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Estimating organ dose for clinical patients requires accurate modeling of the patient anatomy and the dose field of the CT exam. The modeling of patient anatomy can be achieved using a library of representative computational phantoms (Samei et al 2014 Pediatr. Radiol. 44 460-7). The modeling of the dose field can be challenging for CT exams performed with a tube current modulation (TCM) technique. The purpose of this work was to effectively model the dose field for TCM exams using a convolution-based method. A framework was further proposed for prospective and retrospective organ dose estimation in clinical practice. The study included 60 adult patients (age range: 18-70 years, weight range: 60-180 kg). Patient-specific computational phantoms were generated based on patient CT image datasets. A previously validated Monte Carlo simulation program was used to model a clinical CT scanner (SOMATOM Definition Flash, Siemens Healthcare, Forchheim, Germany). A practical strategy was developed to achieve real-time organ dose estimation for a given clinical patient. CTDIvol-normalized organ dose coefficients ([Formula: see text]) under constant tube current were estimated and modeled as a function of patient size. Each clinical patient in the library was optimally matched to another computational phantom to obtain a representation of organ location/distribution. The patient organ distribution was convolved with a dose distribution profile to generate [Formula: see text] values that quantified the regional dose field for each organ. The organ dose was estimated by multiplying [Formula: see text] with the organ dose coefficients ([Formula: see text]). To validate the accuracy of this dose estimation technique, the organ dose of the original clinical patient was estimated using Monte Carlo program with TCM profiles explicitly modeled. The discrepancy between the estimated organ dose and dose simulated using TCM Monte Carlo program was quantified. We further compared the convolution-based organ dose estimation method with two other strategies with different approaches of quantifying the irradiation field. The proposed convolution-based estimation method showed good accuracy with the organ dose simulated using the TCM Monte Carlo simulation. The average percentage error (normalized by CTDIvol) was generally within 10% across all organs and modulation profiles, except for organs located in the pelvic and shoulder regions. This study developed an improved method that accurately quantifies the irradiation field under TCM scans. The results suggested that organ dose could be estimated in real-time both prospectively (with the localizer information only) and retrospectively (with acquired CT data).

Purpose: With the increasing clinical use of shift-variant CT protocols involving tube current modulation (TCM), variable pitch or pitch modulation (PM), and variable aperture a(t), the interpretation of the scanner-reported CTDI(vol) is called into question. This was addressed for TCM in their previous paper published by Dixon and Boone [Med. Phys. 40, 111920 (14pp.) (2013)] and is extended to PM and concurrent TCM/PM as well as variable aperture in this work., Methods: Rigorous convolution equations are derived to describe the accumulated dose distributions for TCM, PM, and concurrent TCM/PM. A comparison with scanner-reported CTDI(vol) formulae clearly identifies the source of their differences with the traditional CTDI(vol). Dose distribution simulations using the convolution are provided for a variety of TCM and PM scenarios including a helical shuttle used for perfusion studies (as well as constant mA)-all having the same scanner-reported CTDI(vol). These new convolution simulations for TCM are validated by comparison with their previous discrete summations., Results: These equations show that PM is equivalent to TCM if the pitch variation p(z) is proportional to 1/i(z), where i(z) is the local tube current. The simulations show that the local dose at z depends only weakly on the local tube current i(z) or local pitch p(z) due to scatter from all other locations along z, and that the "local CTDI(vol)(z)" or "CTDI(vol) per slice" do not represent a local dose but rather only a relative i(z) or p(z). The CTDI-paradigm does not apply to shift-variant techniques and the scanner-reported CTDI(vol) for the same lacks physical significance and relevance., Conclusions: While the traditional CTDI(vol) at constant tube current and pitch conveys useful information (the peak dose at the center of the scan length), CTDI(vol) for shift-variant techniques (TCM or PM) conveys no useful information about the associated dose distribution it purportedly represents. On the other hand, the total energy absorbed E ("integral dose") as well as its surrogate DLP remain robust (invariant) with respect to shift-variance, depending only on the total mAs = 〈i〉t0 accumulated during the total beam-on time t0 and aperture a, where 〈i〉 is the average current.

Purpose: Anomalous, scanner-reported values of CTDIvol for stationary phantom/table protocols (having elevated values of CTDIvol over 300% higher than the actual dose to the phantom) have been observed; which are well-beyond the typical accuracy expected of CTDIvol as a phantom dose. Recognition of these outliers as "bad data" is important to users of CT dose index tracking systems (e.g., ACR DIR), and a method for recognition and correction is provided., Methods: Rigorous methods and equations are presented which describe the dose distributions for stationary-table CT. A comparison with formulae for scanner-reported values of CTDIvol clearly identifies the source of these anomalies., Results: For the stationary table, use of the CTDI100 formula (applicable to a moving phantom only) overestimates the dose due to extra scatter and also includes an overbeaming correction, both of which are nonexistent when the phantom (or patient) is held stationary. The reported DLP remains robust for the stationary phantom., Conclusions: The CTDI-paradigm does not apply in the case of a stationary phantom and simpler nonintegral equations suffice. A method of correction of the currently reported CTDIvol using the approach-to-equilibrium formula H(a) and an overbeaming correction factor serves to scale the reported CTDIvol values to more accurate levels for stationary-table CT, as well as serving as an indicator in the detection of "bad data."

Purpose: The scanner-reported CTDI(vol) for automatic tube current modulation (TCM) has a different physical meaning from the traditional CTDI(vol) at constant mA, resulting in the dichotomy "CTDI(vol) of the first and second kinds" for which a physical interpretation is sought in hopes of establishing some commonality between the two., Methods: Rigorous equations are derived to describe the accumulated dose distributions for TCM. A comparison with formulae for scanner-reported CTDI(vol) clearly identifies the source of their differences. Graphical dose simulations are also provided for a variety of TCM tube current distributions (including constant mA), all having the same scanner-reported CTDI(vol)., Results: These convolution equations and simulations show that the local dose at z depends only weakly on the local tube current i(z) due to the strong influence of scatter from all other locations along z, and that the "local CTDI(vol)(z)" does not represent a local dose but rather only a relative i(z) ≡ mA(z). TCM is a shift-variant technique to which the CTDI-paradigm does not apply and its application to TCM leads to a CTDI(vol) of the second kind which lacks relevance., Conclusions: While the traditional CTDI(vol) at constant mA conveys useful information (the peak dose at the center of the scan length), CTDI(vol) of the second kind conveys no useful information about the associated TCM dose distribution it purportedly represents and its physical interpretation remains elusive. On the other hand, the total energy absorbed E ("integral dose") as well as its surrogate DLP remain robust between variable i(z) TCM and constant current i0 techniques, both depending only on the total mAs = {i}t0 = i0t0 during the beam-on time t0.

Purpose: Knowledge of the complete axial dose profile f(z), including its long scatter tails, provides the most complete (and flexible) description of the accumulated dose in CT scanning. The CTDI paradigm (including CTDIvol) requires shift-invariance along z (identical dose profiles spaced Sat equal intervals), and is therefore inapplicable to many of the new and complex shift-variant scan protocols, e.g., high dose perfusion studies using variable (or zero) pitch. In this work, a convolustion-based beam model developed by Dixon et al. [Med. Phys. 32, 3712-3728, (2005)] updated with a scatter LSF kernel (or DSF) derived from a Monte Carlo simulation by Boone [Med. Phys. 36, 4547-4554 (2009)] is used to create an analytical equation for the axial dose profile f(z) in a cylindrical phantom. Using f(z), equations are derived which provide the analytical description of Sconventional (axial and helical) dose, demonstrating its physical underpinnings; and likewise for the peak axial dose f(0) appropriate to stationary phantom cone beam CT, (SCBCT). The methodology can also be applied to dose calculations in shift-variant scan protocols. This paper is an extension of our recent work Dixon and Boone [Med. Phys. 37, 2703-2718 (2010)], which dealt only with the properties of the peak dose f(0), its relationship to CTDI, and its appropriateness to SCBCT., Methods: The experimental beam profile data f(z) of Mori et al. [Med. Phys. 32, 1061-1069 (2005)] from a 256 channel prototype cone beam scanner for beam widths (apertures) ranging from a = 28 to 138 mm are used to corroborate the theoretical axial profiles in a 32 cm PMMA body phantom., Results: The theoretical functions f(z) closely-matched the central axis experimental profile data for all apertures (a = 28 -138 mm). Integration of f(z) likewise yields analytical equations for all the (CTDI-based) dosimetric quantities of conventional CT (including CTDIL itself) in addition to the peak dose f(0) relevant to SCBCT (allowing direct cross-comparison between CT scan modes and mathematical proofs of several hypotheses of practical utility in CT dosimetry). A fast, analytical dose simulator6 is also demonstrated-successfully matching complex dose distributions measured using OSL and film dosimetry., Conclusions: The model described allows one to obtain analytical functions describing both the primary and scatter components of the axial dose profile. This model (using no empirical functions or adjustable fit parameters) provides a good match to the experimental data, as well as a complete analytical description of dose for both conventional (axial and helical) CT and cone beam CT. An efficient method whereby the complete data set for both modalities can be obtained from a single measurement of either CTDI100 or f(0) is illustrated. This method is also flexible--allowing calculation of heretofore unattainable doses for recently-introduced shift-variant protocols [e.g., variable pitch (irregular scan spacing), variable aperture, shuttle mode acquisition, and mA modulation schemes].

Computed tomography (CT) scanning is now the standard imaging technique for detecting and characterizing urolithiasis. However, the potential effects of the radiation exposure from CT examinations have raised concern among some physicians. This article describes the radiation doses for various urolithiasis-imaging techniques and describes techniques to optimize and reduce radiation exposures to patients undergoing these examinations. Potential future directions in radiologic techniques used for studying urolithiasis are also discussed.

