Spatial resolution measurement for iterative reconstruction by use of image-averaging techniques in computed tomography

The purpose of our study was to investigate the validity of a spatial resolution measuring method that uses a combination of a bar-pattern phantom and an image-averaging technique, and to evaluate the spatial resolution property of iterative reconstruction (IR) images with middle-contrast (50 HU) objects. We used computed tomography (CT) images of the bar-pattern phantom reconstructed by the IR technology Adaptive Iterative Dose Reduction 3D (AIDR 3D), which was installed in the multidetector CT system Aquilion ONE (Toshiba Medical Systems, Otawara, Japan). The contrast of the bar-pattern image was set to 50 HU, which is considered to be a middle contrast that requires higher spatial resolution clinically. We employed an image-averaging technique to eliminate the influence of image noise, and we obtained averaged images of the bar-pattern phantom with sufficiently low noise. Modulation transfer functions (MTFs) were measured from the images. The conventional wire method was also used for comparison; in this method, AIDR 3D showed MTF values equivalent to those of filtered back projection. For the middle-contrast condition, the results showed that the MTF of AIDR 3D decreased with the strength of IR processing. Further, the MTF of AIDR 3D decreased with dose reduction. The image-averaging technique used was effective for correct evaluation of the spatial resolution for middle-contrast objects in IR images. The results obtained by our method clarified that the resolution preservation of AIDR 3D was not sufficient for middle-contrast objects.     

Evaluation of low-contrast detectability in low-dose chest computed tomography

We investigated whether low-contrast resolution evaluation can be applied to chest (lung field) computed tomography (CT) images that are commonly reconstructed using filter kernels with strong frequency emphasis and displayed at wide window widths. We assumed low dose chest CT examinations and set the current-time product to 5 and 10 mAs. The visual detection study was performed by five radiological technologists using water phantom images in which the simulated low contrast objects (disc objects) were implanted. In addition, values of the low contrast detectability index (LCDI) based on a signal-to-noise ratio theorem were calculated from the modulation transfer factor, noise power spectrum, and the object's spectrum for each combination of object size, contrast, current-time product, and kernel. The detectability results correlated well with the LCDI values and correlated less well with the contrast-to-noise ratio results. These results were consistent with the results of past reports on abdomen images. Therefore, our results indicated that low contrast sensitivity can be applied to chest CT images as well as abdominal ones.     

Xenon versus ceramics: a comparison of two CT X-ray detector systems.

PURPOSE: The purpose of this work was to compare image quality in phantom and patient CT scans acquired by xenon and ceramic CT detector systems. METHOD: High and low contrast resolution and image noise were determined with a standard CT phantom for both detector systems. In patient CT images, the effect on image noise was measured in anatomical regions of interest in the head, lumbar spine, chest, and abdomen. RESULTS: In phantom studies, image noise was significantly lower using ceramic versus xenon detectors. Also, in images of the head and lumbar spine, the signal-to-noise ratio was significantly higher with ceramic than with xenon detectors. In chest scans, ceramic significantly reduced beam-hardening artifacts caused by the thoracic spine. However, in abdominal images, the signal-to-noise ratio was not significantly different between ceramic and xenon detector systems. CONCLUSION: For reduced image noise in CT images of the head, lumbar spine, and chest and high resolution CT, ceramic detector systems appear to be superior to xenon detector systems.     

Spatial variation of resolution and noise in multi-detector row spiral CT

RATIONALE AND OBJECTIVES: The authors performed this study to evaluate an approach for measuring the variations of three-dimensional spatial resolution and image noise throughout a field of view imaged with multi-detector row spiral computed tomographic (CT) scanners. MATERIALS AND METHODS: The authors designed a phantom (diameter, 320 mm) that contained 37 metallic spheres (diameter, approximately 0.8 mm) positioned between two disks made of a material with attenuation being that of water. One sphere was located at the isocenter of the phantom, and the rest were evenly spaced in three concentric rings with diameters of 100, 200, and 300 mm, respectively. The phantom was imaged with two widely used multi-detector row CT scanners by using a standard protocol and four variations of that protocol. Because a recently developed theory holds that image resolution should be proportional to the square root of the trace of the covariance matrix of a point spread function, the authors developed a software package to segment high-attenuation spheres from the CT image volume and compute point spread functions from blurred images of the spheres. Three-dimensional spatial resolution and image noise were calculated as a function of radial distance within the field of view. RESULTS: Resolution and noise were quantified in the resultant CT image volumes and found to be nonisotropic, with worse resolution and less noise occurring at the periphery of the field of view. CONCLUSION: The method enabled measurement of variations in spatial resolution and of their distribution on images obtained with multi-detector row CT scanners. These findings may contribute to the development of an improved algorithm for image reconstruction.     

