Computer-controlled flow simulator for MR flow studies.

A novel computer-controlled flow simulator for use in magnetic resonance (MR) flow experiments was evaluated. The accuracy in constant-flow mode was better than 1%. The accuracy in pulsatile-flow mode was found to be dependent on the interconnecting tubing. The short-term and long-term reproducibilities of pulsatile waveforms were less than or equal to 0.4 mL/sec (1 standard deviation). Increased response times due to the lengths of tubing required in MR flow experiments were surmounted by using a modified tubing configuration and precompensated waveforms. Piston reversal was found not to cause major difficulties in MR flow experiments.     

Anthropomorphic breast phantoms for assessing ultrasonic imaging system performance and for training ultrasonographers: Part I

Three prototype anthropomorphic breast phantoms are discussed. The phantoms were constructed using ultrasonically tissue-mimicking materials; these materials mimic various tissue parenchymae in terms of attenuation, speed of sound, density, and scatter level. Realistic artifacts related to refraction and reflection at interfaces between different simulated parenchymae are produced. The phantoms represent premenopausal breasts, and they complement one another. Two of them represent the dense breasts of women under 30 years of age, and one represents that of a woman between 35 and 40 years of age. Of the former two, one produces what is apparently above-average refraction effects in the region of the peripheral fat layer; the other produces more typical refraction effects. Simulated tumors, cysts, and calcifications of various sizes are suspended in the glandular regions. Such phantoms are valuable for use in developmental testing of state-of-the-art ultrasound machines, quality assurance testing of clinical machines, and training of sonographers in breast imaging.     

Gelatine-alginate complex gel: A new acoustically tissue-equivalent material

Methods are described for the preparation of gelatine alginate complex gels and measurements are reported of certain of their acoustic and physical properties relevant to their use as tissue-equivalent phantoms in medical ultrasonics applications. Speed of sound at 20°C is 1520ms^?1 with a coefficient of + 2.6 ms^?1/°C. Attenuation coefficient in unloaded gel is 0.12 dB cm^?1 MHz^?1 (varying approximately linearly with frequency) but can readily be increased to at least 0.5 dB cm^?1 MHz^?1 by loading the gel with polyethylene or lipid microspheres. Volume stability under conditions of water immersion without an impermeable boundary layer is within about ±2% over a 300 day period, an improvement by a factor of at least 25 on both simple and cross-linked gelatine gels, and stability against chemical or bacterial degradation can also readily be maintained.     

New Developed DR Detector Performs Radiographs of Hand,Pelvic and Premature Chest Anatomies at a Lower Radiation Dose and/or a Higher Image Quality

A newly developed Digital Radiography (DR) detector has smaller pixel size and higher fill factor than earlier detector models. These technical advantages should theoretically lead to higher sensitivity and higher spatial resolution, thus making dose reduction possible without scarifying image quality compared to previous DR detector versions. To examine whether the newly developed Canon CXDI-70C DR detector provides an improved image quality and/or allows for dose reductions in hand and pelvic bone examinations as well as premature chest examinations, compared to the previous (CXDI-55C) DR detector version. A total of 450 images of a technical Contrast-Detail phantom were imaged on a DR system employing various kVp and mAs settings, providing an objective image quality assessment. In addition, 450 images of anthropomorphic phantoms were taken and analyzed by three specialized radiologists using Visual Grading Analysis (VGA). The results from the technical phantom studies showed that the image quality expressed as IQF_INV values was on average approximately 45 % higher with the CXDI-70C detector compared to the CXDI-55C detector. Consistently, the VGA results from the anatomical phantom studies indicated that by using the CXDI-70C detector, diagnostic image quality could be maintained at a dose reduction of in average 30 %, depending on anatomy and kVp level. This indicates that the CXDI-70C detector is significantly more sensitive than the previous model, and supports a better clinical image quality. By using the newly developed DR detector a significant dose reduction is possible while maintaining image quality.     

Accuracy of phase-contrast flow measurements in the presence of partial-volume effects

The accuracy of volume flow rate measurements obtained with phase-contrast methods was assessed by means of computer simulation and in vitro experiments. Factors studied include (a) the partial-volume effect due to voxel dimensions relative to vessel dimensions and orientation and (b) intravoxel phase dispersion. It is shown that limited resolution (partial-volume effect) is the major obstacle to accurate flow measurement for both laminar and plug flow. The results show that at least 16 voxels must cover the cross section of the vessel lumen to obtain a measurement accuracy to within 10%. Measurement accuracy also greatly depends on the relative signal intensity of stationary tissue and is better for laminar flow than plug flow.     

MR imaging of blood vessels with an intravascular coil.

A method for producing high-resolution magnetic resonance (MR) images of blood vessel walls is described. The authors review a theoretical analysis of receiver-coil design and present a coil well suited for intravascular MR imaging. The design is based on two coaxial solenoids separated by a gap region and with current driven in opposite directions. Placement of this receiver coil within the vascular space is shown to provide a substantial increase in sensitivity over that external surface coils. Experimental verification of these predictions was obtained in a vessel phantom in which a 13-cm surface coil was compared with a 3.5-mm-diameter opposed-solenoid intravascular coil. This intravascular coil had a cylindric region of high sensitivity that offered a 10-fold improvement in signal-to-noise ratio over that of an external coil near the vessel wall. The performance of this coil was also tested in the jugular vein of a swine.     

