Performance characteristics and quality assurance aspects of kilovoltage cone-beam CT on medical linear accelerator.

A medical linear accelerator equipped with optical position tracking, ultrasound imaging, portal imaging, and radiographic imaging systems was installed at University of Pittsburgh Cancer Institute for the purpose of performing image-guided radiation therapy (IGRT) and image-guided radiosurgery (IGRS) in October 2005. We report the performance characteristics and quality assurance aspects of the kilovoltage cone-beam computed tomography (kV-CBCT) technique. This radiographic imaging system consists of a kilovoltage source and a large-area flat panel amorphous silicon detector mounted on the gantry of the medical linear accelerator via controlled arms. The performance characteristics and quality assurance aspects of this kV-CBCT technique involves alignment of the kilovoltage imaging system to the isocenter of the medical linear accelerator and assessment of (a) image contrast, (b) spatial accuracy of the images, (c) image uniformity, and (d) computed tomography (CT)-to-electron density conversion relationship were investigated. Using the image-guided tools, the alignment of the radiographic imaging system was assessed to be within a millimeter. The low-contrast resolution was found to be a 6-mm diameter hole at 1% contrast level and high-contrast resolution at 9 line pairs per centimeter. The spatial accuracy (50 mm +/- 1%), slice thickness (2.5 mm and 5.0 mm +/- 5%), and image uniformity (+/- 20 HU) were found to be within the manufacturer's specifications. The CT-to-electron density relationship was also determined. By using well-designed procedures and phantom, the number of parameter checks for quality assurance of the IGRT system can be carried out in a relatively short time.     

A quality control test of the image obtained with electronic portal imaging devices]

INTRODUCTION: A quality control of the digital image obtained from two electronic portal imaging devices is discussed. The devices are used to verify the radiotherapic treatment setup by comparing online images of the irradiated volume with those of the simulation devices. MATERIAL AND METHODS: Both iView and Target View devices, respectively installed on a dual energy SLi Precise Elekta and Saturne 42 Ge linear accelerators, consist of highly efficient phosphor screen and high quality videocamera, controlled by a workstation, able to generate digital portal images from few cGy doses. A phantom and software package are used to assess the spatial resolution and signal to noise ratio, and to compare tha data obtained. Spatial resolution and signal to noise ratio of both systems were studied as a function of energy, gantry angle and image acquisition parameters. RESULTS: The mean spatial resolutions obtained from the first 30 measurements were 0,265+/-0,012 and 0,220+/-0,010 lp/mm respectively for 6 and 18 MV of Saturne 42 (Target View) and 0,241+/-0,006 and 0,239+/-0,005 lp/mm for 4 and 6 MV of SLi (iView). Spatial resolution decreases as a function of energy, meanwhile there are no significant statistical differences as regards of the acquisition parameters; signal to noise ratio, instead, increases with integration time. Different values of the spatial resolution as a function of gantry angle are due to changes in the screen-camera distance and flexing of the detector housing. The quality control test is performed every 15 days by the technicians of our Radiotherapy Department. We set the reject level of spatial resolution and signal to noise ratio to be three standard deviations below the mean value obtained during the initial EPID characterization: if the measures fall below these values preventative maintenance is scheduled. CONCLUSIONS: The efficacy of the use electronic portal imaging devices for visualizing and quantifying the relative positions of anatomical structures within the radiation field depends on the image quality. It is therefore essential to devise quality control tests for the devices themselves, to guarantee an optimal level of system performance in a fast and efficient manner.     

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.     

不同DR成像系统成像性能比较研究

目的:将平板探测器相同的两个成像系统成像性能相比较,以期为临床诊断提供更好的图像质量。方法:使用分辨率测试仪器测量两系统的分辨率,比较两系统对细小影像信息的检出能力。使用胸片评价法,指定若干区域,由5位独立观察者分别阅读影像,统计分析后比较两系统的综合成像能力。结果:分辨率测试结果为B系统优于A系统;胸片评价结果为B系统大部分优于A系统。结论:数字图像的成像性能和重建算法在图像的空间分辨率和成像质量上同样具有非常重要的地位,希望能引起大家的重视。     

