Localizer Radiograph Research Articles

Investigating the CT localizer radiograph: acquisition parameters, patient centring and their combined influence on radiation dose.

To systematically investigate the effect of CT localizer radiograph acquisition on the tube current modulation and thus radiation dose of the subsequent diagnostic scan. Localizer radiographs of an abdominal section CT phantom were taken, and the resulting volume CT dose index (CTDIvol) for the diagnostic scan was recorded. Variables included tube potential, the phantom's alignment within the CT scanner gantry in both the vertical and horizontal directions and the X-ray source angle at which the localizer was acquired. Diagnostic scan CTDIvol decreased with increasing tube potential. Vertical (table height) movement was found to affect radiation dose more than horizontal movement, with ±50 mm table movement resulting in a standard deviation in the diagnostic scan CTDIvol of 4.4 mGy, compared with 2.5 mGy with ±50 mm horizontal movement. Correspondingly, localizer angles of 90° or 270° (3 o'clock and 9 o'clock X-ray source positions) were less sensitive overall to alignment errors, with a standard deviation of 2.5 mGy, compared with a 0° or 180° angle, which had a standard deviation of 3.8 mGy. To achieve a consistently optimized radiation dose, the localizer protocol should be paired with the diagnostic acquisition protocol. A final acquisition angle of 90° should be used when possible to minimize dose variation resulting from alignment errors. Localizer parameters that affect radiation output were identified for this scanner system. The importance of tube potential and acquisition angle was highlighted.

Effect of Localizer Radiograph on Radiation Dose Associated With Automatic Exposure Control

Effect of Localizer Radiograph on Radiation Dose Associated With Automatic Exposure Control

SU‐E‐I‐48: The Behavior of AEC in Scan Regions Outside the Localizer Radiograph FOV: An In Phantom Study of CT Systems From Four Vendors

Purpose:This review of scanners from 4 major manufacturers examines the clinical impact of performing CT scans that extend into areas of the body that were not acquired in the CT localizer radiograph.Methods:Anthropomorphic chest and abdomen phantoms were positioned together on the tables of CT scanners from 4 different vendors. All of the scanners offered an Automatic Exposure Control (AEC) option with both lateral and axial tube current modulation. A localizer radiograph was taken covering the entire extent of both phantoms and then the scanner's Chest‐Abdomen‐Pelvis (CAP) study was performed with the clinical AEC settings employed and the scan and reconstruction range extending from the superior portion of the chest phantom through the inferior portion of the abdomen phantom. A new study was then initiated with a localizer radiograph extending the length of the chest phantom (not covering the abdomen phantom). The same CAP protocol and AEC settings were then used to scan and reconstruct the entire length of both phantoms. Scan parameters at specific locations in the abdomen phantom from both studies were investigated using the information contained in the DICOM metadata of the reconstructed images.Results:The AEC systems on all scanners utilized different tube current settings in the abdomen phantom for the scan completed without the full localizer radiograph. The AEC system behavior was also scanner dependent with the default manual tube current, the maximum tube current and the tube current at the last known position observed as outcomes.Conclusion:The behavior of the AEC systems of CT scanners in regions not covered by the localizer radiograph is vendor dependent. To ensure optimal image quality and radiation exposure it is important to include the entire planned scan region in the localizer radiograph.

MO‐E‐17A‐01: BEST IN PHYSICS (IMAGING) – Calculating SSDE From CT Exams Using Size Data Available in the DICOM Header of CT Localizer Radiographs

