Resolution and contrast recovery at depth in planar nuclear images

Resolution and contrast recovery in planar nuclear images at depth with a high purity germanium (HPGe) camera has been achieved through use of a weighted back projection (WBP) method. The algorithm can be derived from Bayes' theorem using the point spread function of the camera. The variations in the formulation of WBP (one single pass and two iterative procedures) are presented with the characteristics and performance of each method. The evaluation procedure determines the behaviour of signal-to-noise ratio, contrast and texture after application of the algorithm. Both real and simulated cold lesions obtained with the HPGe camera are studied with sizes ranging from 3 mm to 17 mm and background count densities from 100 to 6400 counts cm2. Application of WBP is shown to increase spatial resolution and contrast without a concomitant reduction in signal-to-noise ratio. Images obtained with the HPGe camera and processed with WBP are presented. The algorithm has been applied to the scintillation camera, yielding significant resolution and contrast recovery despite the presence of scatter and textured noise present in the HPGe images.     

Generalized dual-energy-window scatter compensation in spatially varying media for SPECT

The detection of scattered photons in the photopeak energy window hinders accurate activity estimation in single-photon-emission computed tomography (SPECT). To compensate for photons scattered in spatially varying media, a framework for generalized dual-energy-window scatter subtraction has been developed. Generalized scatter subtraction factors are introduced, and these factors are decomposed into terms dependent on the uniform (average) and spatially varying components of the source activity distribution. The variation of these factors with projection pixel location and gamma camera position is analysed for a simulated myocardial perfusion study with a 99Tc(m) source radionuclide and a non-uniform thorax model. Monte Carlo methods are used to model photon transport and detection. The application of pixel-dependent scatter subtraction factors for scatter compensation is evaluated in an image reconstruction experiment for this simulated myocardial perfusion study. Generalized matrix inverses with noise-dependent regularization are used for image reconstruction. For this simulation, use of a pixel-dependent scatter subtraction factor and a constant scatter subtraction factor are effective for scatter compensation. Activity estimates within the left ventricular myocardium for these two methods are practically the same as those obtained from image reconstructions where the detection of Compton-scattered photons is included in the system matrix.     

Testing of local gamma-ray scatter fractions determined by spectral fitting

The spectral-fitting method of correction for gamma-ray Compton scattering within objects separates the unscattered and scattered components of locally measured energy spectra. Here, we employ a third-order polynomial for the scattering and an approximately constant fitting window. A scatter fraction, defined as total scattered over total unscattered counts within a 20% window, is calculated for each point in our Anger camera images. These scatter fractions are tested against those from Monte-Carlo simulation for 99mTc and against results from semiconductor detector measurements for 131I. A radioactive sphere at several locations within a non-radioactive cylinder and the inverse are imaged for the testing. For one case, reproducibility of the spectral-fitting scatter fraction as a function of the number of unscattered counts within the 20% acceptance window was also determined. With 99mTc, for all cases, the agreement between spectral fitting and the standard estimation method is within 16%. With 131I, for the 'hot' sphere at two locations, the agreement is within 21%. For the 'hot' sphere at the third location (off the cylinder axis towards the camera), the dependence of scatter fraction on transverse distance is good although the absolute values are too large. Scatter fraction reproducibility is within 10% for 1000 or more counts. Therefore, further testing of spectral fitting and initial application to realistic clinical images seem to be in order.     

Quantitation in SPECT using an effective model of the scattering

A new method for correcting simultaneously the attenuation, scatter and resolution effects in SPECT has been developed for the case of a homogeneous medium. It is based on an effective model of the scatter process. This model depends on only four parameters which are determined experimentally and remain independent of the geometry and of the dimensions of the scatter medium. The method uses the data from the peak events and does not need additional energy windows on the scattered events. An original filter is proposed to remove the noise due to the poor statistics of clinical data. Tests on phantoms varying in size and activity show that the model allows absolute activity determination with an accuracy of a few per cent.     

High accuracy multiple scatter modelling for 3D whole body PET

A new technique for modelling multiple-order Compton scatter which uses the absolute probabilities relating the image space to the projection space in 3D whole body PET is presented. The details considered in this work give a valuable insight into the scatter problem, particularly for multiple scatter. Such modelling is advantageous for large attenuating media where scatter is a dominant component of the measured data, and where multiple scatter may dominate the total scatter depending on the energy threshold and object size. The model offers distinct features setting it apart from previous research: (1) specification of the scatter distribution for each voxel based on the transmission data, the physics of Compton scattering and the specification of a given PET system; (2) independence from the true activity distribution; (3) in principle no scaling or iterative process is required to find the distribution; (4) explicit multiple scatter modelling; (5) no scatter subtraction or addition to the forward model when included in the system matrix used with statistical image reconstruction methods; (6) adaptability to many different scatter compensation methods from simple and fast to more sophisticated and therefore slower methods; (7) accuracy equivalent to that of a Monte Carlo model. The scatter model has been validated using Monte Carlo simulation (SimSET).     

