Impact of miscentering on patient dose and image noise in x-ray CT imaging: Phantom and clinical studies

The operation of the bowtie filter in x-ray CT is correct if the object being scanned is properly centered in the scanner’s field-of-view. Otherwise, the dose delivered to the patient and image noise will deviate from optimal setting. We investigate the effect of miscentering on image noise and surface dose on three commercial CT scanners. Six cylindrical phantoms with different size and material were scanned on each scanner. The phantoms were positioned at 0, 2, 4 and 6 cm below the isocenter of the scanner’s field-of-view. Regression models of surface dose and noise were produced as a function of miscentering magnitude and phantom’s size. 480 patients were assessed using the calculated regression models to estimate the influence of patient miscentering on image noise and patient surface dose in seven imaging centers. For the 64-slice CT scanner, the maximum increase of surface dose using the CTDI-32 phantom was 13.5%, 33.3% and 51.1% for miscenterings of 2, 4 and 6 cm, respectively. The analysis of patients’ scout scans showed miscentering of 2.2 cm in average below the isocenter. An average increase of 23% and 7% was observed for patient dose and image noise, respectively. The maximum variation in patient miscentering derived from the comparison of imaging centers using the same scanner was 1.6 cm. Patient miscentering may substantially increase surface dose and image noise. Therefore, technologists are strongly encouraged to pay greater attention to patient centering.     

CT dosimetry considerations in non typical conditions: The effect of scan field of view and table height selection

PurposeTo experimentally investigate the effect of the scan field of view (SFOV) selection and table height settings on the Computed Tomography Dose Index (CTDI) and the implications concerning patient effective and skin dose.MethodsAir-kerma length product (AKLP) measurements were carried out in a helical CT scanner using a pencil type dosimeter positioned in air and inside the holes of a head and a body phantom, using all available SFOV selections and different table height settings. Furthermore, using radiotherapy verification films placed on the CT table surface, the entrance surface air kerma (ESAK) profiles were derived with different SFOV and table height selections, both with and without a phantom on top of the films.ResultsThe AKLP is strongly dependent on the SFOV selection and the table height settings. Different SFOV selections correspond to the selection of different bowtie filters that shape the X-ray beam intensity, resulting in different ESAK values at the isocenter and at the other points within the scanning plane. With the off-center positioning the calculated CTDI values within the center and the periphery of the phantom change also, as a result of the different intensity and width of the X-ray beam to which are exposed to.ConclusionsThe existing protocols for calculating effective dose are limited to only two patient anatomy-SFOV combinations and cannot account for off-center positioning. Therefore, more work will be required to estimate the effective and skin dose for non-standard SFOV-patient anatomy combinations and off-center patient positioning.     

Comparison of final height in monozygotic twins,one with idiopathic and isolated growth hormone deficiency treated with low dose of growth hormone

OBJECTIVE: We report final heights in a pair of monozygotic twins, one unaffected and the other affected with idiopathic and isolated growth hormone (GH) deficiency treated with human GH, and discuss the effect of GH dosage on the attainment of the genetic height potential in GH deficiency. PATIENTS: Male monozygotic twins were born at 35 weeks of gestation; birth weights were 1,876 g in the unaffected and 1,510 g in the affected twin. At 4.9 years of age, the affected twin was studied for short stature (-3.38 SD) and was diagnosed as having idiopathic and isolated GH deficiency, whereas the unaffected twin was normal in height (+/- 0 SD). GH treatment was started at the age of 5.7 years and continued throughout childhood and adolescence. The average dose of GH administered during the treatment period was 0.35 IU (0.12 mg)/kg/week. The affected twin appeared to grow normally without other hormone replacement and achieved a final height of 165.6 cm (-0.86 SD) compared with that of 166.4 cm (-0.71 SD) in the unaffected twin at 17.5 years of age. CONCLUSION: Our results indicate that a relatively low dose of GH treatment started at an early age may preserve genetic height potential in patients with isolated GH deficiency.     

