scan film radiorgraphy

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1986, Th e British Journal of Radiology, 59 , 365-371 APRIL 1986

A method for increasing the resolution of scanned projectionradiography and other digital X-ray systemsBy D. R. Checkley, M.Sc, X. P. Zhu, M.D., D. S. Hickey, Ph.D., J. K. Hughes, J. B. Carter, M.Sc.and I. Isherwood, M.R.C.P., F.R.C.R.Department of Diagnostic Radiology, The Medical School, University of Manchester, Oxford Road,Manchester M13 9PT(Received April 1985 and in revised form September 1985)

ABSTRACTA method for increasing th e sampling frequency of digital X-ray systems is described. The method employs discrete,stepwise magnetic deflection of the focal spot and therecombination of the resulting displaced images. The techniquewas applied to a GE CT/T 8800 scanner operating in "ScoutView" mode. The hardware an d software modifications wereminor. Th e transverse resolution wa s shown to improve from0.63 to 0.9 line pairs per mm, using a test phantom. Th e effectof the improvement in resolution is also illustrated in thehuman. Th e possibility of further improving th e resolution ofth e system is discussed.This paper describes the development of a method forincreasing the sampling frequency of digital X-raydetector systems by deflection of the X-ray focal spot.The aim is to demonstrate that an increase in spatialresolution of digital images can be obtained usingsimple software and hardware modifications on acommercial CT scanner. The method was implementedusing the scanogram, a line-scanned digital radio-graphic facility usually employed to locate axialsections. The good contrast resolution of somescanogram systems (Sommer & Brody, 1982) hasstimulated a general interest in their direct use fordiagnosis. Initial results were disappointing (Lawson etal , 1980; Baker, 1981) but with modifications moreencouraging data have been obtained (Foley et al, 1979;Casselet al, 1981).A limitation of the scanogram is its poor spatialresolution. An increase in resolution would provideuseful diagnostic images, particularly in limbs, the head,and the spine, where movement is not a problem.Transverse and longitudinal resolution in scanograms iscontrolled by different factors (Katragadda et al, 1979).In our study the transverse sampling frequency andspatial resolution were increased by electron-beamdeflection. To increase resolution in the longitudinaldirection, the X-ray beam was collimated and theinterpulse cradle travel reduced. These techniques aredescribed and high resolution images are presented.

MATERIALS AND METHODSAll experiments were performed on a GE CT/T 8800scanner in scanogram, i.e. "Scout View" mode.Attaching prototype magnetic coils to the X-ray

generator required a minor hardware modification. Allimages were produced using a standard Maxiray 100X-ray tube. The machine was in full-time clinical useand the high resolution facilities were introducedwithout major disruption.High contrast spatial resolution was measured with a0.1 mm thick lead-bar pha ntom . This phan tom offereda convenient visual assessment of resolution in therange of 0.5 to 5.0 line pairs per mm (lp/mm). Initially itwas used to estimate normal scanogram resolution inthe transverse and longitudinal directions. Thesemeasurements were obtained on separate scanogramswith the phantom mounted on the patient cradle in thecentre of the field. For transverse measurements thelead bars in the phantom were aligned parallel to thescanner axis, whilst for the longitudinal measurementsthey were at right angles to it.Increasing the transverse resolutionThe principle of the technique involves discretedeflections of the focal spot. These deflections move theX-ray image in small steps across the effectivelystationary detectors. A digital line image is recorded foreach position of the focal spot. Recombination of twoor more spatially displaced images then results inimproved resolution. The size of the steps between thepositions of the image is manipulated such that they area fraction of the detector width for the geometry of theselected object plane.To increase the transverse resolution we considered ahorizontal plane in the centre of the image field of thescann er 780 mm from the focal spot and 320 mm fromthe central detector. For simplicity, we considered firsta focal-spot deflection of 1.46 mm at the anode surface,which would produce an image shift of half a detectorwidth (0.6 mm ). Other deflections may be treated by asimple extension of the geometry.Lateral beam deflection for the image plane wasequivalent to lateral displacement of the curved detectorarray. Out-of-plane tomographic blurring was ignored,since for a slice effectively 4 00 m m th ick t he blu rrin gextends over less tha n 0.12 mm , or o nly 10% of thedetector width. The intensity distributions sampled bythe detectors are thus effectively the same for thedifferent positions of the focal spot.

