Lower tube voltage reduces contrast material and radiation doses on 16-MDCT aortography

Current status of low dose multi-detector CT in the urinary tract

Over the past several years, advances in the technical domain of computed tomography (CT) have influenced the trend of imaging modalities used in the clinical evaluation of the urinary system. Renal collecting systems can be illustrated more precisely with the advent of multi-detector row CT through thinner slices, high speed acquisitions, and enhanced longitudinal spatial resolution resulting in improved reformatted coronal images. On the other hand, a significant increase in exposure to ionizing radiation, especially in the radiosensitive organs, such as the gonads, is a concern with the increased utilization of urinary tract CT. In this article, we discuss the strategies and techniques available for reducing radiation dose for a variety of urinary tract CT protocols with metabolic clinical examples. We also reviewed CT for hematuria evaluation and related scan parameter optimization such as, reducing the number of acquisition phases, CT angiography of renal donors and lowering tube potential, when possible.

Abdominal dynamic CT in patients with renal dysfunction: contrast agent dose reduction with low tube voltage and high tube current-time product settings at 256-detector row CT

PURPOSE

To evaluate the feasibility of a low-contrast agent dose protocol at abdominal dynamic computed tomography (CT) with a low tube voltage high tube current-time product technique and a 256-detector row CT unit.

MATERIALS AND METHODS

This prospective study received institutional review board approval; written informed consent to participate was obtained from all patients. The study included 151 patients; 117 had an estimated glomerular filtration rate (eGFR) greater than or equal to 60 mL/min/1.73 m(2). These patients were examined with the conventional 120-kVp protocol. The other 34 patients underwent scanning with an 80-kVp tube voltage, a high tube current-time product, and a 40% reduction in contrast agent dose. Effective dose (ED), image noise, attenuation, contrast-to-noise ratio (CNR), and figure of merit of the aorta in the arterial phase and of the portal vein and hepatic parenchyma in the portal venous phase in the two groups were compared with the Student t test.

RESULTS

Estimated ED was about 20% lower with the 80-kVp protocol than with the 120-kVp protocol. There were no significant differences in CNR in any region of interest between the 80-kVp protocol and the 120-kVp protocol (abdominal aorta: 36.9 ± 9.7 [standard deviation] vs 36.1 ± 8.1, P = .63; portal vein: 13.4 ± 3.2 vs 13.1 ± 3.2, P = .65; hepatic parenchyma: 6.4 ± 2.6 vs 6.7 ± 2.3, P = .51).

CONCLUSION

Contrast dose at hepatic dynamic 256-detector row CT in patients with renal dysfunction can be decreased by 40% with this protocol by using the 80-kVp setting and a high tube current-time product.

A low tube voltage technique reduces the radiation dose at retrospective ECG-gated cardiac computed tomography for anatomical and functional analyses

RATIONALE AND OBJECTIVES

To investigate the effect of low-tube-voltage technique on a cardiac computed tomography (CT) for coronary arterial and cardiac functional analyses and radiation dose in slim patients.

MATERIALS AND METHODS

We enrolled 80 patients (52women, 28 men; mean age, 68.7 ± 8.9 years) undergoing retrospective electrocardiogram-gated 64-slice cardiac CT. Forty were subjected to the low (80-kV) and 40 to the standard (120-kV) tube-voltage protocol. Quantitative parameters of the coronary arteries (ie, CT attenuation, image noise, and the contrast-to-noise ratio [CNR]) were calculated, as were the effective radiation dose and the figure of merit (FOM). Each coronary artery segment was visually evaluated using a 5-point scale. Cardiac function calculated by using low-tube-voltage cardiac CT was compared with that on echocardiographs.

RESULTS

CT attenuation and image noise were significantly higher at 80- than 120-kV (P < .01). CNR of the left and right coronary artery was 18.4 ± 3.8 and 18.5 ± 3.3, respectively, at 80 kV; these values were 19.7 ± 2.7 and 19.8 ± 2.8 at 120 kV; the difference was not significant. The estimated effective radiation dose was significantly lower at 80 than 120 kV (6.3 ± 0.6 vs. 13.9 ± 1.1 mSv, P < .01) and FOM was significantly higher at 80 than 120 kV (P < .01). At visual assessment, 99% of the coronary segments were diagnostic quality; the two protocols did not differ significantly. We observed a strong correlation and good agreement between low-tube-voltage cardiac CT and echocardiography for cardiac functional analyses.

