Automatic exposure control at MDCT based on the contrast-to-noise ratio: theoretical background and phantom study

Optimizing image quality using automatic exposure control based on the signal-difference-to-noise ratio: a phantom study

This study proposes to adjust the sensitivity of automatic exposure control (AEC) for achieving consistent image quality over a range of subject thicknesses in abdominal radiography simulations. The relation between image quality and subject thickness was investigated using a digital radiography system with 10-, 15-, 20-, and 25-cm-thick acrylic phantom. Simple pixel signal-to-noise ratio (SNR) was measured to check the default AEC accuracy for each thickness, and image quality was evaluated using the signal-difference-to-noise ratio (SDNR) with an additional acrylic plate and bone-equivalent material. Based on the figure of merit theory, dose ratios to obtain constant image quality regardless of the subject thickness were calculated from SDNR results. The AEC setup was manually modified using this dose ratio, and visibility was examined using a CDRAD 2.0 contrast-detail analysis phantom. Moreover, the entrance surface dose (ESD) was estimated as an index of exposure dose using exposure parameters. The default AEC setup provided a constant simple pixel SNR for each subject thickness with a high accuracy. SDNRs decreased with an increase in the subject thickness. The calculated dose ratios relative to the results for 20 cm thickness were 0.424, 0.647, and 1.43 for 10, 15 and 25 cm, respectively, and a > 25% decrease in ESD was observed for smaller patients. CDRAD analysis using the modified AEC setup showed almost identical visibility for each thickness. Adjusting the sensitivity of AEC according to subject thickness can contribute toward the optimization of the exposure condition.

Usefulness of diluted contrast medium for test-scanning of infants scheduled for contrast-enhanced cardiovascular computed tomography angiography

OBJECTIVE:

To compare the ability of test scans with undiluted and diluted contrast medium (CM) to predict contrast enhancement (CE) on cardiovascular CT angiography (CCTA) images of infants.

METHODS:

We divided 120 consecutive infants who had undergone CCTA on a 64-MDCT scanner into two equal groups. In one group, the test bolus consisted of undiluted CM [protocol 1 (P1): injection volume = total  body weight × 1.2  ml, injection time 5  s], in the other (P2) it was total  body weight × 4.0  ml (CM 15%, saline 85%, injection time 16  s). CE on the test scans was recorded on a 3-point visual scale. We investigated the relation for CE in the pulmonary artery and ascending aorta between the P1 or P2 test scans and CCTA images.

RESULTS:

While peak CE was observed on all test scans performed with P2, in approximately 10 % of test scans obtained under P1, peak CE was not visualized. There was a strong positive linear correlation for CE of the pulmonary artery and ascending aorta on P2 images (r = 0.61  and r = 0.63, p < 0.01); under P1 the correlation was weak (r = 0.26  and r = 0.33, p < 0.01).

CONCLUSION:

Test-scanning with diluted CM revealed the optimal CE peak time and was useful for predicting CE on CCTA scans of the pulmonary artery and ascending aorta in infants with congenital heart disease.

ADVANCES IN KNOWLEDGE:

Diluted test scans help to select the optimal scan parameters for the CCTA study of infants by using contrast-to-noise-based scanning.

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.

Radiation dose reduction based on CNR index with low-tube voltage scan for pediatric CT scan: experimental study using anthropomorphic phantoms

Background

To figure out the relationship between image noise and contrast noise ratio (CNR) at different tube voltages, using anthropomorphic new-born and 1-year-old phantoms, and to discuss the feasibility of radiation dose reduction, based on the obtained CNR index from image noise. We performed helical scans of the anthropomorphic new-born and 1-year-old phantoms. The CT numbers of the simulated aorta and image noise of the simulated mediastinum were measured; then CNR was calculated on 80, 100, and 120-kVp images reconstructed with filtered back projection (FBP) and iterative reconstruction (IR). We also measured the center and surface dose in the case of CNR of 14 using radio-photoluminescence glass dosimeters.

