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Sampling Renal Flow - USA

This course includes information on the normal renal Doppler exam. 
Successful completion of this training is eligible for American Society of Radiology Technician (ASRT) Category A continuing education units (CEU).

Multiple studies show a global increase in the incidence of high blood pressure or hypertension (HTN).1, 2  The World Health Organization (WHO) reports show the extent of the problem with over half a billion individuals with some degree of HTN.2 Though not the only cause of renal failure, HTN and the resulting vessel damage is a controllable process. If the renal artery becomes damaged or in the presence of disease, the clinician cannot control the hypertension which leads to renal failure. Upon completion of this course, you will be able to:   Compare direct and indirect evaluation of renal flow Predict spectral Doppler changes seen with renovascular narrowing Use spectral Doppler velocity measurements to evaluate renal flow Doppler sampling of flow in the extra- and intra-renal vascular systems, provides a method of diagnosing and monitoring vessel narrowing. Before beginning this course, review 2D-mode anatomy, renal measurements, and normal color and spectral Doppler findings in the urinary system. Download and print a copy of the Kidneys, Our Bodies Trash Collectors course review. Vessel Narrowing of the Renal Artery Learn more about vessel narrowing of the renal artery. Tab TitleTextBlood Pressure3Blood pressure indicates how hard the heart needs to contract to move blood through our vessels. In the normal vessel, there is minimal resistance to flow and our blood pressure falls within normal ranges. Expressed as the systolic pressure over the diastolic pressure, the values show the contraction of the heart and relaxation of the heart.   Blood pressure (BP) describes the force of flow against the vessel wall. This diagram shows the result of BP changes due to vessel diameter changes.  The top diagram shows a normal diameter vessel and flow.  The middle has a normal diameter vessel, however, the increased flow results in an increase in BP. On the bottom, the vessel narrows, further increasing BP.  FMD4 ​Medial fibromuscular dysplasia is a series of abnormal growths along the renal artery. As the second most common cause for RAS, lesions form on the mid-to-distal section of the renal artery.  Imaging studies (i.e., angiography, ultrasound) show segmental concentric dilatation and narrowing of the vessel.    This diagram shows the series of abnormal growths in the mid-to-distal renal artery. Atherosclerosis4 Atherosclerosis is a fatty deposit within a vessel and is the most common cause of high blood pressure. HTN often results in damage to the vessel endothelium. This continued damage slowly increases wall thickness, and thus, decreasing the vessel lumen.   Atherosclerosis progression leads to increasing blockage of the renal artery. When the vessels become occluded the echogenicity and size of the kidney changes. In the acute obstruction, such as in the case of an emboli, the kidney may appear normal in size. With a prolonged (chronic) obstruction, the kidney becomes damaged, begins to decrease in size, and may develop collateral flow.     Chronic renal artery obstruction results in atrophy of the affected kidney. Indications for the renal exam include the following:1, 2, 4 Hypertension (HTN) Female Age Audible abdominal bruit Aortic dissection Microscopic or gross hematuria (blood in the urine) White blood cells or protein with a urinalysis Trauma Abnormal BUN (Blood, Urea, Nitrogen) Flank or back pain History of renal disease Urinary tract infection (UTI) Decreased urine output due to renal insufficiency Follow up of stent placement Direct sampling of the renal vasculature is a challenge due to multiple factors, such as body habitus, the presence of disease, and location in the body. To aid in imaging, we often have the patient fast up to 10-hours before the exam. This may help decrease bowel gas, however, in the patient with diabetes, a light meal of toast and clear liquids prevents hypoglycemia.4, 5 Abstaining from smoking or chewing gum also helps reduce swallowed air that hinders the exam.4 If the renal artery cannot be imaged, obtaining segmental waveforms may provide a diagnosis.6   Changing the patient position helps place the renal vessels in the optimal area of the sound beam to obtain both 2D-mode and Doppler images. The right kidney has the overlying liver to provide a window; however, rolling the patient onto their left side, may move bowel gas and fatty tissue.7   Coronal imaging often brings the kidney closer to the transducer, provides improved Doppler angles, and allows use of the kidney as a window for sampling of the main renal artery.  