Weighted CT Dose Index Research Articles

SU‐FF‐I‐17: Methodology for Image Quality Characterization of Cone‐Beam CT Systems Using a Novel Adaptation of Fundamental Imaging Metrics

Purpose: A methodology to characterize reconstructed image quality in cone‐beam computed tomography (CBCT) is presented in which, imaging performance is characterized using fundamental imaging metrics: modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Traditionally, imaging MTF, NPS and DQE have only been used to describe spatial resolution, noise and contrast in 2D imaging. In a novel adaptation to 3D imaging, the MTF and NPS are measured using a conventional CatPhan phantom. An effective DQE parameter is then evaluated by normalization to incident beam flunece characteristics using a dose index. Method and Materials: The modulations from CatPhan line‐pairs are used to calculate the MTF using a technique that we recently validated for megavoltage imaging. Blank/uniform slices are used to calculate the NPS, while the DQE is obtained from MTF and NPS measurements normalized by a quantum fluence term. While the fluence normalization is relatively straightforward with unscattered beams in 2D imaging, its evaluation in CBCT imaging is complicated by the effects of collimators, compensators, and scatter. In our calculations, we obtain an effective incident fluence term for each beam combination based on its weighted CT dose index (CTDIw) and by using Monte Carlo simulations to generate a transmitted fluence to CTDI dose ratio. The methodology was implemented for clinical CBCT systems to observe the influence of image acquisition parameters. Results: CatPhan based MTF, NPS and DQE measurements of clinical CBCT systems were found to be useful in evaluating the effects of various image acquisition and reconstruction parameters. The technique was extended to near automated metric analysis with minimal hindrances from scatter and associated CBCT artifacts. Conclusions: The use of fundamental imaging metrics to characterize CBCT image quality can be effective in optimizing image acquisition parameters and ensure optimal imaging performance at the lowest possible dose.

Clinical evaluation of dual-energy bone removal in CT angiography of the head and neck: comparison with conventional bone-subtraction CT angiography

To evaluate the bone-subtraction effect of dual-energy bone removal in computed tomography angiography (CTA) of the head and neck in comparison with conventional bone-subtraction CTA. The study comprised 52 patients who were divided into two groups at random, and examined using dual-source CT for head and neck CTA. Dual-energy bone removal CTA and conventional bone-subtraction CTA were applied to each of the two groups, respectively. The bone subtraction was performed automatically in both methods. Vascular structures, as well as brain tissue remained visible. The subtracted images were further processed with maximum intensity projection (MIP) and volume-rendering technique (VRT) for image evaluation. Two experienced radiologists reviewed the resulting subtracted and non-subtracted volume data with respect to the delineation and detection of image quality and vascular pathology. The means of the weighted CT dose index (CTDIvol) for bone-removal dual-energy CTA and conventional bone-subtraction CTA were 20.56+/-0.01 mGy and 25.57+/-0.56 mGy, respectively. There was a significant difference between them. The percentage of carotid and vertebral arteries and all other vessels that could be successfully assessed with these two methods were 87.8, 68, and 83%, and 93.5, 91.8, and 92.6%, respectively. There were no significant differences in the visualization of the carotid arteries; however, there were significant differences in the visualization of the vertebral arteries. Compared with conventional bone-subtraction CTA, dual-energy bone-removal CTA had a lower radiation dose. It eliminated most bones in the head and neck successfully; however, the bone subtraction effect around the vertebral artery was unsatisfactory. Dual-energy bone-removal CTA provides a new method for detecting vascular diseases in routine clinical work.

