Optimal kV Research Articles

Optimal Spectral Performance on Pediatric Photon-Counting CT: Investigating Phantom-Based Size-Dependent kV Selection for Spectral Body Imaging.

The comprehensive evaluation of kV selection on photon-counting computed tomography (PCCT) has yet to be performed. The aim of the study is to evaluate and determine the optimal kV options for variable pediatric body sizes on the PCCT unit. In this study, 4 phantoms of variable sizes were utilized to represent abdomens of newborn, 5-year-old, 10-year-old, and adult-sized pediatric patients. One solid water and 4 solid iodine inserts with known concentrations (2, 5, 10, and 15 mg I/mL) were inserted into phantoms. Each phantom setting was scanned on a PCCT system (Siemens Alpha) with 4 kV options (70 and 90 kV under Quantum Mode, 120 and 140 kV under QuantumPlus Mode) and clinical dual-source (3.0 pitch) protocol. For each phantom setting, radiation dose (CTDIvol) was determined by clinical dose settings and matched for all kV acquisitions. Sixty percent clinical dose images were also acquired. Reconstruction was matched across all acquisitions using Qr40 kernel and QIR level 3. Virtual monoenergetic images (VMIs) between 40 and 80 keV with 10 keV interval were generated on the scanner. Low-energy and high-energy images were reconstructed from each scan and subsequently used to generate an iodine map (IM) using an image-based 2-material decomposition method. Image noise of VMIs from each kV acquisition was calculated and compared between kV options. Absolute percent error (APE) of iodine CT number accuracy in VMIs was calculated and compared. Root mean square error (RMSE) and bias of iodine quantification from IMs were compared across kV options. At the newborn size and 50 keV VMI, noise is lower at low kV acquisitions (70 kV: 10.5 HU, 90 kV: 10.4 HU), compared with high kV acquisitions (120 kV: 13.8 HU, 140 kV: 13.9 HU). At the newborn size and 70 keV VMI, the image noise from different kV options is comparable (9.4 HU for 70 kV, 8.9 HU for 90 kV, 9.7 HU for 120 kV, 10.2 HU for 140 kV). For APE of VMI, high kV (120 or 140 kV) performed overall better than low kV (70 or 90 kV). At the 5-year-old size, APE of 90 kV (median: 3.6%) is significantly higher (P < 0.001, Kruskal-Wallis rank sum test with Bonferroni correction) than 140 kV (median: 1.6%). At adult size, APE of 70 kV (median: 18.0%) is significantly higher (P < 0.0001, Kruskal-Wallis rank sum test with Bonferroni correction) than 120 kV (median: 1.4%) or 140 kV (median: 0.8%). The high kV also demonstrated lower RMSE and bias than the low kV across all controlled conditions. At 10-year-old size, RMSE and bias of 120 kV are 1.4 and 0.2 mg I/mL, whereas those from 70 kV are 1.9 and 0.8 mg I/mL. The high kV options (120 or 140 kV) on the PCCT unit demonstrated overall better performance than the low kV options (70 or 90 kV), in terms of image quality of VMIs and IMs. Our results recommend the use of high kV for general body imaging on the PCCT.

Visual and Quantitative Evaluation of Low-Concentration Bismuth in Dual-Contrast Imaging of Iodine and Bismuth Using Clinical Photon-Counting CT.

Simultaneous dual-contrast imaging of iodine and bismuth has shown promise in prior phantom and animal studies utilizing spectral CT. However, it is noted that in previous studies, Pepto-Bismol has frequently been employed as the source of bismuth, exceeding the recommended levels for human subjects. This investigation sought to assess the feasibility of visually differentiating and precisely quantifying low-concentration bismuth using clinical dual-source photon-counting CT (PCCT) in a scenario involving both iodinated and bismuth-based contrast materials. Four bismuth samples (0.6, 1.3, 2.5, and 5.1 mg/mL) were prepared using Pepto-Bismol, alongside three iodine rods (1, 2, and 5 mg/mL), inserted into multi-energy CT phantoms with three different sizes, and scanned on a PCCT system at three tube potentials (120, 140, and Sn140 kV). A generic image-based three-material decomposition method generated iodine and bismuth maps, with mean mass concentrations and noise levels measured. The root-mean-square errors for iodine and bismuth determined the optimal tube potential. The tube potential of 140 kV demonstrated optimal quantification performance when both iodine and bismuth were considered. Distinct differentiation of iodine rods with all three concentrations and bismuth samples with mass concentrations ≥ 1.3 mg/mL was observed across all phantom sizes at the optimal kV setting.

