Fluoroscopy: patient radiation exposure issues.

Fluoroscopic procedures (particularly prolonged interventional procedures) may involve high patient radiation doses. The radiation dose depends on the type of examination, the patient size, the equipment, the technique, and many other factors. The performance of the fluoroscopy system with respect to radiation dose is best characterized by the receptor entrance exposure and skin entrance exposure rates, which should be assessed at regular intervals. Management of patient exposure involves not only measurement of these rates but also clinical monitoring of patient doses. Direct monitoring of patient skin doses during procedures is highly desirable, but current methods still have serious limitations. Skin doses may be reduced by using intermittent exposures, grid removal, last image hold, dose spreading, beam filtration, pulsed fluoroscopy, and other dose reduction techniques. Proper training of fluoroscopic operators, understanding the factors that influence radiation dose, and use of various dose reduction techniques may allow effective management of patient dose.     

Computer frame freezing of fluoroscopic images

Simple but flexible frame freezing of fluoroscopic images can improve the utility of real-time x-ray imaging. A frame-freeze system based on a personal computer has been developed. The system is easily installed using standard hardware that is non-invasive of the fluoroscopic imaging chain. The frozen images can be used as a reference or for hard-copy documentation of real-time or videotaped procedures.     

全自动X线曝光控制的设计与应用

目的实现全自动X线曝光控制，简化人工操作手续，进一步提高影像质量。方法设计5个或7个硅光电池式X线探测器并改制X线主机的夹片盘，通过机械传动装置组合为一体，形成多测量野自动变形式X线探测架。结果根据摄片部位选用片盒时，使探测器自动变形出与部位感兴趣区最吻合的测量野。测量9张照片．光密度值均在电压1V附近，临床使用拍摄照片10000张，优质率达98％。结论X线自动曝光控制技术的关键是X线探测技术，通过它可充分提高影像质量。     

X线照片自动曝光电路的设计与应用

目的 为无自动曝光装置的Ｘ线设计一种摄影自动曝光控制电路,以改进设备性能,提高摄片质量。方法 采用串联控制方式,用射线传感器检测曝光过程中穿过人体到达胶体片的射线量,经积分、放大、比较等处理后,驱动执行机构,自动控制曝光时间。结果 该控制电路安装于国产Ｆ７８－Ⅲ型３００ｍＡ、Ｆ９９－ⅡｍＡ等Ｘ线机上,经６家医院实际应用,效果良好。结论 该设计结构简单,对Ｘ线机电路改动少,效果明显,具有实用价值。     

Tuning of automatic exposure control strength in lumbar spine CT

Objective:

To investigate the impact of tuning the automatic exposure control (AEC) strength curve (specific to Care Dose 4D®; Siemens Healthcare, Forchheim, Germany) from “average” to “strong” on image quality, radiation dose and operator dependency during lumbar spine CT examinations.

Methods:

Two hospitals (H1, H2), both using the same scanners, were considered for two time periods (P1 and P2). During P1, the AEC curve was “average” and radiographers had to select one of two protocols according to the body mass index (BMI): “standard” if BMI <30.0 kg m⁻² (120 kV–330 mAs) or “large” if BMI >30.0 kg m⁻² (140 kV–280 mAs). During P2, the AEC curve was changed to “strong”, and all acquisitions were obtained with one protocol (120 kV and 270 mAs). Image quality was scored and patients'' diameters calculated for both periods.

Results:

497 examinations were analysed. There was no significant difference in mean diameters according to hospitals and periods (p > 0.801) and in quality scores between periods (p > 0.172). There was a significant difference between hospitals regarding how often the “large” protocol was assigned [13 (10%)/132 patients in H1 vs 37 (28%)/133 in H2] (p < 0.001). During P1, volume CT dose index (CTDIvol) was higher in H2 (+13%; p = 0.050). In both hospitals, CTDIvol was reduced between periods (−19.2% in H1 and −29.4% in H2; p < 0.001).