A spectral tracing obtained of the renal artery has a characteristic appearance. There is a rapid upstroke in systole, an early compliance peak with a gradual decrease to end diastole.4 Subjective evaluation of the waveform shape prevents determination of flow changes seen with vessel narrowing. Multiple methods to quantify the spectral tracing help the clinician determine the extent of disease. These include measuring the peak systolic and end diastolic velocities, calculating the resistive index, acceleration time, and renal artery to aortic ratio. Learn More About Measuring Waveforms Learn more about measuring waveforms. Tab TitleTextWaveform This shows a line diagram of a renal Doppler waveform. The rapid upslope (bracket) occurs with systole. A sharp systolic peak (ESP) or compliance peak (arrow) occurs before the gradual decrease to diastole (arrowhead).4, 5 Normal antegrade flow remains above the baseline throughout the cardiac cycle.5RIThe resistive index (RI) uses the peak systolic velocity and end diastolic velocity of flow providing a numeric value for flow.5, 8 This waveform diagram demonstrates the areas to measure for calculating the RI. The peak systolic velocity (PSV) is the highest upstroke (blue arrowhead) of the waveform. End diastolic velocity (EDV) measurements use the lowest portion of the waveform (yellow arrowhead).RI Formula PSV –EDV      PSV PSV = Peak systolic velocity EDV = End diastolic velocity RI = Resistive Index As vascular obstruction increases within the vessel, resistance to flow also increases. The RI measures the resistance (impedance) to flow and is a unitless number.9 Multiple studies confirm that a normal value is less than 0.70, with a value above 0.80 indicating disease.4, 5  Care must be taken when using the RI with renal stenosis as the value increases with age due to vessel stiffening.5 As blood pressure values increase, RI value also increases. Both processes increase the chance of a false positive renal stenosis finding. Peak and End VelocitiesThe PSV increases with increasing vascular disease allowing the use of velocity measurements to determine severity. Stenosis of greater than 60 percent raises the renal artery flow velocities to 180 to 200 cm / sec for the peak systolic measurement.4-7      This spectral tracing shows the increased velocity resulting from a renal artery stenosis. Measurements (left, enlarged) show a PSV (PS; right arrow) compatible with a stenosis of greater than 60 percent. ED (down arrow) – End Diastolic Two methods of measuring flow velocity in renal vessels, direct and indirect, provide necessary information to identify vascular stenosis. Direct testing is the sampling of the renal artery between the aorta and each kidney. Indirect testing, uses spectral Doppler samples from the intrarenal vessels (i.e., arcuate, segmental) using an acceleration index (AI).4    Always sample the renal venous system to document the presence or absence of flow. Learn More about Direct Testing Learn more about direct testing. Tab TitleTextRenal / Aortic Velocity Ratio The ability to measure velocities in vessels allows us to calculate a ratio between the aorta and main renal artery. This ratio, the RAR, uses a unitless value to help determine the amount of stenosis. Classification of a value less than 3.5 indicates minimal disease while a higher ratio suggests renal artery stenosis greater than 60 percent.4, 6, 7 This assessment of determining stenosis is referred to as a direct method of evaluation with the sample velocity occurring in the suprarenal aorta. Use this formula to calculate a ratio. RAR = Renal Artery Velocity / Aortic Velocity6Aortic Velocity   This image shows flow sampling within the proximal aorta at the level of the renal artery origin. Peak systolic velocity (asterisk) is approximately 125 cm / sec. This spectral tracing demonstrates a high-resistance waveform.RA Velocity This spectral tracing shows a low-resistive waveform from the mid, right renal artery. When measuring the peak systole (asterisk) use a waveform with the sharpest