Performance evaluation for CT-AEC(CT automatic exposure control)systems

Although many current CT scanners incorporate CT-AEC, performance evaluation is not standardized. This study evaluates the performance of the latest CT-AEC of each manufacturer with the aim of establishing a standard CT-AEC performance evaluation method. The design of the phantoms was based upon the operation characteristics of different CT-AECs. A cone, an ellipse, a variable-shaped ellipse, stepped phantoms, and their analysis software were devised and carried out the field test. The targets were LightSpeed VCT 64 with 2D and 3D Auto mA(GE), Aquilion 64M with Real-EC and Volume-EC(Toshiba), Sensation 64 with CARE Dose and CARE Dose 4D(Siemens), and Bulliance 16P with Dose Right(Philips). Data was acquired while varying the typical abdominal CT(with CT-AEC)scanning conditions (120 kV, 5 mm slice, standard function for abdomen, scanning range 200 mm). The acquired images were converted to the DICOM format and image noise(SD) was calculated using dedicated software. All 4 CT-AECs reduced exposure dose. For GE and Toshiba, image noise was constant and met the target. For Siemens, noise was independent of phantom shape but varied uniformly with phantom size. For Philips, noise varied with phantom size and shape, and variation degree depended on phantom thickness in scanogram direction. The results reflect the basic concept and performance characteristics of the methods. Standardization of CT-AEC performance evaluation is possible using these phantoms.     

Effect of number of projections on image quality of local CT

OBJECTIVES: To determine the image quality of the local CT imaging procedure, in terms of resolution, contrast, and noise as a function of the number of projections. METHODS: The contrast and resolution of the images was determined with a phantom object consisting of three rods of different materials, as well as a phantom human head embedded in soft tissue equivalent material. In addition, slices reconstructed from computed sinograms were used for comparison. RESULTS: Sharpness, contrast and noise were determined as a function of the number of projections. The number of projections was found to affect the contrast and the noise most, and had much less influence on resolution. CONCLUSIONS: Judging from the images of the phantom head and the numerical data, it seems that the minimum number of projections needed to obtain images of useful quality in the geometry used is about 33. Improved image quality (at any number of projections) can best be achieved through noise suppression.     

Image quality evaluation of new image reconstruction methods applying the iterative reconstruction

The purpose of this study was to evaluate the image quality of an iterative reconstruction method, the iterative reconstruction in image space (IRIS), which was implemented in a 128-slices multi-detector computed tomography system (MDCT), Siemens Somatom Definition Flash (Definition). We evaluated image noise by standard deviation (SD) as many researchers did before, and in addition, we measured modulation transfer function (MTF), noise power spectrum (NPS), and perceptual low-contrast detectability using a water phantom including a low-contrast object with a 10 Hounsfield unit (HU) contrast, to evaluate whether the noise reduction of IRIS was effective. The SD and NPS were measured from the images of a water phantom. The MTF was measured from images of a thin metal wire and a bar pattern phantom with the bar contrast of 125 HU. The NPS of IRIS was lower than that of filtered back projection (FBP) at middle and high frequency regions. The SD values were reduced by 21%. The MTF of IRIS and FBP measured by the wire phantom coincided precisely. However, for the bar pattern phantom, the MTF values of IRIS at 0.625 and 0.833 cycle/mm were lower than those of FBP. Despite the reduction of the SD and the NPS, the low-contrast detectability study indicated no significant difference between IRIS and FBP. From these results, it was demonstrated that IRIS had the noise reduction performance with exact preservation for high contrast resolution and slight degradation of middle contrast resolution, and could slightly improve the low contrast detectability but with no significance.     