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VOL. 59, No. 700

View 1

View 2(displaced\ detectorwidth)

Combinedviews -pseudo \widthdetectors

Thus, aseries ofsimultaneousequations isproduced

D. R. Checkley, X. P. Zhu, D. S. Hickey, J. K. Hughes, J. B. Carter and I. Isherwoodfreedom from the propagation of noise. For eachdetector we set,i A2 A3

B i B 2 B 3

Ci c2 c3 C 5 c6 c7

Ai=Ci+C 2Bi= C2+C3A 2= C3B 2=

FIG. 1.Pseudodetector response. When a detector with a rectangularresponse function is displaced laterally between two radio-grap hs of the same object, the two responses A, and B, can berecombined to form a set of pseudodetector responses C y.These pseudodetectors have half the width and therefore twicethe sampling frequency of the original detector array.

The focal-spot intensity structure was assumed to beGaussian (Barrett & Swindell, 1981). This form haspreviously been used to describe the GE CT/T 8800focal spot in investigations of the resolution of CTscanne rs (Glover & Eisn er, 1979; Blumenfield &Glover, 1981). The xenon gas detectors were eachconsidered to have an approximately rectangularresponse function across a width of 1.2 mm.In the image reconstruction pairs of detector readingswere combined to produce pseudodetector values, asshown in Fig. 1. The result is a series of simultaneousequations relating m pseudodetectors to the n detectorvalues. For simplicity, we consider recombination oftwo images (m = 2n ) displaced relative to each other byhalf the width of a detector, thus:

"i+l = (1)where A ; and B, are the response of the ith detector andits deflected value respectively, and C ; is the jth.pseudodetector value. There are several methods thatcan be used to solve this form of equation althoughimage noise can invalidate direct solutions. For thisstudy we chose a simple, locally acting iteration. Thismethod had the advantages of simplicity and relative

an dD a = A,/2D,2 = B,/2

where D is a dummy array. Then it follows that anestimate of the pseudodetector values isan d (2)(3)

where e is an error term. This error is calculated anddistributed over the dummy array such that:' A i / 2

^ 1 2 ^12 1 " t B (/2 VT-After two or three iterations of the array usingequations (2), (3), and (4), the image data converged.The coil produced distortion in the displaced focalspots, which necessitated the use of simple smoothingand grey-level normalisation procedures to produceartefact-free images.Deflecting the n detectors by half a detector width isthus equivalent to sampling the X-ray intensitydistribution using In half width detectors. Otherpossibilities, such as deflection of the spot to fourpositions and effectively overlapping the shifteddetectors by one quarter of a detector length, involveonly simple modifications to the mathematics. Thetechnique may be used with non-rectangular detectorresponse functions by a simple extension, but the valuesof the integral over the appropriate overlap regionsmust be known.Recombination of images spatially separated by afraction of the width of a single detector was modelledby the convolution of the detector response functionwith a comb fraction. The result was convolved with thefocal-spot intensity distribution to give a ModulationTransfer Function (MTF) for the system. The algorithmemployed for the sampling process is essentially similarto that described by Barrett and Swindell (1981).Coil designThe coil prototype was designed for use with thepulsed X-ray system of the scanner. This rectangularcoil had 200 turns and 1 ohm d.c. resistance and wasdesigned to fit around the initial collimator of the X-raytube, as shown in Fig. 2. The distance between the coilcentre an d the electron b eam was 10 cm. Curren tswitching for the coil was triggered by the couchelectronics of the scanner. These were modified toprovide a pulse that reversed the polarity of the currentin the coil with each X-ray pulse (25 times/s). Alternateviews recorded from the xenon detector array were thusdisplaced by half the width of a detector.

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APRIL 1986Increasing the resolution of digital X-ray systems

FOCAL SPOT

COLLIMATOR"BOX

200 TURN COIL1 2 -1 6 a m ps A C

100c

XENONDETECTORS

FIG. 2.The position of the 200-turn coil, around the initial collimatorbox. The experimental conditions for obtaining pinhole imagesof the focal spot are also illustrated.