An electronically collimated gamma camera for single photon emission computed tomography. Part II: Image reconstruction and preliminary experimental measurements

Iterative algorithms have been investigated for reconstructing images from data acquired with a new type of gamma camera based upon an electronic method of collimating gamma radiation. The camera is composed of two detection systems which record a sequential interaction of the emitted gamma radiation. Coincident counting in accordance with Compton scattering kinematics leads to a localization of activity upon a multitude of conical surfaces throughout the object. A two-stage reconstruction procedure in which conical line projection images as seen by each position sensing element of the first detector are reconstructed in the first stage, and tomographic images are reconstructed in the second stage, has been developed. Computer simulation studies of both stages and first-stage reconstruction studies with preliminary experimental data are reported. Experimental data were obtained with one detection element of a prototype germanium detector. A microcomputer based circuit was developed to record coincident counts between the germanium detector and an uncollimated conventional scintillation camera. Point sources of Tc-99m and Cs-137 were used to perform preliminary measurements of sensitivity and point spread function characteristics of electronic collimation.     

Derivation of the distribution of extrafocal radiation for head scatter factor calculation

Head scatter factors for high energy photon beams from linear accelerators can be modeled using a two-source model consisting of focal and extrafocal radiation. The focal radiation can be approximated as a point source, and the distribution of the extrafocal radiation is a two-dimensional (2D) radial symmetric function. Various methods, including analytical, Monte Carlo, and empirical trial functions, have been used to determine the radial symmetric function of extrafocal radiation distribution. This article describes a method for directly determining the extrafocal radiation distribution without assuming any empirical trial function. The extrafocal radiation distribution is determined with measured head scatter factors for rectangular fields defined by the lower jaw (X) fixed at 40 cm and the upper jaw (Y) varying from 3 to 40 cm. The derivatives of the measured head scatter factors, with respect to the Y jaw position projected in the plane of extrafocal radiation, are proportional to the one-dimensional (1D) projection (also called the line spread function) of the extrafocal radiation distribution. Two methods are used to solve the radial function of extrafocal radiation from the 1D projection. The first method uses a 2D filtered backprojection algorithm, originally developed for parallel beam computed tomography reconstruction, to directly derive the radial dependence of the extrafocal radiation distribution. The method has been applied to 6 and 18 MV photon beams from a Siemens linear accelerator and has been tested by comparing measured and calculated head scatter factors for square and rectangular fields. The second method uses a Fourier transform followed by a Fourier-Bessel transform to solve the problem. The distributions of extrafocal radiation derived from these two methods are virtually identical.     

Energy and spatial distribution of multiple order Compton scatter in SPECT: a Monte Carlo investigation

Energy and spatial projection distributions were simulated for gamma camera imaging of multiple order Compton scattered photons. SPECT imaging of a line source of radioactivity located in a water filled cylindrical phantom was modelled using Monte Carlo techniques. Photon trajectories were followed from emission to detection including the effects of all physical interactions and the resulting energy spectra and spatial projections were sorted as a function of the number of times the photon underwent Compton scattering before detection. Analysis of energy spectra demonstrates that Compton events up to second order overlap with the non-scattered events and distributions are peaked at lower energies as the scattering order increases. Analysis of spatial projections shows that, with increasing order, Compton events produce tails on the line spread function which progress from roughly exponential to nearly flat distributions. The use of Monte Carlo modelling thus allows a detailed investigation of the spatial and energy distribution of Compton scatter which could not be performed using present experimental techniques.     

Attenuation and scatter correction for in-beam positron emission tomography monitoring of tumour irradiations with heavy ions

An in-beam dual-head positron camera is used to monitor the dose application in situ during the tumour irradiation with carbon ion beams at the experimental heavy ion therapy facility at GSI Darmstadt. Therefore, a positron emission tomograph has been mounted directly at the treatment site. A fully 3D reconstruction algorithm based on the maximum likelihood expectation maximization (MLEM) algorithm has been developed and adapted to this spatially varying imaging situation. The scatter and attenuation correction are included in the forward projection step of the maximum likelihood image reconstruction. This requires an attenuation map containing the information on the material composition and densities. This information is derived from the x-ray computed tomograms of the patient and the patient fixation system including the head-rest. The normalization of scattered events relative to the unscattered events is done by a global scatter fraction factor which is estimated by means of a Monte Carlo simulation. The feasibility of the proposed algorithm is shown by means of computer simulations, phantom measurements as well as patient data.     