Results

The CT number of the simulated aorta was increased with decreasing tube voltage from 120 to 80 kVp (362.5–535.1 HU for the new-born, 358.9–532.6 HU for the 1-year-old). At CNR of 14, the center dose was 0.4, 0.6 and 0.9 mGy at FBP and 0.5, 0.6 and 0.9 mGy at IR and with the new-born phantom acquired at 80, 100 and 120 kVp, respectively. The center dose for FBP image was reduced by 56% at 80 kVp, 34% at 100 kVp for the new-born and 36% at 80 kVp, 22% at 100 kVp for the 1-year-old compared with that at 120 kVp. We obtained a relationship between image noise and CNR at different tube voltages using the anthropomorphic new-born and 1-year-old phantoms.

Conclusion

The use of index of CNR with low-tube voltage may achieve further radiation dose reduction in pediatric CT examination.

[Study of CT Automatic Exposure Control System (CT-AEC) Optimization in CT Angiography of Lower Extremity Artery by Considering Contrast-to-Noise Ratio]

PURPOSE

To evaluate the image quality and effect of radiation dose reduction by setting for computed tomography automatic exposure control system (CT-AEC) in computed tomographic angiography (CTA) of lower extremity artery.

METHODS

Two methods of setting were compared for CT-AEC [conventional and contrast-to-noise ratio (CNR) methods]. Conventional method was set noise index (NI): 14and tube current threshold: 10-750 mA. CNR method was set NI: 18, minimum tube current: (X+Y)/2 mA (X, Y: maximum X (Y)-axis tube current value of leg in NI: 14), and maximum tube current: 750 mA. The image quality was evaluated by CNR, and radiation dose reduction was evaluated by dose-length-product (DLP).

RESULTS

In conventional method, mean CNRs for pelvis, femur, and leg were 19.9±4.8, 20.4±5.4, and 16.2±4.3, respectively. There was a significant difference between the CNRs of pelvis and leg (P<0.001), and between femur and leg (P<0.001). In CNR method, mean CNRs for pelvis, femur, and leg were 15.2±3.3, 15.3±3.2, and 15.3±3.1, respectively; no significant difference between pelvis, femur, and leg (P=0.973) in CNR method was observed. Mean DLPs were 1457±434 mGy⋅cm in conventional method, and 1049±434 mGy·cm in CNR method. There was a significant difference in the DLPs of conventional method and CNR method (P<0.001).

CONCLUSION

CNR method gave equal CNRs for pelvis, femur, and leg, and was beneficial for radiation dose reduction in CTA of lower extremity artery.

On the relationship of minimum detectable contrast to dose and lesion size in abdominal CT

CT dose optimization is typically guided by pixel noise or contrast-to-noise ratio that does not delineate low contrast details adequately. We utilized the statistically defined low contrast detectability to study its relationship to dose and lesion size in abdominal CT. A realistically shaped medium sized abdomen phantom was customized to contain a cylindrical void of 4 cm diameter. The void was filled with a low contrast (1% and 2%) insert containing six groups of cylindrical targets ranging from 1.2 mm to 7 mm in size. Helical CT scans were performed using a Siemens 64-slice mCT and a GE Discovery 750 HD at various doses. After the subtractions between adjacent slices, the uniform sections of the filtered backprojection reconstructed images were partitioned to matrices of square elements matching the sizes of the targets. It was verified that the mean values from all the elements in each matrix follow a Gaussian distribution. The minimum detectable contrast (MDC), quantified by the mean signal to background difference equal to the distribution's standard deviation multiplied by 3.29, corresponding to 95% confidence level, was found to be related to the phantom specific dose and the element size by a power law (R^2  >  0.990). Independent readings on the 5 mm and 7 mm targets were compared to the measured contrast to the MDC ratios. The results showed that 93% of the cases were detectable when the measured contrast exceeds the MDC. The correlation of the MDC to the pixel noise and target size was also identified and the relationship was found to be the same for the scanners in the study. To quantify the impact of iterative reconstructions to the low contrast detectability, the noise structure was studied in a similar manner at different doses and with different ASIR blending fractions. The relationship of the dose to the blending fraction and low contrast detectability is presented.