Comparison of radiation doses between cone beam CT and multi detector CT: TLD measurements

In recent years, wide beam computed tomography (CT) technique has become standard in radiation oncology whereas there is little information about radiation dose assessments for the technique. A point dose measurement method was employed to assess the radiation doses of cone beams CT (CBCT) and multi detectors CT (MDCT). The radiation doses of both modalities were measured using thermoluminescence dosemeters (TLDs) in head and body CT phantoms. Four TLD chips were placed at the centre and each peripheral channel to measure the doses. From the measurements, the weighted CT dose index for CBCT (CTDI(w)(CBCT)) and volume CT dose index for MDCT (CTDI(vol)(MDCT)) were derived. In the results, the CTDI(w)(CBCT) was 89.7 +/- 4.0 mGy and the CTDI(vol)(MDCT) was 137.0 +/- 7.4 mGy for the head scan. For the body scan, they were 37.9 +/- 1.4 and 74.3 +/- 5.3 mGy, respectively. In conclusion, CTDI(w)(CBCT) for the head scan was 35% lower than CTDI(vol)(MDCT), and CTDI(w)(CBCT) for the body scan was also 49% lower than CTDI(vol)(MDCT).

Effects of 16-multi-detector row CT aortography with lower tube current

To study the effects of 16-multi-detector row CT (MDCT) aortography with lower tube current in diagnosis of aortic diseases. The study was conducted in 2 steps. In the first step, 58 patients with suspicious aortic disease or after operation on the aorta underwent 16-MDCT aortography for 70 times. Ten of them underwent scanning with conventional tube current of 175 mAs and the other 60 patients were divided into 3 groups according to their bogy weights: <65 kg group (n=20) receiving the lowest tube current of 25 mAs, 65-75 kg group (n=20) receiving the lowest tube current of 50 mAs, and >75 kg group (n=20) receiving the lowest tube current of 75 mAs. In the second step 60 patients with dissecting aneurysm, underwent 16-MDST aortography and were divided into 3 groups according to their body weight too: <65 kg group (n=20) receiving the lowest tube current of 50 mAs, 65-75 kg group (n=20) receiving the lowest tube current of 75 mAs, and >75 kg group (n=20) receiving the lowest tube current of 100 mAs, all 25 mAs more compared with the corresponding groups in the first step. The weighted CT dose index (CTDI), scanning length, and dose length produce (DLP) were recorded. The diagnostic accuracy rates of the images from the low dose groups were compared with those of the higher dose groups. The data of the first step showed that the CTDI values of the patients who received 25 mAs, 50 mAs, and 75 mAs tube current were 11.3%, 29.0%, and 42.7% that of the conventional tube current group (all P < 0.001) and the DLP values of the 3 low dose groups were also significantly lower than that of the conventional tube current group (all P < 0.001). The diagnostic accuracy rate of the 25 mAs, 50 mAs, and 75 mAs groups were 60% , 85%, and 85% that of the conventional tube current group. The results of the second step showed that the CTDI values of the 50 mAs, 75 mAs, and 100 mAs groups were 29.0%, 42.7%, and 57.3% that of the conventional tube current group respectively, and the DLP values of the 3 low dose groups were also significantly lower than that of the conventional group ( all P < 0. 001); and the 50 mAs, 75 mAs, and 100 mAs groups all showed good three-dimensional reconstruction imaging qualities, all with the diagnostic accuracy rate of 100%. The crossing section and three-dimensional images all showed excellent diagnostic image quality. 16-MDCT aortography with the tube current at the doses 50 mAs to 100 mAs suffices to diagnose aortic diseases in patients with different body weights. Higher tube current should be used in dissecting aneurysm. The tube current at the dose of 100 mAs satisfies the imaging and diagnosing of all kinds of aortic diseases in the patients with any body weight.

The development of a diagnostic reference level on patient dose for CT examination in Korea

The purpose of this research is to develop a diagnostic reference level for computed tomography (CT) suitable for Korean medical purposes. The direction of CT application and details on patient dose were investigated by survey, and the dose measurement is targeted in general hospitals registered with the Korean Hospital Association. The dose measurement was done with head and body phantom, and an accurate dosimeter was utilised in medical institutions that participated in the survey. The survey and dose measurements on the patient dose showed that the 75th percentiles in the distribution of weighted CT dose index (CTDIw) were 50.5 and 45.4 mGy for the head and 14.4 and 25.30 mGy for the body portions, respectively. Based on these research data, the target patient doses that are believed to be achievable for CT examination in Korean circumstances are about 48 +/- 4 mGy in the head and 20 +/- 8 mGy in the body.

