CT of the Abdomen with Reduced Tube Voltage in Adults: A Practical Approach 1

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1 GASTROINTESTINAL IMAGING 1922 CT of the Abdomen with Reduced Tube Voltage in Adults: A Practical Approach 1 Adeel R. Seyal, MD Atilla Arslanoglu, MD Samir F. Abboud, MD Azize Sahin, MD Jeanne M. Horowitz, MD Vahid Yaghmai, MD RadioGraphics 2015; 35: Published online /rg Content Codes: 1 From the Department of Radiology, Northwestern University Feinberg School of Medicine, 676 N Saint Clair St, Suite 800, Chicago, IL Recipient of a Certificate of Merit award for an education exhibit at the 2014 RSNA Annual Meeting. Received March 5, 2015; revision requested April 9 and received May 14; accepted July 2. For this journal-based SA-CME activity, the authors A.R.S. and A.A. have provided disclosures (see p 1937); all other authors, the editor, and the reviewers have disclosed no relevant relationships. Address correspondence to V.Y. ( v-yaghmai@northwestern.edu). RSNA, 2015 This copy is for personal use only. To order printed copies, contact reprints@rsna.org SA-CME LEARNING OBJECTIVES After completing this journal-based SA-CME activity, participants will be able to: Explain the benefits of lowering the tube voltage setting at CT. Discuss the approach to the design of protocols that use low tube voltage in modern CT scanners. Describe the various clinical applications of low tube voltage CT of the abdomen, as well as its pitfalls. See Recent innovations in computed tomographic (CT) hardware and software have allowed implementation of low tube voltage imaging into everyday CT scanning protocols in adults. CT at a low tube voltage setting has many benefits, including (a) radiation dose reduction, which is crucial in young patients and those with chronic medical conditions undergoing serial CT examinations for disease management; and (b) higher contrast enhancement. For the latter, increased attenuation of iodinated contrast material improves the evaluation of hypervascular lesions, vascular structures, intestinal mucosa in patients with bowel disease, and CT urographic images. Additionally, the higher contrast enhancement may provide diagnostic images in patients with renal dysfunction receiving a reduced contrast material load and in patients with suboptimal peripheral intravenous access who require a lower contrast material injection rate. One limitation is that noisier images affect image quality at a low tube voltage setting. The development of denoising algorithms such as iterative reconstruction has made it possible to perform CT at a low tube voltage setting without compromising diagnostic confidence. Other potential pitfalls of low tube voltage CT include (a) photon starvation artifact in larger patients, (b) accentuation of streak artifacts, and (c) alteration of the CT attenuation value, which may affect evaluation of lesions on the basis of conventional enhancement thresholds. CT of the abdomen with a low tube voltage setting is an excellent radiation reduction technique when properly applied to imaging of select patients in the appropriate clinical setting. RSNA, 2015 radiographics.rsna.org Introduction Since the inception of computed tomography (CT) in the 1970s, its use has increased rapidly because of its important role in clinical diagnosis and its widespread availability (1). Although CT represents only 15% of medical imaging procedures, it is responsible for at least one-half of the radiation exposure from medical imaging (2,3), and CT is the largest contributor to medical radiation exposure in the United States and Europe (4). It is estimated that as many as 2% of the cancer cases in the United States may be attributed to radiation exposure at CT examinations (1). This issue has led to the optimization of CT imaging protocols with radiation doses as low as reasonably achievable without compromising diagnostic image quality (5). Performing CT examinations at low tube voltage in routine clinical practice is an emerging technique for dose optimization. The most substantial benefits of low tube voltage at CT examinations include (a) a reduction in the radiation dose delivered to the patient and (b) an increase in the image contrast for structures with a high effective atomic number, such as calcium and iodine (6). Therefore, low

2 RG Volume 35 Number 7 Seyal et al 1923 TEACHING POINTS The most substantial benefits of low tube voltage at CT examinations include (a) a reduction in the radiation dose delivered to the patient and (b) an increase in the image contrast for structures with a high effective atomic number, such as calcium and iodine. If the tube current and the exposure time are kept constant, a lower tube potential results in both lower photon flux and lower average energy of the x-ray beam and, thus, lower radiation dose delivered. As the applied tube voltage is reduced, the average photon energy (in kiloelectron volts) decreases and approaches the k edge of iodine (33.2 kev). This decrease results in a greater proportion of photon interactions with an iodine-based contrast material by way of the photoelectric effect, causing greater attenuation of the x-ray beam and higher contrast enhancement. Radiologists rely on the difference in the contrast of an abdominal lesion compared with its surroundings for its accurate identification and characterization. This reliance is different from that for chest imaging, which has a much higher inherent contrast because of the presence of air and the lower physical density of the lungs. The basic principle behind the use of low tube voltage at CT enterography is the higher iodine attenuation achieved at low tube voltage, which results in pronounced mucosal hyperenhancement and mural stratification of the inflamed bowel segments, coupled with potential radiation dose savings. tube voltage CT may be preferred for evaluation of the abdominal vasculature and hypervascular enhancing lesions because of the higher attenuation for iodine and the increased contrast-tonoise ratio (7). Factors such as the patient s size, the diagnostic task at hand, and the availability of iterative reconstruction greatly influence the use of low tube voltage at abdominal CT. The purpose of this article is to show how abdominal CT examinations performed with low tube voltage in appropriately selected adult patients can substantially reduce radiation dose while providing diagnostic-quality CT images. First, the benefits of low tube voltage CT are described, followed by the approach to such imaging. Then the various clinical applications of low tube voltage CT are detailed. Finally, the pitfalls of low tube voltage CT are covered. Benefits of Low Tube Voltage CT Radiation Dose Reduction The spectrum of x-ray photons generated by an x-ray tube is defined by the electric potential applied across the tube. CT with low tube voltage results in a reduction of the radiation dose compared with that from imaging with a higher tube voltage (8 10). Radiation output is generally accepted to be proportional to the square of the x- Figure 1. Liver metastasis from colorectal cancer in a 60-yearold woman. (a) Axial contrast-enhanced CT image was obtained from an abdominal CT examination performed at 120 kv and 220 mas, which delivered an effective dose of 17.6 msv. The attenuation values of the hepatic parenchyma in one region of interest (ROI) (large circle) and of the portal vein in another ROI (small circle) were 133 HU and 231 HU, respectively. (b) Axial contrast-enhanced follow-up CT image was obtained from an abdominal CT examination performed at 100 kv and 241 mas, which delivered a reduced effective dose of 8.9 msv. The hepatic parenchymal ROI (large circle) and the portal vein ROI (small circle) show increased attenuation values of 159 HU and 280 HU, respectively. Both CT scans were performed with a 70-second delay after administration of intravenous iodinated contrast material (125 ml of iopamidol-370 [370 mg of iodine per milliliter]). ray tube voltage, such that even small decreases in the tube voltage can result in substantial radiation dose reductions (11,12). If the tube current and the exposure time are kept constant, a lower tube potential results in both lower photon flux and lower average energy of the x-ray beam and, thus, lower radiation dose delivered (Fig 1). Reducing the tube voltage from 120 kv to 100 kv results in a 33% dose reduction, whereas reducing the tube voltage from 120 kv to 80 kv yields a 65% dose reduction at a constant tube current (13,14). Unlike the tube voltage, the tube current has a linear relationship with the radiation dose. Reducing the tube voltage, therefore, allows a greater dose reduction when compared with that for a similar percent reduction in the tube current. Even when