profile. The velocity for this renal artery is approximately 90 cm / sec. Acceleration Time Another measurement used in both direct and indirect testing is the acceleration time. To measure acceleration time, start at the first peak which is beginning of systole (left line). The second measurement aligns with the ESP (right line). A normal finding is less than 0.07 seconds or 70 milliseconds.6, 7Spectral Tracing Example In this image, sampling of the right renal artery, we see a normal waveform. To measure acceleration time, place the first cursor at the first upstroke of systole (left bar) with the second at peak systole (right bar). The time displays in the upper left of the image as 58 milliseconds (ms). Calculating the seconds is as simple as dividing ms by 1000 for a value of 0.058 seconds. Learn More About Indirect Testing Learn more about indirect testing. Tab TitleTextWaveform Changes A normal renal arterial flow displays in a characteristic flow pattern called a tardus-parvus waveform. Though not unique to the kidney, the term describes the waveform that shows a delayed systolic peak (tardus) with a reduction in amplitude (parvus).9, 10 Multiple research studies show changes in the renal artery waveform in the presence of disease when sampling intrarenal vessels. Observation of the renal artery waveform provides a subjective method to evaluate the spectral tracing. The normal renal waveform has a steep, quick upslope with an early systolic peak (left). The middle waveform shows a delayed acceleration time and a decrease in waveform height resulting in the tardus-parvus waveform. The right example, has no ESP and a further reduction of velocity.7, 11Caution!   Accessory arteries, collateralization, and distance from the main renal artery stenosis changes the appearance of the tardus-parvus waveform. Use color Doppler to help identify all vessels entering the kidney and ensure appropriate waveform acquisition.11 Peak and End Velocities This image of a normal segmental artery demonstrates measurement of the peak systolic (1) and end diastolic (2) measurements. Values display on the left side of the image (box). This spectral tracing demonstrates a low-resistance flow pattern. Your Turn Your turn. Instructions:Flash File:HTML5 File:/content/generator/Course_90022087/Sampling_renal_flow-interaction-V3/index.htmlPDF File: Explore the links below for the Glossary, References, and Further Reading opportunities. Glossary Glossary. Acceleration time – The amount of time to go from baseline to peak systole.   Angiography – Radiographic examination of vessels using a radiopaque material.   Antegrade – Blood flowing in a forward direction.   Atrophy – Degeneration and the resulting size decrease seen with degeneration of an organ.   Baseline – The point of zero flow when using Doppler imaging.   Blood pressure – The amount of pressure in the vascular system that includes the heart rate, force of the heart contractions, the diameter and elasticity of the arterial system.   Bruit – Abnormal sound heard through a stethoscope.   Collateral flow – Blood supply of an organ via minor vessels due to disease in the major artery.   Coronal – Plane dividing the body into anterior and posterior segments.   Dilatation - Enlargement   Direct evaluation – Sampling of arteries and vessels supplying an organ i.e., renal artery and vein.   Echogenicity – Term used to describe the brightness of tissues.   Emboli – Thrombus (blood clot) moving within the circulatory system.   Extrarenal – Outside of the kidney.  Indirect evaluation - Sampling of arteries and vessels within an organ i.e., renal arcuate or interlobular vessels.   Intrarenal – Within the kidney.   Proximal – At the origin i.e., the proximal renal arteries originate at the lateral aorta.   Renal insufficiency – Decreased function of the kidneys due to a reduction in blood flow.   Stenosis – Narrowing of a vessel.   Stent – Tubular support placed within a vessel to aid in flow decreased by disease i.e., arteriosclerosis.   References / Further Reading References / Further reading. 1. Chen, J. (2010). Epidemiology of hypertension and chronic kidney disease in China. Current opinion in nephrology and hypertension. 19(3): 278-282.   2. Luyckx, V.A., Tonelli, M., and Stanifer, J.W. (2018). The global burden of kidney disease and the sustainable development goals. Bull World Health Organ. 