Determination of the weighted CT dose index in modern multi-detector CT scanners

The aim of the present study was to (a) evaluate the underestimation in the value of the free-in-air (CTDIair) and the weighted CT dose index (CTDIw) determined with the standard 100 mm pencil chamber, i.e. the CTDI100 concept, for the whole range of nominal radiation beam collimations selectable in a modern multi-slice CT scanner, (b) estimate the optimum length of the pencil-chamber and phantoms for accurate CTDIw measurements and (c) provide CTDIw values normalized to free-in-air CTDI for different tube-voltage, nominal radiation beam collimations and beam filtration values. The underestimation in the determination of CTDIair and CTDIw using the CTDI100 concept was determined from measurements obtained with standard polymethyl-methacrylate (PMMA) phantoms and arrays of thermoluminescence dosimeters. The Monte Carlo N-Particle transport code was used to simulate standard CTDI measurements on a 16-slice CT scanner. The optimum pencil-chamber length for accurate determination of CTDIw was estimated as the minimum chamber length for which a further increase in length does not alter the value of the CTDI. CTDIw/CTDIair ratios were determined using Monte Carlo simulation and the optimum detector length for all selectable tube-voltage values and for three different values of beam filtration. To verify the Monte Carlo results, measured values of CTDIw/CTDIair ratios using the standard 100 mm pencil ionization chamber were compared with corresponding values calculated with Monte Carlo experiments. The underestimation in the determination of CTDIair using the 100 mm pencil chamber was less than 1% for all beam collimations. The underestimation in CTDIw was 15% and 27% for head and body phantoms, respectively. The optimum detector length for accurate CTDIw measurements was found to be 50 cm for the beam collimations commonly employed in modern multi-detector (MD) CT scanners. The ratio of CTDIw/CTDIair determined using the optimum detector length was found to be independent of beam collimation. Percentage differences between measured and calculated corresponding CTDIw/CTDIair ratios were always less than 8% for head and less than 5% for body PMMA phantoms. In conclusion, the CTDIair of MDCT scanners may be measured accurately with a 100 mm pencil chamber. However, the CTDI100 concept was found to be inadequate for accurate CTDIw determination for the wide beam collimations commonly used in MDCT scanners. Accurate CTDIw determination presupposes the use of a pencil chamber and PMMA phantoms at least 50 cm long.

Patient dose measurements in diagnostic radiology procedures in Korea

This study is the first nationwide investigation aimed at estimating the patient dose for radiographic examinations in Korea including gastrointestinal studies, computed tomography and mammography. The survey data from 161 hospitals and the dose data from 32 hospitals were analysed. The third quartile entrance surface dose, dose area product (DAP), weighted CT dose index (CTDIw) and mean glandular dose (MGD) were reported. All the estimated doses were less than the stated International Atomic Energy Agency (IAEA) reference levels for radiographic examinations. However, DAPs for the fluoroscopic examinations had higher dose values than the IAEA reference levels. In addition, the CTDIw and MGD were lower than the IAEA reference levels.

Material differentiation by dual energy CT: initial experience

The aim of this study was to assess the feasibility of a differentiation of iodine from other materials and of different body tissues using dual energy CT. Ten patients were scanned on a SOMATOM Definition Dual Source CT (DSCT; Siemens, Forchheim, Germany) system in dual energy mode at tube voltages of 140 and 80 kVp and a ratio of 1:3 between tube currents. Weighted CT Dose Index ranged between 7 and 8 mGy, remaining markedly below reference dose values for the respective body regions. Image post-processing with three-material decomposition was applied to differentiate iodine or collagen from other tissue. The results showed that a differentiation and depiction of contrast material distribution is possible in the brain, the lung, the liver and the kidneys with or without the underlying tissue of the organ. In angiographies, bone structures can be removed from the dataset to ease the evaluation of the vessels. The differentiation of collagen makes it possible to depict tendons and ligaments. Dual energy CT offers a more specific tissue characterization in CT and can improve the assessment of vascular disease. Further studies are required to draw conclusions on the diagnostic value of the individual applications.