Practicality of NEMA performance specification measurements for user-based acceptance testing and routine quality assurance

National Electrical Manufactures Association ( NEMA ) performance specifications provide the only standardized and traceable measurements of scintillation-camera performance that are widely accepted by manufacturers. The NEMA publication describing the performance specifications suggests that elaborate equipment beyond a standard imaging computer is required for the measurements. For this reason the tests are currently unsuitable for both user-based acceptance testing and daily quality assurance. We have implemented five of the eight NEMA performance measurements as routine quality-assurance procedures on our computerized scintillation cameras. In addition, we have shown that seven of the eight NEMA measurements can be performed in a manner traceable to NEMA , with energy resolution as the single exception. With a standard imaging computer, NEMA phantom, and minor modification to NEMA collection and analysis constraints, we have analyzed images for intrinsic uniformity, resolution, linearity, and multiple-window spatial registration as well as for system spatial resolution both with and without scatter.     

Portal Verification Using the KODAK ACR 2000 RT Storage Phosphor Plate System and EC® Films

BACKGROUND AND PURPOSE: The suitability of the storage phosphor plate system ACR 2000 RT (Eastman Kodak Corp., Rochester, MN, USA), that is destined for portal verification as well as for portal simulation imaging in radiotherapy, had to be proven by the comparison with a highly sensitive verification film. MATERIAL AND METHODS: The comparison included portal verification images of different regions (head and neck, thorax, abdomen, and pelvis) irradiated with 6- and 15-MV photons and electrons. Each portal verification image was done at the storage screen and the EC film as well, using the EC-L cassettes (both: Eastman Kodak Corp., Rochester, MN, USA) for both systems. The soft-tissue and bony contrast and the brightness were evaluated and compared in a ranking of the two compared images. Different phantoms were irradiated to investigate the high- and low-contrast resolution. To account for quality assurance application, the short-time exposure of the unpacked and irradiated storage screen by green and red room lasers was also investigated. RESULTS: In general, the quality of the processed ACR images was slightly higher than that of the films, mostly due to cases of an insufficient exposure to the film. The storage screen was able to verify electron portals even for low electron energies with only minor photon contamination. The laser lines were sharply and clearly visible on the ACR images. CONCLUSION: The ACR system may replace the film without any noticeable decrease in image quality thereby reducing processing time and saving the costs of films and avoiding incorrect exposures.     

电子射野影像系统在放疗中的应用进展

电子射野影像系统已成为放疗质量控制和质量保证的重要设备之一，可用于摆位误差验证分析、加速器本身日常质量控制、剂量验证、实时剂量验证等放疗质控指标的采集分析，以保障放疗实施的准确性。笔者就电子射野影像系统在放疗中的应用研究进行简要综述。     

Intra-arterial digital subtraction angiography

DSA is an imaging technique that should be integrated into a state-of-the-art angiographic system for proper application and utilization. This results in improvements in both efficiency and diagnostic accuracy. However, from the physician's viewpoint, the proper use of a DSA imaging facility mandates not only an understanding of angiographic principles as applied with conventional film-screen systems but also an in-depth understanding of the factors that affect DSA performance. In particular, factors affecting spatial resolution and contrast sensitivity are crucial. This knowledge has to be applied interactively and "on-line" to achieve optimal IA-DSA image quality.     

Assessment of monitor conditions for the display of radiological diagnostic images and ambient lighting

Diagnostic efficacy is related to viewing conditions. An increasing number of radiology departments are using workstations for reporting and it was the aim of this study to assess monitor performance and ambient lighting in areas allocated to soft-copy reporting and review. The study was performed in 4 Dublin hospitals and 20 monitors were examined. Using a Society of Motion Pictures and Television Engineers' (SMPTE) test pattern, maximum luminance, spatial uniformity of luminance, temporal luminance stability, brightness and contrast resolution (gamma), geometry and ambient lighting was assessed. The results demonstrated that although temporal luminance stability and spatial uniformity appeared to be at acceptable levels, maximum luminance and gamma value variations were noted, with maximum luminance and geometry values often not complying with published guidelines. Cleaning the monitor face had no impact. 90% of viewing areas had acceptable ambient lighting levels. The data presented demonstrate that monitors examined were not operating at optimal levels for all performance parameters and inclusion of regular assessments of monitors should be part of an imaging department's ongoing quality assurance programme.     