A number of experiments were required initially todetermine the relationship between coil current andfocal-spot displacement. From this calibration the coilcurrent required for deflection of the image by onequa rter of a detector width was determ ined (12 A).Images obtained with focal-spot deflection were in effectdouble sampled.The result of combining four laterally displacedimages was investigated using a single strip of the barphantom as the object. This phantom was positioned inthe centre of the field on a travelling microscope andmoved laterally in steps, each of which corresponded toan image displacement of one quarter of the detectorwidth.Improving longitudinal resolution

Spatial resolution in the longitudinal direction wasincreased by placing a 0.5 mm collimator centrally overthe output of the standard collimator box. Thisproduced an X-ray beam 3.5 mm wide at the detectors,i.e. substantially narrow er than the 5 mm width for thenormal scanogram.A further increase in resolution was obtained byhalving the interpulse cradle increment. The number ofcradle-encoder output pulses for each cm of movementwas doubled for the combination of the two images.Focal-spot structureThe effect of deflection on the form of the focal-spot

structure was determined using the pinhole techniqueshown in Fig. 2. Pinho le images w ere recorded ondental film using 80 kVp, 320 mA, and pulse width code3 (a 4 ms p ulse w idth). Precise alignment for theseimages was achieved with simple software modificationswhich allowed the projection data to be read for eachview. It was then possible for the pinhole or otherobjects to be positioned accurately over the centraldetector in the array. All objects placed within thescanner ring were positioned by this method.Data processingNormal scanogram data are corrected for offsets,normalised using the reference detectors, calibratedusing data from an axial air scan and filtered. For thehigh resolution images we employed the normal offsetand reference data corrections, but used calibrationdata from an air scanogram instead of an axial scan.The air-calibration scanogram was carried out with the0.5 mm collimator in place and with the coil producinglateral beam deflection. The normal scanogram lowfrequency suppression filter was not used.Calibrated projections of the object were stored inscanogram raw data files. Since in each interpulseinterval the cradle moved longitudinally, temporallyconsecutive projections were not spatially equivalentand could not be directly combined. Linearinterpolation, between adjacent projection values withthe same coil current direction, was therefore employed.In the final image each interpolated projection wascombined with the equivalent real projection.

RESULTSTheoretical MTF curves calculated from theGaussian focal spot and rectangular detector response

Normal sampling< 2 sampling; 4 sampling

Spacing frequency cycles mmFIG. 3.

System MTF curves calculated for a Gaussian focal spot and arectangular detector response function. The curves correspondto normal, two-image and four-image recombinations.367


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V O L . 59, No. 700D. R. Checkley, X. P. Zhu, D. S. Hickey, J. K. Hughes, J. B. Carter and I. Isherwood

A normal scanogram and a combined image of thebar phantom, reconstructed with half-width pseudo-detectors, are shown in Fig. 5. An apparen timprovement in the transverse resolution can be seen inthe combined image. The four lead bars and five spacescan be resolved at 0.63 lp/mm in the normal image and0.9 lp/mm in the improved image. This is in agreementwith data derived from the predicted MTFs, whichindicated an improvement of 72% in the spatialresolution.The normal scanogram and combined image of thebar phantom, with bars aligned transversely, are shownin Fig. 6. An increase in longitud inal resolution fromless tha n 0.5 lp/mm to 0.7 lp/mm can b e seen in thecombined image. Theoretically the decreased detectorlength should produce up to 40% improvement inspatial resolution. This appears to have been achieved.Normal and beam deflected scanogram images of ahuman ankle are given in Fig. 7. The combined image isgenerally sharper and shows a number of featuresresulting from the improvement in resolution. Thisimage demonstrates that despite the presence ofsubstantial focal-spot distortion, artefact-free imagescan be produced.Our preliminary results from the combination of four

F I G . 4.Pinhole images of the deflected focal spot obtained using the200-turn coil.

functions are shown in Fig. 3 for normal, two-imageand four-image combinations. No further improvementwas obtained with eight-image combinations due tolimitation by the MTF of the focal spot.Deflected pinhole images produced by the 200-turnrapidly switching coil are given in F ig. 4. T hedisplacement produced by this coil differed at the topand bottom of the pinhole images.

F I G . 5.Image of lead bar phantom to show transverse resolution, (A) The norm al unfiltered sca nog ram , resolution limit 0.63 lp/mm , (B )with electron beam deflection and image combination, resolution limit 0.9 lp/mm.

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FIG. 6.Image of lead bar phantom to show longitudinal resolution, (A) Normal scanogram, resolution limit less than 0.5 lp/mm, (B ) withoutput collimation and decreased interpulse cradle movements, resolution limit 0.7 lp/mm.

FIG. 7.(A) Normal scanogram and (B) increased resolution images of the human ankle. The images are similar but the combined imageboth has an improved edge definition and appears less noisy. It is clear that an acceptable image quality was obtained despite thedistortion of the deflected focal spots in the prototype system.

