Review Article

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Ann Phlebology 2022; 20(2): 64-67

Published online December 31, 2022

https://doi.org/10.37923/phle.2022.20.2.64

© Annals of phlebology

Image Optimization in Venous Ultrasound Examination

Hyangkyoung Kim, M.D., Ph.D.1 and Nicos Labropoulos, Ph.D.2

1Department of Surgery, Kyung Hee University Hospital at Gangdong, Kyung Hee University School of Medicine, Seoul, Korea, 2Department of Surgery, Stony Brook University Medical Center, Stony Brook, NY, USA

Correspondence to : Hyangkyoung Kim, 892 Dongnam-ro, Gangdong-gu, Seoul 05278, Korea, Department of Surgery, Kyung Hee University Hospital at Gangdong, Kyung Hee University School of Medicine
Tel: 02-440-7370, Fax: 02-440-6296
E-mail: cindycrow7456@gmail.com

Ultrasound is a diagnostic tool of choice for venous disease. As treatment is based on ultrasound results, accurate examination is essential and image optimization is necessary to provide objective information. Understanding the physical properties and knobology of ultrasound helps to obtain good images. To begin with, comfortable placement of the ultrasound for the examiner to use and the appropriate positioning of the patient is also important. The next step for an optimal ultrasound image is to attain good resolution to a given depth. It is also essential to appropriately utilize the provocation maneuvers when diagnosing a suspected disease, especially for the chronic venous disease.

Keywords Ultrasound, Venous disease, Duplex, Doppler, Provocation

Ultrasound is used to diagnose many diseases due to its non-invasive nature, but its importance is much greater in the diagnosis of venous diseases. To provide objective information, knowledge of anatomical structure and venous physiology is mandatory. In addition, acquisition of best image quality by means of ultrasound machine manipulation is crucial. In-depth knowledge of the instrument functions and underlying physical properties is essential for optimal image adjustment and documentation (1). In this review, we purposed to review techniques to optimize ultrasound machine to acquire best image quality.

Image optimization should be done in following manner; 1. Examiner and patient’s position, 2. Room setting, 3. B mode setting, 4. Doppler setting, and 5. provocation maneuvers.

1) Examiner’s and patient’s position

The ultrasound machine should be set up on the opposite side of the patient from the operator with the screen at the operator’s eye level. The position of the monitor should allow for a near-perpendicular viewing angle for the examiner. The height of the patient table, ultrasound machine keyboard, and examiner’s chair is set so that the examiner can relax his shoulders and reduce fatigue. Therefore, to achieve this the patient table, ultrasound monitor and examiners chair should be adjustable. For venous diseases, patient’s position is more important to obtain clear best image while dynamic testing to avoid displacement of the probe from the lesion or motion artifact. When testing for reflux or chronic infrainguinal obstruction the patient should be in the standing position. It is acceptable for patient comfort to do the veins in the groin area and the posterior thigh in the standing position while the rest of the exam can be done in the sitting position.

2) Room setting

Adjust the lights in the room to view the ultrasound machine and examination site adequately, as well as for safe monitoring of the patient. Dim lighting optimizes visualiza-tion of the image on the screen. When the room is too bright, the contrast of the image on the screen is reduced, resulting in overgain (Fig. 1). The room should be spacious to allow for maneuvering and comfortable examination. The room temperature should be warm. Cold room can induce vessel constriction leading to inaccurate findings while in hot room the patient main faint while standing.

Fig. 1. B-mode image of common femoral artery and vein with different depth setting. (A) Depth is set too deep. (B) The target vein should be at the center of ultra-sound image.

3) B mode setting

(1) Transducer and acoustic gel

The ultrasound transducer with the optimal frequency range should be selected to best visualize the target veins. Ultrasound energy is absorbed gradually by the transmitted tissue; the higher the frequency of ultrasound, the more rapid the absorption and the lesser the penetration. The lower the frequency of ultrasound, the more the penetration and the lower image resolution. Highest frequency that allows adequate depth penetration is required for best image quality. For deep veins or obese patients, a low-frequency transducer is used to scan structures at a deeper location or very obese patients, in expense of reduced image resolution.

The orientation of the transducer needs to match with the medial-lateral orientation of the patient. Usually, there is a marker corresponding the marker on the screen. Instead, touching one edge of the transducer is also helpful the medial-lateral orientation on the patient corresponds to that on the screen.

