Contrast-detail Performance Research Articles

Contrast-Specific Spherical Lesion Phantoms and Ancillary Analysis Software for the Objective Evaluation of Transrectal Ultrasound System Contrast Detectability.

Brachytherapy is an efficacious treatment option because of its benefits for patient recovery, dose localization and conformity, but these favorable outcomes can be ensured only if the transrectal ultrasound (TRUS) system is optimized for the specific application of ultrasound-guided prostate brachytherapy. The ability to delineate the prostate from surrounding tissue during TRUS-guided prostate brachytherapy is vital for treatment planning, and consequently, so is the contrast resolution. This study describes the development of task-specific contrast-detail phantoms with clinically relevant contrast and spherical target sizes for contrast-detail performance evaluation of TRUS systems used in the brachytherapy procedure. The procedure for objective assessment of the contrast detectability of the TRUS systems is also described; a program was developed in MATLAB (R2017a, The MathWorks, Natick, MA, USA) to quantitatively analyze image quality in terms of the lesion signal-to-noise ratio (LSNR) and validated with representative control test images. The LSNR of the Hitachi EUB-7500A (2013, Hitachi, Ltd, Tokyo, Japan) TRUS system was measured on sagittal and transverse TRUS images of the contrast-detail phantoms described in this work. Results revealed the efficacy of the device as an image quality evaluation tool and the impact of the size, depth and relative contrast of the targets to the surrounding tissue on the contrast detectability of a TRUS system for both transducer arrays. The MATLAB program objectively measured the contrast detectability of the TRUS system and has the potential to determine optimized imaging parameters that could be designed as part of standardization of the imaging protocol used in TRUS-guided prostate brachytherapy for prostate cancer.

Development and assessment of an x-ray tube-based multi-beam x-ray scatter projection imaging system.

Coherent scatter x-ray imaging systems are sensitive to material structure and chemical composition, and generate soft-material images with contrast superior to conventional transmission x-ray imaging. For practicality in medical or security applications, the image data acquisition time should be <10 min. Our approach is a multi-beam projection imaging design. Previously, as a development stage, we implemented a synchrotron-based system with five coplanar pencil beams and continuous motion of the object. In the work reported here, we developed a more practical coherent scatter projection imaging system using a conventional x-ray tube source. The object is irradiated by an array of up to three rows by five columns of pencil beams, and motorized stages translate the object through the beams for step-and-shoot acquisition. For the same tube loading, broad spectrum beams, such as 110 kVp filtered with 2.25 mm Al, were found to provide a higher signal-difference-to-noise ratio between soft materials in scatter images than lower kVp, more heavily filtered beams that have a narrower, lower intensity spectrum. The shortest acquisition time for a 6.0 × 10.0 cm2 object with 6000 pixels was 8.8 min. The width of a sharp edge in the scatter image was consistent with the pencil beam diameter. Contrast-detail performance was similar to our synchrotron-based system. In this first x-ray tube-based system, for simplicity, the transmitted x rays are measured through attenuators using the same flat-panel detector that measures scattered x rays. As a result, the primary image quality was reduced.

Validation of radiographic quality of simulation software - ImaSim.

Virtual radiographic simulation has been found educationally effective for students to practice their clinical examinations remotely or online. A free available virtual simulator-ImaSim has received particular attention for radiographic science education because of its portability, free of charge and no constrain of location and physical facility. However, it lacks evidence to validate this virtual simulation software to faithfully reproduce radiographs comparable to that taken from a real X-ray machine to date. To evaluate image quality of the virtual radiographs produced by the ImaSim. Thus, the deployment of this radiographic simulation software for teaching and experimental studying of radiography can be justified. A real medical X-ray examination machine is employed to scan three standard QC phantoms to produce radiographs for comparing to the corresponding virtual radiographs generated by ImaSim software. The high and low range of radiographic contrast and comprehensive contrast-detail performance are considered to characterize the radiographic quality of the virtual simulation software. ImaSim software can generate radiographs with a contrast ranging from 30% to 0.8% and a spatial resolution as low as 0.6mm under the selected exposure setting condition. The characteristics of contrast and spatial resolution of virtual simulation generally agree with that of real medical X-ray examination machine. ImaSim software can be used to simulate a radiographic imaging process to generate radiographs with contrast and detail detectability comparable to those produced by a real X-ray imaging machine. Therefore, it can be adopted as a flexible educational tool for proof of concept and experimental design in radiography.

