A Dutta

Influence of phantom diameter, kVp and scan mode upon computed tomography dose index.

The computed tomography (CT) radiation dose to pediatric patients has received considerable attention recently. Moreover, it is important to be able to determine CT radiation doses for various patient sizes ranging from infants to large adults. The current AAPM protocol only measures CT radiation dose using a 16 cm acrylic phantom to represent an adult head and a 32 cm acrylic phantom to represent an adult body. The goal of this paper is to study the dependence of the computed tomography dose index (CTDI) upon the size of the phantom, the kVp selected and the scan mode employed. Our measurements were done on phantom sizes ranging from 6 cm to 32 cm. The x-ray tube potential ranged from 80 to 140 kVp. The scan modes utilized for the measurements included: consecutive axial scans, single-slice helical scans with variable pitch and multislice helical scans with variable pitch. The results were consolidated into simplified equations which related the phantom diameter and kVp to the measured CTDI. Some generalizations were made about the relationship between the scan modes of the various CT units to the measured radiation doses. The CTDI appears to be an exponential function of phantom diameter. For the same kVp and mAs, the radiation doses for smaller phantoms are much greater than for larger sizes. The derived relationship can be used to estimate the radiation doses for a variety of scan conditions and modes from measurements with the two standard reference phantoms. A method was also given for converting axial CT dose measurements to appropriate MSAD values for helical CT scans.

Comparison of computed radiography and film/screen combination using a contrast-detail phantom

The purpose of this work is to compare computed radiography (Kodak CR 400) and film/screen combination (Speed 400) systems in regards of patient dose, technique settings, and contrast‐detail detectability. A special contrast‐detail phantom with drilled holes of varying diameter (detail) and varying depth (contrast) was utilized. Various thicknesses of the Lucite sheets were utilized to simulate scattering tissues. Images of the phantom were acquired using a range of 60–120 kVp for film/screen and CR with a conventional x‐ray tube and then for CR with additional 2 mm aluminum added filtration to the x‐ray beam. The patient entrance skin dose was measured while maintaining 1.6 o.d. for film/screen images and 1900 Exposure Index for CR images. CR phantom images were displayed on the diagnostic workstation for soft copy reading as well as printed on films for hard copy reading on viewbox. Four physicists evaluated the images by scoring the threshold target depth along the row of the same target diameter. Detection ratio was calculated by counting the number of detectable targets divided by the total number of targets in the phantom. The overall score was related to the patient entrance skin dose, kVp, and the thickness of the scattering material. The patient entrance skin dose was reduced as the additional aluminum filter was added to the x‐ray beam. Our findings suggested using a higher kVp setting and additional added filtration would reduce the patient entrance skin dose without compromising the contrast‐detail detectability, which was compensated by the contrast manipulation on soft‐copy display workstations.

New automated fluoroscopic systems for pediatric applications

Pediatric patients are at higher risk to the adverse effects from exposure to ionizing radiation than adults. The smaller sizes of the anatomy and the reduced X‐ray attenuation of the tissues provide special challenges. The goal of this effort is to investigate strategies for pediatric fluoroscopy in order to minimize the radiation exposure to these individuals, while maintaining effective diagnostic image quality. Modern fluoroscopy systems are often entirely automated and computer controlled. In this paper, various selectable and automated modes are examined to determine the influence of the fluoroscopy parameters upon the patient radiation exposures and image quality. These parameters include variable X‐ray beam filters, automatic brightness control programs, starting kilovolt peak levels, fluoroscopic pulse rates, and other factors. Typical values of radiation exposure rates have been measured for a range of phantom thicknesses from 5 cm to 20 cm of acrylic. Other factors that have been assessed include spatial resolution, low‐contrast discrimination, and temporal resolution. The selection menu for various procedures is based upon the examination type, anatomical region, and patient size. For pediatric patients, the automated system can employ additional filtration, special automatic brightness control curves, pulsed fluoroscopy, and other features to reduce the patient radiation exposures without significantly compromising the image quality. The benefits gained from an optimal selection of automated programs and settings for fluoroscopy include ease of operation, better image quality, and lower patient radiation exposures. PACS numbers: 87.59.‐e, 87.62.+n

