The measurement of radiation dose profiles for electron-beam computed tomography using film dosimetry.

The unique geometry of electron-beam CT (EBCT) scanners produces radiation dose profiles with widths which can be considerably different from the corresponding nominal scan width. Additionally, EBCT scanners produce both complex (multiple-slice) and narrow (3 mm) radiation profiles. This work describes the measurement of the axial dose distribution from EBCT within a scattering phantom using film dosimetry methods, which offer increased convenience and spatial resolution compared to thermoluminescent dosimetry (TLD) techniques. Therapy localization film was cut into 8 x 220 mm strips and placed within specially constructed light-tight holders for placement within the cavities of a CT Dose Index (CTDI) phantom. The film was calibrated using a conventional overhead x-ray tube with spectral characteristics matched to the EBCT scanner (130 kVp, 10 mm A1 HVL). The films were digitized at five samples per mm and calibrated dose profiles plotted as a function of z-axis position. Errors due to angle-of-incidence and beam hardening were estimated to be less than 5% and 10%, respectively. The integral exposure under film dose profiles agreed with ion-chamber measurements to within 15%. Exposures measured along the radiation profile differed from TLD measurements by an average of 5%. The film technique provided acceptable accuracy and convenience in comparison to conventional TLD methods, and allowed high spatial-resolution measurement of EBCT radiation dose profiles.

Performance evaluation of a multi-slice CT system.

Our purpose in this study was to characterize the performance of a recently introduced multi-slice CT scanner (LightSpeed QX/i, Version 1.0, General Electric Medical Systems) in comparison to a single-slice scanner from the same manufacturer (HiSpeed CT/i, Version 4.0). To facilitate this comparison, a refined definition of pitch is introduced which accommodates multi-slice CT systems, yet maintains the existing relationships between pitch, patient dose, and image quality. The following performance parameters were assessed: radiation and slice sensitivity profiles, low-contrast and limiting spatial resolution, image uniformity and noise, CT number and geometric accuracy, and dose. The multi-slice system was tested in axial (1, 2, or 4 images per gantry rotation) and HQ (Pitch = 0.75) and HS (Pitch = 1.5) helical modes. Axial and helical acquisition speed and limiting spatial resolution (0.8-s exposure) were improved on the multi-slice system. Slice sensitivity profiles, image noise, CT number accuracy and uniformity, and low-contrast resolution were similar. In some HS-helical modes, helical artifacts and geometric distortion were more pronounced with a different appearance. Radiation slice profiles and doses were larger on the multi-slice system at all scan widths. For a typical abdomen and pelvis exam, the central and surface body doses for 5-mm helical scans were higher on the multi-slice system by approximately 50%. The increase in surface CTDI values (with respect to the single-slice system) was greatest for the 4 x 1.25-mm detector configuration (190% for head, 240% for body) and least for the 4 x 5-mm configuration (53% for head, 76% for body). Preliminary testing of version 1.1 software demonstrated reduced doses on the multi-slice scanner, where the increase in body surface CTDI values (with respect to the single-slice system) was 105% for the 4 x 1.25-mm detector configuration and 10% for the 4 x 5-mm configuration. In summary, the axial and HQ-helical modes of the multi-slice system provided excellent image quality and a substantial reduction in exam time and tube loading, although at varying degrees of increased dose relative to the single-slice scanner.

Measurement of half-value layer in x-ray CT: a comparison of two noninvasive techniques.

