (58) showed that the relationship between reconstructed CT numbers and tissue electron density was outstandingly linear. The measured resolution was some 4 mm and a signal to noise ratio greater than 100 was achieved. The plastic scintillator was soon abandoned in favor of a detector fabricated from 96 bismuth germanate crystals. These had a superior physical density (7.13 g cmm3 instead of 1.1 g cm-3 for the plastic) and a stopping power of 83% instead of 25%. The distance to the detector from the source was also increased from 140 cm to 180 cm, decreasing the scatter contribution to the data. The recon-

Non-standard CT scanners 0 S.

showed once again a linearity of better than 1% between CT number and electron density, a spatial resolution of 3 mm, and signal to noise in excess of 100. The dose delivered was some 10 cGy and indicated that there was still room for improvement in the data gathering and handling. Also, annoying circular artifacts were seen caused by electronic drift. The data were, however, clearly adequate for treatment planning and verification purposes and Swindell et (11.(65) showed a variety of in viva images, mainly of head structure in view of the relatively small field-of-view (38-32 cm). They concluded that the limit on spatial resolution in such a system would probably be set by the finite spot size of the x-ray source, and they felt it was still an open question as to whether megavoltage CT would allow tumor visualization. As with the systems of Smith et al., (see section 2.6) this work was really approaching standards beyond those of NDCT scanners and more akin to DCT scanners. Lewis and Swindell(49a) and Lewis (49b) report setting up a very similar system to the second above at the Royal Marsden Hospital Sutton. Precise details of the differences with the new system are shown in Table 1. The major improvements include (a) image formation with a factor of 5 less dose but with no reduction of image quality, (b) image reconstruction during the time it takes to scan and a shorter reconstruction time, (c) the detector takes just a few minutes to attach instead of the half an hour for the prototype making working with the system much more convenient. and (d) the system has been put into clinical use. Images from this system are shown in Figure 16. structions

Discussion:

M %ut is the rake qf’data, fkm

NDCT

I603

WEBB

has also been variable. To some extent, one gains the impression that the usefulness of each machine has depended partly on factors other than its physical performance. For example, where a center has not had regular access to a DCT scanner, use of the NDCT scanner has been popular. If one sought to make general statements. one might conclude that most of the successful NDCT scanners achieve a spatial resolution of some 5 mm or better and a density resolution of some 5- 10% or better. This enables good differentiation of soft tissue structures from large air spaces and bones. In particular, imaging of the thorax is very successful, the pictures showing not only good external outline but the average position of the lungs in relation to the contour and to bony structure. The prime need for this information is for properly positioning the beams for external therapy and for enabling tissue inhomogeneity corrections to be made. Since most of the NDCT scanners yield only one or a few sections (and not a full 3D image of the region to be treated), the tissue inhomogeneity corrections are confined to those methods that are Z-dimensional. There are many such methods but. for example. it may easily be shown that these performance characteristics are consistent with obtaining dose at a point to better than 3%. In Figure 17 we illustrate this remark. A simple model was constructed of a single beam irradiating a volume of material comprising IO cm of water overlaying 10 cm of lung overlaying 10 cm of water. In these circumstances the much-used “Batho correction factor” CFM, giving the ratio of the dose at point M to the corresponding dose had all the material been water, is (76):

scanners in rudiotherup~~.?

From our review of individual systems, it may thus be appreciated that the methodology of developing NDCT imaging for radiotherapy purposes has been varied. Not surprisingly. the performance of these machines is nonuniform as a result. Moreover, the degree to which the equipment has been brought into useful clinical service

where the beam traverses

M pixels (labelled

(a) Fig. 16. Figures from RMH Megavoltage

imager (a) pelvic scan (b) thoracic

scan.

by i). the i-

1. J. Radiation Oncology ??Biology 0 Physics

1604

-155

Lung

/

lb-

)

water

1 14

Fig. 17. Batho correction

18

22

dePthM26

factors for a simplified

t130

J

30

tissue model.

The solid line shows the factor calculated assuming that tissue boundaries and densities are precisely known. The other curves show the effect of systematic errors in the tissue boundaries and densities and random density variations (key as in text).

th pixel being at depth xi where the density within the ith pixel is pi and T(Xi) is the tissue air ratio at depth xi. CL,,is the density of water. This correction factor is plotted in Figure 17 as a solid line, The effect on the correction factor of a wrong estimate of the boundary between water and lung (supposedly at depth 10 cm) was calculated by placing this boundary at a number of “incorrectly located” depths, including 9.4 and 10.6 cm (solid lines with one dot and two dots interspersed respectively). Nowhere did the correction factor deviate by more than 3% of the true Batho factor. Next the boundary was considered to be correctly located, but the tissue density was subjected to + 16% systematic variation about the nominal value (0.3 in lung) with the same conclusion regarding dose (The dashed curve is for the - 16% and the dashed-dot curve is for the + 16%). Finally, the effect of random variations in density was determined. In this case the correction factor varies very little from the true value (isolated circles). This little calculation is, of course, oversimplistic because in reality dose distributions arise from multiple beams passing through more inhomogeneous regions and in irregular geometries. However, it serves to make the point that data from NDCT scanners are adequate for

