Misoperation of CT automatic tube current modulation systems with inappropriate patient centering: phantom studies. AJR Am J Roentgenol. 2009;192:862-865. [PMID: 19304687 DOI: 10.2214/ajr.08.1472]

The influence of miscentering on radiation dose during computed tomography head examinations and the role of localiser orientation: A phantom study

INTRODUCTION

Computed Tomography (CT) chest, abdomen and pelvis research demonstrates a relationship between vertical phantom positioning and radiation dose. Moving the phantom closer or further from the x-ray source results in magnification or minimisation of the localiser. As automatic tube current modulation (ATCM) algorithms use localisers to estimate patient size and calculate required tube current, magnification or minimisation results in the incorrect provision of radiation dose. Radiation dose changes also depend on localiser orientation, changes with anteroposterior (AP) and posteroanterior (PA) localisers demonstrating an inverse relationship. However, within CT head literature often attributes radiation dose changes on impaired function of the bow-tie filter instead. The current study investigated the role of miscentering on ATCM function within CT head, paying particular attention to localiser orientation.

METHODS

Head scanning was performed with an anthropomorphic phantom at the isocentre, alongside ten vertically miscentered positions. This was performed three times, with an AP, PA and lateral localiser. CT dose index values at each miscentered level were compared across conditions.

RESULTS

Vertical miscentering altered radiation dose in both AP and PA conditions, radiation dose linearly increasing (up to 17.05%) when positioning the phantom closer to the x-ray source and decreasing when positioning away (up to -13.13%). Changes across AP and PA conditions demonstrated an inverse relationship. Radiation dose was unaffected in the lateral condition.

CONCLUSIONS

Miscentering during CT head alters ATCM function due to magnification/minimisation of the localiser image, causing ATCM algorithms to misinterpret patient size and miscalculate required tube current.

IMPLICATIONS FOR PRACTICE

Radiographers should be accurate when centering for CT head, avoiding any potential radiation dose changes. Further research into vertical miscentering and image quality during CT head is recommended.

Use of a 3D camera for automated patient positioning for chest-abdomen-pelvis CT scans: Effect on positioning accuracy and patient dose

INTRODUCTION

3D positioning cameras that automate the positioning of patients with respect to the CT isocentre have been developed and are in common use in CT departments. This study aimed to compare the performance of radiographers and a 3D camera system with respect to positioning accuracy and the effect on patient radiation dose for chest-abdomen-pelvis scans.

METHODS

Patient positioning and dose data obtained from a dose management system was evaluated over a two-month period for patients positioned with (CAMon) and without (CAMoff) the positioning camera. Median vertical and lateral offset values were compared between the groups whilst doses were evaluated as a function of patient water equivalent diameter (WED) for the thorax and abdomen-pelvis acquisitions for both cohorts.

RESULTS

Radiographers demonstrated high levels of positioning accuracy, however significant improvements in median vertical offset were identified for the CAMon cohort for both thorax (8 mm vs. 17 mm (p = 0.001)) and abdomen-pelvis (7 mm vs. 16 mm (p = 0.003)) scans. The percentage of patients positioned within 5 mm of the isocentre was 39.0% and 16.1% for the CAMon and CAMoff cohorts. For CAMoff scans, 77.4% of patients were positioned below the isocentre, but this was reduced to 45.8% for CAMon scans. No significant changes in dose as a function of WED were identified related to the camera use (thorax: p = 0.569, abdomen-pelvis: p = 0.760).

CONCLUSION

Use of a 3D camera delivered significant improvements in the accuracy and reproducibility of patient positioning when compared with radiographers.

IMPLICATIONS FOR PRACTICE

Improvements in positioning accuracy were observed at the research site and hence positioning camera use has the potential to become standard practice in CT to help ensure appropriate doses are delivered to patients according to their size.

Evaluation of overexposure risk when there is a space between the subject and the couch in computed tomography: a phantom study

The purpose of this study was to investigate the risk of overexposure associated with automatic tube current modulation (ATCM) and automatic couch height positioning compensation mechanism (AHC) in computed tomography (CT) systems, particularly in scenarios involving a gap between the subject and the couch. Results revealed that when AHC was enabled, CT dose index volume (CTDIvol) increased by approximately 10% at 2.5 cm, 20% at 5.0 cm, and 40% at 10.0 cm gaps compared to close contact conditions. While the AHC function ensures consistent exposure doses and image quality regardless of subject positioning relative to the CT gantry isocenter, the study highlights a potential risk of overexposure when a gap exists between the subject and the couch. These findings offer valuable insights for optimizing CT imaging protocols and underscore the importance of carefully considering subject positioning in clinical practice.

Post-mortem feasibility of dual-energy computed tomography in the detection of bone edema-like lesions in the equine foot: a proof of concept

Introduction

In this proof-of-concept study, the post-mortem feasibility of dual-energy computed tomography (DECT) in the detection of bone edema-like lesions in the equine foot is described in agreement with the gold standard imaging technique, which is magnetic resonance imaging (MRI).

Methods

A total of five equine cadaver feet were studied, of which two were pathological and three were within normal limits and served as references. A low-field MRI of each foot was performed, followed by a DECT acquisition. Multiplanar reformations of DECT virtual non-calcium images were compared with MRI for the detection of bone edema-like lesions. A gross post-mortem was performed, and histopathologic samples were obtained of the navicular and/or distal phalanx of the two feet selected based on pathology and one reference foot.

Results

On DECT virtual non-calcium imaging, the two pathological feet showed diffuse increased attenuation corresponding with bone edema-like lesions, whereas the three reference feet were considered normal. These findings were in agreement with the findings on the MRI. Histopathology of the two pathologic feet showed abnormalities in line with bone edema-like lesions. Histopathology of the reference foot was normal.

Conclusion

DECT virtual non-calcium imaging can be a valuable diagnostic tool in the diagnosis of bone edema-like lesions in the equine foot. Further examination of DECT in equine diagnostic imaging is warranted in a larger cohort, different locations, and alive animals.

Patient Positioning Assistive Technology Applicable to the Existing Computed Tomography System: Estimation by Pixel Value of Scout Image

ABSTRACT

This study aimed to propose a patient positioning assistive technique using computed tomography (CT) scout images. A total of 210 patients who underwent CT scans in a single center, including on the upper abdomen, were divided into a study set of 127 patients for regression and 83 patients for verification. Linear regression analysis was performed to determine the R2 coefficient and the linear equation related to the mean pixel value of the scout image and ideal table height (TH ideal ). The average pixel values of the scout image were substituted into the regression equation to estimate the TH ideal . To verify the accuracy of this method, the distance between the estimated table height (TH est ) and TH ideal was measured. The medians of age (in years), gender (male/female), height (in centimeters), and body weight (in kilograms) for the regression and verification groups were 68 versus 70, 85/42 versus 55/28, 163.8 versus 163.0, and 59.9 versus 61.9, respectively. Linear regression analysis indicated a high coefficient of determination ( R2 = 0.91) between the mean pixel value of the scout image and TH ideal . The correlation coefficient between TH ideal and TH est was 0.95 (95% confidence interval, 0.92-0.97; P < 0.0001), systematic bias was 0.2 mm, and the limits of agreement were -5.4 to 5.9 ( P = 0.78). The offset of the table height with TH est was 2.8 ± 2.1 mm. The proposed estimation method using scout images could improve the automatic optimization of table height in CT, and it can be used as a general-purpose automatic positioning technique.

