Estimating size specific dose estimate from computed tomography radiograph localizer with radiation risk assessment.

BACKGROUND: Quantifying radiation burden is necessary for optimizing imaging protocols. The normalized dose coefficient (NDC) is determined from the water-equivalent diameter (WED) and is used to scale the CTDIvol based on body habitus to determine the size specific dose estimate (SSDE). In this study we determine the SSDE prior to the CT scan and how sensitive the SSDE from WED is to the lifetime attributable risk (LAR) from BEIR VII. METHOD: For calibration, phantom images are used to relate the mean pixel values along a profile ( PPV ¯ $\overline {{\rm{PPV}}} $ ) of the CT localizer to the water-equivalent area (AW ) of the CT axial scan at the same z-location. Images of the CTDIvol phantoms (32 cm, 16 cm, and ∼1 cm) and ACR phantom (Gammex 464) were acquired on four scanners. The relationship between the AW and PPV ¯ $\overline {{\rm{PPV}}} $ was used to calculate the WED from the CT localizer for patient scans. A total of 790 CT examinations of the chest and abdominopelvic regions were used in this study. The effective diameter (ED) was calculated from the CT localizer. The LAR was calculated based on the patient chest and abdomen using the National Cancer Institute Dosimetry System for Computed Tomography (NCICT). The radiation sensitivity index (RSI) and risk differentiability index (RDI) were calculated for SSDE and CTDIvol. RESULTS: The WED from CT localizers and CT axials scans show good correlation (R2  = 0.96) with the maximum percentage difference being 13.45%. The NDC from WED correlates poorly with LAR for lungs (R2  = 0.18) and stomach (R2  = 0.19), however that is the best correlation. CONCLUSION: The SSDE can be determined within 20% as recommended by the report of AAPM TG 220. The CTDIvol and SSDE are not good surrogates for radiation risk, however the sensitivity for SSDE improves when using WED instead of ED.

Fetal Dose from PET and CT in Pregnant Patients.

When pregnancy is discovered during or after a diagnostic examination, the physician or the patient may request an estimate of the radiation dose received by the fetus as per guidelines and standard operating procedures. This study provided the imaging community with dose estimates to the fetus from PET/CT with protocols that are adapted to University of Michigan low-dose protocols for patients known to be pregnant. Methods: There were 9 patients analyzed with data for the first, second, and third trimesters, the availability of which is quite rare. These images were used to calculate the size-specific dose estimate (SSDE) from the CT scan portion and the SUV and 18F-FDG uptake dose from the PET scan portion using the MIRD formulation. The fetal dose estimates were tested for correlation with each of the following independent measures: gestational age, fetal volume, average water-equivalent diameter of the patient along the length of the fetus, SSDE, SUV, and percentage of dose from 18F-FDG. Stepwise multiple linear regression analysis was performed to assess the partial correlation of each variable. To our knowledge, this was the first study to determine fetal doses from CT and PET images. Results: Fetal self-doses from 18F for the first, second, and third trimesters were 2.18 mGy (single data point), 0.74-1.82 mGy, and 0.017-0.0017 mGy, respectively. The combined SSDE and fetal self-dose ranged from 1.2 to 8.2 mGy. These types of images from pregnant patients are rare. Conclusion: Our data indicate that the fetal radiation exposure from 18F-FDG PET and CT performed, when medically necessary, on pregnant women with cancer is low. All efforts should be made to minimize fetal radiation exposure by modifying the protocol.

CT-Based Modeling of the Dentition for Craniomaxillofacial Surgical Planning.

ABSTRACT: Historically, the accuracy of imaging teeth by computed tomography (CT) has been suboptimal and deemed inadequate for surgical planning of orthognathic procedures. However, recent advances in CT hardware and software have significantly improved the accuracy of imaging occlusal anatomy. This technical note describes a quantitative means of evaluating the accuracy of CT-based modeling of teeth. Three-dimensional models of the dentition were created from a CT scan obtained of a craniomaxillofacial skeleton. Multiple reconstruction algorithms and modeling parameters were applied. The dentition of the same skeleton was scanned using a handheld optical scanning device to serve as the "gold standard." Semi-automated registrations of CT and optically acquired models were performed and deviation analysis was conducted. On average, the deviation of the CT model with the optical scan measured 0.19 to 0.25 mm across the various reconstruction and modeling parameters, with a mean of 0.22 mm. Computed tomography underestimated contours at cusp tips, while overestimating contours in occlusal groves. The use of bone reconstruction algorithms and decreased model smoothing resulted in more accurate models, though greater surface noise. Future studies evaluating the clinical effectiveness of CT-based occlusal splints should take this finding into account.