Development and assessment of a multi-beam continuous-phantom-motion x-ray scatter projection imaging system.

X-ray image formation using scattered radiation can yield a superior contrast-to-noise ratio compared to conventional transmission x-ray imaging. A barrier to practical implementation of scatter imaging systems has been slow image acquisition. We have developed a projection imaging system which uses five monoenergetic pencil beams in combination with continuous phantom motion to achieve acquisition times that are practical for medical and security applications. The system was configured at the Canadian Light Source synchrotron and consists of a primary collimator, motorized stages for phantom translation, a flat-panel x-ray detector for measuring scattered x rays, and photodiodes for simultaneously measuring transmitted x rays. Image generation requires several corrections to raw data artifacts arising from the nature of the detector, x-ray source, and acquisition procedure. We developed a novel correction for pixel location inaccuracy arising from continuous phantom motion. A five-beam system had nearly five times faster acquisition than a single-beam system. Continuous motion acquisition was approximately 30 times faster than step-and-shoot acquisition. The total acquisition time for a 9 cm × 5 cm phantom with 8425 pixels was just over 2 min. Image quality was also assessed, in part to determine its relation to acquisition speed. The width of sharp material boundaries was found to be at a minimum equal to the pencil beam width (1.75 mm) and to have an additional width equal to the product of the phantom translation speed and the acquisition time per pixel (up to 1.0 mm in our experiments). Contrast-detail performance was independent of acquisition speed, depending only on phantom entrance x-ray fluence. Pixel signal-to-noise ratio measurements indicate that detector readout noise is important for the scatter data, even for phantom air kerma as high as 30 mGy. Images could be improved with a detector having lower readout noise and higher sensitivity. Its spatial resolution could be moderate. We confirmed that for the same range of λ-1 sin(θ/2), where λ is the x-ray wavelength and θ is the scattering angle, scatter images acquired using different beam energies (33-70 keV) had nearly identical contrast.

Characterization of on-site digital mammography systems: Direct versus indirect conversion detectors

We investigated the performances of two digital mammography systems. The systems use a cesium-iodide (CsI) scintillator and an amorphous selenium (a-Se) photoconductor for X-ray detection and are installed in the same hospital. As physical metrics, we measured the modulationtransfer function (MTF), the noise-power spectrum (NPS), and the detective quantum efficiency (DQE). In addition, we analyzed the contrast-detail performances of the two systems by using a commercial contrast-detail phantom. The CsI-based indirect conversion detector provided better MTF and DQE performances than the a-Se-based direct conversion detector whereas the former provided a poorer NPS performance than the latter. These results are explained by the fact that the CsI-based detector used an MTF restoration preprocessing algorithm. The a-Se-based detector showed better contrast-detail performance than the CsI-based detector. We believe that the highfrequency noise characteristic of a detector is more responsible for the visibility of small details than its spatial-resolution performance.

Performance evaluation of contrast-detail in full field digital mammography systems using ideal (Hotelling) observer vs. conventional automated analysis of CDMAM images for quality control of contrast-detail characteristics