The purpose of this paper is to determine the accuracy and reproducibility of two noninvasive methods of measuring half-value layer (HVL), ring and localization, compared with an invasive technique (suspending tube rotation). The ring method uses concentric aluminum rings about a CTDI ionization chamber at isocenter. Data were acquired using axial CT protocols (rotating x-ray tube, stationary patient table). The localization technique uses square aluminum sheets secured to the gantry shroud to filter the radiation beam, and a CTDI chamber suspended externally at isocenter. Data were acquired using localization image protocols (stationary x-ray tube, moving patient table). The invasive technique was similar to the localization technique except that the ion chamber was placed on the patient table and the tube rotation disabled using service software. Data for all techniques were collected on the same CT system. Independent data sets were collected to determine reproducibility. Sensitivity to ionization chamber lateral displacement from isocenter was investigated. Measured HVLs (mm aluminum, mean+/-std, n=4) were 7.19+/-0.03 (ring); 7.17+/-0.04 (localization); and 7.24+/-0.02 (service mode), which were not significantly different (p = 0.05). Displacing the chamber from isocenter changes the HVL, depending on the bow-tie filter, by as much as 5 mm aluminum. Aluminum filter to ion chamber distances of 25-35 cm provided accurate results. Both noninvasive techniques were accurate and reproducible at isocenter. However, the measured HVL was dependent upon the bow-tie filter and the lateral displacement of the ionization chamber with respect to isocenter. Greater than 2 cm off of isocenter, the ring technique did not provide accurate HVL measurements.

Artefacts found in computed radiography.

Artefacts on radiographic images are distracting and may compromise accurate diagnosis. Although most artefacts that occur in conventional radiography have become familiar, computed radiography (CR) systems produce artefacts that differ from those found in conventional radiography. We have encountered a variety of artefacts in CR images that were produced from four different models plate reader. These artefacts have been identified and traced to the imaging plate, plate reader, image processing software or laser printer or to operator error. Understanding the potential sources of CR artefacts will aid in identifying and resolving problems quickly and help prevent future occurrences.

X-ray tubes.

The x-ray tube serves the function of creating x-ray photons from electric energy supplied by the x-ray generator. The process of creating the x-ray beam is very inefficient, with only 1% of the electric energy converted to x-ray photons and the remaining 99% converted to heat in the x-ray tube assembly. Thus, to produce sufficient x-ray output for diagnostic imaging, the x-ray tube must withstand and dissipate a substantial heat load, a requirement that affects the design and composition of the x-ray tube. The major x-ray tube components are the cathode and anode assemblies, the tube envelope, the rotor and stator (for rotating anode systems), and the tube housing. The design of the x-ray tube determines the basic characteristics of the x-ray beam such as focal spot size, x-ray field uniformity, and the x-ray energy spectrum. These x-ray beam characteristics are important because they affect radiologic parameters such as spatial resolution, image contrast, and patient dose.

Radiation dosimetry for electron beam CT.

PURPOSE: To measure the radiation dose profile, multiple-scan average dose (MSAD), and computed tomography dose index (CTDI) for electron beam CT and to determine the accuracy of ionization-chamber and manufacturer estimates of patient dose. MATERIALS AND METHODS: High-resolution dose profiles along the longitudinal axis were acquired at several positions within the scan plane with use of radiographic film. The full-width-at-half-maximum values, peak radiation dose, CTDI, and MSAD were calculated from the digitized film profiles. CTDI was also measured with an ionization chamber. RESULTS: The full-width-half-maximum value of the radiation profiles were significantly wider than the nominal scan width for the 6-mm single-section and 8-mm multisection modes. In the single-section mode, the CTDI underestimated the MSAD by 15%-30%. The multisection radiation profile was nonuniform and asymmetric. CONCLUSION: Patient doses in electron beam CT are approximately 125% of the ionization-chamber CTDI measurements in the single-section mode. For the multisection mode, the average patient dose over the scan volume is approximately 70%-85% of the ionization-chamber CTDI measurements.

Motion artifacts in subsecond conventional CT and electron-beam CT: pictorial demonstration of temporal resolution.

To visually demonstrate the effective temporal resolution of subsecond conventional (slip-ring) and electron-beam computed tomographic (CT) systems, two phantoms containing high-contrast test objects were scanned with a slip-ring CT system (effective exposure time, 0.5 second) and an electron-beam CT system (exposure time, 0.1 second). Images were acquired of each phantom at rest, during translation along the x axis at speeds of 10-100 mm/sec, and during rotation about isocenter at speeds of 0.1 and 0.5 revolution per second. Motion artifacts and loss of spatial resolution were judged to be absent, noticeable, or severe. For 0.5-second conventional CT images, motion artifacts and loss of spatial resolution were noticeable at 10 mm/sec and 0.1 revolution per second and were severe at speeds greater than or equal to 20 mm/sec and at 0.5 revolution per second. For 0.1-second electron-beam CT scans, noticeable, but not severe, motion artifacts and loss of spatial resolution occurred at speeds between 40 and 100 mm/sec and at 0.5 revolution per second. Over the range of physiologic speeds examined, the images provide visually compelling evidence of the effect of improving temporal resolution in CT.