Scatter-to-primary based scatter fractions for transmission-dependent convolution subtraction of SPECT images

In single photon emission computed tomography (SPECT), transmission-dependent convolution subtraction has been shown to be useful when correcting for scattered events. The method is based on convolution subtraction, but includes a matrix of scatter fractions instead of a global scatter fraction. The method can be extended to iteratively improve the scatter estimate, but in this note we show that this requires a modification of the theory to use scatter-to-total scatter fractions for the first iteration only and scatter-to-primary fractions thereafter. To demonstrate this, scatter correction is performed on a Monte Carlo simulated image of a point source of activity in water. The modification of the theory is compared to corrections where the scatter fractions are based on the scatter-to-total ratio, using one and ten iterations. The resulting ratios of subtracted to original counts are compared to the true scatter-to-total ratio of the simulation and the most accurate result is found for our modification of the theory.     

Experimentally measured scatter fractions and energy spectra as a test of Monte Carlo simulations

A method for the validation of Monte Carlo photon transport calculations is presented, with particular emphasis on the scatter component of such calculations. The method is based on a quantitative comparison of calculated and experimental scatter fractions. In addition, the method includes a qualitative comparison of point spread functions and energy spectra. An application of the method is demonstrated by comparing the results of an existing Monte Carlo code with experimental results obtained with a gamma camera viewing a point source of 99Tcm (140 keV gamma rays) centred within a water-filled cylinder. The results of the comparisons show good agreement between experiment and calculation. These results allow the code to be used with increased confidence in a variety of situations, and they define more precisely the region of applicability of the code. In addition, the determination of scatter fractions and energy spectra is useful for other applications. For example, scatter fractions can be a useful parameter for evaluating possible techniques for scatter compensation.     

Model-based compensation for quantitative 123I brain SPECT imaging

Previously we have developed a model-based method that can accurately estimate downscatter contamination from high-energy photons in 123I imaging. In this work we combined the model-based method with iterative reconstruction-based compensations for other image-degrading factors such as attenuation, scatter, the collimator-detector response function (CDRF) and partial volume effects to form a comprehensive method for performing quantitative 123I SPECT image reconstruction. In the model-based downscatter estimation method, photon scatter inside the object was modelled using the effective source scatter estimation (ESSE) technique, including contributions from all the photon emissions. The CDRFs, including the penetration and scatter components due to the high-energy 123I photons, were estimated using Monte Carlo (MC) simulations of point sources in air at various distances from the face of the collimator. The downscatter contamination was then compensated for during the iterative reconstruction by adding the estimated results to the projection steps. The model-based downscatter compensation (MBDC) was evaluated using MC simulated and experimentally acquired projection data. From the MC simulation, we found about 39% of the total counts in the energy window of 123I were attributed to the downscatter contamination, which reduced image contrast and caused a 1.5% to 10% overestimation of activities in various brain structures. Model-based estimates of the downscatter contamination were in good agreement with the simulated data. Compensation using MBDC removed the contamination and improved the image contrast and quantitative accuracy to that of the images obtained from 159 keV photons. The errors in absolute quantitation were reduced to within +/-3.5%. The striatal specific binding potential calculated based on the activity ratio to the background was also improved after MBDC. The errors were reduced from -4.5% to -10.93% without compensation to -0.55% to 4.87% after compensation. The model-based method provided accurate downscatter estimation and, when combined with iterative reconstruction-based compensations, accurate quantitation was obtained with minimal loss of precision.     

Projection correlation based view interpolation for cone beam CT: primary fluence restoration in scatter measurement with a moving beam stop array