Optimisation of imaging parameters during CT guided interventional procedures in patients with total hip arthroplasty

ObjectiveTo determine the optimal KV and mAs settings during CT guided injections in the presence of a total hip arthroplasty so that image quality is maintained whist keeping the dose as low as reasonably achievable. MethodsA total hip arthroplasty phantom with needles with differing gauges was scanned using different CT parameters (from low dose to high dose) and evaluating if this had any effect on needle conspicuity. Conspicuity was graded from 1 to 3 by 2 independent blinded reviewers. ResultsIrrespective of the CT settings used (high dose or low dose parameters) needle conspicuity was not adversely affected by the THA for either scorer, therefore a kVp of 100 mA and a. In addition the needle gauge did not affect the conspicuity of the needle. ConclusionCT guided injections in this total hip arthroplasty phantom model can be performed without any adverse effect on the conspicuity of the needle tip on low dose CT settings. Advancement in knowledgeThis paper enables one to optimise the kV and mA while performing interventional procedures.

Evaluation of optimal kilovoltage-cone beam technique on image quality, registration accuracy, time of imaging and relative dose for head radiotherapy: A phantom study

Background: Image-guided radiation therapy (IGRT) has improved geometry accuracy of patient positioning in radiotherapy. Nowadays, a kilo-voltage cone-beam computed tomography (kV-CBCT) is widely applied in IGRT to confirm daily patient positioning. However, one problem with IGRT is the increased dose to normal tissue outside the target area. Reduction in the dose of kV-CBCT verification imaging leads to deterioration in image quality and registration results. Objectives: The objective of this study was to evaluate an optimal kV cone-beam technique on image quality, registration accuracy, relative dose, and imaging time using different combinations of angular range, angular separation, and mA (in total eight protocols) in the head phantom. Materials and methods: Catphan® 503 phantom was used to evaluate image quality and a PIXY Anthropomorphic Training/Teaching Phantom was used to verify image registration accuracy. The kV-CBCT imaging was performed using an X-ray volumetric imaging (XVI) system mounted on an Elekta Versa HD linear accelerator. The absorbed dose of kV-CBCT imaging was determined using the head phantom with an ionization chamber. The eight protocols were analyzed for image quality, image registration, relative dose change, and imaging delivery time. Results: Image quality parameter results showed that maximum contrast to noise ratio (CNR) increased by approximately 65% at 100 kV, 20 mA, θ=360°, and Δθ=0.54°, while uniformity compared with 100 kV, 10 mA, θ=200°, Δθ=0.54° (default protocol) varied within 7%. The spatial resolution showed no change (0.167 cm), with geometric distortions of less than 0.2 mm. Image registration errors were within 0.2 cm, with the highest magnitude of error seen in the vertical direction. Imaging dose can be reduced using 100 kV, 10 mA, θ=200°, Δθ=1.09° about 50% and 100 kV, 10 mA, θ=360°, Δθ=1.09° about 15% with similar CNR of default protocol. Imaging time decreased by approximately 2-folds, while Δθ increased by 2-folds. Conclusion: The suggested protocols suitable for optimal kV-CBCT image quality, accuracy of image registration, decreased imaging time, and reduced image dose in the head region were 1) 10 mA, θ=200°, Δθ=1.09° 2) 10 mA, θ=360°, Δθ=1.09°, and 3) 20 mA, θ=360°, Δθ=1.09°. These protocols decreased imaging dose about 50%, 15%, and 2%, respectively. The developing kV-CBCT technique should be counterbalanced by careful consideration of imaging dose, image quality, imaging time, and accuracy of image registration.

Optimization dose and image quality enhancement of ct scan with back projection filters on the use of automatic exposure control

Dose and image quality are two important parameters in CT Scan examination. The radiation dose produced at the CT scan is far greater than the radiation dose of the conventional radiological examination, so the utilization must be as good as possible. On Siemens CT Scan there is an automatic exposure control software, with expectations that the radiation dose received by the patient can be reduced because the software is an auto mA that is adjusted to the thickness of the object, but the impact of using the software causes a decrease in image quality, so it needs to be done image improvement using a back projection filter (BPF). Experimental research by applying a back projection filter to the Scan Head image on axial sections. Image assessment using 2 radiologists with data analysis using statistical processing and image post processing. CT Scan image information Head axial pieces experience differences in image information between before and after the back projection, with the mean value in each group experiencing an increase in image quality in the group after a back projection filter. Where the image after the back projection filter by applying automatic exposure control with kV 110 is the most optimal kV.

