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INSTITUTE OF PHYSICS PUBLISHING Phys. Med. Biol. 5 (5) 39 339 PHYSICS IN MEICINE AN BIOLOGY doi:.88/3-955/5// An FK-like cone-beam SPECT reconstruction algorithm for non-uniform attenuated projections acquired using a circular trajectory Q Huang, G L Zeng,JYou 3 and G T Gullberg 4 epartment of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84, USA epartment of Radiology, University of Utah, Salt Lake City, UT 848, USA 3 epartment of Radiology, State University of New York, Stony Brook, NY 794, USA 4 E O Lawrence Berkeley National Laboratory, One Cyclotron Road, Mail Stop 55R, Berkeley, CA 947, USA E-mail: qhuang@ece.utah.edu, larry@ucair.med.utah.edu, jshyou@hotmail.com and gtgullberg@lbl.gov Received 6 February 5, in final form March 5 Published 5 May 5 Online at stacks.iop.org/pmb/5/39 Abstract In this paper, Novikov s inversion formula of the attenuated two-dimensional () Radon transform is applied to the reconstruction of attenuated fanbeam projections acquired with equal detector spacing and of attenuated cone-beam projections acquired with a flat planar detector and circular trajectory. The derivation of the fan-beam algorithm is obtained by transformation from parallel-beam coordinates to fan-beam coordinates. The cone-beam reconstruction algorithm is an extension of the fan-beam reconstruction algorithm using Feldkamp avis Kress s (FK) method. Computer simulations indicate that the algorithm is efficient and is accurate in reconstructing slices close to the central slice of the cone-beam orbit plane. When the attenuation map is set to zero the implementation is equivalent to the FK method. Reconstructed images are also shown for noise corrupted projections.. Introduction Single photon emission computed tomography (SPECT) enables us to visualize functional information on human organs or body systems with the use of radioactive isotopes (radiopharmaceuticals) introduced into the body. Unlike x-ray CT, SPECT provides information on organ function, not just structure, and thus can be used to diagnose diseases and cancers much earlier. SPECT can be used in imaging of the bone, heart, liver/spleen, 3-955/5/39+$3. 5 IOP Publishing Ltd Printed in the UK 39
33 Q Huang et al brain, renal, thyroid and other organs. This work concerns three-dimensional (3) image reconstruction in SPECT using cone-beam collimators with a circular trajectory. Cone-beam CT with a circular scanning trajectory is known for its fast data acquisition and minimum mechanical complexity. Much work has been done about 3 cone-beam reconstruction in the past two decades (Feldkamp et al 984, Grangeat 99, Smith 985, Tuy 983), among which the FK method (Feldkamp et al 984) is famous for its simplicity of implementation. In SPECT, however, the attenuation should be compensated for in quantitative studies. The most representative works in image reconstruction in 3 SPECT include different inversion algorithms for 3 exponential parallel-beam transform (Palamodov 996, Wagner and Noo, Kunyansky 4). Zeng et al (99) developed an algorithm for 3 cone-beam image reconstruction with non-uniform attenuation map; however the algorithm was an iterative one. To date, the analytical 3 cone-beam reconstruction with non-uniform attenuation has not yet been established. Thanks to Novikov s explicit inversion formula (Novikov, Natterer ) for attenuated Radon transform, the 3 reconstruction with non-uniform attenuation becomes feasible. By a change of variables, Novikov s formula can be applied to fan-beam geometry. Cone-beam reconstruction is then straightforward by extending the implementation to a 3 implementation. When attenuation is zero, our cone-beam