Purpose:To demonstrate the feasibility of using existing data stored within the DICOM header of certain CT localizer radiographs as a patient size metric for calculating CT size‐specific dose estimates (SSDE).Methods:For most Siemens CT scanners, the CT localizer radiograph (topogram) contains a private DICOM field that stores an array of numbers describing AP and LAT attenuation‐based measures of patient dimension. The square root of the product of the AP and LAT size data, which provides an estimate of water‐equivalent‐diameter (WED), was calculated retrospectively from topogram data of 20 patients who received clinically‐indicated abdomen/pelvis (n=10) and chest (n=10) scans (WED‐topo). In addition, slice‐by‐slice water‐equivalent‐diameter (WED‐image) and effective diameter (ED‐image) values were calculated from the respective image data. Using TG‐204 lookup tables, size‐dependent conversion factors were determined based upon WED‐topo, WED‐image and ED‐image values. These conversion factors were used with the reported CTDIvol to calculate slice‐by‐slice SSDE for each method. Averaging over all slices, a single SSDE value was determined for each patient and size metric. Patientspecific SSDE and CTDIvol values were then compared with patientspecific organ doses derived from detailed Monte Carlo simulations of fixed tube current scans.Results:For abdomen/pelvis scans, the average difference between liver dose and CTDIvol, SSDE(WED‐topo), SSDE(WED‐image), and SSDE(ED‐image) was 18.70%, 8.17%, 6.84%, and 7.58%, respectively. For chest scans, the average difference between lung dose and CTDIvol, SSDE(WED‐topo), SSDE(WED‐image), and SSDE(ED‐image) was 25.80%, 3.33%, 4.11%, and 7.66%, respectively.Conclusion:SSDE calculated using WED derived from data in the DICOM header of the topogram was comparable to SSDE calculated using WED and ED derived from axial images; each of these estimated organ dose to within 10% for both abdomen/pelvis and chest CT examinations. The topogrambased method has the advantage that WED data are already provided and therefore available without additional post‐processing of the image data.Funding Support: NIH Grant R01‐EB017095; Disclosures – Michael McNitt‐Gray: Institutional Research Agreement, Siemens AG; Research Support, Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski; Disclosures – Cynthia McCollough: Research Grant, Siemens Healthcare

Techniques and Tactics for Optimizing CT Dose in Adults and Children: State of the Art and Future Advances

With growing concern over radiation exposure from CT, dose reduction and optimization have become important considerations. Many protocol factors and CT technologies influence this dose reduction effort, and as such, users should maintain a working knowledge of developments in the field. Individual patient factors and scanner-specific details also require care and expertise, which are vital to the success of any dose reduction effort. The authors review the content of the Virtual Symposium on Radiation Safety in Computed Tomography (University of California Dose Optimization and Standardization Endeavor), specifically that pertaining to the more practical aspects of dose optimization. These range from prescan tips to postscan factors, as well as protocol definition itself. Topics discussed include localizer radiograph acquisition, tube current modulation, reconstruction methods, and pediatric considerations, with the content biased toward a CT technologist or protocol manager. Near-term innovations, including new iterative reconstruction methods, tube potential modulation, and dual-energy CT, are presented, and their capability for dose reduction is briefly discussed.

Effect of Localizer Radiograph on Radiation Dose Associated With Automatic Exposure Control

To evaluate the effect of localizing radiograph on computed tomography (CT) radiation dose associated with automatic exposure control with a human cadaver and patient study. Institutional review board approved the study with a waiver of informed consent. Two chest CT image series with fixed tube current and combined longitudinal-angular automatic exposure control (AEC) were acquired in a human cadaver (64-year-old man) after each of the 8 combinations of localizer radiographs (anteroposterior [AP], AP lateral, AP-posteroanterior [PA], lateral AP, lateral PA, PA, PA-AP, and PA lateral). Applied effective milliampere second, volume CT dose index (CTDIvol) and image noise were recorded for all 24-image series. Volume CT dose indexes were also recorded in 20 patients undergoing chest and abdominal CT after PA and PA-lateral radiographs with the use of AEC. Data were analyzed using analysis of variance and linear correlation tests. With AEC, the CTDIvol fluctuates with the number and projection of localizer radiographs (P < 0.0001). Lowest CTDIvol values are seen when 2 orthogonal localizer radiographs are acquired, whereas highest values are seen when single PA or AP-PA projection localizer radiographs are acquired for planning (P < 0.0001). In 20 patients, CT scanning with AEC after acquisition of 2 orthogonal projection localizer radiographs was associated with significant reduction in radiation dose compared to PA projection radiographs alone (P < 0.0001). When scanning with AEC, acquisition of 2 orthogonal localizer radiographs is associated with lower CTDIvol compared to a single localizer radiograph.