CONCLUSION

Low-tube-voltage cardiac CT significantly reduced the radiation dose by approximately 55% in slim patients while maintaining anatomical image quality and accuracy of cardiac functional analysis.

A review of patient dose and optimisation methods in adult and paediatric CT scanning

An increasing number of publications and international reports on computed tomography (CT) have addressed important issues on optimised imaging practice and patient dose. This is partially due to recent technological developments as well as to the striking rise in the number of CT scans being requested. CT imaging has extended its role to newer applications, such as cardiac CT, CT colonography, angiography and urology. The proportion of paediatric patients undergoing CT scans has also increased. The published scientific literature was reviewed to collect information regarding effective dose levels during the most common CT examinations in adults and paediatrics. Large dose variations were observed (up to 32-fold) with some individual sites exceeding the recommended dose reference levels, indicating a large potential to reduce dose. Current estimates on radiation-related cancer risks are alarming. CT doses account for about 70% of collective dose in the UK and are amongst the highest in diagnostic radiology, however the majority of physicians underestimate the risk, demonstrating a decreased level of awareness. Exposure parameters are not always adjusted appropriately to the clinical question or to patient size, especially for children. Dose reduction techniques, such as tube-current modulation, low-tube voltage protocols, prospective echocardiography-triggered coronary angiography and iterative reconstruction algorithms can substantially decrease doses. An overview of optimisation studies is provided. The justification principle is discussed along with tools that assist clinicians in the decision-making process. There is the potential to eliminate clinically non-indicated CT scans by replacing them with alternative examinations especially for children or patients receiving multiple CT scans.

The optimal contrast media policy in CT of the liver. Part I: Technical notes

Latest developments of multidetector computed tomography (MDCT), which is today considered a real volumetric technique, have revolutionized abdominal imaging. Technological improvements such as higher spatial resolution, larger volume coverage and higher temporal resolution, have reduced scan times allowing CT studies of the abdomen within a single breath-hold. Furthermore, the increased number of slices, the submillimetric collimation, and the use of multiple dynamic post-contrast phases per single examination, may all contribute to increase the radiation exposure of single patients. The aim of this review is to discuss different parameters affecting contrast media enhancement, as vascular enhancement, parenchymal enhancement and timing, in order to minimize the amount of contrast medium injected and the radiation exposure.

Dual-energy computed tomography imaging of the aorta

There are 2 inseparable and complimentary technical advantages of dual-energy computed tomography (CT) imaging of the thoracic aorta. One advantage stems from the simultaneous availability of low and high peak kilovoltage (kVp) spectra data and, in particular, the benefits conferred by the improved conspicuity of iodinated contrast media at lower kVp CT imaging. This, in turn, permits improved aortic visualization or, alternatively, reduction in the volume or rate of contrast administration. Image noise at low kilovoltage does not appear to be a significant issue, with the backup availability of simultaneously acquired high kVp images a distinct advantage over single, low kVp imaging techniques. The second advantage of dual-energy CT imaging stems from the potential to calculate material-specific images derived mathematically from the simultaneous availability of attenuation measurements at 2 distinct energies. These material-specific data sets include virtual noncontrast images, virtual contrast, or "bone-subtracted" angiographic-like images. These techniques may confer significant advantages in the evaluation of patients with aortic disease, improving interpretation and reducing reconstruction time, while potentially reducing the need for, and associated radiation burden of, precontrast or postcontrast multiphasic imaging.

State of the art low-dose CT angiography of the body

This review summarizes current evidence based on pertinent literature on low-dose computed tomography angiography (CTA) of the body. Various strategies for optimizing CTA protocols with the aim to lower the radiation dose while maintaining the diagnostic accuracy of the examination are summarized. To date, various publications have demonstrated that CTA of the body can be performed at a low radiation dose while providing high quality information. Nevertheless, a number of questions still need to be answered, including the optimal combination of tube voltage and tube current settings, as well as the appropriate protocol parameters in relation to the body physiognomy and the specific body region imaged.