Conclusion:

An operator dependency in protocol selection, unexplained by patient diameters or highlighted by image quality scores, has been observed. Tuning the AEC curve from average to strong enables suppression of the operator dependency in protocol selection and related dose increase, while preserving image quality.

Advances in knowledge:

CT acquisition protocols based on weight are responsible for biases in protocol selection. Using an appropriate AEC strength curve reduces the number of protocols to one. Operator dependency of protocol selection is thereby eliminated.Lower-back pain and sciatica affect up to 40% of people in western countries. These conditions are major causes of work-related disability and are costly to society.^1–4 As lumbar disc herniation is the most common cause of sciatica, cross-sectional imaging, either with CT or MRI, is routinely performed. Despite requiring ionizing radiation, CT is frequently used first because it is readily available and less expensive than MRI.^5,6 The challenge with CT is to minimize the radiation dose delivered,⁷ to maintain diagnostic accuracy (and/or image quality) and to tailor the dose to patient body habitus. Adequate adaptation can be reached either through manual selection of tube current according to the body mass index (BMI) or through automatic exposure control (AEC) systems that adapt tube current to patient absorption measured on scout view.^5,8 However, the relationship between tube current and patient absorption differs within and between AEC systems as illustrated in Figure 1.^9–12 AEC systems, such as Auto mA® (GE Healthcare, Milwaukee, WI), are based on a single straight–linear relationship that maintains constant noise, whereas other systems, such as Care Dose 4D® (Siemens Healthcare, Forchheim, Germany), are based on several curvilinear relationships, named “strength curves” by the manufacturer, and are labelled “very weak”, “weak”, “average”, “strong” or “very strong”. These curvilinear relationships are characterized by increased noise in large patients and reduced noise in small patients, compared with normal individuals. Most importantly, increased noise is a limiting factor of lumbar spine CT scanning, as it is associated with increased intra- and interobserver variability.⁵Open in a separate windowFigure 1.Automatic exposure control strengths: different settings for modulation are possible, those used by Siemens Healthcare (Forchheim, Germany) are curvilinear and enable a fine tuning of the strength of the attenuation based mAs-adaptation depending on the user preferences. One single strength could be selected for all body regions with the VA10 release during P1, whereas one strength per acquisition mode could be selected with the latest VA11 release during P2. The attenuation-based mA adaptation of GE (General Electric Healthcare, Waukesha, MI) warranting a constant noise is displayed as a straight line.The default strength curve recommended by the manufacturer and selected at the time of unit installation is most often set to “average”. Although this setting provides satisfactory dose modulation for scanning the chest and abdomen regardless of patient size, it is not so for the lumbar spine because of the increased noise in large individuals.⁸ In these individuals undergoing lumbar spine CT, the radiographer needs to select a specific higher dose protocol. However, this degree of individual dose adjustment also brings risk to normal sized patients and, if the higher dose protocol is inadvertently selected, these patients will be exposed to unnecessary high levels of radiation. In a recent annual local survey on CT radiation dose intended to provide data to the Federal Radiation Protection Agency, we observed that a series of consecutive patients had been scanned with higher dose protocols regardless of their weight. Although we had already drawn the attention of our radiographers to the need to adhere to written local guidelines for protocol selection, we thus suspected that there might still remain an operator dependency in this selection.In 2011, the CT manufacturer (Siemens Healthcare) delivered an updated AEC release (Care Dose 4D VA11) enabling a user to assign a AEC strength curve specifically to each body region. This study, therefore, aimed first to solve the primary issue of operator dependency in protocol selection by replacing two weight-based protocols for lumbar spine CT by a single strength-curve-based protocol, and second to test whether the necessary image quality could be maintained with this protocol in all patients without increasing the radiation dose.     

Cassette videotape recording of computer images

Direct videotape recording of images from computer display terminals is often not possible because of the nonstandard outputs of these systems. A video-sync mixer is described that permits black-and-white recording on standard 3/4-in. videotape cassettes.     