Scatter correction is an open problem in x-ray cone beam (CB) CT. The measurement of scatter intensity with a moving beam stop array (BSA) is a promising technique that offers a low patient dose and accurate scatter measurement. However, when restoring the blocked primary fluence behind the BSA, spatial interpolation cannot well restore the high-frequency part, causing streaks in the reconstructed image. To address this problem, we deduce a projection correlation (PC) to utilize the redundancy (over-determined information) in neighbouring CB views. PC indicates that the main high-frequency information is contained in neighbouring angular projections, instead of the current projection itself, which provides a guiding principle that applies to high-frequency information restoration. On this basis, we present the projection correlation based view interpolation (PC-VI) algorithm; that it outperforms the use of only spatial interpolation is validated. The PC-VI based moving BSA method is developed. In this method, PC-VI is employed instead of spatial interpolation, and new moving modes are designed, which greatly improve the performance of the moving BSA method in terms of reliability and practicability. Evaluation is made on a high-resolution voxel-based human phantom realistically including the entire procedure of scatter measurement with a moving BSA, which is simulated by analytical ray-tracing plus Monte Carlo simulation with EGSnrc. With the proposed method, we get visually artefact-free images approaching the ideal correction. Compared with the spatial interpolation based method, the relative mean square error is reduced by a factor of 6.05-15.94 for different slices. PC-VI does well in CB redundancy mining; therefore, it has further potential in CBCT studies.     

The influence of the single-event energy-deposition distribution on the signal-to-noise ratio in x-ray images in the presence of scattered radiation

The single-event energy-deposition distribution has been included in the signal-to-noise ratio for the detection of low-contrast detail in x-ray projection imaging. Calculations for an ideal detector show that this results in a higher signal-to-noise ratio for energy proportional detection than for the counting technique when scattered radiation is present, in contradiction to what has previously been assumed. The difference is less than 20% for photon spectra typical of medical radiography. If scattered radiation is reduced, the difference between energy proportional detection and counting diminishes. For very efficient scatter reduction, counting is slightly better than energy proportional detection.     

Scatter radiation in digital tomosynthesis of the breast

Digital tomosynthesis of the breast is being investigated as one possible solution to the problem of tissue superposition present in planar mammography. This imaging technique presents various advantages that would make it a feasible replacement for planar mammography, among them similar, if not lower, radiation glandular dose to the breast; implementation on conventional digital mammography technology via relatively simple modifications; and fast acquisition time. One significant problem that tomosynthesis of the breast must overcome, however, is the reduction of x-ray scatter inclusion in the projection images. In tomosynthesis, due to the projection geometry and radiation dose considerations, the use of an antiscatter grid presents several challenges. Therefore, the use of postacquisition software-based scatter reduction algorithms seems well justified, requiring a comprehensive evaluation of x-ray scatter content in the tomosynthesis projections. This study aims to gain insight into the behavior of x-ray scatter in tomosynthesis by characterizing the scatter point spread functions (PSFs) and the scatter to primary ratio (SPR) maps found in tomosynthesis of the breast. This characterization was performed using Monte Carlo simulations, based on the Geant4 toolkit, that simulate the conditions present in a digital tomosynthesis system, including the simulation of the compressed breast in both the cranio-caudal (CC) and the medio-lateral oblique (MLO) views. The variation of the scatter PSF with varying tomosynthesis projection angle, as well as the effects of varying breast glandular fraction and x-ray spectrum, was analyzed. The behavior of the SPR for different projection angle, breast size, thickness, glandular fraction, and x-ray spectrum was also analyzed, and computer fit equations for the magnitude of the SPR at the center of mass for both the CC and the MLO views were found. Within mammographic energies, the x-ray spectrum was found to have no appreciable effect on the scatter PSF and on the SPR. Glandular fraction and compressed breast size were found to have a small effect, while compressed breast thickness and projection angle, as expected, introduced large variations in both the scatter PSF and SPR. The presence of the breast support plate and the detector cover plate in the simulations introduced important effects on the SPR, which are also relevant to the scatter content in planar mammography.     

基于非线性统计估计技术的PET散射校正

散射事件是影响正电子发射断层扫描术 (Positron emission tomography,PET)图像重建质量的一个重要因素。我们根据投影图像的分布特征 ,基于泊松数据模型 ,利用最大似然期望值法 (Maximum likelihoodexpectation maximization,ML EM)对正弦图进行散射校正。比较采用 ML EM方法和去卷积法进行散射校正后的正弦图以及重建图像 ,结果表明我们的方法在进行散射补偿的同时 ,增加了图像的对比度 ,效果优于传统的校正方法     

The PETRRA positron camera: design, characterization and results of a physical evaluation