New method of evaluating edge-preserving adaptive filters for computed tomography (CT): digital phantom method

To evaluate the characteristics of edge-preserving adaptive filters for selectively eliminating noise without affecting resolution in low-dose scanning, we have developed a digital phantom image and evaluated noise statistical values, noise characteristics, and resolution characteristics. The results confirmed that edge-preserving adaptive filters function as smoothing filters in low-contrast regions containing noise, permitting the density resolution to be improved, while the strength of the smoothing filter is reduced to maintain spatial resolution in high-contrast regions containing small structures. It has therefore been confirmed that edge-preserving adaptive filters function as filters for selectively eliminating only the noise elements that are increased when the exposure dose is reduced and that such filters are effective for improving image quality. Using such digital phantom images, images acquired using conditions that are difficult to set in actual CT scanning can be obtained and images specifically for the evaluation target can easily be generated. In addition, the noise level, frequency distribution of the noise, and resolution characteristics of the objects present in the input image can be freely set. It is concluded that evaluation of processing using a digital phantom image is effective for evaluating image processing.     

CT机的性能检测和临床照片评估

目的：探讨ＣＴ 机的性能检测方法。材料和方法：ＣＴ 设备２０ 台。性能检测参数为层厚、ＣＴ 值线性与对比度标度、均匀性、噪声、空间分辨力、密度分辨力、诊视床定位光精度、诊视床运动精度、剂量指数（ ＣＴＤＩ） 等九项。用Ｃａｔｐｈａｎ体模以及ＲＡＤＣＡＬ 剂量测试设备对ＣＴ 机进行性能参数检测。用近２ 个月多部位照片进行临床照片评估。结果：９ 台ＣＴ 机全部参数合格，其中有３ 台照片质量评估为１ 等，６ 台照片质量评估为２ 等，７ 台ＣＴ 机有１ 项参数不合格，照片质量评估为２ 等，４ 台ＣＴ 机有３ 项以上参数不合格，照片质量评估为２ 等。结论：该检测方法可对ＣＴ 机的各项参数作精确的检测，能够满足ＣＴ 机质量保证性能检测的要求。     

Characteristics of a newly developed dentomaxillofacial X-ray cone beam CT scanner (CB MercuRay): system configuration and physical properties

OBJECTIVE: The purpose of this paper is to present the system configuration and physical properties of a new dentomaxillofacial X-ray cone beam CT system (CB MercuRay) being developed. METHODS: The system consists of an image intensifier and a cone beam X-ray source. There are two different models of this system, each with a different size image intensifier, 9" or 12". Each system has three field of view (FOV) modes. The 12" system has facial (F), panoramic (P) and implant (I) FOV modes. The 9" system has P, I and dental (D) modes. Images produced by these systems consist of 512 x 512 x 512 isotropic voxels. Physical properties such as resolution, noise and distortion of the images were evaluated in this study. Modulation transfer function (MTF) was measured using Boone's method. Image noise was measured as the standard deviation of the CT value in water. Circularity of the axial images yielded by the two models was measured using an 8 mm diameter acrylic pipe phantom. RESULTS: The resolving power at a MTF of 0.1 in the D mode was over 2.0 lp mm(-1), suggesting that this system yields images of high resolution. The standard deviation of the CT value in water was approximately 80, which is thought to be greater than that of conventional CT. The circularity of images of the pipe phantom was 99% of the ideal value. CONCLUSION: This study shows that our newly developed cone beam CT system produces high resolution three-dimensional volumetric images that will be useful for the examination of dentomaxillofacial disorders.     

Evaluation of low contrast resolution in cone beam CT using FPD

We examined the low contrast resolution of cone beam CT (CBCT) equipped with an indirect-type flat panel detector and compared it with a commercial CT unit (Robusto) . In CBCT, the X-ray tube voltage of 110 kV was used, and in the Robusto, the usual 120 kV was used for examinations. The computed tomography dose index (CTDI) of the two systems was measured, and images scanned at about the same exposure to radiation were compared. The modulation transfer factors of the two systems were measured, and the convolution kernel that was the nearest to the characteristic of CBCT was chosen among kernels of the Robusto. A water phantom with a diameter of 200 mm was scanned, Wiener spectra were calculated, and signal-to-noise ratios were compared. The low contrast resolution phantom was scanned, and detectability and contrast-to-noise ratio (CNR) were measured. In addition, we placed diluted contrast medium into a phantom, scanned the phantom, and measured the detectability and CNR. When the X-ray irradiation condition of CBCT was 75 mAs at 110 kV, the equal dose of radioactivity in the Robusto was 50 mAs at 120 kV. In the low contrast resolution phantom, detectability was 8.7%mm in CBCT, and 9.4%mm in the Robusto. In the low contrast resolution evaluation phantom, CNR was 1.39 in CBCT, and 2.69 in the Robusto. With diluted contrast medium, CNR was 1.28 in CBCT, and 0.60 in the Robusto. CBCT was inferior to the Robusto in a low contrast resolution phantom, but CBCT was superior to the Robusto using diluted contrast medium. We found that CBCT was useful in examinations using contrast media.     

