KEYWORDS: Computed tomography, Reconstruction algorithms, Image-guided intervention, Medical imaging, Physics, Current controlled current source, CT reconstruction, Data acquisition, Computer simulations
Cone-beam computed tomography (CBCT) has been increasingly used during surgical procedures for providing accurate three-dimensional anatomical information for intra-operative navigation and verification. High-quality CBCT images are in general obtained through reconstruction from projection data acquired at hundreds of view angles, which is associated with a non-negligible amount of radiation exposure to the patient. In this work, we have applied a novel image-reconstruction algorithm, the adaptive-steepest-descent-POCS (ASD-POCS) algorithm, to reconstruct CBCT images from projection data at a significantly reduced number of view angles. Preliminary results from experimental studies involving both simulated data and real data show that images of comparable quality to those presently available in clinical image-guidance systems can be obtained by use of the ASD-POCS algorithm from a fraction of the projection data that are currently used. The result implies potential value of the proposed reconstruction technique for low-dose intra-operative CBCT imaging applications.
Twenty-five paired chest images (twenty Posterior-Anterior views and five Lateral views) were obtained under identical x- ray exposure conditions with both an amorphous silicon digital detector system and a thoracic film-screen system (InSight HC screen and InSight IT film; Kodak, Rochester, NY). The digital image data was transformed using non-linear algorithms to obtain a gray-scale appearance comparable to conventional film images. The digital images were printed at full resolution onto laser film (3M, 969). Each hard copy clinical image pair was evaluated by six radiologist experts in a blinded study. Two thirds of the radiologists' ratings showed the digital detector images 'Better' or 'Much Better' than screen-film images (p less than 0.05); Ninety-eight percent of the radiologists' ratings indicated the digital 'Equivalent,' 'Better' or 'Much Better' (p equals 0.0004) than analog film. The digital image system improvement can be explained by significantly higher DQE and an extended dynamic range. The study clearly demonstrates that clinically the new digital x- ray detector radiographic system can produce diagnostic images equivalent or superior to conventional screen film system.
We have developed a neutron detection system for accelerator based neutron radiography and tomography based on a combination of scintillation screen and large aperture optics combined with a cooled CCD camera. The system is capable of detecting single neutron events and can therefore be considered as a possible detector for neutron scattering as well as conventional imaging. The system has a resolution of 0.1 mm or 1242 by 1152 pixels. The limit of image size is set by the light output of the scintillator, the light collection of the optical system, the size of the CCD and the desired signal to noise ratio. The lower limit on neutron flux is determined by the dark current of the chip. Equations for these limits have been derived and can be used to predict and optimize performance. The scintillation light output per incident neutron is large enough to permit the use of lens coupled systems with their increased flexibility and ease of implementation. The system can approach a quantum limited noise level, depending on the particular geometry used. For our current system, based on the use of NE 426 scintillator,3 a 1242 by 1152 pixel EEV CCD operating at -50C, and using a 100 mm focal length, F/0.9 lens, the maximum size for the imaging screen is 0.5 m, and the lower limit for flux is 1 n/pixel/s based on this size screen and a typical dark current of 10 e/pixel/s. We are now investigating a new type of imaging technology based on large amorphous silicon sensor arrays being developed by Xerox and others. A typical device is 200 by 250 mm with a pixel size of 127 micrometers and the entire array with all electronics is in a 400 mm by 37 mm package. Major advantages of this device are the high light coupling between scintillator screen and the sensor as well as the more compact nature of such an array, since no lens systems is required and, potentially, a much lower cost. Currently, the noise performance is worse than that of CCDs, largely due to the current electronic readout but should be adequate to produce quantum limited images if electronics can be improved.
Transportable accelerator based sources are the only means by which large structures such as aircraft or industrial components can be inspected practically using neutron radiography. Since such sources are generally considerably lower in flux than reactors, the entire system must be designed for high imaging efficiency. We describe an imaging system for accelerator based thermal neutron tomography and radiography based on the use of a radiofrequency quadrupole (RFQ) using a Be target. Fast neutrons, produced through the reaction 9Be4(d,n)10B5, are moderated with typical thermalized neutron fluence rates of 103 to 104 n/cm2/s, depending on the collimation. The camera system consists of a 1242 by 1152 pixel CCD cooled to -50 degrees C viewing a 20 by 20 cm NE-426 scintillator through an F/0.9 100 mm focal length lends. Our experiments show that neutron statistical noise and not the performance of the CCD system is the dominant limitation in the camera. We have used the system for imaging corrosion in lap joints of KC-135 aircraft and have been able to quantify corrosion within the joint.
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