Algorithm development for automatic laser alignment assessment on an ACR CT phantom and its evaluation on sixteen CT scanners

Purpose. The aim of this study is to develop software to automatically assess the laser alignment on the ACR CT phantom and evaluate its accuracy on sixteen CT scanners.Methods. Software for an automated method of laser alignment assessment on the ACR CT phantom was developed. Laser alignment assessment was based on the positions of the ball-bearing markers at the edge of the ACR CT phantom. The automatic assessment was performed using several steps, including segmentation to acquire the coordinates of the ball-bearing markers and determination of the distances between lines connecting them with lines through the center of the image. A comparison of the results from the automatic method with those from the manual method was performed. The manual measurements were carried out using MicroDicom Viewer. A Mann-Whitney U test was performed to determine the statistical difference between both methods. The evaluation was performed on images of the ACR CT phantom scanned with 16 CT scanners from 5 different CT manufacturers.Results. The results confirmed that our software successfully segments the ball-bearing markers and determines the laser alignment assessment on the ACR CT phantom. Evaluation of the algorithm with images from the 16 CT scanners revealed that the difference between the results from automatic and manual methods were about 0.2 mm with apvalue of around 0.7 (no statistical difference). Misalignment in they-axis was larger than the misalignment in the x-axisfor the majority of the scanners tested. It was found that the phantom tended to be placed 2 mm higher than the iso-center.Conclusions. Software to automatically assess CT laser alignment with the ACR CT phantom was successfully developed and evaluated. The automatic assessment was comparable to manual assessment. In addition, the automatic method was user independent and fast.

Optimized Camera-Based Patient Positioning in CT: Impact on Radiation Exposure

OBJECTIVE

The aim of this study was to evaluate whether a 3-dimensional (3D) camera can outperform highly trained technicians in precision of patient positioning and whether this transforms into a reduction in patient exposure.

MATERIALS AND METHODS

In a single-center study, 3118 patients underwent computer tomography (CT) scans of the chest and/or abdomen on a latest generation single-source CT scanner supported with an automated patient positioning system by 3D camera. One thousand five hundred fifty-seven patients were positioned laser-guided by a highly trained radiographer (camera off) and 1561 patients with 3D camera (camera on) guidance. Radiation parameters such as effective dose, organ doses, CT dose index, and dose length product were analyzed and compared. Isocenter accuracy and table height were evaluated between the 2 groups.

RESULTS

Isocenter positioning was significantly improved with the 3D camera ( P < 0.001) as compared with visual laser-guided positioning. Absolute table height differed significantly ( P < 0.001), being higher with camera positioning (165.6 ± 16.2 mm) as compared with laser-guided positioning (170.0 ± 20.4 mm). Radiation exposure decreased using the 3D camera as indicated by dose length product (321.1 ± 266.6 mGy·cm; camera off: 342.0 ± 280.7 mGy·cm; P = 0.033), effective dose (3.3 ± 2.7 mSv; camera off: 3.5 ± 2.9; P = 0.053), and CT dose index (6.4 ± 4.3 mGy; camera off: 6.8 ± 4.6 mGy; P = 0.011). Exposure of radiation-sensitive organs such as colon ( P = 0.015) and red bone marrow ( P = 0.049) were also lower using the camera.

CONCLUSIONS

The introduction of a 3D camera improves patient positioning in the isocenter of the scanner, which results in a lower and also better balanced dose reduction for the patients.

The influence of patient positioning on radiation dose in CT imaging: A narrative review

BACKGROUND AND PURPOSE

Although it is fundamental for optimal scanner operation, it is generally accepted that accurate patient centring cannot always be achieved. This review aimed to examine the reported knowledge of the negative impact of patient positioning on radiation dose and image quality during CT imaging. Furthermore, the study evaluated the current optimisation tools and techniques used to improve patient positioning relative to the gantry iso-center.

METHODOLOGY

A comprehensive search through the databases PubMed, Ovid, and Google Scholar was performed. Keywords included patient off-centring, patient positioning, localiser radiograph orientation, radiation dose, and automatic patient positioning (including synonyms). The search was limited to full-text articles that were written in English. After initial title and abstract screening, a total of 52 articles were identified to address the aim of the review. No limitations were imposed on the year of publication.

RESULTS

Vertical off-centring was reported in up to 95% of patients undergoing chest and abdominal CT examinations, showing a significant influence on radiation dose. Depending on the scanner model and vendor, localiser orientation, bowtie filter used, and patient size, radiation dose varied from a decrease of 36% to an increase of 91%. A significant dose reduction was demonstrated when utilising an AP localiser, aligning with the trend for radiographers to off-center patients below the gantry iso-centre. Utilizing a 3D camera for body contour detection allowed for more accurate patient positioning and promoted further dose reduction.

CONCLUSION

Patient positioning has shown significant effects on radiation dose and image quality in CT. Developing a good understanding of the key factors influencing patient dose (off-centring direction, localiser orientation, patient size and bowtie filter selection) is critical in optimising CT scanning practices. Utilising a 3D camera for body contour detection is strongly recommended to improve patient positioning accuracy, image quality and to minimise patient dose.

The impacts of vertical off-centring, tube voltage, and phantom size on computed tomography numbers: An experimental study

INTRODUCTION

This experimental study explored the effect of vertical off-centring on computed tomography (CT) numbers in combination with various tube voltages and phantom sizes for two CT units.

METHODS

CIRS Model 062 Electron Density and system performance phantoms were imaged on Siemens Emotion 16-slice CT and GEMINI-GXL scanners, respectively. Uniformity and accuracy were evaluated as a function of vertical off-centring (20, 40, 60, and 80 mm above the gantry isocentre) using different water phantom sizes (18, 20, and 30 cm) and tube voltages (80, 90, 110, 120, 130 and 140 kVp).

RESULTS

Vertical off-centring and phantom size accounted for 92% of the recorded variance and the resultant change in CT numbers. The uniformity test recorded maximum changes of 14 and 27.2 HU for peripheral ROIs across the X- and Y-axes for an 80 mm phantom shift above the gantry isocentre on the GEMINI GXL and Siemens scanners, respectively. The absolute CT number differences between the superior and inferior ROIs were 13.7 HU for the 30 cm phantom and 4.8 HU for the 20 cm phantom for 80 mm vertical off-centring. The largest differences were observed at lower tube voltages.

CONCLUSIONS

It is essential to highlight the significance of CT number variation in clinical decision-making. Phantom off-centring affected the uniformity of these numbers, which were further impacted by the ROI position in this experimental study. CT number variation was more evident in peripheral phantom areas, lower tube voltages and larger phantom sizes.

IMPLICATIONS FOR PRACTICE

CT number is observed to be a variable under certain common conditions. This significantly impacts several applications where clinical decisions depend on CT number accuracy for tissue lesion characterisation.

Feasibility Study of Dose Modulation for Reducing Radiation Dose with Arms-Down Patient Position in Abdominal Computed Tomography

This study was carried out to demonstrate whether the radiation dose for patients in arms-down position can be reduced without affecting the diagnosis on abdominal computed tomography (CT). The patients were divided into two groups: group A, which included patients with arms-down position using dose modulation on, and group B, which included patients with arms-down position using dose modulation turned off. Quantitative evaluation was compared using Hounsfield units, standard deviation, and signal-to-noise ratio of the four regions. The qualitative evaluation was assessed for overall image quality, subjective image noise, and beam hardening artifacts. Dose evaluation for CT dose index (CTDI) and dose length product (DLP) was compared by comparing the CT images with dose modulation turned on and off. In the quantitative and qualitative evaluation, there was no statistically significant difference between groups A and B (p > 0.05). In the dose evaluation, the CT images with dose modulation turned off had significantly lower CTDI and DLP than the CT images with dose modulation turned on (p < 0.05). Our results suggest that, for the GE Revolution EVO CT scanner, turning off dose modulation and increasing the tube voltage can reduce the radiation dose for patients with the arms-down position without affecting the diagnosis. This study did not consider the change of tube potential according to the use of dose modulation, and we plan to conduct additional research in the future.