PurposeThe purpose of this work was to evaluate the contrast-detail performance of full field digital mammography (FFDM) systems using ideal (Hotelling) observer Signal-to-Noise Ratio (SNR) methodology and ascertain whether it can be considered an alternative to the conventional, automated analysis of CDMAM phantom images. MethodsFive FFDM units currently used in the national breast screening programme were evaluated, which differed with respect to age, detector, Automatic Exposure Control (AEC) and target/filter combination. Contrast-detail performance was analysed using CDMAM and ideal observer SNR methodology. The ideal observer SNR was calculated for input signal originating from gold discs of varying thicknesses and diameters, and then used to estimate the threshold gold thickness for each diameter as per CDMAM analysis. The variability of both methods and the dependence of CDMAM analysis on phantom manufacturing discrepancies also investigated. ResultsResults from both CDMAM and ideal observer methodologies were informative differentiators of FFDM systems' contrast-detail performance, displaying comparable patterns with respect to the FFDM systems' type and age. CDMAM results suggested higher threshold gold thickness values compared with the ideal observer methodology, especially for small-diameter details, which can be attributed to the behaviour of the CDMAM phantom used in this study. In addition, ideal observer methodology results showed lower variability than CDMAM results. ConclusionThe Ideal observer SNR methodology can provide a useful metric of the FFDM systems' contrast detail characteristics and could be considered a surrogate for conventional, automated analysis of CDMAM images.

Assessment of Double Hole Contrast Detail Phantom for Image Displacement Evaluation

Contrast Detail phantom is used to evaluate the Contrast Detail performance of imaging system. The wall of drilled hole in commercial Contrast Detail phantom will give effect to the penetration of x-ray beam divergence to pass through the base of each hole. This effect will lead to false appearance of image but it does not visualised in the radiograph. In this study, a new design of Contrast Detail phantom’s hole has been developed. The hole feature differs from the current Contrast Detail phantom. The hole consists of combination of different hole's diameters, whereby the small hole diameter (3 mm) is placed on top of large hole diameter of different measurements (5 mm, 7 mm and 10 mm). The thickness of double hole acrylic blocks vary from 10 mm, 15 mm, 20 mm, 25 mm and 30 mm. A tiny size of iron which is used as an indicator was placed at the bottom of each holes. Results shows that, as the value of SID values decrease from 2000 mm, 1800 mm, 1600 mm, 1400 mm, 1200 mm, 1000 mm to 800 mm, the size of upper hole image in different thickness of acrylic block increases from the original size (3mm). In different SID setting, variable acrylic block heights (10 mm, 15 mm, 20 mm, 25 mm and 30 mm) which are located at 100 mm from the center show that, the heighest acrylic block visualises the greatest image displacement. Image displacement which occurs due to magnification can be compensated by increasing the level of SID value and also by reducing the height of acrylic block. The increment in image displacement occurs when the hole is situated far from the center of x-ray beam The new design of hole shows the effect of displacement of object from the x-ray focal length in a well manner.

Comparative performance evaluation of contrast-detail in full field digital mammography (FFDM) systems using ideal (Hotelling) observer versus automated CDMAM analysis

The purpose of our work was to evaluate contrast-detail performance for a range of full field digital mammography systems using Hotelling observer SNR analysis and ascertain whether it can be considered an alternative to CDMAM evaluation. Five FFDM systems were evaluated, which differed in generation (age), Automatic Exposure Control (AEC) behaviour, tube/target combination and detector type. Contrast-detail performance was first analysed using CDMAM phantom analysis and then using the Hotelling observer SNR methodology. The Hotelling observer SNR was calculated for input signal originating from gold discs of varying thicknesses and diameters and then used to estimate the threshold gold thicknesses for each diameter as per CDMAM analysis. There were small differences between the two techniques, especially in small diameter details, which can be attributed to structural characteristics of the CDMAM phantom. The Hotelling observer SNR technique showed lower variability than results from CDMAM analysis. Overall, the Hotelling observer SNR methodology showed variations in the FFDM systems performance consistent with previous findings, demonstrating its value as a performance assessment metric.

