Low Tube Current Research Articles

CT-Guided Lumbar Nerve Root Injections: Are We Using the Correct Radiation Dose Settings?

Selecting a lower tube current for CT fluoroscopic spine injections is a method of radiation dose reduction. Ideally tube current should be tailored to the patient's body habitus, but a greater influence on tube current may be the proceduralist's personal preference. The purpose of this study was to compare tube current and fluoroscopy time of different proceduralists for lumbar spine CT-guided selective nerve root blocks, and to correlate image quality to patient diameter and tube current. Eighty CT-guided SNRBs performed by 4 proceduralists were retrospectively reviewed for tube current and fluoroscopy time. Patient body habitus was evaluated by measuring anteroposterior diameters on scout images. Image quality was evaluated objectively and subjectively: noise was measured in the psoas muscle and images were graded on a 3-point scale. The mean tube current was 59 ± 20 mA and mean fluoroscopy time was 10.4 ± 7.5 seconds. The mean tube current between proceduralists differed by almost 2-fold, and there was greater than 2-fold difference in mean fluoroscopy time (P < .0001 and .01, respectively). Mean AP body size was 27 ± 5 cm. When categoric data of tube current and AP diameter were analyzed, only AP diameter was a statistically significant variable influencing image noise (P = .009). Twenty of 23 patients with AP diameter ≤30 cm had adequate to excellent image quality, even with lower tube current of ≤40 mA. Wide variability in tube current selection between proceduralists calls for a more objective method of selecting tube current to minimize radiation dose. Body size, measured by AP diameter, had the greatest influence on image quality. This could be used to identify patients for lower tube current selection.

Phantom study of the impact of adaptive statistical iterative reconstruction (ASiR&amp;lt;sup&amp;gt;TM&amp;lt;/sup&amp;gt;) on image quality for paediatric computed tomography

Quantitative analysis of image quality will be helpful for designing ASiRTM-enhanced paediatric CT protocols, balancing image quality and radiation dose. Catphan600 phantom studies were performed on a GE Discovery HD750 64-slice CT scanner. Images were reconstructed with 0% - 100% ASiRTM (tube current 150 mA, variable kVp 80 - 140) in order to determine the optimal ASiRTM-Filtered Back Projection (FBP) blend. Images reconstructed with a 50% ASiRTM-50% FBP blend were compared to FBP images (0% ASiRTM) over a wide range of kVp (80 - 140) and mA (10 - 400) values. Measurements of image noise, CT number accuracy and uniformity, spatial and contrast resolution, and low contrast detectability were performed on axial and reformatted coronal images. Improvements in CNR, low contrast detectability and radial uniformity were observed in ASiRTM images compared to FBP images. 50% ASiRTM was associated with a 26% - 30% reduction in image noise. Changes in noise texture were observed at higher % ASiRTM blends with impact on visualisation of low and high contrast objects. A small decrease in limiting spatial resolution was detected with addition of ASiRTM, more appreciable at very low tube currents. The preferred blend for paediatric body protocols in our study, as determined by the image quality parameters investigated, was 50% ASiRTM when used with tube currents greater than 50 mA.

Abdominal Dynamic CT in Patients with Renal Dysfunction: Contrast Agent Dose Reduction with Low Tube Voltage and High Tube Current–Time Product Settings at 256–Detector Row CT

To evaluate the feasibility of a low-contrast agent dose protocol at abdominal dynamic computed tomography (CT) with a low tube voltage high tube current-time product technique and a 256-detector row CT unit. This prospective study received institutional review board approval; written informed consent to participate was obtained from all patients. The study included 151 patients; 117 had an estimated glomerular filtration rate (eGFR) greater than or equal to 60 mL/min/1.73 m(2). These patients were examined with the conventional 120-kVp protocol. The other 34 patients underwent scanning with an 80-kVp tube voltage, a high tube current-time product, and a 40% reduction in contrast agent dose. Effective dose (ED), image noise, attenuation, contrast-to-noise ratio (CNR), and figure of merit of the aorta in the arterial phase and of the portal vein and hepatic parenchyma in the portal venous phase in the two groups were compared with the Student t test. Estimated ED was about 20% lower with the 80-kVp protocol than with the 120-kVp protocol. There were no significant differences in CNR in any region of interest between the 80-kVp protocol and the 120-kVp protocol (abdominal aorta: 36.9 ± 9.7 [standard deviation] vs 36.1 ± 8.1, P = .63; portal vein: 13.4 ± 3.2 vs 13.1 ± 3.2, P = .65; hepatic parenchyma: 6.4 ± 2.6 vs 6.7 ± 2.3, P = .51). Contrast dose at hepatic dynamic 256-detector row CT in patients with renal dysfunction can be decreased by 40% with this protocol by using the 80-kVp setting and a high tube current-time product.