The reliability of CT numbers as absolute values for diagnostic scanning, dental imaging, and radiation therapy simulation: A narrative review

BACKGROUND AND PURPOSE

The purpose of this review was to examine the reported factors that affect the reliability of Computed Tomography (CT) numbers and their impact on clinical applications in diagnostic scanning, dental imaging, and radiation therapy dose calculation.

METHODS

A comprehensive search of the literature was conducted using Medline (PubMed), Google Scholar, and Ovid databases which were searched using the keywords CT number variability, CT number accuracy and uniformity, tube voltage, patient positioning, patient off-centring, and size dependence. A narrative summary was used to compile the findings under the overarching theme.

DISCUSSION

A total of 47 articles were identified to address the aim of this review. There is clear evidence that CT numbers are highly dependent on the energy level applied based on the effective atomic number of the scanned tissue. Furthermore, body size and anatomical location have also indicated an influence on measured CT numbers, especially for high-density materials such as bone tissue and dental implants. Patient off-centring was reported during CT imaging, affecting dose and CT number reliability, which was demonstrated to be dependent on the shaping filter size.

CONCLUSION

CT number accuracy for all energy levels, body sizes, anatomical locations, and degrees of patient off-centring is observed to be a variable under certain common conditions. This has significant implications for several clinical applications. It is crucial for those involved in CT imaging to understand the limitations of their CT system to ensure radiologists and operators avoid potential pitfalls associated with using CT numbers as absolute values for diagnostic scanning, dental imaging, and radiation therapy dose calculation.

Impacts of Phantom Off-Center Positioning on CT Numbers and Dose Index CTDIv: An Evaluation of Two CT Scanners from GE

The purpose of this work is to evaluate the impacts of body off-center positioning on CT numbers and dose index CTDIv of two scanners from GE. HD750 and APEX scanners were used to acquire a PBU60 phantom of Kagaku and a 062M phantom of CIRS respectively. CT images were acquired at various off-center positions under automatic tube current modulation using various peak voltages. CTDIv were recorded for each of the acquisitions. An abdomen section of the PBU60 phantom was used for CT number analysis and tissue inserts of the 062M phantom were filled with water balloons to mimic the human abdomen. CT numbers of central regions of interests were averaged using the Fiji software. As phantoms were lifted above the iso-center, both CTDIv and CT numbers were increased for the HD750 scanner whilst they were approximately constant for the APEX scanner. The measured sizes of anterior-posterior projection images were also increased for both scanners whilst the sizes of lateral projection images were increased for the HD750 scanner but decreased for the APEX scanner. Off-center correction algorithms were implemented in the APEX scanner. Matching the X-ray projection center with the system's iso-center could improve the accuracy of CT imaging.

An assessment of Sri Lankan radiographer's knowledge and awareness of radiation protection and imaging parameters related to patient dose and image quality in computed tomography (CT)

INTRODUCTION

As computed tomography (CT) examinations have considerably risen, safe operation is essential to reduce the patients' dose. The main objective of this study was to evaluate the level of knowledge and awareness regarding the CT exposure parameters and radiation protection in CT imaging among Sri Lankan radiographers.

METHODS

An online survey-based study was devised and distributed among the Sri Lankan CT radiographers working in 63 CT units. Questions were divided into three subsections that collected data on the participants' demographic features, knowledge of the radiation protection, and imaging parameters.

RESULTS

Eighty-eight radiographers from 32 CT units (out of 63 CT units) distributed across 11 districts (out of 27 districts) participated in this survey.The percentages of correct responses for the questions related to radiation protection, imaging parameters, noise, Diagnostic Reference Level (DRL), and CT dosimetric parameters were 71%, 79%, 87%, 50%, and 66%, respectively. Although the years of experience did not influence any of above aspects, the level of education significantly impacted the knowledge about radiation protection, exposure parameters, and noise.

CONCLUSION

The radiographer's knowledge of radiation protection and most imaging parameters associated with patient safety and image quality is satisfactory. However, findings also show that participants should fill the knowledge gap in radiation-related risks, CT exposure parameters, dosimetric parameters, and DRL.

IMPLICATIONS FOR PRACTICE

The study suggests the necessity of initiating continuous education programs for radiographers in line with national radiation protection legislation requirements that can be linked with code of practice.

Influence of breathing state on the accuracy of automated patient positioning in thoracic CT using a 3D camera for body contour detection

Objective

To assess the influence of breathing state on the accuracy of a 3D camera for body contour detection and patient positioning in thoracic CT.

Materials and methods

Patients who underwent CT of the thorax with both an inspiratory and expiratory scan were prospectively included for analysis of differences in the ideal table height at different breathing states. For a subgroup, an ideal table height suggestion based on 3D camera images at both breathing states was available to assess their influence on patient positioning accuracy. Ideal patient positioning was defined as the table height at which the scanner isocenter coincides with the patient’s isocenter.

Results

The mean (SD) difference of the ideal table height between the inspiratory and the expiratory breathing state among the 64 included patients was 10.6 mm (4.5) (p < 0.05). The mean (SD) positioning accuracy, i.e., absolute deviation from the ideal table height, within the subgroup (n = 43) was 4.6 mm (7.0) for inspiratory scans and 7.1 mm (7.7) for expiratory scans (p < 0.05) when using corresponding 3D camera images. The mean (SD) accuracy was 14.7 mm (7.4) (p < 0.05) when using inspiratory camera images on expiratory scans; vice versa, the accuracy was 3.1 mm (9.5) (p < 0.05).

Conclusion

A 3D camera allows for accurate and precise patient positioning if the camera image and the subsequent CT scan are acquired in the same breathing state. It is recommended to perform an expiratory planning image when acquiring a thoracic CT scan in both the inspiratory and expiratory breathing state.

Key Points

• A 3D camera for body contour detection allows for accurate and precise patient positioning if the camera image and the subsequent CT scan are acquired in the same breathing state.

• It is recommended to perform an expiratory planning image when acquiring a thoracic CT scan in both the inspiratory and expiratory breathing state.

A comparison of automatic and manual compensation methods for the calculation of tube currents during off-centered patient positioning with a noise-based automatic exposure control system in computed tomography

Automatic exposure control (AEC) is used to optimize the X-ray tube output during computed tomography (CT) scans. However, calculation of the tube current by AEC can be affected when a patient is not aligned with the rotational center of the X-ray tube. An automatic couch height-positioning compensation mechanism provides a corrective function when the patient is off-center. In this study, we aimed to (a) evaluate the performance characteristics of the positioning compensation mechanism and (b) confirm whether our proposed compensation method can be properly applied to a noise-based AEC system even if the CT device is not equipped with a positioning compensation mechanism. An elliptical phantom was scanned at various table heights on systems without/with the positioning compensation mechanism. Expressions describing the offset from the gantry's isocenter and adjusted standard deviation settings were derived and used in our proposed compensation method. A phantom was scanned at various table heights with our proposed compensation method, and volume CT dose index (CTDI_vol) and image noise levels were obtained. An anthropomorphic chest phantom was also scanned using the proposed compensation method to verify its accuracy. When the positioning compensation mechanism was used, it yielded a constant CTDI_vol and image noise levels at various table heights tested. A comparison between our proposed method and the positioning compensation mechanism for both the elliptical and chest phantoms yielded similar CTDI_vol. Therefore, both automatic and manual positioning compensation methods are useful for avoiding AEC miscalculations in off-centered patient positioning cases.