利用噪声等价图像和深度学习方法对低剂量CT降噪

目的:研究在常规剂量扫描情况下模拟低剂量CT图像的方法,以此生成训练数据集中与常规剂量CT具有对应关系的低剂量CT图像,并建立深度学习模型,用于低剂量CT图像的降噪。方法:使用Philip Brilliance CT Big Bore模拟定位机,其不同算法重建的CT图像具有不同的噪声水平,其中iDose4算法噪声较大,...     

Examination of motion artifacts for helical and non-helical scanning modes in head CT

For head computed tomography (CT), non-helical scanning has been recommended even in the widely used multi-slice CT (MSCT). Also, an acute stroke imaging standardization group has recommended the non-helical mode in Japan. However, no detailed comparison has been reported for current MSCT with more than 16 slices. In this study, we compared the non-helical and helical modes for head CT, focusing on temporal resolution and motion artifacts. The temporal resolution was evaluated by using temporal sensitivity profiles (TSPs) measured using a temporal impulse method. In both modes, the TSPs and temporal modulation transfer factors (MTFs) were measured for various pitch factors using 64-slice CT (Aquilion 64, Toshiba). Two motion phantoms were scanned to evaluate motion artifacts, and then quantitative analyses for motion artifacts and helical artifacts were performed by measuring multiple regions of interest (ROIs) in the phantom images. In addition, the rates of artifact occurrence for retrospective clinical cases were compared. The temporal resolution increased as the pitch factor was increased. Remarkable streak artifacts appeared in the non-helical images of the motion phantom, in spite of the equivalent effective temporal resolution. In clinical analysis, results consistent with the phantom studies were shown. These results indicated that the low pitch helical mode would be effective for emergency head CT with patient movement.     

Quantitative evaluation of low contrast detectability in a brain computed tomography: investigation for the effect of window width on recognition of hyperacute ischemic stroke

In diagnostic brain computed tomography (CT) imaging of hyperacute ischemic stroke, the recognition of subtle lesions is difficult and may be missed. The aim of this study was to evaluate quantitatively the effect of the window width on low contrast detectability by using digital phantom images intended for detection of hyperacute ischemic stroke, and clinical CT images. We first derived the digital phantom images by subtracting the simulated signal data (intensity=1-3 hounsfield units (HU), diameter=10-30 mm) from the water phantom images at various mAs values. Observer studies were then performed under the various window widths at 20, 40, 60, and 80 HU by using the 30 digital phantom images and the 30 water phantom images. In addition, observer studies on brain CT images with 30 abnormal cases and 30 normal cases were performed under the window widths at 20 and 80 HU. Studies were also performed under the simultaneous display of the set of brain CT images on each window width. As a result of evaluation by receiver operating characteristic analysis, narrowing of the window width can improve the low contrast detectability in CT images with noticeable noise, and can decrease the variation in the interpretation skills between observers in clinical CT images. Moreover, the interpretation accuracy was improved by displaying simultaneously the clinical CT image set on window widths at 20 and 80 HU.     

Novel noise reduction filter for improving visibility of early computed tomography signs of hyperacute stroke: evaluation of the filter's performance--preliminary clinical experience

Purpose The aim of this study was to evaluate the performance of a novel noise reduction filter for improving the visibility of early computed tomography (CT) signs of hyperacute stroke on nonenhanced CT images. Material and methods Fourteen patients with a middle cerebral artery occlusion within 4.5 h after onset were evaluated. The signal-to-noise ratio (SNR) of the processed images with the noise reduction filter and that of original images were measured. Two neuroradiologists visually rated all the processed and original images on the visibility of normal and abnormal gray–white matter interfaces. Results The SNR value of the processed images was approximately eight times as high as that of the original images, and a 87% reduction of noise was achieved using this technique. For the visual assessment, the results showed that the visibility of normal gray–white matter interface and that of the loss of the gray–white matter interface were significantly improved using the proposed method (P < 0.05). Conclusion The noise reduction filter proposed in the present study has the potential to improve the visibility of early CT signs of hyperacute stroke on nonenhanced CT images.     