3 1924 November-December 2015 radiographics.rsna.org Figure 2. Lymphoma (arrow) in a 24-year-old woman. (a) Axial contrast-enhanced pretreatment CT image was obtained from an abdominal CT examination performed at 120 kv and 220 mas, which delivered an effective dose of 16.1 msv. (b) Axial posttreatment follow-up CT image after an interval of 5 months was obtained from an abdominal CT examination performed at 100 kv and 241 mas, which delivered an effective dose of 10.2 msv. Note that the effective dose was reduced 36.6% with the low tube voltage protocol. allowing the tube current and the exposure time to vary, it has been shown that in select patients, imaging with a lower tube voltage may result in clinically diagnostic images with a considerably reduced radiation dose (8 10). Reducing Radiation Exposure in Young Patients. One of the important determinants of the risk of radiation exposure is the age at exposure. The Committee on the Biological Effects of Ionizing Radiation reported that lifetime cancer risk after a single exposure to 0.1 Gy of radiation is estimated to be three times higher in early childhood than it is after the age of 35 years (15). Young patients are more radiosensitive and have much longer life expectancies, so performing low tube voltage CT for the purpose of radiation dose reduction in this group would be beneficial. Reducing Radiation Exposure in Patients with Chronic Medical Conditions. In the results of a 22-year study at a tertiary-care academic medical center, Sodickson et al (3) reported that 33% of the patients who underwent multiple CT examinations accrued potentially high cumulative effective doses. Most of the patients had underlying disease that required serial CT examinations. The risk of cumulative radiation exposure needs to be assessed along with the anticipated benefit from recurrent imaging (3). Adopting low tube voltage CT imaging protocols, as opposed to low tube current protocols, in patients with chronic medical conditions (such as malignancy, Crohn disease, and renal colic) may be especially beneficial (Fig 1) (3,16,17). This approach is particularly relevant in younger adults with medical conditions that may warrant serial CT imaging (Fig 2) (18). Higher Contrast Enhancement As the applied tube voltage is reduced, the average photon energy (in kiloelectron volts) decreases and approaches the k edge of iodine (33.2 kev). This decrease results in a greater proportion of photon interactions with an iodine-based contrast material by way of the photoelectric effect, causing greater attenuation of the x-ray beam and higher contrast enhancement (7,9,10,19,20). In the results of a phantom experiment, Yu et al (21) showed that on average, the attenuation of iodine at 80-kV CT increased by 70% and 100% when compared with 120-kV and 140-kV CT, respectively (Fig 1). The contrast enhancement at a given tube voltage is proportional to the iodine concentration of the given contrast material. An increase in the iodine concentration by 1 mg of iodine per milliliter at 120 kv, 100 kv, and 80 kv will proportionally increase the contrast enhancement by approximately 26 HU, 30 HU, and 40 HU, respectively (22). In fact, using low tube voltage settings without adjusting the iodine flux in routine abdominal CT examinations may result in a hyperattenuating appearance of contrast material enhanced structures, requiring adjustment of the window width and level to depict the anatomic structures and diagnose abnormalities (23). Reducing the Contrast Material Load in Patients with Renal Dysfunction. The smallest diagnostically appropriate amount of intravenous contrast material should be used in patients with compromised renal function, to reduce the risk of contrast material induced nephropathy. Because of the higher iodine attenuation, low tube voltage CT may allow lower volumes of contrast material to be used in patients with compromised renal function (Fig 3) (9,10,19,20). Reduced tube voltage abdominal CT examinations have been

4 RG Volume 35 Number 7 Seyal et al 1925 Figure 3. Images of a 75-year-old man who presented with hematuria and had a body mass index of (a, b) Axial nephrographic phase (a) and coronal excretory phase (b) CT images were obtained from an abdominal contrast-enhanced CT examination performed at 120 kv with 125 ml of intravenous iodinated contrast material (iopamidol-370 [370 mg of iodine per milliliter]). The estimated glomerular filtration rate (GFR) was more than 60 ml/min/1.73 m 2. (c, d) Axial nephrographic phase (c) and coronal excretory phase (d) CT images were obtained from a repeat abdominal contrast-enhanced CT examination performed at 100 kv for another episode of hematuria. In the interval, the patient s renal function worsened (estimated GFR of about 49 ml/min/1.73 m 2 ); therefore, the repeat CT scan was performed with a reduced dose of contrast material (75 ml of iopamidol-370) and provided diagnostic-quality images. Hence, CT with a lower tube voltage in patients with reduced renal function allows lower amounts of iodinated contrast material to be used. reported to allow a 40% reduction in the intravenous contrast material dose and a 20% reduction in radiation dose while delivering diagnostically acceptable images in patients with renal dysfunction (24). Improved Enhancement with a Low Injection Rate of Contrast Material. Use of a lower tube voltage, such as 80 kv, increases the peak attenuation for a given concentration of iodine and also reduces the time to peak enhancement when compared with a 120-kV examination (22,25). This finding may be highly beneficial for patients in whom intravenous access is challenging or tenuous (Fig 4). Slower contrast material injection rates in such patients through a smaller-gauge peripheral intravenous access route may still result in diagnostically acceptable CT images at low tube voltage, particularly when contrast material with a higher concentration of iodine is administered. Approach to Low Tube Voltage CT Patient Selection As a guiding principle in selecting the optimal tube voltage for an abdominal CT examination, the patient s body habitus, the specific diagnostic goal, and the availability of denoising algorithms must be taken into account. Patient Body Habitus. Patient size is an important factor in determining the optimal tube voltage

