Kent M. Ogden

Converting Dose-Length Product to Effective Dose at CT

PURPOSE To determine effective dose (ED) per unit dose-length product (DLP) conversion factors for computed tomographic (CT) dosimetry. MATERIALS AND METHODS A CT dosimetry spreadsheet was used to compute patient ED values and corresponding DLP values. The ratio of ED to DLP was determined with 16-section CT scanners from four vendors, as well as with five models from one manufacturer that spanned more than 25 years. ED-to-DLP ratios were determined for 2-cm scan lengths along the patient axis, as well as for typical scan lengths encountered at head and body CT examinations. The dependence of the ratio of ED to DLP on x-ray tube voltage (in kilovolts) was investigated, and the values obtained with the spreadsheet were compared with those obtained by using two other commercially available CT dosimetry software packages. RESULTS For 2-cm scan lengths, changes in the scan region resulted in differences to ED of a factor of 30, but much lower variation was obtained for typical scan lengths at clinical head and body imaging. Inter- and intramanufacturer differences for ED/DLP were generally small. Representative values of ED/DLP at 120 kV were 2.2 microSv/mGy x cm (head scans), 5.4 microSv/mGy x cm (cervical spine scans), and 18 microSv/mGy x cm (body scans). For head scans, ED/DLP was approximately independent of x-ray tube voltage, but for body scans, the increase from 80 to 140 kV increased the ratio of ED to DLP by approximately 25%. Agreement in ED/DLP data for all three software packages was generally very good, except for cervical spine examinations where one software package determined an ED/DLP ratio that was approximately double that of the other two. CONCLUSION This article describes a method of providing CT users with a practical and reliable estimate of adult patient EDs by using the DLP displayed on the CT console at the end of any given examination.

An exposure indicator for digital radiography

Digital radiographic imaging systems, such as those using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD, and MOSFET technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital postprocessing of the acquired image data. Overexposure and underexposures are not readily recognizable. As a result, patient dose has a tendency to gradually increase over time after a department converts from screen/film-based imaging to digital radiographic imaging. The purpose of this report is to recommend a standard indicator which reflects the radiation exposure that is incident on a detector after every exposure event and that reflects the noise levels present in the image data. The intent is to facilitate the production of consistent, high quality digital radiographic images at acceptable patient doses. This should be based not on image optical density or brightness but on feedback regarding the detector exposure provided and actively monitored by the imaging system. A standard beam calibration condition is recommended that is based on RQA5 but uses filtration materials that are commonly available and simple to use. Recommendations on clinical implementation of the indices to control image quality and patient dose are derived from historical tolerance limits and presented as guidelines.

Experimental investigation of the dose and image quality characteristics of a digital mammography imaging system

Our purpose in this study was to investigate the image quality and absorbed dose characteristics of a digital mammography imaging system with a CsI scintillator, and to identify an optimal x-ray tube voltage for imaging simulated masses in an average size breast with 50% glandularity. Images were taken of an ACR accreditation phantom using a LORAD digital mammography system with a Mo target and a Mo filter. In one experiment, exposures were performed at 80 mAs with x-ray tube voltages varying between 24 and 34 kVp. In a second experiment, the x-ray tube voltage was kept constant at 28 kVp and the technique factor was varied between 5 and 500 mAs. The average glandular dose at each x-ray tube voltage was determined from measurements of entrance skin exposure and x-ray beam half-value layer. Image contrast was measured as the fractional digital signal intensity difference for the image of a 4 mm thick acrylic disk. Image noise was obtained from the standard deviation in a uniformly exposed region of interest expressed as a fraction of the background intensity. The measured digital signal intensity was proportional to the mAs and to the kVp5.8. Image contrast was independent of mAs, and dropped by 21% when the x-ray tube voltage increased from 24 to 34 kVp. At a constant x-ray tube voltage, image noise was shown to be approximately proportional to (mAs)(-05), which permits the image contrast to noise ratio (CNR) to be modified by changing the mAs. At 80 mAs, increasing the x-ray tube voltage from 24 to 34 kVp increased the CNR by 78%, and increased the average glandular dose by 285%. At a constant lesion CNR, the lowest average glandular dose value occurred at 27.3 kVp. Increasing or decreasing the x-ray tube voltage by 2.3 kVp from the optimum kVp increased the average glandular dose values by 5%. These results show that imaging simulated masses in a 4.2 cm compressed breast at approximately 27 kVp with a Mo/Mo target/filter results in the lowest average glandular dose.