Hepatocellular nodules in liver cirrhosis: state of the art CT evaluation (perfusion CT/volume helical shuttle scan/dual-energy CT, etc.)

The purpose of this article is to explain the role of advanced liver CT imaging, including perfusion CT, dual-energy CT, and volume helical shuttle (VHS) scanning, with regard to its clinical applications. Perfusion CT is a promising method for calculating hepatic blood flow and portal blood flow, including microcirculation, using a color-encoded display of parameters obtained from the liver time-density curve, with iodine contrast agent. Tumor angiogenesis and assessment of the response to antiangiogenesis treatment (e.g., Sorafenib) can be analyzed by perfusion CT of the liver. VHS scan has very high temporal resolution due to the reciprocating movement employed during scanning, enabling the acquisition of 24 scans of the whole liver in the arterial dominant phase during a 40-s breath hold, and a reduction in radiation dose. Dual-energy CT enables differentiation of materials and tissues based on their CT density values, using two different energy spectra. This method includes a low tube voltage CT technique that increases the contrast enhancement of vascular structures while simultaneously reducing radiation dose. Images obtained at the preferred settings of low tube voltage and high tube current, with dose reduction in the hepatic arterial phase, are useful for detecting hypervascular hepatocellular carcinoma.

In vitrocomparison of digital and conventional bitewing radiographs

Free AccessLetter to the editorIn vitro comparison of digital and conventional bitewing radiographsJohan KM Aps, Erwin Berkhout and Arthur D GorenJohan KM ApsSearch for more papers by this author, Erwin BerkhoutSearch for more papers by this author and Arthur D GorenSearch for more papers by this authorPublished Online:28 Jan 2014https://doi.org/10.1259/dmfr/69119164SectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail AboutWe would like to comment on the recent DMFR article: In vitro comparison of digital and conventional bitewing radiographs for the detection of approximal caries in primary teeth exposed and viewed by a new wireless handheld unit.1We are worried that the erroneous statements the authors make could lead practitioners to use the device improperly and without regard to operator and patient safety. Namely, there are no dosimetry studies supporting their conclusions. The authors have only looked at the image quality in an in vitro environment, but even more important, in our opinion, is the safety of the handheld device for the operator and patient. Holding the device can never assure a safe distance behind the X-ray machine and, furthermore, can never prevent the operator being exposed to backscatter radiation from the patient. It is for the latter that we do not agree with the conclusion of this article, especially the recommendation that the device can be used in nursing homes, paediatric dentistry and with disabled patients. What's more, this recommendation cannot be a conclusion of the article because the use of the device is not studied at all!We are very concerned that the authors recommend the device to be used in a paediatric dental setting. The device's focus-to-skin distance is only 10 cm instead of the required minimal 20 cm. This is, therefore, not a safe X-ray device. The manufacturer should provide a longer spacer cone, a tripod to mount the X-ray machine and a remote control. Only then will an acceptable situation be provided for the patient and the operator. However, because of the low tube current of 1 mA, the exposure time will increase tremendously with greater focus-to-skin distance, increasing the risk of motion unsharpness, especially in the patient groups the authors recommend this device for.The exposure times mentioned in the article seem convenient, but this is only due to the excessively short focus-to-skin distance and probably the absence of any soft tissue equivalent material in the setup of the study. So, in vivo much longer exposure times will be needed, resulting in many adverse effects for operator and patient. This issue is not discussed and in our opinion is of great interest.We also question the fact that the focal spot is 0.8 × 0.8 mm. This is of minor quality compared with recent X-ray machines from various manufacturers. The focal spot should be as small as possible to obtain a sharper image as less penumbra will be available to degrade the image. This issue is not discussed and, in our opinion, is not of minor interest.We agree devices like these can, in certain circumstances, be the only choice, for instance, in humanitarian operations, forensic investigations in the field and in the military (field operations). However, the use of these kind of devices for “current” dental radiography should at all times be discouraged, as it gives the operators and dental practitioners a false sense of safety, while in fact they are probably prone to quite substantial amounts of backscatter radiation. The stochastic effects of these low energy photons should never be underestimated or neglected.In the European Union the problem, as mentioned above, with these kinds of machines is recognized. Member states of the Union aim to abandon these machines for general dental use, including paediatric dentistry and for disabled patients. In the USA some states have regulations stating that the operator must be at least six feet away from the X-ray generator unless a waiver can be obtained showing that the device is both safe to the operator and the patient.We hope that the authors, in their future research with these kinds of devices, take our concerns in mind.Reference1 Ulusu T, , Bodur H, , Odabas ME and . In vitro comparison of digital and conventional bitewing radiographs for the detection of approximal caries in primary teeth exposed and viewed by a new wireless handheld unit. Dentomaxillofac Radiol 2010;39:91–94. Link ISI, Google Scholar Previous article Next article FiguresReferencesRelatedDetails Volume 39, Issue 5July 2010Pages: 257-322 2010 The British Institute of Radiology History Published onlineJanuary 28,2014 Metrics Download PDF