Development of a neonate X-ray phantom for 2D imaging applications using single-tone inkjet printing.

PURPOSE: Inkjet printers can be used to fabricate anthropomorphic phantoms by the use of iodine-doped ink. However, challenges persist in implementing this technique. The calibration from grayscale to ink density is complex and time-consuming. The purpose of this work is to develop a printing methodology that requires a simpler calibration and is less dependent on printer characteristics to produce the desired range of x-ray attenuation values. METHODS: Conventional grayscale printing was substituted by single-tone printing; that is, the superposition of pure black layers of iodinated ink. Printing was performed with a consumer-grade inkjet printer using ink made of potassium-iodide (KI) dissolved in water at 1 g/ml. A calibration for the attenuation of ink was measured using a commercial x-ray system at 70 kVp. A neonate radiograph obtained at 70 kVp served as an anatomical model. The attenuation map of the neonate radiograph was processed into a series of single-tone images. Single-tone images were printed, stacked, and imaged at 70 kVp. The phantom was evaluated by comparing attenuation values between the printed phantom and the original radiograph; attenuation maps were compared using the structural similarity index measure (SSIM), while attenuation histograms were compared using the Kullback-Leibler (KL) divergence. A region of interest (ROI)-based analysis was also performed, where the attenuation distribution within given ROIs was compared between phantom and patient. The phantom sharpness was evaluated in terms of modulation transfer function (MTF) estimates and signal spread profiles of high spatial resolution features in the image. RESULTS: The printed phantom required 36 pages. The printing queue was automated and it took about 2 h to print the phantom. The radiograph of the printed phantom demonstrated a close resemblance to the original neonate radiograph. The SSIM of the phantom with respect to that of the patient was 0.53. Both patient and phantom attenuation histograms followed similar distributions, and the KL divergence between such histograms was 0.20. The ROI-based analysis showed that the largest deviations from patient attenuation values were observed at the higher and lower ends of the attenuation range. The limiting resolution of the proposed methodology was about 1 mm. CONCLUSION: A methodology to generate a neonate phantom for 2D imaging applications, using single-tone printing, was developed. This method only requires a single-value calibration and required less than 2 h to print a complete phantom.

Neck CT angiography examinations for pediatric oropharyngeal trauma: diagnostic yield and proposal of a new targeted technique.

BACKGROUND: Neck computed tomography (CT) angiography is commonly ordered for pediatric patients with soft palate trauma to exclude vascular injury. Debate exists regarding what type of imaging is indicated in this setting, particularly amid growing concern that standard neck CT angiography results in considerable radiation exposure. OBJECTIVE: To assess the diagnostic yield and estimated dose reduction of a novel targeted protocol extending from the skull base to the hyoid bone to evaluate pediatric oropharyngeal trauma. MATERIALS AND METHODS: A retrospective imaging and medical chart review was performed of patients for whom a neck CT angiography was obtained for an indication of oropharyngeal trauma between 2008 and 2018. Effective dose and size-specific dose estimates (SSDEs) were estimated for standard and targeted neck CT angiography protocols with calculation of percent dose reduction of the targeted exams. RESULTS: Ninety-eight CT angiography examinations were reviewed. No cases were positive for neurological or major vessel injury; one case was positive for small vessel extravasation. Clinically significant nonvascular findings included phlegmonous change, retained foreign body, retropharyngeal/mediastinal air and pterygoid process fracture. With the exception of mediastinal air, all findings would have been included in the targeted protocol. Effective dose and SSDE were calculated for all cases where CTDIvol (volume CT dose index) had been reported (n=72). There was a statistically significant reduction in dose for the targeted protocol with an effective dose decrease of 69.7%±10.5% (P=0.009) and SSDE decrease of 53.9%±14.7% (P=0.01). Limiting ionizing radiation to the lung apices, esophagus and thyroid gland provided the greatest dose savings. CONCLUSION: Based on low diagnostic yield and high radiation dose associated with standard neck CT angiography for evaluating oropharyngeal trauma, a targeted protocol is recommended, resulting in significantly less dose to the neck, while preserving diagnostic yield.