Appropriate patient selection at abdominal dual-energy CT using 80 kV: relationship between patient size, image noise, and image quality

PURPOSE

To determine the computed tomographic (CT) detector configuration, patient size, and image noise limitations that will result in acceptable image quality of 80-kV images obtained at abdominal dual-energy CT.

MATERIALS AND METHODS

The Institutional Review Board approved this HIPAA-compliant retrospective study from archival material from patients consenting to the use of medical records for research purposes. A retrospective review of contrast material-enhanced abdominal dual-energy CT scans in 116 consecutive patients was performed. Three gastrointestinal radiologists noted detector configuration and graded image quality and artifacts at specified levels-midliver, midpancreas, midkidneys, and terminal ileum-by using two five-point scales. In addition, an organ-specific enhancement-to-noise ratio and background noise were measured in each patient. Patient size was measured by using the longest linear dimension at the level of interest, weight, lean body weight, body mass index, and body surface area. Detector configuration, patient sizes, and image noise levels that resulted in unacceptable image quality and artifact rankings (score of 4 or higher) were determined by using multivariate logistic regression.

RESULTS

A 14 × 1.2-mm detector configuration resulted in fewer images with unacceptable quality than did the 64 × 0.6-mm configuration at all anatomic levels (P = .004, .01, and .02 for liver, pancreas, and kidneys, respectively). Image acceptability for the kidneys and ileum was significantly greater than that for the liver for all readers and detector configurations (P < .001). For the 14 × 1.2-mm detector configuration, patient longest linear dimensions yielding acceptable image quality across readers ranged from 34.9 to 35.8 cm at the four anatomic levels.

CONCLUSION

An 80-kV abdominal CT can be performed with appropriate diagnostic quality in a substantial percentage of the population, but it is not recommended beyond the described patient size for each anatomic level. The 14 × 1.2-mm detector configuration should be preferred.

Development of low-dose protocols for thin-section CT assessment of cystic fibrosis in pediatric patients

PURPOSE

To develop low-dose thin-section computed tomographic (CT) protocols for assessment of cystic fibrosis (CF) in pediatric patients and determine the clinical usefulness thereof compared with chest radiography.

MATERIALS AND METHODS

After institutional review board approval and informed consent from patients or guardians were obtained, 14 patients with CF and 11 patients without CF (16 male, nine female; mean age, 12.6 years ± 5.4 [standard deviation]; range, 3.5-25 years) who underwent imaging for clinical reasons underwent low-dose thin-section CT. Sections 1 mm thick (protocol A) were used in 10 patients, and sections 0.5 mm thick (protocol B) were used in 15 patients at six levels at 120 kVp and 30-50 mA. Image quality and diagnostic acceptability were scored qualitatively and quantitatively by two radiologists who also quantified disease severity at thin-section CT and chest radiography. Effective doses were calculated by using a CT dosimetry calculator.

RESULTS

Low-dose thin-section CT was performed with mean effective doses of 0.19 mSv ± 0.03 for protocol A and 0.14 mSv ± 0.04 for protocol B (P < .005). Diagnostic acceptability and depiction of bronchovascular structures at lung window settings were graded as almost excellent for both protocols, but protocol B was inferior to protocol A for mediastinal assessment (P < .02). Patients with CF had moderate lung disease with a mean Bhalla score of 9.2 ± 5.3 (range, 0-19), compared with that of patients without CF (1.1 ± 1.4; P < .001). There was excellent correlation between thin-section CT and chest radiography (r = 0.88-0.92; P < .001).

CONCLUSION

Low-dose thin-section CT can be performed at lower effective doses than can standard CT, approaching those of chest radiography. Low-dose thin-section CT could be appropriate for evaluating bronchiectasis in pediatric patients, yielding appropriate information about lung parenchyma and bronchovascular structures.

Effect of automatic tube current modulation on radiation dose and image quality for low tube voltage multidetector row CT angiography: phantom study

RATIONALE AND OBJECTIVES

To evaluate the effect of automatic tube current modulation on radiation dose and image quality for low tube voltage computed tomography (CT) angiography.