Acoustic impedance is physical characteristics of tissue that defines how much opposition an ultrasound beam experiences as it goes through a tissue. Acoustic gel can replace air between the transducer and the patient’s skin because ultrasound waves have difficulty in traveling through the air due to very small acoustic impedance (2). Acoustic gel also can provide lubrication to the skin to aid the movement of the transducer on the skin. Enough gel is applied to either the transducer or the patient’s skin to allow for transmission of the ultrasound. Sterile gel or a copious amount of water is being used during the ultrasound guided venous procedures (3). Insufficient quality of gel will decrease reflection/absorption rates and may result in unclear images.

(2) Depth, focus and zoom

The depth of the vein is the first consideration for ultrasound scanning. Lower extremity veins have a great variation of depth, which depends on patients’ body habitus; the optimal depth setting can provide good focusing properties for imaging. The target vein should be at the center of ultrasound image to allow not only for better resolution but also reveals other anatomical structures in the vicinity (Fig. 2). The depth is reduced as much as possible to increase frame rate having also the largest dimension of structures. When the width of the image is reduced, it results in increase image-resolution as well. Lateral resolution can be improved by focusing the ultrasound beam. Focal zone is adjustable in depth and number. In clinical practice, the focus is adjusted at the level of the target vein; the best image quality for a given vein is obtained by choosing an appropriate frequency transducer and the focal zone (4). When possible, selecting no more than two focus zones yields better image because multiple focal zones can slow the frame rate and decrease the temporal resolution. It is also possible to zoom in on regions of interest. Zooming it improves the resolution in a particular area. Focusing adjusts the location with the best resolution while zooming encloses a specific region of the image.

Fig. 2. Brightness of the room and the ultrasound image. When the room is too bright, the contrast of the image on the screen is reduced (A), resulting in overgain (B).

(3) Gain

In B-mode, brightness can be adjusted manually by two function buttons—gain and time-gain compensation (TGC). Gain regulates the overall gain for all reflected sound waves, while TGC adjusts the gain at specific levels of the ultrasound field to compensate for the attenuation that occurs with increasing depth. Gain function amplifies the reflected ultrasound waves such that each object appears whiter on the image. Screen excessive or inadequate gain can cause blurring of tissue boundaries and loss of information. Optimal gain for scanning peripheral veins is typically the gain at which the best contrast is obtained between the vein and the adjacent connective tissue. In normal vein, lumen should appear echolucent. In addition, increasing gain below the focus works well with the TGC control to visualize both the target vein and the structures below it. TGC sliders aligned in a curve can lead to a desirable image with appropriate gain.

(4) Imaging modes

Common sonographic imaging modes used for medical diagnostics, such as, conventional imaging, compound imaging, and tissue harmonic imaging (THI) can all be utilized in imaging of venous examination. Conventional imaging is generated from a single-element angle beam at a primary frequency designated by the transducer. Compound imaging is implemented by acquiring several (usually three to nine) overlapping frames from different frequencies or from different angles. In compound imaging, spatial compounding combines multiple frames from different angles and frequency compounding combines two images with different frequencies decreasing the speckle. Harmonic frequencies are multiples of the primary frequency, and different tissues will have varying degrees of harmonic generation. THI acquires the information from harmonic frequencies generated by ultrasound beam trans-mission through tissue. THI improves axial resolution and boundary detection by suppression of scattering signals from tissue interfaces, especially for obese patients. Currently, THI has been set as the default mode by many, if not most, US manufacturers. Compound imaging with THI can provide images with better resolution, penetration, and interfaces and margin enhancement compared with conventional sonography.

4) Doppler mode setting

Doppler ultrasound is used to differentiate vessel from other tissue, confirm vessel patency or detect flow related information. In venous reflux test, flow direction is most important information. In color Doppler mode, color gain should be set appropriately. If the gain is set too high, non-vascular structure may appear to be filled with color or produce background noise. To optimize the image, the color gain setting should be decreased until color noise is eliminated. Small color box increases frame rate and therefore improves the temporal resolution.