Method of determining geometric patient size surrogates using localizer images in CT.

PURPOSE: Size-specific dose estimates (SSDE) requires accurate estimates of patient size surrogates. AAPM Report 204 shows that the SSDE is the product of CTDIvol and a scaling factor, the normalized dose coefficient (NDC) which depends on patient size surrogates for CT axial images. However, SSDE can be determined from CT localizer prior to CT scanning. AAPM Report 220 charges that a magnification correction is needed for geometric patient size-surrogates. In this study, we demonstrate a novel "model-based" magnification correction on patient data. METHODS: 573 patient scans obtained from a clinical CT system including 229 adult abdomen, 284 adult chest, 48 pediatric abdomen, and 12 pediatric chest exams. LAT and AP dimensions were extracted from CT localizers using a threshold extraction method (the ACR DIR). The model-based magnification correction was applied to the AP and LAT dimensions extracted using the ACR DIR. NDC was calculated using the effective diameter for the ACR DIR only, the model-based localizer-based and axial-based approaches. The LAT and AP dimensions were extracted from the "gold" standard CT axial scans. Outliers are defined as points outside the 95% confidence intervals and were analyzed. RESULTS: NDC estimates for the localizer-based model-based approach had an excellent correlation (R2  = 0.92) with the gold standard approach. The effective diameter for ACR DIR and model-based approaches are 8.0% and 1.0% greater than the gold standard respectively. Outliers were determined to be primarily patient truncation, with arms down or with devices. ACR DIR size extraction method fails for bariatric patients where the threshold is too high and some of their anatomy was included in the CT couch, and small patients due to the CT couch being included in the size measurement. CONCLUSION: The model-based magnification method gives an accurate estimate of patient size surrogates extracted from CT localizers that are needed for calculating NDC to achieve accurate SSDE.

Prototype system for interventional dual-energy subtraction angiography.

Dual-energy subtraction angiography (DESA) using fast kV switching has received attention for its potential to reduce misregistration artifacts in thoracic and abdominal imaging where patient motion is difficult to control; however, commercial interventional solutions are not currently available. The purpose of this work was to adapt an x-ray angiography system for 2D and 3D DESA. The platform for the dual-energy prototype was a commercially available x-ray angiography system with a flat panel detector and an 80 kW x-ray tube. Fast kV switching was implemented using custom x-ray tube control software that follows a user-defined switching program during a rotational acquisition. Measurements made with a high temporal resolution kV meter were used to calibrate the relationship between the requested and achieved kV and pulse width. To enable practical 2D and 3D imaging experiments, an automatic exposure control algorithm was developed to estimate patient thickness and select a dual-energy switching technique (kV and ms switching) that delivers a user-specified task CNR at the minimum air kerma to the interventional reference point. An XCAT-based simulation study conducted to evaluate low and high energy image registration for the scenario of 30-60 frame/s pulmonary angiography with respiratory motion found normalized RMSE values ranging from 0.16% to 1.06% in tissue-subtracted DESA images, depending on respiratory phase and frame rate. Initial imaging in a porcine model with a 60 kV, 10 ms, 325 mA / 120 kV, 3.2 ms, 325 mA switching technique demonstrated an ability to form tissue-subtracted images from a single contrast-enhanced acquisition.

Technical Note: Model-based magnification/minification correction of patient size surrogates extracted from CT localizers.