SU-GG-I-07: Monitoring Radiation Dose in Cerebral CT Perfusion Exams

PURPOSE: This paper is to study how to monitor and manage the radiation dose in cerebral CT perfusion exams with various CTscanners including a 10 slice MDCT, a 64 slice VCT and a 320 detector row volume scanner. METHODS & MATERIALS: Several cerebral CT perfusion protocols have been studied with various tube current settings. Radiation dose measurements were done on an anthropormorphic head phantom using an optically stimulated luminescence(OSL) dosimeter device (MicroStar, Landauer) and an ion chamber dosimeter (Accu‐Pro, RadCal). All the measurements were done at the skin surface. Several OSLs were used to cover the X‐ray slice beam width. The readout of OSLs was performed right on site with a portable reading device. RESULTS: The accuracy of the OSLs was verified through a side‐by‐side comparison with an ion chamber dosimeter. It is shown that the OSL sensor is angle independent that makes it advantageous over those solid state personal dosimeters for real‐time CT dose measurements. For our study, 80 kVp was utilized for the cerebral CT perfusion exams. The tube current varied from 80 mA to 200 mA. The radiation dose varied directly with the selection of tube voltage and current settings. Considerable variations were also observed among different types of CTscanners. Conclusion: Since there is a potential risk for the skin dose to be accumulated beyond the threshold for determinant effects such as skin erythema and epilation in CT perfusion exams, it is important to optimize the protocol and monitor the patient dose closely. This paper verifies a tool to do that. It can be utilized during the initial set‐up of the protocol. Also, it will be useful for individual patient dose monitoring from case to case. It adds the safeguard to high dose CT scans such as cerebral CT perfusion exams.

SU‐FF‐I‐09: Pediatric Patient Dose Management From a 64 Slice VCT

Purpose: As the clinical utilization of CT grows, it becomes more important to manage patient dose without compromising image quality, especially in children. A special effort should be made to reduce pediatric patient dose through age‐ and size‐specific protocols. Methods & Materials: A pediatric 64 slice VCT scanner(GE Lightspeed) was tested with a group of cylindrical acrylic phantoms with diameters ranging from 6 – 32 cm. In addition, anthropomorphic phantoms (CIRS adult and pediatric dosimetry verification phantoms) were employed to correlate the CTDI values with the skin doses measured by a solid state dosimeter (Unfors PSD). The dose affecting factors included: kVp, mAs, beam filtration, beam collimation, pitch, patient size, detector configuration and dose reduction techniques such as mA modulation and post‐processing. Various techniques and their combinations were included in this study. Finally, clinical protocols for pediatric applications were evaluated and adjusted based upon the measured patient dose and image quality. Results: The automated CTDI values displayed on the system agreed with our measurements when the standard phantom sizes were used, i.e., 16 cm in diameter for head, 32 cm for adult body and 16 cm for pediatric body. However, the measured dose differed from the automated CTDI by a factor of 1.72 for a reduced head phantom size of 6 cm in diameter and a factor of 3.2 for a reduced body phantom size of 10 cm in diameter. Patient age also played an important role in estimating effective dose. The changing beam filtration caused a variation of up to 42% in the in‐air dose output. Concurrently, noise from the phantom images was evaluated. Conclusion: The clinical protocols were established based upon the dose level corresponding to the patient size and age as well as the tolerable noise level corresponding to the specific clinical applications.

SU‐FF‐I‐09: Comparison of Radiation Dose Indexes For CT Scanners: Measured Verses Automated Scanner Calculations

Purpose: Modern CTscanners provide radiation dose indexes such as CTDI and DLP for every patient based upon the selected scan parameters. Our goal is to study the potential sources of errors for these automated calculations. Method and Materials: The measured and automated radiation dose indexes were compared on five different CT models from two manufacturers. Acrylic cylindrical phantoms with diameters ranging from 6 – 32 cm were utilized for the study. Measurements were made at all available kVps and typical scan techniques in both axial and helical modes. The measured CTDI and DLP values were compared with the displayed automated values. Results: The automated displayed CTDI and DLP values are derived from system calibration using fixed size phantoms representing a typical adult head (16 cm) and adult body (32 cm). They vary with parameters such as kVp, mAs, pitch, x‐ray beam collimation and selected protocols. For typical adult head, our measured CTDI and DLP values agreed with the automated values to an average of 7.8% discrepancy. Similarly, an average of 14% discrepancy was observed for typical adult body. However, the discrepancies widened significantly for those measured on other sizes. For pediatric body scans using a 16 cm diameter phantom on table top, the measured CTDI values are more than doubled compared to the displayed automated values. Conclusion: It is important to realize that the automated radiation dose indexes by CTscanners only represent estimates based upon standardized adult body and head sizes. No considerations are given to patient age, size, weight and tissue composition. Especially, the values for pediatric scans are not represented correctly by the automated CTDIs. Future calculated radiation dose assessments needs to be incorporated with more patient factors.

Performance evaluation of a high-strip-density grid using a contrast-detail phantom

A contrast-detail phantom was used to evaluate grid performance. The phantom is constructed of 1 cm of plastic. Holes of varying diameter (detail) and varying depth (contest) were drilled into the contrast-detail phantom. The phantom was placed next to the bucky assembly. A 15 cm block of lucite was placed between the x ray tube and the phantom. A set of radiographs were taken of the phantom at different kVps and different phantom thicknesses. This was done both with and without the grid in place. An ion chamber was used so that the bucky factor could be determined. This entire procedure was repeated for the conventional, reciprocating grid. Contrast-detail curves were generated from the data. As would be expected the reciprocating grid had a lower bucky factor. The contrast improvement factor (contrast with grid/contrast without grid) was higher for the reciprocating grid. The contrast improvement factor for the high-strip- density grid was comparable to that for the reciprocating grid at high kVps and also when a thinner block of lucite was used. Grid lines were seen on the radiographs of the high-strip-density grid.