Doppler velocity scale is set to reduce aliasing of color Doppler imaging and artifacts of color (5). When flow is not identified, it is important to be able to differentiate the true absence of vascular flow from suboptimal image optimization. When the sample volume gate is set small, flow information in the specified small area will be given. An appropriate size of gate not only can exclude distractive signals from adjacent tissues but also can improve temporal resolution by increasing the frame rate. Typically, in vein imaging a lower pulse repetition frequency at 1500 Hz or less is being used. The color box and/or the pulsed wave Doppler should be placed parallel to the channel of flow for obtaining optimal color filling and Doppler signals. The baseline and Doppler scale should be adjusted to contain waveform. In Doppler mode, proper gain setting is important to detect accurate flow information. Doppler gain should be adjusted so that there is no color aliasing in the normal vein in color mode and the background of the waveform is black in the pulse wave mode (Fig. 3). Excessive gain results in false positive test by motion artifacts, while too low gain results in false negative.

Fig. 3. Pulse wave mode image of common femoral vein. (A) Through an appropriate Doppler gain setting, background of the waveform should be black. (B) In case of overgain, noise is observed on the waveform background.

5) Provocation maneuver

Provocation maneuvers are often required for the diagnosis of venous diseases, such as thoracic outlet syndrome, popliteal entrapment and testing for venous reflux (6). Provocation to induce reflux is required when testing patients with chronic venous disease or postthrom-botic syndrome (7). There are several methods for inducing reflux in the lower extremity venous examination: the Valsalva maneuver, distal compression and active dorsi/ plantar flexion of the foot. The Valsalva maneuver is recommended only for the inguinal area. The examiner should carefully instruct the patient of how to perform the Valsalva maneuver to obtain the best results. Manual compression with sudden release distal to the examining site or automated rapid inflation/deflation pneumatic cuffs can be used for most of cases to test valve competency; compression produces flow augmentation then sudden release of the compression allows reflux when the valve is incompetent. In patients with venous ulcer of severe edema, the veins in the lower calf and ankle are best tested by applying pressure in the foot or better with active dorsi/plantar flexion.

In order to obtain optimal ultrasound images, it is necessary to be familiar with ultrasound physical principles, machine properties and adjust several parameters continu-ously during the examination. User adjustment of ultrasound parameters including gain, depth, focus and image modes will give the optimal image quality.

The authors declare no conflicts of interest.

  1. Zander D, Hüske S, Hoffmann B, Cui XW, Dong Y, Lim A, et al. Ultrasound Image Optimization ("Knobology"): B-Mode. Ultrasound Int Open. 2020;6:E14-E24.
  2. Afzal S, Zahid M, Rehan ZA, Shakir HMF, Javed H, Aljohani MMH, et al. Preparation and evaluation of polymer-based ultrasound gel and its application in ultrasonography. Gels. 2022;6(8):42.
  3. Douglas BR, Charboneau JW, Reading CC. Ultrasound- guided intervention: expanding horizons. Radiol Clin North Am. 2001;39:415-28.
  4. Gunabushanam G, Scoutt LM. Ultrasound Image Optimi-zation for the Interventional Radiologist. Techniques in Vascular and Interventional Radiology. 2021;24:100766.
  5. Löwe A, Jenssen C, Hüske S, Zander D, Ignee A, Lim A, et al. "Knobology". in Doppler Ultrasound. 2021;23:7.
  6. Hu J, Biederman R, Kashyap K, Wilson JT, Farah V, Franco T, et al. Duplex ultrasound in the evaluation of venous and arterial thoracic outlet syndrome. JRSM Open. 2021;12:2054270420983101.
  7. Mendoza E, Lattimer CR. Flow Provocation Manoeuvres for the Diagnosis of Venous Disease Using Duplex Ultrasound. In: Mendoza E, Lattimer CR, Morrison N, editors. Duplex Ultrasound of Superficial Leg Veins. . Berlin, Heidelberg: Springer Berlin Heidelberg; 2014. p. 105-18.

Review Article

Ann Phlebology 2022; 20(2): 64-67

Published online December 31, 2022 https://doi.org/10.37923/phle.2022.20.2.64

Copyright © Annals of phlebology.