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V O L . 59, No. 700D. R. Checkley, X. P. Zhu, D. S. Hickey, J. K. Hughes, J. B. Carter and I. Isherwood

A

B

inI 202-8

Iff 0-5 line pairs /mm4 0

F I G . 8.Th e potential of beam deflection, for further improving th eresolution of a scanogram. A single strip of the bar phantom isshown with (A) single-image, (B) two-image and (c) four-imagerecombinations. For these images th e object, not the focal

spot, was moved.

images show that further substantial increases inresolution are possible. In Fig. 8 three images of thesame strip of the bar phantom are shown. These imagesillustrate normal transverse resolution and the effect ofcombining two and four deflected images. These datawere obtained by moving th e object relative to astationary X-ray beam. Since focal-spot distortion wasno t a problem in this experiment only a simple,unweighted moving average procedure was required forsmoothing.

DISCUSSIONWe have demonstrated that an increase in thetransverse and longitudinal resolution of a scanogramsystem can be obtained by making relatively minorchanges to a conventional CT scanner. The technique ofbeam deflection could have general applications indigital radiography and CT.

A specific application of beam deflection may be indigital tomosynthesis systems (Maravilla et al, 1983). Indigital tomosynthesis the detector array is movedrelative to the patient, and a series of images iscollected. Tomographic planes within th e patient arethen formed by simple manipulation of the data. Thetechniques and algorithm described here could beapplied to increase resolution.Deflecting the electron beam, and therefore the X-raybeam, with a magnetic field is relatively simple andinexpensive. The technique can be used to achieveincreased sampling frequencies, and in systems limitedby detector width these modifications will improve thespatial resolution. Resolution also depends upon th eintensity distribution and structure of the focal spot(Glover & Eisner, 1979). If beam deflection isemployed, resolution is restricted by focal-spot size. Ourresults are consistent with th e published analyses ofoversampling (Glover & Eisner, 1979; Blumenfield &Glover, 1981), which also indicate that the increase inspatial resolution with oversampling on the CT/T 8800is limited by the size of the focal spot. An estimate of theeffective width of the detector aperture and thus th esuitability of the beam deflection method may beobtained from:

Xe[ = [X 2 + (M-l)2S2~\yMwhere Xe{ is the effective detector aperture width, M isthe magnification, X is the actual detector aperturewidth, and S the spot size (Yester & Barnes, 1977).The disparity in transverse and longitudinalresolution, although not readily apparent in the images,could be rectified by tube and detector-box collimation.This would be an effective method for decreasing th ewidth of the X-ray beam, but would result in wastedpatient dose. Normal scanogram images do not haveequal transverse and longitudinal resolution. The entrydose for a full-length scanogram is approximately2.5 mG y, a dose which it is anticipated would beproportionately increased by decreasing th e interpulsetravel.The reconstruction algorithm effectively smooths theimages in the longitudinal direction and removes someof th e linear artefact caused by pulse-to-pulsedifferences in photon flux. Pulse-to-pulse variationsmay be caused by deflecting the focal spot and changingthe electron beam track on the anode surface (Braun,1979). Electron-beam focusing may also be a cause,although our results suggest that non-uniformity of theapplied magnetic field is the major problem. There isconsiderable potential in the design of coils to produce amore uniform X-ray output with alternately deflectedpulses. Improved calibration procedures should reducepulse-to-pulse variations and improve scanogramimages still further.Some focal-spot distortion was present, but it did notaffect our production of higher resolution images. Thisdistortion was not present when a larger coil, situatedfurther from the electron beam, was used for deflection.

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A computer simulation of the magnetic field in theregion of the electron beam was performed. Thissimulation used the Bio-Savart formula (e.g. Kip, 1969)and involved a summation of the vector field for 600elements of the coil wire in each of the 200 turns in thesmall coil, and 100 elem ents in each of 4000 turns in thelarger coil. Non-circular geometry in the small coilwrapped round the collimator box was included in thesimulation. The results indicated a 25% change in themagnitude of the field across the electron beam in thesmall coil an d 1% with the large coil, consistent withour experimental results and the apparent cause of focal-spot distortion. Two coils, one on either side of the tubecasing, could be balanced to produce a more uniformfield ac ross the electron beam.Detector dimensions, an d hence the samplingfrequency, are factors which limit the resolution ofmany digital systems, including line-scanned projectionradiographic systems (Brody et al, 1980) and CTscanners (Barnes et al, 1979). A method which achievedincreased image sampling rates, and hence an improvedspatial resolution, without detector or other majorhardware modifications, could have widespreadapplications in digital radiology.The increased resolution obtainable by combiningoverlapped images is limited by the focal-spot intensitydistribution. This distribution is convolved with theimages obtained and produces smoothing and blurring.Deconvolution in real space by direct matrix inversion(Bracewell, 1978) could provide a suitable method forremoval of this focal-spot limitation.We have shown in principle that reconstructions fromfour deflected images is possible as indicated in Fig. 8.Production of improved resolution images bycombination of four deflected data sets is only slightlymore involved than the technique used for thecombination of two. The transverse resolution expectedfrom combination of four images is close to 1.4 lp/mm.Such resolution may be sufficient for a digitalalternative to conventional imaging in a number ofclinical situations.