5 1926 November-December 2015 radiographics.rsna.org Figure 4. Suboptimal intravenous access in a 72-year-old man. (a) Sagittal reformatted CT image was obtained from a contrast-enhanced CT examination performed with 120 kv (effective dose, 10.2 msv) and with a contrast material flow rate of 3 ml/sec. (b) Sagittal reformatted CT image was obtained 8 months later from a repeat contrast-enhanced CT examination performed with 100 kv (effective dose, 7.9 msv) and with a reduced contrast material flow rate of 1.5 ml/sec secondary to suboptimal intravenous access. Note the higher intravascular contrast enhancement and adequate image quality in b with low tube voltage and a low contrast material flow rate, compared with a (higher tube voltage and higher contrast material flow rate). for the desired image quality. The tube voltage has an exponential relationship with radiation dose, whereas the tube current has a linear relationship, as previously discussed. For large adult patients, a higher average energy of the x-ray photons is required for adequate penetration and image quality; thus, use of a higher tube voltage may be preferable. Similarly, using a lower tube voltage may be optimal for thin patients. Newer scanners with powerful generators and denoising algorithms can produce sufficient compensatory increase in the tube current (milliamperage) at low tube voltage, allowing dose reduction without compromising image quality even in average-sized adults. Many newer CT scanner generators have power reserves as high as 120 kw, with the potential to increase the tube current to as much as 1300 ma, allowing lower tube voltage imaging with compensatory higher tube currents. This approach results in a lower radiation dose for patients who could not otherwise be imaged with a low tube voltage setting by using scanners with less-powerful generators. In scanners with less-powerful generators, adequate tube current may not be achieved with low tube voltage settings, resulting in poor image quality. Therefore, low tube voltage abdominal imaging may not be possible in average-sized adult patients imaged with less-powerful scanner generators. It is important to tailor scanner protocols on the basis of the power reserves of scanner generators. Clinical Task. For abdominal CT, low-contrast detectability is of paramount importance in solid organ evaluation. Radiologists rely on the difference in the contrast of an abdominal lesion compared with its surroundings for its accurate identification and characterization (8). This reliance is different from that for chest imaging, which has a much higher inherent contrast because of the presence of air and the lower physical density of the lungs (8,14). Therefore, for evaluation of solid abdominal organs, scanning at extremely low tube voltage may yield noisier images that are inadequate for interpretation. In this situation, a greater compensatory increase in tube current or the application of iterative reconstruction may improve image quality and preserve lesion conspicuity at lower tube voltage. Therefore, finding a balance between image noise and low-contrast detectability is essential for low tube voltage CT of solid organs. For example, if the primary task for CT is evaluation of the vascular structures (eg, CT angiography) or hypervascular enhancing lesions, radiologists may tolerate noisier images because of higher contrast. As a result, diagnostically acceptable images may be obtained despite the increase in image noise with low tube voltage scanning (26). Optimal Tube Voltage Selection Selecting an optimal tube potential to achieve radiation dose reduction while providing diagnostically adequate image quality is challenging, given the interrelationships between the tube voltage and other factors such as the tube current time product (in milliampere-seconds), the tube current limits, and iodine attenuation. Two commonly used

6 RG Volume 35 Number 7 Seyal et al 1927 Table 1: Technique Chart for Manual Selection of Optimal Tube Voltage for Routine Contrastenhanced Abdominal CT on the Basis of Patient Size Body Mass Index (kg/m 2 )* Lateral Width (cm) Peak Tube Voltage Setting (kv) Quality Reference Tube Current Time Product (mas) AEC Iterative Reconstruction Gantry Rotation (sec) Helical Pitch Section Interval/ Thickness (mm) Collimation,20, On Yes / On Yes / On Yes / On Yes / Note. This protocol applies to the latest generation of CT scanners that do not have automated attenuationbased tube voltage selection. Either the body mass index or the lateral width of the patient should be used for manual selection of optimal tube voltage. AEC = automatic exposure control. *Categories for body mass index used at our institution. Categories for lateral width adapted, with permission, from reference 29. The first number is the number of detector rows, and the second number is the collimation thickness in millimeters. methods for selecting the optimal tube voltage are (a) manual tube voltage selection through the use of a technique chart showing the tube voltage and the tube current time product, and (b) automatic tube voltage selection with use of a software tool on the scanner. Manual Tube Voltage Selection. For manual selection of the tube voltage, patient weight based, body mass index based, or size (lateral width) based charts of the tube voltage and the tube current time product may be used that are based on empirical evaluation or quantitative measurement of phantoms (27 29). In the results of recent studies, investigators have reported radiation dose reduction and acceptable overall image quality when selection of the optimal tube voltage was based on the patient s body mass index (30) or weight (31). The lateral width (diameter) of the patient, manually measured at mid liver by the technologist on the initial topogram, is regarded as a better predictor of acceptable image quality at abdominal CT examinations than the other empirical methods (6,27). An example of a tube voltage selection technique chart for routine contrast material enhanced abdominal CT examinations that is based on patient size is shown in Table 1 (29). Table 2 provides examples of various types of abdominal CT examinations that may be performed at different tube potentials incorporating patient size and the level of image noise that may be tolerated by the radiologist for each type of examination (27,29). Education of the technologists and radiologists may help facilitate the adoption of low tube voltage CT into the everyday workflow. Attenuation-based Automated Tube Voltage Selection. A pitfall of using lateral width based, weight-based, or body mass index based tube voltage selection is the approximation of the patient s attenuation level, which may not be the true attenuation of the patient at various anatomic levels in the scanning range (27). As a result, the tube voltage may be set too high for a patient with a high body mass index in whom the scanned area does not attenuate the x-ray photons, thus unnecessarily delivering a high radiation dose. Similarly, if the tube voltage is set too low on the basis of these empirical methods, it may lead to excessive noise and poor image quality (18). Recently, through advances in hardware and software technology, the selection of optimal tube voltage is provided as a software tool implemented in the CT scanner. The tool uses the attenuation information obtained from the initial projection radiograph (topogram) and the userspecified contrast-to-noise ratio (based on the diagnostic task) to select the optimal tube potential and reference tube current time product, thus providing optimal image quality at reduced radiation doses (18,27). Hence, patient weight or body mass index is not used to determine the optimal tube voltage with the automated tube voltage selection tool. Therefore, little input is needed from the CT technologist or radiologist, which results in a more-efficient workflow. In our experience, use of this technology results in an approximately 30% increase in utilization of low tube voltage imaging of the abdomen, compared with body weight based selection of the tube voltage. The attenuation-based automated tube voltage selection tool is reported to lead to a 20% 40% reduction in effective dose at abdominal CT (32). A simplified flowchart for selecting optimal tube voltage for abdominal CT examinations is shown in Figure 5.