Inadequate object positioning and improvement of automatic exposure control system calculations based on an empirical algorithm

When using automatic exposure control (AEC) systems in computed tomography (CT), miscalculation of tube current occurs when a patient is not aligned with the rotational center of the X-ray tube. A positioning compensation mechanism provides a corrective function when the patient is off-center; however, not all CT systems are equipped with this mechanism. AEC systems can broadly be divided into noise- and empirical-based. The authors studied empirical-based AEC systems to derive a compensation process to achieve an equivalent effect to that offered by the mechanism and to verify the accuracy of this process. A relational equation was derived to keep the tube current constant with variations in table height and quality reference milliampere-seconds (QRmAs), and this was adopted as the proposed compensation method. The radiation dose and image quality were evaluated for phantom imaging with and without the proposed compensation method using AEC and varying table heights. The output radiation dose and image quality were also evaluated for anthropomorphic chest phantom imaging to verify the compensatory effect of the proposed method. With the proposed compensation method, changes in the table height resulted in only small changes in the output radiation dose and noise level. Conversely, when the proposed compensation method was not used, changes in the table height resulted in widely varying output radiation dose and noise level. Imaging the anthropomorphic chest phantom with the proposed compensation method also yielded a stable output radiation dose.

Effect of scan projection radiography coverage on tube current modulation in pediatric and adult chest CT

PURPOSE

To investigate the effect of scan projection radiography (SPR) coverage on tube current modulation in pediatric and adult thoracic CT examinations.

METHODS

Sixty pediatric and 60 adult chest CT examinations were retrospectively studied to determine the incidence rate of examinations involving SPRs that did not include the entire image volume (IV) or the entire primarily exposed body volume (PEBV). The routine chest CT acquisition procedure on a modern 64-slice CT system was imitated on five anthropomorphic phantoms of different size. SPRs of varying length were successively acquired. The same IV was prescribed each time and the computed tube current modulation plan was recorded. The SPR boundaries were altered symmetrically by several steps of ±10mm with respect to the IV boundaries.

RESULTS

The upper IV boundary was found to be excluded from SPR in 52% of pediatric and 40% adult chest CT examinations. The corresponding values for the lower boundary were 15% and 20%, respectively. The computed tube current modulation was found to be considerably affected when the SPR did not encompass the entire IV. SPR deficit of 3cm was found to induce up to 46% increase in the computed tube current value to be applied during the first tube rotations over lung apex.

CONCLUSIONS

The tube current modulation mechanism functions properly only if the IV set by the operator is entirely included in the localizing SPR image. Operators should cautiously set the SPR boundaries to avoid partial exclusion of prescribed IV from SPRs and thus achieve optimum tube current modulation.

Vertical Off-Centering in Reduced Dose Chest-CT: Impact on Effective Dose and Image Noise Values

OBJECTIVES

To assess the effect of vertical off-centering in tube current modulation (TCM) on effective-dose and image-noise in reduced-dose (RD) chest-CT.

METHODS

One-hundred consecutive patients (36 female; mean age 56 years) were scanned on a 192-slice CT scanner with a standard-dose (ND) and a RD chest-CT protocol using tube current modulation. Image-noise was evaluated by placing circular regions of interest in the apical, middle, and lower lung regions. Two independent readers evaluated image quality. Study population was stratified according to patient position in the gantry: positioned in the gantry isocenter (i), higher than the gantry isocenter (ii), and lower than the gantry isocenter, (iii). Pearson correlation was used to determine the correlation between effective radiation dose and vertical off-centering. Student's t test was used to evaluate for differences in image-noise between groups (i-iii).

RESULTS

Mean vertical off-centering was of 10.6 mm below the gantry-isocenter (range -45.0-27.9 mm). Effective radiation dose varied in a linear trend, with the highest doses noted below gantry isocenter, and the lowest doses noted above gantry isocenter (ND: r = -0.296; p = 0.003 - RD: r = -0.258; p = 0.010). Lowest image-noise was observed where patients were positioned below the gantry isocenter, and highest in patients positioned above (ND: 79.35 HU vs. 94.86 HU - RD: 143.44 HU vs. 160.13 HU). Subjective image quality was not significantly affected by patient-position (p > 0.05). Overall, there was no over-proportional noise-increase from the ND to the RD protocol in patients which were positioned off-center.

CONCLUSION

Vertical off-centering influences effective radiation dose and image-noise on ND and RD protocols.

ADVANCES IN KNOWLEDGE

There is no over-proportional noise increase in RD compared to ND protocols when patients are positioned off-center.

CT-based radiomics for preoperative prediction of early recurrent hepatocellular carcinoma: technical reproducibility of acquisition and scanners

PURPOSE

To test the technical reproducibility of acquisition and scanners of CT image-based radiomics model for early recurrent hepatocellular carcinoma (HCC).

METHODS

We included primary HCC patient undergone curative therapies, using early recurrence as endpoint. Four datasets were constructed: 109 images from hospital #1 for training (set 1: 1-mm image slice thickness), 47 images from hospital #1 for internal validation (sets 2 and 3: 1-mm and 10-mm image slice thicknesses, respectively), and 47 images from hospital #2 for external validation (set 4: vastly different from training dataset). A radiomics model was constructed. Radiomics technical reproducibility was measured by overfitting and calibration deviation in external validation dataset. The influence of slice thickness on reproducibility was evaluated in two internal validation datasets.

RESULTS

Compared with set 1, the model in set 2 indicated favorable prediction efficiency (the area under the curve 0.79 vs. 0.80, P = 0.47) and good calibration (unreliability statistic U: P = 0.33). However, in set 4, significant overfitting (0.63 vs. 0.80, P < 0.01) and calibration deviation (U: P < 0.01) were observed. Similar poor performance was also observed in set 3 (0.56 vs. 0.80, P = 0.02; U: P < 0.01).

CONCLUSIONS

CT-based radiomics has poor reproducibility between centers. Image heterogeneity, such as slice thickness, can be a significant influencing factor.

Impact of patient centering in CT on organ dose and the effect of using a positioning compensation system: Evidence from OSLD measurements in postmortem subjects

The purpose of this study was to investigate the frequency and impact of vertical mis-centering on organ doses in computed tomography (CT) exams and evaluate the effect of a commercially available positioning compensation system (PCS). Mis-centering frequency and magnitude was retrospectively measured in 300 patients examined with chest-abdomen-pelvis CT. Organ doses were measured in three postmortem subjects scanned on a CT scanner at nine different vertical table positions (maximum shift ± 4 cm). Organ doses were measured with optically stimulated luminescent dosimeters inserted within organs. Regression analysis was performed to determine the correlation between organ doses and mis-centering. Methods were repeated using a PCS that automatically detects the table offset to adjust tube current output accordingly. Clinical mis-centering was >1 cm in 53% and 21% of patients in the vertical and lateral directions, respectively. The 1-cm table shifts resulted in organ dose differences up to 8%, while 4-cm shifts resulted in organ dose differences up to 35%. Organ doses increased linearly with superior table shifts for the lung, colon, uterus, ovaries, and skin (R²  = 0.73-0.99, P < 0.005). When the PCS was utilized, organ doses decreased with superior table shifts and dose differences were lower (average 5%, maximum 18%) than scans performed without PCS (average 9%, maximum 35%) at all table shifts. Mis-centering occurs frequently in the clinic and has a significant effect on patient dose. While accurate patient positioning remains important for maintaining optimal imaging conditions, a PCS has been shown to reduce the effects of patient mis-centering.

The effect of vertical centering and scout direction on automatic tube voltage selection in chest CT: a preliminary phantom study on two different CT equipments

Purpose

To determine the effect of patient's vertical off-centering and scout direction on the function of automatic tube voltage selection (ATVS) and tube current modulation (TCM) in chest computed tomography (CT).

Methods

Chest phantom was scanned with Siemens and GE CT systems using three clinical chest CT protocols exploiting ATVS and a fixed 120 kVp chest protocol. The scans were performed at five vertical positions of the phantom (-6 to +6 cm from the scanner isocenter). The effects of scout direction (posterior-to-anterior, anterior-to-posterior, and lateral) and vertical off-centering on the function of ATVS and TCM were studied by examining changes in selected voltage, radiation dose (volume CT dose index, CTDI_vol), and image noise and contrast.