A Comparison of three commercially available Contrast-Detail phantoms and evaluation of the effect of Instrument Settings on the performance

Contrast detail is an important characteristic of an ultrasound imaging system. Contrast detail is the measure of a systems ability to distinguish between different intensities or gray levels within an image. A range of Contrast-Detail phantoms are currently commercially available from a variety of manufacturers although no comparison of the usefulness of these phantoms for contrast-detail performance has been carried out. The aim of this study was to compare three such commercial available phantoms two produced by Dansk Phantoms Inc. and one by Gammex-RMI. In this study a range of instrument settings were investigated to determine which had the greatest impact on contrast-detail performance. The effect of damaged crystals was also investigated by simulating dropped elements on one of the transducers. The ultrasound machines used in this project were the Philips HDI 3000 and the Siemens Sonoline Antares. The two machines each had a linear array, curvilinear array and phased array transducer. The default settings on each machine were only used initially to acquire the baseline images of the phantoms. The data collection method involved optimising the conventional image of the appropriate targets and storing the image for off-line analysis on a PC using a contrast-details image analysis program which evaluated the Contrast_to_Noise Ratio, the Lesion Signal_to_Noise Ratio and the Contrast Ratio. The results obtained from each of the phantoms will be presented with a recommendation to the contrast detail measurement which is most sensitive to changes in contrast detail performance.

Factors Affecting Contrast-Detail Performance in Computed Tomography: A Review

This article reports on recent research findings into the factors that influence the detectability performance of different systems of computed tomography (CT) scanners. These systems include multidetector CT (MDCT) of different slice numbers, dual-source CT (DSCT), and cone-beam CT (CBCT). The introduction of more slices for MDCT, DSCT, and the new technology of CBCT increases the need to optimize the image quality and to examine the potential reduction of radiation doses to the patient. Low-contrast detail detectability is a method that has proven to be an appropriate evaluation method for this purpose. However, it is essential to recognize factors that affect detectability performance and understand how these factors influence image quality and radiation dose. It is argued that deep understanding of the influences of these factors is the key to image quality optimization in terms of contrast-detail detectability and radiation dose reduction. The purpose of this article is, therefore, to specify these factors and to explain their influence on detectability performance and hence on CT image quality. Further low-contrast detail studies are required to optimize imaging performance of different CT systems and scanners.

A comparative contrast‐detail study of five medical displaysa)

The objective of this study was to compare the contrast-detail performance of five different commercial liquid crystal displays (LCDs) to other LCD and cathode-ray tube (CRT) displays for medical applications. A contrast-detail phantom, supplemented with 5 in. of acrylic, was imaged on a commercial digital radiographic system using techniques comparable to chest radiography. The phantom design enabled observer evaluation by a four-alternative forced choice paradigm. The acquired images were independently scored by five observers on five medical display devices: a 5 megapixel monochrome LCD, a 3 megapixel monochrome LCD, a 9 megapixel color LCD, a 5 megapixel monochrome CRT, and a mammographic-grade monochrome CRT. The data were analyzed using the method suggested by the manufacturer based on a nearest neighbor correction technique. They were further analyzed using a logistic regression response model with a natural threshold using an overall chi-square test for display type followed by pairwise comparisons for individual display performance. The differences between the display devices were small. The standard analysis of the results based on the manufacturer-recommended method did not yield any statistically discernible trend among displays. The logistic regression analysis, however, indicated that the 5 megapixel monochrome LCD was statistically significantly (p <0.0001) superior to the others, followed by the 3 megapixel monochrome LCD (p<0.0001). The three other displays exhibited lower but generally similar performance characteristics. The findings suggest that 5 and 3 megapixel monochrome LCDs provide comparable but subtly superior contrast detectability than other tested displays, with the former performing slightly better in the detection of subtle and fine details.

An investigation into the effects of suboptimal viewing conditions in screen-film mammography

The ability of an observer to detect objects on a radiograph is influenced by the conditions under which the image is viewed. Therefore, to ensure that as much diagnostically relevant information as possible can be extracted from the image, it is important that satisfactory viewing conditions for the task are established and maintained. Factors that are thought to be important are the luminance of the image, glare and ambient light. Together, these factors lead to the formation of reflection on the image, which may degrade the observer's performance. The purpose of this study was to explore, in a systematic manner, the effect of the factors outlined above on the performance of an observer conducting a threshold contrast-detail diameter test. Each factor was investigated separately with attempts made to minimize the confounding effects of other factors. When examined individually, viewing box luminance, ambient light and glare were found to have little effect on the contrast-detail performance of the observers. Reflection was found to have a significant effect, particularly non-uniform reflection, and the magnitude of the effect was related to the contrast degradation factor and reflection modulation. These quantities, which are derived from basic photometric measurements, may be used to develop a protocol to assess viewing conditions in screen-film mammography.