Towards establishment of the national reference dose levels from computed tomographyexaminations in Tanzania

Without the knowledge of reference dose levels (RDLs) from computed tomography (CT)examinations, the optimal dose to patients undergoing CT examinations cannot be realised.The aim of this study was therefore to assess the radiation dose levels from CT examinationsaccording to reference dose quantities proposed by the European Commission (EC) guidelines.The dosimetric quantities proposed in the EC for CT are weighted CT dose index(CTDIw) for a single slice and dose–length product (DLP) for a complete examination. The RDLs fromfive common CT examinations were obtained from eight hospitals. The RDLs in terms ofCTDIw and DLP were estimated from measurements of CTDI in standardphantoms using typical exposure parameters. Mean values ofCTDIw for head and lumbar spine had a range of 25–77 and 18–47 mGy, respectively, while thosefor chest, abdomen and pelvis had a range of about 11–25 mGy, respectively. Meanvalues of DLP for head, chest and abdomen had a range of 610–1684, 496–992 and717–1428 mGy cm, respectively, while those for lumbar spine and pelvis had arange of 200–382 and 526–1302 mGy cm, respectively. Wide variations of meanCTDIw and DLP values among hospitals observed for similar CT examinations were mainlyattributed to the variations of CT scanning protocols and scanner types. The meanCTDIw values per examination for almost all hospitals were below proposed RDLs, while the meanDLP values per examination were almost all above the proposed RDLs for all except onehospital. These were mainly influenced by the large scan length used in Tanzanianhospitals. In order to achieve the required level of dose for establishment of the nationalRDLs, it was concluded that further investigation of optimization of scanning protocols isneeded.

Comparison of the Image Quality on Low-dose Urinary Tract Multidetector CT Examinations with 80 kVp and 120 kVp (Original Research)

Purpose: To compare the image quality on low-dose urinary tract multidetector CT examinations with 80 kVp and 120 kVp. Materials and Methods: Seventy-six patients with urinary tract symptoms were evaluated for urinary tract calculi by unenhanced multidetector CT. The examinations were performed with a four-row detector CT scanner. In all patients, two low-dose CT examinations with different kVp and mAs values (120 kVp, 30 mAs and 80 kVp, 95 mAs) were performed. The other scanning parameters were kept constant. Weighted-CT dose index was equal in both examinations. Both CT examinations were quantitatively compared with calculating the HU values of kidneys, subcutaneous fat, calculus, and phleboliths and the HU ratio of urinary calculi to the kidney. Both scans were also qualitatively compared for image quality of upper and lower abdomen. Results: One hundred and two calculi were detected in 50 of 76 patients. One or more phleboliths were present in 20 patients. HU values of the phleboliths and urinary calculi were higher on images obtained with 80 kVp than those obtained with 120 kVp. HU ratio of the calculi to the kidney was higher on scans with 80 kVp. Adipose tissue was less noisy and had lower HU value on 80 kVp images. Image quality of the upper abdomen and major pelvis were better on 80 kVp images, and the majority of these patients were normal (n=28) and overweight (n=20). Minor pelvis mostly (60%) revealed poorer image quality on 80 kVp images. However, the majority were overweight (n=22) or obese (n=6). In patients with osteoporosis, image quality was relatively better when compared with similar weighed patients without osteoporosis on 80 kVp images. Small renal, proximal and middle ureteral calculi were better demarcated on 80 kVp images. Conclusion: Low-dose urinary tract multidetector CT examination with 80 kVp, compared to 120 kVp provides better image quality in the upper abdomen and major pelvis. In overweight and obese patients, minor pelvis may not be evaluated thoroughly on 80 kVp images. Detection of small renal, proximal and middle ureteral calculi could be improved by using 80 kVp setting. Keywords: urinary calculi, low-dose multidetector ct, lower kilovoltage, 80 kvp