A C K N O W L E D G M E N T SWe extend ou r sincere thanks to Brian Pullan for his helpand would like to acknowledge grants from the North WesternRegional Health Authority and the Peter Kershaw Trust. We

also thank I.G.E. for technical help with this study.R E F E R E N C E S

BAKER, H. L., 1981. Screening for brain lesions with digital

rad iography of the head using a CT scanner. Journal ofComputer Assisted Tomography, 5, 54-59.BARNES, G. T., YESTER, M. W. & K I N G , M. A., 1979.Optimizing computed tomography scanner geometry. Anapplication of optical instrumentation in medicine VII.Proceedings of the Society of Photo-optical Instrumentation

Engineers, 173, 225-237 .BARRETT, H. H. & SWINDELL, W., 1981. In RadiologicalImaging Vol. 1, The Theory of Image Formation, Detectionand Processing. (Academic Press, London), pp . 290-296.BLUMENFIELD, S. M. & GLOVER, G., 1981. Spatial resolution inComputed Tomography . In Radiology of the Skull andBrain: Technical Aspects of Computed Tomography. Vol. 5.Ed . by T. H. Newton , and D. G. Potts (C. V. Mosby , StLouis), pp . 3918-3940.BRACEWELL, R. N., 1978. In The Fourier Transform and ItsApplications. 2nd edit. (McG raw Hill , Tokyo ), pp . 24-48 .BRAUN, M., 1979. Physics of X-ray tubes for CT scanners.IEEE Transactions of Nuclear Science, 26, 2840-2844.BRODY, W. R., M AC OVSKI , A. , LEHMAN, L. , DIBIANCA, F. A.,

V O L Z , D. & EDELHEIT, L. S., 1980. Intravenous angiographyusing scanned projection radiog raphy: Preliminaryinvestigation of a new method . Investigative Radiology, 15,220-223 .CASSEL, D. M., Y O U N G , S. W., BRODY, W. R. & H A L L , A. L.,

1981. Cancer imaging by scanned projection radiography.Journal of Computer Assisted Tomography, 5, 557-562.FOLEY, W. D., L AW SON, T. L., SCANLON, G. T., HEESCHEN,R. C. & DIBIANCA, F. , 1979. Digital radiography of the chestusing a computed tomography instrument. Radiology, 133,231-234.GLOVER, G. H. & EISNER, R. L., 1979. Theoretical resolutionof computed tomog raphy systems. Journal of ComputerAssisted Tomography, 3, 8 5 -9 1 .KATR AGADDA, C. S., F OGEL, S. R., C OHE N, G., W A G N E R , L. K.,

M O R G A N , C , H A N D E L , S. F., AMTEY, S. R. & LESTER, R. G.,1979. Digital radiog raph y using a computed tomograph icinstrument. Radiology, 133, 83-87 .KI P , A. F., 1969. In Fundamentals of Electricity and

Magnetism. (McGraw Hill , New York) , p. 241.LAW S ON, T. L., FOLEY, W. D., IMRAY, T. J., STEWART, E. T.,

WILSON, C. R. & YOUKER, J. E., 1980. Abdominal computedradiography: Evaluation of low-contrast lesions.Investigative Radiology, 15, 215-219.M AR AVILLA, K. R., M U R R Y , R. C. & HOR NE R , S., 1983. Digitaltomosynthesis: A technique for electronic reconstructivetomography . American Journal of Roentgenology, 141,497-502.SOMMER, G. F. & BRODY, W. R., 1982. Contrast resolution ofl ine-scanned digital radiography. Journal of ComputerAssisted Tomography, 6, 373-377.YESTER, M. W. & BARNES, G. T., 1977. Geome trical limitationsof computed tomograp hy (CT) scanner resolution.

Proceedings SPIE, Applied Optical Instruments in Medicine,127, 296-303 .

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