7 1928 November-December 2015 radiographics.rsna.org Table 2: Optimal Tube Voltage Selection for Various Types of Abdominal CT Examinations on the Basis of Patient Size and Image Noise Tolerance Recommended CT Examinations Body Mass Index (kg/m 2 )* Lateral Width (cm) Image Noise Tolerance Peak Tube Voltage (kv) Routine Unenhanced CT Routine Contrastenhanced CT CT Urography and CT Angiography of Abdominal Aorta Branch Vessels CT Enterography CT Angiography of Aorta,20,31 Low 100 X X X High 80 X X Low 120 X X High 100 X X X Low 120 X X High 100 X X X Any 140 X X X X X Note. When automated tube voltage selection is not available, either the body mass index or the lateral width of the patient should be used for manual selection of optimal tube voltage. X = recommended. *Categories for body mass index used at our institution. Categories for lateral width adapted, with permission, from reference 29. Categories are based on the radiologists image noise tolerance for a given CT examination at our institution and may vary among different institutions. When the body mass index is used for tube voltage selection, consider selecting 120 kv if the body mass index is between 35 and 40 kg/m 2. Figure 5. Flowchart shows a simplified approach for the selection of optimal tube potential for abdominal CT examinations. BMI = body mass index. Image Quality and Iterative Reconstruction Image Quality. Radiation dose reduction should be achieved with the condition that the diagnostic image quality is not compromised. The change in the image noise is approximately inversely proportional to the change in the tube voltage (33). The contrast-to-noise ratio and the signal-to-noise ratio are common metrics used to objectively quantify the overall image quality. Thus, if all other parameters are kept constant, lowering the tube voltage will increase image noise and may decrease the image quality parameters (34). Therefore, the benefit of higher contrast enhancement with low tube voltage may not be seen because of increased image noise. This observation is particularly true in large patients because the images produced at low tube potential may be much noisier if sufficiently high tube currents cannot be applied. Because the abdominal region inherently demonstrates lower contrast, the signal-to-noise ratio for a lesion usually decreases with increasing image noise at low tube voltage, provided other acquisition parameters are kept constant (Fig 6). Iterative Reconstruction. Filtered backprojection has been the predominant method for image reconstruction since the advent of CT. Filtered backprojection is a fast and computationally effective method for image reconstruction which relies on certain assumptions that result in excessive image noise at decreased

8 RG Volume 35 Number 7 Seyal et al 1929 Figure 6. Images from contrast-enhanced triphasic renal CT examinations of a 62-year-old woman. (a) Axial excretory phase CT image was obtained from a contrast-enhanced triphasic renal CT examination performed at 120 kv and reconstructed with filtered backprojection; the examination delivered an effective dose of 8.5 msv. The image noise was 14.5 HU (standard deviation of the region-of-interest [ROI] placed over the abdominal subcutaneous fat). The contrast-to-noise ratio and signal-to-noise ratio were 14.3 and 5.3, respectively. (b) Axial excretory phase CT image was obtained from a contrast-enhanced triphasic renal CT examination performed at 100 kv and reconstructed with filtered backprojection; the examination delivered a reduced effective dose of 6.4 msv. The image noise increased to 16.6 HU (standard deviation of the ROI placed over the abdominal subcutaneous fat). The contrast-tonoise ratio and signal-to-noise ratio decreased to 13.4 and 4.1, respectively. (c) Axial excretory phase CT image obtained with the same CT examination as in b with iterative reconstruction showed substantially lower image noise of 10.2 HU (standard deviation of the ROI placed over the abdominal subcutaneous fat) and improved values for contrast-to-noise ratio and signal-to-noise ratio of 21.9 and 6.6, respectively, when compared with the image in a (120-kV examination with filtered backprojection). radiation doses (35). Although many different techniques have been developed for controlling noise on CT images by using optimally designed data processing and reconstruction methods (36), iterative reconstruction for CT has recently received much attention with regard to maintaining image quality after radiation dose reduction (37). Iterative reconstruction uses a correction loop in the reconstruction of an image from the raw image data. Various iterative reconstruction algorithms are available from CT scanner manufacturers. These algorithms primarily differ in their reconstruction methods, although the specific details are beyond the scope of this review. Iterative reconstruction significantly reduces image noise and provides high-contrast images at abdominal CT examinations performed with low tube voltage (18), affording radiation dose reduction without deteriorating image quality (Fig 6). For example, an abdominal CT examination of an average-sized adult performed with 100 kv, a quality reference tube current time product of 241 mas, and iterative reconstruction will typically have a lower radiation dose and less image noise than an examination performed with 120 kv, a reference tube current time product of 175 mas, and a filtered backprojection algorithm. Extensive and timeconsuming computational tasks have been the biggest limitation for iterative reconstruction, although efforts are being made to make it more feasible for routine clinical CT. Optimal Strategy. When the tube voltage is reduced, the images become noisier, as discussed previously. To produce images of diagnostic quality with a low tube voltage, either the tube current is increased alone, or it is increased coupled with iterative reconstruction (which, in turn, allows smaller increases in the tube current and a greater radiation dose reduction) as a denoising strategy (26). Clinical Applications of Low Tube Voltage CT Solid Organ Imaging A reduction in the tube potential during performance of a contrast-enhanced abdominal CT examination increases the iodine attenuation and substantially improves the conspicuity of hyper- and hypovascular lesions within

