Current commercial micro-CT scanners have the capability of imaging objects ex vivo with high spatial resolution, but performing in vivo micro-CT on free-breathing small animals is still challenging because their physiological motions are non-periodic and much faster than those of humans. In this paper, we present a prototype physiologically gated micro-computed tomography (micro-CT) scanner based on a carbon nanotube field emission micro-focus x-ray source. The novel x-ray source allows x-ray pulses and imaging sequences to be readily synchronized and gated to non-periodic physiological signals from small animals. The system performance is evaluated using phantoms and sacrificed and anesthetized mice. Prospective respiratory-gated micro-CT images of anesthetized free-breathing mice were collected using this scanner at 50 ms temporal resolution and 6.2 lp mm(-1) at 10% system MTF. The high spatial and temporal resolutions of the micro-CT scanner make it well suited for high-resolution imaging of free-breathing small animals.

Digital breast tomosynthesis ͑DBT͒ is a limited angle computed tomography technique that can distinguish tumors from its overlying breast tissues and has potentials for detection of cancers at a smaller size and earlier stage. Current prototype DBT scanners are based on the regular full-field digital mammography systems and require partial isocentric motion of an x-ray tube over certain angular range to record the projection views. This prolongs the scanning time and, in turn, degrades the imaging quality due to motion blur. To mitigate the above limitations, the concept of a stationary DBT ͑s-DBT͒ scanner has been recently proposed based on the newly developed spatially distributed multibeam field emission x-ray ͑MBFEX͒ source technique using the carbon nanotube. The purpose of this article is to evaluate the performance of the 25-beam MBFEX source array that has been designed and fabricated for the s-DBT system. The s-DBT system records all the projection images by electronically activating the multiple x-ray beams from different viewing angles without any mechanical motion. The configuration of the MBFEX source is close to the published values from the Siemens Mammomat system. The key issues including the x-ray flux, focal spot size, spatial resolution, scanning time, beam-to-beam consistency, and reliability are evaluated using the standard procedures. In this article, the authors describe the design and performance of a distributed x-ray source array specifically designed for the s-DBT system. They evaluate the emission current, current variation, lifetime, and focal spot sizes of the source array. An emission current of up to 18 mA was obtained at 0.5ϫ 0.3 mm effective focal spot size. The experimentally measured focal spot sizes are comparable to that of a typical commercial mammography tube without motion blurring. Trade-off between the system spatial resolution, x-ray flux, and scanning time are also discussed. Projection images of a breast phantom were collected using the x-ray source array from 25 different viewing angles without motion. These preliminary results demonstrate the feasibility of the proposed s-DBT scanner. The technology has the potential to increase the resolution and reduce the imaging time for DBT. experimentally the feasibility of achieving 11 s scanning time at full detector resolution with 0.5 ϫ 0.3 mm source resolution without motion blur. The flexibility in configuration of the x-ray source array will also allow system designers to consider imaging geometries that are difficult to achieve with the conventional single-source rotating approach.

A stationary tomosynthesis mammography system with a carbon nanotube-based x-ray source array can shorten imaging time and improve image quality.Mammography is currently the most effective screening and diagnostic tool for early breast cancer detection. In fact, the recent reduction in the breast cancer mortality rate has been attributed to increased mammographic screening 1 . However, mammography suffers from several limitations. It is very difficult to distinguish cancer from overlying breast tissues on two-dimensional mammograms, and radiologists' interpretation of the images may vary. There are also higher rates of false-positive and falsenegative test results because dense tissues interfere with the identification of abnormalities associated with tumors. To solve this problem, researchers in the late 1990s developed a novel technique called x-ray digital breast tomosynthesis (DBT).DBT is a three-dimensional imaging technique that uses a series of projection images acquired at different angles to provide reconstruction planes in the breast. Several commercial vendors, including GE 2 , Hologic 3 , and Siemens 4 , have manufactured prototype DBT scanners that are based on full-field digital mammography (FFDM) systems. To generate the series of projection images, a conventional x-ray tube mounted on a rotating gantry fixes the imaging beam on the breast, while the tube moves in an arc to generate images at multiple angles. A typical tomosynthesis scan can take anywhere from 20 seconds to more than 1 minute. Compared with conventional mammography, the prolonged imaging time introduces patient motion blur on the images. Moreover, gantry motion leads to a larger effective x-ray focal spot size, which degrades the image quality.To overcome this problem, we proposed a stationary digital breast tomosynthesis system using a carbon nanotube-based field emission x-ray source array 5,6 . The device, called Argus, uses spatially distributed x-ray sources, so it acquires the projection images without source or detector movement. It reduces the total imaging time and potentially improves image quality.We have designed and constructed a prototype system composed of a 25-pixel x-ray source array, a flat panel detector for full-field mammography, a control unit for x-ray sources, and a computer work station. As shown in Figure 1, the geometry of the Argus system, including source-to-object distance, angle coverage, and view number, is comparable to that of conventional mammography and DBT systems. Table 1 shows a comparison between Argus and other prototype systems. Our target goal is the acquisition of 25 projection images in 11 seconds at 0.2mm resolution. By contrast, the Siemens system at the same dose requires 20 seconds to take 25 images with ∼0.3mm focal spot size-and additional blur due to gantry motion ranges from 0.2mm to 1mm depending on the rotation speed.The key component of the Argus system is the 25-pixel x-ray source array. Other medical applications like micro-CT 7 have Continued on next page

The IQ-SPECT system was introduced by Siemens in 2010 to significantly improve the efficiency of myocardial perfusion imaging (MPI) using conventional, large field-of-view (FOV) SPECT and SPECT-CT systems. With IQ-SPECT, it is possible to perform MPI scans in one-fourth the time or using one-fourth the administered dose as compared to a standard protocol using parallel-hole collimators. This improvement is achieved by means of a proprietary multifocal collimator that rotates around the patient in a cardio-centric orbit resulting in a four-fold magnification of the heart while keeping the entire torso in the FOV. The data are reconstructed using an advanced reconstruction algorithm that incorporates measured values for gantry deflections, collimator-hole angles, and system point response function. This article explores the boundary conditions of IQ-SPECT imaging, as measured using the Data Spectrum® cardiac torso phantom with the cardiac insert. Impact on reconstructed image quality was evaluated for variations in positioning of the myocardium relative to the sweet spot, scan-arc limitations, and for low-dose imaging protocols. Reconstructed image quality was assessed visually using the INVIA 4DMSPECT and quantitatively using Siemens internal IQ assessment software. The results indicated that the IQ-SPECT system is capable of tolerating possible mispositioning of the myocardium relative to the sweet spot by the operator, and that no artifacts are introduced by the limited angle coverage. We also found from the study of multiple low dose protocols that the dwell time will need to be adjusted in order to acquire data with sufficient signal-to-noise ratio for good reconstructed image quality.

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