An exposure indicator for digital radiography: AAPM Task Group 116 (Executive Summary)

Digital radiographic imaging systems, such as those using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD, and MOSFET technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital postprocessing of the acquired image data. Overexposure and underexposures are not readily recognizable. As a result, patient dose has a tendency to gradually increase over time after a department converts from screen/film-based imaging to digital radiographic imaging. The purpose of this report is to recommend a standard indicator which reflects the radiation exposure that is incident on a detector after every exposure event and that reflects the noise levels present in the image data. The intent is to facilitate the production of consistent, high quality digital radiographic images at acceptable patient doses. This should be based not on image optical density or brightness but on feedback regarding the detector exposure provided and actively monitored by the imaging system. A standard beam calibration condition is recommended that is based on RQA5 but uses filtration materials that are commonly available and simple to use. Recommendations on clinical implementation of the indices to control image quality and patient dose are derived from historical tolerance limits and presented as guidelines.

How Good is the ACR Accreditation Phantom for Assessing Image Quality in Digital Mammography

RATIONALE AND OBJECTIVES The purpose of this study was to evaluate the American College of Radiology (ACR) accreditation phantom for assessing image quality in digital mammography. MATERIALS AND METHODS Digital images were obtained of an ACR accreditation phantom at varying mAs (constant kVp) and varying kVp (constant mAs). The average glandular dose for a breast with 50% glandularity was determined for each technique factor. Images were displayed on a 5 mega-pixel monitor, with the window width and level settings individually optimized for viewing the fibers, specks, and masses in the ACR phantom. Digital images of the ACR phantom were presented in a random manner to eight observers, each of whom indicated the number of objects visible in each image. RESULTS Intraobserver variability was greater than interobserver variability for the detection of fibers and specks, but the reverse was true for the detection of masses. As the mAs increased, the number of fibers visible increased from less than one at 5 mAs to all six being visible at 80 mAs. The corresponding number of visible specks increased from 12 to 24, and the number of visible masses increased from 1.25 to about four. Above 26 kVp, object visibility was constant with increasing x-ray tube voltage. Reducing the x-ray tube voltage to 24 kVp, however, reduced the number of visible fibers from six to five, the number of visible specks from 24 to 21.1, and the number of visible masses from four to 3.1. Observer performance was approximately constant for average glandular doses greater than 1.6 mGy, so that the range of lesion detectability in the ACR phantom occurs at doses lower than those normally encountered in clinical practice. CONCLUSION The current design of the ACR phantom is unsatisfactory for assessing image quality in digital mammography.

Patient size and x-ray transmission in body CT.

Abstract— Physical characteristics were obtained for 196 patients undergoing chest and abdomen computed tomography (CT) examinations. Computed tomography sections for these patients having no evident pathology were analyzed to determine patient dimensions (AP and lateral), together with the average attenuation coefficient. Patient weights ranged from approximately 3 kg to about 120 kg. For chest CT, the mean Hounsfield unit (HU) fell from about −120 HU for newborns to about −300 HU for adults. For abdominal CT, the mean HU for children and normal-sized adults was about 20 HU, but decreased to below −50 HU for adults weighing more than 100 kg. The effective photon energy and percent energy fluence transmitted through a given patient size and composition was calculated for representative x-ray spectra at 80, 100, 120, and 140 kV tube potentials. A 70-kg adult scanned at 120 kVp transmits 2.6% of the energy fluence for chest and 0.7% for abdomen CT examinations. Reducing the patient size to 10 kg increases transmission by an order of magnitude. For 70 kg patients, effective energies in body CT range from ∼50 keV at 80 kVp to ∼67 keV at 140 kVp; increasing patient size from 10 to 120 kg resulted in an increase in effective photon energy of ∼4 keV. The x-ray transmission data and effective photon energy data can be used to determine CT image noise and image contrast, respectively, and information on patient size and composition can be used to determine patient doses.

Factors Affecting Dimensional Accuracy of 3-D Printed Anatomical Structures Derived from CT Data

Additive manufacturing and bio-printing, with the potential for direct fabrication of complex patient-specific anatomies derived from medical scan data, are having an ever-increasing impact on the practice of medicine. Anatomic structures are typically derived from CT or MRI scans, and there are multiple steps in the model derivation process that influence the geometric accuracy of the printed constructs. In this work, we compare the dimensional accuracy of 3-D printed constructs of an L1 vertebra derived from CT data for an ex vivo cadaver T-L spine with the original vertebra. Processing of segmented structures using binary median filters and various surface extraction algorithms is evaluated for the effect on model dimensions. We investigate the effects of changing CT reconstruction kernels by scanning simple geometric objects and measuring the impact on the derived model dimensions. We also investigate if there are significant differences between physical and virtual model measurements. The 3-D models were printed using a commercial 3-D printer, the Replicator 2 (MakerBot, Brooklyn, NY) using polylactic acid (PLA) filament. We found that changing parameters during the scan reconstruction, segmentation, filtering, and surface extraction steps will have an effect on the dimensions of the final model. These effects need to be quantified for specific situations that rely on the accuracy of 3-D printed models used in medicine or tissue engineering applications.