MATERIALS AND METHODS

An anthropomorphic phantom was scanned with a 64-section CT scanner using following tube voltages: 140 kVp (Protocol A), 120 kVp (Protocol B), 100 kVp (Protocol C), and 80 kVp (Protocol D). To achieve similar noise, combined z-axis and xy-axes automatic tube current modulation was applied. Effective dose (ED) for the four tube voltages was assessed. Three plastic vials filled with different concentrations of iodinated solution were placed on the phantom's abdomen to obtain attenuation measurements. The signal-to-noise ratio (SNR) was calculated and a figure of merit (FOM) for each iodinated solution was computed as SNR(2)/ED.

RESULTS

The ED was kept similar for the four different tube voltages: (A) 5.4 mSv +/- 0.3, (B) 4.1 mSv +/- 0.6, (C) 3.9 mSv +/- 0.5, and (D) 4.2 mSv +/- 0.3 (P > .05). As the tube voltage decreased from 140 to 80 kVp, image noise was maintained (range, 13.8-14.9 HU) (P > .05). SNR increased as the tube voltage decreased, with an overall gain of 119% for the 80-kVp compared to the 140-kVp protocol (P < .05). The FOM results indicated that with a reduction of the tube voltage from 140 to 120, 100, and 80 kVp, at constant SNR, ED was reduced by a factor of 2.1, 3.3, and 5.1, respectively, (P < .001).

CONCLUSIONS

As tube voltage decreases, automatic tube current modulation for CT angiography yields either a significant increase in image quality at constant radiation dose or a significant decrease in radiation dose at a constant image quality.

The image quality and radiation dose of 100-kVp versus 120-kVp ECG-gated 16-slice CT coronary angiography

Objective

This study was conducted to assess the feasibility of performing 100-kVp electrocardiogram (ECG)-gated coronary CT angiography, as compared to 120-kVp ECG-gated coronary CT angiography.

Materials and Methods

We retrospectively evaluated one hundred eighty five gender- and body mass index-matched 16-slice coronary CT sets of data, which were obtained using either 100 kVp and 620 effective mAs or 120 kVp and 500 effective mAs. The density measurements (image noise, vessel density, signal-to-noise ratio [SNR] and contrast-to-noise ratio [CNR]) and the estimated radiation dose were calculated. As a preference test, two image readers were independently asked to choose one image from each pair of images. The results of both protocols were compared using the paired t-test or the Wilcoxon signed rank test.

Results

The 100-kVp images showed significantly more noise and a significantly higher vessel density than did the 120-kVp images. There were no significant differences in the SNR and CNR. The estimated reduction of the radiation dose for the 100-kVp protocol was 24%; 7.8 ± 0.4 mSV for 100-kVp and 10.1 ± 1.0 mSV for 120-kVp (p < 0.001). The readers preferred the 100-kVp images for reading (reader 1, p = 0.01; reader 2, p = 0.06), with their preferences being stronger when the subject's body mass index was less than 25.

Conclusion

Reducing the tube kilovoltage from 120 to 100 kVp allows a significant reduction of the radiation dose without a significant change in the SNR and the CNR.

Cone-beam technique for 64-MDCT of lung: image quality comparison with stepwise (step-and-shoot) technique

OBJECTIVE

The purpose of this study was to use phantom and patient data acquired with 64-MDCT to compare the image quality and characteristics of helical high-resolution CT images obtained with cone-beam reconstruction with those of stepwise high-resolution CT images obtained with fan-beam reconstruction.

SUBJECTS AND METHODS

We reconstructed helical high-resolution CT images with cone-beam technique and stepwise high-resolution CT images with fan-beam technique. In the phantom study, we measured high-contrast spatial resolution and image noise using a phantom. Streak artifact was evaluated by five radiologists using the phantom. In the clinical phase of the study, two radiologists independently evaluated high-resolution helical and stepwise CT images of the lung fields of 30 patients with diffuse lung disease. Using a 3-point ordinal scale, the radiologists assessed the sharpness of peripheral vessels and interlobular fissures, artifacts, and graininess in the lung fields; overall image quality; and the sharpness of the contour of the left ventricle.