WE‐A‐218‐06: A Practical Method of CT Radiation Dose Adjustment for Patient Size and Tissue Composition

Purpose: The CTDIvol displayed on CTscanners is not patient specific. A practical method to estimate CT dose corrected for patient size, tissue composition, and anatomical location is presented. Methods: The measured CTDIvol of acrylic cylindrical phantoms of diameters from 6–32 cm on a CTscanner revealed a logarithmic relationship with the phantom mass. The relationship has been used in 25 cases of each category of adult head, chest, abdomen and pelvic CT exams. Actual patient CTimages have been used to measure the average CT numbers within image slices which are subsequently related to the total tissue mass. By applying the relationship between the actual CTDIvol and the total mass, the CT radiation dose has been adjusted for patient size and tissue composition. Results: Overall, dose underestimation by the scanner dose report is well demonstrated in all 25 cases of adult chest exams (ranging from 9% to 80% under‐estimation) because the lungtissue density is significantly lower. Even in the abdomen, which has a more consistent soft tissue density, there are adipose tissues and voids that complicate the dosimetry. The pelvis contains considerable amounts of bone, which has a much higher attenuation coefficient than soft tissues, complicating the estimation of radiation doses. Also, there is a significant variation in actual patient sizes even among adult patients; thus leads to a spread in actual CTDIvol delivered to these patients. Conclusions: There is a nationwide effort to monitor patient CT dose. Because the current scanner‐reported dose may underestimate the patient dose when the patient size is smaller than the assumed standard or the tissue density is lower, excessive dose outliers may be undetected. This is more likely to happen to pediatric patients. We present a practical method to provide a more accurate estimate of patient CT radiation dose.

SU‐E‐I‐71: Susceptibility Weighted Imaging (SWI) Software for Post‐Processing of SWI Data

PURPOSE To develop open source software for post processing of susceptibility weighted (SWI) MR images using magnitude and phase data. METHODS SWI data was acquired using Philips MRI 3T scanner with the following parameter: 3D T1 FFE axial with TR=40ms, TE=25ms, FOV=22 cm, acquisition matrix of 440×440 and 40 slices. Both magnitude and phase data was stored for SWI post processing. The SWI homodyne filtering is performed by converting the magnitude and phase image to complex real and imaginary images. The SWI software was implemented in C++ using ITK (Image registration and segmentation toolkit) toolkit. To generate SWI maps the user needs to provide the DICOM data directory, the series number of DICOM SWI series, low pass filter size and the weighting factor of phase mask. This outputted SWI series is saved as DICOM and appended to the patient series and can be viewed in any DICOM compatible viewer. The software also outputs SWI filtered phase maps which can be further used for iron quantification in organs like brain, liver etc. RESULTS An open source implementation of SWI post-processing tool using ITK was provided. The SWI processed phase weighted data can be used for qualitative assessment of iron deposits. The filtered phase map outputted can be used for quantitative iron measurements. CONCLUSIONS SWI post processing software is implemented here to provide qualitative SWI maps of iron deposits in brain and other organs. The post processed images can also be useful for MR Venography with minimum intensity projection. This tool would be useful to study disease processes involved with accumulation of iron in different organs.

SU‐E‐I‐81: Radiation Dose for a 320 Slice Cone Beam CT: Methods and Analysis

Purpose: The new 320 slice CT scanners can irradiate x‐ray beam widths from 2.0 mm to 160 mm length in one rotation, and they present challenges for the measurement and analysis of radiation doses. Various methods to perform and analyze the radiation dose data were examined. Methods: These CT scanners have various modes of operation such as: 320 detector axial volume mode, 64 detector axial and helical scan, and scan mode with 4 narrow axial slices ranging from 4*0.5 mm to 4*8 mm. There are many selectable scan parameters such as: four kVp settings (80, 100, 120 and 135), three x‐ray beam filters, 50 to 550 mA, several scan time settings of 0.35 to 3.0 seconds, various pitch values and automatic mA modulation. These options complicate the radiation dose measurement and analysis procedures. Longer CT dose phantoms were evaluated. Several ionization chambers (100 mm pencil chamber and 0.6 cc thimble) and OSL dosimeters were utilized to perform the measurements and evaluated. Results: The radiation dose profiles were measured, and the narrowest beam width resulted in the largest radiation doses. In the 320 slice volume mode, the utilization of 3 radiation dose phantoms stacked together only changes the measured radiation dose by less than 10%. The large amount of data was reduced to a few simple equations and graphs that could be used to estimate the CT radiation dose for any of the clinical procedures. Both thimble ionization and OSLradiation detectors provide useful contributions in CTdosimetry. Conclusions: Limitations and approaches to CT radiation dosimetry for large cone beam CT scanners are reviewed and guidance is provided. Current dosimetry methods can be employed without significant errors.