Size-specific dose estimates: Localizer or transverse abdominal computed tomography images?

To investigate effect of body dimensions obtained from localizer radiograph and transverse abdominal computed tomography (CT) images on Size Specific Dose Estimate. This study was approved by Institutional Review Board and was compliant with Health Insurance Portability and Accountability Act. Fifty patients with abdominal CT examinations (58 ± 13 years, Male:Female 28:22) were included in this study. Anterior-posterior (AP) and lateral (Lat) diameters were measured at 5 cm intervals from the CT exam localizer radiograph (simple X-ray image acquired for planning the CT exam before starting the scan) and transverse CT images. Average of measured AP and Lat diameters, as well as maximum, minimum and mid location AP and Lat were measured on both image sets. In addition, off centering of patients from the gantry iso-center was calculated from the localizers. Conversion factors from American Association of Physicists in Medicine (AAPM) report 204 were obtained for AP, Lat, AP + Lat, and effective diameter (√ AP * Lat) to determine size specific dose estimate (SSDE) from the CT dose index volume (CTDIvol) recorded from the dose reports. Data were analyzed using SPSS v19. Total number of 5376 measurements was done. In some patients entire body circumference was not covered on either projection radiograph or transverse CT images; hence accurate measurement of AP and Lat diameters was not possible in 11% (278/2488) of locations. Forty one patients were off-centered with mean of 1.9 ± 1.8 cm (range: 0.4-7 cm). Conversion factors for attained diameters were not listed on AAPM look-up tables in 3% (80/2488) of measurements. SSDE values were significantly different compared to CTDIvol, ranging from 32% lower to 74% greater than CTDIvol. There is underestimation and overestimation of dose comparing SSDE values to CTDIvol. Localizer radiographs are associated with overestimation of patient size and therefore underestimation of SSDE.

Bony Landmarks on Computed Tomographic Localizer Radiographs to Prescribe a Reduced Scan Range in Patients Undergoing Multidetector Computed Tomography for Suspected Urolithiasis

The objective of this study was to determine bony landmarks that can be used on computed tomographic (CT) localizer radiographs to prescribe a reduced scan range for noncontrast CT (NCCT) for suspected urolithiasis. This retrospective review comprised 243 patients who underwent NCCT for suspected urolithiasis. Scout line mode was used to correlate axial images with the z-axis location on CT localizer radiographs. The most inferior vertebral body end plate (superior or inferior) that was above the superiormost aspect of the highest kidney was recorded. The superior or inferior border of the pubic symphysis was determined to be below the inferiormost aspect of the bladder. The CT studies were then viewed to identify any findings that would be missed above or below the defined bony landmarks. Using the top or bottom of T10 included both kidneys in their entirety in 243 (100%) of 243 cases. Using the top or bottom of T11 included both kidneys in 241 (99%) of the 243 and 189 (78%) of the 243 cases, respectively. Using the top or bottom of the pubic symphysis included the entire bladder in 158 (65%) of the 243 and 243 (100%) of the 243 cases, respectively. The mean reduction in scan length was 5.17 cm (11.3%) above the bottom of T10 and 2.89 cm (6.3%) below the pubic symphysis, for a total reduction of 8.06 cm (17.7%). Reducing the scan length is an effective technique to reduce radiation dose in patients undergoing NCCT for suspected urolithiasis. The bottom of T10 and the bottom of the pubic symphysis should be used as landmarks on the CT localizer radiograph.