The influence of operator position,height and body orientation on eye lens dose in interventional radiology and cardiology: Monte Carlo simulations versus realistic clinical measurements

ObjectiveThis paper aims to provide some practical recommendations to reduce eye lens dose for workers exposed to X-rays in interventional cardiology and radiology and also to propose an eye lens correction factor when lead glasses are used.MethodsMonte Carlo simulations are used to study the variation of eye lens exposure with operator position, height and body orientation with respect to the patient and the X-ray tube. The paper also looks into the efficiency of wraparound lead glasses using simulations. Computation results are compared with experimental measurements performed in Spanish hospitals using eye lens dosemeters as well as with data from available literature.ResultsSimulations showed that left eye exposure is generally higher than the right eye, when the operator stands on the right side of the patient. Operator height can induce a strong dose decrease by up to a factor of 2 for the left eye for 10-cm-taller operators. Body rotation of the operator away from the tube by 45°–60° reduces eye exposure by a factor of 2. The calculation-based correction factor of 0.3 for wraparound type lead glasses was found to agree reasonably well with experimental data.ConclusionsSimple precautions, such as the positioning of the image screen away from the X-ray source, lead to a significant reduction of the eye lens dose. Measurements and simulations performed in this work also show that a general eye lens correction factor of 0.5 can be used when lead glasses are worn regardless of operator position, height and body orientation.     

Fruit size, crop mass, and plant height explain differential fruit choice of primates and birds

Seed dispersal by animals is an important ecological process shaping plant regeneration. In general, seed dispersers are highly variable and often opportunistic in their fruit choice. Despite much research, the factors that can explain patterns of fruit consumption among different animal groups remain contentious. Here, we analysed the interactions between 81 animal species feeding on the fruits of 30 plant species in Kakamega Forest, Kenya, during 840 h of observations. Our aim was to determine whether plant characteristics, fruit morphology, fruit colours and/or fruit compounds such as water, sugar, phenols and tannins explained the relative importance of fruit consumption by the two most important consumer groups, primates and birds. We found significant differences in fruit choice between both groups. Primates fed on larger fruits and on higher trees that had larger fruit crops, whereas birds were observed feeding on smaller fruits and on smaller plants producing fewer fruits. Fruit colours did not differ between fruits consumed by primates and those consumed by birds. However, differences in the fruit choice among frugivorous birds were associated with differences in fruit colours. Smaller plants with smaller fruits produced red fruits which contrasted strongly with the background; these fruits were dispersed by a distinct set of bird species. The contents of water, sugar, phenols and tannins did not differ between fruits eaten by primates and those eaten by birds. Some phylogenetic patterns were apparent; primates fed preferentially on a phylogenetically restricted subsample of large plants with large fruits of the subclass Rosidae. We discuss why the observed primate dispersal syndrome is most likely explained by a process of ecological fitting.     

Effects of size asymmetry on aggression and food acquisition in Arctic charr

The feeding rate of a group (DS) of two small, two medium and two large 1+ Arctic charr Salvelinus alpinus was higher than in a group of six fish of the same size (SS), suggesting that size distribution had an effect on feeding behaviour, with a correlation between the size of the fish and the amount of food eaten–the largest fish eating the most. Against predictions, rate of aggression was not higher in the SS groups; rather there was a modest tendency of higher aggression among individuals in the DS groups.     

The effect of tube focal spot size and acquisition mode on task-based image quality performance of a GE revolution HD dual energy CT scanner

PurposeTo assess the task-based performance of images obtained under different focal spot size and acquisition mode on a dual-energy CT scanner.MethodsAxial CT image series of the Catphan phantom were obtained using a tube focus at different sizes. Acquisitions were performed in standard single-energy, high resolution (HR) and dual-energy modes. Images were reconstructed using conventional and high definition (HD) kernels. Task-based transfer function at the 50% level (TTF_50%) for teflon, delrin, low density polyethylene (LDPE) and acrylic, as well as image noise and noise texture, were assessed across all focal spots and acquisition modes using Noise Power Spectrum (NPS) analysis. A non-prewhitening mathematical observer model was used to calculate detectability index ($d_{\mathit{NPW}}^{\prime }$).ResultsTTF_50% degraded with increasing focal spot size. TTF_50% ranged from 0.67 mm⁻¹ for teflon to 0.25 mm⁻¹ for acrylic. For standard kernel, image noise and NPS-determined average spatial frequency were 8.3 HU and 0.29 mm⁻¹, respectively in single-energy, 12.0 HU and 0.37 mm⁻¹ in HR, and 7.9 HU and 0.26 mm⁻¹ in dual-energy mode. For standard kernel, $d_{\mathit{NPW}}^{\prime }$ was 61 in single-energy and HR mode and reduced to 56 in dual-energy mode.ConclusionsThe task-based image quality assessment metrics have shown that spatial resolution is higher for higher image contrast materials and detectability is higher in the standard single-energy mode compared to HR and dual-energy mode. The results of the current study provide CT operators the required knowledge to characterize their CT system towards the optimization of its clinical performance.     