Multileafkollimator: Erweiterte Qualitätssicherung an einem GE-Linearbeschleuniger

BACKGROUND: In comparison to a conventional collimator, a multileaf collimator demands a great deal of quality assurance procedures due to its large number of leaves. A concept for daily quality assurance is presented, mainly concerning the positioning accuracy of the leaves. MATERIAL AND METHODS: Two leaf configurations including maximal opening as well as overtravel of single leaves, at a maximal opening of the jaws, are transmitted online in daily exchange from our record- and verify system to the linac. Aiming at a special test phantom a visual control of the positioning accuracy is performed. The leaf positioning is documented by an electronic portal imaging system and is compared with a reference shot by superposition of a grid. RESULTS: This method of quality assurance offers a fast and effective possibility to guarantee the proper function of the whole system by simulating the routine treatment situation. CONCLUSIONS: Compared to a conventional collimator only a slightly greater workload is needed for quality assurance of a multileaf collimator.     

Basic imaging properties of a large image intensifier-TV digital chest radiographic system

The basic imaging properties of a large (57 cm) image intensifier (I.I.)-TV digital imaging system were examined to determine the effects of various physical parameters on the quality of the digital chest images obtained, and also to explore the clinical usefulness of the system. The characteristic curve of the digital system, which relates the output pixel value to the input relative x-ray intensity, was measured with an aluminum stepwedge. MTFs were determined using slit images, and the veiling-glare fraction was measured with a lead-disk technique. Noise Wiener spectra were obtained from uniformly exposed images. The current limitations of the large II-TV digital chest system are its low spatial resolution, and the presence of large amounts of veiling glare and structure mottle. Advantages of this system over other digital chest imaging systems include the high speed of image data acquisition and the capability of "real-time" dynamic imaging of the chest at a radiation dose comparable to that in conventional radiography of the chest.     

Proposal for the standardisation of multi-centre trials in nuclear medicine imaging: prerequisites for a European 123I-FP-CIT SPECT database

Purpose

Multi-centre trials are an important part of proving the efficacy of procedures, drugs and interventions. Imaging components in such trials are becoming increasingly common; however, without sufficient control measures the usefulness of these data can be compromised. This paper describes a framework for performing high-quality multi-centre trials with single photon emission computed tomography (SPECT), using a pan-European initiative to acquire a normal control dopamine transporter brain scan database as an example.

Methods

A framework to produce high-quality and consistent SPECT imaging data was based on three key areas: quality assurance, the imaging protocol and system characterisation. Quality assurance was important to ensure that the quality of the equipment and local techniques was good and consistently high; system characterisation helped understand and where possible match the performance of the systems involved, whereas the imaging protocol was designed to allow a degree of flexibility to best match the characteristics of each imaging device.

Results

A total of 24 cameras on 15 sites from 8 different manufacturers were evaluated for inclusion in our multi-centre initiative. All results matched the required level of specification and each had their performance characterised. Differences in performance were found between different system types and cameras of the same type. Imaging protocols for each site were modified to match their individual characteristics to produce comparable high-quality SPECT images.

Conclusion

A framework has been designed to produce high-quality data for multi-centre SPECT studies. This framework has been successfully applied to a pan-European initiative to acquire a healthy control dopamine transporter image database.     

An elliptical SPECT system with slit-slat collimation for cardiac imaging

Cardiac studies are a good candidate for SPECT (single photon emission computed tomography) because of the large clinical demand and the need for improved image quality. But SPECT imaging suffers from poor spatial resolution and high statistical noise. A new SPECT system with slit-slat collimation arranged on an elliptical arc for cardiac imaging is proposed in this paper. Simulated emission computed tomography data are generated along an elliptical moving orbit with system configuration parameters. The iterative reconstruction techniques are used to implement the cardiac imaging of the proposed SPECT system. Image reconstruction can be done using the OS-EM algorithm from the data collected. This system is developed to improve the reconstructed image if attenuation correction and depth-dependent correction are included in the reconstruction, and the body contour is used in the reconstruction for severely truncated data. The simulation results show that the present methods can provide significant improvement in the spatial resolution and the image quality of SPECT sets.     