Image Optimization in Venous Ultrasound Examination

Hyangkyoung Kim, M.D., Ph.D.1 and Nicos Labropoulos, Ph.D.2

1Department of Surgery, Kyung Hee University Hospital at Gangdong, Kyung Hee University School of Medicine, Seoul, Korea, 2Department of Surgery, Stony Brook University Medical Center, Stony Brook, NY, USA

Correspondence to:Hyangkyoung Kim, 892 Dongnam-ro, Gangdong-gu, Seoul 05278, Korea, Department of Surgery, Kyung Hee University Hospital at Gangdong, Kyung Hee University School of Medicine
Tel: 02-440-7370, Fax: 02-440-6296
E-mail: cindycrow7456@gmail.com

Abstract

Ultrasound is a diagnostic tool of choice for venous disease. As treatment is based on ultrasound results, accurate examination is essential and image optimization is necessary to provide objective information. Understanding the physical properties and knobology of ultrasound helps to obtain good images. To begin with, comfortable placement of the ultrasound for the examiner to use and the appropriate positioning of the patient is also important. The next step for an optimal ultrasound image is to attain good resolution to a given depth. It is also essential to appropriately utilize the provocation maneuvers when diagnosing a suspected disease, especially for the chronic venous disease.

Keywords: Ultrasound, Venous disease, Duplex, Doppler, Provocation

INTRODUCTION

Ultrasound is used to diagnose many diseases due to its non-invasive nature, but its importance is much greater in the diagnosis of venous diseases. To provide objective information, knowledge of anatomical structure and venous physiology is mandatory. In addition, acquisition of best image quality by means of ultrasound machine manipulation is crucial. In-depth knowledge of the instrument functions and underlying physical properties is essential for optimal image adjustment and documentation (1). In this review, we purposed to review techniques to optimize ultrasound machine to acquire best image quality.

IMAGE OPTIMIZATION

Image optimization should be done in following manner; 1. Examiner and patient’s position, 2. Room setting, 3. B mode setting, 4. Doppler setting, and 5. provocation maneuvers.

1) Examiner’s and patient’s position

The ultrasound machine should be set up on the opposite side of the patient from the operator with the screen at the operator’s eye level. The position of the monitor should allow for a near-perpendicular viewing angle for the examiner. The height of the patient table, ultrasound machine keyboard, and examiner’s chair is set so that the examiner can relax his shoulders and reduce fatigue. Therefore, to achieve this the patient table, ultrasound monitor and examiners chair should be adjustable. For venous diseases, patient’s position is more important to obtain clear best image while dynamic testing to avoid displacement of the probe from the lesion or motion artifact. When testing for reflux or chronic infrainguinal obstruction the patient should be in the standing position. It is acceptable for patient comfort to do the veins in the groin area and the posterior thigh in the standing position while the rest of the exam can be done in the sitting position.

2) Room setting

Adjust the lights in the room to view the ultrasound machine and examination site adequately, as well as for safe monitoring of the patient. Dim lighting optimizes visualiza-tion of the image on the screen. When the room is too bright, the contrast of the image on the screen is reduced, resulting in overgain (Fig. 1). The room should be spacious to allow for maneuvering and comfortable examination. The room temperature should be warm. Cold room can induce vessel constriction leading to inaccurate findings while in hot room the patient main faint while standing.

Figure 1. B-mode image of common femoral artery and vein with different depth setting. (A) Depth is set too deep. (B) The target vein should be at the center of ultra-sound image.

3) B mode setting

(1) Transducer and acoustic gel

The ultrasound transducer with the optimal frequency range should be selected to best visualize the target veins. Ultrasound energy is absorbed gradually by the transmitted tissue; the higher the frequency of ultrasound, the more rapid the absorption and the lesser the penetration. The lower the frequency of ultrasound, the more the penetration and the lower image resolution. Highest frequency that allows adequate depth penetration is required for best image quality. For deep veins or obese patients, a low-frequency transducer is used to scan structures at a deeper location or very obese patients, in expense of reduced image resolution.

The orientation of the transducer needs to match with the medial-lateral orientation of the patient. Usually, there is a marker corresponding the marker on the screen. Instead, touching one edge of the transducer is also helpful the medial-lateral orientation on the patient corresponds to that on the screen.

Acoustic impedance is physical characteristics of tissue that defines how much opposition an ultrasound beam experiences as it goes through a tissue. Acoustic gel can replace air between the transducer and the patient’s skin because ultrasound waves have difficulty in traveling through the air due to very small acoustic impedance (2). Acoustic gel also can provide lubrication to the skin to aid the movement of the transducer on the skin. Enough gel is applied to either the transducer or the patient’s skin to allow for transmission of the ultrasound. Sterile gel or a copious amount of water is being used during the ultrasound guided venous procedures (3). Insufficient quality of gel will decrease reflection/absorption rates and may result in unclear images.