Assessment of patient dose from CT localizer radiographs

With recently introduced technical innovations for CT systems, the dose of CT scan acquisitions has been substantially reduced; even effective dose values below 1 mSv have been reported. Due to this development, dose of the localizer radiograph may contribute substantially to dose of the whole CT examination. Since there are only limited data in the literature regarding patient dose for the different types of localizer radiographs, patient dose values were estimated in our study by measurements and Monte Carlo simulations and compared to dose values of typical CT examinations. First, dose distributions were measured in anthropomorphic phantoms for three different body regions (head, thorax, abdomen-pelvic) and three positions of the x-ray tube (AP, PA, and lateral views); measured values were compared to simulated data using Monte Carlo techniques for validation purposes. Second, organ and effective dose values for the various investigated localizer radiograph scenarios were calculated and compared with published dose values for standard CT and low-dose CT examinations. For the anthropomorphic phantom, deviations of the dose values between measured and calculated results were in the range of 15%. Organ and effective dose values showed a strong dependence on the tube position. The largest differences were observed for chest localizer radiographs in the female phantom for the dose to the breast (AP: 1.01 mGy vs PA: 0.24 mGy). Overall effective dose values were in the range of 0.04-0.42 mSv per localizer radiograph acquisition. In view of the technical dose-reducing innovations in CT, localizer radiographs may substantially contribute to the total dose of the whole CT examination, particularly in the case of dedicated low-dose scans used, e.g., for young patients or screening purposes. Optimization of dose in localizer radiographs should be pursued further in the same way as it was done in CT.

SU‐E‐I‐55: An Automated Technique to Determine Patient Size for Obtaining Size Specific Dose Estimates From System Reported CT Dose Index

Purpose: Size‐Specific Dose Estimate (SSDE) uses patient size to produce an estimate of the radiation dose to the patient resulting from a CT scan. The purpose of this work was to develop an automated technique to determine patient size based on localizer radiograph and the center slice of the scanned volume for SSDE calculations. Methods: This retrospective study was approved by the Institutional Review Board. Abdominal CT studies of 38 patients (age range:12−86 years) were evaluated. An automated software algorithm was developed in Matlab (The Mathworks Inc.). Using information from the dose report images, series that resulted in radiation dose to the patient and the central slice of each series were identified. The lateral dimensions for the patient size were obtained from the localizer radiograph and the central transaxial slice by calculating the FWHM of the Gaussian function fit using a linear least squares estimation to the central row profile in each image. Similarly the anterior posterior (AP) patient dimensions were calculated from the central column profile of the central transaxial slice. These dimensions were compared with manual measurements performed by a radiology resident blinded to the algorithm results. Results: A significant difference was observed between manual size measurements (range:27.1–49.9cm) and automated measurements (range:24.2–45.5cm) techniques (p&lt;0.001, mean difference: 6.0cm, 95% CI: 4.7 to 7.3cm) for lateral measurements from the localizer radiograph. For measurements from the center transaxial slice images, the mean difference between the manual (range:16.6– 42.4cm) and automated techniques (range:16.4–42.5cm) was 1cm (95% CI: 0.4 to 1.7cm; p&lt;0.001) for the lateral dimension and was 0cm (95% CI: −0.4 to 0.4cm; p&gt;0.821) for the AP dimension. Conclusion: This automated algorithm may be used to obtain patient dimensions from CT images for calculation of the size specific dose estimate, and may eliminate the need to perform these measurements manually.

SU‐E‐I‐35: Using Six‐Sigma Methodology for Practice Quality Improvement Projects

Purpose: To illustrate the utility of Six‐Sigma tools for Practice Quality Improvement Projects Methods: Practice Quality Improvement (PQI) Projects are required as a component of Maintenance of Certification (MOC). The methodology of Define, Measure, Analyze, Improve, and Control (DMAIC) is particularly well suited for PQI. An example of the DMAIC approach is presented using the CT pulmonary embolism protocol. Within DMAIC the Define phase consists of stating the problem and the processes involved in the task. During Measure phase the baseline performance of the process is established. Analyze phase involves looking at the data collected and proposing methods to enhance the process. The Improve phase comprises the changes that are made to the process. Control phase consists of follow‐up to insure that improvements are robust and remain in place over time. For the CT pulmonary embolism example:Define: For dose reduction purposes kV is switched from 120 to 100 resulting in higher mA values than desired by the Radiologist. Manual mA is utilized instead of mA modulation techniques.Measure: Baseline CTDIvol values across patient sizes for pulmonary embolism.Analyze: What modulation algorithm will Result in similar doses? Improve: Pilot studies with adjusted mA modulation parameters. Control: Establish a database to continually monitor doses from pulmonary embolism protocol. Results: Baseline measurements demonstrate four dose levels 2.1 mGy, 3.9 mGy, 4.9 mGy, and 10.9 mGy, depending on patient size measured laterally on a localizer radiograph. After a size‐based protocol chart is implemented dose values are variable ranging from 1.02 mGy to 12.19 mGy. Conclusion: The DMAIC approach can successfully be implemented during Practice Quality Improvement Projects.

