Correcting organ motion artifacts in x-ray CT medical imaging systems by adaptive processing. I. Theory

X-ray CT scanners provide images of transverse cross sections of the human body from a large number of projections. During the data acquisition process, which usually takes about 1 s, motion effects such as respiration, cardiac motion, and patient restlessness produce artifacts that appear as blurring, doubling, and distortion in the reconstructed images, and may lead to inaccurate diagnosis. To address this problem several processing techniques have been proposed that require a priori knowledge of the motion characteristics. This paper proposes a method, which makes no assumptions about the properties of the motion, to eliminate the motion artifacts. The approach in this paper uses a spatial overlap correlator scheme to accurately track organ motion in computed tomography imaging systems. Then, it is shown that as optimum processing scheme to remove organ motion effects is to apply adaptive interference cancellation (AIC) methods, which treat the output of the spatial overlap correlator as noise interference at the input of the AIC process. Furthermore, an AIC method does not require any kind of periodicity of the motion effects. Synthetic data tests demonstrate the validity of this approach and show that hardware modifications are essential for its implementation in x-ray CT medical imaging systems.     

Mitigation of motion artifacts in CBCT of lung tumors based on tracked tumor motion during CBCT acquisition

An algorithm capable of mitigating respiratory motion blurring artifacts in cone-beam computed tomography (CBCT) lung tumor images based on the motion of the tumor during the CBCT scan is developed. The tumor motion trajectory and probability density function (PDF) are reconstructed from the acquired CBCT projection images using a recently developed algorithm Lewis et al (2010 Phys. Med. Biol. 55 2505-22). Assuming that the effects of motion blurring can be represented by convolution of the static lung (or tumor) anatomy with the motion PDF, a cost function is defined, consisting of a data fidelity term and a total variation regularization term. Deconvolution is performed through iterative minimization of this cost function. The algorithm was tested on digital respiratory phantom, physical respiratory phantom and patient data. A clear qualitative improvement is evident in the deblurred images as compared to the motion-blurred images for all cases. Line profiles show that the tumor boundaries are more accurately and clearly represented in the deblurred images. The normalized root-mean-squared error between the images used as ground truth and the motion-blurred images are 0.29, 0.12 and 0.30 in the digital phantom, physical phantom and patient data, respectively. Deblurring reduces the corresponding values to 0.13, 0.07 and 0.19. Application of a -700 HU threshold to the digital phantom results in tumor dimension measurements along the superior-inferior axis of 2.8, 1.8 and 1.9 cm in the motion-blurred, ground truth and deblurred images, respectively. Corresponding values for the physical phantom are 3.4, 2.7 and 2.7 cm. A threshold of -500 HU applied to the patient case gives measurements of 3.1, 1.6 and 1.7 cm along the SI axis in the CBCT, 4DCT and deblurred images, respectively. This technique could provide more accurate information about a lung tumor's size and shape on the day of treatment.     

基于投影图像和CT容积图像的三维冠状动脉运动跟踪新方法

目的利用双平面X射线投影图像序列和参考CT容积图像进行冠状动脉的三维运动跟踪建模。方法①提取投影图像序列中的冠状动脉树;②利用多尺度滤波和血管函数提取CT容积图像中的动脉血管;③采用基于B样条配准的方法进行三维运动建模。结果将双投影图像中血管的运动估计结果的三维重建形态与三维运动模型预测出的结果进行了量化比较,调整配准的B样条参数后,两者误差处于有效范围之内。结论由此表明采用投影图像和对应容积图像的联合先验知识进行冠状动脉的三维运动建模是有效的。     

Minimizing artifacts resulting from respiratory and cardiac motion by optimization of the transmission scan in cardiac PET/CT