PURPOSE: Patient size-specific dose estimate (SSDE) calculations require knowledge of a patient's size. Errors in patient size propagate through SSDE calculations. AAPM Reports 204 and 220 recommend that a magnification correction be applied to patient size surrogates extracted from CT localizer radiographs. This technical note presents a novel approach for such a magnification correction. METHODS: In our model-based magnification correction, we assume that the patient's cross sections are elliptical with minor and major axes defined using the anterior-posterior (AP) and lateral (LAT) patient dimensions. We parameterize the problem by modeling a line emanating from the source, grazing the patient (i.e., the ellipse), and then terminating onto the detector plane. We model tangent lines on each side of the ellipse on both the LAT and AP CT localizer radiographs. We also account for vertical mispositioning with table offset. We compared our correction model to the actual AP and LAT dimensions to the vendor-supplied CT localizer images that only received a geometric magnification correction, and to other methods described in the literature. We compare our model to the others using direct size to size comparisons as well as SSDE conversion factor. RESULTS: Our model-based method provides consistent accurate results (less than 1.8% error for absolute size and 1.2% error for SSDE for all measurement conditions) for all positions and patient sizes. Existing literature-based methods had maximum errors for absolute size and SSDE of 7.5% and 5.2%, and for the vendor, they were 30.9% and 17.0%, respectively. CONCLUSION: We presented a new model-based geometric size correction method that outperforms a simple geometric correction as well as other methods presented in the literature. By modeling the patient cross section and beam geometry using information all derived from the DICOM header and CT localizer views, we demonstrated SSDE correction factor improvements from 17.0% (vendor correction) to 1.2% (model base). These changes correspond directly into changes in SSDE itself and also represent clinically realistic patient sizes and mispositioning amounts.

Evaluation of AAPM Reports 204 and 220: Estimation of effective diameter, water-equivalent diameter, and ellipticity ratios for chest, abdomen, pelvis, and head CT scans.

PURPOSE: To confirm AAPM Reports 204/220 and provide data for the future expansion of these reports by: (a) presenting the first large-scale confirmation of the reports using clinical data, (b) providing the community with size surrogate data for the head region which was not provided in the original reports, and additionally providing the measurements of patient ellipticity ratio for different body regions. METHOD: A total of 884 routine scans were included in our analysis including data from the head, thorax, abdomen, and pelvis for adults and pediatrics. We calculated the ellipticity ratio and all of the size surrogates presented in AAPM Reports 204/220. We correlated the purely geometric-based metrics with the "gold standard" water-equivalent diameter (DW ). RESULTS: Our results and AAPM Reports 204/220 agree within our data's 95% confidence intervals. Outliers to the AAPM reports' methods were caused by excess gas in the GI tract, exceptionally low BMI, and cranial metaphyseal dysplasia. For the head, we show lower correlation (R2 = 0.812) between effective diameter and DW relative to other body regions. The ellipticity ratio of the shoulder region was the highest at 2.28 ± 0.22 and the head the smallest at 0.85 ± 0.08. The abdomen pelvis, chest, thorax, and abdomen regions all had ellipticity values near 1.5. CONCLUSION: We confirmed AAPM reports 204/220 using clinical data and identified patient conditions causing discrepancies. We presented new size surrogate data for the head region and for the first time presented ellipticity data for all regions. Future automatic exposure control characterization should include ellipticity information.

Energy subtraction angiography is comparable to digital subtraction angiography in terms of iodine Rose SNR.

PURPOSE: X-ray digital subtraction angiography (DSA) is widely used for vascular imaging. However, motion artifacts render it largely unsuccessful for some applications including cardiac imaging. Dual-energy imaging using fast kV switching was proposed in the past to provide the benefits of DSA with fewer motion artifacts, but image quality was inferior to DSA. This study compares the iodine Rose SNR that can be achieved using dual-energy methods, called energy-subtraction angiography (ESA), with that of DSA and examines the technical conditions required to achieve near-optimal SNR. METHODS: A Rose SNR model is described, experimentally validated, and used to compare ESA with DSA. The model considers detector quantum efficiency, readout noise (quantum-limit exposure), and scatter-to-primary ratio. RESULTS: The theoretical Rose SNR showed excellent agreement with experimental results for both ESA and DSA images, and shows that near-optimal SNR is harder to achieve with ESA than DSA. In comparison to DSA, ESA requires: (1) high detector quantum efficiency at a higher energy (120 kV); (2) lower detector readout noise by a factor of four (approximately 0.005 µGy air KERMA or lower); and (3) lower scatter-to-primary ratio by a factor of three (approximately 0.05 or lower). These conditions were not achievable in the past, and remain difficult but not impossible to achieve at present. CONCLUSIONS: ESA and DSA can provide similar iodine Rose SNR for the same patient exposure, but only when satisfying the above conditions. This may explain why dual-energy methods have been unsuccessful in the past and suggests ESA methods may offer a viable alternative to DSA when implemented under optimal conditions.