December 1990. Volume 19. Number 6

tissue inhomogeneity corrections involving large air spaces. The calculations of Geiss and McCullough (20) are much cited also as showing that quite large spatial and density imaging inaccuracies can be tolerated without important errors arising from their use in dose calculation. Wilkinson and Redpath (9 1) made similar comments that for Cobalt energy 1 cm errors in boundaries were acceptable, giving only 3% dose error. Hendee (26) wrote, “As Geiss and McCullough have shown, present photon beam inhomogeneity correction methods can tolerate uncertainties in electron densities of 10% and in distances of 5 mm with errors of no greater than 2% in corrected dose distributions. Cannot this level of accuracy be met simply by using a digitizing device to trace the inhomogeneities from a CT display service, and by assigning some average value for the electron density within the inhomogeneity without requiring an element by element knowledge of the electron density distribution?” Kijewski and Bjarngard (3 1) also wrote that “CT techniques can provide quantitative information about the body composition with high accuracy and with a resolution exceeding what can reasonably be required for radiotherapy dose calculations.” Perhaps one should not forget that one of the most important pieces of information is patient contour in the treatment position and that this is easily obtained from NDCT scanners with no danger of compressing soft tissue or the inconvenience of using lead wire or plaster of paris. The data of Redpath (54) showed that differences between outlines obtained by NDCT means and those by conventional means could be as large as 3 cm. Our own data on breast imaging support a strong contention that to plan breast radiotherapy accurately, the outlines at several levels are needed. Moreover. the internal air spaces cannot simply be derived from some average chest wall thickness. It should not, of course, be forgotten that there have been many attempts to build devices that give the external contour only by a non-contact method. These were reviewed by Sternick (63) and include a variety of optical and ultrasound methods (which generally only achieve the anterior contour) as well as methods involving the use of orthogonal radiographs of the patient with a lead shot ribbon taped around the contour. The accuracy of radiotherapy may well, as Hendee has said (26) depend ultimately on whether the imaging geometry for treatment planning matches the imaging geometry for treatment delivery. To date, there has not been a great deal of activity towards solving this problem with the exception of the work of Swindell. There is a historical precedent at least on paper as follows. A very interesting development of an adjunct for tomographic imaging was patented by Froggatt (19). This concerned the well-known problem, when delivering radiotherapy via rotation of the source relative to the patient, that the patient (and thus the tumor being treated) may

Non-standard CT scanners 0 S. WEBB

move during the therapeutic exposure by small but significant distances. Froggatt provided a detector array, based on the use of scintillators or a fluoroscopic screen, which recorded a transmission projection during the therapy. Such projections were able to be computed repeatedly for each orientation of the beam relative to the patient. Prior to this, a diagnostic CT scan of the patient in the same position had been made. From this a computer calculated the projection data for all angles to be visited by the therapy beam. The projections during treatment with the gantry at some-and all-orientations were compared with those calculated from the therapy scan. From the comparison, the required beam shift was executed to reregister the treatment beam with the target. The patent goes on to point out that 2D projection data may be likewise calculated and used to make out-of-plane adjustments. This problem, described 10 years ago, is still considered by some to be largely unaddressed in practical terms. In conclusion, one must perceive that the role of NDCl

1605

scanners in radiotherapy is an adjunct to that of diagnostic CT scanners. There are some problems for which NDCT scanners cannot be of help and there are others for which they are ideally suited. Very few manufacturers have expressed interest in NDCT machines, which is a pity; this may well be because the experience with them has not reached a “critical mass” and radiological history is full of good inventions being ignored in this way. The time is right for a reappraisal. In summary, one may conclude that the principal limitation of NDCT scanners is that they generally produce single slices, rather slowly and sometimes with a poor FOV. NDCT has come a long way but to find a ready place in the clinic, it is necessary to address the multislice problem and to remember the need for a wide FOV. Spatial and density resolutions are probably adequate, although improvements would reduce the limitation of being unable to visualize tumors on NDCT images. However, one must be mindful of the potential danger of simply re-inventing the diagnostic CT scanner.

REFERENCES 1. 2.

3.

4. 5.

6.

7.

8.

9.

10.

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Non-standard CT scanners: their role in radiotherapy.

In the past 10 years a number of groups worldwide have investigated the extent to which the imaging requirements for radiotherapy planning may be met ...
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