优化数字乳腺断层摄影曝光条件的体模显像

目的比较数字乳腺断层摄影(DBT)不同摄影条件下对乳腺体模病灶的检出率。方法对ACR-156体模及MPW-01体模进行DBT,使用有机玻璃板组合模拟5个不同厚度,首先采用自动曝光控制模式(AEC)进行曝光,然后保持相同管电压值,管电流设为AEC曝光时的80%、60%、40%和20%。比较不同厚度、不同曝光条件下辐射剂量及病灶检出率。结果随管电流下降,平均腺体剂量和皮肤吸收剂量及总体病灶检出率下降(P均0.01);相同厚度下,管电流降低20%时,肿块样病灶及钙化样病灶检出率下降平缓甚至无明显变化,纤维样病灶检出率下降较明显,管电流降低20%时,各病灶检出率均随管电流下降而下降;厚度增加对肿块样病灶检出率有一定影响。结论乳腺筛查中适当降低管电流可有效减低辐射剂量而不影响病灶检出率。     

肺部容积高分辨CT低剂量扫描方案的体模与临床应用研究

目的 研究肺部容积高分辨CT(volumetric high-resolution CT,VHRCT)的低剂量扫描方案,评价其诊断价值.方法 采用120 kV和10～250 mAs对Catphan 500体模行VHRCT扫描,层厚0.625 mm,记录图像的空间分辨力、密度分辨力、噪声及扫描剂量,制定低剂量VHRCT的扫描方案;105例在本院行常规剂量VHRCT检查的肺弥漫病变患者,复诊时行低剂量VHRCT,比较常规剂量与低剂量VHRCT对于肺弥漫病变的显示情况.结果 体模研究中,管电压120 kV,管电流120～250 mAs时,VHRCT图像的空间分辨力均为9 LP/cm;低于120 mAs时,随着管电流降低,VHRCT图像的空间分辨力和密度分辨力下降而噪声增加.临床研究中,对于肺弥漫病变各种征象的显示,低剂量VHRCT(120 kV,120 mAs)与常规剂量VHRCT(120 kV,250 mAs)比较差异无统计学意义(P＞0.05).扫描剂量较常规VHRCT降低52％.结论 采用120 kV和120 mAs行低剂量VHRCT,可以在保持图像的分辨能力及诊断价值的前提下显著降低放射剂量,其取代常规剂量VHRCT具有可行性.     

Subjective image quality assessment of cross sectional imaging methods for the symphyseal region of the mandible prior to dental implant placement

Objectives

A three dimensional appreciation of the form of the anterior mandible is required to place dental implants safely in the region. This study compared the subjective image quality of four available methods of cross sectional imaging of the symphyseal region, the lateral cephalometric view, cone beam computed tomography (CBCT), spiral tomography and the transymphyseal X-ray view.

Methods

An experimental water phantom was developed to reproduce human soft tissue around the mandible. Images were taken of four mandibles by the four X-ray techniques. Three different CBCT machines were included.The source of each image was disguised by displaying all images in the same format on the same computer screen. A protocol was developed to process the images for viewing whilst preserving their image quality.A panel of observers of ten dentists viewed the images and rated their image quality by recording their agreement with six statements on a five point Likert scale.

Results

The results showed a statistically significant difference in image quality between imaging methods. There were clear differences in the ratings between the three cone beam computed tomography machines. Small volume, high resolution CBCT provided images with the highest scores for subjective image quality.

Conclusions

Within the limitations of this study, significant differences in subjective image quality were found between imaging systems used for cross sectional imaging for the symphyseal region of the mandible prior to dental implant placement.     

64层VCT冠状动脉成像的实验研究

目的定量评估64层容积CT(VCT)显示冠状动脉狭窄及支架内再狭窄的能力和可靠性。材料与方法应用心脏动态体模,设定心率0、50、70、90次/min,对内径3mm和5mm的模拟冠状动脉(内设25%、50%、75%3段狭窄)及内径4mm的模拟带支架冠状动脉(支架段内设50%、75%2段狭窄)分别进行VCT成像,将VCT对应数据与标准值进行对比分析。结果(1)VCT对3mm和5mm模拟冠状动脉25%、50%、75%3段狭窄及管腔内径的平均测量值分别为(0.61±0.02)mm、(1.11±0.06)mm、(1.68±0.05)mm、(2.28±0.06)mm(单样本P值0.001、0.001、0.000、0.000)和(1.14±0.04)mm、(2.30±0.04)mm、(3.50±0.04)mm、(4.71±0.04)mm(单样本P值0.010、0.003、0.001、0.001);3段狭窄的平均百分比测量值分别为(26.8±0.6)%、(48.6±0.7)%、(73.8±0.9)%(单样本P值0.009、0.145、0.075)和(24.1±0.6)%、(48.9±0.8)%、(74.4±0.7)%(单样本P值0.066、0.071、0.139)。(2)VCT图像伪影随心率增快而增加,3mm模拟冠状动脉更严重;心率70次/min以上影响变明显。(3)VCT可同时显示支架及支架内狭窄,但显示支架内狭窄能力有限,对4mm模拟冠状动脉50%狭窄显示为46.4%(0次/min)和43.6%(50次/min),与标准值相比差异有统计学意义(P=0.000);支架段的内腔径〔平均(2.16±0.13)mm〕<正常段〔平均(3.22±0.10)mm〕,两者差异有统计学意义。(4)心率90次/min时,机架转速0.42s/r较0.35s/r测量效果好(配对t检验,P=0.036)。结论(1)VCT评价冠状动脉狭窄很可靠,但受心率的影响大,时间分辨率仍有待提高。(2)VCT在管径较粗和低心率条件下评价支架内再狭窄有一定价值。(3)优化扫描参数同后处理一样可以提高成像质量。