Computed tomography radiation dose optimization: scanning protocols and clinical applications of automatic exposure control

As multi-detector-row computed tomography (CT) technology evolves, manifold applications of CT scanning have been adopted in clinical practice and optimization of scanning protocols to comply with an "as low as reasonably achievable" radiation dose have become more complex. Automatic exposure control techniques, which have been recently introduced on most state-of-the-art CT equipment, aid in radiation dose optimization at a selected image quality. The present article reviews the fundamentals of automatic exposure control techniques in CT, along with the scanning protocols and associated radiation dose reduction.     

Reduction of fluoroscopic exposure for the air-contrast barium enema

In a fluoroscopic imaging system, image quality and patient dose are both affected by the optical system linking the image intensifier with the video camera. The effect on patient exposure of increasing the optical iris aperture size over that required for other procedures performed on the same imaging system was investigated for the air-contrast barium enema examination. Using a large-area transmission ionisation chamber to monitor the Roentgen-area-product of entrance exposure, a decrease in fluoroscopic radiation of greater than 50% was clinically documented for a fluoroscopic system utilising kVp and mA variable automatic brightness control. For this iris change, the video image was of acceptable quality for positioning and monitoring the patient, and no deleterious effect was detected in the conduct of the air-contrast exam. The availability of a variable-sized operator-selectable iris diaphragm would permit this dose-reduction approach to be extended to other fluoroscopic procedures.     

Performance evaluation for CT-AEC(CT automatic exposure control)systems

Although many current CT scanners incorporate CT-AEC, performance evaluation is not standardized. This study evaluates the performance of the latest CT-AEC of each manufacturer with the aim of establishing a standard CT-AEC performance evaluation method. The design of the phantoms was based upon the operation characteristics of different CT-AECs. A cone, an ellipse, a variable-shaped ellipse, stepped phantoms, and their analysis software were devised and carried out the field test. The targets were LightSpeed VCT 64 with 2D and 3D Auto mA(GE), Aquilion 64M with Real-EC and Volume-EC(Toshiba), Sensation 64 with CARE Dose and CARE Dose 4D(Siemens), and Bulliance 16P with Dose Right(Philips). Data was acquired while varying the typical abdominal CT(with CT-AEC)scanning conditions (120 kV, 5 mm slice, standard function for abdomen, scanning range 200 mm). The acquired images were converted to the DICOM format and image noise(SD) was calculated using dedicated software. All 4 CT-AECs reduced exposure dose. For GE and Toshiba, image noise was constant and met the target. For Siemens, noise was independent of phantom shape but varied uniformly with phantom size. For Philips, noise varied with phantom size and shape, and variation degree depended on phantom thickness in scanogram direction. The results reflect the basic concept and performance characteristics of the methods. Standardization of CT-AEC performance evaluation is possible using these phantoms.     

Dose reduction by automatic exposure control in multidetector computed tomography: comparison between measurement and calculation

The aim of this study was to investigate the potential of dose reduction in multidetector computed tomography (MDCT) by current-modulated automatic exposure control (AEC) and to test the reliability of the dose estimation by the conventional CT dosimetry program CT-EXPO, when an average tube current is used. Phantom measurements were performed at a CT system with 64 detector rows for four representative examination protocols, each without and with current-modulated AEC. Organ and effective doses were measured by thermoluminescence dosimeters (TLD) at an anthropomorphic Alderson phantom and compared with those given by the calculation with CT-EXPO. The application of AEC yielded dose reductions between 27 and 40% (TLD measurements). While good linearity was observed between measured and computed effective dose values both without and with AEC, the organ doses showed large deviations between measurement and calculation. The dose to patients undergoing a MDCT examination can be reduced considerably by applying a current-modulated AEC. Dosimetric algorithms using a constant current-time product provide reliable estimates of the effective dose.