Results

Both scout direction and vertical off-centering affected ATVS. The effect differed between the vendors for the studied geometry, demonstrating differences in technical approaches. The greatest observed increase in CTDI_vol due to off-centering was 91%. Anterior-to-posterior scout produced highest doses at the uppermost table position, whereas posterior-to-anterior scout produced highest doses at the lowermost table position. Dose varied least using lateral scouts. Vertical off-centering impacted image noise and contrast due to the combined effect of ATVS, TCM, structural noise, and bowtie filters.

Conclusions

Patient vertical off-centering and scout direction affected substantially the CTDI_vol and image quality in chest CT examinations. Vertical off-centering caused variation also in the selected tube voltage. The function of ATVS and TCM methods differ significantly between the CT vendors, resulting in differences in CTDI_vol and image noise characteristics.

Knowledge and practice of computed tomography exposure parameters amongst radiographers in Jordan

OBJECTIVE

To investigate the knowledge and practice of computed tomography (CT) radiographers working in Jordan.

MATERIALS AND METHODS

This Institutional Review Board (IRB) approved study disseminated a questionnaire via social media and recruited 54 Jordanian CT radiographers. The questionnaire comprised 36 questions divided into four sections: demographics; an evaluation of knowledge regarding CT exposure; modifications to CT exposure for paediatric patients; dose units and diagnostic reference levels (DRLs). Descriptive and inferential statistics including Chi-square tests, Mann-Whitney U tests, independent samples t-tests and Kruskal-Wallis H tests were employed. Statistical significance was considered below p < 0.05.

RESULTS

The 54 participants had various qualifications, with the majority holding a Bachelor's degree (n = 35, 64.8%) and the rest holding a Diploma (n = 19, 35.2%). In order to pass the questionnaire, participants needed to score 13 correct answers. The overall number of radiographers who correctly passed the questionnaire was 48 (88.9%). None of the participants correctly stated all the DRL values for chest, abdomen and brain CT. However, four out of 54 respondents (7.4%) knew the chest DRL value, three (5.6%) participants correctly estimated the abdominal DRL value but only two (3.7%) knew the DRL for the brain.

CONCLUSION

Good general knowledge was found amongst radiographers regarding the relationship of each exposure parameter to the image quality and patient dose. However, there was poor knowledge of diagnostic reference levels and the order of the organ radiation sensitivity. The need for CT radiographers to undertake further education that focuses on radiation exposure in CT is highlighted.

Relationship between size-specific dose estimates and image quality in computed tomography depending on patient size

This study investigates the relationship between contrast-to-noise ratio (CNR) and size-specific dose estimate (SSDE) in computed tomography (CT) depending on patient size. In addition, the relationship to the auto exposure control (AEC) techniques is examined. A tissue-equivalent material having human-liver energy dependence is developed and used to evaluate these relationships. Three exposure dose levels (constant CT dose index, constant SSDE, and with AEC) are tested using four different phantom sizes (diameter: 15, 20, 25 and 30 cm) in two different CT scanners (SOMATOM Definition Flash, Siemens, and LightSpeed VCT, GE). The contrast-to-noise ratios (CNRs) are measured using the developed phantom. It is found that the CNR increases with decreasing phantom size at constant SSDE, although the increase ratio is smaller than that of the constant CT dose index. This result indicates that the image characteristics differ even when the patient dose received from the CT examination is equivalent for each patient size. In the case of AEC use, the CNR results of the Siemens scanner exhibit a similar trend to those obtained for constant SSDE, for each phantom size. This suggests that the AEC technique that maintains a constant image quality (CARE Dose 4D) for each patient size corresponds well to the image quality obtained for constant SSDE. These findings facilitate further understanding of the relationship between image quality and exposure CT dose depending on patient size.

Effect of arm position, presence of medical devices, and off-centering during acquisition of scout image on automatic tube voltage selection and current modulation in pediatric chest CT

PURPOSE

To evaluate the patients' morphologic factors affecting radiation dose in pediatric chest CT.

MATERIALS AND METHODS

From November 2013 to May 2015, 315 pediatric chest CT scans were obtained using a CT scanner, and classified into 5 groups according to the patients' age. For each age group, the chest CT scans were divided into two subgroups. A cut-off value used was the 75th percentile of size-specific dose estimates (SSDE), age-specific diagnostic reference level (DRL): less than the 75th percentile of SSDE (Group A, n = 238) and greater than the 75th percentile of SSDE (Group B, n = 77). All CT scans were performed with the same protocol using automatic tube voltage selection and current modulation techniques. The morphologic factors of the patients including body mass index (BMI), arm angles, presence of medical devices in the scan field, and degree of off-centering within the CT gantry were compared between groups A and B.

RESULTS

Group B showed narrower arm angles on scout and coronal reformatted images, higher frequency of the presence of devices and higher BMI than group A (P < 0.001, P < 0.001; P = 0.018, and P < 0.001, respectively). In multivariate analysis, narrower arm angles, the presence of devices on the scout images and higher BMI were independently associated with higher SSDE (P = 0.001, P = 0.037, and P < 0.001, respectively).

CONCLUSIONS

During acquisition of the scout images, arms-down position and the presence of medical devices were associated with a high radiation dose above age-specific DRLs in pediatric chest CT, regardless of repositioning before the actual scanning. In addition, off-centering had no clinical impact on radiation dose in the routine practice.

Radiation dose management for pediatric cardiac computed tomography: a report from the Image Gently 'Have-A-Heart' campaign

Children with congenital or acquired heart disease can be exposed to relatively high lifetime cumulative doses of ionizing radiation from necessary medical imaging procedures including radiography, fluoroscopic procedures including diagnostic and interventional cardiac catheterizations, electrophysiology examinations, cardiac computed tomography (CT) studies, and nuclear cardiology examinations. Despite the clinical necessity of these imaging studies, the related ionizing radiation exposure could pose an increased lifetime attributable cancer risk. The Image Gently "Have-A-Heart" campaign is promoting the appropriate use of medical imaging studies in children with congenital or acquired heart disease while minimizing radiation exposure. The focus of this manuscript is to provide a comprehensive review of radiation dose management and CT performance in children with congenital or acquired heart disease.

A CONSORT-compliant prospective randomized controlled trial: radiation dose reducing in computed tomography using an additional lateral scout view combined with automatic tube current modulation: Phantom and patient study

BACKGROUND

Radiation exposure has been a hot point in research field of computed tomography (CT). Recently, automated tube current modulation (ATCM) has emerged as an important technique to reduce radiation exposure. Many studies have shown that the difference in scout view would affect modulation. This prospective randomized controlled study is aimed to investigate the impact of an additional lateral scout view on radiation dose and image quality in CT using ACTM.

METHODS

Combined with ATCM (Care Dose 4D) on multidetector CT, 2 thoracic phantom CT image series were acquired in which planning was conducted with either an anteroposterior (AP) or an AP-lateral scout view. Also, 410 patients underwent thoracic CT examinations using Care Dose 4D modulation and were randomized to either a scan planned with an AP-lateral scout or a single AP scout. Effects of the different scout views on applied effective milliampere seconds (mAs), volume CT dose index (CTDIvol) and dose-length-product (DLP) were analyzed. The quality of patient CT images was also assessed. Data were analyzed using independent t tests and linear correlation analysis.

RESULTS

Compared with AP groups, the mean CTDIvol (phantom, 0.89 ± 0.08 vs 1.36 ± 0.26 mGy, P < .001; in patients, 1.12 [0.96, 1.34] vs 2.16 [1.66, 2.64] mGy, P < .001) and DLP (in phantom, 26 [23.25, 28] vs 40 [34.25, 48] mGy×cm, P < .001; in patients, 41 [33, 41] vs 77 [60.5, 99.5] mGy×cm, P < .001) were significantly reduced by approximately 50% in AP-lateral scout view group. With the AP-lateral topogram, the radiation dose on different off-center positions was essentially equal (CTDIvol: 0.76-0.99 mGy; DLP: 22-28 mGy×cm effective dose: 0.31-0. 39 mSv). For image quality, contrast-to-noise ratio and signal-to-noise ratio values in the AP group were similar to those of AP-lateral scout view group.