Monte Carlo calculation of radiation dose in CT examinations using phantom and patient tomographic models

By using a voxel-based Monte Carlo simulation technique, we developed and validated a method to calculate radiation-absorbed dose in the computed tomography (CT) examinations from the images of phantoms and patients. The ionising radiation transport was simulated using the EGS4 code system. The geometry of the X-ray beam (focus-to-axis distance, field of view, collimation, and primary and beam-shaper filtration) and the X-ray spectral distribution (HiSpeed LX/i) were included in the simulation. Each axial CT image was reduced to a 256 x 256 matrix and stacked in a volume. The patient images were segmented before the simulation of radiation transport by using four categories of materials, such as air, lung, muscle and bone. To test the voxel-based method, the values of the radiation dose derived from a simulated CT exposure were calculated and compared with those obtained from the measurements performed within the dosimetry phantoms. To complete the scope of the work, series of CT scans of the trunk of an anthropomorphic phantom and patients were simulated to calculate the average dose in each 1-cm-wide transverse slice (ADS). The comparison between the simulated and measured dose data for the CT indices showed a difference of <5% in all the cases. The estimated mean values of ADS from the chest, abdomen and pelvis of the anthropomorphic phantom were approximately 1.7-2 times the weighted CT dose index (CTDI(w)) value, whereas the mean ADS values for these anatomical areas were 1.3-2 times the CTDI(w) of patients. The voxel-based simulation method provided a technique for estimating the individual patient doses in the CT examinations.

Survey of computed tomography techniques and absorbed dose in Italian hospitals: a comparison between two methods to estimate the dose–length product and the effective dose and to verify fulfilment of the diagnostic reference levels

The aim of this study was the production of the first Italian survey of radiation dose in computed tomography (CT) prior to the widespread adoption of multislice CT, in order to have a reference point to facilitate later investigation of dose exposure changes brought by this new CT modality. The collected dose data were compared with diagnostic reference levels (DRLs). The agreement between experimental dose evaluation and Monte Carlo (MC) simulations was investigated. The survey was carried out in 29 Italian hospitals, covered 48 CT scanners and 232 examinations. The dose-length product (DLP) and effective dose (E) values were estimated based on MC simulations for seven clinical protocols using the CT-Dose program. Statistical analysis showed a significant difference (p<0.01) in the DLP between the two methods, with MC values being greater than the experimental ones. For E, the MC values were greater in routine head (8.2%), cervical spine (2.7%) and lumbar spine (2.9%) studies. The weighted CT dose index, the DLP and E were always below the DRLs set by the European Community. This dose survey gives a good but incomplete picture of the Italian CT dose situation and may be useful as a reference baseline for defining clinical multislice protocols in the near future.

Feasibility of MDCT Colonography in Ultra-Low-Dose Technique in the Detection of Colorectal Lesions: Comparison with High-Resolution Video Colonoscopy

The purpose of this study was to assess the feasibility of MDCT colonography in an ultra-low-dose technique in the detection of endoluminal colonic lesions compared with high-resolution video colonoscopy. After standard bowel cleansing, 137 patients (77 men, 60 women; mean [+/- SD] age, 57.1 +/- 11.3 years) underwent high-resolution video colonoscopy within 2 hr after ultra-low-dose MDCT colonography had been performed. Ultra-low-dose MDCT colonography was performed with patients in the supine position only using 10 mAs (effective weighted CT dose index, 0.94 mGy). After mathematic noise reduction by nonlinear gaussian filter chains, using dedicated software (ECCET), images were analyzed by two blinded observers in simultaneously displayed interactive 2D and 3D modes. Findings of ultra-low-dose MDCT colonography were compared with the results obtained with high-resolution video colonoscopy. Calculated effective doses were 0.7 and 1.2 mSv for men and women, respectively. Ultra-low-dose MDCT colonography detected 84 (62%) of 135 lesions: 11 (78.6%) of 14 large polyps (> 10 mm), 12 (85.7%) of 14 medium polyps (9.9-5 mm), and 61 (57%) of 107 small polyps (< 5 mm). On a patient-by-patient basis, an overall sensitivity of 70.3% with a specificity of 80.8% was calculated. False-positive findings were seen mostly for small lesions (eight medium and 29 small lesions). Two of the three false-negative lesions were retrospectively detected in contrast-enhanced cleansing fluid; one was a flat lipoma not detectable on ultra-low-dose MDCT colonography. Despite an effective dose of approximately 1 mSv, MDCT colonography using an ultra-low-dose technique performs as well as MDCT colonography with a standard dose, according to published data. After mathematic noise reduction, 82% of polyps larger than 5 mm can be detected.