Evaluation of Gamma Cameras for Use in Dedicated Breast Imaging

Due to the limitations of conventional gamma cameras for breast imaging, many researchers are developing dedicated imagers for use in scintimammography that can be positioned closer to the breast, thereby improving spatial resolution. The purpose of this paper was to compare the performance characteristics of several dedicated gamma cameras with two different types of position sensitive photomultiplier tubes (PSPMTs), and four different NaI crystal arrays. Quantities evaluated include intrinsic spatial resolution, spatial resolution versus source-to-crystal distance, energy resolution, intrinsic nonuniformity, and system sensitivity. In order to assess the impact of changing crystal size on lesion detectability, a contrast-detail study was also performed. Our studies demonstrate that the camera with the newer PSPMTs (Hamamatsu model H8500) shows superior performance in terms of uniformity and energy resolution. The contrast-detail performance of the camera with the highest spatial resolution (1.4-mm crystal pitch with a high resolution collimator) was poor, even with relatively high input fluence. However, the use of a high-efficiency collimator significantly improves object detectability. The camera with the largest (3.2 mm) crystal pitch performed better than the others tested, with both high resolution and high efficiency collimators. However, its performance for small lesion size was better with the high-resolution collimator. Hence, based on the results of the contrast-detail study, of the cameras evaluated here, the camera with the 3.2-mm crystal pitch and the high-resolution collimator is best suited for imaging breast tumors

A comparison of low contrast performance for amorphous Silicon/caesium iodide direct radiography with a computed radiography: A contrast detail phantom study

A range of digital image acquisition devices exists in diagnostic radiology. This study compares contrast performance of two such systems: an amorphous Silicon/caesium iodide (a-Si:CsI) based flat panel (DR) digital chest radiography system and a computed radiography (CR) system. Images of a contrast detail resolution phantom were acquired at a range of radiation doses. Three observers assessed all hardcopy images using a four-alternative forced choice observer perception technique. Contrast detail performance was calculated and low contrast performance quantified. The DR system demonstrated significantly better low contrast performance and potential dose savings of up to 75% compared to the CR system. Threshold levels of contrast detail resolution were defined and levels of under- and over-exposure, compared to the threshold level, were highlighted. Both systems were noise limited at lower exposures and latitude limited at higher exposures. The results demonstrate that the DR system should perform better than the CR system under typical clinical conditions relevant to chest radiography particularly for the detection of low contrast details such as lung metastases or pneumothoraces.

Contrast-detail evaluation and dose assessment of eight digital chest radiography systems in clinical practice

The purpose of this study was to assess contrast-detail performance and effective dose of eight different digital chest radiography systems. Digital chest radiography systems from different manufacturers were included: one storage phosphor system, one selenium-coated drum system, and six direct readout systems including four thin-film transistor (TFT) systems and two charge-coupled device (CCD) systems. For measuring image quality, a contrast-detail test object was used in combination with a phantom that simulates the primary and scatter transmission through lung fields (LucAl). Six observers judged phantom images of each modality by soft-copy reading in a four-alternative-forced-choice experiment. The entrance dose was also measured, and the effective dose was calculated for an average patient. Contrast-detail curves were constructed from the observer data. The blocked two-way ANOVA test was used for statistical analysis. Significant difference in contrast-detail performance was found between the systems. Best contrast-detail performance was shown by a CCD system with slot-scan technology, and the selenium-coated drum system was compared to the other six systems (p values <or=0.003). Calculated effective dose varied between 0.010 mSv and 0.032 mSv. Significant differences in contrast-detail performance and effective dose levels were found between different digital chest radiography systems in clinical practice.