CONCLUSION

AP combined with an additional lateral scout view using ACTM can significantly reduce the radiation dose without compromising image quality in chest screening CT.

The effects of patient positioning when interpreting CT dose metrics: A phantom study

PURPOSE

Review of dose metrics as part of the routine evaluation of CT protocols has become commonplace and is required by the Joint Commission and the American College of Radiology for accreditation. Most CT quality assurance programs include a review of CTDI_vol and/or SSDE, both of which are affected by changes in mAs and kV. mAs, and sometimes kV, are largely determined by the Tube Current Modulation (TCM) functions of the scanner. TCM, in turn, relies on localizer scans to provide an accurate estimate of patient size. When patient size estimates are inaccurate, TCM and SSDE calculations are affected, leading to errors in both. It is important that those who are involved in reviewing CT dose indices recognize these effects to properly direct quality improvement initiatives.

METHODS

An anthropomophic phantom was scanned on four clinical CT scanners using AP and PA localizers and the institution's routine abdomen protocol. Scans were repeated with the phantom at various heights relative to scanner isocenter. For each height, the projected phantom width, as shown by the localizer scans, was measured and normalized by the width of the helical scan. After each localizer scan, the TCM algorithm determined the mAs to be used for the helical scan. The scanner-reported average CTDI_vol was recorded for each helical scan, and the SSDE was calculated from the projected phantom size and the scanner-reported CTDI_vol at each phantom height. Last, the phantom was augmented with a lipid-gel bolus material to simulate different body mass distributions and investigate the effect of differing body habitus on projected phantom size. The results were considered in the context of optimizing dose in CT imaging, with particular attention paid to the effect on dose to breast tissue.

RESULTS

Vertical mis-positioning of the phantom within the scanner led to errors in estimated phantom size of up to a factor of 1.5. These effects were more severe when localizers were acquired in the PA orientation compared with the AP orientation. Minification effects were more pronounced for AP localizers. As a consequence of inaccuracies in estimated phantom size, TCM resulted in changes in CTDI_vol and SSDE of as much as a factor of 4.4 and 2.7, respectively. The effect was more pronounced when the TCM function used data from the PA, rather than the AP, localizer.

CONCLUSIONS

Proper patient positioning plays a large role in the function of TCM, and hence CTDI_vol and SSDE. In addition, body mass distribution may affect how patients ought to be positioned within the scanner. Understanding these effects is critical in optimizing CT scanning practices.

Variation in CT Number and Image Noise Uniformity According to Patient Positioning in MDCT

OBJECTIVE

Many algorithms for clinical decision making rely on assessment of the CT number (expressed as Hounsfield units); however, to our knowledge, few, if any, studies have addressed how CT numbers change as a function of patient positioning within the scanner.

MATERIALS AND METHODS

An anthropomorphic phantom underwent imaging with varying amounts of vertical orientation misalignment with respect to isocenter. CT number and noise were measured using ROIs in the upper thorax, mid thorax, and abdomen. The degree of noise nonuniformity and changes in the CT number were assessed by comparing values obtained in the anterior versus posterior ROIs. To add clinical relevance, data on vertical mispositioning were collected from 20,316 clinical abdominal CT scans. Box-and-whisker plot analysis was used to identify the range of patient positioning.

RESULTS

Absolute CT number changes of more than 20 HU were observed for some ROIs at phantom positions of 10 cm from isocenter, with important differences noted between the thoracic and abdominal regions. Noise uniformity varied by more than twofold for all regions at 10 cm below isocenter. On clinical CT examinations, off-centering of more than 1, 2, 4, and 6 cm occurred for 41%, 19%, 1.9%, and 0.3% of patients, respectively.

CONCLUSION

Radiologists should treat CT number measurements with caution when patients are grossly mispositioned in the scanner. The substantial changes in attenuation values shown in the present study are large enough to warrant further investigation.

Tracking Patterns of Nonadherence to Prescribed CT Protocol Parameters

PURPOSE

Quantification of the frequency, understanding the motivation, and documentation of the changes made by CT technologists at scan time are important components of monitoring a quality CT workflow.

METHODS

CT scan acquisition data were collected from one CT scanner for a period of 1 year. The data included all relevant acquisition parameters needed to define the technical side of a CT protocol. An algorithm was created to sort these data in groups of irradiation events with the same combinations of scan acquisition parameters. For scans modified at scan time, it was hypothesized that these examinations would show up only once in the organized data. A classification scheme was developed to place each "one-off" examination into a category related to what motivated the scan-time change.

RESULTS

A total of 132,707 irradiation events were organized into 434 groups of unique scan acquisition parameters. One hundred forty-four irradiation events had acquisition parameters that showed up only once in the data. These "one-offs" were classified as follows: 25% represented rarely used protocols, 17% were due to service scans, 16% were changed for unknown and therefore undesired reasons, 15% were changed by technologists trying to adapt protocol to patient size, 12% were allowable scan-time changes, 8% of scans had tube current maxed out, and 6% of scans were changed to a higher dose mode as requested by radiologists.

CONCLUSIONS

The outcome of this study suggests many areas of needed technologist training and chances for optimizing this institution's CT protocols.

The impact of x-ray tube stabilization on localized radiation dose in axial CT scans: initial results in CTDI phantoms

Rise, fall, and stabilization of the x-ray tube output occur immediately before and after data acquisition on some computed tomography (CT) scanners and are believed to contribute additional dose to anatomy facing the x-ray tube when it powers on or off. In this study, we characterized the dose penalty caused by additional radiation exposure during the rise, stabilization, and/or fall time (referred to as overscanning). A 32 cm CT dose-index (CTDI) phantom was scanned on three CT scanners: GE Healthcare LightSpeed VCT, GE Healthcare Discovery CT750 HD, and Siemens Somatom Definition Flash. Radiation exposure was detected for various x-ray tube start acquisition angles using a 10 cm pencil ionization chamber placed in the peripheral chamber hole at the phantom's 12 o'clock position. Scan rotation time, ionization chamber location, phantom diameter, and phantom centering were varied to quantify their effects on the dose penalty caused by overscanning. For 1 s single, axial rotations, CTDI at the 12 o'clock chamber position (CTDI_{100, 12:00}) was 6.1%, 4.0%, and 4.4% higher when the start angle of the x-ray tube was aligned at the top of the gantry (12 o'clock) versus when the start angle was aligned at 9 o'clock for the Siemens Flash, GE CT750 HD, and GE VCT scanner, respectively. For the scanners' fastest rotation times (0.285 s for the Siemens and 0.4 s for both GE scanners), the dose penalties increased to 22.3%, 10.7%, and 10.5%, respectively, suggesting a trade-off between rotation speed and the dose penalty from overscanning. In general, overscanning was shown to have a greater radiation dose impact for larger diameter phantoms, shorter rotation times, and to peripheral phantom locations. Future research is necessary to determine an appropriate method for incorporating the localized dose penalty from overscanning into standard dose metrics, as well as to assess the impact on organ dose.