CT doses in children: a multicentre study

We evaluated examination protocols used for common CT procedures of paediatric patients at different hospitals in Belgium in order to determine whether adjustments related to patient size are made in scanning parameters, and to compare patient doses with proposed reference levels. Three paediatric hospitals and one non-paediatric hospital participated in the study. Weighted CT dose-index (CTDI(w)), dose-length product (DLP) and effective dose (E) were evaluated for three patient ages (1 year, 5 years and 10 years) and three common procedures (brain, thorax and abdomen). CTDI(w) and DLP values higher than the reference levels were found for all types of evaluated examination. E ranged from 0.4 mSv to 2.3 mSv, 1.1 mSv to 6.6 mSv, and 2.3 mSv to 19.9 mSv for brain, thorax and abdomen examinations, respectively. All centres but one adapted their protocols as a function of patient size. However, no common trend in the selection of protocols was observed. Some centres divided the whole range of patient size into only two/three groups by age, while others classified the patients into six groups by weight. It was also observed that some centres used the same mAs for the total range of patient sizes and decreased the pitch factor for small children, which resulted in higher doses. This indicates the importance of careful selection of technical scan parameters. If CT parameters used for paediatric patients are not adjusted on the basis of examination type, age and/or size of the child, then some patients will be exposed to an unnecessarily high radiation dose during CT examinations.

Living donor kidneys: usefulness of multi-detector row CT for comprehensive evaluation.

To evaluate in living renal donors the usefulness of multi-detector row computed tomography (CT) in the assessment of renal vasculature and the upper urinary tract. Four-channel multi-detector row CT scans were obtained in 77 patients. Vascular phase scans were used for CT angiography; excretory phase scans, for CT urography. At CT angiography, two independent observers evaluated the number of arteries and veins and the presence of early-branching arteries. CT urographic images were evaluated with regard to the opacification of the urinary tract and for abnormalities. Findings of CT angiography and urography were compared with surgical findings. Interobserver agreement between CT angiographic and surgical findings was quantified with weighted kappa statistics. Sensitivity and specificity of CT angiography in identifying supernumerary vessels and early-branching arteries were also evaluated. To evaluate the radiation dose to patients, weighted CT dose index (DI) was assessed for each scan. Agreement between CT angiographic and surgical findings was excellent for the number of renal arteries (kappa = 0.896) and veins (kappa = 0.843). Detection rate of CT angiography was 98% (89 of 91) for arteries and 98% (83 of 85) for veins. The respective sensitivity and specificity of CT angiography were 86% (12 of 14) and 100% (65 of 65) for supernumerary arteries, 100% (11 of 11) and 100% (66 of 66) for early-branching arteries, and 75% (six of eight) and 100% (69 of 69) for supernumerary veins. At CT urography, collecting systems and proximal ureters were well opacified in all patients; two patients had underrotated kidneys without obstruction. The weighted CT DI was 10.19 mGy for unenhanced and excretory phase scans and 12.88 mGy for the vascular phase scan. Multi-detector row CT can help assess well the renal vasculature and the urinary tract of living renal donors.

Multi-detector row CT: radiation dose characteristics.