Apport du TDM en imagerie multimodalité : le point de vue du physicien médical

The recent introduction of hybrid systems SPECT/CT and PET/CT in nuclear medicine, greatly improved the diagnostic accuracy for particular clinical indications, due to the possible attenuation correction of functional images and the availability of helpful anatomic information. The introduction of CT in the nuclear diagnostic process results in a significant increase of the patient dose. This increase should be justified and optimized considering both the clinical question and the CT settings available on these systems. The choice of CT settings directly affects the effective dose. It varies basically as the square of the tube voltage, linearly with the length of the scan and the product of the current by the rotation time of the tube. It is also inversely proportional to the pitch. For attenuation correction, the literature shows that it is possible to use a low CT tube current without significant effect on tumor FDG uptake or lesion size. Conversely low CT voltage must be used with caution, depending on the algorithm implemented in the CT hybrid device to transform CT Hounsfield units to the attenuation map at the appropriate energy. The radiation dose for anatomic correlation can be substantially lower than for diagnostic-quality CT. It is possible to reduce the patient's radiation dose by a factor of 2 or 3 by acquiring a low-dose PET/CT scan for anatomic correlation of adequate image quality if compared with diagnostic 18FDG PET/CT. Using specific CT settings, the effective dose can range 7.3–11.3 mSv depending on the patient weight and age.

WE-E-201B-03: Compressed Sensing with a First-Order Method for Low-Dose Cone-Beam CT Reconstruction

Purpose: This work considers the problem of reconstructing cone‐beam computed tomography(CBCT)images from a set of under‐sampled and potentially noisy projection measurements. Materials and Methods: We cast the reconstruction as a compressed sensing problem based on l 1 norm minimization constrained by statistically weighted least‐squares of CBCT projection data. For accurate modeling, the noise characteristics of CBCT projection data are used to determine the relative importance of each projection measurement. To solve the compressed sensing problem, we employ a method minimizing total‐variation norm satisfying a pre‐specified level of measurement consistency using a first‐order method developed by Nesterov.Results: The method converges fastly to the optimal solution without excessive memory requirement, thanks to the method of iterative forward and back projections. The performance of the proposed algorithm is demonstrated through a series of anthropomorphic phantom studies. It is found that high quality CBCTimage can be reconstructed from under‐sampled and potentially noisy projection data by using the proposed method. Both sparse sampling and decreasing x‐ray tube current (i.e., noisy projection data), lead to reduction of radiation dose in CBCTimaging.Conclusions: It is demonstrated that compressed sensing outperforms the traditional algorithm when dealing with sparse, and potentially noisy, CBCT projection views and relatively high noise due to low tube current.