Setting up computed tomography automatic tube current modulation systems

Automatic tube current modulation (ATCM) on CT scanners can yield significant reductions in patient doses. Modulation is based on x-ray beam attenuation in body tissues obtained from scan projection radiographs (SPRs) and aims to maintain the same level of image quality throughout a scan. Noise level is important in judging image quality, but tissues in larger patients exhibit higher contrast resulting from the presence of fat. CT scanner manufacturers use different metrics to assess image quality. Some employ a simple measure of image noise, while others adopt a measure related to a reference image that accepts higher noise levels in more attenuating parts with higher contrast. At the present time there is no standard method for testing ATCM. This paper reviews the operation of different ATCM systems, considers options for testing, and sets out a framework that could be used for optimizing clinical protocols. If dose and image quality can be established for a reference phantom, the modulation performed by ATCM systems can be characterised using anatomical phantoms or geometrical elliptical phantoms which may be conical or include sections of varying dimension. For scanners using a reference image or mAs, selection of the image quality reference determines other factors. However, for scanners using a noise reference, a higher noise level should be selected for larger patients to avoid high doses, and the operator should ensure that appropriate limits are set for mA modulation. Other factors that need to be considered include the SPRs used to plan the ATCM and image thickness. Users should be aware of the mode of operation of the ATCM system on their CT scanner, and be familiar with the effects of changing different protocol parameters. The behaviour of ATCM systems should be established through testing of each CT scanner with suitable phantoms during commissioning.

IMPLICATIONS OF PATIENT CENTRING ON ORGAN DOSE IN COMPUTED TOMOGRAPHY

Automatic exposure control (AEC) in computed tomography (CT) facilitates optimisation of dose absorbed by the patient. The use of AEC requires appropriate 'patient centring' within the gantry, since positioning the patient off-centre may affect both image quality and absorbed dose. The aim of this experimental study was to measure the variation in organ and abdominal surface dose during CT examinations of the head, neck/thorax and abdomen. The dose was compared at the isocenter with two off-centre positions-ventral and dorsal to the isocenter. Measurements were made with an anthropomorphic adult phantom and thermoluminescent dosemeters. Organs and surfaces for ventral regions received lesser dose (5.6-39.0 %) than the isocenter when the phantom was positioned +3 cm off-centre. Similarly, organ and surface doses for dorsal regions were reduced by 5.0-21.0 % at -5 cm off-centre. Therefore, correct vertical positioning of the patient at the gantry isocenter is important to maintain optimal imaging conditions.

CT dose reduction: approaches, strategies and results from a province-wide program in Quebec

Many studies have shown a statistically significant increase of life-time risk of radiation-induced cancer from CT examinations. In this context, in Canada, the Quebec's provincial clinical center of expertise in radiation safety (CECR) has led a province-wide tour of 180 CT installations in order to: (i) evaluate the technical and functional performance of CT scanners, (ii) evaluate and improve radiation safety practices and (iii) initiate, with local teams, a CT dose optimization process. The CT tour consisted of a two day visit of CT installations by a CECR multidisciplinary team of medical physicists, engineers and medical imaging technologists (MITs) carried out in close collaboration with local teams composed of MITs, radiologists, physicists, engineers and managers. The CECR has evaluated 112 CT scanners since 2011. Optimization of CT protocols was performed in all centers visited. The average dose reduction obtained from optimization was [Formula: see text], [Formula: see text] and [Formula: see text] for adult head, thorax and abdomen-pelvis, respectively. The main recommendations often made by the CECR experts were: (1) the implementation of low-dose protocols for the follow-up of pulmonary nodules and for renal calculi, (2) the compliance to the prescribed scan range as defined by local guidelines, (3) the correct positioning of patients and (4) the use of bismuth shielding to reduce the dose to radiosensitive organs. The CECR approach to optimize CT doses to patients is based on the active participation of local stakeholders and takes into account the performance of CT scanners. The clinical requirements as expressed by radiologists remain at the core of the optimization process.

Improvement in CT image resolution due to the use of focal spot deflection and increased sampling

When patient anatomy is positioned away from a CT scanner's isocenter, scans of limited diagnostic value may result. Yet in some cases, positioning of patient anatomy far from isocenter is unavoidable. This study examines the effect of position and reconstruction algorithm on image resolution achieved by a CT scanner operating in a high resolution (HR) scan mode which incorporates focal spot deflection and acquires an increased number of projections per rotation. Images of a metal bead contained in a phantom were acquired on a GE CT750 HD scanner with multiple reconstruction algorithms, in the normal and HR scan mode, and at two positions, scanner isocenter and 15 cm directly above isocenter. The images of the metal bead yielded two‐dimensional point spread functions which were averaged along two perpendicular directions to yield line spread functions. Fourier transforms of the line spread functions yielded radial and azimuthal modulation transfer functions (MTFs). At isocenter, the radial and azimuthal MTFs were averaged. MTF improvement depended on image position and modulation direction. The results from a single algorithm, Edge, can be generalized to other algorithms. At isocenter, the 10% MTF cutoff was 14.4 cycles/cm in normal and HR mode. At 15 cm above isocenter, the 10% cutoff was 6.0 and 8.5 cycles/cm for the azimuthal and radial MTFs in normal mode. In HR mode, the azimuthal and radial MTF 10% cutoff was 8.3 and 10.3 cycles/cm. Our results indicate that the best image resolution is achieved at scanner isocenter and that the azimuthal resolution degrades more significantly than the radial resolution. For the GE CT750 HD CT scanner, the resolution is significantly enhanced by the HR scan mode away from scanner isocenter, and the use of the HR scan mode has much more of an impact on image resolution away from isocenter than the choice of algorithm.

PACS number(s): 87.57.Q‐

Patient Vertical Centering and Correlation with Radiation Output in Adult Abdominopelvic CT

The purpose of this study was to determine if there is a significant effect, independent of patient size, of patient vertical centering on the current-modulated CT scanner radiation output in adult abdominopelvic CT. A phantom was used to evaluate calculation of vertical positioning and effective diameter at five different table heights. In addition, 656 consecutive contrast-enhanced abdominopelvic scans using the same protocol and automatic tube current modulation settings on a Philips Brilliance 64 MDCT scanner were retrospectively evaluated. The vertical position of the patient center of mass and the average effective diameter of the scanned patient were computed using the reconstructed images. The average volume CT dose index (CTDIvol) for each scan was recorded. The mean patient center of mass y coordinate ranged from -3.7 to 6.7 cm (mean ± SD, 2.8 ± 1.2 cm), indicating that patients were on average positioned slightly below the scanner isocenter. There was a slight tendency for smaller patients to be mis-centered lower than larger patients. Average CTDIvol closely fit a quadratic regression curve with respect to mean effective diameter. However, the value of the regression coefficient relating CTDIvol to the patient's vertical position was nearly zero, indicating only a very slight increase in CTDIvol with patient mis-centering for the scanner used in this study. The techniques used here may be useful both for automated evaluation of proper patient positioning in CT and for estimating the radiation dose effects of patient mis-centering for any CT scanner.

Increased Computed Tomography Dose Due to Miscentering With Use of Automated Tube Voltage Selection: Phantom and Patient Study

The purpose of the article is to determine if miscentering affected dose with use of automated tube voltage selection software. An anthropomorphic phantom was imaged at different table heights (centered in the computed tomography [CT] gantry, and -6, -3, +3, and +5.7cm relative to the centered position). Topogram magnification, tube voltage selection, and dose were assessed. Effect of table height on dose also was assessed retrospectively in human subjects (n = 50). When the CT table was positioned closer to the x-ray source, subjects appeared up to 33% magnified in topogram images. When subjects appeared magnified in topogram images, automated software selected higher tube potentials and tube currents that were based on the magnified size of the subject rather than the subject׳s true size. Table height strongly correlated with CT dose index (r = 0.98, P < 0.05) and dose length product (r = 0.98, P < 0.05) in the phantom study. Transverse dimension in the topogram highly correlated with dose in human subjects (r = 0.75-0.87, P <0.05). Miscentering results in increased dose due to topogram magnification with automated voltage selection software.

Investigating the CT localizer radiograph: acquisition parameters, patient centring and their combined influence on radiation dose

OBJECTIVE

To systematically investigate the effect of CT localizer radiograph acquisition on the tube current modulation and thus radiation dose of the subsequent diagnostic scan.

