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WO2008065394A1 - Procédé et appareil pour réduire une distorsion dans une image de tomographie assistée par ordinateur - Google Patents

Procédé et appareil pour réduire une distorsion dans une image de tomographie assistée par ordinateur Download PDF

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Publication number
WO2008065394A1
WO2008065394A1 PCT/GB2007/004557 GB2007004557W WO2008065394A1 WO 2008065394 A1 WO2008065394 A1 WO 2008065394A1 GB 2007004557 W GB2007004557 W GB 2007004557W WO 2008065394 A1 WO2008065394 A1 WO 2008065394A1
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Prior art keywords
implant
reconstruction
projection
initial
correspondence
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PCT/GB2007/004557
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English (en)
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Julian J. Liu
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Oxford University Innovation Ltd
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Oxford University Innovation Ltd
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    • G—PHYSICS
    • G06—COMPUTING OR CALCULATING; COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T11/00—2D [Two Dimensional] image generation
    • G06T11/003—Reconstruction from projections, e.g. tomography
    • G06T11/005—Specific pre-processing for tomographic reconstruction, e.g. calibration, source positioning, rebinning, scatter correction, retrospective gating
    • G—PHYSICS
    • G06—COMPUTING OR CALCULATING; COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T2211/00—Image generation
    • G06T2211/40—Computed tomography
    • G06T2211/448—Computed tomography involving metal artefacts, streaking artefacts, beam hardening or photon starvation

