Computed Tomography Scanner Model Research Articles

Comparison of two types of adult phantoms in terms of organ doses from diagnostic CT procedures

The rapidly increasing number of diagnostic computed tomography (CT) procedures in the recent decades has spurred heightened concern over the potential risk to patients. Although an accurate organ dose assessment tool has now become highly desirable, existing software packages depend on stylized computational phantoms that were originally developed more than 40 years ago, exhibiting very large discrepancies when compared with phantoms that are anatomically realistic. However, past comparative studies did not focus on CT protocols for adult patients. This study was designed to quantitatively compare two types of phantoms, the stylized phantoms and a pair of recently developed RPI-adult male and adult female (RPI-AM and RPI-AF) phantoms, for various CT scanning protocols involving the chest, abdomen–pelvis and chest–abdomen–pelvis. Organ doses were based on Monte Carlo simulations using the MCNPX code and a detailed CT scanner model for the GE LightSpeed 16. Results are presented as ratios of organ doses from the stylized phantoms to those from the RPI phantoms. It is found that, for most organs contained in the scan volume, the ratios were within the range of 0.75–1.16. However, the stomach doses are significantly different and the ratio is found to be up to 1.86 in male phantoms and 2.29 in the female phantoms due to the anatomical differences between the two types of phantoms. Organs that lie near a scan boundary also exhibit a significant relative difference in organ doses between the two types of phantoms. This study concludes that, due to relatively low x-ray energies, CT doses are very sensitive to organ shape, size and position, and thus anatomically realistic phantoms should be used to avoid the dose uncertainties caused by the lack of anatomical realism. The new phantoms, such as the RPI-AM and AF phantoms that are designed using advanced surface meshes, are deformable and will make it possible to match the anatomy of a specific patient leading to further improvement in dose and risk assessments for patients undergoing CT examinations.

Method for evaluating bow tie filter angle‐dependent attenuation in CT: Theory and simulation results

Dosimetry in computed tomography (CT) is increasingly based on Monte Carlo studies that define the dose in the patient (in mGy) as a function of air kerma (free in air) at isocenter (mGy). The accuracy of Monte Carlo studies depends in part on the accuracy of the characterization of the bow tie filter for a given CT scanner model. A simple method for characterizing the bow tie filter attenuation profile in CT scanners would therefore be very useful. The theory behind such a method is proposed. A measurement protocol is discussed mathematically and demonstrated using computer simulation. The proposed method requires the placement of a radiation monitor at the periphery of the CT field, and the time domain signal (kerma rate versus time) is measured with good temporal resolution (-200 Hz or better) and with all other objects (e.g., patient couch) retracted from the field of view. Knowledge of the source to isocenter distance (or alternately, the isocenter to probe distance) is required. The stationary detector records the kerma rate versus time signal as the gantry rotates through several revolutions. From this temporal data, signal processing techniques are used to extract in-phase peaks, as well as out-of-phase kerma rate levels. From these data, the distance from isocenter to the probe can be determined (or, alternatively, the source to isocenter distance), and the angle-dependent bow tie filter attenuation can be computed. By measuring the angle-dependent bow tie filter attenuation at several kVp settings, the bow tie composition versus fan angle can be computed using basis decomposition techniques. The simulations illustrated that with 2% added noise in the kerma rate versus time signal, the attenuation properties of a hypothetical two component (aluminum and polymethyl methacrylate) bow tie filter could be determined (r2 > 0.99). Although the computed basis material thicknesses were not exactly equal to the actual thicknesses, their combined attenuation factors matched that of the actual filter across kVp's to within an average of 0.057%. It is concluded that the proposed method may provide a simple noninvasive approach to characterizing the performance of bow tie filters in CT systems; however, experimental validation is necessary.

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We developed a method for quantitative measurement of cerebral blood flow (CBF) using multi slice dynamic computed tomography (CT) and iodinated contrast material based on the box-MTF model. We investigated whether it is possible to determine an indication for acute thrombolytic therapy using this method. Initially we examined the correlation between CBF measured by this method and that by SPECT using 99mTC-ECD in 10 patients with chronic ischemic cerebrovascular diseases. The correlationship between them showed CBF (SPECT)=0.79×CBF (Perfusion CT) and its correlation coefficient was 0.60. To examine the reproducibility, CBF was measured twice at 5-minute intervals including 1 overlapping slice in 9 patients. The CBF value in the overlapped slice was compared between 2 examinations. Finally, the correlation between CBF and back pressure was examined in 10 patients with acute cerebral embolism. A fairly good correlation was obtained from these 2 studies. The correlation coefficient of the latter 2 studies was 0.80 and 0.86 respectively. It takes nearly 1 minute for dynamic scan, and about 15 minutes for data analysis. This is a significant advantage for the evaluation of CBF in acute cerebrovascular disease. We conclude that our method to measure CBF using a multi slice dynamic CT scanner and box-MTF model is useful to determine the indication for acute thrombolytic therapy.

Investigations of the thermal fatigue behaviour of single-crystal nickel-based superalloys SRR 99 and CMSX-4: Meyer-Olbersleben, F., Rezai-Aria F. and Goldschmidt, D. Proc. Conf. Superalloys 1991, Champion, Pennsylvania, USA, 20–24 Sept. 1992 (The Minerals, Metals & Materials Society, 420 Commonwealth Dr., Warrendale, PA 15086, USA) 1992 pp 785–794

Patient-specific phantomless calibration of computed tomography (CT) scans has the potential to simplify and expand the use of pre-existing clinical CT for quantitative bone densitometry and bone strength analysis for diagnostic and monitoring purposes. In this study, we quantified the inter-operator reanalysis precision errors for a novel implementation of patient-specific phantomless calibration, using air and either aortic blood or hip adipose tissue as internal calibrating reference materials, and sought to confirm the equivalence between phantomless and (traditional) phantom-based measurements. CT scans of the spine and hip for 25 women and 15 men (mean ± SD age of 67 ± 9 years, range 41–86 years), one scan per anatomic site per patient, were analyzed independently by two analysts using the VirtuOst software (O.N. Diagnostics, Berkeley, CA). The scans were acquired at 120 kVp, with a slice thickness/increment of 3 mm or less, on nine different CT scanner models across 24 different scanners. The main parameters assessed were areal bone mineral density (BMD) at the hip (total hip and femoral neck), trabecular volumetric BMD at the spine, and vertebral and femoral strength by finite element analysis; other volumetric BMD measures were also assessed. We found that the reanalysis precision errors for all phantomless measurements were ≤ 0.5%, which was as good as for phantom calibration. Regression analysis indicated equivalence of the phantom- versus phantomless-calibrated measurements (slope not different than unity, R2 ≥ 0.98). Of the main parameters assessed, non-significant paired mean differences (n = 40) between the two measurements ranged from 0.6% for hip areal BMD to 1.1% for mid-vertebral trabecular BMD. These results indicate that phantom-equivalent measurements of both BMD and finite element-derived bone strength can be reliably obtained from CT scans using patient-specific phantomless calibration.