Effect of dual-source cardiac computed tomography on patient radiation dose in a clinical setting: Comparison to single-source imaging

Dual-source computed tomography (DSCT) was introduced with significant hardware and software changes compared with single-source CT (SSCT), resulting in improved temporal resolution (83 ms) and the potential for improved image quality. The effect of these changes on radiation dose requirements for coronary CT angiography in clinical practice has not been investigated. We evaluated patient radiation dose and image quality of electrocardiogram (ECG)-gated helical techniques, using DSCT compared with SSCT for clinical imaging of the coronary arteries. DSCT data from 160 patients were evaluated; 82 patients (DSCT group 1) were imaged with early software, and 78 patients (DSCT group 2) were imaged with a later software version. Patients imaged with SSCT (n = 124) were the control group. Effective radiation dose values were estimated for all patients. Image noise was measured, and image quality was evaluated on a 5-point scale. Effective dose values for DSCT group 2 (11.7 +/- 4.0 mSv) were not different from those for SSCT group (10.9 +/- 2.9 mSv); the highest doses, 13.2 +/- 3.2 mSv, were recorded for DSCT group 1 (P < 0.001). A decrease in image noise was observed for DSCT compared with SSCT (P <or= 0.001) as was an increase in image quality (P < 0.01). With optimized DSCT imaging, lower dose values were associated with (1) shorter scan range, (2) lower maximum tube current, and (3) lower fraction of R-R interval receiving maximum tube current. ECG-gated helical DSCT can provide images of the coronary arteries with improved image quality and decreased noise without an increase in radiation dose compared with SSCT in clinical patient groups.

Effect of X-ray Tube Parameters, Iodine Concentration, and Patient Size on Image Quality in Pulmonary Computed Tomography Angiography: A Chest-Phantom-Study

The aim of this phantom study was to evaluate the contrast-to-noise ratio (CNR) in pulmonary computed tomography (CT)-angiography for 300 and 400 mg iodine/mL contrast media using variable x-ray tube parameters and patient sizes. We also analyzed the possible strategies of dose reduction in patients with different sizes. The segmental pulmonary arteries were simulated by plastic tubes filled with 1:30 diluted solutions of 300 and 400 mg iodine/mL contrast media in a chest phantom mimicking thick, intermediate, and thin patients. Volume scanning was done with a CT scanner at 80, 100, 120, and 140 kVp. Tube current-time products (mAs) varied between 50 and 120% of the optimal value given by the built-in automatic dose optimization protocol. Attenuation values and CNR for both contrast media were evaluated and compared with the volume CT dose index (CTDI(vol)). Figure of merit, calculated as CNR/CTDIvol, was used to quantify image quality improvement per exposure risk to the patient. Attenuation of iodinated contrast media increased both with decreasing tube voltage and patient size. A CTDIvol reduction by 44% was achieved in the thin phantom with the use of 80 instead of 140 kVp without deterioration of CNR. Figure of merit correlated with kVp in the thin phantom (r = -0.897 to -0.999; P < 0.05) but not in the intermediate and thick phantoms (P = 0.09-0.71), reflecting a decreasing benefit of tube voltage reduction on image quality as the thickness of the phantom increased. Compared with the 300 mg iodine/mL concentration, the same CNR for 400 mg iodine/mL contrast medium was achieved at a lower CTDIvol by 18 to 40%, depending on phantom size and applied tube voltage. Low kVp protocols for pulmonary embolism are potentially advantageous especially in thin and, to a lesser extent, in intermediate patients. Thin patients profit from low voltage protocols preserving a good CNR at a lower exposure. The use of 80 kVp in obese patients may be problematic because of the limitation of the tube current available, reduced CNR, and high skin dose. The high CNR of the 400 mg iodine/mL contrast medium together with lower tube energy and/or current can be used for exposure reduction.