Utility of fixed-quantity analysis in evaluating image noise and resolution in multi-slice CT

We analyzed the spatial frequency of images generated with multi-slice CT (Z-axis) and estimated the definition and noise characteristics. The noise characteristics were calculated with Wiener spectra from multi planar reconstruction (MPR) images, and, in the evaluation of definition characteristics, the modulation transfer function (MTF) was calculated using section sensitivity profile (SSP) with the base method. The Wiener spectra showed a tendency for noise to decrease from a low frequency domain as the effective slice thickness increased. In addition, Wiener spectra were not much different in helical pitch because effective slice thickness became thicker with increases in helical pitch. Examination results suggest that quantitative analysis of the characteristics of image noise and resolution in multi-slice CT images may provide an optical parameter for improving the quality of images in clinical data.     

A digital fluoroscopic imaging system for verification during external beam radiotherapy

A digital fluoroscopic (DF) imaging system has been constructed to obtain portal images for verification during external beam radiotherapy. The imaging device consists of a fluorescent screen viewed by a highly sensitive video camera through a mirror. The video signal is digitized and processed by an image processor which is linked on-line with a host microcomputer. The image quality of the DF system was compared with that of film for portal images of the Burger phantom and the Alderson anthropomorphic phantom using 10 MV X-rays. Contrast resolution of the DF image integrated for 8.5 sec. was superior to the film resolution, while spatial resolution was slightly inferior. The DF image of the Alderson phantom processed by the adaptive histogram equalization was better in showing anatomical landmarks than the film portal image. The DF image integrated for 1 sec. which is used for movie mode can show patient movement during treatment.     

CT图像空间分辨力的实用检测方法

目的:设计应用于检测CT图像空间分辨力的实用方法及体模。方法:检测体模由两块纯有机玻璃中嵌入约0.1 mm厚的铝箔制成,扫描时使之与扫描面垂直,得到系统的冲击响应,即线扩展函数,经相干平均和傅立叶变换等步骤获得系统的调制传输函数,从而评价设备的空间分辨力。受检设备为单层面和多层面CT扫描仪各一台,设置不同扫描条件,共做4组实验。结果:调制传输函数曲线与所采用的扫描条件一致,截止频率与采用国际通用的CAPHAN 500体模测量的结果相当。结论:与线对法相比,通过测量线扩展函数计算调制传输函数来评价CT图像空间分辨力的方法减少了人工干预,结果准确、可靠。     

Comparison of noise equivalent count rate and image quality for two-dimensional and three-dimensional PET scans

The aim of this study was to investigate the correlation between noise equivalent count (NEC) rates and the signal-to-noise ratio (S/N) in reconstructed images. The NEC rates were determined using uniform 20 cm and 70 cm tall, 20 cm diameter cylinders filled with 11C. The phantoms were scanned in both two-dimensional and three-dimensional modes. The reconstructed image noise was evaluated using FBP and OSEM algorithms (4 iterations and 8 subsets). The images were filtered to a final image resolution of 6.5 mm. From the reconstructed image sets, averages and standard deviations of images were generated, from which the average image S/N (=average/standard deviation) was calculated within an 18 cm central ROI. The S/N of a central slice and an end slice was compared with the NEC. The NEC was found to have a linear relationship to the image S/N of all slices, depending on differences in noise properties specific to the reconstruction algorithm. In two-dimensional mode, although the image S/N of the central slice and the edge slice showed a linear relationship with the NEC, in three-dimensional mode, the S/N of the central slice did not show a relationship with the NEC. The linear relationship was also found in both two- and three-dimensional acquisition modes, as well as for the different activity distributions. These results indicate that the NEC is not only a measure for comparing the count rate performance of imaging systems. However, an absolute evaluation is impossible to depend on reconstruction algorithm, slice number, and phantom type.     

CT机质量控制检测的探讨

目的:CT机质量控制检测方法的探讨及应用。方法:用美国模体实验室的Catphan 500模体和瑞典奥利科公司的Solidose 400剂量仪检测CT机扫描架的定位光精度、扫描床运动精度、层厚、CT值线性、视野均匀性、噪声、高对比度分辨率、低对比度分辨率和CT剂量指数(CTDI)。结果:所检测CT机作为整机均合格,但部分指标不合格。结论:通过对噪声、CT线性、对比度分辨率等图像性能参数的检测,可有效保证系统性能良好和维持最优化的图像质量;对CTDI的检测,可及时了解辐射危险水平,进而优化扫描方案,在保证诊断的前提下降低病人受照剂量。