Image quality optimization using a narrow vertical detector dental cone-beam CT.

To determine the optimized kV setting for a narrow detector cone-beam CT (CBCT) unit. Clinical (CL) and quantitative (QUANT) evaluations of image quality were performed using an anthropomorphic phantom. Technical (TECH) evaluation was performed with a polymethyl methacrylate phantom. Images were obtained using a PaX-i3D Green CBCT (Vatech, Hwaseong, Korea) device, with a large 21 × 19 and a medium 12 × 9 cm field of view (FOV), and high-dose (HD-ranging from 85 to 110 kV) and low-dose (LD-ranging from 75 to 95 kV) protocols, totaling four groups (21 × 19 cm HD, 21 × 19 cm LD, 12 × 9 cm HD, 12 × 9 cm LD). The radiation dose within each group was fixed by adapting the mA according to a predetermined dose-area product. For CL evaluation, three observers assessed images based on overall quality, sharpness, contrast, artefacts, and noise. For QUANT evaluation, mean gray value shift, % increase of standard deviation (SD), % of beam hardening and contrast-to-noise ratio (CNR) were calculated. For TECH evaluation, segmentation accuracy, CNR, metal artefact SD, metal object area, and sharpness were measured. Representative parameters were chosen for CL, QUANT, and TECH evaluations to determine the optimal kV based on biplot graphs. kV values of the same protocol were compared by the bootstrapping approach. The ones that had statistical differences with the best kV were considered as worse quality. Overall, kV values within the same group showed similar quality (p > 0.05), except for 110 kV in 21 × 19 cm HD and 85 kV in 12 × 9 cm HD of CL score; also 85, 90 kV in 21 × 19 cm HD and 75, 80 kV in 21 × 19 cm LD of QUANT score which were worse (p < 0.05). At a constant dose, low and high kV protocols yield acceptable image quality for a narrow-detector CBCT unit.

Impact of Photon Counting Detector Technology on kV Selection and Diagnostic Workflow in CT.

The purpose of this study is to determine the optimal iodine contrast-to-noise ratio (CNR) achievable for different patient sizes using virtual-monoenergetic-images (VMIs) and a universal acquisition protocol on photon-counting-detector CT (PCD-CT), and to compare results to those from single-energy (SE) and dual-source-dual-energy (DSDE) CT. Vials containing 3 concentrations of iodine were placed in torso-shaped water phantoms of 5 sizes and scanned on a 2nd generation DSDE scanner with both SE and DE modes. Tube current was automatically adjusted based on phantom size with CTDIvol ranging from 5.1 to 22.3 mGy. PCD-CT scans were performed at 140 kV, 25 and 75 keV thresholds, with CTDIvol matched to the SE scans. DE VMIs were created and CNR was calculated for SE images and DE VMIs. The optimal kV (SE) or keV (DE VMI) was chosen at the point of highest CNR with no noticeable artifacts. For 10 mgI/cc vials in the 35 cm phantom, the optimal CNR of VMIs on PCD (22.6@50keV) was comparable to that of the best DSDE protocol (23.9@50keV) and was higher than that of the best SE protocol (19.7@80kV). In general, the difference of optimal CNR between PCD and SE increased with phantom size, with PCD 50 keV VMIs having an equivalent CNR (0.6% difference) with that of SE at the 25 cm phantom and 57% higher CNR at the 45 cm phantom. PCD-CT demonstrated comparable iodine CNR of VMIs to that of DSDE across patient sizes. Whereas SE and DSDE CT exams require use of patient-size-specific acquisitions settings, our findings point to the ability of PCD-CT to simplify protocol selection, using a single VMI keV setting (50 keV), acquisition kV (140 kV), and energy thresholds (25 and 75 keV) for all patient sizes, while achieving optimal or near optimal iodine CNR values.

Statistical model based iterative reconstruction in clinical CT systems. Part III. Task-based kV/mAs optimization for radiation dose reduction.