SU-E-I-43: Reducing Scanogram Dose Without Affecting the Planned Tube Current Modulation of a Diagnostic Abdomen/pelvis CT Scan.

In tube current modulated CT scans, scanograms (localizer radiographs) are needed prior to the diagnostic scan to obtain the attenuation properties of each patient which is then used to plan the mA modulation of the diagnostic scan. The purpose of this study was to investigate whether lowering the dual-scanogram dose affects the mA modulation scheme. Although scanograms use relatively low dose, the goal of this study is to reduce radiation dose from scanograms even further in keeping with the ALARA principle. Abdomen-pelvis CT exams were performed on an anthropomorphic phantom (Pixy RS102 phantom, Radiology Support Devices) using a Toshiba Aquilion CT scanner with SUREExposure tube current modulation. Various combinations of kVp and mAs were used to acquire dual-scanograms (AP and lateral); parameters for the diagnostic scan following the scanograms were fixed. For each kVp, the mA was reduced in intervals from a baseline 100% mA level, including 80%, 60%, 40%, 20% of the baseline, and a minimum level of 10mA. For each exam the resulting CTDIvol and total mAs were compared. When scanogram kVp was reduced from 120kVp (default) to 100kVp and 80kVp, both CTDIvol and total mAs of the diagnostic scan increased slightly (up to 4.3%). When scanogram mA was decreased in intervals representing 80%, 60%, 40%, and 20% of baseline, there was little to no effect (less than 3%) on CTDIvol and total mAs for the diagnostic scan. For the minimum mA scanograms, CTDIvol was affected by up to 5.5% and total mAs was affected by up to 6.1% (at 80kVp). It should be noted that for 120kVp, the decrease of both CTDIvol and total mAs was within 1% at all reduced scanogram dose levels. It was suggested to use the default 120kVp scanogram to minimize the dose from the diagnostic scan. However, the mA of dual-scanograms may be significantly reduced without affecting planned mA of the exam.

上肢体位の影響を考慮した胸部CT撮影条件最適化の検討

Computed tomography automatic exposure control (CT-AEC) technique is calculated from a localizer radiograph. When we perform neck and chest CT examination, at first, we acquire localizer radiograph and neck images by placing the arm in a lowered position. Next, the arm is raised for the chest scan. Therefore, the localizer radiograph and subject information are different in the chest scan. In this situation, the chest scan with the use of the CT-AEC causes radiation over-dose. The purpose of this study is to optimize the CT-AEC by controlling noise index (NI), and make a chest CT scan condition considering the position of the arms. We measured the image noise (SD) in the phantom by using CT-AEC. In addition, dose length product (DLP) was recorded. Moreover, we examined the correlation with the clinical images. The results of our experiments show that radiation dose can be reduced with the image quality kept by controlling NI.

Ablation margin assessment of liver tumors with intravenous contrast-enhanced C-arm computed tomography

To determine the influence of anthropomorphic parameters on the relationship between patient centering, mean computed tomography (CT) numbers and quantitative image noise in abdominal CT. Our Institutional Review Board approved study included 395 patients (age range 21-108, years; male:female = 195:200) who underwent contrast-enhanced abdominal CT on a 16-section multi-detector row scanner (GE LightSpeed 16). Patient centering in the gantry isocenter was measured from the lateral localizer radiograph (off center S = patient off centered superior to isocenter; off center I = patient off centered inferior to isocenter). Mean CT numbers (Hounsfield Units: HU) and noise (standard deviation of CT numbers: SD) were measured in the anterior (aHU, aSD) and posterior (pHU, pSD) abdominal wall subcutaneous fat and liver parenchyma (LivHU, LivSD) at the level of the porta hepatis. Patients' age, gender, weight, body mass index and maximal anteroposterior diameter were recorded. The data were analyzed using linear regression analysis. Most patients (81%; 320/395) were not correctly centered in the gantry isocenter for abdominal CT scanning. Mean CT numbers in the abdominal wall increased significantly with an increase in the off-centering distance, regardless of the direction of the off-center (P < 0.05). There was a substantial increase in pSD (P = 0.01) and LivSD (P = 0.017) with off-centering. Change in mean CT numbers and image noise along the off-center distance was influenced by the patient size (P < 0.01). Inappropriate patient centering for CT scanning adversely affects the reliability of mean CT numbers and image noise.