METHODS

Localizer radiographs of an abdominal section CT phantom were taken, and the resulting volume CT dose index (CTDIvol) for the diagnostic scan was recorded. Variables included tube potential, the phantom's alignment within the CT scanner gantry in both the vertical and horizontal directions and the X-ray source angle at which the localizer was acquired.

RESULTS

Diagnostic scan CTDIvol decreased with increasing tube potential. Vertical (table height) movement was found to affect radiation dose more than horizontal movement, with ±50 mm table movement resulting in a standard deviation in the diagnostic scan CTDIvol of 4.4 mGy, compared with 2.5 mGy with ±50 mm horizontal movement. Correspondingly, localizer angles of 90° or 270° (3 o'clock and 9 o'clock X-ray source positions) were less sensitive overall to alignment errors, with a standard deviation of 2.5 mGy, compared with a 0° or 180° angle, which had a standard deviation of 3.8 mGy.

CONCLUSION

To achieve a consistently optimized radiation dose, the localizer protocol should be paired with the diagnostic acquisition protocol. A final acquisition angle of 90° should be used when possible to minimize dose variation resulting from alignment errors.

ADVANCES IN KNOWLEDGE

Localizer parameters that affect radiation output were identified for this scanner system. The importance of tube potential and acquisition angle was highlighted.

Variability of MDCT dose due to technologist performance: impact of posteroanterior versus anteroposterior localizer image and table height with use of automated tube current modulation

OBJECTIVE

The purpose of this study was to determine MDCT dose variability due to technologist variability in performing CT studies.

MATERIALS AND METHODS

Fifty consecutive adult patients who underwent two portal venous phase CT examinations of the abdomen and pelvis on the same 64-MDCT scanner between January and December 2011 were retrospectively identified. Tube voltage (kVp), tube current (mA), use of automated tube current modulation (ATCM), dose-length product (DLP), volume CT dose index (CTDIvol), table height, whether the localizer image was obtained using the posteroanterior or the anteroposterior technique, arm position, and number of overscanned slices were recorded.

RESULTS

For a given patient, the total examination DLP difference comparing the two MDCT studies ranged from 0.1% to 238.0%. For the same patient, total examination DLP was always higher when the localizer image was obtained with the posteroanterior compared with the anteroposterior technique. When table position was closer to the x-ray source, patients appeared magnified in the posteroanterior localizer image (8-29%; average, 14%) and higher tube currents were selected with ATCM. Localizer technique, table height, arm position, number of overscanned slices, and technologist were all significant predictors of dose.

CONCLUSION

Patient off-centering closer to the x-ray source resulted in patient magnification in the posteroanterior localizer image, leading to higher tube currents with ATCM and increased DLP. Differences in technologist, arm position, and overscanning also resulted in dose variability.

Effect of patient centering on patient dose and image noise in chest CT

OBJECTIVE

The objective of our study was to evaluate the effect of vertical centering on dose and image noise in chest MDCT of different-sized patients using anthropomorphic phantoms and retrospectively studying examinations of clinical patients.

MATERIALS AND METHODS

Three different anthropomorphic phantoms were scanned using different vertical centering (offset ± 6 cm) and were assessed with radiation dose-monitoring software. The effect of vertical positioning on the radiation dose was studied using the volume CT dose index, dose-length product, and size-specific dose estimates for different-sized phantoms. Image noise was determined from CT number histograms. Vertical positioning for chest CT examinations of 112 patients ranging from neonates to adults were retrospectively assessed.

RESULTS

Radiation doses were highest when using the posteroanterior scout image for automatic exposure control (AEC) and when phantoms were set in the lowest table position, and radiation doses were lowest when phantoms were set in the uppermost table position. For the adult phantom, relative doses increased by 38% in the lowest table position and decreased by 23% in the highest table position. Similarly, doses for pediatric 5-year-old and newborn phantoms were 21% and 12% higher in the lowest table position and 12% and 8% lower in the highest table position, respectively. The effect decreased when a lateral scout image was used for AEC. The relative noise was lowest when the phantoms were properly centered and increased with vertical offset. In clinical patients, we observed offset with a median value varying from 25 to 35 mm below the isocenter.

CONCLUSION

Regardless of patient size, most patients in this study were positioned too low, which negatively affected both patient dose and image noise. Miscentering was more pronounced in smaller pediatric patients.

Size-specific dose estimates: Localizer or transverse abdominal computed tomography images?

AIM: To investigate effect of body dimensions obtained from localizer radiograph and transverse abdominal computed tomography (CT) images on Size Specific Dose Estimate.

METHODS: This study was approved by Institutional Review Board and was compliant with Health Insurance Portability and Accountability Act. Fifty patients with abdominal CT examinations (58 ± 13 years, Male:Female 28:22) were included in this study. Anterior-posterior (AP) and lateral (Lat) diameters were measured at 5 cm intervals from the CT exam localizer radiograph (simple X-ray image acquired for planning the CT exam before starting the scan) and transverse CT images. Average of measured AP and Lat diameters, as well as maximum, minimum and mid location AP and Lat were measured on both image sets. In addition, off centering of patients from the gantry iso-center was calculated from the localizers. Conversion factors from American Association of Physicists in Medicine (AAPM) report 204 were obtained for AP, Lat, AP + Lat, and effective diameter (√ AP * Lat) to determine size specific dose estimate (SSDE) from the CT dose index volume (CTDI_vol) recorded from the dose reports. Data were analyzed using SPSS v19.

RESULTS: Total number of 5376 measurements was done. In some patients entire body circumference was not covered on either projection radiograph or transverse CT images; hence accurate measurement of AP and Lat diameters was not possible in 11% (278/2488) of locations. Forty one patients were off-centered with mean of 1.9 ± 1.8 cm (range: 0.4-7 cm). Conversion factors for attained diameters were not listed on AAPM look-up tables in 3% (80/2488) of measurements. SSDE values were significantly different compared to CTDI_vol, ranging from 32% lower to 74% greater than CTDI_vol.

CONCLUSION: There is underestimation and overestimation of dose comparing SSDE values to CTDI_vol. Localizer radiographs are associated with overestimation of patient size and therefore underestimation of SSDE.

Effect of localizer radiograph on radiation dose associated with automatic exposure control: human cadaver and patient study

PURPOSE

To evaluate the effect of localizing radiograph on computed tomography (CT) radiation dose associated with automatic exposure control with a human cadaver and patient study.

MATERIALS AND METHODS

Institutional review board approved the study with a waiver of informed consent. Two chest CT image series with fixed tube current and combined longitudinal-angular automatic exposure control (AEC) were acquired in a human cadaver (64-year-old man) after each of the 8 combinations of localizer radiographs (anteroposterior [AP], AP lateral, AP-posteroanterior [PA], lateral AP, lateral PA, PA, PA-AP, and PA lateral). Applied effective milliampere second, volume CT dose index (CTDIvol) and image noise were recorded for all 24-image series. Volume CT dose indexes were also recorded in 20 patients undergoing chest and abdominal CT after PA and PA-lateral radiographs with the use of AEC. Data were analyzed using analysis of variance and linear correlation tests.

RESULTS

With AEC, the CTDIvol fluctuates with the number and projection of localizer radiographs (P < 0.0001). Lowest CTDIvol values are seen when 2 orthogonal localizer radiographs are acquired, whereas highest values are seen when single PA or AP-PA projection localizer radiographs are acquired for planning (P < 0.0001). In 20 patients, CT scanning with AEC after acquisition of 2 orthogonal projection localizer radiographs was associated with significant reduction in radiation dose compared to PA projection radiographs alone (P < 0.0001).

CONCLUSIONS

When scanning with AEC, acquisition of 2 orthogonal localizer radiographs is associated with lower CTDIvol compared to a single localizer radiograph.