9 1930 November-December 2015 radiographics.rsna.org Figure 7. Hypervascular hepatocellular carcinoma in a 61-year-old man. (a) Axial late arterial phase CT image obtained from an abdominal contrast-enhanced CT examination performed at 120 kv and reconstructed with filtered backprojection shows a hypervascular hepatocellular carcinoma (arrow) in the left hepatic lobe. The effective dose was 7.3 msv, and the signal-to-noise ratio and contrast-to-noise ratio were 4.6 and 12.6, respectively. (b) Axial late arterial phase CT image obtained from a follow-up contrastenhanced CT examination performed after an interval of 2 months with 100 kv and iterative reconstruction shows increased conspicuity of the lesion (arrow). The effective dose was reduced to 4.8 msv, and the signal-to-noise ratio and contrast-to-noise ratio were 6.2 and 16.8, respectively. Note that performing a liver CT examination with iterative reconstruction at a lower tube voltage may improve lesion conspicuity with a reduced radiation dose. Figure 8. Pancreatic adenocarcinoma in a 79-year-old man. (a) Axial pancreatic phase CT image obtained from a contrast-enhanced abdominal CT examination performed at 140 kv and reconstructed with filtered backprojection shows a hypovascular pancreatic mass (arrow). The volume CT dose index (CTDI vol ) was 13.3 mgy, and the contrast-to-noise ratio and signal-to-noise ratio were 11 and 3.9, respectively. (b) Axial follow-up CT image obtained from a contrast-enhanced CT examination performed after an interval of 2 months with 100 kv and iterative reconstruction shows improved conspicuity of the hypoattenuating pancreatic lesion (arrow) at a reduced dose (CTDI vol of 10.1 mgy). The contrast-to-noise ratio and signal-to-noise ratio were 19.4 and 8.6, respectively. Note the markedly improved solid organ and vascular enhancement with lower tube voltage in b, compared with a (higher tube voltage). abdominal organs (liver, pancreas, kidneys, spleen) because of the differential iodine distribution (36,38,39). This improvement is because the relative contrast difference increases at low tube voltage imaging, which improves lesion detectability. A higher tumor-to-liver contrast-to-noise ratio and improved conspicuity of malignant hypervascular liver tumors (hepatocellular carcinoma and metastatic neuroendocrine, breast, and renal tumors) (Fig 7) (40,41), as well as improved conspicuity and detection of hypovascular pancreatic adenocarcinoma, have been reported at low tube voltage CT, with significant radiation dose savings (P,.0001) (42) (Fig 8). Normal pancreatic parenchyma maximally enhances during the pancreatic phase of CT. On the other hand, pancreatic adenocarcinoma is typically hypoattenuating because of its fibrotic nature. Performing CT with a lower tube voltage would therefore increase the difference in attenuation between the hypoattenuating pancreatic cancer and the surrounding enhancing normal pancreatic tissue, hence improving the detection of the cancer (42). Also, low tube voltage CT will accentuate the difference in attenuation between the pancreatic duct and the parenchyma, which in turn improves depiction of the pancreatic duct when its caliber is altered by an isoattenuating mass.

10 RG Volume 35 Number 7 Seyal et al 1931 Figure 9. Volume-rendered CT urographic images of a 62-yearold woman. Both the coronal volume-rendered CT urographic image obtained with 120 kv and reconstructed with filtered backprojection (a) and the coronal volume-rendered CT urographic image obtained with 100 kv with iterative reconstruction (b) are diagnostic-quality images. The use of lower tube voltage (reconstructed with iterative reconstruction) reduced the effective dose from 8.5 msv at 120 kv to 6.4 msv at 100 kv for the excretory phase while maintaining diagnostic image quality. Genitourinary Imaging CT Urography. CT urography with multiplanar imaging is an excellent method for evaluation of the urinary tract. CT urography involves a multiphasic imaging protocol (unenhanced and nephrographic and excretory phases), which may result in higher mean effective doses that are as much as 70% greater than those at conventional excretory urography (43). Lowering the tube potential may result in a substantial reduction in the radiation dose while increasing the opacification of the urinary system. Application of low tube voltage (80 kv) to the excretory phase of CT urography was recently reported to generate diagnostically adequate images at reduced radiation doses despite higher image noise in average-sized patients (43). Yanaga et al (44) applied an adaptive noise reduction filter to a low tube voltage CT urographic technique to reduce noise and achieve radiation dose reduction without appreciable degradation of image quality for evaluation of the upper part of the urinary system. The use of denoising algorithms such as iterative reconstruction, therefore, may play an important role in improving image quality for CT urography performed at low radiation doses (Fig 9) (45). Low tube voltage CT is particularly useful for detection of enhancing urothelial lesions if image noise is kept at levels similar to that of standard-dose examinations (43). Low tube voltage CT also reduces the attenuation of fat (46) within renal lesions such as angiomyolipoma, which makes the detection of fat potentially easier. Evaluation of Renal Colic. Unenhanced CT has been regarded as the reference modality for imaging for diagnosis of renal colic (30). The primary limitation is the absorption of a greater radiation dose compared with the doses at abdominal radiography and intravenous excretory urography (30,47). Because renal colic (a) may occur in both pediatric and adult populations, (b) has a relatively high rate of relapse (35% at 10 years after diagnosis), and (c) may require multiple CT examinations, dose reduction techniques are essential for CT examinations performed in patients suspected of having renal colic (30). Renal stones, because of their inherent high contrast, are particularly well suited for depiction with lower-dose CT examinations (48). In several studies of patients with renal colic, investigators have shown diagnostic-quality CT images that were obtained with lower radiation dose when the tube current is reduced (49,50). Lowering the tube potential is also an effective technique for substantial reduction of the radiation dose and, similar to tube current reduction, degrades the image quality because of higher noise. Recently, low tube voltage CT has been performed in conjunction with iterative reconstruction, allowing dose reductions and diagnostic-quality images for reliable detection of urolithiasis (Fig 10) (30,47). Gastrointestinal Imaging CT Enterography. CT enterography is a noninvasive imaging technique used for evaluation of small bowel pathologic findings. CT enterography allows better evaluation and depiction of small bowel disease compared with routine abdominal CT (51). Crohn disease is the most common indication for performing CT enterography. Because Crohn disease often has a chronic remitting-recurring clinical course with frequent complications and mainly affects younger individuals, it is likely that these patients may undergo multiple CT examinations during the