96: 414-422C.   3. Campese, V.M., Weir, M., and Oritz, E. (2014). High blood pressure & kidney disease: what is high blood pressure? 2014 [cited 2019 May 30]; Available from: https://www.niddk.nih.gov/health-information/kidney-disease/high-blood-pressure.   4. Neumyer, M.M. (2018). The renal vasculature. 2 ed. Diagnostic medical sonography: The vascular system, ed. Kapinski, A.M., Philadelphia: Wolters Kluwer. 335-351.   5. Baker, S.M. and Walker, D.C. (2017). The kidneys. In Kawamura, D. and Nolan, T., (Eds.), Diagnostic medical sonography: Abdomen and superficial structures (pp. 271-334). Philadelphia: Wolters Kluwer. 6. Weber, T.M., Robbin, M.L., and Lockhart, M.E. (2013). The kidneys. In Pozniak, M.A. and Allan, P.L., (Eds.), Clinical Doppler ultrasound (pp. 193-213). Edinburgh: Churchill Livingstone Elsevier.   7. Lee, H. and Grant, E.G. (2002). Sonography in renovascular hypertension. Journal of Ultrasound in Medicine. 21(4): 431-441.   8. Li, J.-c., Yuan, Y., Qin, W., Wang, L., Dai, Q., Qi, Z.-h., . . . Jiang, Y.-x. (2007). Evaluation of the tardus-parvus pattern in patients with atherosclerotic and nonatherosclerotic renal artery stenosis. Journal of Ultrasound in Medicine. 26(4): 419-426.   9. Granata, A., Fiorini, F., Andrulli, S., Logias, F., Gallieni, M., Romano, G., . . . Fiore, C.E. (2009). Doppler ultrasound and renal artery stenosis: An overview. Journal of ultrasound. 12(4): 133-143.   10. Stafford, W.L., Stevens, S.D., Krohmer, S., and DiSantis, D. (2016). ‘Tardus-Parvus’ waveform. Abdominal Radiology. 41(2): 344-346.   11. Stavros, A.T., Parker, S.H., Yakes, W.F., Chantelois, A.E., Burke, B.J., Meyers, P.R., and Schenck, J.J. (1992). Segmental stenosis of the renal artery: pattern recognition of tardus and parvus abnormalities with duplex sonography. Radiology. 184(2): 487-492.   Congratulations! You have completed the Sampling Renal Flow course. Listed below are the key points presented in this course. Take time to review the material before you try the final assessment.   Download and print a copy of the detailed Course Review In this course you have learned to: Compare direct and indirect evaluation of renal flow Predict spectral Doppler changes seen with renovascular narrowing Use spectral Doppler velocity measurements to evaluate renal flow The reproduction, transmission or distribution of this training or its contents is not permitted without express written authority. Offenders will be liable for damages.   All names and data of patients, parameters and configuration dependent designations are fictional and examples only. All rights, including rights created by patent grant or registration of a utility model or design, are reserved.   Please note that the learning material is for training purposes only!   For the proper use of the software or hardware, please always use the Operator Manual or Instructions for Use (hereinafter collectively “Operator Manual”) issued by Siemens Healthineers. This material is to be used as training material only and shall by no means substitute the Operator Manual. Any material used in this training will not be updated on a regular basis and does not necessarily reflect the latest version of the software and hardware available at the time of the training.   The Operator Manual shall be used as your main reference, in particular for relevant safety information like warnings and cautions. Note: Some functions shown in this material are optional and might not be part of your system. The information in this material contains general technical descriptions of specifications and options as well as standard and optional features that do not always have to be present in individual cases.   Certain products, product related claims or functionalities described in the material (hereinafter collectively “Functionality”) may not (yet) be commercially available in your country. Due to regulatory requirements, the future availability of said Functionalities in any specific country is not guaranteed. Please contact your local Siemens Healthineers sales representative for the most current information.   Copyright © Siemens Healthcare GmbH, 2019

  • ASRT
  • CME
  • CEU
  • Renal
  • Doppler
  • renal Doppler
  • color
  • renal artery
  • direct testing
  • indirect testing
  • RAS
  • renal artery stenosis
  • RI
  • resistive index
  • AT
  • acceleration time
  • PSV
  • EDV
  • peak systolic velocity
  • end diastolic velocity
  • RAR
  • renal artery ratio
  • tardus-parvus
  • tardus parvus