(2) Depth, focus and zoom

The depth of the vein is the first consideration for ultrasound scanning. Lower extremity veins have a great variation of depth, which depends on patients’ body habitus; the optimal depth setting can provide good focusing properties for imaging. The target vein should be at the center of ultrasound image to allow not only for better resolution but also reveals other anatomical structures in the vicinity (Fig. 2). The depth is reduced as much as possible to increase frame rate having also the largest dimension of structures. When the width of the image is reduced, it results in increase image-resolution as well. Lateral resolution can be improved by focusing the ultrasound beam. Focal zone is adjustable in depth and number. In clinical practice, the focus is adjusted at the level of the target vein; the best image quality for a given vein is obtained by choosing an appropriate frequency transducer and the focal zone (4). When possible, selecting no more than two focus zones yields better image because multiple focal zones can slow the frame rate and decrease the temporal resolution. It is also possible to zoom in on regions of interest. Zooming it improves the resolution in a particular area. Focusing adjusts the location with the best resolution while zooming encloses a specific region of the image.

Figure 2. Brightness of the room and the ultrasound image. When the room is too bright, the contrast of the image on the screen is reduced (A), resulting in overgain (B).

(3) Gain

In B-mode, brightness can be adjusted manually by two function buttons—gain and time-gain compensation (TGC). Gain regulates the overall gain for all reflected sound waves, while TGC adjusts the gain at specific levels of the ultrasound field to compensate for the attenuation that occurs with increasing depth. Gain function amplifies the reflected ultrasound waves such that each object appears whiter on the image. Screen excessive or inadequate gain can cause blurring of tissue boundaries and loss of information. Optimal gain for scanning peripheral veins is typically the gain at which the best contrast is obtained between the vein and the adjacent connective tissue. In normal vein, lumen should appear echolucent. In addition, increasing gain below the focus works well with the TGC control to visualize both the target vein and the structures below it. TGC sliders aligned in a curve can lead to a desirable image with appropriate gain.

(4) Imaging modes

Common sonographic imaging modes used for medical diagnostics, such as, conventional imaging, compound imaging, and tissue harmonic imaging (THI) can all be utilized in imaging of venous examination. Conventional imaging is generated from a single-element angle beam at a primary frequency designated by the transducer. Compound imaging is implemented by acquiring several (usually three to nine) overlapping frames from different frequencies or from different angles. In compound imaging, spatial compounding combines multiple frames from different angles and frequency compounding combines two images with different frequencies decreasing the speckle. Harmonic frequencies are multiples of the primary frequency, and different tissues will have varying degrees of harmonic generation. THI acquires the information from harmonic frequencies generated by ultrasound beam trans-mission through tissue. THI improves axial resolution and boundary detection by suppression of scattering signals from tissue interfaces, especially for obese patients. Currently, THI has been set as the default mode by many, if not most, US manufacturers. Compound imaging with THI can provide images with better resolution, penetration, and interfaces and margin enhancement compared with conventional sonography.

4) Doppler mode setting

Doppler ultrasound is used to differentiate vessel from other tissue, confirm vessel patency or detect flow related information. In venous reflux test, flow direction is most important information. In color Doppler mode, color gain should be set appropriately. If the gain is set too high, non-vascular structure may appear to be filled with color or produce background noise. To optimize the image, the color gain setting should be decreased until color noise is eliminated. Small color box increases frame rate and therefore improves the temporal resolution.

Doppler velocity scale is set to reduce aliasing of color Doppler imaging and artifacts of color (5). When flow is not identified, it is important to be able to differentiate the true absence of vascular flow from suboptimal image optimization. When the sample volume gate is set small, flow information in the specified small area will be given. An appropriate size of gate not only can exclude distractive signals from adjacent tissues but also can improve temporal resolution by increasing the frame rate. Typically, in vein imaging a lower pulse repetition frequency at 1500 Hz or less is being used. The color box and/or the pulsed wave Doppler should be placed parallel to the channel of flow for obtaining optimal color filling and Doppler signals. The baseline and Doppler scale should be adjusted to contain waveform. In Doppler mode, proper gain setting is important to detect accurate flow information. Doppler gain should be adjusted so that there is no color aliasing in the normal vein in color mode and the background of the waveform is black in the pulse wave mode (Fig. 3). Excessive gain results in false positive test by motion artifacts, while too low gain results in false negative.