Definitions

  • the present invention relates to a method and apparatus for reconstructing an image for X- ray computed tomography robust to metal artefact, thorax/pelvic streaking and lower dose streaking, with particular but not necessarily exclusive, emphasis on one or more of errorless forward modelling, beam hardening analysis and approximation, and noise identification and isolation prior to reconstruction.
  • Metal implants are one of the main reasons for the occurrence of image artefact, which is regarded as one of the key factors impeding post-operative evaluation using Computed Tomography (CT). Since the attenuation coefficient of metal is much higher than that of human tissue, a metal implant causes a low signal to noise ratio, beam hardening and scatter, all of which contribute to the projection data received from CT scans to be corrupted, by virtue of the production of a star-burst artefact and thus presenting an obstacle to the proper reconstruction of the image.
  • the filtered back-projection (FBP) method is the most widely used method for image reconstruction since it gives high computational efficiency whilst maintaining reasonable accuracy [I].
  • Star-burst artefacts have been reduced by substituting the data corresponding to the projection lines through metal objects with data from the neighbourhood [2], or synthetic data using linear, polynomial, or other interpolation strategies [3,4], or by adjusting the wavelet decomposition coefficients [5]. These methods are very effective for removing streaking generated by metal implants and do not increase the amount of computation significantly. Iterative algorithms have been regarded as a potential method of providing high quality CT reconstructions, especially for metal artefact reduction [6-8]. An alternative strategy for avoiding metal artefact was investigated by using a low attenuation material such as titanium [9]. Currently, the standard reconstruction strategy in clinical practice is still a filtered back projection with linear interpolation, which is reasonable for some general applications.
  • the present inventor has realised that a new method for computed tomography, parallel and equal to Radon transformation but with pixel voxel based description based on sinusoidal rather than line integrals could achieve the reduction of metal artefacts without the burden of computational costs, as well as eliminate streaking in upper-thorax/pelvic imaging and lower dose imaging to improve the quality of image.
  • a first specific object of the present invention is to reduce the artefact caused by the metal implant during CT scanning.
  • a second specific object of the present invention is to improve significantly the quality of the scanned image.
  • a method for reducing distortion caused by an implant and/or thorax /pelvic streaking in a CT scanned image of an object comprising the steps of : performing an initial reconstruction of the object using an iterative process; locating the region affected by an implant ; detecting and analysing inconsistencies caused by the implant inside said scanned object; approximating the real correspondence of the implant, updating said initial reconstruction; and isolating the approximated correspondence of the implant, updating said initial reconstruction and isolating the approximated correspondence of the implant; synthesising the implant- free reconstruction and the location of said implant; analysing reliability of the updated reconstruction; and displaying the reliability analysis as pixels of the reconstruction.
  • an apparatus for reducing distortion caused by an implant and/or thorax/pelvic streaking in a CT scanned image of an object the apparatus being arranged and configured to comprise the steps of: perform an initial reconstruction of the object using ain iterative process; locate the region affected by the implant; detect and analyse inconsistencies caused by the implant inside the scanned object; approximate the real correspondence of the implant, update said initial reconstruction, and isolate the approximated correspondence of the implant; synthesise the implant-free reconstruction and the location of said implant; analyse reliability of the updated reconstruction; and display the reliability analysis as pixels of the reconstruction.
  • FIG. 1 is an analytically geometrical description of errorless forward modelling.
  • FIG 2 shows a diagram of the method and apparatus of the present invention.
  • FIG 3 (a) shows an observed projection from a conventional CT scanner.
  • FIG 3 (b) shows a default reconstruction of a conventional CT scanner.
  • FIG 4 depicts the inconsistency of the projection of a conventional CT scanner.
  • FIG 5 (a) shows an ideal correspondence of the metal implant in the projection.
  • FIG 5 (b) shows the maximum value of each view of the ideal correspondence of the metal implant, which strongly correlates to the inconsistency description shown in FIG 4.
  • FIG 6 (a) depicts the approximated correspondence of the metal implant.
  • FIG 6 (b) depicts the compensation for beam hardening.
  • FIG 7 depicts the detected high energy noises in projection.
  • FIG 8 (a) is a projection amended using the method and apparatus of the present invention.
  • FIG 8 (b) is a reconstruction using the method and apparatus of the present invention.
  • FIG 9 (a) is a sagittal reconstruction of the original output of a conventional CT scanner.
  • FIG 9 (b) is a sagittal reconstruction using the method and apparatus of the present invention, where clearer image has been obtained.
  • FIG 10 (a) is a default output of a conventional CT scanner for pelvic imaging.
  • FIG 10 (b) is a reconstructions using the method and apparatus of the present invention, where streaking has been eliminated.
  • FIG 11 (a) is a default output of a CT scanner for lower dose imaging.
  • FIG 11 (b) is a reconstruction of the same lower dose scan using the method and apparatus of the present invention, where the image quality has been significantly improved.
  • the method according to the present invention comprises the steps of first pre-processing raw data from the scanner to obtain a projection and a multiple-order (2-7) polynomial is automatically identified to form a baseline and to correct the projection.
  • the attenuation values of a projection smoothed using a multiple phase wavelet decomposition are accumulated to detect an initial global inconsistency baseline.
  • data from this local couch-caused inconsistency which is normally detected in a small area where the attenuation has a local maximum, is accumulated in the direction of detectors.
  • the inconsistency caused by the scanned body (also called “scanning-object-inconsistency description") is then obtained by accumulating a smoothed projection in the direction of detectors, where the initial global inconsistency baseline, the local couch-caused inconsistency, and high energy noise caused by metal implant, detected using multiple phase wavelet decomposition, are removed.
  • the initial reconstruction the location of the metal implant is first identified and then the correspondence of the metal implant is generated so the region affected is detected.
  • the initial reconstruction is obtained by iterating several times and analysing the errors.
  • the ideal correspondence of the metal implant in the projection data set is a group of extended sinusoidal curves and is generated by an errorless forward modelling algorithm to be used as a basic function to approximate beam hardening.
  • the original projection can therefore be represented as
  • r / are the residuals, which are ideally expected as white noise.
  • rj may include more information rather than just white noise.
  • the filter back projection for example, aims at reducing the accumulated error causing the boundary of different tissues to blur, however, the effect could not be completely obtained.
  • An iterative algorithm can be effective to overcome this kind of limitation of reconstruction algorithm, which is described as follows:
  • equation (2.k) can be presented as
  • the iterative can be terminated when is smaller than a given positive small number or start to oscillate after a dramatic decreasing period, and the output is expressed as
  • the residuals Po - F(C) are most likely not white noise due to beam hardening and high energy noise.
  • the reconstruction is an ill-posed inverse problem.
  • the reconstruction will not be completely reliable because it may be based on the wrong information, and the objective of the reconstruction can be a minimum variance of the residual.
  • the projection can be divided into a high reliability region and a low reliability region.
  • Beam hardening is regarded as one of the key factors causing artefact and impeding a detailed post operation evaluation using CT, when metal implants are present.
  • the projection of CT scan is the integration alone the X-ray beam path, but because of high attenuation, the beam of X-ray becomes harder and causes nonlinearities, especially in the region affected by metal implants.
  • the accuracy of the forward modelling is sensitive to the approximation of beam hardening.
  • An errorless forward modelling algorithm is implemented by extending the sinusoidal description, combining analytical computation with discrete computation, and substituting a pixel/voxel with a unit square/cube.
  • Errorless forward modelling for parallel beam is described as follows, which can be easily extended to fan beam, axial cone beam, and spiral cone beam. If the descriptions of the errorless forward modelling for fan beam, axial cone beam, and spiral cone beam are interested, please contact the author.
  • the X-rays through this unit square can be detected by one, or two, or even three detectors, and the attenuation values can be calculated according to the corresponding areas of the unit square determined by the angle of the view ⁇ * and the distance between the mapping of the centre of the unit square and the boundary of the detector.
  • the location where the centre of the unit square is mapped on the corresponding detector can be represented by the distance to the boundaries of the detector a and is floor function of /.
  • the unit square is therefore divided into four parts: the area above 1 + denoted as Si, the area between / and 1 denoted as S 2 i, the area betwee nd /, denoted as .S 22 , and the area below denoted as S 3 .
  • the attenuation values on the three corresponding detectors are S ⁇ , S 22 , and Si, respectively.
  • Si, S 21 , S 22 , and S 3 can be calculated as follows, (see figure 1).
  • P ⁇ cm be the ideal correspondence of the initially detected metal implant
  • V ⁇ cm ⁇ k denote the views containing the areas of P ⁇ cm (i,j) ⁇ k - max(P lcm )/ K m , and
  • V (k) be the views that
  • a kernel function can be identified with features as
  • ⁇ and x 0 are parameters to be identified.
  • - max represents the compensation for beam hardening caused by metal implant
  • Q denotes the compensation of beam hardening for the biological tissues in the shadow of metal implant.
  • the parameter k r ⁇ max can be directly computed, and the parameters of x 0 and a are automatically identified with a large set of initial values to avoid local minimum.
  • the number of the initial values is sensitive to computational efficiency, as well as the accuracy of the approximation. Therefore the number of the initial values needs to be balanced with computational cost.
  • the supervision function plays a key role to the control of beam hardening approximation, which is formed by combining the summation of the correspondence of the scanning-object in the direction of detectors and the moving average values.
  • the error caused by beam hardening can be adjusted, where x is the ideal correspondence of the metal implants, and K ⁇ is the approximated compensation value corresponding to beam hardening.
  • the projection is amended by the compensation, and reconstructed similar to the initial reconstruction discussed above.
  • the detected locations of metal implant are compared with the previous detections which will be updated if different, and repeat the steps following initial reconstruction.
  • the region affected by the metal implants and its neighbourhood can be smoothed by decomposing the projection with the isolation of the approximated real correspondence of the metal implants into several components using wavelet in MATLAB toolbox.
  • the two highest frequency bands are focused on, and the phase of the wavelet base function in the highest frequency band and the second highest frequency band are shifted V 2 and 1 A, respectively.
  • the high reliable region and the low reliable region are analysed statistically, and a function can be derived to map the variance and the distribution in the low reliable region into the same levels as in the high reliable region.
  • the statistically amended components corresponding to each phase of wavelet functions are reconstructed separately.
  • An amended projection can be generated by the synthesis of all the wavelet reconstructions, and the high energy noises caused by the metal implant are eliminated.
  • Phantom for upper thorax/pelvic can be scanned, as well as simulated using errorless forward modelling to generate references for noise analysis.
  • the smoothed projection can be decomposed into subsets according to the measurement function. In each subset the noses are statistically analysed, identified, and shrunk in wavelet domain. The streaking can then be eliminated from reconstruction. This method can be directly extended to lower dose imaging.
  • the final, the reconstructed image is obtained by synthesizing the implant -free reconstruction of the amended projection data and the position of the metal implant.
  • This procedure has the potential to be developed into a new reconstruction algorithm, based on errorless forward modelling.
  • the reconstruction is processed to match the expected attenuation values and output with dicom format.
  • the reliability analysis is displayed as pixels of the reconstruction.
  • the projections are collected from a CT scanner, GE Lightspeedl ⁇ .
  • Example 1 a patient had both knees replaced was scanned with the parameters as 12OkV and 100mA.
  • the inconsistency can be shown by summing up the attenuation values in each view in the smoothed projection, and part of the inconsistency is caused by beam hardening, as shown in figure 4.
  • the ideal correspondence of the metal implant in projection is generated by errorless forward modelling, based on the initial reconstruction using the global iterative, as shown in figure 5(a).
  • the maximum of the ideal correspondence of the metal implant in each view is shown in figure 5(b), where strong similarity with the inconsistency description shown in figure 4 can be detected.
  • the high energy noises are detected using multiple phase wavelet decomposition, as shown in figure 7.
  • the approximated correspondence of the metal implant and the detected high energy noises are isolated from the projection, as shown in figure 8 (a).
  • the amended projection is reconstructed in figure 8(b), where the metal implant and the adjacent area are clearly imaged.
  • Example 2 a pelvic imaging
  • FIG 10(a) A patient was scanned for pelvic imaging with parameters as 14OkV and 32OmA.
  • the default output of the CT scanner is shown in Figure 10(a), where streaking in horizontal direction affects the image.
  • noises in the projection are analysed, identified, and shrunk using the method and apparatus of the present invention.
  • the streaking has been eliminated, and the quality of the image has been obviously improved, as shown in Figure 10(b).
  • a CT performance phantom 76-410 was amounted in a plastic container filled with water and Niopam 300 iodinated contrast, 10 ml per litter, to simulate upper thorax or pelvic imaging.
  • the scanning parameters are 14OkV and 80mA, much lower than a normal clinical application.

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  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
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  • Apparatus For Radiation Diagnosis (AREA)

Abstract

L'invention concerne un procédé et un appareil pour réduire un artéfact provoqué par un implant métallique, des traînées du thorax/pelviennes, et des traînées de dose inférieure dans des images de reconstruction. Le procédé comporte les étapes ou les moyens pour prétraiter la projection observée pour une correction de ligne de base par reconstruction initiale de l'objet à l'aide d'un procédé itératif et génération d'une correspondance idéale de l'implant; pour localiser la région affectée par l'implant par la détection et l'analyse des incohérences provoquées par l'implant à l'intérieur d'un objet scanné, approximation de la correspondance réelle de l'implant et mise à jour de ladite reconstruction initiale, isolation de la correspondance approximée de l'implant, synthétisation de la reconstruction sans implant et la localisation dudit implant, analyse de la fiabilité de la reconstruction mise à jour, et affichage de l'analyse de fiabilité sous forme de pixels de la reconstruction. Les techniques clés de la présente invention incluent la modélisation directe sans erreur de balayage CT, l'analyse et l'approximation par durcissement de faisceau et l'identification et l'isolement des bruits avant reconstruction.
PCT/GB2007/004557 2006-11-28 2007-11-28 Procédé et appareil pour réduire une distorsion dans une image de tomographie assistée par ordinateur Ceased WO2008065394A1 (fr)

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2010131131A1 (fr) * 2009-05-13 2010-11-18 Koninklijke Philips Electronics, N.V. Procede et systeme d'imagerie de patient au moyen d'un dispositif medical personnel
US20150224320A1 (en) * 2014-02-10 2015-08-13 Cardiac Pacemakers, Inc. Multi-chamber leadless pacemaker system with inter-device communication
EP2975578A2 (fr) 2014-06-23 2016-01-20 PaloDEx Group Oy Systeme et procede de correction d'artefacts en imagerie 3d
CN106204726A (zh) * 2016-07-08 2016-12-07 贵港市人民医院 一种再造指模型的模拟方法
WO2018128630A1 (fr) * 2017-01-09 2018-07-12 Carestream Dental Technology Topco Limited Système pour la détection et l'affichage de régions obscurcies par un métal dans une tomodensitométrie à faisceau conique

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
LIU J J ET AL: "Reconstruction of computed tomography with metal implants", 2005 27TH ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY (IEEE CAT. NO.05CH37611C) IEEE PISCATAWAY, NJ, USA, 1 September 2005 (2005-09-01) - 4 September 2005 (2005-09-04), pages 3340 - 3343, XP010908523, ISBN: 0-7803-8740-6 *

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2010131131A1 (fr) * 2009-05-13 2010-11-18 Koninklijke Philips Electronics, N.V. Procede et systeme d'imagerie de patient au moyen d'un dispositif medical personnel
CN102422291A (zh) * 2009-05-13 2012-04-18 皇家飞利浦电子股份有限公司 对具有个人医疗装置的患者进行成像的方法和系统
CN102422291B (zh) * 2009-05-13 2015-06-24 皇家飞利浦电子股份有限公司 对具有个人医疗装置的患者进行成像的方法和系统
US20150224320A1 (en) * 2014-02-10 2015-08-13 Cardiac Pacemakers, Inc. Multi-chamber leadless pacemaker system with inter-device communication
EP2975578A2 (fr) 2014-06-23 2016-01-20 PaloDEx Group Oy Systeme et procede de correction d'artefacts en imagerie 3d
US9592020B2 (en) 2014-06-23 2017-03-14 Palodex Group Oy System and method of artifact correction in 3D imaging
US10939887B2 (en) 2014-06-23 2021-03-09 Palodex Group Oy System and method of artifact correction in 3D imaging
CN106204726A (zh) * 2016-07-08 2016-12-07 贵港市人民医院 一种再造指模型的模拟方法
WO2018128630A1 (fr) * 2017-01-09 2018-07-12 Carestream Dental Technology Topco Limited Système pour la détection et l'affichage de régions obscurcies par un métal dans une tomodensitométrie à faisceau conique

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