To determine the dose characteristics of multi-detector row computed tomography (CT) and to provide tabulated dose values and rules of thumb that assist in minimizing the radiation dose at multi-detector row CT. Weighted CT dose index (CTDI100w) values were obtained from three multi-detector row CT scanners (LightSpeed; GE Medical Systems, Milwaukee, Wis) for both head and body CT modes by using standard CT-dose phantoms. The CTDI100w was determined as a function of x-ray tube voltage (80, 100, 120, 140 kVp), tube current (range, 50-380 mA), tube rotation time (0.5-4.0 seconds), radiation profile width (RPW) (5, 10, 15, 20 mm), and acquisition mode (helical high-quality and high-speed modes and axial one-, two-, and four-section modes). Statistical regression was performed to characterize the relationships between CTDI100w and various technique factors. The CTDI100w (milligray) increased linearly with tube current: in head mode, CTDI100w = (0.391 mGy/mA +/- 0.004) x tube current (milliampere) (r2 = 0.999); in body mode, CTDI100w = (0.162 mGy/mA +/- 0.002) x tube current (milliampere) (r2 = 0.999). The CTDI100w increased linearly with rotation time: in head mode, CTDI100w = (34.7 mGy/sec +/- 0.2) x rotation time (seconds) (r2 = 1.0); in body mode, CTDI100w = (13.957 mGy/sec +/- 0.005) x rotation time (seconds) (r2 = 1.0). The relationship of normalized CTDI100w (milligrays per 100 mAs) with tube voltage followed a power law: in head mode, CTDI100w = (0.00016 mGy/100 mAs. kVp +/- 0.00007) x (tube voltage)(2.5+/-0.1) (r2 = 0.997); in body mode, CTDI100w = (0.000012 mGy/100 mAs. kVp +/- 0.000007) x (tube voltage)(2.8+/-0.1) (r2 = 0.996). In all scanning modes, CTDI100w decreased when RPW increased. CTDI100w was 10% higher in head mode and 13% lower in body mode compared with the value suggested by the manufacturer, which is displayed at the scanner console. When deposited power exceeded 24 kW, CTDI100w increased by 10% as a result of use of the large focal spot. The authors provide a set of tables of radiation dose as a function of imaging protocol to facilitate implementation of radiation dose-efficient studies.

Optimizing dosage in thoracic computerized tomography

Radiation exposure in computed tomography (CT) by far exceeds radiation dose in chest radiography. Dose requirements in CT of the chest, however, are much smaller than for the abdomen because of low x-ray absorption in the lungs. This article describes scanner parameters that influence patient exposure and image quality. Suitable compromises will be explained that allow for dose reduction in chest CT without jeopardising image quality. Dose reduction in chest CT should be performed depending on the clinical indication and requires active reduction of mAs settings. For helical CT, a pitch of 1.5 to 2 should be employed. Dedicated low-dose techniques for screening of bronchogenic carcinoma are described. Dose reduction decreases image quality but the detrimental effects can be reduced by applying proper parameters for scanning and image reconstruction. Thus, images of the mediastinum should be reconstructed with a smoothing filter, while a higher resolution filter should be used for the lungs. In multislice CT, reconstruction of thicker axial or multiplanar sections retains spatial resolution but keeps image noise and thus dose requirements low. The effective, weighted CT dose index (CTDIw,eff) can serve as a rough estimate of the effective patient dose (E) in helical or multislice CT of the whole chest: for most scanners and age groups the conversion factors are 0.5 mSv/mGy for females and 0.4 mSv/mGy for males.

Quality criteria implementation for brain and lumbar spine CT examinations.