Contrast-detail phantom study for x-ray spectrum optimization regarding chest radiography using a cesium iodide-amorphous silicon flat-panel detector.

The purpose of this study evaluating a cesium iodide-amorphous silicon-based flat-panel detector was to optimize the x-ray spectrum for chest radiography combining excellent contrast-detail visibility with reduced patient exposure. A Lucite plate with 36 drilled holes of varying diameter and depth was used as contrast-detail phantom. For 3 scatter body thicknesses (7.5 cm, 12.5 cm, 21.5 cm Lucite) images were obtained at 113 kVp, 117 kVp, and 125 kVp with additional copper filter of 0.2 and 0.3 mm, respectively. For each setting, radiographs acquired with 125 kVp and no copper filter were taken as standard of reference. On soft-copy displays, 3 observers blinded to the exposure technique evaluated the detectability of each aperture in each image according to a 5-point scale. The number of points given to all 36 holes per image was added. The scores of images acquired with filtration were compared with the standard images by means of a multivariate analysis of variance. Radiation burden was approximated by referring to the entrance dose and calculated using Monte Carlo method. All 6 evaluated x-ray spectra resulted in a statistically equivalent contrast-detail performance when compared with the standard of reference. The combination 125 kVp with 0.3 mm copper was most favorable in terms of dose reduction (approximately 33%). Within the constraints of the presented contrast-detail phantom study simulating chest radiography, the CsI/a-Si system enables an addition of up to 0.3 mm copper filtration without the need for compensatory reduction of the tube voltage for providing constant image quality. Beam filtration reduces radiation burden by about 33%.

Experimental in vitro study to evaluate a flat-panel detector system

To evaluate the contrast-detail performance of a flat-panel detector system, we performed a comparative study of this flat-panel system versus storage phosphor and conventional screen-film systems. Bone models made of human humeri were prepared with foreign bodies, fracture lines and drilled holes to create artificial fractures, osteolyses or metastases. Immersed in a water bath, hard copy images of these models were acquired with the same exposure dose (55 kV; 3.2 mAs) on the flat-panel detector, two state-of-the-art storage phosphor systems (PCR, ADC) and two conventional screen-film systems (Insight, T-Mat). Using a standardized protocol with a 4-point scale for a lesion, 220 different images were analyzed as to their appearance by 5 independent radiologists. The statistical significance of the differences between the used modalities and the observers was determined with a sign test. Especially the drilled holes showed differences between the modalities. The flat-panel detector showed significantly slightly better results locally than the two storage phosphor systems. The same applied to the Insight screen-film systems, but the intraindividual differences between the modalities were not clinically relevant. Under the conditions of the chosen experimental design, the flat-panel detector showed results comparable to the conventional screen-film and the state of the art storage phosphor radiographs. The use in skeletal radiography is possible.

Measurement and correction of the effects of lag on contrast–detail test results in fluoroscopy

Persistence of the video signal between TV frames, an effect also known as image lag, can lead to anomalously good contrast–detail test results for fluoroscopy systems. In this practical paper, a simple method is described which quantifies lag in fluoroscopy systems and corrects for its effect on threshold contrast. A digital framestore was used to acquire temporally contiguous fluoroscopy images. Correlation of the variance between an initial base TV frame and successive later frames was then measured via the correlation coefficient. Plotted against time, this function defines a time constant which characterizes the rate at which the initial variance pattern is replaced by incoming quantum noise. A survey of seven fluoroscopy units incorporating vacuum TV camera tubes found a mean time constant of 0.06 s.The relative change in contrast–detail performance was then measured as a function of applied digital frame averaging for two separate fluoroscopy units. A time constant was found for each frame averaging mode using the correlation of variance between frames. These measurements were used to derive a function which corrects contrast–detail results obtained for a unit with a measured nominal time constant to the typical vacuum camera tube time constant of 0.06 s. The correction is shown to significantly reduce the spread of contrast–detail results obtained over a range of temporal filtration settings.