reconstruction is equivalent to the FK method. It is well known, unfortunately, that cone-beam projections acquired with a circular trajectory are not sufficient for exact reconstruction according to Tuy s data sufficiency condition (Tuy 983). Therefore, in this paper the reconstruction method is an approximation. Our cone-beam reconstruction is efficient and is accurate in the regions close to the cone-beam trajectory plane. This paper is organized as follows. In section, Novikov s inversion formula for the attenuated Radon transform is reviewed and then applied to the fan-beam geometry by changing variables. An FK-like cone-beam SPECT reconstruction algorithm is then derived based on the fan-beam SPECT reconstruction algorithm. Section 3 provides simulation results to validate these algorithms. Conclusions are given with a short discussion in section 4.. Algorithm.. Novikov s inversion formula Assume x = (x, y) is a vector in a two-dimensional Euclidean space. Let f(x) denote the distribution of radiopharmaceutical concentration and µ(x) the attenuation map of the tissue, which can be non-uniform. The mathematical model of the attenuated Radon transform in a parallel-beam geometry (see figure ) is (Gullberg 979) (R µ f )(θ, s) = f(sθ + tθ ) e (µ)(sθ+tθ, θ ) dt, () where θ = ( ) cos θ sin θ,θ = ( ) sin θ cos θ and (µ)(x,φ) is the divergent beam transform of µ(x) in the direction of φ = (cos(φ), sin(φ)) defined as (µ)(x,φ) = µ(x + pφ) dp. Without attenuation, equation () becomes the Radon transform (Rf )(θ, s) = f(sθ+ tθ ) dt. Assuming µ(x) is known, Novikov () gave an explicit inversion formula to reconstruct f(x) from the parallel projection data g(θ, s) = (R µ f )(θ, s): f(x) = { π } 4π Re θ{e h(θ,s)+(µ)(x, θ ) H e h(θ,s) g(θ, s)} s=x θ dθ, ()
An FK-like cone-beam SPECT reconstruction algorithm 33 y t (x,y) s θ O x detector where Figure. Parallel-beam geometry. h(θ, s) = (I +ih )(Rµ)(θ, s), H ϕ(θ, s) = π p.v. ϕ(θ,s ) s s ds, with i =. I is the identity operator. H is the Hilbert transform with respect to the second parameter throughout this paper and p.v. denotes the Cauchy principal value of the integral. The symbol stands for the divergence defined as V = V x x + V y (3) y for vector V = (V x,v y )... Fan-beam reconstruction with equal detector spacing Fan-beam data acquisition geometry with equally spaced detectors is shown in figure. The fan-beam focal point S is located on a circle of radius. Without loss of generality, let us put the detector passing through the centre O and perpendicular to SO. enote the projection data acquired at position u on the detector with view angle β by g(β,u) = ( µ f )(β, u), where the operator ( µ f )(β, u) is given by ( µ f )(β, u) = f(x S + tα) e (µ)(x S +tα,α) dt. (4) x S = (x S,y S ) is the focal point location and α is the unit vector shown in figure. It is clear that g(β,u) is equal to projection data g(θ, s) acquired at (θ, s) in the parallel-beam geometry if i.e., we have s = u u +, ( θ = β u ) +tan, (5) ( ( g β +tan u ) ) u, = g(β,u). (6) u +
33 Q Huang et al y S α σ β s u detector O θ x Figure. Fan-beam geometry. You et al proved that the Hilbert transform of any non-attenuated parallel-beam data k(θ, s) with respect to the second variable s can be evaluated by the following integral of their fan-beam counterpart k(β,u) (You et al 5) Hk(θ,s) = π/ π p.v. k(β, tan(σ )) dσ, (7) π/ sin(σ σ ) where (5) is assumed and σ = tan ( ) u. Changing variable σ to u, with u = tan σ and du = dσ = u + cos σ dσ, (7) becomes Hk(θ,s) = π p.v. = π p.v. u + = p.v. π k(β,u ) sin ( tan ( ) u ( )) tan u k(β,u ) (u u ) u + u + u + du k(β,u ) (u u ) u + du u + du = H k(β,u), (8) where H is the Hilbert transform in the fan-beam geometry and is defined as H ϕ(β,u) = ( ) u + H ϕ(β,u). (9) u + Changing variable in (), with ds dθ = 3 ( + u ) 3/ du dβ, we obtain the inversion formula for the fan-beam geometry as follows: { f(x) = { } } π 4π Re β e h(β,u)+(µ)(x, β ) e h(β,u) g(β,u) H dβ () U + u with U = + x sin β y cos β, ( ( )) cos β +tan u β = sin ( β +tan ), u u=u