11 1932 November-December 2015 radiographics.rsna.org Figure 10. Renal colic in a 54-year-old man. (a) Axial unenhanced CT image obtained with 120 kv and reconstructed with filtered backprojection shows a 3-mm left ureteral stone (arrow). The effective dose delivered was 14.8 msv. (b) Axial unenhanced CT image obtained later for another episode of renal colic from a repeat CT examination performed with 100 kv and iterative reconstruction again shows a left ureteral stone (arrow). The effective dose delivered decreased to 9.6 msv. CT at lower tube voltage with iterative reconstruction affords diagnostic image quality (because of the higher inherent contrast of the calculus) at a reduced radiation dose. Figure 11. Crohn disease in a 32-year-old man. (a) Coronal CT enterographic image from an examination performed with 120 kv shows inflammation and mild enhancement of the terminal ileum (arrowhead). (b) Coronal repeat CT enterographic image from an examination performed with 80 kv shows higher mucosal enhancement of the terminal ileum (arrowhead), compared with the prior higher tube voltage image in a, while substantially reducing the radiation dose from an effective dose of 10.8 msv with 120 kv to 3.6 msv with 80 kv. course of their illness, accruing a high cumulative radiation dose (29). Another notable indication for CT enterography includes evaluation of small bowel vascular lesions, which are reported to account for as many as 40% of cases of obscure gastrointestinal bleeding, in which a triphasic acquisition protocol for CT enterography is responsible for a greater effective dose to the patient than routine abdominal CT (29). Applying low tube voltage to CT enterography can potentially reduce the radiation dose by 16% 30% with 80 or 100 kv, compared with 120 kv (29). The basic principle behind the use of low tube voltage at CT enterography is the higher iodine attenuation achieved at low tube voltage, which results in pronounced mucosal hyperenhancement and mural stratification of the inflamed bowel segments, coupled with potential radiation dose savings (Fig 11). The higher image noise with low tube voltage imaging can be reduced by increasing the tube current or by using denoising techniques such as iterative reconstruction (29). CT Colonography. CT colonography is a useful screening tool for colorectal cancer, although it is not without the risks of radiation exposure (Fig 12). Intrinsic higher contrast between the luminal air and the surrounding soft tissue on CT colonographic images may allow radiologists to tolerate more noise and reduce the radiation dose without adversely affecting detection of colonic abnormalities (eg, pedunculated or sessile polyps, diverticula) (52). Chang et al (53) recently reported that a decrease in the tube voltage from 120 kv to 100 kv resulted in significant radiation dose reduction (P,.001) while maintaining the contrast-to-noise ratio of the tagged endoluminal fluid and the three-dimensional image quality. Thus, imaging at a lower tube voltage improves the visibility of colonic polyps covered by material tagged with iodine or barium. More recently, investigators have reported submillisievert radiation doses for CT colonography performed at low tube voltage and coupled with iterative reconstruction (52,54).

12 RG Volume 35 Number 7 Seyal et al 1933 Figure 12. CT colonography of a 61-year-old man. CT colonographic image obtained at 100 kv with iterative reconstruction shows preservation of image quality with use of a lower tube potential. Figure 13. Abdominal aortic aneurysm in a 92-year-old woman. (a) Coronal maximum intensity projection (MIP) image was obtained at abdominal CT angiography performed with 120 kv and administration of 125 ml of iopamidol-370 (370 mg of iodine per milliliter), which delivered an effective dose of 13 msv. (b) Coronal MIP image obtained at repeat CT angiography performed with 100 kv and a lesser amount of contrast material (75 ml of iopamidol-370) shows comparable image quality with a lower effective dose of 9 msv. Vascular Imaging Use of a low tube voltage technique at abdominal CT angiography has the dual benefits of achieving radiation dose reduction and increasing intravascular contrast enhancement. The technique also has the potential to allow a decrease in the total grams of iodine in the contrast material (Fig 13) (55). Nakayama et al (10) reported substantial radiation dose savings (22% 25.2%) and similar qualitative image quality scores with a protocol combining low tube voltage and a reduced amount of contrast material (90 kv and 40 ml of contrast material) for body CT angiography, compared with a standard protocol (120 kv and 100 ml of contrast material). In the findings from several other studies, investigators have also stressed the importance of using low tube voltage for abdominal CT angiographic examinations (56,57). Aggressive tube voltage reduction in the range of kv will lead to excellent intravascular contrast enhancement at the cost of an appreciable increase in image noise. To improve the image quality when an increase in image noise interferes with the radiologist s interpretation, increased tube current or iterative reconstruction should be included as part of the low tube voltage CT angiographic technique (58,59). CT-guided Interventions Abdominal CT-guided interventions (eg, biopsy, abscess drainage) can be a considerable source of radiation exposure to the patient, and adequate measures should be used to decrease the radiation exposure. Among other parameters (gantry rotation, number of scans, scan length, section thickness, pitch, tube current), adjusting the tube potential can also help reduce the radiation dose delivered to the patient (Fig 14) (60).