Dental CT: image quality and radiation exposure in relation to scan parameters

To develop a scan protocol for dental-CT which guarantees good image quality at the lowest possible radiation dose. In an experimental investigation Dental-CT (HSA, GE, Milwaukee, USA) of the mandible of two human skeletons positioned in a water tank were performed in order to define the most advantageous scan protocol. Tube currents ranged from 40 to 200 mA and the scan technique was modified (axial mode or helical mode with pitches of 1 to 3 and corresponding increments of 0.4 to 1.0 mm). 39 patients underwent a dental-CT with decreased current (80 mA) in the helical scan mode (pitch 2, slice thickness 1 mm). Dose measurements were performed for two different scan protocols (A: axial, 130 mAs, B: helical, 80 mA, pitch 2). The preliminary investigations of image quality showed only a minor effect of the applied current. For the helical scan mode, pitches of more than 2 impaired image quality. A low increment had no advantages. There were no disadvantages in clinical practice using protocol B with decreased tube current. Absorbed radiation dose of dental CT performed with protocol B was decreased to one third in comparison to protocol A. A scan protocol with a low tube current (e.g., 80 mA, for a rotation time of 1 s) and a helical scan mode (e.g., for a slice thickness of 1 mm with a pitch of 2 and an increment of 1 mm) is recommended for performing dental-CT.

Optimisation of protocol for low dose CT-derived attenuation correction in myocardial perfusion SPECT imaging

In clinical routine, attenuation correction (AC) using X-ray CT is a relatively new method for reducing attenuation artefacts. We evaluated the quality of attenuation maps generated with very low tube current to minimise exposure due to transmission scanning. SPECT/CT acquisitions were performed with a Millenium VG3 gamma camera with the Hawkeye CT device (GE Medical Systems). In phantom studies, determination of linear absorption coefficients (mu) for air, water and Teflon was carried out. The attenuation maps in both stress and resting studies from 62 patients (21 females and 41 males, age 63.7 +/- 11.0 years, BMI 30.0 +/- 5.7 kg/m(2)) were compared. All patients underwent exercise or pharmacologic stress testing and a resting study for comparison using Tc-99m MIBI or Tc-99m Tetrofosmin. AC in stress studies was performed using 2.5 mA tube current (set as default), whereas 1.0 mA was used in resting studies. In both phantom and patient studies, differences of linear absorption coefficients were not significant (p > 0.05). Effective dose decreased from 0.90 mSv down to 0.36 mSv, respectively. Our results indicate that reliable attenuation maps (mu-maps) of the thorax can be obtained even with the use of very low tube current. In our study, radiation exposure in CT-based AC for myocardial perfusion SPECT was substantially lowered (60% reduction). This is of particular importance in high-risk patients who may have to undergo follow-up scans and in research studies on volunteers. The procedure introduced is relatively simple and can be transferred to other SPECT/CT devices, which allow adjustment of tube current.

Appendicitis in Children: Low-Dose CT with a Phantom-based Simulation Technique—Initial Observations

To retrospectively determine the accuracy of low-dose (20-mAs) computed tomography (CT) in the diagnosis of acute appendicitis in children by using a technique that enables the simulation of human CT scans acquired at a lower tube current given the image acquired at a standard dose. Institutional review board approval was obtained, informed consent was not required, and the study was HIPAA compliant. The authors reviewed 100 standard-dose pediatric abdominal-pelvic CT scans (50 positive and 50 negative scans) obtained in 100 patients and corresponding simulated low-dose (20-mAs) scans. The standard-dose scans were obtained for evaluation in patients suspected of having appendicitis. Scans were reviewed in randomized order by four experienced pediatric radiologists. The patients with positive findings included 21 girls (mean age, 9.2 years) and 29 boys (mean age, 8.4 years). The patients with negative findings included 28 girls (mean age, 9.2 years) and 22 boys (mean age, 8.4 years). Simulation was achieved by adding noise patterns from repeated 20-mAs scans of a pediatric pelvis phantom to the original scans obtained with a standard tube current. Observers recorded their confidence in the diagnosis of appendicitis by using a six-point scale. Dose-related changes were analyzed with generalized estimating equations and the nonparametric sign test. There was a statistically significant (P < .001, sign test) decrease in both sensitivity and accuracy with a lower tube current, from 91.5% with the original tube current to 77% with the lower tube current. A low dose was the only statistically significant (P < .001) risk factor for a false-negative result. The specificity was unchanged at 94% for both the images obtained with the original tube current and the simulated low-dose images. The overall accuracy decreased from 92% with the original dose to 86% with the low dose. Preliminary findings indicate that it is feasible to optimize the CT dose used to evaluate appendicitis in children by using phantom-based computer simulations.