The introduction of positron emission/computed tomography (PET/CT) systems coupled with multidetector CT arrays has greatly increased the amount of clinical information in myocardial perfusion studies. The CT acquisition serves the dual role of providing high spatial anatomical detail and attenuation correction for PET. However, the differences between the interaction of respiratory and cardiac cycles in the CT and PET acquisitions presents a challenge when using the CT to determine PET attenuation correction. Three CT attenuation correction protocols were tested for their ability to produce accurate emission images: gated, a step mode acquisition covering the diastolic heart phase; normal, a high-pitch helical CT; and slow, a low-pitch, low-temporal-resolution helical CT. The amount of cardiac tissue in the emission image that overlaid lung tissue in the transmission image was used as the measure of mismatch between acquisitions. Phantom studies simulating misalignment of the heart between the transmission and emission sequences were used to correlate the amount of mismatch with the artificial defect changes in the emission image. Consecutive patients were studied prospectively with either paired gated (diastolic phase, 120 kVp, 280 mA, 2.6 s) and slow CT (0.562:1 pitch, 120 kVp, Auto-mA, 16 s) or paired normal (0.938:1 pitch, 120 kVp, Auto-mA, 4.8 s) and slow CT protocols, prior to a Rb-82 perfusion study. To determine the amount of mismatch, the transmission and emission images were converted to binary representations of attenuating tissue and cardiac tissue and overlaid using their native registration. The number of cardiac tissue pixels from the emission image present in the CT lung field yielded the magnitude of misalignment represented in terms of volume, of where a small volume indicates better registration. Acquiring a slow CT improved registration between the transmission and emission acquisitions compared to the gated and normal CT protocols. The volume of PET cardiac tissue in the CT lung field was significantly lower (p < 0.03) for the slow CT protocol in both the rest and stress emission studies. Phantom studies showed that an overlaying volume greater than 2.6 mL would produce significant artificial defects as determined by a quantitative software package that employs a normal database. The percentage of patient studies with overlaying volume greater than 2.6 mL was reduced from 71% with the normal CT protocol to 28% with the slow CT protocol. The remaining 28% exhibited artifacts consistent with heart drift and patient motion that could not be corrected by adjusting the CT acquisition protocol. The low pitch of the slow CT protocol provided the best match to the emission study and is recommended for attenuation correction in cardiac PET/CT studies. Further reduction in artifacts arising from cardiac drift is required and warrants an image registration solution.     

Modelling the dosimetric consequences of organ motion at CT imaging on radiotherapy treatment planning

Treatment planning algorithms usually assume that the correct or at least the mean organ position is derived from the CT imaging procedure, and that this position is reproduced throughout the treatment. In reality a mobile organ is unlikely to be in its exact mean position at the time of imaging, causing the treatment to be planned with an organ off-set from its assumed mean position. This introduces an extra 'CT uncertainty' into the treatment. A Monte Carlo (MC) model is used to simulate organ translations at imaging and evaluate the effect of this uncertainty (above the treatment delivery uncertainties) on the dose distribution. An underdose by 4 Gy in a 60 Gy treatment is calculated in the penumbral region of a single-field dose distribution as a result of the CT uncertainty. The effect is reduced to less then 0.5 Gy when the organ position at planning is derived as the average from multiple pretreatment CT scans. It is shown that a convolution method can be applied to predict the effect of CT uncertainty on the dose distribution for a patient population. Additionally, a variation kernel for a convolution method is derived that incorporates uncertainty at both imaging and treatment.     

Cone beam volume CT image artifacts caused by defective cells in x-ray flat panel imagers and the artifact removal using a wavelet-analysis-based algorithm

The application of x-ray flat panel imagers (FPIs) in cone beam volume CT (CBVCT) has attracted increasing attention. However, due to a deficient semiconductor array manufacturing process, defective cells unavoidably exist in x-ray FPIs. These defective cells cause their corresponding image pixels in a projection image to behave abnormally in signal gray level, and result in severe streak and ring artifacts in a CBVCT image reconstructed from the projection images. Since a three-dimensional (3-D) back-projection is involved in CBVCT, the formation of the streak and ring artifacts is different from that in the two-dimensional (2-D) fan beam CT. In this paper, a geometric analysis of the abnormality propagation in the 3D back-projection is presented, and the morphology of the streak and ring artifacts caused by the abnormality propagation is investigated through both computer simulation and phantom studies. In order to calibrate those artifacts, a 2D wavelet-analysis-based statistical approach to correct the abnormal pixels is proposed. The approach consists of three steps: (1) the location-invariant defective cells in an x-ray FPI are recognized by applying 2-D wavelet analysis on flat-field images, and a comprehensive defective cell template is acquired; (2) based upon the template, the abnormal signal gray level of the projection image pixels corresponding to the location-invariant defective cells is replaced with the interpolation of that of their normal neighbor pixels; (3) that corresponding to the isolated location-variant defective cells are corrected using a narrow-windowed median filter. The CBVCT images of a CT low-contrast phantom are employed to evaluate this proposed approach, showing that the streak and ring artifacts can be reliably eliminated. The novelty and merit of the approach are the incorporation of the wavelet analysis whose intrinsic multi-resolution analysis and localizability make the recognition algorithm robust under variable x-ray exposure levels between 30% and 70% of the dynamic range of an x-ray FPI.     