Moral Complexity

An attending orthopaedic surgeon, an orthopaedic resident, and a spine Fellow are scheduled to perform a multilevel spinal decompression and instrumentation from L3 to S1. The case is delayed for several hours and begins in the late afternoon. The attending orthopaedist chooses not to wait for localization radiographs and proceeds with the decompression and four-level pedicle screw instrumentation with posterior-lateral fusion. The Fellow suggests several times during the procedure that radiographs should be obtained, but the attending surgeon is late for a meeting and declines to obtain radiographs. The attending surgeon leaves the resident and Fellow to close the wound after spinal instrumentation and fusion is complete. Radiographs are obtained prior to closure of the wound, and these indicate that the decompression has been performed at the appropriate levels but that the spinal instrumentation and fusion was from L2 to L5 rather than from L3 to S1. The Fellow notifies the attending surgeon, who has already left the hospital. The Fellow offers to remove the L2 screws, place S1 screws, and reconfigure the instrumentation prior to closing. The attending surgeon forbids the Fellow to do this and tells her, "Everything will be fine." He also states that he will discuss the surgery with the patient and family in the morning. As ordered, the Fellow and the resident close the wound.

Systematic offset of kV and MV localization systems as a function of gantry angle

Mechanical flex of the gantry and mounted imaging panels leads to systematic offsets in localization image isocenter as a function of gantry angle for linear accelerator‐mounted image guidance systems. Subsequently, object positions obtained from localization radiographs may be offset, resulting in greater target positioning uncertainty. While current QA procedures measure kV/MV image agreement, these measurements do not provide insight to apparent isocenter position for either single imaging system as a function of gantry rotation. This study measures offset as a function of gantry angle in kV and MV imaging systems on four treatment machines to investigate the magnitude of systematic offsets and their reproducibility between systems and machines, as well as over time. It is shown that each machine and energy has a reproducible pattern of offset as a function of gantry angle that is independent of kV/MV agreement, and it varies by machine. kV and MV offset ranges are on the order of 1.5 mm in the R/L and A/P directions, and 0.5 mm in the S/I direction. Variability of kV‐MV agreement is on the order of 0.7 mm. At certain angles, combinations of localization images could show a compounded offset of over 2 mm, exceeding the desired certainty threshold. Since these trends are persistent over time for each machine, online correction for image offsets as a function of gantry angle could improve the margin of positioning uncertainty.PACS number: 87.55.Qr

Misoperation of CT Automatic Tube Current Modulation Systems with Inappropriate Patient Centering: Phantom Studies

Inappropriate patient centering on the gantry changes the size of the localizer radiographs used for CT examinations, influencing the operation of CT automatic tube current modulation because tube current is controlled with information from localizer radiographs. The purpose of this study was to examine the influence of inappropriate patient centering on the gantry isocenter on automatic tube current modulation. An elliptical phantom was scanned with four automatic tube current modulation techniques after acquisition of localizer radiographs in the horizontal and vertical directions with the phantom center shifted from the gantry isocenter in the vertical direction. After scanning, the magnification rate of the frontal localizer radiographs, tube current-time product, and image noise were examined. On phantom studies, the magnification rate of localizer radiographs showed a linear relation to the vertical deviation of the phantom from the gantry isocenter. From 50 mm above to 50 mm below the gantry isocenter, tube current-time products ranged from 75% to 141% compared with those at the gantry isocenter. In addition, increases and decreases in the amount of image noise related to changes in tube current-time product were confirmed. Inappropriate patient centering causes misoperation of automatic tube current modulation systems, in which tube current is controlled with information from localizer radiographs, and thus causes increases in tube current or image noise.