TU‐D‐J‐6C‐02: Radiation Dose Reduction in 4D Computed Tomography

Purpose: 4D CT is useful clinically for detailed abdominal and thoracic imaging over the course of the respiratory cycle. However, it usually delivers 10∼15 times more radiation dose to the patient as compared to the standard 3D CT, since multiple scans at each couch position are required to obtain the temporal information. In this work we propose a method to obtain high quality 4D CT with low tube current, hence reducing the radiation exposure of patients. Method and Materials: The improvement of the signal‐to‐noise ratio (SNR) of the CT image at a given phase was achieved by superposing the imaging information from other phases with the use of a deformable image registration model. To further reduce the statistical noise caused by low tube current, we developed a novel 4D penalized weighted least square (4D‐PWLS) method to smooth the data spatially and temporally. The method was validated by motion‐phantom and patient studies using a GE Discovery‐ST PET/CT scanner. A Varian RPM respiratory gating system was used to track the motion and to facilitate the phase binning of the 4D CT data. Results: We calculated the SNRs for both studies. The average SNR of 10 mA phantom images increased by more than three‐fold from 0.051 to 0.165 after the proposed 4D‐PWLS processing, without noticeable resolution loss. The patient images acquired at 90mA showed an increase from 2.204 to 4.558 for the end‐inspiration phase, and from 1.741 to 3.862 for the end‐expiration phase, respectively. By examining the subtraction images before and after processing, good edge preservation was also observed in the patient study. Conclusion: By appropriately utilizing the temporal information in 4D‐CT, the proposed method effectively suppresses the noise while preserving the resolution. The technique provides a useful way to reduce the patient dose during 4D CT and is thus valuable for 4D‐radiotherapy.

Low-dose spiral CT: application to surface-rendered three-dimensional imaging of central airways.

The purpose of this work was to assess the utility of low-dose spiral CT for three-dimensional imaging of the central airways. Spiral CT was performed in 15 patients using two tube currents (50 and 240 mA), producing 30 CT data sets. Surface-rendered virtual bronchoscopy (VB) and shaded surface display (SSD) images were assessed by three radiologists for image quality. The radiologists were also asked to compare 15 matched pairs each of 50 and 240 mA VB and SSD images, blindly select the 240 mA image, and record differences in diagnostic quality between the matched images. No significant difference in image quality was noted with 50 or 240 mA. Only 51.1% of 240 mA SSD and 51.1% of 240 mA VB images were correctly identified. Differences in diagnostic quality, noted in 84.4% of SSD and 33% of VB image pairs, were not ascribed to image noise. Image quality of surface-rendered three-dimensional images of the central airways is preserved using a lower tube current.

Virtual endoscopy of the tympanic cavity based on high-resolution multislice computed tomographic data.

This study was designed to assess the value of high-resolution multislice computed tomography (MSCT) data of the petrous bone for the virtual endoscopic visualization of the tympanic cavity. The recently introduced MSCT technology has improved spatial resolution in the z axis as well as scan speed in computed tomography. Three-dimensional rendering of high-resolution MSCT data of the petrous bone may be expected to provide endoluminal views of superior image quality, thus competing with transtympanic endoscopy (otoendoscopy). This study was conducted at a university teaching hospital. Cadaveric phantom studies in a MSCT scanner were performed to define a data acquisition protocol, combining adequate detail resolution with low tube current. Subsequently, the cadaveric phantom underwent otoendoscopy. The postprocessing parameters of the three-dimensional rendering protocol were chosen to produce views closely resembling the corresponding otoendoscopic images. High-resolution data from 18 patients with pathologic conditions of the middle ear, as suggested by clinical findings and assessment of cross-sectional data, were postprocessed using the volume rendering technique to generate standardized virtual endoscopic views. A total of 36 virtual endoscopic scans of the tympanic cavity were generated. With regard to intermediate and high-density structures, virtual endoscopic images, based on MSCT data, yielded endoluminal views closely resembling corresponding otoendoscopic views. Virtual endoscopy seems useful for imaging ossicular pathologic conditions such as dysplasia and chain disruption as well as for assessing patient status before and after otosurgery. MSCT data sets allow for generating virtual endoscopic views closely resembling otoendoscopic images. The technique is especially useful when ossicular pathologic changes are present as well as for preoperative and postoperative imaging of otologic procedures.