Figure 3. Pulse wave mode image of common femoral vein. (A) Through an appropriate Doppler gain setting, background of the waveform should be black. (B) In case of overgain, noise is observed on the waveform background.

5) Provocation maneuver

Provocation maneuvers are often required for the diagnosis of venous diseases, such as thoracic outlet syndrome, popliteal entrapment and testing for venous reflux (6). Provocation to induce reflux is required when testing patients with chronic venous disease or postthrom-botic syndrome (7). There are several methods for inducing reflux in the lower extremity venous examination: the Valsalva maneuver, distal compression and active dorsi/ plantar flexion of the foot. The Valsalva maneuver is recommended only for the inguinal area. The examiner should carefully instruct the patient of how to perform the Valsalva maneuver to obtain the best results. Manual compression with sudden release distal to the examining site or automated rapid inflation/deflation pneumatic cuffs can be used for most of cases to test valve competency; compression produces flow augmentation then sudden release of the compression allows reflux when the valve is incompetent. In patients with venous ulcer of severe edema, the veins in the lower calf and ankle are best tested by applying pressure in the foot or better with active dorsi/plantar flexion.

CONCLUSION

In order to obtain optimal ultrasound images, it is necessary to be familiar with ultrasound physical principles, machine properties and adjust several parameters continu-ously during the examination. User adjustment of ultrasound parameters including gain, depth, focus and image modes will give the optimal image quality.

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.

Fig 1.

Figure 1.B-mode image of common femoral artery and vein with different depth setting. (A) Depth is set too deep. (B) The target vein should be at the center of ultra-sound image.
Annals of Phlebology 2022; 20: 64-67https://doi.org/10.37923/phle.2022.20.2.64

Fig 2.

Figure 2.Brightness of the room and the ultrasound image. When the room is too bright, the contrast of the image on the screen is reduced (A), resulting in overgain (B).
Annals of Phlebology 2022; 20: 64-67https://doi.org/10.37923/phle.2022.20.2.64

Fig 3.

Figure 3.Pulse wave mode image of common femoral vein. (A) Through an appropriate Doppler gain setting, background of the waveform should be black. (B) In case of overgain, noise is observed on the waveform background.
Annals of Phlebology 2022; 20: 64-67https://doi.org/10.37923/phle.2022.20.2.64

References

  1. Zander D, Hüske S, Hoffmann B, Cui XW, Dong Y, Lim A, et al. Ultrasound Image Optimization ("Knobology"): B-Mode. Ultrasound Int Open. 2020;6:E14-E24.
  2. Afzal S, Zahid M, Rehan ZA, Shakir HMF, Javed H, Aljohani MMH, et al. Preparation and evaluation of polymer-based ultrasound gel and its application in ultrasonography. Gels. 2022;6(8):42.
  3. Douglas BR, Charboneau JW, Reading CC. Ultrasound- guided intervention: expanding horizons. Radiol Clin North Am. 2001;39:415-28.
  4. Gunabushanam G, Scoutt LM. Ultrasound Image Optimi-zation for the Interventional Radiologist. Techniques in Vascular and Interventional Radiology. 2021;24:100766.
  5. Löwe A, Jenssen C, Hüske S, Zander D, Ignee A, Lim A, et al. "Knobology". in Doppler Ultrasound. 2021;23:7.
  6. Hu J, Biederman R, Kashyap K, Wilson JT, Farah V, Franco T, et al. Duplex ultrasound in the evaluation of venous and arterial thoracic outlet syndrome. JRSM Open. 2021;12:2054270420983101.
  7. Mendoza E, Lattimer CR. Flow Provocation Manoeuvres for the Diagnosis of Venous Disease Using Duplex Ultrasound. In: Mendoza E, Lattimer CR, Morrison N, editors. Duplex Ultrasound of Superficial Leg Veins. . Berlin, Heidelberg: Springer Berlin Heidelberg; 2014. p. 105-18.
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