Optimization of dental CBCT exposures through mAs reduction

OBJECTIVES

To investigate the effect of tube current-exposure time (mAs) reduction on clinical and technical image quality for different CBCT scanners, and to determine preliminary minimally acceptable values for the mAs and contrast-to-noise ratio (CNR) in CBCT.

METHODS

A polymethyl methacrylate (PMMA) phantom and an anthropomorphic skull phantom, containing a human skeleton embedded in polyurethane, were scanned using four CBCT devices, including seven exposure protocols. For all protocols, the mAs was varied within the selectable range. Using the PMMA phantom, the CNRAIR was measured and corrected for voxel size. Eight axial slices and one coronal slice showing various anatomical landmarks were selected for each CBCT scan of the skull phantom. The slices were presented to six dentomaxillofacial radiologists, providing scores for various anatomical and diagnostic parameters.

RESULTS

A hyperbolic relationship was seen between CNRAIR and mAs. Similarly, a gradual reduction in clinical image quality was seen at lower mAs values; however, for several protocols, image quality remained acceptable for a moderate or large mAs reduction compared with the standard exposure setting, depending on the clinical application. The relationship between mAs, CNRAIR and observer scores was different for each CBCT device. Minimally acceptable values for mAs were between 9 and 70, depending on the criterion and clinical application.

CONCLUSIONS

Although noise increased at a lower mAs, clinical image quality often remained acceptable at exposure levels below the manufacturer's recommended setting, for certain patient groups. Currently, it is not possible to determine minimally acceptable values for image quality that are applicable to multiple CBCT models.

CT image quality over time: comparison of image quality for six different CT scanners over a six-year period

UNSCEAR concluded that increased use of CT scanning caused dramatic changes in population dose. Therefore, international radiation protection authorities demand: 1) periodical quality assurance tests with respect to image quality and radiation dose, and 2) optimization of all examination protocols with respect to image quality and radiation dose. This study aimed to evaluate and analyze multiple image quality parameters and variability measured throughout time for six different CT scanners from four different vendors, in order to evaluate the current methodology for QA controls of CT systems. The results from this study indicate that there is minor drifting in the image noise and uniformity and in the spatial resolution over time for CT scanners, independent of vendors. The HU for different object densities vary between different CT scanner models from different vendors, and over time for one specific CT scanner. Future tests of interphantom and intraphantom variations, along with inclusion of more CT scanners, are necessary to establish robust baselines and recommendations of methodology for QA controls of CT systems, independent of model and vendor.

PACS number: 07, 87

Prediction of aortic enhancement on coronary CTA images using a test bolus of diluted contrast material

RATIONALE AND OBJECTIVES

The purpose of our study was to compare test bolus techniques using undiluted or diluted contrast material for their ability to predict aortic enhancement on coronary computed tomographic angiography (c-CTA) images.

MATERIALS AND METHODS

We divided 200 consecutive patients who underwent c-CTA on a 64-MDCT scanner into two groups. In group A (n = 100), we used a test bolus of undiluted contrast material and in group B (n = 100), the contrast material was diluted. The injection volume was body weight × 0.2 (contrast material 100%) in group A and body weight × 0.7 (contrast material 30%, saline 70%) in group B. We then compared the CT number in the ascending aorta on c-CTA images obtained with undiluted and diluted contrast media to the CT number on c-CTA images.

RESULTS

The mean CT number in the ascending aorta was significantly higher in group B than group A (217.1 vs. 157.4 HU, P < .001). There was a significant difference in the correlation between the CT number of the ascending aorta on c-CTA images and on images acquired with the test bolus using undiluted or diluted test bolus (P < .001). In group B, the correlation had a strong positive linear relationship (r = 0.72, P < .001), whereas in group A the positive linear relationship was weak (r = 0.36).

CONCLUSIONS

The test bolus with diluted contrast material was useful for predicting aortic enhancement before c-CTA scanning.