Objective evaluation of the correction by non-rigid registration of abdominal organ motion in low-dose 4D dynamic contrast-enhanced CT

We objectively evaluate a straightforward registration method for correcting respiration-induced movement of abdominal organs in CT perfusion studies by measuring the distributions of alignment errors between corresponding landmark pairs. We introduce the concept and describe the advantages of using the surface-normal component of distance between pairs of corresponding landmarks selected so that their surface normal is in one of the three coordinate axis directions, and show that such landmarks can be precisely placed with respect to the surface normal. Using a large population of landmark pairs on a substantial quantity of 4D dynamic contrast-enhanced CT volume data, we quantify the average alignment errors of abdominal organs that remain uncorrected by registration.     

Sinusoidal forearm tracking with delayed visual feedback II. Dependence of the relative phase on the relative delay

The influence of delayed visual feedback on the phase relationships between target and response signal during a sinusoidal tracking task was analysed in five normal subjects. Target frequency was varied systematically between 0.3 and 1.5 Hz, and the delay between 0 and 120% of the target cycle duration. For each subject, 63 trials were recorded. Phase parameters (relative phase, absolute relative phase and percentage of positive relative phases) revealed a clear dependence on relative delay but not on target frequency. With relative delays close to 0 and 100% of the target cycle duration, subjects successfully tracked the target signal with a small phase lag. With delays in the 30–90% range, larger phase differences were observed. Furthermore, the (delayed) response signal usually preceded the target signal in this delay range. These findings provide further evidence for a dependence of tracking error on external delays in the visual feedback loop, and indicate that delays of about 50% of the movement cycle are more difficult to handle than are smaller or larger delays. The results are discussed with regard to the influence of different control strategies (feedback, feedforward) and different types of feedback (such as vision and proprioception) on motor control.     

An experimental study on the noise properties of x-ray CT sinogram data in Radon space

Computed tomography (CT) has been well established as a diagnostic tool through hardware optimization and sophisticated data calibration. For screening purposes, the associated x-ray exposure risk must be minimized. An effective way to minimize the risk is to deliver fewer x-rays to the subject or lower the mAs parameter in data acquisition. This will increase the data noise. This work aims to study the noise property of the calibrated or preprocessed sinogram data in Radon space as the mAs level decreases. An anthropomorphic torso phantom was scanned repeatedly by a commercial CT imager at five different mAs levels from 100 down to 17 (the lowest value provided by the scanner). The preprocessed sinogram datasets were extracted from the CT scanner to a laboratory computer for noise analysis. The repeated measurements at each mAs level were used to test the normality of the repeatedly measured samples for each data channel using the Shapiro-Wilk statistical test merit. We further studied the probability distribution of the repeated measures. Most importantly, we validated a theoretical relationship between the sample mean and variance at each channel. It is our intention that the statistical test and particularly the relationship between the first and second statistical moments will improve low-dose CT image reconstruction for screening applications.     

阈下条件电刺激心室抑制作用空间限制性的实验研究

临床上,已利用阈下条件刺激心室抑制作用的空间限制性对室性心律失常异位激动起源点进行定位。为了解这种方法定位的精确性,本工作采用各种观察阈下条件刺激的心室抑制作用及其空间限制性的方法,并首次设计了阈下条件刺激心室起搏节律抑制模型,进行了详细研究。结果表明,阈下条件刺激的心室抑制作用具有很强的空间限制性,这一特性可用来对室性心律失常异位激动起源点进行精确定位。     

Automatic detection of motion artifacts in the ballistocardiogram measured on a modified bathroom scale