The practical application of signal detection theory to image qualityassessment in x-ray image intensifier-TV fluoroscopy

This paper applies a published version of signal detection theory to x-rayimage intensifier fluoroscopy data and compares the results with moreconventional subjective image quality measures. An eight-bit digitalframestore was used to acquire temporally contiguous frames of fluoroscopydata from which the modulation transfer function (MTF(u)) and noisepower spectrum were established. These parameters were then combined to givedetective quantum efficiency (DQE(u)) and used in conjunction withsignal detection theory to calculate contrast-detail performance. DQE(u) was found to lie between 0.1 and 0.5 for a range of fluoroscopysystems.Two separate image quality experiments were then performed in order to assessthe correspondence between the objective and subjective methods. First, imagequality for a given fluoroscopy system was studied as a function of doserateusing objective parameters and a standard subjective contrast-detail method.Following this, the two approaches were used to assess three differentfluoroscopy units. Agreement between objective and subjective methods wasgood; doserate changes were modelled correctly while both methods ranked thethree systems consistently.

A radiographic and tomographic imaging system integrated into a medical linear accelerator for localization of bone and soft-tissue targets

Purpose: Dose escalation in conformal radiation therapy requires accurate field placement. Electronic portal imaging devices are used to verify field placement but are limited by the low subject contrast of bony anatomy at megavoltage (MV) energies, the large imaging dose, and the small size of the radiation fields. In this article, we describe the in-house modification of a medical linear accelerator to provide radiographic and tomographic localization of bone and soft-tissue targets in the reference frame of the accelerator. This system separates the verification of beam delivery (machine settings, field shaping) from patient and target localization. Materials and Methods: A kilovoltage (kV) x-ray source is mounted on the drum assembly of an Elekta SL-20 medical linear accelerator, maintaining the same isocenter as the treatment beam with the central axis at 90° to the treatment beam axis. The x-ray tube is powered by a high-frequency generator and can be retracted to the drum-face. Two CCD-based fluoroscopic imaging systems are mounted on the accelerator to collect MV and kV radiographic images. The system is also capable of cone-beam tomographic imaging at both MV and kV energies. The gain stages of the two imaging systems have been modeled to assess imaging performance. The contrast-resolution of the kV and MV systems was measured using a contrast-detail (C-D) phantom. The dosimetric advantage of using the kV imaging system over the MV system for the detection of bone-like objects is quantified for a specific imaging geometry using a C-D phantom. Accurate guidance of the treatment beam requires registration of the imaging and treatment coordinate systems. The mechanical characteristics of the treatment and imaging gantries are examined to determine a localizing precision assuming an unambiguous object. MV and kV radiographs of patients receiving radiation therapy are acquired to demonstrate the radiographic performance of the system. The tomographic performance is demonstrated on phantoms using both the MV and the kV imaging system, and the visibility of soft-tissue targets is assessed. Results and Discussion: Characterization of the gains in the two systems demonstrates that the MV system is x-ray quantum noise-limited at very low spatial frequencies; this is not the case for the kV system. The estimates of gain used in the model are validated by measurements of the total gain in each system. Contrast-detail measurements demonstrate that the MV system is capable of detecting subject contrasts of less than 0.1% (at 6 and 18 MV). A comparison of the kV and MV contrast-detail performance indicates that equivalent bony object detection can be achieved with the kV system at significantly lower doses (factors of 40 and 90 lower than for 6 and 18 MV, respectively). The tomographic performance of the system is promising; soft-tissue visibility is demonstrated at relatively low imaging doses (3 cGy) using four laboratory rats. Conclusions: We have integrated a kV radiographic and tomographic imaging system with a medical linear accelerator to allow localization of bone and soft-tissue structures in the reference frame of the accelerator. Modeling and experiments have demonstrated the feasibility of acquiring high-quality radiographic and tomographic images at acceptable imaging doses. Full integration of the kV and MV imaging systems with the treatment machine will allow on-line radiographic and tomographic guidance of field placement. Keywords: