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Computing effective dose in cardiac CT

We present a method of estimating effective doses in cardiac CT that accounts for selected techniques (kV mAs(-1)), anatomical location of the scan and patient size. A CT dosimetry spreadsheet (ImPACT CT Patient Dosimetry Calculator) was used to estimate effective doses (E) using ICRP 103 weighting factors for a 70 kg patient undergoing cardiac CT examinations. Using dose length product (DLP) for the same scans, we obtained values of E/DLP for three CT scanners used in cardiac imaging from two vendors. E/DLP ratios were obtained as a function of the anatomical location in the chest and for x-ray tube voltages ranging from 80 to 140 kV. We also computed the ratio of the average absorbed dose in a water cylinder modeling a patient weighing W kg to the corresponding average absorbed dose in a water cylinder equivalent to a 70 kg patient. The average E/DLP for a 16 cm cardiac heart CT scan was 26 microSv (mGy cm)(-1), which is about 70% higher than the current E/DLP values used for chest CT scans (i.e. 14-17 microSv (mGy cm)(-1)). Our cardiac E/DLP ratios are higher because the cardiac region is approximately 30% more radiosensitive than the chest, and use of the ICRP 103 tissue weighting factors increases cardiac CT effective doses by approximately 30%. Increasing the x-ray tube voltage from 80 to 140 kV increases the E/DLP conversion factor for cardiac CT by 17%. For the same incident radiation at 120 kV, doses in 45 kg adults were approximately 22% higher than those in 70 kg adults, whereas doses in 120 kg adults were approximately 28% lower. Accurate estimates of the patient effective dose in cardiac CT should use ICRP 103 tissue weighting factors, and account for a choice of scan techniques (kV mAs(-1)), exposed scan region, as well as patient size.

Organ doses to adult patients for chest CT

PURPOSE The goal of this study was to estimate organ doses for chest CT examinations using volume computed tomography dose index (CTDIvol) data as well as accounting for patient weight. METHODS A CT dosimetry spreadsheet (ImPACT CT patient dosimetry calculator) was used to compute organ doses for a 70 kg patient undergoing chest CT examinations, as well as volume computed tomography dose index (CTDIvol) in a body CT dosimetry phantom at the same CT technique factors. Ratios of organ dose to CTDIvoI (f(organ)) were generated as a function of anatomical location in the chest for the breasts, lungs, stomach, red bone marrow, liver, thyroid, liver, and thymus. Values of f(organ) were obtained for x-ray tube voltages ranging from 80 to 140 kV for 1, 4, 16, and 64 slice CT scanners from two vendors. For constant CT techniques, we computed ratios of dose in water phantoms of differing diameter. By modeling patients of different weights as equivalent water cylinders of different diameters, we generated factors that permit the estimation of the organ doses in patients weighing between 50 and 100 kg who undergo chest CT examinations relative to the corresponding organ doses received by a 70 kg adult. RESULTS For a 32 cm long CT scan encompassing the complete lungs, values of f(organ) ranged from 1.7 (thymus) to 0.3 (stomach). Organs that are directly in the x-ray beam, and are completely irradiated, generally had f(organ), values well above 1 (i.e., breast, lung, heart, and thymus). Organs that are not completely irradiated in a total chest CT scan generally had f(organ) values that are less than 1 (e.g., red bone marrow, liver, and stomach). Increasing the x-ray tube voltage from 80 to 140 kV resulted in modest increases in f(organ) for the heart (9%) and thymus (8%), but resulted in larger increases for the breast (19%) and red bone marrow (21%). Adult patient chests have been modeled by water cylinders with diameters between approximately 20 cm for a 50 kg patient and approximately 28 cm for a 100 kg patient. At constant x-ray techniques, a 50 kg patient is expected to have doses that are approximately 18% higher than those in a 70 kg adult, whereas a 100 kg patient will have doses that are apparoximately 18% lower. CONCLUSIONS We describe a practical method to use CTDI data provided by commercial CT scanners to obtain patient and examination specific estimates of organ dose for chest CT examinations.

RELATIONSHIP BETWEEN RADIOGRAPHIC TECHNIQUES (KILOVOLT AND MILLIAMPERE-SECOND) AND CTDIvol

To investigate the relationship between radiographic techniques (i.e. kilovolt and milliampere-second) and the corresponding volume computed tomography dose index (CTDI(vol)). Data were obtained for CTDI(vol) for head and body phantoms from the ImPACT CT patient dosimetry calculator for 43 scanners from four major vendors of medical imaging equipment (i.e. GE, Philips, Siemens and Toshiba). CTDI(vol) were obtained with the largest X-ray beam width, and using a CT pitch of unity. For each scanner, relative values of CTDI(vol) were also computed as a function of X-ray tube voltage, normalised to unity at 120 kV. The average CTDI(vol) for 43 commercial scanners was 167 + or - 44 microGy (mA s)(-1) for the head phantom and 78 + or - 22 microGy (mA s)(-1) for the body phantom. The 90th percentile CTDI(vol) values are approximately twice the corresponding 10th percentile values for both head and body phantoms. Over the last 20 y, the head phantom CTDI(vol) has increased approximately 50 % and the body phantom CTDI(vol) has increased approximately 90 %. For both, the head phantom and the body phantom, CTDI(vol) is proportional to kilovolt(2.6). CT output must be specified using CTDI(vol) because for a fixed kilovolt and milliampere-second, CT scanner outputs (CTDI(vol)) differ by about a factor of 2. Increasing the X-ray tube voltage from 80 to 140 kV increases CTDI(vol) by about a factor of 4.

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.

X-ray beam filtration, dosimetry phantom size and CT patient dose conversion factors

We examine how the choice of CT x-ray beam filtration and phantom size influences patient dose (D) to computed tomography dose index (CTDI) conversion factors (i.e. D/CTDI). The ratio of head to body phantom CTDI(w) for a defined scan technique is alpha, and the ratio of organ dose when the body filter is changed to the head filter is beta. CTDI and organ doses were obtained using the ImPACT CT patient dosimetry calculator, and values of alpha and beta were determined for 39 CT scanners. The average value of alpha for the 39 CT scanners covering a 20 year period was 1.99 +/- 0.23, but 30% of scanners had alpha values that differed by more than 10% from the average. For GE, the value of alpha has been approximately constant at approximately 2.0. Both Philips and Siemens show a definite upward trend from values well below 2.0 in the early 1990s to well over 2.0 for their latest models. The data for Toshiba show no overall trend with time with half the data points below 2.0 and the remainder above this value. The average value of beta was 1.09 +/- 0.25. All vendors showed a downward trend in the beta parameter, and where the most recent scanners from each vendor had a beta value close to unity. Our results show that average D/CTDI conversion factors for a body phantom/filter combination are typically double those appropriate for a head phantom/filter combination.

Iterative reconstruction in image space (IRIS) and lesion detection in abdominal CT

The purpose of this study was to compare lesion detection in images reconstructed using standard filtered back projection (FBP) with those reconstructed using a new CT reconstruction algorithm called Iterative Reconstruction in Image Space (IRIS). Detection performance was experimentally measured using a 2- AFC software package that computes the lesion intensity corresponding to a detection accuracy of 92% (i.e., I92%). Abdominal images were acquired on a Siemens Somaton Definition Flash CT scanner and reconstructed at four slice thickness values ranging from 1.5 mm to 10 mm. Detection of three lesion sizes was investigated, whose diameters ranged from 5 mm to 10 mm. AFC experiments were performed using FBP and IRIS reconstructed images that were presented to observers in a random manner. For any lesion in a given image, we obtained an Enhancement Factor (EF) defined as the I92% using FBP divided by the corresponding I92% using IRIS. In 9 out of 12 paired results, EF values were significantly greater than 1.0, and in the remaining three cases, EF values were approximately 1.0. EF was independent of CT image slice thickness, with an average value of 1.17 ± 0.12. Values of EF increased with decreasing lesion size, and were about 20% greater for 5 mm lesions than 10 mm lesions. The results of this pilot study show that IRIS improved lesion detection compared to conventional FBP, with an average increase in signal to noise ratio of 17%. For the smallest lesions, improvements in signal to noise ratio approached 30%. Our results suggest that radiation dose reductions of one third might be achievable for abdominal imaging without any loss in signal to noise ratio.

SCAN REGION AND ORGAN DOSES IN COMPUTED TOMOGRAPHY

The purpose of this study was to investigate how the choice of the scanned region affects organ doses in CT. ImPACT CT Patient Dosimetry Calculator (version 1.0) was used to compute absorbed doses to eight organs of interest in medical radiation dosimetry. For 13 dosimetry data sets, the authors calculated the maximum organ dose (D(max)) as well as the corresponding organ dose for a scan with selected length D(L). These data permitted the relative dose (D(r) = D(L)/D(max)) to be determined for varying scan lengths. Computations were performed for a nominal X-ray tube current of 100 mA, a rotation time of 1 s and a CT pitch of 1. The authors also determined values of D(max)/CTDI(vol), where CTDI(vol) is obtained in a 32-cm diameter CT dosimetry phantom using the same radiographic techniques. For each organ, D(r) was independent of the type of scanner, and increased monotonically to unity with increasing scan length. Relative doses for a scan restricted to the organ length ranged from 0.65 D(max) for the bladder to 0.86 D(max) for the lungs. There was good correlation (r = 0.64) between relative organ dose and the corresponding organ length. At 120 kV, the lowest value of D(max)/CTDI(vol) was 1.23 for the breast and the highest was 2.22 for the thyroid. Varying the X-ray tube voltage between 100 and 130 kV results in changes in D(max)/CTDI(vol) of no more than 4 %. CT scans limited to the direct irradiation of an average-sized organ results in an absorbed dose of ~0.75 D(max).

What is the preferred strength setting of the sinogram-affirmed iterative reconstruction algorithm in abdominal CT imaging?

Our primary objective in this study was to determine the preferred strength setting for the sinogram-affirmed iterative reconstruction algorithm (SAFIRE) in abdominal computed tomography (CT) imaging. Sixteen consecutive clinical CT scans of the abdomen were reconstructed by use of traditional filtered back projection (FBP) and 5 SAFIRE strengths: S1–S5. Six readers of differing experience were asked to rank the images on preference for overall diagnostic quality. The contrast-to-noise ratio was not significantly different between SAFIRE S1 and FBP, but increased with increasing SAFIRE strength. For pooled data, S2 and S3 were preferred equally but both were preferred over all other reconstructions. S5 was the least preferred, with FBP the next least preferred. This represents a marked disparity between the image quality based on quantitative parameters and qualitative preference. Care should be taken to factor in qualitative in addition to quantitative aspects of image quality when one is optimizing iterative reconstruction images.

Thyroid Doses and Risks to Adult Patients Undergoing Neck CT Examinations

OBJECTIVE The purpose of this study was to estimate absorbed thyroid dose and consequent cancer risks in adult patients undergoing neck CT examinations. MATERIALS AND METHODS We used data from neck CT examinations of 68 consecutive adult patients to calculate the thyroid dose and estimate the corresponding cancer risk. Age and sex were recorded along with the volume CT dose index (CTDIvol) that was used to perform the examination. CTDIvol values were used to estimate thyroid doses in the mathematic phantom used in the ImPACT patient CT dosimetry calculator. Corresponding doses in patients were estimated by modeling each patients neck as an equivalent cylinder of water and applying correction factors for varying neck size and scanning length and the variation of radiation intensity due to automatic exposure control. RESULTS The mean (± SD) adult patient age was 59 ± 16 years, and the mean equivalent water cylinder diameter used for modeling the patient neck was 19.4 ± 4.2 cm. The average adult patient neck size was about 3 cm larger than the mathematic anthropomorphic phantom (16.5 cm), decreasing the estimated thyroid doses by 15%. Thyroid doses were independent of age and sex, with an average of 50 ± 23 mGy. The average cancer risk for a 20-year-old woman was six times higher than the corresponding risk for a 20-year-old man. Increasing patient age of either sex from 40 to 60 years reduced the cancer risk by approximately an order of magnitude. CONCLUSION Patient sex and age are the most important factors in determining thyroid cancer risk, with the thyroid dose being secondary.

Computation of thyroid doses and carcinogenic radiation risks to patients undergoing neck CT examinations.

The aim of the study was to investigate how differences in patient anatomy and CT technical factors in neck CT impact on thyroid doses and the corresponding carcinogenic risks. The CTDIvol and dose-length product used in 11 consecutive neck CT studies, as well as data on automatic exposure control (AEC) tube current variation(s) from the image DICOM header, were recorded. For each CT image that included the thyroid, the mass equivalent water cylinder was estimated based on the patient cross-sectional area and average relative attenuation coefficient (Hounsfield unit, HU). Patient thyroid doses were estimated by accounting for radiation intensity at the location of the patients thyroid, patient size and the scan length. Thyroid doses were used to estimate thyroid cancer risks as a function of patient demographics using risk factors in BEIR VII. The length of the thyroid glands ranged from 21 to 54 mm with an average length of 42 ± 12 mm. Water cylinder diameters corresponding to the central slice through the patient thyroid ranged from 18 to 32 cm with a mean of 25 ± 5 cm. The average CTDIvol (32-cm phantom) used to perform these scans was 26 ± 6 mGy, but the use of an AEC increased the tube current by an average of 44 % at the thyroid mid-point. Thyroid doses ranged from 29 to 80 mGy, with an average of 55 ± 19 mGy. A 20-y-old female receiving the highest thyroid dose of 80 mGy would have a thyroid cancer risk of nearly 0.1 %, but radiation risks decreased very rapidly with increasing patient age. The key factors that affect thyroid doses in neck CT examinations are the radiation intensity at the thyroid location and the size of the patient. The corresponding patient thyroid cancer risk is markedly influenced by patient sex and age.