Typetesting of physical characteristics of digital mammography systems for screening within the Flemish breast cancer screening programme

To investigate compliance with the acceptance criteria of the European guidelines for quality assurance in breast cancer screening, a typetesting programme of the physical characteristics of digital mammography systems based on direct readout (DR) technology or computed radiography (CR) was organised and executed within the Flemish breast cancer screening programme. While in general image quality/dose characteristics of flat panel DR systems passed the acceptance criteria more easily than CR systems, the slit-scanning direct photon counting system included in present study was outstanding in combining a very low dose with a good image quality. The data obtained up to now indicate the necessity of retuning the AEC for DR systems according to constant contrast to noise ratio (CNR) over the whole range of PMMA thicknesses (20–70 mm) to improve image quality in imaging breasts of large thickness at the cost of higher doses. For the two CR systems which passed the typetesting procedure dose levels do not allow a similar improvement of CNR for thick objects for these systems. The obtained results highlight the importance of the use of high Z target/filter combinations in X-ray generating systems for imaging thick objects to meet the image quality/dose criteria. With respect to image display aspects high-quality 3-megapixel LCD monitors succeeded also in the typetesting procedure in addition to 5-megapixel monitors. However, as zooming and scrolling are necessary for 3-megapixel monitors to get the full resolution capabilities of the image capture system, 5-megapixel monitors are preferred in a busy screening environment.     

Cardiac spiral dual-source CT with high pitch: a feasibility study

Increase of pitch in spiral CT decreases data acquisition time; dual-source CT (DSCT) systems provide improved temporal resolution. We evaluated the combination of these two features. Measurements were performed using a commercial DSCT system equipped with prototype software allowing pitch factors from p?=?0.35 to 3.0. We measured slice sensitivity profiles as a function of pitch to assess spatial resolution in the z-direction and the contrast of structures moved periodically to measure temporal resolution. Additionally we derived modulation transfer functions to provide objective parameters; both spatial and temporal resolution were essentially unchanged even at high pitch. CT of the cardiac region of three pigs was performed at p?=?3.0. In vivo CT images confirmed good image quality; direct comparison with standard low-pitch phase-correlated CT image datasets showed no significant difference. For a normalized z-axis acquisition of 12 cm, the corresponding effective dose value was 2.0 mSv for the high-pitch CT protocol. We conclude that spiral DSCT imaging with a pitch of 3.0 can provide unimpaired image quality with respect to spatial and temporal resolution. Applications to cardiac and thoracic imaging with effective dose below 1 mSv are possible.     

磁共振成像设备的状态检测和临床照片评估

目的探讨我国ＭＲＩ设备的状态检测方法。方法ＭＲＩ设备７台,其中有超低磁场强度电磁型３台,低磁场强度永磁型２台,中、高磁场强度超导型各１台,７台中有二手机１台。状态检测的参数为信噪比、均匀度、线性度、层厚、空间分辨率和低对比度分辨率等６项。用美国体模实验室（ＴｈｅＰｈａｎｔｏｍＬａｂｏｒａｔｏｒｙ）和医学物理学家Ｇｏｏｄｅｎｏｕｇｈ博士研制的Ｍａｇｐｈａｎ体模进行性能参数检测。用随机抽取近２周的头部、胸部、腹部、腰椎的Ｔ１、Ｔ２横断面照片进行临床照片质量评估。照片质量从优到劣分为１、２、３等。结果３台超低磁场电磁型磁共振成像设备的６项参数分别有５项以上不合格,照片质量评估为３等;１台低磁场强度永磁型设备除信噪比不合格外,其余５项参数均合格,照片质量为２等;１台低磁场强度永磁型设备和１台二手中磁场强度超导型设备均有２项参数不合格,其中空间分辨率不合格,照片质量为２等。１台高磁场强度超导型设备６项参数均合格,照片质量为１－等。结论该状态检测方法借鉴了国外的经验,同时又根据实际需要进行补充完善,能满足从整体上评价设备的性能质量的要求     