RESULTS

In high-contrast spatial resolution, the contrast curves in each spatial frequency were similar on the helical and stepwise images. In the clinical study, there was no statistically significant difference between helical and stepwise images with respect to sharpness of the contour of the left ventricle, peripheral vessels, or interlobular fissures (p>0.05). With respect to streak artifacts and graininess in the lung fields, helical images received a significantly higher quality grade than did stepwise images (p<0.05).

CONCLUSION

Our phantom and clinical evaluation showed that the quality of high-resolution CT images of the lung obtained with helical scanning was comparable with the quality of stepwise scans.

Radiation dose savings for adult pulmonary embolus 64-MDCT using bismuth breast shields, lower peak kilovoltage, and automatic tube current modulation

OBJECTIVE

The purpose of this study was to assess whether radiation dose savings using a lower peak kilovoltage (kVp) setting, bismuth breast shields, and automatic tube current modulation could be achieved while preserving the image quality of MDCT scans obtained to assess for pulmonary embolus (PE).

MATERIALS AND METHODS

CT angiography (CTA) examinations were performed to assess for the presence or absence of pulmonary artery emboli using a 64-MDCT scanner with automatic tube current modulation (noise level=10 HU), two kVp settings (120 and 140 kVp), and bismuth breast shields. Absorbed organ doses were measured using anthropomorphic phantoms and metal oxide semiconductor field effect transistor (MOSFET) detectors. Image quality was assessed quantitatively as well as qualitatively in various anatomic sites of the thorax.

RESULTS

Using a lower kVp (120 vs 140 kVp) and automatic tube current modulation resulted in a dose savings of 27% to the breast and 47% to the lungs. The use of a lower kVp (120 kVp), automatic tube current modulation, and bismuth shields placed directly on the anterior chest wall reduced absorbed breast and lung doses by 55% and 45%, respectively. Qualitative assessment of the images showed no change in image quality of the lungs and mediastinum when using a lower kVp, bismuth shields, or both.

CONCLUSION

The use of bismuth breast shields together with a lower kVp and automatic tube current modulation will reduce the absorbed radiation dose to the breast and lungs without degradation of image quality to the organs of the thorax for CTA detection of PE.

Multiphase CT angiography versus single-phase CT angiography: comparison of image quality and radiation dose

BACKGROUND AND PURPOSE

Conventional CT angiography (CTA) is acquired during only a short interval in the arterial phase, which limits its ability to evaluate the cerebral circulation. Our aim was to compare the image quality and radiation dose of conventional single-phase CTA (SP-CTA) with a multiphase CTA (MP-CTA) algorithm reconstructed from a perfusion CT (PCT) dataset.

MATERIALS AND METHODS

Fifty consecutive patients undergoing head CTA and PCT in 1 examination were enrolled. The PCT dataset was obtained with 40.0-mm-detector coverage, 5.0-mm axial thickness, 80 kilovolt peak (kVp), 180 mA, and 30 mL of contrast medium. MP-CTA was reconstructed from the same PCT dataset with an axial thickness of 0.625 mm by using a new axial reconstruction algorithm. A conventional SP-CTA dataset was obtained with 0.625-mm axial thickness, 120 kVp, 350 mA, and 60 mL of contrast medium. We compared image quality, vascular enhancement, and radiation dose.

RESULTS

SP-CTA and MP-CTA of 50 patients (male/female ratio, 31/19; mean age, 59.25 years) were analyzed. MP-CTA was significantly better than SP-CTA in vascular enhancement (P = .002), in the absence of venous contamination (P = .006), and was significantly higher in image noise (P < .001). MP-CTA used less contrast medium than SP-CTA and could demonstrate hemodynamic information. The effective dose of MP-CTA was 5.73 mSv, which was equal to that in conventional PCT, and it was 3.57 mSv in SP-CTA.

CONCLUSION

It is feasible that MP-CTA may provide both CTA and PCT results. Compared with SP-CTA, MP-CTA provides comparable image quality, better vascular enhancement, hemodynamic information, and more noise with less detail visibility with a lower tube voltage. The radiation dose of MP-CTA is higher than that of SP-CTA, but the dose can be reduced by altering the sampling interval.