In this paper, the performance of focused lamellar anti-scatter grids, which are currently used in fluoroscopy, is studied in order to determine guidelines of grid usage for flat detector based cone beam CT. The investigation aims at obtaining the signal to noise ratio improvement factor by the use of anti-scatter grids. First, the results of detailed Monte Carlo simulations as well as measurements are presented. From these the general characteristics of the impinging field of scattered and primary photons are derived. Phantoms modeling the head, thorax and pelvis regions have been studied for various imaging geometries with varying phantom size, cone and fan angles and patient-detector distances. Second, simulation results are shown for ideally focused and vacuum spaced grids as best case approach as well as for grids with realistic spacing materials. The grid performance is evaluated by means of the primary and scatter transmission and the signal to noise ratio improvement factor as function of imaging geometry and grid parameters. For a typical flat detector cone beam CT setup, the grid selectivity and thus the performance of anti-scatter grids is much lower compared to setups where the grid is located directly behind the irradiated object. While for small object-to-grid distances a standard grid improves the SNR, the SNR for geometries as used in flat detector based cone beam CT is deteriorated by the use of an anti-scatter grid for many application scenarios. This holds even for the pelvic region. Standard fluoroscopy anti-scatter grids were found to decrease the SNR in many application scenarios of cone beam CT due to the large patient-detector distance and have, therefore, only a limited benefit in flat detector based cone beam CT.

Our aim was to determine whether fast MR imaging using the half-Fourier acquisition single-shot turbo spin-echo (HASTE) MR sequence is accurate for diagnosis of small-bowel obstruction. MATERIALS AND METHODS. Forty-three nonconsecutive patients with suspected small-bowel obstruction were evaluated with HASTE MR imaging during a I 2-month period. Images were retrospectively assessed for the presence, level, and cause of bowel obstruction. Data were compared with results of conventional radiography and findings at surgery when available. RESULTS. Of the 43 patients imaged.

HASTE MRI has a high degree of accuracy in diagnosing acute cholecystitis based on the single finding of pericholecystic MR high signal. A similar level of accuracy is demonstrated in detecting gallbladder stones. Biliary duct calculi are detected with even greater accuracy than with sonography in patients with acute cholecystitis. Invasive preoperative endoscopic retrograde cholangiography may therefore be limited to only those patients with acute cholecystitis and CBD stones demonstrated on HASTE MRI. These features make HASTE MRI and ideal imaging modality in the initial evaluation of acute biliary pain and may ultimately replace sonography in the preoperative evaluation of acute cholecystitis.

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In this paper, soft tissue contrast visibility in neural applications is investigated for volume imaging based on flat X-ray detector cone-beam CT. Experiments have been performed on a high precision bench-top system with rotating object table and fixed X-ray tube-detector arrangement. Several scans of a post mortem human head specimen have been performed under various conditions. Hereby two different flat X-ray detectors with 366 x 298mm 2 (Trixell Pixium 4700) and 176 x 176mm 2 (Trixell Pixium 4800) active area have been employed. During a single rotation up to 720 projections have been acquired. For reconstruction of the 3D images a Feldkamp algorithm has been employed. Reconstructed images of the head of human cadaver demonstrate that added soft tissue contrast down to 10 HU is detectable for X-ray dose comparable to CT. However, the limited size of the smaller detector led to truncation artifacts, which were partly compensated by extrapolation of the projections outside the field of view. To reduce cupping artifacts resulting from scattered radiation and to improve visibility of low contrast details, a novel homogenization procedure based on segmentation and polynomial fitting has been developed and applied on the reconstructed voxel data. Even for narrow HU-Windows, limitations due to scatter induced cupping artifacts are no longer noticeable after applying the homogenization procedure.