Image resolution of the Lunar Expert-XL.

The Lunar Expert-XL is an example of the latest generation of fan beam densitometers, with the X-ray source and detector array mounted on a C-arm to enable supine lateral imaging. Image resolution for anteroposterior (AP) spine, femur, hand, forearm and lateral morphometry on the Expert-XL were assessed in vitro with the 07-541 Nuclear Associates line pair test pattern. Each scan type was investigated at all available tube currents and scan speeds, and at the maximum, minimum and default bed heights. The effect of soft tissue thickness on resolution was investigated by using varying amounts of Perspex attenuator. The in vitro median lateral (x-axis) resolutions at the default bed height for the default scan types were 0.9 line pairs (lps)/mm for the 5 mA fast AP spine and femur scans, and l.0 lps/mm for 1 mA fast hand, forearm and 5 mA fast morphometry scans. This equates to a resolution of about 1 mm. The best resolution achieved was 1.2 lps/mm (0.83 mm), obtainable on all scan modes with the bed at maximum elevation, but only consistently with the forearm mode. Lower tube current did not affect resolution but did change the range of soft tissue thickness over which an image could be resolved. Turbo scan modes greatly reduced longitudinal (y-axis) resolution but had little effect on lateral resolution. This study demonstrates the importance of including an assessment of resolution when validating new equipment, especially if morphometric investigations are to be conducted.

Optimal technique factors for magnification mammography.

Use of small focal spots with low x-ray tube currents may result in very long exposure times and thus result in motion blur in magnification mammography. The authors investigated the reduction in exposure time with increasing x-ray tube kVp and the corresponding decrease in perceived visibility of low-contrast objects in phantom images. Exposure times required to radiograph an RMI 156 phantom in a magnification geometry were measured as a function of x-ray tube kVp when operated under automatic exposure control. Magnification images of the RMI 156 phantom were obtained at x-ray tube voltages ranging from 28 to 34 kVp. Five radiology residents ranked the visibility of two borderline fibers and six borderline microcalcification specks using a 5-point scale ranging from excellent to barely visible. Between 28 and 34 kVp, the density of the RMI phantom images was nearly constant with a mean value of 1.32 +/- 0.04. Increasing the x-ray tube voltage from 28 kVp to 34 kVp reduced the exposure time from 1.27 seconds to 0.66 seconds. Image quality at 30 and 32 kVp was not significantly worse than that achieved at 28 kVp. Increasing the x-ray tube voltage to 34 kVp, however, resulted in a statistically significant (P < 0.001) deterioration in the relative visibility of fibers and microcalcification specks. Magnification mammography performed at 32 kVp will decrease exposure times significantly and result in a microcalcification and fiber visibility that is similar to that achieved at 28 kVp.

Determination of the spectral intensity distribution of the incident beam in energy-dispersive X-ray diffractometry

The measurement of the spectral intensity distribution of the incident beam in energy-dispersive X-ray diffractometry has been examined by several methods. The spectral distribution estimated from the integrated intensity of a well defined reflection at several energies is the most reliable. Three direct measurements of the spectral distribution by a solid-state detector are also examined. Of these only the measurement at very low tube current can be used in structure factor determination, but the accuracy is about 5% even for strong reflections.

Correction for spectral artifacts in cross-sectional reconstruction from x rays.

A monoenergetic response correction is described which, along with adequate filtration, may be used to remove the spectral shift artifact encountered in three-dimensional reconstruction from x rays. Reconstructions were carried out by means of a convolution algorithm for simulated data using this method. These are compared with reconstructions obtained using fixed-length water-bath scans as a remedy for the special artifact. These studies suggest that the spectral artifact can be successfully eliminated from computerized cross-sectional scans without resorting to the use of the water bath while, at the same time, improving quantum statistics and/or permitting operation at a lower tube current.