The PETRRA positron camera is a large-area (600 mm x 400 mm sensitive area) prototype system that has been developed through a collaboration between the Rutherford Appleton Laboratory and the Institute of Cancer Research/Royal Marsden Hospital. The camera uses novel technology involving the coupling of 10 mm thick barium fluoride scintillating crystals to multi-wire proportional chambers filled with a photosensitive gas. The performance of the camera is reported here and shows that the present system has a 3D spatial resolution of approximately 7.5 mm full-width-half-maximum (FWHM), a timing resolution of approximately 3.5 ns (FWHM), a total coincidence count-rate performance of at least 80-90 kcps and a randoms-corrected sensitivity of approximately 8-10 kcps kBq(-1) ml. For an average concentration of 3 kBq ml(-1) as expected in a patient it is shown that, for the present prototype, approximately 20% of the data would be true events. The count-rate performance is presently limited by the obsolete off-camera read-out electronics and computer system and the sensitivity by the use of thin (10 mm thick) crystals. The prototype camera has limited scatter rejection and no intrinsic shielding and is, therefore, susceptible to high levels of scatter and out-of-field activity when imaging patients. All these factors are being addressed to improve the performance of the camera. The large axial field-of-view of 400 mm makes the camera ideally suited to whole-body PET imaging. We present examples of preliminary clinical images taken with the prototype camera. Overall, the results show the potential for this alternative technology justifying further development.     

Spectral reconstruction by scatter analysis for a linear accelerator photon beam

Pre-existing methods for photon beam spectral reconstruction are briefly reviewed. An alternative reconstruction method by scatter analysis for linear accelerators is introduced. The method consists in irradiating a small plastic phantom at standard 100 cm SSD and inferring primary beam energy spectral information based on the measurement with a standard Farmer chamber of scatter around the phantom at several specific scatter angles: a scatter curve is measured which is indicative of the primary spectrum at hand. A Monte Carlo code is used to simulate the scatter measurement set-up and predict the relative magnitude of scatter measurements for mono-energetic primary beams. Based on mono-energetic primary scatter data, measured scatter curves are analysed and the spectrum unfolded as the sum of mono-energetic individual energy bins using the Schiff bremsstrahlung model. The method is applied to an Elekta/SL18 6 MV photon beam. The reconstructed spectrum matches the Monte Carlo calculated spectrum for the same beam within 6.2% (average error when spectra are compared bin by bin). Depth dose values calculated for the reconstructed spectrum agree with physically measured depth dose data to within 1%. Scatter analysis is preliminarily shown to have potential as a practical spectral reconstruction method requiring few measurements under standard 100 cm SSD and feasible in any radiotherapy department using a phantom and a Farmer chamber.     

Scoliosis evaluation utilising truncal cross-sections

In scoliosis, two basic structural changes, i.e. lateral flexion and axial rotation, result in the characteristic spinal deformity. Radiography has proved satisfactory in measuring the lateral flexion deformity. However, at present there is no satisfactory method to measure the rotational deformity. A method to measure the latter deformity by drawing out cross-sections of the trunk has been developed: the projection of a horizontal laser beam on the body surface is detected by an inclined line sensor camera. The laser and line sensor camera are simultanelously moved around the patient and the camera output data are converted, by a minicomputer, into the cross-section on a graphic display or an x-y plotter. To evaluate the deformity of the cross-sections quantitatively, we introduced deformity indices and proposed algorithms for their calculation. Measuring crosssections of 59 scoliosis patients, we analysed the relationship between the deformity indices and the Cobb angle.     

Monte Carlo simulations of a scintillation camera using GATE: validation and application modelling

Geant4 application for tomographic emission (GATE) is a recently developed simulation platform based on Geant4, specifically designed for PET and SPECT studies. In this paper we present validation results of GATE based on the comparison of simulations against experimental data, acquired with a standard SPECT camera. The most important components of the scintillation camera were modelled. The photoelectric effect. Compton and Rayleigh scatter are included in the gamma transport process. Special attention was paid to the processes involved in the collimator: scatter, penetration and lead fluorescence. A LEHR and a MEGP collimator were modelled as closely as possible to their shape and dimensions. In the validation study, we compared the simulated and measured energy spectra of different isotopes: 99mTc, 22Na, 57Co and 67Ga. The sensitivity was evaluated by using sources at varying distances from the detector surface. Scatter component analysis was performed in different energy windows at different distances from the detector and for different attenuation geometries. Spatial resolution was evaluated using a 99mTc source at various distances. Overall results showed very good agreement between the acquisitions and the simulations. The clinical usefulness of GATE depends on its ability to use voxelized datasets. Therefore, a clinical extension was written so that digital patient data can be read in by the simulator as a source distribution or as an attenuating geometry. Following this validation we modelled two additional camera designs: the Beacon transmission device for attenuation correction and the Solstice scanner prototype with a rotating collimator. For the first setup a scatter analysis was performed and for the latter design. the simulated sensitivity results were compared against theoretical predictions. Both case studies demonstrated the flexibility and accuracy of GATE and exemplified its potential benefits in protocol optimization and in system design.