Eugene Mah

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.

Performance evaluation of computed radiography systems.

Recommended methods to test the performance of computed radiography (CR) digital radiographic systems have been recently developed by the AAPM Task Group No. 10. Included are tests for dark noise, uniformity, exposure response, laser beam function, spatial resolution, low-contrast resolution, spatial accuracy, erasure thoroughness, and throughput. The recommendations may be used for acceptance testing of new CR devices as well as routine performance evaluation checks of devices in clinical use. The purpose of this short communication is to provide a tabular summary of the tests recommended by the AAPM Task Group, delineate the technical aspects of the tests, suggest quantitative measures of the performance results, and recommend uniform quantitative criteria for the satisfactory performance of CR devices. The applicability of the acceptance criteria is verified by tests performed on CR systems in clinical use at five different institutions. This paper further clarifies the recommendations with respect to the beam filtration to be used for exposure calibration of the system, and the calibration of automatic exposure control systems.

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.

Radiation-Related Cancer Risks in a Clinical Patient Population Undergoing Cardiac CT

OBJECTIVE The purpose of our study was to estimate cancer induction risk and generate risk conversion factors in cardiac CT angiography. MATERIALS AND METHODS Under an institutional review board waiver and in compliance with HIPAA, we collected characteristics for a consecutive cohort of 100 patients (60 men and 40 women; mean age, 59 ± 11 years) who had previously undergone ECG-gated cardiac CT angiography on a 64-slice CT scanner. The volume CT Dose Index (CTDI(vol)) and dose-length product (DLP) were recorded and used with the ImPACT CT Patient Dosimetry Calculator to compute organ and effective doses in a standard 70 kg phantom. Patient-specific organ and effective doses were obtained by applying a weight-based correction factor. Radiation doses to radiosensitive organs were converted to risks using age- and sex-specific data published in BEIR VII. RESULTS Median values were 62 mGy for CTDI(vol), 1,084 mGy-cm for DLP, and 17 cm for scan length. Effective doses ranged from 20 mSv (10th percentile) to 31 mSv (90th percentile). Median cancer induction risks in sensitive organs for men and women were 0.065% and 0.17%, respectively. For men and women, the range of risks was about a factor of 2. In men and women, about three quarters of the cancer risk was from lung cancer. Inclusion of the remaining less sensitive organs exposed during cardiac CT angiography examinations would likely increase the cancer induction risk by ∼20%. CONCLUSION The average cancer induction risk in sensitive organs from cardiac CT angiography for our patient cohort was 0.13%, with a female to male cancer induction risk ratio of 2.6.

Embryo Dose Estimates in Body CT

OBJECTIVE The purpose of this article is to develop a method for estimating embryo doses in CT. MATERIALS AND METHODS Absorbed doses to the uterus (embryo) of a 70-kg woman were estimated using the ImPACT CT Patient Dosimetry Calculator. For a particular CT scan length, relative uterus doses and normalized plateau uterus doses were determined for a range of commercial CT scanners. Patient size characteristics were obtained from cross-sectional axial images of 100 consecutive patients (healthy women undergoing unenhanced pelvic CT examinations). For each patient, the diameter of a water cylinder with the same mass as the patients pelvis was computed. Relative dose values were generated for cylinder diameters ranging from 16 to 36 cm at x-ray tube voltages between 80 and 140 kV. RESULTS Values of relative uterus dose increased monotonically with increasing scan length, independently of scanner model, and reached a plateau for scan lengths greater than approximately 50 cm. The average normalized plateau uterus dose for all scanners was approximately 1.4 and showed interscanner differences of less than 10% for modern scanners operated at 120 kV. Normalized plateau doses show little dependence on the x-ray tube voltage used to perform the CT examination. Our results show that the uterus dose estimate in an abdominal or pelvis CT examination performed on a 70-kg patient is about 40% higher than the reported value of the volume CT dose index (CTDI(vol)). The pelvis of a 70-kg patient may be modeled as a water cylinder with a diameter of 28 cm and has an average anteroposterior dimension of 22 cm. For constant CT technique factors, embryo dose estimates for a 45-kg patient would be approximately 18% higher than those for a 70-kg patient, whereas the corresponding dose estimates in a 120-kg patient would be approximately 37% lower. CONCLUSION Embryo doses can be estimated using relative uterus doses, normalized plateau uterus doses, and CTDI(vol) data with correction factors for patient size.