13 1934 November-December 2015 radiographics.rsna.org Figure 14. CT-guided drainage of a pelvic abscess in a 46-year-old woman. (a) Axial contrast-enhanced CT image from a diagnostic pretreatment CT examination shows a cm pelvic abscess (arrows). (b) Axial CT image from a low-dose CT examination performed with 100 kv after placing the patient in a prone position shows transgluteal placement of a self-retaining drain within the pelvic abscess (arrows). Use of lower tube voltage during performance of the interventional procedure substantially reduces radiation exposure. The higher image noise with lower tube voltage may be tolerated because of the availability of the prior diagnostic CT examination. In the results of a recent study, Rezazadeh et al (61) reported a high technical success rate for abdominal, pelvic, and chest CT-guided interventions performed with 80 or 100 kv, compared with the rate for those performed with 120 kv, along with a significant reduction in the effective dose (57% 73%) delivered (P,.01). Reducing the tube potential for the preprocedural planning CT may substantially lower the radiation exposure for the entire procedure the rationale being that patients, in most cases, have already had diagnostic imaging performed, and high-quality planning CT may not be required. Also, the use of intermittent CT fluoroscopy, preferably with lower tube voltage, may achieve further radiation dose reduction during the procedure (60). Other Applications Pediatric Imaging. Owing to the smaller size of pediatric patients, a greater degree of radiation dose reduction may be achieved with low tube voltage CT without a marked increase in image noise, thus maintaining diagnostic quality. Appropriate reduction in the tube potential in the pediatric population is dependent on patient size and the diagnostic task (36). A review of reduced tube potential CT in the pediatric population is beyond the scope of this review, and the topic has been discussed elsewhere (21,36). Dual-Energy CT. Dual-energy CT acquires data at two different tube potentials and provides more diagnostic information than a comparable single-energy conventional CT examination for the same radiation dose. Dualenergy CT has a number of clinical applications, including iodine quantification, bone removal, and kidney stone characterization. Creation of a virtual unenhanced image from a contrast-enhanced dual-energy CT examination is another important application of dual-energy CT. This application has an important implication for radiation dose reduction, because it may potentially replace the precontrast scan performed as a part of various abdominal CT protocols (36,38). Pitfalls of Low Tube Voltage CT The benefits of low tube voltage imaging for abdominal CT examinations have been covered in previous sections: Low tube voltage CT of the abdomen (a) reduces the radiation dose delivered, (b) increases the attenuation of iodinated contrast material, and (c) helps lower the dose of iodinated contrast material. The following sections focus on the pitfalls of low tube voltage imaging for abdominal CT examinations, which include (a) photon starvation; (b) accentuation of streak artifact; (c) alteration of the attenuation of iodine, which may affect the characterization of lesions; (d) requirement for window width and level adjustment on a picture archiving and communication system, to optimize visual contrast; and (e) extremely low tube voltage selection and aggressive radiation dose reduction limited by the scanner s tube current limit. Photon Starvation in Large Patients Imaging of obese patients is complicated by increased image noise and scatter with routine radiation doses (62). Even when the radiation dose is increased to improve tissue penetration, photon starvation may still occur if the required energy flux is greater than the maximal output of the x-ray tubes. Lowering the tube potential decreases the average peak energies and the

14 RG Volume 35 Number 7 Seyal et al 1935 Figure 15. Contrast-enhanced CT examinations of a 69-year-old man with morbid obesity. (a) Axial CT image obtained with 120 kv and reconstructed with filtered backprojection shows degraded image quality secondary to the photon deficiency artifact that is related to the patient s large body habitus (image noise of 33.1 HU obtained by placing a region of interest [ROI] over the abdominal subcutaneous fat). Note the large left inguinal hernia. (b) Axial repeat CT image obtained with 140 kv and iterative reconstruction shows improved image quality (image noise of 14 HU obtained by placing an ROI over the abdominal subcutaneous fat). Note the left inguinal hernia repair. Figure 16. Coil embolization and stent placement across hepatic and splenic arterial aneurysms in a 77-year-old man. Axial contrastenhanced CT images obtained with 100 kv (a) and 80 kv (b) show streak artifact. Streak artifact is accentuated (arrows in b) with lower tube voltage and may further compromise the diagnostic image quality. number of photons produced and may further increase photon starvation and result in more image noise and scatter, potentially obscuring normal anatomic structures or mimicking disease (63) (Fig 15). Applying iterative reconstruction to the low tube voltage abdominal CT performed in obese patients may allow equal image quality if the image noise is not considerable, thus reducing the long-term risks related to cumulative radiation exposure (18,64). Accentuation of Streak Artifact To improve the diagnostic quality of CT images when metallic artifact is present, a higher setting for the tube voltage is typically used. Reduced penetration of the x-ray photons at lower tube voltage tends to accentuate streak artifacts caused by beam hardening adjacent to structures with higher attenuation, such as bone and metal hardware (Fig 16). This adverse effect may be reduced by using an iterative reconstruction algorithm (65). Change in the CT Attenuation Value with Low Tube Voltage Lowering the tube voltage results in higher attenuation of calcified structures and iodinated contrast material, as the photoelectric effect increases and the Compton scattering decreases (26,66). For contrast-enhanced CT examinations performed with low tube voltage, this higher attenuation may lead to a higher incidence and magnitude of pseudoenhancement in the renal cystic lesions surrounded by