For a given imaging task and patient size, the optimal selection of x-ray tube potential (kV) and tube current-rotation time product (mAs) is pivotal in achieving the maximal radiation dose reduction while maintaining the needed diagnostic performance. Although contrast-to-noise (CNR)-based strategies can be used to optimize kV/mAs for computed tomography (CT) imaging systems employing the linear filtered backprojection (FBP) reconstruction method, a more general framework needs to be developed for systems using the nonlinear statistical model-based iterative reconstruction (MBIR) method. The purpose of this paper is to present such a unified framework for the optimization of kV/mAs selection for both FBP- and MBIR-based CT systems. The optimal selection of kV and mAs was formulated as a constrained optimization problem to minimize the objective function, Dose(kV,mAs), under the constraint that the achievable detectability index d'(kV,mAs) is not lower than the prescribed value of d'R for a given imaging task. Since it is difficult to analytically model the dependence of d' on kV and mAs for the highly nonlinear MBIR method, this constrained optimization problem is solved with comprehensive measurements of Dose(kV,mAs) and d'(kV,mAs) at a variety of kV-mAs combinations, after which the overlay of the dose contours and d' contours is used to graphically determine the optimal kV-mAs combination to achieve the lowest dose while maintaining the needed detectability for the given imaging task. As an example, d' for a 17 mm hypoattenuating liver lesion detection task was experimentally measured with an anthropomorphic abdominal phantom at four tube potentials (80, 100, 120, and 140 kV) and fifteen mA levels (25 and 50-700) with a sampling interval of 50 mA at a fixed rotation time of 0.5 s, which corresponded to a dose (CTDIvol) range of [0.6, 70] mGy. Using the proposed method, the optimal kV and mA that minimized dose for the prescribed detectability level of d'R=16 were determined. As another example, the optimal kV and mA for an 8 mm hyperattenuating liver lesion detection task were also measured using the developed framework. Both an in vivo animal and human subject study were used as demonstrations of how the developed framework can be applied to the clinical work flow. For the first task, the optimal kV and mAs were measured to be 100 and 500, respectively, for FBP, which corresponded to a dose level of 24 mGy. In comparison, the optimal kV and mAs for MBIR were 80 and 150, respectively, which corresponded to a dose level of 4 mGy. The topographies of the iso-d' map and the iso-CNR map were the same for FBP; thus, the use of d'- and CNR-based optimization methods generated the same results for FBP. However, the topographies of the iso-d' and iso-CNR map were significantly different in MBIR; the CNR-based method overestimated the performance of MBIR, predicting an overly aggressive dose reduction factor. For the second task, the developed framework generated the following optimization results: for FBP, kV = 140, mA = 350, dose = 37.5 mGy; for MBIR, kV = 120, mA = 250, dose = 18.8 mGy. Again, the CNR-based method overestimated the performance of MBIR. Results of the preliminary in vivo studies were consistent with those of the phantom experiments. A unified and task-driven kV/mAs optimization framework has been developed in this work. The framework is applicable to both linear and nonlinear CT systems such as those using the MBIR method. As expected, the developed framework can be reduced to the conventional CNR-based kV/mAs optimization frameworks if the system is linear. For MBIR-based nonlinear CT systems, however, the developed task-based kV/mAs optimization framework is needed to achieve the maximal dose reduction while maintaining the desired diagnostic performance.

TU‐EF‐204‐03: Task‐Based KV and MAs Optimization for Radiation Dose Reduction in CT: From FBP to Statistical Model‐Based Iterative Reconstruction (MBIR)

Purpose:The introduction of the highly nonlinear MBIR algorithm to clinical CT systems has made CNR an invalid metric for kV optimization. The purpose of this work was to develop a task‐based framework to unify kV and mAs optimization for both FBP‐ and MBIR‐based CT systems.Methods:The kV‐mAs optimization was formulated as a constrained minimization problem: to select kV and mAs to minimize dose under the constraint of maintaining the detection performance as clinically prescribed. To experimentally solve this optimization problem, exhaustive measurements of detectability index (d’) for a hepatic lesion detection task were performed at 15 different mA levels and 4 kV levels using an anthropomorphic phantom. The measured d’ values were used to generate an iso‐detectability map; similarly, dose levels recorded at different kV‐mAs combinations were used to generate an iso‐dose map. The iso‐detectability map was overlaid on top of the iso‐dose map so that for a prescribed detectability level d’, the optimal kV‐mA can be determined from the crossing between the d’ contour and the dose contour that corresponds to the minimum dose.Results:Taking d’=16 as an example: the kV‐mAs combinations on the measured iso‐d’ line of MBIR are 80–150 (3.8), 100–140 (6.6), 120–150 (11.3), and 140–160 (17.2), where values in the parentheses are measured dose values. As a Result, the optimal kV was 80 and optimal mA was 150. In comparison, the optimal kV and mA for FBP were 100 and 500, which corresponded to a dose level of 24 mGy. Results of in vivo animal experiments were consistent with the phantom results.Conclusion:A new method to optimize kV and mAs selection has been developed. This method is applicable to both linear and nonlinear CT systems such as those using MBIR. Additional dose savings can be achieved by combining MBIR with this method.This work was partially supported by an NIH grant R01CA169331 and GE Healthcare. K. Li, D. Gomez‐Cardona, M. G. Lubner: Nothing to disclose. P. J. Pickhardt: Co‐founder, VirtuoCTC, LLC Stockholder, Cellectar Biosciences, Inc. G.‐H. Chen: Research funded, GE Healthcare; Research funded, Siemens AX.