Ballistocardiography (BCG) is a non-invasive technique used to measure the ejection force of blood into the aorta which can be used to estimate cardiac output and contractility change. In this work, a noise sensor was embedded in a BCG measurement system to detect excessive motion from standing subjects. For nine healthy subjects, the cross-correlation of the motion signal to the BCG noise—estimated using a simultaneously acquired electrocardiogram and statistics of the BCG signal—was found to be 0.94 and 0.87, during periods of standing still and with induced motion artifacts, respectively. In a separate study, where 35 recordings were taken from seven subjects, a threshold-based algorithm was used to flag motion-corrupted segments of the BCG signal using only the auxiliary motion sensor. Removing these flagged segments enhanced the BCG signal-to-noise ratio (SNR) by an average of 14 dB (P < 0.001). This integrated motion-sensing technique addresses a gap in methods available to identify and remove noise in standing BCG recordings due to movement, in a practical manner that does not require user intervention or obtrusive sensing.     

The influence of reflection artifacts on apparent phase velocity measurements in the trachea

Models of the pulmonary airway network were analyzed to determine the effect of reflections on apparent phase velocity measurements in the trachea of dogs. A stiffly constrained elastic tube filled with a viscous fluid was used to model each airway segment, and a series resistance-capacitance combination served as a terminal element for each pathway. Reflection, transmission and attenuation coefficients at each branching site and reflection coefficients at each termination were computed. Composite sinusoidal pressure waves at two sites in the trachea where computed for frequencies from 5 to 150 Hz, and crosscorrelograms were used to determine apparent phase velocities. In an asymmetrical model, the randomness of the airway diameters caused phase shifts in the reflection coefficients to vary between approximately 0° and 180° resulting in significant destructive interference between the reflected waves. Calculated apparent phase velocities agreed with the assumed values within 40% at frequencies of 70 Hz and below and within 20% at higher frequencies in this model.     

Effects of motion on optical properties in the spatial frequency domain

Spatial frequency domain imaging (SFDI) is a noncontact and wide-field optical imaging technology currently being used to study the optical properties and chromophore concentrations of in vivo skin including skin lesions of various types. Part of the challenge of developing a clinically deployable SFDI system is related to the development of effective motion compensation strategies, which in turn, is critical for recording high fidelity optical properties. Here we present a two-part strategy for SFDI motion correction. After verifying the effectiveness of the motion correction algorithm on tissue-simulating phantoms, a set of skin-imaging data was collected in order to test the performance of the correction technique under real clinical conditions. Optical properties were obtained with and without the use of the motion correction technique. The results indicate that the algorithm presented here can be used to render optical properties in moving skin surfaces with fidelities within 1.5% of an ideal stationary case and with up to 92.63% less variance. Systematic characterization of the impact of motion variables on clinical SFDI measurements reveals that until SFDI instrumentation is developed to the point of instantaneous imaging, motion compensation is necessary for the accurate localization and quantification of heterogeneities in a clinical setting.     

AngⅡ诱导大鼠BMSCs联合脱细胞牛心包体外构建工程化心肌组织的实验研究

目的：探讨血管紧张素Ⅱ（AngⅡ）诱导骨髓间充质干细胞（BMSCs）分化为心肌细胞的能力,及诱导后BMSCs体外联合脱细胞牛心包构建工程化组织心肌的可行性。方法分离培养大鼠BMSCs,用含0.1μmol/L AngⅡ的完全培养基诱导培养第3代BMSCs 24 h,然后换用完全培养基连续培养;对照组采用完全培养基连续培养。采用免疫荧光法检测诱导后的BMSCs表达心肌特异蛋白cTnT、β-MHC情况;透射电镜观察诱导后的细胞超微结构。用去污剂-酶消化法对新鲜牛心包行脱细胞处理,然后将诱导后4周的BMSCs接种于脱细胞牛心包生物支架上培养3 d后,对其进行观察检测。结果 AngⅡ诱导后的BMSCs向心肌细胞分化,可表达cTnT和β-MHC,对照组则不表达cTnT和β-MHC。电镜结果显示诱导后的细胞间可见类肌节组织及明显的桥粒连接;新鲜的牛心包经去污剂-酶联合四步法脱细胞处理及京尼平交联后,HE染色观察脱细胞的效率接近100%。扫描电镜观察发现脱细胞处理后的牛心包表面无细胞残留且表面排列杂乱,放大后可见有2μm小孔。观察体外构建的工程化心肌显示诱导后的BMSCs可良好地黏附于脱细胞牛心包生物支架表面并生长增殖,且少量可渗透到支架内部。结论 AngⅡ诱导的大鼠BMSCs可向心肌细胞方向分化,表达心肌特异蛋白cTnT和β-MHC。将其种植于脱细胞牛心包生物支架上,表面黏附良好,且可渗透到支架内部及血管周围,有望用于构建具有血管网络化的组织工程化心肌。     