15 1936 November-December 2015 radiographics.rsna.org Figure 17. Pseudoenhancement of a renal lesion in a 46-year-old man. (a) Axial contrast-enhanced CT image obtained with 120 kv shows a lesion in the right kidney with an attenuation coefficient of 10.5 HU in the region of interest (ROI) (circle). The attenuation coefficient was 9.3 HU on a corresponding unenhanced image (not shown). (b) Axial contrast-enhanced CT image obtained after an interval of 7 months from a CT examination performed with 100 kv shows a considerable increase in the attenuation coefficient of the renal lesion to 40.3 HU in the ROI (circle), whereas the attenuation coefficient on the corresponding unenhanced image was 11.2 HU (not shown). Note that lowering the tube voltage may potentially result in pseudoenhancement of a renal lesion. enhancing parenchyma (Fig 17) (67). Also, the conventional enhancement thresholds (largely based on a 120-kV setting) that are used for categorization of lesions may not be applicable to lower tube voltage CT, and new diagnostic standards may need to be set (68). Image Display Low tube voltage will result in increased attenuation of anatomic structures that contain iodine. Caution should be exercised when these images are evaluated with a picture archiving and communication system (PACS). For example, a subtle vascular abnormality such as aortic dissection or disease in the renal collecting system may be missed unless window and level settings are adjusted (Fig 18). Nakaura et al (23) used a window width-level ratio of 280:46 at 100 kv and a ratio of 420:60 at 80 kv as the optimal display settings for interpretation of abdominal CT examinations (23). Further research is needed to define optimal display settings for interpretation of low tube voltage imaging with PACS. CT Scanner Tube Current Limit Reducing the tube potential tends to increase the image noise and decrease the image quality (69). For adequate x-ray penetration and preservation of image quality, when the tube potential is decreased, there needs to be a compensatory increase in the tube current (milliamperage). Therefore, it is essential to account for the tube current limit of the scanner while selecting the tube potential. Scanners with lower-power generators may not be able to handle the demand for higher tube current settings and may produce images with considerable photon starvation (Fig 19). Conclusion The key conclusions are summarized as follows: (a) Low tube voltage imaging is a robust method for radiation dose reduction in abdominal CT examinations. (b) Attenuation-based automated tube voltage selection offers greater reduction in the radiation dose delivered to the patient, compared with empirical methods (based on lateral width, body mass index, etc). (c) If available, denoising algorithms (such as iterative reconstruction) should be used in conjunction with low tube voltage to maintain image quality at a reduced radiation dose. (d) The higher attenuation of iodine allows CT to be performed with a reduced contrast material load in patients with renal failure and with a reduced contrast material flow rate for patients with suboptimal peripheral intravenous access. (e) Increased contrast enhancement allows better characterization of certain abdominal malignancies at abdominal CT and improves diagnostic performance for CT urography, CT enterography, and abdominal CT angiography in appropriately selected cases. (f) Low tube voltage imaging helps decrease radiation exposure at CT colonography and CT-guided interventions in appropriately selected patients. The appropriateness of low tube voltage for abdominal CT examinations depends on careful consideration of the clinical task, the size of the patient, and the generator power of the scanner to obtain an optimal trade-off between image quality and the radiation dose. The combined use of a low tube potential and iterative reconstruction at abdominal CT examinations may lead to substantial reductions in the radiation dose while maintaining objective image quality.

16 RG Volume 35 Number 7 Seyal et al 1937 Figure 18. CT urography performed at 70 kv in an 82-year-old woman with a history of bladder cancer, bilateral hydroureteronephrosis, and bilateral indwelling ureteral stents. (a) Axial contrast-enhanced CT urographic images obtained before (left) and after (right) adjusting the routine window and level settings in the image viewer show that such an adjustment allows detection of floating debris within the right renal pelvis (arrow). Using routine window and level settings may obscure pertinent findings because of the increased iodine attenuation at 70 kv. (b) Coronal volume-rendered CT urographic image shows bilateral hydroureteronephrosis with bilateral indwelling ureteral stents. Note the preservation of image quality at extremely low tube voltage of 70 kv. Disclosures of Conflicts of Interest. A.A. Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: educational grant from Siemens Healthcare. Other activities: disclosed no relevant relationships. A.R.S. Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: educational grant from Siemens Healthcare. Other activities: disclosed no relevant relationships. References 1. Brenner DJ, Hall EJ. Computed tomography: an increasing source of radiation exposure. N Engl J Med 2007;357(22): Schauer DA, Linton OW. National Council on Radiation Protection and Measurements report shows substantial medical exposure increase. Radiology 2009;253(2): Sodickson A, Baeyens PF, Andriole KP, et al. Recurrent CT, cumulative radiation exposure, and associated radiation-induced cancer risks from CT of adults. Radiology 2009;251(1): Mayer C, Meyer M, Fink C, et al. Potential for radiation dose savings in abdominal and chest CT using automatic tube voltage selection in combination with automatic tube current modulation. AJR Am J Roentgenol 2014;203(2): International Commission on Radiological Protection. Radiological protection in medicine: ICRP publication 105. Ann ICRP 2007;37(6): Guimarães LS, Fletcher JG, Harmsen WS, et al. Appropriate patient selection at abdominal dual-energy CT using 80 kv: relationship between patient size, image noise, and image quality. Radiology 2010;257(3): Kalva SP, Sahani DV, Hahn PF, Saini S. Using the k-edge to improve contrast conspicuity and to lower radiation dose with a 16-MDCT: a phantom and human study. J Comput Assist Tomogr 2006;30(3): Funama Y, Awai K, Nakayama Y, et al. Radiation dose reduction without degradation of low-contrast detectability at abdominal multisection CT with a low-tube voltage technique: phantom study. Radiology 2005;237(3): Nakayama Y, Awai K, Funama Y, et al. Abdominal CT with low tube voltage: preliminary observations about radiation dose, contrast enhancement, image quality, and noise. Radiology 2005;237(3): Nakayama Y, Awai K, Funama Y, et al. Lower tube voltage reduces contrast material and radiation doses on 16-MDCT aortography. AJR Am J Roentgenol 2006;187(5):W490 W Ende JF, Huda W, Ros PR, Litwiller AL. Image mottle in abdominal CT. Invest Radiol 1999;34(4): Huda W, Lieberman KA, Chang J, Roskopf ML. Patient size and x-ray technique factors in head computed tomography examinations. I. Radiation doses. Med Phys 2004;31(3): Raman SP, Johnson PT, Deshmukh S, Mahesh M, Grant KL, Fishman EK. CT dose reduction applications: available tools on the latest generation of CT scanners. J Am Coll Radiol 2013;10(1): Sigal-Cinqualbre AB, Hennequin R, Abada HT, Chen X, Paul JF. Low-kilovoltage multi detector row chest CT in adults: feasibility and effect on image quality and iodine dose. Radiology 2004;231(1): Royal HD. Effects of low level radiation: what s new? Semin Nucl Med 2008;38(5): Katz SI, Saluja S, Brink JA, Forman HP. Radiation dose associated with unenhanced CT for suspected renal colic: impact of repetitive studies. AJR Am J Roentgenol 2006;186(4): Jaffe TA, Gaca AM, Delaney S, et al. Radiation doses from small-bowel follow-through and abdominopelvic MDCT in Crohn s disease. AJR Am J Roentgenol 2007;189(5): Gonzalez-Guindalini FD, Ferreira Botelho MP, Töre HG, Ahn RW, Gordon LI, Yaghmai V. MDCT of chest, abdomen, and pelvis using attenuation-based automated tube voltage selection in combination with iterative reconstruction: an intrapatient study of radiation dose and image quality. AJR Am J Roentgenol 2013;201(5):

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