Effect of diagnostic cone-beam computed tomography protocols on image quality,patient dose,and lesion detection

ObjectiveTo evaluate the effect of cone-beam computed tomography (CBCT) image acquisition protocols on image quality, lesion detection, delineation, and patient dose.Methods100-patients and a CTDI phantom combined with an electron density phantom were examined using four different CBCT-image acquisition protocols during image-guided transarterial chemoembolization (TACE). Protocol-1 (time: 6 s, tube rotation: 360°), protocol-2 (5 s, 300°), protocol-3 (4 s, 240°) and protocol-4 (3 s, 180°) were used. The protocols were first investigated using a phantom. The protocols that were found to be clinically appropriate in terms of image quality and radiation dose were then assessed on patients. A higher radiation dose and/or a poor image quality were inappropriate for the patient imaging. Patient dose (patient-entrance dose and dose-area product), image quality (Hounsfield Unit, noise, signal-to-noise ratio and contrast-to-noise ratio), and lesion delineation (tumor-liver contrast) were assessed and compared using appropriate statistical tests. Lesion detectability, sensitivity, and predictive values were estimated for CBCT-image data using pre-treatment patient magnetic resonance imaging.ResultsThe estimated patient dose showed no statistical significance (p > 0.05) between protocols-2 and -3; the assessed image quality between these protocols manifested insignificant difference (p > 0.05). Two other phantom protocols were not considered for patient imaging due to significantly higher dose (protocols-1) and poor image quality (protocol-4). Lesion delineation and detection were insignificant (p > 0.05) between protocols-2 and -3. Lesion sensitivities generated were 81–89% (protocol-2) and 81–85% (protocol-3) for different lesion types.ConclusionData acquisition using protocols-2 and -3 provided good image quality, lesion detection and delineation with acceptable patient dose during CBCT-imaging mainly due to similar frame numbers acquired.     

Experimental assessment of the influence of beam hardening filters on image quality and patient dose in volumetric 64-slice X-ray CT scanners

Beam hardening filters have long been employed in X-ray Computed Tomography (CT) to preferentially absorb soft and low-energy X-rays having no or little contribution to image formation, thus allowing the reduction of patient dose and beam hardening artefacts. In this work, we studied the influence of additional copper (Cu) and aluminium (Al) flat filters on patient dose and image quality and seek an optimum filter thickness for the GE LightSpeed VCT 64-slice CT scanner using experimental phantom measurements. Different thicknesses of Cu and Al filters (0.5–1.6 mm Cu, 0.5–4 mm Al) were installed on the scanner’s collimator. A planar phantom consisting of 13 slabs of Cu having different thicknesses was designed and scanned to assess the impact of beam filtration on contrast in the intensity domain (CT detector’s output). To assess image contrast and image noise, a cylindrical phantom consisting of a polyethylene cylinder having 16 holes filled with different concentrations of K₂HPO₄ solution mimicking different tissue types was used. The GE performance and the standard head CT dose index (CTDI) phantoms were also used to assess image resolution characterized by the modulation transfer function (MTF) and patient dose defined by the weighted CTDI. A 100 mm pencil ionization chamber was used for CTDI measurement. Finally, an optimum filter thickness was determined from an objective figure of merit (FOM) metric. The results show that the contrast is somewhat compromised with filter thickness in both the planar and cylindrical phantoms. The contrast of the K₂HPO₄ solutions in the cylindrical phantom was degraded by up to 10% for a 0.68 mm Cu filter and 6% for a 4.14 mm Al filter. It was shown that additional filters increase image noise which impaired the detectability of low density K₂HPO₄ solutions. It was found that with a 0.48 mm Cu filter the 50% MTF value is shifted by about 0.77 lp/cm compared to the case where the filter is not used. An added Cu filter with approximately 0.5 mm thickness accounts for 50% reduction in radiation-absorbed dose as measured by the weighted CTDI. The FOM results indicate that with an additional filter of 0.5 mm Cu or minimum 4 mm Al, a good compromise between image quality and patient dose is achieved for CT images acquired at tube voltages of 120 and 140 kVp. The results seem to indicate that an optimum filter for high kVp acquisitions, routinely used in cardiovascular imaging, should be 0.5 mm copper or 4 mm aluminium minimum.     