A study was undertaken to implement the quality criteria proposed by the European Commission for brain general and lumbar spine (disc herniation) CT examinations. The proposed criteria were evaluated for samples including 93 brain and 86 lumbar spine CT examinations, with special emphasis on the diagnostic and radiation dose requirements. The extent to which the image criteria had been achieved was evaluated after two independent observers had each read the images twice. Dose measurements were conducted in parallel to estimate the proposed dose quantities needed to obtain the images. For brain examinations, we found that a group of image criteria were largely met, and met uniformly in all sites. One criterion (1.2.5) was frequently fulfilled but had intermediate values for two sites; the remaining criteria were fulfilled to different extents, although for criteria 1.2.1 and 1.2.2, scores were lower than 50% and 70%, respectively. The mean percentage image quality score had values between 57% and 78%, with variation coefficients in the range 30-68%. Mean values of the dose quantities were in the ranges 44-74 mGy for weighted CT dose index (CTDIw), 497-1018 mGy cm for dose-length product (DLP) and 1.1-2.2 mSv for effective dose (E). CTDIw and DLP were not correlated because of significant variations in the scanned length, whereas DLP and E were strongly correlated. A weak relationship between image quality score and DLP was found for the sample as a whole. For lumbar spine examinations, none of the critical reproduction image criteria was systematically achieved. One group of criteria (1.2.7, 1.2.8 and 1.2.9) was fulfilled to a large extent in many departments, but fulfilment of the remainder varied widely. The mean score fluctuated in the range 39-88%, with three groups of differences: low (39-51%), intermediate (67-71%) and high (85-88%). Mean values of the CTDIw varied between sites in the range 27-48 mGy. Mean DLP values varied between 188 mGy cm and 333 mGy cm, and the mean effective dose ranged between 3 mSv and 5 mSv. There were significant differences in effective dose between men and women. By sites, there was no relationship between DLP and mean score, with the highest image score associated with intermediate dose values. The percentage disagreement among the observers about a given criterion ranged between 2% and 22% for brain, and between 3% and 46% for lumbar spine.

Application of draft European Commission reference levels to a regional CT dose survey.

A survey of CT doses in Northern Ireland in the period between October 1995 and March 1997 was carried out. The survey included all but one of the 10 scanners in use at the time, and, additionally, two others that were replacement machines. The method used was to study standard protocols and calculate doses to the NRPB mathematical phantom, so that a direct comparison could be made with other surveys carried out in a similar fashion elsewhere. The survey addressed the patient radiation dose but not image quality or clinical outcomes. It is estimated that in Northern Ireland the contribution to collective dose to the population from CT is about 40% of that from all medical X-rays. The proposed European Commission reference quantities, weighted CT dose index and dose-length product were computed and their potential use evaluated. A full study of mean values of effective dose per examination revealed the average dose per examination was not significantly different from that found in the 1989 UK survey, although several procedures gave rise to doses that were high enough to be investigated with a view to justification or reduction. One of the scanners was found to give consistently high doses. It is likely that a revision of the mAs values used on this scanner will produce a significant reduction in patient doses without compromising image quality. When compared with the draft EC reference levels, fewer procedures were found to have excessively high dose values. The proposed EC reference levels would therefore be useful for continual monitoring of CT dose status, but do not appear to provide as comprehensive an assessment of patient exposure as that given by consideration of effective doses.

An anthropomorphic phantom for receiver operating characteristic studies in CT imaging of liver lesions.

The purpose of the study was to develop a methodology that allowed quantitative assessment of image quality in CT and its relationship to dose. An anthropomorphic phantom was designed for use in receiver operating characteristic (ROC) studies of the detectability of liver lesions with CT. The lesions were simulated by different mixtures of glycerol and water that were filled into holes of different diameters in a liver tissue substitute. A pilot study was carried out on five different scanners that were operated at various exposure settings. A positive correlation was demonstrated for each of the scanners between the weighted CT dose index (CTDIW) and the area under the ROC curve. For the exposure settings used in the clinical routine in the five laboratories, the CTDIW ranged from 15 to 31 mGy. Three observers who read the corresponding set of five phantom images agreed, as judged from the areas under the ROC curves, that there was a marked difference in quality between the three best images and the other two. The two newest scanners in the study had the lowest CTDIW, and at the same time the best ROC results. The phantom and the ROC methodology may, with a set of suggested improvements, be used for comparison of the performance in different CT laboratories, and to establish the dose needed to ensure adequate image quality for a particular scanner.