An FK-like cone-beam SPECT reconstruction algorithm 333 Figure 3. Cone-beam geometry. h(β, u) = (I +i H)( µ)(β, u) = (I +i H) u = (x cos β + y sin β) + x sin β y cos β. µ(x S + tα) dt, The above inversion formula can be discretized to reconstruct the image from fan-beam projections. The Hilbert transform and divergence operator are implemented as two separate filtrations. irect discretization of () results in a filtering backprojection-filtering algorithm..3. Cone-beam reconstruction with circular scanning trajectory The cone-beam geometry is described in figure 3. The cone-beam focal point S is on a circle centred at O with radius and in the plane z =. Assume the detector plane is perpendicular to SO at O. efine the coordinates system (u, ξ) centred at O on the detector plane such that the u-axis is parallel to the tangent of the trajectory and the ξ-axis is parallel to the z-axis (i.e. the axis of rotation). enote g(β,u,ξ) as the attenuated projection data acquired at view angle β and at point (u, ξ) on the detector plane and ( µ)(β,u,ξ)as the unattenuated projection of the attenuation map. Introducing the rotated coordinates s = x cos β + y sin β t = xsin β + y cos β () z = z and following the derivation of the FK method (Feldkamp et al 984), we obtain the reconstruction formula at x = (x,y,z)as { f(x) ˆ = π 4π Re + ξ t β e h(β,u,ξ)+(µ)(x, β ) H e h(β,u,ξ) g(β,u,ξ) + u + ξ u=u dβ } ()
334 Q Huang et al.8.6.4 µ=... µ=.5.8.6.4. Figure 4. Non-uniform attenuation map for the fan-beam study. with cos ( β +tan u β = sin ( β +tan ), (3) u +ξ +ξ ) h(β, u, ξ) = (I +i H)( µ)(β, u, ξ), (4) u = s t, (5) ξ = z t, (6) Hk(β,u,ξ)= u + + ξ H k(β,u,ξ) u + + ξ. (7) This formula gives an approximate algorithm to reconstruct an image from cone-beam projection data. Artefacts are introduced in the reconstructed image except for the central slice. However, the inversion formula is easy to implement and efficient. The reconstruction of f(x) from formula () can be implemented in the following three steps: Step. Filtration (i) Multiply the projection data, g(β,u,ξ), bythee h(β,u,ξ) to obtain the modified projection data. Weight the modified projection data by a factor. +u +ξ (ii) Filter the weighted modified projection data by doing the Hilbert transform defined in section.. (iii) Post-weight the result from (ii) by multiplying by β e h(β,u,ξ)+(µ)(x, β) to obtain a vector Q. Step. Backprojection Weight Q by +ξ and backproject it over the 3 reconstruction grid. t
An FK-like cone-beam SPECT reconstruction algorithm 335 9 8 7 6 5 4 3 Figure 5. Emission phantom for the fan-beam study. 3 5 5 5 Figure 6. Attenuated fan-beam projections. Step 3. Filtration The divergence can be seen as a filtration after the backprojection. component of the result and scale it by 4π. Keep the real The Hilbert transform and divergence operator are both one-dimensional () filtration performed in the spatial domain. The backprojection is performed as a 3 cone-beam backprojection. The implementation is thus a filtering backprojection-filtering algorithm. In the case where there is no attenuation, h(β, u, ξ) and (µ)(x, β ) both vanish. Exchanging the order of divergence and backprojection (Natterer ) and combining the derivative and the Hilbert transform together, we obtain a ramp filtration along the row direction in the detector plane. Our algorithm is thus reduced to the FK method if µ(x) =.