Effect of dose metrics and radiation risk models when optimizing CT x-ray tube voltage

We investigated the effect of different CT dose metrics, as well as the implications of various radiation risk models, on the optimization of x-ray tube voltage (kV) in CT. Soft tissue attenuation characteristics and noise levels, obtained from CT scans of a Rando phantom, were used to compute contrast-to-noise ratios (CNR) at x-ray tube voltages between 80 and 140 kV. Four CT dose metrics were evaluated: (a) CTDIair, (b) weighted CTDIw, (c) organ dose (Dorgan), and (d) effective dose (E). All doses were obtained using the ImPACT CT Dosimetry software package. Soft tissue CNR was adjusted by the modification of the mAs by assuming that CNR(2) was proportional to mAs. Optimization criteria were: (a) maintaining a constant CNR at each kV and identifying the value that minimizes patient dose; and (b) maintaining a constant dose at each kV and identifying the value that maximizes CNR. We also investigated the implication for optimization strategies assuming that radiation risk is proportional to En, with n varying between 0 and 2. Optimizing with respect to phantom measurements (i.e., CTDIair and CTDIw) could generate results that differed quantitatively and qualitatively from those obtained using patient doses (i.e., Dorgan and E). For head CT scans, 140 kV offered the lowest patient doses as well as the highest CNR, whereas in abdominal scans 80 kV was optimal. Use of an optimal kV for CT imaging over current practice of using 120 kV might reduce patient doses by 10-15%, or improve CNR by 5-10%. Assuming that the risk was proportional to En made no difference to the optimal kV for positive values of n up to 2. We conclude that (a) CT optimization with respect to kV should generally be performed with respect to the patient effective dose, (b) neither CTDIair nor the body CTDIw are appropriate for use in CT optimization, (c) the range of current radiation risk models should not affect the optimal kV value in CT imaging.

Use of the VIP-Man model to calculate energy imparted and effective dose for x-ray examinations.

Abstract— A male human tomographic model was used to calculate values of energy imparted (&egr;) and effective dose (E) for monoenergetic photons (30–150 keV) in radiographic examinations. Energy deposition in the organs and tissues of the human phantom were obtained using Monte Carlo simulations. Values of E/&egr; were obtained for three common projections [anterior-posterior (AP), posterior-anterior (PA), and lateral (LAT)] of the head, cervical spine, chest, and abdomen, respectively. For head radiographs, all three projections yielded similar E/&egr; values. At 30 keV, the value of E/&egr; was ∼1.6 mSv J−1, which is increased to ∼7 mSv J−1 for 150 keV photons. The AP cervical spine was the only projection investigated where the value of E/&egr; decreased with increasing photon energy. Above 70 keV, cervical spine E/&egr; values showed little energy dependence and ranged between ∼8.5 mSv J−1 for PA projections and ∼17 mSv J−1 for AP projections. The values of E/&egr; for AP chest examinations showed very little variation with photon energy, and had values of ∼23 mSv J−1. Values of E/&egr; for PA and LAT chest projections were substantially lower than the AP projections and increased with increasing photon energy. For abdominal radiographs, differences between the PA and LAT projections were very small. All abdomen projections showed an increase in the E/&egr; ratio with increasing photon energy, and reached a maximum value of ∼13.5 mSv J−1 for AP projections, and ∼9.5 mSv J−1 for PA/lateral projections. These monoenergetic E/&egr; values can generate values of E/&egr; for any x-ray spectrum, and can be used to convert values of energy imparted into effective dose for patients undergoing common head and body radiological examinations.

A METHOD TO OBTAIN MEAN ORGAN DOSES IN A RANDO PHANTOM

A quantitative method of obtaining average organ dose from point measurements made in the male RANDO phantom is described for 24 compact organs of interest in patient dosimetry. A three-dimensional Cartesian coordinate system was created by considering each of the 36 RANDO phantom sections as the z coordinate, and using a rectangular frame to assign x and y coordinates relative to the center of each section. Anatomical atlases and clinical experience were used to identify the location and extent of each organ and tissue in the RANDO phantom. This proposed scheme is comparable to one used in a commercial phantom and offers investigators a comprehensive protocol for obtaining mean organ doses in the RANDO phantom.