The influence of vessel height and top-section size on the hydrodynamic characteristics of airlift fermentors

Fermentations of the yeast Saccharomyces cerevisiae were carried out in a 90 to 250-L working volume concentric tube airlift fermentor. Measurements of liquid circulation velocity, gas hold-up, and liquid mixing were made under varying conditions of gas flowrate, vessel height, and top-section size. Both liquid circulation velocity and mixing time increased with vessel height. Liquid velocity varied approximately in proportion to the square root of column height, supporting a theoretically based relationship. The effect of vessel height on gas hold-up was negligible. The height of the top-section had a significant effect on liquid mixing. Mixing time decreased with increasing size of the top-section up to a critical height. As the top-section was expanded beyond this height, little improvement in mixing was seen. This indicated the presence of a two-zone flow pattern in the top-section. Liquid velocity and gas hold-up were essentially independent of top-section height. (c) 1994 John Wiley & Sons, Inc.     

Maternal height,shoe size,and outcome of labour in white primigravidas: a prospective anthropometric study.

A total of 563 white primigravid patients at Raigmore Hospital, Inverness, were recruited in a prospective study to examine the association between maternal height, shoe size, and the outcome of labour. There was a significantly increased caesarean section rate in women of short stature but no association between mode of delivery and shoe size. Babies born vaginally had heavier birth weights with increasing height and shoe size. Babies born by caesarean section were heavier than those born vaginally, but their birthweight showed no relation with either height or shoe size. Shoe size is not a useful clinical predictor for the probability of cephalopelvic disproportion, and, although maternal height is a better clinical guide to pelvic adequacy in labour, 80% of mothers less than 160 cm tall delivered vaginally. A well conducted trial of labour should be considered in all primigravid patients with cephalic presentation irrespective of maternal height or shoe size if no obstetric complication exists.     

Trade-offs between flowering time,plant height,and seed size within and across 11 communities of a QingHai-Tibetan flora

Flowering timing is of fundamental biological importance for its tight association with pre-flower growth states and subsequent reproduction success. Here, we selected plant height and seed size to represent plant growth and reproduction states, respectively, and analyzed their associations with flowering time in 11 communities together representing a QingHai-Tibetan flora. Trait associations were examined using Pearson correlation analyses (TIPs) and phylogenetically independent contrasts (PICs) within individual communities and meta-analyses across all communities. The results of TIPs-based and PICs-based analyses were generally congruent, although fewer contrasts were significant with PICs, probably because of low statistical power. Overall, flowering time was negatively correlated with seed size and plant height (i.e., plants with larger seeds and stature started flowering earlier) in various woody communities, but correlations were neutral or positive in herbaceous communities. The seed size–flowering time relationship was negative for woody and herbaceous perennials but not for annual herbs in most communities. The relationship between plant height and flowering time was negative for woody but positive for herbaceous plants. Moreover, the lack of difference in time–size relationships between anemophilous and entomophilous plants suggests that pollination type may only be a secondary force in controlling flowering phenology. Our studies demonstrate that environmental conditions, community structure, and plant life history strategies may affect community flowering time singly or in combination.     

A Monte Carlo simulation for the estimation of patient dose in rest and stress cardiac computed tomography with a 320-detector row CT scanner

PurposeTo estimate organ dose and effective dose for patients for cardiac CT as applied in an international multicenter study (CORE320) with a 320-Detector row CT scanner using Monte Carlo (MC) simulations and voxelized phantoms. The effect of positioning of the arms, off-centering the patient and heart rate on patient dose was analyzed.MethodsA MC code was tailored to simulate the geometry and characteristics of the CT scanner. The phantoms representing the adult reference male and female were implemented according to ICRP 110. Effective dose and organ doses were obtained for CT acquisition protocols for calcium scoring, coronary angiography and myocardial perfusion.ResultsFor low heart rate, the normalized effective dose (E) for cardiac CT was higher for female (5.6 mSv/100 mAs) compared to male (2.2 mSv/100 mAs) due to the contribution of female breast tissue. Averaged E for female and male was 11.3 mSv for the comprehensive cardiac protocol consisting of calcium scoring (1.9 mSv); coronary angiography including rest cardiac perfusion (5.1 mSv) and stress cardiac perfusion (4.3 mSv). These values almost doubled at higher heart rates (20.1 mSv). Excluding the arms increased effective dose by 6–8%, centering the patient showed no significant effect. The k-factor (0.028 mSv/mGy.cm) derived from this study leads to effective doses up to 2–3 times higher than the values obtained using now outdated methodologies.ConclusionMC modeling of cardiac CT examinations on realistic voxelized phantoms allowed us to assess patient doses accurately and we derived k-factors that are well above those published previously.