336 Q Huang et al 9 8 7 6 5 4 3 Figure 7. Reconstructed image from noise-free fan-beam projections. x.8.6.4 µ=.5 µ=....8.6.4. y Figure 8. Attenuation map in the x y plane for the cone-beam study. 3. Numerical results In our computer simulation, for the fan-beam geometry, we choose the attenuation map as in figure 4 and the emission phantom as in figure 5. The focal length is set to be 8 units where one unit is the image voxel size. The attenuated noise-free projections with 8 views over 36 and the reconstructed image in a 8 8 array are shown in figures 6 and 7, respectively. This simulation result validates the reconstruction formula () derived for fan-beam projections. For the cone-beam geometry, the attenuation map is defined in a 9 9 9 array with maximum coefficient of. per unit and is composed of two spheres. The central slice of the attenuation map is shown in figure 8. If we choose one unit as.7 cm, the image is approximately the size of a human skull and the attenuation coefficient. per unit is the same as that of water at 4 kev. The noise-free projections are analytically generated at 8 views
An FK-like cone-beam SPECT reconstruction algorithm 337 O x O y O x y (z= ) z (x=) z (y=) Figure 9. Original emission phantom (first row), reconstructed image from noise-free projections (second row) and reconstructed image from noisy projections (third row) for z = (first column), x = (second column) and y = (third column)..5.5.5.5.5.5.5.5 4 6 8 4 4 6 8 4 (a) x=, z= (b) y=4, z= Figure. Profiles: dashdot line is for the original phantom, solid line for reconstructed image and dashed line for reconstructed image without attenuation correction. over 36 and at 9 9 equally spaced detector grid. The focal length is set to be units. The reconstructed images at the three orthogonal central slices are shown in figure 9. ue to the random nature of radioactivity and measurement error, there is inevitable noise in the acquired data. We model the noise in the projections with Poisson distributions. Figure 9 also shows the reconstructed images at the three orthogonal central slices for noisy
338 Q Huang et al.5.5.5.5.5.5.5.5 4 6 8 4 4 6 8 4 (a) x=, z= (b) y=4, z= Figure. Profiles in a noisy projection case: dashdot line is for the original phantom and solid line for reconstructed image. projections. The Poisson noise is such that the total count calculated from projections of the x y plane is 6. Profiles drawn across the phantom and reconstructed image are shown in figure for a noise-free case and figure for a noisy case. 4. Conclusion In this paper we derived an FK-like cone-beam reconstruction algorithm for non-uniform attenuated projections acquired using a circular trajectory. This algorithm is a filtering backprojection-filtering algorithm and gives an approximate reconstruction of the image. Simulation results showed the efficiency and accuracy of the reconstruction from noise-free projection data. From Tuy s data sufficiency condition, we know it is impossible to reconstruct exactly the image from projections acquired in the geometry described above. However, in regions close to the orbit plane images can be reconstructed with acceptable artefacts. In order to do exact reconstruction, a non-planar trajectory should be adopted. Adding either an arc or a vertical line to the circular trajectory is a solution. Using a helix trajectory is another. Future work is required to extend Novikov s formula to other trajectories to obtain exact reconstruction. Acknowledgments This work was supported by the National Institutes of Biomedical Imaging and Bioengineering, and National Cancer Institute of the National Institutes of Health under grants CA 8, EB and by the irector, Office of Science, Office of Biological and Environmental Research, Medical Sciences ivision of US epartment of Energy under contract E-AC3-76SF98. References Feldkamp L A, avis L C and Kress J K 984 Practical cone-beam algorithm J. Opt. Soc. Am. A 6 9 Grangeat P 99 Mathematical framework of cone beam 3 reconstruction via the first derivative of the Radon transform Math. Methods Tomogr. 497 66 97 Gullberg G T 979 The attenuated Radon transform: theory and application in medicine and biology Ph Thesis p66
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