SU-E-I-29: Care KV: Dose It Influence Radiation Dose in Non-Contrast Examination of CT Abdomen/pelvis?

Purpose: CARE kV is a tool that automatically recommends optimal kV setting for individual patient for specific CT examination. The use of CARE kV depends on topogram and the user-selected contrast behavior. CARE kV is expected to reduce radiation dose while improving image quality. However, this may work only for certain groups of patients and/or certain CT examinations. This study is to investigate the effects of CARE kV on radiation dose of non-contrast examination of CT abdomen/pelvis. Methods: Radiation dose (CTDIvol and DLP) from patients who underwent abdomen/pelvis non-contrast examination with and without CARE kV were retrospectively reviewed. All patients were scanned in the same scanner (Siemens Somatom AS64). To mitigate any possible influences due to technologists’ unfamiliarity with the CARE kV, the data with CARE kV were retrieved 1.5 years after the start of CARE kV usage. T-test was used for significant difference in radiation dose. Results: Volume CTDIs and DLPs from 18 patients before and 24 patients after the use of CARE kV were obtained in a duration of one month. There is a slight increase in both average CTDIvol and average DLP with CARE kV compared to those without CARE kV (25.52 mGy vs. 22.65 mGy formore » CTDIvol; 1265.81 mGy-cm vs. 1199.19 mGy-cm). Statistically there was no significant difference. Without CARE kV, 140 kV was used in 9 of 18 patients, while with CARE KV, 140 kV was used in 15 of 24 patients. 80kV was not used in either group. Conclusion: The use of CARE kV may save time for protocol optimization and minimize variability among technologists. Radiation dose reduction was not observed in non-contrast examinations of CT abdomen/pelvis. This was partially because our CT protocols were tailored according to patient size before CARE kV and partially because of large size patients.« less

Combined MV + kV inverse treatment planning for optimal kV dose incorporation in IGRT

Despite the existence of real-time kV intra-fractional tumor tracking strategies for many years, clinical adoption has been held back by concern over the excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This work aims to solve this problem by investigating, for the first time, the use of convex optimization tools to optimally integrate this excess kV imaging dose into the MV therapeutic dose in order to make real-time kV tracking clinically feasible. Phase space files modeling both a 6 MV treatment beam and a kV on-board-imaging beam of a commercial LINAC were generated with BEAMnrc, and used to generate dose influence matrices in DOSXYZnrc for ten previously treated lung cancer patients. The dose optimization problem for IMRT, formulated as a quadratic problem, was modified to include additional constraints as required for real-time kV intra-fractional tracking. An interior point optimizer was used to solve the modified optimization problem. It was found that when using large kV imaging apertures during fluoroscopic tracking, combined MV + kV optimization lead to a 0.5%–5.17% reduction in the total number of monitor units assigned to the MV beam due to inclusion of the kV dose over the ten patients. This was accompanied by a reduction of up to 42% of the excess kV dose compared to standard MV IMRT with kV tracking. For all kV field sizes considered, combined MV + kV optimization provided prescription dose to the treatment volume coverage equal to the no-imaging case, yet superior to standard MV IMRT with non-optimized kV fluoroscopic tracking. With combined MV + kV optimization, it is possible to quantify in a patient specific way the dosimetric effect of real-time imaging on the patient. Such information is necessary when substantial kV imaging is performed.