Conventional chest radiography vs dual-energy computed radiography in the detection and characterization of pulmonary nodules.

OBJECTIVE: We evaluated a single-exposure, phosphor-plate, dual-energy imaging device that produces, in addition to conventional chest radiographs, both tissue- and bone-selective images. Our purpose was to determine whether dual-energy radiography was more accurate than routine chest radiography for detection and characterization of pulmonary nodules. SUBJECTS AND METHODS: Two hundred patients undergoing chest CT were asked to volunteer to have dual-energy and conventional chest radiographs obtained immediately before or after their CT scan. Radiographs from a subset of 50 of these patients with 116 CT-detected nodules and 10 patients with normal findings on CT scans of the chest were presented to the observers for the nodule detection study. Similarly, radiographs from a subset of 29 patients with 20 calcified and 20 uncalcified nodules were presented to five observers to determine nodule calcification. Dual-energy images were produced by filtering the X-ray tube output with a gadolinium sheet while using a multiple phosphor plate receptor. A dual-energy triad of images consisting of a conventional image, a tissue-selective image, and a bone-selective image were produced. The conventional chest radiographs and dual-energy image sets were presented to observers in random order. Data from a free response receiver operating curve and a receiver operating curve were generated for nodule detection and characterization, respectively. RESULTS: By using the dual-energy images, all five observers improved their ability to diagnose pulmonary nodules (p = .0005) and to characterize nodules as calcified (p = .005). CONCLUSION: By eliminating rib shadows with tissue-selective images and enhancing calcified structures with bone-selective images, dual-energy chest radiography improved the ability of all observers, regardless of expertise, to detect and characterize pulmonary nodules.

Blood flow velocity measurements: a comparison of 25 clinical ultrasonographic units.

A blood-mimicking flow phantom was used to evaluate the precision of velocity measurements acquired using 25 pulsed Doppler ultrasonographic units from four vendors. Measurements were made at four constant flow rates (12 to 50 cm/s peak velocity). The average standard deviation values of the peak and time-averaged velocities among all units and all flow rates were found to be 7 and 9% of the mean, respectively, while the corresponding values for a subgroup of 20 identical units were 5 and 8%. Considered in conjunction with other published data, this suggests that units should be calibrated to an institutional standard at the time of acceptance testing.

Benefits and safety of CT fluoroscopy in interventional radiologic procedures.

PURPOSE: To determine the benefits and safety of computed tomographic (CT) fluoroscopy when compared with conventional CT for the guidance of interventional radiologic procedures. MATERIALS AND METHODS: Data on 203 consecutive percutaneous interventional procedures performed with use of CT fluoroscopic guidance and 99 consecutive procedures with conventional CT guidance were obtained from a questionnaire completed by the radiologists and CT technologists who performed the procedures. The questionnaire specifically addressed radiation dose measurements to patients and personnel, total procedure time, total CT fluoroscopy time, mode of CT fluoroscopic guidance (continuous versus intermittent), success of procedure, major complications, type of procedure (biopsy, aspiration, or drainage), site of procedure, and level of operator experience. RESULTS: The median calculated patient absorbed dose per procedure and the median procedure time with CT fluoroscopy were 94% less and 32% less, respectively, than those measurements with conventional CT scanning (P <.05). An intermittent mode of image acquisition was used in 97% of the 203 cases. This resulted in personnel radiation dosimetric readings below measurable levels in all cases. CONCLUSION: As implemented at the authors' institution, use of CT fluoroscopy for the guidance of interventional radiologic procedures markedly decreased patient radiation dose and total procedure time compared with use of conventional CT guidance.