This article is an experimental demonstration and authentication of a new method of computed tomography dosimetry [R. L. Dixon, Med. Phys. 30, 1272-1280 (2003)], which utilizes a short, conventional ion chamber rather than a pencil chamber, and which is more versatile than the latter. The value of CTDI100 correctly predicts the accumulated dose only for a total scan length L equal to 100 mm and underestimates the limiting equilibrium dose approached for longer, clinically relevant body scan lengths [R. L. Dixon, Med. Phys. 30, 1272-1280 (2003); K. D. Nakonechny, B. G. Fallone, and S. Rathee, Med. Phys. 32, 98-109 (2005); S. Mori, M. Endo, K. Nishizawa, T. Tsunoo, T. Aoyama, H. Fujiwara, and K. Murase, Med. Phys. 32, 1061-1069 (2005); R. L. Dixon, M. T. Munley, and E. Bayram, Med. Phys. 32, 3712-3728 (2005); R. L. Dixon, Med. Phys. 33, 3973-3976 (2006)]. Dixon [Med. Phys. 30, 1272-1280 (2003)] originally proposed an alternative using a short ion chamber and a helical scan acquisition to collect the same integral for any scan length L (and not limited 100 mm). The primary purpose of this work is to demonstrate experimentally the implementation, robustness, and versatility of this small ion chamber method in measuring the accumulated dose in the body phantom for any desired scan length L (up to the available phantom length) including the limiting equilibrium dose (symbolically CTDIinfinity), and validation of the method against the pencil chamber methodology. Additionally, a simple and robust method for independently verifying the active length of a pencil chamber is described. The results of measurements made in a 400 mm long, 32 cm diameter polymethylmethacrylate body phantom using a small Farmer-type ion chamber and two pencil chambers of lengths l=100 and 150 mm confirm that the two methodologies provide the same dose values at the corresponding scan lengths L=l. The measured equilibrium doses obtained for GE MDCT scanners at 120 kVp are CTDIinfinity = 1.75 CTDI100 on the central axis and 1.22 CTDI100 on the peripheral axes, illustrating a nontrivial shortfall of CTDI100 in that regard and in good agreement with comparable data [S. Mori, M. Endo, K. Nishizawa, T. Tsunoo, T. Aoyama, H. Fujiwara, and K. Murase, Med. Phys. 32, 1061-1069 (2005); J. M. Boone, Med. Phys. 34, 1364-1371 (2007)].

Increased volume and density of the skull makes computed tomography differentiation of gray and white matter (GM and WM, respectively) more difficult. The purpose of this investigation was to study the effects of skull volume and bone density on GM and WM differentiation. A total of 21 patients with thick skulls and 22 controls were included in this study. Three consecutive slices from the computed tomography scan were analyzed. The basal ganglia had to be visualized on at least 1 slice. Calvarial volume measurement, mean pixel value in each slice, and Hounsfield unit difference between WM and GM, were compared between the thick-skulled and control groups. The mean bone volume of each slice in the thick-skulled group was 55.7, 54.3, and 56 mL, whereas the mean volume of each slice in the normal group was 39.3, 38.5, and 39.9 mL (P < 0.001). In our series, patients with thick skulls had 41% more bone volume than the normal group. The mean skull pixel value in each slice was 935.9 in patients with thick skulls and 987 in patients in the normal group. There was no difference between right and left sides of the same group of patients. Patients with larger volumes of skull have significant decrease in the Hounsfield unit of the GM and WM compared with the control group. As a result, diagnosing any low-contrast brain abnormality including early/subtle infarction in subjects with a thicker calvarium may be more difficult.

The use of noncontrast helical CT (NHCT) to assess patients with acute flank pain and hematuria for potential urinary tract stone disease was first reported in 1995. After several years of experience with the technique, sensitivity and specificity of NHCT has proven to be better than intravenous urography for evaluating ureteral stones. NHCT imaging findings for urinary calculi and the differential diagnosis are discussed in this article. Various extraurinary diseases found while using NHCT in searching for stone disease are addressed and illustrated. As experience with the use of NHCT has increased, clinicians have broadened the indications for this technique, which has a lower charge than standard CT, beyond the specific evaluation of urinary colic. This indication creep has increased the number of NHCT examinations ordered. It has also reduced the rate of stone positivity and increased the diagnostic yield for extraurinary disease.

The authors describe a film dosimetry system for use in calculating the surface dose delivered by a CT scanner. Kodak XV-2 film is wrapped around a cylindrical water-filled phantom and the dose distribution is recorded. This system is easier to use than thermoluminescent dosimetry (TLD) and provides a detailed map of the dose distribution. Comparison with TLD measurements for a variety of CT scanners indicates that an accuracy of +/-15% can be achieved using this system. Dose distributions obtained with several scanners are shown.