Chest CT Performed with 3D and z-Axis Automatic Tube Current Modulation Technique : Breast and Effective Doses

Chest computed tomographic (CT) scans are the most effective examinations for detecting lung cancer at an early stage. In chest CT examinations, it is important to consider the reduction of radiation dose, particularly to the mammary gland. The objective of this study was to assess breast doses and effective doses on chest CT examinations between three-dimensional and z-axis automatic tube current modulation (ATCM) techniques. Absorbed dose to the breast, lung, mediastinum, and skin was evaluated with an anthropomorphic phantom and radiophotoluminescence glass dosimeters using two different CT scanners. The dosimeters were placed inside and outside the phantom. The phantom was scanned using three-dimensional and z-axis ATCM techniques after scanning localizer radiographs from the horizontal and vertical directions. After scanning, each organ dose was calculated. Moreover, the dose-length product recorded in the dose reports was examined, and each effective dose was calculated. Compared with z-axis ATCM, three-dimensional ATCM reduced breast dose by 0.7% to 18.6% and effective dose by 4.9% to 10.2%. In particular, three-dimensional ATCM reduced frontal breast dose. For other organs, three-dimensional ATCM reduced absorbed doses by 3.4% to 13.6% compared to z-axis ATCM. Three-dimensional ATCM can reduce absorbed doses to the breast and other organs, in addition to reducing effective dose, compared to z-axis ATCM.

Dose reduction in computed tomography by attenuation-based on-line modulation of tube current: evaluation of six anatomical regions

This study investigated the potential of attenuation-based on-line modulation of tube current to reduce the dose of computed tomography (in milliamperes) without loss in image quality. The dose can be reduced for non-circular patient cross-sections by reducing the tube current at the angular positions at which the diameter through the patient diameter is smallest. We investigated a new technical approach with attenuation-based on-line modulation of tube current. Computed tomographic projection data were analyzed to determine the optimal milliampere values for each projection angle in real time, instead of performing prior measurements with localizer radiographs. We compared image quality, noise pattern, and dose for standard scans and for scans with attenuation-based on-line modulation of tube current in a group of 30 radiation therapy patients. Six different anatomical regions were examined: head, shoulder, thorax, abdomen, pelvis, and extremities (knee). Image quality was evaluated by four radiologists in a blinded fashion. We found the dose to be reduced typically by 15-50 %. In general, no deterioration in image quality was observed. Thus the dose in computed tomography be reduced substantially by technical measures without sacrificing image quality. Attenuation-based on-line modulation of tube current is an efficient and practical means for this.

Changes in high-dose-rate tandem and ovoid applicator positions during treatment in an unfixed brachytherapy system.

To measure the change in applicator position during treatment in an unfixed high-dose-rate (HDR) brachytherapy system and to evaluate the effect of the shifts on dose calculations. Posttreatment localization radiographs were obtained for 47 HDR treatments (26 tandem and ovoid applicators, 21 ovoids-only applicators). The authors measured the change in applicator position relative to the patient's bone anatomy. Doses to the target and critical structures were calculated for posttreatment applicator positions for comparison. Average displacements of the tandem and ovoid applicators in anteroposterior dimension were 5 and 4 mm for the tandem and ovoids, respectively. Anterior displacement occurred twice as often as posterior displacement. The average lateral and longitudinal shifts were less. Less displacement was observed with ovoids-only insertions. The largest displacement for ovoids-only applicators was 3 mm in the anteroposterior dimension. A high bladder dose difference (17.4%) for tandem and ovoid applications correlated with anterior shifts of the applicators. Patient movement in an unfixed HDR brachytherapy system can displace the applicators, especially the tandem. Anterior shifts correlate with high bladder dose differences. Immobilization of the patient's hips and legs, as well as stabilization of applicators, would reduce these shifts.