Evaluation of a flat panel digital radiographic system for low-dose portable imaging of neonates.

The purpose of this study was to evaluate the clinical utility of an investigational flat-panel digital radiography system for low-dose portable neonatal imaging. Thirty image-pairs from neonatal intensive care unit patients were acquired with a commercial Computed Radiography system (Agfa, ADC 70), and with the investigational system (Varian, Paxscan 2520) at one-quarter of the exposure. The images were evaluated for conspicuity and localization of the endings of ancillary catheters and tubes in two observer performance experiments with three pediatric radiologists and three neonatologists serving as observers. The results indicated no statistically significant difference in diagnostic quality between the images from the investigational system and from CR. Given the investigational systems superior resolution and noise characteristics, observer results suggest that the high detective quantum efficiency of flat-panel digital radiography systems can be utilized to decrease the radiation dose/exposure to neonatal patients, although post-processing of the images remains to be optimized. The rapid availability of flat-panel images in portable imaging was found to be an added advantage for timely clinical decision-making.

Evaluation of a quality control phantom for digital chest radiography

Rationale and Objectives: To examine the effectiveness and suitability of a quality control (QC) phantom for a routine QC program in digital radiography. Materials and Methods: The chest phantom consists of copper and aluminum cutouts arranged to resemble the appearance of a chest. Performance of the digital radiography (DR) system is evaluated using high and low contrast resolution objects placed in the “heart,” “lung,” and “subdiaphragm” areas of the phantom. In addition, the signal levels from these areas were compared to similar areas from clinical chest radiographs. Results: The test objects included within the phantom were effective in assessing image quality except within the subdiaphragm area, where most of the low contrast disks were visible. Spatial resolution for the DR systems evaluated with the phantom ranged from 2.6 lp/mm to 4 lp/mm, falling within the middle of the line pair range provided. The signal levels of the heart and diaphragm regions relative to the lung region of the phantom were significantly higher than in clinical chest radiographs (0.67 versus 0.21 and 0.28 versus 0.10 for the heart and diaphragm regions, respectively). The heart‐to‐diaphragm signal level ratio, however, was comparable to those in clinical radiographs. Conclusion: The findings suggest that the attenuation characteristics of the phantom are somewhat different from actual chests, but this did not appear to affect the post‐processing used by the imaging systems and usefulness for QC of these systems. The qualitative and quantitative measurements on the phantom for different systems were similar, suggesting that a single phantom can be used to evaluate system performance in a routine QC program for a wide range of digital radiography systems. This makes the implementation of a uniform QC program easier for institutions with a mixture of different digital radiography systems. PACS number(s): 87.57.–s, 87.62.+n

Improved Nuclear Medicine Uniformity Assessment with Noise Texture Analysis

Because γ cameras are generally susceptible to environmental conditions and system vulnerabilities, they require routine evaluation of uniformity performance. The metrics for such evaluations are commonly pixel value–based. Although these metrics are typically successful at identifying regional nonuniformities, they often do not adequately reflect subtle periodic structures; therefore, additional visual inspections are required. The goal of this project was to develop, test, and validate a new uniformity analysis metric capable of accurately identifying structures and patterns present in nuclear medicine flood-field uniformity images. Methods: A new uniformity assessment metric, termed the structured noise index (SNI), was based on the 2-dimensional noise power spectrum (NPS). The contribution of quantum noise was subtracted from the NPS of a flood-field uniformity image, resulting in an NPS representing image artifacts. A visual response filter function was then applied to both the original NPS and the artifact NPS. A single quantitative score was calculated on the basis of the magnitude of the artifact. To verify the validity of the SNI, an observer study was performed with 5 expert nuclear medicine physicists. The correlation between the SNI and the visual score was assessed with Spearman rank correlation analysis. The SNI was also compared with pixel value–based assessment metrics modeled on the National Electrical Manufacturers Association standard for integral uniformity in both the useful field of view (UFOV) and the central field of view (CFOV). Results: The SNI outperformed the pixel value–based metrics in terms of its correlation with the visual score (ρ values for the SNI, integral UFOV, and integral CFOV were 0.86, 0.59, and 0.58, respectively). The SNI had 100% sensitivity for identifying both structured and nonstructured nonuniformities; for the integral UFOV and CFOV metrics, the sensitivities were only 62% and 54%, respectively. The overall positive predictive value of the SNI was 87%; for the integral UFOV and CFOV metrics, the positive predictive values were only 67% and 50%, respectively. Conclusion: The SNI accurately identified both structured and nonstructured flood-field nonuniformities and correlated closely with expert visual assessment. Compared with traditional pixel value–based analysis, the SNI showed superior performance in terms of its correlation with visual perception. The SNI method is effective for detecting and quantifying visually apparent nonuniformities and may reduce the need for more subjective visual analyses.

