PubMed · 14864018
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E GRENDEL. 1951-07-21. [Tempora mutantur].. https://pubmed.ncbi.nlm.nih.gov/14864018/
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Dose assessment in computed tomography (CT) is challenging due to the vast variety of CT scanners and imaging protocols in use. In the present study, the accurateness of a theoretical formalism implemented in the PC program CT-EXPO for dose calculation was evaluated by means of phantom measurements. Phantom measurements were performed with four 1-slice, four 4-slice and two 16-slice spiral CT scanners. Firstly, scanner-specific nCTDIw values were measured and compared with the corresponding standard values used for dose calculation. Secondly, effective doses were determined for three CT scans (head, chest and pelvis) performed at each of the ten installations from readings of thermoluminescent dosimeters distributed inside an anthropomorphic Alderson phantom and compared with the corresponding dose values computed with CT-EXPO. Differences between standard and individually measured nCTDIw values were less than 16%. Statistical analysis yielded a highly significant correlation (P < 0.001) between calculated and measured effective doses. The systematic and random uncertainty of the dose values calculated using standard nCTDIw values was about -9 and +/- 11%, respectively. The phantom measurements and model calculations were carried out for a variety of CT scanners and representative scan protocols validate the reliability of the dosimetric formalism considered-at least for patients with a standard body size and a tube voltage of 120 kV selected for the majority of CT scans performed in our study.
Inhomogeneous magnetic fields produce artifacts in MR images including signal dropout and spatial distortion. A novel perturbative method for calculating the magnetic field to first order (error is second order) within and around nonconducting objects is presented. The perturbation parameter is the susceptibility difference between the object and its surroundings (for example, approximately 10 ppm in the case of brain tissue and air). This method is advantageous as it is sufficiently accurate for most purposes, can be implemented as a simple convolution with a voxel-based object model, and is linear. Furthermore, the method is simple to use and can quickly calculate the field for any orientation of an object using a set of precalculated basis images.
PURPOSE: The right internal jugular (RIJ) is commonly used to provide central venous access, and success of cannulation shows a positive correlation with the vein's diameter. The purpose of this study is to establish the patient position resulting in the largest RIJ diameter. METHOD: 2D ultrasound was used to measure RIJ diameter, in varying body positions, in 21 healthy volunteers. RESULTS: In the neutral position (table flat, head on the table in midline) the RIJ diameter was (mean +/- standard deviation) 9.2 +/- 2.18 mm. A small pillow under the head increased RIJ diameter (10.6 +/- 2.16 mm, P < 0.001). Trendelenburg tilt of 15 degrees increased RIJ diameter (12.1 +/- 2.34 mm, P < 0.001). In the Trendelenburg position (15 degrees of tilt), a small pillow under the head further increased RIJ diameter (13.3 +/- 2.26, mm P < 0.001), palpating for the carotid artery decreased RIJ diameter (8.2 +/- 1.98 mm, P < 0.001), and rotation of the head 45 degrees to the left did not reduce RIJ diameter significantly (11.7 +/- 2.52 mm, P = 0.12). CONCLUSION: The patient position to achieve maximal RIJ diameter cannulation is: 15 degrees of Trendelenburg tilt; a small pillow or head ring under the head; the head in or close to midline; and after palpation of the carotid artery, it should be released prior to vein cannulation.