Diagnostic quality of CT pulmonary angiography in pulmonary thromboembolism: A comparison of three different kV values

BackgroundOur purpose was to evaluate the effectiveness of different kilovolt (kV) uses in computed tomography pulmonary angiography (CTPA) in the diagnosis of pulmonary thromboembolism (PTE). We also aimed to establish the optimal kV value and investigate the possibility of obtaining appropriate imaging quality with minimal radiation dose.Material/MethodsWe compared 120, 100, and 80 kV CTPA for 90 patients in whom PTE was clinically considered. The examinations were carried out using a 128 multislice CT device (Definition AS, Siemens Medical Solutions, Forchheim, Germany). Each kV value was used on 30 patients in 3 groups. Patients in all groups were compared with respect to the mean radiation dose they received, pulmonary arterial attenuation values, image quality, and motion artefacts.ResultsWith respect to pulmonary arterial attenuation values, imaging with 80 kV yielded significantly higher values (p<0.05). However, no difference was found between 120 kV, 100 kV, and 80 kV with respect to image quality. Similarly, no significant difference was detected between the groups with respect to pulmonary artery contrasting and motion artefacts. Statistically significant differences were present in DLP values and effective dose among all 3 groups (p<0.001).ConclusionsUsing 80 kV as the low value in CTPA imaging for patients pre-diagnosed with PTE will increase the density of pulmonary arteries and decrease the amount of radiation received.

TU-A-201B-06: Automatic Tube Potential Selection for Radiation Dose Reduction in Abdominal CT: Techniques and Experimental Study

Purpose: The appropriateness of using lower tube potential (kV) is highly dependent on patient size and diagnostic task. We proposed a general strategy that allows automatic adaptation of kV as a function of patient size and diagnostic task. The purpose of this work is to implement this general strategy in adult abdominal CT. Method and Materials: The automatic tool was developed based on a novel image quality index, noise constrained iodine contrast to noise ratio. The noise constraint parameter can be adjusted for each exam type. Seven abdominal phantoms were constructed to represent adult abdomens with lateral dimensions of 25, 30, 35, 40, 45, 50, and 55 cm. Three iodine samples were placed in the central region of the phantoms and scanned on a dual-source CT scanner at all available kVs (80, 100, 120, 140). A database including the measured image noise and sample contrast, phantom size, and CTDIvol was created. The relative dose at each kV was calculated as a function of phantom size and a noise constraint parameter that was adjustable for different exam types. Results: The proposed strategy provided a quantitative way to select the most dose-efficient kV for different patient sizes and exam types. For angiographic exams, the most dose-efficient kVs were 80, 80, 100, 100, 100, 120, and 140 for phantoms from 25 to 55 cm, respectively. For routine contrast-enhanced abdomen exams, the optimal kVs were 100, 100, 100, 100, 120, 120, and 140. For the 55cm phantom size, 140 kV was always the optimal kV. Conclusion: We implemented an automatic kV selection strategy in adult abdominal CT. Dependent on the patient size and exam type, the use of the optimal kV resulted in various dose reduction levels. These results can be directly translated into clinical practice for guiding the kV selection and dose reduction.

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 CNR2 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.

SU‐FF‐I‐06: Determination of Optimal KV for Imaging Iodine and Bone in Computed Tomography

Purpose: To determine the kV that will yield optimal contrast to noise ratio (CNR) at a constant patient dose for iodine/ soft tissue and bone/ soft tissue contrast in head and small body studies. Method and Materials: A head phantom containing water equivalent material and a bone ring to simulate the skull (for head studies) was used to obtain the data. To measure the contrast of iodine to water and bone to water, cylindrical plugs were inserted into the phantom: two plugs contained different dilutions of iodine contrast (Omnipauqe 300mgI/ml), one contained bone equivalent material, and one contained water. This phantom was scanned at kV values ranging from 80 to 140 kV. The contrast was measured as the difference in CT number between the iodine and bone plugs and the water plug. The noise measurements were taken from the water plug. Noise was determined at constant dose (CTDI w), so that the mAs was increased as the kV decreased. Both image noise and image contrast increased as the kV decreased. The relevant parameter for determining optimal kV is the contrast to noise ratio (CNR) at a constant dose as a function of kV. Results: The optimal kV, within the range of 80 to 140 kV, for the imaging of both iodine and bone is 80 kV. Limitations to this kV selection include the possible presence of increased artifacts in bone imaging, and CT scanner limitations in mAs that might not allow low kV with an adequate level of image noise. Conclusion: The common use of 120 to 140 kV in CT scanning may not be optimal for many studies whose primarily objective is the discernment of iodine or bone contrast, such as in brain perfusion studies. The use of lower kV settings may be advantageous in these cases.