Monte Carlo modeling of the scatter radiation doses in IR

Purpose: To use Monte Carlo techniques to compute the scatter radiation dose distribution patterns around patients undergoing Interventional Radiological (IR) examinations. Method: MCNP was used to model the scatter radiation air kerma (AK) per unit kerma area product (KAP) distribution around a 24 cm diameter water cylinder irradiated with monoenergetic x-rays. Normalized scatter fractions (SF) were generated defined as the air kerma at a point of interest that has been normalized by the Kerma Area Product incident on the phantom (i.e., AK/KAP). Three regions surrounding the water cylinder were investigated consisting of the area below the water cylinder (i.e., backscatter), above the water cylinder (i.e., forward scatter) and to the sides of the water cylinder (i.e., side scatter). Results: Immediately above and below the water cylinder and in the side scatter region, values of normalized SF decreased with the inverse square of the distance. For z-planes further away, the decrease was exponential. Values of normalized SF around the phantom were generally less than 10-4. Changes in normalized SF with x-ray energy were less than 20% and generally decreased with increasing x-ray energy. At a given distance from region where the x-ray beam enters the phantom, the normalized SF was higher in the backscatter regions, and smaller in the forward scatter regions. The ratio of forward to back scatter normalized SF was lowest at 60 keV and highest at 120 keV. Conclusion: Computed SF values quantify the normalized fractional radiation intensities at the operator location relative to the radiation intensities incident on the patient, where the normalization refers to the beam area that is incident on the patient. SF values can be used to estimate the radiation dose received by personnel within the procedure room, and which depend on the imaging geometry, patient size and location within the room. Monte Carlo techniques have the potential for simulating normalized SF values for any arrangement of imaging geometry, patient size and personnel location and are therefore an important tool for minimizing operator doses in IR.

Review of Kerma-Area Product and total energy incident on patients in radiography, mammography and CT.

This study estimated the energy incident on patients in radiography, mammography and CT using data related to X-ray beam quantity and quality. The total X-ray beam quantity is the average Air Kerma multiplied by the X-ray beam area and expressed as the Kerma-Area Product (Gy cm(-2)). The X-ray beam quality primarily depends on the target material (and anode angle), X-ray voltage (and ripple) as well as X-ray beam filtration. For any X-ray spectra, dividing total energy (fluence × mean energy) by the X-ray beam Kerma-Area Product yields the energy per Kerma-Area Product value (ε/KAP). Published data on X-ray spectra characteristics and energy fluence per Air Kerma conversion factors were used to determine ε/KAP factors. In radiography, ε/KAP increased from 6 mJ Gy(-1) cm(-2) at the lowest X-ray tube voltage (50 kV) to 25 mJ Gy(-1) cm(-2) at the highest X-ray tube voltage (140 kV). ε/KAP values ranged between 1 and 5 mJ Gy(-1) cm(-2) in mammography and between 24 and 42 mJ Gy(-1) cm(-2) in CT. Changes in waveform ripple resulted in variations in ε/KAP of up to 15 %, similar to the effect of changes resulting in the choice of anode angle. For monoenergetic X-ray photons, there was a sigmoidal-type increase in ε/KAP from 2 mJ Gy(-1) cm(-2) at 20 keV to 42 mJ Gy(-1) cm(-2) at 80 keV. However, between 80 and 150 keV, the ε/KAP shows variations with changing photon energy of <10 %. Taking the average spectrum energy to consist of monoenergetic X rays generally overestimates the true value of ε/KAP. This study illustrated that the energy incident on a patient in any area of radiological imaging can be estimated from the total X-ray beam intensity (KAP) when X-ray beam quality is taken into account. Energy incident on the patient can be used to estimate the energy absorbed by the patient and the corresponding patient effective dose.