Experimental evaluation of a simple algorithm to enhance the spatial resolution in scanned radiographic systems

In order to ensure an early diagnosis of breast cancer, an imaging system must fulfil extremely stringent requirements in terms of dynamic range, contrast resolution and spatial resolution. Furthermore, in order to reduce the dose delivered to the patient, a high efficiency of the detector device should be provided. In this paper the SYRMEP/FRONTRAD (SYnchrotron Radiation for MEdical Physics/FRONTier RADiology) mammography project, based on synchroton radiation and a novel solid state pixel detector, is briefly described. Particular relevance is given to the fact that the radiographic image is obtained by means of a scanning technique, which allows the possibility of utilizing a scanning step smaller than the pixel size. With this procedure, a convolution between the real image and the detector point spread function (PSF) is actually acquired: by carefully measuring the detector PSF, it is possible to apply a post-processing procedure (filtered deconvolution), which reconstructs images with enhanced spatial resolution. The image acquisition modality and the deconvolution algorithm are herein described, and some test object images, with spatial resolution enhanced by means of the filtered deconvolution procedure, are presented. As discussed in detail in this paper, this procedure allows us to obtain a spatial resolution determined by the scanning step, rather than by the pixel size.     

Influence of respiration-induced organ motion on dose distributions in treatments using enhanced dynamic wedges.

The mean velocity of respiration-induced organ motion in cranio-caudal direction is of the same magnitude as the velocity of the moving jaw during a treatment with an enhanced dynamic wedge. Therefore, if organ motion is present during collimator movement, the resulting dose distribution in wedge direction may differ from that obtained for the static case, i.e., without organ motion. The position as a function of time of the moving jaw has been derived from a log-file generated during each treatment. Parameters for the respiratory cycle and information about respiration-induced motion for organs in the upper abdomen were taken from the literature. Both movements were superimposed and the resulting monitor unit distribution has been calculated in the intrinsic coordinate system of the organ. The deviations from the static case have been studied as a function of wedge angle, amplitude of organ motion, respiratory rate, asymmetry of the respiratory cycle, beam energy, and the dose rate. If an amplitude of 30 mm and a respiratory rate of 10 min(-1) are assumed, the maximum deviation in monitor units is 2.5% for a 10 degees wedge, 7% for a 30 degrees wedge, and 16% for a 60 degrees wedge. Furthermore, a dose distribution for an organ undergoing respiration-induced motion has been generated and we found dose deviations of the same magnitude as calculated with the monitor unit distribution.     

CTA图像后处理技术对血管管径影响的实验研究

目的:探讨CT血管造影(CT angiography,CTA)后处理技术容积重建(volume rendering,VR)中阈值、最大密度投影(maximum intensity projection,MIP)中窗宽及窗位、视野(Field of view,FOV)对血管管径测量的影响及相关关系.方法:建立9个不同密度的血管模型,使用不同的FOV、阈值(VR)、窗宽(MIP)测鼍不同密度血管模型管径,分析标准阈值、窗宽与血管模型管径的关系,并与CT值进行相关性分析.统计临床脑血管CTA MIP重建病例30例的窗宽及窗位,与实验结果进行比较.结果:VR重建,阈值和血管模型管径呈明显负相关(r=-0.847,P=0.000);MIP重建,窗宽和血管模型管径呈明显正相关(r=0.983.P=0.000):FOV和血管模型管径无相关性(r=-0.042,P=0.381).临床脑血管CTA MIP重建窗宽明显增宽,夸大了血管真实管径.结论:CTA血管管径测量的准确性受VR重建阈值、MIP重建窗宽窗位的影响,根据血管内对比剂密度对这些参数进行调整,保证测量血管管径的可靠性和准确性.