Evolution of the imaging tests in hemophilia with emphasis on radiography and magnetic resonance imaging

Even before the discovery of X-rays in 1895 attempts were being made to classify the joint destruction that occurs in hemophilic arthropathy. The advent of radiography added impetus to the search for the optimum classification system. Subsequent attempts have included advanced imaging methods, especially magnetic resonance imaging (MRI). Because of its high spatial resolution and ability to visualize soft tissue abnormalities, MRI is ideally suited as the best way to classify arthropathy. A literature search was carried out to document and list all the imaging methods for hemophilic arthropathy that have been published up to the present. All published classification systems are discussed and listed in tables. MRI has superior imaging capability and will probably become the most important modality for radiological classification of hemophilic arthropathy in the future.     

Accuracy of prostate radiation therapy using a fiducial point-pair registration technique based on the computer-assisted portal imaging quality assurance program PIPSpro

The aim of this study was to assess portal imaging for quality assurance of patient positioning in external beam radiotherapy. We present a retrospective study of the variability of patient position in the treatment of 34 prostate cancer patients who were treated with whole pelvic irradiation followed by arc therapy or boost field (Series I) and 25 patients treated by 'small' pelvic 4-field box technique (Series II). Weekly anteroposterior-posteranterior (AP-PA) and left-lateral portal images were compared to simulation films by using a fiducial point-pair registration technique based on the computer-assisted portal imaging quality assurance program PIPSpro, developed specifically for the verification of treatment positioning in radiation therapy. Series I consisted of 34 patients and 194 portal films (97 AP-PA and 97 left-lateral). Overirradiated (OA) and underirradiated (UA) areas were computed in terms of percentage of the reference field size. For the AP-PA portals, the average OA was 2.75% and average UA was 2.74%. For left-lateral portals, an average OA of 2.49% and UA of 2.78% were measured. Series II consisted of 25 patients and 194 portal films (98 AP-PA and 96 left-lateral). The average OA was 0.88% and average UA was 0.86% in AP-PA portals, and 1.03 and 0.82% for left-lateral portals, respectively. The accuracy of patient positioning in irradiation of prostate cancer in our institution is in the range of 2.69% for whole pelvic fields and 1.0% for small fields. We conclude that PIPSpro is an effective and useful tool for quality assurance in radiotherapy.     

Physical and technical aspects of digital mammography

The establishment of digital mammography systems constitutes a slow process, the reason for this being the general need of particular image quality in mammography. This article provides an overview of the physical basis of digital mammography with high image quality and dose as low as reasonably achievable. The trade-off of high contrast resolution and effective quantum efficiency of the imaging system on the one hand, and the demand of high spatial resolution or very small pixel size on the other hand is discussed. The actual status of the available digital detector technology for mammography is described. The digital systems presently available are superior to conventional screen-film mammographic systems with respect to contrast resolution. An outlook on possible further developments in the field of digital mammography is presented.     

Multislice helical CT of the pancreas and spleen

Multislice helical CT (MSCT) with its multidetector technology and faster rotation times, has led to new dimensions in spatial and temporal resolution in CT imaging. In contrast to single-slice CT, smaller slice collimations can be applied that lead to almost isotropic voxels and allow high quality multiplanar and 3-D image reconstructions. The high speed of multislice CT can be used to reduce the time needed to cover a given volume, to increase the spatial resolution along the z-axis by applying thinner slice collimations, and to cover longer anatomic volumes. The speed of MSCT allows organ imaging in clearly defined perfusion phases, e.g. the arterial, parenchymal, and portal venous perfusion phases. Contrast agents with higher iodine concentrations (400 mg iodine per ml compared with 300 mg iodine per ml) lead to higher contrast enhancement of the pancreas (arterial+portal venous phases), the kidneys (arterial+portal venous phases), the spleen (arterial phase), the wall of the small intestine (arterial+portal venous phases), the larger and smaller arteries (arterial phase), and the portal vein (portal venous phase). All of these advancements lead to improved visualization of small structures and of various pathologies, such as pancreatic tumors, liver metastases, vessel infiltration, and vascular diseases.